Assessment of a mutation in the H5 domain of Girk2 as a candidate for the weaver mutation.

Abstract

A mutation in the GIRK2 inwardly rectifying K ÷ channel was mapped recently to the region of mouse chromosome 16 containing the wv gene and shown to occur in mutant but not in wild-type mice. We demonstrate tight linkage of the Girk2 mutation to the wv phenotype and refine the localization of the weaver (wv) gene on recombinational and physical maps. This linkage between Girk2 and ~ has existed since at least 1988 in descendants of the original mutation maintained in CS7BLI6 animals. Giric2. is shown to be transcribed in brain before the first recognized manifestation of the wv phenotype and in cultures of granule cells (GCs} isolated from cerebellum at postnatal day 8. Wild-type GCs grown in this culture system display an important developmental property--the ability to extend neurites. However, no inwardly rectifying K + current is detected in GCs cultured from either wv/~ or +/+ cerebellum under a variety of conditions that activate related channels in other tissues. This suggests that if the Gir~ mutation is responsible for the wv phenotype, it does not act by altering these electrical properties of developing GCs.The weaver (wv) mutation in mice was first characterized in 1973 as a recessive mutation resulting in hypoplasia of the cerebellar internal granule layer (IGL) (Rakic and Sidman 1973). As a consequence, wv/wv mice display severe ataxia. Heterozygous (+/wv) individuals are not ataxic, but develop a significantly smaller cerebellum than do control animals. The wv gene maps to a region of mouse chromosome 16 (Chr 16) that is highly conserved with human chromosome 21 (HSA21), suggesting that its normal human homolog is at dosage imbalance in Down syndrome (DS) . The human homolog of wv may also be involved in some forms of Parkinson's disease, as wv/wv mice display a deficit in the dopaminergic projection to the caudoputamen traced to degeneration of the tyrosine hydroxylase-positive cells of the substantia nigra beginning at postnatal day 7 (P7) (Schmidt et al. 1982). In addition to these central nervous system (CNS) anomalies, sperm in the wv/wv testis progress to the late spermatid stage, but mature sperm are never released (Harrison and RofflerTarlov 1993).The best-characterized aspect of the wv phenotype is the ataxia that results from a failure of granule cells (GC) to mature and migrate to form the cerebellar IGL. wv/wv individuals are first identifiable at the day of birth (P0) by an increased number of pyknotic nuclei in the external granule layer (EGL) of the cerebellum . By 8 days of age, both the lack of GC migration in mutant animals and differences in cerebellar size and organization among +/+, wv/+, and wv/wv mice are evident on histological analysis. Two distinct lines of investigation suggest that the wv defect is GC specific, but give rise to conflicting conclusions about whether the defect is extrinsic or intrinsic. Aggregate cultures of neurons (presumptive GC) cultured from the EGL of young normal mice extend neurites, whereas wv/wv aggregates do not. wv/wv cells will extend neurit...

Neuropathologic findings in a case of OFDS type VI (V�radi syndrome)

“…For example, defects in granule cell migration and Bergmann glial fibers leading to loss of granule cells characterize the homozygous weaver mutant mouse [Rakic et al, 1973a,b;Smeyne and Goldowitz, 1989]. A candidate gene, Girk2, homologous to a human gene localized to the long arm of chromosome 21, was proposed as the causative gene in the weaver mouse [Mjaatvedt et al, 1995;Bandmann et al, 1996]. Loss of dopaminecontaining neurons in the substantia nigra and other nuclei was also observed in weaver mutant mice, suggesting a more widespread neuronal defect [Graybiel et al, 1990], although no definite evidence of cell loss in the substantia nigra was observed in this case.…”

BAPTA-AM and ethanol protect cerebellar granule neurons from the destructive effect of the weaver gene

“…However, these studies have been confronted by findings that the GIRK2 channel is not functional in the cultured weaver neurons (Mjaatvedt et al, 1995;Surmeier et al, 1996). Our earlier results indicate that neurite outgrowth of the weaver granule neurons can be restored by drugs that reduce proteolysis and accumulation of neurotoxic peptides derived from the B2 chain of laminin along the neuronal cell surfaces (Murtomäki et al, 1995;Liesi and Wright, 1996).…”

Gain of Function Mutants: Ion Channels and G Protein-Coupled Receptors

“…There is, of course, ample precedent for the idea that ion channels mutated in the permeation pathway display more stable open states, presumably because the permeation pathway is also in contact with the regions of the channel thought to govern gating (Revah et al 1991, De Biasi et al 1993, Labarca et al 1995, but the argument is less direct than for the permeation pathway. Furthermore, several reports showed no constitutive activation of the wvGIRK2 channel-and no GIRK currents at all-in weaver mice (Mjaatvedt et al 1995, Surmeier et al 1996, Lauritzen et al 1997, and constitutive activation was incomplete when only low levels of wvGIRK2 were expressed in oocytes (Slesinger et al 1996). Also, wvGIRK2 channels were insensitive to G protein subunits in excised inside-out patches from CHO cells (Navarro et al 1996).…”