apoptosis in neural development and disease

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Annu. Rev. Neurosci. 2000. 23:73–87Copyright 2000 by Annual Reviews. All rights reserved

0147–006X/00/0301–0073$12.00 73

APOPTOSIS IN NEURAL DEVELOPMENT

AND DISEASE

Deepak Nijhawan, Narimon Honarpour, andXiaodong Wang

Howard Hughes Medical Institute and Department of Biochemistry, University of Texas

Southwestern Medical Center at Dallas, Dallas, Texas 75235; e-mail:

[email protected], [email protected],

[email protected]

Key Words caspase, Bcl-2, cytochrome c, Apaf-1, neurodegenerative

Abstract Cell death via apoptosis is a prominent feature in mammalian neuraldevelopment. Recent studies into the basic mechanism of apoptosis have revealedbiochemical pathways that control and execute apoptosis in mammalian cells. Proteinfactors in these pathways play important roles during development in regulating the

balance between neuronal life and death. Additionally, mounting evidence indicatessuch pathways may also be activated during several neurodegenerative diseases,resulting in improper loss of neurons.

INTRODUCTION

In 1972, Kerr et al coined the term apoptosis, after the Greek word meaning

leaves falling from a tree, to describe an intrinsic cell suicide program involved

in the normal turnover of hepatocytes (Kerr et al 1972). Cell morphologic man-

ifestations of apoptosis include condensation of cell contents, nuclear membrane

breakdown, and the formation of apoptotic bodies that are small membrane-bound

vesicles phagocytosed by neighboring cells. Molecular components of the apop-totic pathway were first described in two important studies. Genetic studies in

Caenorhabditis elegans revealed three genes, ced3, ced4, and ced9, that specifi-

cally function in a pathway that controls developmental specific cell death (Ellis

et al 1991). Second, Bcl-2, a human oncogene overexpressed in follicular lym-

phoma, was found to influence cell apoptotic response (Adams & Cory 1998).

These discoveries ignited an explosion of research into apoptosis that in the past

decade has unveiled a complex, yet cohesive, picture of this intrinsic cell suicide

program. Apoptotic signals, both intracellular and extracellular, converge to acti-

vate a group of apoptotic-specific cysteine proteases termed caspases that cleave

their substrates with signature specificity after aspartic acid residues (Figure 1;

see color insert) (Thornberry & Lazebnik 1998). Chromatin condensation, DNA



74 NIJHAWAN HONARPOUR WANG

fragmentation into nucleosomal fragments, nuclear membrane breakdown, and

the formation of apoptotic bodies are direct consequences of caspase activity.

In this review article, we focus on current understanding of biochemical path-

ways upstream and downstream of caspase activation in mammalian neuronal

development and human neurological diseases. The review is divided into three

sections: (a) a current description of caspase activation and DNA fragmentation

in the apoptotic pathway, (b) the role of pro- and anti-apoptotic proteins in neural

development, and (c) evidence implicating apoptosis in neurodegenerative

disease.

BIOCHEMICAL MECHANISMS OF APOPTOSIS

DNA Fragmentation and Chromatin CondensationDuring Apoptosis

The fragmentation of DNA into nucleosomal fragments was one of the first iden-

tified cellular features of apoptosis, and it is commonly used as a biochemical

marker for apoptosis (Wyllie 1980). In vivo, nucleosomal DNA fragmentation is

assayed by the TUNEL (TdT-mediated dUTP-biotin nick end labeling) stain: FreeDNA ends are end labeled with biotinylated poly-dUTP by terminal deoxytrans-

ferase and then stained using avidin-conjugated peroxidase (Gavrieli et al 1992).

