Distinct neuropathologic phenotypes after disrupting the chloride transport proteins ClC-6 or ClC-7/Ostm1

Mutations in CLCN6 as a Novel Genetic Cause of Neuronal Ceroid Lipofuscinosis

OBJECTIVE

The aim of this study was to explore the pathogenesis of CLCN6-related disease and to assess whether its Cl-/H+-exchange activity is crucial for the biological role of ClC-6.

METHODS

We performed whole-exome sequencing on a girl with development delay, intractable epilepsy, behavioral abnormities, retinal dysfunction, progressive brain atrophy, suggestive of neuronal ceroid lipofuscinoses (NCLs). We generated and analyzed the first knock-in mouse model of a patient variant (p.E200A) and compared it with a Clcn6-/- mouse model. Additional functional tests were performed with heterologous expression of mutant ClC-6.

RESULTS

We identified a de novo heterozygous p.E200A variant in the proband. Expression of disease-causing ClC-6E200A or ClC-6Y553C mutants blocked autophagic flux and activated transcription factors EB (TFEB) and E3 (TFE3), leading to autophagic vesicle and cholesterol accumulation. Such alterations were absent with a transport-deficient ClC-6E267A mutant. Clcn6E200A/+ mice developed severe neurodegeneration with typical features of NCLs. Mutant ClC-6E200A, but not loss of ClC-6 in Clcn6-/- mice, increased lysosomal biogenesis by suppressing mTORC1-TFEB signaling, blocked autophagic flux through impairing lysosomal function, and increased apoptosis. Carbohydrate and lipid deposits accumulated in Clcn6E200A/+ brain, while only lipid storage was found in Clcn6-/- brain. Lysosome dysfunction, autophagy defects, and gliosis were early pathogenic events preceding neuron loss.

INTERPRETATION

CLCN6 is a novel genetic cause of NCLs, highlighting the importance of considering CLCN6 mutations in the diagnostic workup for molecularly undefined forms of NCLs. Uncoupling of Cl- transport from H+ countertransport in the E200A mutant has a dominant effect on the autophagic/lysosomal pathway. ANN NEUROL 2024.

Pathological Functions of Lysosomal Ion Channels in the Central Nervous System

Lysosomes are highly dynamic organelles that maintain cellular homeostasis and regulate fundamental cellular processes by integrating multiple metabolic pathways. Lysosomal ion channels such as TRPML1-3, TPC1/2, ClC6/7, CLN7, and TMEM175 mediate the flux of Ca2+, Cl-, Na+, H+, and K+ across lysosomal membranes in response to osmotic stimulus, nutrient-dependent signals, and cellular stresses. These ion channels serve as the crucial transducers of cell signals and are essential for the regulation of lysosomal biogenesis, motility, membrane contact site formation, and lysosomal homeostasis. In terms of pathophysiology, genetic variations in these channel genes have been associated with the development of lysosomal storage diseases, neurodegenerative diseases, inflammation, and cancer. This review aims to discuss the current understanding of the role of these ion channels in the central nervous system and to assess their potential as drug targets.

Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease

Together with its β-subunit OSTM1, ClC-7 performs 2Cl-/H+ exchange across lysosomal membranes. Pathogenic variants in either gene cause lysosome-related pathologies, including osteopetrosis and lysosomal storage. CLCN7 variants can cause recessive or dominant disease. Different variants entail different sets of symptoms. Loss of ClC-7 causes osteopetrosis and mostly neuronal lysosomal storage. A recently reported de novo CLCN7 mutation (p.Tyr715Cys) causes widespread severe lysosome pathology (hypopigmentation, organomegaly, and delayed myelination and development, "HOD syndrome"), but no osteopetrosis. We now describe two additional HOD individuals with the previously described p.Tyr715Cys and a novel p.Lys285Thr mutation, respectively. Both mutations decreased ClC-7 inhibition by PI(3,5)P2 and affected residues lining its binding pocket, and shifted voltage-dependent gating to less positive potentials, an effect partially conferred to WT subunits in WT/mutant heteromers. This shift predicts augmented pH gradient-driven Cl- uptake into vesicles. Overexpressing either mutant induced large lysosome-related vacuoles. This effect depended on Cl-/H+-exchange, as shown using mutants carrying uncoupling mutations. Fibroblasts from the p.Y715C patient also displayed giant vacuoles. This was not observed with p.K285T fibroblasts probably due to residual PI(3,5)P2 sensitivity. The gain of function caused by the shifted voltage-dependence of either mutant likely is the main pathogenic factor. Loss of PI(3,5)P2 inhibition will further increase current amplitudes, but may not be a general feature of HOD. Overactivity of ClC-7 induces pathologically enlarged vacuoles in many tissues, which is distinct from lysosomal storage observed with the loss of ClC-7 function. Osteopetrosis results from a loss of ClC-7, but osteoclasts remain resilient to increased ClC-7 activity.

The Cl- transporter ClC-7 is essential for phagocytic clearance by microglia

Microglia, professional phagocytic cells of the brain, rely upon the appropriate activation of lysosomes to execute their immune and clearance functions. Lysosomal activity is, in turn, modulated by a complex network of over 200 membrane and accessory proteins that relay extracellular cues to these key degradation centers. The ClC-7 chloride (Cl-)-proton (H+) antiporter (also known as CLCN7) is localized to the endolysosomal compartments and mutations in CLCN7 lead to osteopetrosis and neurodegeneration. Although the functions of ClC-7 have been extensively investigated in osteoclasts and neurons, its role in microglia in vivo remains largely unexamined. Here, we show that microglia and embryonic macrophages in zebrafish clcn7 mutants cannot effectively process extracellular debris in the form of apoptotic cells and β-amyloid. Despite these functional defects, microglia develop normally in clcn7 mutants and display normal expression of endosomal and lysosomal markers. We also find that mutants for ostm1, which encodes the β-subunit of ClC-7, have a phenotype that is strikingly similar to that of clcn7 mutants. Together, our observations uncover a previously unappreciated role of ClC-7 in microglia and contribute to the understanding of the neurodegenerative phenotypes that accompany mutations in this channel.

