scherer and arroyo

7/27/2019 Scherer and Arroyo

1/12

Journal of the Peripheral Nervous System 7:112 (2002)

2002 Peripheral Nerve Society, Inc. 1

BUNGE MEMORIAL LECTURE

Recent progress on the molecular organization

of myelinated axons

Steven S. Scherer and Edgardo J. Arroyo

Department of Neurology, Room 460 Stemmler Hall, 36th Street and Hamilton Walk, The University of Pennsylvania

Medical Center, Philadelphia

Abstract

The structure of myelinated axons was well described 100 years ago by Ramn

y Cajal, and now their molecular organization is being revealed. The basal lamina of myeli-

nating Schwann cells contains laminin-2, and their abaxonal/outer membrane contains two

laminin-2 receptors,

6

4 integrin and dystroglycan. Dystroglycan binds utrophin, a short

dystrophin isoform (Dp116), and dystroglycan-related protein 2 (DRP2), all of which are part

of a macromolecular complex. Utrophin is linked to the actin cytoskeleton, and DRP2 bindsto periaxin, a PDZ domain protein associated with the cell membrane. Non-compact my-

elinfound at incisures and paranodescontains adherens junctions, tight junctions, and

gap junctions. Nodal microvilli contain F-actin, ERM proteins, and cell adhesion molecules

that may govern the clustering of voltage-gated Na

channels in the nodal axolemma.

Na

v

1.6 is the predominant voltage-gated Na

channel in mature nerves, and is linked to the

spectrin cytoskeleton by ankyrin

G

. The paranodal glial loops contain neurofascin 155, which

likely interacts with heterodimers composed of contactin and Caspr/paranodin to form sep-

tate-like junctions. The juxtaparanodal axonal membrane contains the potassium channels

Kv1.1 and Kv1.2, their associated

2 subunit, as well as Caspr2. Kv1.1, Kv1.2, and Caspr2

all have PDZ binding sites and likely interact with the same PDZ binding protein. Like Caspr,

Caspr2 has a band 4.1 binding domain, and both Caspr and Caspr2 probably bind to the

band 4.1B isoform that is specifically found associated with the paranodal and juxtaparan-

odal axolemma. When the paranode is disrupted by mutations (in cgt

-, contactin

-, and

Caspr

-null mice), the localization of these paranodal and juxtaparanodal proteins is altered:

Kv1.1, Kv1.2, and Caspr2 are juxtaposed to the nodal axolemma, and this reorganization is

associated with altered conduction of myelinated fibers. Understanding how axon-Schwann

interactions create the molecular architecture of myelinated axons is fundamental and al-

most certainly involved in the pathogenesis of peripheral neuropathies.

Key words:

junctions, Schwann cells, cell adhesion molecules, neuropathy, channels

Introduction

Myelinating Schwann cells differentiate from im-

mature Schwann cells in response to as yet undeter-

mined axonal signals (Mirsky and Jessen, 1999)

. The ac-

quisition of a myelinating phenotype is accompanied by

altered expression of numerous genes, many of which

encode components of the myelin sheath. Moreover,

the maintenance of the myelinating phenotype appears

to depend on a continuous relationship with an axon, as

axotomy results in the down-regulation of myelin-related

genes and the dedifferentiation of previously myelinat-

ing Schwann cells (Scherer and Salzer, 1996)

.

Myelinating Schwann cells, in turn, organize the ax-

onal membrane. Voltage-gated Na

channels accumulate

at the ends of developing myelin sheaths, and as two

adjacent internodes elongate, these clusters fuse to

form a node of Ranvier (Vabnick and Shrager, 1998)

. The

Shaker

-type K

channels, Kv1.1 and Kv1.2, are subse-

Address correspondence to: Steven S. Scherer, M.D., Ph.D., De-partment of Neurology, Room 460 Stemmler Hall, 36th Street andHamilton Walk, The University of Pennsylvania Medical Center, Phil-adelphia, PA 19104, USA. Tel: 215-573-3198; Fax: 215-573-4454;E-mail: [email protected]


2/12

Scherer and Arroyo Journal of the Peripheral Nervous System 7:112 (2002)

2

quently excluded from the nodal axolemma and seques-

tered beneath the myelin sheath by the developing paran-

ode (Vabnick et al., 1999)

. The analysis of mutations that

affect the integrity of paranode, cgt

-, contactin

, and Caspr

-

null mice, highlights the importance of this structure for

the proper function of myelinated fibers (Bhat et al., 2001;

Boyle et al., 2001; Poliak et al., 2001; Popko, 2000)

.

In this review, we consider some of the recent

findings relating to structure and function of myelinated

axons in the PNS. We have focused on three emerging

topics: the laminin-2 receptors of myelinating Schwann

cells, the differences between compact and non-com-

pact myelin, and the structure and function of the nodal

region. We have compared some of these findings to

those of Ramn y Cajal, and have emphasized how the

molecular organization of myelinated fibers is related to

various peripheral neuropathies.

Myelinating Schwann cells are polarized

According to Ramn y Cajal (1928), The medullated

nerve fibre, then, consists of an external solid mem-

brane

sheath of Schwannof a tubular cellSchwann

cellof a fatty sheath

, and of an axon. He understood

that the sheath was not part of the Schwann cell itself,

and that it continued across nodes of Ranvier. With the

advent of electron microscopy, the sheath of Schwann

was shown to be composed of a basal lamina and asso-

ciated collagen fibers (Thomas and Olsson, 1993)

, and

subsequently to contain laminin-2 (also known as merosin)

and other associated proteins (Bunge, 1993)

. A series of in-

vestigations, largely in the laboratory of Richard and Mary

Bunge (Bunge, 1993)

, indicate that axon-Schwann cells in-

teractions are required for Schwann cells to assemble their

extracellular matrix, and that extracellular matrix may be

required for myelination (c.f., Podratz et al., 1998)

.

Laminins are trimers of

,

, and

subunits, each of

which belongs to a gene family; the composition of lami-

nin-2 is

2

1

1. Laminin-2 is found in the basal laminae of

skeletal muscle and Schwann cells, and is required for

their normal development, as shown by the phenotype of

dystrophic

mice. These mice lack the

2 chain because

they are homozygous for lama2

mutations (Sunada et al.,

1994; Xu et al., 1994)

, and have a mild dysmyelinating neu-

ropathy in addition to a myopathy, that are both related to

defective basal laminae. In humans, LamA2

mutations are

the commonest cause of congenital myopathies. Becausesome of these patients also have mildly slowed conduc-

tion velocities (Shorer et al., 1995)

, laminin-2 is likely to be

essential for the normal myelination in humans, too.

Myelinating Schwann cells have two receptors for

laminin-2,

6

4 integrin (Einheber et al., 1993; Feltri et al.,

1994)

and dystroglycan (Matsumura et al., 1997; Yamada

et al., 1996)

. As shown in Fig. 1,

6

4 integrin is ex-

Figure 1. Myelinating Schwann cells are polarized. The left panel is a confocal image of a transverse section of rat sciaticnerve, double-labeled with a rabbit antiserum against 4 integrin (FITC) and a mouse monoclonal antibody against MAG(TRITC). 4 integrin is localized around the entire circumference of the outer/abaxonal membrane, and MAG is localized onthe inner/adaxonal membrane. Compact myelin is not stained and thus appears black. The circumferential organization of amyelinated axon is shown schematically on the right panel.


3/12


3

pressed around the entire circumference of the abaxonal/

outer membrane of myelinating Schwann cells. Mice lack-

ing either the

6 or the

4 integrin subunit die shortly after

birth because of a profound skin defect, but myelination in

co-cultures established from

4-null mice appears normal

(Frei et al., 1999)

. In epithelia such as the skin,

6

4 het-

erodimers are found in hemi-desmosomes, which adhere

epithelial cells to their basal lamina via intermediate fila-

ments. Schwann cells, however, do not have hemi-des-

mosomes and

6

4 integrin may be linked to the actin cy-

toskeleton and not to intermediate filaments.

