Smn-Deficiency Increases the Intrinsic Excitability of Motoneurons

Fatigue in Spinal Muscular Atrophy: a fundamental open issue

Hereditary proximal 5q Spinal Muscular Atrophy (SMA) is a severe neuromuscular disorder with onset mainly in infancy or childhood. The underlying pathogenic mechanism is the loss of alpha motor neurons in the anterior horns of spine, due to deficiency of the survival motor neuron (SMN) protein as a consequence of the deletion of the SMN1 gene. Clinically, SMA is characterized by progressive loss of muscle strength and motor function ranging from the extremely severe, the neonatal onset type 1, to the mild type 4 arising in the adult life. All the clinical variants share the same molecular defect, the difference being driven mainly by the copy number of SMN2 gene, a centromeric gene nearly identical to SMN1 with a unique C to T transition in Exon 7 that results in exclusion of Exon 7 during post-transcriptional processing. In all the types of SMA the clinical picture is characterized by hypotonia, weakness and areflexia. Clinical severity can vary a lot between the four main recognized types of SMA. As for the most of patients affected by different neuromuscular disorders, also in SMA fatigability is a major complaint as it is frequently reported in common daily activities and negatively impacts on the overall quality of life. The increasing awareness of fatigability as an important dimension of impairment in Neuromuscular Disorders and particularly in SMA, is making it both a relevant subject of study and identifies it as a fundamental therapeutic target. In this review, we aimed to overview the current literature articles concerning this problem, in order to highlight what is known and what deserves further research.

Impaired communication at the neuromotor axis during Degenerative Cervical Myelopathy

Degenerative Cervical Myelopathy (DCM) is a progressive neurological condition characterized by structural alterations in the cervical spine, resulting in compression of the spinal cord. While clinical manifestations of DCM are well-documented, numerous unanswered questions persist at the molecular and cellular levels. In this study, we sought to investigate the neuromotor axis during DCM. We use a clinically relevant mouse model, where after 3 months of DCM induction, the sensorimotor tests revealed a significant reduction in both locomotor activity and muscle strength compared to the control group. Immunohistochemical analyses showed alterations in the gross anatomy of the cervical spinal cord segment after DCM. These changes were concomitant with the loss of motoneurons and a decrease in the number of excitatory synaptic inputs within the spinal cord. Additionally, the DCM group exhibited a reduction in the endplate surface, which correlated with diminished presynaptic axon endings in the supraspinous muscles. Furthermore, the biceps brachii (BB) muscle exhibited signs of atrophy and impaired regenerative capacity, which inversely correlated with the transversal area of remnants of muscle fibers. Additionally, metabolic assessments in BB muscle indicated an increased proportion of oxidative skeletal muscle fibers. In line with the link between neuromotor disorders and gut alterations, DCM mice displayed smaller mucin granules in the mucosa layer without damage to the epithelial barrier in the colon. Notably, a shift in the abundance of microbiota phylum profiles reveals an elevated Firmicutes-to-Bacteroidetes ratio-a consistent hallmark of dysbiosis that correlates with alterations in gut microbiota-derived metabolites. Additionally, treatment with short-chain fatty acids stimulated the differentiation of the motoneuron-like NSC34 cell line. These findings shed light on the multifaceted nature of DCM, resembling a synaptopathy that disrupts cellular communication within the neuromotor axis while concurrently exerting influence on other systems. Notably, the colon emerges as a focal point, experiencing substantial perturbations in both mucosal barrier integrity and the delicate balance of intestinal microbiota.

Diminished motor neuron activity driven by abnormal astrocytic EAAT1 glutamate transporter activity in spinal muscular atrophy is not fully restored after lentiviral SMN delivery

