Distribution of plasma membrane Ca2+ pump (PMCA) isoforms in the gerbil brain: effect of ischemia-reperfusion injury

Calcium dysregulation and compensation in cortical pyramidal neurons of the R6/2 mouse model of Huntington's disease

Huntington's disease (HD) is a fatal, hereditary neurodegenerative disorder that predominantly affects striatal medium-sized spiny neurons and cortical pyramidal neurons (CPNs). It has been proposed that perturbations in Ca²⁺ homeostasis could play a role in CPN alterations. To test this hypothesis, we used the R6/2 mouse model of juvenile HD at different stages of disease progression; presymptomatic, early symptomatic, and late symptomatic. We combined whole-cell patch-clamp recordings of layer 2/3 CPNs with two-photon laser scanning microscopy to image somatic and dendritic Ca²⁺ transients associated with evoked action potentials (APs). We found that the amplitude of AP-induced Ca²⁺ transients recorded at the somata of CPNs was significantly reduced in presymptomatic and late symptomatic R6/2 mice compared with wild-type (WT) littermates. However, reduced amplitudes were compensated by increases in decay times, so that Ca²⁺ transient areas were similar between genotypes. AP-induced Ca²⁺ transients in CPN proximal dendrites were variable and differences did not reach statistical significance, except for reduced areas in the late symptomatic group. In late symptomatic mice, a specific store-operated Ca²⁺ channel antagonist, EVP4593, reduced somatic Ca²⁺ transient amplitude similarly in WT and R6/2 CPNs. In contrast, dantrolene, a ryanodine receptor (RyR) antagonist, and nifedipine, an L-type Ca²⁺ channel blocker, significantly reduced both somatic Ca²⁺ transient amplitude and area in R6/2 but not WT CPNs. These findings demonstrate that perturbations of Ca²⁺ homeostasis and compensation occur in CPNs before and after the onset of overt symptoms, and suggest RyRs and L-type Ca²⁺ channels as potential targets for therapeutic intervention.NEW & NOTEWORTHY We used two-photon microscopy to examine calcium influx induced by action potentials in cortical pyramidal neurons from a mouse model of Huntington's disease (HD), the R6/2. The amplitude of somatic calcium transients was reduced in R6/2 mice compared with controls. This reduction was compensated by increased decay times, which could lead to reduced calcium buffering capacity. L-type calcium channel and ryanodine receptor blockers reduced calcium transient area in HD neurons, suggesting new therapeutic avenues.

The role of Plasma Membrane Calcium ATPases (PMCAs) in neurodegenerative disorders

Selective degeneration of differentiated neurons in the brain is the unifying feature of neurodegenerative disorders such as Parkinson's disease (PD) or Alzheimer's disease (AD). A broad spectrum of evidence indicates that initially subtle, but temporally early calcium dysregulation may be central to the selective neuronal vulnerability observed in these slowly progressing, chronic disorders. Moreover, it has long been evident that excitotoxicity and its major toxic effector mechanism, neuronal calcium overload, play a decisive role in the propagation of secondary neuronal death after acute brain injury from trauma or ischemia. Under physiological conditions, neuronal calcium homeostasis is maintained by a fine-tuned interplay between calcium influx and releasing mechanisms (Ca²⁺-channels), and calcium efflux mechanisms (Ca²⁺-pumps and -exchangers). Central functional components of the calcium efflux machinery are the Plasma Membrane Calcium ATPases (PMCAs), which represent high-affinity calcium pumps responsible for the ATP-dependent removal of calcium out of the cytosol. Beyond a growing body of experimental evidence, it is their high expression level, their independence of secondary ions or membrane potential, their profound redox regulation and autoregulation, their postsynaptic localization in close proximity to the primary mediators of pathological calcium influx, i.e. NMDA receptors, as well as evolutionary considerations which all suggest a pivotal role of the PMCAs in the etiology of neurodegeneration and make them equally challenging and alluring candidates for drug development. This review aims to summarize the recent literature on the role of PMCAs in the pathogenesis of neurodegenerative disorders.

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.

