Astrocytic production of nerve growth factor in motor neuron apoptosis: implications for amyotrophic lateral sclerosis

Growth Factors and Their Application in the Therapy of Hereditary Neurodegenerative Diseases

Hereditary neurodegenerative diseases (hNDDs) such as Alzheimer's, Parkinson's, Huntington's disease, and others are primarily characterized by their progressive nature, severely compromising both the cognitive and motor abilities of patients. The underlying genetic component in hNDDs contributes to disease risk, creating a complex genetic landscape. Considering the fact that growth factors play crucial roles in regulating cellular processes, such as proliferation, differentiation, and survival, they could have therapeutic potential for hNDDs, provided appropriate dosing and safe delivery approaches are ensured. This article presents a detailed overview of growth factors, and explores their therapeutic potential in treating hNDDs, emphasizing their roles in neuronal survival, growth, and synaptic plasticity. However, challenges such as proper dosing, delivery methods, and patient variability can hinder their clinical application.

Skin innervation across amyotrophic lateral sclerosis clinical stages: new prognostic biomarkers

Over recent decades, peripheral sensory abnormalities, including the evidence of cutaneous denervation, have been reported among the non-motor manifestations in amyotrophic lateral sclerosis (ALS). However, a correlation between cutaneous innervation and clinical features has not been found. The aims of this study were to assess sensory involvement by applying a morpho-functional approach to a large population of ALS patients stratified according to King's stages and correlate these findings with the severity and prognosis of the disease. We recruited 149 ALS patients and 41 healthy controls. Patients undertook clinical questionnaires for small fibre neuropathy symptoms (Small Fiber Neuropathy Symptoms Inventory Questionnaire) and underwent nerve conductions studies (NCS) and 3-mm punch skin biopsies from leg, thigh and fingertip. We assessed intraepidermal nerve fibre (IENF) and Meissner corpuscle (MC) density by applying an indirect immunofluorescence technique. Moreover, a subset of 65 ALS patients underwent a longitudinal study with repeat biopsies from the thigh at 6- and 12-month follow-ups. Serum NfL levels were measured in 40 patients. Sensory symptoms and sensory NCS abnormalities were present in 32.2% and 24% of patients, respectively, and increased across clinical stages. Analogously, we observed a progressive reduction in amplitude of the sensory and motor ulnar nerve potential from stage 1 to stage 4. Skin biopsy showed a significant loss of IENFs and MCs in ALS compared with healthy controls (all P < 0.001). Across the clinical stages, we found a progressive reduction in MCs (P = 0.004) and an increase in IENFs (all P < 0.027). The increase in IENFs was confirmed by the longitudinal study. Interestingly, the MC density inversely correlated with NfL level (r = -0.424, P = 0.012), and survival analysis revealed that low MC density, higher NfL levels and increasing IENF density over time were associated with a poorer prognosis (all P < 0.024). To summarize, in patients with ALS, peripheral sensory involvement worsens in parallel with motor disability. Furthermore, the correlation between skin innervation and disease activity may suggest the use of skin innervation as a putative prognostic biomarker.

FABP7 drives an inflammatory response in human astrocytes and is upregulated in Alzheimer's disease

Alzheimer's disease (AD), the most common cause of dementia in the elderly, is characterized by the accumulation of intracellular neurofibrillary tangles, extracellular amyloid plaques, and neuroinflammation. In partnership with microglial cells, astrocytes are key players in the regulation of neuroinflammation. Fatty acid binding protein 7 (FABP7) belongs to a family of conserved proteins that regulate lipid metabolism, energy homeostasis, and inflammation. FABP7 expression is largely restricted to astrocytes and radial glia-like cells in the adult central nervous system. We observed that treatment of primary hippocampal astrocyte cultures with amyloid β fragment 25-35 (Aβ25-35) induces FABP7 upregulation. In addition, FABP7 expression is upregulated in the brain of APP/PS1 mice, a widely used AD mouse model. Co-immunostaining with specific astrocyte markers revealed increased FABP7 expression in astrocytes. Moreover, astrocytes surrounding amyloid plaques displayed increased FABP7 staining when compared to non-plaque-associated astrocytes. A similar result was obtained in the brain of AD patients. Whole transcriptome RNA sequencing analysis of human astrocytes differentiated from induced pluripotent stem cells (i-astrocytes) overexpressing FABP7 identified 500 transcripts with at least a 2-fold change in expression. Gene Ontology enrichment analysis identified (i) positive regulation of cytokine production and (ii) inflammatory response as the top two statistically significant overrepresented biological processes. We confirmed that wild-type FABP7 overexpression induces an NF-κB-driven inflammatory response in human i-astrocytes. On the other hand, the expression of a ligand-binding impaired mutant FABP7 did not induce NF-κB activation. Together, our results suggest that the upregulation of FABP7 in astrocytes could contribute to the neuroinflammation observed in AD.

Astrocyte-Neuron Interactions Contributing to Amyotrophic Lateral Sclerosis Progression

Amyotrophic lateral sclerosis (ALS) is a complex disease impacting motor neurons of the brain, brainstem, and spinal cord. Disease etiology is quite heterogeneous with over 40 genes causing the disease and a vast ~90% of patients having no prior family history. Astrocytes are major contributors to ALS, particularly through involvement in accelerating disease progression. Through study of genetic forms of disease including SOD1, TDP43, FUS, C9orf72, VCP, TBK1, and more recently patient-derived cells from sporadic individuals, many biological mechanisms have been identified to cause intrinsic or glial-mediated neurotoxicity to motor neurons. Overall, many of the normally supportive and beneficial roles that astrocytes contribute to neuronal health and survival instead switch to become deleterious and neurotoxic. While the exact pathways may differ based on disease-origin, altered astrocyte-neuron communication is a common feature of ALS. Within this chapter, distinct genetic forms are examined in detail, along with what is known from sporadic patient-derived cells. Overall, this chapter highlights the interplay between astrocytes and neurons in this complex disease and describes the key features underlying: astrocyte-mediated motor neuron toxicity, excitotoxicity, oxidative/nitrosative stress, protein dyshomeostasis, metabolic imbalance, inflammation, trophic factor withdrawal, blood-brain/blood-spinal cord barrier involvement, disease spreading, and the extracellular matrix/cell adhesion/TGF-β signaling pathways.

Neuroprotective Effects and Therapeutic Potential of Dichloroacetate: Targeting Metabolic Disorders in Nervous System Diseases

Dichloroacetate (DCA) is an investigational drug used to treat lactic acidosis and malignant tumours. It works by inhibiting pyruvate dehydrogenase kinase and increasing the rate of glucose oxidation. Some studies have documented the neuroprotective benefits of DCA. By reviewing these studies, this paper shows that DCA has multiple pharmacological activities, including regulating metabolism, ameliorating oxidative stress, attenuating neuroinflammation, inhibiting apoptosis, decreasing autophagy, protecting the blood‒brain barrier, improving the function of endothelial progenitor cells, improving mitochondrial dynamics, and decreasing amyloid β-protein. In addition, DCA inhibits the enzyme that metabolizes it, which leads to peripheral neurotoxicity due to drug accumulation that may be solved by individualized drug delivery and nanovesicle delivery. In summary, in this review, we analyse the mechanisms of neuroprotection by DCA in different diseases and discuss the causes of and solutions to its adverse effects.

Astrocytes in Parkinson's Disease: From Role to Possible Intervention

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the loss of dopaminergic neurons. While neuronal dysfunction is central to PD, astrocytes also play important roles, both positive and negative, and such roles have not yet been fully explored. This literature review serves to highlight these roles and how the properties of astrocytes can be used to increase neuron survivability. Astrocytes normally have protective functions, such as releasing neurotrophic factors, metabolizing glutamate, transferring healthy mitochondria to neurons, or maintaining the blood-brain barrier. However, in PD, astrocytes can become dysfunctional and contribute to neurotoxicity, e.g., via impaired glutamate metabolism or the release of inflammatory cytokines. Therefore, astrocytes represent a double-edged sword. Restoring healthy astrocyte function and increasing the beneficial effects of astrocytes represents a promising therapeutic approach. Strategies such as promoting neurotrophin release, preventing harmful astrocyte reactivity, or utilizing regional astrocyte diversity may help restore neuroprotection.

