Defective Acetylcholine Receptor Subunit Switch Precedes Atrophy of Slow-Twitch Skeletal Muscle Fibers Lacking ERK1/2 Kinases in Soleus Muscle

To test the role of extracellular-signal regulated kinases 1 and 2 (ERK1/2) in slow-twitch, type 1 skeletal muscle fibers, we studied the soleus muscle in mice genetically deficient for myofiber ERK1/2. Young adult mutant soleus was drastically wasted, with highly atrophied type 1 fibers, denervation at most synaptic sites, induction of “fetal” acetylcholine receptor gamma subunit (AChRγ), reduction of “adult” AChRε, and impaired mitochondrial biogenesis and function. In weanlings, fiber morphology and mitochondrial markers were mostly normal, yet AChRγ upregulation and AChRε downregulation were observed. Synaptic sites with fetal AChRs in weanling muscle were ~3% in control and ~40% in mutants, with most of the latter on type 1 fibers. These results suggest that: (1) ERK1/2 are critical for slow-twitch fiber growth; (2) a defective γ/ε-AChR subunit switch, preferentially at synapses on slow fibers, precedes wasting of mutant soleus; (3) denervation is likely to drive this wasting, and (4) the neuromuscular synapse is a primary subcellular target for muscle ERK1/2 function in vivo

Improvement of Neuromuscular Synaptic Phenotypes without Enhanced Survival and Motor Function in Severe Spinal Muscular Atrophy Mice Selectively Rescued in Motor Neurons

In the inherited childhood neuromuscular disease spinal muscular atrophy (SMA), lower motor neuron death and severe muscle weakness result from the reduction of the ubiquitously expressed protein survival of motor neuron (SMN). Although SMA mice recapitulate many features of the human disease, it has remained unclear if their short lifespan and motor weakness are primarily due to cell-autonomous defects in motor neurons. Using Hb9(Cre) as a driver, we selectively raised SMN expression in motor neurons in conditional SMAΔ7 mice. Unlike a previous study that used choline acetyltransferase (ChAT(Cre+) ) as a driver on the same mice, and another report that used Hb9(Cre) as a driver on a different line of conditional SMA mice, we found no improvement in survival, weight, motor behavior and presynaptic neurofilament accumulation. However, like in ChAT(Cre+) mice, we detected rescue of endplate size and mitigation of neuromuscular junction (NMJ) denervation status. The rescue of endplate size occurred in the absence of an increase in myofiber size, suggesting endplate size is determined by the motor neuron in these animals. Real time-PCR showed that the expression of spinal cord SMN transcript was sharply reduced in Hb9(Cre+) SMA mice relative to ChAT(Cre+) SMA mice. This suggests that our lack of overall phenotypic improvement is most likely due to an unexpectedly poor recombination efficiency driven by Hb9(Cre) . Nonetheless, the low levels of SMN were sufficient to rescue two NMJ structural parameters indicating that these motor neuron cell autonomous phenotypes are very sensitive to changes in motoneuronal SMN levels. Our results directly suggest that even those therapeutic interventions with very modest effects in raising SMN in motor neurons may provide mitigation of neuromuscular phenotypes in SMA patients

Temporal evolution of the microbiome, immune system and epigenome with disease progression in ALS mice

Amyotrophic lateral sclerosis (ALS) is a terminal neurodegenerative disease. Genetic predisposition, epigenetic changes, aging and accumulated life-long environmental exposures are known ALS risk factors. The complex and dynamic interplay between these pathological influences plays a role in disease onset and progression. Recently, the gut microbiome has also been implicated in ALS development. In addition, immune cell populations are differentially expanded and activated in ALS compared to healthy individuals. However, the temporal evolution of both the intestinal flora and the immune system relative to symptom onset in ALS is presently not fully understood. To better elucidate the timeline of the various potential pathological factors, we performed a longitudinal study to simultaneously assess the gut microbiome, immunophenotype and changes in ileum and brain epigenetic marks relative to motor behavior and muscle atrophy in the mutant superoxide dismutase 1 (SOD1G93A) familial ALS mouse model. We identified alterations in the gut microbial environment early in the life of SOD1G93A animals followed by motor dysfunction and muscle atrophy, and immune cell expansion and activation, particularly in the spinal cord. Global brain cytosine hydroxymethylation was also altered in SOD1G93A animals at disease end-stage compared to control mice. Correlation analysis confirmed interrelationships with the microbiome and immune system. This study serves as a starting point to more deeply comprehend the influence of gut microorganisms and the immune system on ALS onset and progression. Greater insight may help pinpoint novel biomarkers and therapeutic interventions to improve diagnosis and treatment for ALS patients. This article has an associated First Person interview with the joint first authors of the paper