Dystroglycan is composed of

and

subunits;

these are products of the same gene. In skeletal muscle,

dystroglycan binds to dystrophin, which, in turn, binds to

the actin cytoskeleton; dystroglycan also forms a com-

plex with

,

,

, and

sacroglycans and sarcospan

(Ozawa et al., 1998; Straub and Campbell, 1997)

. Re-

cently, Sherman and colleagues (Sherman et al., 2001)

showed that dystroglycan-related protein 2 (DRP2) and

periaxin form a complex with the dystroglycan receptorin myelinating Schwann cells (Fig. 2). As shown in Fig.

3A, DRP2 is found in large blocks separated by longitudi-

nal and transverse bands that can be immunostained for

caveolin-1, as previously shown by Mikol et al. (1999).

These anatomical features were well known to Ramn y

Cajal (1928) (Fig. 3B). Thus, in contrast to the continuous

localization of

6

4 integrin, DRP2 is specifically local-

ized to the Schwann cell cytoplasm that directly apposes

the myelin sheath (Sherman et al., 2001)

.

As depicted in Fig. 2, in myelinating Schwann cells

DRP2/periaxin complexes are linked to a complex con-

taining a short dystropin isoform (Dp116), utrophin,

,

,and

sarcoglycan, but not

or

sarcoglycan or sar-

cospan (Imumura et al., 2000; Sherman et al., 2001)

.

Whereas utrophin is probably directly linked to the actin

cytoskeleton, Dp116 and DRP2 lack an actin-binding do-

main. The importance of this complex in myelinating

Schwann cells remains to be fully elucidated, but it is

clear that dystrophin as well as

,

,

, and

sarcoglycanare essential for skeletal muscle cells (Ozawa et al.,

1998; Straub and Campbell, 1997)

. Dp116 may be es-

sential for proper myelination, as a splice site mutation in

the human dystrophin gene that abolishes dystrophin ex-

pression in peripheral nerve has been reported to cause

a demyelinating neuropathy (Comi et al., 1995)

. Periaxin is

clearly essential, as periaxin

-null mice develop a demyeli-

nating neuropathy (Gillespie et al., 2000)

, and recessive

PRX

mutations in humans cause inherited demyelinating

neuropathy (Boerkoel et al., 2000; Guilbot et al., 2001)

.

Dystroglycan also binds to an isoform of agrin that is

found in the basal laminae of Schwann cells (Yamada

et al., 1996)

. Finally, dystroglycan has another important

connection to peripheral neuropathy, as it appears to be

a receptor for Mycobacterium leprae

, an intracellular

pathogen of Schwann cells (Rambukkana, 2001)

.

The PNS myelin sheath

The organization of a myelinated fiber is shown in Fig.

4A. It depicts two internodes; one has been unrolled to re-

veal its trapezoidal shape. The figure shows that myelin

sheath itself can be divided into two domains, compact

Figure 2. The dystroglycan receptor complex of myelinatingSchwann cells. The dystroglycan receptor is linked to threemembers of the dystrophin family in myelinating Schwanncells, utrophin, DRP2, and Dp116. Of these, only utrophinhas an actin-binding domain, whereas DPR2 binds periaxin,which is shown binding to itself via its PDZ domains. Threesacroglycans, , , and , associate with dystroglycan. Thefigure was modified from one kindly provided by Drs. DianeSherman and Peter Brophy.

Figure 3. Cytoplasmic domains in myelinating Schwanncells. Panel A is a confocal image of three teased fibers froma rat sciatic nerve, double-labeled with a rabbit antiserumagainst DRP2 (gift of Dr. Diane Sherman; FITC) and a mousemonoclonal antibody against caveolin-1 (TRITC). A node (ap-posed arrowheads) and two nuclei (n) are indicated. Panel B

is a drawing made by Ramn y Cajal (1928) of teased fibersfrom an adult cat, stained with reduced silver (used with per-mission of Oxford University Press). He subdivided theSchwann cell cytoplasm into a perinuclear mass, longitu-dinal stripes, and transverse trabeculae; as shown inpanel A, the Schwann cell membrane in these regions con-tains caveolin-1 but not DRP2. Scale bar: 10 m.


4/12


4

and non-compact myelin, each containing a non-overlap-

ping set of proteins (Fig. 4B). Compact myelin forms the

bulk of the myelin sheath, and Fig. 5 depicts its molecular

organization. Interaction of P0 tetramers in cisand trans

(Shapiro et al., 1996)are essential for myelin compaction,

as the extracellular space (between the intraperiod lines) is

widened in Mpz-null myelin sheaths (Giese et al., 1992).

Even mice that are heterozygous for a null Mpzmutation

have focal areas of widened myelin (Samsam et al., 2002);

this is also a hallmark of some human MPZmutations

(Gabreels-Festen et al., 1996). Hence, both MPZ/Mpz al-

leles may be required for proper myelination; the loss of

one allele causes haplotype insufficiency.

Duplication or deletion of PMP22 is the most com-

mon cause of inherited demyelinating neuropathy (Mu-

rakami et al., 1996; Suter and Snipes, 1995), causing

Charcot-Marie-Tooth disease type 1A (CMT1A; affected

individuals have 3 copies of PMP22) and hereditary neu-

ropathy with liability to pressure palsies (HNPP; affected

individuals have 1 copy of PMP22), respectively. Al-

though the function PMP22 is unknown, these results

provide strong evidence that the amount of PMP22 in

compact myelin is critical, which has been directly con-

firmed (Vallat et al., 1996). Because perturbations in the

stoichiometry of either P0 or PMP22 appears to alter the

stability of the myelin sheath, compact myelin has been

Figure 4. The organiza-tion of a myelinated axon.Panel A depicts an un-rolled myelinating Schwanncell, revealing the regions

that form compact andnon-compact myelin. Tightjunctions are depicted astwo continuous (green)lines; these form a cir-cumferential belt and arealso found in incisures.Gap junctions are depictedas orange ovals; these arefound between the rowsof tight junctions, and aremore numerous in theinner aspects of incisuresand paranodes. Adherensjunctions are depicted as

purple ovals; these are morenumerous in the outer as-pects of incisures and paran-odes. The nodal, paranodal,and juxtaparanodal regionsof the axonal membraneare colored blue, red, andgreen, respectively. Thefigure was modified fromArroyo and Scherer (2000),with permission of Springer-Verlag. Panel B depictsthe proteins of compactand non-compact myelin.Compact myelin contains

P0, PMP22, and MBP; non-compact myelin containsE-cadherin, MAG, DM20,Cx32, and an unknownclaudin.


5/12


5

likened to a liquid crystal composed of highly ordered pro-

teins and lipids. How other point mutations of MPZand

PMP22 cause inherited demyelinating neuropathies re-

mains to be determined, as these mutant proteins havetoxic effects that are not seen in null alleles. The main

finding to date in this regard is that most mutant PMP22

proteins are retained in the endoplasmic reticulum, and

do not reach compact myelin (DUrso et al., 1998; Naef

et al., 1997; Naef and Suter, 1999; Notterpek et al., 1997;

Tobler et al., 1999). Protein-protein interactions may also

play a role, as P0 forms tetramers and PMP22 forms

dimers; PMP22 may also interact with P0 (DUrso et al.,

1999; Shapiro et al., 1996; Tobler et al., 1999).

