Ionic leakage underlies a gain-of-function effect of dominant disease mutations affecting diverse P-type ATPases

Cation leak through the ATP1A3 pump causes spasticity and intellectual disability

ATP1A3 encodes the α3 subunit of the sodium-potassium ATPase, one of two isoforms responsible for powering electrochemical gradients in neurons. Heterozygous pathogenic ATP1A3 variants produce several distinct neurological syndromes, yet the molecular basis for phenotypic variability is unclear. We report a novel recurrent variant, ATP1A3(NM_152296.5):c.2324C>T; p.(Pro775Leu), in nine individuals associated with the primary clinical features of progressive or non-progressive spasticity and developmental delay/intellectual disability. No patients fulfil diagnostic criteria for ATP1A3-associated syndromes, including alternating hemiplegia of childhood, rapid-onset dystonia-parkinsonism or cerebellar ataxia-areflexia-pes cavus-optic atrophy-sensorineural hearing loss (CAPOS), and none were suspected of having an ATP1A3-related disorder. Uniquely among known ATP1A3 variants, P775L causes leakage of sodium ions and protons into the cell, associated with impaired sodium binding/occlusion kinetics favouring states with fewer bound ions. These phenotypic and electrophysiologic studies demonstrate that ATP1A3:c.2324C>T; p.(Pro775Leu) results in mild ATP1A3-related phenotypes resembling complex hereditary spastic paraplegia or idiopathic spastic cerebral palsy. Cation leak provides a molecular explanation for this genotype-phenotype correlation, adding another mechanism to further explain phenotypic variability and highlighting the importance of biophysical properties beyond ion transport rate in ion transport diseases.

ATP1A3-related phenotypes in Chinese children: AHC, CAPOS, and RECA

The aim of this research is to study the phenotype, genotype, treatment strategies, and short-term prognosis of Chinese children with ATP1A3 (Na⁺/K⁺-ATPase alpha 3 gene)-related disorders in Southwest China. Patients with pathogenic ATP1A3 variants identified using next-generation sequencing were registered at the Children's Hospital of Chongqing Medical University from December 2015 to May 2019. We followed them as a cohort and analyzed their clinical data. Eleven patients were identified with de novo pathogenic ATP1A3 heterozygous variants. One (c.2542 + 1G > T, splicing) has not been reported. Eight patients with alternating hemiplegia of childhood (AHC), one with cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss (CAPOS), and two with relapsing encephalopathy with cerebellar ataxia (RECA) were included. The initial manifestations of AHC included hemiplegia, oculomotor abnormalities, and seizures, and the most common trigger was an upper respiratory tract infection without fever. All patients had paroxysmal hemiplegic attacks during their disease course. The brain MRI showed no abnormalities. Six out of eight AHC cases reached a stable disease state after treatment. The initial symptom of the patient with CAPOS was ataxia followed by developmental regression, seizures, deafness, visual impairment, and dysarthria, and the brain MRI indicated mild cerebellar atrophy. No fluctuation was noted after using Acetazolamide. The initial manifestations of the two RECA cases were dystonia and encephalopathy, respectively. One manifested a rapid-onset course of dystonia triggered by a fever followed by dysarthria and action tremors, and independent walking was impossible. The brain MRI image was normal. The other one presented with disturbance of consciousness, seizures, sleep disturbance, tremor, and dyskinesias. The EEG revealed a slow background (δ activity), and the brain MRI result was normal. No response to Flunarizine was noted for them, and it took 61 and 60 months for them to reach a stable disease state, respectively.

CONCLUSION

Pathogenic ATP1A3 variants play an essential role in the pathogenesis of Sodium-Potassium pump disorders, and AHC is the most common phenotype. The treatment strategies and prognosis depend on the phenotype categories caused by different variation sites and types. The correlation between the genotype and phenotype requires further exploration.

WHAT IS KNOWN

• Pathogenic heterozygous ATP1A3 variants cause a spectrum of neurological phenotypes, and ATP1A3-disorders are viewed as a phenotypic continuum presenting with atypical and overlapping features. • The genotype-phenotype correlation of ATP1A3-disorders remains unclear.

WHAT IS NEW

• In this study, the genotypes and phenotypes of ATP1A3-related disorders from Southwest of China were described. The splice-site variation c.2542+1G>T was detected for the first time in ATP1A3-related disorders. • The prognosis of twins with AHC p. Gly947Arg was more serious than AHC cases with other variants, which was inconsistent with previous reports. The phenomenon indicated the diversity of the correlation between the genotype and phenotype.

