In Huntington's disease (HD), mutant Huntingtin (mHtt) protein causes striatal neuron dysfunction, synaptic loss, and eventual neurodegeneration. To understand the mechanisms responsible for synaptic loss in HD, we developed a corticostriatal coculture model that features age-dependent dendritic spine loss in striatal medium spiny neurons (MSNs) from YAC128 transgenic HD mice. Age-dependent spine loss was also observed in vivo in YAC128 MSNs. To understand the causes of spine loss in YAC128 MSNs, we performed a series of mechanistic studies. We previously discovered that mHtt protein binds to type 1 inositol (1,4,5)-trisphosphate receptor (InsP 3 R1) and increases its sensitivity to activation by InsP 3 . We now report that the resulting increase in steady-state InsP 3 R1 activity reduces endoplasmic reticulum (ER) Ca 2ϩ levels. Depletion of ER Ca 2ϩ leads to overactivation of the neuronal store-operated Ca 2ϩ entry (nSOC) pathway in YAC128 MSN spines. The synaptic nSOC pathway is controlled by the ER resident protein STIM2. We discovered that STIM2 expression is elevated in aged YAC128 striatal cultures and in YAC128 mouse striatum. Knock-down of InsP 3 R1 expression by antisense oligonucleotides or knock-down or knock-out of STIM2 resulted in normalization of nSOC and rescue of spine loss in YAC128 MSNs. The selective nSOC inhibitor EVP4593 was identified in our previous studies. We now demonstrate that EVP4593 reduces synaptic nSOC and rescues spine loss in YAC128 MSNs. Intraventricular delivery of EVP4593 in YAC128 mice rescued age-dependent striatal spine loss in vivo. Our results suggest EVP4593 and other inhibitors of the STIM2-dependent nSOC pathway as promising leads for HD therapeutic development.

Spinocerebellar ataxia 2 (SCA2) is a neurodegenerative disorder characterized by progressive ataxia. SCA2 results from a polyglutamine (polyQ) expansion in the cytosolic protein ataxin-2 (Atx2). Cerebellar Purkine cells (PC) are primarily affected in SCA2, but the cause of PC dysfunction and death in SCA2 is poorly understood. In previous studies, we reported that mutant but not wild type Atx2 specifically binds the inositol 1,4,5-trisphosphate receptor (InsP3R) and increases its sensitivity to activation by InsP3. We further proposed that the resulting supranormal calcium (Ca2+) release from the PC endoplasmic reticulum (ER) plays a key role in the development of SCA2 pathology. To test this hypothesis, we achieved a chronic suppression of InsP3R-mediated Ca2+ signaling by adeno-associated virus (AAV)-mediated expression of the inositol 1,4,5-phosphatase (Inpp5a) enzyme (5PP) in PCs of a SCA2 transgenic mouse model. We determined that recombinant 5PP overexpression alleviated age-dependent dysfunction in the firing pattern of SCA2 PCs. We further discovered that chronic 5PP overexpression also rescued age-dependent motor incoordination and PC death in SCA2 mice. Our findings further support the important role of supranormal Ca2+ signaling in SCA2 pathogenesis and suggest that partial inhibition of InsP3-mediated Ca2+ signaling could provide therapeutic benefit for the patients afflicted with SCA2 and possibly other SCAs.

Spinocerebellar ataxia type 2 (SCA2) is a neurodegenerative disorder caused by a polyglutamine expansion within the Ataxin-2 (Atxn2) protein. Purkinje cells (PC) of the cerebellum fire irregularly and eventually die in SCA2. We show here that the type 2 small conductance calcium-activated potassium channel (SK2) play a key role in control of normal PC activity. Using cerebellar slices from transgenic SCA2 mice we demonstrate that SK channel modulators restore regular pacemaker activity of SCA2 PCs. Furthermore, we also show that oral delivery of a novel selective positive modulator of SK2/3 channels (NS13001) alleviates behavioural and neuropathological phenotypes of aging SCA2 transgenic mice. We conclude that SK2 channels constitute a novel target for SCA2 treatment and that the developed selective SK2/3 modulator NS13001 holds promise as a potential therapeutic agent for treatment of SCA2 and possibly other cerebellar ataxias.

