Loss of the chloride channel ClC-7 leads to lysosomal storage disease and neurodegeneration

Regulatory-auxiliary subunits of CLC chloride channel-transport proteins

The CLC family of chloride channels and transporters is composed by nine members, but only three of them, ClC-Ka/b, ClC-7 and ClC-2, have been found so far associated with auxiliary subunits. These CLC regulatory subunits are small proteins that present few common characteristics among them, both structurally and functionally, and their effects on the corresponding CLC protein are different. Barttin, a protein with two transmembrane domains, is essential for the membrane localization of ClC-K proteins and their activity in the kidney and inner ear. Ostm1 is a protein with a single transmembrane domain and a highly glycosylated N-terminus. Unlike the other two CLC auxiliary subunits, Ostm1 shows a reciprocal relationship with ClC-7 for their stability. The subcellular localization of Ostm1 depends on ClC-7 and not the other way around. ClC-2 is active on its own, but GlialCAM, a transmembrane cell adhesion molecule with two extracellular immunoglobulin (Ig)-like domains, regulates its subcellular localization and activity in glial cells. The common theme for these three proteins is their requirement for a proper homeostasis, since their malfunction leads to distinct diseases. We will review here their properties and their role in normal chloride physiology and the pathological consequences of their improper function.

Therapeutic Approaches to Genetic Ion Channelopathies and Perspectives in Drug Discovery

In the human genome more than 400 genes encode ion channels, which are transmembrane proteins mediating ion fluxes across membranes. Being expressed in all cell types, they are involved in almost all physiological processes, including sense perception, neurotransmission, muscle contraction, secretion, immune response, cell proliferation, and differentiation. Due to the widespread tissue distribution of ion channels and their physiological functions, mutations in genes encoding ion channel subunits, or their interacting proteins, are responsible for inherited ion channelopathies. These diseases can range from common to very rare disorders and their severity can be mild, disabling, or life-threatening. In spite of this, ion channels are the primary target of only about 5% of the marketed drugs suggesting their potential in drug discovery. The current review summarizes the therapeutic management of the principal ion channelopathies of central and peripheral nervous system, heart, kidney, bone, skeletal muscle and pancreas, resulting from mutations in calcium, sodium, potassium, and chloride ion channels. For most channelopathies the therapy is mainly empirical and symptomatic, often limited by lack of efficacy and tolerability for a significant number of patients. Other channelopathies can exploit ion channel targeted drugs, such as marketed sodium channel blockers. Developing new and more specific therapeutic approaches is therefore required. To this aim, a major advancement in the pharmacotherapy of channelopathies has been the discovery that ion channel mutations lead to change in biophysics that can in turn specifically modify the sensitivity to drugs: this opens the way to a pharmacogenetics strategy, allowing the development of a personalized therapy with increased efficacy and reduced side effects. In addition, the identification of disease modifiers in ion channelopathies appears an alternative strategy to discover novel druggable targets.

Anion homeostasis is important for non-lytic release of BK polyomavirus from infected cells

BK polyomavirus (BKPyV) is a member of a family of potentially oncogenic viruses, whose reactivation can cause severe pathological conditions in transplant patients, leading to graft rejection. As with many non-enveloped viruses, it is assumed that virus release occurs through lysis of the host cell. We now show the first evidence for a non-lytic release pathway for BKPyV and that this pathway can be blocked by the anion channel inhibitor DIDS. Our data show a dose-dependent effect of DIDS on the release of BKPyV virions. We also observed an accumulation of viral capsids in large LAMP-1-positive acidic organelles within the cytoplasm of cells upon DIDS treatment, suggesting potential late endosome or lysosome-related compartments are involved in non-lytic BKPyV release. These data highlight a novel mechanism by which polyomaviruses can be released from infected cells in an active and non-lytic manner, and that anion homeostasis regulation is important in this pathway.

Lysosomal dysfunction and impaired autophagy in a novel mouse model deficient for the lysosomal membrane protein Cln7

CLN7 disease is an autosomal recessive, childhood-onset neurodegenerative lysosomal storage disorder caused by the defective lysosomal membrane protein CLN7. We have disrupted the Cln7/Mfsd8 gene in mice by targeted deletion of exon 2 generating a novel knockout (KO) mouse model for CLN7 disease, which recapitulates key features of human CLN7 disease pathology. Cln7 KO mice showed increased mortality and a neurological phenotype including hind limb clasping and myoclonus. Lysosomal dysfunction in the brain of mutant mice was shown by the storage of autofluorescent lipofuscin-like lipopigments, subunit c of mitochondrial ATP synthase and saposin D and increased expression of lysosomal cathepsins B, D and Z. By immunohistochemical co-stainings, increased cathepsin Z expression restricted to Cln7-deficient microglia and neurons was found. Ultrastructural analyses revealed large storage bodies in Purkinje cells of Cln7 KO mice containing inclusions composed of irregular, curvilinear and rectilinear profiles as well as fingerprint profiles. Generalized astrogliosis and microgliosis in the brain preceded neurodegeneration in the olfactory bulb, cerebral cortex and cerebellum in Cln7 KO mice. Increased levels of LC3-II and the presence of neuronal p62- and ubiquitin-positive protein aggregates suggested that impaired autophagy represents a major pathomechanism in the brain of Cln7 KO mice. The data suggest that loss of the putative lysosomal transporter Cln7 in the brain leads to lysosomal dysfunction, impaired constitutive autophagy and neurodegeneration late in disease.

ClC-7 Deficiency Impairs Tooth Development and Eruption

CLCN7 gene encodes the voltage gated chloride channel 7 (ClC-7) in humans. The mutations in CLCN7 have been associated with osteopetrosis in connection to the abnormal osteoclasts functions. Previously, we found that some osteopetrosis patients with CLCN7 mutations suffered from impacted teeth and root dysplasia. Here we set up two in vivo models under a normal or an osteoclast-poor environment to investigate how ClC-7 affects tooth development and tooth eruption. Firstly, chitosan-Clcn7-siRNA nanoparticles were injected around the first maxillary molar germ of newborn mice and caused the delay of tooth eruption and deformed tooth with root dysplasia. Secondly, E13.5 molar germs infected with Clcn7 shRNA lentivirus were transplanted under the kidney capsule and presented the abnormal changes in dentin structure, periodontal tissue and cementum. All these teeth changes have been reported in the patients with CLCN7 mutation. In vitro studies of ameloblasts, odontoblasts and dental follicle cells (DFCs) were conducted to explore the involved mechanism. We found that Clcn7 deficiency affect the differentiation of these cells, as well as the interaction between DFCs and osteoclasts through RANKL/OPG pathway. We conclude that ClC-7 may affect tooth development by directly targeting tooth cells, and regulate tooth eruption through DFC mediated osteoclast pathway.

X-exome sequencing of 405 unresolved families identifies seven novel intellectual disability genes

X-linked intellectual disability (XLID) is a clinically and genetically heterogeneous disorder. During the past two decades in excess of 100 X-chromosome ID genes have been identified. Yet, a large number of families mapping to the X-chromosome remained unresolved suggesting that more XLID genes or loci are yet to be identified. Here, we have investigated 405 unresolved families with XLID. We employed massively parallel sequencing of all X-chromosome exons in the index males. The majority of these males were previously tested negative for copy number variations and for mutations in a subset of known XLID genes by Sanger sequencing. In total, 745 X-chromosomal genes were screened. After stringent filtering, a total of 1297 non-recurrent exonic variants remained for prioritization. Co-segregation analysis of potential clinically relevant changes revealed that 80 families (20%) carried pathogenic variants in established XLID genes. In 19 families, we detected likely causative protein truncating and missense variants in 7 novel and validated XLID genes (CLCN4, CNKSR2, FRMPD4, KLHL15, LAS1L, RLIM and USP27X) and potentially deleterious variants in 2 novel candidate XLID genes (CDK16 and TAF1). We show that the CLCN4 and CNKSR2 variants impair protein functions as indicated by electrophysiological studies and altered differentiation of cultured primary neurons from Clcn4(-/-) mice or after mRNA knock-down. The newly identified and candidate XLID proteins belong to pathways and networks with established roles in cognitive function and intellectual disability in particular. We suggest that systematic sequencing of all X-chromosomal genes in a cohort of patients with genetic evidence for X-chromosome locus involvement may resolve up to 58% of Fragile X-negative cases.

