The fate of heterotopically grafted neural precursor cells in the normal and dystrophic adult mouse retina

PURPOSE. To study the integration and differentiation of heterotopically transplanted neural precursor cells in the retina of adult mouse mutants displaying apoptotic degeneration of photoreceptor cells. METHODS. Neural precursor cells were isolated from the spinal cord of transgenic mouse embryos ubiquitously expressing enhanced green fluorescent protein. Cells were expanded in vitro and transplanted into the retina of adult wild-type and age-matched ␤2/␤1 knock-in mice. ␤2/␤1 knock-in mutants display apoptotic death of photoreceptor cells and were generated by placing the cDNA of the ␤1 subunit into the gene of the ␤2 subunit of Na,K-ATPase. The integration and differentiation of grafted cells in recipient retinas was studied 1 or 6 months after transplantation. RESULTS. Mutant retinas contained more donor-derived cells than wild-type hosts. Moreover, in mutants, donor cells integrated into deeper retinal layers. In both genotypes, grafted cells differentiated into astrocytes and oligodendrocytes. Only a few ganglion cell axons were myelinated by donor-derived oligodendrocytes 1 month after transplantation, whereas extensive myelination of the nerve fiber layer was observed 6 months after transplantation. Unequivocal evidence for differentiation of grafted cells into neurons was not obtained. CONCLUSIONS. Heterotopically transplanted neural precursor cells are capable of integrating, surviving, and differentiating into neural cell types in normal and dystrophic retinas of adult mice. The particular environment of a pathologically altered retina facilitates integration of transplanted precursor cells. In principle, neural precursors may thus be useful to substitute for or replace dysfunctional or degenerated cell types. Results of the present study also indicate that replacement of retinal cell types is likely to require more appropriate donor cells, such as retinal precursor cells. (Invest Ophthalmol Vis Sci. 2001;42:3311-3319) I nherited retinal dystrophies are a heterogeneous group of disorders characterized by progressive retinal degeneration. Effective therapeutic treatments of retinal dystrophies in humans are currently not available. However, animal experiments have shown beneficial effects of various therapeutic strategies, including gene therapy to substitute for the pathogenic gene, application of growth factors to minimize cell degeneration, or transplantation of committed cell types to replace dysfunctional or degenerated cells. 10 -12 Neural precursors have been isolated from the developing and adult brain and can be massively expanded in vitro, providing, in principle, unlimited amounts of cell material for transplantation (different from primary retinal cells). When transplanted into the developing or adult brain, they have been demonstrated to integrate extensively into the recipient tissue, to survive for extended periods, and to eventually differentiate into those cell types that are affected in the host. 22 These cells were expanded in vitro in the presence of mitogens and subsequently transplanted into the retina of adult wild-type mice and mouse mutants displaying apoptotic degeneration of photoreceptor cells. As a mutant host, we used ␤2/␤1 knock-in mice. 23 ␤1 and ␤2 are subunits of Na,K-ATPase, a heterodimeric ion pump additionally consisting of a catalytic ␣-subunit. 23,24 ␤-subunits play a pivotal role for the formation of functional Na,K-ATPases as exemplified, for instance, by the severe phenotype of ␤2-deficient mice. 25 Such mice display a variety of severe defects in the central nervous system (CNS), including massive apoptotic degeneration of photoreceptor cells, and die at the end of the third postnatal week. 23 To obtain information about the fate of heterotopically transplanted neural precursor cells in the normal and dystrophic mouse retina, we isolated such cells from the spinal cord of EGFP transgenic mice and transplanted them into the retinas of adult wild-type mice and ␤2/␤1 knock-in mutants. Heterotopically transplanted neural precursor cells integrated into the mutant retina without disrupting the histoarchitecture of the host tissue. Quantitative investigations revealed that donorderived cells were more numerous and more widely distributed in mutant retinas than in retinas of age-matched wild-type From th