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.

Natural Products: Biochemical-Chemical Characterization, and Evidence for Therapeutic Potential

This Special Issue follows from a previous Issue entitled “Mechanism-Based Development of Natural Products for Human Health” arising from an inaugural conference on Natural Products Development held at Whistler Mountain, Canada from September 21-22, 2012. The timeliness of this issue reflects the continued growth in Natural Product research and the success of the First Issue, as judged from feedback we received and citations received. We were also encouraged by the interest in natural products research in recent years, as exemplified by the recent publication on Traditional Medicine in the prestigious journal Science (1, 2).

Deletion of the intestinal plasma membrane calcium pump, isoform 1, Atp2b1, in mice is associated with decreased bone mineral density and impaired responsiveness to 1, 25-dihydroxyvitamin D3

The physiological importance of the intestinal plasma membrane calcium pump, isoform 1, (Pmca1, Atp2b1), in calcium absorption and homeostasis has not been previously demonstrated in vivo. Since global germ-line deletion of the Pmca1 in mice is associated with embryonic lethality, we selectively deleted the Pmca1 in intestinal absorptive cells. Mice with loxP sites flanking exon 2 of the Pmca1 gene (Pmca1(fl/fl)) were crossed with mice expressing Cre recombinase in the intestine under control of the villin promoter to give mice in which the Pmca1 had been deleted in the intestine (Pmca1(EKO) mice). Pmca1(EKO) mice were born at a reduced frequency and were small at the time of birth when compared to wild-type (Wt) littermates. At two months of age, Pmca1(EKO) mice fed a 0.81% calcium, 0.34% phosphorus, normal vitamin D diet had reduced whole body bone mineral density (P < 0.037), and reduced femoral bone mineral density (P < 0.015). There was a trend towards lower serum calcium and higher serum parathyroid hormone (PTH) and 1α,25-dihydroxyvitamin D3 (1α,25(OH)2D3) concentrations in Pmca1(EKO) mice compared to Wt mice but the changes were not statistically significant. The urinary phosphorus/creatinine ratio was increased in Pmca1(EKO) mice (P < 0.004). Following the administration of 200 ng of 1α,25(OH)2D3 intraperitoneally to Wt mice, active intestinal calcium transport increased ∼2-fold, whereas Pmca1(EKO) mice administered an equal amount of 1α,25(OH)2D3 failed to show an increase in active calcium transport. Deletion of the Pmca1 in the intestine is associated with reduced growth and bone mineralization, and a failure to up-regulate calcium absorption in response to 1α,25(OH)2D3.

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.

Expression and secretion of plasma membrane Ca2+-ATPase 4a (PMCA4a) during murine estrus: association with oviductal exosomes and uptake in sperm

PMCA4, a membrane protein, is the major Ca²⁺ efflux pump in murine sperm where its deletion leads to a severe loss of hyperactivated motility and to male infertility. We have previously shown that the PMCA4b splice variant interacts with CASK (Ca^2+/CaM-dependent serine kinase) in regulating sperm Ca²⁺. More recently we detected that PMCA4a isoform, in addition to its presence in testis, is secreted in the epididymal luminal fluid and transferred to sperm. Here we show that Pmca4 mRNA is expressed in both the 4a and 4b variants in the vagina, uterus, and oviduct. Immunofluorescence reveals that PMCA4a is similarly expressed and is elevated during estrus, appearing in the glandular and luminal epithelia. Western analysis detected PMCA4a in all tissues and in the luminal fluids (LF) of the vagina (VLF), uterus (ULF), and the oviduct (OLF) collected during estrus. It was ~9- and 4-fold higher in OLF than in VLF and ULF, and only marginally present in LF collected at metestrus/diestrus. Fractionation of the LF collected at estrus, via ultracentrifugation, revealed that 100% of the PMCA4a resides in the vesicular fraction of the ULF and OLF. Transmission electron microscopy (TEM) revealed that OLF vesicles have an exosomal orientation (with the cytoplasmic-side inward), a size range of 25-100 nm, with the characteristic CD9 biomarker. Thus, we dubbed these vesicles “oviductosomes”, to which PMCA4a was immunolocalized. Incubation of caudal sperm in the combined LF or exosomes resulted in up to a ~3-fold increase of sperm PMCA4a, as detected by flow cytometry, indicating in vitro uptake. Our results are consistent with the increased requirement of Ca²⁺ efflux in the oviduct. They show for the first time the presence of oviductal exosomes and highlight their role, along with uterosomes and vaginal exosomes, in post-testicular sperm acquisition of PMCA4a which is essential for hyperactivated motility and fertility.

Plasma membrane Ca2+-ATPase 4 in murine epididymis: secretion of splice variants in the luminal fluid and a role in sperm maturation

Plasma membrane Ca(2+)-ATPase isoform 4 (PMCA4) is the primary Ca(2+) efflux pump in murine sperm, where it regulates motility. In Pmca4 null sperm, motility loss results in infertility. We have shown that murine sperm PMCA4b interacts with Ca(2+)/CaM-dependent serine kinase (CASK) in regulating Ca(2+) homeostasis and motility. However, recent work indicated that the bovine PMCA4a splice variant (missing in testis) is epididymally expressed, along with 4b, and may be transferred to sperm. Here we show, via conventional and in situ RT-PCR, that both the splice variants of Pmca4 mRNA are expressed in murine testis and throughout the epididymis. Immunofluorescence localized PMCA4a to the apical membrane of the epididymal epithelium, and Western analysis not only confirmed its presence but showed for the first time that PMCA4a and PMCA4b are secreted in the epididymal luminal fluid (ELF), from which epididymosomes containing PMCA4a were isolated. Flow cytometry indicated the presence of PMCA4a on mature caudal sperm where it was increased ~5-fold compared to caput sperm (detected by Western blotting) and ~2-fold after incubation in ELF, revealing in vitro uptake and implicating PMCA4a in epididymal sperm maturation. Coimmunoprecipitation using pan-PMCA4 antibodies, revealed that both variants associate with CASK, suggesting their presence in a complex. Because they have different kinetic properties for Ca(2+) transport and different abilities to bind to CASK, our study suggests a mechanism for combining the functional attributes of both PMCA4 variants, leading to heightened efficiency of the pump in the maintenance of Ca(2+) homeostasis, which is crucial for normal motility and male fertility.

