Bi-allelic LETM1 variants perturb mitochondrial ion homeostasis leading to a clinical spectrum with predominant nervous system involvement.

Leucine zipper-EF-hand containing transmembrane protein 1 (LETM1) encodes an inner mitochondrial membrane protein with an osmoregulatory function controlling mitochondrial volume and ion homeostasis. The putative association of LETM1 with a human disease was initially suggested in Wolf-Hirschhorn syndrome, a disorder that results from de novo monoallelic deletion of chromosome 4p16.3, a region encompassing LETM1. Utilizing exome sequencing and international gene-matching efforts, we have identified 18 affected individuals from 11 unrelated families harboring ultra-rare bi-allelic missense and loss-of-function LETM1 variants and clinical presentations highly suggestive of mitochondrial disease. These manifested as a spectrum of predominantly infantile-onset (14/18, 78%) and variably progressive neurological, metabolic, and dysmorphic symptoms, plus multiple organ dysfunction associated with neurodegeneration. The common features included respiratory chain complex deficiencies (100%), global developmental delay (94%), optic atrophy (83%), sensorineural hearing loss (78%), and cerebellar ataxia (78%) followed by epilepsy (67%), spasticity (53%), and myopathy (50%). Other features included bilateral cataracts (42%), cardiomyopathy (36%), and diabetes (27%). To better understand the pathogenic mechanism of the identified LETM1 variants, we performed biochemical and morphological studies on mitochondrial K+/H+ exchange activity, proteins, and shape in proband-derived fibroblasts and muscles and in Saccharomyces cerevisiae, which is an important model organism for mitochondrial osmotic regulation. Our results demonstrate that bi-allelic LETM1 variants are associated with defective mitochondrial K+ efflux, swollen mitochondrial matrix structures, and loss of important mitochondrial oxidative phosphorylation protein components, thus highlighting the implication of perturbed mitochondrial osmoregulation caused by LETM1 variants in neurological and mitochondrial pathologies.

Calcium signaling in mitochondrial intermembrane space.

The mitochondrial intermembrane space (IMS) is a highly protected compartment, second only to the matrix. It is a crucial bridge, coordinating mitochondrial activities with cellular processes such as metabolites, protein, lipid, and ion exchange. This regulation influences signaling pathways for metabolic activities and cellular homeostasis. The IMS harbors various proteins critical for initiating apoptotic cascades and regulating reactive oxygen species production by controlling the respiratory chain. Calcium (Ca2+), a key intracellular secondary messenger, enter the mitochondrial matrix via the IMS, regulating mitochondrial bioenergetics, ATP production, modulating cell death pathways. IMS acts as a regulatory site for Ca2+ entry due to the presence of different Ca2+ sensors such as MICUs, solute carriers (SLCs); ion exchangers (LETM1/SCaMCs); S100A1, mitochondrial glycerol-3-phosphate dehydrogenase, and EFHD1, each with unique Ca2+ binding motifs and spatial localizations. This review primarily emphasizes the role of these IMS-localized Ca2+ sensors concerning their spatial localization, mechanism, and molecular functions. Additionally, we discuss how these sensors contribute to the progression and pathogenesis of various human health conditions and diseases.

The Mysteries of LETM1 Pleiotropy.

LETM1 is a nuclear-encoded protein located in the inner mitochondrial membrane, playing a critical role in regulating mitochondrial cation and volume homeostasis. However, numerous studies on functional features, molecular interactions, and disease-associated effects of LETM1 revealed that LETM1 is also involved in other metabolic functions including glucose utilization, mitochondrial DNA and ribosome organization, cristae architecture and respiratory complex stability. Undisputedly, osmoregulatory processes are essential for mitochondrial functionality, but the pleiotropic aspects of LETM1 challenges us to understand the core function of LETM1, which still remains elusive. In this review, we provide an overview of the current knowledge and latest developments regarding the activities involving LETM1. We highlight various findings that offer different functional perspectives and ideas on the core function of LETM1. Specifically, we emphasize data supporting LETM1's role as a mitochondrial translational factor, K+/H+ exchanger, or Ca2+/H+ exchanger, along with recent findings on its interaction with ATAD3A and TMBIM5. We also present the severe clinical implications of LETM1 deficiency. Finally, we discuss emerging questions raised by the different views on LETM1, which need to be addressed to guide future research directions and ultimately resolve the function of this essential protein and develop targeted therapeutic strategies.

