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

The role of cellular senescence in neurodegenerative diseases

Increasing evidence has revealed that cellular senescence drives NDs, including Alzheimer's disease (AD) and Parkinson's disease. Different senescent cell populations secrete senescence-associated secretory phenotypes (SASP), including matrix metalloproteinase-3, interleukin (IL)-1α, IL-6, and IL-8, which can harm adjacent microglia. Moreover, these cells possess high expression levels of senescence hallmarks (p16 and p21) and elevated senescence-associated β-galactosidase activity in in vitro and in vivo ND models. These senescence phenotypes contribute to the deposition of β-amyloid and tau-protein tangles. Selective clearance of senescent cells and SASP regulation by inhibiting p38/mitogen-activated protein kinase and nuclear factor kappa B signaling attenuate β-amyloid load and prevent tau-protein tangle deposition, thereby improving cognitive performance in AD mouse models. In addition, telomere shortening, a cellular senescence biomarker, is associated with increased ND risks. Telomere dysfunction causes cellular senescence, stimulating IL-6, tumor necrosis factor-α, and IL-1β secretions. The forced expression of telomerase activators prevents cellular senescence, yielding considerable neuroprotective effects. This review elucidates the mechanism of cellular senescence in ND pathogenesis, suggesting strategies to eliminate or restore senescent cells to a normal phenotype for treating such diseases.

Molecular hallmarks of ageing in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal, severely debilitating and rapidly progressing disorder affecting motor neurons in the brain, brainstem, and spinal cord. Unfortunately, there are few effective treatments, thus there remains a critical need to find novel interventions that can mitigate against its effects. Whilst the aetiology of ALS remains unclear, ageing is the major risk factor. Ageing is a slowly progressive process marked by functional decline of an organism over its lifespan. However, it remains unclear how ageing promotes the risk of ALS. At the molecular and cellular level there are specific hallmarks characteristic of normal ageing. These hallmarks are highly inter-related and overlap significantly with each other. Moreover, whilst ageing is a normal process, there are striking similarities at the molecular level between these factors and neurodegeneration in ALS. Nine ageing hallmarks were originally proposed: genomic instability, loss of telomeres, senescence, epigenetic modifications, dysregulated nutrient sensing, loss of proteostasis, mitochondrial dysfunction, stem cell exhaustion, and altered inter-cellular communication. However, these were recently (2023) expanded to include dysregulation of autophagy, inflammation and dysbiosis. Hence, given the latest updates to these hallmarks, and their close association to disease processes in ALS, a new examination of their relationship to pathophysiology is warranted. In this review, we describe possible mechanisms by which normal ageing impacts on neurodegenerative mechanisms implicated in ALS, and new therapeutic interventions that may arise from this.

IRF3 activates RB to authorize cGAS-STING-induced senescence and mitigate liver fibrosis

Cytosolic double-stranded DNA surveillance by cyclic GMP-AMP synthase (cGAS)-Stimulator of Interferon Genes (STING) signaling triggers cellular senescence, autophagy, biased mRNA translation, and interferon-mediated immune responses. However, detailed mechanisms and physiological relevance of STING-induced senescence are not fully understood. Here, we unexpectedly found that interferon regulatory factor 3 (IRF3), activated during innate DNA sensing, forms substantial endogenous complexes in the nucleus with retinoblastoma (RB), a key cell cycle regulator. The IRF3-RB interaction attenuates cyclin-dependent kinase 4/6 (CDK4/6)-mediated RB hyperphosphorylation that mobilizes RB to deactivate E2 family (E2F) transcription factors, thereby driving cells into senescence. STING-IRF3-RB signaling plays a notable role in hepatic stellate cells (HSCs) within various murine models, pushing activated HSCs toward senescence. Accordingly, IRF3 global knockout or conditional deletion in HSCs aggravated liver fibrosis, a process mitigated by the CDK4/6 inhibitor. These findings underscore a straightforward yet vital mechanism of cGAS-STING signaling in inducing cellular senescence and unveil its unexpected biology in limiting liver fibrosis.

Emerging role of senescent microglia in brain aging-related neurodegenerative diseases

Brain aging is a recognized risk factor for neurodegenerative diseases like Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS, Lou Gehrig's disease), but the intricate interplay between brain aging and the pathogenesis of these conditions remains inadequately understood. Cellular senescence is considered to contribute to cellular dysfunction and inflammaging. According to the threshold theory of senescent cell accumulation, the vulnerability to neurodegenerative diseases is associated with the rates of senescent cell generation and clearance within the brain. Given the role of microglia in eliminating senescent cells, the accumulation of senescent microglia may lead to the acceleration of brain aging, contributing to inflammaging and increased vulnerability to neurodegenerative diseases. In this review, we propose the idea that the senescence of microglia, which is notably vulnerable to aging, could potentially serve as a central catalyst in the progression of neurodegenerative diseases. The senescent microglia are emerging as a promising target for mitigating neurodegenerative diseases.

Cellular senescence in brain aging and neurodegeneration

Cellular senescence is a state of terminal cell cycle arrest associated with various macromolecular changes and a hypersecretory phenotype. In the brain, senescent cells naturally accumulate during aging and at sites of age-related pathologies. Here, we discuss the recent advances in understanding the accumulation of senescent cells in brain aging and disorders. Here we highlight the phenotypical heterogeneity of different senescent brain cell types, highlighting the potential importance of subtype-specific features for physiology and pathology. We provide a comprehensive overview of various senescent cell types in naturally occurring aging and the most common neurodegenerative disorders. Finally, we critically discuss the potential of adapting senotherapeutics to improve brain health and reduce pathological progression, addressing limitations and future directions for application and development.

C9orf72 poly(PR) mediated neurodegeneration is associated with nucleolar stress

The ALS/FTD-linked intronic hexanucleotide repeat expansion in the C9orf72 gene is aberrantly translated in the sense and antisense directions into dipeptide repeat proteins, among which poly proline-arginine (PR) displays the most aggressive neurotoxicity in-vitro and in-vivo. PR partitions to the nucleus when heterologously expressed in neurons and other cell types. We show that by lessening the nuclear accumulation of PR, we can drastically reduce its neurotoxicity. PR strongly accumulates in the nucleolus, a nuclear structure critical in regulating the cell stress response. We determined that, in neurons, PR caused nucleolar stress and increased levels of the transcription factor p53. Downregulating p53 levels also prevented PR-mediated neurotoxicity both in in-vitro and in-vivo models. We investigated if PR could induce the senescence phenotype in neurons. However, we did not observe any indications of such an effect. Instead, we found evidence for the induction of programmed cell death via caspase-3 activation.

