Differential effect of Fas activation on spinal muscular atrophy motoneuron death and induction of axonal growth

Amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA) are the most common motoneuron diseases affecting adults and infants, respectively. ALS and SMA are both characterized by the selective degeneration of motoneurons. Although different in their genetic etiology, growing evidence indicates that they share molecular and cellular pathogenic signatures that constitute potential common therapeutic targets. We previously described a motoneuron-specific death pathway elicited by the Fas death receptor, whereby vulnerable ALS motoneurons show an exacerbated sensitivity to Fas activation. However, the mechanisms that drive the loss of SMA motoneurons remain poorly understood. Here, we describe an in vitro model of SMA-associated degeneration using primary motoneurons derived from Smn2B/- SMA mice and show that Fas activation selectively triggers the death of the proximal motoneurons. Fas-induced death of SMA motoneurons has the molecular signature of the motoneuron-selective Fas death pathway that requires activation of p38 kinase, caspase-8, -9 and -3 as well as upregulation of collapsin response mediator protein 4 (CRMP4). In addition, Rho-associated Kinase (ROCK) is required for Fas recruitment. Remarkably, we found that exogenous activation of Fas also promotes axonal elongation in both wildtype and SMA motoneurons. Axon outgrowth of motoneurons promoted by Fas requires the activity of ERK, ROCK and caspases. This work defines a dual role of Fas signaling in motoneurons that can elicit distinct responses from cell death to axonal growth

Synaptic Transmission and Motoneuron Excitability Defects in Amyotrophic Lateral Sclerosis

International audienceAmyotrophic lateral sclerosis is a fatal adult-onset neurodegenerative disease characterized by progressive muscular weakness and atrophy. The primary feature of amyotrophic lateral sclerosis is the selective loss of motoneurons in the brain and spinal cord. However, changes in synaptic transmission and motoneuron excitability are among the first events that take place during development and accompany the relentless deterioration of motor circuitry. This chapter aims to summarize the current understanding of defects in intrinsic electrophysiological properties of motoneurons, local GABAergic and glycinergic inhibitory as well as cholinergic modulatory interneuron networks, and long-range glutamatergic excitatory input neurons that can precede disease onset or occur during the progression of the disease. We summarize evidence that therapeutic options that target synaptic transmission and intrinsic features of motoneurons might represent novel effective strategies for patients with amyotrophic lateral sclerosis

Spinal Motoneuron TMEM16F Acts at C-boutons to Modulate Motor Resistance and Contributes to ALS Pathogenesis

International audienceNeuronal Ca2+ entry elicited by electrical activity contributes to information coding via activation of K+ and Cl- channels. While Ca2+-dependent K+ channels have been extensively studied, the molecular identity and role of Ca2+-activated Cl- channels (CaCCs) remain unclear. Here, we demonstrate that TMEM16F governs a Ca2+-activated Cl- conductance in spinal motoneurons. We show that TMEM16F is expressed in synaptic clusters facing pre-synaptic cholinergic C-boutons in α-motoneurons of the spinal cord. Mice with targeted exon deletion in Tmem16f display decreased motor performance under high-demanding tasks attributable to an increase in the recruitment threshold of fast α-motoneurons. Remarkably, loss of TMEM16F function in a mouse model of amyotrophic lateral sclerosis (ALS) significantly reduces expression of an activity-dependent early stress marker and muscle denervation, delays disease onset, and preserves muscular strength only in male ALS mice. Thus, TMEM16F controls motoneuron excitability and impacts motor resistance as well as motor deterioration in ALS

NKCC1 Phosphorylation Stimulates Neurite Growth of Injured Adult Sensory Neurons: NKCC1 and regenerative growth

