NF-M is an essential target for the myelin-directed “outside-in” signaling cascade that mediates radial axonal growth

Neurofilaments are essential for acquisition of normal axonal calibers. Several lines of evidence have suggested that neurofilament-dependent structuring of axoplasm arises through an “outside-in” signaling cascade originating from myelinating cells. Implicated as targets in this cascade are the highly phosphorylated KSP domains of neurofilament subunits NF-H and NF-M. These are nearly stoichiometrically phosphorylated in myelinated internodes where radial axonal growth takes place, but not in the smaller, unmyelinated nodes. Gene replacement has now been used to produce mice expressing normal levels of the three neurofilament subunits, but which are deleted in the known phosphorylation sites within either NF-M or within both NF-M and NF-H. This has revealed that the tail domain of NF-M, with seven KSP motifs, is an essential target for the myelination-dependent outside-in signaling cascade that determines axonal caliber and conduction velocity of motor axons

The Amyotrophic Lateral Sclerosis M114T PFN1 Mutation Deregulates Alternative Autophagy Pathways and Mitochondrial Homeostasis

International audienceMutations in profilin 1 (PFN1) have been identified in rare familial cases of Amyotrophic Lateral Sclerosis (ALS). PFN1 is involved in multiple pathways that could intervene in ALS pathology. However, the specific pathogenic role of PFN1 mutations in ALS is still not fully understood. We hypothesized that PFN1 could play a role in regulating autophagy pathways and that PFN1 mutations could disrupt this function. We used patient cells (lymphoblasts) or tissue (post-mortem) carrying PFN1 mutations (M114T and E117G), and designed experimental models expressing wild-type or mutant PFN1 (cell lines and novel PFN1 mice established by lentiviral transgenesis) to study the effects of PFN1 mutations on autophagic pathway markers. We observed no accumulation of PFN1 in the spinal cord of one E117G mutation carrier. Moreover, in patient lymphoblasts and transfected cell lines, the M114T mutant PFN1 protein was unstable and deregulated the RAB9-mediated alternative autophagy pathway involved in the clearance of damaged mitochondria. In vivo, motor neurons expressing M114T mutant PFN1 showed mitochondrial abnormalities. Our results demonstrate that the M114T PFN1 mutation is more deleterious than the E117G variant in patient cells and experimental models and suggest a role for the RAB9-dependent autophagic pathway in ALS

Misfolded SOD1 Associated with Motor Neuron Mitochondria Alters Mitochondrial Shape and Distribution Prior to Clinical Onset

Mutations in superoxide dismutase (SOD1) are causative for inherited amyotrophic lateral sclerosis. A proportion of SOD1 mutant protein is misfolded onto the cytoplasmic face of mitochondria in one or more spinal cord cell types. By construction of mice in which mitochondrially targeted enhanced green fluorescent protein is selectively expressed in motor neurons, we demonstrate that axonal mitochondria of motor neurons are primary in vivo targets for misfolded SOD1. Mutant SOD1 alters axonal mitochondrial morphology and distribution, with dismutase active SOD1 causing mitochondrial clustering at the proximal side of Schmidt-Lanterman incisures within motor axons and dismutase inactive SOD1 producing aberrantly elongated axonal mitochondria beginning pre-symptomatically and increasing in severity as disease progresses. Somal mitochondria are altered by mutant SOD1, with loss of the characteristic cylindrical, networked morphology and its replacement by a less elongated, more spherical shape. These data indicate that mutant SOD1 binding to mitochondria disrupts normal mitochondrial distribution and size homeostasis as early pathogenic features of SOD1 mutant-mediated ALS

New insights on the disease contribution of neuroinflammation in ALS

International audiencePurpose of review: Amyotrophic lateral sclerosis (ALS) is a degenerative motor neuron disease with a strong neuroinflammatory component. This review summarizes how the connection between neurodegeneration and the immune system is strengthened by new discoveries from ALS genetics and the analysis of subpopulations of immune cells in ALS.Recent findings: Recent genes identified in ALS encode for proteins with direct immune roles, which when mutated lead to deregulation of immune functions, potentially influencing the disease. Although neuroinflammation in the central nervous system (CNS) of ALS patients has been well documented, new evidence suggests also direct malfunctions of immune cells in the CNS and at the periphery. Although CD4+ T-regulatory lymphocytes are protective in ALS, their number and function are altered over the disease course. CD8+ T cells are detrimental for motor neurons in the CNS but show some protective roles at the periphery. Similarly, the presence of mast cells in muscles of ALS models and patients and impairments of monocyte functions reveal potential new players in ALS disease progression.Summary: Although motor neuron degeneration is considered the prime event in ALS, dysfunctions in immune processes can impact the disease, highlighting that targeting specific immune components is a strategy for developing biomarkers and ultimately new drugs

Toxicity from different SOD1 mutants dysregulates the complement system and the neuronal regenerative response in ALS motor neurons

Global, age-dependent changes in gene expression from rodent models of inherited ALS caused by dominant mutations in superoxide-dismutase 1 (SOD1) were identified by using gene arrays and RNAs isolated from purified embryonic and adult motor neurons. Comparison of embryonic motor neurons expressing a dismutase active ALS-linked mutant SOD1 with those expressing comparable levels of wild-type SOD1 revealed the absence of mutant-induced mRNA changes. An age-dependent mRNA change that developed presymptomatically in adult motor neurons collected by laser microdissection from mice expressing dismutase active ALS-linked mutants was dysregulation of the d/l-serine biosynthetic pathway, previously linked to both excitotoxic and neurotrophic effects. An unexpected dysregulation common to motor neurons expressing either dismutase active or inactive mutants was induction of neuronally derived components of the classic complement system and the regenerative/injury response. Alteration of these mutant SOD1-induced pathways identified a set of targets for therapies for inherited ALS

C1q induction and global complement pathway activation do not contribute to ALS toxicity in mutant SOD1 mice

Accumulating evidence from mice expressing ALS-causing mutations in superoxide dismutase (SOD1) has implicated pathological immune responses in motor neuron degeneration. This includes microglial activation, lymphocyte infiltration, and the induction of C1q, the initiating component of the classic complement system that is the protein-based arm of the innate immune response, in motor neurons of multiple ALS mouse models expressing dismutase active or inactive SOD1 mutants. Robust induction early in disease course is now identified for multiple complement components (including C1q, C4, and C3) in spinal cords of SOD1 mutant-expressing mice, consistent with initial intraneuronal C1q induction, followed by global activation of the complement pathway. We now test if this activation is a mechanistic contributor to disease. Deletion of the C1q gene in mice expressing an ALS-causing mutant in SOD1 to eliminate C1q induction, and complement cascade activation that follows from it, is demonstrated to produce changes in microglial morphology accompanied by enhanced loss, not retention, of synaptic densities during disease. C1q-dependent synaptic loss is shown to be especially prominent for cholinergic C-bouton nerve terminal input onto motor neurons in affected C1q-deleted SOD1 mutant mice. Nevertheless, overall onset and progression of disease are unaffected in C1q- and C3-deleted ALS mice, thus establishing that C1q induction and classic or alternative complement pathway activation do not contribute significantly to SOD1 mutant-mediated ALS pathogenesis in mice