Structure of the human plasma membrane Ca2+-ATPase 1 in complex with its obligatory subunit neuroplastin

Increased expression of the neuroplastin 65 protein is involved in neurofibrillary tangles and amyloid beta plaques in Alzheimer’s disease

BACKGROUND

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder currently lacking effective therapeutic interventions. Pathological hallmarks of AD include intracellular neurofibrillary tangles (NFTs) and extracellular amyloid beta (Aβ) plaques. Neuroplastin 65 (NP65), highly expressed in the brain, has been previously shown to mitigate cognitive impairments and decrease Aβ plaques in the AD mouse model, suggesting that NP65 is involved in AD neuropathology. However, direct evidence linking NP65 expression to AD pathogenesis in human brain remains absent.

AIM

To quantify NP65 isoform expression gradients across distinct neuroanatomical regions in the healthy brain and investigate the alterations of NP65 expression in the AD brain.

METHODS

Immunohistochemical, immunofluorescent and western blot analyses were used to investigate NP65 expression in 19 postmortem brains (AD = 10, controls = 9). Double immunostaining with 6E10 and or phosphorylated-microtubule-associated protein tau (AT-8, a marker for NFT) markers was performed to assess NP65 colocalization with Aβ plaques and NFTs.

RESULTS

In controls, NP65 was highly expressed in a wide-range of brain areas. AD cases showed significantly increased NP65 immunoreactivity across multiple brain regions, including the frontal and temporal cortex, hippocampus, and cerebellum, compared to controls. Western blot analysis consistently confirmed significantly elevated NP65 expression in the hippocampus of AD patients relative to controls. Double immunostaining demonstrated partial colocalization of NP65 with NFTs and Aβ plaques in AD brain tissue.

CONCLUSION

Our findings demonstrate a significant increase of NP65 protein, which colocalizes with NFTs and Aβ plaques in AD brains, providing direct evidence supporting a critical role of NP65 expression in the neuropathological mechanisms of this disease.

A Comparison Between Calcium and Strontium Transport by the (Ca2+ + Mg2+)ATPase of the Basolateral Plasma Membrane of Renal Proximal Convoluted Tubules

In this work, the utilization of calcium and strontium by the (Ca2+ + Mg2+)ATPase of the basolateral plasma membrane of renal proximal convoluted tubules were compared. [90Sr]Sr2+ and [45Ca]Ca2+ uptake by vesicles derived from this membrane were strictly dependent on ATP and Mg2+, and no other nucleotide was able to support the transport. Each cation inhibited the uptake of the other one in a purely competitive fashion (the same Vmax; increased K0.5), without causing a significant change in the influx rate. These results indicate that both cations bind at the same transport site on the enzyme, facing the cytosolic surface of the cell. The K0.5 for Sr2+ obtained for (Sr2+ + Mg2+)ATPase activity was 13.1 ± 0.2 µM and for Sr2+ uptake was 13.4 ± 0.1 µM. They were higher than K0.5 for Ca2+ obtained for (Ca2+ + Mg2+)ATPase activity (0.42 ± 0.03 µM) and for Ca2+ uptake (0.28 ± 0.02 µM). It is postulated that the lower ATPase affinity for Sr2+ is associated with greater steric difficulties for the occupation by this cation of the binding and transport sites, as a consequence of its greater crystal ionic radius (1.13 Å for Sr2+ against 0.99 Å for Ca2+).

Structure of the [Ca]E2P intermediate of Ca2+-ATPase 1 from Listeria monocytogenes

Active transport by P-type Ca2+-ATPases maintain internal calcium stores and a low cytosolic calcium concentration. Structural studies of mammalian sarco/endoplasmic reticulum Ca2+-ATPases (SERCA) have revealed several steps of the transport cycle, but a calcium-releasing intermediate has remained elusive. Single-molecule FRET studies of the bacterial Ca2+-ATPase LMCA1 revealed an intermediate of the transition between so-called [Ca]E1P and E2P states and suggested that calcium release from this intermediate was the essentially irreversible step of transport. Here, we present a 3.5 Å resolution cryo-EM structure for a four-glycine insertion mutant of LMCA1 in a lipid nanodisc obtained under conditions with calcium and ATP and adopting such an intermediate state, denoted [Ca]E2P. The cytosolic domains are positioned in the E2P-like conformation, while the calcium-binding transmembrane (TM) domain adopts a calcium-bound E1P-ADP-like conformation. Missing density for the E292 residue at the calcium site (the equivalent of SERCA1a E309) suggests flexibility and a site poised for calcium release and proton uptake. The structure suggests a mechanism where ADP release and re-organization of the cytoplasmic domains precede calcium release.

Non-hyperbolic enzyme kinetics: the case of P-type ATPases

Many enzymes operate through mechanisms that comply with the Michaelis-Menten equation (hyperbolic kinetics). The theoretical framework for analyzing these enzymes, widely developed in the literature, is largely based on the ability to linearize the equation and apply linear regression to experimental data. However, certain systems, such as P-type ATPases, present mechanisms that do not fit into hyperbolic models, requiring the development of more complex equations. This study explores the underlying causes of the non-hyperbolic behavior observed for P-type ATPases and reviews some methodologies used for their analysis. Here, we propose to employ rational equations, whose form limits the range of possible kinetic models applicable to the system, offering a structured approach to its analysis.

A role for plasma membrane Ca2+ ATPases in regulation of cellular Ca2+ homeostasis by sphingosine kinase-1

Sphingosine-1-phosphate (S1P) is a ubiquitous lipid mediator, acting via specific G-protein-coupled receptors (GPCR) and intracellularly. Previous work has shown that deletion of S1P lyase caused a chronic elevation of cytosolic [Ca2+]i and enhanced Ca2+ storage in mouse embryonic fibroblasts. Here, we studied the role of sphingosine kinase (SphK)-1 in Ca2+ signaling, using two independently generated EA.hy926 cell lines with stable knockdown of SphK1 (SphK1-KD1/2). Resting [Ca2+]i and thapsigargin-induced [Ca2+]i increases were reduced in both SphK1-KD1 and -KD2 cells. Agonist-induced [Ca2+]i increases, measured in SphK1-KD1, were blunted. In the absence of extracellular Ca2+, thapsigargin-induced [Ca2+]i increases declined rapidly, indicating enhanced removal of Ca2+ from the cytosol. In agreement, plasma membrane Ca2+ ATPase (PMCA)-1 and -4 and their auxiliary subunit, basigin, were strongly upregulated. Activation of S1P-GPCR by specific agonists or extracellular S1P did not rescue the effects of SphK1 knockdown, indicating that S1P-GPCR were not involved. Lipid measurements indicated that not only S1P but also dihydro-sphingosine, ceramides, and lactosylceramides were markedly depleted in SphK1-KD2 cells. SphK2 and S1P lyase were upregulated, suggesting enhanced flux via the sphingolipid degradation pathway. Finally, histone acetylation was enhanced in SphK1-KD2 cells, and the histone deacetylase inhibitor, vorinostat, induced upregulation of PMCA1 and basigin on mRNA and protein levels in EA.hy926 cells. These data show for the first time a transcriptional regulation of PMCA1 and basigin by S1P metabolism. It is concluded that SphK1 knockdown in EA.hy926 cells caused long-term alterations in cellular Ca2+ homeostasis by upregulating PMCA via increased histone acetylation.

Dephosphorylation and ion binding in prokaryotic calcium transport

Calcium (Ca2+) signaling is fundamental to cellular processes in both eukaryotic and prokaryotic organisms. While the mechanisms underlying eukaryotic Ca2+ transport are well documented, an understanding of prokaryotic transport remains nascent. LMCA1, a Ca2+ adenosine triphosphatase (ATPase) from Listeria monocytogenes, has emerged as a prototype for elucidating structure and dynamics in prokaryotic Ca2+ transport. Here, we used a multidisciplinary approach integrating kinetics, structure, and dynamics to unravel the intricacies of LMCA1 function. A cryo-electron microscopy (cryo-EM) structure of a Ca2+-bound E1 state showed ion coordination by Asp720, Asn716, and Glu292. Time-resolved x-ray solution scattering experiments identified phosphorylation as the rate-determining step. A cryo-EM E2P state structure exhibited remarkable similarities to a SERCA1a E2-P* state, which highlights the essential role of the unique P-A domain interface in enhancing dephosphorylation rates and reconciles earlier proposed mechanisms. Our study underscores the distinctiveness between eukaryotic and prokaryotic Ca2+ ATPase transport systems and positions LMCA1 as a promising drug target for developing antimicrobial strategies.

