Molecular mechanisms and therapeutic strategies for neuromuscular diseases

Neuromuscular diseases encompass a heterogeneous array of disorders characterized by varying onset ages, clinical presentations, severity, and progression. While these conditions can stem from acquired or inherited causes, this review specifically focuses on disorders arising from genetic abnormalities, excluding metabolic conditions. The pathogenic defect may primarily affect the anterior horn cells, the axonal or myelin component of peripheral nerves, the neuromuscular junction, or skeletal and/or cardiac muscles. While inherited neuromuscular disorders have been historically deemed not treatable, the advent of gene-based and molecular therapies is reshaping the treatment landscape for this group of condition. With the caveat that many products still fail to translate the positive results obtained in pre-clinical models to humans, both the technological development (e.g., implementation of tissue-specific vectors) as well as advances on the knowledge of pathogenetic mechanisms form a collective foundation for potentially curative approaches to these debilitating conditions. This review delineates the current panorama of therapies targeting the most prevalent forms of inherited neuromuscular diseases, emphasizing approved treatments and those already undergoing human testing, offering insights into the state-of-the-art interventions.

Amphiphysin-2 (BIN1) functions and defects in cardiac and skeletal muscle

Amphiphysin-2 is a ubiquitously expressed protein also known as bridging integrator 1 (BIN1), playing a critical role in membrane remodeling, trafficking, and cytoskeleton dynamics in a wide range of tissues. Mutations in the gene encoding BIN1 cause centronuclear myopathies (CNM), and recent evidence has implicated BIN1 in heart failure, underlining its crucial role in both skeletal and cardiac muscle. Furthermore, altered expression of BIN1 is linked to an increased risk of late-onset Alzheimer's disease and several types of cancer, including breast, colon, prostate, and lung cancers. Recently, the first proof-of-concept for potential therapeutic strategies modulating BIN1 were obtained for muscle diseases. In this review article, we discuss the similarities and differences in BIN1's functions in cardiac and skeletal muscle, along with its associated diseases and potential therapies.

Phosphoinositide switches in cell physiology - From molecular mechanisms to disease

Phosphoinositides are amphipathic lipid molecules derived from phosphatidylinositol that represent low abundance components of biological membranes. Rather than serving as mere structural elements of lipid bilayers, they represent molecular switches for a broad range of biological processes, including cell signaling, membrane dynamics and remodeling, and many other functions. Here, we focus on the molecular mechanisms that turn phosphoinositides into molecular switches and how the dysregulation of these processes can lead to disease.

Transcriptomic Profiles of Pectoralis major Muscles Affected by Spaghetti Meat and Woody Breast in Broiler Chickens

Spaghetti meat (SM) and woody breast (WB) are breast muscle myopathies of broiler chickens, characterized by separation of myofibers and by fibrosis, respectively. This study sought to investigate the transcriptomic profiles of breast muscles affected by SM and WB. Targeted sampling was conducted on a flock to obtain 10 WB, 10 SM, and 10 Normal Pectoralis major muscle samples from 37-day-old male chickens. Total RNA was extracted, cDNA was used for pair-end sequencing, and differentially expressed genes (DEGs) were determined by a false discovery rate of <0.1 and a >1.5-fold change. Principal component and heatmap cluster analyses showed that the SM and WB samples clustered together. No DEGs were observed between SM and WB fillets, while a total of 4018 and 2323 DEGs were found when comparing SM and WB, respectively, against Normal samples. In both the SM and WB samples, Gene Ontology terms associated with extracellular environment and immune response were enriched. The KEGG analysis showed enrichment of cytokine-cytokine receptor interaction and extracellular matrix-receptor interaction pathways in both myopathies. Although SM and WB are macroscopically different, the similar transcriptomic profiles suggest that these conditions may share a common pathogenesis. This is the first study to compare the transcriptomes of SM and WB, and it showed that, while both myopathies had profiles different from the normal breast muscle, SM and WB were similar, with comparable enriched metabolic pathways and processes despite presenting markedly different macroscopic features.