DNA fragmentation is mediated by a heterodimeric factor of 40 and 45 kDa,

respectively, in humans [DNA fragmentation factor (DFF) 40 and 45] and in mice

[caspase activated DNase (CAD) and inhibitor of caspase-activated DNase

(ICAD)] (Liu et al 1997, 1998; Enari et al 1998). DFF40/CAD and DFF45/ICAD

are encoded by novel genes and do not share sequence homology with other

proteins with known functions. In apoptotic cells, DFF45, which has two caspase

cleavage sites, is cleaved into three smaller fragments. Cleaved DFF45 dissociates

from DFF40, inducing oligomerization of DFF40 into a large protein complex

that has DNase activty (Liu et al 1999). DFF activity can only be reconstituted

by coexpressing the two subunits together (Liu et al 1998, Enari et al 1998).

When expressed alone, DFF40 has lower expression and no DNase activity, whichsuggests that DFF45 functions as a specific molecular chaperone important for

DFF40 activation and synthesis (Liu et al 1998). Unlike other DNases, DFF40 is

significantly stimulated by internucleosomal, chromatin-associated proteins such

as high mobility group (HMG)-1, -2, and -14 and histone H1 but not core histones

(Liu et al 1998, 1999). HMGs and histone H1 may target DFF40 to the internu-

cleosomal linker region, resulting in the exquisite pattern of internucleosomal

DNA fragmentation commonly detected during apoptosis. The multimeric nature

of the active DFF40 may also contribute to apoptotic chromatin condensation by

pulling cleaved nucleosomal fragments together. After treatment with active

DFF40, nuclei stained with a DNA dye exhibit bright particles, an apoptotic

hallmark indicative of chromatin condensation (Liu et al 1998). Thymocytes and



NEURAL APOPTOSIS 75

splenocytes from mice deficient in ICAD die by apoptosis but fail to condense

chromatin or fragment DNA (Zhang et al 1998). ICAD null mice develop nor-

mally and are fertile, indicating that DNA fragmentation and chromatin conden-

sation during apoptosis is not essential for normal development of a mouse (Zhang

et al 1998).

Caspase Activation PathwaysIn living cells, caspases exist as inactive zymogens that, like DFF, are activated

by proteolytic cleavage (Thornberry & Lazebnik 1998). There are two relatively

well-studied pathways that lead to caspase activation (Figure 1). One pathway

involves death receptors, such as Fas, and a tumor necrosis factor (TNF) receptor

at the cell surface, leading to the activation of caspase-8 intracellularly (Ashkenazi

& Dixit 1998). Fas ligand and TNF, which usually exist as trimers, bind and

activate their receptors by inducing receptor trimerization (Nagata 1997). Acti-

vated receptors recruit adaptor molecules such as FADD/MORT1 (Fas-associat-

ing protein with death domain), which recruit procaspase-8 to the receptor

complex, where it undergoes autocatalytic activation (Boldin et al 1995, 1996;

Chinnaiyan et al 1995; Muzio et al 1996; Srinivasula et al 1996). Activated cas-

pase-8 will cleave and activate other downstream caspases, such as caspase-3,

caspase-6, and caspase-7, constituting the main caspase activity of apoptotic cells

(Boldin et al 1996, Muzio et al 1996, Srinivasula et al 1996).

Another means of caspase activation is through the release of cytochrome c

from the mitochondria. Cytochrome c is a 13-kDa soluble electron transfer protein

exclusively located in the mitochondrial intermembrane space. During apoptosis,

however, the outer membrane of mitochondria becomes permeable to cytochrome

c (Liu et al 1996), which binds to Apaf-1.

Apaf-1 is a 130-kDa cytosolic monomer consisting of three distinctive

domains: a caspase recruitment domain, CED4 homologous domain, and a series

of WD40 repeats (Zou et al 1997). On induction of apoptosis, Apaf-1 forms a

multimeric complex with cytochrome c (Li et al 1997, Zou et al 1999). Apaf-1/

cytochrome c complexes are sufficient to recruit and activate procaspase-9. Acti-

vated caspase-9 released from the complex activates downstream caspases suchas caspase-3, caspase-6, and caspase-7.