Non-compact myelin is found in paranodes (the lat-

eral borders of the myelin sheath) and in Schmidt-Lan-

terman incisures. As depicted in Fig. 4, non-compact

myelin contains junctional specializations between the

layers of the myelin sheath, so-called reflexive or au-

totypic junctions (Arroyo and Scherer, 2000). Some of

these reflexive junctions are also found in inner and outer

mesaxons. Adherens junctions are most numerous in the

outer mesaxon as well as in outermost layers of the para-

nodes and incisures; these contain E-cadherin, -catenin,and -catenin, and are likely linked to the actin cytoskele-ton (Fannon et al., 1995; Hall and Williams, 1970). Tight

junction strands enclosing gap junction-like plaques have

been found by freeze fracture electron microscopy (San-

dri et al., 1982). The claudin(s) forming these tight junc-

tions remains to be determined.

The gap junctions in myelinating Schwann cells con-

tain connexin32 (Cx32) (Bergoffen et al., 1993; Chandross

et al., 1996; Scherer et al., 1995). A role for gap junctions

in the myelin sheath was not established until it was

discovered that mutations in the gene encoding Cx32,

GJB1, cause X-linked CMT (CMTX) (Bergoffen et al., 1993).

Dye transfer studies in living myelinated fibers provide

functional evidence that gap junctions mediate a radial

pathway of diffusion across incisures (Balice-Gordon et al.,

1998). A radial pathway would be advantageous as it pro-

vides a much shorter pathway (up to 1000-fold), owing

to the geometry of the myelin sheath. Disruption of this

radial pathway may be the reason that GJB1 mutations

cause CMTX. However, the pathway and the rate of 5,6-

carboxyfluorescein diffusion in Gjb1/cx32-null mice did not

appear to be different than in wild type mice (Balice-

Gordon et al., 1998), implying that another connexin(s)

forms functional gap junctions in PNS myelin sheaths. Theexistence of a radial pathway provides further evidence

that the resistance of myelin is not as high as com-

monly conceived (Funch and Faber, 1984).

The lateral borders of the Schwann cell cytoplasm

have microvilli (Fig. 6). Microvilli contain F-actin (Trapp

et al., 1989), and as shown in Figs. 6 and 7, ezrin, radixin,

and moesin (Hayashi et al., 1999; Melendez-Vasquez et al.,

2001; Scherer et al., 2001), the defining members of the

ERM family of proteins. ERM proteins bind to actin fila-

ments mainly via their C-termini, and can associate with

a number of different integral membrane proteins via

their N-termini. ERM proteins also form head-to-tail oli-gomers with themselves and with merlin, the product

of the NF2locus. It remains to be determined whether

ERM proteins are associated with integral membrane

proteins in Schwann cell microvilli, and whether their

phosphorylation regulates their function in this setting

(Hayashi et al., 1999). As shown in Fig. 8A, ERM proteins

are also colocalized with F-actin in incisures and in the

inner mesaxon (Scherer et al., 2001; Trapp et al., 1989).

Strands of ERM/F-actin staining within the non-compact

myelin appear to form a spiral pattern (Scherer et al.,

2001), owing to the spiral nature of the myelin sheath

itself, strikingly reminiscent of what was once known as

the apparatus of Rezzonico (Fig. 8B).

Specializations at nodes

In spite of the differences between myelinating

Schwann cells and oligodendrocytes and their myelin

sheaths, the organization of the axon itself is quite sim-

ilar in the PNS and CNS (Fig. 6A). Voltage-gated Na

channels are highly concentrated in the nodal axo-

lemma (Ellisman and Levinson, 1982; Haimovich et al.,

1984); these belong to a multi-gene family, but the

Figure 5. The localization of PNS myelin proteins in com-pact myelin. The left panel is an electron micrograph of com-pact myelin, which consists of alternating layers known as

the intraperiod line (which is actually a double line) and themajor dense line. The right panel is a schematic depiction ofhow apposed cell membranes create the intraperiod andmajor dense lines. The disposition of P0 tetramers, PMP22dimers, and MBP monomers, as well as the glycolipids ga-lactocerebroside and sulfatide are shown. The approximatethickness of the lipid bilayers, as well as the intracellular andextracellular spaces is indicated (Vonasek et al., 2001).


6/12


6

Nav1.6 channel appears to be the main one expressed

at nodes (Caldwell et al., 2000). The gene encoding this

channel is mutated in mice with motor endplate dis-

ease (med), a recessively inherited disease that cause

respiratory paralysis (Burgess et al., 1995). Nav1.6 is prob-

ably not the only voltage-gated Na channel at nodes, be-

cause mixed nerve conduction velocity is only minimally

slowed in medmice (Duchen and Stefani, 1971; Rieger

et al., 1984); indeed, Nav1.2, Nav1.8, and Nav1.9 have

also been reported in nodes (Boiko et al., 2001; Fjell

et al., 2000; Kaplan et al., 2001).

Two splice variants of ankyrinG, 480 and 270 kDa,

anchor voltage-gated Na channels at nodes (Kordeli et al.,

1990; Kordeli et al., 1995). These isoforms are distin-

guished by their membrane-binding domain composed of

ANK repeats, a spectrin-binding domain, and a serine/

threonine-rich domain (Zhang and Bennett, 1996). As

depicted in Fig. 6B, ankyrinG also interacts with the cy-

toplasmic domains of neurofascin 186 kDa (NF186) and

Nr-CAM (Davis and Bennett, 1994; Davis et al., 1996;

Davis et al., 1993; Srinivasan et al., 1988), and with

spectrin (Koenig and Repasky, 1985; Trapp et al., 1989).

A splice variant of spectrin IV (IV4) is specifically lo-calized to nodes (Berghs et al., 2000), a number of differ-

ent spontaneous mutations in the murine spectrin 4

gene cause quivering, in which altered ion channel dis-

tribution has been noted (Parkinson et al., 2001). In

keeping with the idea that ankyrinG links voltage-gated

Na channels, NF186, and Nr-CAM to the spectrin cyto-

skeleton, inactivation of the ankyrinG gene in the cere-

bellum reduces the amount of voltage-gated Na chan-

nels and neurofascin in the initial segments of granule

cells and Purkinje cells, respectively (Zhou et al., 1998a).

Although these workers were unable to visualize a re-

duced number of Na channels in Purkinje cell initial

segments, the diminished ability of these cells to initiate

axon potentials is consistent with this idea. Since initial

segments and nodes share many molecular characteris-

Figure 7. ERM proteins in Schwann cell microvilli. PanelsA-D show a confocal reconstruction of a teased fiber from arat sciatic nerve (fixed for 60 minutes in 4% paraformalde-hyde), labeled with a pan-ERM antiserum (A), a mousemonoclonal antibody against Na channels (B), and a ratmonoclonal antibody against a phosphorylated epitope ofneurofilament heavy (NF-H, C); the merged image is shownin panel D. The insets in panels A and B show the superim-posed NF-H staining. At the node (double arrowheads), notethat the ERM antibody stains a larger diameter disk thandoes the Na channel antibody. Panel E shows a single 0.5m thick optical section from a section through a node ofRanvier; taken from an unfixed ventral root, double-labeledwith a rabbit antiserum against ezrin (TRITC) and a mousemonoclonal antibody against Na channels (FITC). Note thatthe ring of ERM staining is larger than the ring of Na chan-nel staining. Scale bar: A-D, 10 m; E, 1 m. From Schereret al. (2001), with permission of Wiley-Liss.

Figure 6. Nodal specializations in PNS myelin sheaths.Panel A is a schematic depiction of the node, paranode, andjuxtaparanode. Panel B is a schematic drawing of possiblecis and trans interactions between the molecular compo-

nents of nodes (modified from Arroyo and Scherer (2000),used with permission of Springer-Verlag).


7/12


7

tics, the nodal membranes of Purkinje cells are probably

similarly affected.