Case report: ocular manifestations of a gain-of-function mutation in CLCN6, a newly diagnosed disease

BACKGROUND

In 2020, a new disease was reported by Polovitskaya et al., caused by a monoallelic, gain-of-function mutation in CLCN6, encoding the ClC-6 Cl-/H±exchanger.

METHODS

Here, we report the ophthalmic findings of one of the first three patients with this disease (the proband) and review the findings in the other two patients in the literature.

RESULTS

The CLCN6 gene is part of the voltage-dependent chloride channel protein family. It functions as either a chloride channel aiding in cell-volume regulation and acidification of intracellular organelles or as an antiporter, which are membrane proteins involved in the transport of molecules across a phospholipid membrane. This particular gene is found in late endosomes. Ion transport across endosome membranes is essential for endosomal function. The proband carried a de novo c.1658A>G (p.Tyr553Cys) mutation in CLCN6. The patient reported herein has a notable optic nerve appearance. The nerve initially appeared elevated. Over time, the optic nerve elevation appearance decreased, associated with progressive vision loss with a visual acuity of 20/470 at last follow-up.

CONCLUSION

While Clcn6-/- mice have been found to have a mild neuronal lysosomal storage phenotype, the three reported children with a de novo c.1658A>G (p.Tyr553Cys) variant displayed significant developmental delay and neurodegeneration.

Vesicular CLC chloride/proton exchangers in health and diseases

Chloride is one of the most abundant anions in the human body; it is implicated in several physiological processes such as the transmission of action potentials, transepithelial salt transport, maintenance of cellular homeostasis, regulation of osmotic pressure and intracellular pH, and synaptic transmission. The balance between the extracellular and intracellular chloride concentrations is controlled by the interplay of ion channels and transporters embedded in the cellular membranes. Vesicular members of the CLC chloride protein family (vCLCs) are chloride/proton exchangers expressed in the membrane of the intracellular organelles, where they control vesicular acidification and luminal chloride concentration. It is well known that mutations in CLCs cause bone, kidney, and lysosomal genetic diseases. However, the role of CLC exchangers in neurological disorders is only now emerging with the identification of pathogenic CLCN gene variants in patients with severe neuronal and intellectual dysfunctions. This review will provide an overview of the recent advances in understanding the role of the vesicular CLC chloride/proton exchangers in human pathophysiology.

Lysosomal chloride transporter CLH-6 protects lysosome membrane integrity via cathepsin activation

Lysosomal integrity is vital for cell homeostasis, but the underlying mechanisms are poorly understood. Here, we identify CLH-6, the C. elegans ortholog of the lysosomal Cl-/H+ antiporter ClC-7, as an important factor for protecting lysosomal integrity. Loss of CLH-6 affects lysosomal degradation, causing cargo accumulation and membrane rupture. Reducing cargo delivery or increasing CPL-1/cathepsin L or CPR-2/cathepsin B expression suppresses these lysosomal defects. Inactivation of CPL-1 or CPR-2, like CLH-6 inactivation, affects cargo digestion and causes lysosomal membrane rupture. Thus, loss of CLH-6 impairs cargo degradation, leading to membrane damage of lysosomes. In clh-6(lf) mutants, lysosomes are acidified as in wild type but contain lower chloride levels, and cathepsin B and L activities are significantly reduced. Cl- binds to CPL-1 and CPR-2 in vitro, and Cl- supplementation increases lysosomal cathepsin B and L activities. Altogether, these findings suggest that CLH-6 maintains the luminal chloride levels required for cathepsin activity, thus facilitating substrate digestion to protect lysosomal membrane integrity.

The clinical features of OSTM1-associated malignant infantile osteopetrosis: A retrospective, single-center experience over one decade

Mutation in OSTM1 give rise to the rarest and most lethal subtype of malignant infantile osteopetrosis (MIOP), and an improved understanding of OSTM1-associated MIOP would help with informed decision-making regarding symptom management and early palliative care referral. This retrospective study describes the clinical and laboratory features of patients with a genetic diagnosis of OSTM1 MIOP made between January 2011 and December 2021 in the Department of Pediatrics, Al-Adan Hospital, Kuwait. Twenty-two children had confirmed homozygous deletion in OSTM1 (13 females, nine males). Consanguinity was reported in almost all parents. 72.7% were diagnosed before the age of two months, most commonly incidentally with a high clinical suspicion. All 22 patients developed upper respiratory symptoms, hepatosplenomegaly, poor feeding, and had severe developmental delay. 80% of patients developed pain and/or irritability, and 40.9% were diagnosed with primary seizures. Bone fractures developed in 27% of patients, most likely iatrogenic, and some patients had hernia and gum abnormalities. The mean survival was 10.9 months. The clinical presentation, symptomatology, and mortality of our cohort were compared with other cases of OSTM1 MIOP identified through a comperhensive search of the PubMed database. The findings conclude that OSTM1 MIOP is a multi-systemic disease with distinct clinical features, of which neurological complications are the most severe and include nociplastic pain and irritability. Although orthopedic complications influence the trajectory of most patients with other forms of osteopetrosis, OSTM1 MIOP is driven by its neurological complications. Hence, OSTM1 should be regarded as a neurodegenerative disease with osteopetrosis as a comorbidity that warrants early palliative care referral.

Molecular Abnormalities in BTBR Mice and Their Relevance to Schizophrenia and Autism Spectrum Disorders: An Overview of Transcriptomic and Proteomic Studies

Animal models of psychopathologies are of exceptional interest for neurobiologists because these models allow us to clarify molecular mechanisms underlying the pathologies. One such model is the inbred BTBR strain of mice, which is characterized by behavioral, neuroanatomical, and physiological hallmarks of schizophrenia (SCZ) and autism spectrum disorders (ASDs). Despite the active use of BTBR mice as a model object, the understanding of the molecular features of this strain that cause the observed behavioral phenotype remains insufficient. Here, we analyzed recently published data from independent transcriptomic and proteomic studies on hippocampal and corticostriatal samples from BTBR mice to search for the most consistent aberrations in gene or protein expression. Next, we compared reproducible molecular signatures of BTBR mice with data on postmortem samples from ASD and SCZ patients. Taken together, these data helped us to elucidate brain-region-specific molecular abnormalities in BTBR mice as well as their relevance to the anomalies seen in ASDs or SCZ in humans.

