Chimeric analysis of the site of Lurcher gene action with respect to glial enzyme expression

Immunocytochemical analysis of aggregation chimeras made using either of the mutants, Lurcher or Purkinje cell degeneration, previously showed that only Bergmann glia close to surviving Purkinje cells expressed an apparently normal level of the enzyme, glycerol-3-phosphate dehydrogenase (GPDH) (Fisher and Mullen, 1988). In the present study, aggregation chimeras were made using embryos from a B6D2 hybrid mouse strain carrying the Lurcher (Lc/+) mutation and homozygous for an allele specifying a high level of beta-glucuronidase activity and embryos from a wild-type C3H strain with low beta-glucuronidase activity. Chimeric cerebella were analyzed immunocytochemically and histochemically to determine whether Lurcher mutant Bergmann glia could express a normal, high level of GPDH. Comparisons between pairs of alternately stained, 2 microns thick serial sections showed that Bergmann glia with high level of beta-glucuronidase (i.e., Lurcher) expressed normal GPDH immunoreactivity only when close to surviving, wild-type Purkinje cells. Interestingly, in Purkinje cell-free areas of cerebellar cortex that were sufficiently large that GPDH expression was diminished, Bergmann glia also showed a reduced level of histochemically detectable beta-glucuronidase activity, as do Bergmann glia in adult Lc/+ mice. The results indicate that Lc/+ mutant Bergmann glia are not intrinsically defective with regard to expression of the enzymes, GPDH and beta-glucuronidase, but rather, expression of these enzymes depends on glial interaction with wild-type Purkinje cells.