Biomaterials to change astrocyte behaviour and morphology for brain repair

Abstract

The incapacity of the central nervous system (CNS) to regenerate is a barrier to the effective treatment of neurodegenerative diseases and traumatic injuries. Of particular importance in treating traumatic injuries is the CNS’s inflammatory response, which is a complex response that does not effectively transition from the growth-inhibitory and protective phase to a growth supportive phase that would allow for tissue repair and remodelling. Therefore, the astrocyte response to injury presents a valuable therapeutic target as they perform both cytotrophic and cytotoxic functions, sometimes concomitantly, after injury. The chronic persistence of scar-forming astrocytes presents a significant barrier to regeneration and hence, functional recovery. As such, understanding how particular cues, and when they are presented, affect astrocyte behaviour is of interest in developing tissue engineering solutions for traumatic brain injury (TBI). Biomaterials present an attractive solution candidate as they can mimic the extracellular matrix of the brain, present biologically relevant cues, and facilitate cell growth. Through understanding how biomaterials and the cues they can present impact astrocyte behaviour and morphology, we seek to inform the future design of biomaterials to harness the cytotrophic aspects of astrocytes and their response to injury to improve reparative outcomes. In this thesis, the development and biological evaluation of nanofibrous biomaterial systems functionalized with galactose moieties or the anti-inflammatory polysaccharide, fucoidan are described. Electrospun poly(ε-caprolactone) nanofibre scaffolds were fabricated and functionalized with biologically relevant heparin (anti-inflammatory) and poly(L-lysine) (PLL), or the novel galactose-presenting poly(l-lysine)-lactobionic acid (PLL-LBA). The research reported here demonstrates the functionalization and materials characterization, as well as biological evaluation in vitro and in vivo to elucidate the impact of nanofibrous morphology and the galactose moieties on astrocytes in culture as well as after TBI. The galactose-presenting scaffold could maintain a reduced inflammatory profile of astrocytes in vitro and resulted in neuroprotection at 7 days post injury in mice. These findings were extended upon by transitioning to the Fmoc-capped self-assembled peptide (SAP) hydrogel, Fmoc-DIKVAV, which can effectively fill a brain lesion, whilst also providing bioactive cues on the surface of the nanofibres within the hydrogel. This system was co-assembled with fucoidan to present anti-inflammatory cues after TBI, where it was found that structural support and no additional functionalization was required to reduce the primary astrocyte scar by ~50% compared to the stab control 7 days post injury. The presentation of fucoidan on the fibrils of Fmoc-DIKVAV increased the organization of astrocytes within the primary scar and also altered the morphology of the astrocytes far away from the lesion site. This demonstrates the ability of fucoidan to alter the morphology, and potentially the phenotype of reactive astrocyte after injury. Finally, this SAP system was evaluated as a 3-dimensional (3D) cell culture environment to enhance the understanding of astrocyte behaviour in culture as well as after lipopolysaccharide (LPS) or interleukin-1α (IL-1α) stimulation. Fucoidan delivered via the hydrogel system significantly reduced the proliferation of LPS-stimulated astrocytes compared to soluble fucoidan or the control, and exposure to the hydrogel resulted in significant reorganization of astrocyte networks in vitro, which was also observed in vivo. Thus, this SAP hydrogel is promising as a 3D biomimetic cell culture environment for future studies of astrocytes. Here, we have engineered functionalized nanofibrous biomaterial scaffolds that can be used in vitro and in vivo to impact astrocyte behaviour and morphology after injury or stimulation. The results presented can be used to better inform the design of future tissue engineering strategies that can manipulate the inflammatory response to improve functional recovery outcomes. Show more