Morphological screens using aged primary adult neuronal, microglial, and astrocytic cultures to find novel neurotherapeutics

The average age of a patient with neurotraumatic injuries or neurodegenerative diseases has been increasing worldwide. The preclinical live animal models used for neurotrauma and neurodegenerative diseases are typically young adults, failing to represent the age of humans in the clinic. This dichotomy in age between human populations and animal models is likely to impede the understanding of the pathological mechanisms of most neurological disorders and the translation of their respective promising therapies. This lack of cohesion between animal models and patients in the clinic begins prior to in vivo testing, it starts during the in vitro drug screening phase. Conventional screening methods typically involve the use of stem cell derived neural cells, with some researchers using embryonic derived neural cells instead. These cells lack the fundamental characteristics present in aged neural cells, such as age-induced changes in process length and branching in microglia and how astrocytes respond to various insults. Various technologies and techniques have been developed recently that can help researchers use age-appropriate neural cells for their drug discovery endeavors. The use of age-appropriate neural cells during screening phases is hypothesized to significantly increase the translation rate of the hits to the geriatric patients suffering from neurotraumatic and neurodegenerative diseases.

Cellular senescence and neurodegeneration

Advancing age is a major risk factor of Alzheimer's disease (AD). The worldwide prevalence of AD is approximately 50 million people, and this number is projected to increase substantially. The molecular mechanisms underlying the aging-associated susceptibility to cognitive impairment in AD are largely unknown. As a hallmark of aging, cellular senescence is a significant contributor to aging and age-related diseases including AD. Senescent neurons and glial cells have been detected to accumulate in the brains of AD patients and mouse models. Importantly, selective elimination of senescent cells ameliorates amyloid beta and tau pathologies and improves cognition in AD mouse models, indicating a critical role of cellular senescence in AD pathogenesis. Nonetheless, the mechanisms underlying when and how cellular senescence contributes to AD pathogenesis remain unclear. This review provides an overview of cellular senescence and discusses recent advances in the understanding of the impact of cellular senescence on AD pathogenesis, with brief discussions of the possible role of cellular senescence in other neurodegenerative diseases including Down syndrome, Parkinson's disease, multiple sclerosis, and amyotrophic lateral sclerosis.

Revisiting the neuroinflammation hypothesis in Alzheimer's disease: a focus on the druggability of current targets

Alzheimer's disease (AD) is the most common form of neurodegenerative disease and disability in the elderly; it is estimated to account for 60%-70% of all cases of dementia worldwide. The most relevant mechanistic hypothesis to explain AD symptoms is neurotoxicity induced by aggregated amyloid-β peptide (Aβ) and misfolded tau protein. These molecular entities are seemingly insufficient to explain AD as a multifactorial disease characterized by synaptic dysfunction, cognitive decline, psychotic symptoms, chronic inflammatory environment within the central nervous system (CNS), activated microglial cells, and dysfunctional gut microbiota. The discovery that AD is a neuroinflammatory disease linked to innate immunity phenomena started in the early nineties by several authors, including the ICC´s group that described, in 2004, the role IL-6 in AD-type phosphorylation of tau protein in deregulating the cdk5/p35 pathway. The "Theory of Neuroimmunomodulation", published in 2008, proposed the onset and progression of degenerative diseases as a multi-component "damage signals" phenomena, suggesting the feasibility of "multitarget" therapies in AD. This theory explains in detail the cascade of molecular events stemming from microglial disorder through the overactivation of the Cdk5/p35 pathway. All these knowledge have led to the rational search for inflammatory druggable targets against AD. The accumulated evidence on increased levels of inflammatory markers in the cerebrospinal fluid (CSF) of AD patients, along with reports describing CNS alterations caused by senescent immune cells in neuro-degenerative diseases, set out a conceptual framework in which the neuroinflammation hypothesis is being challenged from different angles towards developing new therapies against AD. The current evidence points to controversial findings in the search for therapeutic candidates to treat neuroinflammation in AD. In this article, we discuss a neuroimmune-modulatory perspective for pharmacological exploration of molecular targets against AD, as well as potential deleterious effects of modifying neuroinflammation in the brain parenchyma. We specifically focus on the role of B and T cells, immuno-senescence, the brain lymphatic system (BLS), gut-brain axis alterations, and dysfunctional interactions between neurons, microglia and astrocytes. We also outline a rational framework for identifying "druggable" targets for multi-mechanistic small molecules with therapeutic potential against AD.

Molecular mechanisms of cellular senescence in neurodegenerative diseases

Neurodegenerative diseases, such as Alzheimer's and Parkinson's, are characterized by several pathological features, including selective neuronal loss, aggregation of specific proteins, and chronic inflammation. Aging is the most critical risk factor of these disorders. However, the mechanism by which aging contributes to the pathogenesis of neurodegenerative diseases is not clearly understood. Cellular senescence is a cell state or fate in response to stimuli. It is typically associated with a series of changes in cellular phenotypes such as abnormal cellular metabolism and proteostasis, reactive oxygen species (ROS) production, and increased secretion of certain molecules via senescence-associated secretory phenotype (SASP). In this review, we discuss how cellular senescence contributes to brain aging and neurodegenerative diseases, and the relationship between protein aggregation and cellular senescence. Finally, we discuss the potential of senescence modifiers and senolytics in the treatment of neurodegenerative diseases.

Connecting the Dots: Neuronal Senescence, Stress Granules, and Neurodegeneration

Cellular senescence increases with aging. While senescence is associated with an exit of the cell cycle, there is ample evidence that post-mitotic cells including neurons can undergo senescence as the brain ages, and that senescence likely contributes significantly to the progression of neurodegenerative diseases (ND) such as Alzheimer's Disease (AD) and Amyotrophic Lateral Sclerosis (ALS). Stress granules (SGs) are stress-induced cytoplasmic biomolecular condensates of RNA and proteins, which have been linked to the development of AD and ALS. The SG seeding hypothesis of NDs proposes that chronic stress in aging neurons results in static SGs that progress into pathological aggregates Alterations in SG dynamics have also been linked to senescence, though studies that link SGs and senescence in the context of NDs and the aging brain have not yet been performed. In this Review, we summarize the literature on senescence, and explore the contribution of senescence to the aging brain. We describe senescence phenotypes in aging neurons and glia, and their links to neuroinflammation and the development of AD and ALS. We further examine the relationships of SGs to senescence and to ND. We propose a new hypothesis that neuronal senescence may contribute to the mechanism of SG seeding in ND by altering SG dynamics in aged cells, thereby providing additional aggregation opportunities within aged neurons.

