Selective rather than inductive mechanisms favour specific replacement of Purkinje cells by embryonic cerebellar cells transplanted to the cerebellum of adult Purkinje cell degeneration (pcd) mutant mice.

Cell replacement after neuronal degeneration in the adult CNS depends on the availability of specific cues to direct specification, differentiation and integration of newly born neurons into mature circuits. Following recent reports indicating that neurogenic signals may be reactivated in the adult injured CNS, here we asked whether such signals are expressed in the cerebellum after Purkinje cell degeneration. Thus, we compared the fate of embryonic cerebellar cells transplanted to the cerebella of adult wild-type and Purkinje cell degeneration (pcd) mutant mice. Donor cells were dissected from beta-actin-enhanced green fluorescent protein (EGFP) transgenic mice and transplanted as a single cell suspension. In both hosts, grafted cells generated all major cerebellar phenotypes, with a precise localization in the recipient cortex or white matter. Nevertheless, the phenotypic distributions showed striking quantitative differences. Most notably, in the pcd cerebellum there was a higher amount of Purkinje cells, while other phenotypes were less frequent. Analysis of cell proliferation by 5-bromo-2'-deoxyuridine (BrDU) incorporation revealed that in both hosts mitotic activity was strongly reduced shortly after transplantation, and virtually all donor Purkinje cells were actually generated before grafting. Together, these results indicate that some compensatory mechanisms operate in the pcd environment. However, the very low mitotic rate of transplanted cells suggests that the adult cerebellum, either wild-type or mutant, does not provide instructive neurogenic cues to direct the specification of uncommitted progenitors. Rather, specific replacement in mutant hosts is achieved through selective mechanisms that favour the survival and integration of donor Purkinje cells at the expense of other phenotypes.