INTRODUCTION - Mechanism-based Development of Natural Products for Human Health

This Special Issue of the Journal of Pharmacy and Pharmaceutical Sciences arises from an inaugural conference on “Mechanism-Based Natural Product Development” held at Whistler Mountain, Canada on September 21-22, 2012. The aim of the conference was to bring together scientists from various disciplines to discuss the development of new therapeutic products from natural medicines based on mechanistic and related scientific studies. It provided an opportunity to explore new directions in natural medicine research and development, with the ultimate objective of leading to greater integration of natural and conventional synthetic pharmaceutical medicines for the health of the community worldwide. The concept for this conference and the Special Issue has come from the growing interest internationally in traditional and natural medicinal health products in recent years. It may, at first, appear surprising that developed countries are re-embracing natural product research and development. There are of course many reasons for this, including growing community interest in natural products providing improved health and wellbeing, the growing difficulty and cost of maintaining a pipeline of effective and, above all, safe new products for chronic diseases in the mainstream pharmaceutical industry, and the knowledge that many of our small drug pharmaceutical medicines have come from plants and other organisms. But as traditional natural product medicines increasingly enter the mainstream, the call for evidence to support their use also grows louder. Essential to the acceptance of natural medicines are the validation of their traditional uses and identification, isolation and structural characterization of their active components, together with the elucidation of their mechanisms of biological action, adverse effects, and identification of their molecular targets. These requirements provide the focus of this Special Issue. Scientific investigation and development of new health products requires the joining together of many disciplines, including chemistry, pharmacology, pharmacognosy and cell and molecular biology, as well as integration with clinical medicine. Natural product medicines are expected to be multi-component and multi-targeted. Are they effective, safe and properly standardized in their existing formulations? Are there opportunities to isolate single active components for standardization and conventional drug discovery and development? Answering these questions requires collaboration between scientific disciplines focused on a common goal. In line with the aims of the conference, the Special Issue has incorporated review and original research articles related to mechanisms of action in a number of therapeutic areas, mostly from invited speakers at the Whistler conference. Other articles were unsolicited submissions to the Journal that satisfy the scope of the issue. Articles range from reports on efforts to work with traditional owners in the appropriate cultural context, to develop new therapeutics based on traditional literature, to discover new medicinal products, to develop new pharmaceuticals based on the isolation of active chemical components, to develop new methods of delivery, and to identify mechanisms of action. The medical 'territory' includes cancer, heart disease, diabetes and related chronic inflammatory diseases, pain pathways, deafness and infertility. Other articles investigate the quality and safety of products by the application of current analytical methods, the potential for interactions of natural products (e.g., cranberry) with pharmaceutical medicines and the variability of a selection of similar natural product medicines with regard to their contents of therapeutically beneficial and marker compounds, as claimed on the product labels. In addition, the issue includes the abstracts of posters that formed an important part of the conference, especially from postgraduate students and postdoctoral fellows. These are included as Proceedings. We are indebted to the scientists who willingly gave their time and resources to attend the Whistler conference and, in many cases, submitted manuscripts for inclusion in this Special Issue. The delegates and contributors came from many places, near and far, to make this inaugural conference on mechanism-based natural product development a success. Such a conference and the subsequent proceedings in the Special issue are not possible without the work of many. We thank the organizing and scientific committee for their support and valuable suggestions. A conference of this scope would not be possible without sponsorship; this includes especially the National Health Products Research Society of Canada, the Canadian Society for Pharmaceutical Sciences and several Universities. Major support, without program or editorial input, was provided by SOHO Flordis International (SFI). We commend them for believing in the quest for clinically proven and research-based products based on an understanding of underlying mechanisms. The support of Purapharm International is also gratefully acknowledged. We would like to dedicate this Special Issue to Professor Allan SY Lau of the University of Hong Kong, who contributed enthusiastically to the organization of the conference and its program, but was ultimately unable to attend due to his untimely passing. Professor Lau was a pioneer in the integration of natural and orthodox medical studies and his inspiration will be greatly missed. Basil D Roufogalis, Arthur D Conigrave and Emanuel E Strehler Co-Guest Editors

Plasma membrane calcium ATPase activity is regulated by actin oligomers through direct interaction

As recently described by our group, plasma membrane calcium ATPase (PMCA) activity can be regulated by the actin cytoskeleton. In this study, we characterize the interaction of purified G-actin with isolated PMCA and examine the effect of G-actin during the first polymerization steps. As measured by surface plasmon resonance, G-actin directly interacts with PMCA with an apparent 1:1 stoichiometry in the presence of Ca(2+) with an apparent affinity in the micromolar range. As assessed by the photoactivatable probe 1-O-hexadecanoyl-2-O-[9-[[[2-[(125)I]iodo-4-(trifluoromethyl-3H-diazirin-3-yl)benzyl]oxy]carbonyl]nonanoyl]-sn-glycero-3-phosphocholine, the association of PMCA to actin produced a shift in the distribution of the conformers of the pump toward a calmodulin-activated conformation. G-actin stimulates Ca(2+)-ATPase activity of the enzyme when incubated under polymerizing conditions, displaying a cooperative behavior. The increase in the Ca(2+)-ATPase activity was related to an increase in the apparent affinity for Ca(2+) and an increase in the phosphoenzyme levels at steady state. Although surface plasmon resonance experiments revealed only one binding site for G-actin, results clearly indicate that more than one molecule of G-actin was needed for a regulatory effect on the pump. Polymerization studies showed that the experimental conditions are compatible with the presence of actin in the first stages of assembly. Altogether, these observations suggest that the stimulatory effect is exerted by short oligomers of actin. The functional interaction between actin oligomers and PMCA represents a novel regulatory pathway by which the cortical actin cytoskeleton participates in the regulation of cytosolic Ca(2+) homeostasis.

Differential effects of G- and F-actin on the plasma membrane calcium pump activity

We have previously shown that plasma membrane calcium ATPase (PMCA) pump activity is affected by the membrane protein concentration (Vanagas et al., Biochim Biophys Acta 1768:1641-1644, 2007). The results of this study provided evidence for the involvement of the actin cytoskeleton. In this study, we explored the relationship between the polymerization state of actin and its effects on purified PMCA activity. Our results show that PMCA associates with the actin cytoskeleton and this interaction causes a modulation of the catalytic activity involving the phosphorylated intermediate of the pump. The state of actin polymerization determines whether it acts as an activator or an inhibitor of the pump: G-actin and/or short oligomers activate the pump, while F-actin inhibits it. The effects of actin on PMCA are the consequence of direct interaction as demonstrated by immunoblotting and cosedimentation experiments. Taken together, these findings suggest that interactions with actin play a dynamic role in the regulation of PMCA-mediated Ca(2+) extrusion through the membrane. Our results provide further evidence of the activation-inhibition phenomenon as a property of many cytoskeleton-associated membrane proteins where the cytoskeleton is no longer restricted to a mechanical function but is dynamically involved in modulating the activity of integral proteins with which it interacts.