Among Gerontogens, Heavy Metals Are a Class of Their Own: A Review of the Evidence for Cellular Senescence

Advancements in modern medicine have improved the quality of life across the globe and increased the average lifespan of our population by multiple decades. Current estimates predict by 2030, 12% of the global population will reach a geriatric age and live another 3-4 decades. This swelling geriatric population will place critical stress on healthcare infrastructures due to accompanying increases in age-related diseases and comorbidities. While much research focused on long-lived individuals seeks to answer questions regarding how to age healthier, there is a deficit in research investigating what aspects of our lives accelerate or exacerbate aging. In particular, heavy metals are recognized as a significant threat to human health with links to a plethora of age-related diseases, and have widespread human exposures from occupational, medical, or environmental settings. We believe heavy metals ought to be classified as a class of gerontogens (i.e., chemicals that accelerate biological aging in cells and tissues). Gerontogens may be best studied through their effects on the "Hallmarks of Aging", nine physiological hallmarks demonstrated to occur in aged cells, tissues, and bodies. Evidence suggests that cellular senescence-a permanent growth arrest in cells-is one of the most pertinent hallmarks of aging and is a useful indicator of aging in tissues. Here, we discuss the roles of heavy metals in brain aging. We briefly discuss brain aging in general, then expand upon observations for heavy metals contributing to age-related neurodegenerative disorders. We particularly emphasize the roles and observations of cellular senescence in neurodegenerative diseases. Finally, we discuss the observations for heavy metals inducing cellular senescence. The glaring lack of knowledge about gerontogens and gerontogenic mechanisms necessitates greater research in the field, especially in the context of the global aging crisis.

Senescent cells in the brain and where to find them

Cellular senescence is a process in which cells change their characteristic phenotype in response to stress and enter a state of prolonged cell cycle arrest accompanied by a distinct secretory phenotype. Cellular senescence has both beneficial and detrimental outcomes. With age, senescent cells progressively accumulate in tissues and might be the bridge connecting ageing to many age-related pathologies. In recent years, evidence emerged supporting the accumulation of brain senescent cells during neurological disorders and ageing. Here, we will discuss the different brain cell populations that exhibit a senescent phenotype. Subsequently, we will explore several senolytic strategies which have been developed to eliminate senescent cells. Finally, we will examine their potential to directly eliminate these senescent brain cells.

Untangling senescent and damage-associated microglia in the aging and diseased brain

Microglial homeostasis has emerged as a critical mediator of health and disease in the central nervous system. In their neuroprotective role as the predominant immune cells of the brain, microglia surveil the microenvironment for debris and pathogens, while also promoting neurogenesis and performing maintenance on synapses. Chronological ageing, disease onset, or traumatic injury promotes irreparable damage or deregulated signaling to reinforce neurotoxic phenotypes in microglia. These insults may include cellular senescence, a stable growth arrest often accompanied by the production of a distinctive pro-inflammatory secretory phenotype, which may contribute to age- or disease-driven decline in neuronal health and cognition and is a potential novel therapeutic target. Despite this increased scrutiny, unanswered questions remain about what distinguishes senescent microglia and non-senescent microglia reacting to insults occurring in ageing, disease, and injury, and how central the development of senescence is in their pivot from guardian to assailant. To intelligently design future studies to untangle senescent microglia from other primed and reactionary states, specific criteria must be developed that define this population and allow for comparisons between different model systems. Comparing microglial activity seen in homeostasis, ageing, disease, and injury allows for a more coherent understanding of when and how senescent and other harmful microglial subpopulations should be targeted.

C9orf72 poly(PR) mediated neurodegeneration is associated with nucleolar stress

The ALS/FTD-linked intronic hexanucleotide repeat expansion in the C9orf72 gene is translated into dipeptide repeat proteins, among which poly-proline-arginine (PR) displays the most aggressive neurotoxicity in-vitro and in-vivo . PR partitions to the nucleus when expressed in neurons and other cell types. Using drosophila and primary rat cortical neurons as model systems, we show that by lessening the nuclear accumulation of PR, we can drastically reduce its neurotoxicity. PR accumulates in the nucleolus, a site of ribosome biogenesis that regulates the cell stress response. We examined the effect of nucleolar PR accumulation and its impact on nucleolar function and determined that PR caused nucleolar stress and increased levels of the transcription factor p53. Downregulating p53 levels, either genetically or by increasing its degradation, also prevented PR-mediated neurotoxic phenotypes both in in-vitro and in-vivo models. We also investigated whether PR could cause the senescence phenotype in neurons but observed none. Instead, we found induction of apoptosis via caspase-3 activation. In summary, we uncovered the central role of nucleolar dysfunction upon PR expression in the context of C9-ALS/FTD.

Is microglial dystrophy a form of cellular senescence? An analysis of senescence markers in the aged human brain

Aging can cause morphological transformation in human microglia indicative of cell senescence, termed microglial dystrophy. However, cellular senescence is characterized by additional changes, such as an irregular cell cycle arrest, and a variety of metabolic and molecular changes including a senescence-associated secretory phenotype, dysfunction of degradation mechanisms, and altered DNA damage response. Here, we tested whether dystrophic microglia display customary markers of cell senescence by performing double and triple staining in sections of the temporal lobe and brain stem from 14 humans. We found that markers related to oxidative damage, such as upregulation of 8-hydroxy-2'-deoxyguanosine (8-OHdG), hemeoxygenase-1 (HO-1), and y-H2AX, as well as inclusion of lipofuscin, do not or only exceptionally colocalize with dystrophic microglia. Further, we did not observe a decline in lamin B1 around nuclear laminae in either dystrophic or ramified microglia within the same microscopic field. Only ferritin expression, which is known to increase with aging in CNS microglia, was frequently observed in dystrophic, but rarely in ramified microglial cells. We conclude that neither dystrophic nor ramified microglia in human brain exhibit significant expression of conventional senescence markers associated with oxidative stress, and that ferritin is the dominant immunophenotypic change related to microglial aging. We suggest that multiple pathogenic mechanisms other than those driving cellular senescence contribute to dystrophic transformation of microglia.