Inhibitory effect of parthenolide on peripheral nerve degeneration

Traumatic axonal damage disrupts connections between neurons, leading to the loss of motor and sensory functions. Although damaged peripheral nerves can regenerate, recovery depends on the variety and severity of nerve damage. Thus, many phytochemicals have been studied for their ability to reduce peripheral nerve degeneration, and among them, Parthenolide (PTL), which is extracted from Feverfew has effects against production of free radicals, inflammation, and apoptosis. Thus, we conducted a study to investigate whether PTL has an inhibitory effect on peripheral nerve degeneration during peripheral nerve damage. To verify the effect of PTL on peripheral nerve degeneration process, a morphological comparison of peripheral nerves with and without PTL was performed. PTL significantly reduced the quantity of fragmented ovoid formations at 3DIV (days in vitro). Immunostaining for MBP revealed that the ratio of intact myelin sheaths increased significantly in sciatic nerve with PTL compared with absence of PTL at 3DIV. Furthermore, nerve fibers in the presence of PTL maintained the continuity of Neurofilament (NF) compared to those without at 3DIV. Immunostaining for LAMP1 and p75 NTR showed that the expression of LAMP1 and p75 NTR decreased in the nerve after PTL addition at 3DIV. Lastly, immunostaining for anti-Ki67 revealed that PTL inhibited Ki67 expression at 3DIV compared to without PTL. These results confirm that PTL inhibits peripheral nerve degenerative processes. PTL may be a good applicant to inhibit peripheral nerve degeneration. Our study examined the effect of Parthenolide in preventing degeneration of peripheral nerves by inhibiting the breakdown of peripheral axons and myelin, also inhibiting Schwann cell trans-dedifferentiation and proliferation.

Neurotrophic factors in the physiology of motor neurons and their role in the pathobiology and therapeutic approach to amyotrophic lateral sclerosis

The discovery of the neurotrophins and their potent survival and trophic effects led to great enthusiasm about their therapeutic potential to rescue dying neurons in neurodegenerative diseases. The further discovery that brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF) and glial cell line-derived neurotrophic factor (GDNF) had potent survival-promoting activity on motor neurons led to the proposal for their use in motor neuron diseases such as amyotrophic lateral sclerosis (ALS). In this review we synthesize the literature pertaining to the role of NGF, BDNF, CNTF and GDNF on the development and physiology of spinal motor neurons, as well as the preclinical studies that evaluated their potential for the treatment of ALS. Results from the clinical trials of these molecules will also be described and, with the aid of decades of hindsight, we will discuss what can reasonably be concluded and how this information can inform future clinical development of neurotrophic factors for ALS.

Assessing causal relationship between circulating cytokines and age-related neurodegenerative diseases: a bidirectional two-sample Mendelian randomization analysis

Numerous studies have reported that circulating cytokines (CCs) are linked to age-related neurodegenerative diseases (ANDDs); however, there is a lack of systematic investigation for the causal association. A two-sample bidirectional Mendelian Randomisation (MR) method was utilized to evaluate the causal effect. We applied genetic variants correlated with concentrations of CCs from a genome-wide association study meta-analysis (n = 8293) as instrumental variables. Summary data of three major ANDDs [Alzheimer's disease (AD), Parkinson's disease (PD), and Amyotrophic lateral sclerosis (ALS)] were identified from the IEU OpenGWAS platform (n = 627, 266). Inverse-variance weighted method is the main approach to analyse causal effect, and MR results are verified by several sensitivity and pleiotropy analyses. In directional MR, it suggested that several CCs were nominally correlated with the risk of ANDDs, with a causal odds ratio (OR) of Interleukin (IL)-5 of 0.909 for AD; OR of IL-2 of 1.169 for PD; and OR of Beta nerve growth factor of 1.142 for ALS). In reverse MR, there were some suggestively causal effects of ANDDs on CCs (AD on increased Basic fibroblast growth factor and IL-12 and decreased Stem cell growth factor beta; PD on decreased Monokine induced by interferon-gamma; ALS on decreased Basic fibroblast growth factor and IL-17). The findings were stable across sensitivity and pleiotropy analyses. However, after Bonferroni correction, there is no statistically significant association between CCs and ANDDs. Through the genetic epidemiological approach, our study assessed the role and presented possible causal associations between CCs and ANDDs. Further studies are warranted to verify the causal associations.

Nitric Oxide/Nitric Oxide Synthase System in the Pathogenesis of Neurodegenerative Disorders-An Overview

Nitric oxide, a ubiquitous molecule found throughout the natural world, is a key molecule implicated in many central and benefic molecular pathways and has a well-established role in the function of the central nervous system, as numerous studies have previously shown. Dysregulation of its metabolism, mainly the upregulation of nitric oxide production, has been proposed as a trigger and/or aggravator for many neurological affections. Increasing evidence supports the implication of this molecule in prevalent neurodegenerative diseases, such as Parkinson's disease, Alzheimer's disease, or amyotrophic lateral sclerosis. The mechanisms proposed for its neurotoxicity mainly center around the increased quantities of nitric oxide that are produced in the brain, their cause, and, most importantly, the pathological metabolic cascades created. These cascades lead to the formation of neuronal toxic substances that impair the neurons' function and structure on multiple levels. The purpose of this review is to present the main causes of increased pathological production, as well as the most important pathophysiological mechanisms triggered by nitric oxide, mechanisms that could help explain a part of the complex picture of neurodegenerative diseases and help develop targeted therapies.

Involvement of Lipids in the Pathogenesis of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the progressive degeneration of upper and lower motor neurons. To study its underlying mechanisms, a variety of models are currently used at the cellular level and in animals with mutations in multiple ALS associated genes, including SOD1, C9ORF72, TDP-43, and FUS. Key mechanisms involved in the disease include excitotoxicity, oxidative stress, mitochondrial dysfunction, neuroinflammatory, and immune reactions. In addition, significant metabolism alterations of various lipids classes, including phospholipids, fatty acids, sphingolipids, and others have been increasingly recognized. Recently, the mechanisms of programmed cell death (apoptosis), which may be responsible for the degeneration of motor neurons observed in the disease, have been intensively studied. In this context, sphingolipids, which are the most important sources of secondary messengers transmitting signals for cell proliferation, differentiation, and apoptosis, are gaining increasing attention in the context of ALS pathogenesis given their role in the development of neuroinflammatory and immune responses. This review describes changes in lipids content and activity of enzymes involved in their metabolism in ALS, both summarizing current evidence from animal models and clinical studies and discussing the potential of new drugs among modulators of lipid metabolism enzymes.

SOD1G93A Astrocyte-Derived Extracellular Vesicles Induce Motor Neuron Death by a miRNA-155-5p-Mediated Mechanism

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by upper and lower motor neuron (MN) degeneration. Astrocytes surrounding MNs are known to modulate ALS progression. When cocultured with astrocytes overexpressing the ALS-linked mutant Cu/Zn superoxide dismutase (SOD1G93A) or when cultured with conditioned medium from SOD1G93A astrocytes, MN survival is reduced. The exact mechanism of this neurotoxic effect is unknown. Astrocytes secrete extracellular vesicles (EVs) that transport protein, mRNA, and microRNA species from one cell to another. The size and protein markers characteristic of exosomes were observed in the EVs obtained from cultured astrocytes, indicating their abundance in exosomes. Here, we analyzed the microRNA content of the exosomes derived from SOD1G93A astrocytes and evaluated their role in MN survival. Purified MNs exposed to SOD1G93A astrocyte-derived exosomes showed reduced survival and neurite length compared to those exposed to exosomes derived from non-transgenic (non-Tg) astrocytes. Analysis of the miRNA content of the exosomes revealed that miR-155-5p and miR-582-3p are differentially expressed in SOD1G93A exosomes compared with exosomes from non-Tg astrocytes. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis indicates that miR-155-5p and miR-582-3p predicted targets are enriched in the neurotrophin signaling pathway. Importantly, when levels of miR-155-5p were reduced by incubation with a specific antagomir, SOD1G93A exosomes did not affect MN survival or neurite length. These results demonstrate that SOD1G93A-derived exosomes are sufficient to induce MN death, and miRNA-155-5p contributes to this effect. miRNA-155-5p may offer a new therapeutic target to modulate disease progression in ALS.