The Genetic Background of Abnormalities in Metabolic Pathways of Phosphoinositides and Their Linkage with the Myotubular Myopathies, Neurodegenerative Disorders, and Carcinogenesis

Myo-inositol belongs to one of the sugar alcohol groups known as cyclitols. Phosphatidylinositols are one of the derivatives of Myo-inositol, and constitute important mediators in many intracellular processes such as cell growth, cell differentiation, receptor recycling, cytoskeletal organization, and membrane fusion. They also have even more functions that are essential for cell survival. Mutations in genes encoding phosphatidylinositols and their derivatives can lead to many disorders. This review aims to perform an in-depth analysis of these connections. Many authors emphasize the significant influence of phosphatidylinositols and phosphatidylinositols' phosphates in the pathogenesis of myotubular myopathies, neurodegenerative disorders, carcinogenesis, and other less frequently observed diseases. In our review, we have focused on three of the most often mentioned groups of disorders. Inositols are the topic of many studies, and yet, there are no clear results of successful clinical trials. Analysis of the available literature gives promising results and shows that further research is still needed.

MTM1 overexpression prevents and reverts BIN1-related centronuclear myopathy

Centronuclear and myotubular myopathies (CNM) are rare and severe genetic diseases associated with muscle weakness and atrophy as well as intracellular disorganization of myofibres. The main mutated proteins control lipid and membrane dynamics and are the lipid phosphatase myotubularin (MTM1), and the membrane remodelling proteins amphiphysin 2 (BIN1) and dynamin 2 (DNM2). There is no available therapy. Here, to validate a novel therapeutic strategy for BIN1- and DNM2-CNM, we evaluated adeno-associated virus-mediated MTM1 (AAV-MTM1 ) overexpression in relevant mouse models. Early systemic MTM1 overexpression prevented the development of the CNM pathology in Bin1mck-/- mice, while late intramuscular MTM1 expression partially reverted the established phenotypes after only 4 weeks of treatment. However, AAV-MTM1 injection did not change the DNM2-CNM mouse phenotypes. We investigated the mechanism of the rescue of the myopathy in BIN1-CNM and found that the lipid phosphatase activity of MTM1 was essential for the rescue of muscle atrophy and myofibre hypotrophy but dispensable for the rescue of myofibre disorganization including organelle mis-position and T-tubule defects. Furthermore, the improvement of T-tubule organization correlated with normalization of key regulators of T-tubule morphogenesis, dysferlin and caveolin. Overall, these data support the inclusion of BIN1-CNM patients in an AAV-MTM1 clinical trial.

One transgene, two myopathies: an MTM1 'cross gene therapy' for BIN1 deficiency?

This scientific commentary refers to ‘MTM1 overexpression prevents and reverts BIN1-related centronuclear myopathy’ by Giraud et al. (https://doi.org/10.1093/brain/awad251).

Therapeutic approaches in different congenital myopathies

Congenital myopathies are rare and severe genetic diseases affecting the skeletal muscle function in children and adults. They present a variable spectrum of phenotypes and a genetic heterogeneity. Subgroups are defined according to the clinical and histopathological features and encompass core myopathy, centronuclear myopathy, nemaline myopathy and other rare congenital myopathies. No approved treatment exists to date for any congenital myopathies. To tackle this important unmet need, an increased number of proof-of-concept studies recently assessed the therapeutic potential of various strategies, either pharmacological or genetic-based, aiming at counteracting muscle weakness or/and cure the pathology. Here, we list the implicated genes and cellular pathways, and review the therapeutic approaches preclinically tested and the ongoing/completed clinical trials for the different types of congenital myopathies.

