Fast motor axon loss in SMARD1 does not correspond to morphological and functional alterations of the NMJ

Disease Mechanisms and Therapeutic Approaches in SMARD1-Insights from Animal Models and Cell Models

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a fatal childhood motoneuron disease caused by mutations in the IGHMBP2 gene. It is characterized by muscle weakness, initially affecting the distal extremities due to the degeneration of spinal α-motoneurons, and respiratory distress, due to the paralysis of the diaphragm. Infantile forms with a severe course of the disease can be distinguished from juvenile forms with a milder course. Mutations in the IGHMBP2 gene have also been found in patients with peripheral neuropathy Charcot-Marie-Tooth type 2S (CMT2S). IGHMBP2 is an ATP-dependent 5'→3' RNA helicase thought to be involved in translational mechanisms. In recent years, several animal models representing both SMARD1 forms and CMT2S have been generated to initially study disease mechanisms. Later, the models showed very well that both stem cell therapies and the delivery of the human IGHMBP2 cDNA by AAV9 approaches (AAV9-IGHMBP2) can lead to significant improvements in disease symptoms. Therefore, the SMARD1 animal models, in addition to the cellular models, provide an inexhaustible source for obtaining knowledge of disease mechanisms, disease progression at the cellular level, and deeper insights into the development of therapies against SMARD1.

Therapy development for spinal muscular atrophy: perspectives for muscular dystrophies and neurodegenerative disorders

BACKGROUND

Major efforts have been made in the last decade to develop and improve therapies for proximal spinal muscular atrophy (SMA). The introduction of Nusinersen/Spinraza™ as an antisense oligonucleotide therapy, Onasemnogene abeparvovec/Zolgensma™ as an AAV9-based gene therapy and Risdiplam/Evrysdi™ as a small molecule modifier of pre-mRNA splicing have set new standards for interference with neurodegeneration.

MAIN BODY

Therapies for SMA are designed to interfere with the cellular basis of the disease by modifying pre-mRNA splicing and enhancing expression of the Survival Motor Neuron (SMN) protein, which is only expressed at low levels in this disorder. The corresponding strategies also can be applied to other disease mechanisms caused by loss of function or toxic gain of function mutations. The development of therapies for SMA was based on the use of cell culture systems and mouse models, as well as innovative clinical trials that included readouts that had originally been introduced and optimized in preclinical studies. This is summarized in the first part of this review. The second part discusses current developments and perspectives for amyotrophic lateral sclerosis, muscular dystrophies, Parkinson's and Alzheimer's disease, as well as the obstacles that need to be overcome to introduce RNA-based therapies and gene therapies for these disorders.

CONCLUSION

RNA-based therapies offer chances for therapy development of complex neurodegenerative disorders such as amyotrophic lateral sclerosis, muscular dystrophies, Parkinson's and Alzheimer's disease. The experiences made with these new drugs for SMA, and also the experiences in AAV gene therapies could help to broaden the spectrum of current approaches to interfere with pathophysiological mechanisms in neurodegeneration.

Models for IGHMBP2-associated diseases: an overview and a roadmap for the future

Models are practical tools with which to establish the basic aspects of a diseases. They allow systematic research into the significance of mutations, of cellular and molecular pathomechanisms, of therapeutic options and of functions of diseases associated proteins. Thus, disease models are an integral part of the study of enigmatic proteins such as immunoglobulin mu-binding protein 2 (IGHMBP2). IGHMBP2 has been well defined as a helicase, however there is little known about its role in cellular processes. Notably, it is unclear why changes in such an abundant protein lead to specific neuronal disorders including spinal muscular atrophy with respiratory distress type 1 (SMARD1) and Charcot-Marie-Tooth type 2S (CMT2S). SMARD1 is caused by a loss of motor neurons in the spinal cord that results in muscle atrophy and is accompanied by rapid respiratory failure. In contrast, CMT2S manifests as a severe neuropathy, but typically without critical breathing problems. Here, we present the clinical manifestation of IGHMBP2 mutations, function of protein and models that may be used for the study of IGHMBP2-associated disorders. We highlight the strengths and weaknesses of specific models and discuss the orthologs of IGHMBP2 that are found in different systems with regard to their similarity to human IGHMBP2.

