Multifaceted roles of microRNAs: From motor neuron generation in embryos to degeneration in spinal muscular atrophy

Two crucial questions in neuroscience are how neurons establish individual identity in the developing nervous system and why only specific neuron subtypes are vulnerable to neurodegenerative diseases. In the central nervous system, spinal motor neurons serve as one of the best-characterized cell types for addressing these two questions. In this review, we dissect these questions by evaluating the emerging role of regulatory microRNAs in motor neuron generation in developing embryos and their potential contributions to neurodegenerative diseases such as spinal muscular atrophy (SMA). Given recent promising results from novel microRNA-based medicines, we discuss the potential applications of microRNAs for clinical assessments of SMA disease progression and treatment.

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Molecular Mechanisms of Neurodegeneration in Spinal Muscular Atrophy

Journal of Experimental Neuroscience ◽

10.4137/jen.s33122 ◽

2016 ◽

Vol 10 ◽

pp. JEN.S33122 ◽

Cited By ~ 36

Author(s):

Saif Ahmad ◽

Kanchan Bhatia ◽

Annapoorna Kannan ◽

Laxman Gangwani

Keyword(s):

Spinal Muscular Atrophy ◽

Motor Neuron ◽

Motor Neurons ◽

Molecular Mechanisms ◽

Muscular Atrophy ◽

Survival Motor Neuron ◽

Skeletal Muscle Atrophy ◽

Spinal Motor Neurons ◽

Low Levels ◽

Smn Protein

Spinal muscular atrophy (SMA) is an autosomal recessive motor neuron disease with a high incidence and is the most common genetic cause of infant mortality. SMA is primarily characterized by degeneration of the spinal motor neurons that leads to skeletal muscle atrophy followed by symmetric limb paralysis, respiratory failure, and death. In humans, mutation of the Survival Motor Neuron 1 (SMN1) gene shifts the load of expression of SMN protein to the SMN2 gene that produces low levels of full-length SMN protein because of alternative splicing, which are sufficient for embryonic development and survival but result in SMA. The molecular mechanisms of the (a) regulation of SMN gene expression and (b) degeneration of motor neurons caused by low levels of SMN are unclear. However, some progress has been made in recent years that have provided new insights into understanding of the cellular and molecular basis of SMA pathogenesis. In this review, we have briefly summarized recent advances toward understanding of the molecular mechanisms of regulation of SMN levels and signaling mechanisms that mediate neurodegeneration in SMA.

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Genomic Variability in the Survival Motor Neuron Genes (SMN1 and SMN2): Implications for Spinal Muscular Atrophy Phenotype and Therapeutics Development

International Journal of Molecular Sciences ◽

10.3390/ijms22157896 ◽

2021 ◽

Vol 22 (15) ◽

pp. 7896

Author(s):

Matthew E. R. Butchbach

Keyword(s):

Spinal Muscular Atrophy ◽

Motor Neuron ◽

Motor Neurons ◽

Muscular Atrophy ◽

Survival Motor Neuron ◽

Smn1 Gene ◽

Spinal Motor Neurons ◽

Copy Numbers ◽

Smn Protein ◽

High Degree

Spinal muscular atrophy (SMA) is a leading genetic cause of infant death worldwide that is characterized by loss of spinal motor neurons leading to muscle weakness and atrophy. SMA results from the loss of survival motor neuron 1 (SMN1) gene but retention of its paralog SMN2. The copy numbers of SMN1 and SMN2 are variable within the human population with SMN2 copy number inversely correlating with SMA severity. Current therapeutic options for SMA focus on increasing SMN2 expression and alternative splicing so as to increase the amount of SMN protein. Recent work has demonstrated that not all SMN2, or SMN1, genes are equivalent and there is a high degree of genomic heterogeneity with respect to the SMN genes. Because SMA is now an actionable disease with SMN2 being the primary target, it is imperative to have a comprehensive understanding of this genomic heterogeneity with respect to hybrid SMN1–SMN2 genes generated by gene conversion events as well as partial deletions of the SMN genes. This review will describe this genetic heterogeneity in SMA and its impact on disease phenotype as well as therapeutic efficacy.

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The atlas of RNase H antisense oligonucleotide distribution and activity in the CNS of rodents and non-human primates following central administration

Nucleic Acids Research ◽

10.1093/nar/gkaa1235 ◽

2020 ◽

Cited By ~ 1

Author(s):

Paymaan Jafar-nejad ◽

Berit Powers ◽

Armand Soriano ◽

Hien Zhao ◽

Daniel A Norris ◽

...