OSTM1 pleiotropic roles from osteopetrosis to neurodegeneration

Autosomal recessive osteopetroses (ARO) are rare genetic skeletal disorders of high clinical and molecular heterogeneity with an estimated frequency of 1:250,000 worldwide. The manifestations are diverse and although individually rare, the various forms contribute to the prevalence of a significant number of affected individuals with considerable morbidity and mortality. Among the ARO classification, the most severe form is the autosomal recessive-5 (OPTB5) osteopetrosis (OMIM 259720) that results from homozygous mutation in the OSTM1 gene (607649). OSTM1 mutations account for approximately 5 % of instances of autosomal recessive osteopetrosis and lead to a highly debilitating form of the disease in infancy and death within the first few years of life (Sobacchi et al., 2013) [1].

Regulation of Aging and Longevity by Ion Channels and Transporters

Despite significant advances in our understanding of the mechanisms that underlie age-related physiological decline, our ability to translate these insights into actionable strategies to extend human healthspan has been limited. One of the major reasons for the existence of this barrier is that with a few important exceptions, many of the proteins that mediate aging have proven to be undruggable. The argument put forth here is that the amenability of ion channels and transporters to pharmacological manipulation could be leveraged to develop novel therapeutic strategies to combat aging. This review delves into the established roles for ion channels and transporters in the regulation of aging and longevity via their influence on membrane excitability, Ca2+ homeostasis, mitochondrial and endolysosomal function, and the transduction of sensory stimuli. The goal is to provide the reader with an understanding of emergent themes, and prompt further investigation into how the activities of ion channels and transporters sculpt the trajectories of cellular and organismal aging.

Correlation of age of onset and clinical severity in Niemann-Pick disease type C1 with lysosomal abnormalities and gene expression

Niemann-Pick disease type C1 (NPC1) is a rare, prematurely fatal lysosomal storage disorder which exhibits highly variable severity and disease progression as well as a wide-ranging age of onset, from perinatal stages to adulthood. This heterogeneity has made it difficult to obtain prompt diagnosis and to predict disease course. In addition, small NPC1 patient sample sizes have been a limiting factor in acquiring genome-wide transcriptome data. In this study, primary fibroblasts from an extensive cohort of 41 NPC1 patients were used to validate our previous findings that the lysosomal quantitative probe LysoTracker can be used as a predictor for age of onset and disease severity. We also examined the correlation between these clinical parameters and RNA expression data from primary fibroblasts and identified a set of genes that were significantly associated with lysosomal defects or age of onset, in particular neurological symptom onset. Hierarchical clustering showed that these genes exhibited distinct expression patterns among patient subgroups. This study is the first to collect transcriptomic data on such a large scale in correlation with clinical and cellular phenotypes, providing a rich genomic resource to address NPC1 clinical heterogeneity and discover potential biomarkers, disease modifiers, or therapeutic targets.

The Role of the Lysosomal Cl−/H+ Antiporter ClC-7 in Osteopetrosis and Neurodegeneration

CLC proteins comprise Cl⁻ channels and anion/H⁺ antiporters involved in several fundamental physiological processes. ClC-7 is a lysosomal Cl⁻/H⁺ antiporter that together with its beta subunit Ostm1 has a critical role in the ionic homeostasis of lysosomes and of the osteoclasts’ resorption lacuna, although the specific underlying mechanism has so far remained elusive. Mutations in ClC-7 cause osteopetrosis, but also a form of lysosomal storage disease and neurodegeneration. Interestingly, both loss-of- and gain-of-function mutations of ClC-7 can be pathogenic, but the mechanistic implications of this finding are still unclear. This review will focus on the recent advances in our understanding of the biophysical properties of ClC-7 and of its role in human diseases with a focus on osteopetrosis and neurodegeneration.

Altered Expression of Ion Channels in White Matter Lesions of Progressive Multiple Sclerosis: What Do We Know About Their Function?

Despite significant advances in our understanding of the pathophysiology of multiple sclerosis (MS), knowledge about contribution of individual ion channels to axonal impairment and remyelination failure in progressive MS remains incomplete. Ion channel families play a fundamental role in maintaining white matter (WM) integrity and in regulating WM activities in axons, interstitial neurons, glia, and vascular cells. Recently, transcriptomic studies have considerably increased insight into the gene expression changes that occur in diverse WM lesions and the gene expression fingerprint of specific WM cells associated with secondary progressive MS. Here, we review the ion channel genes encoding K⁺, Ca²⁺, Na⁺, and Cl^- channels; ryanodine receptors; TRP channels; and others that are significantly and uniquely dysregulated in active, chronic active, inactive, remyelinating WM lesions, and normal-appearing WM of secondary progressive MS brain, based on recently published bulk and single-nuclei RNA-sequencing datasets. We discuss the current state of knowledge about the corresponding ion channels and their implication in the MS brain or in experimental models of MS. This comprehensive review suggests that the intense upregulation of voltage-gated Na⁺ channel genes in WM lesions with ongoing tissue damage may reflect the imbalance of Na⁺ homeostasis that is observed in progressive MS brain, while the upregulation of a large number of voltage-gated K⁺ channel genes may be linked to a protective response to limit neuronal excitability. In addition, the altered chloride homeostasis, revealed by the significant downregulation of voltage-gated Cl^- channels in MS lesions, may contribute to an altered inhibitory neurotransmission and increased excitability.