Plasma membrane calcium ATPases and related disorders

The plasma membrane Ca(2+) ATPases (PMCA pumps) cooperate with other transport systems in the plasma membrane and in the organelles in the regulation of cell Ca(2+). They have high Ca(2+) affinity and are thus the fine tuners of cytosolic Ca(2+). They belong to the superfamily of P-type ATPases: their four basic isoforms share the essential properties of the reaction cycle and the general membrane topography motif of 10 transmembrane domains and three large cytosolic units. However they also differ in other important properties, e.g., tissue distribution and regulatory mechanisms. Their chief regulator is calmodulin, that removes their C-terminal cytosolic tail from autoinhibitory binding sites next to the active site of the pump, restoring activity. The number of pump isoforms is increased to over 30 by alternative splicing of the transcripts at a N-terminal site (site A) and at site C within the C-terminal calmodulin binding domain: the splice variants are tissue specific and developmentally regulated. The importance of PMCAs in the maintenance of cellular Ca(2+) homeostasis is underlined by the disease phenotypes, genetic or acquired, caused by their malfunction. Non-genetic PMCA deficiencies have long been considered possible causative factors in disease conditions as important as cancer, hypertension, or neurodegeneration. Those of genetic origin are better characterized: some have now been discovered in humans as well. They concern all four PMCA isoforms, and range from cardiac dysfunctions, to deafness, to hypertension, to cerebellar ataxia.

Atherosclerosis and atherosensitivity in two southwest Algerian desert rodents, Psammomys obesus and Gerbillus gerbillus, and in Rattus norvegicus

Cardiovascular disease, including atherosclerosis, is the leading cause of death in patients with diabetes worldwide; thus, it is a major medical concern. The endothelium contributes to the control of many vascular functions, and clinical observations show that it is a primary target for diabetic syndrome. To get better insight into the mechanisms underlying atherosclerosis, we studied the interspecific differences in the arterial metabolisms of two, Psammomys obesus and Gerbillus gerbillus, as well as Rattus norvegicus (Wistar rat), well known for its atheroresistance. Twenty-two enzymatic activities and six macromolecular substances were histochemically compared in the two desert species and in Wistar aortas (abdominal and thoracic) and arteries (femoral and caudal) embedded in a common block. In the healthy adult rodents, enzyme activities were very intense. They demonstrated that aortic myocytes are capable of various synthesis and catabolism processes. However, considering the frequency of atherosclerosis and its phenotypes, significant differences appeared between the species studied. Our comparative study shows that aortic atherosensitive animals have several common metabolic characteristics, which are found in Psammomys rich in metachromatic glycosaminoglycans (involved in the inhibition of lipolysis and in calcification of the organic matrix), reduced activity in enzymes related to the Krebs cycle (weakening energetic power), and low lipolytic enzyme, adenosine triphosphatase, and adenosine diphosphatase activities. However, the most fundamental pathophysiological difference is the low lipolytic power of the aorta of Psammomys when compared to Wistar rats. This characteristic determines its atherosensitivity and makes this animal model more applicable to the experimental development of atherosclerosis.

Plasma membrane Ca2+-ATPases in the nervous system during development and ageing

Calcium signaling is used by neurons to control a variety of functions, including cellular differentiation, synaptic maturation, neurotransmitter release, intracellular signaling and cell death. This review focuses on one of the most important Ca²⁺ regulators in the cell, the plasma membrane Ca²⁺-ATPase (PMCA), which has a high affinity for Ca²⁺ and is widely expressed in brain. The ontogeny of PMCA isoforms, linked to specific requirements of Ca²⁺ during development of different brain areas, is addressed, as well as their function in the adult tissue. This is based on the high diversity of variants in the PMCA family in brain, which show particular kinetic differences possibly related to specific localizations and functions of the cell. Conversely, alterations in the activity of PMCAs could lead to changes in Ca²⁺ homeostasis and, consequently, to neural dysfunction. The involvement of PMCA isoforms in certain neuropathologies and in brain ageing is also discussed.