Molecular interactions between Schwann cells and

axons may define the locations of nodes. Bennett and

colleagues (Bennett et al., 1997; Davis et al., 1996;

Lambert et al., 1997)have proposed that NF186 and Nr-CAM have heterophilic interactions with other CAMs

on the microvilli as depicted in Fig. 6B. This suggestion

is in accord with the ultrastructural data showing tether-

ing of the microvilli to the nodal axolemma (Ichimura

and Ellisman, 1991; Raine, 1982). Neurofascin and Nr-

CAM appear to be localized at presumptive nodes be-

fore ankyrinG and voltage-dependent Na channels

(Lambert et al., 1997). The observation that ERM pro-

teins are associated with clusters of voltage-gated Na

channels in developing nerves also supports this idea

(Melendez-Vasquez et al., 2001). Other molecular inter-

actions that may serve to localize Na channels: the ex-

tracellular domain of the Na channel 2 subunit mayinteract in with tenascin-C and tenascin-R (Xiao et al.,

1999), and extracellular matrix molecules that have

been reported to be localized to the nodal region in the

PNS and CNS (Bartsch et al., 1993; Ffrench-Constant et

al., 1986; Rieger et al., 1986).

Specializations at paranodes

At paranodes, the lateral edge of the myelin sheath

spirals around the axon, forming the axoglial junctions

(Ichimura and Ellisman, 1991; Sandri et al., 1982; Tho-

mas et al., 1993). In freeze-fracture, the paranodal loops

of the myelin sheath contain rows of large particles that

are in register with a double row of smaller particles on

the axolemma; these particles are thought to connect

the terminal loops to the axon. These particles correspond

to the so-called terminal bands seen by transmission

electron microscopy, and have been more recently

termed septate-like junctions, as they resemble inverte-

brate septate junctions (Einheber et al., 1997). Septate

junctions may function similarly to vertebrate tight junc-

tions, forming intercellular junctions that prevent the diffu-

sion of small molecules and ions. Septate-like junctions,

however, do not prevent the diffusion of lanthanum or

even microperoxidase (molecular mass 5 kDa) into the

periaxonal space (Feder, 1971; Hirano et al., 1969; Mac-

Kenzie et al., 1984); hence are not tight in the con-

ventional sense.

The molecular organization of the paranode is de-

picted in Fig. 6B. Heterodimers of contactin and Caspr arelocalized to the paranodal axolemma in myelinated fibers

of the PNS and CNS (Einheber et al., 1997; Menegoz et al.,

1997; Rios et al., 2000). These contactin/Caspr het-

erodimers co-localize with an isoform of neurofascin,

NF155 (Tait et al., 2000). All of these molecules are likely

to be components of septate-like junctions, as in both

contactin- and Caspr-null mice, the absence of either

contactin or Caspr results in the loss of septate-like

junctions (Bhat et al., 2001; Boyle et al., 2001). A neuro-

fascin-null mouse has not yet been reported, but the

targeted disruption of UDP-galactose ceramide galacto-

syltransferase gene (cgt) have provided a glimpse of thelikely phenotype in myelinating Schwann cells (Bosio

et al., 1996; Coetzee et al., 1996; 1998). CGT is neces-

sary for the synthesis of galactocerebroside and sul-

fatide, which are glycolipids found in the myelin sheath

(Fig. 5). Besides lacking these glycolipids, the paran-

odes in cgt-null mice lack septate-like junctions (Bosio

et al., 1998; Dupree et al., 1998).

Specializations at juxtaparanodes

By freeze-fracture electron microscopy, the axo-

lemma in the region extending 10-15 m from the para-

node contains clusters of 5-6 particles (Miller and Da Silva,1977; Rosenbluth, 1976; Stolinski et al., 1981; Stolinski

et al., 1985; Tao-Cheng and Rosenbluth, 1984). The dis-

tribution of these juxtaparanodal particles corresponds to

the distribution of delayed rectifying K channels (Chiu

and Ritchie, 1980), Kv1.1 and Kv1.2 and their associ-

ated 2 subunit (Gulbis et al., 1999; Mi et al., 1995;Rasband et al., 1998; Vabnick and Shrager, 1998; Wang

et al., 1993; Zhou et al., 1998b). Kv1.1 and Kv1.2 subunits

can freely mix in varying proportions to form tetramers,

the functional channels (Hopkins et al., 1994), and the size

Figure 8. Internodal localization of ERM proteins in myelinatingSchwann cells. Panel A is a confocal reconstruction of twoteased fibers from adult rat sciatic nerves immunolabeledwith a pan ERM rabbit antiserum. The inner mesaxon is indi-

cated (arrows). Scale bar: 10 m. From Scherer et al. (2001),with permission of Wiley-Liss. Panel B is a drawing made byRamn y Cajal (1928) of teased fibers following silver im-pregnation after fixation in formol-uranium . . . showing theapparatus of Rezzonico (used with permission of OxfordUniversity Press).


8/12


8

of the particles (10 nm in diameter) compares well to the

expected size of a tetramer (Kreusch et al., 1998). Al-

though Kv1.1 and Kv1.2 channels appear to be concealed

under the myelin sheath (Hildebrand et al., 1994; Kocsis

et al., 1983), juxtaparanodal K channels are thought to

have an important physiological function, dampening the

excitability of myelinated fibers. The finding that Kv1.1-null

mice have abnormal impulse generators near the neu-

romuscular junctions supports this idea (Smart et al.,

1998; Zhou et al., 1998b). Similarly, mutations in the hu-

man Kv1.1 gene, KCNA1, cause a form of familial episodic

ataxia that is associated with ectopic impulse generators

somewhere within the peripheral nerve (Adelman et al.,

1995; Browne et al., 1994; Brunt and Van Weerden, 1990;

Van Dyke et al., 1975; Zerr et al., 1998).

A homologue of Caspr, Caspr2 is localized to the jux-

taparanodes of myelinated fibers in both the CNS and the

PNS, colocalizing with Kv1.1/1.2/2 (Poliak et al., 1999).Caspr2 and Caspr have similar structures especially in the

extracellular region, but only Caspr2 has an intracellularPDZ domain. Kv1.1 and Kv1.2, both of which also have

PDZ domains, form a complex with Caspr2, probably me-

diated by a protein with multiple PDZ binding sites. Al-

though transcellular connections between the juxtaparan-

odal axonal membrane and the myelin sheath have not

been described, it is possible that Caspr2 has a binding

partner (Peles and Salzer, 2000). Caspr and Caspr2 both

have a cytoplasmic Band 4.1 protein binding site, and

Band 4.1B protein is specifically localized to paranodes

and juxtaparanodes (Ohara et al., 2000; Parra et al., 2000).

The internodal regionThe internodal axonal membrane lacks the conspic-

uous specializations of the nodal region. Nevertheless, in

the PNS, intramembranous particles similar to those of

the juxtaparanodal region are found apposing the internal

mesaxon and incisures of the myelin sheath (Stolinski

and Breathnach, 1982; Stolinski et al., 1981; Stolinski

et al., 1985). In accord with these findings, the intern-

odal membranes of PNS axons have a tripartite strand

(consisting of a central strand of Caspr/contactin staining,

flanked by strands of Kv1.1/1.2/2/Caspr2-immunoreac-tivity) that apposes the inner mesaxon and the inner-

most aspect of incisures (Arroyo et al., 1999; Rios et al.,2000). These results suggest that Caspr2 could be lo-

calized to the juxtaparanodal and internodal membrane

by a trans-interaction with a protein expressed by the

myelinating Schwann cell.

Disrupted septate-like junctions anddisorganized axonal membranes

The lack of septate-like junctions in contactin-, Caspr,

and cgt-null mice leads to a profound reorganization of the

axonal membrane (Fig. 9). Contactin and Caspr are notrestricted to the paranode, but are more diffusely localized

in the internodal membrane, and NF155 is not restricted

to the paranodal loops (Bhat et al., 2001; Boyle et al.,

2001; Dupree et al., 1999; Poliak et al., 2001). Further,

Kv1.1, Kv1.2, and Caspr2 are mislocalized to the paranodal

axonal membrane. The apposition of Kv1.1 and Kv1.2

likely results in the inefficient axonal conduction of my-

elinated fibers, as conduction velocities are slowed in

contactin-, Caspr, and cgt-null mice (Bosio et al., 1996;

Coetzee et al., 1996). In keeping with this suggestion,

K channel blockers have extraordinary effects in these

mutant mice (Bhat et al., 2001; Boyle et al., 2001; Coet-

zee et al., 1996).