Regulation of Cytochrome c Release by the Bcl-2 Familyof Proteins

A major regulatory step for caspase activation is at the level of cytochrome c

release from mitochondria to cytosol. Cytochrome c release not only initiates

caspase activation by activating Apaf-1, it also breaks the electron transfer chain

resulting in reduced energy generation and more reactive oxygen species due to

incomplete reduction of atomic oxygen (Reed 1997). The release of cytochrome

c is regulated by the Bcl-2 family of proteins, including anti-apoptotic members




Bcl-2 and Bcl-xL and pro-apoptotic Bak, Bim, Bad, and Bax (Adams & Cory

1998). Overexpression of anti-apoptotic Bcl-2, or its close homologue Bcl-xL,

blocks cytochrome c release induced by a variety of apoptotic stimuli (Kim et al

1997, Kluck et al 1997, Yang et al 1997). In contrast, Bax, Bak, and Bid have

been shown to directly cause cytochrome c release both in vivo and in vitro

(Jurgensmeier et al 1998, Kuwana et al 1998, Li et al 1998, Luo et al 1998, Rosse

et al 1998). The mechanism of cytochrome c release and its regulation by the

Bcl-2 family of proteins is not known. One possibility is that changes in mito-

chondrial membrane permeability induce mitochondrial swelling, causing outer

membrane rupture (Kroemer et al 1997; Vander Heiden et al 1997, 1999). On the

other hand, increases in outer membrane permeability may occur independent of

swelling.

Intrinsic or extrinsic death signals may be transmitted to the mitochondria by

the translocation of pro-apoptotic Bcl-2 family members to the mitochondria from

different cellular compartments. Extracellular death signals such as Fas ligand or

TNF activate caspase-8 intracellularly. Activated caspase-8 cleaves and activates

Bid, which translocates to the mitochondria and induces cytochrome c release,

amplifying the caspase activation signal (Li et al 1998, Luo et al 1998). Extra-

cellular survival signals inhibit apoptosis by activating the phosphotidylinositol-

3 kinase/Akt pathway, leading to Bad phosphorylation. Phosphorylated Bad binds14–3-3 protein and is sequestered in the cytoplasm, whereas dephosphorylated

Bad translocates to the mitochondria (Zha et al 1996). Conversely, Ca2 may

induce apoptosis by activating the calcineurin-dependent phosphatase that

dephosphorylates Bad (Wang et al 1999). Other intrinsic death signals may reg-

ulate Bim translocation. In normal living cells, Bim, a pro-apoptotic Bcl-2 family

member, binds to LC8, a cytoskeletal component. After cells are induced to die

by apoptosis, the Bim/LC8 complex dissociates from the cytoskeleton and trans-

locates to the mitochondria (Puthalakath et al 1999). Bax has also been shown to

translocate from the cytoplasm to the mitochondria during apoptosis (Wolter et

al 1997). Apoptotic signals may activate the translocation of these factors to the

mitochondria, which then trigger cytochrome c release, inducing caspase

activation.

APOPTOSIS IN NEURAL DEVELOPMENT

Apoptosis occurs throughout the nervous system in neuron, glial, and neural pro-

genitor cells. It is estimated that at least half of the original cell population is

eliminated as a result of apoptosis in the developing nervous system (Oppenheim

1981, Burek & Oppenheim 1999). The potential role of apoptosis during neural

development includes optimization of synaptic connections, removal of unnec-

essary neurons, and pattern formation (Burek & Oppenheim 1999). A neuron’s

chance for survival during development is believed to directly depend on the

extent of its connections to a postsynaptic target, which suggests that neurons are



NEURAL APOPTOSIS 77

initially overproduced and then compete for target-derived neurotrophic factors

(Cowan et al 1984). Neurons also receive trophic support from glial cells, pre-

synaptic cells, and steroid hormones (Lindsay 1979, Okado & Oppenheim 1984,

Nordeen et al 1985, Linden 1994).