Immunolocalization of the tumor-sensitive calmodulin-like protein CALML3 in normal human skin and hyperproliferative skin disorders

BACKGROUND AND OBJECTIVE

Calmodulin-like protein CALML3 is an epithelial-specific protein regulated during keratinocyte differentiation in vitro. CALML3 expression is downregulated in breast cancers and transformed cell lines making it an attractive marker for tumor formation. The objective of this study was to survey CALML3 localization in normal epidermis and in hyperproliferative skin diseases including actinic keratosis, squamous and basal cell carcinoma as well as verruca and psoriasis and to compare CALML3 immunoreactivity with the proliferation marker Ki-67.

METHODS

Paraffin-embedded tissue sections from normal human skin and hyperproliferative skin disorders were examined by immunohistochemistry and analyzed for localization and expression of CALML3 and Ki-67.

RESULTS

CALML3 was strongly expressed in differentiating layers of normal skin, staining the periphery in suprabasal cells and exhibiting nuclear localization in the stratum granulosum. CALML3 nuclear localization was inversely correlated to Ki-67 staining in each disease, indicating that CALML3 nuclear presence is related to terminal cell differentiation and postmitotic state.

CONCLUSIONS

Increased CALML3 expression in suprabasal layers is characteristic for differentiating keratinocytes in normal epidermis, and nuclear expression of CALML3 inversely correlates with expression of the proliferation marker Ki-67. This suggests that CALML3 is a useful marker for normal and benign hyperplastic epidermal development, whereas the loss of nuclear CALML3 indicates progression to a proliferative and potentially malignant phenotype.

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.

Alternative pathways for association and dissociation of the calmodulin-binding domain of plasma membrane Ca(2+)-ATPase isoform 4b (PMCA4b)

The calmodulin (CaM)-binding domain of isoform 4b of the plasma membrane Ca(2+) -ATPase (PMCA) pump is represented by peptide C28. CaM binds to either PMCA or C28 by a mechanism in which the primary anchor residue Trp-1093 binds to the C-terminal lobe of the extended CaM molecule, followed by collapse of CaM with the N-terminal lobe binding to the secondary anchor Phe-1110 (Juranic, N., Atanasova, E., Filoteo, A. G., Macura, S., Prendergast, F. G., Penniston, J. T., and Strehler, E. E. (2010) J. Biol. Chem. 285, 4015-4024). This is a relatively rapid reaction, with an apparent half-time of ~1 s. The dissociation of CaM from PMCA4b or C28 is much slower, with an overall half-time of ~10 min. Using targeted molecular dynamics, we now show that dissociation of Ca(2+)-CaM from C28 may occur by a pathway in which Trp-1093, although deeply embedded in a pocket in the C-terminal lobe of CaM, leaves first. The dissociation begins by relatively rapid release of Trp-1093, followed by very slow release of Phe-1110, removal of C28, and return of CaM to its conformation in the free state. Fluorescence measurements and molecular dynamics calculations concur in showing that this alternative path of release of the PMCA4b CaM-binding domain is quite different from that of binding. The intermediate of dissociation with exposed Trp-1093 has a long lifetime (minutes) and may keep the PMCA primed for activation.

About and beyond the Henri-Michaelis-Menten rate equation for single-substrate enzyme kinetics

For more than a century the simple single-substrate enzyme kinetics model and related Henri-Michaelis-Menten (HMM) rate equation have been thoroughly explored in various directions. In the present paper we are concerned with a possible generalization of this rate equation recently proposed by F. Kargi (BBRC 382 (2009) 157-159), which is assumed to be valid both in the case that the total substrate or enzyme is in excess and the quasi-steady-state is achieved. We demonstrate that this generalization is grossly inadequate and propose another generalization based on application of the quasi-steady-state condition and conservation equations for both enzyme and substrate. The standard HMM equation is derived by (a) assuming the quasi-steady-state condition, (b) applying the conservation equation only for the enzyme, and (c) assuming that the substrate concentration at quasi-steady-state can be approximated by the total substrate concentration [S](0). In our formula the rate is already expressed through [S](0), and we only assume that when quasi-steady-state is achieved the amount of product formed is negligible compared to [S](0). Numerical simulations show that our formula is generally more accurate than the HMM formula and also can provide a good approximation when the enzyme is in excess, which is not the case for the HMM formula. We show that the HMM formula can be derived from our expression by further assuming that the total enzyme concentration is negligible compared to [S](0).

Calcium occlusion in plasma membrane Ca2+-ATPase

In this work, we set out to identify and characterize the calcium occluded intermediate(s) of the plasma membrane Ca(2+)-ATPase (PMCA) to study the mechanism of calcium transport. To this end, we developed a procedure for measuring the occlusion of Ca(2+) in microsomes containing PMCA. This involves a system for overexpression of the PMCA and the use of a rapid mixing device combined with a filtration chamber, allowing the isolation of the enzyme and quantification of retained calcium. Measurements of retained calcium as a function of the Ca(2+) concentration in steady state showed a hyperbolic dependence with an apparent dissociation constant of 12 ± 2.2 μM, which agrees with the value found through measurements of PMCA activity in the absence of calmodulin. When enzyme phosphorylation and the retained calcium were studied as a function of time in the presence of La(III) (inducing accumulation of phosphoenzyme in the E(1)P state), we obtained apparent rate constants not significantly different from each other. Quantification of EP and retained calcium in steady state yield a stoichiometry of one mole of occluded calcium per mole of phosphoenzyme. These results demonstrate for the first time that one calcium ion becomes occluded in the E(1)P-phosphorylated intermediate of the PMCA.

Regulation of apical membrane enrichment and retention of plasma membrane Ca ATPase splice variants by the PDZ-domain protein NHERF2

The localization of plasma membrane calcium ATPase (PMCA) isoforms in specified membrane compartments is crucial for their function in local Ca(2+) handling. PMCA2w/b is present in the apical membrane whereas alternative splice variants PMCA2x/b and 2z/b reside in the basolateral membrane in polarized epithelial cells. Here we found that the apical scaffolding protein NHERF2 greatly enhances the apical concentration of PMCA2w/b by tethering the pump to the underlying actin cytoskeleton. The interaction requires the C-terminal PDZ binding sequence in PMCA2b and results in increased membrane retention and decreased lateral mobility of the pump. In contrast, PMCA2x/b remains exclusively basolateral even when NHERF2 is overexpressed. Our results suggest that the alternatively spliced intracellular loop in PMCA2 imposes dominant membrane targeting information. NHERF2-mediated recruitment may be an effective means for polarized cells to regulate the abundance of PMCA2w/b in the apical membrane to meet an increased demand for local Ca(2+) extrusion.

