Spinal anterior horn has the capacity to self-regenerate in amyotrophic lateral sclerosis model mice

Fibrotic Scar in Neurodegenerative Diseases

The process of uncontrolled internal scarring, called fibrosis, is now emerging as a pathological feature shared by both peripheral and central nervous system diseases. In the CNS, damaged neurons are not replaced by tissue regeneration, and scar-forming cells such as endothelial cells, inflammatory immune cells, stromal fibroblasts, and astrocytes can persist chronically in brain and spinal cord lesions. Although this process was extensively described in acute CNS damages, novel evidence indicates the involvement of a fibrotic reaction in chronic CNS injuries as those occurring during neurodegenerative diseases, where inflammation and fibrosis fuel degeneration. In this mini review, we discuss recent advances around the role of fibrotic scar formation and function in different neurodegenerative conditions, particularly focusing on the rising role of scarring in the pathogenesis of amyotrophic lateral sclerosis, multiple sclerosis, and Alzheimer's disease and highlighting the therapeutic relevance of targeting fibrotic scarring to slow and reverse neurodegeneration.

Astrocyte-Derived Exosomes Treated With a Semaphorin 3A Inhibitor Enhance Stroke Recovery via Prostaglandin D 2 Synthase

Background and Purpose—

Exosomes play a pivotal role in neurogenesis. In the peri-infarct area after stroke, axons begin to regenerate but are inhibited by astrocyte scar formation. The direct effect and underlying molecular mechanisms of astrocyte-derived exosomes on axonal outgrowth after ischemia are not known.

Methods—

Using a semaphorin 3A (Sema3A) inhibitor, we explored neuronal signaling during axonal outgrowth after ischemia in rats subjected to middle cerebral artery occlusion and in cultured cortical neurons challenged with oxygen-glucose deprivation. Furthermore, we assessed whether this inhibitor suppressed astrocyte activation and regulated astrocyte-derived exosomes to enhance axonal outgrowth after ischemia.

Results—

In rats subjected to middle cerebral artery occlusion, we administered a Sema3A inhibitor into the peri-infarct area from 7 to 21 days after occlusion. We found that phosphorylated high-molecular weight neurofilament-immunoreactive axons were increased, glial fibrillary acidic protein–immunoreactive astrocytes were decreased, and functional recovery was promoted at 28 days after middle cerebral artery occlusion. In cultured neurons, the Sema3A inhibitor decreased Rho family GTPase 1, increased R-Ras, which phosphorylates Akt and glycogen synthase kinase 3β (GSK-3β), selectively increased phosphorylated GSK-3β in axons, and thereby enhanced phosphorylated high-molecular weight neurofilament-immunoreactive axons after oxygen-glucose deprivation. In cultured astrocytes, the Sema3A inhibitor suppressed activation of astrocytes induced by oxygen-glucose deprivation. Exosomes secreted from ischemic astrocytes treated with the Sema3A inhibitor further promoted axonal elongation and increased prostaglandin D 2 synthase expression on microarray analysis. GSK-3β + and prostaglandin D 2 synthase + neurons were robustly increased after treatment with the Sema3A inhibitor in the peri-infarct area.

Conclusions—

Neuronal Rho family GTPase 1/R-Ras/Akt/GSK-3β signaling, axonal GSK-3β expression, and astrocyte-derived exosomes with prostaglandin D 2 synthase expression contribute to axonal outgrowth and functional recovery after stroke.

Impaired hypoxic sensor Siah-1, PHD3, and FIH system in spinal motor neurons of an amyotrophic lateral sclerosis mouse model

We recently reported spinal blood flow-metabolism uncoupling in the Cu/Zn-superoxide dismutase 1 (SOD1)-transgenic (Tg) mouse model of amyotrophic lateral sclerosis (ALS), suggesting relative hypoxia in the spinal cord. However, the hypoxic stress sensor pathway in ALS has not been well studied. In the present work, we examined the temporal and spatial changes of hypoxic stress sensor proteins (Siah-1, PHD3, and FIH) following motor neuron (MN) degeneration in the spinal cord of normoxic ALS mice. The expression of Siah-1 and PHD3 proteins progressively increased in the surrounding glial cells of presymptomatic Tg mice (10 weeks, 10 weeks) compared with the large MN of the anterior horn. In contrast, a significant reduction in Siah-1 and PHD3 protein expression was evident in end-stage ALS mice (18 weeks, 18 weeks). Double-immunofluorescence analysis revealed PHD3 plus Siah-1 double-positive cells in the surrounding glia of symptomatic Tg mice (14-18 weeks), with no change in the large MNs. In contrast, FIH protein expression decreased in the surrounding glial cells of Tg mice at end-stage ALS (18 weeks). The present study suggests a partial loss in the neuroprotective response of spinal MNs in ALS results from a relative hypoxia through the Siah-1, PHD3, and FIH system under normoxic conditions. This response could be an important mechanism of neurodegeneration in ALS.