The contribution of an imbalanced redox signalling to neurological and neurodegenerative conditions

Nitric oxide and other redox active molecules such as oxygen free radicals provide essential signalling in diverse neuronal functions, but their excess production and insufficient scavenging induces cytotoxic redox stress which is associated with numerous neurodegenerative and neurological conditions. A further component of redox signalling is mediated by a homeostatic regulation of divalent metal ions, the imbalance of which contributes to neuronal dysfunction. Additional antioxidant molecules such as glutathione and enzymes such as super oxide dismutase are involved in maintaining a physiological redox status within neurons. When cellular processes are perturbed and generation of free radicals overwhelms the antioxidants capacity of the neurons, a resulting redox damage leads to neuronal dysfunction and cell death. Cellular sources for production of redox-active molecules may include NADPH oxidases, mitochondria, cytochrome P450 and nitric oxide (NO)-generating enzymes, such as endothelial, neuronal and inducible NO synthases. Several neurodegenerative and developmental neurological conditions are associated with an imbalanced redox state as a result of neuroinflammatory processes leading to nitrosative and oxidative stress. Ongoing research aims at understanding the causes and consequences of such imbalanced redox homeostasis and its role in neuronal dysfunction.

Remodeling of astrocyte secretome in amyotrophic lateral sclerosis: uncovering novel targets to combat astrocyte-mediated toxicity

Amyotrophic lateral sclerosis (ALS) is an adult-onset paralytic disease characterized by progressive degeneration of upper and lower motor neurons in the motor cortex, brainstem and spinal cord. Motor neuron degeneration is typically caused by a combination of intrinsic neuronal (cell autonomous) defects as well as extrinsic (non-cell autonomous) factors such as astrocyte-mediated toxicity. Astrocytes are highly plastic cells that react to their microenvironment to mediate relevant responses. In neurodegeneration, astrocytes often turn reactive and in turn secrete a slew of factors to exert pro-inflammatory and neurotoxic effects. Various efforts have been carried out to characterize the diseased astrocyte secretome over the years, revealing that pro-inflammatory chemokines, cytokines and microRNAs are the main players in mediating neuronal death. As metabolomic technologies mature, these studies begin to shed light on neurotoxic metabolites such as secreted lipids. In this focused review, we will discuss changes in the astrocyte secretome during ALS. In particular, we will discuss the components of the reactive astrocyte secretome that contribute to neuronal death in ALS.

P75 neurotrophin receptor as a therapeutic target for drug development to treat neurological diseases

The interaction of neurotrophins with their receptors is involved in the pathogenesis and progression of various neurological diseases, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, spinal cord injury and acute and chronic cerebral damage. The p75 neurotrophin receptor (p75NTR) plays a pivotal role in the development of neurological dysfunctions as a result of its high expression, abnormal processing and signalling. Therefore, p75NTR represents as a vital therapeutic target for the treatment of neurodegeneration, neuropsychiatric disorders and cerebrovascular insufficiency. This review summarizes the current research progress on the p75NTR signalling in neurological deficits. We also summarize the present therapeutic approaches by genetically and pharmacologically targeting p75NTR for the attenuation of pathological changes. Based on the evolving knowledge, the role of p75NTR in the regulation of tau hyperphosphorylation, Aβ metabolism, the degeneration of motor neurons and dopaminergic neurons has been discussed. Its position as a biomarker to evaluate the severity of diseases and as a druggable target for drug development has also been elucidated. Several prototype small molecule compounds were introduced to be crucial in neuronal survival and functional recovery via targeting p75NTR. These small molecule compounds represent desirable agents in attenuating neurodegeneration and cell death as they abolish activation-induced neurotoxicity of neurotrophins via modulating p75NTR signalling. More comprehensive and in-depth investigations on p75NTR-based drug development are required to shed light on effective treatment of numerous neurological disorders.

Astrocytes and major depression: The purinergic avenue

Major depressive disorder (MDD) is one of the most prevalent psychiatric illnesses worldwide which impairs the social functioning of the afflicted patients. Astrocytes promote homeostasis of the CNS and provide defense against various types of harmful influences. Increasing evidence suggests that the number, morphology and function of astrocytes are deteriorated in the depressed brain and the malfunction of the astrocytic purinergic system appears to participate in the pathophysiology of MDD. Adenosine 5'-triphosphate (ATP) released from astrocytes modulates depressive-like behavior in animal models and probably also clinical depression in patients. Astrocytes possess purinergic receptors, such as adenosine A_2A receptors (Rs), and P2X7, P2Y₁, and P2Y₁₁Rs, which mediate neuroinflammation, neuro(glio)transmission, and synaptic plasticity in depression-relevant areas of the brain (e.g. medial prefrontal cortex, hippocampus, amygdala nuclei). By contrast, astrocytic A₁Rs are neuroprotective and immunosuppressive. In the present review, we shall discuss the release of purines from astrocytes, and the expression/function of astrocytic purinergic receptors. Subsequently, we shall review in more detail novel evidence indicating that the dysregulation of astrocytic purinergic signaling actively contributes to the pathophysiology of depression and shall discuss possible therapeutic options based on knowledge recently acquired in this field.

Structure and Function of the Mammalian Neuromuscular Junction

The mammalian neuromuscular junction (NMJ) comprises a presynaptic terminal, a postsynaptic receptor region on the muscle fiber (endplate), and the perisynaptic (terminal) Schwann cell. As with any synapse, the purpose of the NMJ is to transmit signals from the nervous system to muscle fibers. This neural control of muscle fibers is organized as motor units, which display distinct structural and functional phenotypes including differences in pre- and postsynaptic elements of NMJs. Motor units vary considerably in the frequency of their activation (both motor neuron discharge rate and duration/duty cycle), force generation, and susceptibility to fatigue. For earlier and more frequently recruited motor units, the structure and function of the activated NMJs must have high fidelity to ensure consistent activation and continued contractile response to sustain vital motor behaviors (e.g., breathing and postural balance). Similarly, for higher force less frequent behaviors (e.g., coughing and jumping), the structure and function of recruited NMJs must ensure short-term reliable activation but not activation sustained for a prolonged period in which fatigue may occur. The NMJ is highly plastic, changing structurally and functionally throughout the life span from embryonic development to old age. The NMJ also changes under pathological conditions including acute and chronic disease. Such neuroplasticity often varies across motor unit types. © 2022 American Physiological Society. Compr Physiol 12:1-36, 2022.