17β-estradiol induces hyperresponsiveness in guinea pig airway smooth muscle by inhibiting the plasma membrane Ca2+-ATPase

High serum estrogen concentrations are associated with asthma development and severity, suggesting a link between estradiol and airway hyperresponsiveness (AHR). 17β-estradiol (E2) has non-genomic effects via Ca2+ regulatory mechanisms; however, its effect on the plasma membrane Ca2+-ATPases (PMCA1 and 4) and sarcoplasmic reticulum Ca2+-ATPase (SERCA) is unknown. Hence, in the present study, we aim to demonstrate if E2 favors AHR by increasing intracellular Ca2+ concentrations in guinea pig airway smooth muscle (ASM) through a mechanism involving Ca2+-ATPases. In guinea pig ASM, Ca2+ microfluorometry, muscle contraction, and Western blot were evaluated. Then, we performed molecular docking analysis between the estrogens and Ca2+ ATPases. In tracheal rings, E2 produced AHR to carbachol. In guinea pig myocytes, acute exposure to physiological levels of E2 modified the transient Ca2+ peak induced by caffeine to a Ca2+ plateau. The incubation with PMCA inhibitors (lanthanum and carboxyeosin, CE) partially reversed the E2-induced sustained plateau in the caffeine response. In contrast, cyclopiazonic acid (SERCA inhibitor), U-0126 (an inhibitor of ERK 1/2), and choline chloride did not modify the Ca2+ plateau produced by E2. The mitochondrial uniporter activity and the capacitative Ca2+ entry were unaffected by E2. In guinea pig ASM, Western blot analysis demonstrated PMCA1 and PMCA4 expression. The results from the docking modeling demonstrate that E2 binds to both plasma membrane ATPases. In guinea pig tracheal smooth muscle, inhibiting the PMCA with CE, induced hyperresponsiveness to carbachol. 17β-estradiol produces hyperresponsiveness by inhibiting the PMCA in the ASM and could be one of the mechanisms responsible for the increase in asthmatic crisis in women.

Dual role of neuroplastin in pancreatic β cells: Regulating insulin secretion and promoting islet inflammation

Mesencephalic astrocyte-derived neurotrophic factor (MANF) is an endoplasmic reticulum (ER)-resident secretory protein that reduces inflammation and promotes proliferation in pancreatic β cells. Numerous studies have highlighted the potential of MANF as a therapeutic agent for diabetes mellitus (DM), making it essential to understand the mechanisms underlying MANF's functions. In our previous search for a molecule that mediates MANF signaling, we identified Neuroplastin (NPTN) as a binding partner of MANF that localizes on the cell surface. However, the roles of NPTN in pancreatic β cells remain unclear. In this study, we generated β cell-specific Nptn knockout (KO) mice and conducted metabolic characterization. NPTN deficiency improved glucose tolerance by increasing insulin secretion and β cell mass in the pancreas. Moreover, proliferation and mitochondrial numbers in β cells increased in Nptn KO islets. These phenotypes resulted from elevated cytosolic Ca2+ levels and subsequent activation of downstream molecules. Simultaneously, we demonstrated that NPTN induces the expression of proinflammatory cytokines via the TRAF6-NF-κB axis in β cells. Additionally, NPTN deficiency conferred resistance to streptozotocin-induced diabetic phenotypes. Finally, exogenous MANF treatment in islets or β cells led to similar phenotypes as those observed in NPTN-deficient models. These results indicate that NPTN plays important roles in the regulation of insulin secretion, proliferation, and mitochondrial quantity, as well as proinflammatory responses, which are antagonized by MANF treatment. Thus, targeting the MANF-NPTN interaction may lead to a novel treatment for improving β cell functions in DM.

Deafness causing neuroplastin missense variants fail to promote plasma membrane Ca2+-ATPase levels and Ca2+ transient regulation in brain neurons

Hearing, the ability to sense sounds, and the processing of auditory information are important for perception of the world. Mice lacking expression of neuroplastin (Np), a type-1 transmembrane glycoprotein, display deafness, multiple cognitive deficiencies, and reduced expression of plasma membrane calcium (Ca2+) ATPases (PMCAs) in cochlear hair cells and brain neurons. In this study, we transferred the deafness causing missense mutations pitch (C315S) and audio-1 (I122N) into human Np (hNp) constructs and investigated their effects at the molecular and cellular levels. Computational molecular dynamics show that loss of the disulfide bridge in hNppitch causes structural destabilization of immunoglobulin-like domain (Ig) III and that the novel asparagine in hNpaudio-1 results in steric constraints and an additional N-glycosylation site in IgII. Additional N-glycosylation of hNpaudio-1 was confirmed by PNGaseF treatment. In comparison to hNpWT, transfection of hNppitch and hNpaudio-1 into HEK293T cells resulted in normal mRNA levels but reduced the Np protein levels and their cell surface expression due to proteasomal/lysosomal degradation. Furthermore, hNppitch and hNpaudio-1 failed to promote exogenous PMCA levels in HEK293T cells. In hippocampal neurons, expression of additional hNppitch or hNpaudio-1 was less efficient than hNpWT to elevate endogenous PMCA levels and to accelerate the restoration of basal Ca2+ levels after electrically evoked Ca2+ transients. We propose that mutations leading to pathological Np variants, as exemplified here by the deafness causing Np mutants, can affect Np-dependent Ca2+ regulatory mechanisms and may potentially cause intellectual and cognitive deficits in humans.

Analysis of cholesterol-recognition motifs of the plasma membrane Ca2+-ATPase

The plasma membrane Ca2+-ATPase (PMCA) is crucial for the fine tuning of intracellular calcium levels in eukaryotic cells. In this study, we show the presence of CARC sequences in all human and rat PMCA isoforms and we performed further analysis by molecular dynamics simulations. This analysis focuses on PMCA1, containing three CARC motifs, and PMCA4, with four CARC domains. In PMCA1, two CARC motifs reside within transmembrane domains, while the third is situated at the intracellular interface. The simulations depict more stable RMSD values and lower RMSF fluctuations in the presence of cholesterol, emphasizing its potential stabilizing effect. In PMCA4, a distinct dynamic was found. Notably, the total energy differences between simulations with cholesterol and phospholipids are pronounced in PMCA4 compared to PMCA1. RMSD values for PMCA4 indicate a more energetically favorable conformation in the presence of cholesterol, suggesting a robust interaction between CARCs and this lipid in the membranes. Furthermore, RMSF analysis for CARCs in both PMCA isoforms exhibit lower values in the presence of cholesterol compared to POPC alone. The analysis of H-bond occupancy and total energy values strongly suggests the potential interaction of CARCs with cholesterol. Given the crucial role of PMCAs in physiological calcium regulation and their involvement in diverse pathological processes, this study underscores the significance of CARC motifs and their interaction with cholesterol in elucidating PMCA function. These insights into the energetic preferences associated with CARC-cholesterol interactions offer valuable implications for understanding PMCA function in maintaining calcium homeostasis and addressing potential associated pathologies.

Plasma membrane Ca2+ pump isoform 4 function in cell migration and cancer metastasis

The Ca2+ ion is a universal second messenger involved in many vital physiological functions including cell migration and development. To fulfil these tasks the cytosolic Ca2+ concentration is tightly controlled, and this involves an intricate functional balance between a variety of channels and pumps of the Ca2+ signalling machinery. Among these proteins, plasma membrane Ca2+ ATPases (PMCAs) represent the major high-affinity Ca2+ extrusion systems in the cell membrane that are effective in maintaining free Ca2+ concentration at exceedingly low cytosolic levels, which is essential for normal cell function. An imbalance in Ca2+ signalling can have pathogenic consequences including cancer and metastasis. Recent studies have highlighted the role of PMCAs in cancer progression and have shown that a particular variant, PMCA4b, is downregulated in certain cancer types, causing delayed attenuation of the Ca2+ signal. It has also been shown that loss of PMCA4b leads to increased migration and metastasis of melanoma and gastric cancer cells. In contrast, an increased PMCA4 expression has been reported in pancreatic ductal adenocarcinoma that coincided with increased cell migration and shorter patient survival, suggesting distinct roles of PMCA4b in various tumour types and/or different stages of tumour development. The recently discovered interaction of PMCAs with basigin, an extracellular matrix metalloproteinase inducer, may provide further insights into our understanding of the specific roles of PMCA4b in tumour progression and cancer metastasis.

Cardiac energy metabolism disorder mediated by energy substrate imbalance and mitochondrial damage upon tebuconazole exposure

Tebuconazole exposure has been described as an increasing hazard to human health. An increasing number of recent studies have shown a positive association between tebuconazole exposure and cardiovascular disease risk, which is characterized by the reduction of adenosine triphosphate (ATP) synthesis. However, researches on the damage of tebuconazole exposure to energy metabolism and the related molecular mechanisms are limited. In the present study, male C57BL/6 mice were treated with tebuconazole at different low concentrations for 4 weeks. The results indicated that tebuconazole could accumulate in the heart and further induce the decrease of ATP content in the mouse heart. Importantly, tebuconazole induced an obvious shift in substrate utilization of fatty acid and glucose by disrupting their corresponding transporters (GLUT1, GLUT4, CD36, FABP3 and FATP1) expression, and significantly repressed the expression of mitochondrial biogenesis (Gabpa and Tfam) and oxidative phosphorylation (CS, Ndufa4, Sdhb, Cox5a and Atp5b) related genes in a dose-dependent manner. Further investigation revealed that these alterations were related to the IRS1/AKT and PPARγ/RXRα pathways. These findings contribute to a better understanding of triazole fungicide-induced cardiovascular disease by revealing the key indicators associated with this phenomenon.

