Reduced expression of the Ca(2+) transporter protein PMCA2 slows Ca(2+) dynamics in mouse cerebellar Purkinje neurones and alters the precision of motor coordination

Pre-ataxic loss of intrinsic plasticity and motor learning in a mouse model of SCA1

Spinocerebellar ataxias are neurodegenerative diseases, the hallmark symptom of which is the development of ataxia due to cerebellar dysfunction. Purkinje cells, the principal neurons of the cerebellar cortex, are the main cells affected in these disorders, but the sequence of pathological events leading to their dysfunction is poorly understood. Understanding the origins of Purkinje cells dysfunction before it manifests is imperative to interpret the functional and behavioural consequences of cerebellar-related disorders, providing an optimal timeline for therapeutic interventions. Here, we report the cascade of events leading to Purkinje cells dysfunction before the onset of ataxia in a mouse model of spinocerebellar ataxia 1 (SCA1). Spatiotemporal characterization of the ATXN1[82Q] SCA1 mouse model revealed high levels of the mutant ATXN1[82Q] weeks before the onset of ataxia. The expression of the toxic protein first caused a reduction of Purkinje cells intrinsic excitability, which was followed by atrophy of Purkinje cells dendrite arborization and aberrant glutamatergic signalling, finally leading to disruption of Purkinje cells innervation of climbing fibres and loss of intrinsic plasticity of Purkinje cells. Functionally, we found that deficits in eyeblink conditioning, a form of cerebellum-dependent motor learning, precede the onset of ataxia, matching the timeline of climbing fibre degeneration and reduced intrinsic plasticity. Together, our results suggest that abnormal synaptic signalling and intrinsic plasticity during the pre-ataxia stage of spinocerebellar ataxias underlie an aberrant cerebellar circuitry that anticipates the full extent of the disease severity. Furthermore, our work indicates the potential for eyeblink conditioning to be used as a sensitive tool to detect early cerebellar dysfunction as a sign of future disease.

Cerebellar granule cell signaling is indispensable for normal motor performance

Within the cerebellar cortex, mossy fibers (MFs) excite granule cells (GCs) that excite Purkinje cells (PCs), which provide outputs to the deep cerebellar nuclei (DCNs). It is well established that PC disruption produces motor deficits such as ataxia. This could arise from either decreases in ongoing PC-DCN inhibition, increases in the variability of PC firing, or disruption of the flow of MF-evoked signals. Remarkably, it is not known whether GCs are essential for normal motor function. Here we address this issue by selectively eliminating calcium channels that mediate transmission (CaV2.1, CaV2.2, and CaV2.3) in a combinatorial manner. We observe profound motor deficits but only when all CaV2 channels are eliminated. In these mice, the baseline rate and variability of PC firing are unaltered, and locomotion-dependent increases in PC firing are eliminated. We conclude that GCs are indispensable for normal motor performance and that disruption of MF-induced signals impairs motor performance.

Crosstalk among Calcium ATPases: PMCA, SERCA and SPCA in Mental Diseases

Calcium in mammalian neurons is essential for developmental processes, neurotransmitter release, apoptosis, and signal transduction. Incorrectly processed Ca²⁺ signal is well-known to trigger a cascade of events leading to altered response to variety of stimuli and persistent accumulation of pathological changes at the molecular level. To counterbalance potentially detrimental consequences of Ca²⁺, neurons are equipped with sophisticated mechanisms that function to keep its concentration in a tightly regulated range. Calcium pumps belonging to the P-type family of ATPases: plasma membrane Ca²⁺-ATPase (PMCA), sarco/endoplasmic Ca²⁺-ATPase (SERCA) and secretory pathway Ca²⁺-ATPase (SPCA) are considered efficient line of defense against abnormal Ca²⁺ rises. However, their role is not limited only to Ca²⁺ transport, as they present tissue-specific functionality and unique sensitive to the regulation by the main calcium signal decoding protein—calmodulin (CaM). Based on the available literature, in this review we analyze the contribution of these three types of Ca²⁺-ATPases to neuropathology, with a special emphasis on mental diseases.

Estrogen receptors in pain modulation: cellular signaling

Sensory perception and emotional disorders are disproportionally represented in men and women and are thus thought to be modulated by different sex hormones in various conditions. Among the most important hormones perceived to affect sensory processing and transduction is estrogen. Numerous previous researchers have endeavored to demonstrate that estrogen is capable of modulating the activity of sensory neurons in peripheral and central sites in female, male, or castrated animals. However, the underlying mechanisms of its modulation of neuronal activity are somewhat unclear. In the present review, we discuss the possible cellular and molecular mechanisms involved in the modulation of nociception by estrogen.