Muscle fiber-type selective propensity to pathology in the nmd mouse model of SMARD1

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is an autosomal recessive disease that causes distal limb muscle atrophy, due to motor neuron degeneration. Similar to other motor neuron diseases, SMARD1 shows differential vulnerability to denervation in various muscle groups, which is recapitulated in the nmd mouse, a model of SMARD1. In multiple neurodegenerative disease models, transcriptomic analysis has identified differentially expressed genes between vulnerable motor neuron populations, but the mechanism leading to susceptibility is largely unknown. To investigate if denervation vulnerability is linked to intrinsic muscle properties, we analyzed muscle fiber-type composition in muscles from motor units that show different degrees of denervation in nmd mice: gastrocnemius, tibialis anterior (TA), and extensor digitorum longus (EDL). Our results revealed that denervation vulnerability correlated with atrophy and loss of MyHC-IIb and MyHC-IIx muscle fiber types. Interestingly, increased vulnerability also correlated with an increased abundance of MyHC-I and MyHC-IIa muscle fibers. These results indicated that MyHC-IIx muscle fibers are the most vulnerable to denervation, followed by MyHC-IIb muscle fibers. Moreover, our data indicate that type MyHC-IIa and MyHC-IIb muscle fibers show resistance to denervation and compensate for the loss of MyHC-IIx and MyHC-IIb muscle fibers in the most vulnerable muscles. Taken together these results provide a basis for the selective vulnerability to denervation of specific muscles in nmd mice and identifies new targets for potential therapeutic intervention.

Selective vulnerability in neuronal populations in nmd/SMARD1 mice

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is an autosomal recessive motor neuron disease causing distal limb muscle atrophy that progresses proximally and is accompanied by diaphragmatic paralysis. Neuromuscular junction (NMJ) alterations have been reported in muscles of SMARD1 model mice, known as nmd mice, with varying degrees of severity, suggesting that different muscles are specifically and selectively resistant or susceptible to denervation. To evaluate the extent of NMJ pathology in a broad range of muscles, a panel of axial and appendicular muscles were isolated and immunostained from nmd mice. These analyses revealed that selective distal appendage muscles were highly vulnerable to denervation. Susceptibility to pathology was not limited to NMJ alterations, but included defects in myelination within those neurons innervating susceptible muscles. Interestingly, end plate fragmentation was present within all muscles independent of the extent of NMJ alterations, suggesting that end plate fragmentation is an early hallmark of SMARD1 pathogenesis. Expressing the full-length IGHMBP2 cDNA using an adeno-associated virus (AAV9) significantly decreased all aspects of muscle and nerve disease pathology. These results shed new light onto the pathogenesis of SMARD1 by identifying specific motor units that are resistant and susceptible to neurodegeneration in an important model of SMARD1.

Sarm1 Deletion, but Not WldS, Confers Lifelong Rescue in a Mouse Model of Severe Axonopathy

Studies with the Wld^S mutant mouse have shown that axon and synapse pathology in several models of neurodegenerative diseases are mechanistically related to injury-induced axon degeneration (Wallerian degeneration). Crucially, an absence of SARM1 delays Wallerian degeneration as robustly as Wld^S, but their relative capacities to confer long-term protection against related, non-injury axonopathy and/or synaptopathy have not been directly compared. While Sarm1 deletion or Wld^S can rescue perinatal lethality and widespread Wallerian-like axonopathy in young NMNAT2-deficient mice, we report that an absence of SARM1 enables these mice to survive into old age with no overt phenotype, whereas those rescued by Wld^S invariantly develop a progressive neuromuscular defect in their hindlimbs from around 3 months of age. We therefore propose Sarm1 deletion as a more reliable tool than Wld^S for investigating Wallerian-like mechanisms in disease models and suggest that SARM1 blockade may have greater therapeutic potential than WLD^S-related strategies.

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Rescue of an axonopathy model by Sarm1 deletion or Wld^S compared in an aging study

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Young adult NMNAT2-deficient mice rescued by Wld^S develop a hindlimb motor defect

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NMNAT2-deficient mice rescued by Sarm1 deletion are overtly normal up to 24 months

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SARM1 depletion/inhibition may have analytical and therapeutic advantages over WLD^S

ANKK1 is found in myogenic precursors and muscle fibers subtypes with glycolytic metabolism

Ankyrin repeat and kinase domain containing 1 (ANKK1) gene has been widely related to neuropsychiatry disorders. The localization of ANKK1 in neural progenitors and its correlation with the cell cycle has suggested its participation in development. However, ANKK1 functions still need to be identified. Here, we have further characterized the ANKK1 localization in vivo and in vitro, by using immunolabeling, quantitative real-time PCR and Western blot in the myogenic lineage. Histologic investigations in mice and humans revealed that ANKK1 is expressed in precursors of embryonic and adult muscles. In mice embryos, ANKK1 was found in migrating myotubes where it shows a polarized cytoplasmic distribution, while proliferative myoblasts and satellite cells show different isoforms in their nuclei and cytoplasm. In vitro studies of ANKK1 protein isoforms along the myogenic progression showed the decline of nuclear ANKK1-kinase until its total exclusion in myotubes. In adult mice, ANKK1 was expressed exclusively in the Fast-Twitch muscles fibers subtype. The induction of glycolytic metabolism in C2C12 cells with high glucose concentration or treatment with berberine caused a significant increase in the ANKK1 mRNA. Similarly, C2C12 cells under hypoxic conditions caused the increase of nuclear ANKK1. These results altogether show a relationship between ANKK1 gene regulation and the metabolism of muscles during development and in adulthood. Finally, we found ANKK1 expression in regenerative fibers of muscles from dystrophic patients. Future studies in ANKK1 biology and the pathological response of muscles will reveal whether this protein is a novel muscle disease biomarker.