Keyword(s):

Motor Neurons ◽

Muscular Atrophy ◽

Neurological Diseases ◽

Cell Types ◽

Rnase H ◽

Central Administration ◽

Spinal Motor Neurons ◽

New Class ◽

Wide Range ◽

Therapeutic Development

Abstract Antisense oligonucleotides (ASOs) have emerged as a new class of drugs to treat a wide range of diseases, including neurological indications. Spinraza, an ASO that modulates splicing of SMN2 RNA, has shown profound disease modifying effects in Spinal Muscular Atrophy (SMA) patients, energizing efforts to develop ASOs for other neurological diseases. While SMA specifically affects spinal motor neurons, other neurological diseases affect different central nervous system (CNS) regions, neuronal and non-neuronal cells. Therefore, it is important to characterize ASO distribution and activity in all major CNS structures and cell types to have a better understanding of which neurological diseases are amenable to ASO therapy. Here we present for the first time the atlas of ASO distribution and activity in the CNS of mice, rats, and non-human primates (NHP), species commonly used in preclinical therapeutic development. Following central administration of an ASO to rodents, we observe widespread distribution and target RNA reduction throughout the CNS in neurons, oligodendrocytes, astrocytes and microglia. This is also the case in NHP, despite a larger CNS volume and more complex neuroarchitecture. Our results demonstrate that ASO drugs are well suited for treating a wide range of neurological diseases for which no effective treatments are available.

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Dopamine modulation of gill reflex behavior in Aplysia

Canadian Journal of Physiology and Pharmacology ◽

10.1139/y79-051 ◽

1979 ◽

Vol 57 (3) ◽

pp. 329-332 ◽

Cited By ~ 9

Author(s):

Peter Ruben ◽

Ken Lukowiak

Keyword(s):

Central Nervous System ◽

Nervous System ◽

Motor Neuron ◽

Peripheral Nervous System ◽

Motor Neurons ◽

Withdrawal Reflex ◽

Dopaminergic Pathway ◽

The Central Nervous System ◽

Dopamine Modulation

We have studied the effects of dopamine on the gill withdrawal reflex evoked by tactile siphon stimulation in the margine mollusc Aplysia. Physiological concentrations of dopamine (diluted in seawater) were perfused through the gill during siphon stimulation series. The amplitude of the reflex was potentiated by dopamine and habituation of the reflex was prevented. This occurred with no change in the activity evoked in central motor neurons. These results lead us to conclude that the dopaminergic motor neuron L9 is modulating habituation in the periphery and that the central nervous system facilitatory control of the peripheral nervous system may act via a dopaminergic pathway.

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Muscle regulates mTOR dependent axonal local translation in motor neurons via CTRP3 secretion: implications for a neuromuscular disorder, spinal muscular atrophy

Acta Neuropathologica Communications ◽

10.1186/s40478-019-0806-3 ◽

2019 ◽

Vol 7 (1) ◽

Cited By ~ 5

Author(s):

Wiebke A. Rehorst ◽

Maximilian P. Thelen ◽

Hendrik Nolte ◽

Clara Türk ◽

Sebahattin Cirak ◽

...

Keyword(s):

Protein Synthesis ◽

Spinal Muscular Atrophy ◽

Motor Neuron ◽

Motor Neurons ◽

Muscular Atrophy ◽

Neuromuscular Disorder ◽

Survival Motor Neuron ◽

Synthesis Rate ◽

Protein Synthesis Rate ◽

Metabolic Labeling

Abstract Spinal muscular atrophy (SMA) is an inherited neuromuscular disorder, which causes dysfunction/loss of lower motor neurons and muscle weakness as well as atrophy. While SMA is primarily considered as a motor neuron disease, recent data suggests that survival motor neuron (SMN) deficiency in muscle causes intrinsic defects. We systematically profiled secreted proteins from control and SMN deficient muscle cells with two combined metabolic labeling methods and mass spectrometry. From the screening, we found lower levels of C1q/TNF-related protein 3 (CTRP3) in the SMA muscle secretome and confirmed that CTRP3 levels are indeed reduced in muscle tissues and serum of an SMA mouse model. We identified that CTRP3 regulates neuronal protein synthesis including SMN via mTOR pathway. Furthermore, CTRP3 enhances axonal outgrowth and protein synthesis rate, which are well-known impaired processes in SMA motor neurons. Our data revealed a new molecular mechanism by which muscles regulate the physiology of motor neurons via secreted molecules. Dysregulation of this mechanism contributes to the pathophysiology of SMA.