Null mutation of chloride channel 7 (Clcn7) impairs dental root formation but does not affect enamel mineralization

ClC-7, located in late endosomes and lysosomes, is critical for the function of osteoclasts. Secretion of Cl(-) by the ruffled border of osteoclasts enables H(+) secretion by v-H(+)-ATPases to dissolve bone mineral. Mice lacking ClC-7 show altered lysosomal function that leads to severe lysosomal storage. Maturation ameloblasts are epithelial cells with a ruffled border that secrete Cl(-) as well as endocytose and digest large quantities of enamel matrix proteins during formation of dental enamel. We tested the hypothesis that ClC-7 in maturation ameloblasts is required for intracellular digestion of matrix fragments to complete enamel mineralization. Craniofacial bones and developing teeth in Clcn7(-/-) mice were examined by micro-CT, immunohistochemistry, quantified histomorphometry and electron microscopy. Osteoclasts and ameloblasts in wild-type mice stained intensely with anti-ClC-7 antibody but not in Clcn7(-/-) mice. Craniofacial bones in Clcn7(-/-) mice were severely osteopetrotic and contained 1.4- to 1.6-fold more bone volume, which was less mineralized than the wild-type littermates. In Clcn7(-/-) mice maturation ameloblasts and osteoclasts highly expressed Ae2 as in wild-type mice. However, teeth failed to erupt, incisors were much shorter and roots were disfigured. Molars formed a normal dental crown. In compacted teeth, dentin was slightly less mineralized, enamel did not retain a matrix and mineralized fairly normal. We concluded that ClC-7 is essential for osteoclasts to resorb craniofacial bones to enable tooth eruption and root development. Disruption of Clcn7 reduces bone and dentin mineral density but does not affect enamel mineralization.

Zinc-Induced Effects on Osteoclastogenesis Involves Activation of Hyperpolarization-Activated Cyclic Nucleotide Modulated Channels via Changes in Membrane Potential

Zinc is a trace element in the mammalian body, and increasing evidence shows its critical role in bone development and osteoclastogenesis. The relationships between zinc and voltage-gated ion channels have been reported; however, the effects of zinc on membrane potential and the related ion channels remain unknown. In this study, we found that zinc-induced hyperpolarization in RAW264.7 cells (RAW) was promoted by inhibition of hyperpolarization-activated cyclic nucleotide modulated channels (HCNs). In electrophysiological experiments with RAW-derived osteoclasts, HCNs were functional and generated hyperpolarization-activated inward currents (Ih) with properties similar to the Ih recorded in excitable cells such as neurons and cardiomyocytes. Quantitative PCR of HCN subunits HCN1 and HCN4 in RAW cells showed detectable levels of HCN1 mRNA and HCN4 expression was the highest of all four subunits. HCN4 knockdown decreased osteoclastic Ih and promoted osteoclastogenesis in the presence of zinc, but not in the absence of zinc. To determine the effect of membrane hyperpolarization on osteoclastogenesis, we developed a light-controllable membrane potential system in RAW cells by stably expressing the light-driven outward proton pump, Archaerhodopsin3 (Arch). Arch activation by yellow-green light hyperpolarizes the cell membrane. Light-induced hyperpolarization accelerated osteoclast differentiation in the presence of receptor activator of nuclear factor kappa-B ligand (RANKL). Thus, HCN activation reduced the hyperpolarization-related promotion of osteoclast differentiation in the presence of zinc. This study revealed the novel role of HCN and membrane potential in non-excitable osteoclasts.

Discovery of CLC transport proteins: cloning, structure, function and pathophysiology

After providing a personal description of the convoluted path leading 25 years ago to the molecular identification of the Torpedo Cl(-) channel ClC-0 and the discovery of the CLC gene family, I succinctly describe the general structural and functional features of these ion transporters before giving a short overview of mammalian CLCs. These can be categorized into plasma membrane Cl(-) channels and vesicular Cl(-) /H(+) -exchangers. They are involved in the regulation of membrane excitability, transepithelial transport, extracellular ion homeostasis, endocytosis and lysosomal function. Diseases caused by CLC dysfunction include myotonia, neurodegeneration, deafness, blindness, leukodystrophy, male infertility, renal salt loss, kidney stones and osteopetrosis, revealing a surprisingly broad spectrum of biological roles for chloride transport that was unsuspected when I set out to clone the first voltage-gated chloride channel.

A pH-independent DNA nanodevice for quantifying chloride transport in organelles of living cells

The concentration of chloride ions in the cytoplasm and subcellular organelles of living cells spans a wide range (5-130 mM), and is tightly regulated by intracellular chloride channels or transporters. Chloride-sensitive protein reporters have been used to study the role of these chloride regulators, but they are limited to a small range of chloride concentrations and are pH-sensitive. Here, we show that a DNA nanodevice can precisely measure the activity and location of subcellular chloride channels and transporters in living cells in a pH-independent manner. The DNA nanodevice, called Clensor, is composed of sensing, normalizing and targeting modules, and is designed to localize within organelles along the endolysosomal pathway. It allows fluorescent, ratiometric sensing of chloride ions across the entire physiological regime. We used Clensor to quantitate the resting chloride concentration in the lumen of acidic organelles in Drosophila melanogaster. We showed that lumenal lysosomal chloride, which is implicated in various lysosomal storage diseases, is regulated by the intracellular chloride transporter DmClC-b.

A tale of two CLCs: biophysical insights toward understanding ClC-5 and ClC-7 function in endosomes and lysosomes

The CLC protein family comprises both Cl(-) channels and H(+) -coupled anion transporters. The understanding of the critical role of CLC proteins in a number of physiological functions has greatly contributed to a revision of the classical paradigm that attributed to Cl(-) ions only a marginal role in human physiology. The endosomal ClC-5 and the lysosomal ClC-7 are the best characterized human CLC transporters. Their dysfunction causes Dent's disease and osteopetrosis, respectively. It had been originally proposed that they would provide a Cl(-) shunt conductance allowing efficient acidification of intracellular compartments. However, this model seems to conflict with the transport properties of these proteins and with recent physiological evidence. Currently, there is no consensus on their specific physiological role. CLC proteins present also a number of peculiar biophysical properties, such as the dimeric architecture, the co-existence of intrinsically different thermodynamic modes of transport based on similar structural principles, and the gating mechanism recently emerging for the transporters, just to name a few. This review focuses on the biophysical properties and physiological roles of ClC-5 and ClC-7.