The PfRCR complex bridges malaria parasite and erythrocyte during invasion

The symptoms of malaria occur during the blood stage of infection, when parasites invade and replicate within human erythrocytes. The PfPCRCR complex1, containing PfRH5 (refs. 2,3), PfCyRPA, PfRIPR, PfCSS and PfPTRAMP, is essential for erythrocyte invasion by the deadliest human malaria parasite, Plasmodium falciparum. Invasion can be prevented by antibodies3-6 or nanobodies1 against each of these conserved proteins, making them the leading blood-stage malaria vaccine candidates. However, little is known about how PfPCRCR functions during invasion. Here we present the structure of the PfRCR complex7,8, containing PfRH5, PfCyRPA and PfRIPR, determined by cryogenic-electron microscopy. We test the hypothesis that PfRH5 opens to insert into the membrane9, instead showing that a rigid, disulfide-locked PfRH5 can mediate efficient erythrocyte invasion. We show, through modelling and an erythrocyte-binding assay, that PfCyRPA-binding antibodies5 neutralize invasion through a steric mechanism. We determine the structure of PfRIPR, showing that it consists of an ordered, multidomain core flexibly linked to an elongated tail. We also show that the elongated tail of PfRIPR, which is the target of growth-neutralizing antibodies6, binds to the PfCSS-PfPTRAMP complex on the parasite membrane. A modular PfRIPR is therefore linked to the merozoite membrane through an elongated tail, and its structured core presents PfCyRPA and PfRH5 to interact with erythrocyte receptors. This provides fresh insight into the molecular mechanism of erythrocyte invasion and opens the way to new approaches in rational vaccine design.

Structure, function and biogenesis of the fungal proton pump Pma1

The fungal plasma membrane proton pump Pma1 is an integral plasma membrane protein of the P-type ATPase family. It is an essential enzyme responsible for maintaining a constant cytosolic pH and for energising the plasma membrane to secondary transport processes. Due to its importance for fungal survival and absence from animals, Pma1 is also a highly sought-after drug target. Until recently, its characterisation has been limited to functional, mutational and localisation studies, due to a lack of high-resolution structural information. The determination of three cryo-EM structures of Pma1 in its unique hexameric state offers a new level of understanding the molecular mechanisms underlying the protein's stability, regulated activity and druggability. In light of this context, this article aims to review what we currently know about the structure, function and biogenesis of fungal Pma1.

Neuroplastin Expression in Male Mice Is Essential for Fertility, Mating, and Adult Testosterone Levels

Male reproduction depends on hormonally driven behaviors and numerous genes for testis development and spermatogenesis. Neuroplastin-deficient (Nptn-/-) male mice cannot sire offspring. By immunohistochemistry, we characterized neuroplastin expression in the testis. Breeding, mating behavior, hormonal regulation, testicular development, and spermatogenesis were analyzed in cell-type specific neuroplastin mutant mice. Leydig, Sertoli, peritubular myoid, and germ cells express Np, but spermatogenesis and sperm number are not affected in Nptn-/- males. Neuroplastin lack from CNS neurons or restricted to spermatogonia or Sertoli cells permitted reproduction. Normal luteinizing hormone (LH) and follicle-stimulating hormone (FSH) blood levels in Nptn-/- males support undisturbed hormonal regulation in the brain. However, Nptn-/- males lack mounting behavior accompanied by low testosterone blood levels. Testosterone rise from juvenile to adult blood levels is absent in Nptn-/- males. LH-receptor stimulation raising intracellular Ca2+ in Leydig cells triggers testosterone production. Reduced Plasma Membrane Ca2+ ATPase 1 (PMCA1) in Nptn-/- Leydig cells suggests that Nptn-/- Leydig cells produce sufficient testosterone for testis and sperm development, but a lack of PMCA-Np complexes prevents the increase from reaching adult blood levels. Behavioral immaturity with low testosterone blood levels underlies infertility of Nptn-/- males, revealing that Np is essential for reproduction.

Monomeric α-synuclein activates the plasma membrane calcium pump

Alpha-synuclein (aSN) is a membrane-associated and intrinsically disordered protein, well known for pathological aggregation in neurodegeneration. However, the physiological function of aSN is disputed. Pull-down experiments have pointed to plasma membrane Ca2+ -ATPase (PMCA) as a potential interaction partner. From proximity ligation assays, we find that aSN and PMCA colocalize at neuronal synapses, and we show that calcium expulsion is activated by aSN and PMCA. We further show that soluble, monomeric aSN activates PMCA at par with calmodulin, but independent of the autoinhibitory domain of PMCA, and highly dependent on acidic phospholipids and membrane-anchoring properties of aSN. On PMCA, the key site is mapped to the acidic lipid-binding site, located within a disordered PMCA-specific loop connecting the cytosolic A domain and transmembrane segment 3. Our studies point toward a novel physiological role of monomeric aSN as a stimulator of calcium clearance in neurons through activation of PMCA.

SLC3A2 N-glycosylation and Golgi remodeling regulate SLC7A amino acid exchangers and stress mitigation

Proteostasis requires oxidative metabolism (ATP) and mitigation of the associated damage by glutathione, in an increasingly dysfunctional relationship with aging. SLC3A2 (4F2hc, CD98) plays a role as a disulfide-linked adaptor to the SLC7A5 and SLC7A11 exchangers which import essential amino acids and cystine while exporting Gln and Glu, respectively. The positions of N-glycosylation sites on SLC3A2 have evolved with the emergence of primates, presumably in synchrony with metabolism. Herein, we report that each of the four sites in SLC3A2 has distinct profiles of Golgi-modified N-glycans. N-glycans at the primate-derived site N381 stabilized SLC3A2 in the galectin-3 lattice against coated-pit endocytosis, while N365, the site nearest the membrane promoted glycolipid-galectin-3 (GL-Lect)-driven endocytosis. Our results indicate that surface retention and endocytosis are precisely balanced by the number, position, and remodeling of N-glycans on SLC3A2. Furthermore, proteomics and functional assays revealed an N-glycan-dependent clustering of the SLC3A2∗SLC7A5 heterodimer with amino-acid/Na+ symporters (SLC1A4, SLC1A5) that balances branched-chain amino acids and Gln levels, at the expense of ATP to maintain the Na+/K+ gradient. In replete conditions, SLC3A2 interactions require Golgi-modified N-glycans at N365D and N381D, whereas reducing N-glycosylation in the endoplasmic reticulum by fluvastatin treatment promoted the recruitment of CD44 and transporters needed to mitigate stress. Thus, SLC3A2 N-glycosylation and Golgi remodeling of the N-glycans have distinct roles in amino acids import for growth, maintenance, and metabolic stresses.

P-type ATPases: Many more enigmas left to solve

P-type ATPases constitute a large ancient super-family of primary active pumps that have diverse substrate specificities ranging from H+ to phospholipids. The significance of these enzymes in biology cannot be overstated. They are structurally related, and their catalytic cycles alternate between high- and low-affinity conformations that are induced by phosphorylation and dephosphorylation of a conserved aspartate residue. In the year 1988, all P-type sequences available by then were analyzed and five major families, P1 to P5, were identified. Since then, a large body of knowledge has accumulated concerning the structure, function, and physiological roles of members of these families, but only one additional family, P6 ATPases, has been identified. However, much is still left to be learned. For each family a few remaining enigmas are presented, with the intention that they will stimulate interest in continued research in the field. The review is by no way comprehensive and merely presents personal views with a focus on evolution.

Absence of Embigin accelerates hearing loss and causes sub-viability, brain and heart defects in C57BL/6N mice due to interaction with Cdh23ahl

Mouse studies continue to help elaborate upon the genetic landscape of mammalian disease and the underlying molecular mechanisms. Here, we have investigated an Embigintm1b allele maintained on a standard C57BL/6N background and on a co-isogenic C57BL/6N background in which the Cdh23ahl allele has been "repaired." The hypomorphic Cdh23ahl allele is present in several commonly used inbred mouse strains, predisposing them to progressive hearing loss, starting in high-frequency regions. Absence of the neural cell adhesion molecule Embigin on the standard C57BL/6N background leads to accelerated hearing loss and causes sub-viability, brain and cardiac defects. Contrastingly, Embigintm1b/tm1b mice maintained on the co-isogenic "repaired" C57BL/6N background exhibit normal hearing and viability. Thus Embigin genetically interacts with Cdh23. Importantly, our study is the first to demonstrate an effect of the common Cdh23ahl allele outside of the auditory system, which has important ramifications for genetic studies involving inbred strains carrying this allele.