Rejuvenation of the aged brain immune cell landscape in mice through p16-positive senescent cell clearance

Cellular senescence is a plausible mediator of inflammation-related tissue dysfunction. In the aged brain, senescent cell identities and the mechanisms by which they exert adverse influence are unclear. Here we used high-dimensional molecular profiling, coupled with mechanistic experiments, to study the properties of senescent cells in the aged mouse brain. We show that senescence and inflammatory expression profiles increase with age and are brain region- and sex-specific. p16-positive myeloid cells exhibiting senescent and disease-associated activation signatures, including upregulation of chemoattractant factors, accumulate in the aged mouse brain. Senescent brain myeloid cells promote peripheral immune cell chemotaxis in vitro. Activated resident and infiltrating immune cells increase in the aged brain and are partially restored to youthful levels through p16-positive senescent cell clearance in female p16-InkAttac mice, which is associated with preservation of cognitive function. Our study reveals dynamic remodeling of the brain immune cell landscape in aging and suggests senescent cell targeting as a strategy to counter inflammatory changes and cognitive decline.

The motor neuron disease mouse model hSOD1-G93A shows a non-canonical profile of senescence biomarkers

To evaluate senescence mechanisms, including senescence-associated secretory phenotype (SASP), in the motor-neuron disease model hSOD1-G93A, we quantified the expression of p16 and p21 and the senescence-associated β galactosidase (SA-β-gal) in nervous tissue. As SASP markers, we measured the mRNA levels of Il1a, Il6, Ifna, and Ifnb. Furthermore, we explored if an alteration of alternative splicing is associated with senescence by measuring the Adipor2 cryptic exon inclusion levels, a specific splicing variant repressed by TAR-DNA binding of 43 kDa (Tdp-43). Transgenic mice show an atypical senescence profile with high p16 and p21 mRNA and protein in glia, without the canonical increase in SA-β-gal activity. Consistent with SASP, there is an increase in Il1a and Il6 expression, associated with increased TNFR and M-CSF protein levels, with females being partially protected. TDP-43 splicing activity is compromised in this model. Senolytic drug Navitoclax does not alter the present 'model's disease progression. This lack of effect is reproduced in vitro, in contrast with Dasatinib and quercetin, which diminish p16 and p21. Our findings show a non-canonical profile of senescence biomarkers in the model hSOD1-G93A.

Age-Related Midbrain Inflammation and Senescence in Parkinson’s Disease

Immune responses are arising as a common feature of several neurodegenerative diseases, such as Parkinson’s disease (PD), Alzheimer’s disease (AD), and Amyotrophic Lateral Sclerosis (ALS), but their role as either causative or consequential remains debated. It is evident that there is local inflammation in the midbrain in PD patients even before symptom onset, but the underlying mechanisms remain elusive. In this mini-review, we discuss this midbrain inflammation in the context of PD and argue that cellular senescence may be the cause for this immune response. We postulate that to unravel the relationship between inflammation and senescence in PD, it is crucial to first understand the potential causative roles of various cell types of the midbrain and determine how the possible paracrine spreading of senescence between them may lead to observed local immune responses. We hypothesize that secretion of pro-inflammatory factors by senescent cells in the midbrain triggers neuroinflammation resulting in immune cell-mediated killing of midbrain dopaminergic (DA) neurons in PD.

NF-κB, a culprit of both inflamm-ageing and declining immunity?

NF-κB is generally recognized as an important regulator of ageing, through its roles in cellular senescence and inflammatory pathways. Activated in virtually all cell-cell communication networks of the immune system, NF-κB is thought to affect age-related defects of both innate and adaptive immune cells, relevant to inflamm-ageing and declining adaptive immunity, respectively. Moreover, the family of NF-κB proteins that exist as heterodimers and homodimers exert their function beyond the immune system. Given their involvement in diverse areas such as DNA damage to metabolism, NF-κB has the potential to serve as linkages between known hallmarks of ageing. However, the complexity of NF-κB dimer composition, dynamic signaling, and tissue-specific actions has received relatively little attention in ageing research. Here, we discuss some areas where further research may bear fruit in our understanding the impact of NF-κB in healthy ageing and longevity.

Aged Microglia in Neurodegenerative Diseases: Microglia Lifespan and Culture Methods

Microglia have been recognized as macrophages of the central nervous system (CNS) that are regarded as a culprit of neuroinflammation in neurodegenerative diseases. Thus, microglia have been considered as a cell that should be suppressed for maintaining a homeostatic CNS environment. However, microglia ontogeny, fate, heterogeneity, and their function in health and disease have been defined better with advances in single-cell and imaging technologies, and how to maintain homeostatic microglial function has become an emerging issue for targeting neurodegenerative diseases. Microglia are long-lived cells of yolk sac origin and have limited repopulating capacity. So, microglial perturbation in their lifespan is associated with not only neurodevelopmental disorders but also neurodegenerative diseases with aging. Considering that microglia are long-lived cells and may lose their functional capacity as they age, we can expect that aged microglia contribute to various neurodegenerative diseases. Thus, understanding microglial development and aging may represent an opportunity for clarifying CNS disease mechanisms and developing novel therapies.

Neuroinflammation in Amyotrophic Lateral Sclerosis and Frontotemporal Dementia and the Interest of Induced Pluripotent Stem Cells to Study Immune Cells Interactions With Neurons

Inflammation is a shared hallmark between amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). For long, studies were conducted on tissues of post-mortem patients and neuroinflammation was thought to be only bystander result of the disease with the immune system reacting to dying neurons. In the last two decades, thanks to improving technologies, the identification of causal genes and the development of new tools and models, the involvement of inflammation has emerged as a potential driver of the diseases and evolved as a new area of intense research. In this review, we present the current knowledge about neuroinflammation in ALS, ALS-FTD, and FTD patients and animal models and we discuss reasons of failures linked to therapeutic trials with immunomodulator drugs. Then we present the induced pluripotent stem cell (iPSC) technology and its interest as a new tool to have a better immunopathological comprehension of both diseases in a human context. The iPSC technology giving the unique opportunity to study cells across differentiation and maturation times, brings the hope to shed light on the different mechanisms linking neurodegeneration and activation of the immune system. Protocols available to differentiate iPSC into different immune cell types are presented. Finally, we discuss the interest in studying monocultures of iPS-derived immune cells, co-cultures with neurons and 3D cultures with different cell types, as more integrated cellular approaches. The hope is that the future work with human iPS-derived cells helps not only to identify disease-specific defects in the different cell types but also to decipher the synergistic effects between neurons and immune cells. These new cellular tools could help to find new therapeutic approaches for all patients with ALS, ALS-FTD, and FTD.

Senolytics: A Novel Strategy for Neuroprotection in ALS?