Salt, chloride, bleach, and innate host defense

Salt provides 2 life-essential elements: sodium and chlorine. Chloride, the ionic form of chlorine, derived exclusively from dietary absorption and constituting the most abundant anion in the human body, plays critical roles in many vital physiologic functions, from fluid retention and secretion to osmotic maintenance and pH balance. However, an often overlooked role of chloride is its function in innate host defense against infection. Chloride serves as a substrate for the generation of the potent microbicide chlorine bleach by stimulated neutrophils and also contributes to regulation of ionic homeostasis for optimal antimicrobial activity within phagosomes. An inadequate supply of chloride to phagocytes and their phagosomes, such as in CF disease and other chloride channel disorders, severely compromises host defense against infection. We provide an overview of the roles that chloride plays in normal innate immunity, highlighting specific links between defective chloride channel function and failures in host defense.

Chloride transporters and receptor-mediated endocytosis in the renal proximal tubule

KEY POINTS

The reabsorptive activity of renal proximal tubule cells is mediated by receptor-mediated endocytosis and polarized transport systems that reflect final cell differentiation. Loss-of-function mutations of the endosomal chloride-proton exchanger ClC-5 (Dent's disease) cause a major trafficking defect in proximal tubule cells, associated with lysosomal dysfunction, oxidative stress and dedifferentiation/proliferation. A similar but milder defect is associated with mutations in CFTR (cystic fibrosis transmembrane conductance regulator). Vesicular chloride transport appears to be important for the integrity of the endolysosomal pathway in epithelial cells.

ABSTRACT

The epithelial cells lining the proximal tubules of the kidney reabsorb a large amount of filtered ions and solutes owing to receptor-mediated endocytosis and polarized transport systems that reflect final cell differentiation. Dedifferentiation of proximal tubule cells and dysfunction of receptor-mediated endocytosis characterize Dent's disease, a rare disorder caused by inactivating mutations in the CLCN5 gene that encodes the endosomal chloride-proton exchanger, ClC-5. The disease is characterized by a massive urinary loss of solutes (renal Fanconi syndrome), with severe metabolic complications and progressive renal failure. Investigations of mutations affecting the gating of ClC-5 revealed that the proximal tubule dysfunction may occur despite normal endosomal acidification. In addition to defective endocytosis, proximal tubule cells lacking ClC-5 show a trafficking defect in apical receptors and transporters, as well as lysosomal dysfunction and typical features of dedifferentiation, proliferation and oxidative stress. A similar but milder defect is observed in mouse models with defective CFTR, a chloride channel that is also expressed in the endosomes of proximal tubule cells. These data suggest a major role for endosomal chloride transport in the maintenance of epithelial differentiation and reabsorption capacity of the renal proximal tubule.

Novel targets for the prevention of osteoporosis - lessons learned from studies of metabolic bone disorders

INTRODUCTION

Osteoporosis is a major health care problem, and whereas efficacious treatments for vertebral fracture reduction are available for osteoporosis patients, these therapies are still limited with respect to capacity for restoration of bone loss, as well as efficacy on non-vertebral fractures, such as hip fractures, which are the source of morbidity and mortality.

AREAS COVERED

Studies of rare bone diseases in humans, such as osteopetrosis, sclerosteosis, pycnodysostosis and more, have shed light on a series of drug targets in bone that have the potential to result in therapies for osteoporosis with novel mechanisms of action, and the potential to improve the standard of care substantially. We focus on how they are separated from classic treatments for osteoporosis, in terms of novel modes of action, additional beneficial effects on bone turnover and importantly also safety. We focus on the status of anti-sclerostin antibodies, novel parathyroid hormone-related protein analogs, inhibitors of cathepsin K and ClC-7 in osteoclasts, all of which are currently in development.

EXPERT OPINION

There is a good possibility that the treatment of osteoporosis will be greatly improved within the coming years; however, with numerous effective and safe drugs already available careful attention to the safety of these novel candidates is crucial.

The neuronal ceroid lipofuscinoses: Opportunities from model systems

The neuronal ceroid lipofuscinoses are a group of severe and progressive neurodegenerative disorders, generally with childhood onset. Despite the fact that these diseases remain fatal, significant breakthroughs have been made in our understanding of the genetics that underpin these conditions. This understanding has allowed the development of a broad range of models to study disease processes, and to develop new therapeutic approaches. Such models have contributed significantly to our knowledge of these conditions. In this review we will focus on the advantages of each individual model, describe some of the contributions the models have made to our understanding of the broader disease biology and highlight new techniques and approaches relevant to the study and potential treatment of the neuronal ceroid lipofuscinoses. This article is part of a Special Issue entitled: "Current Research on the Neuronal Ceroid Lipofuscinoses (Batten Disease)".

Loss of Clcc1 results in ER stress, misfolded protein accumulation, and neurodegeneration

Folding of transmembrane and secretory proteins occurs in the lumen of the endoplasmic reticulum (ER) before transportation to the cell surface and is monitored by the unfolded protein response (UPR) signaling pathway. The accumulation of unfolded proteins in the ER activates the UPR that restores ER homeostasis by regulating gene expression that leads to an increase in the protein-folding capacity of the ER and a decrease in the ER protein-folding load. However, prolonged UPR activity has been associated with cell death in multiple pathological conditions, including neurodegeneration. Here, we report a spontaneous recessive mouse mutation that causes progressive cerebellar granule cell death and peripheral motor axon degeneration. By positional cloning, we identify the mutation in this strain as a retrotransposon insertion in the Clcc1 gene, which encodes a putative chloride channel localized to the ER. Furthermore, we demonstrate that the C3H/HeSnJ inbred strain has late onset cerebellar degeneration due to this mutation. Interestingly, acute knockdown of Clcc1 expression in cultured cells increases sensitivity to ER stress. In agreement, GRP78, the major HSP70 family chaperone in the ER, is upregulated in Clcc1-deficient granule cells in vivo, and ubiquitinated proteins accumulate in these neurons before their degeneration. These data suggest that disruption of chloride homeostasis in the ER disrupts the protein-folding capacity of the ER, leading to eventual neuron death.

Functional characterization of Na+/H+ exchangers of intracellular compartments using proton-killing selection to express them at the plasma membrane

Endosomal acidification is critical for a wide range of processes, such as protein recycling and degradation, receptor desensitization, and neurotransmitter loading in synaptic vesicles. This acidification is described to be mediated by proton ATPases, coupled to ClC chloride transporters. Highly-conserved electroneutral protons transporters, the Na+/H+ exchangers (NHE) 6, 7 and 9 are also expressed in these compartments. Mutations in their genes have been linked with human cognitive and neurodegenerative diseases. Paradoxically, their roles remain elusive, as their intracellular localization has prevented detailed functional characterization. This manuscript shows a method to solve this problem. This consists of the selection of mutant cell lines, capable of surviving acute cytosolic acidification by retaining intracellular NHEs at the plasma membrane. It then depicts two complementary protocols to measure the ion selectivity and activity of these exchangers: (i) one based on intracellular pH measurements using fluorescence video microscopy, and (ii) one based on the fast kinetics of lithium uptake. Such protocols can be extrapolated to measure other non-electrogenic transporters. Furthermore, the selection procedure presented here generates cells with an intracellular retention defective phenotype. Therefore these cells will also express other vesicular membrane proteins at the plasma membrane. The experimental strategy depicted here may therefore constitute a potentially powerful tool to study other intracellular proteins that will be then expressed at the plasma membrane together with the vesicular Na+/H+ exchangers used for the selection.