Genetic determinants of fibro-osseous lesions in aged inbred mice

Fibro-osseous lesions in mice are progressive aging changes in which the bone marrow is replaced to various degrees by fibrovascular stroma and bony trabeculae in a wide variety of bones. The frequency and severity varied greatly among 28 different inbred mouse stains, predominantly affecting females, ranging from 0% for 10 strains to 100% for KK/HlJ and NZW/LacJ female mice. Few lesions were observed in male mice and for 23 of the strains, no lesions were observed in males for any of the cohorts. There were no significant correlations between strain-specific severities of fibro-osseous lesions and ovarian (r=0.11; P=0.57) or endometrial (r=0.03; P=0.89) cyst formation frequency or abnormalities in parathyroid glands. Frequency of fibro-osseous lesions was most strongly associated (P<10(-6)) with genome variations on chromosome (Chr) 8 at 90.6 and 90.8Mb (rs33108071, rs33500669; P=5.0·10(-10), 1.3·10(-6)), Chr 15 at 23.6 and 23.8Mb (rs32087871, rs45770368; P=7.3·10(-7), 2.7·10(-6)), and Chr 19 at 33.2, 33.4, and 33.6Mb (rs311004232, rs30524929, rs30448815; P=2.8·10(-6), 2.8·10(-6), 2.8·10(-6)) in genome-wide association studies (GWAS). The relatively large number of candidate genes identified in the GWAS analyses suggests that this may be an extremely complex polygenic disease. These results indicate that fibro-osseous lesions are surprisingly common in many inbred strains of laboratory mice as they age. While this presents little problem in most studies that utilize young animals, it may complicate aging studies, particularly those focused on bone.

Clinical and molecular features and therapeutic perspectives of spinal muscular atrophy with respiratory distress type 1

Spinal muscular atrophy with respiratory distress (SMARD1) is an autosomal recessive neuromuscular disease caused by mutations in the IGHMBP2 gene, encoding the immunoglobulin μ-binding protein 2, leading to motor neuron degeneration. It is a rare and fatal disease with an early onset in infancy in the majority of the cases. The main clinical features are muscular atrophy and diaphragmatic palsy, which requires prompt and permanent supportive ventilation. The human disease is recapitulated in the neuromuscular degeneration (nmd) mouse. No effective treatment is available yet, but novel therapeutical approaches tested on the nmd mouse, such as the use of neurotrophic factors and stem cell therapy, have shown positive effects. Gene therapy demonstrated effectiveness in SMA, being now at the stage of clinical trial in patients and therefore representing a possible treatment for SMARD1 as well. The significant advancement in understanding of both SMARD1 clinical spectrum and molecular mechanisms makes ground for a rapid translation of pre-clinical therapeutic strategies in humans.

Lack of GDAP1 induces neuronal calcium and mitochondrial defects in a knockout mouse model of charcot-marie-tooth neuropathy

Mutations in GDAP1, which encodes protein located in the mitochondrial outer membrane, cause axonal recessive (AR-CMT2), axonal dominant (CMT2K) and demyelinating recessive (CMT4A) forms of Charcot-Marie-Tooth (CMT) neuropathy. Loss of function recessive mutations in GDAP1 are associated with decreased mitochondrial fission activity, while dominant mutations result in impairment of mitochondrial fusion with increased production of reactive oxygen species and susceptibility to apoptotic stimuli. GDAP1 silencing in vitro reduces Ca2+ inflow through store-operated Ca2+ entry (SOCE) upon mobilization of endoplasmic reticulum (ER) Ca2+, likely in association with an abnormal distribution of the mitochondrial network. To investigate the functional consequences of lack of GDAP1 in vivo, we generated a Gdap1 knockout mouse. The affected animals presented abnormal motor behavior starting at the age of 3 months. Electrophysiological and biochemical studies confirmed the axonal nature of the neuropathy whereas histopathological studies over time showed progressive loss of motor neurons (MNs) in the anterior horn of the spinal cord and defects in neuromuscular junctions. Analyses of cultured embryonic MNs and adult dorsal root ganglia neurons from affected animals demonstrated large and defective mitochondria, changes in the ER cisternae, reduced acetylation of cytoskeletal α-tubulin and increased autophagy vesicles. Importantly, MNs showed reduced cytosolic calcium and SOCE response. The development and characterization of the GDAP1 neuropathy mice model thus revealed that some of the pathophysiological changes present in axonal recessive form of the GDAP1-related CMT might be the consequence of changes in the mitochondrial network biology and mitochondria-endoplasmic reticulum interaction leading to abnormalities in calcium homeostasis.