TMBIM5 is the Ca2+ /H+ antiporter of mammalian mitochondria.

Mitochondrial Ca2+ ions are crucial regulators of bioenergetics and cell death pathways. Mitochondrial Ca2+ content and cytosolic Ca2+ homeostasis strictly depend on Ca2+ transporters. In recent decades, the major players responsible for mitochondrial Ca2+ uptake and release have been identified, except the mitochondrial Ca2+ /H+ exchanger (CHE). Originally identified as the mitochondrial K+ /H+ exchanger, LETM1 was also considered as a candidate for the mitochondrial CHE. Defining the mitochondrial interactome of LETM1, we identify TMBIM5/MICS1, the only mitochondrial member of the TMBIM family, and validate the physical interaction of TMBIM5 and LETM1. Cell-based and cell-free biochemical assays demonstrate the absence or greatly reduced Na+ -independent mitochondrial Ca2+ release in TMBIM5 knockout or pH-sensing site mutants, respectively, and pH-dependent Ca2+ transport by recombinant TMBIM5. Taken together, we demonstrate that TMBIM5, but not LETM1, is the long-sought mitochondrial CHE, involved in setting and regulating the mitochondrial proton gradient. This finding provides the final piece of the puzzle of mitochondrial Ca2+ transporters and opens the door to exploring its importance in health and disease, and to developing drugs modulating Ca2+ exchange.

LETM1: Essential for Mitochondrial Biology and Cation Homeostasis?

Mitochondrial function is essential for life. Therefore, it is unsurprising that perturbations in mitochondrial function have wide-ranging consequences in the cell. High-throughput screening has identified essential genes required for cellular survival and fitness. One such gene is LETM1. The undisputed function of LETM1 from yeast to human is to maintain the mitochondrial osmotic balance. Osmotic imbalance has been demonstrated to affect mitochondrial morphology, dynamics, and, more recently, metabolism. Whether conservation of osmotic homeostasis by LETM1 occurs by extrusion of excess mitochondrial potassium (K+), calcium (Ca2+), or both has been a matter of dispute over the past 10 years. In this Opinion, we report and discuss recent findings on LETM1 structure, essentiality, and function and its involvement in Wolf-Hirschhorn syndrome (WHS) and seizures.

Circ_0061140 Contributes to Ovarian Cancer Progression by Targeting miR-761/LETM1 Signaling.

Previous studies have suggested that circular RNAs (circRNAs) play important regulatory roles in cancer progression. Previous evidence exhibited the aberrant upregulation of circ_0061140 in ovarian cancer. However, the detailed role of circ_0061140 in ovarian cancer progression and its associated mechanism remain largely unknown and need further exploration. The expression of circ_0061140, microRNA-761 (miR-761) and leucine zipper and EF-hand containing transmembrane protein 1 (LETM1) was checked by reverse transcription-quantitative polymerase chain reaction (RT-qPCR) or western blot. Cell Counting Kit-8 (CCK8), colony formation, 5-Ethynyl-2'-deoxyuridine (EdU), flow cytometry, wound healing, transwell, and tube formation assays were conducted to assess cell functions. Dual-luciferase reporter assay and RNA immunoprecipitation (RIP) assay were performed to confirm the interaction between miR-761 and circ_0061140 or LETM1. Xenograft tumor model was established to analyze the role of circ_0061140 in tumor growth in vivo. Circ_0061140 expression was notably up-regulated in ovarian cancer tissues and cell lines. Circ_0061140 knockdown suppressed the proliferation, migration, invasion, and angiogenesis and triggered the apoptosis of ovarian cancer cells. Circ_0061140 directly interacted with miR-761, and circ_0061140 silencing-mediated anti-tumor effects were partly abolished by miR-761 knockdown in ovarian cancer cells. LETM1 was a direct target of miR-761, and LETM1 overexpression partly counteracted miR-761-induced anti-tumor effects. Circ_0061140 could up-regulate LETM1 expression by sponging miR-761. Circ_0061140 knockdown significantly suppressed xenograft tumor growth in vivo. Circ_0061140 aggravated ovarian cancer progression through miR-761-dependent regulation of LETM1.