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Circulating microRNAs as potential biomarkers and therapeutic targets in spinal muscular atrophy

Therapeutic Advances in Neurological Disorders ◽

10.1177/1756286420979954 ◽

2020 ◽

Vol 13 ◽

pp. 175628642097995

Author(s):

Tai-Heng Chen

Keyword(s):

Spinal Muscular Atrophy ◽

Motor Neurons ◽

Molecular Mechanisms ◽

Muscular Atrophy ◽

Therapeutic Targets ◽

Homozygous Deletion ◽

Survival Motor Neuron ◽

Novel Mirna ◽

Potential Applications ◽

Smn Protein

Spinal muscular atrophy (SMA), a leading genetic cause of infant death, is a neurodegenerative disease characterized by the selective loss of particular groups of motor neurons (MNs) in the anterior horn of the spinal cord with progressive muscle wasting. SMA is caused by a deficiency of the survival motor neuron (SMN) protein due to a homozygous deletion or mutation of the SMN1 gene. However, the molecular mechanisms whereby the SMN complex regulates MN functions are not fully elucidated. Emerging studies on SMA pathogenesis have turned the attention of researchers to RNA metabolism, given that increasingly identified SMN-associated modifiers are involved in both coding and non-coding RNA (ncRNA) processing. Among various ncRNAs, microRNAs (miRNAs) are the most studied in terms of regulation of posttranscriptional gene expression. Recently, the discovery that miRNAs are critical to MN function and survival led to the study of dysregulated miRNAs in SMA pathogenesis. Circulating miRNAs have drawn attention as a readily available biomarker due to their property of being clinically detectable in numerous human biofluids through non-invasive approaches. As there are recent promising findings from novel miRNA-based medicines, this article presents an extensive review of the most up-to-date studies connecting specific miRNAs to SMA pathogenesis and the potential applications of miRNAs as biomarkers and therapeutic targets for SMA.

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Ultrastructural characterization of peripheral denervation in a mouse model of Type III spinal muscular atrophy

Journal of Neural Transmission ◽

10.1007/s00702-021-02353-9 ◽

2021 ◽

Author(s):

Federica Fulceri ◽

Francesca Biagioni ◽

Fiona Limanaqi ◽

Carla L. Busceti ◽

Larisa Ryskalin ◽

...

Keyword(s):

Mouse Model ◽

Spinal Muscular Atrophy ◽

Motor Neuron ◽

Motor Neurons ◽

Muscular Atrophy ◽

Disease Onset ◽

Muscle Spindles ◽

Neuromuscular Disorder ◽

Type Iii ◽

Smn Protein

AbstractSpinal muscular atrophy (SMA) is a heritable, autosomal recessive neuromuscular disorder characterized by a loss of the survival of motor neurons (SMN) protein, which leads to degeneration of lower motor neurons, and muscle atrophy. Despite SMA being nosographically classified as a motor neuron disease, recent advances indicate that peripheral alterations at the level of the neuromuscular junction (NMJ), involving the muscle, and axons of the sensory-motor system, occur early, and may even precede motor neuron loss. In the present study, we used a mouse model of slow progressive (type III) SMA, whereby the absence of the mouse SMN protein is compensated by the expression of two human genes (heterozygous SMN1A2G, and SMN2). This leads to late disease onset and prolonged survival, which allows for dissecting slow degenerative steps operating early in SMA pathogenesis. In this purely morphological study carried out at transmission electron microscopy, we extend the examination of motor neurons and proximal axons towards peripheral components, including distal axons, muscle fibers, and also muscle spindles. We document remarkable ultrastructural alterations being consistent with early peripheral denervation in SMA, which may shift the ultimate anatomical target in neuromuscular disease from the spinal cord towards the muscle. This concerns mostly mitochondrial alterations within distal axons and muscle, which are quantified here through ultrastructural morphometry. The present study is expected to provide a deeper knowledge of early pathogenic mechanisms in SMA.

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Central and peripheral delivery of AAV9-SMN target different pathomechanisms in a mouse model of spinal muscular atrophy

10.1101/2021.11.08.467795 ◽

2021 ◽

Author(s):

Aoife N Reilly ◽

Marco Deguise ◽

Ariane Beauvais ◽

Rebecca Yaworski ◽

Simon Thebault ◽

...