Emanuel Strehler’s work on calcium pumps and calcium signaling

Cells are equipped with mechanisms to control tightly the influx, efflux and resting level of free calcium (Ca²⁺). Inappropriate Ca²⁺ signaling and abnormal Ca²⁺ levels are involved in many clinical disorders including heart disease, Alzheimer’s disease and stroke. Ca²⁺ also plays a major role in cell growth, differentiation and motility; disturbances in these processes underlie cell transformation and the progression of cancer. Accordingly, research in the Strehler laboratory is focused on a better understanding of the molecular “toolkit” needed to ensure proper Ca²⁺ homeostasis in the cell, as well as on the mechanisms of localized Ca²⁺ signaling. A long-term focus has been on the plasma membrane calcium pumps (PMCAs), which are linked to multiple disorders including hearing loss, neurodegeneration, and heart disease. Our work over the past 20 years or more has revealed a surprising complexity of PMCA isoforms with different functional characteristics, regulation, and cellular localization. Emerging evidence shows how specific PMCAs contribute not only to setting basal intracellular Ca²⁺ levels, but also to local Ca²⁺ signaling and vectorial Ca²⁺ transport. A second major research area revolves around the calcium sensor protein calmodulin and an enigmatic calmodulin-like protein (CALML3) that is linked to epithelial differentiation. One of the cellular targets of CALML3 is the unconventional motor protein myosin-10, which raises new questions about the role of CALML3 and myosin-10 in cell adhesion and migration in normal cell differentiation and cancer.

Diving Into the Lipid Bilayer to Investigate the Transmembrane Organization and Conformational State Transitions of P-type Ion ATPases

Although membrane proteins constitute more than 20% of the total proteins, the structures of only a few are known in detail. An important group of integral membrane proteins are ion-transporting ATPases of the P-type family, which share the formation of an acid-stable phosphorylated intermediate as part of their reaction cycle. There are several crystal structures of the sarcoplasmic reticulum Ca(2+) pump (SERCA) revealing different conformations, and recently, crystal structures of the H(+)-ATPase and the Na(+)/K(+)-ATPase were reported as well. However, there are no atomic resolution structures for other P-type ATPases including the plasma membrane calcium pump (PMCA), which is integral to cellular Ca(2+) signaling. Crystallization of these proteins is challenging because there is often no natural source from which the protein can be obtained in large quantities, and the presence of multiple isoforms in the same tissue further complicates efforts to obtain homogeneous samples suitable for crystallization. Alternative techniques to study structural aspects and conformational transitions in the PMCAs (and other P-type ATPases) have therefore been developed. Specifically, information about the structure and assembly of the transmembrane domain of an integral membrane protein can be obtained from an analysis of the lipid-protein interactions. Here, we review recent efforts using different hydrophobic photo-labeling methods to study the non-covalent interactions between the PMCA and surrounding phospholipids under different experimental conditions, and discuss how the use of these lipid probes can reveal valuable information on the membrane organization and conformational state transitions in the PMCA, Na(+)/K(+)-ATPase, and other P-type ATPases.

Switch of PMCA4 splice variants in bovine epididymis results in altered isoform expression during functional sperm maturation

Ca(2+) and Ca(2+)-dependent signals are essential for sperm maturation and fertilization. In mouse sperm the plasma membrane Ca(2+)-ATPase (PMCA) isoform 4 plays a crucial role in Ca(2+) transport. The two major splice variants of PMCA4 are PMCA4a and PMCA4b. PMCA4a differs from PMCA4b in the mechanism of calmodulin binding and activation. PMCA4a shows a much higher basal activity and is more effective than PMCA4b in returning Ca(2+) to resting levels. Knock-out mice carrying a PMCA4-null mutation are infertile because their sperm cannot achieve a hyperactivated state of motility. As sperm reach functional maturity during their transit through the epididymis, the expression of PMCA4a and 4b was assessed in bull testis and epididymis. Quantitative PCR revealed that PMCA4b is the major splice variant in testis, caput, and corpus epididymidis. In contrast, PMCA4a is the major splice variant in cauda epididymidis, whereas sperm are transcriptionally silent. Immunohistochemical staining using a new antibody against bovine PMCA4a located the PMCA4a to the apical membrane of the epithelium of cauda epididymidis, whereas testis, caput, and corpus epididymidis were negative. Western blotting of testis, epididymis, and sperm isolated from caput and cauda epididymidis showed a much higher level of PMCA4a in cauda epididymidis and sperm from cauda epididymidis compared with testis membranes and sperm from caput epididymidis. These findings suggest that PMCA4a is transferred to bovine sperm membranes in cauda epididymidis. This isoform switch may facilitate a higher calcium turnover in sperm necessary to traverse the female genital tract.

Kinetic analysis reveals differences in the binding mechanism of calmodulin and calmodulin-like protein to the IQ motifs of myosin-10

Myo10 is an unconventional myosin with important functions in filopodial motility, cell migration, and cell adhesion. The neck region of Myo10 contains three IQ motifs that bind calmodulin (CaM) or the tissue-restricted calmodulin-like protein (CLP) as light chains. However, little is known about the mechanism of light chain binding to the IQ motifs in Myo10. Binding of CaM and CLP to each IQ motif was assessed by nondenaturing gel electrophoresis and by stopped-flow experiments using fluorescence-labeled CaM and CLP. Although the binding kinetics are different in each case, there are similarities in the mechanism of binding of CaM and CLP to IQ1 and IQ2: for both IQ motifs Ca(2+) increased the binding affinity, mainly by increasing the rate of the forward steps. The general kinetic mechanism comprises a two-step process, which in some cases may involve the binding of a second IQ motif with lower affinity. For IQ3, however, the kinetics of CaM binding is very different from that of CLP. In both cases, binding in the absence of Ca(2+) is poor, and addition of Ca(2+) decreases the K(d) to below 10 nM. However, while the CaM binding kinetics are complex and best fitted by a multistep model, binding of CLP is fitted by a relatively simple two-step model. The results show that, in keeping with growing structural evidence, complexes between CaM or CaM-like myosin light chains and IQ motifs are highly diverse and depend on the specific sequence of the particular IQ motif as well as the light chain.