Trypanosoma cruzi Letm1 is involved in mitochondrial Ca2+ transport, and is essential for replication, differentiation, and host cell invasion.

Leucine zipper-EF-hand containing transmembrane protein 1 (Letm1) is a mitochondrial inner membrane protein involved in Ca2+ and K+ homeostasis in mammalian cells. Here, we demonstrate that the Letm1 orthologue of Trypanosoma cruzi, the etiologic agent of Chagas disease, is important for mitochondrial Ca2+ uptake and release. The results show that both mitochondrial Ca2+ influx and efflux are reduced in TcLetm1 knockdown (TcLetm1-KD) cells and increased in TcLetm1 overexpressing cells, without alterations in the mitochondrial membrane potential. Remarkably, TcLetm1 knockdown or overexpression increases or does not affect mitochondrial Ca2+ levels in epimastigotes, respectively. TcLetm1-KD epimastigotes have reduced growth, and both overexpression and knockdown of TcLetm1 cause a defect in metacyclogenesis. TcLetm1-KD also affected mitochondrial bioenergetics. Invasion of host cells by TcLetm1-KD trypomastigotes and their intracellular replication is greatly impaired. Taken together, our findings indicate that TcLetm1 is important for Ca2+ homeostasis and cell viability in T cruzi.

LETMD1 is required for mitochondrial structure and thermogenic function of brown adipocytes.

Obesity and metabolic disorders caused by energy surplus pose an increasing concern within the global population. Brown adipose tissue (BAT) dissipates energy through mitochondrial non-shivering thermogenesis, thus representing a powerful agent against obesity. Here we explore the novel role of a mitochondrial outer membrane protein, LETM1-domain containing 1 (LETMD1), in BAT. We generated a knockout (Letmd1KO ) mouse model and analyzed BAT morphology, function and gene expression under various physiological conditions. While the Letmd1KO mice are born normally and have normal morphology and body weight, they lose multilocular brown adipocytes completely and have diminished mitochondrial abundance, DNA copy number, cristae structure, and thermogenic gene expression in the intrascapular BAT, associated with elevated reactive oxidative stress. In consequence, the Letmd1KO mice fail to maintain body temperature in response to acute cold exposure without food and become hypothermic within 4 h. Although the cold-exposed Letmd1KO mice can maintain body temperature in the presence of food, they cannot upregulate expression of uncoupling protein 1 (UCP1) and convert white to beige adipocytes, nor can they respond to adrenergic stimulation. These results demonstrate that LETMD1 is essential for mitochondrial structure and function, and thermogenesis of brown adipocytes.

Suppression of LETM1 inhibits the proliferation and stemness of colorectal cancer cells through reactive oxygen species-induced autophagy.

Leucine zipper-EF-hand-containing transmembrane protein 1 (LETM1) is a mitochondrial inner membrane protein that is highly expressed in various cancers. Although LETM1 is known to be associated with poor prognosis in colorectal cancer (CRC), its roles in autophagic cell death in CRC have not been explored. In this study, we examined the mechanisms through which LETM1 mediates autophagy in CRC. Our results showed that LETM1 was highly expressed in CRC tissues and that down-regulation of LETM1 inhibited cell proliferation and induced S-phase arrest. LETM1 silencing also suppressed cancer stem cell-like properties and induced autophagy in CRC cells. Additionally, the autophagy inhibitor 3-methyladenine reversed the inhibitory effects of LETM1 silencing on proliferation and stemness, whereas the autophagy activator rapamycin had the opposite effects. Mechanistically, suppression of LETM1 increased the levels of reactive oxygen species (ROS) and mitochondrial ROS by regulation of SOD2, which in turn activated AMP-activated protein kinase (AMPK)/mammalian target of rapamycin (mTOR), initiated autophagy, and inhibited proliferation and stemness. Our findings suggest that silencing LETM1 induced autophagy in CRC cells by triggering ROS-mediated AMPK/mTOR signalling, thus blocking CRC progression, which will enhance our understanding of the molecular mechanism of LETM1 in CRC.

