Deletion of TDP-43 down-regulates Tbc1d1, a gene linked to obesity, and alters body fat metabolism. Proc Natl Acad Sci U S A. 2010;107:16320-16324. [PMID: 20660762 DOI: 10.1073/pnas.1002176107]

Alterations to metabolic hormones in amyotrophic lateral sclerosis and frontotemporal dementia postmortem human tissue

Metabolic changes are observed in patients with both amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Although regulation of metabolic processes in the CNS is predominantly carried out within the hypothalamus, extra-hypothalamic CNS areas contain metabolic hormone receptors, including those for leptin (LEPR), insulin (INSR), and neuropeptide Y (NPY), indicating that they may play a role in biological processes underlying pathogenic disease processes. The status of these hormones within regions vulnerable in ALS/FTD is not well described. This study sought to determine whether the expression of these hormones and their receptors is altered in pathology-rich regions in cases of human FTD (superior frontal gyrus and insular cortex) and ALS (primary motor cortex and lumbar spinal cord) with TDP-43 pathology compared to matched healthy controls. LEPR mRNA was increased within the superior frontal gyrus of FTD cases and within primary motor cortex and lumbar spinal cord of ALS cases; INSR mRNA was increased in superior frontal gyrus and insular cortex of FTD cases. NPY protein was decreased in primary motor cortex and lumbar spinal cord of ALS cases. Our results demonstrate that metabolic hormones undergo complex alterations in ALS and FTD and suggest that these hormones could play critical roles in the pathogenesis of these diseases.

Dysregulated FOXO1 activity drives skeletal muscle intrinsic dysfunction in amyotrophic lateral sclerosis

Amyotrophic Lateral Sclerosis (ALS) is a multisystemic neurodegenerative disorder, with accumulating evidence indicating metabolic disruptions in the skeletal muscle preceding disease symptoms, rather than them manifesting as a secondary consequence of motor neuron (MN) degeneration. Hence, energy homeostasis is deeply implicated in the complex physiopathology of ALS and skeletal muscle has emerged as a key therapeutic target. Here, we describe intrinsic abnormalities in ALS skeletal muscle, both in patient-derived muscle cells and in muscle cell lines with genetic knockdown of genes related to familial ALS, such as TARDBP (TDP-43) and FUS. We found a functional impairment of myogenesis that parallels defects of glucose oxidation in ALS muscle cells. We identified FOXO1 transcription factor as a key mediator of these metabolic and functional features in ALS muscle, via gene expression profiling and biochemical surveys in TDP-43 and FUS-silenced muscle progenitors. Strikingly, inhibition of FOXO1 mitigated the impaired myogenesis in both the genetically modified and the primary ALS myoblasts. In addition, specific in vivo conditional knockdown of TDP-43 or FUS orthologs (TBPH or caz) in Drosophila muscle precursor cells resulted in decreased innervation and profound dysfunction of motor nerve terminals and neuromuscular synapses, accompanied by motor abnormalities and reduced lifespan. Remarkably, these phenotypes were partially corrected by foxo inhibition, bolstering the potential pharmacological management of muscle intrinsic abnormalities associated with ALS. The findings demonstrate an intrinsic muscle dysfunction in ALS, which can be modulated by targeting FOXO factors, paving the way for novel therapeutic approaches that focus on the skeletal muscle as complementary target tissue.

Elevated nuclear TDP-43 induces constitutive exon skipping

BACKGROUND

Cytoplasmic inclusions and loss of nuclear TDP-43 are key pathological features found in several neurodegenerative disorders, suggesting both gain- and loss-of-function mechanisms of disease. To study gain-of-function, TDP-43 overexpression has been used to generate in vitro and in vivo model systems.

METHODS

We analyzed RNA-seq datasets from mouse and human neurons overexpressing TDP-43 to explore species specific splicing patterns. We explored the dynamics between TDP-43 levels and exon repression in vitro. Furthermore we analyzed human brain samples and publicly available RNA datasets to explore the relationship between exon repression and disease.

RESULTS

Our study shows that excessive levels of nuclear TDP-43 protein lead to constitutive exon skipping that is largely species-specific. Furthermore, while aberrant exon skipping is detected in some human brains, it is not correlated with disease, unlike the incorporation of cryptic exons that occurs after loss of TDP-43.

CONCLUSIONS

Our findings emphasize the need for caution in interpreting TDP-43 overexpression data and stress the importance of controlling for exon skipping when generating models of TDP-43 proteinopathy.

New developments in pre-clinical models of ALS to guide translation

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder in which selective death of motor neurons leads to muscle weakness and paralysis. Most research has focused on understanding and treating monogenic familial forms, most frequently caused by mutations in SOD1, FUS, TARDBP and C9orf72, although ALS is mostly sporadic and without a clear genetic cause. Rodent models have been developed to study monogenic ALS, but despite numerous pre-clinical studies and clinical trials, few disease-modifying therapies are available. ALS is a heterogeneous disease with complex underlying mechanisms where several genes and molecular pathways appear to play a role. One reason for the high failure rate of clinical translation from the current models could be oversimplification in pre-clinical studies. Here, we review advances in pre-clinical models to better capture the heterogeneous nature of ALS and discuss the value of novel model systems to guide translation and aid in the development of precision medicine.

Endogenous TDP-43 mislocalization in a novel knock-in mouse model reveals DNA repair impairment, inflammation, and neuronal senescence

TDP-43 mislocalization and aggregation are key pathological features of motor neuron diseases (MND) including amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). However, transgenic hTDP-43 WT or ∆NLS-overexpression animal models mainly capture late-stages TDP-43 proteinopathy, and do not provide a complete understanding of early motor neuron-specific pathology during pre-symptomatic phases. We have now addressed this shortcoming by generating a new endogenous knock-in (KI) mouse model using a combination of CRISPR/Cas9 and FLEX Cre-switch strategy for the conditional expression of a mislocalized Tdp-43∆NLS variant of mouse Tdp-43. This variant is either expressed conditionally in whole mice or specifically in the motor neurons. The mice exhibit loss of nuclear Tdp-43 concomitant with its cytosolic accumulation and aggregation in targeted cells, leading to increased DNA double-strand breaks (DSBs), signs of inflammation and DNA damage-associated cellular senescence. Notably, unlike WT Tdp43 which functionally interacts with Xrcc4 and DNA Ligase 4, the key DSB repair proteins in the non-homologous end-joining (NHEJ) pathway, the Tdp-43∆NLS mutant sequesters them into cytosolic aggregates, exacerbating neuronal damage in mice brain. The mutant mice also exhibit myogenic degeneration in limb muscles and distinct motor deficits, consistent with the characteristics of MND. Our findings reveal progressive degenerative mechanisms in motor neurons expressing endogenous Tdp-43∆NLS mutant, independent of TDP-43 overexpression or other confounding etiological factors. Thus, this unique Tdp-43 KI mouse model, which displays key molecular and phenotypic features of Tdp-43 proteinopathy, offers a significant opportunity to further characterize the early-stage progression of MND and also opens avenues for developing DNA repair-targeted approaches for treating TDP-43 pathology-linked neurodegenerative diseases.

Endothelial TDP-43 controls sprouting angiogenesis and vascular barrier integrity, and its deletion triggers neuroinflammation

TAR DNA-binding protein 43 (TDP-43) is a DNA/RNA-binding protein that regulates gene expression, and its malfunction in neurons has been causally associated with multiple neurodegenerative disorders. Although progress has been made in understanding the functions of TDP-43 in neurons, little is known about its roles in endothelial cells (ECs), angiogenesis, and vascular function. Using inducible EC-specific TDP-43-KO mice, we showed that TDP-43 is required for sprouting angiogenesis, vascular barrier integrity, and blood vessel stability. Postnatal EC-specific deletion of TDP-43 led to retinal hypovascularization due to defects in vessel sprouting associated with reduced EC proliferation and migration. In mature blood vessels, loss of TDP-43 disrupted the blood-brain barrier and triggered vascular degeneration. These vascular defects were associated with an inflammatory response in the CNS with activation of microglia and astrocytes. Mechanistically, deletion of TDP-43 disrupted the fibronectin matrix around sprouting vessels and reduced β-catenin signaling in ECs. Together, our results indicate that TDP-43 is essential for the formation of a stable and mature vasculature.

