Assessment of biomaterial microcarriers for sustained neurotrophic factor delivery in the context of cell replacement strategies in Parkinson’s disease

The dopaminergic cells transplanted into the brains for cell replacement therapy (CRT), as a potential treatment strategy for Parkinson’s disease (PD), have shown survival and functionality in several studies. While extensive evidence exists for the beneficial effects of fetal/embryonic-derived cells in both preclinical animal models and clinical trials, there is relatively lesser evidence of the long-term effects of stem cell-derived cells in clinical trials. Data from several preclinical studies of stem cell-derived dopaminergic transplants indicate highly variable graft survival with poor dopaminergic differentiation in vivo. This is in part due to the growth factor deprivation faced by the cells when they are lifted from a neurotrophin-rich culture and transplanted into the adult diseased brain. The protocol for terminal differentiation of the dopaminergic precursor cells indicates that a sustained availability of neurotrophic factors (NTFs) like glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) might be needed by the cells for long periods of time post-transplantation, which cannot be conferred by a single bolus administration, due to their short half-lives and rapid metabolism in vivo. Biomaterial microcarriers capable of sustained delivery of GDNF & BDNF to the brain, have the potential to address this limitation. Thus, the central aim of the research in this thesis was to identify biocompatible biomaterial microcarriers capable of sustained GDNF and BDNF delivery to the brain, and assess if co-engrafting them with ventral mesencephalic dopamine precursor (vmDAP) cells (rat fetal or human induced pluripotent stem cell (iPSC)-derived) into rat brains would result in an enhanced in vivo survival and differentiation. First, through a series of in vitro and in vivo studies, the safety/biocompatibility of polyhedrin-based delivery systems (termed PODS®) was assessed, in addition to the kinetics of NTF release from them. Following this, the impact of PODS® co-crystals on the survival of co-transplanted rat embryonic primary dopaminergic neurons was determined. Following the assessment of PODS®, cryogel microspheres made of poly(ethyleneglycol) diacrylate (PEGDA) and 3-sulfopropyl acrylate (SPA) were assessed for their ability to load NTFs. Then, through a series of in vitro and in vivo studies, their NTF delivery, retention, and release profiles were assessed, in addition to their safety/biocompatibility. Finally, the impact of the PEGDA-SPA cryogel spheres was studied on the a) survival and functionality of, and striatal reinnervation by, the co-transplanted rat embryonic primary dopaminergic neurons, and b) survival and differentiation of the co-transplanted human iPSC-DAPs in rat models of PD. In brief, we found that BDNF PODS®, although cytocompatible, showed no evident BDNF release, either in in vitro cultures, or in vivo in the brain, whereas GDNF PODS® in addition to being cytocompatible, showed detectable release of GDNF, in small yet steadily increasing amounts, in in vitro cultures. However, when delivered along with rat primary dopaminergic neurons in the brain, GDNF PODS® displayed an immunogenic tendency, with no benefit on the survival of the co-transplanted primary dopaminergic neurons. Following this, the PEGDA-SPA cryogel spheres displayed a) efficient and optimal loading of GDNF and BDNF, with b) sustained release of the loaded NTFs, and c) cytocompatibility with in vitro cultures and functional benefit on the metabolic activities of ex vivo rat embryonic ventral midbrain cultures. When implanted into rat brains, the cryogel spheres were a) biocompatible and induced no higher neuroimmune response when compared to implants of vehicle alone, and b) able to deliver, retain, and release NTFs for long time periods. Finally, the cryogel spheres had a beneficial impact on the survival of, and reinnervation by, the co-engrafted rat primary dopaminergic neurons. However, their impact on the survival and differentiation of the co-transplanted human iPSC-DAPs could not be determined due to the very poor in vivo survival of the DAPs, differentiated from the specific iPSC cell line (NAS) used in the study, even when transplanted on their own. In conclusion, NTF-loaded cryogel microspheres have a great potential to improve the efficacy of primary dopaminergic cell transplants, by providing them with a long-term sustained availability of NTFs upon transplantation in the brain. However, further studies are needed to determine their potential to enhance stem cell-based CRT in PD.

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University of Galway

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Attribution-NonCommercial-NoDerivatives 4.0 International

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University of Galway Theses (PhD Theses)