GSK3α phosphorylates dynamin-2 to promote GLUT4 endocytosis in muscle cells

Insulin-stimulated translocation of glucose transporter 4 (GLUT4) to plasma membrane of skeletal muscle is critical for postprandial glucose uptake; however, whether the internalization of GLUT4 is also regulated by insulin signaling remains unclear. Here, we discover that the activity of dynamin-2 (Dyn2) in catalyzing GLUT4 endocytosis is negatively regulated by insulin signaling in muscle cells. Mechanistically, the fission activity of Dyn2 is inhibited by binding with the SH3 domain of Bin1. In the absence of insulin, GSK3α phosphorylates Dyn2 to relieve the inhibition of Bin1 and promotes endocytosis. Conversely, insulin signaling inactivates GSK3α and leads to attenuated GLUT4 internalization. Furthermore, the isoform-specific pharmacological inhibition of GSK3α significantly improves insulin sensitivity and glucose tolerance in diet-induced insulin-resistant mice. Together, we identify a new role of GSK3α in insulin-stimulated glucose disposal by regulating Dyn2-mediated GLUT4 endocytosis in muscle cells. These results highlight the isoform-specific function of GSK3α on membrane trafficking and its potential as a therapeutic target for metabolic disorders.

Antagonistic control of active surface integrins by myotubularin and phosphatidylinositol 3-kinase C2β in a myotubular myopathy model

X-linked centronuclear myopathy (XLCNM) is a severe human disease without existing therapies caused by mutations in the phosphoinositide 3-phosphatase MTM1. Loss of MTM1 function is associated with muscle fiber defects characterized by impaired localization of β-integrins and other components of focal adhesions. Here we show that defective focal adhesions and reduced active β-integrin surface levels in a cellular model of XLCNM are rescued by loss of phosphatidylinositiol 3-kinase C2β (PI3KC2β) function. Inactivation of the Mtm1 gene impaired myoblast differentiation into myotubes and resulted in reduced surface levels of active β1-integrins as well as corresponding defects in focal adhesions. These phenotypes were rescued by concomitant genetic loss of Pik3c2b or pharmacological inhibition of PI3KC2β activity. We further demonstrate that a hitherto unknown role of PI3KC2β in the endocytic trafficking of active β1-integrins rather than rescue of phosphatidylinositol 3-phosphate levels underlies the ability of Pik3c2b to act as a genetic modifier of cellular XLCNM phenotypes. Our findings reveal a crucial antagonistic function of MTM1 and PI3KC2β in the control of active β-integrin surface levels, thereby providing a molecular mechanism for the adhesion and myofiber defects observed in XLCNM. They further suggest specific pharmacological inhibition of PI3KC2β catalysis as a viable treatment option for XLCNM patients.

X-linked myotubular myopathy is associated with epigenetic alterations and is ameliorated by HDAC inhibition

X-linked myotubular myopathy (XLMTM) is a fatal neuromuscular disorder caused by loss of function mutations in MTM1. At present, there are no directed therapies for XLMTM, and incomplete understanding of disease pathomechanisms. To address these knowledge gaps, we performed a drug screen in mtm1 mutant zebrafish and identified four positive hits, including valproic acid, which functions as a potent suppressor of the mtm1 zebrafish phenotype via HDAC inhibition. We translated these findings to a mouse XLMTM model, and showed that valproic acid ameliorates the murine phenotype. These observations led us to interrogate the epigenome in Mtm1 knockout mice; we found increased DNA methylation, which is normalized with valproic acid, and likely mediated through aberrant 1-carbon metabolism. Finally, we made the unexpected observation that XLMTM patients share a distinct DNA methylation signature, suggesting that epigenetic alteration is a conserved disease feature amenable to therapeutic intervention.

Dynamin-2 reduction rescues the skeletal myopathy of SPEG-deficient mouse model

Striated preferentially expressed protein kinase (SPEG), a myosin light chain kinase, is mutated in centronuclear myopathy (CNM) and/or dilated cardiomyopathy. No precise therapies are available for this disorder, and gene replacement therapy is not a feasible option due to the large size of SPEG. We evaluated the potential of dynamin-2 (DNM2) reduction as a potential therapeutic strategy because it has been shown to revert muscle phenotypes in mouse models of CNM caused by MTM1, DNM2, and BIN1 mutations. We determined that SPEG-β interacted with DNM2, and SPEG deficiency caused an increase in DNM2 levels. The DNM2 reduction strategy in Speg-KO mice was associated with an increase in life span, body weight, and motor performance. Additionally, it normalized the distribution of triadic proteins, triad ultrastructure, and triad number and restored phosphatidylinositol-3-phosphate levels in SPEG-deficient skeletal muscles. Although DNM2 reduction rescued the myopathy phenotype, it did not improve cardiac dysfunction, indicating a differential tissue-specific function. Combining DNM2 reduction with other strategies may be needed to target both the cardiac and skeletal defects associated with SPEG deficiency. DNM2 reduction should be explored as a therapeutic strategy against other genetic myopathies (and dystrophies) associated with a high level of DNM2.