Cerebellar Purkinje cells (PCs) are primarily affected in many spinocerebellar ataxias (SCA). In this study we investigated functional activity of PCs in transgenic mouse model of SCA2, a polyglutamine neurodegenerative hereditary disorder. In our studies we used extracellular single-unit recording method to compare spontaneous activity of PCs in age-matched wild-type mice and SCA2-58Q transgenic mice. We discovered that the fraction of PCs with bursting and an irregular pattern of spontaneous activity dramatically increases in aged SCA2-58Q mice compared with wild-type littermates. Small-conductance calcium-activated potassium (SK) channels play an important role in determining firing rate of PCs. Indeed, we demonstrated that intraperitoneal (IP) injection of SK channel inhibitor NS8593 induces an irregular pattern of PC activity in wild-type mice. Furthermore, we demonstrated that IP injection of SK channel-positive modulator chlorzoxazone (CHZ) decreases spontaneous firing rate of cerebellar PCs. Finally, we have shown that IP injections with CHZ normalize firing activity of cerebellar PCs from aging SCA2-58Q mice. We propose that alterations in PC firing patterns is one of potential causes of ataxic symptoms in SCA2 and in other SCAs and that positive modulators of SK channels can be used to normalize activity of PCs and alleviate ataxic phenotype in patients with SCA.

The inositol 1,4,5-trisphosphate receptor (IP 3 R) is an intracellular ion channel that mediates the release of calcium ions from the endoplasmic reticulum. It plays a role in basic biological functions, such as cell division, differentiation, fertilization and cell death, and is involved in developmental processes including learning, memory and behavior. Deregulation of neuronal calcium signaling results in disturbance of cell homeostasis, synaptic loss and dysfunction, eventually leading to cell death. Three IP 3 R subtypes have been identified in mammalian cells and the predominant isoform in neurons is IP 3 R type 1. Dysfunction of IP 3 R type 1 may play a role in the pathogenesis of certain neurodegenerative diseases as enhanced activity of the IP 3 R was observed in models of Huntington's disease, spinocerebellar ataxias and Alzheimer's disease. These results suggest that IP 3 R-mediated signaling is a potential target for treatment of these disorders. In this review we discuss the structure, functions and regulation of the IP 3 R in healthy neurons and in conditions of neurodegeneration. IntroductionInositol 1,4,5-trisphosphate receptors (IP 3 Rs) are a family of intracellular ion channels that mediate calcium (Ca 2+ ) release from the endoplasmic reticulum (ER) following stimulation by the second messenger inositol 1,4,5-trisphosphate (IP 3 ). Generation of IP 3 molecules is caused by the activation of phospholipase C in response to the activation of G proteincoupled receptors such as serotonergic receptors, adrenergic receptors, calcitonin receptors, histamine receptors, metabotropic glutamate receptors (mGluRs), Abbreviations AAV, adeno-associated virus; AD, Alzheimer's disease; ALG-2, apoptosis-linked gene 2; ARM, armadillo; Ab, beta amyloid; BH, Bcl-2 homology; CaBP1, calcium-binding protein 1; CTD, C-terminal domain; EM, electron microscopy; ER, endoplasmic reticulum; FAD, familial Alzheimer's disease; GRP78, 78-kDa glucose regulated protein; HAP1, huntingtin-associated protein 1; HD, Huntington's disease; HelD, helical domain; Htt, huntingtin; IBC, IP 3 -binding core; IICR, inositol 1,4,5-trisphosphate-induced calcium release; ILD, 'intervening lateral' domain; IP 3 , inositol 1,4,5-trisphosphate; IP 3 R1, IP 3 R type 1; IP 3 R, inositol 1,4,5-trisphosphate receptor; IRBIT, IP 3 R binding protein released with IP 3 ; KO, knock-out; LBD, ligand-binding domain; LNK, helical linker domain; MCI, mild cognitive impairment; mGluR, metabotropic glutamate receptor; mPTP, mitochondrial permeability transition pore; MSN, medium spiny neuron; NMDA, N-methyl-D-aspartate; NTR, Nterminal region; Opt, opisthotonos; PC, Purkinje cell; polyQ, polyglutamine; PS, presenilin; RyR, ryanodine receptor; SAD, sporadic Alzheimer's disease; SCA, spinocerebellar ataxia; SD, IP 3 -binding suppressor domain; SUMF1, sulfatase modifying factor 1; TG2, transglutaminase type 2; TMD, transmembrane domain; WT, wild-type; YAC, yeast artificial chromosome; b-TF, b-trefoil. 3547The (Fig. 1). Upon extracellular stimulation -by various ...