Temperature instability of a mutation at a multidomain junction in Na,K-ATPase isoform ATP1A3 (p.Arg756His) produces a fever-induced neurological syndrome

ATP1A3 encodes the α3 isoform of Na,K-ATPase. In the brain, it is expressed only in neurons. Human ATP1A3 mutations produce a wide spectrum of phenotypes, but particular syndromes are associated with unique substitutions. For arginine 756, at the junction of membrane and cytoplasmic domains, mutations produce encephalopathy during febrile infections. Here we tested the pathogenicity of p.Arg756His (R756H) in isogenic mammalian cells. R756H protein had sufficient transport activity to support cells when endogenous ATP1A1 was inhibited. It had half the turnover rate of wildtype, reduced affinity for Na⁺, and increased affinity for K⁺. There was modest endoplasmic reticulum retention during biosynthesis at 37 °C but little benefit from the folding drug phenylbutyrate (4-PBA), suggesting a tolerated level of misfolding. When cells were incubated at just 39 °C, however, α3 protein level dropped without loss of β subunit, paralleled by an increase of endogenous α1. Elevated temperature resulted in internalization of α3 from the surface along with some β subunit, accompanied by cytoplasmic redistribution of a marker of lysosomes and endosomes, lysosomal-associated membrane protein 1. After return to 37 °C, α3 protein levels recovered with cycloheximide-sensitive new protein synthesis. Heating in vitro showed activity loss at a rate 20- to 30-fold faster than wildtype, indicating a temperature-dependent destabilization of protein structure. Arg756 appears to confer thermal resistance as an anchor, forming hydrogen bonds among four linearly distant parts of the Na,K-ATPase structure. Taken together, our observations are consistent with fever-induced symptoms in patients.

Visceral organ morphogenesis via calcium-patterned muscle constrictions

Organ architecture is often composed of multiple laminar tissues arranged in concentric layers. During morphogenesis, the initial geometry of visceral organs undergoes a sequence of folding, adopting a complex shape that is vital for function. Genetic signals are known to impact form, yet the dynamic and mechanical interplay of tissue layers giving rise to organs' complex shapes remains elusive. Here, we trace the dynamics and mechanical interactions of a developing visceral organ across tissue layers, from subcellular to organ scale in vivo. Combining deep tissue light-sheet microscopy for in toto live visualization with a novel computational framework for multilayer analysis of evolving complex shapes, we find a dynamic mechanism for organ folding using the embryonic midgut of Drosophila as a model visceral organ. Hox genes, known regulators of organ shape, control the emergence of high-frequency calcium pulses. Spatiotemporally patterned calcium pulses trigger muscle contractions via myosin light chain kinase. Muscle contractions, in turn, induce cell shape change in the adjacent tissue layer. This cell shape change collectively drives a convergent extension pattern. Through tissue incompressibility and initial organ geometry, this in-plane shape change is linked to out-of-plane organ folding. Our analysis follows tissue dynamics during organ shape change in vivo, tracing organ-scale folding to a high-frequency molecular mechanism. These findings offer a mechanical route for gene expression to induce organ shape change: genetic patterning in one layer triggers a physical process in the adjacent layer – revealing post-translational mechanisms that govern shape change.

Restoration of Sarco/Endoplasmic Reticulum Ca2+-ATPase Activity Functions as a Pivotal Therapeutic Target of Anti-Glutamate-Induced Excitotoxicity to Attenuate Endoplasmic Reticulum Ca2+ Depletion

Glutamate-induced excitotoxicity is a pathological basis of many acute/chronic neurodegenerative diseases. Sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA2b) is a membrane-embedded P-type ATPase pump that manages the translocation of calcium ions (Ca²⁺) from cytosol into the lumen of the endoplasmic reticulum (ER) calcium stores. It participates in a wide range of biological functions in the central nervous system (CNS). However, the role of SERCA2b in glutamate-induced excitotoxicity and its mechanism must be elucidated. Herein, we demonstrate that SERCA2b mutants exacerbate the excitotoxicity of hypo-glutamate stimulation on HT22 cells. In this study, SERCA2b mutants accelerated Ca²⁺ depletion through loss-of-function (reduced pumping capacity) or gain-of-function (acquired leakage), resulting in ER stress. In addition, the occurrence of ER Ca²⁺ depletion increased mitochondria-associated membrane formation, which led to mitochondrial Ca²⁺ overload and dysfunction. Moreover, the enhancement of SERCA2b pumping capacity or inhibition of Ca²⁺ leakage attenuated Ca²⁺ depletion and impeded excitotoxicity in response to hypo-glutamate stimulation. In conclusion, SERCA2b mutants exacerbate ER Ca²⁺-depletion-mediated excitotoxicity in glutamate-sensitive HT22 cells. The mechanism of disruption is mainly related to the heterogeneity of SERCA2b mutation sites. Stabilization of SRECA2b function is a critical therapeutic approach against glutamate-induced excitotoxicity. These data will expand understanding of organelle regulatory networks and facilitate the discovery and creation of drugs against excitatory/inhibitory imbalance in the CNS.