Dental and Cranial Pathologies in Mice Lacking the Cl(-) /H(+) -Exchanger ClC-7

ClC-7 is a 2Cl(-) /1H(+) -exchanger expressed at late endosomes and lysosomes, as well as the ruffled border of osteoclasts. ClC-7 deficiencies in mice and humans lead to impaired osteoclast function and therefore osteopetrosis. Failure of tooth eruption is also apparent in ClC-7 mutant animals, and this has been attributed to the osteoclast dysfunction and the subsequent defect in alveolar bone resorptive activity surrounding tooth roots. Ameloblasts also express ClC-7, and this study aims to determine the significance of ClC-7 in enamel formation by examining the dentitions of ClC-7 mutant mice. Micro-CT analysis revealed that the molar teeth of 3-week old ClC-7 mutant mice had no roots, and the incisors were smaller than their age-matched controls. Despite these notable developmental differences, the enamel and dentin densities of the mutant mice were comparable to those of the wild-type littermates. Scanning electron microscopy showed normal enamel crystallite and prismatic organization in the ClC-7 mutant mice, although the enamel was thinner (hypoplastic) than in controls. These results suggested that ClC-7 was not critical to enamel and dentin formation, and the observed tooth defects may be related more to a resulting alveolar bone phenotype. Micro-CT analysis also revealed abnormal features in the calvarial bones of the mutant mice. The cranial sutures in ClC-7 mutant mice remained open compared to the closed sutures seen in the control mice at 3 weeks. These data demonstrate that ClC-7 deficiency impacts the development of the dentition and calvaria, but does not significantly disrupt amelogenesis.

LAMP-2 deficiency leads to hippocampal dysfunction but normal clearance of neuronal substrates of chaperone-mediated autophagy in a mouse model for Danon disease

The Lysosomal Associated Membrane Protein type-2 (LAMP-2) is an abundant lysosomal membrane protein with an important role in immunity, macroautophagy (MA) and chaperone-mediated autophagy (CMA). Mutations within the Lamp2 gene cause Danon disease, an X-linked lysosomal storage disorder characterized by (cardio)myopathy and intellectual dysfunction. The pathological hallmark of this disease is an accumulation of glycogen and autophagic vacuoles in cardiac and skeletal muscle that, along with the myopathy, is also present in LAMP-2-deficient mice. Intellectual dysfunction observed in the human disease suggests a pivotal role of LAMP-2 within brain. LAMP-2A, one specific LAMP-2 isoform, was proposed to be important for the lysosomal degradation of selective proteins involved in neurodegenerative diseases such as Huntington’s and Parkinson’s disease.

To elucidate the neuronal function of LAMP-2 we analyzed knockout mice for neuropathological changes, MA and steady-state levels of CMA substrates. The absence of LAMP-2 in murine brain led to inflammation and abnormal behavior, including motor deficits and impaired learning. The latter abnormality points to hippocampal dysfunction caused by altered lysosomal activity, distinct accumulation of p62-positive aggregates, autophagic vacuoles and lipid storage within hippocampal neurons and their presynaptic terminals. The absence of LAMP-2 did not apparently affect MA or steady-state levels of selected CMA substrates in brain or neuroblastoma cells under physiological and prolonged starvation conditions.

Our data contribute to the understanding of intellectual dysfunction observed in Danon disease patients and highlight the role of LAMP-2 within the central nervous system, particularly the hippocampus.

Approaches for detecting lysosomal alkalinization and impaired degradation in fresh and cultured RPE cells: evidence for a role in retinal degenerations

Lysosomes contribute to a multitude of cellular processes, and the pH of the lysosomal lumen plays a central mechanistic role in many of these functions. In addition to controlling the rate of enzymatic degradation for material delivered through autophagic or phagocytotic pathways, lysosomal pH regulates events such as lysosomal fusion with autophagosomes and the release of lysosomal calcium into the cytoplasm. Disruption of either the steady state lysosomal pH or of the regulated manipulations to lysosomal pH may be pathological. For example, chloroquine elevates the lysosomal pH of retinal pigmented epithelial (RPE) cells and triggers a retinopathy characterized by the accumulation of lipofuscin-like material in both humans and animals. Compensatory responses to restore lysosomal pH are observed; new data illustrate that chronic chloroquine treatment increases mRNA expression of the lysosomal/autophagy master transcription factor TcFEB and of the vesicular proton pump vHATPase in the RPE/choroid of mice. An elevated lysosomal pH with upregulation of TcFEB and vHATPase resembles the pathology in fibroblasts of patients with mutant presenilin 1 (PS1), suggesting a common link between age-related macular degeneration (AMD) and Alzheimer's disease. While the absolute rise in pH is often small in these disorders, elevations of only a few tenths of a pH unit can have a major impact on both lysosomal function and the accumulation of waste over decades. Accurate measurement of lysosomal pH can be complex, and imprecise measurements have clouded the field. Protocols to optimize pH measurement from fresh and cultured cells are discussed, and indirect measurements to confirm changes in lysosomal pH and degradative capacity are addressed. The ability of reacidifying treatments to restore degradative function confirms the central role of lysosomal pH in these disorders and identifies potential approaches to treat diseases of lysosomal accumulation like AMD and Alzheimer's disease. In summary, various approaches to determine lysosomal pH in fresh and cultured cells, as well as the potential to restore pH levels to an optimal range, can help identify and repair pathologies associated with lysosomal defects in RPE cells and perhaps also suggest new approaches to treat lysosomal storage diseases throughout the body.

Transport activity and presence of ClC-7/Ostm1 complex account for different cellular functions

Loss of the lysosomal ClC-7/Ostm1 2Cl(-)/H(+) exchanger causes lysosomal storage disease and osteopetrosis in humans and additionally changes fur colour in mice. Its conversion into a Cl(-) conductance in Clcn7(unc/unc) mice entails similarly severe lysosomal storage, but less severe osteopetrosis and no change in fur colour. To elucidate the basis for these phenotypical differences, we generated Clcn7(td/td) mice expressing an ion transport-deficient mutant. Their osteopetrosis was as severe as in Clcn7(-/-) mice, suggesting that the electric shunt provided by ClC-7(unc) can partially rescue osteoclast function. The normal coat colour of Clcn7(td/td) mice and their less severe neurodegeneration suggested that the ClC-7 protein, even when lacking measurable ion transport activity, is sufficient for hair pigmentation and that the conductance of ClC-7(unc) is harmful for neurons. Our in vivo structure-function analysis of ClC-7 reveals that both protein-protein interactions and ion transport must be considered in the pathogenesis of ClC-7-related diseases.

Emerging therapeutic targets for osteoporosis treatment

INTRODUCTION

To date, osteoporosis still remains a major public health burden especially for the aging populations. Over the last few decades treatments for osteoporosis have largely focused on anti-resorptive agents represented by bisphosphonates and estrogen therapy that dominated the market. Unsatisfactory efficacy, non-specificity and long-term safety of current therapies necessitate the need for new targets effectively preventing and treating of osteoporosis.

AREAS COVERED

This review expatiates on the mechanism of osteoporosis occurrence and bone remodeling cycle in detail. New targets of antiresorptive and anabolic agents based on the functions of osteoblasts and osteoclasts as well as associated signaling pathways are outlined.

EXPERT OPINION

Advanced understanding in the fields of bone remodeling, functions of osteoblasts, osteoclasts and osteocytes associated with osteoporosis occurrence offers the emerging bone-resorptive or bone-formative targets. Currently, molecules involving RANK-RANKL-OPG system and Wnt/β-catenin signaling pathway act as the most promising targets.