Conclusion

In summary, the intricate localization of numerous

axonal proteins is highly related to the structure of the

overlying myelin sheath. This organization is disrupted

by a number of mutations that affect various compo-

nents of the myelinated axons, and functional conse-

quences have been established. How the molecular ar-

chitecture of myelinated fibers is disrupted in other

Figure 9. Altered localization of paranodal and juxtaparanodalin cgt-null mice. Panel A is a photomicrograph of a ventralroot from a cgt-null mouse, double-labeled with a rabbit anti-serum against Caspr (TRITC) and a mouse monoclonal anti-body against Kv1.2 (FITC). Scale bar: 10 m. Panel B is aschematic representation of the reorganization of the axonalmembrane in cgt-null mice. In cgt-null mice, Kv1.2 is local-ized to paranodes, whereas Caspr is diffusely localizedthroughout the internode. Septate-like junctions/transversebands are absent in both the CNS and the PNS, and axoglialjunctions are disorganized in the CNS.


9/12


9

inherited and acquired neuropathies may provide in-

sight into their pathogenesis.

Acknowledgements

This paper is based on The Third Richard P. Bunge

Memorial Lecture, given by S.S.S. to the Peripheral

Nerve Society. Our work is supported by the NIH(NS37100, NS34528, and NS08075), the Juvenile Dia-

betes Foundation, and the Charcot-Marie-Tooth Associ-

ation. We thank Drs. Rita Balice-Gordon, Linda Bone,

Peter Brophy, Bill Chiu, Suzanne Deschnes, Laura Feltri,

Kurt Fischbeck, and David Gutmann, Albee Messing,

Dan Mikol, David Paul, Ori Peles, Brian Popko, Jim

Salzer, Diane Sherman, Larry Wrabetz, and Lei Zhou for

their contributions to various aspects of the work re-

ported here.

ReferencesAdelman JP, Bond CT, Pessia M, Maylie J (1995). Episodic

ataxia results from voltage-dependent potassium channels

with altered functions. Neuron 15:14491454.

Arroyo EJ, Scherer SS (2000). On the molecular architecture of

myelinated fibers. Histochem Cell Biol 113:118.

Arroyo EJ, Xu Y-T, Zhou L, Messing A, Peles E, Chiu SY,

Scherer SS (1999). Myelinating Schwann cells determine the

internodal localization of Kv1.1, Kv1.2, Kv2, and Caspr. JNeurocytol 28:333347.

Balice-Gordon RJ, Bone LJ, Scherer SS (1998). Functional gap

junctions in the Schwann cell myelin sheath. J Cell Biol 142:

10951104.

Bartsch U, Pesheva P, Raff M, Schachner M (1993). Expression

of janusin (J1-160/180) in the retina and optic nerve of thedeveloping and adult mouse. Glia 9:5769.

Bennett V, Lambert S, Davis JQ, Zhang X (1997). Molecular ar-

chitecture of the specialized axonal membrane at the node

of Ranvier. Soc Gen Physiol 52:107120.

Berghs S, Aggujaro D, Dirkx R, Maksimova E, Stabach P, Her-

mel JM, Zhang JP, Philbrick W, Slepnev V, Ort T, Solimena

M (2000). Beta IV spectrin, a new spectrin localized at axon

initial segments and nodes of Ranvier in the central and pe-

ripheral nervous system. J Cell Biol 151:9851001.

Bergoffen J, Scherer SS, Wang S, Oronzi-Scott M, Bone L, Paul

DL, Chen K, Lensch MW, Chance P, Fischbeck K (1993).

Connexin mutations in X-linked Charcot-Marie-Tooth dis-

ease. Science 262:20392042.

Bhat MA, Rios JC, Lu Y, Garcia-Fresco GP, Ching W, St. MartinM, Li JJ, Einheber S, Chesler M, Rosenbluth J, Salzer JL,

Bellen HJ (2001). Axon-glia interactions and the domain or-

ganization of myelinated axons requires Neurexin IV/Caspr/

Paranodin. Neuron 30:369383.

Boerkoel CF, Takashima H, Stankiexicz P, Garcia CA, Leber SM,

Rhee-Morris L, Lupski JR (2000). Periaxin mutations cause

recessive Dejerine-Sottas neuropathy. Am J Hum Genet 68:

325333.

Boiko T, Rasband MN, Levinson SR, Coldwell JH, Mandel G,

Trimmer JS, Matthews G (2001). Compact myelin dictates

the differential targeting of two sodium channel isoforms in

the same axon. Neuron 30:91104.

Bosio A, Binczek E, Stoffel W (1996). Functional breakdown of

the lipid bilayer of the myelin membrane in central and pe-

ripheral nervous system myelin by disrupted galactocerebro-

side synthesis. Proc Natl Acad Sci USA 93:1328013285.

Bosio A, Bussow H, Adam J, Stoffel W (1998). Galactosphin-

golipids and axon-glial interaction in myelin of the central

nervous system. Cell Tissue Res 292:199210.

Boyle MET, Berglund EO, Murai KK, Weber L, Peles E, Ranscht

B (2001). Contactin orchestrates assembly of the septate-

like junctions at the paranode in myelinated peripheral nerve.

Neuron 30:385397.

Browne DL, Gancher ST, Nutt JG, Brunt ERP, Smith EA, Kramer

P, Litt M (1994). Episodic ataxia/myokymia syndrome is as-

sociated with point mutations in the human potassium chan-

nel gene, KCNA1. Nature Genet 8:136140.

Brunt ERP, Van Weerden TW (1990). Familial paroxysmal kine-

sigenic ataxia and continuous myokymia. Brain 113:1361

1382.

Bunge MB (1993). Schwann Cell Regulation of Extracellular Ma-

trix Biosynthesis and Assembly. In: Peripheral Neuropathy.

Dyck PJ, Thomas PK, Low PA, Poduslo JF (Eds). W.B. Saun-

ders, Philadelphia, Vol. 1, pp 229316.

Burgess DL, Kohrman DC, Galt J, Plummer NW, Jones JM,Spear B, Meisler MH (1995). Mutation of a new sodium

channel gene, Scn8a, in the mouse mutant motor endplate

disease. Nature Genet 10:461465.

Caldwell JH, Schaller KL, Lasher RS, Peles E, Levinson SR

(2000). Sodium channel NaV1.6 is localized at nodes of Ran-

vier, dendrites, and synapses. Proc Nat Acad Sci USA 97:

56165620.

Chandross KJ, Kessler JA, Cohen RI, Simburger E, Spray DC,

Bieri P, Dermietzel R (1996). Altered connexin expression af-

ter peripheral nerve injury. Mol Cell Neurosci 7:501518.

Chiu SY, Ritchie JM (1980). Potassium channels in nodal and in-

ternodal axonal membrane of mammalian myelinated fibres.

Nature 284:170171.

Coetzee T, Dupree JL, Popko B (1998). Demyelination and al-tered expression of myelin-associated glycoprotein isoforms

in the central nervous system of galactolipid-deficient mice.

J Neurosci Res 54:613622.

Coetzee T, Fujita N, Dupree J, Shi R, Blight A, Suzuki K, Suzuki K,

Popko B (1996). Myelination in the absence of galactocere-

broside and sulfatide: normal structure with abnormal func-

tion and regional instability. Cell 86:209219.