BIN1 modulation in vivo rescues dynamin-related myopathy

The mechanoenzyme dynamin 2 (DNM2) is crucial for intracellular organization and trafficking. DNM2 is mutated in dominant centronuclear myopathy (DNM2-CNM), a muscle disease characterized by defects in organelle positioning in myofibers. It remains unclear how the in vivo functions of DNM2 are regulated in muscle. Moreover, there is no therapy for DNM2-CNM to date. Here, we overexpressed human amphiphysin 2 (BIN1), a membrane remodeling protein mutated in other CNM forms, in Dnm2 ^RW/+ and Dnm2 ^RW/RW mice modeling mild and severe DNM2-CNM, through transgenesis or with adeno-associated virus (AAV). Increasing BIN1 improved muscle atrophy and main histopathological features of Dnm2 ^RW/+ mice and rescued the perinatal lethality and survival of Dnm2 ^RW/RW mice. In vitro experiments showed that BIN1 binds and recruits DNM2 to membrane tubules, and that the BIN1-DNM2 complex regulates tubules fission. Overall, BIN1 is a potential therapeutic target for dominant centronuclear myopathy linked to DNM2 mutations.

Cardiac Transverse Tubules in Physiology and Heart Failure

In mammalian cardiac myocytes, the plasma membrane includes the surface sarcolemma but also a network of membrane invaginations called transverse (t-) tubules. These structures carry the action potential deep into the cell interior, allowing efficient triggering of Ca2+ release and initiation of contraction. Once thought to serve as rather static enablers of excitation-contraction coupling, recent work has provided a newfound appreciation of the plasticity of the t-tubule network's structure and function. Indeed, t-tubules are now understood to support dynamic regulation of the heartbeat across a range of timescales, during all stages of life, in both health and disease. This review article aims to summarize these concepts, with consideration given to emerging t-tubule regulators and their targeting in future therapies.

Cold Exposure Affects Lipid Metabolism, Fatty Acids Composition and Transcription in Pig Skeletal Muscle

Cold exposure promotes glucose oxidation and modulates the lipid metabolism in adipose tissue, but it is still not fully clear whether cold exposure could affect meat quality and fatty acid metabolism in skeletal muscle of pig in vivo. Here, we kept finishing pigs under cold or room temperature overnight and determined the effects of cold exposure on meat quality, fatty acids composition and transcriptional changes in skeletal muscle of pigs. We found that cold exposure significantly reduced the meat colour_{24 h} and pH_{24 h}, without affecting carcass characteristics and other meat quality traits. Considerable changes were found in the proportions of individual fatty acids and the total content of saturated fatty acid, polyunsaturated fatty acids, monounsaturated fatty acid and n3-fatty acids. RNA-seq results showed upregulated fatty acid biosynthesis genes and downregulated mitochondrial beta-oxidation genes. The lipid metabolism in cold-treated longissimus dorsi muscle might be regulated by functions of the lipoprotein particle, the extracellular matrix, and the PPAR signaling pathways. Our study revealed the potential of cold exposure to regulate the lipid metabolism and fatty acid composition in skeletal muscle of farmed animals.

Common Pathogenic Mechanisms in Centronuclear and Myotubular Myopathies and Latest Treatment Advances