Losing the Beat: Contribution of Purkinje Cell Firing Dysfunction to Disease, and Its Reversal

The cerebellum is a brain structure that is highly interconnected with other brain regions. There are many contributing factors to cerebellar-related brain disease, such as altered afferent input, local connectivity, and/or cerebellar output. Purkinje cells (PC) are the principle cells of the cerebellar cortex, and fire intrinsically; that is, they fire spontaneous action potentials at high frequencies. This review paper focuses on PC intrinsic firing activity, which is altered in multiple neurological diseases, including ataxia, Huntington Disease (HD) and autism spectrum disorder (ASD). Notably, there are several cases where interventions that restore or rescue PC intrinsic activity also improve impaired behavior in these mouse models of disease. These findings suggest that rescuing PC firing deficits themselves may be sufficient to improve impairment in cerebellar-related behavior in disease. We propose that restoring PC intrinsic firing represents a good target for drug development that might be of therapeutic use for several disorders.

The Puzzling Role of Neuron-Specific PMCA Isoforms in the Aging Process

The aging process is a physiological phenomenon associated with progressive changes in metabolism, genes expression, and cellular resistance to stress. In neurons, one of the hallmarks of senescence is a disturbance of calcium homeostasis that may have far-reaching detrimental consequences on neuronal physiology and function. Among several proteins involved in calcium handling, plasma membrane Ca²⁺-ATPase (PMCA) is the most sensitive calcium detector controlling calcium homeostasis. PMCA exists in four main isoforms and PMCA2 and PMCA3 are highly expressed in the brain. The overall effects of impaired calcium extrusion due to age-dependent decline of PMCA function seem to accumulate with age, increasing the susceptibility to neurotoxic insults. To analyze the PMCA role in neuronal cells, we have developed stable transfected differentiated PC12 lines with down-regulated PMCA2 or PMCA3 isoforms to mimic age-related changes. The resting Ca²⁺ increased in both PMCA-deficient lines affecting the expression of several Ca²⁺-associated proteins, i.e., sarco/endoplasmic Ca²⁺-ATPase (SERCA), calmodulin, calcineurin, GAP43, CCR5, IP₃Rs, and certain types of voltage-gated Ca²⁺ channels (VGCCs). Functional studies also demonstrated profound changes in intracellular pH regulation and mitochondrial metabolism. Moreover, modification of PMCAs membrane composition triggered some adaptive processes to counterbalance calcium overload, but the reduction of PMCA2 appeared to be more detrimental to the cells than PMCA3.

Calcium Dyshomeostasis Alters CCL5 Signaling in Differentiated PC12 Cells

Background

Plasma membrane Ca²⁺-ATPase (PMCA) is the most sensitive cellular calcium detector. It exists in four main isoforms (PMCA1-4), among which PMCA2 and PMCA3 are considered as fast-acting neuron-specific forms. In the brain, PMCA function declines progressively during aging; thereby impaired calcium homeostasis may contribute to some neurodegenerative diseases. These destructive processes can be propagated by proinflammatory chemokines, including chemokine CCL5, which causes phospholipase C-mediated liberation of Ca²⁺ from endoplasmic reticulum by IP₃-gated channels.

Methods

To mimic the changes in aged neurons we used stable transfected differentiated PC12 cells with downregulated PMCA2 or PMCA3 and analyzed the effect of CCL5 on calcium transients with Fluo-4 reagent. Chemokine receptors were evaluated using Western blot, and IP₃ receptors expression level was assessed using qRT-PCR and Western blot.

Results

In PMCA-reduced cell lines, CCL5 released more Ca²⁺ by IP₃-sensitive receptors, and the time required for Ca²⁺ clearance was significantly longer. Also, in these lines we detected altered expression level of CCR5 and IP₃ receptors.

Conclusion

Although modification of PMCAs composition could provide some protection against calcium overload, reduction of PMCA2 appeared to be more detrimental to the cells than deficiency of PMCA3. Under pathological conditions, including inflammatory CCL5 action and long-lasting Ca²⁺ dyshomeostasis, insufficient cell protection may result in progressive degeneration and death of neurons.

Peroxisome proliferator-activated receptor-γ activation attenuates harmaline-induced cognitive impairments in rats

Cognitive and motor disturbances are serious concerns of the tremors induced by motor disorders. Despite the lack of effective clinical treatment, some potential therapeutic agents have been used to alleviate the cognitive symptoms in the animal models of tremor. Recent studies have shown that PPAR-γ agonists have neuroprotective effects. In the current study, the effects of pioglitazone (PIO), a peroxisome proliferator-activated receptor gamma agonist, on harmaline-induced motor and cognitive impairment were studied. Male Wistar rats were divided into vehicle (normal saline), PIO (20 mg/kg i.p.), harmaline (10 mg/kg, i.p.) and PIO + harmaline (PIO injected 2 h before harmaline) groups. Open field, rotarod, wire grip, foot print and Morris water maze tests were used to evaluate the motor and cognitive performance. The results indicated that administration of PIO attenuated harmaline-induced locomotor, anxiety-like behaviors, and spatial learning and memory impairments, but it partially decreased the tremor score. The neuroprotective and anxiolytic effects of PIO demonstrated in the current study can offer the PPAR-γ receptor agonism as a potential therapeutic agent in the treatment of patients with tremor that manifest mental dysfunction.