Neurodegeneration Upon Dysfunction of Endosomal/Lysosomal CLC Chloride Transporters

The regulation of luminal ion concentrations is critical for the function of, and transport between intracellular organelles. The importance of the acidic pH in the compartments of the endosomal-lysosomal pathway has been well-known for decades. Besides the V-ATPase, which pumps protons into their lumen, a variety of ion transporters and channels is involved in the regulation of the organelles' complex ion homeostasis. Amongst these are the intracellular members of the CLC family, ClC-3 through ClC-7. They localize to distinct but overlapping compartments of the endosomal-lysosomal pathway, partially with tissue-specific expression. Functioning as 2Cl⁻/H⁺ exchangers, they can support the vesicular acidification and accumulate luminal Cl⁻. Mutations in the encoding genes in patients and mouse models underlie severe phenotypes including kidney stones with CLCN5 and osteopetrosis or hypopigmentation with CLCN7. Dysfunction of those intracellular CLCs that are expressed in neurons lead to neuronal defects. Loss of endosomal ClC-3, which heteromerizes with ClC-4, results in neurodegeneration. Mutations in ClC-4 are associated with epileptic encephalopathy and intellectual disability. Mice lacking the late endosomal ClC-6 develop a lysosomal storage disease with reduced pain sensitivity. Human gene variants have been associated with epilepsy, and a gain-of-function mutation causes early-onset neurodegeneration. Dysfunction of the lysosomal ClC-7 leads to a lysosomal storage disease and neurodegeneration in mice and humans. Reduced luminal chloride, as well as altered calcium regulation, has been associated with lysosomal storage diseases in general. This review discusses the properties of endosomal and lysosomal Cl⁻/H⁺ exchange by CLCs and how various alterations of ion transport by CLCs impact organellar ion homeostasis and function in neurodegenerative disorders.

Loss of Chloride Channel 6 (CLC-6) Affects Vascular Smooth Muscle Contractility and Arterial Stiffness via Alterations to Golgi Calcium Stores

Genome-wide association studies have found a number of potential genes involved in blood pressure regulation; however, the functional role of many of these candidates has yet to be established. One such candidate gene is CLCN6, which encodes the transmembrane protein, chloride channel 6 (ClC-6). Although the CLCN6 locus has been widely associated with human blood pressure regulation, the mechanistic role of ClC-6 in blood pressure homeostasis at the molecular, cellular, and physiological levels is completely unknown. In this study, we demonstrate that rats with a functional knockout of ClC-6 on the Dahl Salt-Sensitive rat background (SS-Clcn6) have lower diastolic but not systolic blood pressures. The effect of diastolic blood pressure attenuation was independent of dietary salt exposure in knockout animals. Moreover, SS-Clcn6 rats are protected from hypertension-induced cardiac hypertrophy and arterial stiffening; however, they have impaired vasodilation and dysregulated intracellular calcium handling. ClC-6 is highly expressed in vascular smooth muscle cells where it is targeted to the Golgi apparatus. Using bilayer electrophysiology, we provide evidence that recombinant human ClC-6 protein can function as a channel. Last, we demonstrate that loss of ClC-6 function reduces Golgi calcium stores, which may play a previously unidentified role in vascular contraction and relaxation signaling in vascular smooth muscle cells. Collectively, these data indicate that ClC-6 may modulate blood pressure by regulating Golgi calcium reserves, which in turn contribute to vascular smooth muscle function.

A Recurrent Gain-of-Function Mutation in CLCN6, Encoding the ClC-6 Cl-/H+-Exchanger, Causes Early-Onset Neurodegeneration

Dysfunction of the endolysosomal system is often associated with neurodegenerative disease because postmitotic neurons are particularly reliant on the elimination of intracellular aggregates. Adequate function of endosomes and lysosomes requires finely tuned luminal ion homeostasis and transmembrane ion fluxes. Endolysosomal CLC Cl-/H+ exchangers function as electric shunts for proton pumping and in luminal Cl- accumulation. We now report three unrelated children with severe neurodegenerative disease, who carry the same de novo c.1658A>G (p.Tyr553Cys) mutation in CLCN6, encoding the late endosomal Cl-/H+-exchanger ClC-6. Whereas Clcn6-/- mice have only mild neuronal lysosomal storage abnormalities, the affected individuals displayed severe developmental delay with pronounced generalized hypotonia, respiratory insufficiency, and variable neurodegeneration and diffusion restriction in cerebral peduncles, midbrain, and/or brainstem in MRI scans. The p.Tyr553Cys amino acid substitution strongly slowed ClC-6 gating and increased current amplitudes, particularly at the acidic pH of late endosomes. Transfection of ClC-6Tyr553Cys, but not ClC-6WT, generated giant LAMP1-positive vacuoles that were poorly acidified. Their generation strictly required ClC-6 ion transport, as shown by transport-deficient double mutants, and depended on Cl-/H+ exchange, as revealed by combination with the uncoupling p.Glu200Ala substitution. Transfection of either ClC-6Tyr553Cys/Glu200Ala or ClC-6Glu200Ala generated slightly enlarged vesicles, suggesting that p.Glu200Ala, previously associated with infantile spasms and microcephaly, is also pathogenic. Bafilomycin treatment abrogated vacuole generation, indicating that H+-driven Cl- accumulation osmotically drives vesicle enlargement. Our work establishes mutations in CLCN6 associated with neurological diseases, whose spectrum of clinical features depends on the differential impact of the allele on ClC-6 function.

Cryo-EM structure of the lysosomal chloride-proton exchanger CLC-7 in complex with OSTM1

The chloride-proton exchanger CLC-7 plays critical roles in lysosomal homeostasis and bone regeneration and its mutation can lead to osteopetrosis, lysosomal storage disease and neurological disorders. In lysosomes and the ruffled border of osteoclasts, CLC-7 requires a β-subunit, OSTM1, for stability and activity. Here, we present electron cryomicroscopy structures of CLC-7 in occluded states by itself and in complex with OSTM1, determined at resolutions up to 2.8 Å. In the complex, the luminal surface of CLC-7 is entirely covered by a dimer of the heavily glycosylated and disulfide-bonded OSTM1, which serves to protect CLC-7 from the degradative environment of the lysosomal lumen. OSTM1 binding does not induce large-scale rearrangements of CLC-7, but does have minor effects on the conformation of the ion-conduction pathway, potentially contributing to its regulatory role. These studies provide insights into the role of OSTM1 and serve as a foundation for understanding the mechanisms of CLC-7 regulation.