Impaired response of hypoxic sensor protein HIF-1α and its downstream proteins in the spinal motor neurons of ALS model mice

We have recently reported spinal blood flow-metabolism uncoupling in an amyotrophic lateral sclerosis (ALS) animal model using Cu/Zn-superoxide dismutase 1 (SOD1)-transgenic (Tg) mice, suggesting a relative hypoxia in the spinal cord. However, the hypoxic stress sensor pathway has not been well studied in ALS. Here, we examined temporal and spatial changes of the hypoxic stress sensor proteins HIF-1α and its downstream proteins (VEGF, HO-1, and EPO) during the normoxiccourse of motor neuron (MN) degeneration in the spinal cord of these ALS model mice. We found that HIF-1α protein expression progressively increased both in the anterior large MNs and the surrounding glial cells in Tg mice from early symptomatic 14 week (W) and end stage 18 W. Double immunofluorescence analysis revealed that HIF-1α, plus GFAP and Iba-1 double-positive surrounding glial cells, progressively increased from 14 W to 18 W, although the immunohistochemistry in large MNs did not change. Expression levels of VEGF and HO-1 also showed a progressive increase but were significant only in the surrounding glial cells at 18 W. In contrast, EPO protein expression was decreased in the surrounding glial cells of Tg mice at 18 W. Because HIF1-α serves as an important mediator of the hypoxic response, these findings indicate that MNs lack the neuroprotective response to hypoxic stress through the HIF-1α system, which could be an important mechanism of neurodegeneration in ALS.

Differential gene expression in the axotomized facial motor nucleus of presymptomatic SOD1 mice

Previously, we compared molecular profiles of one population of wild-type (WT) mouse facial motoneurons (FMNs) surviving with FMNs undergoing significant cell death after axotomy. Regardless of their ultimate fate, injured FMNs respond with a vigorous pro-survival/regenerative molecular response. In contrast, the neuropil surrounding the two different injured FMN populations contained distinct molecular differences that support a causative role for glial and/or immune-derived molecules in directing contrasting responses of the same cell types to the same injury. In the current investigation, we utilized the facial nerve axotomy model and a presymptomatic amyotrophic lateral sclerosis (ALS) mouse (SOD1) model to experimentally mimic the axonal die-back process observed in ALS pathogenesis without the confounding variable of disease onset. Presymptomatic SOD1 mice had a significant decrease in FMN survival compared with WT, which suggests an increased susceptibility to axotomy. Laser microdissection was used to accurately collect uninjured and axotomized facial motor nuclei of WT and presymptomatic SOD1 mice for mRNA expression pattern analyses of pro-survival/pro-regeneration genes, neuropil-specific genes, and genes involved in or responsive to the interaction of FMNs and non-neuronal cells. Axotomized presymptomatic SOD1 FMNs displayed a dynamic pro-survival/regenerative response to axotomy, similar to WT, despite increased cell death. However, significant differences were revealed when the axotomy-induced gene expression response of presymptomatic SOD1 neuropil was compared with WT. We propose that the increased susceptibility of presymptomatic SOD1 FMNs to axotomy-induced cell death and, by extrapolation, disease progression, is not intrinsic to the motoneuron, but rather involves a dysregulated response by non-neuronal cells in the surrounding neuropil.

Impaired antioxydative Keap1/Nrf2 system and the downstream stress protein responses in the motor neuron of ALS model mice

The Kelch-like ECH-associated protein 1 (Keap1)/Nuclear erythroid 2-related factor 2 (Nrf2) system is the major cellular defense mechanism under oxidative stress, but the role in motor neuron degeneration under amyotrophic lateral sclerosis (ALS) pathology has not yet been fully elucidated. Here we examined temporal and spatial changes of Keap1, Nrf2, and their downstream stress response proteins heme oxgenase-1 (HO-1), glutathione, thioredoxin (TRX), and heat shock protein 70 (HSP70) throughout the course of motor neuron (MN) degeneration in the spinal cord of ALS model mice. Keap1 protein levels progressively decreased in the MN and anterior lumbar cord of ALS mice to 63% at early symptomatic 14 weeks and 58% at end symptomatic 18 weeks, while Nrf2 dramatically increased in the anterior lumbar cord with accumulation in the MN nucleus to 229% at 14 weeks and 471% at 18 weeks when glial like cells became also positive. In contrast, downstream stress response proteins such as HO-1, glutathione, TRX, and HSP70 showed only a small increase in MN with a significant increase to 149% to 280% in the number of glial-like cells after symptomatic 14 weeks. Our present observation suggests that MN selectively lost inductions of these important downstream protective proteins without regard to the Keap1/Nrf2 system activation, which could be a pivotal mechanism of neurodegenerative processes of ALS.

NO orchestrates the loss of synaptic boutons from adult "sick" motoneurons: modeling a molecular mechanism

Synapse elimination is the main factor responsible for the cognitive decline accompanying many of the neuropathological conditions affecting humans. Synaptic stripping of motoneurons is also a common hallmark of several motor pathologies. Therefore, knowledge of the molecular basis underlying this plastic process is of central interest for the development of new therapeutic tools. Recent advances from our group highlight the role of nitric oxide (NO) as a key molecule triggering synapse loss in two models of motor pathologies. De novo expression of the neuronal isoform of NO synthase (nNOS) in motoneurons commonly occurs in response to the physical injury of a motor nerve and in the course of amyotrophic lateral sclerosis. In both conditions, this event precedes synaptic withdrawal from motoneurons. Strikingly, nNOS-synthesized NO is "necessary" and "sufficient" to induce synaptic detachment from motoneurons. The mechanism involves a paracrine/retrograde action of NO on pre-synaptic structures, initiating a downstream signaling cascade that includes sequential activation of (1) soluble guanylyl cyclase, (2) cyclic guanosine monophosphate-dependent protein kinase, and (3) RhoA/Rho kinase (ROCK) signaling. Finally, ROCK activation promotes phosphorylation of regulatory myosin light chain, which leads to myosin activation and actomyosin contraction. This latter event presumably contributes to the contractile force to produce ending axon retraction. Several findings support that this mechanism may operate in the most prevalent neurodegenerative diseases.