The Interplay Between Neuroinfections, the Immune System and Neurological Disorders: A Focus on Africa

Neurological disorders related to neuroinfections are highly prevalent in Sub-Saharan Africa (SSA), constituting a major cause of disability and economic burden for patients and society. These include epilepsy, dementia, motor neuron diseases, headache disorders, sleep disorders, and peripheral neuropathy. The highest prevalence of human immunodeficiency virus (HIV) is in SSA. Consequently, there is a high prevalence of neurological disorders associated with HIV infection such as HIV-associated neurocognitive disorders, motor disorders, chronic headaches, and peripheral neuropathy in the region. The pathogenesis of these neurological disorders involves the direct role of the virus, some antiretroviral treatments, and the dysregulated immune system. Furthermore, the high prevalence of epilepsy in SSA (mainly due to perinatal causes) is exacerbated by infections such as toxoplasmosis, neurocysticercosis, onchocerciasis, malaria, bacterial meningitis, tuberculosis, and the immune reactions they elicit. Sleep disorders are another common problem in the region and have been associated with infectious diseases such as human African trypanosomiasis and HIV and involve the activation of the immune system. While most headache disorders are due to benign primary headaches, some secondary headaches are caused by infections (meningitis, encephalitis, brain abscess). HIV and neurosyphilis, both common in SSA, can trigger long-standing immune activation in the central nervous system (CNS) potentially resulting in dementia. Despite the progress achieved in preventing diseases from the poliovirus and retroviruses, these microbes may cause motor neuron diseases in SSA. The immune mechanisms involved in these neurological disorders include increased cytokine levels, immune cells infiltration into the CNS, and autoantibodies. This review focuses on the major neurological disorders relevant to Africa and neuroinfections highly prevalent in SSA, describes the interplay between neuroinfections, immune system, neuroinflammation, and neurological disorders, and how understanding this can be exploited for the development of novel diagnostics and therapeutics for improved patient care.

Motoneuron Diseases

Motoneuron diseases (MNDs) represent a heterogeneous group of progressive paralytic disorders, mainly characterized by the loss of upper (corticospinal) motoneurons, lower (spinal) motoneurons or, often both. MNDs can occur from birth to adulthood and have a highly variable clinical presentation, even within gene-positive forms, suggesting the existence of environmental and genetic modifiers. A combination of cell autonomous and non-cell autonomous mechanisms contributes to motoneuron degeneration in MNDs, suggesting multifactorial pathogenic processes.

Therapeutic Potential of Astrocyte Transplantation

Cell transplantation is an attractive treatment strategy for a variety of brain disorders, as it promises to replenish lost functions and rejuvenate the brain. In particular, transplantation of astrocytes has come into light recently as a therapy for amyotrophic lateral sclerosis (ALS); moreover, grafting of astrocytes also showed positive results in models of other conditions ranging from neurodegenerative diseases of older age to traumatic injury and stroke. Despite clear differences in etiology, disorders such as ALS, Parkinson's, Alzheimer's, and Huntington's diseases, as well as traumatic injury and stroke, converge on a number of underlying astrocytic abnormalities, which include inflammatory changes, mitochondrial damage, calcium signaling disturbance, hemichannel opening, and loss of glutamate transporters. In this review, we examine these convergent pathways leading to astrocyte dysfunction, and explore the existing evidence for a therapeutic potential of transplantation of healthy astrocytes in various models. Existing literature presents a wide variety of methods to generate astrocytes, or relevant precursor cells, for subsequent transplantation, while described outcomes of this type of treatment also differ between studies. We take technical differences between methodologies into account to understand the variability of therapeutic benefits, or lack thereof, at a deeper level. We conclude by discussing some key requirements of an astrocyte graft that would be most suitable for clinical applications.

Urinary p75ECD levels in patients with amyotrophic lateral sclerosis: a meta-analysis

Objective: p75 neurotrophin receptor (p75^NTR) is associated with the pathogenesis of amyotrophic lateral sclerosis (ALS). However, its role is not fully understood. The aim of this study was to evaluate the association between ALS and the extracellular domain of p75^NTR(p75^ECD) in urine. Methods: We conducted a comprehensive literature search using keywords in the PubMed, Embase, Science, and the Cochrane Library, and identified five case control studies, with the latest date of search being 18 April 2021. Results: The results showed that urinary p75^ECD levels were significantly higher in patients with ALS compared to non-neurological control (weighted mean difference (WMD) = 4.18, 95% CI [2.525, 6.990], p < 0.001), and other neurological diseases (WMD = 6.005, 95% CI [1.596, 10.414], p = 0.008). Increased urinary p75^ECD levels were inversely associated with ALSFRS-R in ALS patients (r = -0.32, 95% CI [-0.43, -0.21], p < 0.001). Conclusions: Given the associations between p75^ECD and ALS found in this meta-analysis, urinary p75^ECD levels have potential to be used as a diagnostic biomarker and a progression indicator in the future.

Streptococcal meningitis reveals the presence of residual streptococci and down-regulated aquaporin 4 in the brain

The pathology of streptococcal meningitis is poorly understood, even though streptococcal infection induces meningitis. The aim of this study was to clarify the relationship between streptococcal meningitis and aquaporin 4 (AQP4) in the mouse brain. After Streptococcus suis infection, the streptococcal number was calculated, and AQP4 mRNA expression in the brain was quantified at 2 and 7 days after infection. At 7-day post-infection, mice with neurological symptoms showed significantly higher S. suis levels in the brain than mice without neurological symptoms. AQP4 expression was significantly decreased in mice with neurological symptoms than in mice without neurological symptoms. Image analysis demonstrated that S. suis progressed to invade the white matter. Pathological analysis revealed that infected mouse brains had higher inflammation and neurological damage scores than uninfected mouse brains. Therefore, mice with neurological symptoms caused by streptococcal meningitis had high S. suis levels in the brain and reduced AQP4 expression.

[The role of sphingolipids in pathogenesis of amyotrophic lateral sclerosis]

Amyotrophic lateral sclerosis (ALS) is an incurable neurodegenerative disease characterized by selective degeneration of motor neurons of the spinal cord and motor cortex and brain stem. The key features of the course of this disease are excitotoxicity, oxidative stress, mitochondrial dysfunction, neuro-inflammatory and immune reactions. Recently, the mechanisms of programmed cell death (apoptosis), which may be responsible for the degeneration of motor neurons in this disease, have been intensively studied. In this regard, sphingolipids, which are the most important sources of secondary messengers that transmit cell proliferation, differentiation and apoptosis signals, and are involved in the development of neuroinflammatory and immune responses, are of particular interest in the context of ALS pathogenesis. The review provides information from domestic and foreign authors on the involvement of various sphingolipids (sphingomyelins, ceramides, sphingosine, sphinganin, sphingosine-1-phosphate, galactosylceramides, glucosylceramides, gangliosides) in the development of pro-inflammatory reactions and apoptosis of motor neurons in ALS. The authors discuss the prospects of using new drugs that control the metabolism of sphingolipids for the treatment of ALS.

Is there a role for the p75 neurotrophin receptor in mediating degeneration during oxidative stress and after hypoxia?

Cholinergic basal forebrain (cBF) neurons are particularly vulnerable to degeneration following trauma and in neurodegenerative conditions. One reason for this is their characteristic expression of the p75 neurotrophin receptor (p75^NTR ), which is up-regulated and mediates neuronal death in a range of neurological and neurodegenerative conditions, including dementia, stroke and ischaemia. The signalling pathway by which p75^NTR signals cell death is incompletely characterised, but typically involves activation by neurotrophic ligands and signalling through c-Jun kinase, resulting in caspase activation via mitochondrial apoptotic signalling pathways. Less well appreciated is the link between conditions of oxidative stress and p75^NTR death signalling. Here, we review the literature describing what is currently known regarding p75^NTR death signalling in environments of oxidative stress and hypoxia to highlight the overlap in signalling pathways and the implications for p75^NTR signalling in cBF neurons. We propose that there is a causal relationship and define key questions to test this assertion.

Non-cell-autonomous pathogenic mechanisms in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is the most common adult-onset paralytic disorder, characterized mainly by a loss of motor neurons (MNs) in the CNS. Over the past decades, thanks to intense investigations performed in both in vivo and in vitro models of ALS, major progress has been made toward gaining insights into the pathobiology of this incurable, fatal disorder. Among these advances is the growing recognition that non-neuronal cells participate in the degeneration of MNs in ALS, which could transform our understanding of the neurobiology of disease and the ability to devise effective disease-modifying therapies. In this review, we examine the contribution of non-cell-autonomous processes to the pathogenesis of ALS, with a focus on glial cells and in particular on astrocytes.