A link between plasma membrane calcium ATPase 2 (PMCA2), estrogen and estrogen receptor α signaling in mechanical pain

Earlier studies on genetically modified mice indicated that plasma membrane calcium ATPase 2 (PMCA2), a calcium extrusion pump, plays a novel and sex-dependent role in mechanical pain responses: female, but not male, PMCA2^+/− mice manifest increased mechanical pain compared to female PMCA2^+/+ mice. The goal of the present studies was to determine the contribution of ovarian steroids to the genotype- and sex-dependent manifestation of mechanical pain in PMCA2^+/+ versus PMCA2^+/− mice. Ovariectomy increased mechanical pain sensitivity and 17β-estradiol (E2) replacement restored it to basal levels in PMCA2^+/+ mice, but not in PMCA2^+/− littermates. Intrathecal administration of an estrogen receptor alpha (ERα) agonist induced ERα signaling in the dorsal horn (DH) of female PMCA2^+/+ mice, but was ineffective in PMCA2^+/− mice. In male PMCA2^+/+ and PMCA2^+/− mice, E2 treatment following orchidectomy did not recapitulate the genotype-dependent differential pain responses observed in females and the agonist did not elicit ERα signaling. These findings establish a novel, female-specific link between PMCA2, ERα and mechanical pain. It is postulated that PMCA2 is essential for adequate ERα signaling in the female DH and that impaired ERα signaling in the female PMCA2^+/− mice hinders the analgesic effects of E2 leading to increased sensitivity to mechanical stimuli.

A V1143F mutation in the neuronal-enriched isoform 2 of the PMCA pump is linked with ataxia

The fine regulation of intracellular calcium is fundamental for all eukaryotic cells. In neurons, Ca²⁺ oscillations govern the synaptic development, the release of neurotransmitters and the expression of several genes. Alterations of Ca²⁺ homeostasis were found to play a pivotal role in neurodegenerative progression. The maintenance of proper Ca²⁺ signaling in neurons demands the continuous activity of Ca²⁺ pumps and exchangers to guarantee physiological cytosolic concentration of the cation. The plasma membrane Ca²⁺ATPases (PMCA pumps) play a key role in the regulation of Ca²⁺ handling in selected sub-plasma membrane microdomains. Among the four basic PMCA pump isoforms existing in mammals, isoforms 2 and 3 are particularly enriched in the nervous system. In humans, genetic mutations in the PMCA2 gene in association with cadherin 23 mutations have been linked to hearing loss phenotypes, while those occurring in the PMCA3 gene were associated with X-linked congenital cerebellar ataxias. Here we describe a novel missense mutation (V1143F) in the calmodulin binding domain (CaM-BD) of the PMCA2 protein. The mutant pump was present in a patient showing congenital cerebellar ataxia but no overt signs of deafness, in line with the absence of mutations in the cadherin 23 gene. Biochemical and molecular dynamics studies on the mutated PMCA2 have revealed that the V1143F substitution alters the binding of calmodulin to the CaM-BD leading to impaired Ca²⁺ ejection.

Survival strategies for mouse cerebellar Purkinje neurons lacking PMCA2

Expression of the fast calcium extrusion protein, PMCA2, in the cerebellum is amongst the highest found throughout the central nervous system, and unsurprisingly PMCA2 knockout mice exhibit cerebellar ataxia or loss of controlled movement. The sole output neurons of the cerebellar cortex, Purkinje neurons, are functionally compromised in these knockout mice, yet remarkably these neurons survive. In this mini-review we review and speculate on the importance of multiple PMCA2 dependent actions at cellular and synaptic sites within the cerebellar Purkinje neuron network. We also explore how loss of PMCA2^-/- can lead to the ataxic phenotype, but can paradoxically also minimise calcium rises in cerebellar Purkinje neurons, thereby ensuring their resilience and survival.

Evidence for a role of plasma membrane calcium pumps in neurodegenerative disease: Recent developments

Plasma membrane Ca2+ ATPases (PMCAs) are a major system for calcium extrusion from all cells. Different PMCA isoforms and splice variants are involved in the precise temporal and spatial handling of Ca2+ signals and the re-establishment of resting Ca2+ levels in the nervous system. Lack or inappropriate expression of specific PMCAs leads to characteristic neuronal phenotypes, which may be reciprocally exacerbated by genetic predisposition through alleles in other genes that modify PMCA interactions, regulation, and function. PMCA dysfunction is often poorly compensated in neurons and may lead to changes in synaptic transmission, altered excitability and, with long-term calcium overload, eventual cell death. Decrease and functional decline of PMCAs are hallmarks of neurodegeneration during aging, and mutations in specific PMCAs are responsible for neuronal dysfunction and accelerated neurodegeneration in many sensory and cognitive diseases.