Neuroplastin exerts antiepileptic effects through binding to the α1 subunit of GABA type A receptors to inhibit the internalization of the receptors

BACKGROUND

Seizures are associated with a decrease in γ-aminobutyric type A acid receptors (GABAaRs) on the neuronal surface, which may be regulated by enhanced internalization of GABAaRs. When interactions between GABAaR subunit α-1 (GABRA1) and postsynaptic scaffold proteins are weakened, the α1-containing GABAaRs leave the postsynaptic membrane and are internalized. Previous evidence suggested that neuroplastin (NPTN) promotes the localization of GABRA1 on the postsynaptic membrane. However, the association between NPTN and GABRA1 in seizures and its effect on the internalization of α1-containing GABAaRs on the neuronal surface has not been studied before.

METHODS

An in vitro seizure model was constructed using magnesium-free extracellular fluid, and an in vivo model of status epilepticus (SE) was constructed using pentylenetetrazole (PTZ). Additionally, in vitro and in vivo NPTN-overexpression models were constructed. Electrophysiological recordings and internalization assays were performed to evaluate the action potentials and miniature inhibitory postsynaptic currents of neurons, as well as the intracellular accumulation ratio of α1-containing GABAaRs in neurons. Western blot analysis was performed to detect the expression of GABRA1 and NPTN both in vitro and in vivo. Immunofluorescence co-localization analysis and co-immunoprecipitation were performed to evaluate the interaction between GABRA1 and NPTN.

RESULTS

The expression of GABRA1 was found to be decreased on the neuronal surface both in vivo and in vitro seizure models. In the in vitro seizure model, α1-containing GABAaRs showed increased internalization. NPTN expression was found to be positively correlated with GABRA1 expression on the neuronal surface both in vivo and in vitro seizure models. In addition, NPTN overexpression alleviated seizures and NPTN was shown to bind to GABRA1 to form protein complexes that can be disrupted during seizures in both in vivo and in vitro models. Furthermore, NPTN was found to inhibit the internalization of α1-containing GABAaRs in the in vitro seizure model.

CONCLUSION

Our findings provide evidence that NPTN may exert antiepileptic effects by binding to GABRA1 to inhibit the internalization of α1-containing GABAaRs.

Erythrocyte invasion-neutralising antibodies prevent Plasmodium falciparum RH5 from binding to basigin-containing membrane protein complexes

Basigin is an essential host receptor for invasion of Plasmodium falciparum into human erythrocytes, interacting with parasite surface protein PfRH5. PfRH5 is a leading blood-stage malaria vaccine candidate and a target of growth-inhibitory antibodies. Here, we show that erythrocyte basigin is exclusively found in one of two macromolecular complexes, bound either to plasma membrane Ca2+-ATPase 1/4 (PMCA1/4) or to monocarboxylate transporter 1 (MCT1). PfRH5 binds to each of these complexes with a higher affinity than to isolated basigin ectodomain, making it likely that these are the physiological targets of PfRH5. PMCA-mediated Ca2+ export is not affected by PfRH5, making it unlikely that this is the mechanism underlying changes in calcium flux at the interface between an erythrocyte and the invading parasite. However, our studies rationalise the function of the most effective growth-inhibitory antibodies targeting PfRH5. While these antibodies do not reduce the binding of PfRH5 to monomeric basigin, they do reduce its binding to basigin-PMCA and basigin-MCT complexes. This indicates that the most effective PfRH5-targeting antibodies inhibit growth by sterically blocking the essential interaction of PfRH5 with basigin in its physiological context.

Physiology of intracellular calcium buffering

Calcium signaling underlies much of physiology. Almost all the Ca2+ in the cytoplasm is bound to buffers, with typically only ∼1% being freely ionized at resting levels in most cells. Physiological Ca2+ buffers include small molecules and proteins, and experimentally Ca2+ indicators will also buffer calcium. The chemistry of interactions between Ca2+ and buffers determines the extent and speed of Ca2+ binding. The physiological effects of Ca2+ buffers are determined by the kinetics with which they bind Ca2+ and their mobility within the cell. The degree of buffering depends on factors such as the affinity for Ca2+, the Ca2+ concentration, and whether Ca2+ ions bind cooperatively. Buffering affects both the amplitude and time course of cytoplasmic Ca2+ signals as well as changes of Ca2+ concentration in organelles. It can also facilitate Ca2+ diffusion inside the cell. Ca2+ buffering affects synaptic transmission, muscle contraction, Ca2+ transport across epithelia, and the killing of bacteria. Saturation of buffers leads to synaptic facilitation and tetanic contraction in skeletal muscle and may play a role in inotropy in the heart. This review focuses on the link between buffer chemistry and function and how Ca2+ buffering affects normal physiology and the consequences of changes in disease. As well as summarizing what is known, we point out the many areas where further work is required.

Resting membrane state as an interplay of electrogenic transporters with various pumps

In this study, a new idea that electrogenic transporters determine cell resting state is presented. The previous assumption was that pumps, especially the sodium one, determine it. The latter meets difficulties, because it violates the law of conservation of energy; also a significant deficit of pump activity is reported. The amount of energy carried by a single ATP molecule reflects the potential of the inner mitochondrial membrane, which is about -200 mV. If pumps enforce a resting membrane potential that is more than twice smaller, then the majority of energy stored in ATP would be dissipated by each pump turning. However, this problem could be solved if control is transferred from pumps to something else, e.g., electrogenic transporters. Then pumps would transfer the energy to the ionic gradient without losses, while the cell surface membrane potential would be associated with the reversal potential of some electrogenic transporters. A minimal scheme of this type would include a sodium-calcium exchanger as well as sodium and calcium pumps. However, note that calcium channels and pumps are positioned along both intracellular organelles and the surface membrane. Therefore, the above-mentioned scheme would involve them as well as possible intercellular communications. Such schemes where various kinds of pumps are assumed to work in parallel may explain, to a great extent, the slow turning rate of the individual members. Interaction of pumps and transporters positioned at distant biological membranes with various forms of energy transfer between them may thus result in hypoxic/reperfusion injury, different kinds of muscle fatigue, and nerve-glia interactions.

Plastic recognition and electrogenic uniport translocation of 1st-, 2nd-, and 3rd-row transition and post-transition metals by primary-active transmembrane P1B-2-type ATPase pumps

Transmembrane P1B-type ATPase pumps catalyze the extrusion of transition metal ions across cellular lipid membranes to maintain essential cellular metal homeostasis and detoxify toxic metals. Zn(ii)-pumps of the P1B-2-type subclass, in addition to Zn2+, select diverse metals (Pb2+, Cd2+ and Hg2+) at their transmembrane binding site and feature promiscuous metal-dependent ATP hydrolysis in the presence of these metals. Yet, a comprehensive understanding of the transport of these metals, their relative translocation rates, and transport mechanism remain elusive. We developed a platform for the characterization of primary-active Zn(ii)-pumps in proteoliposomes to study metal selectivity, translocation events and transport mechanism in real-time, employing a "multi-probe" approach with fluorescent sensors responsive to diverse stimuli (metals, pH and membrane potential). Together with atomic-resolution investigation of cargo selection by X-ray absorption spectroscopy (XAS), we demonstrate that Zn(ii)-pumps are electrogenic uniporters that preserve the transport mechanism with 1st-, 2nd- and 3rd-row transition metal substrates. Promiscuous coordination plasticity, guarantees diverse, yet defined, cargo selectivity coupled to their translocation.

Ca2+ Homeostasis by Plasma Membrane Ca2+ ATPase (PMCA) 1 Is Essential for the Development of DP Thymocytes

The strength of Ca²⁺ signaling is a hallmark of T cell activation, yet the role of Ca²⁺ homeostasis in developing T cells before expressing a mature T cell receptor is poorly understood. We aimed to unveil specific functions of the two plasma membrane Ca²⁺ ATPases expressed in T cells, PMCA1 and PMCA4. On a transcriptional and protein level we found that PMCA4 was expressed at low levels in CD4^-CD8^- double negative (DN) thymocytes and was even downregulated in subsequent stages while PMCA1 was present throughout development and upregulated in CD4⁺CD8⁺ double positive (DP) thymocytes. Mice with a targeted deletion of Pmca1 in DN3 thymocytes had an almost complete block of DP thymocyte development with an accumulation of DN4 thymocytes but severely reduced numbers of CD8⁺ immature single positive (ISP) thymocytes. The DN4 thymocytes of these mice showed strongly elevated basal cytosolic Ca²⁺ levels and a pre-mature CD5 expression, but in contrast to the DP thymocytes they were only mildly prone to apoptosis. Surprisingly, mice with a germline deletion of Pmca4 did not show any signs of altered progression through the developmental thymocyte stages, nor altered Ca²⁺ homeostasis throughout this process. PMCA1 is, therefore, non-redundant in keeping cellular Ca²⁺ levels low in the early thymocyte development required for the DN to DP transition.

Neuroplastin 65 deficiency reduces amyloid plaque formation and cognitive deficits in an Alzheimer's disease mouse model

Introduction

Alzheimer's disease (AD) is characterized by increasing cognitive dysfunction, progressive cerebral amyloid beta (Aβ) deposition, and neurofibrillary tangle aggregation. However, the molecular mechanisms of AD pathologies have not been completely understood. As synaptic glycoprotein neuroplastin 65 (NP65) is related with synaptic plasticity and complex molecular events underlying learning and memory, we hypothesized that NP65 would be involved in cognitive dysfunction and Aβ plaque formation of AD. For this purpose, we examined the role of NP65 in the transgenic amyloid precursor protein (APP)/presenilin 1 (PS1) mouse model of AD.