Early role for a Na+,K+-ATPase (ATP1A3) in brain development

By evaluating children with a malformed cerebral cortex, we identified an ATPase pump (ATP1A3) with an early role in brain development. The ATP1A3 pump maintains the physiological concentration of sodium and potassium ions in cells, a process critical for osmotic equilibrium and membrane potential across several developing cell populations. We employed single-cell sequencing approaches to identify key enrichments for ATP1A3 expression during human cortex development. Unravelling this early cell-type–specific pathophysiology in the developing brain offers a potential basis for the treatment of ATP1A3-related diseases affecting prenatal and early childhood development.

Osmotic equilibrium and membrane potential in animal cells depend on concentration gradients of sodium (Na⁺) and potassium (K⁺) ions across the plasma membrane, a function catalyzed by the Na⁺,K⁺-ATPase α-subunit. Here, we describe ATP1A3 variants encoding dysfunctional α3-subunits in children affected by polymicrogyria, a developmental malformation of the cerebral cortex characterized by abnormal folding and laminar organization. To gain cell-biological insights into the spatiotemporal dynamics of prenatal ATP1A3 expression, we built an ATP1A3 transcriptional atlas of fetal cortical development using mRNA in situ hybridization and transcriptomic profiling of ∼125,000 individual cells with single-cell RNA sequencing (Drop-seq) from 11 areas of the midgestational human neocortex. We found that fetal expression of ATP1A3 is most abundant to a subset of excitatory neurons carrying transcriptional signatures of the developing subplate, yet also maintains expression in nonneuronal cell populations. Moving forward a year in human development, we profiled ∼52,000 nuclei from four areas of an infant neocortex and show that ATP1A3 expression persists throughout early postnatal development, most predominantly in inhibitory neurons, including parvalbumin interneurons in the frontal cortex. Finally, we discovered the heteromeric Na⁺,K⁺-ATPase pump complex may form nonredundant cell-type–specific α-β isoform combinations, including α3-β1 in excitatory neurons and α3-β2 in inhibitory neurons. Together, the developmental malformation phenotype of affected individuals and single-cell ATP1A3 expression patterns point to a key role for α3 in human cortex development, as well as a cell-type basis for pre- and postnatal ATP1A3-associated diseases.

Oxidative modification impairs SERCA activity in Drosophila and human cell models of Parkinson's disease

DJ-1 is a causative gene for familial Parkinson's disease (PD) with different functions, standing out its role against oxidative stress (OS). Accordingly, PD model flies harboring a mutation in the DJ-1β gene (the Drosophila ortholog of human DJ-1) show high levels of OS markers like protein carbonylation, a common post-translational modification that may alter protein function. To increase our understanding of PD pathogenesis as well as to discover potential therapeutic targets for pharmacological intervention, we performed a redox proteomic assay in DJ-1β mutant flies. Among the proteins that showed increased carbonylation levels in PD model flies, we found SERCA, an endoplasmic reticulum Ca²⁺ channel that plays an important role in Ca²⁺ homeostasis. Interestingly, several studies have supported the involvement of Ca²⁺ dyshomeostasis in PD. Thus, we decided to study the relation between SERCA activity and PD physiopathology. Our results showed that SERCA enzymatic activity is significantly reduced in DJ-1β mutant flies, probably as a consequence of OS-induced carbonylation, as well as in a human cell PD model based on DJ-1-deficiency. Indeed, higher carbonylation levels of SERCA were also observed in DJ-1-deficient cells compared to controls. In addition, the specific activator of SERCA, CDN1163, was also able to restore PD-related phenotypes in both familial PD models by increasing SERCA activity. Taken together, our results indicate that impaired SERCA activity due to oxidative modification may play a role in PD physiopathology. Furthermore, we demonstrate that therapeutic strategies addressing SERCA activation could be beneficial to treat this disease as shown for CDN1163.

Neuronal modeling of alternating hemiplegia of childhood reveals transcriptional compensation and replicates a trigger-induced phenotype

Alternating hemiplegia of childhood (AHC) is a rare neurodevelopmental disease caused by heterozygous de novo missense mutations in the ATP1A3 gene that encodes the neuronal specific α3 subunit of the Na,K-ATPase (NKA) pump. Mechanisms underlying patient episodes including environmental triggers remain poorly understood, and there are no empirically proven treatments for AHC. In this study, we generated patient-specific induced pluripotent stem cells (iPSCs) and isogenic controls for the E815K ATP1A3 mutation that causes the most phenotypically severe form of AHC. Using an in vitro iPSC-derived cortical neuron disease model, we found elevated levels of ATP1A3 mRNA in AHC lines compared to controls, without significant perturbations in protein expression. Microelectrode array analyses demonstrated that in cortical neuronal cultures, ATP1A3^+/E815K iPSC-derived neurons displayed less overall activity than neurons differentiated from isogenic mutation-corrected and unrelated control cell lines. However, induction of cellular stress by elevated temperature revealed a hyperactivity phenotype following heat stress in ATP1A3^+/E815K neurons compared to control lines. Treatment with flunarizine, a drug commonly used to prevent AHC episodes, did not impact this stress-triggered phenotype. These findings support the use of iPSC-derived neuronal cultures for studying complex neurodevelopmental conditions such as AHC and provide a platform for mechanistic discovery in a human disease model.