Niemann-Pick type C disease: cellular pathology and pharmacotherapy

Niemann-Pick type C disease (NPCD) was first described in 1914 and affects approximately 1 in 150 000 live births. It is characterized clinically by diverse symptoms affecting liver, spleen, motor control, and brain; premature death invariably results. Its molecular origins were traced, as late as 1997, to a protein of late endosomes and lysosomes which was named NPC1. Mutation or absence of this protein leads to accumulation of cholesterol in these organelles. In this review, we focus on the intracellular events that drive the pathology of this disease. We first introduce endocytosis, a much-studied area of dysfunction in NPCD cells, and survey the various ways in which this process malfunctions. We briefly consider autophagy before attempting to map the more complex pathways by which lysosomal cholesterol storage leads to protein misregulation, mitochondrial dysfunction, and cell death. We then briefly introduce the metabolic pathways of sphingolipids (as these emerge as key species for treatment) and critically examine the various treatment approaches that have been attempted to date.

A Mathematical Model of Lysosomal Ion Homeostasis Points to Differential Effects of Cl- Transport in Ca2+ Dynamics

The establishment and maintenance of ion gradients between the interior of lysosomes and the cytosol are crucial for numerous cellular and organismal functions. Numerous ion transport proteins ensure the required variation in luminal concentrations of the different ions along the endocytic pathway to fit the needs of the organelles. Failures in keeping proper ion homeostasis have pathological consequences. Accordingly, several human diseases are caused by the dysfunction of ion transporters. These include osteopetrosis, caused by the dysfunction of Cl-/H+ exchange by the lysosomal transporter ClC-7. To better understand how chloride transport affects lysosomal ion homeostasis and how its disruption impinges on lysosomal function, we developed a mathematical model of lysosomal ion homeostasis including Ca2+ dynamics. The model recapitulates known biophysical properties of ClC-7 and enables the investigation of its differential activation kinetics on lysosomal ion homeostasis. We show that normal functioning of ClC-7 supports the acidification process, is associated with increased luminal concentrations of sodium, potassium, and chloride, and leads to a higher Ca2+ uptake and release. Our model highlights the role of ClC-7 in lysosomal acidification and shows the existence of differential Ca2+ dynamics upon perturbations of Cl-/H+ exchange and its activation kinetics, with possible pathological consequences.

Extra-skeletal manifestations in mice affected by Clcn7-dependent autosomal dominant osteopetrosis type 2 clinical and therapeutic implications

Autosomal dominant osteopetrosis type 2 (ADO2) is a high-density brittle bone disease characterized by bone pain, multiple fractures and skeletal-related events, including nerve compression syndrome and hematological failure. We demonstrated that in mice carrying the heterozygous Clcn7^G213R mutation, whose human mutant homolog CLCN7^G215R affects patients, the clinical impacts of ADO2 extend beyond the skeleton, affecting several other organs. The hallmark of the extra-skeletal alterations is a consistent perivascular fibrosis, associated with high numbers of macrophages and lymphoid infiltrates. Fragmented clinical information in a small cohort of patients confirms extra-skeletal alterations consistent with a systemic disease, in line with the observation that the CLCN7 gene is expressed in many organs. ADO2 mice also show anxiety and depression and their brains exhibit not only perivascular fibrosis but also β-amyloid accumulation and astrogliosis, suggesting the involvement of the nervous system in the pathogenesis of the ADO2 extra-skeletal alterations. Extra-skeletal organs share a similar cellular pathology, confirmed also in vitro in bone marrow mononuclear cells and osteoclasts, characterized by an impairment of the exit pathway of the Clcn7 protein product, ClC7, through the Golgi, with consequent reduced ClC7 expression in late endosomes and lysosomes, associated with high vesicular pH and accumulation of autophagosome markers. Finally, an experimental siRNA therapy, previously proven to counteract the bone phenotype, also improves the extra-skeletal alterations. These results could have important clinical implications, supporting the notion that a systematic evaluation of ADO2 patients for extra-skeletal symptoms could help improve their diagnosis, clinical management, and therapeutic options.

Intravenous administration of scAAV9-Hexb normalizes lifespan and prevents pathology in Sandhoff disease mice

Sandhoff disease (SD) is a rare inherited disorder caused by a deficiency of β-hexosaminidase activity which is fatal because no effective treatment is available. A mouse model of Hexb deficiency reproduces the key pathognomonic features of SD patients with severe ubiquitous lysosomal dysfunction, GM2 accumulation, neuroinflammation and neurodegeneration, culminating in death at 4 months. Here, we show that a single intravenous neonatal administration of a self-complementary adeno-associated virus 9 vector (scAAV9) expressing the Hexb cDNA in SD mice is safe and sufficient to prevent disease development. Importantly, we demonstrate for the first time that this treatment results in a normal lifespan (over 700 days) and normalizes motor function assessed by a battery of behavioral tests, with scAAV9-treated SD mice being indistinguishable from wild-type littermates. Biochemical analyses in multiple tissues showed a significant increase in hexosaminidase A activity, which reached 10-15% of normal levels. AAV9 treatment was sufficient to prevent GM2 and GA2 storage almost completely in the cerebrum (less so in the cerebellum), as well as thalamic reactive gliosis and thalamocortical neuron loss in treated Hexb-/- mice. In summary, this study demonstrated a widespread protective effect throughout the entire CNS after a single intravenous administration of the scAAV9-Hexb vector to neonatal SD mice.

CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease

CLC anion transporters are found in all phyla and form a gene family of eight members in mammals. Two CLC proteins, each of which completely contains an ion translocation parthway, assemble to homo- or heteromeric dimers that sometimes require accessory β-subunits for function. CLC proteins come in two flavors: anion channels and anion/proton exchangers. Structures of these two CLC protein classes are surprisingly similar. Extensive structure-function analysis identified residues involved in ion permeation, anion-proton coupling and gating and led to attractive biophysical models. In mammals, ClC-1, -2, -Ka/-Kb are plasma membrane Cl−channels, whereas ClC-3 through ClC-7 are 2Cl−/H+-exchangers in endolysosomal membranes. Biological roles of CLCs were mostly studied in mammals, but also in plants and model organisms like yeast and Caenorhabditis elegans. CLC Cl−channels have roles in the control of electrical excitability, extra- and intracellular ion homeostasis, and transepithelial transport, whereas anion/proton exchangers influence vesicular ion composition and impinge on endocytosis and lysosomal function. The surprisingly diverse roles of CLCs are highlighted by human and mouse disorders elicited by mutations in their genes. These pathologies include neurodegeneration, leukodystrophy, mental retardation, deafness, blindness, myotonia, hyperaldosteronism, renal salt loss, proteinuria, kidney stones, male infertility, and osteopetrosis. In this review, emphasis is laid on biophysical structure-function analysis and on the cell biological and organismal roles of mammalian CLCs and their role in disease.

