Abnormal phosphorylation of synapsin I predicts a neuronal transmission impairment in the R6/2 Huntington's disease transgenic mice

Calcineurin in memory and bidirectional plasticity

The molecular mechanisms of learning and memory, and the underlying bidirectional changes in synaptic plasticity that sustain them largely implicate protein kinases and phosphatases. Specifically, Ca(2+)-dependent kinases and phosphatases actively control neuronal processing by forming a tightly regulated balance in which they oppose each other. In this balance, calcineurin (PP2B) is a critical protein phosphatase whose main function is to negatively modulate learning, memory, and plasticity. It acts by dephosphorylating numerous substrates in different neuronal compartments. This review outlines some of CN neuronal targets and their implication in synaptic functions, and describes the role of CN in the acquisition, storage, retrieval, and extinction of memory, as well as in bidirectional plasticity.

Cysteine String Protein (CSP) Inhibition of N-type Calcium Channels Is Blocked by Mutant Huntingtin

Cysteine string protein (CSP), a 34-kDa molecular chaperone, is expressed on synaptic vesicles in neurons and on secretory vesicles in endocrine, neuroendocrine, and exocrine cells. CSP can be found in a complex with two other chaperones, the heat shock cognate protein Hsc70, and small glutamine-rich tetratricopeptide repeat domain protein (SGT). CSP function is vital in synaptic transmission; however, the precise nature of its role remains controversial. We have previously reported interactions of CSP with both heterotrimeric GTP-binding proteins (G proteins) and N-type calcium channels. These associations give rise to a tonic G protein inhibition of the channels. Here we have examined the effects of huntingtin fragments (exon 1) with (huntingtin(exon1/exp)) and without (huntingtin(exon1/nonexp)) expanded polyglutamine (polyQ) tracts on the CSP chaperone system. In vitro huntingtin(exon1/exp) sequestered CSP and blocked the association of CSP with G proteins. In contrast, huntingtin(exon1/nonexp) did not interact with CSP and did not alter the CSP/G protein association. Similarly, co-expression of huntingtin(exon1/exp) with CSP and N-type calcium channels eliminated CSP's tonic G protein inhibition of the channels, while coexpression of huntingtin(exon1/nonexp) did not alter the robust inhibition promoted by CSP. These results indicate that CSP's modulation of G protein inhibition of calcium channel activity is blocked in the presence of a huntingtin fragment with expanded polyglutamine tracts.

Huntington's disease: a synaptopathy?

Huntington's disease (HD) is caused by a polyglutamine expansion in the protein huntingtin. In its terminal stage, HD is characterized by widespread neuronal death in the neocortex and the striatum. Classically, this neuronal death has been thought to underlie most of the symptoms of the disease. Accumulating evidence suggests, however, that cellular dysfunction is important in the pathogenesis of HD. We propose that specific impairment of the exocytosis and endocytosis machinery contributes to the development of HD. We also suggest that abnormal synaptic transmission underlies the early symptoms of HD and can contribute to the triggering of cell death in later stages of the disease.

The role of serine/threonine protein phosphatases in exocytosis

Modulation of exocytosis is integral to the regulation of cellular signalling, and a variety of disorders (such as epilepsy, hypertension, diabetes and asthma) are closely associated with pathological modulation of exocytosis. Emerging evidence points to protein phosphatases as key regulators of exocytosis in many cells and, therefore, as potential targets for the design of novel therapies to treat these diseases. Diverse yet exquisite regulatory mechanisms have evolved to direct the specificity of these enzymes in controlling particular cell processes, and functionally driven studies have demonstrated differential regulation of exocytosis by individual protein phosphatases. This Review discusses the evidence for the regulation of exocytosis by protein phosphatases in three major secretory systems, (1) mast cells, in which the regulation of exocytosis of inflammatory mediators plays a major role in the respiratory response to antigens, (2) insulin-secreting cells in which regulation of exocytosis is essential for metabolic control, and (3) neurons, in which regulation of exocytosis is perhaps the most complex and is essential for effective neurotransmission.

[Huntington chorea in Drosophila and mice: toward new therapeutic steps]

Huntington's disease is an hereditary dominant neurodegenerative disorder clinically characterised by progressive dyskinesia, cognitive decline and psychiatric disturbances. One decade after the identification of the gene whose mutation is responsible for the disease, this pathology remains incurable. However, major insights into early cellular and molecular basis of Huntington's disease have arisen from transgenic models. Transcriptional dysregulation, abnormal degradation of misfolded proteins as well as excitotoxic processes and mitochondrial dysfunction are involved in Huntington's disease. The present review discusses the recent insights gained from mouse and Drosophila models towards the understanding of pathogenesis and the development of new therapeutic tools.

Transient and progressive electrophysiological alterations in the corticostriatal pathway in a mouse model of Huntington's disease

Alterations in the corticostriatal pathway may precede symptomatology and striatal cell death in Huntington's disease (HD) patients. Here we examined spontaneous EPSCs in striatal medium-sized spiny neurons in slices from a mouse model of HD (R6/2). Spontaneous EPSC frequency was similar in young (3-4 weeks) transgenics and controls but decreased significantly in transgenics when overt behavioral symptoms began (5-7 weeks) and was most pronounced in severely impaired transgenics (11-15 weeks). These differences were maintained after bicuculline or tetrodotoxin, indicating they were specific to glutamatergic input and likely presynaptic in origin. Decreases in presynaptic and postsynaptic protein markers, synaptophysin and postsynaptic density-95, occurred in 11-15 week R6/2 mice, supporting the electrophysiological results. Furthermore, isolated, large-amplitude synaptic events (>100 pA) occurred more frequently in transgenic animals, particularly at 5-7 weeks, suggesting additional dysregulation of cortical inputs. Large events were blocked by tetrodotoxin, indicating a possible cortical origin. Addition of bicuculline and 4-aminopyridine facilitated the occurrence of large events. Riluzole, a compound that decreases glutamate release, reduced these events. Together, these observations indicate that both progressive and transient alterations occur along the corticostriatal pathway in experimental HD. These alterations are likely to contribute to the selective vulnerability of striatal medium-sized spiny neurons.