Neural shutdown under stress: an evolutionary perspective on spreading depolarization

Neural function depends on maintaining cellular membrane potentials as the basis for electrical signaling. Yet, in mammals and insects, neuronal and glial membrane potentials can reversibly depolarize to zero, shutting down neural function by the process of spreading depolarization (SD) that collapses the ion gradients across membranes. SD is not evident in all metazoan taxa with centralized nervous systems. We consider the occurrence and similarities of SD in different animals and suggest that it is an emergent property of nervous systems that have evolved to control complex behaviors requiring energetically expensive, rapid information processing in a tightly regulated extracellular environment. Whether SD is beneficial or not in mammals remains an open question. However, in insects, it is associated with the response to harsh environments and may provide an energetic advantage that improves the chances of survival. The remarkable similarity of SD in diverse taxa supports a model systems approach to understanding the mechanistic underpinning of human neuropathology associated with migraine, stroke, and traumatic brain injury.

Primary Active Ca2+ Transport Systems in Health and Disease

Calcium ions (Ca²⁺) are prominent cell signaling effectors that regulate a wide variety of cellular processes. Among the different players in Ca²⁺ homeostasis, primary active Ca²⁺ transporters are responsible for keeping low basal Ca²⁺ levels in the cytosol while establishing steep Ca²⁺ gradients across intracellular membranes or the plasma membrane. This review summarizes our current knowledge on the three types of primary active Ca²⁺-ATPases: the sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) pumps, the secretory pathway Ca²⁺- ATPase (SPCA) isoforms, and the plasma membrane Ca²⁺-ATPase (PMCA) Ca²⁺-transporters. We first discuss the Ca²⁺ transport mechanism of SERCA1a, which serves as a reference to describe the Ca²⁺ transport of other Ca²⁺ pumps. We further highlight the common and unique features of each isoform and review their structure-function relationship, expression pattern, regulatory mechanisms, and specific physiological roles. Finally, we discuss the increasing genetic and in vivo evidence that links the dysfunction of specific Ca²⁺-ATPase isoforms to a broad range of human pathologies, and highlight emerging therapeutic strategies that target Ca²⁺ pumps.

Movement disorders in pregnancy

Movement disorders in women during pregnancy are uncommon. Therefore, high quality studies are limited, and guidelines are lacking for the treatment of movement disorders in pregnancy, thus posing a significant therapeutic challenge for the treating physicians. In this chapter, we discuss movement disorders that arise during pregnancy and the preexisting movement disorders during pregnancy. Common conditions encountered in pregnancy include but are not limited to restless legs syndrome, chorea gravidarum, Parkinson disease, essential tremor, and Huntington disease as well as more rare movement disorders (Wilson's disease, dystonia, etc.). This chapter summarizes the published literature on movement disorders and pharmacologic and surgical considerations for neurologists and physicians in other specialties caring for patients who are pregnant or considering pregnancy.

Characterization of Hailey-Hailey Disease-mutants in presence and absence of wild type SPCA1 using Saccharomyces cerevisiae as model organism

Hailey-Hailey disease is an autosomal genetic disease caused by mutations in one of the two ATP2C1 alleles encoding the secretory pathway Ca²⁺/Mn²⁺-ATPase, hSPCA1. The disease almost exclusively affects epidermis, where it mainly results in acantholysis of the suprabasal layers. The etiology of the disease is complex and not well understood. We applied a yeast based complementation system to characterize fourteen disease-causing ATP2C1 missense mutations in presence or absence of wild type ATP2C1 or ATP2A2, encoding SERCA2. In our yeast model system, mutations in ATP2C1 affected Mn²⁺ transport more than Ca²⁺ transport as twelve out of fourteen mutations were unable to complement Mn²⁺ sensitivity while thirteen out of fourteen to some extent complemented the high Ca²⁺requirement. Nine out of fourteen mutations conferred a cold sensitive complementation capacity. In absence of a wild type ATP2C1 allele, twelve out of fourteen mutations induced an unfolded protein response indicating that in vivo folding of hSPCA1 is sensitive to disease causing amino acid substitutions and four of the fourteen mutations caused the hSPCA1 protein to accumulate in the vacuolar membrane. Co-expression of either wild type ATP2C1 or ATP2A2 prevented induction of the unfolded protein response and hSPCA1 mis-localization.