How do such developmental signals as limited trophic factor support induce

apoptosis during neural development? Neurotrophin mediated survival is linked

to the regulation of cytochrome c release and caspase activation. Following NGF

withdrawal from sympathetic neurons, cytochrome c is released from the mito-

chondria to the cytosol, and apoptosis ensues (Deshmukh & Johnson 1998). In

Bax-deficient sympathetic neurons, however, there is a delay in cytochrome c

release when NGF is removed, implying that Bax catalyzes the release of cyto-

chrome c on withdrawal of nerve growth factor (NGF) (Easton et al 1997, Desh-

mukh & Johnson 1998, Neame et al 1998).

The balance between pro- and anti-apoptotic Bcl-2 family members is impor-

tant for neural survival during development. Mice deficient in Bcl-xL, which is

widely expressed throughout the developing nervous system, die at embryonic

day 13 (E13) with extensive apoptotsis in the postmitotic, differentiating imma-

ture neurons of the brain, spinal cord, and dorsal root ganglia (Motoyama et al

1995). Bcl-2 is widely expressed in the developing nervous system but only

persists at high levels in the adult peripheral nervous system (Merry et al 1994).Postnatal Bcl-2 knockout mice have significant degeneration of the facial motor

neurons, dorsal root ganglion (L3) sensory neurons, and sympathetic neurons in

the superior cervical ganglion, indicating that Bcl-2 is necessary for peripheral

nervous system survival postnataly but not for neuronal survival during central

nervous system development (Michaelidis et al 1996). Consistently, ectopic

expression of Bcl-2 in neurons results in brain hypertrophy, with more neurons

in the mesencephalic nucleus of the trigeminal nerve, the facial nucleus, the fifth

lumbar dorsal root ganglion, and the retinal ganglion cell layer (Martinou et al

1994, Farlie et al 1995). Deletion of Bax, which is predominantly expressed in

the neonatal cortex, superior cervical ganglion, and facial motor nucleus, results

in a 51% increase in facial motor neurons and yields 2.5-fold more superior

cervical ganglion neurons (Deckwerth et al 1996, Vekrellis et al 1997). Notably,

mice heterozygous for Bax disruption possess more neurons in the facial motor

nucleus and superior cervical ganglion than do wild-type mice but fewer than

Bax null mice, indicative of a gene dosing effect (Deckwerth et al 1996).

Alterations in Bcl-2 family members also regulate cell death following axo-

tomy. Bax, but not Bcl-2, is important for mediating axotomy-induced apoptosis.

Motor neurons do not survive after axotomy in Bcl-2–deficient or wild-type mice;

however, in response to axotomy, 86% of the facial motor neurons in neonatal

Bax-deficient mice survive (Deckwerth et al 1996). Following axotomy, over-

expression of Bcl-2 reduces death probably by tilting the balance of pro- and anti-

apoptotic Bcl-2 family members in favor of survival (Allsopp et al 1993,

Dubois-Dauphin et al 1994, Farlie et al 1995, Michaelidis et al 1996). Target-




derived trophic factors may promote survival by influencing Bax activity, poten-

tially preventing its translocation from the cytosol to the mitochondria.

The essential role of apoptosis in neural development is dramatically illustrated

by mice deficient in Apaf-1, caspase-9, and caspase-3 (Kuida et al 1996, Cecconi

et al 1998, Hakem et al 1998, Kuida et al 1998, Yoshida et al 1998). Despite

their ubiquitous expression, pathology resulting from the disruption of these genes

mostly affects the developing brain and is lethal. In all three cases, knockout mice

have prominent protrusions of the forebrain with exencephaly (Figure 2; see color

insert). These mice also possess craniofacial malformations and ventriclular

obstruction by supernumerary mitotic and differentiating neurons (Cecconi et al

1998, Yoshida et al 1998). These studies further support in vitro data describing

a linear cytochrome c–dependent caspase activation pathway: No activated cas-

pase-3 is detected in the brains of Apaf-1 null mice (Cecconi et al 1998).