Mitochondria: A Promising Convergent Target for the Treatment of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease characterized by the progressive loss of motor neurons, for which current treatment options are limited. Recent studies have shed light on the role of mitochondria in ALS pathogenesis, making them an attractive therapeutic intervention target. This review contains a very comprehensive critical description of the involvement of mitochondria and mitochondria-mediated mechanisms in ALS. The review covers several key areas related to mitochondria in ALS, including impaired mitochondrial function, mitochondrial bioenergetics, reactive oxygen species, metabolic processes and energy metabolism, mitochondrial dynamics, turnover, autophagy and mitophagy, impaired mitochondrial transport, and apoptosis. This review also highlights preclinical and clinical studies that have investigated various mitochondria-targeted therapies for ALS treatment. These include strategies to improve mitochondrial function, such as the use of dichloroacetate, ketogenic and high-fat diets, acetyl-carnitine, and mitochondria-targeted antioxidants. Additionally, antiapoptotic agents, like the mPTP-targeting agents minocycline and rasagiline, are discussed. The paper aims to contribute to the identification of effective mitochondria-targeted therapies for ALS treatment by synthesizing the current understanding of the role of mitochondria in ALS pathogenesis and reviewing potential convergent therapeutic interventions. The complex interplay between mitochondria and the pathogenic mechanisms of ALS holds promise for the development of novel treatment strategies to combat this devastating disease.

Implications of TDP-43 in non-neuronal systems

TAR DNA-binding protein 43 (TDP-43) is a versatile RNA/DNA-binding protein with multifaceted processes. While TDP-43 has been extensively studied in the context of degenerative diseases, recent evidence has also highlighted its crucial involvement in diverse life processes beyond neurodegeneration. Here, we mainly reviewed the function of TDP-43 in non-neurodegenerative physiological and pathological processes, including spermatogenesis, embryonic development, mammary gland development, tumor formation, and viral infection, highlighting its importance as a key regulatory factor for the maintenance of normal functions throughout life. TDP-43 exhibits diverse and sometimes opposite functionality across different cell types through various mechanisms, and its roles can shift at distinct stages within the same biological system. Consequently, TDP-43 operates in both a context-dependent and a stage-specific manner in response to a variety of internal and external stimuli. Video Abstract.

Skeletal muscle in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS), the major adult-onset motor neuron disease, has been viewed almost exclusively as a disease of upper and lower motor neurons, with muscle changes interpreted as a consequence of the progressive loss of motor neurons and neuromuscular junctions. This has led to the prevailing view that the involvement of muscle in ALS is only secondary to motor neuron loss. Skeletal muscle and motor neurons reciprocally influence their respective development and constitute a single functional unit. In ALS, multiple studies indicate that skeletal muscle dysfunction might contribute to progressive muscle weakness, as well as to the final demise of neuromuscular junctions and motor neurons. Furthermore, skeletal muscle has been shown to participate in disease pathogenesis of several monogenic diseases closely related to ALS. Here, we move the narrative towards a better appreciation of muscle as a contributor of disease in ALS. We review the various potential roles of skeletal muscle cells in ALS, from passive bystanders to active players in ALS pathophysiology. We also compare ALS to other motor neuron diseases and draw perspectives for future research and treatment.

Uncovering Critical Roles for RNA in Neurodegeneration

RNA-binding proteins, in particular TDP-43, are key players in neurodegenerative disorders, mainly amyotrophic lateral sclerosis and frontotemporal dementia. We aim to elucidate how TDP-43 dysfunction alters cell metabolism and to identify mechanisms linked to aberrant behavior. We find that RNA binding plays a key role in maintaining TDP-43 homeostasis and in controlling cellular organization, two processes of essential importance to TDP-43 pathology. This research will provide insight into pathogenesis and help develop therapeutic interventions.

Maternal TDP-43 interacts with RNA Pol II and regulates zygotic genome activation

Zygotic genome activation (ZGA) is essential for early embryonic development. However, the regulation of ZGA remains elusive in mammals. Here we report that a maternal factor TDP-43, a nuclear transactive response DNA-binding protein, regulates ZGA through RNA Pol II and is essential for mouse early embryogenesis. Maternal TDP-43 translocates from the cytoplasm into the nucleus at the early two-cell stage when minor to major ZGA transition occurs. Genetic deletion of maternal TDP-43 results in mouse early embryos arrested at the two-cell stage. TDP-43 co-occupies with RNA Pol II as large foci in the nucleus and also at the promoters of ZGA genes at the late two-cell stage. Biochemical evidence indicates that TDP-43 binds Polr2a and Cyclin T1. Depletion of maternal TDP-43 caused the loss of Pol II foci and reduced Pol II binding on chromatin at major ZGA genes, accompanied by defective ZGA. Collectively, our results suggest that maternal TDP-43 is critical for mouse early embryonic development, in part through facilitating the correct RNA Pol II configuration and zygotic genome activation.

Targeting the glycine-rich domain of TDP-43 with antibodies prevents its aggregation in vitro and reduces neurofilament levels in vivo

Cytoplasmic aggregation and concomitant nuclear clearance of the RNA-binding protein TDP-43 are found in ~ 90% of cases of amyotrophic lateral sclerosis and ~ 45% of patients living with frontotemporal lobar degeneration, but no disease-modifying therapy is available. Antibody therapy targeting other aggregating proteins associated with neurodegenerative disorders has shown beneficial effects in animal models and clinical trials. The most effective epitopes for safe antibody therapy targeting TDP-43 are unknown. Here, we identified safe and effective epitopes in TDP-43 for active and potential future passive immunotherapy. We prescreened 15 peptide antigens covering all regions of TDP-43 to identify the most immunogenic epitopes and to raise novel monoclonal antibodies in wild-type mice. Most peptides induced a considerable antibody response and no antigen triggered obvious side effects. Thus, we immunized mice with rapidly progressing TDP-43 proteinopathy ("rNLS8" model) with the nine most immunogenic peptides in five pools prior to TDP-43ΔNLS transgene induction. Strikingly, combined administration of two N-terminal peptides induced genetic background-specific sudden lethality in several mice and was therefore discontinued. Despite a strong antibody response, no TDP-43 peptide prevented the rapid body weight loss or reduced phospho-TDP-43 levels as well as the profound astrogliosis and microgliosis in rNLS8 mice. However, immunization with a C-terminal peptide containing the disease-associated phospho-serines 409/410 significantly lowered serum neurofilament light chain levels, indicative of reduced neuroaxonal damage. Transcriptomic profiling showed a pronounced neuroinflammatory signature (IL-1β, TNF-α, NfκB) in rNLS8 mice and suggested modest benefits of immunization targeting the glycine-rich region. Several novel monoclonal antibodies targeting the glycine-rich domain potently reduced phase separation and aggregation of TDP-43 in vitro and prevented cellular uptake of preformed aggregates. Our unbiased screen suggests that targeting the RRM2 domain and the C-terminal region of TDP-43 by active or passive immunization may be beneficial in TDP-43 proteinopathies by inhibiting cardinal processes of disease progression.

Loss of TDP-43 causes ectopic endothelial sprouting and migration defects through increased fibronectin, vcam 1 and integrin α4/β1

Aggregation of the Tar DNA-binding protein of 43 kDa (TDP-43) is a pathological hallmark of amyotrophic lateral sclerosis and frontotemporal dementia and likely contributes to disease by loss of nuclear function. Analysis of TDP-43 function in knockout zebrafish identified an endothelial directional migration and hypersprouting phenotype during development prior lethality. In human umbilical vein cells (HUVEC) the loss of TDP-43 leads to hyperbranching. We identified elevated expression of FIBRONECTIN 1 (FN1), the VASCULAR CELL ADHESION MOLECULE 1 (VCAM1), as well as their receptor INTEGRIN α4β1 (ITGA4B1) in HUVEC cells. Importantly, reducing the levels of ITGA4, FN1, and VCAM1 homologues in the TDP-43 loss-of-function zebrafish rescues the angiogenic defects indicating the conservation of human and zebrafish TDP-43 function during angiogenesis. Our study identifies a novel pathway regulated by TDP-43 important for angiogenesis during development.