The Plasma Membrane Calcium ATPases and Their Role as Major New Players in Human Disease

The Ca²⁺ extrusion function of the four mammalian isoforms of the plasma membrane calcium ATPases (PMCAs) is well established. There is also ever-increasing detail known of their roles in global and local Ca²⁺ homeostasis and intracellular Ca²⁺ signaling in a wide variety of cell types and tissues. It is becoming clear that the spatiotemporal patterns of expression of the PMCAs and the fact that their abundances and relative expression levels vary from cell type to cell type both reflect and impact on their specific functions in these cells. Over recent years it has become increasingly apparent that these genes have potentially significant roles in human health and disease, with PMCAs1-4 being associated with cardiovascular diseases, deafness, autism, ataxia, adenoma, and malarial resistance. This review will bring together evidence of the variety of tissue-specific functions of PMCAs and will highlight the roles these genes play in regulating normal physiological functions and the considerable impact the genes have on human disease.

A positive feedback loop linking enhanced mGluR function and basal calcium in spinocerebellar ataxia type 2

Metabotropic glutamate receptor 1 (mGluR1) function in Purkinje neurons (PNs) is essential for cerebellar development and for motor learning and altered mGluR1 signaling causes ataxia. Downstream of mGluR1, dysregulation of calcium homeostasis has been hypothesized as a key pathological event in genetic forms of ataxia but the underlying mechanisms remain unclear. We find in a spinocerebellar ataxia type 2 (SCA2) mouse model that calcium homeostasis in PNs is disturbed across a broad range of physiological conditions. At parallel fiber synapses, mGluR1-mediated excitatory postsynaptic currents (EPSCs) and associated calcium transients are increased and prolonged in SCA2 PNs. In SCA2 PNs, enhanced mGluR1 function is prevented by buffering [Ca²⁺] at normal resting levels while in wildtype PNs mGluR1 EPSCs are enhanced by elevated [Ca²⁺]. These findings demonstrate a deleterious positive feedback loop involving elevated intracellular calcium and enhanced mGluR1 function, a mechanism likely to contribute to PN dysfunction and loss in SCA2.

DOI:http://dx.doi.org/10.7554/eLife.26377.001

The ataxia related G1107D mutation of the plasma membrane Ca2+ ATPase isoform 3 affects its interplay with calmodulin and the autoinhibition process

The plasma membrane Ca²⁺ ATPases (PMCA pumps) have a long, cytosolic C-terminal regulatory region where a calmodulin-binding domain (CaM-BD) is located. Under basal conditions (low Ca²⁺), the C-terminal tail of the pump interacts with autoinhibitory sites proximal to the active center of the enzyme. In activating conditions (i.e., high Ca²⁺), Ca²⁺-bound CaM displaces the C-terminal tail from the autoinhibitory sites, restoring activity. We have recently identified a G1107D replacement within the CaM-BD of isoform 3 of the PMCA pump in a family affected by X-linked congenital cerebellar ataxia. Here, we investigate the effects of the G1107D replacement on the interplay of the mutated CaM-BD with both CaM and the pump core, by combining computational, biochemical and functional approaches. We provide evidence that the affinity of the isolated mutated CaM-BD for CaM is significantly reduced with respect to the wild type (wt) counterpart, and that the ability of CaM to activate the pump in vitro is thus decreased. Multiscale simulations support the conclusions on the detrimental effect of the mutation, indicating reduced stability of the CaM binding. We further show that the G1107D replacement impairs the autoinhibition mechanism of the PMCA3 pump as well, as the introduction of a negative charge perturbs the contacts between the CaM-BD and the pump core. Thus, the mutation affects both the ability of the pump to optimally transport Ca²⁺ in the activated state, and the autoinhibition mechanism in its resting state.

Cellular and circuit mechanisms underlying spinocerebellar ataxias

Degenerative ataxias are a common form of neurodegenerative disease that affect about 20 individuals per 100,000. The autosomal dominant spinocerebellar ataxias (SCAs) are caused by a variety of protein coding mutations (single nucleotide changes, deletions and expansions) in single genes. Affected genes encode plasma membrane and intracellular ion channels, membrane receptors, protein kinases, protein phosphatases and proteins of unknown function. Although SCA-linked genes are quite diverse they share two key features: first, they are highly, although not exclusively, expressed in cerebellar Purkinje neurons (PNs), and second, when mutated they lead ultimately to the degeneration of PNs. In this review we summarize ataxia-related changes in PN neurophysiology that have been observed in various mouse knockout lines and in transgenic models of human SCA. We also highlight emerging evidence that altered metabotropic glutamate receptor signalling and disrupted calcium homeostasis in PNs form a common, early pathophysiological mechanism in SCAs. Together these findings indicate that aberrant calcium signalling and profound changes in PN neurophysiology precede PN cell loss and are likely to lead to cerebellar circuit dysfunction that explains behavioural signs of ataxia characteristic of the disease.

Plasma membrane calcium ATPases: From generic Ca(2+) sump pumps to versatile systems for fine-tuning cellular Ca(2.)