Amyotrophic lateral sclerosis (ALS) is a progressive motor neurodegenerative disease that currently has no cure and has few effective treatments. On a cellular level, ALS manifests through significant changes in the proper function of astrocytes, microglia, motor neurons, and other central nervous system (CNS) cells, leading to excess neuroinflammation and neurodegeneration. Damage to the upper and lower motor neurons results in neural and muscular dysfunction, leading to death most often due to respiratory paralysis. A new therapeutic strategy is targeting glial cells affected by senescence, which contribute to motor neuron degeneration. Whilst this new therapeutic approach holds much promise, it is yet to be trialled in ALS-relevant preclinical models and needs to be designed carefully to ensure selectivity. This review summarizes the pathways involved in ALS-related senescence, as well as known senolytic agents and their mechanisms of action, all of which may inform strategies for ALS-focused drug discovery efforts.

Aberrant NLRP3 Inflammasome Activation Ignites the Fire of Inflammation in Neuromuscular Diseases

Inflammasomes are molecular hubs that are assembled and activated by a host in response to various microbial and non-microbial stimuli and play a pivotal role in maintaining tissue homeostasis. The NLRP3 is a highly promiscuous inflammasome that is activated by a wide variety of sterile triggers, including misfolded protein aggregates, and drives chronic inflammation via caspase-1-mediated proteolytic cleavage and secretion of proinflammatory cytokines, interleukin-1β and interleukin-18. These cytokines further amplify inflammatory responses by activating various signaling cascades, leading to the recruitment of immune cells and overproduction of proinflammatory cytokines and chemokines, resulting in a vicious cycle of chronic inflammation and tissue damage. Neuromuscular diseases are a heterogeneous group of muscle disorders that involve injury or dysfunction of peripheral nerves, neuromuscular junctions and muscles. A growing body of evidence suggests that dysregulation, impairment or aberrant NLRP3 inflammasome signaling leads to the initiation and exacerbation of pathological processes associated with neuromuscular diseases. In this review, we summarize the available knowledge about the NLRP3 inflammasome in neuromuscular diseases that affect the peripheral nervous system and amyotrophic lateral sclerosis, which affects the central nervous system. In addition, we also examine whether therapeutic targeting of the NLRP3 inflammasome components is a viable approach to alleviating the detrimental phenotype of neuromuscular diseases and improving clinical outcomes.

Aging is associated with glial senescence in the brainstem - implications for age-related sympathetic overactivity

Accumulating evidence suggests that the sympathetic nervous system (SNS) overactivity plays a crucial role in age-related increase in the risk for cardiovascular diseases such as hypertension, myocardial infarction, stroke and heart diseases. Previous studies indicate that neuroinflammation in key brainstem regions that regulate sympathetic outflow plays a pathogenic role in aging-mediated sympathoexcitation. However, the molecular mechanisms underlying this phenomenon are not clear. While senescent cells and their secretory phenotype (SASP) have been implicated in the pathogenesis of several age-related diseases, their role in age-related neuroinflammation in the brainstem and SNS overactivity has not been investigated. To test this, we isolated brainstems from young (2-4 months) and aged (24 months) male C57BL/6J mice and assessed senescence using a combination of RNA-in situ hybridization, PCR analysis, multiplex assay and SA-β gal staining. Our results show significant increases in p16Ink4a expression, increased activity of SA-β gal and increases in SASP levels in the aged brainstem, suggesting age-induced senescence in the brainstem. Further, analysis of senescence markers in glial cells enriched fraction from fresh brainstem samples demonstrated that glial cells are more susceptible to senesce with age in the brainstem. In conclusion, our study suggests that aging induces glial senescence in the brainstem which likely causes inflammation and SNS overactivity.

Cellular senescence and its role in white adipose tissue

Cell senescence is defined as a state of irreversible cell cycle arrest combined with DNA damage and the induction of a senescence-associated secretory phenotype (SASP). This includes increased secretion of many inflammatory agents, proteases, miRNA's, and others. Cell senescence has been widely studied in oncogenesis and has generally been considered to be protective, due to cell cycle arrest and the inhibition of proliferation. Cell senescence is also associated with ageing and extensive experimental data support its role in generating the ageing-associated phenotype. Senescent cells can also influence proximal "healthy" cells through SASPs and, e.g., inhibit normal development of progenitor/stem cells, thereby preventing tissue replacement of dying cells and reducing organ functions. Recent evidence demonstrates that SASPs may also play important roles in several chronic diseases including diabetes and cardiovascular disease. White adipose tissue (WAT) cells are highly susceptible to becoming senescent both with ageing but also with obesity and type 2 diabetes, independently of chronological age. WAT senescence is associated with inappropriate expansion (hypertrophy) of adipocytes, insulin resistance, and dyslipidemia. Major efforts have been made to identify approaches to delete senescent cells including the use of "senolytic" compounds. The most established senolytic treatment to date is the combination of dasatinib, an antagonist of the SRC family of kinases, and the antioxidant quercetin. This combination reduces cell senescence and improves chronic disorders in experimental animal models. Although only small and short-term studies have been performed in man, no severe adverse effects have been reported. Hopefully, these or other senolytic agents may provide novel ways to prevent and treat different chronic diseases in man. Here we review the current knowledge on cellular senescence in both murine and human studies. We also discuss the pathophysiological role of this process and the potential therapeutic relevance of targeting senescence selectively in WAT.

Senescent Microglia: The Key to the Ageing Brain?

Ageing represents the single biggest risk factor for development of neurodegenerative disease. Despite being such long-lived cells, microglia have been relatively understudied for their role in the ageing process. Reliably identifying aged microglia has proven challenging, not least due to the diversity of cell populations, and the limitations of available models, further complicated by differences between human and rodent cells. Consequently, the literature contains multiple descriptions and categorisations of microglia with neurotoxic phenotypes, including senescence, without any unifying markers. The role of microglia in brain homeostasis, particularly iron storage and metabolism, may provide a key to reliable identification.