Evidence of p75 Neurotrophin Receptor Involvement in the Central Nervous System Pathogenesis of Classical Scrapie in Sheep and a Transgenic Mouse Model

Neurotrophins constitute a group of growth factor that exerts important functions in the nervous system of vertebrates. They act through two classes of transmembrane receptors: tyrosine-kinase receptors and the p75 neurotrophin receptor (p75^NTR). The activation of p75^NTR can favor cell survival or apoptosis depending on diverse factors. Several studies evidenced a link between p75^NTR and the pathogenesis of prion diseases. In this study, we investigated the distribution of several neurotrophins and their receptors, including p75^NTR, in the brain of naturally scrapie-affected sheep and experimentally infected ovinized transgenic mice and its correlation with other markers of prion disease. No evident changes in infected mice or sheep were observed regarding neurotrophins and their receptors except for the immunohistochemistry against p75^NTR. Infected mice showed higher abundance of p75^NTR immunostained cells than their non-infected counterparts. The astrocytic labeling correlated with other neuropathological alterations of prion disease. Confocal microscopy demonstrated the co-localization of p75^NTR and the astrocytic marker GFAP, suggesting an involvement of astrocytes in p75^NTR-mediated neurodegeneration. In contrast, p75^NTR staining in sheep lacked astrocytic labeling. However, digital image analyses revealed increased labeling intensities in preclinical sheep compared with non-infected and terminal sheep in several brain nuclei. This suggests that this receptor is overexpressed in early stages of prion-related neurodegeneration in sheep. Our results confirm a role of p75^NTR in the pathogenesis of classical ovine scrapie in both the natural host and in an experimental transgenic mouse model.

Functional Importance of Transient Receptor Potential (TRP) Channels in Neurological Disorders

Transient receptor potential (TRP) channels are transmembrane protein complexes that play important roles in the physiology and pathophysiology of both the central nervous system (CNS) and the peripheral nerve system (PNS). TRP channels function as non-selective cation channels that are activated by several chemical, mechanical, and thermal stimuli as well as by pH, osmolarity, and several endogenous or exogenous ligands, second messengers, and signaling molecules. On the pathophysiological side, these channels have been shown to play essential roles in the reproductive system, kidney, pancreas, lung, bone, intestine, as well as in neuropathic pain in both the CNS and PNS. In this context, TRP channels have been implicated in several neurological disorders, including Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis, and epilepsy. Herein, we focus on the latest involvement of TRP channels, with a special emphasis on the recently identified functional roles of TRP channels in neurological disorders related to the disruption in calcium ion homeostasis.

Neuroimmune crosstalk and evolving pharmacotherapies in neurodegenerative diseases

Neurodegeneration is characterized by gradual onset and limited availability of specific biomarkers. Apart from various aetiologies such as infection, trauma, genetic mutation, the interaction between the immune system and CNS is widely associated with neuronal damage in neurodegenerative diseases. The immune system plays a distinct role in disease progression and cellular homeostasis. It induces cellular and humoral responses, and enables tissue repair, cellular healing and clearance of cellular detritus. Aberrant and chronic activation of the immune system can damage healthy neurons. The pro-inflammatory mediators secreted by chief innate immune components, the complement system, microglia and inflammasome can augment cytotoxicity. Furthermore, these inflammatory mediators accelerate microglial activation resulting in progressive neuronal loss. Various animal studies have been carried out to unravel the complex pathology and ascertain biomarkers for these harmful diseases, but have had limited success. The present review will provide a thorough understanding of microglial activation, complement system and inflammasome generation, which lead the healthy brain towards neurodegeneration. In addition to this, possible targets of immune components to confer a strategic treatment regime for the alleviation of neuronal damage are also summarized.

Glial Metabolic Reprogramming in Amyotrophic Lateral Sclerosis

ALS is a human neurodegenerative disorder that induces a progressive paralysis of voluntary muscles due to motor neuron loss. The causes are unknown, and there is no curative treatment available. Mitochondrial dysfunction is a hallmark of ALS pathology; however, it is currently unknown whether it is a cause or a consequence of disease progression. Recent evidence indicates that glial mitochondrial function changes to cope with energy demands and critically influences neuronal death and disease progression. Aberrant glial cells detected in the spinal cord of diseased animals are characterized by increased proliferation rate and reduced mitochondrial bioenergetics. These features can be compared with cancer cell behavior of adapting to nutrient microenvironment by altering energy metabolism, a concept known as metabolic reprogramming. We focus on data that suggest that aberrant glial cells in ALS undergo metabolic reprogramming and profound changes in glial mitochondrial activity, which are associated with motor neuron death in ALS. This review article emphasizes on the association between metabolic reprogramming and glial reactivity, bringing new paradigms from the area of cancer research into neurodegenerative diseases. Targeting glial mitochondrial function and metabolic reprogramming may result in promising therapeutic strategies for ALS.

How Degeneration of Cells Surrounding Motoneurons Contributes to Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurological disorder characterized by the progressive degeneration of upper and lower motoneurons. Despite motoneuron death being recognized as the cardinal event of the disease, the loss of glial cells and interneurons in the brain and spinal cord accompanies and even precedes motoneuron elimination. In this review, we provide striking evidence that the degeneration of astrocytes and oligodendrocytes, in addition to inhibitory and modulatory interneurons, disrupt the functionally coherent environment of motoneurons. We discuss the extent to which the degeneration of glial cells and interneurons also contributes to the decline of the motor system. This pathogenic cellular network therefore represents a novel strategic field of therapeutic investigation.

Nerve Growth Factor is a Potential Treated Target in Tg(SOD1*G93A)1Gur Mice

Nerve growth factor (NGF) is a protective factor of neural cells; the possible relationship between the NGF and the pathogenesis of amyotrophic lateral sclerosis (ALS) hasn't been completely known. In this study, we observed and analyzed the expression and distribution of NGF, as well as the possible relationship between the NGF expression and distribution and the neural cell death in both SOD1 wild-type (WT) and Tg(SOD1*G93A)1Gur (TG) mice applying the fluorescence immunohistochemistry method. The results showed that the expression and distribution of NGF in the anterior horn (AH), the lateral horn (LH), and the surrounding central canal (CC) significantly increased at the supper early stage of ALS (Pre-onset stage) and the early stage (Onset stage), but the NGF expression and distribution in the AH, the LH, and the surrounding CC significantly reduced at the progression stage. The astrocyte, neuron, and oligodendrocyte produced the NGF and the neural precursor cells (NPCs) produced the NGF. The neural cell death gradually increased accompanying with the reduction of NGF expression and distribution. Our data suggested that the NGF was a protective factor of neural cells, because the neural cells in the AH, the LH, and the surrounding CC produced more NGF at the supper early and early stage of ALS; moreover, the NPCs produced the NGF. It implied that the NGF exerted the protective effect of neural cells, prevented from the neural cell death and aroused the potential of self-repair in the development of ALS.

Systematic elucidation of neuron-astrocyte interaction in models of amyotrophic lateral sclerosis using multi-modal integrated bioinformatics workflow

Cell-to-cell communications are critical determinants of pathophysiological phenotypes, but methodologies for their systematic elucidation are lacking. Herein, we propose an approach for the Systematic Elucidation and Assessment of Regulatory Cell-to-cell Interaction Networks (SEARCHIN) to identify ligand-mediated interactions between distinct cellular compartments. To test this approach, we selected a model of amyotrophic lateral sclerosis (ALS), in which astrocytes expressing mutant superoxide dismutase-1 (mutSOD1) kill wild-type motor neurons (MNs) by an unknown mechanism. Our integrative analysis that combines proteomics and regulatory network analysis infers the interaction between astrocyte-released amyloid precursor protein (APP) and death receptor-6 (DR6) on MNs as the top predicted ligand-receptor pair. The inferred deleterious role of APP and DR6 is confirmed in vitro in models of ALS. Moreover, the DR6 knockdown in MNs of transgenic mutSOD1 mice attenuates the ALS-like phenotype. Our results support the usefulness of integrative, systems biology approach to gain insights into complex neurobiological disease processes as in ALS and posit that the proposed methodology is not restricted to this biological context and could be used in a variety of other non-cell-autonomous communication mechanisms.