APOPTOSIS IN NEURODEGENERATIVE DISEASE

If neurons die by apoptosis during development, could the same pathways be

activated in neurodegenerative disease? Recently, investigation into neurodegen-

erative disease has focused on this question in the hope that apoptosis may ulti-mately serve as a valuable therapeutic target for several currently untreatable

diseases. We focus on three well-studied disorders with strong implications of

apoptosis in neurodegeneration: Alzheimer’s disease, Huntington’s disease, and

amyotrophic lateral sclerosis. All these diseases have the following in common:

(a) a familial form of the disease with a mendelian inheritance pattern, (b) selec-

tive degeneration of particular neuronal subtypes, and (c) disease-associated cel-

lular or extracellular aggregates.

Alzheimer’s Disease

Alzheimer disease (AD) is the most common cause of dementia among the

elderly. AD is the result of damage to selective neuronal circuits in the neocortex,

hippocampus, and basal forebrain cholinergic system. Some forms of AD, pri-marily early onset, are familial (FAD) and show an autosomal dominant inheri-

tance pattern. Missense mutations in three genes, amyloid precursor protein

( APP), presenilin-1 (PS1), and presenilin-2 (PS2) are associated with FAD (Price

& Sisodia 1998). In AD, postmortem histopathology reveals either one or both

of the following: (a) dystrophic neurites and intracellular neurofibrillary tangles,

which are composed of the tau protein (Goedert et al 1996); (b) extracellular

senile plaques composed of the 42- to 43-amino acid b-amyloid peptide (Ab),

which is a minor proteolytic product of APP. Ab is neurotoxic to primary neuronal

cells, and overexpression of the Ab peptide intracellularly in transgenic mice

causes neurodegeneration. Additionally, transgenic mice overexpressing FAD

mutant APP (V717F), wild-type PS1, or both develop senile plaques composed



NEURAL APOPTOSIS 79

of Ab aggregates and dystrophic neurites similar to the histopathology seen in

AD patients (Games et al 1995, Masliah et al 1996, Holcomb et al 1998). Degen-

erating neurons in APP V717F b-amyloid mice show chromatin segmentation

and condensation and increased TUNEL staining, which suggests an apoptotic

death.

COS or F11 cells (a hybrid of a primary rat dorsal root ganglion neuron and

a mouse neuroblastoma cell line) overexpressing FAD mutant APP (V642I,

V642F, or V642G) have increased DNA fragmentation and TUNEL staining, i.e.

inhibited by Bcl-2 coexpression (Yamatsuji et al 1996a,b). In a similar manner,

wild-type PS2, FAD mutant PS2 (N141I), or PS1 (A246E) overexpression sen-

sitizes PC12 cells to apoptosis after b-amyloid treatment or trophic factor with-

drawal (Deng et al 1996, Wolozin et al 1996). In contrast, using a herpes simplex

virus, Bursztajn and colleagues (1998) found that overexpression of PS1 or PS1

(A246E) in primary mouse cortical culture does not enhance apoptosis, which

suggests that the apoptogenic effects of mutant presenlin in culture are cell type

specific. The importance of apoptosis in AD pathogenesis is supported by evi-

dence describing increased TUNEL staining and activated caspases in postmor-

tem analysis of AD brain (Dragunow et al 1995, Lassmann et al 1995, Smale et

al 1995, Bancher et al 1996, Gervais et al 1999).

Little is known about which molecules mediate AD-associated apoptosis. Oneimportant player might be Par-4, which was first identified in prostate cancer cells

undergoing apoptosis (Sells et al 1994, 1997). Par-4 is expressed at high levels

in regions of the brain affected by AD, including the hippocampus. Apoptosis in

hippocampal neurons exposed to b-amyloid in culture requires Par-4 up-regula-

tion because neurons transfected with Par-4 antisense message survive treatment

(Guo et al 1998).