Cardioprotection via modulation of calcium homeostasis by thiopental in hypoxia-reoxygenated neonatal rat cardiomyocytes

PURPOSE

Ca(2+) homeostasis plays an important role in myocardial cell injury induced by hypoxia-reoxygenation, and prevention of intracellular Ca(2+) overload is key to cardioprotection. Even though thiopental is a frequently used anesthetic agent, little is known about its cardioprotective effects, particularly in association with Ca(2+) homeostasis. We investigated whether thiopental protects cardiomyocytes against hypoxia-reoxygenation injury by regulating Ca(2+) homeostasis.

MATERIALS AND METHODS

Neonatal rat cardiomyocytes were isolated. Cardiomyocytes were exposed to different concentrations of thiopental and immediately replaced in the hypoxic chamber to maintain hypoxia. After 1 hour of exposure, a culture dish was transferred to the CO(2) incubator and cells were incubated at 37 for 5 hours. At the end of the experiments, the authors assessed cell protection using immunoblot analysis and caspase activity. The mRNA of genes involved in Ca(2+) homeostasis, mitochondrial membrane potential, and cellular Ca(2+) levels were examined.

RESULTS

In thiopental-treated cardiomyocytes, there was a decrease in expression of the proapoptotic protein Bax, caspase-3 activation, and intracellular Ca(2+) content. In addition, both enhancement of anti-apoptotic protein Bcl-2 and activation of Erk concerned with survival were shown. Furthermore, thiopental attenuated alterations of genes involving Ca(2+) regulation and significantly modulated abnormal changes of NCX and SERCA2a genes in hypoxia-reoxygenated neonatal cardiomyocytes. Thiopental suppressed disruption of mitochondrial membrane potential (DeltaPsi(m)) induced by hypoxia-reoxygenation.

CONCLUSION

Thiopental is likely to modulate expression of genes that regulate Ca(2+) homeostasis, which reduces apoptotic cell death and results in cardioprotection.

Calcium Pumps in Health and Disease

Ca2+-ATPases (pumps) are key actors in the regulation of Ca2+ in eukaryotic cells and are thus essential to the correct functioning of the cell machinery. They have high affinity for Ca2+ and can efficiently regulate it down to very low concentration levels. Two of the pumps have been known for decades (the SERCA and PMCA pumps); one (the SPCA pump) has only become known recently. Each pump is the product of a multigene family, the number of isoforms being further increased by alternative splicing of the primary transcripts. The three pumps share the basic features of the catalytic mechanism but differ in a number of properties related to tissue distribution, regulation, and role in the cellular homeostasis of Ca2+. The molecular understanding of the function of the pumps has received great impetus from the solution of the three-dimensional structure of one of them, the SERCA pump. These spectacular advances in the structure and molecular mechanism of the pumps have been accompanied by the emergence and rapid expansion of the topic of pump malfunction, which has paralleled the rapid expansion of knowledge in the topic of Ca2+-signaling dysfunction. Most of the pump defects described so far are genetic: when they are very severe, they produce gross and global disturbances of Ca2+ homeostasis that are incompatible with cell life. However, pump defects may also be of a type that produce subtler, often tissue-specific disturbances that affect individual components of the Ca2+-controlling and/or processing machinery. They do not bring cells to immediate death but seriously compromise their normal functioning.

Region-specific sensitivity of the spinal cord to ischemia/reperfusion: the dynamic of changes in catalytic NOS activity

This study was designed in order to consider whether the release of neuronally derived nitric oxide (NO) in the lumbosacral spinal cord during ischemia/reperfusion is region-specific and whether changes in Ca(2+)-dependent NO synthase (cNOS) activity paralell with functional outcome. The cNOS activity was measured in the spinal cord regions after 13-, 15- and 17-min ischemia alone and that followed by 24 h of reperfusion. In addition, the Tarlov's criteria were applied to define the neurological consequences of ischemia/reperfusion in experimental animals. Based on the results, it is evident that only the 17-min ischemia alone was quite sufficient to cause changes in cNOS activity, however, without alterations in functional outcomes. On the other hand, the ischemic episodes followed by reperfusion caused dynamic, region-specific alterations in cNOS activity and consequently led to deterioration of motor function of hindlimbs in affected animals. Our results indicate that the motoneurons in the ventral horns respond more sensitively to ischemia/reperfusion than do neurons localized in the other spinal cord regions and that changes in cNOS activity may also influence the axonal conductance in the white matter and account for the impairment of motoneuronal activity in affected animals.