The Cellular Senescence Stress Response in Post-Mitotic Brain Cells: Cell Survival at the Expense of Tissue Degeneration

In 1960, Rita Levi-Montalcini and Barbara Booker made an observation that transformed neuroscience: as neurons mature, they become apoptosis resistant. The following year Leonard Hayflick and Paul Moorhead described a stable replicative arrest of cells in vitro, termed "senescence". For nearly 60 years, the cell biology fields of neuroscience and senescence ran in parallel, each separately defining phenotypes and uncovering molecular mediators to explain the 1960s observations of their founding mothers and fathers, respectively. During this time neuroscientists have consistently observed the remarkable ability of neurons to survive. Despite residing in environments of chronic inflammation and degeneration, as occurs in numerous neurodegenerative diseases, often times the neurons with highest levels of pathology resist death. Similarly, cellular senescence (hereon referred to simply as "senescence") now is recognized as a complex stress response that culminates with a change in cell fate. Instead of reacting to cellular/DNA damage by proliferation or apoptosis, senescent cells survive in a stable cell cycle arrest. Senescent cells simultaneously contribute to chronic tissue degeneration by secreting deleterious molecules that negatively impact surrounding cells. These fields have finally collided. Neuroscientists have begun applying concepts of senescence to the brain, including post-mitotic cells. This initially presented conceptual challenges to senescence cell biologists. Nonetheless, efforts to understand senescence in the context of brain aging and neurodegenerative disease and injury emerged and are advancing the field. The present review uses pre-defined criteria to evaluate evidence for post-mitotic brain cell senescence. A closer interaction between neuro and senescent cell biologists has potential to advance both disciplines and explain fundamental questions that have plagued their fields for decades.

The microglial component of amyotrophic lateral sclerosis

Microglia are the primary immune cells of the CNS, carrying out key homeostatic roles and undergoing context-dependent and temporally regulated changes in response to injury and neurodegenerative diseases. Microglia have been implicated in playing a role in amyotrophic lateral sclerosis (ALS), a neurodegenerative disease characterized by extensive motor neuron loss leading to paralysis and premature death. However, as the pathomechansims of ALS are increasingly recognized to involve a multitude of different cell types, it has been difficult to delineate the specific contribution of microglia to disease. Here, we review the literature of microglial involvement in ALS and discuss the evidence for the neurotoxic and neuroprotective pathways that have been attributed to microglia in this disease. We also discuss accumulating evidence for spatiotemporal regulation of microglial activation in this context. A deeper understanding of the role of microglia in the ‘cellular phase’ of ALS is crucial in the development of mechanistically rationalized therapies.

Evidence of the Cellular Senescence Stress Response in Mitotically Active Brain Cells-Implications for Cancer and Neurodegeneration

Cellular stress responses influence cell fate decisions. Apoptosis and proliferation represent opposing reactions to cellular stress or damage and may influence distinct health outcomes. Clinical and epidemiological studies consistently report inverse comorbidities between age-associated neurodegenerative diseases and cancer. This review discusses how one particular stress response, cellular senescence, may contribute to this inverse correlation. In mitotically competent cells, senescence is favorable over uncontrolled proliferation, i.e., cancer. However, senescent cells notoriously secrete deleterious molecules that drive disease, dysfunction and degeneration in surrounding tissue. In recent years, senescent cells have emerged as unexpected mediators of neurodegenerative diseases. The present review uses pre-defined criteria to evaluate evidence of cellular senescence in mitotically competent brain cells, highlights the discovery of novel molecular regulators and discusses how this single cell fate decision impacts cancer and degeneration in the brain. We also underscore methodological considerations required to appropriately evaluate the cellular senescence stress response in the brain.

Similarities and interplay between senescent cells and macrophages

Senescence is a cellular program that prevents the replication of old, damaged, or cancerous cells. Senescent cells become growth arrested and undergo changes in their morphology, chromatin organization, and metabolism, and produce a bioactive secretome. This secretome, the senescence-associated secretory phenotype (SASP), mediates many of the pathophysiological effects associated with senescent cells, for example, recruiting and activating immune cells such as macrophages. The relation between senescent cells and macrophages is intriguing: senescent cells recruit macrophages, can induce them to undergo senescence, or can influence their polarization. Senescent cells and macrophages share multiple phenotypic characteristics; both have a high secretory status, increased lysosome numbers, or the ability to activate the inflammasome. Senescent cells accumulate during aging and disease, and killing them results in widespread benefits. Here we discuss similarities between senescent cells and macrophages and interpret the latest developments in macrophage biology to understand the molecular mechanisms of cellular senescence. We describe evidence and effects of senescence in macrophages and speculate on the ontogeny of the senescent-like state in macrophages. Finally, we examine the macrophage-senescent cell interplay and its impact on macrophage effector functions during inflammatory conditions and in the tumor microenvironment.

Whole-body senescent cell clearance alleviates age-related brain inflammation and cognitive impairment in mice

Cellular senescence is characterized by an irreversible cell cycle arrest and a pro-inflammatory senescence-associated secretory phenotype (SASP), which is a major contributor to aging and age-related diseases. Clearance of senescent cells has been shown to improve brain function in mouse models of neurodegenerative diseases. However, it is still unknown whether senescent cell clearance alleviates cognitive dysfunction during the aging process. To investigate this, we first conducted single-nuclei and single-cell RNA-seq in the hippocampus from young and aged mice. We observed an age-dependent increase in p16Ink4a senescent cells, which was more pronounced in microglia and oligodendrocyte progenitor cells and characterized by a SASP. We then aged INK-ATTAC mice, in which p16Ink4a -positive senescent cells can be genetically eliminated upon treatment with the drug AP20187 and treated them either with AP20187 or with the senolytic cocktail Dasatinib and Quercetin. We observed that both strategies resulted in a decrease in p16Ink4a exclusively in the microglial population, resulting in reduced microglial activation and reduced expression of SASP factors. Importantly, both approaches significantly improved cognitive function in aged mice. Our data provide proof-of-concept for senolytic interventions' being a potential therapeutic avenue for alleviating age-associated cognitive impairment.

A Nitroalkene Benzoic Acid Derivative Targets Reactive Microglia and Prolongs Survival in an Inherited Model of ALS via NF-κB Inhibition

Motor neuron degeneration and neuroinflammation are the most striking pathological features of amyotrophic lateral sclerosis (ALS). ALS currently has no cure and approved drugs have only a modest clinically therapeutic effect in patients. Drugs targeting different deleterious inflammatory pathways in ALS appear as promising therapeutic alternatives. Here, we have assessed the potential therapeutic effect of an electrophilic nitroalkene benzoic acid derivative, (E)-4-(2-nitrovinyl) benzoic acid (BANA), to slow down paralysis progression when administered after overt disease onset in SOD1G93A rats. BANA exerted a significant inhibition of NF-κB activation in NF-κB reporter transgenic mice and microglial cell cultures. Systemic daily oral administration of BANA to SOD1G93A rats after paralysis onset significantly decreased microgliosis and astrocytosis, and significantly reduced the number of NF-κB-p65-positive microglial nuclei surrounding spinal motor neurons. Numerous microglia bearing nuclear NF-κB-p65 were observed in the surrounding of motor neurons in autopsy spinal cords from ALS patients but not in controls, suggesting ALS-associated microglia could be targeted by BANA. In addition, BANA-treated SOD1G93A rats after paralysis onset showed significantly ameliorated spinal motor neuron pathology as well as conserved neuromuscular junction innervation in the skeletal muscle, as compared to controls. Notably, BANA prolonged post-paralysis survival by ~30%, compared to vehicle-treated littermates. These data provide a rationale to therapeutically slow paralysis progression in ALS using small electrophilic compounds such as BANA, through a mechanism involving microglial NF-κB inhibition.

