Essentials of Glycobiology

May 1st, 2008

Ajit VarkiLecture 11

Chapter 12 : Sialic Acids

Chapter 32 : I-type Lectins

Major Glycan Classes in Vertebrate Cells

General Questions for Lecture 11

1. Compare and contrast the structure of sialic acids with other vertebrate monosaccharides

2. What advantages does sialic acid diversity provide in vertebrate systems?

3. What are the unique features of the sialic acid biosynthetic pathways in comparison to those of other vertebrate monosaccharides?

4. How would you determine if a previously unstudied organism contains sialic acids?

5. Contrast the addition of lpha2-6-linked sialic acids to O-GalNAc glycans and N-linked glycans and their recognition by sialic acid-binding lectins.

6. Why do plants and invertebrates that do not express sialic acids have sialic acid binding proteins?

7. There are now more than a dozen human Siglecs known.  Why were these and other sialic acid binding proteins not discovered until very recently?

8. Compare the potential function of Siglecs with inhibitory motifs in their cytosolic tails with those that can recruit activatory motifs

9. Why are Siglec homologs found primarily in "higher" animals

10. Why are some Siglecs evolving rapidly?

11. What changes in sialic acid biology occurred during human evolution?

Two common “primary” sialic acids.

1. Compare and contrast the structure of sialic acids with other vertebrate monosaccharides

Biological Roles of Sialic Acids

EXTRINSIC RECEPTOREXTRINSIC RECEPTOR

M

Ligands forIntrinsic

Receptors

SiglecsFactor HSelectinsL1CAM

Laminins?

SIALYLATED GLYCAN =

M = Micro-organism/Toxin

InfluenzaMalariaCholera

HelicobacterMycoplasma

RotavirusSV40 virus

CoronavirusPertussis

Tetanus etc.

Ligands for Extrinsic

Receptors

Structural/Physical Roles

MolecularMimicry Meningococcus

E.Coli K1Gonococcus

CampylobacterTrypanosoma

Group BStreptococcus

Etc.

SELFSELF

INTRINSIC RECEPTORINTRINSIC RECEPTOR

SELF

Natural Diversity in the Sialic Acids

O

COO

O

O

OO

O 1

2

345

69 8

7

R1

R2

R4

R7R8

R9

R5

Natural Diversity in the Sialic Acids

R1 = H, dissociation at physiological pH gives negative charge; lactones with -OH groups on same molecule or other glycans; lactams with a free amino group at C5; or tauryl group.

R2 = H in free Sia; alpha linkage to Gal(3/4/6), GalNAc(6), GlcNAc(4/6), Sia (8/9) or 5-O-Neu5Gc; oxygen linked to C7 in 2,7-anhydro molecule; anomeric hydroxyl eliminated in Neu2en5Ac (double bond to C3)

R4 = H, O-acetyl, anhydro to C8, Fuc, Gal

R5 = Amino, N-acetyl, N-glycolyl, hydroxyl, N-acetimidoyl, N-glycolyl-O-acetyl, N-glycolyl-O-methyl, N-glycolyl-5-O-2-Neu5Gc

R7 = H, O-acetyl, anhydro to C2; substituted by amino and N-acetyl in Leg

R8 = H, O-acetyl, anhydro to C4, O-methyl, O-sulfate, Sia, Glc

R9 = OH, O-acetyl, O-lactyl, O-phosphate, O-sulfate, Sia: OH substituted by H in Leg.

2. What advantages does sialic acid diversity provide in vertebrate systems?

Terminal, oligo-, and poly-sialic acids, and the enzymes that can degrade them

Genes and pathways involved in the biology of sialic acids.3. What are the unique features of the sialic acid biosynthetic pathways in comparison to those of other vertebrate monosaccharides?

Examples of terminal glycan sequences recognized by some sialic-acid-binding proteins.

5. Contrast the addition of lpha2-6-linked sialic acids to O-GalNAc glycans and N-linked glycans and their recognition by sialic acid-binding lectins.

6. Why do plants and invertebrates that do not express sialic acids have sialic acid binding proteins?

Biological Roles of Sialic Acids

EXTRINSIC RECEPTOREXTRINSIC RECEPTOR

M

Ligands forIntrinsic

Receptors

SiglecsFactor HSelectinsL1CAM

Laminins?