The plasma membrane calcium ATPases (PMCAs) are ATP-driven primary ion pumps found in all eukaryotic cells. They are the major high-affinity calcium extrusion system for expulsion of Ca(2+) ions from the cytosol and help restore the low resting levels of intracellular [Ca(2+)] following the temporary elevation of Ca(2+) generated during Ca(2+) signaling. Due to their essential role in the maintenance of cellular Ca(2+) homeostasis they were initially thought to be "sump pumps" for Ca(2+) removal needed by all cells to avoid eventual calcium overload. The discovery of multiple PMCA isoforms and alternatively spliced variants cast doubt on this simplistic assumption, and revealed instead that PMCAs are integral components of highly regulated multi-protein complexes fulfilling specific roles in calcium-dependent signaling originating at the plasma membrane. Biochemical, genetic, and physiological studies in gene-manipulated and mutant animals demonstrate the important role played by specific PMCAs in distinct diseases including those affecting the peripheral and central nervous system, cardiovascular disease, and osteoporosis. Human PMCA gene mutations and allelic variants associated with specific disorders continue to be discovered and underline the crucial role of different PMCAs in particular cells, tissues and organs.

Regulation of GAP43/calmodulin complex formation via calcineurin-dependent mechanism in differentiated PC12 cells with altered PMCA isoforms composition

Several lines of evidence suggest the contribution of age-related decline in plasma membrane calcium pump (PMCA) to the onset of neurodegenerative diseases. From four PMCA isoforms, PMCA2, and PMCA3 respond to a rapid removal of Ca²⁺ and are expressed predominantly in excitable cells. We have previously shown that suppression of neuron-specific PMCAs in differentiated PC12 cells accelerated cell differentiation, but increased apoptosis in PMCA2-deficient line. We also demonstrated that altered expression of voltage-dependent calcium channels correlated with their higher contribution to Ca²⁺ influx, which varied between PMCA-reduced lines. Here, we propose a mechanism unique for differentiated PC12 cells by which PMCA2 and PMCA3 regulate pGAP43/GAP43 ratio and the interaction between GAP43 and calmodulin (CaM). Although down-regulation of PMCA2 or PMCA3 altered the content of GAP43/pGAP43, of paramount importance for the regulatory mechanism is a disruption of isoform-specific inhibitory PMCA/calcineurin interaction. In result, higher endogenous calcineurin (CaN) activity leads to hypophosphorylation of GAP43 in PMCA2- or PMCA3-deficient lines and intensification of GAP43/CaM complex formation, thus potentially limiting the availability of free CaM. In overall, our results indicate that both “fast” PMCA isoforms could actively regulate the local CaN function and CaN-downstream processes. In connection with our previous observations, we also suggest a negative feedback of cooperative action of CaM, GAP43, and CaN on P/Q and L-type channels activity. PMCAs- and CaN-dependent mechanism presented here, may signify a protective action against calcium overload in neuronal cells during aging, as well a potential way for decreasing neuronal cells vulnerability to neurodegenerative insults.

A Novel Mutation in Isoform 3 of the Plasma Membrane Ca2+ Pump Impairs Cellular Ca2+ Homeostasis in a Patient with Cerebellar Ataxia and Laminin Subunit 1α Mutations

The particular importance of Ca(2+) signaling to neurons demands its precise regulation within their cytoplasm. Isoform 3 of the plasma membrane Ca(2+) ATPase (the PMCA3 pump), which is highly expressed in brain and cerebellum, plays an important role in the regulation of neuronal Ca(2+). A genetic defect of the PMCA3 pump has been described in one family with X-linked congenital cerebellar ataxia. Here we describe a novel mutation in the ATP2B3 gene in a patient with global developmental delay, generalized hypotonia and cerebellar ataxia. The mutation (a R482H replacement) impairs the Ca(2+) ejection function of the pump. It reduces the ability of the pump expressed in model cells to control Ca(2+) transients generated by cell stimulation and impairs its Ca(2+) extrusion function under conditions of low resting cytosolic Ca(2+) as well. In silico analysis of the structural effect of the mutation suggests a reduced stabilization of the portion of the pump surrounding the mutated residue in the Ca(2+)-bound state. The patient also carries two missense mutations in LAMA1, encoding laminin subunit 1α. On the basis of the family pedigree of the patient, the presence of both PMCA3 and laminin subunit 1α mutations appears to be necessary for the development of the disease. Considering the observed defect in cellular Ca(2+) homeostasis and the previous finding that PMCAs act as digenic modulators in Ca(2+)-linked pathologies, the PMCA3 dysfunction along with LAMA1 mutations could act synergistically to cause the neurological phenotype.