Relapsing encephalopathy with cerebellar ataxia are caused by variants involving p.Arg756 in ATP1A3

Mutations in ATP1A3 lead to different phenotypes having in common acute neurological decompensation episodes triggered by a specific circumstance and followed by sequelae. Alongside Alternating Hemiplegia of Childhood (AHC), Rapid-onset Dystonia Parkinsonism (RDP) and Cerebellar ataxia, Areflexia, Pes cavus, Optic atrophy, Sensorineural hearing loss syndrome (CAPOS), a new Relapsing Encephalopathy with Cerebellar Ataxia (RECA) phenotype was published in 2015. We describe herein eight new pediatric cases. Most of them had no specific history when the first neurological decompensation episode occurred, before the age of 5 years, triggered by fever with severe paralytic hypotonia followed by ataxia with or without abnormal movements. Neurological sequelae with ataxia as the predominant symptom were present after the first episode in three cases and after at least one subsequent relapse in five cases. Five of the eight cases had a familial involvement with one of the two parents affected. The phenotype-genotype correlation is unequivocal with the causal substitution always located at position 756. The pathophysiology of the dysfunctions of the mutated ATPase pump, triggered by fever is unknown. Severe recurrent neurological decompensation episodes triggered by fever, without any metabolic cause, should lead to the sequencing of ATP1A3.

Fever-related ataxia: a case report of CAPOS syndrome

Background

CAPOS (Cerebellar ataxia, Areflexia, Pes cavus, Optic atrophy and Sensorineural hearing loss) syndrome is caused by the heterozygous mutation, c.2452G > A, in the ATP1A3 gene. Other mutations in this gene can cause a spectrum of overlapping phenotypes including alternating hemiplegia of childhood, rapid onset dystonia parkinsonism, early infantile epileptic encephalopathy and fever induced paroxysmal weakness and encephalopathy. The phenotype is still mistaken for mitochondrial/metabolic disorders and follow up studies are scare.

Case presentation

We report a 20 year old Norwegian male with ataxia, sensorineural deafness and visual loss. Before the age of five he experienced three fever related episodes of acute neurological deterioration when he temporarily lost his acquired motor skills and developed persistent gait and limb ataxia. In childhood, he developed bilateral optic atrophy and bilateral sensorineural hearing loss. Motor skills improved and at age 20 the patient showed a mild ataxia, hearing loss and reduced vision. A c.2452G > A mutation in the ATP1A3 gene was identified and CAPOS syndrome was confirmed.

Conclusions

This is the first Norwegian patient reported with CAPOS syndrome. Our patient had a de novo, previously identified ATP1A3 mutation. The combination of recurrent episodes of fever related ataxia, loss of motor skills in early childhood, and early onset hearing and vision loss is typical of CAPOS syndrome. Previous reports suggest a gradual progression of the disease after the initial episodes, while this patient showed a good outcome with improvement of motor skills from adolescence long after the last deterioration episode.

Genotype-structure-phenotype relationships diverge in paralogs ATP1A1, ATP1A2, and ATP1A3

Objective

We tested the assumption that closely related genes should have similar pathogenic variants by analyzing >200 pathogenic variants in a gene family with high neurologic impact and high sequence identity, the Na,K-ATPases ATP1A1, ATP1A2, and ATP1A3.

Methods

Data sets of disease-associated variants were compared. Their equivalent positions in protein crystal structures were used for insights into pathogenicity and correlated with the phenotype and conservation of homology.

Results

Relatively few mutations affected the corresponding amino acids in 2 genes. In the membrane domain of ATP1A3 (primarily expressed in neurons), variants producing milder neurologic phenotypes had different structural positions than variants producing severe phenotypes. In ATP1A2 (primarily expressed in astrocytes), membrane domain variants characteristic of severe phenotypes in ATP1A3 were absent from patient data. The known variants in ATP1A1 fell into 2 distinct groups. Sequence conservation was an imperfect indicator: it varied among structural domains, and some variants with demonstrated pathogenicity were in low conservation sites.

Conclusions

Pathogenic variants varied between genes despite high sequence identity, and there is a genotype-structure-phenotype relationship in ATP1A3 that correlates with neurologic outcomes. The absence of "severe" pathogenic variants in ATP1A2 patients predicts that they will manifest either in a different tissue or by death in utero and that new ATP1A1 variants will produce additional phenotypes. It is important that some variants in poorly conserved amino acids are nonetheless pathogenic and could be incorrectly predicted to be benign.