Reductions in motor unit number estimates (MUNE) precede motor neuron loss in the plasma membrane calcium ATPase 2 (PMCA2)-heterozygous mice

The potential of MUNE as a unique electrophysiological tool to detect early motor unit abnormalities during a clinically silent period was investigated in the plasma membrane calcium ATPase 2 (PMCA2)-heterozygous mice. There was a significant reduction in MUNE in the PMCA2-heterozygous mice as compared to the wild type littermates at two months of age. In contrast, the compound motor action potential (CMAP) was not altered. The conduction velocity (CV) of the sensory nerve and sensory nerve action potentials (SNAP) were not modified indicating lack of major sensory deficits. Interestingly, despite a decline in MUNE at this age, no changes were detected in choline acetyl transferase (ChAT) positive motor neuron number in the ventral horn of the lumbar spinal cord. Hindlimb grip strength, a test that evaluates clinical dysfunction, was also similar to that of the wild type controls. However, motor neuron number significantly decreased by five months suggesting that a drop in MUNE preceded motor neuron loss. In the two-month-old PMCA2-null mice, reduced MUNE measurements coincided with lower motor neuron number and decreased hindlimb grip strength. The fall in motor neuron number was already detectable at three weeks, the earliest time studied, and became more pronounced by five months. Our results show that even partial reductions in PMCA2 levels are sufficient to cause delayed death of motor neurons and that MUNE may be a reliable and sensitive approach to detect pathology prior to cell loss and in the absence of overt clinical signs.

Mechanistic role of calpains in postischemic neurodegeneration

The calpain family of proteases is causally linked to postischemic neurodegeneration. However, the precise mechanisms by which calpains contribute to postischemic neuronal death have not been fully elucidated. This review outlines the key features of the calpain system, and the evidence for its causal role in postischemic neuronal pathology. Furthermore, the consequences of specific calpain substrate cleavage at various subcellular locations are explored. Calpain substrates within synapses, plasma membrane, endoplasmic reticulum, lysosomes, mitochondria, and the nucleus, as well as the overall effect of postischemic calpain activity on calcium regulation and cell death signaling are considered. Finally, potential pathways for calpain-mediated neurodegeneration are outlined in an effort to guide future studies aimed at understanding the downstream pathology of postischemic calpain activity and identifying optimal therapeutic strategies.

Quantitative proteomic profiling of membrane proteins from the mouse brain cortex, hippocampus, and cerebellum using the HysTag reagent: mapping of neurotransmitter receptors and ion channels

Analysis of the brain proteome and studying brain diseases through clinical biopsies and animal disease models require methods of quantitative proteomics that are sensitive and allow identification and quantification of low abundant membrane proteins from minute amount of tissue. Taking advantage of recently developed methods for isolation of membrane proteins from 10-20 mg brain tissue [Nielsen, P.Aa., Olsen, J.V., Podtelejnokov, A.V., Andersen, J.R., Mann, M., Wiśniewski, J.R., 2005. Proteomic mapping of brain plasma membrane proteins. Mol. Cell. Proteomics 4, 402--408] and the HysTag-quantification method [Olsen, J.V., Andersen, J.R., Nielsen, P.Aa., Nielsen, M.L., Figeys, D., Mann, M., Wiśniewski, J.R., 2004. HysTag---A novel proteomic qualification tool applied to differential analysis of membrane proteins from distinct areas of mouse brain. Mol. Cell. Proteomics 3, 82--92] we performed quantitative proteomic analysis of three functionally distinct compartments of mouse brain: cortex, hippocampus, and cerebellum. In total, 976 unique peptides corresponding to 555 unique proteins were quantified. Up to 20-fold differences in the levels of some proteins between brain areas were measured. For many quantified proteins--as for glutamate receptors, calcium channel subunits, and ATP-ases--an excellent correlation between our proteomic data and previously published mRNA expression levels or intensity of immunostaining was found. Our results clearly demonstrate differences in levels of membrane proteins mapped in distinct brain compartments and offer a technology that allows in depth study of brain membrane proteomes, such as mouse models of neurological diseases.