ClC Channels and Transporters: Structure, Physiological Functions, and Implications in Human Chloride Channelopathies

The discovery of ClC proteins at the beginning of the 1990s was important for the development of the Cl^- transport research field. ClCs form a large family of proteins that mediate voltage-dependent transport of Cl^- ions across cell membranes. They are expressed in both plasma and intracellular membranes of cells from almost all living organisms. ClC proteins form transmembrane dimers, in which each monomer displays independent ion conductance. Eukaryotic members also possess a large cytoplasmic domain containing two CBS domains, which are involved in transport modulation. ClC proteins function as either Cl^- channels or Cl^-/H⁺ exchangers, although all ClC proteins share the same basic architecture. ClC channels have two gating mechanisms: a relatively well-studied fast gating mechanism, and a slow gating mechanism, which is poorly defined. ClCs are involved in a wide range of physiological processes, including regulation of resting membrane potential in skeletal muscle, facilitation of transepithelial Cl^- reabsorption in kidneys, and control of pH and Cl^- concentration in intracellular compartments through coupled Cl^-/H⁺ exchange mechanisms. Several inherited diseases result from C1C gene mutations, including myotonia congenita, Bartter's syndrome (types 3 and 4), Dent's disease, osteopetrosis, retinal degeneration, and lysosomal storage diseases. This review summarizes general features, known or suspected, of ClC structure, gating and physiological functions. We also discuss biophysical properties of mammalian ClCs that are directly involved in the pathophysiology of several human inherited disorders, or that induce interesting phenotypes in animal models.

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.

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.

Single nucleotide variations in CLCN6 identified in patients with benign partial epilepsies in infancy and/or febrile seizures

Nucleotide alterations in the gene encoding proline-rich transmembrane protein 2 (PRRT2) have been identified in most patients with benign partial epilepsies in infancy (BPEI)/benign familial infantile epilepsy (BFIE). However, not all patients harbor these PRRT2 mutations, indicating the involvement of genes other than PRRT2. In this study, we performed whole exome sequencing analysis for a large family affected with PRRT2-unrelated BPEI. We identified a non-synonymous single nucleotide variation (SNV) in the voltage-sensitive chloride channel 6 gene (CLCN6). A cohort study of 48 BPEI patients without PRRT2 mutations revealed a different CLCN6 SNV in a patient, his sibling and his father who had a history of febrile seizures (FS) but not BPEI. Another study of 48 patients with FS identified an additional SNV in CLCN6. Chloride channels (CLCs) are involved in a multitude of physiologic processes and some members of the CLC family have been linked to inherited diseases. However, a phenotypic correlation has not been confirmed for CLCN6. Although we could not detect significant biological effects linked to the identified CLCN6 SNVs, further studies should investigate potential CLCN6 variants that may underlie the genetic susceptibility to convulsive disorders.

A novel, long-lived, and highly engraftable immunodeficient mouse model of mucopolysaccharidosis type I

Mucopolysaccharidosis type I (MPS I) is an inherited α-L-iduronidase (IDUA, I) deficiency in which glycosaminoglycan (GAG) accumulation causes progressive multisystem organ dysfunction, neurological impairment, and death. Current MPS I mouse models, based on a NOD/SCID (NS) background, are short-lived, providing a very narrow window to assess the long-term efficacy of therapeutic interventions. They also develop thymic lymphomas, making the assessment of potential tumorigenicity of human stem cell transplantation problematic. We therefore developed a new MPS I model based on a NOD/SCID/Il2rγ (NSG) background. This model lives longer than 1 year and is tumor-free during that time. NSG MPS I (NSGI) mice exhibit the typical phenotypic features of MPS I including coarsened fur and facial features, reduced/abnormal gait, kyphosis, and corneal clouding. IDUA is undetectable in all tissues examined while GAG levels are dramatically higher in most tissues. NSGI brain shows a significant inflammatory response and prominent gliosis. Neurological MPS I manifestations are evidenced by impaired performance in behavioral tests. Human neural and hematopoietic stem cells were found to readily engraft, with human cells detectable for at least 1 year posttransplantation. This new MPS I model is thus suitable for preclinical testing of novel pluripotent stem cell-based therapy approaches.

Autophagy in the physiology and pathology of the central nervous system

Neurons are highly specialized postmitotic cells that depend on dynamic cellular processes for their proper function.These include among others, neuronal growth and maturation, axonal migration, synapse formation and elimination, all requiring continuous protein synthesis and degradation. Therefore quality-control processes in neurons are directly linked to their physiology. Autophagy is a tightly regulated cellular degradation pathway by which defective or superfluouscytosolic proteins, organelles and other cellular constituents are sequestered in autophagosomes and delivered to lysosomes for degradation. Here we present emerging evidence indicating that constitutive autophagic fluxin neurons has essential roles in key neuronal processes under physiological conditions.Moreover, we discuss how perturbations of the autophagic pathway may underlie diverse pathological phenotypes in neurons associated with neurodevelopmental and neurodegenerative diseases.

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.