Centronuclear myopathies (CNM) are rare congenital disorders characterized by muscle weakness and structural defects including fiber hypotrophy and organelle mispositioning. The main CNM forms are caused by mutations in: the MTM1 gene encoding the phosphoinositide phosphatase myotubularin (myotubular myopathy), the DNM2 gene encoding the mechanoenzyme dynamin 2, the BIN1 gene encoding the membrane curvature sensing amphiphysin 2, and the RYR1 gene encoding the skeletal muscle calcium release channel/ryanodine receptor. MTM1, BIN1, and DNM2 proteins are involved in membrane remodeling and trafficking, while RyR1 directly regulates excitation-contraction coupling (ECC). Several CNM animal models have been generated or identified, which confirm shared pathological anomalies in T-tubule remodeling, ECC, organelle mispositioning, protein homeostasis, neuromuscular junction, and muscle regeneration. Dynamin 2 plays a crucial role in CNM physiopathology and has been validated as a common therapeutic target for three CNM forms. Indeed, the promising results in preclinical models set up the basis for ongoing clinical trials. Another two clinical trials to treat myotubular myopathy by MTM1 gene therapy or tamoxifen repurposing are also ongoing. Here, we review the contribution of the different CNM models to understanding physiopathology and therapy development with a focus on the commonly dysregulated pathways and current therapeutic targets.

Multi-omics comparisons of different forms of centronuclear myopathies and the effects of several therapeutic strategies

Omics analyses are powerful methods to obtain an integrated view of complex biological processes, disease progression, or therapy efficiency. However, few studies have compared different disease forms and different therapy strategies to define the common molecular signatures representing the most significant implicated pathways. In this study, we used RNA sequencing and mass spectrometry to profile the transcriptomes and proteomes of mouse models for three forms of centronuclear myopathies (CNMs), untreated or treated with either a drug (tamoxifen), antisense oligonucleotides reducing the level of dynamin 2 (DNM2), or following modulation of DNM2 or amphiphysin 2 (BIN1) through genetic crosses. Unsupervised analysis and differential gene and protein expression were performed to retrieve CNM molecular signatures. Longitudinal studies before, at, and after disease onset highlighted potential disease causes and consequences. Main pathways in the common CNM disease signature include muscle contraction, regeneration and inflammation. The common therapy signature revealed novel potential therapeutic targets, including the calcium regulator sarcolipin. We identified several novel biomarkers validated in muscle and/or plasma through RNA quantification, western blotting, and enzyme-linked immunosorbent assay (ELISA) assays, including ANXA2 and IGFBP2. This study validates the concept of using multi-omics approaches to identify molecular signatures common to different disease forms and therapeutic strategies.

Symptomatic heterozygous X-Linked myotubular myopathy female patient with a large deletion at Xq28 and decrease expression of normal allele

X-linked myotubular myopathy (XLMTM; OMIM 310400) is a centronuclear congenital muscular disorder of X-linked recessive inheritance. Although female carriers are typically asymptomatic, affected heterozygous females have been described. Here, we describe the case of a sporadic female patient with suspicion of centronuclear myopathy and a heterozygous large deletion at Xq28 encompassing the MAMLD1, MTM1, MTMR1, CD99L2, and HMGB3 genes. The deletion was first detected using a custom next generation sequencing (NGS)-based multigene panel and finally characterized by comparative genomic hybridization array and multiplex ligation probe assay techniques. In this patient we have confirmed, by MTM1 mRNA quantification, a MTM1 gene expression less than the expected 50 percent in patient muscle. The significant 20% reduction in MTM1 mRNA expression in muscle, precludes low level of the normal myotubularin protein as the cause of the phenotype in this heterozygous female. We have also found that BIN1 expression in patient muscle biopsy was significantly increased, and postulate that BIN1 expression will be increased in XLMTM patient muscle as an attempt to maintain muscle function.

SPEG binds with desmin and its deficiency causes defects in triad and focal adhesion proteins

