Depolarization and Ca(2+) down regulate CB1 receptors and CB1-mediated signaling in cerebellar granule neurons

CB1 cannabinoid receptors increase neuronal precursor proliferation through AKT/glycogen synthase kinase-3beta/beta-catenin signaling

The endocannabinoid system is involved in the regulation of many physiological effects in the central and peripheral nervous system. Recent findings have demonstrated the presence of a functional endocannabinoid system within neuronal progenitors located in the hippocampus and ventricular/subventricular zone that participates in the regulation of cell proliferation. It is presently unknown whether the endocannabinoid system exerts a widespread effect on neuronal precursors from different neurogenic regions, and very little is known about the signaling by which it regulates neuronal precursor proliferation. Herein, we demonstrate the presence of cannabinoid CB(1) receptors in granule cell precursors (GCPs) during early cerebellar development. Activation of CB(1) receptors by HU-210 promoted GCP proliferation in vitro, an effect that was prevented by a selective CB(1) antagonist. Accordingly, in vivo experiments showed that GCP proliferation was increased by chronic HU-210 treatment and that in CB(1)-deficient mice cell proliferation was significantly lower than in wild-type littermates, indicating that the endocannabinoid system is physiologically involved in regulation of GCP proliferation. The pro-proliferative effect of cannabinoids in GCPs was mediated through the CB(1)/AKT/glycogen synthase kinase-3beta/beta-catenin pathway. Involvement of this pathway was also observed in cultures of neuronal precursors from the subventricular zone, suggesting that this pathway may be a general mechanism by which endocannabinoids regulate proliferation of neuronal precursors. These observations suggest that endocannabinoids constitute a new family of lipid signaling cues that may exert a widespread effect on neuronal precursor proliferation during brain development.

Activity-dependent repression of Cbln1 expression: mechanism for developmental and homeostatic regulation of synapses in the cerebellum

Cbln1, which belongs to the C1q/tumor necrosis factor superfamily, is released from cerebellar granule cells and plays a crucial role in forming and maintaining excitatory synapses between parallel fibers (PFs; axons of granule cells) and Purkinje cells not only during development but also in the adult cerebellum. Although neuronal activity is known to cause morphological changes at synapses, how Cbln1 signaling is affected by neuronal activity remains unclear. Here, we show that chronic stimulation of neuronal activity by elevating extracellular K(+) levels or by adding kainate decreased the expression of cbln1 mRNA within several hours in mature granule cells in a manner dependent on L-type voltage-dependent Ca(2+) channels and calcineurin. Chronic activity also reduced Cbln1 protein levels within a few days, during which time the number of excitatory synapses on Purkinje cell dendrites was reduced; this activity-induced reduction of synapses was prevented by the addition of exogenous Cbln1 to the culture medium. Therefore, the activity-dependent downregulation of cbln1 may serve as a new presynaptic mechanism by which PF-Purkinje cell synapses adapt to chronically elevated activity, thereby maintaining homeostasis. In addition, the expression of cbln1 mRNA was prevented when immature granule cells were maintained in high-K(+) medium. Since immature granule cells are chronically depolarized before migrating to the internal granule layer, this depolarization-dependent regulation of cbln1 mRNA expression may also serve as a developmental switch to facilitate PF synapse formation in mature granule cells in the internal granule layer.

Ca2+ -dependent splicing of neurexin IIalpha

Neurexins are synaptic adhesion proteins encoded by 3 genes (NRXN1, NRXN2, and NRXN3) each transcribed from 2 promoters to yield longer (alpha) and shorter (beta) forms. The primary gene transcripts undergo extensive alternative splicing leading to products that may differ in synaptic coupling properties. Here we show that depolarization of neurons modulates splicing of NRXN2alpha, particularly at splice sites 1 and 3. Furthermore, we demonstrate that exclusion of exon 11 at splice site 3 is calcium-dependent. These data indicate neuronal activity-dependent splicing of NRXN2alpha. This dynamic process may be important for maintenance of mature neuronal circuits.

Transcriptional profiling of depolarization-dependent phenotypic alterations in primary cultures of developing granule neurons

Rat cerebellar granule neurons cultured in medium supplemented with elevated KCl are extensively used as a model to examine the coupling between neural activity and Ca(2+)-dependent gene expression. Elevated (25 mM) KCl is believed to mimic endogenous neural activity because it promotes depolarization and Ca(+2)-dependent survival and some aspects of maturation. By comparison, at least half of the granule neurons grown in standard medium containing 5 mM KCl undergo apoptosis beginning approximately 4 days in vitro. However, accumulating evidence suggests that chronic depolarization induces phenotypic abnormalities whereas growth in chemically defined medium containing 5 mM KCl more closely resembles the constitutive phenotype. To examine this, oligonucleotide microarrays and RT-PCR of selected mRNAs were used to compare transcription profiles of cultures grown in 5 mM and 25 mM KCl. In some cases, N-methyl-D-aspartate (NMDA) which, like elevated KCl, promotes long-term survival was also tested. Robust changes in several gene groups were observed and indicated that growth in elevated KCl: induces expression of mRNAs that are not normally observed; represses expression of mRNAs that should be present; maintains expression of mRNAs that are markers of immature neurons. Supplementation of the growth medium with NMDA instead of elevated KCl produces similar abnormalities. Altogether, these data indicate that growth in 5 mM KCl more closely mimics survival and maturation of granule neurons in vivo and should therefore be adopted in future studies.