Phase 1/2a clinical trial in ALS with ropinirole, a drug candidate identified by iPSC drug discovery

iPSC-based drug discovery led to a phase 1/2a trial of ropinirole in ALS. 20 participants with sporadic ALS received ropinirole or placebo for 24 weeks in the double-blind period to evaluate safety, tolerability, and therapeutic effects. Adverse events were similar in both groups. During the double-blind period, muscle strength and daily activity were maintained, but a decline in the ALSFRS-R, which assesses the functional status of ALS patients, was not different from that in the placebo group. However, in the open-label extension period, the ropinirole group showed significant suppression of ALSFRS-R decline and an additional 27.9 weeks of disease-progression-free survival. iPSC-derived motor neurons from participants showed dopamine D2 receptor expression and a potential involvement of the SREBP2-cholesterol pathway in therapeutic effects. Lipid peroxide represents a clinical surrogate marker to assess disease progression and drug efficacy. Limitations include small sample sizes and high attrition rates in the open-label extension period, requiring further validation.

Elevated nuclear TDP-43 induces constitutive exon skipping

Cytoplasmic inclusions and loss of nuclear TDP-43 are key pathological features found in several neurodegenerative disorders, suggesting both gain- and loss-of-function mechanisms of disease. To study gain-of-function, TDP-43 overexpression has been used to generate in vitro and in vivo model systems. Our study shows that excessive levels of nuclear TDP-43 protein lead to constitutive exon skipping that is largely species-specific. Furthermore, while aberrant exon skipping is detected in some human brains, it is not correlated with disease, unlike the incorporation of cryptic exons that occurs after loss of TDP-43. Our findings emphasize the need for caution in interpreting TDP-43 overexpression data, and stress the importance of controlling for exon skipping when generating models of TDP-43 proteinopathy. Understanding the subtle aspects of TDP-43 toxicity within different subcellular locations is essential for the development of therapies targeting neurodegenerative disease.

TDP-43 knockdown in mouse model of ALS leads to dsRNA deposition, gliosis, and neurodegeneration in the spinal cord

Transactive response DNA binding protein 43 kilodaltons (TDP-43) is a DNA and RNA binding protein associated with severe neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS), primarily affecting motor neurons in the brain and spinal cord. Partial knockdown of TDP-43 expression in a mouse model (the amiR-TDP-43 mice) leads to progressive, age-related motor dysfunction, as observed in ALS patients. Work in Caenorhabditis elegans suggests that TDP-43 dysfunction can lead to deficits in chromatin processing and double-stranded RNA (dsRNA) accumulation, potentially activating the innate immune system and promoting neuroinflammation. To test this hypothesis, we used immunostaining to investigate dsRNA accumulation and other signs of CNS pathology in the spinal cords of amiR-TDP-43 mice. Compared with wild-type controls, TDP-43 knockdown animals show increases in dsRNA deposition in the dorsal and ventral horns of the spinal cord. Additionally, animals with heavy dsRNA expression show markedly increased levels of astrogliosis and microgliosis. Interestingly, areas of high dsRNA expression and microgliosis overlap with regions of heavy neurodegeneration, indicating that activated microglia could contribute to the degeneration of spinal cord neurons. This study suggests that loss of TDP-43 function could contribute to neuropathology by increasing dsRNA deposition and subsequent innate immune system activation.

APOE2 Exacerbates TDP-43 Related Toxicity in the Absence of Alzheimer Pathology

OBJECTIVE

Recent evidence supports a link between increased TDP-43 burden and the presence of an APOE4 gene allele in Alzheimer's disease (AD); however, it is difficult to conclude the direct effect of APOE on TDP-43 pathology due to the presence of mixed AD pathologies. The goal of this study is to address how APOE isoforms impact TDP-43 pathology and related neurodegeneration in the absence of typical AD pathologies.

METHODS

We overexpressed human TDP-43 via viral transduction in humanized APOE2, APOE3, APOE4 mice, and murine Apoe-knockout (Apoe-KO) mice. Behavior tests were performed across ages. Animals were harvested at 11 months of age and TDP-43 overexpression-related neurodegeneration and gliosis were assessed. To further address the human relevance, we analyzed the association of APOE with TDP-43 pathology in 160 postmortem brains from autopsy-confirmed amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with motor neuron disease (FTLD-MND) in the Mayo Clinic Brain Bank.

RESULTS

We found that TDP-43 overexpression induced motor function deficits, neuronal loss, and gliosis in the motor cortex, especially in APOE2 mice, with much milder or absent effects in APOE3, APOE4, or Apoe-KO mice. In the motor cortex of the ALS and FTLD-MND postmortem human brains, we found that the APOE2 allele was associated with more severe TDP-43-positive dystrophic neurites.

INTERPRETATION

Our data suggest a genotype-specific effect of APOE on TDP-43 proteinopathy and neurodegeneration in the absence of AD pathology, with the strongest association seen with APOE2. ANN NEUROL 2023;93:830-843.

New AAV tools fail to detect Neurod1-mediated neuronal conversion of Müller glia and astrocytes in vivo

BACKGROUND

Reprogramming resident glial cells to convert them into neurons in vivo represents a potential therapeutic strategy that could replenish lost neurons, repair damaged neural circuits, and restore function. AAV (adeno-associated virus)-based expression systems are powerful tools for in vivo gene delivery in glia-to-neuron reprogramming, however, recent studies show that AAV-based gene delivery of Neurod1 into the mouse brain can cause severe leaky expression into endogenous neurons leading to misinterpretation of glia-to-neuron conversion.

METHODS

AAV-based delivery systems were modified for improved in vivo delivery of Neurod1, Math5, Ascl1, and Neurog2 in the adult mouse retina and brain. To examine whether bona fide glia-to-neuron conversion occurs, stringent fate mapping experiments were performed to trace the lineage of glial cells.

FINDINGS

The neuronal leakage is prevalent after AAV-GFAP-mediated delivery of Neurod1, Math5, Ascl1, and Neurog2. The transgene-dependent leakage cannot be corrected after lowering the AAV doses, using alterative AAV serotypes or injection routes. Importantly, we report the development of two new AAV-based tools that can significantly reduce neuronal leakage. Using the new AAV-based tools, we provide evidence that Neurod1 gene transfer fails to convert lineage traced glial cells into neurons.

INTERPRETATION

Stringent fate mapping techniques independently of an AAV-based expression system are the golden standard for tracing the fate of glia cells during neuronal reprogramming. The newly developed AAV-based systems are invaluable tools for glia-to-neuron reprogramming in vivo.

FUNDING

The work in Chen lab was supported by National Institutes of Health (NIH) grants R01 EY024986 and R01 EY028921, an unrestricted challenge grant from Research to Prevent Blindness, the New York Eye and Ear Infirmary Foundation, and The Harold W. McGraw, Jr. Family Foundation for Vision Research. The work in Zhang lab was supported by NIH (R01 NS127375 and R01 NS117065) and The Decherd Foundation.

Advances in sequencing technologies for amyotrophic lateral sclerosis research

Amyotrophic lateral sclerosis (ALS) is caused by upper and lower motor neuron loss and has a fairly rapid disease progression, leading to fatality in an average of 2-5 years after symptom onset. Numerous genes have been implicated in this disease; however, many cases remain unexplained. Several technologies are being used to identify regions of interest and investigate candidate genes. Initial approaches to detect ALS genes include, among others, linkage analysis, Sanger sequencing, and genome-wide association studies. More recently, next-generation sequencing methods, such as whole-exome and whole-genome sequencing, have been introduced. While those methods have been particularly useful in discovering new ALS-linked genes, methodological advances are becoming increasingly important, especially given the complex genetics of ALS. Novel sequencing technologies, like long-read sequencing, are beginning to be used to uncover the contribution of repeat expansions and other types of structural variation, which may help explain missing heritability in ALS. In this review, we discuss how popular and/or upcoming methods are being used to discover ALS genes, highlighting emerging long-read sequencing platforms and their role in aiding our understanding of this challenging disease.