Abnormal RNA Stability in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) share key features, including accumulation of the RNA-binding protein TDP-43. TDP-43 regulates RNA homeostasis, but it remains unclear whether RNA stability is affected in these disorders. We use Bru-seq and BruChase-seq to assess genome-wide RNA stability in ALS patient-derived cells, demonstrating profound destabilization of ribosomal and mitochondrial transcripts. This pattern is recapitulated by TDP-43 overexpression, suggesting a primary role for TDP-43 in RNA destabilization, and in postmortem samples from ALS and FTD patients. Proteomics and functional studies illustrate corresponding reductions in mitochondrial components and compensatory increases in protein synthesis. Collectively, these observations suggest that TDP-43 deposition leads to targeted RNA instability in ALS and FTD, and may ultimately cause cell death by disrupting energy production and protein synthesis pathways

Vascular defects and spinal cord hypoxia in spinal muscular atrophy

Acknowledgment S.H.P. is funded by The Euan MacDonald Center for Motor Neurone Disease Research and The SMA Trust. T.H.G. is funded by Muscular Dystrophy UK and The SMA Trust. K.T. is funded by The SMA Trust and the Motor Neurone Disease Association. H.Z. is funded by National Institute for Health Research and Great Ormond Street Hospital Biomedical Research Center, and F.M. is funded by the Medical Research Council and Great Ormond Street Hospital Charity. The MRC Center for Neuromuscular Diseases BioBank London (CNMD_BBL) is gratefully acknowledged.Peer reviewedPostprintPostprin

Expression of microRNAs in human post-mortem amyotrophic lateral sclerosis spinal cords provides insight into disease mechanisms

Amyotrophic lateral sclerosis is a late-onset and terminal neurodegenerative disease. The majority of cases are sporadic with unknown causes and only a small number of cases are genetically linked. Recent evidence suggests that post-transcriptional regulation and epigenetic mechanisms, such as microRNAs, underlie the onset and progression of neurodegenerative disorders; therefore, altered microRNA expression may result in the dysregulation of key genes and biological pathways that contribute to the development of sporadic amyotrophic lateral sclerosis. Using systems biology analyses on postmortem human spinal cord tissue, we identified dysregulated mature microRNAs and their potential targets previously implicated in functional process and pathways associated with the pathogenesis of ALS. Furthermore, we report a global reduction of mature microRNAs, alterations in microRNA processing, and support for a role of the nucleotide binding protein, TAR DNA binding protein 43, in regulating sporadic amyotrophic lateral sclerosis-associated microRNAs, thereby offering a potential underlying mechanism for sporadic amyotrophic lateral sclerosis.http://deepblue.lib.umich.edu/bitstream/2027.42/193091/2/nihms-748924.pdfPublished versio

Hb9-Cre recombination fails to improve lifespan, weight gain and motor behavior in SMAΔ7 mice.

<p><b>A.</b> Kaplan-Meier curves demonstrate no increase in survival, p = 0.6436, log-rank test. Mean life span: Hb9(Cre⁺)SMA: 13.41±3.73 days, n = 37. Hb9(Cre⁻)SMA mice: 14.6±2.64 days; n = 20. Controls: n = 130. <b>B</b>. While controls (n = 107) gained weight as expected, no statistical differences in weight were found during the lifespan of Hb9(Cre⁺)SMA and Hb9(Cre⁻)SMA mice except at P12 (*p = 0.004, F = 9.78, ANOVA). Hb9(Cre⁺)SMA: n = 10–27 per time point. Hb9(Cre⁻)SMA: n = 3–16 per time point. <b>C and </b><b>D</b>. Motor behavior was assessed by the righting reflex (C) and hindlimb (tube) suspension (D) tests. Only the latency to fall was recorded for the tube assays. While controls improved their performance with age, SMA mice motor behavior deteriorated as they approached end stage. For the righting reflex assays: Controls: n = 107 per time point. Hb9(Cre⁺)SMA: n = 7–27 per time point. Hb9(Cre⁻)SMA: n = 4–16 per time point. For the tube test: Controls: n = 148 per time point. Hb9(Cre⁺)SMA: n = 7–28 per time point. Hb9(Cre⁻)SMA: n = 5–17 per time point.</p

Schematic representation of the <i>Smn</i> WT allele and the <i>Smn^Res</i> conditional hybrid mutant allele before and after Hb9-Cre recombination.

<p>Blue boxes represent mouse <i>Smn</i> exons. Red boxes represent human <i>SMN2</i> exons. Arrows within boxes display orientation relative to transcription start. Green stars show approximate location of <i>loxP</i> sites (<i>lox71, lox 66</i>) in <i>Smn^Res</i>. Black lines connecting exons show splicing pattern for the predominant transcript encoded by each allele. Protein products are named to the right. <i>SMN67m8h</i> is the transcript encoded by the repaired <i>Smn^Res</i> allele. Not drawn to scale.</p