Cerebellar Purkinje neurons (PNs) possess a well characterized propensity to fuse with bone marrow-derived cells (BMDCs), producing heterokaryons with Purkinje cell identities. This offers the potential to rescue/repair at risk or degenerating PNs in the inherited ataxias, including Spinocerebellar Ataxia 1 (SCA1), by introducing therapeutic factors through BMDCs to potentially halt or reverse disease progression. In this study, we combined gene therapy and a stem cell-based treatment to attempt repair of at-risk PNs through cell-cell fusion in a Sca1154Q/2Q knock-in mouse model. BMDCs enriched for the hematopoietic stem cell (HSC) population were genetically modified using adeno-associated viral vector 7 (AAV7) to carry SCA1 modifier genes and transplanted into irradiated Sca1154Q/2Q mice. Binucleated Purkinje heterokaryons with sex-mismatched donor Y chromosomes were detected and successfully expressed the modifier genes in vivo. Potential effects of the new genome within Purkinje heterokaryons were evaluated using nuclear inclusions (NIs) as a biological marker to reflect possible modifications of the SCA1 disease process. An overall decrease in number of NIs and an increase in the number of surviving PNs were observed in treated Sca1154Q/2Q. Furthermore, Bergmann glia were found to have fusogenic potential with the donor population and reveal another potential route of therapeutic entry into at-risk cells of the SCA1 cerebellum. This study presents a first step towards a proof of principle that combines somatic cellular fusion events with a neuroprotective gene therapy approach for providing potential neuronal protection/repair in a variety of neurodegenerative disorders.

Numerous studies have explored the potential of different stem and progenitor cells to replace at-risk neuronal populations in a variety of neurodegenerative disease models. This study presents data from a side by side approach of engrafting two different stem/progenitor cell populations within the postnatal cerebellum of the weaver neurological mutant mouse: cerebellar-derived multipotent astrocytic stem cells (MASCs) and embryonic stem cell-derived neural precursor (ESNPs), for comparative analysis. We show here that both donor populations survive, migrate, and appear to initiate differentiation into neurons within the granuloprival host environment. Neither of these disparate stem/progenitor cell populations adopted significant region-specific identities despite earlier studies that suggested the potential of these cells to respond to in vivo cues when placed in a permissive/instructive milieu. However, data presented here suggest that molecular and cellular deficits present within weaver homozygous or heterozygous brains may promote a slightly more positive donor cell response towards acquisition of neuronal phenotype. Hence it is likely that a fine balance exists between a compromised host environment that is amenable to cell replacement and that of a degenerating cellular milieu where it is perhaps too deleterious to support extensive neuronal differentiation and functional cellular integration. These findings join a growing list of studies that show successful cell replacement depends largely on the interplay between the potentiality of the donor cells and the specific pathological conditions of the recipient environment, and that emergent therapies for neurological disorders involving the use of neural stem cells still require refinement.

Neural transplantation has been a long-standing goal for the treatment of neurological injury and disease. The recent discovery of persistent pools of neural stem cells within the adult mammalian brain has re-ignited interest in transplant therapeutics. Since neural stem cells are self-renewing, it may be possible to culture and expand neural stem cells and their progenitor cell progeny to sufficient numbers for use in autologous, self-repair strategies. Such approaches will require optimized cultivation protocols, as well as extensive testing of candidate donor cells to assess their capacity for engraftment, survival, and integration. In this chapter, we describe the transplantation of neural stem/progenitor cells-cultivated as either neurospheres or neurogenic astrocyte monolayers-into the persistently neurogenic olfactory bulb system of the adult mouse forebrain, and into the cerebellum of neonatal mutant mice.