Traumatic injury of the spinal cord and nitric oxide

In the current report, we summarize our findings related to the involvement of nitric oxide (NO) in the pathology of spinal cord trauma. We initially studied the distribution of nitric oxide synthase (NOS)-immunolabeled and/or nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd; which is highly colocalized with NOS)-stained somata and fibers in the spinal cord of the rabbit. Segmental and laminar distribution of NADPHd-stained neurons in the rabbit revealed a large number of NADPHd-stained neurons in the spinal cord falling into six categories, N1-N6, while others could not be classified. Large numbers of NADPHd-stained neurons were identified in the superficial dorsal horn and around the central canal. Four morphologically distinct kinds of NADPHd-stained axons 2.5-3.5 microm in diameter were identified throughout the white matter in the spinal cord. Moreover, a massive occurrence of axonal NADPHd-staining was detected in the juxtagriseal layer of the ventral funiculus along the rostrocaudal axis. The prominent NADPHd-stained fiber bundles were identified in the mediobasal and central portion of the ventral funiculus. The sulcomarginal fasciculus was found in the basal and medial portion of the ventral funiculus in all cervical and thoracic segments. Since the discovery that NO may act as a neuronal transmitter, an increasing interest has focused on its ability to modulate synaptic function. NO passes through cell membranes without specific release or uptake mechanisms inducing changes in signal-related functions by several means. In particular, the activation of the soluble guanylyl cyclases (sGC), the formation of cyclic guanosine 3',5'-monophosphate (cGMP) and the action of cGMP-dependent protein kinases has been identified as the main signal transduction pathways of NO in the nervous system including spinal cord. It is known that the intracellular level of cGMP is strictly controlled by its rate of synthesis via guanylyl cyclases (GC) and/or by the rate of its degradation via 3',5'-cyclic nucleotide phosphodiesterases (PDE). GC can be divided into two main groups, i.e., the membrane-bound or particular guanylyl cyclase (pGC) and the cytosolic or sGC. In the spinal cord, the activation of pGC has only been demonstrated for natriuretic peptides, which stimulate cGMP accumulation in GABA-ergic structures in laminae I-III of the rat cervical spinal cord. These neurons are involved in controlling the action of the locomotor circuit. In view of the abundance of NO-responsive structures in the brain, it is proposed that NO-cGMP signaling will be part of neuronal information processing at many levels. In relation to this, we found that surgically induced Th7 constriction of 24 h duration stimulated both the constitutive NOS activity and cGMP level by 120 and 131%, respectively, in non-compartmentalized white matter of Th8-Th9 segments, located just caudally to the site of injury. NO-mediated cGMP formation was only slightly increased in the dorsal funiculus of Th5-Th9 segments. There are some other sources that may influence the NO-mediated cGMP formation in spinal cord. A high level of glutamate produced at the site of the lesion and an excessive accumulation of intracellular Ca2+ may stimulate NOS activity and create suitable conditions for NO synthesis and its adverse effect on white matter. An increased interest has focused on the role of NO at the site of injury and in areas located close to the epicenter of the impact site and, in these connections an upregulation of NOS was noted in neurons and interneurons. However, the upregulation of NOS expression was also seen in interneurons located just rostrally and caudally to the lesion. A quantitative analysis of laminar distribution of multiple cauda equina constriction (MCEC) induced NADPHd-stained neurons revealed a considerable increase in these neurons in laminae VIII-IX 8h postconstriction, and a highly statistically significant increase of such neurons in laminae VII-X 5 days postconstriction in the lumbosacral segments. Concurrently, the number of NADPHd-stained neurons on laminae I-II in LS segments was greatly reduced. It is concluded that a greater understanding of NO changes after spinal cord trauma is essential for the possibility of targeting this pathway therapeutically.