Severe neurodegeneration with impaired autophagy mechanism triggered by ostm1 deficiency

Loss of Ostm1 leads to the most severe form of osteopetrosis in mice and humans. Because functional rescue of the osteopetrotic defect in these mice extended their lifespan from ∼3 weeks to 6 weeks, this unraveled a second essential role of Ostm1. We discovered that Ostm1 is highly expressed in the mouse brain in neurons, microglia, and astrocytes. At 3-4 weeks of age, mice with Ostm1 loss showed 3-10-fold stimulation of reactive gliosis, with an increased astrocyte cell population and microglia activation. This inflammatory response was associated with marked retinal photoreceptor degeneration and massive neuronal loss in the brain. Intracellular characterization of neurons revealed abnormal storage of carbohydrates, lipids, and ubiquitinated proteins, combined with marked accumulation of autophagosomes that causes frequent axonal swelling. Stimulation of autophagy was provided by specific markers and by significant down-regulation of the mammalian target of rapamycin signaling, identifying a cellular pathologic mechanism. A series of transgenic mouse lines specifically targeted to distinct central nervous system cell subpopulations determined that Ostm1 has a primary and autonomous role in neuronal homeostasis. Complete functional complementation demonstrated that the development of severe and rapid neurodegeneration in these mice is independent of the hematopoietic lineage and has clinical implications for treatment of osteopetrosis. Importantly, this study establishes a novel neurodegenerative mouse model critical for understanding the multistep pathogenic cascade of cellular autophagy disorders toward therapeutic strategy design.

Differentially expressed genes in autosomal dominant osteopetrosis type II osteoclasts reveal known and novel pathways for osteoclast biology

Autosomal dominant osteopetrosis type II (ADO II) is a rare, heritable bone disorder characterized by a high bone mass and insufficient osteoclast activity. Mutations in the CLCN7 gene have been reported to cause ADO II. To gain novel insights into the pathways dysregulated in ADOII osteoclasts, we identified changes in gene expression in osteoclasts from patients with a heterozygous mutation of CLCN7. To do this, we carried out a transcriptomic study comparing gene expression in the osteoclasts of patients with ADO II and healthy donors. Our data show that, according to our selection criteria, 182 genes were differentially expressed in osteoclasts from patients and controls. From the 18 displaying the highest change in microarray, we confirmed differential expression for seven by qPCR. Although two of them have previously been found to be expressed in osteoclasts (ITGB5 and SERPINE2), the other five (CES1 (carboxyl esterase 1), UCHL1 (ubiquitin carboxy-terminal esterase L1, also known as ubiquitin thiolesterase), WARS (tryptophanyl-tRNA synthetase), GBP4 (guanylate-binding protein 4), and PRF1) are not yet known to have a role in this cell type. At the protein level, we confirmed elevated expression of ITGB5 and reduced expression of WARS, PRF1, and SERPINE2. Transfection of ClC-7 harboring the G215R mutation into osteoclasts resulted in an increased ITGB5 and reduced PRF1 expression of borderline significance. Finally, we observed that the ADO II patients presented a normal or increased serum level of bone formation markers, demonstrating a coupling between dysfunctional osteoclasts and osteoblasts. Sphingosine kinase 1 mRNA was expressed at the same level in ADO II and control osteoclasts. In conclusion, these data suggest that in addition to an acidification dysfunction caused by the CLCN7 mutation, a change in ITGB5, PRF1, WARS, and SERPINE2 expression could be part of the osteoclastic phenotype of ADO II.

Generation of the first autosomal dominant osteopetrosis type II (ADO2) disease models

Autosomal dominant osteopetrosis type II (ADO2) is a heritable osteosclerotic disorder dependent on osteoclast impairment. In most patients it results from heterozygous missense mutations in the chloride channel 7 (CLCN7) gene, encoding for a 2Cl(-)/1H(+) antiporter. By a knock-in strategy inserting a missense mutation in the Clcn7 gene, our two research groups independently generated mouse models of ADO2 on different genetic backgrounds carrying the homolog of the most frequent heterozygous mutation (p.G213R) in the Clcn7 gene found in humans. Our results demonstrate that the heterozygous model holds true presenting with higher bone mass, increased numbers of poorly resorbing osteoclasts and a lethal phenotype in the homozygous state. Considerable variability is observed in the heterozygous mice according with the mouse background, suggesting that modifier genes could influence the penetrance of the disease gene.

A novel computational biostatistics approach implies impaired dephosphorylation of growth factor receptors as associated with severity of autism

The prevalence of autism spectrum disorders (ASDs) has increased 20-fold over the past 50 years to >1% of US children. Although twin studies attest to a high degree of heritability, the genetic risk factors are still poorly understood. We analyzed data from two independent populations using u-statistics for genetically structured wide-locus data and added data from unrelated controls to explore epistasis. To account for systematic, but disease-unrelated differences in (non-randomized) genome-wide association studies (GWAS), a correlation between P-values and minor allele frequency with low granularity data and for conducting multiple tests in overlapping genetic regions, we present a novel study-specific criterion for 'genome-wide significance'. From recent results in a comorbid disease, childhood absence epilepsy, we had hypothesized that axonal guidance and calcium signaling are involved in autism as well. Enrichment of the results in both studies with related genes confirms this hypothesis. Additional ASD-specific variations identified in this study suggest protracted growth factor signaling as causing more severe forms of ASD. Another cluster of related genes suggests chloride and potassium ion channels as additional ASD-specific drug targets. The involvement of growth factors suggests the time of accelerated neuronal growth and pruning at 9-24 months of age as the period during which treatment with ion channel modulators would be most effective in preventing progression to more severe forms of autism. By extension, the same computational biostatistics approach could yield profound insights into the etiology of many common diseases from the genetic data collected over the last decade.

Vacuolar H+-ATPase: An Essential Multitasking Enzyme in Physiology and Pathophysiology

Vacuolar H+-ATPases (V-ATPases) are large multisubunit proton pumps that are required for housekeeping acidification of membrane-bound compartments in eukaryotic cells. Mammalian V-ATPases are composed of 13 different subunits. Their housekeeping functions include acidifying endosomes, lysosomes, phagosomes, compartments for uncoupling receptors and ligands, autophagosomes, and elements of the Golgi apparatus. Specialized cells, including osteoclasts, intercalated cells in the kidney and pancreatic beta cells, contain both the housekeeping V-ATPases and an additional subset of V-ATPases, which plays a cell type specific role. The specialized V-ATPases are typically marked by the inclusion of cell type specific isoforms of one or more of the subunits. Three human diseases caused by mutations of isoforms of subunits have been identified. Cancer cells utilize V-ATPases in unusual ways; characterization of V-ATPases may lead to new therapeutic modalities for the treatment of cancer. Two accessory proteins to the V-ATPase have been identified that regulate the proton pump. One is the (pro)renin receptor and data is emerging that indicates that V-ATPase may be intimately linked to renin/angiotensin signaling both systemically and locally. In summary, V-ATPases play vital housekeeping roles in eukaryotic cells. Specialized versions of the pump are required by specific organ systems and are involved in diseases.

ClC-7 expression levels critically regulate bone turnover, but not gastric acid secretion

Mutations in the 2Cl(-)/1H(+)-exchanger ClC-7 impair osteoclast function and cause different types of osteoclast-rich osteopetrosis. However, it is unknown to what extent ClC-7 function has to be reduced to become rate-limiting for bone resorption. In osteoclasts from osteopetrosis patients expression of the mutated ClC-7 protein did not correlate with disease severity and resorption impairment. Therefore, a series of transgenic mice expressing ClC-7 in osteoclasts at different levels was generated. Crossing of these mice with Clcn7(-/-) mutants rescued the osteopetrotic phenotype to variable degrees. One resulting double transgenic line mimicked human autosomal dominant osteopetrosis. The trabecular bone of these mice showed a reduction of osteoblast numbers, osteoid, and osteoblast marker gene expression indicative of reduced osteoblast function. In osteoclasts from these mutants ClC-7 expression levels were 20 to 30% of wildtype levels. These reduced levels not only impaired resorptive activity, but also increased numbers, size and nucleus numbers of osteoclasts differentiated in vitro. Although ClC-7 was expressed in the stomach and PTH levels were high in Clcn7(-/-) mutants loss of ClC-7 did not entail a relevant elevation of gastric pH. In conclusion, we show that in our model a reduction of ClC-7 function by approximately 70% is sufficient to increase bone mass, but does not necessarily enhance bone formation. ClC-7 does not appear to be crucially involved in gastric acid secretion, which explains the absence of an osteopetrorickets phenotype in CLCN7-related osteopetrosis.