Upper and Lower Motor Neurons and the Skeletal Muscle: Implication for Amyotrophic Lateral Sclerosis (ALS)

The relationships between motor neurons and the skeletal muscle during development and in pathologic contexts are addressed in this Chapter.We discuss the developmental interplay of muscle and nervous tissue, through neurotrophins and the activation of differentiation and survival pathways. After a brief overview on muscular regulatory factors, we focus on the contribution of muscle to early and late neurodevelopment. Such a role seems especially intriguing in relation to the epigenetic shaping of developing motor neuron fate choices. In this context, emphasis is attributed to factors regulating energy metabolism, which may concomitantly act in muscle and neural cells, being involved in common pathways.We then review the main features of motor neuron diseases, addressing the cellular processes underlying clinical symptoms. The involvement of different muscle-associated neurotrophic factors for survival of lateral motor column neurons, innervating MyoD-dependent limb muscles, and of medial motor column neurons, innervating Myf5-dependent back musculature is discussed. Among the pathogenic mechanisms, we focus on oxidative stress, that represents a common and early trait in several neurodegenerative disorders. The role of organelles primarily involved in reactive oxygen species scavenging and, more generally, in energy metabolism-namely mitochondria and peroxisomes-is discussed in the frame of motor neuron degeneration.We finally address muscular involvement in amyotrophic lateral sclerosis (ALS), a multifactorial degenerative disorder, hallmarked by severe weight loss, caused by imbalanced lipid metabolism. Even though multiple mechanisms have been recognized to play a role in the disease, current literature generally assumes that the primum movens is neuronal degeneration and that muscle atrophy is only a consequence of such pathogenic event. However, several lines of evidence point to the muscle as primarily involved in the disease, mainly through its role in energy homeostasis. Data from different ALS mouse models strongly argue for an early mitochondrial dysfunction in muscle tissue, possibly leading to motor neuron disturbances. Detailed understanding of skeletal muscle contribution to ALS pathogenesis will likely lead to the identification of novel therapeutic strategies.

Induction of whole-body gene deletion via R26-regulated tamoxifen-inducible Cre recombinase activity

Germline deletion of certain genes causes embryonic lethality, therefore, understanding the effect of deletion of such genes on mammalian pathophysiology remains challenging. Tamoxifen (TAM)-inducible Cre recombinase is widely used for tissue-specific and temporal induction of gene deletion in mice. However, the tamoxifen treatment regimen for the generation of whole-body deletion of a gene is not yet fully standardized for the majority of organs/tissues. Accordingly, we employed GtROSA26 (R26) promoter-regulated Cre and a reporter gene expression strategy. GtROSA26 (R26) is an ubiquitous promoter and mice carrying the R26^Cre-ERT2 transgene express Cre-ERT2 in all the cells. Similarly, mice carrying the R26^mTOM-mEGFP transgene express mTOM (membrane-targeted tdTomato), in the absence of Cre or mEGFP (membrane-targeted enhanced green fluorescent protein), in the presence of Cre, in all the cells. The progeny carrying one allele of both transgenes were subjected to different TAM regimens, i.e., IP injections (4 injections; 1.35 mg/injection), diet (400 mg TAM-citrate/kg food), or diet (400 mg TAM-citrate/kg food) combined with either TAM-oral gavage (4 gavages; 1.35 mg/gavage) or TAM IP injections (4 injections; 1.35 mg/injection) for 2-weeks beginning at postnatal day (PND) 21 and the extent of Cre recombination in different tissues was determined at PND35. Tamoxifen administration resulted in a transient loss of body weight in all the treatment regimens with a relatively slower rate of weight gain in the TAM-diet plus TAM-oral gavage group compared to other groups. While the efficiency of Cre recombination, as determined by the expression of mEGFP protein, was variable among tissues, major tissues such as the liver, heart, lungs, spleen, and thymus-showed almost complete recombination. No recombination was evident in any of the tissues examined from the control mice. In general, the efficiency of Cre recombination was better with a combined regimen of TAM-diet with either TAM-injections or TAM-oral gavage compared to TAM-diet alone or TAM-injections alone. Our results demonstrate that a combination of TAM-diet with either TAM-injections or TAM-oral gavage can be employed for the efficient deletion of a gene in the whole body. Our findings will provide technical expertise to the researchers employing TAM-inducible Cre for the deletion of floxed genes in varied tissues.

Sertoli cells require TDP-43 to support spermatogenesis†

TAR DNA binding protein of 43 kD (TDP-43) is an evolutionarily conserved, ubiquitously expressed transcription factor and RNA-binding protein with major human health relevance. TDP-43 is present in Sertoli and germ cells of the testis and is aberrantly expressed in the sperm of infertile men. Sertoli cells play a key role in spermatogenesis by offering physical and nutritional support to male germ cells. The current study investigated the requirement of TDP-43 in Sertoli cells. Conditional knockout (cKO) of TDP-43 in mouse Sertoli cells caused failure of spermatogenesis and male subfertility. The cKO mice showed decreased testis weight, and low sperm count. Testis showed loss of germ cell layers, presence of vacuoles, and sloughing of round spermatids, suggesting loss of contact with Sertoli cells. Using a biotin tracer, we found that the blood-testis barrier (BTB) was disrupted as early as postnatal day 24 and worsened in adult cKO mice. We noted aberrant expression of the junction proteins connexin-43 (gap junction) and N-cadherin (ectoplasmic specialization). Oil Red O staining showed a decrease in lipid droplets (phagocytic function) in tubule cross-sections, Sertoli cells cytoplasm, and in the lumen of seminiferous tubules of cKO mice. Finally, qRT-PCR showed upregulation of genes involved in the formation and/or maintenance of Sertoli cell junctions as well as in the phagocytic pathway. Sertoli cells require TDP-43 for germ cell attachment, formation and maintenance of BTB, and phagocytic function, thus indicating an essential role for TDP-43 in the maintenance of spermatogenesis.

Transcriptomic and epigenomic landscapes of Alzheimer's disease evidence mitochondrial-related pathways

Alzheimers disease (AD) is the main cause of dementia and it is defined by cognitive decline coupled to extracellular deposit of amyloid-beta protein and intracellular hyperphosphorylation of tau protein. Historically, efforts to target such hallmarks have failed in numerous clinical trials. In addition to these hallmark-targeted approaches, several clinical trials focus on other AD pathological processes, such as inflammation, mitochondrial dysfunction, and oxidative stress. Mitochondria and mitochondrial-related mechanisms have become an attractive target for disease-modifying strategies, as mitochondrial dysfunction prior to clinical onset has been widely described in AD patients and AD animal models. Mitochondrial function relies on both the nuclear and mitochondrial genome. Findings from omics technologies have shed light on AD pathophysiology at different levels (e.g., epigenome, transcriptome and proteome). Most of these studies have focused on the nuclear-encoded components. The first part of this review provides an updated overview of the mechanisms that regulate mitochondrial gene expression and function. The second part of this review focuses on evidence of mitochondrial dysfunction in AD. We have focused on published findings and datasets that study AD. We analyzed published data and provide examples for mitochondrial-related pathways. These pathways are strikingly dysregulated in AD neurons and glia in sex-, cell- and disease stage-specific manners. Analysis of mitochondrial omics data highlights the involvement of mitochondria in AD, providing a rationale for further disease modeling and drug targeting.