Molecular alterations in the cerebellum of the plasma membrane calcium ATPase 2 (PMCA2)-null mouse indicate abnormalities in Purkinje neurons

PMCA2, a major calcium pump, is expressed at particularly high levels in Purkinje neurons. Accordingly, PMCA2-null mice exhibit ataxia suggesting cerebellar pathology. It is not yet known how changes in PMCA2 expression or activity affect molecular pathways in Purkinje neurons. We now report that the levels of metabotropic glutamate receptor 1 (mGluR1), which plays essential roles in motor coordination, synaptic plasticity, and associative learning, are reduced in the cerebellum of PMCA2-null mice as compared to wild type littermates. The levels of inositol 1,4,5-triphosphate receptor type 1 (IP3R1), an effector downstream to mGluR1, which mediates intracellular calcium signaling, and the expression of Homer 1b/c and Homer 3, scaffold proteins that couple mGluR1 to IP3R1, are also reduced in somata and dendrites of some Purkinje cell subpopulations. In contrast, no alterations occur in the levels of mGluR1 and its downstream effectors in the hippocampus, indicating that the changes are region specific. The reduction in cerebellar mGluR1, IP3R1 and Homer 3 levels are neither due to a generic decrease in Purkinje proteins nor extensive dendritic loss as immunoreactivity to total and non-phosphorylated neurofilament H (NFH) is increased in Purkinje dendrites and microtubule associated protein 2 (MAP2) staining reveals a dense dendritic network in the molecular layer of the PMCA2-null mouse cerebellum. PMCA2 coimmunoprecipitates with mGluR1, Homer 3 and IP3R1, suggesting that the calcium pump is a constituent of the mGluR1 signaling complex. Our results suggest that the decrease in the expression of mGluR1 and its downstream effectors and perturbations in the mGluR1 signaling complex in the absence of PMCA2 may cumulatively result in aberrant metabotropic glutamate receptor signaling in Purkinje neurons leading to cerebellar deficits in the PMCA2-null mouse.

Calcium pumps in the central nervous system

Two families of Ca2+ transport ATPases are involved in the maintenance of Ca2+ homeostasis in the nervous system, the plasma membrane Ca2+-ATPase that pumps Ca2+ to the extracellular medium and the intracellular sarco/endoplasmic reticulum Ca2+-ATPase that transports Ca2+ from the cytosol to the endoplasmic reticulum. Both types of calcium pumps show precise regulatory properties and they are localized in specific subcellular regions. In this review, we describe the functional and regulatory properties of both families of calcium pumps, their distribution in nerve cells, and their involvement in neurological disorders. The functional characterization of neuronal calcium pumps is very important in order to understand the biochemical processes involved in the maintenance of intracellular calcium in synaptic terminals.

Localization of intracellular and plasma membrane Ca2+-ATPases in the cerebellum

The sarco-endoplasmic reticulum Ca2+-ATPase and the plasma membrane Ca2+-ATPase contribute to the regulation of the intracellular Ca2+ concentration. These proteins transport Ca2+ ions into the endoplasmic reticulum and to the extracellular medium, respectively. A different localization of the two families of Ca2+-ATPases has been shown in concrete subcellular areas of Purkinje cells and in other neuronal elements from cerebellum. In the light of the actual knowledge of Ca2+-ATPases, this strict distribution suggests the existence of different demands on Ca2+ homeostasis in these cerebellar and cellular subregions.

Ca2+-ATPase isoforms are expressed in neuroprotection in rat, but not human, neurons

Glutamate excitotoxicity has been implicated in neuronal death and damage in many neurodegenerative disorders. The potential neuroprotective role of the plasma membrane calcium ATPase (PMCA) and the NMDA receptor were investigated in rat and human brain neurons after a glutamate insult. Investigation of potential mechanisms of neuronal survival revealed that surviving rat cerebellar granule cells expressed the mRNA of new PMCA isoforms 2b and 2c. There was no observable change in expression of PMCA isoforms or NMDA receptor NR2 subtypes in human cortical neurons. This study shows that subsets of rat and human neurons are resistant to glutamate-induced excitotoxicity and the mechanisms employed to enable survival differ between rat and human neurons.