Cellular and subcellular localization of the neuron-specific plasma membrane calcium ATPase PMCA1a in the rat brain

Regulation of intracellular calcium is crucial both for proper neuronal function and survival. By coupling ATP hydrolysis with Ca(2+) extrusion from the cell, the plasma membrane calcium-dependent ATPases (PMCAs) play an essential role in controlling intracellular calcium levels in neurons. In contrast to PMCA2 and PMCA3, which are expressed in significant levels only in the brain and a few other tissues, PMCA1 is ubiquitously distributed, and is thus widely believed to play a "housekeeping" function in mammalian cells. Whereas the PMCA1b splice variant is predominant in most tissues, an alternative variant, PMCA1a, is the major form of PMCA1 in the adult brain. Here, we use immunohistochemistry to analyze the cellular and subcellular distribution of PMCA1a in the brain. We show that PMCA1a is not ubiquitously expressed, but rather is confined to neurons, where it concentrates in the plasma membrane of somata, dendrites, and spines. Thus, rather than serving a general housekeeping function, our data suggest that PMCA1a is a calcium pump specialized for neurons, where it may contribute to the modulation of somatic and dendritic Ca(2+) transients.

Apical scaffolding protein NHERF2 modulates the localization of alternatively spliced plasma membrane Ca2+ pump 2B variants in polarized epithelial cells

The membrane localization of the plasma membrane Ca(2+)-ATPase isoform 2 (PMCA2) in polarized cells is determined by alternative splicing; the PMCA2w/b splice variant shows apical localization, whereas the PMCA2z/b and PMCA2x/b variants are mostly basolateral. We previously reported that PMCA2b interacts with the PDZ protein Na(+)/H(+) exchanger regulatory factor 2 (NHERF2), but the role of this interaction for the specific membrane localization of PMCA2 is not known. Here we show that co-expression of NHERF2 greatly enhanced the apical localization of GFP-tagged PMCA2w/b in polarized Madin-Darby canine kidney cells. GFP-PMCA2z/b was also redirected to the apical membrane by NHERF2, whereas GFP-PMCA2x/b remained exclusively basolateral. In the presence of NHERF2, GFP-PMCA2w/b co-localized with the actin-binding protein ezrin even after disruption of the actin cytoskeleton by cytochalasin D or latrunculin B. Surface biotinylation and fluorescence recovery after photobleaching experiments demonstrated that NHERF2-mediated anchorage to the actin cytoskeleton reduced internalization and lateral mobility of the pump. Our results show that the specific interaction with NHERF2 enhances the apical concentration of PMCA2w/b by anchoring the pump to the apical membrane cytoskeleton. The data also suggest that the x/b splice form of PMCA2 contains a dominant lateral targeting signal, whereas the targeting and localization of the z/b form are more flexible and not fully determined by intrinsic sequence features.

A plasma membrane Ca2+ ATPase isoform at the postsynaptic density

Most excitatory input in the hippocampus impinges on dendritic spines. Entry of Ca(2+) into spines through NMDA receptors can trigger a sequence of biochemical reactions leading to sustained changes in synaptic efficacy. To provide specificity, dendritic spines restrict the diffusion of Ca(2+) signaling and downstream molecules. The postsynaptic density (PSD) (the most prominent subdomain within the spine) is the site of Ca(2+) entry through NMDA receptors. We here demonstrate that Ca(2+) can also be removed via pumps embedded in the PSD. Using light- and electron-microscopic immunohistochemistry, we find that PMCA2w, a member of the plasma membrane Ca(2+)-ATPase (PMCA) family, concentrates at the PSD of most hippocampal spines. We propose that PMCA2w may be recruited into supramolecular complexes at the postsynaptic density, thus helping to regulate Ca(2+) nanodomains at subsynaptic sites. Taken together, these results suggest a novel function for PMCAs as modulators of Ca(2+) signaling at the synapse.

Calmodulin wraps around its binding domain in the plasma membrane Ca2+ pump anchored by a novel 18-1 motif

Using solution NMR spectroscopy, we obtained the structure of Ca(2+)-calmodulin (holoCaM) in complex with peptide C28 from the binding domain of the plasma membrane Ca(2+)-ATPase (PMCA) pump isoform 4b. This provides the first atomic resolution insight into the binding mode of holoCaM to the full-length binding domain of PMCA. Structural comparison of the previously determined holoCaM.C20 complex with this holoCaM.C28 complex supports the idea that the initial binding step is represented by (holoCaM.C20) and the final bound complex by (holoCaM.C28). This affirms the existing multi-step kinetic model of PMCA4b activation by CaM. The complex exhibits a new binding motif in which holoCaM is wrapped around helical C28 peptide using two anchoring residues from the peptide at relative positions 18 and 1. The anchors correspond to Phe-1110 and Trp-1093, respectively, in full-length PMCA4b, and the peptide and CaM are oriented in an anti-parallel manner. This is a greater sequence distance between anchors than in any of the known holoCaM complexes with a helical peptide. Analysis of the geometry of holoCaM-peptide binding for the cases where the target peptide adopts an alpha(D)-helix with its anchors buried in the main hydrophobic pockets of the two CaM lobes establishes that only relative sequential positions of 10, 14, 17, and 18 are allowed for the second anchor.

Determination of the dissociation constants for Ca2+ and calmodulin from the plasma membrane Ca2+ pump by a lipid probe that senses membrane domain changes

The purpose of this work was to obtain information about conformational changes of the plasma membrane Ca(2+)-pump (PMCA) in the membrane region upon interaction with Ca(2+), calmodulin (CaM) and acidic phospholipids. To this end, we have quantified labeling of PMCA with the photoactivatable phosphatidylcholine analog [(125)I]TID-PC/16, measuring the shift of conformation E(2) to the auto-inhibited conformation E(1)I and to the activated E(1)A state, titrating the effect of Ca(2+) under different conditions. Using a similar approach, we also determined the CaM-PMCA dissociation constant. The results indicate that the PMCA possesses a high affinity site for Ca(2+) regardless of the presence or absence of activators. Modulation of pump activity is exerted through the C-terminal domain, which induces an apparent auto-inhibited conformation for Ca(2+) transport but does not modify the affinity for Ca(2+) at the transmembrane domain. The C-terminal domain is affected by CaM and CaM-like treatments driving the auto-inhibited conformation E(1)I to the activated E(1)A conformation and thus modulating the transport of Ca(2+). This is reflected in the different apparent constants for Ca(2+) in the absence of CaM (calculated by Ca(2+)-ATPase activity) that sharply contrast with the lack of variation of the affinity for the Ca(2+) site at equilibrium. This is the first time that equilibrium constants for the dissociation of Ca(2+) and CaM ligands from PMCA complexes are measured through the change of transmembrane conformations of the pump. The data further suggest that the transmembrane domain of the PMCA undergoes major rearrangements resulting in altered lipid accessibility upon Ca(2+) binding and activation.

