Neurons and glial cells acquire a senescent signature after repeated mild traumatic brain injury in a sex-dependent manner

More than a number: Incorporating the aged phenotype to improve in vitro and in vivo modeling of neurodegenerative disease

Age is the number one risk factor for developing a neurodegenerative disease (ND), such as Alzheimer's disease (AD) or Parkinson's disease (PD). With our rapidly ageing world population, there will be an increased burden of ND and need for disease-modifying treatments. Currently, however, translation of research from bench to bedside in NDs is poor. This may be due, at least in part, to the failure to account for the potential effect of ageing in preclinical modelling of NDs. While ageing can impact upon physiological response in multiple ways, only a limited number of preclinical studies of ND have incorporated ageing as a factor of interest. Here, we evaluate the aged phenotype and highlight the critical, but unmet, need to incorporate aspects of this phenotype into both the in vitro and in vivo models used in ND research. Given technological advances in the field over the past several years, we discuss how these could be harnessed to create novel models of ND that more readily incorporate aspects of the aged phenotype. This includes a recently described in vitro panel of ageing markers, which could help lead to more standardised models and improve reproducibility across studies. Importantly, we cannot assume that young cells or animals yield the same responses as seen in the context of ageing; thus, an improved understanding of the biology of ageing, and how to appropriately incorporate this into the modelling of ND, will ensure the best chance for successful translation of new therapies to the aged patient.

From spreading depolarization to blood-brain barrier dysfunction: navigating traumatic brain injury for novel diagnosis and therapy

Considerable strides in medical interventions during the acute phase of traumatic brain injury (TBI) have brought improved overall survival rates. However, following TBI, people often face ongoing, persistent and debilitating long-term complications. Here, we review the recent literature to propose possible mechanisms that lead from TBI to long-term complications, focusing particularly on the involvement of a compromised blood-brain barrier (BBB). We discuss evidence for the role of spreading depolarization as a key pathological mechanism associated with microvascular dysfunction and the transformation of astrocytes to an inflammatory phenotype. Finally, we summarize new predictive and diagnostic biomarkers and explore potential therapeutic targets for treating long-term complications of TBI.

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.

Failure of senolytic treatment to prevent cognitive decline in a female rodent model of aging

There are sex differences in vulnerability and resilience to the stressors of aging and subsequent age-related cognitive decline. Cellular senescence occurs as a response to damaging or stress-inducing stimuli. The response includes a state of irreversible growth arrest, the development of a senescence-associated secretory phenotype, and the release of pro-inflammatory cytokines associated with aging and age-related diseases. Senolytics are compounds designed to eliminate senescent cells. Our recent work indicates that senolytic treatment preserves cognitive function in aging male F344 rats. The current study examined the effect of senolytic treatment on cognitive function in aging female rats. Female F344 rats (12 months) were treated with dasatinib (1.2 mg/kg) + quercetin (12 mg/kg) or ABT-263 (12 mg/kg) or vehicle for 7 months. Examination of the estrus cycle indicated that females had undergone estropause during treatment. Senolytic treatment may have increased sex differences in behavioral stress responsivity, particularly for the initial training on the cued version of the watermaze. However, pre-training on the cue task reduced stress responsivity for subsequent spatial training and all groups learned the spatial discrimination. In contrast to preserved memory observed in senolytic-treated males, all older females exhibited impaired episodic memory relative to young (6-month) females. We suggest that the senolytic treatment may not have been able to compensate for the loss of estradiol, which can act on aging mechanisms for anxiety and memory independent of cellular senescence.

The Sixth Sense: Self-nucleic acid sensing in the brain

Our innate immune system uses pattern recognition receptors (PRRs) as a first line of defense to detect microbial ligands and initiate an immune response. Viral nucleic acids are key ligands for the activation of many PRRs and the induction of downstream inflammatory and antiviral effects. Initially it was thought that endogenous (self) nucleic acids rarely activated these PRRs, however emerging evidence indicates that endogenous nucleic acids are able to activate host PRRs in homeostasis and disease. In fact, many regulatory mechanisms are in place to finely control and regulate sensing of self-nucleic acids by PRRs. Sensing of self-nucleic acids is particularly important in the brain, as perturbations to nucleic acid sensing commonly leads to neuropathology. This review will highlight the role of nucleic acid sensors in the brain, both in disease and homeostasis. We also indicate the source of endogenous stimulatory nucleic acids where known and summarize future directions for the study of this growing field.

Bi-directional neuro-immune dysfunction after chronic experimental brain injury

BACKGROUND

It is well established that traumatic brain injury (TBI) causes acute and chronic alterations in systemic immune function and that systemic immune changes contribute to posttraumatic neuroinflammation and neurodegeneration. However, how TBI affects bone marrow (BM) hematopoietic stem/progenitor cells chronically and to what extent such changes may negatively impact innate immunity and neurological function has not been examined.

METHODS

To further understand the role of BM cell derivatives on TBI outcome, we generated BM chimeric mice by transplanting BM from chronically injured or sham (i.e., 90 days post-surgery) congenic donor mice into otherwise healthy, age-matched, irradiated CD45.2 C57BL/6 (WT) hosts. Immune changes were evaluated by flow cytometry, multiplex ELISA, and NanoString technology. Moderate-to-severe TBI was induced by controlled cortical impact injury and neurological function was measured using a battery of behavioral tests.