Comi G, Ciafaloni E, de Silva HAR, Prelle A, Bardoni A, Rigoletto

C, Robotti M, Bresolin N, Moggio M, Fortunato F, Ciscato P,

Turconi A, Roses AD, Scarlato G (1995). A G1 A transver-

sion at the 5 splice site of intron 69 of the dystrophin gene

causing the absence of peripheral nerve Dp116 and severe

clinical involvement in a DMD patient. Hum Mol Gen 4:

21712174.DUrso D, Ehrhardt P, Muller HW (1999). Peripheral myelin pro-

tein 22 and protein zero: a novel association in peripheral

nervous system myelin. J Neurosci 19:33963403.

DUrso D, Prior R, Greiner-Petter R, Gabreels-Festen AAWM,

Muller HW (1998). Overloaded endoplasmic reticulum-Golgi

compartments, a possible pathomechanism of peripheral

neuropathies caused by mutations of the peripheral myelin

protein PMP22. J Neurosci 18:731740.

Davis JQ, Bennett V (1994). Ankyrin binding activity shared by

the neurofascin/L1/NrCAM family of nervous system cell ad-

hesion molecules. J Biol Chem 269:2716327166.

Davis JQ, Lambert S, Bennett V (1996). Molecular composition of


10/12


10

the node of Ranvier: identification of ankyrin-binding cell adhe-

sion molecules neurofascin (mucin third FNIII domain-) andNrCAM at nodal axon segments. J Cell Biol 135:13551367.

Davis JQ, McLaughlin T, Bennett V (1993). Ankyrin-binding pro-

teins related to nervous system cell adhesion molecules:

candidates to provide transmembrane and intercellular con-

nections in adult brain. J Cell Biol 121:121133.

Duchen LW, Stefani E (1971). Electrophysiological studies of

neuromuscular transmission hereditary motor end-plate dis-

ease of the mouse. J Physiol 212:535548.

Dupree JL, Coetzee T, Blight A, Suzuki K, Popko B (1998). My-

elin galactolipids are essential for proper node of Ranvier for-

mation in the CNS. J Neurosci 18:16421649.

Dupree JL, Girault JA, Popko B (1999). Axo-glial interactions

regulate the localization of axonal paranodal proteins. J Cell

Biol 147:11451151.

Einheber S, Milner T, Giancotti F, Salzer J (1993). Axonal regula-

tion of Schwann cell integrin expression suggests a role for

64 in myelination. J Cell Biol 123:625638.

Einheber S, Zanazzi G, Ching W, Scherer SS, Milner TA, Peles

E, Salzer JL (1997). The axonal membrane protein Caspr/

Neurexin IV is a component of the septate-like paranodal

junctions that assemble during myelination. J Cell Biol 139:14951506.

Ellisman MH, Levinson SR (1982). Immunocytochemical local-

ization of sodium channel distributions in the excitable mem-

branes of Electrophorus electricus. Proc Natl Acad Sci USA

79:67076711.

Fannon AM, Sherman DL, Ilyina-Gragerova G, Brophy PJ, Friedrich

VL, Colman DR (1995). Novel E-cadherin mediated adhesion

in peripheral nerve: Schwann cell architecture is stabilized by

autotypic adherens junctions. J Cell Biol 129:189202.

Feder N (1971). Microperoxidasean ultrastructural tracer of

low molecular weight. J Cell Biol 51:339343.

Feltri ML, Scherer SS, Nemni R, Kamholz J, Vogelbacker H,

Oronzi-Scott M, Canal N, Quaranta V, Wrabetz L (1994). 4 in-

tegrin expression in myelinating Schwann cells is ab-axonallypolarized, developmentally-regulated, and axonally-dependent.

Development 120:12871301.

Ffrench-Constant C, Miller RH, Kruse J, Schachner M, Raff MC

(1986). Molecular specialization of astrocyte processes at

nodes of Ranvier in rat optic nerve. J Cell Biol 102:844852.

Fjell J, Hjelmstrom P, Hormuzdiar W, Milenkovic M, Aglieco F,

Tyrrell L, Dib-Haji S, Waxman SG, Black JA (2000). Localiza-

tion of the tetrodotoxin-resistant sodium channel NaN in no-

ciceptors. Neuroreport 11:199202.

Frei R, Dowling J, Carenini S, Fuchs E, Martini R (1999). Myelin

formation by Schwann cells in the absence of 4 integrin.

Glia 27:269274.

Funch PG, Faber DS (1984). Measurement of myelin sheath re-

sistances: implications for axonal conduction and pathophys-iology. Science 225:538540.

Gabreels-Festen AAWM, Hoogendijk JE, Meijerink PHS, Gabreels

FJM, Bolhuis PA, Vanbeersum S, Kulkens T, Nelis E, Jennek-

ens FGI, Devisser M, Vanengelen BGM, Van Broeckhoven C,

Mariman ECM (1996). Two divergent types of nerve pathology

in patients with different P0 mutations in Charcot-Marie-Tooth

disease. Neurology 47:761765.

Giese KP, Martini R, Lemke G, Soriano P, Schachner M (1992).

Mouse P0 gene disruption leads to hypomyelination, abnor-

mal expression of recognition molecules, and degeneration

of myelin and axons. Cell 71:565576.

Gillespie CS, Sherman DL, Fleetwood-Walker SM, Cottrell DF,

Tait S, Garry EM, Wallace VCJ, Ure J, Griffiths IR, Smith A,

Brophy PJ (2000). Peripheral demyelination and neuropathic

pain behavior in periaxin-deficient mice. Neuron 26:523531.

Guilbot A, Williams A, Ravise N, Verny C, Brice A, Sherman DL,

Brophy PJ, LeGuern E, Delague V, Bareil C, Megarbane A,

Claustres M (2001). A mutation in periaxin is responsible for

CMT4F, an autosomal recessive form of Charcot-Marie-

Tooth disease. Hum Mol Genet 10:415421.

Gulbis JM, Mann S, MacKinnon R (1999). Structure of a voltage-

dependent K channel beta subunit. Cell 97:943952.

Haimovich B, Bonilla E, Casadei J, Barchi RL (1984). Immunocy-

tochemical localization of the mammalian voltage-dependent

sodium channel using polyclonal antibodies against the puri-

fied protein. J Neurosci 4:22592268.

Hall SM, Williams PL (1970). Studies on the incisures of

Schmidt and Lanterman. J Cell Sci 6:767791.

Hayashi K, Yonemura S, Matsui T, Tsukita S, Tsukita S (1999).

Immunofluorescence detection of ezrin/radixin/moesin (ERM)

proteins with their carboxyl-terminal threonine phosphorylated

in cultured cells and tissues. Application of a novel fixation

protocol using trichloroacetic acid (TCA) as a fixative. J Cell

Sci 112:11491158.

Hildebrand C, Bowe CM, Remahl IN (1994). Myelination andmyelin sheath remodelling in normal and pathological PNS

nerve fibres. Prog Neurobiol 43:85141.

Hirano A, Becker NH, Zimmerman HM (1969). Isolation of the

periaxonal space of the central myelinated nerve fiber with

regard to the diffusion of peroxidase. J Histochem Cy-

tochem 17:512516.

Hopkins WF, Allen ML, Mouamed KM, Tempel BL (1994). Prop-

erties of voltage-gated K currents expressed in Xenopus

oocytes by mKv1.1, mKv1.2 and their heteromultimers as

revealed by mutagenesis of the dendrotoxin-binding site in

Kv1.1. Pflgers Arch 428:382390.

Ichimura T, Ellisman MH (1991). Three-dimensional fine struc-

ture of cytoskeletal-membrane interactions at nodes of Ran-

vier. J Neurocytol 20:667681.Imumura M, Araishi K, Noguchi S, Ozawa E (2000). A sarcogly-

can-dystrophin complex anchors DP116 and utrophin in the

peripheral nervous system. Hum Mol Genet 9:30913100.