Monogene Ionenkanalerkrankungen des Knochens

Obwohl Ionenkanäle eher mit der Generierung von Aktionspotenzialen in Verbindung gebracht werden, können sie auch in unterschiedlichster Weise die Entwicklung und Funktion von Knochenzellen und -gewebe beeinflussen, was durch die hier vorgestellten Skeletterkrankungen verdeutlicht werden soll. Jeder der grundlegenden Zelltypen, Chondrozyten, Osteoblasten, Osteozyten, Osteoklasten, kann in die Pathogenese involviert sein und in vielen Fällen ist das Zusammenspiel der verschiedenen zellulären Defekte nicht verstanden. Connexin 43 und TRPV4, 2 der genannten Membranproteine, transportieren v. a. Kalzium und stehen jeweils mit einem Spektrum an Skelettphänotypen in Verbindung. Hierbei scheint Connexin 43 v. a. als Regulator in Osteoblasten und Osteozyten zu fungieren, während TRPV4 eine wichtige Rolle in Chondrozyten spielt. Die anderen beiden Beispiele sind die chloridtransportierenden Proteine ANO5 und ClC-7, deren Defekt die gnathodiaphysäre Dysplasie bzw. die Osteopetrose nach sich zieht. Während die Funktion von ANO5 noch unklar ist, konnte die Funktion von ClC-7 in Osteoklasten detailliert beschrieben werden.

A model of lysosomal pH regulation

Lysosomes must maintain an acidic luminal pH to activate hydrolytic enzymes and degrade internalized macromolecules. Acidification requires the vacuolar-type H(+)-ATPase to pump protons into the lumen and a counterion flux to neutralize the membrane potential created by proton accumulation. Early experiments suggested that the counterion was chloride, and more recently a pathway consistent with the ClC-7 Cl(-)/H(+) antiporter was identified. However, reports that the steady-state luminal pH is unaffected in ClC-7 knockout mice raise questions regarding the identity of the carrier and the counterion. Here, we measure the current-voltage characteristics of a mammalian ClC-7 antiporter, and we use its transport properties, together with other key ion regulating elements, to construct a mathematical model of lysosomal pH regulation. We show that results of in vitro lysosome experiments can only be explained by the presence of ClC-7, and that ClC-7 promotes greater acidification than Cl(-), K(+), or Na(+) channels. Our models predict strikingly different lysosomal K(+) dynamics depending on the major counterion pathways. However, given the lack of experimental data concerning acidification in vivo, the model cannot definitively rule out any given mechanism, but the model does provide concrete predictions for additional experiments that would clarify the identity of the counterion and its carrier.

Cell biology and physiology of CLC chloride channels and transporters

Proteins of the CLC gene family assemble to homo- or sometimes heterodimers and either function as Cl(-) channels or as Cl(-)/H(+)-exchangers. CLC proteins are present in all phyla. Detailed structural information is available from crystal structures of bacterial and algal CLCs. Mammals express nine CLC genes, four of which encode Cl(-) channels and five 2Cl(-)/H(+)-exchangers. Two accessory β-subunits are known: (1) barttin and (2) Ostm1. ClC-Ka and ClC-Kb Cl(-) channels need barttin, whereas Ostm1 is required for the function of the lysosomal ClC-7 2Cl(-)/H(+)-exchanger. ClC-1, -2, -Ka and -Kb Cl(-) channels reside in the plasma membrane and function in the control of electrical excitability of muscles or neurons, in extra- and intracellular ion homeostasis, and in transepithelial transport. The mainly endosomal/lysosomal Cl(-)/H(+)-exchangers ClC-3 to ClC-7 may facilitate vesicular acidification by shunting currents of proton pumps and increase vesicular Cl(-) concentration. ClC-3 is also present on synaptic vesicles, whereas ClC-4 and -5 can reach the plasma membrane to some extent. ClC-7/Ostm1 is coinserted with the vesicular H(+)-ATPase into the acid-secreting ruffled border membrane of osteoclasts. Mice or humans lacking ClC-7 or Ostm1 display osteopetrosis and lysosomal storage disease. Disruption of the endosomal ClC-5 Cl(-)/H(+)-exchanger leads to proteinuria and Dent's disease. Mouse models in which ClC-5 or ClC-7 is converted to uncoupled Cl(-) conductors suggest an important role of vesicular Cl(-) accumulation in these pathologies. The important functions of CLC Cl(-) channels were also revealed by human diseases and mouse models, with phenotypes including myotonia, renal loss of salt and water, deafness, blindness, leukodystrophy, and male infertility.

A missense mutation accelerating the gating of the lysosomal Cl-/H+-exchanger ClC-7/Ostm1 causes osteopetrosis with gingival hamartomas in cattle

Chloride-proton exchange by the lysosomal anion transporter ClC-7/Ostm1 is of pivotal importance for the physiology of lysosomes and bone resorption. Mice lacking either ClC-7 or Ostm1 develop a lysosomal storage disease and mutations in either protein have been found to underlie osteopetrosis in mice and humans. Some human disease-causing CLCN7 mutations accelerate the usually slow voltage-dependent gating of ClC-7/Ostm1. However, it has remained unclear whether the fastened kinetics is indeed causative for the disease. Here we identified and characterized a new deleterious ClC-7 mutation in Belgian Blue cattle with a severe symptomatology including perinatal lethality and in most cases gingival hamartomas. By autozygosity mapping and genome-wide sequencing we found a handful of candidate variants, including a cluster of three private SNPs causing the substitution of a conserved tyrosine in the CBS2 domain of ClC-7 by glutamine. The case for ClC-7 was strengthened by subsequent examination of affected calves that revealed severe osteopetrosis. The Y750Q mutation largely preserved the lysosomal localization and assembly of ClC-7/Ostm1, but drastically accelerated its activation by membrane depolarization. These data provide first evidence that accelerated ClC-7/Ostm1 gating per se is deleterious, highlighting a physiological importance of the slow voltage-activation of ClC-7/Ostm1 in lysosomal function and bone resorption.

Osteopetrosis: genetics, treatment and new insights into osteoclast function

Osteopetrosis is a genetic condition of increased bone mass, which is caused by defects in osteoclast formation and function. Both autosomal recessive and autosomal dominant forms exist, but this Review focuses on autosomal recessive osteopetrosis (ARO), also known as malignant infantile osteopetrosis. The genetic basis of this disease is now largely uncovered: mutations in TCIRG1, CLCN7, OSTM1, SNX10 and PLEKHM1 lead to osteoclast-rich ARO (in which osteoclasts are abundant but have severely impaired resorptive function), whereas mutations in TNFSF11 and TNFRSF11A lead to osteoclast-poor ARO. In osteoclast-rich ARO, impaired endosomal and lysosomal vesicle trafficking results in defective osteoclast ruffled-border formation and, hence, the inability to resorb bone and mineralized cartilage. ARO presents soon after birth and can be fatal if left untreated. However, the disease is heterogeneous in clinical presentation and often misdiagnosed. This article describes the genetics of ARO and discusses the diagnostic role of next-generation sequencing methods. The management of affected patients, including guidelines for the indication of haematopoietic stem cell transplantation (which can provide a cure for many types of ARO), are outlined. Finally, novel treatments, including preclinical data on in utero stem cell treatment, RANKL replacement therapy and denosumab therapy for hypercalcaemia are also discussed.