Striated preferentially expressed gene (SPEG), a member of the myosin light chain kinase family, is localized at the level of triad surrounding myofibrils in skeletal muscles. In humans, SPEG mutations are associated with centronuclear myopathy and cardiomyopathy. Using a striated muscle-specific Speg-knockout (KO) mouse model, we have previously shown that SPEG is critical for triad maintenance and calcium handling. Here, we further examined the molecular function of SPEG and characterized the effects of SPEG deficiency on triad and focal adhesion proteins. We used yeast two-hybrid assay, and identified desmin, an intermediate filament protein, to interact with SPEG and confirmed this interaction by co-immunoprecipitation. Using domain-mapping assay, we defined that Ig-like and fibronectin III domains of SPEG interact with rod domain of desmin. In skeletal muscles, SPEG depletion leads to desmin aggregates in vivo and a shift in desmin equilibrium from soluble to insoluble fraction. We also profiled the expression and localization of triadic proteins in Speg-KO mice using western blot and immunofluorescence. The amount of RyR1 and triadin were markedly reduced, whereas DHPRα1, SERCA1 and triadin were abnormally accumulated in discrete areas of Speg-KO myofibers. In addition, Speg-KO muscles exhibited internalized vinculin and β1 integrin, both of which are critical components of the focal adhesion complex. Further, β1 integrin was abnormally accumulated in early endosomes of Speg-KO myofibers. These results demonstrate that SPEG-deficient skeletal muscles exhibit several pathological features similar to those seen in MTM1 deficiency. Defects of shared cellular pathways may underlie these structural and functional abnormalities in both types of diseases.

Nanobar Array Assay Revealed Complementary Roles of BIN1 Splice Isoforms in Cardiac T-Tubule Morphogenesis

Bridging integrator-1 (BIN1) is a family of banana-shaped molecules implicated in cell membrane tubulation. To understand the curvature sensitivity and functional roles of BIN1 splicing isoforms, we engineered vertical nanobars on a cell culture substrate to create high and low curvatures. When expressed individually, BIN1 isoforms with phosphoinositide-binding motifs (pBIN1) appeared preferentially at high-curvature nanobar ends, agreeing well with their membrane tubulation in cardiomyocytes. In contrast, the ubiquitous BIN1 isoform without phosphoinositide-binding motif (uBIN1) exhibited no affinity to membranes around nanobars but accumulated along Z-lines in cardiomyocytes. Importantly, in pBIN1-uBIN1 coexpression, pBIN1 recruited uBIN1 to high-curvature membranes at nanobar ends, and uBIN1 attached the otherwise messy pBIN1 tubules to Z-lines. The complementary cooperation of BIN1 isoforms (comboBIN1) represents a novel mechanism of T-tubule formation along Z-lines in cardiomyocytes. Dysregulation of BIN1 splicing, e.g., during myocardial infarction, underlied T-tubule disorganization, and correction of uBIN1/pBIN1 stoichiometry rescued T-tubule morphology in heart disease.

Insights into wild-type dynamin 2 and the consequences of DNM2 mutations from transgenic zebrafish

Dynamin 2 (DNM2) encodes a ubiquitously expressed large GTPase with membrane fission capabilities that participates in the endocytosis of clathrin-coated vesicles. Heterozygous mutations in DNM2 are associated with two distinct neuromuscular disorders, Charcot-Marie-Tooth disease (CMT) and autosomal dominant centronuclear myopathy (CNM). Despite extensive investigations in cell culture, the role of dynamin 2 in normal muscle development is poorly understood and the consequences of DNM2 mutations at the molecular level in vivo are not known. To address these gaps in knowledge, we developed transgenic zebrafish expressing either wild-type dynamin 2 or dynamin 2 with either a CNM or CMT mutation. Taking advantage of the live imaging capabilities of the zebrafish embryo, we establish the localization of wild-type and mutant dynamin 2 in vivo, showing for the first time distinctive dynamin 2 subcellular compartments. Additionally, we demonstrate that CNM-related DNM2 mutations are associated with protein mislocalization and aggregation. Lastly, we define core phenotypes associated with our transgenic mutant fish, including impaired motor function and altered muscle ultrastructure, making them the ideal platform for drug screening. Overall, using the power of the zebrafish, we establish novel insights into dynamin 2 localization and dynamics and provide the necessary groundwork for future studies examining dynamin 2 pathomechanisms and therapy development.