Kaplan MR, Cho M-H, Ullian EM, Isom LL, Levinson SR, Barres

BA (2001). Differential control of clustering of the sodium

channels Nav1.2 and Nav1.6 at developing CNS nodes of

Ranvier. Neuron 40:105119.

Kocsis JD, Ruiz JA, Waxman SG (1983). Maturation of mamma-

lian myelinated fibers: changes in action-potential character-

istics following 4-aminopyridine application. J Neurophysiol

50:449463.

Koenig E, Repasky E (1985). A regional analysis of -spectrin inthe isolated Mauthner neuron and in isolated axons of the

goldfish and rabbit. J Neurosci 5:705714.Kordeli E, Davis J, Trapp B, Bennett V (1990). An isoform of

ankyrin is localized at nodes of Ranvier in myelinated axons

of central and peripheral nerves. J Cell Biol 110:13411352.

Kordeli E, Lambert S, Bennett V (1995). AnkyrinG: a new ankyrin

gene with neural-specific isoforms localized at the axonal ini-

tial segment and node of Ranvier. J Biol Chem 270:2352

2359.

Kreusch A, Pfaffinger PJ, Stevens CF, Choe S (1998). Crystal

structure of the tetramerization domain of the Shakerpotas-

sium channel. Nature 392:945948.

Lambert S, Davis JQ, Bennett V (1997). Morphogenesis of the

node of Ranvier: co-clusters of ankyrin and ankyrin-binding


11/12


11

integral proteins define early developmental intermediates. J

Neurosci 17:70257036.

MacKenzie ML, Ghabriel MN, Allt G (1984). Nodes of Ranvier

and Schmidt-Lanterman incisures: an in vivo lanthanum

tracer study. J Neurocytol 13:10431055.

Matsumura K, Yamada H, Saito F, Sunada Y, Shimizu T (1997).

Peripheral nerve involvement in merosin-deficient congenital

muscular dystrophy and dy mouse. Neuromuscular Disord 7:

712.

Melendez-Vasquez CV, Rios JC, Zanazzi G, Lambert S,

Bretscher A, Salzer JL (2001). Nodes of Ranvier form in as-

sociation with ezrin-radixin-moesin (ERM)-positive Schwann

cell processes. Proc Nat Acad Sci USA 98:12351240.

Menegoz M, Gaspar P, Le Bert M, Galvez T, Burgaya F, Palfrey

C, Ezan P, Arnos F, Girault J-A (1997). Paranodin, a glycopro-

tein of neuronal paranodal membranes. Neuron 19:319331.

Mi HY, Deerinck TJ, Ellisman MH, Schwarz TL (1995). Differen-

tial distribution of closely related potassium channels in rat

Schwann cells. J Neurosci 15:37613774.

Mikol DD, Hong HL, Cheng HL, Feldman EL (1999). Caveolin-1

expression in Schwann cells. Glia 27:3952.

Miller RG, Da Silva PP (1977). Particle rosettes in the periaxonal

Schwann cell membrane and particle clusters in the axo-lemma of rat sciatic nerve. Brain Res 130:135141.

Mirsky R, Jessen KR (1999). The neurobiology of Schwann

cells. Brain Pathol 9:293311.

Murakami T, Garcia CA, Reiter LT, Lupski JR (1996). Charcot-

Marie-Tooth disease and related inherited neuropathies. Med-

icine 75:233250.

Naef R, Adlkofer K, Lescher B, Suter U (1997). Aberrant protein

trafficking in Trembler suggests a disease mechanism for

hereditary human peripheral neuropathies. Mol Cell Neuro-

sci 9:1325.

Naef R, Suter U (1999). Impaired intracellular trafficking is a

common disease mechanism of PMP22 point mutations in

peripheral neuropathies. Neurobiol Disease 6:114.

Notterpek L, Shooter EM, Snipes GJ (1997). Upregulation of theendosomal-lysosomal pathway in the Trembler-J neuro-

pathy. J Neurosci 17:41904200.

Ohara R, Yamakawa H, Nakayama M, Ohara O (2000). Type II

brain 4.1 (4.1B/KIAA0987), a member of the protein 4.1 family,

is localized to neuronal paranodes. Mol Brain Res 85:4152.

Ozawa E, Noguchi S, Mizuno Y, Hagiwara Y, Yoshida M (1998).

From dystrophinopathy to sarcoglycanopathy: evolution of the

concept of muscular dystrophy. Muscle Nerve 21:421438.

Parkinson NJ, Olsson CL, Hallows JL, McKee-Johnson J, Keogh

BP, Noben-Trauth K, Kujawa SG, Tempel BL (2001). Mutant

beta-spectrin 4 causes auditory and motor neuropathies in

quiveringmice. Nat Genet 29:6165.

Parra M, Gascard P, Walensky LD, Gimm JA, Blackshaw S,

Chan N, Takakuwa Y, Berger T, Lee G, Chasis JA, SnyderSH, Mohandas N, Conboy JG (2000). Molecular and func-

tional characterization of protein 4.1B, a novel member of

the protein 4.1 family with high level, focal expression in

brain. J Biol Chem 275:32473255.

Peles E, Salzer JL (2000). Molecular domains of myelinated fi-

bers. Curr Opin Neurobiol 10:558565.

Podratz JL, Rodriguez EH, DiNonno ES, Windebank AJ (1998).

Myelination by Schwann cells in the absence of extracellular

matrix assembly. Glia 23:383388.

Poliak S, Gollan L, Martinez R, Custer A, Einheber S, Salzer JL,

Trimmer J, Shrager P, Peles E (1999). Caspr2, a new member

of the neurexin superfamily is localized at the juxtaparanodes

of myelinated axons and associates with K channels. Neuron

24:10371047.

Poliak S, Gollan L, Salomon D, Berglund EO, Ranscht B, Peles E

(2001). Localization of Caspr2 in myelinated nerves depends

on axon-glia interactions and the generation of barriers along

the axon. J Neurosci 21:75687575.

Popko B (2000). Myelin galactolipids: Mediators of axon-glial in-

teractions? Glia 29:149153.

Raine CS (1982). Differences between the nodes of Ranvier of

large and small diameter fibres in the P.N.S. J Neurocytol

11:935947.

Rambukkana A (2001). Molecular basis for the peripheral nerve

predilection of Mycobacterium leprae. Curr Opin Microbiol 4:

2127.

Ramn y Cajal S (1928). Degeneration and Regeneration of the

Nervous System. May RM (Ed). Oxford University Press,

Oxford, p 769.

Rasband M, Trimmer JS, Schwarz TL, Levinson SR, Ellisman

MH, Schachner M, Shrager P (1998). Potassium channel dis-

tribution, clustering, and function in remyelinating rat axons.

J Neurosci 18:3647.

Rieger F, Daniloff JK, Pincon-Raymond M, Crossin KC, Grumet

M, Edelman GM (1986). Neuronal cell adhesion moleculesand cytotactin are colocalized at the node of Ranvier. J Cell

Biol 103:379391.

Rieger F, Pincon-Raymond M, Lombet A, Ponzio G, Lazdunski

M, Sidman RL (1984). Paranodal dysmyelination and in-

crease in tetrodotoxin binding sites in the sciatic nerve of

the motor end-plate disease (med/med) mouse during post-

natal development. Dev Biol 101:401409.

Rios JC, Melandez-Vasquez CV, Einheber S, Lustig M, Grumet

M, Hemperly J, Peles E, Salzer JL (2000). Contactin-associ-

ated protein (Caspr) and contactin form a complex that is tar-

geted to the paranodal junctions during myelination. J Neu-

rosci 20:83548364.

Rosenbluth J (1976). Intramembranous particle distribution at

the node of Ranvier and adjacent axolemma in myelinatedaxons of the frog brain. J Neurocytol 5:731745.