Neuronal calcium signaling: function and dysfunction

Calcium (Ca(2+)) is an universal second messenger that regulates the most important activities of all eukaryotic cells. It is of critical importance to neurons as it participates in the transmission of the depolarizing signal and contributes to synaptic activity. Neurons have thus developed extensive and intricate Ca(2+) signaling pathways to couple the Ca(2+) signal to their biochemical machinery. Ca(2+) influx into neurons occurs through plasma membrane receptors and voltage-dependent ion channels. The release of Ca(2+) from the intracellular stores, such as the endoplasmic reticulum, by intracellular channels also contributes to the elevation of cytosolic Ca(2+). Inside the cell, Ca(2+) is controlled by the buffering action of cytosolic Ca(2+)-binding proteins and by its uptake and release by mitochondria. The uptake of Ca(2+) in the mitochondrial matrix stimulates the citric acid cycle, thus enhancing ATP production and the removal of Ca(2+) from the cytosol by the ATP-driven pumps in the endoplasmic reticulum and the plasma membrane. A Na(+)/Ca(2+) exchanger in the plasma membrane also participates in the control of neuronal Ca(2+). The impaired ability of neurons to maintain an adequate energy level may impact Ca(2+) signaling: this occurs during aging and in neurodegenerative disease processes. The focus of this review is on neuronal Ca(2+) signaling and its involvement in synaptic signaling processes, neuronal energy metabolism, and neurotransmission. The contribution of altered Ca(2+) signaling in the most important neurological disorders will then be considered.

A new Atp2b2 deafwaddler allele, dfw(i5), interacts strongly with Cdh23 and other auditory modifiers

Tight regulation of calcium (Ca2+) concentrations in the stereocilia bundles of auditory hair cells of the inner ear is critical to normal auditory transduction. The plasma membrane Ca2+ ATPase 2 (PMCA2), encoded by the Atp2b2 gene, is the primary mechanism for clearance of Ca2+ from auditory stereocilia, keeping intracellular levels low, and also contributes to maintaining adequate levels of extracellular Ca2+ in the endolymph. This study characterizes a novel null Atp2b2 allele, dfw(i5), by examining cochlear anatomy, vestibular function and auditory physiology in mutant mice. Loss of auditory function in PMCA2 mutants can be attributed to dysregulation of intracellular Ca2+ inside the stereocilia bundles. However, extracellular Ca2+ ions surrounding the stereocilia are also required for rigidity of cadherin 23, a component of the stereocilia tip-link encoded by the Cdh23 gene. This study further resolves the interaction between Atp2b2 and Cdh23 in a gene dosage and frequency-dependent manner, and finds that low frequencies are significantly affected by the interaction. In +/dfw(i5) mice, one mutant copy of Cdh23 is sufficient to cause broad frequency hearing impairment. Additionally, we report another modifying interaction with Atp2b2 on auditory sensitivity, possibly caused by an unidentified hearing loss gene in mice.

The evidence for association of ATP2B2 polymorphisms with autism in Chinese Han population

Background

Autism is a neurodevelopmental disorder with a high estimated heritability. ATP2B2, located on human chromosome 3p25.3, encodes the plasma membrane calcium-transporting ATPase 2 which extrudes Ca²⁺ from cytosol into extracellular space. Recent studies reported association between ATP2B2 and autism in samples from Autism Genetic Resource Exchange (AGRE) and Italy. In this study, we investigated whether ATP2B2 polymorphisms were associated with autism in Chinese Han population.

Methods

We performed a family based association study between five SNPs (rs35678 in exon, rs241509, rs3774180, rs3774179, and rs2278556 in introns) in ATP2B2 and autism in 427 autism trios of Han Chinese descent. All SNPs were genotyped using the Sequenom genotyping platform. The family-based association test (FBAT) program was used to perform association test for SNPs and haplotype analyses.

Results

This study demonstrated a preferential transmission of T allele of rs3774179 to affected offsprings under an additive model (T>C, Z = 2.482, p = 0.013). While C allele of rs3774179 showed an undertransmission from parents to affected children under an additive and a dominant model, respectively (Z = −2.482, p = 0.013; Z = −2.591, p = 0.0096). Haplotype analyses revealed that three haplotypes were significantly associated with autism. The haplotype C-C (rs3774180–rs3774179) showed a significant undertransmission from parents to affected offsprings both in specific and global haplotype FBAT (Z = −2.037, p = 0.042; Global p = 0.03). As for the haplotype constructed by rs3774179 and rs2278556, C-A might be a protective haplotype (Z = −2.206, p = 0.027; Global p = 0.04), while T-A demonstrated an excess transmission from parents to affected offsprings (Z = 2.143, p = 0.032). These results were still significant after using the permutation method to obtain empirical p values.

Conclusions

Our research suggested that ATP2B2 might play a role in the etiology of autism in Chinese Han population.

Plasma membrane calcium ATPases as novel candidates for therapeutic agent development

Plasma membrane Ca2+ ATPases (PMCAs) are highly regulated transporters responsible for Ca2+ extrusion from all eukaryotic cells. Different PMCA isoforms are implicated in various tasks of Ca2+ regulation including bulk Ca2+ transport and localized Ca2+ signaling in specific membrane microdomains. Accumulating evidence shows that loss, mutation or inappropriate expression of different PMCAs is associated with pathologies ranging from hypertension, low bone density and male infertility to hearing loss and cerebellar ataxia. Compared to Ca2+ influx channels, PMCAs have lagged far behind as targets for drug development, mainly due to the lack of detailed understanding of their structure and specific function. This is rapidly changing thanks to integrated efforts combining biochemical, structural, cellular and physiological studies suggesting that selective modulation of PMCA isoforms may be of therapeutic value in the management of different and complex diseases. Both structurally informed rational design and high-throughput small molecule library screenings are promising strategies that are expected to lead to specific and isoform-selective modulators of PMCA function. This short review will provide an overview of the diverse roles played by PMCA isoforms in different cells and tissues and their emerging involvement in pathophysiological processes, summarize recent progress in obtaining structural information on the PMCAs, and discuss current and future strategies to develop specific PMCA inhibitors and activators for potential therapeutic applications.

