DIFFERENTIATION OF ENGRAFTED MULTIPOTENT NEURAL PROGENITORS TOWARDS REPLACEMENT OF MISSING GRANULE NEURONS IN MEANDER TAIL CEREBELLUM MAY HELP DETERMINE THE LOCUS OF MUTANT-GENE ACTION

Previously we observed that stable clones of multipotent neural progen itor cells, initially isolated and propagated from the external granul ar layer of newborn wild-type mouse cerebellum, could participate appr opriately in cerebellar development when reimplanted into the external granular layer of normal mice. Donor cells could reintegrate and diff erentiate into neurons (including granule cells) and/or glia consisten t with their site of engraftment. These findings suggested that progen itors might be useful for cellular replacement in models of aberrant n eural development or neurodegeneration. We tested this hypothesis by i mplanting clonally related multipotent progenitors into the external g ranular layer of newborn meander tail mice (gene symbol=mea). mea is a n autosomal recessive mutation characterized principally by the failur e of granule cells to develop in the cerebellar anterior lobe; the mec hanism is unknown. We report that similar to 75% of progenitors transp lanted into the granuloprival anterior lobe of neonatal mea mutants di fferentiated into granule cells, partially replacing or augmenting tha t largely absent neuronal population in the internal granular layer of the mature meander tail anterior lobe. (The ostensibly 'normal' meand er tail posterior lobe also benefited from repletion of a more subtle granule cell deficiency.) Donor-derived neurons were well-integrated w ithin the neuropil, suggesting that these progenitors' developmental p rograms for granule cell differentiation were unperturbed. These obser vations permitted several conclusions. (1) That exogenous progenitors could survive transplantation into affected regions of neonatal meande r tail cerebellum and differentiate into the deficient cell type sugge sted that the microenvironment was not inimical to granule cell develo pment. Rather it suggested that nea's deleterious action is intrinsic to the external granular layer cell. (Any cell-extrinsic actions - alb eit unlikely - had to be restricted to readily circumventable prenatal events.) This study, therefore, offers a paradigm for using progenito rs to help determine the site of action of other mutant genes or to te st hypotheses regarding the pathophysiology underlying other anomalies . (2) In the regions most deficient in neurons, a neuronal phenotype w as pursued in preference to other potential cell types, suggesting a ' push' of undifferentiated, multipotent progenitors towards compensatio n for granule cell dearth, These data suggested that progenitors with the potential for multiple fates might differentiate towards repletion of deficient cell types, a possible developmental mechanism with ther apeutic implications. Neural progenitors (donor or endogenous) might e nable cell replacement in some developmental or degenerative diseases - most obviously in cases where a defect is intrinsic to the diseased cell, but also, under certain circumstances, when extrinsic pathologic forces may exist.