Yeast as a Model to Understand Actin-Mediated Cellular Functions in Mammals-Illustrated with Four Actin Cytoskeleton Proteins

The budding yeast Saccharomyces cerevisiae has an actin cytoskeleton that comprises a set of protein components analogous to those found in the actin cytoskeletons of higher eukaryotes. Furthermore, the actin cytoskeletons of S. cerevisiae and of higher eukaryotes have some similar physiological roles. The genetic tractability of budding yeast and the availability of a stable haploid cell type facilitates the application of molecular genetic approaches to assign functions to the various actin cytoskeleton components. This has provided information that is in general complementary to that provided by studies of the equivalent proteins of higher eukaryotes and hence has enabled a more complete view of the role of these proteins. Several human functional homologues of yeast actin effectors are implicated in diseases. A better understanding of the molecular mechanisms underpinning the functions of these proteins is critical to develop improved therapeutic strategies. In this article we chose as examples four evolutionarily conserved proteins that associate with the actin cytoskeleton: (1) yeast Hof1p/mammalian PSTPIP1, (2) yeast Rvs167p/mammalian BIN1, (3) yeast eEF1A/eEF1A1 and eEF1A2 and (4) yeast Yih1p/mammalian IMPACT. We compare the knowledge on the functions of these actin cytoskeleton-associated proteins that has arisen from studies of their homologues in yeast with information that has been obtained from in vivo studies using live animals or in vitro studies using cultured animal cell lines.

BIN1 recovers tauopathy-induced long-term memory deficits in mice and interacts with Tau through Thr348 phosphorylation

The bridging integrator 1 gene (BIN1) is a major genetic risk factor for Alzheimer's disease (AD). In this report, we investigated how BIN1-dependent pathophysiological processes might be associated with Tau. We first generated a cohort of control and transgenic mice either overexpressing human MAPT (TgMAPT) or both human MAPT and BIN1 (TgMAPT;TgBIN1), which we followed-up from 3 to 15 months. In TgMAPT;TgBIN1 mice short-term memory deficits appeared earlier than in TgMAPT mice; however-unlike TgMAPT mice-TgMAPT;TgBIN1 mice did not exhibit any long-term or spatial memory deficits for at least 15 months. After killing the cohort at 18 months, immunohistochemistry revealed that BIN1 overexpression prevents both Tau mislocalization and somatic inclusion in the hippocampus, where an increase in BIN1-Tau interaction was also observed. We then sought mechanisms controlling the BIN1-Tau interaction. We developed a high-content screening approach to characterize modulators of the BIN1-Tau interaction in an agnostic way (1,126 compounds targeting multiple pathways), and we identified-among others-an inhibitor of calcineurin, a Ser/Thr phosphatase. We determined that calcineurin dephosphorylates BIN1 on a cyclin-dependent kinase phosphorylation site at T348, promoting the open conformation of the neuronal BIN1 isoform. Phosphorylation of this site increases the availability of the BIN1 SH3 domain for Tau interaction, as demonstrated by nuclear magnetic resonance experiments and in primary neurons. Finally, we observed that although the levels of the neuronal BIN1 isoform were unchanged in AD brains, phospho-BIN1(T348):BIN1 ratio was increased, suggesting a compensatory mechanism. In conclusion, our data support the idea that BIN1 modulates the AD risk through an intricate regulation of its interaction with Tau. Alteration in BIN1 expression or activity may disrupt this regulatory balance with Tau and have direct effects on learning and memory.

The expanding spectrum of neurological disorders of phosphoinositide metabolism

Phosphoinositides (PIPs) are a ubiquitous group of seven low-abundance phospholipids that play a crucial role in defining localized membrane properties and that regulate myriad cellular processes, including cytoskeletal remodeling, cell signaling cascades, ion channel activity and membrane traffic. PIP homeostasis is tightly regulated by numerous inositol kinases and phosphatases, which phosphorylate and dephosphorylate distinct PIP species. The importance of these phospholipids, and of the enzymes that regulate them, is increasingly being recognized, with the identification of human neurological disorders that are caused by mutations in PIP-modulating enzymes. Genetic disorders of PIP metabolism include forms of epilepsy, neurodegenerative disease, brain malformation syndromes, peripheral neuropathy and congenital myopathy. In this Review, we provide an overview of PIP function and regulation, delineate the disorders associated with mutations in genes that modulate or utilize PIPs, and discuss what is understood about gene function and disease pathogenesis as established through animal models of these diseases.

Summary: This Review highlights the intersection between phosphoinositides and the enzymes that regulate their metabolism, which together are crucial regulators of myriad cellular processes and neurological disorders.