Melatonin reverses the oxidative stress and mitochondrial dysfunction caused by LETM1 silencing.

LETM1 is a mitochondrial inner-membrane protein, which is encoded by a gene present in a locus of 4p, which, in turn, is deleted in the Wolf-Hirschhorn Syndrome, and is assumed to be related to its pathogenesis. The cellular damage caused by the deletion is presumably related to oxidative stress. Melatonin has many beneficial roles in protecting mitochondria by scavenging reactive oxygen species, maintaining membrane potential, and improving functions. The aim of this study was to investigate the effects of melatonin administration to LETM1-silenced mouse embryonic fibroblast cells as a cellular model for LETM1 deficiency. We transfected mouse embryonic fibroblast cells with a pair of siRNA against LETM1 and monitored the oxidative stress and mitochondrial functions with or without melatonin addition. MnSOD expression and aconitase activity decreased and oxidized protein levels increased in LETM1-silenced cells. LETM1 suppression did not alter the expression of OXPHOS complexes, but the oxygen consumption rates decreased significantly; however, this change was not related to complex I but instead involved complex IV and complex II. Melatonin supplementation effectively normalized the parameters studied, including the oxygen consumption rate. Our findings identified a novel effect of LETM1 deficiency on cellular respiration via complex II as well as a potential beneficial role of melatonin treatment. On the other hand, these effects may be specific to the cell line used and need to be verified in other cell lines.

Homozygous variant in OTX2 and possible genetic modifiers identified in a patient with combined pituitary hormone deficiency, ocular involvement, myopathy, ataxia, and mitochondrial impairment.

Here we report on a singleton patient affected by a complicated congenital syndrome characterized by growth delay, retinal dystrophy, sensorineural deafness, myopathy, ataxia, combined pituitary hormone deficiency, associated with mitochondrial impairment. Targeted clinical exome sequencing led to the identification of a homozygous missense variant in OTX2. Since only dominant mutations within OTX2 have been associated with cases of syndromic microphthalmia, retinal dystrophy with or without pituitary dysfunctions, this represents the first report of an OTX2 recessive mutation. Part of the phenotype, including ataxia, myopathy and multiple mitochondrial respiratory chain defects, seemed not related to OTX2. Further analysis of next generation sequencing (NGS) data revealed additional candidate variants: a homozygous variant in LETM1, and heterozygous rare variants in AFG3L2 and POLG. All three genes encode mitochondrial proteins and the last two are known to be associated with ataxia, a neurological sign present also in the father of the proband. With our study, we aim to encourage the integration of NGS data with a detailed analysis of clinical description and family history in order to unravel composite genotypes sometimes associated with complicated phenotypes.

Molecular Mechanisms of Leucine Zipper EF-Hand Containing Transmembrane Protein-1 Function in Health and Disease.

Mitochondrial calcium (Ca2+) uptake shapes cytosolic Ca2+ signals involved in countless cellular processes and more directly regulates numerous mitochondrial functions including ATP production, autophagy and apoptosis. Given the intimate link to both life and death processes, it is imperative that mitochondria tightly regulate intramitochondrial Ca2+ levels with a high degree of precision. Among the Ca2+ handling tools of mitochondria, the leucine zipper EF-hand containing transmembrane protein-1 (LETM1) is a transporter protein localized to the inner mitochondrial membrane shown to constitute a Ca2+/H⁺ exchanger activity. The significance of LETM1 to mitochondrial Ca2+ regulation is evident from Wolf-Hirschhorn syndrome patients that harbor a haplodeficiency in LETM1 expression, leading to dysfunctional mitochondrial Ca2+ handling and from numerous types of cancer cells that show an upregulation of LETM1 expression. Despite the significance of LETM1 to cell physiology and pathophysiology, the molecular mechanisms of LETM1 function remain poorly defined. In this review, we aim to provide an overview of the current understanding of LETM1 structure and function and pinpoint the knowledge gaps that need to be filled in order to unravel the underlying mechanistic basis for LETM1 function.