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 murine model of variant late infantile ceroid lipofuscinosis recapitulates behavioral and pathological phenotypes of human disease

Neuronal ceroid lipofuscinoses (NCLs; also known collectively as Batten Disease) are a family of autosomal recessive lysosomal storage disorders. Mutations in as many as 13 genes give rise to ∼10 variants of NCL, all with overlapping clinical symptomatology including visual impairment, motor and cognitive dysfunction, seizures, and premature death. Mutations in CLN6 result in both a variant late infantile onset neuronal ceroid lipofuscinosis (vLINCL) as well as an adult-onset form of the disease called Type A Kufs. CLN6 is a non-glycosylated membrane protein of unknown function localized to the endoplasmic reticulum (ER). In this study, we perform a detailed characterization of a naturally occurring Cln6 mutant (Cln6^nclf) mouse line to validate its utility for translational research. We demonstrate that this Cln6^nclf mutation leads to deficits in motor coordination, vision, memory, and learning. Pathologically, we demonstrate loss of neurons within specific subregions and lamina of the cortex that correlate to behavioral phenotypes. As in other NCL models, this model displays selective loss of GABAergic interneuron sub-populations in the cortex and the hippocampus with profound, early-onset glial activation. Finally, we demonstrate a novel deficit in memory and learning, including a dramatic reduction in dendritic spine density in the cerebral cortex, which suggests a reduction in synaptic strength following disruption in CLN6. Together, these findings highlight the behavioral and pathological similarities between the Cln6^nclf mouse model and human NCL patients, validating this model as a reliable format for screening potential therapeutics.

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.

Autophagy failure in Alzheimer's disease and the role of defective lysosomal acidification

Autophagy is a lysosomal degradative process which recycles cellular waste and eliminates potentially toxic damaged organelles and protein aggregates. The important cytoprotective functions of autophagy are demonstrated by the diverse pathogenic consequences that may stem from autophagy dysregulation in a growing number of neurodegenerative disorders. In many of the diseases associated with autophagy anomalies, it is the final stage of autophagy-lysosomal degradation that is disrupted. In several disorders, including Alzheimer's disease (AD), defective lysosomal acidification contributes to this proteolytic failure. The complex regulation of lysosomal pH makes this process vulnerable to disruption by many factors, and reliable lysosomal pH measurements have become increasingly important in investigations of disease mechanisms. Although various reagents for pH quantification have been developed over several decades, they are not all equally well suited for measuring the pH of lysosomes. Here, we evaluate the most commonly used pH probes for sensitivity and localisation, and identify LysoSensor yellow/blue-dextran, among currently used probes, as having the optimal profile of properties for measuring lysosomal pH. In addition, we review evidence that lysosomal acidification is defective in AD and extend our original findings, of elevated lysosomal pH in presenilin 1 (PS1)-deficient blastocysts and neurons, to additional cell models of PS1 and PS1/2 deficiency, to fibroblasts from AD patients with PS1 mutations, and to neurons in the PS/APP mouse model of AD.

Cell biology and function of neuronal ceroid lipofuscinosis-related proteins

Neuronal ceroid lipofuscinoses (NCL) comprise a group of inherited lysosomal disorders with variable age of onset, characterized by lysosomal accumulation of autofluorescent ceroid lipopigments, neuroinflammation, photoreceptor- and neurodegeneration. Most of the NCL-related genes encode soluble and transmembrane proteins which localize to the endoplasmic reticulum or to the endosomal/lysosomal compartment and directly or indirectly regulate lysosomal function. Recently, exome sequencing led to the identification of four novel gene defects in NCL patients and a new NCL nomenclature currently comprising CLN1 through CLN14. Although the precise function of most of the NCL proteins remains elusive, comprehensive analyses of model organisms, particularly mouse models, provided new insight into pathogenic mechanisms of NCL diseases and roles of mutant NCL proteins in cellular/subcellular protein and lipid homeostasis, as well as their adaptive/compensatorial regulation at the transcriptional level. This review summarizes the current knowledge on the expression, function and regulation of NCL proteins and their impact on lysosomal integrity. This article is part of a Special Issue entitled: The Neuronal Ceroid Lipofuscinoses or Batten Disease.

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.

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.

Disruption of the autophagy-lysosome pathway is involved in neuropathology of the nclf mouse model of neuronal ceroid lipofuscinosis

Variant late-infantile neuronal ceroid lipofuscinosis, a fatal lysosomal storage disorder accompanied by regional atrophy and pronounced neuron loss in the brain, is caused by mutations in the CLN6 gene. CLN6 is a non-glycosylated endoplasmic reticulum (ER)-resident membrane protein of unknown function. To investigate mechanisms contributing to neurodegeneration in CLN6 disease we examined the nclf mouse, a naturally occurring model of the human CLN6 disease. Prominent autofluorescent and electron-dense lysosomal storage material was found in cerebellar Purkinje cells, thalamus, hippocampus, olfactory bulb and in cortical layer II to V. Another prominent early feature of nclf pathogenesis was the localized astrocytosis that was evident in many brain regions and the more widespread microgliosis. Expression analysis of mutant Cln6 found in nclf mice demonstrated synthesis of a truncated protein with a reduced half-life. Whereas the rapid degradation of the mutant Cln6 protein can be inhibited by proteasomal inhibitors, there was no evidence for ER stress or activation of the unfolded protein response in various brain areas during postnatal development. Age-dependent increases in LC3-II, ubiquitinated proteins, and neuronal p62-positive aggregates were observed, indicating a disruption of the autophagy-lysosome degradation pathway of proteins in brains of nclf mice, most likely due to defective fusion between autophagosomes and lysosomes. These data suggest that proteasomal degradation of mutant Cln6 is sufficient to prevent the accumulation of misfolded Cln6 protein, whereas lysosomal dysfunction impairs constitutive autophagy promoting neurodegeneration.

Patterned expression of ion channel genes in mouse dorsal raphe nucleus determined with the Allen Mouse Brain Atlas

The dorsal raphe nucleus (DR) is the major source of serotonin (5-hydroxytryptamine, 5-HT) in the forebrain and dysfunction of this midbrain structure is implicated in affective disorders. The DR is composed of several types of 5-HT and non-5-HT neurons and their excitable-membrane properties are heterogeneous and overlapping. In order to understand how these properties may be generated, we examined the mRNA expression patterns of voltage- and ligand-gated ion channels in the DR using the Allen Mouse Brain Atlas. Since DR cytoarchitecture is organized with respect to the midline, we sought to identify genes that were expressed in a pattern with respect to the midline, either enriched or depleted, rather than those that were homogenously expressed throughout the DR. Less than 10% of the screened genes for voltage-gated ion channels showed patterned expression within the DR. Identified genes included voltage-gated sodium channel beta subunits, potassium channels, P/Q-, N-type calcium channels, as well as the alpha2/delta-1 calcium channel. Several voltage-gated chloride channels were also identified, although these may function within intracellular compartments. Of the ligand-gated ion channels examined, 20% showed patterned expression. These consisted primarily of glutamate and GABA-A receptor subunits. The identified genes likely contribute to unique excitable properties of different groups of neurons in the DR and may include novel pharmacologic targets for affective disorders.