Methods

Neuroplastin 65-knockout (NP65^-/-) mice crossed with APP/PS1 mice to get the NP65-deficient APP/PS1 mice. In the present study, a separate cohort of NP65-deficient APP/PS1 mice were used. First, the cognitive behaviors of NP65-deficient APP/PS1 mice were assessed. Then, Aβ plaque burden and Aβ levels in NP65-deficient APP/PS1 mice were measured by immunostaining and western blot as well as ELISA. Thirdly, immunostaining and western blot were used to evaluate the glial response and neuroinflammation. Finally, protein levels of 5-hydroxytryptamin (serotonin) receptor 3A and synaptic proteins and neurons were measured.

Results

We found that loss of NP65 alleviated the cognitive deficits of APP/PS1 mice. In addition, Aβ plaque burden and Aβ levels were significantly reduced in NP65-deficient APP/PS1 mice compared with control animals. NP65-loss in APP/PS1 mice resulted in a decrease in glial activation and the levels of pro- and anti-inflammatory cytokines (IL-1β, TNF-α, and IL-4) as well as protective matrix YM-1 and Arg-1, but had no effect on microglial phenotype. Moreover, NP65 deficiency significantly reversed the increase in 5-hydroxytryptamine (serotonin) receptor 3A (Htr3A) expression levels in the hippocampus of APP/PS1 mice.

Discussion

These findings identify a previously unrecognized role of NP65 in cognitive deficits and Aβ formation of APP/PS1 mice, and suggest that NP65 may serve as a potential therapeutic target for AD.

Astrocytes amplify neurovascular coupling to sustained activation of neocortex in awake mice

Functional hyperemia occurs when enhanced neuronal activity signals to increase local cerebral blood flow (CBF) to satisfy regional energy demand. Ca2+ elevation in astrocytes can drive arteriole dilation to increase CBF, yet affirmative evidence for the necessity of astrocytes in functional hyperemia in vivo is lacking. In awake mice, we discovered that functional hyperemia is bimodal with a distinct early and late component whereby arteriole dilation progresses as sensory stimulation is sustained. Clamping astrocyte Ca2+ signaling in vivo by expressing a plasma membrane Ca2+ ATPase (CalEx) reduces sustained but not brief sensory-evoked arteriole dilation. Elevating astrocyte free Ca2+ using chemogenetics selectively augments sustained hyperemia. Antagonizing NMDA-receptors or epoxyeicosatrienoic acid production reduces only the late component of functional hyperemia, leaving brief increases in CBF to sensory stimulation intact. We propose that a fundamental role of astrocyte Ca2+ is to amplify functional hyperemia when neuronal activation is prolonged.

A novel role for ATP2B in ascidians: Ascidian-specific mutations in ATP2B contribute to sperm chemotaxis

Sperm chemotaxis, in which sperms are attracted to conspecific eggs via species-specific attractants, plays an important role in fertilization. This phenomenon has been observed in various animals and species-specific sperm attractants have been reported in some species. However, the mechanisms involved in the reception and recognition of the species-specific attractant by the sperms is poorly studied. Previously, we found that the plasma membrane-type Ca²⁺ /ATPase (PMCA) is the receptor for the sperm-activating and -attracting factor (SAAF) in the ascidian Ciona intestinalis. To determine the role of PMCA in species-specific sperm chemotaxis, we identified the amino acid sequences of PMCAs derived from six Phlebobranchia species. The testis-specific splice variant of PMCA was found to be present in all the species investigated and the ascidian-specific sequence was detected near the 3'-terminus. Moreover, dN/dS analysis revealed that the extracellular loops 1, 2, and 4 in ascidian PMCA underwent a positive selection. These findings suggest that PMCA recognizes the species-specific structure of SAAF at the extracellular loops 1, 2, and 4, and its testis-specific C-terminal region is involved in the activation and chemotaxis of ascidian sperms.

The role of neuroplastin65 in macrophage against E. coli infection in mice

BACKGROUND

Innate immune response constitutes the first line of defense against pathogens. Inflammatory responses involve close contact between different populations of cells. These adhesive interactions mediate migration of cells to sites of infection leading the effective action of cells within the lesions. Cell adhesion molecules are critical to controlling immune response mediating cell adhesion or chemotaxis, as well as coordinating actin-based cell motility during phagocytosis and chemotaxis. Recently, a newly discovered neuroplastin (Np) adhesion molecule is found to play an important role in the nervous system. However, there is limited information on Np functions in immune response. To understand how Np is involved in innate immune response, a mouse model of intraperitoneal infection was established to investigate the effect of Np on macrophage-mediated clearance of E. coli infection and its possible molecular mechanisms.

METHODS

Specific deficiency mice with Nptn gene controlling Np65 isoform were employed in this study. The expression levels of mRNA and proteins were detected by qPCR and western blot, or evaluated by flow cytometry. The expression level of NO and ROS were measured with their specific indicators. Cell cycle and apoptosis were detected by specific detection kits. Acid phosphatase activity was measured by flow cytometry after labelling with LysoRed fluorescent probe. Bone marrow derived macrophages (BMDMs) were isolated from bone marrow of mice hind legs. Cell proliferation was detected by CCK8 assay. Cell migration was measured by wound healing assay or transwell assay.

RESULTS

The lethal dose of E. coli infection in Np65^-/- mice dropped to the half of lethal dose in WT mice. The bacterial load in the spleen, kidney and liver from Np65^-/- mice were significantly higher than that from WT mice, which were due to the dramatic reduction of NO and ROS production in phagocytes from Np65^-/- mice. Np65 gene deficiency remarkably impaired phagocytosis and function of lysosome in macrophage. Furthermore, Np65 molecule was involved in maturation and proliferation, even in migration and chemotaxis of BMDM in vitro.

CONCLUSION

This study for the first time demonstrates that Np is involved in multi-function of phagocytes during bacterial infection, proposing that Np adhesion molecule plays a critical role in clearing pathogen infection in innate immunity.

Ca2+ signaling in postsynaptic neurons: Neuroplastin-65 regulates the interplay between plasma membrane Ca2+ ATPases and ionotropic glutamate receptors

Upon postsynaptic glutamate receptor activation, the cytosolic Ca²⁺ concentration rises and initiates signaling and plasticity in spines. The plasma membrane Ca²⁺ ATPase (PMCA) is a major player to limit the duration of cytosolic Ca²⁺ signals. It forms complexes with the glycoprotein neuroplastin (Np) isoforms Np55 and Np65 and functionally interplays with N-methyl-D-aspartate (NMDA)-type ionotropic glutamate receptors (iGluNRs). Moreover, binding of the Np65-specific extracellular domain to Ca²⁺-permeable GluA1-containing α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type ionotropic glutamate receptors (iGluA1Rs) was found to be required for long-term potentiation (LTP). However, the link between PMCA and iGluRs function to regulate cytosolic Ca²⁺ signals remained unclear. Here, we report that Np65 coordinates PMCA and iGluRs' functions to modulate the duration and amplitude of cytosolic Ca²⁺ transients in dendrites and spines of hippocampal neurons. Using live-cell Ca²⁺ imaging, acute pharmacological treatments, and GCaMP5G-expressing hippocampal neurons, we discovered that endogenous or Np65-promoted PMCA activity contributes to the restoration of basal Ca²⁺ levels and that this effect is dependent on iGluR activation. Super-resolution STED and confocal microscopy revealed that electrical stimulation increases the abundance of synaptic neuroplastin-PMCA complexes depending on iGluR activation and that low-rate overexpression of Np65 doubled PMCA levels and decreased cell surface levels of GluN2A and GluA1 in dendrites and Shank2-positive glutamatergic synapses. In neuroplastin-deficient hippocampi, we observed reduced PMCA and unchanged GluN2B levels, while GluN2A and GluA1 levels were imbalanced. Our electrophysiological data from hippocampal slices argues for an essential interplay of PMCA with GluN2A- but not with GluN2B-containing receptors upon induction of synaptic plasticity. Accordingly, we conclude that Np65 may interconnect PMCA with core players of glutamatergic neurotransmission to fine-tune the Ca²⁺ signal regulation in basal synaptic function and plasticity.

Embigin is a fibronectin receptor that affects sebaceous gland differentiation and metabolism

Stem cell renewal and differentiation are regulated by interactions with the niche. Although multiple cell populations have been identified in distinct anatomical compartments, little is known about niche-specific molecular factors. Using skin as a model system and combining single-cell RNA-seq data analysis, immunofluorescence, and transgenic mouse models, we show that the transmembrane protein embigin is specifically expressed in the sebaceous gland and that the number of embigin-expressing cells is negatively regulated by Wnt. The loss of embigin promotes exit from the progenitor compartment and progression toward differentiation, and also compromises lipid metabolism. Embigin modulates sebaceous niche architecture by affecting extracellular matrix organization and basolateral targeting of monocarboxylate transport. We discover through ligand screening that embigin is a direct fibronectin receptor, binding to the N-terminal fibronectin domain without impairing integrin function. Our results solve the long-standing question of how embigin regulates cell adhesion and demonstrate a mechanism that couples adhesion and metabolism.

