PATTERN DEFORMITIES AND CELL LOSS IN ENGRAILED-2 MUTANT MICE SUGGEST 2 SEPARATE PATTERNING EVENTS DURING CEREBELLAR DEVELOPMENT

Null alleles of the mouse Engrailed-2 gene, a molecular homolog of the fly gene engrailed, have demonstrable effects on the anteroposterior (A/P) patterning of cerebellum as reflected in the disruption of the n ormal process of foliation of the cerebellar cortex and the alteration of transgene expression boundaries in the adult. Engrailed-2 also aff ects the transient mediolateral (M/L) pattern of En-1 and Wnt-7b expre ssion seen in late embryogenesis. We have examined three markers of ce rebellar compartmentation in En-2 mutant mice: the Zebrin II and Ppath monoclonal antibodies and the transgene L7lacZ. In En-2 mutants, the normal temporal pattern of expression is preserved for all three marke rs, although the size and spatial location of various bands differ fro m those of the wild type. Unlike the foliation abnormalities, the M/L pattern disturbances we have found occur in nearly all cerebellar regi ons. Cell counts reveal that all major cell types of the olivocerebell ar circuit are reduced by 30-40%. We propose that these results are be st explained by a model in which the Engrailed-2 gene is involved in t he early specification of the cerebellar field including the number of progenitors. Because each of these progenitors gives rise to a clone of defined size, Engrailed-2 helps specify adult cell number. We furth er postulate that the configuration of the seven Zebrin bands as well as the shapes and locations of the cerebellar lobules are set up by a second patterning event that occurs after neurogenesis is complete.