Apical localization of PMCA2w/b is lipid raft-dependent

Alternative splicing of the first intracellular loop differentially targets plasma membrane calcium ATPase (PMCA) isoform 2 to the apical or basolateral membrane in MDCK cells. To determine if the targeting is affected by lipid interactions, we stably expressed PMCA2w/b and PMCA2z/b in MDCK cells, and analyzed the PMCA distribution by confocal fluorescence microscopy and membrane fractionation. PMCA2w/b showed clear apical and lateral distribution, whereas PMCA2z/b was mainly localized to the basolateral membrane. A significant fraction of PMCA2w/b partitioned into low-density membranes associated with lipid rafts. Depletion of membrane cholesterol by methyl-beta-cyclodextrin resulted in reduced lipid raft association and a striking loss of PMCA2w/b from the apical membrane, whereas the lateral localization of PMCA2z/b remained unchanged. Our data indicate that alternative splicing differentially affects the lipid interactions of PMCA2w/b and PMCA2z/b and that the apical localization of PMCA2w/b is lipid raft-dependent and sensitive to cholesterol depletion.

"Fast" plasma membrane calcium pump PMCA2a concentrates in GABAergic terminals in the adult rat brain

The plasma membrane Ca(2+)-ATPases (PMCA) represent the major high-affinity Ca(2+) extrusion system in the brain. PMCAs comprise four isoforms and over 20 splice variants. Their different functional properties may permit different PMCA splice variants to accommodate different kinds of local [Ca(2+)] transients, but for a specific PMCA to play a unique role in local Ca(2+) handling it must be targeted to the appropriate subcellular compartment. We used immunohistochemistry to study the spatial distribution of PMCA2a-one of the two major carboxyl-terminal splice variants of PMCA2-in the adult rat brain, testing whether this isoform, with especially high basal activity, is targeted to specific subcellular compartments. In striking contrast to the widespread distribution of PMCA2 as a whole, we found that PMCA2a is largely restricted to parvalbumin-positive inhibitory presynaptic terminals throughout the brain. The only major exception to this targeting pattern was in the cerebellar cortex, where PMCA2a also concentrates postsynaptically, in the spines of Purkinje cells. We propose that the fast Ca(2+) activation kinetics and high V(max) of PMCA2a make this pump especially suited for rapid clearance of presynaptic Ca(2+) in fast-spiking inhibitory nerve terminals, which face severe transient calcium loads.

Human calmodulin-like protein (CLP) expression in oral squamous mucosa and in malignant transformation

PURPOSE

The purpose of this study was to test whether calmodulin-like protein (CLP) is expressed in normal human oral mucosal cells and if downregulation of CLP occurs in malignant transformation.

MATERIALS AND METHODS

Oral mucosal tissue was taken from three individuals in a double-blind manner. The samples were cut, measured, and homogenized. Total RNA was extracted and reverse transcribed. Each cDNA sample was subjected to polymerase chain reaction (PCR). PCR fragments were purified, cloned, and sequenced to verify the presence of CLP. Three oral mucosal tissue samples with biopsy-confirmed squamous cell carcinoma were obtained. These samples demonstrated regions of normal epithelial cells as well as invasive squamous cell carcinoma. One normal breast epithelial sample was also obtained for positive control. Sections were stained with an affinity-purified CLP antibody and counterstained with a diluted hematoxylin. Two observers evaluated the specimens for expression of CLP. Staining patterns and intensity were noted in normal oral mucosa, comparing them to the normal breast epithelium sample. Staining patterns and intensity were then observed in squamous tumor cells, comparing them to the patterns of benign squamous mucosa.

RESULTS

CLP coding sequences were positively identified from the normal oral mucosal tissue samples by reverse transcription and polymerase chain reaction (RT-PCR) with 100% identity to the published CLP sequence (accession #M58026). In the three oral mucosa tissue samples with known squamous cell carcinoma, expression of CLP was readily detected in areas of normal oral mucosa, while a notable downregulation of CLP expression occurred in areas of malignant transformation. The staining intensity was equivalent to the staining seen in the benign breast epithelium used as a control. In the areas of squamous cell carcinoma, a decrease in CLP immunoreactivity occurred. There was a sharp contrast in staining quality and clarity between benign and malignant tissue. In the majority of the carcinoma regions, a complete lack of immunoreactivity was noted.

CONCLUSIONS

The RNA for human CLP is found in normal oral mucosa. CLP expression is seen in normal oral mucosa with a downregulation of CLP expression in malignant transformation.

Calmodulin-like protein upregulates myosin-10 in human keratinocytes and is regulated during epidermal wound healing in vivo

Epidermal wound healing is required for normal skin barrier function. Cell motility is a key factor in the ability of keratinocytes to heal epithelial damage. Calmodulin-like protein (CLP) is an epithelial-specific Ca(2+)-binding protein that is regulated during terminal keratinocyte differentiation. CLP is a specific light chain of unconventional myosin-10 (Myo10) and its expression increases filopodial length, filopodial number, and Myo10-dependent cell motility in vitro. However, the effects of CLP expression on keratinocyte motility are unknown. Here we used cultured human keratinocytes to study the role of CLP in regulating Myo10 and the effects of Myo10 and CLP on cell migration. CLP and Myo10 expression were correlated in vitro and required for keratinocyte motility in wound-healing assays. We examined the localization of CLP in wounded skin by immunohistochemistry and found an upregulation and peripheral localization of CLP in the basal and suprabasal cells adjacent to and immediately over the wound bed in vivo. The results suggest that increased CLP expression and CLP-mediated Myo10 function are important for skin differentiation and wound reepithelialization.

Plasma-membrane Ca(2+) pumps: structural diversity as the basis for functional versatility

Plasma-membrane calcium pumps [PMCAs (plasma-membrane Ca(2+)-ATPases)] expel Ca(2+) from eukaryotic cells to maintain overall Ca(2+) homoeostasis and to provide local control of intracellular Ca(2+) signalling. Recent work indicates functional versatility among PMCA isoforms, with specific pumps being essential for cochlear hair cell function, sperm motility, feedback signalling in the heart and pre- and post-synaptic Ca(2+) regulation in neurons. The functional versatility of PMCAs is due to differences in their regulation by CaM (calmodulin), kinases and other signalling proteins, as well as to their differential targeting and retention in defined plasma membrane domains. The basis for this is the structural diversity of PMCAs. In mammals, four genes encode PMCA isoforms 1-4, and each of these has multiple variants generated by alternative RNA splicing. The alternatively spliced regions are intimately involved in the regulatory interactions and differential membrane localization of the pumps. The alternatively spliced C-terminal tail acts as an autoinhibitory domain by interacting with the catalytic core of the pump. The degree of inhibition and the kinetics of interaction with the major activator CaM differ between PMCA variants. This translates into functional differences in how PMCAs handle Ca(2+) signals of different magnitude and frequency. Accumulating evidence thus demonstrates how structural diversity provides functional versatility in the PMCAs.