Huntington’s Disease

Huntington’s disease (HD) is a fatal neurodegenerative disorder with an autoso-

mal dominant pattern of inheritance characterized by hyperkinetic involuntary

movements, slowing of voluntary movements, and cognitive impairment (Harper

1991). The major pathological change in HD patients is the selective degenerationof the cortex and striatum due to a CAG triplet expansion in the first exon of

huntingtin, which forms a polyglutamine expansion in a protein of approximately

350 kDa (Huntington’s Dis. Collab. Res. Group 1993). Normal individuals have

between 10 and 35 repeats whereas HD patients have from 37 to 121. Immuno-

histological analysis of brains from HD patients and the HD mouse model

revealed neuronal nuclear inclusions throughout the brain that are immunoreac-

tive with anti-huntingtin and anti-ubiquitin (Davies et al 1997, DiFiglia et al 1997,

Scherzinger et al 1997, Reddy et al 1998).

Huntingtin shares no significant sequence homology with any other known

genes and is widely expressed throughout the brain in regions both affected and

spared during the course of HD. Mice deficient in huntingtin die between E8.5




and E10.5 (Duyao et al 1995, Nasir et al 1995, Zeitlin et al 1995). In huntingtin

null mice, embryonic ectodermal cells exhibit increased apoptosis,which suggests

an anti-apoptotic role for huntingtin. Transgene expression of full-length hun-

tingtin cDNA with 48 or 89 but not 16 CAG repeats exhibits a progressive neu-

rological phenotype recapitulating many of the clinical features of HD (Reddy et

al 1998). Neuropathological analysis revealed prominent neuronal loss in the

striata, hippocampus, thalamus, and cerebral cortex whereas purkinje cells

remained unaffected, demonstrating selective neuronal loss. These mice also have

intranuclear neuronal inclusions that directly correlate with disease severity.

Finally, neurodegenerative regions of the brain show increased TUNEL staining

compared with age-matched wild-type controls, which is suggestive of apoptotic

death.

Recently, debate has focused on the role of nuclear inclusions in neurodegen-

eration and HD pathogenesis. Saudou et al (1998) demonstrate that mutant hun-

tingtin selectively kills striatal but not purkinje neurons in culture by apoptosis.

Cell death was dependent on nuclear localization but was not associated with

intranuclear inclusions. In fact, they propose that the formation of inclusions

might be a form of cellular defense because conditions that suppress inclusion

formation result in increased apoptosis. On the other hand, Sanchez et al (1999)

show that FADD is recruited to and caspase-8 is activated on inclusions, whichsuggests that inclusions composed of polyglutamine repeats induce apoptosis by

catalyzing caspase activation. These authors induce apoptosis in primary cere-

bellar and striatal neurons by overexpressing a construct containing green fluo-

rescent protein and an expanded CAG repeat. Cotransfection with a dominant

negative form of FADD inhibits apoptosis and reduces recruitment of caspase-8

to inclusions. Furthermore, they identified activated caspase-8 in the insoluble

protein fraction of postmortem HD brain, which suggests that activated caspase-

8 colocalizes with inclusions. Cell culture studies and in vivo studies of mouse

models suggest that cell death induced by mutant huntingtin involves apoptosis.

The role of inclusions in HD-related apoptosis is still unclear. However, if neurons

die apoptotically because of inclusion-induced caspase activation, then drugs that

inhibit inclusion formation or inhibit caspase activation would be of potential

therapeutic benefit.

Amyotrophic Lateral Sclerosis

Patients with amyotrophic lateral sclerosis (ALS) suffer from muscle atrophy and

fatal paralysis as a result of selective degeneration of both upper and lower motor

neurons. Some 10% of ALS cases are familial (FALS), with an autosomal dom-

inant pattern of inheritance (Brown 1995). Some 20% of FALS cases are due to

mutations in the ubiquitously expressed cytoplasmic Cu/Zn superoxide dismutase

(SOD1), which protects cells from oxidative damage induced by the superoxide

anion (Deng et al 1993, Rosen et al 1993). Transgenic mice overexpressing FALS

mutant SOD1 exhibit a phenotype similar to FALS, including selective loss of



NEURAL APOPTOSIS 81

upper and lower motor neurons, muscle atrophy, and paralysis (Gurney et al 1994,

Dal Canto & Gurney 1995, Ripps et al 1995, Bruijn et al 1997). FALS patients

with SOD1 mutations and SOD1 mutant mice have neuronal and astrocytic cyto-

plasmic inclusions that are immunoreactive for SOD1 (Kato et al 1996, 1999;