Samsam M, Frei R, Marziniak M, Martini R, Sommer C (2002).

Impaired sensory function in heterozygous P0 knockout

mice is associated with nodal changes in sensory nerves. J

Neurosci Res 67:167173.

Sandri C, Van Buren JM, Akert K (1982). Membrane morphol-

ogy of the vertebrate nervous system. Prog Brain Res 46:

201265.

Scherer SS, Deschnes SM, Xu Y-T, Grinspan JB, Fischbeck

KH, Paul DL (1995). Connexin32 is a myelin-related protein in

the PNS and CNS. J Neurosci 15:82818294.

Scherer SS, Salzer JL (1996). Axon-Schwann Cell Interactions in

Peripheral Nerve Regeneration. In: Glial Cell Development.

Jessen KR, Richardson WD (Eds). Oxford University Press,Oxford, pp 299330.

Scherer SS, Xu T, Crino P, Arroyo EJ, Gutmann DH (2001).

Ezrin, radixin, and moesin are components of Schwann cell

microvilli. J Neurosci Res 65:150164.

Shapiro L, Doyle JP, Hensley P, Colman DR, Hendrickson WA

(1996). Crystal stucture of the extracellular domain from P 0,

the major structural protein of peripheral nerve myelin. Neu-

ron 17:435449.

Sherman DL, Fabrizi C, Gillespie CS, Brophy PJ (2001). Specific

disruption of a Schwann cell dystrophin-related protein com-

plex in a demyelinating neuropathy. Neuron 30:677687.

Shorer Z, Philpot J, Muntoni F, Sewry C, Dubowitz V (1995). De-


12/12


12

myelinating peripheral neuropathy in merosin-deficient con-

genital muscular dystrophy. J Child Neurol 10:472475.

Smart SL, Lopantsev V, Zhang CL, Robbins CA, Wang H, Chiu

SY, Schwartzkroin PA, Messing A, Tempel BL (1998). Dele-

tion of the Kv1.1 potassium channel causes epilepsy in

mice. Neuron 20:809819.

Srinivasan Y, Elmer L, Davis H, Bennett V, Angelides K (1988).

Ankyrin and spectrin associate with voltage-dependent so-

dium channels in brain. Nature 333:177180.

Stolinski C, Breathnach AS (1982). Freeze-fracture replication of

mammalian peripheral nervea review. J Neurol Sci 57:128.

Stolinski C, Breathnach AS, Martin B, Thomas PK, King RMH,

Gabriel G (1981). Associated particle aggregates in juxtapara-

nodal axolemma and adaxonal Schwann cell membrane of

rat peripheral nerve. J Neurocytol 10:679691.

Stolinski C, Breathnach AS, Thomas PK, Gabriel G, King RMH

(1985). Distribution of particle aggregates in the internodal

axolemma and adaxonal Schwann cell membrane of rodent

peripheral nerve. J Neurol Sci 67:213222.

Straub V, Campbell KP (1997). Muscular dystrophies and the dys-

trophin-glycoprotein complex. Curr Opin Neurol 10:168175.

Sunada Y, Bernier SM, Kozak CA, Yamada Y, Campbell KP (1994).

Deficiency of merosin in dystrophic dymice and genetic link-age of laminin M chain gene to dy locus. J Biol Chem 269:

1372913732.

Suter U, Snipes GJ (1995). Biology and genetics of hereditary mo-

tor and sensory neuropathies. Annu Rev Neurosci 18:4575.

Tait S, Gunn-Moore F, Collinson JM, Huang J, Lubetzki C, Pe-

draza L, Sherman DL, Colman DR, Brophy PJ (2000). An oli-

godendrocyte cell adhesion molecule at the site of assembly

of the paranodal axo-glial junction. J Cell Biol 150:657666.

Tao-Cheng J-H, Rosenbluth J (1984). Extranodal particle accu-

mulations in the axolemma of myelinated frog optic axons.

Brain Res 308:289300.

Thomas PK, Olsson Y (1993). Microscopic Anatomy and Func-

tion of the Connective Tissue Components of Peripheral

Nerve. In: Peripheral Neuropathy. Dyck PJ, Thomas PK, Grif-fin JW, Low PA, Poduslo JF (Eds). W.B. Saunders, Philadel-

phia, pp 97120.

Tobler AR, Notterpek L, Naef R, Taylor V, Suter U, Shooter EM

(1999). Transport of Trembler-J mutant peripheral myelin

protein 22 is blocked in the intermediate compartment and

affects the transport of the wild-type protein by direct inter-

action. J Neurosci 19:20272036.

Trapp BD, Andrews SB, Wong A, OConnell M, Griffin JW (1989).

Co-localization of the myelin-associated glycoprotein and the

microfilament components, F-actin and spectrin, in Schwann

cells of myelinated nerve fibers. J Neurocytol 18:4760.

Vabnick I, Shrager P (1998). Ion channel redistribution and func-

tion during development of the myelinated axon. J Neurobiol

37:8096.

Vabnick I, Trimmer JS, Schwarz TL, Levinson SR, Risal D,

Shrager P (1999). Dynamic potassium channel distributions

during axonal development prevent aberrant firing patterns.

J Neurosci 19:747758.

Vallat JM, Sindou P, Preux PM, Tabaraud F, Milor AM, Couratier P,

Leguern E, Brice A (1996). Ultrastructural PMP22 expression in

inherited demyelinating neuropathies. Ann Neurol 39:813817.

Van Dyke DH, Griggs RC, Murphy MJ, Goldstein MN (1975). He-

reditary myokymia and periodic ataxia. J Neurol Sci 25:109118.

Vonasek E, Mateu L, Luzzati V, Marquez G, Vargas R, Cespedes

G, Cotua M, Borges J (2001). A reversible abnormal form of

myelin: an X-ray scattering study of human sural and rat sci-

atic nerves. Eur Biophys J Biophys Lett 30:1116.

Wang H, Kunkel DD, Martin TM, Schwartkroin PA, Tempel BL

(1993). Heteromultimeric K channels in terminal and juxta-

paranodal regions of neurons. Nature 365:7579.

Xiao ZC, Ragsdale DS, Malhotra JD, Mattei LN, Braun PE,

Schachner M, Isom LL (1999). Tenascin-R is a functional mod-ulator of sodium channel beta subunits. J Biol Chem 274:

2651126517.

Xu H, Wu X-R, Wewer UM, Engvall E (1994). Murine muscular

dystrophy caused by a mutation in the laminin 2 (Lama2)gene. Nature Genet 8:297302.

Yamada H, Denzer AJ, Hori H, Tanaka T, Anderson LVB, Fujita

S, Fukutaohi H, Shimizu T, Ruegg MA, Matsumura K (1996).

Dystroglycan is a dual receptor for agrin and laminin-2 in

Schwann cell membrane. J Biol Chem 271:2341823423.

Zerr P, Adelman JP, Maylie J (1998). Episodic ataxia mutations

in Kv1.1 alter potassium channel function by dominant nega-

tive effects or haploinsufficiency. J Neurosci 18:28422848.

Zhang X, Bennett V (1996). Identification of O-linked N-acetyl-

glucosamine modification of ankyrinG isoforms targeted tonodes of Ranvier. J Biol Chem 271:3139131398.

Zhou DX, Lambert S, Malen PL, Carpenter S, Boland LM, Ben-

nett V (1998a). AnkyrinG is required for clustering of voltage-

gated Na channels at axon initial segments and for normal

action potential firing. J Cell Biol 143:12951304.

Zhou L, Zhang CL, Messing A, Chiu SY (1998b). Temperature-

sensitive neuromuscular transmission in Kv1.1 null mice:

Role of potassium channels under the myelin sheath in

young nerves. J Neurosci 18:72007215.

scherer and arroyo

Documents