Biology of Sialyl Glycans
Part 3: New thinking in function and glycan biosynthesis
Joseph Lau
ST6Gal-1
• A single copy gene (Siat1)
• Transcription starts from six physically distinct promoters
• Generation of the identical ST6Gal-1 protein
• Differential regulation in different tissues and cells
• Feature conserved across mammalian species
X1a X1b
X3
I
II
IV
III
V
VI
heart
brain
α2,6
spleen
H
lung
X2
P2c
liver
L
P2a
muscle
O
P1
kidney
Q
P4 P2b
testes
P3
β1,4
P1
ST6Gal-1 deficient mice have more
severe acute inflammatory response
*
2 00 0
* p <0.003
wt
d P1
*
Cells x 10-3
1 50 0
1 00 0
5 00
0
t ota l
eo s
Eos
Neu
neu
CD 8
Ly6Gpos CCR3pos
Ly6Gpos CCR3neg
CD 4
B
DC
;
Bone niches govern
hematopoietic stem cell activities
-- interactions between “seeds” and “soil”--
;
- from Yin & Li (2006) J Clin Invest. 116: 1195-1201
Hematopoietic stem and progenitor cells
Lin
neg
cKit
lo
Sca-1 lo
;
LSK
Lin neg
cKit ++
Sca-1 +
LK
Lin neg
cKit +
Sca-1 neg
Lin pos
HSPCs from ST6Gal-1 deficient (∆P1) animals
proliferate and differentiate faster ex vivo
LK
*
OPP
CMP
HSC
B
differentiation
platelet
MKEP
MPP
mono
GMP
WT
RBC
neu
cells x 105
LSK
A
∆P1
*
;
*
*
CLP
lymph
differentiation
Total
Lin
Sca1
cKit
+
+
+
+
+
+
-
Recombinant ST6Gal-1 can suppress ex vivo
HSPC proliferation and differentiation !!
LK
platelet
MKEP
MPP
RBC
mono
GMP
rST6Gal
cells x 106
CMP
HSC
no addition
OPP
LSK
differentiation
;
neu
CLP
lymph
differentiation
Total
LSK
LK
CD11b+
cKit +
CD11b+
cKit -
liver
ST6Gal-1 Expression
circulation
;
α2,6 sialylated
hepatic glycoproteins
ST6Gal-1
Proliferation, Differentiation
bone marrow
Challenging the Traditional Paradigm
TT
T
T
T T
T
secreted ST6
TT
T
T
T T
T
Each cell always makes its own glycans
Glycans can be made by non-self enzymes!
;
HSPCs have cell surface α2,6-sialic acids,
but do not express the ST6Gal-1 to make them
Cell surface α2,6 Sia
ST6Gal-1 enzyme activity
;
relative abundance
ST6Gal-1 mRNA
Cell surface α2,6-SA is constructed by
extrinsic ST6Gal-1
Mice strains:
WT= wild-type mice with intact ST6Gal-1
KO= mutant mice with defective ST6Gal-1
KO > KO
donor
donor
donor
donor
KO
WT
WT
KO
WT > KO
WT > WT
KO > WT
;
recipient
recipient
recipient
recipient
KO
KO
WT
WT
α2,6 sialic acid on cells?
Summary
Glycans as mediators between cell (“seed”) and niche (“soil”)
Glycans occupy critical intersection between cells and their outside
environment.
Existence of a novel, “extrinsic” glycosylation pathway
Extrinsic ST6Gal-1 can alter hematopoietic activity
;
Current Questions:
What is mechanism of ST6Gal-1 action?
•
How can this be exploited for therapeutic benefit?
Do other glycosyltransferases act by the “extrinsic” pathway?
•
Do they possess clinically useful properties?