Cryo-EM structure of human glucose transporter GLUT4

GLUT4 is the primary glucose transporter in adipose and skeletal muscle tissues. Its cellular trafficking is regulated by insulin signaling. Failed or reduced plasma membrane localization of GLUT4 is associated with diabetes. Here, we report the cryo-EM structures of human GLUT4 bound to a small molecule inhibitor cytochalasin B (CCB) at resolutions of 3.3 Å in both detergent micelles and lipid nanodiscs. CCB-bound GLUT4 exhibits an inward-open conformation. Despite the nearly identical conformation of the transmembrane domain to GLUT1, the cryo-EM structure reveals an extracellular glycosylation site and an intracellular helix that is invisible in the crystal structure of GLUT1. The structural study presented here lays the foundation for further mechanistic investigation of the modulation of GLUT4 trafficking. Our methods for cryo-EM analysis of GLUT4 will also facilitate structural determination of many other small size solute carriers.

Small solute carriers remain difficult to study by single particle cryo-EM. Here, the authors report the cryo-EM structure of human insulin-responsive glucose transporter GLUT4 (55 kDa) without rigid soluble domains or binders.

De novo variants in ATP2B1 lead to neurodevelopmental delay

Calcium (Ca²⁺) is a universal second messenger involved in synaptogenesis and cell survival; consequently, its regulation is important for neurons. ATPase plasma membrane Ca²⁺ transporting 1 (ATP2B1) belongs to the family of ATP-driven calmodulin-dependent Ca²⁺ pumps that participate in the regulation of intracellular free Ca²⁺. Here, we clinically describe a cohort of 12 unrelated individuals with variants in ATP2B1 and an overlapping phenotype of mild to moderate global development delay. Additional common symptoms include autism, seizures, and distal limb abnormalities. Nine probands harbor missense variants, seven of which were in specific functional domains, and three individuals have nonsense variants. 3D structural protein modeling suggested that the variants have a destabilizing effect on the protein. We performed Ca²⁺ imaging after introducing all nine missense variants in transfected HEK293 cells and showed that all variants lead to a significant decrease in Ca²⁺ export capacity compared with the wild-type construct, thus proving their pathogenicity. Furthermore, we observed for the same variant set an incorrect intracellular localization of ATP2B1. The genetic findings and the overlapping phenotype of the probands as well as the functional analyses imply that de novo variants in ATP2B1 lead to a monogenic form of neurodevelopmental disorder.

Structure, Function and Regulation of the Plasma Membrane Calcium Pump in Health and Disease

In this review, I summarize the present knowledge of the structural and functional properties of the mammalian plasma membrane calcium pump (PMCA). It is outlined how the cellular expression of the different spliced isoforms of the four genes are regulated under normal and pathological conditions.

Retrograde Amnesia - A Question of Disturbed Calcium Levels?

Retrograde amnesia is the inability to remember events or information. The successful acquisition and memory of information is required before retrograde amnesia may occur. Often, the trigger for retrograde amnesia is a traumatic event. Loss of memories may be caused in two ways: either by loss/erasure of the memory itself or by the inability to access the memory, which is still present. In general, memories and learning are associated with a positive connotation although the extinction of unpleasant experiences and memories of traumatic events may be highly welcome. In contrast to the many experimental models addressing learning deficits caused by anterograde amnesia, the incapability to acquire new information, retrograde amnesia could so far only be investigated sporadically in human patients and in a limited number of model systems. Apart from models and diseases in which neurodegeneration or dementia like Alzheimer’s disease result in loss of memory, retrograde amnesia can be elicited by various drugs of which alcohol is the most prominent one and exemplifies the non-specific effects and the variable duration. External or internal impacts like traumatic brain injury, stroke, or electroconvulsive treatments may similarly result in variable degrees of retrograde amnesia. In this review, I will discuss a new genetic approach to induce retrograde amnesia in a mouse model and raise the hypothesis that retrograde amnesia is caused by altered intracellular calcium homeostasis. Recently, we observed that neuronal loss of neuroplastin resulted in retrograde amnesia specifically for associative memories. Neuroplastin is tightly linked to the expression of the main Ca²⁺ extruding pumps, the plasma membrane calcium ATPases (PMCAs). Therefore, neuronal loss of neuroplastin may block the retrieval and storage of associative memories by interference with Ca²⁺ signaling cascades. The possibility to elicit retrograde amnesia in a controlled manner allows to investigate the underlying mechanisms and may provide a deeper understanding of the molecular and circuit processes of memory.

Neuroplastin genetically interacts with Cadherin 23 and the encoded isoform Np55 is sufficient for cochlear hair cell function and hearing

Mammalian hearing involves the mechanoelectrical transduction (MET) of sound-induced fluid waves in the cochlea. Essential to this process are the specialised sensory cochlear cells, the inner (IHCs) and outer hair cells (OHCs). While genetic hearing loss is highly heterogeneous, understanding the requirement of each gene will lead to a better understanding of the molecular basis of hearing and also to therapeutic opportunities for deafness. The Neuroplastin (Nptn) gene, which encodes two protein isoforms Np55 and Np65, is required for hearing, and homozygous loss-of-function mutations that affect both isoforms lead to profound deafness in mice. Here we have utilised several distinct mouse models to elaborate upon the spatial, temporal, and functional requirement of Nptn for hearing. While we demonstrate that both Np55 and Np65 are present in cochlear cells, characterisation of a Np65-specific mouse knockout shows normal hearing thresholds indicating that Np65 is functionally redundant for hearing. In contrast, we find that Nptn-knockout mice have significantly reduced maximal MET currents and MET channel open probabilities in mature OHCs, with both OHCs and IHCs also failing to develop fully mature basolateral currents. Furthermore, comparing the hearing thresholds and IHC synapse structure of Nptn-knockout mice with those of mice that lack Nptn only in IHCs and OHCs shows that the majority of the auditory deficit is explained by hair cell dysfunction, with abnormal afferent synapses contributing only a small proportion of the hearing loss. Finally, we show that continued expression of Neuroplastin in OHCs of adult mice is required for membrane localisation of Plasma Membrane Ca2+ ATPase 2 (PMCA2), which is essential for hearing function. Moreover, Nptn haploinsufficiency phenocopies Atp2b2 (encodes PMCA2) mutations, with heterozygous Nptn-knockout mice exhibiting hearing loss through genetic interaction with the Cdh23ahl allele. Together, our findings provide further insight to the functional requirement of Neuroplastin for mammalian hearing.

Plasma Membrane Calcium ATPase-Neuroplastin Complexes Are Selectively Stabilized in GM1-Containing Lipid Rafts

The recent identification of plasma membrane (Ca²⁺)-ATPase (PMCA)-Neuroplastin (Np) complexes has renewed attention on cell regulation of cytosolic calcium extrusion, which is of particular relevance in neurons. Here, we tested the hypothesis that PMCA-Neuroplastin complexes exist in specific ganglioside-containing rafts, which could affect calcium homeostasis. We analyzed the abundance of all four PMCA paralogs (PMCA1-4) and Neuroplastin isoforms (Np65 and Np55) in lipid rafts and bulk membrane fractions from GM2/GD2 synthase-deficient mouse brains. In these fractions, we found altered distribution of Np65/Np55 and selected PMCA isoforms, namely PMCA1 and 2. Cell surface staining and confocal microscopy identified GM1 as the main complex ganglioside co-localizing with Neuroplastin in cultured hippocampal neurons. Furthermore, blocking GM1 with a specific antibody resulted in delayed calcium restoration of electrically evoked calcium transients in the soma of hippocampal neurons. The content and composition of all ganglioside species were unchanged in Neuroplastin-deficient mouse brains. Therefore, we conclude that altered composition or disorganization of ganglioside-containing rafts results in changed regulation of calcium signals in neurons. We propose that GM1 could be a key sphingolipid for ensuring proper location of the PMCA-Neuroplastin complexes into rafts in order to participate in the regulation of neuronal calcium homeostasis.

Neuroplastin in Neuropsychiatric Diseases

Molecular mechanisms underlying neuropsychiatric and neurodegenerative diseases are insufficiently elucidated. A detailed understanding of these mechanisms may help to further improve medical intervention. Recently, intellectual abilities, creativity, and amnesia have been associated with neuroplastin, a cell recognition glycoprotein of the immunoglobulin superfamily that participates in synapse formation and function and calcium signaling. Data from animal models suggest a role for neuroplastin in pathways affected in neuropsychiatric and neurodegenerative diseases. Neuroplastin loss or disruption of molecular pathways related to neuronal processes has been linked to various neurological diseases, including dementia, schizophrenia, and Alzheimer's disease. Here, we review the molecular features of the cell recognition molecule neuroplastin, and its binding partners, which are related to neurological processes and involved in learning and memory. The emerging functions of neuroplastin may have implications for the treatment of diseases, particularly those of the nervous system.