Biomimetic Approaches for Separated Regeneration of Sensory and Motor Fibers in Amputee People: Necessary Conditions for Functional Integration of Sensory-Motor Prostheses With the Peripheral Nerves

The regenerative capacity of the peripheral nervous system after an injury is limited, and a complete function is not recovered, mainly due to the loss of nerve tissue after the injury that causes a separation between the nerve ends and to the disorganized and intermingled growth of sensory and motor nerve fibers that cause erroneous reinnervations. Even though the development of biomaterials is a very promising field, today no significant results have been achieved. In this work, we study not only the characteristics that should have the support that will allow the growth of nerve fibers, but also the molecular profile necessary for a specific guidance. To do this, we carried out an exhaustive study of the molecular profile present during the regeneration of the sensory and motor fibers separately, as well as of the effect obtained by the administration and inhibition of different factors involved in the regeneration. In addition, we offer a complete design of the ideal characteristics of a biomaterial, which allows the growth of the sensory and motor neurons in a differentiated way, indicating (1) size and characteristics of the material; (2) necessity to act at the microlevel, on small groups of neurons; (3) combination of molecules and specific substrates; and (4) temporal profile of those molecules expression throughout the regeneration process. The importance of the design we offer is that it respects the complexity and characteristics of the regeneration process; it indicates the appropriate temporal conditions of molecular expression, in order to obtain a synergistic effect; it takes into account the importance of considering the process at the group of neuron level; and it gives an answer to the main limitations in the current studies.

CDK5 Targeting as a Therapy for Recovering Neurovascular Unit Integrity in Alzheimer's Disease

The neurovascular unit (NVU) is responsible for synchronizing the energetic demand, vasodynamic changes, and neurochemical and electrical function of the brain through a closed and interdependent interaction of cell components conforming to brain tissue. In this review, we will focus on cyclin-dependent kinase 5 (CDK5) as a molecular pivot, which plays a crucial role in the healthy function of neurons, astrocytes, and the endothelium and is implicated in the cross-talk of cellular adhesion signaling, ion transmission, and cytoskeletal remodeling, thus allowing the individual and interconnected homeostasis of cerebral parenchyma. Then, we discuss how CDK5 overactivation affects the integrity of the NVU in Alzheimer's disease (AD) and cognitive impairment; we emphasize how CDK5 is involved in the excitotoxicity spreading of glutamate and Ca2+ imbalance under acute and chronic injury. Additionally, we present pharmacological and gene therapy strategies for producing partial depletion of CDK5 activity on neurons, astrocytes, or endothelium to recover neuroplasticity and neurotransmission, suggesting that the NVU should be the targeted tissue unit in protective strategies. Finally, we conclude that CDK5 could be effective due to its intervention on astrocytes by its end feet on the endothelium and neurons, acting as an intermediary cell between systemic and central communication in the brain. This review provides integrated guidance regarding the pathogenesis of and potential repair strategies for AD.

Effects of RAGE inhibition on the progression of the disease in hSOD1G93A ALS mice

Astrocytes play a key role in the progression of amyotrophic lateral sclerosis (ALS) by actively inducing the degeneration of motor neurons. Motor neurons isolated from receptor for advanced glycation end products (RAGE)-knockout mice are resistant to the neurotoxic signal derived from ALS-astrocytes. Here, we confirmed that in a co-culture model, the neuronal death induced by astrocytes over-expressing the ALS-linked mutant hSOD1G93A is prevented by the addition of the RAGE inhibitors FPS-ZM1 or RAP. These inhibitors also prevented the motor neuron death induced by spinal cord extracts from symptomatic hSOD1G93A mice. To evaluate the relevance of this neurotoxic mechanism in ALS pathology, we assessed the therapeutic potential of FPS-ZM1 in hSOD1G93A mice. FPS-ZM1 treatment significantly improved hind-limb grip strength in hSOD1G93A mice during the progression of the disease, reduced the expression of atrophy markers in the gastrocnemius muscle, improved the survival of large motor neurons, and reduced gliosis in the ventral horn of the spinal cord. However, we did not observe a statistically significant effect of the drug in symptoms onset nor in the survival of hSOD1G93A mice. Maintenance of hind-limb grip strength was also observed in hSOD1G93A mice with RAGE haploinsufficiency [hSOD1G93A ;RAGE(+/-)], further supporting the beneficial effect of RAGE inhibition on muscle function. However, no benefits were observed after complete RAGE ablation. Moreover, genetic RAGE ablation significantly shortened the median survival of hSOD1G93A mice. These results indicate that the advance of new therapies targeting RAGE in ALS demands a better understanding of its physiological role in a cell type/tissue-specific context.

Nerve growth factor receptor role on rabbit sperm storage

The influence of NGF in male reproduction in some animal species and humans has already been assessed. Many of these effects are mediated by the distribution and abundance of tropomyosin receptor kinase A (TrKA) and p75 neurotrophin (p75NTR) receptors on sperm cells. The aim of this research was to investigate the role of NGF and its receptors, TrKA and p75NTR, in rabbit sperm outcomes during in vitro storage. Major semen traits (kinetic parameters, apoptotic, necrotic and live sperm) were recorded in rabbit semen samples from 0 to 12 h of storage (every 4 h). Three experimental hypotheses were formulated: i) sperm storage changes NGF receptor abundance in rabbit sperm; ii) TrKA and p75NTR differently modulate NGF signalling (assessed by the neutralisation of receptors); iii) NGF-receptor interactions show different responses during storage (evaluated by the addition of exogenous NGF). The results demonstrate that: (i) the receptor number changed in a time-dependent manner with a significant increase in p75NTR after 8-12 h of storage; ii) the neutralisation of NGF receptors largely affected VCL, apoptotic, necrotic and live cells during sperm storage, i.e. blockade of TrKA significantly increased speed, capacitation, necrosis and apoptosis, whereas blockade of p75NTR improved motility and live cells; iii) the addition of exogenous human NGF (100 ng/mL) at different time points of storage (0, 4, 8 h) differently influenced sperm traits i.e. NGF addition at time 0 positively affected all the pro-vital traits (kinetic, live cells) whereas, after 4-8 h, the effect of NGF was null or negative. In conclusion, NGF affects kinetic and other physiological traits (capacitation, apoptosis and necrosis) of rabbit sperm in a time-dependent manner. Most of these modifications are modulated by the receptors involved (TrKA or p75NTR), which changed considerably during sperm storage (increase of p75NTR).

Genetic targeting of astrocytes to combat neurodegenerative disease

Astrocytes, glial cells that interact extensively with neurons and other support cells throughout the central nervous system, have recently come under the spotlight for their potential contribution to, or potential regenerative role in a host of neurodegenerative disorders. It is becoming increasingly clear that astrocytes, in concert with microglial cells, activate intrinsic immunological pathways in the setting of neurodegenerative injury, although the direct and indirect consequences of such activation are still largely unknown. We review the current literature on the astrocyte’s role in several neurodegenerative diseases, as well as highlighting recent advances in genetic manipulation of astrocytes that may prove critical to modulating their response to neurological injury, potentially combatting neurodegenerative damage.