Peripheral nerve section promotes regenerative, elongated neuritic growth of adult sensory neurons. While the role of chloride homeostasis, through the regulation of ionotropic GABA receptors, in the growth status of immature neurons in the central nervous system begins to emerge, nothing is known of its role in the regenerative growth of injured adult neurons. To analyze the intracellular Cl- variation following a sciatic nerve section in vivo, gramicidin perforated-patch recordings were used to study muscimol-induced currents in mice dorsal root ganglion neurons isolated from control and axotomized neurons. We show that the reversal potential of muscimol-induced current, EGABA-A was shifted towards depolarized potentials in axotomized neurons. This was due to Cl- influx since removal of extracellular Cl- prevented this shift. Application of bumetanide, an inhibitor of NKCC1 cotransporter and EGABA-A recordings in sensory neurons from NKCC1-/- mice, identified NKCC1 as being responsible for the increase in intracellular Cl- in axotomized neurons. In addition, we demonstrate with a phospho-NKCC1 antibody that nerve injury induces an increase in the phosphorylated form of NKCC1 in dorsal root ganglia that could account for intracellular Cl- accumulation. Time-lapse recordings of the neuritic growth of axotomized neurons show a faster growth velocity in comparison with control. Both bumetanide, the intrathecal injection of NKCC1 siRNA and the use of NKCC1-/- mice demonstrated that NKCC1 is involved in determining the velocity of elongated growth of axotomized neurons. Our results clearly show that NKCC1-induced increase in intracellular chloride concentration is a major event accompanying peripheral nerve regeneration

Myotube elasticity of an amyotrophic lateral sclerosis mouse model

Abstract Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease that affects the motor system leading to generalized paralysis and death of patients. The understanding of early pathogenic mechanisms will help to define early diagnostics criteria that will eventually provide basis for efficient therapeutics. Early symptoms of ALS usually include muscle weakness or stiffness. Therefore, mechanical response of differentiated myotubes from primary cultures of mice, expressing the ALS-causing SOD1 G93A mutation, was examined by atomic force microscopy. Simultaneous acquisition of topography and cell elasticity of ALS myotubes was performed by force mapping method, compared with healthy myotubes and supplemented with immunofluorescence and qRT-PCR studies. Wild type myotubes reveal a significant difference in elasticity between a narrow and a wide population, consistent with maturation occurring with higher actin expression relative to myosin together with larger myotube width. However, this is not true for SOD1 G93A expressing myotubes, where a significant shift of thin population towards higher elastic modulus values was observed. We provide evidence that SOD1 mutant induces structural changes that occurs very early in muscle development and well before symptomatic stage of the disease. These findings could significantly contribute to the understanding of the role of skeletal muscle in ALS pathogenesis

Differential effect of Fas activation on spinal muscular atrophy motoneuron death and induction of axonal growth.

Amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA) are the most common motoneuron diseases affecting adults and infants, respectively. ALS and SMA are both characterized by the selective degeneration of motoneurons. Although different in their genetic etiology, growing evidence indicates that they share molecular and cellular pathogenic signatures that constitute potential common therapeutic targets. We previously described a motoneuron-specific death pathway elicited by the Fas death receptor, whereby vulnerable ALS motoneurons show an exacerbated sensitivity to Fas activation. However, the mechanisms that drive the loss of SMA motoneurons remains poorly understood. Here, we describe an in vitro model of SMA-associated degeneration using primary motoneurons derived from Smn2B/- SMA mice and show that Fas activation selectively triggers death of the proximal motoneurons. Fas-induced death of SMA motoneurons has the molecular signature of the motoneuron-selective Fas death pathway that requires activation of p38 kinase, caspase-8, -9 and -3 as well as upregulation of collapsin response mediator protein 4 (CRMP4). In addition, Rho-associated Kinase (ROCK) is required for Fas recruitment. Remarkably, we found that exogenous activation of Fas also promotes axonal elongation in both wildtype and SMA motoneurons. Axon outgrowth of motoneurons promoted by Fas requires the activity of ERK, ROCK and caspases. This work defines a dual role of Fas signaling in motoneurons that can elicit distinct responses from cell death to axonal growth