Functional consequences of the CAPOS mutation E818K of Na+,K+-ATPase

The cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss (CAPOS) syndrome is caused by the single mutation E818K of the α3-isoform of Na⁺,K⁺-ATPase. Here, using biochemical and electrophysiological approaches, we examined the functional characteristics of E818K, as well as of E818Q and E818A mutants. We found that these amino acid substitutions reduce the apparent Na⁺ affinity at the cytoplasmic-facing sites of the pump protein and that this effect is more pronounced for the lysine and glutamine substitutions (3-4-fold) than for the alanine substitution. The electrophysiological measurements indicated a more conspicuous, ∼30-fold reduction of apparent Na⁺ affinity for the extracellular-facing sites in the CAPOS mutant, which was related to an accelerated transition between the phosphoenzyme intermediates E₁P and E₂P. The apparent affinity for K⁺ activation of the ATPase activity was unaffected by these substitutions, suggesting that primarily the Na⁺-specific site III is affected. Furthermore, the apparent affinities for ATP and vanadate were WT-like in E818K, indicating a normal E₁-E₂ equilibrium of the dephosphoenzyme. Proton-leak currents were not increased in E818K. However, the CAPOS mutation caused a weaker voltage dependence of the pumping rate and a stronger inhibition by cytoplasmic K⁺ than the WT enzyme, which together with the reduced Na⁺ affinity of the cytoplasmic-facing sites precluded proper pump activation under physiological conditions. The functional deficiencies could be traced to the participation of Glu-818 in an intricate hydrogen-bonding/salt-bridge network, connecting it to key residues involved in Na⁺ interaction at site III.

SERCA is critical to control the Bowditch effect in the heart

The Bowditch effect or staircase phenomenon is the increment or reduction of contractile force when heart rate increases, defined as either a positive or negative staircase. The healthy and failing human heart both show positive or negative staircase, respectively, but the causes of these distinct cardiac responses are unclear. Different experimental approaches indicate that while the level of Ca2+ in the sarcoplasmic reticulum is critical, the molecular mechanisms are unclear. Here, we demonstrate that Drosophila melanogaster shows a negative staircase which is associated to a slight but significant frequency-dependent acceleration of relaxation (FDAR) at the highest stimulation frequencies tested. We further showed that the type of staircase is oppositely modified by two distinct SERCA mutations. The dominant conditional mutation SERCAA617T induced positive staircase and arrhythmia, while SERCAE442K accentuated the negative staircase of wild type. At the stimulation frequencies tested, no significant FDAR could be appreciated in mutant flies. The present results provide evidence that two individual mutations directly modify the type of staircase occurring within the heart and suggest an important role of SERCA in regulating the Bowditch effect.

Novel pregnancy-triggered episodes of CAPOS syndrome

Cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss (CAPOS) syndrome (OMIM# 601338) is a rare autosomal dominant disorder characterized by episodic, fever-induced ataxic encephalopathy in childhood with residual symptoms. All identified patients have the same heterozygous missense variant c.2452G>A (p.Glu818Lys) in the ATP1A3 gene, encoding Na⁺ /K⁺ ATPase α3. We describe a large CAPOS pedigree with three generations of affected members, the first ascertained in the United States. Deafness, optic atrophy, and pes cavus were present in all three members of the family evaluated. In addition, one of the affected individuals experienced markedly worsening features during her three pregnancies and in the immediate postpartum period, a potential element of the natural history of CAPOS previously unreported. We conclude that the triggering factors and clinical spectrum of pathogenic ATP1A3 variants may be broader than previously described. Targeted sequencing of ATP1A3 should be considered in any patient presenting with cerebellar ataxia triggered by febrile illness, or pregnancy and delivery, especially in the presence of sensorineural hearing loss, optic atrophy, pes cavus, or early childhood history of acute encephalopathic ataxia. Prophylactic administration of acetazolamide or flunarizine may prevent acute episodes of ataxia or mitigate neurologic symptoms, although their efficacies have not been well studied.

Of channels and pumps: different ways to boost the aldosterone?

The mineralocorticoid aldosterone is a major factor controlling the salt and water balance and thereby also the arterial blood pressure. Accordingly, primary aldosteronism (PA) characterized by an inappropriately high aldosterone secretion is the most common form of secondary hypertension. The physiological stimulation of aldosterone synthesis in adrenocortical glomerulosa cells by angiotensin II and an increased plasma K⁺ concentration depends on a membrane depolarization and an increase in the cytosolic Ca²⁺ activity. Recurrent gain-of-function mutations of ion channels and transporters have been identified in a majority of cases of aldosterone-producing adenomas and in familial forms of PA. In this review, the physiological role of these genes in the regulation of aldosterone synthesis and the altered function of the mutant proteins as well are described. The specific changes of the membrane potential and the cellular ion homoeostasis in adrenal cells expressing the different mutants are compared, and their impact on autonomous aldosterone production and proliferation is discussed.

Early Diagnosis of CAPOS Syndrome Before Acute-Onset Ataxia-Review of the Literature and a New Family

BACKGROUND

CAPOS syndrome (cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss) is a rare disease that has been reported in 22 patients so far. In all cases, the mutation c.2452G>A (p.Glu818Lys) in the ATP1A3 gene was identified. Patients typically present at an early age with an acute-onset fever-induced episode of ataxia frequently associated with encephalopathy and weakness. They usually present one to three episodes. The acute symptoms improve within days, but most patients show slow progression afterward.

