Increased proteolytic activity of the granule neurons may contribute to neuronal death in the weaver mouse cerebellum

Effect of the weaver mutation on the expression of dopamine membrane transporter, tyrosine hydroxylase and vesicular monoamine transporter in dopaminergic neurons of the substantia nigra and the ventral tegmental area

The adult homozygous weaver mutant mouse (wv/wv) is characterized by a loss of dopamine (DA) neurons in the nigrostriatal pathway. Quantitative in situ hybridization of three different dopaminergic markers: dopamine membrane transporter (DAT), tyrosine hydroxylase (TH), and vesicular monoamine transporter (VMAT2) was performed on individual dopaminergic cells of the substantia nigra pars compacta (SNC) and the ventral tegmental area (VTA) in 2-month-old wv/wv mice, in order to investigate the metabolic state of remaining dopaminergic cell bodies and gain further insight into modifications observed on dopaminergic nerve terminals in the striatum and the nucleus accumbens. Cellular expression of DAT mRNA in remaining dopaminergic cells of both the SNC and the VTA was decreased in the wv/wv mice compared to the wild-type mice (+/+). In contrast, the expression of TH and VMAT2 mRNA remained unchanged in the wv/wv mice. Furthermore, in 7-day-old wv/wv mice, before the onset of cell death in the midbrain. DAT mRNA levels were reduced in dopaminergic neurons in both the SNC and VTA. In these animals, the cellular expression of TH mRNA remained unchanged. These results taken together indicate that DAT expression is one of the first targets in the ventral mesencephalon of the wv mutation, inducing a specific decrease of DA uptake in the striatum and the nucleus accumbens. The alteration of the DA membrane transporter could play a role in the progression of DA neuronal death in the wv mice.

Enhancement of mRNA expression of tissue-type plasminogen activator by L-threo-3,4-dihydroxyphenylserine in association with ocular dominance plasticity

Tissue-type plasminogen activator (tPA) plays important roles in the regulation of synaptic plasticity in the hippocampus and cerebellum. We found that the expression of tPA mRNA in the visual cortex was increased significantly by the peripheral administration of L-threo-3,4-dihydroxyphenylserine (L-threo-DOPS; 100 mg/kg, i.p.), which we had previously shown to have a promotive effect on ocular dominance (OD) plasticity. When plasminogen activator inhibitor-1 (PAI-1; 100 muM in an osmotic minipump) was infused into the kitten visual cortex, OD plasticity was suppressed; i.e. a significantly large number of binocular cells was recorded in the PAI-1 infused cortex following monocular deprivation. These results, therefore, suggest that the PA system is involved in the promotive effect of L-threo-DOPS in OD plasticity.

The weaver mutation changes the ion selectivity of the affected inwardly rectifying potassium channel GIRK2

The weaver mutation in mice has recently been identified as a single base-pair mutation in the Girk2 gene, which encodes a G-protein-activated inwardly rectifying potassium channel, GIRK2. The mutation results in a Gly to Ser substitution at residue 156, in the putative pore-forming region of the potassium channel. In the present study, we used Xenopus oocytes to express mutant GIRK2, and to characterize the effects of the mutation on the channel. The mutation results in a loss of the normal high selectivity for K+ over Na+, with little effect on other channel properties such as activation by the mu opioid receptor. The resulting increase in basal Na+ permeability causes a marked depolarization of oocytes expressing the mutant GIRK2 protein. This result was observed even when the mutant GIRK2 was coexpressed with GIRK1, a situation more analogous to that seen in vivo. Thus, the increased Na+ permeability and resulting depolarization may contribute to the pathology of cerebellar granule cells and substantia nigra dopaminergic neurons observed in the weaver mice.

Weaver granule neurons are rescued by calcium channel antagonists and antibodies against a neurite outgrowth domain of the B2 chain of laminin

The weaver mutation impairs migration of the cerebellar granular neurons and induces neuronal death during the first two weeks of postnatal life. To elucidate the molecular mechanisms for the impaired neuronal migration, we investigated the rescue mechanisms of the weaver (wv/wv) granule neurons in vitro. We found that Fab2 fragments of antibodies against a neurite outgrowth domain of the B2 chain of laminin enhanced neurite outgrowth and neuronal migration of the weaver granule neurons on a laminin substratum and in the established cable culture system. The rescue of the weaver granule neurons by antibodies against the B2 chain of laminin may result from the neutralizing effect of these antibodies against the elevated B2 chain levels of the weaver brain. The L-type calcium channel blocker, verapamil (1-5 microM), also rescued the weaver granule neurons. High concentrations of MK-801 (10-20 microM), a glutamate receptor antagonist and voltage-gated calcium channel blocker, rescued the weaver granule neurons similar to verapamil, but low concentrations of MK-801 (1 microM) had no rescue effect. Simultaneous patch-clamp studies indicated that the weaver granule neurons did not express functional N-methyl-D-aspartate receptors further indicating that the rescue of the weaver granule neurons by MK-801 resulted from its known inhibition of voltage-gated calcium channels. The present results indicate that antibodies against the B2 chain of laminin, verapamil, and high concentrations of MK-801 protect the weaver granule neurons from the otherwise destructive action of the weaver gene. Thus, both the laminin system and calcium channel function contribute to the migration deficiency of the weaver granule neurons.

Functional effects of the mouse weaver mutation on G protein-gated inwardly rectifying K+ channels

The weaver mutation corresponds to a substitution of glycine to serine in the H5 region of a G protein-gated inwardly rectifying K+ channel gene (GIRK2). By studying mutant GIRK2 weaver homomultimeric channels and heteromultimeric channels comprised of GIRK2 weaver and GIRK1 in Xenopus oocytes, we found that GIRK2 weaver homomultimeric channels lose their selectivity for K+ ions, giving rise to inappropriate receptor-activated and basally active Na+ currents, whereas heteromultimers of GIRK2 weaver and GIRK1 appeared to have reduced current. Immunohistochemical localization indicates that GIRK2 and GIRK1 proteins are expressed in the cerebellar neurons of mice at postnatal day 4, at a time when these neurons normally undergo differentiation. Thus, the aberrant behavior of mutant GIRK2 weaver channels could affect the development of weaver mice in at least two distinct ways.