FUS Mutation Causes Disordered Lipid Metabolism in Skeletal Muscle Associated with ALS

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease characterized by dysfunction of the upper and lower motor neurons resulting in muscle weakness and wasting. Recently, several studies on ALS patients and ALS animal models indicated that intramuscular toxicity played a role in ALS disease progression; however, the mechanisms driving this are unknown. In this study, we explored the possible dysfunction of lipid metabolism in myocytes associated with ALS. Initially, skeletal muscle from 41 ALS patients, as well as 53 non-ALS control subjects, was investigated, and we identified that lipid droplet accumulation in the muscle fibers of ALS patients was significantly increased, especially in patients with FUS mutations. A myoblast (C2C12) cell line expressing mutant FUS (FUS-K510Q) was able to induce lipid droplet accumulation and mitochondrial dysfunction. Consistently, transgenic flies expressing FUS-K510Q under a muscle-specific driver showed elevated triglyceride levels in the flight muscles, as well as locomotor defects. Biochemical analysis of C2C12 cells and fly muscle tissues showed upregulation of PLIN2, and downregulation of ATGL and CPT1A, indicating inhibition of lipolysis and fatty acid β-oxidation in muscle cells with FUS mutations. Our study provided a potential explanation for the pathogenesis associated with lipid droplets accumulating in skeletal muscle in ALS. Our data also suggested that disordered lipid metabolism and mitochondrial dysfunction play a crucial role in intramuscular toxicity in ALS.

Status of ALS Treatment, Insights into Therapeutic Challenges and Dilemmas

Amyotrophic lateral sclerosis (ALS) is an extremely heterogeneous disease of motor neurons that eventually leads to death. Despite impressive advances in understanding the genetic, molecular, and pathological mechanisms of the disease, the only drug approved to date by both the FDA and EMA is riluzole, with a modest effect on survival. In this opinion view paper, we will discuss how to address some challenges for drug development in ALS at the conceptual, technological, and methodological levels. In addition, socioeconomic and ethical issues related to the legitimate need of patients to benefit quickly from new treatments will also be addressed. In conclusion, this brief review takes a more optimistic view, given the recent approval of two new drugs in some countries and the development of targeted gene therapies.

Aberrant neural activity in prefrontal pyramidal neurons lacking TDP-43 precedes neuron loss

Mislocalization of TAR DNA binding protein 43 kDa (TARDBP, or TDP-43) is a principal pathological hallmark identified in cases of neurodegenerative disorders such as amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). As an RNA binding protein, TDP-43 serves in the nuclear compartment to repress non-conserved cryptic exons to ensure the normal transcriptome. Multiple lines of evidence from animal models and human studies support the view that loss of TDP-43 leads to neuron loss, independent of its cytosolic aggregation. However, the underlying pathogenic pathways driven by the loss-of-function mechanism are still poorly defined. We employed a genetic approach to determine the impact of TDP-43 loss in pyramidal neurons of the prefrontal cortex (PFC). Using a custom-built miniscope imaging system, we performed repetitive in vivo calcium imaging from freely behaving mice for up to 7 months. By comparing calcium activity in PFC pyramidal neurons between TDP-43 depleted and TDP-43 intact mice, we demonstrated remarkably increased numbers of pyramidal neurons exhibiting hyperactive calcium activity after short-term TDP-43 depletion, followed by rapid activity declines prior to neuron loss. Our results suggest aberrant neural activity driven by loss of TDP-43 as the pathogenic pathway at early stage in ALS and FTD.

Altered TDP-43 Structure and Function: Key Insights into Aberrant RNA, Mitochondrial, and Cellular and Systemic Metabolism in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive and fatal neuromuscular disorder with no cure available and limited treatment options. ALS is a highly heterogeneous disease, whereby patients present with vastly different phenotypes. Despite this heterogeneity, over 97% of patients will exhibit pathological TAR-DNA binding protein-43 (TDP-43) cytoplasmic inclusions. TDP-43 is a ubiquitously expressed RNA binding protein with the capacity to bind over 6000 RNA and DNA targets—particularly those involved in RNA, mitochondrial, and lipid metabolism. Here, we review the unique structure and function of TDP-43 and its role in affecting the aforementioned metabolic processes in ALS. Considering evidence published specifically in TDP-43-relevant in vitro, in vivo, and ex vivo models we posit that TDP-43 acts in a positive feedback loop with mRNA transcription/translation, stress granules, cytoplasmic aggregates, and mitochondrial proteins causing a relentless cycle of disease-like pathology eventuating in neuronal toxicity. Given its undeniable presence in ALS pathology, TDP-43 presents as a promising target for mechanistic disease modelling and future therapeutic investigations.

Metabolic Dysfunction in Motor Neuron Disease: Shedding Light through the Lens of Autophagy

Amyotrophic lateral sclerosis (ALS) patients show a myriad of energetic abnormalities, such as weight loss, hypermetabolism, and dyslipidaemia. Evidence suggests that these indices correlate with and ultimately affect the duration of survival. This review aims to discuss ALS metabolic abnormalities in the context of autophagy, the primordial system acting at the cellular level for energy production during nutrient deficiency. As the primary pathway of protein degradation in eukaryotic cells, the fundamental role of cellular autophagy is the adaptation to metabolic demands. Therefore, autophagy is tightly coupled to cellular metabolism. We review evidence that the delicate balance between autophagy and metabolism is aberrant in ALS, giving rise to intracellular and systemic pathophysiology observations. Understanding the metabolism autophagy crosstalk can lead to the identification of novel therapeutic targets for ALS.

Chchd10 is dispensable for myogenesis but critical for adipose browning

The Chchd10 gene encodes a coiled-coil-helix-coiled-coil-helix-domain containing protein predicted to function in the mitochondrion and nucleus. Mutations of Chchd10 are associated with ALS, dementia and myopathy in humans and animal models, but how knockout of Chchd10 (Chchd10^KO) affects various tissues especially skeletal muscle and adipose tissues remains unclear. Here we show that Chchd10 expression increases as myoblasts and preadipocytes differentiate. During myogenesis, CHCHD10 interacts with TAR DNA binding protein 43 (TDP-43) in regenerating myofibers in vivo and in newly differentiated myotubes ex vivo. Surprisingly, Chchd10^KO mice had normal skeletal muscle development, growth and regeneration, with moderate defects in grip strength and motor performance. Chchd10^KO similarly had no effects on development of brown and white adipose tissues (WAT). However, Chchd10^KO mice had blunted response to acute cold and attenuated cold-induced browning of WAT, with markedly reduced UCP1 levels. Together, these results demonstrate that Chchd10 is dispensable for normal myogenesis and adipogenesis but is required for normal motility and cold-induced, mitochondrion-dependent browning of adipocytes. The data also suggest that human CHCHD10 mutations cause myopathy through a gain-of-function mechanism.

Stage-specific control of oligodendrocyte survival and morphogenesis by TDP-43

Generation of oligodendrocytes in the adult brain enables both adaptive changes in neural circuits and regeneration of myelin sheaths destroyed by injury, disease, and normal aging. This transformation of oligodendrocyte precursor cells (OPCs) into myelinating oligodendrocytes requires processing of distinct mRNAs at different stages of cell maturation. Although mislocalization and aggregation of the RNA-binding protein, TDP-43, occur in both neurons and glia in neurodegenerative diseases, the consequences of TDP-43 loss within different stages of the oligodendrocyte lineage are not well understood. By performing stage-specific genetic inactivation of Tardbp in vivo, we show that oligodendrocyte lineage cells are differentially sensitive to loss of TDP-43. While OPCs depend on TDP-43 for survival, with conditional deletion resulting in cascading cell loss followed by rapid regeneration to restore their density, oligodendrocytes become less sensitive to TDP-43 depletion as they mature. Deletion of TDP-43 early in the maturation process led to eventual oligodendrocyte degeneration, seizures, and premature lethality, while oligodendrocytes that experienced late deletion survived and mice exhibited a normal lifespan. At both stages, TDP-43-deficient oligodendrocytes formed fewer and thinner myelin sheaths and extended new processes that inappropriately wrapped neuronal somata and blood vessels. Transcriptional analysis revealed that in the absence of TDP-43, key proteins involved in oligodendrocyte maturation and myelination were misspliced, leading to aberrant incorporation of cryptic exons. Inducible deletion of TDP-43 from oligodendrocytes in the adult central nervous system (CNS) induced the same progressive morphological changes and mice acquired profound hindlimb weakness, suggesting that loss of TDP-43 function in oligodendrocytes may contribute to neuronal dysfunction in neurodegenerative disease.