Shibata et al 1996; Bruijn et al 1998). In SOD1 mutant mice, inclusions appear

before disease onset and increase in abundance during the course of disease

(Bruijn et al 1998). In FALS, mutant SOD1 may adopt a novel gain of function

that is selectively toxic to motor neurons.

Bcl-2 overexpression in neurons of ALS mice significantly delays the onset

but not the duration of disease (Kostic et al 1997). In contrast, overexpression of

dominant negative caspase-1 (DNcaspase-1) in neurons delays the duration but

not the onset of disease (Friedlander et al 1997). These data may be explained

by the differing effects of Bcl-2 and DNcaspase-1 on apoptosis; upstream, Bcl-2

inhibits cytochrome c release, and downstream, DNcaspase-1 inhibits caspase

activity. Cytochrome c release may be the trigger for the toxic effect of mutant

SOD1, and thus inhibiting release would delay onset. However, once cytochrome

c is released and caspases are activated, the cell is committed to die, so caspase

inhibition will only delay the course of disease. Although these studies provide

in vivo evidence that motor neurons in ALS mice die via apoptosis, they should

be considered with caution for the following reasons: (a) Mice overexpressingBcl-2 in neurons possess more neurons than do normal mice, which might explain

the effect on disease course (Martinou et al 1994), and (b) DN-caspase-1 may not

inhibit other caspases, which presumably would also be activated.

The toxic effects of mutant SOD1 have also been shown in culture. Using an

adenoviral transducing system, Ghadge et al (1997) showed that two different

SOD1 mutations are selectively toxic to PC12 cells, primary sympathetic, and

hippocampal pyramidal mouse neurons but not astrocytes. Dying PC12 cells or

primary neurons exhibit characteristics of an apoptotic death, including shrunken

cell bodies, increased TUNEL staining, and chromatin condensation. Moreover,

PC12 cell death is inhibited by caspase inhibitors and Bcl-2 contransfection.

CONCLUSION

Much evidence has been gathered implicating apoptosis in neurodegenerative

disease. Although these studies provide hope that the apoptotic program is an

effective therapeutic target, important questions about how disease induces apop-

tosis remain: Is apoptosis the cell’s reaction to a permanent neuronal insult

inflicted by disease or is the disease directly involved in activating the apoptotic

program? In the latter case, anti-apoptotic agents such as caspase inhibitors could

be used to inhibit cell death and preserve cellular integrity. In the former case,

however, caspase inhibitors would be less beneficial because cellular integrity

would already be compromised by disease.




All three of the diseases reviewed are associated with intracelluar or extracel-

lular protein aggregation (Kakizuka 1998). Recent reports suggest that neurode-

generation may be the result of protein aggregation that directly activates caspases

and induces apoptosis. It is difficult to imagine how mutated disease genes might

directly activate the apoptotic program, because disease onset occurs in later life.

However, the formation of aggregates later in life might serve as the “rate-limiting

step” in triggering the degenerative disease process. The recruitment of apopto-

genic proteins to protein aggregates may activate apoptotic pathways because

oligomerization steps are required for the formation of active multimeric com-

plexes in FADD/caspase-8, Apaf-1/caspase-9, and DFF pathways. In such cases,

drugs that prevent aggregate formation or inhibit caspase activation might be

effective therapies.

ACKNOWLEDGMENTS

We thank Drs. Razqallah Hakem and Tak Mak for pictures of wild-type, Apaf-

1, caspase-9, and caspase-3 mutant mice. We thank Dr. Roger Rosenberg for

suggestions and critical reading of the manuscript.