The plasma membrane Ca2+ pump isoform 4a differs from isoform 4b in the mechanism of calmodulin binding and activation kinetics: implications for Ca2+ signaling

The inhibition by the regulatory domain and the interaction with calmodulin (CaM) vary among plasma membrane calcium pump (PMCA) isoforms. To explore these differences, the kinetics of CaM effects on PMCA4a were investigated and compared with those of PMCA4b. The maximal apparent rate constant for CaM activation of PMCA4a was almost twice that for PMCA4b, whereas the rates of activation for both isoforms showed similar dependence on Ca2+. The inactivation of PMCA4a by CaM removal was also faster than for PMCA4b, and Ca2+ showed a much smaller effect (2- versus 30-fold modification). The rate constants of the individual steps that determine the overall rates were obtained from stopped-flow experiments in which binding of TA-CaM was observed by changes in its fluorescence. TA-CaM binds to two conformations of PMCA4a, an "open" conformation with high activity, and a "closed" one with lower activity. Compared with PMCA4b (Penheiter, A. R., Bajzer, Z., Filoteo, A. G., Thorogate, R., Török, K., and Caride, A. J. (2003) Biochemistry 41, 12115-12124), the model for PMCA4a predicts less inhibition in the closed form and a much faster equilibrium between the open and closed forms. Based on the available kinetic parameters, we determined the constants to fit the shape of a Ca2+ signal in PMCA4b-overexpressing Chinese hamster ovary cells. Using the constants for PMCA4a, and allowing small variations in parameters of other systems contributing to a Ca2+ signal, we then simulated the effect of PMCA4a on the shape of a Ca2+ signal in Chinese hamster ovary cells. The results reproduce the published data (Brini, M., Coletto, L., Pierobon, N., Kraev, N., Guerini, D., and Carafoli, E. (2003) J. Biol. Chem. 278, 24500-24508), and thereby demonstrate the importance of altered regulatory kinetics for the different functional properties of PMCA isoforms.

Plasma membrane Ca2+ ATPases as dynamic regulators of cellular calcium handling

Plasma membrane Ca2+ ATPases (PMCAs) are essential components of the cellular toolkit to regulate and fine-tune cytosolic Ca2+ concentrations. Historically, the PMCAs have been assigned a housekeeping role in the maintenance of intracellular Ca2+ homeostasis. More recent work has revealed a perplexing multitude of PMCA isoforms and alternative splice variants, raising questions about their specific role in Ca2+ handling under conditions of varying Ca2+ loads. Studies on the kinetics of individual isoforms, combined with expression and localization studies suggest that PMCAs are optimized to function in Ca2+ regulation according to tissue- and cell-specific demands. Different PMCA isoforms help control slow, tonic Ca2+ signals in some cells and rapid, efficient Ca2+ extrusion in others. Localized Ca2+ handling requires targeting of the pumps to specialized cellular locales, such as the apical membrane of cochlear hair cells or the basolateral membrane of kidney epithelial cells. Recent studies suggest that alternatively spliced regions in the PMCAs are responsible for their unique targeting, membrane localization, and signaling cross-talk. The regulated deployment and retrieval of PMCAs from specific membranes provide a dynamic system for a cell to respond to changing needs of Ca2+ regulation.

Rapid downregulation of NCX and PMCA in hippocampal neurons following H2O2 oxidative stress

Na+/Ca2+ exchangers (NCXs) and plasma membrane Ca2+ pumps (PMCAs) are crucial for intracellular Ca2+ homeostasis and Ca2+ signaling. Elevated [Ca2+]i is a hallmark of neurodegenerative disease and stroke. Here we studied the short-term effect of oxidative stress on the plasma membrane Ca2+ extrusion systems in hippocampal neurons (HN) and found that after 2-3 h exposure to 300 microM H2O2, all NCXs and PMCAs were significantly downregulated at the RNA (NCX) and protein (PMCA) level. Rapid internalization and aggregation of the PMCA was also observed. Our data show that the plasma membrane calcium extrusion systems are sensitive early targets of neurotoxic oxidative stress.

Plasma membrane calcium pump activity is affected by the membrane protein concentration: evidence for the involvement of the actin cytoskeleton

Plasma membrane calcium pumps (PMCAs) are integral membrane proteins that actively expel Ca(2+) from the cell. Specific Ca(2+)-ATPase activity of erythrocyte membranes increased steeply up to 1.5-5 times when the membrane protein concentration decreased from 50 microg/ml to 1 microg/ml. The activation by dilution was also observed for ATP-dependent Ca(2+) uptake into vesicles from Sf9 cells over-expressing the PMCA 4b isoform, confirming that it is a property of the PMCA. Dilution of the protein did not modify the activation by ATP, Ca(2+) or Ca(2+)-calmodulin. Treatment with non-ionic detergents did not abolish the dilution effect, suggesting that it was not due to resealing of the membrane vesicles. Pre-incubation of erythrocyte membranes with Cytochalasin D under conditions that promote actin polymerization abolished the dilution effect. Highly-purified, micellar PMCA showed no dilution effect and was not affected by Cytochalasin D. Taken together, these results suggest that the concentration-dependent behavior of the PMCA activity was due to interactions with cytoskeletal proteins. The dilution effect was also observed with different PMCA isoforms, indicating that this is a general phenomenon for all PMCAs.

Calmodulin-like protein increases filopodia-dependent cell motility via up-regulation of myosin-10

Human calmodulin-like protein (CLP) is an epithelial-specific protein that is expressed during cell differentiation but down-regulated in primary cancers and transformed cell lines. Using stably transfected and inducible HeLa cell lines, we found that CLP expression did not alter the proliferation rate and colony-forming potential of these cells. However, remarkable phenotypic changes were observed in CLP-expressing compared with control cells. Soft agar colonies of CLP-expressing cells had rough boundaries, with peripheral cells migrating away from the colony. Cells expressing CLP displayed a striking increase in the number and length of myosin-10-positive filopodia and showed increased mobility in a wound healing assay. This increase in wound healing capacity was prevented by small interference RNA-mediated down-regulation of myosin-10. Fluorescence microscopy and Western blotting revealed that CLP expression results in up-regulation of its target protein, myosin-10. This up-regulation occurs at the protein level by stabilization of myosin-10. Thus, CLP functions by increasing the stability of myosin-10, leading to enhanced myosin-10 function and a subsequent increase in filopodial dynamics and cell migration. In stratified epithelia, CLP may be required during terminal differentiation to increase myosin-10 function as cells migrate toward the upper layers and establish new adhesive contacts.