Expanding the TDP-43 Proteinopathy Pathway From Neurons to Muscle: Physiological and Pathophysiological Functions

TAR DNA-binding protein 43, mostly referred to as TDP-43 (encoded by the TARDBP gene) is strongly linked to the pathogenesis of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). From the identification of TDP-43 positive aggregates in the brains and spinal cords of ALS/FTD patients, to a genetic link between TARBDP mutations and the development of TDP-43 pathology in ALS, there is strong evidence indicating that TDP-43 plays a pivotal role in the process of neuronal degeneration. What this role is, however, remains to be determined with evidence ranging from gain of toxic properties through the formation of cytotoxic aggregates, to an inability to perform its normal functions due to nuclear depletion. To add to an already complex subject, recent studies highlight a role for TDP-43 in muscle physiology and disease. We here review the biophysical, biochemical, cellular and tissue-specific properties of TDP-43 in the context of neurodegeneration and have a look at the nascent stream of evidence that positions TDP-43 in a myogenic context. By integrating the neurogenic and myogenic pathological roles of TDP-43 we provide a more comprehensive and encompassing view of the role and mechanisms associated with TDP-43 across the various cell types of the motor system, all the way from brain to limbs.

Low-level overexpression of wild type TDP-43 causes late-onset, progressive neurodegeneration and paralysis in mice

Modestly increased expression of transactive response DNA binding protein (TDP-43) gene have been reported in amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and other neuromuscular diseases. However, whether this modest elevation triggers neurodegeneration is not known. Although high levels of TDP-43 overexpression have been modeled in mice and shown to cause early death, models with low-level overexpression that mimic the human condition have not been established. In this study, transgenic mice overexpressing wild type TDP-43 at less than 60% above the endogenous CNS levels were constructed, and their phenotypes analyzed by a variety of techniques, including biochemical, molecular, histological, behavioral techniques and electromyography. The TDP-43 transgene was expressed in neurons, astrocytes, and oligodendrocytes in the cortex and predominantly in astrocytes and oligodendrocytes in the spinal cord. The mice developed a reproducible progressive weakness ending in paralysis in mid-life. Detailed analysis showed ~30% loss of large pyramidal neurons in the layer V motor cortex; in the spinal cord, severe demyelination was accompanied by oligodendrocyte injury, protein aggregation, astrogliosis and microgliosis, and elevation of neuroinflammation. Surprisingly, there was no loss of lower motor neurons in the lumbar spinal cord despite the complete paralysis of the hindlimbs. However, denervation was detected at the neuromuscular junction. These results demonstrate that low-level TDP-43 overexpression can cause diverse aspects of ALS, including late-onset and progressive motor dysfunction, neuroinflammation, and neurodegeneration. Our findings suggest that persistent modest elevations in TDP-43 expression can lead to ALS and other neurological disorders involving TDP-43 proteinopathy. Because of the predictable and progressive clinical paralytic phenotype, this transgenic mouse model will be useful in preclinical trial of therapeutics targeting neurological disorders associated with elevated levels of TDP-43.

The Multifunctional Faces of T-Cell Intracellular Antigen 1 in Health and Disease

T-cell intracellular antigen 1 (TIA1) is an RNA-binding protein that is expressed in many tissues and in the vast majority of species, although it was first discovered as a component of human cytotoxic T lymphocytes. TIA1 has a dual localization in the nucleus and cytoplasm, where it plays an important role as a regulator of gene-expression flux. As a multifunctional master modulator, TIA1 controls biological processes relevant to the physiological functioning of the organism and the development and/or progression of several human pathologies. This review summarizes our current knowledge of the molecular aspects and cellular processes involving TIA1, with relevance for human pathophysiology.

Increased levels of TAR DNA-binding protein 43 in the hippocampus of subjects with bipolar disorder: a postmortem study

Bipolar disorder shares symptoms and pathological pathways with other neurodegenerative diseases, including frontotemporal dementia (FTD). Since TAR DNA-binding protein 43 (TDP-43) is a neuropathological marker of frontotemporal dementia and it is involved in synaptic transmission, we explored the role of TDP-43 as a molecular feature of bipolar disorder (BD). Homogenates were acquired from frozen hippocampus of postmortem brains of bipolar disorder subjects. TDP-43 levels were quantified using an ELISA-sandwich method and compared between the postmortem brains of bipolar disorder subjects and age-matched control group. We found higher levels of TDP-43 protein in the hippocampus of BD (n = 15) subjects, when compared to controls (n = 15). We did not find associations of TDP-43 with age at death, postmortem interval, or age of disease onset. Our results suggest that protein TDP-43 may be potentially implicated in behavioral abnormalities seen in BD. Further investigation is needed to validate these findings and to examine the role of this protein during the disease course and mood states.

Inflammasome activation leads to cDC1-independent cross-priming of CD8 T cells by epithelial cell-derived antigen

The innate immune system detects pathogens and initiates adaptive immune responses. Inflammasomes are central components of the innate immune system, but whether inflammasomes provide sufficient signals to activate adaptive immunity is unclear. In intestinal epithelial cells (IECs), inflammasomes activate a lytic form of cell death called pyroptosis, leading to epithelial cell expulsion and the release of cytokines. Here, we employed a genetic system to show that simultaneous antigen expression and inflammasome activation specifically in IECs is sufficient to activate CD8+ T cells. By genetic elimination of direct T cell priming by IECs, we found that IEC-derived antigens were cross-presented to CD8+ T cells. However, cross-presentation of IEC-derived antigen to CD8+ T cells only partially depended on IEC pyroptosis. In the absence of inflammasome activation, cross-priming of CD8+ T cells required Batf3+ dendritic cells (conventional type one dendritic cells [cDC1]), whereas cross-priming in the presence of inflammasome activation required a Zbtb46+ but Batf3-independent cDC population. These data suggest the existence of parallel inflammasome-dependent and inflammasome-independent pathways for cross-presentation of IEC-derived antigens.

A Novel Four-Gene Prognostic Signature for Prediction of Survival in Patients with Soft Tissue Sarcoma

Simple Summary

Soft tissue sarcomas (STS) still lack effective clinical stratification and prognostic models. The aim of this study is to establish a reliable prognostic gene signature in STS. Using 189 STS samples from the TCGA database, a four-gene signature (including DHRS3, JRK, TARDBP and TTC3) and nomograms that can be used to predict the overall survival and relapse free survival of STS patients was developed. The predictive ability for metastasis free survival was externally verified in the GEO cohort. We demonstrated that the novel gene signature provides an attractive platform for risk stratification and prognosis prediction of STS patients, which is of great importance for individualized clinical treatment and long-term management of patients with this rare and severe disease.

Abstract

Soft tissue sarcomas (STS), a group of rare malignant tumours with high tissue heterogeneity, still lack effective clinical stratification and prognostic models. Therefore, we conducted this study to establish a reliable prognostic gene signature. Using 189 STS patients’ data from The Cancer Genome Atlas database, a four-gene signature including DHRS3, JRK, TARDBP and TTC3 was established. A risk score based on this gene signature was able to divide STS patients into a low-risk and a high-risk group. The latter had significantly worse overall survival (OS) and relapse free survival (RFS), and Cox regression analyses showed that the risk score is an independent prognostic factor. Nomograms containing the four-gene signature have also been established and have been verified through calibration curves. In addition, the predictive ability of this four-gene signature for STS metastasis free survival was verified in an independent cohort (309 STS patients from the Gene Expression Omnibus database). Finally, Gene Set Enrichment Analysis indicated that the four-gene signature may be related to some pathways associated with tumorigenesis, growth, and metastasis. In conclusion, our study establishes a novel four-gene signature and clinically feasible nomograms to predict the OS and RFS. This can help personalized treatment decisions, long-term patient management, and possible future development of targeted therapy.

