essential role in fibroblast growth factor (FGF)
signaling (31), and the number of FGF–FGF receptor protomers in a supramolecular assembly
directly correlates with the size of the GAG chain
(32). Opposing HSPG and CSPG effects have
been reported in semaphorin-mediated axon
guidance (22). HSPGs and CSPGs differ in the
chemical composition of their GAG chains. The
distribution of sulfate groups (typically one to
two per disaccharide) along CS chains is relatively uniform, whereas HS has a distinct modular
composition, with high-sulfation regions (three
groups per disaccharide) flanked by intermediately
modified transition zones and variably spaced
by largely unmodified sections almost devoid of
sulfation (33). Our observations suggest a model
in which islands of high sulfation present in HS,
but not CS, may promote close packing of RPTPs
RPTPs clustering would translate into an un-
even distribution of phosphatase activity on the
cell surface, consistent with localization-based mod-
els for receptor action (34). Small regions de-
pleted in phosphatase activity could enhance the
extent and duration of a phosphorylated state for
proteins stimulating neuronal extension (1, 35, 36).
Our solution and cellular data are consistent with
a model in which increasing the CSPG:HSPG ratio
shifts the balance away from growth-promoting
RPTPs clusters, stalling neuronal growth cones
(Fig. 4T). This model predicts that molecules able
to promote RPTPs oligomerization may prove
beneficial in strategies to facilitate plasticity and
regeneration after nervous system injury. More
generally, proteoglycan-binding is a common prop-
erty of many cell surface signaling systems in-
volved in normal biology and disease. Our results
point to a mechanism by which differences in the
structure of GAG chains can serve as a stop/go
molecular switch for cell motility and may provide
a general paradigm in the biology of cell surface
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Acknowledgments: Coordinates and structure factors are
deposited in the Protein Data Bank (see table S2). We
thank K. Harlos, T. Walter, and staff of the European
Synchrotron Radiation Facility and Diamond Light Source
for assistance; the Harvard Medical School Nikon Imaging
Center for resources and advice; C. Serra-Pagès and
A. W. Stoker for RPTPs cDNAs; M. Tremblay and N. Uetani
(McGill Univ.) for the RPTPs–/– mouse; R. J. Gilbert and
R. T. Aplin for analytical ultracentrifugation and mass
spectrometry; and J. Silver, B. Lang, and A. W. Stoker for
advice and discussions. This work was funded by the
Wellcome Trust, the European Research Community Fund
(MRTN-CT-2006-035830), the NIH (grants EY11559 and
HD29417), Cancer Research UK, and the UK Medical
Research Council. C.H.C. was the recipient of a Wellcome
Trust D.Phil. studentship, C.S. is a Wellcome Trust
Research Career Development Fellow, E. Y.J. is a Cancer
Research UK Principal Research Fellow, and A.R.A. is a UK
Medical Research Council Career Development Award
Fellow. Current patent applications related to this work
have been filed by Harvard Univ. A provisional patent
filed by the University College London, related to the
discovery of HSPGs as RPTPs ligands, expired in 2001.
J. T.G. is the founder, director, and majority shareholder
of Iduron, Paterson Institute for Cancer Research,
Univ. of Manchester (Manchester, UK).
Supporting Online Material
Materials and Methods
Figs. S1 to S15
Tables S1 to S3
23 November 2010; accepted 16 March 2011
Published online 31 March 2011;