4p16.3 microdeletions and microduplications detected by chromosomal microarray analysis: New insights into mechanisms and critical regions.

Deletions in the 4p16.3 region cause Wolf-Hirschhorn syndrome, a well known contiguous microdeletion syndrome with the critical region for common phenotype mapped in WHSCR2. Recently, duplications in 4p16.3 were reported in three patients with developmental delay and dysmorphic features. Through chromosomal microarray analysis, we identified 156 patients with a deletion (n = 109) or duplication (n = 47) in 4p16.3 out of approximately 60,000 patients analyzed by Baylor Miraca Genetics Laboratories. Seventy-five of the postnatally detected deletions encompassed the entire critical region, 32 (43%) of which were associated with other chromosome rearrangements, including six patients (8%) that had a duplication adjacent to the terminal deletion. Our data indicate that Wolf-Hirschhorn syndrome deletions with an adjacent duplication occur at a higher frequency than previously appreciated. Pure deletions (n = 14) or duplications (n = 15) without other copy number changes distal to or inside the WHSCR2 were identified for mapping of critical regions. Our data suggest that deletion of the segment from 0.6 to 0.9 Mb from the terminus of 4p causes a seizure phenotype and duplications of a region distal to the previously defined smallest region of overlap for 4p16.3 microduplication syndrome are associated with neurodevelopmental problems. We detected seven Wolf-Hirschhorn syndrome deletions and one 4p16.3 duplication prenatally; all of the seven are either >8 Mb in size and/or associated with large duplications. In conclusion, our study provides deeper insight into the molecular mechanisms, the critical regions and effective prenatal diagnosis for 4p16.3 deletions/ duplications. © 2016 Wiley Periodicals, Inc.

Clinical implication of leucine zipper/EF hand-containing transmembrane-1 overexpression in the prognosis of triple-negative breast cancer.

Triple negative breast cancer (TNBC) is a heterogeneous disease with higher rates of relapse and decreased overall survival in metastatic tumors. Due to its poor prognosis, it is necessary to identify effective biomarkers that are associated with tumor growth and metastasis. The leucine zipper/EF hand-containing transmembrane-1 (LETM1) protein, which is a mitochondrial inner membrane protein, can reduce mitochondrial biogenesis and ATP production. The expression levels of LETM1 were significantly increased in numerous human malignancies. However, the clinicopathological characteristics and prognostic value of LETM1 overexpression in TNBC remains unclear. LETM1 protein was detected in 107 TNBC, 42 ductal carcinoma in situ (DCIS) and 65 adjacent non-tumor breast tissues using immunohistochemical (IHC) staining. Immunofluorescence (IF) staining was also performed to detect the localization of LETM1 protein in MCF-7 BC cells. The correlations between LETM1 overexpression and clinicopathological features of TNBC were evaluated using Chi-squared test and Fisher's exact tests. The survival rate was calculated using the Kaplan-Meier method. LETM1 protein showed cytoplasmic staining patterns in TNBC. The strongly positive rate of LETM1 in TNBC was 69.2% (74/107), which was significantly higher than in both DCIS 35.7% (15/42) and adjacent non-tumor tissues 12.3% (8/65). High-level expression of LETM1 was positively correlated with late clinical stage, poor differentiation, lymph node metastasis, disease-free survival (DFS) and 10-year overall survival (OS) rates in TNBC. Further analysis showed that high LETM1 expression along with clinical stage emerged as significant independent risk factors in patients with TNBC. In conclusion, LETM1 protein overexpression is associated with TNBC progression, and may be a potential biomarker for poor prognostic evaluation of TNBC.