Loss of TDP-43 in male germ cells causes meiotic failure and impairs fertility in mice

Meiotic arrest is a common cause of human male infertility, but the causes of this arrest are poorly understood. Transactive response DNA-binding protein of 43 kDa (TDP-43) is highly expressed in spermatocytes in the preleptotene and pachytene stages of meiosis. TDP-43 is linked to several human neurodegenerative disorders wherein its nuclear clearance accompanied by cytoplasmic aggregates underlies neurodegeneration. Exploring the functional requirement for TDP-43 for spermatogenesis for the first time, we show here that conditional KO (cKO) of the Tardbp gene (encoding TDP-43) in male germ cells of mice leads to reduced testis size, depletion of germ cells, vacuole formation within the seminiferous epithelium, and reduced sperm production. Fertility trials also indicated severe subfertility. Spermatocytes of cKO mice showed failure to complete prophase I of meiosis with arrest at the midpachytene stage. Staining of synaptonemal complex protein 3 and γH2AX, markers of the meiotic synaptonemal complex and DNA damage, respectively, and super illumination microscopy revealed nonhomologous pairing and synapsis defects. Quantitative RT-PCR showed reduction in the expression of genes critical for prophase I of meiosis, including Spo11 (initiator of meiotic double-stranded breaks), Rec8 (meiotic recombination protein), and Rad21L (RAD21-like, cohesin complex component), as well as those involved in the retinoic acid pathway critical for entry into meiosis. RNA-Seq showed 1036 upregulated and 1638 downregulated genes (false discovery rate <0.05) in the Tardbp cKO testis, impacting meiosis pathways. Our work reveals a crucial role for TDP-43 in male meiosis and suggests that some forms of meiotic arrest seen in infertile men may result from the loss of function of TDP-43.

TDP-43 is essential for Eph receptor-class-specific spinal motor axon trajectory into the limb

During the neural circuit formation, neuronal growth cones must be guided precisely to their neuronal or muscle targets, which can be achieved by the activation of membrane-bound guidance receptors at the periphery. However, the mechanisms that regulate the temporal availability of these receptors remain largely unknown. TAR DNA binding protein-43 (TDP-43) has been proposed to bind with the mRNAs of guidance receptors, thus prompting us to investigate its role in axon guidance of the spinal lateral motor column (LMC) neurons into the limb mesenchyme. We first identified the TDP-43 expression in the LMC neurons at the stage of axons growth into the limb using in situ mRNA hybridization. The loss and gain of TDP-43 function in chick LMC neurons redirected their axon trajectory with opposite effects. In mice, a spinal motor neuron-specific TDP-43 deletion led to the misrouting of LMC axons. Further, ectopic TDP-43 expression increased EphB protein levels in LMC neurons, suggesting that TDP-43 mediates LMC pathfinding by regulating EphB expression. Finally, TDP-43 levels influenced the growth preference of LMC neurites against ephrin-B, but not Netrin-1 and Semaphorin ligands. Our results demonstrate that TDP-43 is essential for the ephrinB:EphB signaling-mediated axon trajectory selection of LMC subtypes into the limb.

Alterations in Leptin Signaling in Amyotrophic Lateral Sclerosis (ALS)

Leptin has been suggested to play a role in amyotrophic lateral sclerosis (ALS), a fatal progressive neurodegenerative disease. This adipokine has previously been shown to be associated with a lower risk of ALS and to confer a survival advantage in ALS patients. However, the role of leptin in the progression of ALS is unknown. Indeed, our understanding of the mechanisms underlying leptin's effects in the pathogenesis of ALS is very limited, and it is fundamental to determine whether alterations in leptin's actions take place in this neurodegenerative disease. To characterize the association between leptin signaling and the clinical course of ALS, we assessed the mRNA and protein expression profiles of leptin, the long-form of the leptin receptor (Ob-Rb), and leptin-related signaling pathways at two different stages of the disease (onset and end-stage) in TDP-43^A315T mice compared to age-matched WT littermates. In addition, at selected time-points, an immunoassay analysis was conducted to characterize plasma levels of total ghrelin, the adipokines resistin and leptin, and metabolic proteins (plasminogen activator inhibitor type 1 (PAI-1), gastric inhibitory peptide (GIP), glucagon-like peptide 1 (GLP-1), insulin and glucagon) in TDP-43^A315T mice compared to WT controls. Our results indicate alterations in leptin signaling in the spinal cord and the hypothalamus on the backdrop of TDP-43-induced deficits in mice, providing new evidence about the pathways that could link leptin signaling to ALS.

Extensive phenotypic characterisation of a human TDP-43Q331K transgenic mouse model of amyotrophic lateral sclerosis (ALS)

The majority of preclinical studies in ALS have relied on transgenic models with overexpression of mutant human superoxide dismutase 1 (SOD1), widely regarded to have failed in terms of translation of therapeutic effects. However, there are still no widely accepted models of other genetic subtypes of ALS. The majority of patients show ubiquitinated cytoplasmic inclusions of TAR DNA binding protein of 43 kilodaltons (TDP-43) in spinal motor neurons at the end stage of disease and a small proportion have mutations in TARDBP, the gene encoding TDP-43. TDP-43 transgenic mouse models have been produced, but have not been widely adopted. Here, we characterised one of these models available from the Jackson Laboratory in detail. Compared to TDP-43^WT mice, TDP-43^Q331K mice had 43% less hindlimb muscle mass at 6 months and a 73% reduction in hindlimb compound muscle action potential at 8 months of age. Rotarod and gait analysis indicated motor system decline with elevated weight gain. At the molecular level, the lack of TDP-43 cellular pathology was confirmed with a surprising increase in nuclear TDP-43 in motor neurons. Power analysis indicated group sizes of 12–14 mice are needed to detect 10–20% changes in measured parameters with a power of 80%, providing valid readouts for preclinical testing. Overall, this model may represent a useful component of multi-model pre-clinical therapeutic studies for ALS.

Strategies in the design and development of (TAR) DNA-binding protein 43 (TDP-43) binding ligands

The human transactive responsive (TAR) DNA-binding protein 43 (TDP-43) is involved in a number of physiological processes in the body. Its primary function involves RNA regulation. The TDP-43 protein is also involved in many diseases such as amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), Alzheimer's disease (AD), Parkinson's disease (PD) and even cancers. These TDP-43 mediated diseases are collectively called as TDP-43 proteinopathies. Intense research in the last decade has increased our understanding on TDP-43 structure and function in biology. The three-dimensional structures of TDP-43 domains such as N-terminal domain (NTD), RNA-recognition motif-1 (RRM1), RNA-recognition motif-2 (RRM2) and the C-terminal domain (CTD) or low-complexity domain (LCD) have been solved. These structures have yielded insights into novel binding sites and pockets at various TDP-43 domains, which can be targeted by designing a diverse library of ligands including small molecules, peptides and oligonucleotides as molecular tools to (i) study TDP-43 function, (ii) develop novel diagnostic agents and (iii) discover disease-modifying therapies to treat TDP-43 proteinopathies. This review provides a summary on recent progress in the development of TDP-43 binding ligands and uses the solved structures of various TDP-43 domains to investigate putative ligand binding regions that can be exploited to discover novel molecular probes to modulate TDP-43 structure and function.

Non-neuronal cells in amyotrophic lateral sclerosis - from pathogenesis to biomarkers

The prevailing motor neuron-centric view of amyotrophic lateral sclerosis (ALS) pathogenesis could be an important factor in the failure to identify disease-modifying therapy for this neurodegenerative disorder. Non-neuronal cells have crucial homeostatic functions within the CNS and evidence of involvement of these cells in the pathophysiology of several neurodegenerative disorders, including ALS, is accumulating. Microglia and astrocytes, in crosstalk with peripheral immune cells, can exert both neuroprotective and adverse effects, resulting in a highly nuanced range of neuronal and non-neuronal cell interactions. This Review provides an overview of the diverse roles of non-neuronal cells in relation to the pathogenesis of ALS and the emerging potential of non-neuronal cell biomarkers to advance therapeutic development.