SIALYLATED GLYCAN =

M = Micro-organism/Toxin

InfluenzaMalariaCholera

HelicobacterMycoplasma

RotavirusSV40 virus

CoronavirusPertussis

Tetanus etc.

Ligands for Extrinsic

Receptors

Structural/Physical Roles

MolecularMimicry Meningococcus

E.Coli K1Gonococcus

CampylobacterTrypanosoma

Group BStreptococcus

Etc.

SELFSELF

INTRINSIC RECEPTORINTRINSIC RECEPTOR

SELF

Domain structures of the known Siglecs in humans and mice.

7. There are now more than a dozen human Siglecs known.  Why were these and other sialic acid binding proteins not discovered until very recently?

From: Crocker P, Paulson J. & Varki, A. Nature Reviews Immunol. 7:255-266, 2007.

Biological Interactions Involving Siglecs

From: Crocker P, Paulson J. & Varki, A. Nature Reviews Immunol. 7:255-266, 2007.

Signaling Responses Mediated by Siglecs8. Compare the potential function of Siglecs with inhibitory motifs in their cytosolic tails with those that can recruit activatory motifs

Fig. 32.2

Structural basis of Siglec binding to ligands. X-ray crystal structures of the

V-set domains ofsialoadhesin (Sn) (A) and

Siglec-7 (B) are shown complexed with sialic

acid.

(C,D) Molecular details of

interactions of sialic acid with Sn and Siglec-7.

Biological functions mediated by sialoadhesin: Interactions of sialoadhesin on macrophages with cells and pathogens.

(Right) Red staining shows ring of sialoadhesin expressed by macrophages in marginal zone of spleen and green staining shows

Siglec-H on the plasmacytoid dendritic cells

Proposed Biological functions mediated by CD22: CD22 glycan-dependent homotypic interactions in equilibrium with CD22–BCR interactions.

Biological functions mediated by myelin-associated glycoprotein (MAG)

Proposed Biological functions mediated by CD33-related Siglecs

Chromosomal organization of CD33-related Siglec clusters in some rodents and primates

9. Why are Siglec homologs found primarily in "higher" animals?

10. Why are some Siglecs evolving rapidly?

Proposed Evolutionary Chain of “Red Queen” Effects involving Sialic Acids and CD33-related-Siglecs

Host sialic acids are evolving to evade pathogens

that exploit them as receptors?

Host Sialic acidsPathogens

Pathogens are evolving to utilize host sialic acids

as receptors.

CD33rSiglecs

Siglecs are evolving to adjust to the changes

of host sialic acids?

Host sialic acids are evolving to adjust to the changes

of Siglecs?

SialylatedPathogens

Pathogens expressing sialic acids are evolving to utilize Siglecs as receptors?

Siglecs are evolving to evade sialylated pathogens

that exploit them as receptors?

Based on Varki &Angata Glycobiology 16:1R-27R, 2006.Modified by Takashi Angata: For Glycoforum/Glycowords

PrimaryPrimary““Red Red

QueenQueen””EffectEffect

PrimaryPrimary““Red Red

QueenQueen””Effect?Effect?

SecondarySecondary““Red Red

QueenQueen””Effect?Effect?

Proposed Evolutionary Scenario for Multiple Proposed Evolutionary Scenario for Multiple Human-Specific Changes in Sialic Acid BiologyHuman-Specific Changes in Sialic Acid Biology

Varki A. Nature 446: 1023, 2007

11. What changes in sialic acid biology occurred during human evolution?

Proposed Evolutionary Scenario for Multiple Proposed Evolutionary Scenario for Multiple Human-Specific Changes in Sialic Acid BiologyHuman-Specific Changes in Sialic Acid Biology

Varki A. Nature 446: 1023, 2007

Proposed Evolutionary Scenario for Multiple Proposed Evolutionary Scenario for Multiple Human-Specific Changes in Sialic Acid BiologyHuman-Specific Changes in Sialic Acid Biology

Varki A. Nature 446: 1023, 2007

ST6GAL1ST6GAL1

SIGLEC7SIGLEC7SIGLEC9SIGLEC9

DELETIONAMINO ACID CHANGEEXPRESSION CHANGE

GENE CONVERSION

SIGLEC6SIGLEC6SIGLEC11SIGLEC11SIGLEC5/14SIGLEC5/14SIGLEC13SIGLEC13

CMAHCMAH

SIGLEC12SIGLEC12

SIGLEC1SIGLEC1