Spinal muscular atrophy (SMA) is characterized by the loss of the lower spinal motor neurons due to survival motor neuron (SMN) deficiency. The motor neuron cell autonomous and non-cell autonomous disease mechanisms driving early glutamatergic dysfunction, a therapeutically targetable phenotype prior to motor neuron cell loss, remain unclear. Using microelectrode array analysis, we demonstrate that the secretome and cell surface proteins needed for proper synaptic modulation are likely disrupted in human SMA astrocytes and lead to diminished motor neuron activity. While healthy astrocyte conditioned media did not improve SMA motor neuron activity, SMA motor neurons robustly responded to healthy astrocyte neuromodulation in direct contact cultures. This suggests an important role of astrocyte synaptic-associated plasma membrane proteins and contact-mediated cellular interactions for proper motor neuron function in SMA. Specifically, we identified a significant reduction of the glutamate Na⁺ dependent excitatory amino acid transporter EAAT1 within human SMA astrocytes and SMA lumbar spinal cord tissue. The selective inhibition of EAAT1 in healthy co-cultures phenocopied the diminished neural activity observed in SMA astrocyte co-cultures. Caveolin-1, an SMN-interacting protein previously associated with local translation at the plasma membrane, was abnormally elevated in human SMA astrocytes. Although lentiviral SMN delivery to SMA astrocytes partially rescued EAAT1 expression, limited activity of healthy motor neurons was still observed in SMN-transduced SMA astrocyte co-cultures. Together, these data highlight the detrimental impact of astrocyte-mediated disease mechanisms on motor neuron function in SMA and that SMN delivery may be insufficient to fully restore astrocyte function at the synapse.

Survival motor neuron protein deficiency alters microglia reactivity

Survival motor neuron (SMN) protein deficiency results in loss of alpha motor neurons and subsequent muscle atrophy in patients with spinal muscular atrophy (SMA). Reactive microglia have been reported in SMA mice and depleting microglia rescues the number of proprioceptive synapses, suggesting a role in SMA pathology. Here, we explore the contribution of lymphocytes on microglia reactivity in SMA mice and investigate how SMN deficiency alters the reactive profile of human induced pluripotent stem cell (iPSC)-derived microglia. We show that microglia adopt a reactive morphology in spinal cords of SMA mice. Ablating lymphocytes did not alter the reactive morphology of SMA microglia and did not improve the survival or motor function of SMA mice, indicating limited impact of peripheral immune cells on the SMA phenotype. We found iPSC-derived SMA microglia adopted an amoeboid morphology and displayed a reactive transcriptome profile, increased cell migration, and enhanced phagocytic activity. Importantly, cell morphology and electrophysiological properties of motor neurons were altered when they were incubated with conditioned media from SMA microglia. Together, these data reveal that SMN-deficient microglia adopt a reactive profile and exhibit an exaggerated inflammatory response with potential impact on SMA neuropathology.

Emerging concepts underlying selective neuromuscular dysfunction in infantile-onset spinal muscular atrophy

Infantile-onset spinal muscular atrophy is the quintessential example of a disorder characterized by a predominantly neurodegenerative phenotype that nevertheless stems from perturbations in a housekeeping protein. Resulting from low levels of the Survival of Motor Neuron (SMN) protein, spinal muscular atrophy manifests mainly as a lower motor neuron disease. Why this is so and whether other cell types contribute to the classic spinal muscular atrophy phenotype continue to be the subject of intense investigation and are only now gaining appreciation. Yet, what is emerging is sometimes as puzzling as it is instructive, arguing for a careful re-examination of recent study outcomes, raising questions about established dogma in the field and making the case for a greater focus on milder spinal muscular atrophy models as tools to identify key mechanisms driving selective neuromuscular dysfunction in the disease. This review examines the evidence for novel molecular and cellular mechanisms that have recently been implicated in spinal muscular atrophy, highlights breakthroughs, points out caveats and poses questions that ought to serve as the basis of new investigations to better understand and treat this and other more common neurodegenerative disorders.

Cutaneous silent period in patients with spinal muscular atrophy type 2 and type 3

OBJECTIVE

The aim of this study was to examine cutaneous silent periods (CSPs) in spinal muscular atrophy (SMA) type 2 and type 3 patients.

METHODS

Fourteen patients with SMA and 14 healthy subjects were included. CSPs were recorded from thenar muscles after painful stimulation of the index finger during slight thumb abduction. CSP parameters (including onset latency, duration, index of suppression, long-loop reflex, and post-inhibitory rebound activity) were measured. All parameters were compared between SMA patients and healthy subjects using the Mann-Whitney U test. We then correlated CSP parameters to Hammersmith Functional Motor Scale Expanded (HFMSE) scores in SMA patients by Spearman-rho testing.