Plasma Membrane Ca2+ Pump PMCA4z Is More Active Than Splicing Variant PMCA4x

The plasma membrane Ca²⁺ pumps (PMCA) are P-ATPases that control Ca²⁺ signaling and homeostasis by transporting Ca²⁺ out of the eukaryotic cell. Humans have four genes that code for PMCA isoforms (PMCA1-4). A large diversity of PMCA isoforms is generated by alternative mRNA splicing at sites A and C. The different PMCA isoforms are expressed in a cell-type and developmental-specific manner and exhibit differential sensitivity to a great number of regulatory mechanisms. PMCA4 has two A splice variants, the forms "x" and "z". While PMCA4x is ubiquitously expressed and relatively well-studied, PMCA4z is less characterized and its expression is restricted to some tissues such as the brain and heart muscle. PMCA4z lacks a stretch of 12 amino acids in the so-called A-M3 linker, a conformation-sensitive region of the molecule connecting the actuator domain (A) with the third transmembrane segment (M3). We expressed in yeast PMCA4 variants "x" and "z", maintaining constant the most frequent splice variant "b" at the C-terminal end, and obtained purified preparations of both proteins. In the basal autoinhibited state, PMCA4zb showed a higher ATPase activity and a higher apparent Ca²⁺ affinity than PMCA4xb. Both isoforms were stimulated by calmodulin but PMCA4zb was more strongly activated by acidic lipids than PMCA4xb. The results indicate that a PMCA4 intrinsically more active and more responsive to acidic lipids is produced by the variant "z" of the splicing site A.

Transmembrane Cu(I) P-type ATPase pumps are electrogenic uniporters

Cu(i) P-type ATPases are transmembrane primary active ion pumps that catalyze the extrusion of copper ions across cellular membranes. Their activity is critical in controlling copper levels in all kingdoms of life. Biochemical and structural characterization established the structural framework by which Cu-pumps perform their function. However, the details of the overall mechanism of transport (uniporter vs. cotransporter) and electrogenicity still remain elusive. In this work, we developed a platform to reconstitute the model Cu(i)-pump from E. coli (EcCopA) in artificial lipid bilayer small unilamellar vesicles (SUVs) to quantitatively characterize the metal substrate, putative counter-ions and charge translocation. By encapsulating in the liposome lumen fluorescence detector probes (CTAP-3, pyranine and oxonol VI) responsive to diverse stimuli (Cu(i), pH and membrane potential), we correlated substrate, secondary-ion translocation and charge movement events in EcCopA proteoliposomes. This platform centered on multiple fluorescence reporters allowed study of the mechanism and translocation kinetic parameters in real-time for wild-type EcCopA and inactive mutants. The maximal initial Cu(i) transport rate of 165 nmol Cu(i) mg-1 min-1 and KM, Cu(I) = 0.15 ± 0.07 μM was determined with this analysis. We reveal that Cu(i) pumps are primary-active uniporters and electrogenic. The Cu(i) translocation cycle does not require proton counter-transport resulting in electrogenic generation of transmembrane potential upon translocation of one Cu(i) per ATP hydrolysis cycle. Thus, mechanistic differences between Cu(i) pumps and other better characterized P-type ATPases are discussed. The platform opens the venue to study translocation events and mechanisms of transport in other transition metal P-type ATPase pumps.

Neuroplastin in human cognition: review of literature and future perspectives

Synaptic glycoprotein neuroplastin is involved in synaptic plasticity and complex molecular events underlying learning and memory. Studies in mice and rats suggest that neuroplastin is essential for cognition, as it is needed for long-term potentiation and associative memory formation. Recently, it was found that some of the effects of neuroplastin are related to regulation of calcium homeostasis through interactions with plasma membrane calcium ATPases. Neuroplastin is increasingly seen as a key factor in complex brain functions, but studies in humans remain scarce. Here we summarize present knowledge about neuroplastin in human tissues and argue its genetic association with cortical thickness, intelligence, schizophrenia, and autism; specific immunolocalization depicting hippocampal trisynaptic pathway; potential role in tissue compensatory response in neurodegeneration; and high, almost housekeeping, level of spatio-temporal gene expression in the human brain. We also propose that neuroplastin acts as a housekeeper of neuroplasticity, and that it may be considered as an important novel cognition-related molecule in humans. Several promising directions for future investigations are suggested, which may complete our understanding of neuroplastin actions in molecular basis of human cognition.

Separation of presynaptic Cav2 and Cav1 channel function in synaptic vesicle exo- and endocytosis by the membrane anchored Ca2+ pump PMCA

Synaptic vesicle (SV) release, recycling, and plastic changes of release probability co-occur side by side within nerve terminals and rely on local Ca²⁺ signals with different temporal and spatial profiles. The mechanisms that guarantee separate regulation of these vital presynaptic functions during action potential (AP)-triggered presynaptic Ca²⁺ entry remain unclear. Combining Drosophila genetics with electrophysiology and imaging reveals the localization of two different voltage-gated calcium channels at the presynaptic terminals of glutamatergic neuromuscular synapses (the Drosophila Ca_v2 homolog, Dmca1A or cacophony, and the Ca_v1 homolog, Dmca1D) but with spatial and functional separation. Ca_v2 within active zones is required for AP-triggered neurotransmitter release. By contrast, Ca_v1 localizes predominantly around active zones and contributes substantially to AP-evoked Ca²⁺ influx but has a small impact on release. Instead, L-type calcium currents through Ca_v1 fine-tune short-term plasticity and facilitate SV recycling. Separate control of SV exo- and endocytosis by AP-triggered presynaptic Ca²⁺ influx through different channels demands efficient measures to protect the neurotransmitter release machinery against Ca_v1-mediated Ca²⁺ influx. We show that the plasma membrane Ca²⁺ ATPase (PMCA) resides in between active zones and isolates Ca_v2-triggered release from Ca_v1-mediated dynamic regulation of recycling and short-term plasticity, two processes which Ca_v2 may also contribute to. As L-type Ca_v1 channels also localize next to PQ-type Ca_v2 channels within axon terminals of some central mammalian synapses, we propose that Ca_v2, Ca_v1, and PMCA act as a conserved functional triad that enables separate control of SV release and recycling rates in presynaptic terminals.

Who's in, who's out? Re-evaluation of lipid raft residents

Lipid rafts, membrane microdomains enriched with (glyco)sphingolipids, cholesterol, and select proteins, act as cellular signalosomes. Various methods have been used to separate lipid rafts from bulk (non‐raft) membranes, but most often, non‐ionic detergent Triton X‐100 has been used in their isolation. However, Triton X‐100 is a reported disruptor of lipid rafts. Histological evidence confirmed raft disruption by Triton X‐100, but remarkably revealed raft stability to treatment with a related polyethylene oxide detergent, Brij O20. We report isolation of detergent‐resistant membranes from mouse brain using Brij O20 and its use to determine the distribution of major mammalian brain gangliosides, GM1, GD1a, GD1b and GT1b. A different distribution of gangliosides—classically used as a raft marker—was discovered using Brij O20 versus Triton X‐100. Immunohistochemistry and imaging mass spectrometry confirm the results. Use of Brij O20 results in a distinctive membrane distribution of gangliosides that is not all lipid raft associated, but depends on the ganglioside structure. This is the first report of a significant proportion of gangliosides outside raft domains. We also determined the distribution of proteins functionally related to neuroplasticity and known to be affected by ganglioside environment, glutamate receptor subunit 2, amyloid precursor protein and neuroplastin and report the lipid raft populations of these proteins in mouse brain tissue. This work will enable more accurate lipid raft analysis with respect to glycosphingolipid and membrane protein composition and lead to improved resolution of lipid–protein interactions within biological membranes.

Plasma Membrane Calcium ATPase Regulates Stoichiometry of CD4+ T-Cell Compartments

Immune responses involve mobilization of T cells within naïve and memory compartments. Tightly regulated Ca²⁺ levels are essential for balanced immune outcomes. How Ca²⁺ contributes to regulating compartment stoichiometry is unknown. Here, we show that plasma membrane Ca²⁺ ATPase 4 (PMCA4) is differentially expressed in human CD4⁺ T compartments yielding distinct store operated Ca²⁺ entry (SOCE) profiles. Modulation of PMCA4 yielded a more prominent increase of SOCE in memory than in naïve CD4⁺ T cell. Interestingly, downregulation of PMCA4 reduced the effector compartment fraction and led to accumulation of cells in the naïve compartment. In silico analysis and chromatin immunoprecipitation point towards Ying Yang 1 (YY1) as a transcription factor regulating PMCA4 expression. Analyses of PMCA and YY1 expression patterns following activation and of PMCA promoter activity following downregulation of YY1 highlight repressive role of YY1 on PMCA expression. Our findings show that PMCA4 adapts Ca²⁺ levels to cellular requirements during effector and quiescent phases and thereby represent a potential target to intervene with the outcome of the immune response.