Association of mitochondrial letm1 with epileptic seizures.

Leucine zipper-EF-hand containing transmembrane protein 1 (Letm1) is a mitochondrial protein that is associated with seizure attacks in Wolf-Hirschhorn syndrome. This study aimed to investigate the expression pattern of Letm1 in patients with temporal lobe epilepsy (TLE) and pilocarpine-induced rat model of epilepsy, and to determine if altered Letm1 leads to mitochondrial dysfunction and increased susceptibility to seizures. Using immunohistochemical, immunofluorescent, western blotting, and transmission electron microscopic methods, we have found that Letm1 was significantly decreased in TLE patients, and gradually decreased in experimental rats from 1 to 7 days after onset of seizures. Letm1 knock-down by a lentivirus bearing LV-Letm1-sh resulted in mitochondrial swelling and decreased expression of Letm1 target protein mitochondrially encoded cytochrome B (MT-CYB). Behavioral study revealed that inhibition of Letm1 caused early onset of the first seizure, increased seizure frequency, and duration. However, administration of Letm1 homolog nigericin failed to prevent epilepsy. These results indicate that inhibition of Letm1 and mitochondrial dysfunctions contributes to the development of epileptic seizures. Appropriate Letm1 level may be critical for maintaining normal neuronal functions.

Trypanosome Letm1 protein is essential for mitochondrial potassium homeostasis.

Letm1 is a conserved protein in eukaryotes bearing energized mitochondria. Hemizygous deletion of its gene has been implicated in symptoms of the human disease Wolf-Hirschhorn syndrome. Studies almost exclusively performed in opisthokonts have attributed several roles to Letm1, including maintaining mitochondrial morphology, mediating either calcium or potassium/proton antiport, and facilitating mitochondrial translation. We address the ancestral function of Letm1 in the highly diverged protist and significant pathogen, Trypanosoma brucei. We demonstrate that Letm1 is involved in maintaining mitochondrial volume via potassium/proton exchange across the inner membrane. This role is essential in the vector-dwelling procyclic and mammal-infecting bloodstream stages as well as in Trypanosoma brucei evansi, a form of the latter stage lacking an organellar genome. In the pathogenic bloodstream stage, the mitochondrion consumes ATP to maintain an energized state, whereas that of T. brucei evansi also lacks a conventional proton-driven membrane potential. Thus, Letm1 performs its function in different physiological states, suggesting that ion homeostasis is among the few characterized essential pathways of the mitochondrion at this T. brucei life stage. Interestingly, Letm1 depletion in the procyclic stage can be complemented by exogenous expression of its human counterpart, highlighting the conservation of protein function between highly divergent species. Furthermore, although mitochondrial translation is affected upon Letm1 ablation, it is an indirect consequence of K(+) accumulation in the matrix.

Aberrant LETM1 elevation dysregulates mitochondrial functions and energy metabolism and promotes lung metastasis in osteosarcoma.

Osteosarcoma is a differentiation-deficient disease, and despite the unique advantages and great potential of differentiation therapy, there are only a few known differentiation inducers, and little research has been done on their targets. Cell differentiation is associated with an increase in mitochondrial content and activity. The metabolism of some tumor cells is characterized by impaired oxidative phosphorylation, as well as up-regulation of aerobic glycolysis and pentose phosphate pathways. Leucine-containing zipper and EF-hand transmembrane protein 1 (LETM1) is involved in the maintenance of mitochondrial morphology and is closely associated with tumorigenesis and progression, as well as cancer cell stemness. We found that MG63 and 143B osteosarcoma cells overexpress LETM1 and exhibit abnormalities in mitochondrial structure and function. Knockdown of LETM1 partially restored the mitochondrial structure and function, inhibited the pentose phosphate pathway, promoted oxidative phosphorylation, and led to osteogenic differentiation. It also inhibited spheroid cell formation, proliferation, migration, and invasion in an in vitro model. When LETM1 was knocked down in vivo, there was reduced tumor formation and lung metastasis. These data suggest that mitochondria are aberrant in LETM1-overexpressing osteosarcoma cells, and knockdown of LETM1 partially restores the mitochondrial structure and function, inhibits the pentose phosphate pathway, promotes oxidative phosphorylation, and increases osteogenic differentiation, thereby reducing malignant biological behavior of the cells.