Truncating and missense mutations in IGHMBP2 cause Charcot-Marie Tooth disease type 2

Using a combination of exome sequencing and linkage analysis, we investigated an English family with two affected siblings in their 40s with recessive Charcot-Marie Tooth disease type 2 (CMT2). Compound heterozygous mutations in the immunoglobulin-helicase-μ-binding protein 2 (IGHMBP2) gene were identified. Further sequencing revealed a total of 11 CMT2 families with recessively inherited IGHMBP2 gene mutations. IGHMBP2 mutations usually lead to spinal muscular atrophy with respiratory distress type 1 (SMARD1), where most infants die before 1 year of age. The individuals with CMT2 described here, have slowly progressive weakness, wasting and sensory loss, with an axonal neuropathy typical of CMT2, but no significant respiratory compromise. Segregating IGHMBP2 mutations in CMT2 were mainly loss-of-function nonsense in the 5' region of the gene in combination with a truncating frameshift, missense, or homozygous frameshift mutations in the last exon. Mutations in CMT2 were predicted to be less aggressive as compared to those in SMARD1, and fibroblast and lymphoblast studies indicate that the IGHMBP2 protein levels are significantly higher in CMT2 than SMARD1, but lower than controls, suggesting that the clinical phenotype differences are related to the IGHMBP2 protein levels.

Differentiation defects in primary motoneurons from a SMARD1 mouse model that are insensitive to treatment with low dose PEGylated IGF1

Muscle atrophy and diaphragmatic palsy are the clinical characteristics of spinal muscular atrophy with respiratory distress type 1 (SMARD1), and are well represented in the neuromuscular degeneration (Nmd^2J) mouse, modeling the juvenile form of SMARD1. Both in humans and mice mutations in the IGHMBP2 gene lead to motoneuron degeneration. We could previously demonstrate that treatment with a polyethylene glycol-coupled variant of IGF1 (PEG-IGF1) improves motor functions accompanied by reduced fiber degeneration in the gastrocnemius muscle and the diaphragm, but has no beneficial effect on motoneuron survival. These data raised the question which cell autonomous disease mechanisms contribute to dysfunction and loss of Ighmbp2-deficient motoneurons. An analysis of primary Ighmbp2-deficient motoneurons exhibited differentiation deficits such as reduced spontaneous Ca²⁺ transients and altered axon elongation, which was not compensated by PEG-IGF1. This points to an IGF1 independent mechanism of motoneuron degeneration that deserves treatment approaches in addition to IGF1.

Morphological and functional remodelling of the neuromuscular junction by skeletal muscle PGC-1α

The neuromuscular junction (NMJ) exhibits high morphological and functional plasticity. In the mature muscle, the relative levels of physical activity are the major determinants of NMJ function. Classically, motor neuron-mediated activation patterns of skeletal muscle have been thought of as the major drivers of NMJ plasticity and the ensuing fibre-type determination in muscle. Here we use muscle-specific transgenic animals for the peroxisome proliferator-activated receptor γ co-activator 1α (PGC-1α) as a genetic model for trained mice to elucidate the contribution of skeletal muscle to activity-induced adaptation of the NMJ. We find that muscle-specific expression of PGC-1α promotes a remodelling of the NMJ, even in the absence of increased physical activity. Importantly, these plastic changes are not restricted to post-synaptic structures, but extended to modulation of presynaptic cell morphology and function. Therefore, our data indicate that skeletal muscle significantly contributes to the adaptation of the NMJ subsequent to physical activity.

Transcriptional analysis of apoptotic cerebellar granule neurons following rescue by gastric inhibitory polypeptide

Apoptosis triggered by exogenous or endogenous stimuli is a crucial phenomenon to determine the fate of neurons, both in physiological and in pathological conditions. Our previous study established that gastric inhibitory polypeptide (Gip) is a neurotrophic factor capable of preventing apoptosis of cerebellar granule neurons (CGNs), during its pre-commitment phase. In the present study, we conducted whole-genome expression profiling to obtain a comprehensive view of the transcriptional program underlying the rescue effect of Gip in CGNs. By using DNA microarray technology, we identified 65 genes, we named survival related genes, whose expression is significantly de-regulated following Gip treatment. The expression levels of six transcripts were confirmed by real-time quantitative polymerase chain reaction. The proteins encoded by the survival related genes are functionally grouped in the following categories: signal transduction, transcription, cell cycle, chromatin remodeling, cell death, antioxidant activity, ubiquitination, metabolism and cytoskeletal organization. Our data outline that Gip supports CGNs rescue via a molecular framework, orchestrated by a wide spectrum of gene actors, which propagate survival signals and support neuronal viability.