HIV TAT-mediated microglial senescence: Role of SIRT3-dependent mitochondrial oxidative stress

The advent of combined antiretroviral treatment (cART) as a treatment for HIV-1 infection has not only resulted in a dramatic decrease in the peripheral viral load but has also led to increased life expectancy of the infected individuals. Paradoxically, increased lifespan is accompanied with higher prevalence of age-related comorbidities, including HIV-associated neurocognitive disorders (HAND). Present study was aimed at exploring the role of HIV TAT protein in mediating microglial mitochondrial oxidative stress, ultimately resulting in neuroinflammation and microglial senescence. Our findings demonstrated that exposure of mouse primary microglial cells (mPMs) to HIV TAT protein resulted in a senescence-like phenotype, that was characterized by elevated expression of both p16 and p21 proteins, increased numbers of senescence-associated-β-galactosidase positive cells, augmented cell-cycle arrest, increased release of proinflammatory cytokines and decreased telomerase activity. Additionally, exposure of mPMs to HIV TAT also resulted downregulation of SIRT3 with a concomitant increase in mitochondrial oxidative stress. Dual luciferase reporter assay identified miR-505 as a novel target of SIRT3, which was upregulated in mPMs exposed to HIV TAT. Furthermore, transient transfection of mPMs with either the SIRT3 plasmid or miRNA-505 inhibitor upregulated the expression of SIRT3 and mitochondrial antioxidant enzymes, with a concomitant decrease in microglial senescence. These in vitro findings were also validated in the prefrontal cortices and striatum of HIV transgenic rats as well as cART-treated HIV-infected individuals. In summary, this study underscores a yet undiscovered novel mechanism(s) underlying HIV TAT-mediated induction of senescence phenotype in microglia, involving the miR-505-SIRT3 axis-mediated induction of mitochondrial oxidative stress.

•

HIV TAT induces senescence-like phenotype in microglia.

•

HIV TAT decreases SIRT3 with concomitant increase of mitochondrial ROS.

•

Overexpression of SIRT3 attenuated HIV TAT-mediated microglial senescence.

•

miR-505 negatively regulate SIRT3 expression.

•

miR-505 inhibition prevents SIRT3-mediated mitochondria stress and glial senescence.

Up-regulated inflammatory signatures of the spinal cord in canine degenerative myelopathy

Canine degenerative myelopathy (DM) is an adult-onset fatal disease characterized by progressive degeneration of the spinal cord. Affected dogs have homozygous mutations in superoxide dismutase 1, and thus DM is a potential spontaneous animal model of human familial amyotrophic lateral sclerosis (ALS). Neuroinflammation is the pathological hallmark of ALS, whereby proinflammatory cytokines and chemokines are overproduced by activated glial cells such as astrocytes and microglia. However, the detailed pathogenesis of spinal cord degeneration in DM remains unknown. To further characterize the pathological mechanism of DM, we analyzed the caudal cervical cords of ten Pembroke Welsh Corgis pathologically diagnosed with DM by quantitative real-time reverse transcription polymerase chain reaction, immunohistochemistry (IHC), and double immunofluorescence. Compared to control spinal cord tissues, we found significantly enhanced transcriptions of interleukin-1β, tumor necrosis factor-α, CC motif chemokine ligand (CCL) 2 and vascular cell adhesion molecule -1 mRNA in the spinal cords of DM dogs. Moreover, IHC for the class II major histocompatibility complex molecules HLA-DR and CCL2 indicated that the immunopositive areas of activated macrophages/microglia and CCL2 protein were significantly increased in DM, and CCL2 protein was mainly overproduced by astrocytes. Our results suggest a proinflammatory state of the microenvironment in the DM spinal cord in which activated microglia and astrocytes play important roles by secreting a set of cytokines, chemokines, and expressing adhesion molecules.

Cellular senescence in aging and age-related diseases: Implications for neurodegenerative diseases

Aging is the major predictor for developing multiple neurodegenerative diseases, including Alzheimer's disease (AD) other dementias, and Parkinson's disease (PD). Senescent cells, which can drive aging phenotypes, accumulate at etiological sites of many age-related chronic diseases. These cells are resistant to apoptosis and can cause local and systemic dysfunction. Decreasing senescent cell abundance using senolytic drugs, agents that selectively target these cells, alleviates neurodegenerative diseases in preclinical models. In this review, we consider roles of senescent cells in neurodegenerative diseases and potential implications of senolytic agents as an innovative treatment.

Cellular senescence in vivo: From cells to tissues to pathologies

Senescent cells accumulate during aging in a variety of tissues. Although scarce, they could influence tissue function non-cell-autonomously via secretion of a range of factors in their neighborhood. Recent studies support a role of senescent cells in age-related morbidity, including neurodegenerative diseases, cardiovascular pathologies, cancers, aging-associated nephrological alterations, chronic pulmonary disease and osteoarthritis, indicating that senescent cells could represent an interesting target for therapeutic exploitation across a range of pathophysiological contexts. In this article, we review data available to indicate which cell types can undergo senescence within various mammalian tissue environments and how these processes may contribute to tissue-specific pathologies associated with old age. We also consider markers used to identify senescent cells in vitro and in vivo. The data discussed may serve as an important starting point for an extended definition of molecular and functional characteristics of senescent cells in different organs and may hence promote the development and refinement of targeting strategies aimed at removing senescent cells from aging tissues.

The Diversity of Intermediate Filaments in Astrocytes

Despite the remarkable complexity of the individual neuron and of neuronal circuits, it has been clear for quite a while that, in order to understand the functioning of the brain, the contribution of other cell types in the brain have to be accounted for. Among glial cells, astrocytes have multiple roles in orchestrating neuronal functions. Their communication with neurons by exchanging signaling molecules and removing molecules from extracellular space takes place at several levels and is governed by different cellular processes, supported by multiple cellular structures, including the cytoskeleton. Intermediate filaments in astrocytes are emerging as important integrators of cellular processes. Astrocytes express five types of intermediate filaments: glial fibrillary acidic protein (GFAP); vimentin; nestin; synemin; lamins. Variability, interactions with different cellular structures and the particular roles of individual intermediate filaments in astrocytes have been studied extensively in the case of GFAP and vimentin, but far less attention has been given to nestin, synemin and lamins. Similarly, the interplay between different types of cytoskeleton and the interaction between the cytoskeleton and membranous structures, which is mediated by cytolinker proteins, are understudied in astrocytes. The present review summarizes the basic properties of astrocytic intermediate filaments and of other cytoskeletal macromolecules, such as cytolinker proteins, and describes the current knowledge of their roles in normal physiological and pathological conditions.