Visit the Annual Reviews home page at www.AnnualReviews.org.

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NIJHAWAN s HONARPOUR s WANG C-1

Figure 1 Caspases are activated through two different pathways: Fas/FADD/Caspase-8

and Cytochrome c /Apaf-1/Caspase-9. Cytochrome c release from the mitochondria to the

cytosol is regulated by Bcl-2 family members.

y

g

g

y

gy

p

y



Figure 2 Brain morphology of wild-type, Apaf-1, caspase-9, and caspase-

3 mutant mice demonstrating forebrain extrusions. Cytochrome c /Apaf-

1/Caspase-9 pathway is important for executing apoptosis during neural

development. Courtesy of Razqallah Hakem and Tak Mak.

C-2 NIJHAWAN s HONARPOUR s WANG

y

g

g

y

gy

p

y



Annual Review of Neuroscience

Volume 23, 2000

CONTENTS

Cortical and Subcortical Contributions to Activity-Dependent Plasticity in

Primate Somatosensory Cortex, Edward G. Jones 1

Microtubule-Based Transport Systems in Neurons: The Roles of Kinesinsand Dyneins, Lawrence S. B. Goldstein, Zhaohuai Yang 39

Apoptosis in Neural Development and Disease, Deepak Nijhawan,

Narimon Honarpour, Xiaodong Wang 73

Gain of Function Mutants: Ion Channels and G Protein-Coupled

Receptors, Henry A. Lester, Andreas Karschin 89

The Koniocellular Pathway in Primate Vision, Stewart H. C. Hendry, R.

Clay Reid 127

Emotion Circuits in the Brain, Joseph E. LeDoux 155

Dopaminergic Modulation of Neuronal Excitability in the Striatum and

Nucleus Accumbens, Saleem M. Nicola, D. James Surmeier, Robert C.

Malenka 185

Glutamine Repeats and Neurodegeneration, Huda Y. Zoghbi, Harry T.

Orr 217

Confronting Complexity: Strategies for Understanding the Microcircuitry

of the Retina, Richard H. Masland , Elio Raviola 249

Adaptation in Hair Cells, Ruth Anne Eatock 285

Mechanisms of Visual Attention in the Human Cortex, Sabine Kastner

and Leslie G. Ungerleider 315

The Emergence of Modern Neuroscience: Some Implications for

Neurology and Psychiatry, W. Maxwell Cowan, Donald H. Harter, Eric

R. Kandel 343

Plasticity and Primary Motor Cortex, Jerome N. Sanes, John P.

Donoghue 393

Guanylyl Cyclases as a Family of Putative Odorant Receptors, Angelia D.

Gibson, David L. Garbers 417Neural Mechanisms of Orientation Selectivity in the Visual Cortex, David

Ferster, Kenneth D. Miller 441

Neuronal Coding of Prediction Errors, Wolfram Schultz, Anthony

Dickinson 473

Modular Organization of Frequency Integration in Primary Auditory

Cortex, Christoph E. Schreiner, Heather L. Read, Mitchell L. Sutter 501

Control of Cell Divisions in the Nervous System: Symmetry and

Asymmetry, Bingwei Lu, Lily Jan, Yuh-Nung Jan 531

Consciousness, John R. Searle 557

The Relationship between Neuronal Survival and Regeneration, Jeffrey L.

Goldberg, Ben A. Barres 579

Neural Representation and the Cortical Code, R. Christopher deCharms, Anthony Zador 613

Synaptic Plasticity and Memory: An Evaluation of the Hypothesis, S. J.

Martin, P. D. Grimwood, R. G. M. Morris 649

Molecular Genetics of Circadian Rhythms in Mammals, David P. King,

Joseph S. Takahashi 713

Parallel Pathways for Spectral Coding in Primate Retina, Dennis M.

Dacey 743

Pain Genes?: Natural Variation and Transgenic Mutants, Jeffrey S. Mogil,

Lei Yu, Allan I. Basbaum 777

apoptosis in neural development and disease

Documents