Mice homozygous for the weaver spontaneous mutation (Kcnj6wv) are recognizable in the second postnatal week by their small size, instability of gait, weakness, and hypotonia. Many homozygous mutant mice die at weaning age, but some survive to adulthood, and females may breed. The cerebellum in homozygous mutants is very small, simple, and almost devoid of granule cells, which degenerate during the second week. Heterozygotes behave normally, but they have a smaller than normal cerebellum with a deficiency of granule cells, some of which fail to migrate into the internal granule layer and remain scattered in the molecular layer. Evidence from cultures of mutant and normal cerebellum show that granule cells of Kcnj6wv/Kcnj6wv and Kcnj6wv/+ mice have gene-dosage dependent abnormalities in morphology and cell behavior. Studies using homozygous weaver/wildtype chimeras indicate that the migration defect of granule cells is intrinsic to the granule cells themselves. The disarrangement of Purkinje cells, however, is not caused by intrinsic action of weaver in these cells. Cell mixing experiments also suggest Kcnj6wv acts non-autonomously and encodes a membrane-associated ligand that induces external germinal layer neuron differentiation.