Astrocyte Senescence and Alzheimer's Disease: A Review

Astrocytes are the largest group of glial cells in the brain and participate in several essential functions of the central nervous system (CNS). Disruption of their normal physiological function can lead to metabolism disequilibrium and the pathology of CNS. As an important mechanism of aging, cellular senescence has been considered as a primary inducing factor of age-associated neurodegenerative disorders. Senescent astrocytes showed decreased normal physiological function and increased secretion of senescence-associated secretory phenotype (SASP) factors, which contribute to Aβ accumulation, tau hyperphosphorylation, and the deposition of neurofibrillary tangles (NFTs) in Alzheimer’s disease (AD). Astrocyte senescence also leads to a number of detrimental effects, including induced glutamate excitotoxicity, impaired synaptic plasticity, neural stem cell loss, and blood–brain barrier (BBB) dysfunction. In this review article, we have summarized the growing findings regarding astrocyte senescence and its putative role in the pathologic progress of AD. Additionally, we also focus on the significance of targeting astrocyte senescence as a novel and feasible therapeutic approach for AD.

A Novel Microglia-Specific Transcriptional Signature Correlates With Behavioral Deficits in Neuropsychiatric Lupus

Neuropsychiatric symptoms of systemic lupus erythematosus (NP-SLE) affect over one-half of SLE patients, yet underlying mechanisms remain largely unknown. We demonstrate that SLE-prone mice (CReCOM) develop NP-SLE, including behavioral deficits prior to systemic autoimmunity, reduced brain volumes, decreased vascular integrity, and brain-infiltrating leukocytes. NP-SLE microglia exhibit numerical expansion, increased synaptic uptake, and a more metabolically active phenotype. Microglia from multiple SLE-prone models express a "NP-SLE signature" unrelated to type I interferon. Rather, the signature is associated with lipid metabolism, scavenger receptor activity and downregulation of inflammatory and chemotaxis processes, suggesting a more regulatory, anti-inflammatory profile. NP-SLE microglia also express genes associated with disease-associated microglia (DAM), a subset of microglia thought to be instrumental in neurodegenerative diseases. Further, expression of "NP-SLE" and "DAM" signatures correlate with the severity of behavioral deficits in young SLE-prone mice prior to overt systemic disease. Our data are the first to demonstrate the predictive value of our newly identified microglia-specific "NP-SLE" and "DAM" signatures as a surrogate for NP-SLE clinical outcomes and suggests that microglia-intrinsic defects precede contributions from systemic SLE for neuropsychiatric manifestations.

Pyrola incarnata demonstrates neuroprotective effects against β-amyloid-induced memory impairment in mice

This study aims to investigate the neuroprotective effects of Pyrola incarnata against β-amyloid-induced memory impairment in mice. Ethanol extract of Pyrola incarnata (EPI) was obtained and led to eleven phytochemicals successfully by isolation and purification, which were elucidated by spectroscopic analysis (¹H NMR, ¹³C NMR and HR-ESI-MS). Thereinto, ursolic acid was gained as most abundant monomer. C57BL/6 mice were intracerebroventricular injected with aggregated Aβ_25-35. Open-field test, Barnes maze test and Morris water maze were conducted for evaluating cognition processes of EPI and ursolic acid. EPI significantly improved learning and memory deficits, attenuated the Aβ_25-35 level of deposition immunohistochemically. Further studies revealed that ursolic acid as bioactive phytochemical of P. incarnata improved spatial memory performance and ameliorated Aβ_25-35 accumulation by activating microglia cells and up-regulating Iba1 level in the hippocampus. These findings suggest P. incarnata could improve the cognition of mice and be a promising natural source for the treatment of neurodegenerative disease.

Autophagy, Cellular Aging and Age-related Human Diseases

Macroautophagy/autophagy is a conserved degradation system that engulfs intracytoplasmic contents, including aggregated proteins and organelles, which is crucial for cellular homeostasis. During aging, cellular factors suggested as the cause of aging have been reported to be associated with progressively compromised autophagy. Dysfunctional autophagy may contribute to age-related diseases, such as neurodegenerative disease, cancer, and metabolic syndrome, in the elderly. Therefore, restoration of impaired autophagy to normal may help to prevent age-related disease and extend lifespan and longevity. Therefore, this review aims to provide an overview of the mechanisms of autophagy underlying cellular aging and the consequent disease. Understanding the mechanisms of autophagy may provide potential information to aid therapeutic interventions in age-related diseases.

CD34 Identifies a Subset of Proliferating Microglial Cells Associated with Degenerating Motor Neurons in ALS

Amyotrophic lateral sclerosis (ALS) is characterized by degeneration of upper and lower motor neurons accompanied by proliferation of reactive microglia in affected regions. However, it is unknown whether the hematopoietic marker CD34 can identify a subpopulation of proliferating microglial cells in the ALS degenerating spinal cord. Immunohistochemistry for CD34 and microglia markers was performed in lumbar spinal cords of ALS rats bearing the SOD1^G93A mutation and autopsied ALS and control human subjects. Characterization of CD34-positive cells was also performed in primary cell cultures of the rat spinal cords. CD34 was expressed in a large number of cells that closely interacted with degenerating lumbar spinal cord motor neurons in symptomatic SOD1^G93A rats, but not in controls. Most CD34⁺ cells co-expressed the myeloid marker CD11b, while only a subpopulation was stained for Iba1 or CD68. Notably, CD34⁺ cells actively proliferated and formed clusters adjacent to damaged motor neurons bearing misfolded SOD1. CD34⁺ cells were identified in the proximity of motor neurons in autopsied spinal cord from sporadic ALS subjects but not in controls. Cell culture of symptomatic SOD1^G93A rat spinal cords yielded a large number of CD34⁺ cells exclusively in the non-adherent phase, which generated microglia after successive passaging. A yet unrecognized CD34⁺ cells, expressing or not the microglial marker Iba1, proliferate and accumulate adjacent to degenerating spinal motor neurons, representing an intriguing cell target for approaching ALS pathogenesis and therapeutics.