Common gating of both CLC transporter subunits underlies voltage-dependent activation of the 2Cl-/1H+ exchanger ClC-7/Ostm1

CLC anion transporters form dimers that function either as Cl(-) channels or as electrogenic Cl(-)/H(+) exchangers. CLC channels display two different types of "gates," "protopore" gates that open and close the two pores of a CLC dimer independently of each other and common gates that act on both pores simultaneously. ClC-7/Ostm1 is a lysosomal 2Cl(-)/1H(+) exchanger that is slowly activated by depolarization. This gating process is drastically accelerated by many CLCN7 mutations underlying human osteopetrosis. Making use of some of these mutants, we now investigate whether slow voltage activation of plasma membrane-targeted ClC-7/Ostm1 involves protopore or common gates. Voltage activation of wild-type ClC-7 subunits was accelerated by co-expressing an excess of ClC-7 subunits carrying an accelerating mutation together with a point mutation rendering these subunits transport-deficient. Conversely, voltage activation of a fast ClC-7 mutant could be slowed by co-expressing an excess of a transport-deficient mutant. These effects did not depend on whether the accelerating mutation localized to the transmembrane part or to cytoplasmic cystathionine-β-synthase (CBS) domains of ClC-7. Combining accelerating mutations in the same subunit did not speed up gating further. No currents were observed when ClC-7 was truncated after the last intramembrane helix. Currents and slow gating were restored when the C terminus was co-expressed by itself or fused to the C terminus of the β-subunit Ostm1. We conclude that common gating underlies the slow voltage activation of ClC-7. It depends on the CBS domain-containing C terminus that does not require covalent binding to the membrane domain of ClC-7.

Regulation of lysosome biogenesis and functions in osteoclasts

In order to resorb the mineralized bone extracellular matrix, the osteoclast relies on the generation of a resorption lacuna characterized by the presence of specific proteases and a low pH. Hence, bone resorption by osteoclasts is highly dependent on lysosomes, the organelles specialized in intra- and extracellular material degradation. This is best illustrated by the fact that multiple forms of human osteopetrosis are caused by mutations in genes encoding for lysosomal proteins. Yet, until recently, the molecular mechanisms regulating lysosomal biogenesis and function in osteoclasts were poorly understood. Here we review the latest developments in the study of lysosomal biogenesis and function in osteoclasts with an emphasis on the transcriptional control of these processes.

Autosomal dominant osteopetrosis revisited: lessons from recent studies

Systematic studies of autosomal dominant osteopetrosis (ADO) were followed by the identification of underlying mutations giving unique possibilities to perform translational studies. What was previously designated ADO1 turned out to be a high bone mass phenotype caused by a missense mutation in the first propeller of LRP5, a region of importance for binding inhibitory proteins. Thereby, ADO1 cannot be regarded as a classical form of osteopetrosis but must now be considered a disease of LRP5 activation. ADO (Albers-Schönberg disease, or previously ADO2) is characterized by increased number of osteoclasts and a defect in the chloride transport system (ClC-7) of importance for acidification of the resorption lacuna (a form of Chloride Channel 7 Deficiency Osteopetrosis). Ex vivo studies of osteoclasts from ADO have shown that cells do form normally but have reduced resorption capacity and an expanded life span. Bone formation seems normal despite decreased osteoclast function. Uncoupling of formation from resorption makes ADO of interest for new strategies for treatment of osteoporosis. Recent studies have integrated bone metabolism in whole-body energy homeostasis. Patients with ADO may have decreased insulin levels indicating importance beyond bone metabolism. There seems to be a paradigm shift in the treatment of osteoporosis. Targeting ClC-7 might introduce a new principle of dual action. Drugs affecting ClC-7 could be antiresorptive, still allowing ongoing bone formation. Inversely, drugs affecting the inhibitory site of LRP5 might stimulate bone formation and inhibit resorption. Thereby, these studies have highlighted several intriguing treatment possibilities, employing novel modes of action, which could provide benefits to the treatment of osteoporosis.

Signals from the lysosome: a control centre for cellular clearance and energy metabolism

For a long time, lysosomes were considered merely to be cellular 'incinerators' involved in the degradation and recycling of cellular waste. However, now there is compelling evidence indicating that lysosomes have a much broader function and that they are involved in fundamental processes such as secretion, plasma membrane repair, signalling and energy metabolism. Furthermore, the essential role of lysosomes in autophagic pathways puts these organelles at the crossroads of several cellular processes, with significant implications for health and disease. The identification of a master regulator, transcription factor EB (TFEB), that regulates lysosomal biogenesis and autophagy has revealed how the lysosome adapts to environmental cues, such as starvation, and targeting TFEB may provide a novel therapeutic strategy for modulating lysosomal function in human disease.

Slc26a11 is prominently expressed in the brain and functions as a chloride channel: expression in Purkinje cells and stimulation of V H⁺-ATPase

SLC26A11 (human)/Slc26a11 (mouse), also known as kidney brain anion transporter (KBAT), is a member of the SLC26 anion transporter family and shows abundant mRNA expression in the brain. However, its exact cellular distribution and subcellular localization in the brain and its functional identity and possible physiological roles remain unknown. Expression and immunostaining studies demonstrated that Slc26a11 is abundantly expressed in the cerebellum, with a predominant expression in Purkinje cells. Lower expression levels were detected in hippocampus, olfactory bulb, cerebral cortex, and subcortical structures. Patch clamp studies in HEK293 cells transfected with mouse cDNA demonstrated that Slc26a11 can function as a chloride channel that is active under basal conditions and is not regulated by calcium, forskolin, or co-expression with cystic fibrosis transmembrane regulator. Single and double immunofluorescent labeling studies demonstrated the localization of vacuolar (V) H⁺-ATPase and Slc26a11 (KBAT) in the plasma membrane in Purkinje cells. Functional studies in HEK293 cells indicated that transfection with Slc26a11 stimulated acid transport via endogenous V H⁺-ATPase. We conclude that Slc26a11 (KBAT) is prominently distributed in output neurons of various subcortical and cortical structures in the central nervous system, with specific expression in Purkinje cells and that it may operate as a chloride channel regulating acid translocation by H⁺-ATPase across the plasma membrane and in intracellular compartments.

Osteoimmunology: the study of the relationship between the immune system and bone tissue

Bone tissue is a highly regulated structure, which plays an essential role in various physiological functions. Through autocrine and paracrine mechanisms, bone tissue is involved in hematopoiesis, influencing the fate of hematopoietic stem cells. There are a number of molecules shared by bone cells and immune system cells indicating that there are multiple connections between the immune system and bone tissue. In order to pool all the knowledge concerning both systems, a new discipline known under the term «osteoimmunology» has been developed. Their progress in recent years has been exponential and allowed us to connect and increase our knowledge in areas not seemingly related such as rheumatoid erosion, postmenopausal osteoporosis, bone metastases or periodontal disease. In this review, we have tried to summarize the most important advances that have occurred in the last decade, especially in those areas of interest related to rheumatology.