Early glial activation, synaptic changes and axonal pathology in the thalamocortical system of Niemann-Pick type C1 mice

Niemann–Pick disease type C (NPC) is an inherited lysosomal storage disease characterised by accumulation of cholesterol and glycosphingolipids. NPC patients suffer a progressive neurodegenerative phenotype presenting with motor dysfunction, mental retardation and cognitive decline. To examine the onset and progression of neuropathological insults in NPC we have systematically examined the CNS of a mouse model of NPC1 (Npc1^−/− mice) at different stages of the disease course. This revealed a specific spatial and temporal pattern of neuropathology in Npc1^−/− mice, highlighting that sensory thalamic pathways are particularly vulnerable to loss of NPC1 resulting in neurodegeneration in Npc1^−/− mice. Examination of markers of astrocytosis and microglial activation revealed a particularly pronounced reactive gliosis in the thalamus early in the disease, which subsequently also occurred in interconnected cortical laminae at later ages. Our examination of the precise staging of events demonstrate that the relationship between glia and neurons varies between brain regions in Npc1^−/− mice, suggesting that the cues causing glial reactivity may differ between brain regions. In addition, aggregations of pre-synaptic markers are apparent in white matter tracts and the thalamus and are likely to be formed within axonal spheroids. Our data provide a new perspective, revealing a number of events that occur prior to and alongside neuron loss and highlighting that these occur in a pathway dependent manner.

High expression of disease-related Cln6 in the cerebral cortex, purkinje cells, dentate gyrus, and hippocampal ca1 neurons

Mutations in the CLN6 gene cause a variant form of late infantile neuronal ceroid lipofuscinosis, a relentless neurodegenerative disease that is inherited as an autosomal recessive trait in humans and in the naturally occurring nclf mouse strain. The CLN6 protein is localized in the endoplasmic reticulum, but it has an unknown function. To develop a molecular understanding of neurodegeneration induced by mutations in CLN6, we examined the spatial and temporal distribution of Cln6 mRNA expression in murine brain. By using Northern blot and tissue qPCR array techniques, a single Cln6 transcript was detected throughout the adult brain, with greatest expression in the cerebellum and hypothalamus. Real-time qPCR showed 2.4-4-fold increases in Cln6 mRNA levels in the cortex and cerebellum during the first 28 days of life, with less prominent enhancement of expression in the hippocampus. In situ hybridization analyses demonstrated Cln6 expression in brainstem, dentate gyrus, and hippocampal neurons of newborn P0 mice. From P14 onward, Cln6 expression is widely distributed throughout the brain and is most prominent in cells of cortical layers II-VI, the Purkinje cell layer, dentate gyrus, and hippocampal CA1 region of adult mice. In different regions of the brain in P0 and P28 nclf mice, the Cln6 mRNA abundance was reduced by 30-40% compared with control mice. These findings implicate Cln6 in the survival and maturation of specific neuronal populations during development and make it possible to compare regional Cln6 expression with the distribution of subsequent pathology.

Recurrent partial rhombencephalosynapsis and holoprosencephaly in siblings with a mutation of ZIC2

Rhombencephalosynapsis (RES) is a rare congenital brain malformation typically identified by magnetic resonance imaging and characterized by fusion of the cerebellar hemispheres and dentate nuclei and vermian agenesis or hypogenesis. Although RES is frequently found in conjunction with other brain malformations and/or congenital anomalies, no specific molecular etiology has been discovered to date and no animal models exist. We identified two half sisters with alobar or semi-lobar holoprosencephaly (HPE) and partial RES, suggesting that genes linked to HPE may also contribute to RES. A deletion of seven base pairs in exon one of the ZIC2 gene (c.392_98del7) was identified in each of the two half sisters with HPE and partial RES. To identify genetic causes of RES and to assess whether genes identified in HPE have a role in RES, we tested 11 additional individuals with RES by high-resolution chromosome analysis, chromosomal microarray analysis, and sequencing of four HPE genes. No mutations in ZIC2 or in other genes that cause HPE were identified, suggesting that mutation of ZIC2 is a rare cause of, or contributor to, RES associated with HPE. In addition, an individual with a complex rearrangement of chromosome 22q13.3 and RES was identified, suggesting the presence of a dosage-sensitive gene that may contribute to RES in this region.

ClC-7 is a slowly voltage-gated 2Cl(-)/1H(+)-exchanger and requires Ostm1 for transport activity

Mutations in the ClC-7/Ostm1 ion transporter lead to osteopetrosis and lysosomal storage disease. Its lysosomal localization hitherto precluded detailed functional characterization. Using a mutated ClC-7 that reaches the plasma membrane, we now show that both the aminoterminus and transmembrane span of the Ostm1 β-subunit are required for ClC-7 Cl(-)/H(+)-exchange, whereas the Ostm1 transmembrane domain suffices for its ClC-7-dependent trafficking to lysosomes. ClC-7/Ostm1 currents were strongly outwardly rectifying owing to slow gating of ion exchange, which itself displays an intrinsically almost linear voltage dependence. Reversal potentials of tail currents revealed a 2Cl(-)/1H(+)-exchange stoichiometry. Several disease-causing CLCN7 mutations accelerated gating. Such mutations cluster to the second cytosolic cystathionine-β-synthase domain and potential contact sites at the transmembrane segment. Our work suggests that gating underlies the rectification of all endosomal/lysosomal CLCs and extends the concept of voltage gating beyond channels to ion exchangers.