Over 1 million people in the USA live with spinal cord injury (SCI) for which no current clinical treatment can restore function. Barriers to spinal cord regeneration include loss of functional neural circuits, extensive cell death, astrocyte induced scar formation, and chronic inflammation. Hyaluronic acid (HA)---a viscous fluid carbohydrate that is the main structural component of the extracellular matrix (ECM)--is an important mediator in each of these events. In healthy spinal cord, HA remains in its native, high molecular weight form (nHA, molecular weight > 300 kDA) and acts to maintain tissue structure, protect against oxidative stress, and promote quiescence of astrocytes and neural stem cells (NSCs). Immediately after injury, nHA is degraded into a range of smaller fragments (fHA, molecular weight = 1-100 kDa), which are thought to have size-dependent actions on wound-healing processes including direct effects on astrocyte proliferation, angiogenesis and NSC recruitment. Studies examining the effects of HA biomaterials relative to reducing scarring and promoting NSC differentiation have produced conflicting results. The PI posits that these discrepancies arise from failure to experimentally control for differences in HA molecular weight. Thus, the goal of this project is to provide a fundamental understanding of how the diverse biological effects of HA can be leveraged to engineer biomaterials that effectively direct tissue repair after SCI by investigating: 1) how the molecular weight of HA incorporated into crosslinked hydrogels affects cell-biomaterial interactions and 2) how these molecular weight-dependent effects can be used to design HA biomaterials with improved therapeutic benefits. The integrated research and education plan includes mentored laboratory experiences for high school, undergraduate and graduate students that are designed to generate enthusiasm for science and engineering in medicine that will motivate future career goals, e.g.: 1) fluorescent reporters that enable students to visualize subcellular processes in real-time and 2) the promise of new clinical treatments for spinal cord repair. Research results will be integrated into two UCLA undergraduate bioengineering courses with unique hands-on, laboratory components, including the CAPSTONE design course where groups of seniors work closely with an interdisciplinary team of academic and industry mentors to design and construct new solutions to important biomedical problems.
As part of the long-term goal to engineer improved therapies to promote nerve regeneration, the focus of this project is to improve understanding of how the molecular weight-dependent bioactivities of HA affect spinal cord repair. Specifically, the PI aims to identify the molecular weight-dependent effects of HA on human cells relevant to SCI (NSCs and astrocytes) and apply this information to design HA-releasing biomaterials to guide spinal cord regeneration. Multiple studies have demonstrated that nHA--which degrades to fHA after SCI-- inhibits astrocyte proliferation and reactivity. In opposition, disruption of the nHA matrix promotes astrocyte proliferation and activation and, as a result, increased glial scarring. A few studies have reported similar effects on NSCs. Despite the known dependence of HA bioactivity on molecular weight, HA chain size has not been a significant consideration in the development of regenerative biomaterials. Aim 1 will investigate how the molecular weight of HA crosslinked into hydrogel biomaterials affects CD44 (a receptor for HA)-mediated bioactivity of cells integral to spinal cord repair (NSCs and astrocytes). Aim 2 will characterize the molecular weight profile of HA fragments released from biomaterials during biodegradation and assess the effects of its products on chemotaxis and proliferation of NSCs and astrocytes. Aim 3 will work towards clinical translation by developing HA-based hydrogels that are injectable, support robust cell infiltration and enable temporal control over HA molecular weight. This aim includes the novel fabrication of injectable, macroporous scaffolds with defined HA molecular weights, including nHA, which has not previously been accomplished due to the high viscosity of nHA. Effects of the hydrogels on NSCs and inflammation will be evaluated in a mouse model of acute SCI, whose outcome will be followed 4 weeks post injury. Thus, the project will provide a fundamental understanding of the molecular weight-dependent bioactivities of HA and guide the design of advanced HA-based therapeutics, in particular for spinal cord regeneration but with implications for a broad range of medical applications.