Autophagy and ALS: mechanistic insights and therapeutic implications

Mechanisms of protein homeostasis are crucial for overseeing the clearance of misfolded and toxic proteins over the lifetime of an organism, thereby ensuring the health of neurons and other cells of the central nervous system. The highly conserved pathway of autophagy is particularly necessary for preventing and counteracting pathogenic insults that may lead to neurodegeneration. In line with this, mutations in genes that encode essential autophagy factors result in impaired autophagy and lead to neurodegenerative conditions such as amyotrophic lateral sclerosis (ALS). However, the mechanistic details underlying the neuroprotective role of autophagy, neuronal resistance to autophagy induction, and the neuron-specific effects of autophagy-impairing mutations remain incompletely defined. Further, the manner and extent to which non-cell autonomous effects of autophagy dysfunction contribute to ALS pathogenesis are not fully understood. Here, we review the current understanding of the interplay between autophagy and ALS pathogenesis by providing an overview of critical steps in the autophagy pathway, with special focus on pivotal factors impaired by ALS-causing mutations, their physiologic effects on autophagy in disease models, and the cell type-specific mechanisms regulating autophagy in non-neuronal cells which, when impaired, can contribute to neurodegeneration. This review thereby provides a framework not only to guide further investigations of neuronal autophagy but also to refine therapeutic strategies for ALS and related neurodegenerative diseases.Abbreviations: ALS: amyotrophic lateral sclerosis; Atg: autophagy-related; CHMP2B: charged multivesicular body protein 2B; DPR: dipeptide repeat; FTD: frontotemporal dementia; iPSC: induced pluripotent stem cell; LIR: LC3-interacting region; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MTOR: mechanistic target of rapamycin kinase; PINK1: PTEN induced kinase 1; RNP: ribonuclear protein; sALS: sporadic ALS; SPHK1: sphingosine kinase 1; TARDBP/TDP-43: TAR DNA binding protein; TBK1: TANK-binding kinase 1; TFEB: transcription factor EB; ULK: unc-51 like autophagy activating kinase; UPR: unfolded protein response; UPS: ubiquitin-proteasome system; VCP: valosin containing protein.

TDP-43 maximizes nerve conduction velocity by repressing a cryptic exon for paranodal junction assembly in Schwann cells

TDP-43 is extensively studied in neurons in physiological and pathological contexts. However, emerging evidence indicates that glial cells are also reliant on TDP-43 function. We demonstrate that deletion of TDP-43 in Schwann cells results in a dramatic delay in peripheral nerve conduction causing significant motor deficits in mice, which is directly attributed to the absence of paranodal axoglial junctions. By contrast, paranodes in the central nervous system are unaltered in oligodendrocytes lacking TDP-43. Mechanistically, TDP-43 binds directly to Neurofascin mRNA, encoding the cell adhesion molecule essential for paranode assembly and maintenance. Loss of TDP-43 triggers the retention of a previously unidentified cryptic exon, which targets Neurofascin mRNA for nonsense-mediated decay. Thus, TDP-43 is required for neurofascin expression, proper assembly and maintenance of paranodes, and rapid saltatory conduction. Our findings provide a framework and mechanism for how Schwann cell-autonomous dysfunction in nerve conduction is directly caused by TDP-43 loss-of-function.

Atxn2-CAG100-KnockIn mouse spinal cord shows progressive TDP43 pathology associated with cholesterol biosynthesis suppression

Large polyglutamine expansions in Ataxin-2 (ATXN2) cause multi-system nervous atrophy in Spinocerebellar Ataxia type 2 (SCA2). Intermediate size expansions carry a risk for selective motor neuron degeneration, known as Amyotrophic Lateral Sclerosis (ALS). Conversely, the depletion of ATXN2 prevents disease progression in ALS. Although ATXN2 interacts directly with RNA, and in ALS pathogenesis there is a crucial role of RNA toxicity, the affected functional pathways remain ill defined. Here, we examined an authentic SCA2 mouse model with Atxn2-CAG100-KnockIn for a first definition of molecular mechanisms in spinal cord pathology. Neurophysiology of lower limbs detected sensory neuropathy rather than motor denervation. Triple immunofluorescence demonstrated cytosolic ATXN2 aggregates sequestrating TDP43 and TIA1 from the nucleus. In immunoblots, this was accompanied by elevated CASP3, RIPK1 and PQBP1 abundance. RT-qPCR showed increase of Grn, Tlr7 and Rnaset2 mRNA versus Eif5a2, Dcp2, Uhmk1 and Kif5a decrease. These SCA2 findings overlap well with known ALS features. Similar to other ataxias and dystonias, decreased mRNA levels for Unc80, Tacr1, Gnal, Ano3, Kcna2, Elovl5 and Cdr1 contrasted with Gpnmb increase. Preterminal stage tissue showed strongly activated microglia containing ATXN2 aggregates, with parallel astrogliosis. Global transcriptome profiles from stages of incipient motor deficit versus preterminal age identified molecules with progressive downregulation, where a cluster of cholesterol biosynthesis enzymes including Dhcr24, Msmo1, Idi1 and Hmgcs1 was prominent. Gas chromatography demonstrated a massive loss of crucial cholesterol precursor metabolites. Overall, the ATXN2 protein aggregation process affects diverse subcellular compartments, in particular stress granules, endoplasmic reticulum and receptor tyrosine kinase signaling. These findings identify new targets and potential biomarkers for neuroprotective therapies.

TDP-43 aggregation induced by oxidative stress causes global mitochondrial imbalance in ALS

Amyotrophic lateral sclerosis (ALS) was initially thought to be associated with oxidative stress when it was first linked to mutant superoxide dismutase 1 (SOD1). The subsequent discovery of ALS-linked genes functioning in RNA processing and proteostasis raised the question of how different biological pathways converge to cause the disease. Both familial and sporadic ALS are characterized by the aggregation of the essential DNA- and RNA-binding protein TDP-43, suggesting a central role in ALS etiology. Here we report that TDP-43 aggregation in neuronal cells of mouse and human origin causes sensitivity to oxidative stress. Aggregated TDP-43 sequesters specific microRNAs (miRNAs) and proteins, leading to increased levels of some proteins while functionally depleting others. Many of those functionally perturbed gene products are nuclear-genome-encoded mitochondrial proteins, and their dysregulation causes a global mitochondrial imbalance that augments oxidative stress. We propose that this stress-aggregation cycle may underlie ALS onset and progression.

Sterol auto-oxidation adversely affects human motor neuron viability and is a neuropathological feature of amyotrophic lateral sclerosis

Aberrant cholesterol homeostasis is implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS), a fatal neuromuscular disease that is due to motor neuron (MN) death. Cellular toxicity from excess cholesterol is averted when it is enzymatically oxidized to oxysterols and bile acids (BAs) to promote its removal. In contrast, the auto oxidation of excess cholesterol is often detrimental to cellular survival. Although oxidized metabolites of cholesterol are altered in the blood and CSF of ALS patients, it is unknown if increased cholesterol oxidation occurs in the SC during ALS, and if exposure to oxidized cholesterol metabolites affects human MN viability. Here, we show that in the SOD1^G93A mouse model of ALS that several oxysterols, BAs and auto oxidized sterols are increased in the lumbar SC, plasma, and feces during disease. Similar changes in cholesterol oxidation were found in the cervical SC of sporadic ALS patients. Notably, auto-oxidized sterols, but not oxysterols and BAs, were toxic to iPSC derived human MNs. Thus, increased cholesterol oxidation is a manifestation of ALS and non-regulated sterol oxidation likely contributes to MN death. Developing therapeutic approaches to restore cholesterol homeostasis in the SC may lead to a treatment for ALS.

Hypothalamus and weight loss in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating progressive neurodegenerative disorder. While initially pathophysiology was thought to be restricted to motor deficits, it is increasingly recognized that patients develop prominent changes in weight and eating behavior that result from and mediate the underlying neurodegenerative process. These changes include alterations in metabolism, lipid levels, and insulin resistance. Emerging research suggests that these alterations may be mediated through changes in the hypothalamic function, with atrophy of the hypothalamus shown in both ALS patients and also presymptomatic genetic at-risk patients. This chapter reviews the evidence for hypothalamic involvement in ALS, including melanocortin pathways and potential treatment targets.