Functional integration of calcium regulatory mechanisms at Purkinje neuron synapses

Cerebellar Purkinje neurons receive synaptic inputs from three different sources: the excitatory parallel fibre and climbing fibre synapses as well as the inhibitory synapses from molecular layer stellate and basket cells. These three synaptic systems use distinct mechanisms in order to generate Ca(2+) signals that are specialized for specific modes of neurotransmitter release and post-synaptic signal integration. In this review, we first describe the repertoire of Ca(2+) regulatory mechanisms that generate and regulate the amplitude and timing of Ca(2+) fluxes during synaptic transmission and then explore how these mechanisms interact to generate the unique functional properties of each of the Purkinje neuron synapses.

Gene expression analysis on a single cell level in Purkinje cells of Huntington's disease transgenic mice

Ataxia is a clinical feature of most polyglutamine disorders. Cerebellar neurodegeneration of Purkinje cells (PCs) in Huntington’s Disease (HD) brain was described in the 1980s. PC death in the R6/2 transgenic model for HD was published by Turmaine et al. So far, PCs have not been examined on a single cell level. In order to begin to understand PC dysfunction and degeneration in HD we performed a gene expression study on laser-dissected PC based on a DNA microarray screening and quantitative real time PCR (Q-PCR). We demonstrate downregulation of the retinoid acid receptor-related orphan receptor (ROR) mRNA and ROR-mediated mRNAs, also seen by immunofluorescent staining. As ROR and ROR-dependent transcriptional dysregulation is not only found in the R6/2 model for HD but also in a model for spinocerebellar ataxia type 1 (SCA1) (Serra et al.) the data suggest common pathogenic mechanisms for both polyglutamine diseases.

Downregulation of PMCA2 or PMCA3 reorganizes Ca(2+) handling systems in differentiating PC12 cells

Changes in PMCA2 and PMCA3 expression during neuronal development are tightly linked to structural and functional modifications in Ca(2+) handling machinery. Using antisense strategy we obtained stably transfected PC12 lines with reduced level of PMCA2 or PMCA3, which were then subjected to dibutyryl-cAMP differentiation. Reduced level of neuron-specific PMCAs led to acceleration of differentiation and formation of longer neurites than in control PC12 line. Treatment with dibutyryl-cAMP was associated with retraction of growth cones and intensified formation of varicosities. In PMCA2-reduced cells development of apoptosis and DNA laddering were detected. Higher amounts of constitutive isoforms PMCA1 and PMCA4, their putative extended location to gaps left after partial removal of PMCA2 or PMCA3, together with increased SERCA may indicate the induction of compensatory mechanism in modified cells. Functional studies showed altered expression of certain types of VDCCs in PMCA-reduced cells, which correlated with their higher contribution to Ca(2+) influx. The cell response to PMCAs suppression suggests the interplay between transcription level of two opposite calcium-transporting systems i.e. voltage- and store depletion-activated channels facilitating Ca(2+) influx and calcium pumps responsible for Ca(2+) clearance, as well highlights the role of both neuron-specific PMCA isoforms in the control of PC12 cells differentiation.

Mutation of plasma membrane Ca2+ ATPase isoform 3 in a family with X-linked congenital cerebellar ataxia impairs Ca2+ homeostasis

Ca(2+) in neurons is vital to processes such as neurotransmission, neurotoxicity, synaptic development, and gene expression. Disruption of Ca(2+) homeostasis occurs in brain aging and in neurodegenerative disorders. Membrane transporters, among them the calmodulin (CaM)-activated plasma membrane Ca(2+) ATPases (PMCAs) that extrude Ca(2+) from the cell, play a key role in neuronal Ca(2+) homeostasis. Using X-exome sequencing we have identified a missense mutation (G1107D) in the CaM-binding domain of isoform 3 of the PMCAs in a family with X-linked congenital cerebellar ataxia. PMCA3 is highly expressed in the cerebellum, particularly in the presynaptic terminals of parallel fibers-Purkinje neurons. To study the effects of the mutation on Ca(2+) extrusion by the pump, model cells (HeLa) were cotransfected with expression plasmids encoding its mutant or wild-type (wt) variants and with the Ca(2+)-sensing probe aequorin. The mutation reduced the ability of the PMCA3 pump to control the cellular homeostasis of Ca(2+). It significantly slowed the return to baseline of the Ca(2+) transient induced by an inositol-trisphosphate (InsP(3))-linked plasma membrane agonist. It also compromised the ability of the pump to oppose the influx of Ca(2+) through the plasma membrane capacitative channels.

