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Type of Document	Dissertation
Author	Sakiyama-Elbert, Shelly Elese
URN	etd-08112006-152417
Persistent URL	http://resolver.caltech.edu/CaltechETD:etd-08112006-152417
Title	Biofunctional polymers for the controlled release of growth factors in the peripheral nervous system
Degree	PhD
Option	Chemical Engineering
Advisory Committee	Advisor Name Title J.A. Hubbell Committee Chair David A. Tirrell Committee Member Julia A. Kornfield Committee Member Mark E. Davis Committee Member Scott E. Fraser Committee Member
Keywords	none
Date of Defense	2000-05-01
Availability	restricted
Abstract Fibrin is the natural biomaterial of nerve regeneration. Fibrin possesses the ability to promote cell adhesion and can be degraded locally by cell-regulated proteases. However, fibrin lacks sufficient neuroinductive character to promote nerve regeneration across large gaps. A previously developed method for the incorporation of peptides via the transglutaminase activity of factor XIIIa was used to immobilize cell adhesion sites and growth factors within fibrin matrices that could serve as potential therapeutics for peripheral nerve regeneration. Heparin-binding domains from several proteins have been shown to promote neurite extension on surfaces. Four different heparin-binding domains were immobilized within fibrin matrices using factor XIIIa and all four domains were found to enhance three-dimensional neurite extension through fibrin. The ability of these domains to enhance neurite extension was found to correlate positively with their relative heparin-binding affinity. Prolonged release of exogenous growth factors over the duration of nerve regeneration could potentially enhance regeneration. Two delivery systems were developed to provide controlled release of exogenous growth factors, where the rate of release could be regulated by cellular activity rather than by diffusion. The first system developed was designed to mimic the ability of the extracellular matrix to sequester heparin-binding growth factors based on interactions with heparin sulfate proteoglycans. It was hypothesized that by immobilizing a high excess of heparin sites within fibrin, diffusion of growth factors from the matrix could be slowed, and this would allow release to be dominated by cell-regulated matrix degradation. The ability of such systems to immobilize growth factors and release them in an active form was assayed using neurite extension from dorsal root ganglia (DRGs). A second method of delivery was developed consisting of recombinantly expressed nerve growth factor (NGF) fusion proteins containing an exogenous crosslinking substrate. A plasmin-degradable linker was placed between the crosslinking substrate and the NGF domain. These fusion proteins could be immobilized in fibrin using factor XIIIa and released in a native form by plasmin cleavage. Immobilized NGF fusion proteins were found to enhance neurite extension from DRGs within fibrin matrices versus similar concentrations of native NGF in the medium.
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