RESULTS

No CSP parameter except E3% (i.e., the post-inhibitory rebound activity relative to pre-stimulus baseline) differed significantly between SMA patients and healthy subjects. E3% was significantly smaller in patients than healthy individuals. HFMSE scores of SMA patients correlated negatively with CSP duration and positively with E3%.

DISCUSSION

Smaller E3%, correlating with longer CSP duration, in SMA patients with more severe clinical findings and therefore more marked motoneuron loss suggests that the E3 component following the CSP is directly affected by motoneuron loss.

Highly Efficient Conversion of Motor Neuron-Like NSC-34 Cells into Functional Motor Neurons by Prostaglandin E2

Motor neuron diseases are a group of progressive neurological disorders that degenerate motor neurons. The neuroblastoma × spinal cord hybrid cell line NSC-34 is widely used as an experimental model in studies of motor neuron diseases. However, the differentiation efficiency of NSC-34 cells to neurons is not always sufficient. We have found that prostaglandin E₂ (PGE₂) induces morphological differentiation in NSC-34 cells. The present study investigated the functional properties of PGE₂-differentiated NSC-34 cells. Retinoic acid (RA), a widely-used agent inducing cell differentiation, facilitated neuritogenesis, which peaked on day 7, whereas PGE₂-induced neuritogenesis took only 2 days to reach the same level. Whole-cell patch-clamp recordings showed that the current threshold of PGE₂-treated cell action potentials was lower than that of RA-treated cells. PGE₂ and RA increased the protein expression levels of neuronal differentiation markers, microtubule-associated protein 2c and synaptophysin, and to the same extent, motor neuron-specific markers HB9 and Islet-1. On the other hand, protein levels of choline acetyltransferase and basal release of acetylcholine in PGE₂-treated cells were higher than in RA-treated cells. These results suggest that PGE₂ is a rapid and efficient differentiation-inducing factor for the preparation of functionally mature motor neurons from NSC-34 cells.

The Y172 Monoclonal Antibody Against p-c-Jun (Ser63) Is a Marker of the Postsynaptic Compartment of C-Type Cholinergic Afferent Synapses on Motoneurons

C-bouton-type cholinergic afferents exert an important function in controlling motoneuron (MN) excitability. During the immunocytochemical analysis of the role of c-Jun in MNs with a monoclonal (clone Y172) antibody against phospho (p)-c-Jun (serine [Ser]63), unexpected labeling was identified in the cell body cytoplasm. As predicted for c-Jun in adult spinal cord, very few, if any MNs exhibited nuclear immunoreactivity with the Y172 antibody; conversely, virtually all MNs displayed strong Y172 immunostaining in cytoplasmic structures scattered throughout the soma and proximal dendrites. The majority of these cytoplasmic Y172-positive profiles was closely associated with VAChT-positive C-boutons, but not with other types of nerve afferents contacting MNs. Ultrastructural analysis revealed that cytoplasmic Y172 immunostaining was selectively located at the subsurface cistern (SSC) of C-boutons and also in the inner areas of the endoplasmic reticulum (ER). We also described changes in cytoplasmic Y172 immunoreactivity in injured and degenerating MNs. Moreover, we noticed that MNs from NRG1 type III-overexpressing transgenic mice, which show abnormally expanded SSCs, exhibited an increase in the density and size of peripherally located Y172-positive profiles. A similar immunocytochemical pattern to that of the Y172 antibody in MNs was found with a polyclonal antibody against p-c-Jun (Ser63) but not with another polyclonal antibody that recognizes c-Jun phosphorylated at a different site. No differential band patterns were found by western blotting with any of the antibodies against c-Jun or p-c-Jun used in our study. In cultured MNs, Y172-positive oval profiles were distributed in the cell body and proximal dendrites. The in vitro lentiviral-based knockdown of c-Jun resulted in a dramatic decrease in nuclear Y172 immunostaining in MNs without any reduction in the density of cytoplasmic Y172-positive profiles, suggesting that the synaptic antigen recognized by the antibody corresponds to a C-bouton-specific protein other than p-c-Jun. Our results lay the foundation for further studies aimed at identifying this protein and determining its role in this particular type of synapse.