METHODS

We describe three new patients, a woman and her two sons diagnosed with CAPOS syndrome. A systematic review of literature on previously reported patients was performed.

RESULTS

The first son presented with acute-onset ataxia, encephalopathy, and sensorineural hearing loss, induced by febrile illness. The second one developed generalized areflexia and mild instability without an acute episode. The mother had been previously diagnosed with sensorineural hearing loss and optic nerve atrophy. The c.2452G>A mutation in ATP1A3 was found in all three patients.

CONCLUSION

Only 25 Individuals with CAPOS syndrome have been reported, including our family. This is the first time a Spanish family has been described. The fact that both siblings were assessed before the first acute-onset episode contributes to the description of early symptoms and signs of the disease, which could aid early diagnosis and management before the onset of acute episodes.

Structure/activity relationship of thapsigargin inhibition on the purified Golgi/secretory pathway Ca2+/Mn2+-transport ATPase (SPCA1a)

The Golgi/secretory pathway Ca²⁺/Mn²⁺-transport ATPase (SPCA1a) is implicated in breast cancer and Hailey-Hailey disease. Here, we purified recombinant human SPCA1a from Saccharomyces cerevisiae and measured Ca²⁺-dependent ATPase activity following reconstitution in proteoliposomes. The purified SPCA1a displays a higher apparent Ca²⁺ affinity and a lower maximal turnover rate than the purified sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA1a). The lipids cholesteryl hemisuccinate, linoleamide/oleamide, and phosphatidylethanolamine inhibit and phosphatidic acid and sphingomyelin enhance SPCA1a activity. Moreover, SPCA1a is blocked by micromolar concentrations of the commonly used SERCA1a inhibitors thapsigargin (Tg), cyclopiazonic acid, and 2,5-di-tert-butylhydroquinone. Because tissue-specific targeting of SERCA2b by Tg analogues is considered for prostate cancer therapy, the inhibition of SPCA1a by Tg might represent an off-target risk. We assessed the structure-activity relationship (SAR) of Tg for SPCA1a by in silico modeling, site-directed mutagenesis, and measuring the potency of a series of Tg analogues. These indicate that Tg and the analogues are bound via the Tg scaffold but with lower affinity to the same homologous cavity as on the membrane surface of SERCA1a. The lower Tg affinity may depend on a more flexible binding cavity in SPCA1a, with low contributions of the Tg O-3, O-8, and O-10 chains to the binding energy. Conversely, the protein interaction of the Tg O-2 side chain with SPCA1a appears comparable with that of SERCA1a. These differences define a SAR of Tg for SPCA1a distinct from that of SERCA1a, indicating that Tg analogues with a higher specificity for SPCA1a can probably be developed.

Cellular Pathophysiology of an Adrenal Adenoma-Associated Mutant of the Plasma Membrane Ca(2+)-ATPase ATP2B3

Adrenal aldosterone-producing adenomas (APAs) are a main cause for primary aldosteronism leading to arterial hypertension. Physiologically, aldosterone production in the adrenal gland is stimulated by angiotensin II and high extracellular potassium. These stimuli lead to a depolarization of the plasma membrane and, as a consequence, an increase of intracellular Ca(2+). Mutations of the plasma membrane Ca(2+)-ATPase ATP2B3 have been found in APAs with a prevalence of 0.6%-3.1%. Here, we investigated the effects of the APA-associated ATP2B3(Leu425_Val426del) mutation in adrenocortical NCI-H295R and human embryonic kidney (HEK-293) cells. Ca(2+) measurements revealed a higher basal Ca(2+) level in cells expressing the mutant ATP2B3. This rise in intracellular Ca(2+) was even more pronounced under conditions with high extracellular Ca(2+) pointing to an increased Ca(2+) influx associated with the mutated protein. Furthermore, cells with the mutant ATP2B3 appeared to have a reduced capacity to export Ca(2+) suggesting a loss of the physiological pump function. Surprisingly, expression of the mutant ATP2B3 caused a Na(+)-dependent inward current that strongly depolarized the plasma membrane and compromised the cytosolic cation composition. In parallel to these findings, mRNA expression of the cytochrome P450, family 11, subfamily B, polypeptide 2 (aldosterone synthase) was substantially increased and aldosterone production was enhanced in cells overexpressing mutant ATP2B3. In summary, the APA-associated ATP2B3(Leu425_Val426del) mutant promotes aldosterone production by at least 2 different mechanisms: 1) a reduced Ca(2+) export due to the loss of the physiological pump function; and 2) an increased Ca(2+) influx due to opening of depolarization-activated Ca(2+) channels as well as a possible Ca(2+) leak through the mutated pump.