Glutamate-induced protease-mediated loss of plasma membrane Ca2+ pump activity in rat hippocampal neurons

Ca2+ dysregulation is a hallmark of excitotoxicity, a process that underlies multiple neurodegenerative disorders. The plasma membrane Ca2+ ATPase (PMCA) plays a major role in clearing Ca2+ from the neuronal cytoplasm. Here, we show that the rate of PMCA-mediated Ca2+ efflux from rat hippocampal neurons decreased following treatment with an excitotoxic concentration of glutamate. PMCA-mediated Ca2+ extrusion following a brief train of action potentials exhibited an exponential decay with a mean time constant (tau) of 8.8 +/- 0.2 s. Four hours following the start of a 30 min treatment with 200 microm glutamate, a second population of cells emerged with slowed recovery kinetics (tau = 16.5 +/- 0.3 s). Confocal imaging of cells expressing an enhanced green fluorescent protein (EGFP)-PMCA4b fusion protein revealed that glutamate treatment internalized EGFP and that cells with reduced plasma membrane fluorescence had impaired Ca2+ clearance. Treatment with inhibitors of the Ca2+-activated protease calpain protected PMCA function and prevented EGFP-PMCA internalization. PMCA internalization was triggered by activation of NMDA receptors and was less pronounced for a non-toxic concentration of glutamate relative to one that produces excitotoxicity. PMCA isoform 2 also internalized following exposure to glutamate, although the Na+/K+ ATPase did not. These data suggest that glutamate exposure initiated protease-mediated internalization of PMCAs with a corresponding loss of function that may contribute to the Ca2+ dysregulation that accompanies excitotoxicity.

Rapid turnover of stereocilia membrane proteins: evidence from the trafficking and mobility of plasma membrane Ca(2+)-ATPase 2

We studied the spatial distribution, mobility, and trafficking of plasma membrane Ca2+ATPase-2 (PMCA2), a protein enriched in the hair cell apical membrane and essential for hair cell function. Using immunofluorescence, we determined that PMCA2 is enriched in the stereocilia and present at a relatively low concentration in the kinocilium and in the remaining apical membrane. Using an antibody to the extracellular domain of PMCA2 as a probe, we observed that PMCA2 diffuses laterally from the stereocilia membrane and is internalized at the apical cell border maintaining an estimated half-life of residency in the stereocilia of approximately 5-7 h. A computer simulation of our data indicates that PMCA2 has an estimated global diffusion coefficient of 0.01-0.005 microm2/s. Using a green fluorescent protein tag, we observed that PMCA2 is rapidly delivered to the apical cell border from where it diffuses to the entire stereocilia surface. Fluorescence recovery after photobleaching experiments show that approximately 60% of PMCA2 in the stereocilia exhibit high mobility with a diffusion coefficient of 0.1-0.2 microm2/s, whereas the remaining pool represents a relatively immobile fraction. These results suggest that PMCA2 molecules maintain transient interactions with other components of the stereocilia, and the mobile pool of PMCA2 mediates the exchange between the stereocilia and the removal and delivery sites at the periphery of the apical cell surface. This rapid turnover of a major stereocilia membrane protein matches the previously described rapid turnover of proteins of the stereocilia actin core, further demonstrating that these organelles undergo rapid continuous renewal.

Splice-site A choice targets plasma-membrane Ca2+-ATPase isoform 2 to hair bundles

Localization of mechanotransduction in sensory hair cells to hair bundles requires selective targeting of essential proteins to specific locations. Isoform 2 of the plasma-membrane Ca2+-ATPase (PMCA2), required for hearing and balance, is found exclusively in hair bundles. We determined the contribution of splicing at the two major splicing sites (A and C) to hair-cell targeting of PMCA2. When PMCA2 isoforms were immunoprecipitated from purified hair bundles of rat utricle, 2w was the only site A variant detected; moreover, immunocytochemistry for 2w in rat vestibular and cochlear tissues indicated that this splice form was located solely in bundles. To demonstrate the necessity of the 2w sequence, we transfected hair cells with PMCA2 containing different variants at splice sites A and C. Although native hair bundles exclusively use the 2a form at splice-site C, epitope-tagged PMCA2w/a and PMCA2w/b were both concentrated in bundles, indicating that site C is not involved in bundle targeting. In contrast, PMCA2z/a was excluded from bundles and was instead targeted to the basolateral plasma membrane. Bundle-specific targeting of PMCA2w/a tagged with green fluorescent protein (GFP) was diminished, suggesting that GFP interfered with splice-site A. Together, these data demonstrate that PMCA2w/a is the hair-bundle isoform of PMCA in rat hair cells and that 2w targets PMCA2 to bundles. The 2w sequence is thus the first targeting signal identified for a hair-bundle membrane protein; moreover, the striking distribution of inner-ear PMCA isoforms dictated by selective targeting suggests a critical functional role for segregated pathways of Ca2+ transport.

Molecular determinants for differential membrane trafficking of PMCA1 and PMCA2 in mammalian hair cells

The plasma membrane Ca2+-ATPase-2 (PMCA2) is expressed in stereocilia of hair cells of the inner ear, whereas PMCA1 is expressed in the basolateral plasma membrane of hair cells. Both extrude excess Ca2+ from the cytosol. They are predicted to contain ten membrane-spanning segments, two large cytoplasmic loops as well as cytosolic N- and C-termini. Several isoform variants are generated for both PMCA1 and PMCA2 by alternative splicing, affecting their first cytosolic loop (A-site) and their C-terminal tail. To understand how these isoforms are differentially targeted in hair cells, we investigated their targeting regions and expression in hair cells. Our results show that a Leu-Ile motif in 'b'-tail splice variants promotes PMCA1b and PMCA2b basolateral sorting in hair cells. Moreover, apical targeting of PMCA2 depends on the size of the A-site-spliced insert, suggesting that the conformation of the cytoplasmic loop plays a role in apical targeting.

The Homer-1 protein Ania-3 interacts with the plasma membrane calcium pump

The Homer family of scaffold proteins couples NMDA receptors to metabotropic glutamate receptors and links extracellular signals to calcium release from intracellular stores. Ania-3 is a member of the Homer family and is rapidly inducible in brain in response to diverse stimuli. Here, we report the identification of the plasma membrane Ca2+ ATPase (PMCA) as a novel Ania-3/Homer-associated protein. Ania-3/Homer interacts with the b-splice forms of all PMCAs (PMCA1b, 2b, 3b, and 4b) via their PDZ domain-binding COOH-terminal tail. Ectopically expressed Ania-3 colocalized with the PMCA at the plasma membrane of polarized MDCK epithelial cells, and endogenous Ania-3/Homer and PMCA2 are co-expressed in the soma and dendrites of primary rat hippocampal neurons. The interaction between Ania-3/Homer and PMCAs may represent a novel mechanism by which local calcium signaling and hence synaptic function can be modulated in neurons.