Hyperexcitability precedes motoneuron loss in the Smn2B/- mouse model of spinal muscular atrophy

Spinal motoneuron dysfunction and loss are pathological hallmarks of the neuromuscular disease spinal muscular atrophy (SMA). Changes in motoneuron physiological function precede cell death, but how these alterations vary with disease severity and motoneuron maturational state is unknown. To address this question, we assessed the electrophysiology and morphology of spinal motoneurons of presymptomatic Smn^2B/- mice older than 1 wk of age and tracked the timing of motor unit loss in this model using motor unit number estimation (MUNE). In contrast to other commonly used SMA mouse models, Smn^2B/- mice exhibit more typical postnatal development until postnatal day (P)11 or 12 and have longer survival (~3 wk of age). We demonstrate that Smn^2B/- motoneuron hyperexcitability, marked by hyperpolarization of the threshold voltage for action potential firing, was present at P9-10 and preceded the loss of motor units. Using MUNE studies, we determined that motor unit loss in this mouse model occurred 2 wk after birth. Smn^2B/- motoneurons were also larger in size, which may reflect compensatory changes taking place during postnatal development. This work suggests that motoneuron hyperexcitability, marked by a reduced threshold for action potential firing, is a pathological change preceding motoneuron loss that is common to multiple models of severe SMA with different motoneuron maturational states. Our results indicate voltage-gated sodium channel activity may be altered in the disease process.NEW & NOTEWORTHY Changes in spinal motoneuron physiologic function precede cell death in spinal muscular atrophy (SMA), but how they vary with maturational state and disease severity remains unknown. This study characterized motoneuron and neuromuscular electrophysiology from the Smn^2B/- model of SMA. Motoneurons were hyperexcitable at postnatal day (P)9-10, and specific electrophysiological changes in Smn^2B/- motoneurons preceded functional motor unit loss at P14, as determined by motor unit number estimation studies.

Glial Activation and Central Synapse Loss, but Not Motoneuron Degeneration, Are Prevented by the Sigma-1 Receptor Agonist PRE-084 in the Smn2B/- Mouse Model of Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is characterized by the loss of α-motoneurons (MNs) with concomitant muscle denervation. MN excitability and vulnerability to disease are particularly regulated by cholinergic synaptic afferents (C-boutons), in which Sigma-1 receptor (Sig1R) is concentrated. Alterations in Sig1R have been associated with MN degeneration. Here, we investigated whether a chronic treatment with the Sig1R agonist PRE-084 was able to exert beneficial effects on SMA. We used a model of intermediate SMA, the Smn2B/- mouse, in which we performed a detailed characterization of the histopathological changes that occur throughout the disease. We report that Smn2B/- mice exhibited qualitative differences in major alterations found in mouse models of severe SMA: Smn2B/- animals showed more prominent MN degeneration, early motor axon alterations, marked changes in sensory neurons, and later MN deafferentation that correlated with conspicuous reactive gliosis and altered neuroinflammatory M1/M2 microglial balance. PRE-084 attenuated reactive gliosis, mitigated M1/M2 imbalance, and prevented MN deafferentation in Smn2B/- mice. These effects were also observed in a severe SMA model, the SMNΔ7 mouse. However, the prevention of gliosis and MN deafferentation promoted by PRE-084 were not accompanied by any improvements in clinical outcome or other major pathological changes found in SMA mice.

The Metabolic Disturbances of Motoneurons Exposed to Glutamate

Glutamate-induced excitotoxicity is considered as one of the major pathophysiological factors of motoneuron death in amyotrophic lateral sclerosis and other motoneuron diseases. In order to expand our knowledge on mechanisms of glutamate-induced excitotoxicity, the present study proposes to determine the metabolic consequences of glutamate and astrocytes in primary enriched motoneuron culture. Using liquid chromatography coupled to high-resolution mass spectrometry (LC-HRMS), we showed that the presence of astrocytes and glutamate profoundly modified the metabolic profile of motoneurons. Our study highlights for the first time that crosstalk between astrocytes and enriched motoneuron culture induced alterations in phenylalanine, tryptophan, purine, arginine, proline, aspartate, and glutamate metabolism in motoneurons. We observed that astrocytes modulate the sensitivity of motoneurons to glutamate, since metabolites altered by glutamate in motoneurons cultured alone were different (except 5-hydroxylysine) from those altered in co-cultured motoneurons. Our findings provide new insight into the metabolic alterations associated to astrocytes and glutamate in motoneurons and provide opportunities to identify novel therapeutic targets.