The Genetic Homogeneity of CAPOS Syndrome: Four New Patients With the c.2452G>A (p.Glu818Lys) Mutation in the ATP1A3 Gene

BACKGROUND

The clinical syndrome of cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss (CAPOS) was first described 20 years ago, but it was only recently that whole exome sequencing unveiled the causative mutation in the ATP1A3 gene. We present four patients from the seventh and eighth family identified worldwide, provide a critical review of all patients published thus far, and speculate about the pathophysiologic processes underlying the acute neurological manifestations.

CLINICAL OBSERVATIONS

The individuals presented here experienced one to three paroxysmal, short-lasting episodes in childhood with cerebellar symptoms and signs, hypotonia, ophthalmoparesis, motor weakness, areflexia, and/or lethargy that were consistently associated with febrile illness. An underlying c.2452G>A mutation in the ATP1A3 gene was found in all four individuals. Besides the persisting CAPOS features, other possibly related sequelae included dystonia, myoclonus, and emotional and behavioral changes. After initiation of acetazolamide in two patients, no further episodes occurred.

CONCLUSION

Targeted sequencing of the ATP1A3 gene is recommended in children exhibiting paroxysmal, fever-induced ataxia and in adults with a more or less stationary or slowly progressive cerebellar syndrome since childhood accompanied by mixed combinations of areflexia, pes cavus, profound visual impairment, and/or sensorineural hearing loss. Similar to some other types of episodic ataxia, acetazolamide may be considered in patients with CAPOS syndrome to prevent or attenuate bouts of ataxia, but this requires further study.

ATP1A3 Mutation in Adult Rapid-Onset Ataxia

A 21-year old male presented with ataxia and dysarthria that had appeared over a period of months. Exome sequencing identified a de novo missense variant in ATP1A3, the gene encoding the α3 subunit of Na,K-ATPase. Several lines of evidence suggest that the variant is causative. ATP1A3 mutations can cause rapid-onset dystonia-parkinsonism (RDP) with a similar age and speed of onset, as well as severe diseases of infancy. The patient's ATP1A3 p.Gly316Ser mutation was validated in the laboratory by the impaired ability of the expressed protein to support the growth of cultured cells. In a crystal structure of Na,K-ATPase, the mutated amino acid was directly apposed to a different amino acid mutated in RDP. Clinical evaluation showed that the patient had many characteristics of RDP, however he had minimal fixed dystonia, a defining symptom of RDP. Successive magnetic resonance imaging (MRI) revealed progressive cerebellar atrophy, explaining the ataxia. The absence of dystonia in the presence of other RDP symptoms corroborates other evidence that the cerebellum contributes importantly to dystonia pathophysiology. We discuss the possibility that a second de novo variant, in ubiquilin 4 (UBQLN4), a ubiquitin pathway component, contributed to the cerebellar neurodegenerative phenotype and differentiated the disease from other manifestations of ATP1A3 mutations. We also show that a homozygous variant in GPRIN1 (G protein-regulated inducer of neurite outgrowth 1) deletes a motif with multiple copies and is unlikely to be causative.

Single Enzyme Experiments Reveal a Long-Lifetime Proton Leak State in a Heme-Copper Oxidase

Heme-copper oxidases (HCOs) are key enzymes in prokaryotes and eukaryotes for energy production during aerobic respiration. They catalyze the reduction of the terminal electron acceptor, oxygen, and utilize the Gibbs free energy to transport protons across a membrane to generate a proton (ΔpH) and electrochemical gradient termed proton motive force (PMF), which provides the driving force for the adenosine triphosphate (ATP) synthesis. Excessive PMF is known to limit the turnover of HCOs, but the molecular mechanism of this regulatory feedback remains relatively unexplored. Here we present a single-enzyme study that reveals that cytochrome bo₃ from Escherichia coli, an HCO closely homologous to Complex IV in human mitochondria, can enter a rare, long-lifetime leak state during which proton flow is reversed. The probability of entering the leak state is increased at higher ΔpH. By rapidly dissipating the PMF, we propose that this leak state may enable cytochrome bo₃, and possibly other HCOs, to maintain a suitable ΔpH under extreme redox conditions.

Distinct neurological disorders with ATP1A3 mutations

Genetic research has shown that mutations that modify the protein-coding sequence of ATP1A3, the gene encoding the α3 subunit of Na(+)/K(+)-ATPase, cause both rapid-onset dystonia parkinsonism and alternating hemiplegia of childhood. These discoveries link two clinically distinct neurological diseases to the same gene, however, ATP1A3 mutations are, with one exception, disease-specific. Although the exact mechanism of how these mutations lead to disease is still unknown, much knowledge has been gained about functional consequences of ATP1A3 mutations using a range of in-vitro and animal model systems, and the role of Na(+)/K(+)-ATPases in the brain. Researchers and clinicians are attempting to further characterise neurological manifestations associated with mutations in ATP1A3, and to build on the existing molecular knowledge to understand how specific mutations can lead to different diseases.