Purkinje cell dysfunction and delayed death in plasma membrane calcium ATPase 2-heterozygous mice

Purkinje cell (PC) dysfunction or death has been implicated in a number of disorders including ataxia, autism and multiple sclerosis. Plasma membrane calcium ATPase 2 (PMCA2), an important calcium (Ca(2+)) extrusion pump that interacts with synaptic signaling complexes, is most abundantly expressed in PCs compared to other neurons. Using the PMCA2 heterozygous mouse as a model, we investigated whether a reduction in PMCA2 levels affects PC function. We focused on Ca(2+) signaling and the expression of glutamate receptors which play a key role in PC function including synaptic plasticity. We found that the amplitude of depolarization and 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid receptor (AMPAR)-mediated Ca(2+) transients are significantly higher in cultured PMCA2(+/-) PCs than in PMCA2(+/+) PCs. This is due to increased Ca(2+) influx, since P/Q type voltage-gated Ca(2+) channel (VGCC) expression was more pronounced in PCs and cerebella of PMCA2(+/-) mice and VGCC blockade prevented the elevation in amplitude. Neuronal nitric oxide synthase (nNOS) activity was higher in PMCA2(+/-) cerebella and inhibition of nNOS or the soluble guanylate cyclase (sGC)-cyclic guanosine monophosphate (cGMP) pathway, which mediates nitric oxide (NO) signaling, reduced the amplitude of Ca(2+) transients in PMCA2(+/-) PCs, in vitro. In addition, there was an age-dependent decrease in metabotropic glutamate receptor 1 (mGluR1) and AMPA receptor subunit GluR2/3 transcript and protein levels at 8 weeks of age. These changes were followed by PC loss in the 20-week-old PMCA2(+/-) mice. Our studies highlight the importance of PMCA2 in Ca(2+) signaling, glutamate receptor expression and survival of Purkinje cells.

Maternal exposure to the CB1 cannabinoid agonist WIN 55212-2 produces robust changes in motor function and intrinsic electrophysiological properties of cerebellar Purkinje neurons in rat offspring

The cerebellum, which controls coordinated and rapid movements, is a potential target for the deleterious effects of drugs of abuse including cannabis (i.e. marijuana, cannabinoids). Prenatal exposure to cannabinoids has been documented to cause abnormalities in motor and cognitive development, but the exact mechanism of this effect at the cellular level has not been fully elucidated. Previous studies indicate that cannabinoids are capable of modulating synaptic neurotransmission. In addition to altering synaptic activity, cannabinoid exposure may also change intrinsic neuronal properties. In the present study several different approaches including behavioral assays, extracellular field potential recordings and whole-cell patch clamp recordings, were used to address whether maternal exposure to the CB1 cannabinoid receptor agonist WIN 55-212-2 (WIN) affects the intrinsic electrophysiological properties of Purkinje neurons. WIN treatment of pregnant rats produced a significant decrease in the rearing frequency, total distance moved and mobility of the offspring, but significantly increased the time of the righting reflex, the grooming frequency and immobility. Neuromotor function, as assessed in the grip test and balance beam test, was also significantly impaired in prenatally WIN-treated group. Prenatal exposure to WIN increased the amplitude of population spikes (PS) recorded from the cerebellar Purkinje cell layer of offspring following synaptic blockage. WIN treatment of pregnant rats also profoundly affected the intrinsic properties of Purkinje neurons in offspring. This treatment increased the firing regularity, firing frequency, amplitude of afterhyperpolarization (AHP), the peak amplitude of action potential and the first spike latency, but decreased significantly the time to peak and duration of action potentials, the instantaneous firing frequency, the rate of rebound action potential and the voltage "sag" ratio. These results raise the possibility that maternal exposure to cannabinoids may profoundly affect the intrinsic membrane properties of cerebellar Purkinje neurons of offspring by altering the membrane excitability through modulation of intrinsic ion channels.

Contribution of plasma membrane Ca2+ ATPase to cerebellar synapse function

The cerebellum expresses one of the highest levels of the plasma membrane Ca²⁺ ATPase, isoform 2 in the mammalian brain. This highly efficient plasma membrane calcium transporter protein is enriched within the main output neurons of the cerebellar cortex; i.e. the Purkinje neurons (PNs). Here we review recent evidence, including electrophysiological and calcium imaging approaches using the plasma membrane calcium ATPase 2 (PMCA2) knockout mouse, to show that PMCA2 is critical for the physiological control of calcium at cerebellar synapses and cerebellar dependent behaviour. These studies have also revealed that deletion of PMCA2 throughout cerebellar development in the PMCA2 knockout mouse leads to permanent signalling and morphological alterations in the PN dendrites. Whilst these findings highlight the importance of PMCA2 during cerebellar synapse function and development, they also reveal some limitations in the use of the PMCA2 knockout mouse and the need for additional experimental approaches including cell-specific and reversible manipulation of PMCAs.