Keyword(s):

Spinal Muscular Atrophy ◽

Motor Neuron ◽

Motor Neurons ◽

Liver Toxicity ◽

Muscular Atrophy ◽

Intrathecal Administration ◽

High Titre ◽

High Dose ◽

Peripheral Tissues ◽

Liver And Pancreas

Spinal muscular atrophy (SMA) is a neuromuscular disease caused by loss of the SMN1 gene. Although lower motor neurons are a primary target, there is evidence that peripheral organ defects contribute to SMA. Current SMA gene therapy uses a single, high titre intravenous bolus of AAV9-SMN resulting in impressive, yet limited amelioration of the clinical phenotype. However, risks of this treatment include liver toxicity. Intrathecal administration is under clinical trial but was interrupted due to safety concerns in a concomitant animal study. As there is no direct comparison between the different delivery strategies while avoiding high dose toxicity, we injected SMA mice with low dose scAAV9-cba-SMN either intravenously (IV) for peripheral SMN restoration or intracerebroventricularly (ICV) for CNS-focused SMN restoration. Here, IV injections restored SMN in peripheral tissues but not CNS, while ICV injections mildly increased SMN in the periphery and the CNS. Consequently, only ICV treatment rescued motor neuron degeneration. Surprisingly, both treatments resulted in an impressive rescue of survival, weight, motor function, and peripheral phenotypes including liver and pancreas pathology. Our work highlights independent contributions of peripheral organs to SMA pathology and suggests that treatments should not be restricted to the motor neuron.

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Utilizing gene therapy methods to probe the genetic requirements to prevent spinal muscular atrophy.

10.32469/10355/57243 ◽

2016 ◽

Author(s):

◽

Madeline R. Miller

Keyword(s):

Neuromuscular Junction ◽

Spinal Muscular Atrophy ◽

Motor Neuron ◽

Motor Neurons ◽

Muscular Atrophy ◽

Survival Motor Neuron ◽

Downstream Effects ◽

Smn Protein ◽

Progressive Weakness

Spinal Muscular Atrophy is clinically recognized as a progressive weakness within the trunk and proximal limbs that will lead to breathing failure and death within infants. As a neurodegenerative genetic disease, SMA is caused by loss of motor neurons, which in turn is caused by low levels of the Survival Motor Neuron (SMN) protein. The mechanism by which a ubiquitously expressed protein such as SMN is able to cause the specific death of motor neurons is highly debated and of great interest. Work presented here focuses on understanding the biological requirements of SMN and its downstream effects on the neuromuscular junction. To this end we utilize viral based gene delivery as a powerful tool to assess the effects of genes of interest in vivo. Our findings contribute to the conversation regarding whether SMA is truly a "motor neuron" disease, suggesting that astrocytes play a meaningful role in staving off SMA. Further, we investigate the domains within SMN needed to maintain its function in a mammalian system. We take a novel and challenging approach to identify a minimal domain capable of maintaining function. Finally, we demonstrate the practical use of morophological analysis of the neuromuscular junction as a means to characterize SMA pathology.

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LCM-seq reveals unique transcriptional adaption mechanisms of resistant neurons in spinal muscular atrophy

10.1101/356113 ◽

2018 ◽

Author(s):

S Nichterwitz ◽

H Storvall ◽

J Nijssen ◽

LH Comley ◽

I Allodi ◽

...

Keyword(s):

Spinal Muscular Atrophy ◽

Motor Neuron ◽

Motor Neurons ◽

Muscular Atrophy ◽

Differential Expression Analysis ◽

Red Nucleus ◽

Ocular Motor ◽

Cell Type ◽

Network Analyses ◽

Cell Type Specific

AbstractSomatic motor neurons are selectively vulnerable in spinal muscular atrophy (SMA), a lethal disease caused by a deficiency of the ubiquitously expressed survival of motor neuron (SMN) protein. However, some brainstem motor neuron groups, including oculomotor and trochlear (ocular), which innervate the muscles around the eyes, are for unknown reasons spared. Here, using laser capture microdissection coupled with RNA sequencing (LCM-seq), we investigate the transcriptional dynamics in discrete neuronal populations in health and SMA to reveal mechanisms of vulnerability and resistance. Using gene correlation network analysis, we reveal a p53-mediated stress response that is intrinsic to all somatic motor neurons independent of their vulnerability, but absent in resistant red nucleus and visceral motor neurons. However, our temporal and spatial differential expression analysis across neuron types clearly demonstrates that the majority of SMA-induced modulations are cell-type specific. Notably, using gene ontology and protein-network analyses we show that ocular motor neurons present unique disease-adaptation mechanisms that could explain their resilience. In particular, ocular motor neurons up-regulate; i) Syt1, Syt5 and Cplx2, which modulate neurotransmitter release; ii) the motor neuron survival factors Chl1 and Lif, iii) Aldh4, that can protect cells from oxidative stress and iv) the caspase inhibitor Pak4. In conclusion, our in-depth longitudinal analysis of gene expression changes in SMA reveal novel cell-type specific changes that present compelling targets for future gene therapy studies aimed towards preserving vulnerable motor neurons.

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