Chemical Biology
Another area of interest is the development of high throughput methods to identify small molecule antagonists of heparan sulfate function. One of the compounds discovered in this way is bis-2-methyl-4-amino-quinolyl-6-carbamide, also known as surfen. Fluorescence-based titrations indicate that surfen binds to glycosaminoglycans and neutralizes the anti-Factor Xa activity of heparin-antithrombin complexes. Addition of surfen to cultured cells blocks binding and signaling mediated by FGF2, which depends on formation of ternary complexes of FGF, FGF receptors and cell surface heparan sulfate. Furthermore, surfen blocks cell adhesion to the heparin-binding Hep-II domain of fibronectin and prevents infection by Herpes simplex virus dependent on glycoprotein D binding to cell surface heparan sulfate. Surfen also blocks both FGF and VEGF mediated endothelial sprouting in Matrigel. These findings demonstrate the feasibility of identifying small molecule antagonists of glycosaminoglycans. Analogs are under development to determine how the structure of surfen affects its capacity to bind glycosaminoglycans and specificity.
Another approach to this problem is to target cells for death by exploiting proteoglycans expressed on the cell surface. In collaboration with Yitzhak Tor's group at UCSD, we have tested analogs of aminoglycosides in which the amino groups were converted to guanidinium groups. A derivative of the aminoglycoside antibiotic neomycin can carry large (>300 kDa) bioactive molecules across cell membranes. Delivery occurs at nanomolar transporter concentrations and under these conditions depends entirely on cell surface heparan sulfate proteoglycans. Conjugation of guanidino-neomycin to the plant toxin saporin, a ribosome-inactivating agent, results in proteoglycan-dependent cell toxicity. The high selectivity of guanidino-neomycin for heparan sulfate suggests the possibility of exploiting differences in proteoglycan compositions to target delivery to different cell types. Conjugation chemistry has been worked out to simplify the derivatization of proteins with the carriers to test delivery of cargo to various cell compartments.
Relevant Papers
Brown JR, Crawford BE, Esko JD. (2007) Glycan antagonists and inhibitors: a fount for drug discovery. Crit Rev Biochem Mol Biol. 42:481-515.
Elson-Schwab, L., Garner, O.B., Schuksz, M., Crawford, B.E., Esko, J.D., and Tor, Y. (2007) Guanidinylated-Neomycin delivers large, bioactive cargo into cells through a heparan sulfate dependent pathway. J. Biol. Chem. 282:13585-13591.
Schuksz, M., Fuster, M.M., Brown, J.R., Crawford, B.E., Ditto, D.P., Lawrence, R., Glass, C.A., Wang, L., Tor, Y., and Esko, J.D. (2008) Surfen, a small molecule antagonist of heparan sulfate. Proc. Natl. Acad. Sci. 105:13075-13080.