Examining the relationship between astrocyte dysfunction and neurodegeneration in ALS using hiPSCs

Amyotrophic lateral sclerosis (ALS) is a complex and fatal neurodegenerative disease for which the causes of disease onset and progression remain unclear. Recent advances in human induced pluripotent stem cell (hiPSC)-based models permit the study of the genetic factors associated with ALS in patient-derived neural cell types, including motor neurons and glia. While astrocyte dysfunction has traditionally been thought to exacerbate disease progression, astrocytic dysfunction may play a more direct role in disease initiation and progression. Such non-cell autonomous mechanisms expand the potential targets of therapeutic intervention, but only a handful of ALS risk-associated genes have been examined for their impact on astrocyte dysfunction and neurodegeneration. This review summarizes what is currently known about astrocyte function in ALS and suggests ways in which hiPSC-based models can be used to more effectively study the role of astrocytes in neurodegenerative disease.

Emergence of Microglia Bearing Senescence Markers During Paralysis Progression in a Rat Model of Inherited ALS

Age is a recognized risk factor for amyotrophic lateral sclerosis (ALS), a paralytic disease characterized by progressive loss of motor neurons and neuroinflammation. A hallmark of aging is the accumulation of senescent cells. Yet, the pathogenic role of cellular senescence in ALS remains poorly understood. In rats bearing the ALS-linked SOD1^G93A mutation, microgliosis contribute to motor neuron death, and its pharmacologic downregulation results in increased survival. Here, we have explored whether gliosis and motor neuron loss were associated with cellular senescence in the spinal cord during paralysis progression. In the lumbar spinal cord of symptomatic SOD1^G93A rats, numerous cells displayed nuclear p16^INK4a as well as loss of nuclear Lamin B1 expression, two recognized senescence-associated markers. The number of p16^INK4a-positive nuclei increased by four-fold while Lamin B1-negative nuclei increased by 1,2-fold, respect to non-transgenic or asymptomatic transgenic rats. p16^INK4a-positive nuclei and Lamin B1-negative nuclei were typically localized in a subset of hypertrophic Iba1-positive microglia, occasionally exhibiting nuclear giant multinucleated cell aggregates and abnormal nuclear morphology. Next, we analyzed senescence markers in cell cultures of microglia obtained from the spinal cord of symptomatic SOD1^G93A rats. Although microglia actively proliferated in cultures, a subset of them developed senescence markers after few days in vitro and subsequent passages. Senescent SOD1^G93A microglia in culture conditions were characterized by large and flat morphology, senescence-associated beta-Galactosidase (SA-β-Gal) activity as well as positive labeling for p16^INK4a, p53, matrix metalloproteinase-1 (MMP-1) and nitrotyrosine, suggesting a senescent-associated secretory phenotype (SASP). Remarkably, in the degenerating lumbar spinal cord other cell types, including ChAT-positive motor neurons and GFAP-expressing astrocytes, also displayed nuclear p16^INK4a staining. These results suggest that cellular senescence is closely associated with inflammation and motor neuron loss occurring after paralysis onset in SOD1^G93A rats. The emergence of senescent cells could mediate key pathogenic mechanisms in ALS.

Overview of Lipid Biomarkers in Amyotrophic Lateral Sclerosis (ALS)

Amyotrophic lateral sclerosis (ALS) is a multifactorial neurodegenerative disease involving motor neuron (MN) degeneration in the spinal cord, brain stem and primary motor cortex. The existence of inflammatory processes around MN and axonal degeneration in ALS has been shown. Unfortunately, none of the successful therapies in ALS animal models has improved clinical outcomes in patients with ALS. Therefore, the detection of blood biomarkers to be used as screening tools for disease onset and progression has been an expanding research area with few advances in the development of drugs for the treatment of ALS. In this review, we will address the available data analyzing regarding the relationship of lipid metabolism and lipid derived- products with ALS. We will address the advances on the studies about the role that lipids plays at the onset, progression and lifespan extension of ALS patients.

Phenotypic heterogeneity of astrocytes in motor neuron disease

Accumulating evidence has shown that astrocytes do not just support the function of neurons, but play key roles in maintaining the brain environment in health and disease. Contrary to the traditional understanding of astrocytes as static cells, reactive astrocytes possess more diverse functions and phenotypes than previously predicted. In the present focused review, we summarize the evidence showing that astrocytes are playing profound roles in the disease process of amyotrophic lateral sclerosis. Aberrantly activated astrocytes in amyotrophic lateral sclerosis rodents express microglial molecular markers and provoke toxicities to accelerate disease progression. In addition, TIR domain–containing adapter protein–inducing interferon‐β‐dependent innate immune pathway in astrocytes also has a novel function in terminating glial activation and neuroinflammation. Furthermore, heterogeneity in phenotypes and functions of astrocytes are also observed in various disease conditions, such as other neurodegenerative diseases, ischemia, aging and acute lesions in the central nervous system. Through accumulating knowledge of the phenotypic and functional diversity of astrocytes, these cells will become more attractive therapeutic targets for neurological diseases.

Nitration and Glycation Turn Mature NGF into a Toxic Factor for Motor Neurons: A Role for p75NTR and RAGE Signaling in ALS

INTRODUCTION

Glycating stress can occur together with oxidative stress during neurodegeneration and contribute to the pathogenic mechanism. Nerve growth factor (NGF) accumulates in several neurodegenerative diseases. Besides promoting survival, NGF can paradoxically induce cell death by signaling through the p75 neurotrophin receptor (p75^NTR). The ability of NGF to induce cell death is increased by nitration of its tyrosine residues under conditions associated with increased peroxynitrite formation.

AIMS

Here we investigated whether glycation also changes the ability of NGF to induce cell death and assessed the ability of post-translational modified NGF to signal through the receptor for advanced glycation end products (RAGEs). We also explored the potential role of RAGE-p75^NTR interaction in the motor neuron death occurring in amyotrophic lateral sclerosis (ALS) models.

RESULTS

Glycation promoted NGF oligomerization and ultimately allowed the modified neurotrophin to signal through RAGE and p75^NTR to induce motor neuron death at low physiological concentrations. A similar mechanism was observed for nitrated NGF. We provide evidence for the interaction of RAGE with p75^NTR at the cell surface. Moreover, we observed that post-translational modified NGF was present in the spinal cord of an ALS mouse model. In addition, NGF signaling through RAGE and p75^NTR was involved in astrocyte-mediated motor neuron toxicity, a pathogenic feature of ALS.

INNOVATION

Oxidative modifications occurring under stress conditions can enhance the ability of mature NGF to induce neuronal death at physiologically relevant concentrations, and RAGE is a new p75^NTR coreceptor contributing to this pathway.

CONCLUSION

Our results indicate that NGF-RAGE/p75^NTR signaling may be a therapeutic target in ALS. Antioxid. Redox Signal. 28, 1587-1602.

Diversity of astroglial responses across human neurodegenerative disorders and brain aging

Astrogliopathy refers to alterations of astrocytes occurring in diseases of the nervous system, and it implies the involvement of astrocytes as key elements in the pathogenesis and pathology of diseases and injuries of the central nervous system. Reactive astrocytosis refers to the response of astrocytes to different insults to the nervous system, whereas astrocytopathy indicates hypertrophy, atrophy/degeneration and loss of function and pathological remodeling occurring as a primary cause of a disease or as a factor contributing to the development and progression of a particular disease. Reactive astrocytosis secondary to neuron loss and astrocytopathy due to intrinsic alterations of astrocytes occur in neurodegenerative diseases, overlap each other, and, together with astrocyte senescence, contribute to disease-specific astrogliopathy in aging and neurodegenerative diseases with abnormal protein aggregates in old age. In addition to the well-known increase in glial fibrillary acidic protein and other proteins in reactive astrocytes, astrocytopathy is evidenced by deposition of abnormal proteins such as β-amyloid, hyper-phosphorylated tau, abnormal α-synuclein, mutated huntingtin, phosphorylated TDP-43 and mutated SOD1, and PrPres , in Alzheimer's disease, tauopathies, Lewy body diseases, Huntington's disease, amyotrophic lateral sclerosis and Creutzfeldt-Jakob disease, respectively. Astrocytopathy in these diseases can also be manifested by impaired glutamate transport; abnormal metabolism and release of neurotransmitters; altered potassium, calcium and water channels resulting in abnormal ion and water homeostasis; abnormal glucose metabolism; abnormal lipid and, particularly, cholesterol metabolism; increased oxidative damage and altered oxidative stress responses; increased production of cytokines and mediators of the inflammatory response; altered expression of connexins with deterioration of cell-to-cell networks and transfer of gliotransmitters; and worsening function of the blood brain barrier, among others. Increased knowledge of these aspects will permit a better understanding of brain aging and neurodegenerative diseases in old age as complex disorders in which neurons are not the only players.