The Crystal Structure of the Ca2+-ATPase 1 from Listeria monocytogenes reveals a Pump Primed for Dephosphorylation

Many bacteria export intracellular calcium using active transporters homologous to the sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA). Here we present three crystal structures of Ca²⁺-ATPase 1 from Listeria monocytogenes (LMCA1). Structures with BeF₃^- mimicking a phosphoenzyme state reveal a closed state, which is intermediate between the outward-open E2P and the proton-occluded E2-P* conformations known for SERCA. It suggests that LMCA1 in the E2P state is pre-organized for dephosphorylation upon Ca²⁺ release, consistent with the rapid dephosphorylation observed in single-molecule studies. An arginine side-chain occupies the position equivalent to calcium binding site I in SERCA, leaving a single Ca²⁺ binding site in LMCA1, corresponding to SERCA site II. Observing no putative transport pathways dedicated to protons, we infer a direct proton counter transport through the Ca²⁺ exchange pathways. The LMCA1 structures provide insight into the evolutionary divergence and conserved features of this important class of ion transporters.

Ion Transporters and Osmoregulation in the Kidney of Teleost Fishes as a Function of Salinity

Euryhaline teleosts exhibit major changes in renal function as they move between freshwater (FW) and seawater (SW) environments, thus tolerating large fluctuations in salinity. In FW, the kidney excretes large volumes of water through high glomerular filtration rates (GFR) and low tubular reabsorption rates, while actively reabsorbing most ions at high rates. The excreted product has a high urine flow rate (UFR) with a dilute composition. In SW, GFR is greatly reduced, and the tubules reabsorb as much water as possible, while actively secreting divalent ions. The excreted product has a low UFR, and is almost isosmotic to the blood plasma, with Mg²⁺, SO₄^2–, and Cl^– as the major ionic components. Early studies at the organismal level have described these basic patterns, while in the last two decades, studies of regulation at the cell and molecular level have been implemented, though only in a few euryhaline groups (salmonids, eels, tilapias, and fugus). There have been few studies combining the two approaches. The aim of the review is to integrate known aspects of renal physiology (reabsorption and secretion) with more recent advances in molecular water and solute physiology (gene and protein function of transporters). The renal transporters addressed include the subunits of the Na⁺, K⁺- ATPase (NKA) enzyme, monovalent ion transporters for Na⁺, Cl^–, and K⁺ (NKCC1, NKCC2, CLC-K, NCC, ROMK2), water transport pathways [aquaporins (AQP), claudins (CLDN)], and divalent ion transporters for SO₄^2–, Mg²⁺, and Ca²⁺ (SLC26A6, SLC26A1, SLC13A1, SLC41A1, CNNM2, CNNM3, NCX1, NCX2, PMCA). For each transport category, we address the current understanding at the molecular level, try to synthesize it with classical knowledge of overall renal function, and highlight knowledge gaps. Future research on the kidney of euryhaline fishes should focus on integrating changes in kidney reabsorption and secretion of ions with changes in transporter function at the cellular and molecular level (gene and protein verification) in different regions of the nephrons. An increased focus on the kidney individually and its functional integration with the other osmoregulatory organs (gills, skin and intestine) in maintaining overall homeostasis will have applied relevance for aquaculture.

Neuroplastin expression is essential for hearing and hair cell PMCA expression

Hearing deficits impact on the communication with the external world and severely compromise perception of the surrounding. Deafness can be caused by particular mutations in the neuroplastin (Nptn) gene, which encodes a transmembrane recognition molecule of the immunoglobulin (Ig) superfamily and plasma membrane Calcium ATPase (PMCA) accessory subunit. This study investigates whether the complete absence of neuroplastin or the loss of neuroplastin in the adult after normal development lead to hearing impairment in mice analyzed by behavioral, electrophysiological, and in vivo imaging measurements. Auditory brainstem recordings from adult neuroplastin-deficient mice (Nptn^-/-) show that these mice are deaf. With age, hair cells and spiral ganglion cells degenerate in Nptn^-/- mice. Adult Nptn^-/- mice fail to behaviorally respond to white noise and show reduced baseline blood flow in the auditory cortex (AC) as revealed by single-photon emission computed tomography (SPECT). In adult Nptn^-/- mice, tone-evoked cortical activity was not detectable within the primary auditory field (A1) of the AC, although we observed non-persistent tone-like evoked activities in electrophysiological recordings of some young Nptn^-/- mice. Conditional ablation of neuroplastin in Nptn^{lox/loxEmx1Cre} mice reveals that behavioral responses to simple tones or white noise do not require neuroplastin expression by central glutamatergic neurons. Loss of neuroplastin from hair cells in adult Nptn^{Δlox/loxPrCreERT} mice after normal development is correlated with increased hearing thresholds and only high prepulse intensities result in effective prepulse inhibition (PPI) of the startle response. Furthermore, we show that neuroplastin is required for the expression of PMCA 2 in outer hair cells. This suggests that altered Ca²⁺ homeostasis underlies the observed hearing impairments and leads to hair cell degeneration. Our results underline the importance of neuroplastin for the development and the maintenance of the auditory system.

An extended catalogue of tandem alternative splice sites in human tissue transcriptomes

Tandem alternative splice sites (TASS) is a special class of alternative splicing events that are characterized by a close tandem arrangement of splice sites. Most TASS lack functional characterization and are believed to arise from splicing noise. Based on the RNA-seq data from the Genotype Tissue Expression project, we present an extended catalogue of TASS in healthy human tissues and analyze their tissue-specific expression. The expression of TASS is usually dominated by one major splice site (maSS), while the expression of minor splice sites (miSS) is at least an order of magnitude lower. Among 46k miSS with sufficient read support, 9k (20%) are significantly expressed above the expected noise level, and among them 2.5k are expressed tissue-specifically. We found significant correlations between tissue-specific expression of RNA-binding proteins (RBP), tissue-specific expression of miSS, and miSS response to RBP inactivation by shRNA. In combination with RBP profiling by eCLIP, this allowed prediction of novel cases of tissue-specific splicing regulation including a miSS in QKI mRNA that is likely regulated by PTBP1. The analysis of human primary cell transcriptomes suggested that both tissue-specific and cell-type-specific factors contribute to the regulation of miSS expression. More than 20% of tissue-specific miSS affect structured protein regions and may adjust protein-protein interactions or modify the stability of the protein core. The significantly expressed miSS evolve under the same selection pressure as maSS, while other miSS lack signatures of evolutionary selection and conservation. Using mixture models, we estimated that not more than 15% of maSS and not more than 54% of tissue-specific miSS are noisy, while the proportion of noisy splice sites among non-significantly expressed miSS is above 63%.

Pre-mRNA splicing is an important step in the processing of the genomic information during gene expression. During splicing, introns are excised from a gene transcript, and the remaining exons are ligated. Our work concerns one its particular subtype, which involves the so-called tandem alternative splice sites, a group of closely located exon borders that are used alternatively. We analyzed RNA-seq measurements of gene expression provided by the Genotype-Tissue Expression (GTEx) project, the largest to-date collection of such measurements in healthy human tissues, and constructed a detailed catalogue of tandem alternative splice sites. Within this catalogue, we characterized patterns of tissue-specific expression, regulation, impact on protein structure, and evolutionary selection acting on tandem alternative splice sites. In a number of genes, we predicted regulatory mechanisms that could be responsible for choosing one of many tandem alternative splice sites. The results of this study provide an invaluable resource for molecular biologists studying alternative splicing.

Basigin drives intracellular accumulation of l-lactate by harvesting protons and substrate anions

Transmembrane transport of l-lactate by members of the monocarboxylate transporter family, MCT, is vital in human physiology and a malignancy factor in cancer. Interaction with an accessory protein, typically basigin, is required to deliver the MCT to the plasma membrane. It is unknown whether basigin additionally exerts direct effects on the transmembrane l-lactate transport of MCT1. Here, we show that the presence of basigin leads to an intracellular accumulation of l-lactate 4.5-fold above the substrate/proton concentrations provided by the external buffer. Using basigin truncations we localized the effect to arise from the extracellular Ig-I domain. Identification of surface patches of condensed opposite electrostatic potential, and experimental analysis of charge-affecting Ig-I mutants indicated a bivalent harvesting antenna functionality for both, protons and substrate anions. From these data, and determinations of the cytosolic pH with a fluorescent probe, we conclude that the basigin Ig-I domain drives lactate uptake by locally increasing the proton and substrate concentration at the extracellular MCT entry site. The biophysical properties are physiologically relevant as cell growth on lactate media was strongly promoted in the presence of the Ig-I domain. Lack of the domain due to shedding, or misfolding due to breakage of a stabilizing disulfide bridge reversed the effect. Tumor progression according to classical or reverse Warburg effects depends on the transmembrane l-lactate distribution, and this study shows that the basigin Ig-I domain is a pivotal determinant.

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

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