An optical assay of the transport activity of ClC-7

Osteoporosis, characterized by excessive osteoclast mediated bone resorption, affects millions of people worldwide representing a major public health problem. ClC-7 is a chloride-proton exchanger localized in lysosomes and in the resorption lacuna in osteoclasts where it is essential for bone resorption. Thus, drugs targeted at ClC-7 have been proposed for ameliorating osteoporosis. However, functional assays suited for high throughput screening (HTS) of ClC-7 function are lacking. Here we describe two complementary variants of purely optical assays of the transport activity of ClC-7, redirected to the plasma membrane employing a genetically encoded fluorescent Cl⁻/pH indicator fused to the ClC-7 protein. These simple and robust functional assays of ClC-7 transport are well-suited to be applied in HTS of small-molecule inhibitors and may help to develop drugs suited for the treatment of osteoporosis.

Use of model organisms for the study of neuronal ceroid lipofuscinosis

Neuronal ceroid lipofuscinoses are a group of fatal progressive neurodegenerative diseases predominantly affecting children. Identification of mutations that cause neuronal ceroid lipofuscinosis, and subsequent functional and pathological studies of the affected genes, underpins efforts to investigate disease mechanisms and identify and test potential therapeutic strategies. These functional studies and pre-clinical trials necessitate the use of model organisms in addition to cell and tissue culture models as they enable the study of protein function within a complex organ such as the brain and the testing of therapies on a whole organism. To this end, a large number of disease models and genetic tools have been identified or created in a variety of model organisms. In this review, we will discuss the ethical issues associated with experiments using model organisms, the factors underlying the choice of model organism, the disease models and genetic tools available, and the contributions of those disease models and tools to neuronal ceroid lipofuscinosis research. This article is part of a Special Issue entitled: The Neuronal Ceroid Lipofuscinoses or Batten Disease.

CLC-3 chloride channels moderate long-term potentiation at Schaffer collateral-CA1 synapses

The chloride channel CLC-3 is expressed in the brain on synaptic vesicles and postsynaptic membranes. Although CLC-3 is broadly expressed throughout the brain, the CLC-3 knockout mouse shows complete, selective postnatal neurodegeneration of the hippocampus, suggesting a crucial role for the channel in maintaining normal brain function. CLC-3 channels are functionally linked to NMDA receptors in the hippocampus; NMDA receptor-dependent Ca(2+) entry, activation of Ca(2+)/calmodulin kinase II and subsequent gating of CLC-3 link the channels via a Ca(2+)-mediated feedback loop. We demonstrate that loss of CLC-3 at mature synapses increases long-term potentiation from 135 ± 4% in the wild-type slice preparation to 154 ± 7% above baseline (P < 0.001) in the knockout; therefore, the contribution of CLC-3 is to reduce synaptic potentiation by ∼40%. Using a decoy peptide representing the Ca(2+)/calmodulin kinase II phosphorylation site on CLC-3, we show that phosphorylation of CLC-3 is required for its regulatory function in long-term potentiation. CLC-3 is also expressed on synaptic vesicles; however, our data suggest functionally separable pre- and postsynaptic roles. Thus, CLC-3 confers Cl(-) sensitivity to excitatory synapses, controls the magnitude of long-term potentiation and may provide a protective limit on Ca(2+) influx.

Association of severe autosomal recessive osteopetrosis and structural brain abnormalities: a case report and review of the literature

We describe a fetus with severe osteopetrosis diagnosed on post-mortem radiographs following termination of pregnancy at 29 weeks for major brain malformations detected on ultrasound. SNP microarray confirmed loss of heterozygosity in 5% of the genome, consistent with parental consanguinity. Sequencing of the genes known to cause severe recessive osteopetrosis, TCIRG1, CLCN7, OSTM1 and SNX10, was negative. Brain malformations are not typically considered part of the phenotypic spectrum of osteopetrosis. We review the literature, and propose that this may represent a novel autosomal recessive variant of osteopetrosis.

Chloride in vesicular trafficking and function

Luminal acidification is of pivotal importance for the physiology of the secretory and endocytic pathways and its diverse trafficking events. Acidification by the proton-pumping V-ATPase requires charge compensation by counterion currents that are commonly attributed to chloride. The molecular identification of intracellular chloride transporters and the improvement of methodologies for measuring intraorganellar pH and chloride have facilitated the investigation of the physiology of vesicular chloride transport. New data question the requirement of chloride for pH regulation of various organelles and furthermore ascribe functions to chloride that are beyond merely electrically shunting the proton pump. This review surveys the currently established and proposed intracellular chloride transporters and gives an overview of membrane-trafficking steps that are affected by the perturbation of chloride transport. Finally, potential mechanisms of membrane-trafficking modulation by chloride are discussed and put into the context of organellar ion homeostasis in general.

The Arabidopsis central vacuole as an expression system for intracellular transporters: functional characterization of the Cl-/H+ exchanger CLC-7

Functional characterization of intracellular transporters is hampered by the inaccessibility of animal endomembranes to standard electrophysiological techniques. Here, we used Arabidopsis mesophyll protoplasts as a novel heterologous expression system for the lysosomal chloride–proton exchanger CLC-7 from rat. Following transient expression of a rCLC-7:EGFP construct in isolated protoplasts, the fusion protein efficiently targeted to the membrane of the large central vacuole, the lytic compartment of plant cells. Membrane currents recorded from EGFP-positive vacuoles were almost voltage independent and showed time-dependent activation at elevated positive membrane potentials as a hallmark. The shift in the reversal potential of the current induced by a decrease of cytosolic pH was compatible with a 2Cl(-)/1H(+) exchange stoichiometry. Mutating the so-called gating glutamate into alanine (E245A) uncoupled chloride fluxes from the movement of protons, transforming the transporter into a chloride channel-like protein. Importantly, CLC-7 transport activity in the vacuolar expression system was recorded in the absence of the auxiliary subunit Ostm1, differently to recent data obtained in Xenopus oocytes using a CLC-7 mutant with partial plasma membrane expression. We also show that plasma membrane-targeted CLC-7(E245A) is non-functional in Xenopus oocytes when expressed without Ostm1. In summary, our data suggest the existence of an alternative CLC-7 operating mode, which is active when the protein is not in complex with Ostm1. The vacuolar expression system has the potential to become a valuable tool for functional studies on intracellular ion channels and transporters from animal cells.

Mouse models of neuronal ceroid lipofuscinoses: useful pre-clinical tools to delineate disease pathophysiology and validate therapeutics

The neuronal ceroid lipofuscinoses (NCL, also known as Batten disease) is a devastating neurodegenerative diseases caused by mutations in either soluble enzymes or membrane-associated structural proteins that result in lysosome dysfunction. Different forms of NCL were defined initially by age of onset, affected population and/or type of storage material but collectively represent the most prevalent pediatric hereditary neurovisceral storage disorder. Specific gene mutations are now known for each subclass of NCL in humans that now largely define the disease: cathepsin D (CTSD) for congenital (CLN10 form); palmitoyl protein thioesterase 1 (PPT1) for infantile (CLN1 form); tripeptidyl peptidase 1 (TPP1) for classic late infantile (CLN2 form); variant late infantile-CLN5, CLN6 or CLN8 for variant late infantile forms; and CLN3 for juvenile (CLN3 form). Several mouse models of NCL have been developed, or in some cases exist sporadically, that exhibit mutations producing a progressive neurodegenerative phenotype similar to that observed in human NCL. The study of these mouse models of NCL has dramatically advanced our knowledge of NCL pathophysiology and in some cases has helped delineate the function of proteins mutated in human NCL. In addition, NCL mutant mice have been tested for several different therapeutic approaches and as such they have become important pre-clinical models for validating treatment options. In this review we will assess the current state of mouse models of NCL with regards to their unique pathophysiology and how these mice have helped investigators achieve a better understanding of human NCL disease and therapy.