Decreased Motor Neuron Support by SMA Astrocytes due to Diminished MCP1 Secretion

Spinal muscular atrophy (SMA) is an autosomal-recessive disorder characterized by severe, often fatal muscle weakness due to loss of motor neurons. SMA patients have deletions and other mutations of the survival of motor neuron 1 (SMN1) gene, resulting in decreased SMN protein. Astrocytes are the primary support cells of the CNS and are responsible for glutamate clearance, metabolic support, response to injury, and regulation of signal transmission. Astrocytes have been implicated in SMA as in in other neurodegenerative disorders. Astrocyte-specific rescue of SMN protein levels has been shown to mitigate disease manifestations in mice. However, the mechanism by which SMN deficiency in astrocytes may contribute to SMA is unclear and what aspect of astrocyte activity is lacking is unknown. Therefore, it is worthwhile to identify defects in SMN-deficient astrocytes that compromise normal function. We show here that SMA astrocyte cultures derived from mouse spinal cord of both sexes are deficient in supporting both WT and SMN-deficient motor neurons derived from male, female, and mixed-sex sources and that this deficiency may be mitigated with secreted factors. In particular, SMN-deficient astrocytes have decreased levels of monocyte chemoactive protein 1 (MCP1) secretion compared with controls and MCP1 restoration stimulates outgrowth of neurites from cultured motor neurons. Correction of MCP1 deficiency may thus be a new therapeutic approach to SMA.SIGNIFICANCE STATEMENT Spinal muscular atrophy (SMA) is caused by the loss of motor neurons, but astrocyte dysfunction also contributes to the disease in mouse models. Monocyte chemoactive protein 1 (MCP1) has been shown to be neuroprotective and is released by astrocytes. Here, we report that MCP1 levels are decreased in SMA mice and that replacement of deficient MCP1 increases differentiation and neurite length of WT and SMN-deficient motor-neuron-like cells in cell culture. This study reveals a novel aspect of astrocyte dysfunction in SMA and indicates a possible approach for improving motor neuron growth and survival in this disease.

Retrograde apoptotic signaling by the p75 neurotrophin receptor

Neurotrophins are target-derived factors necessary for mammalian nervous system development and maintenance. They are typically produced by neuronal target tissues and interact with their receptors at axonal endings. Therefore, locally generated neurotrophin signals must be conveyed from the axon back to the cell soma. Retrograde survival signaling by neurotrophin binding to Trk receptors has been extensively studied. However, neurotrophins also bind to the p75 receptor, which can induce apoptosis in a variety of contexts. Selective activation of p75 at distal axon ends has been shown to generate a retrograde apoptotic signal, although the mechanisms involved are poorly understood. The present review summarizes the available evidence for retrograde proapoptotic signaling in general and the role of the p75 receptor in particular, with discussion of unanswered questions in the field. In-depth knowledge of the mechanisms of retrograde apoptotic signaling is essential for understanding the etiology of neurodegeneration in many diseases and injuries.

The Function of FGFR1 Signalling in the Spinal Cord: Therapeutic Approaches Using FGFR1 Ligands after Spinal Cord Injury

Extensive research is ongoing that concentrates on finding therapies to enhance CNS regeneration after spinal cord injury (SCI) and to cure paralysis. This review sheds light on the role of the FGFR pathway in the injured spinal cord and discusses various therapies that use FGFR activating ligands to promote regeneration after SCI. We discuss studies that use peripheral nerve grafts or Schwann cell grafts in combination with FGF1 or FGF2 supplementation. Most of these studies show evidence that these therapies successfully enhance axon regeneration into the graft. Further they provide evidence for partial recovery of sensory function shown by electrophysiology and motor activity evidenced by behavioural data. We also present one study that indicates that combination with additional, synergistic factors might further drive the system towards functional regeneration. In essence, this review summarises the potential of nerve and cell grafts combined with FGF1/2 supplementation to improve outcome even after severe spinal cord injury.

Role and Therapeutic Potential of Astrocytes in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is characterized by the progressive degeneration of motor neurons in the spinal cord, brain stem, and motor cortex. The molecular mechanism underlying the progressive degeneration of motor neuron remains uncertain but involves a non-cell autonomous process. In acute injury or degenerative diseases astrocytes adopt a reactive phenotype known as astrogliosis. Astrogliosis is a complex remodeling of astrocyte biology and most likely represents a continuum of potential phenotypes that affect neuronal function and survival in an injury-specific manner. In ALS patients, reactive astrocytes surround both upper and lower degenerating motor neurons and play a key role in the pathology. It has become clear that astrocytes play a major role in ALS pathology. Through loss of normal function or acquired new characteristics, astrocytes are able to influence motor neuron fate and the progression of the disease. The use of different cell culture models indicates that ALS-astrocytes are able to induce motor neuron death by secreting a soluble factor(s). Here, we discuss several pathogenic mechanisms that have been proposed to explain astrocyte-mediated motor neuron death in ALS. In addition, examples of strategies that revert astrocyte-mediated motor neuron toxicity are reviewed to illustrate the therapeutic potential of astrocytes in ALS. Due to the central role played by astrocytes in ALS pathology, therapies aimed at modulating astrocyte biology may contribute to the development of integral therapeutic approaches to halt ALS progression.

Up-regulated fractalkine (FKN) and its receptor CX3CR1 are involved in fructose-induced neuroinflammation: Suppression by curcumin

Recent studies suggest that diet-induced fractalkine (FKN) stimulates neuroinflammation in animal models of obesity, yet how it occurs is unclear. This study investigated the role of FKN and it receptor, CX3CR1, in fructose-induced neuroinflammation, and examined curcumin's beneficial effect. Fructose feeding was found to induce hippocampal microglia activation with neuroinflammation through the activation of the Toll-like receptor 4 (TLR4)/nuclear transcription factor κB (NF-κB) signaling, resulting in the reduction of neurogenesis in the dentate gyrus (DG) of mice. Serum FKN levels, as well as hypothalamic FKN and CX3CR1 gene expression, were significantly increased in fructose-fed mice with hypothalamic microglia activation. Hippocampal gene expression of FKN and CX3CR1 was also up-regulated at 14d and normalized at 56d in mice fed with fructose, which were consistent with the change of GFAP. Furthermore, immunostaining showed that GFAP and FKN expression was increased in cornu amonis 1, but decreased in DG in fructose-fed mice. In vitro studies showed that GFAP and FKN expression was stimulated in astrocytes, and suppressed in mixed glial cells exposed to 48h-fructose, with the continual increase of pro-inflammatory cytokines. Thus, increased FKN and CX3CR1 may cause a cross-talk between activated glial cells and neurons, playing an important role in the development of neuroinflammation in fructose-fed mice. Curcumin protected against neuronal damage in hippocampal DG of fructose-fed mice by inhibiting microglia activation and suppressed FKN/CX3CR1 up-regulation in the neuronal network. These results suggest a new therapeutic approach to protect against neuronal damage associated with dietary obesity-associated neuroinflammation.