An AI-informed NMR structure reveals an extraordinary LETM1 F-EF-hand domain that functions as a two-way regulator of mitochondrial calcium.

AlphaFold can accurately predict static protein structures but does not account for solvent conditions. Human leucine zipper EF-hand transmembrane protein-1 (LETM1) has one sequence-identifiable EF-hand but how calcium (Ca2+) affects structure and function remains enigmatic. Here, we used highly confident AlphaFold Cα predictions to guide nuclear Overhauser effect (NOE) assignments and structure calculation of the LETM1 EF-hand in the presence of Ca2+. The resultant NMR structure exposes pairing between a partial loop-helix and full helix-loop-helix, forming an unprecedented F-EF-hand with non-canonical Ca2+ coordination but enhanced hydrophobicity for protein interactions compared to calmodulin. The structure also reveals the basis for pH sensing at the link between canonical and partial EF-hands. Functionally, mutations that augmented or weakened Ca2+ binding increased or decreased matrix Ca2+, respectively, establishing F-EF as a two-way mitochondrial Ca2+ regulator. Thus, we show how to synergize AI prediction with NMR data, elucidating a solution-specific and extraordinary LETM1 F-EF-hand.

Distinct Epileptogenic Mechanisms Associated with Seizures in Wolf-Hirschhorn Syndrome.

Seizures are one of the clinical hallmarks of Wolf-Hirschhorn syndrome (WHS), causing a significant impact on the life quality, still in the first years of life. Even that the knowledge about WHS-related seizure candidate genes has grown, cumulative evidence suggests synergic haploinsufficiency of distinct genes within cellular networks that should be better elucidated. Herein, we evaluated common mechanisms between candidate genes from WHS seizure-susceptibility regions (SSR) and genes globally associated with epilepsy. For this purpose, data from 94 WHS patients delineated by chromosomal microarray analysis were integrated into a tissue-specific gene network with gene expression, drugs, and biological processes. We found functional modules and signaling pathways involving candidate and new genes with potential involvement in the WHS-related seizure phenotype. The proximity among the previous reported haploinsufficient candidate genes (PIGG, CPLX1, CTBP1, LETM1) and disease genes associated with epilepsy suggests not just one, but different impaired mechanisms in cellular networks responsible for the balance of neuronal activity in WHS patients, from which neuron communication is the most impaired in WHS-related seizures. Furthermore, CTBP1 obtained the largest number of drug associations, reinforcing its importance for adaptations of brain circuits and its putative use as a pharmacological target for treating seizures/epilepsy in patients with WHS.

Mitochondrial calcium at the synapse.

Mitochondria are dynamic organelles, which serve various purposes, including but not limited to the production of ATP and various metabolites, buffering ions, acting as a signaling hub, etc. In recent years, mitochondria are being seen as the central regulators of cellular growth, development, and death. Since neurons are highly specialized cells with a heavy metabolic demand, it is not surprising that neurons are one of the most mitochondria-rich cells in an animal. At synapses, mitochondrial function and dynamics is tightly regulated by synaptic calcium. Calcium influx during synaptic activity causes increased mitochondrial calcium influx leading to an increased ATP production as well as buffering of synaptic calcium. While increased ATP production is required during synaptic transmission, calcium buffering by mitochondria is crucial to prevent faulty neurotransmission and excitotoxicity. Interestingly, mitochondrial calcium also regulates the mobility of mitochondria within synapses causing mitochondria to halt at the synapse during synaptic transmission. In this review, we summarize the various roles of mitochondrial calcium at the synapse.