RESULTS

TBI induced chronic alterations in the transcriptome of BM lineage-c-Kit+Sca1+ (LSK+) cells in C57BL/6 mice, including modified epigenetic and senescence pathways. After 8 weeks of reconstitution, peripheral myeloid cells from TBI→WT mice showed significantly higher oxidative stress levels and reduced phagocytic activity. At eight months after reconstitution, TBI→WT chimeric mice were leukopenic, with continued alterations in phagocytosis and oxidative stress responses, as well as persistent neurological deficits. Gene expression analysis revealed BM-driven changes in neuroinflammation and neuropathology after 8 weeks and 8 months of reconstitution, respectively. Chimeric mice subjected to TBI at 8 weeks and 8 months post-reconstitution showed that longer reconstitution periods (i.e., time post-injury) were associated with increased microgliosis and leukocyte infiltration. Pre-treatment with a senolytic agent, ABT-263, significantly improved behavioral performance of aged C57BL/6 mice at baseline, although it did not attenuate neuroinflammation in the acutely injured brain.

CONCLUSIONS

TBI causes chronic activation and progressive dysfunction of the BM stem/progenitor cell pool, which drives long-term deficits in hematopoiesis, innate immunity, and neurological function, as well as altered sensitivity to subsequent brain injury.

The cellular senescence response and neuroinflammation in juvenile mice following controlled cortical impact and repetitive mild traumatic brain injury

Traumatic brain injury (TBI) is a leading cause of disability and increases the risk of developing neurodegenerative diseases. The mechanisms linking TBI to neurodegeneration remain to be defined. It has been proposed that the induction of cellular senescence after injury could amplify neuroinflammation and induce long-term tissue changes. The induction of a senescence response post-injury in the immature brain has yet to be characterised. We carried out two types of brain injury in juvenile CD1 mice: invasive TBI using controlled cortical impact (CCI) and repetitive mild TBI (rmTBI) using weight drop injury. The analysis of senescence-related signals showed an increase in γH2AX-53BP1 nuclear foci, p53, p19ARF, and p16INK4a expression in the CCI group, 5 days post-injury (dpi). At 35 days, the difference was no longer statistically significant. Gene expression showed the activation of different senescence pathways in the ipsilateral and contralateral hemispheres in the injured mice. CCI-injured mice showed a neuroinflammatory early phase after injury (increased Iba1 and GFAP expression), which persisted for GFAP. After CCI, there was an increase at 5 days in p16INK4, whereas in rmTBI, a significant increase was seen at 35 dpi. Both injuries caused a decrease in p21 at 35 dpi. In rmTBI, other markers showed no significant change. The PCR array data predicted the activation of pathways connected to senescence after rmTBI. These results indicate the induction of a complex cellular senescence and glial reaction in the immature mouse brain, with clear differences between an invasive brain injury and a repetitive mild injury.

Brain pathology and symptoms linked to concussion history: beyond chronic traumatic encephalopathy

Repeated head trauma acquired through sports injuries has been associated with the development of long-term disabling symptoms that negatively impact the quality of life. In this retrospective case series, 52 male former professional athletes involved in contact sports and with a history of multiple concussions were evaluated for chronic clinical symptoms and post-mortem neuropathological diagnoses. The clinical symptoms of 19 cases were examined in greater detail for symptom type, severity and duration. Information on neurological, psychiatric and physical symptoms, substance use profiles and concussion histories was obtained from the athletes' next of kin and assessed in relation to post-mortem neuropathological diagnoses. Cases were categorized into three different neuropathological groups: no major neuropathological findings, the presence of only chronic traumatic encephalopathy (CTE) and the diagnosis(es) of other neurodegenerative diseases. Age at death and the presence of DNA damage in the post-mortem brains were analysed for correlation with the clinical symptoms. In this case series, 14/52 (26.9%) cases (mean age 48.2 ± 11.4) had neuropathological evidence of low-stage/low-burden CTE. A total of 11/52 (21.2%) cases (mean age 38.7 ± 12.7) presented a similar profile and severity of behavioural symptoms to those with CTE, despite the lack of significant post-mortem neuropathological findings. A total of 27/52 (51.9%) cases (mean age 75.5 ± 8.7) presented with complex post-mortem neurodegenerative diagnoses, including Alzheimer's disease and other mixed pathologies, and clinical symptoms associated with language, memory and sensory dysfunction. The presence of DNA damage in the brain was found in all neuropathological groups, predominantly in the ependymal lining of ventricles, and phosphorylated histone H2AX staining was correlated with higher age at death (r = 0.59) and symptoms of language dysfunction (r = 0.56). Findings from our case series suggest that post-concussive symptoms are not driven by CTE. Our findings show that proteinopathies alone may not account for the complexity of the clinical manifestations and suggest the possibility of other drivers, such as DNA damage, as potentially useful markers of brain trauma. Broadening the search for biological markers that reflect the effects of brain injury, even when proteinopathy is not observed, and taking a symptom-driven approach are therefore advised.

STING signaling in the brain: Molecular threats, signaling activities, and therapeutic challenges

Stimulator of interferon genes (STING) is an innate immune signaling protein critical to infections, autoimmunity, and cancer. STING signaling is also emerging as an exciting and integral part of many neurological diseases. Here, we discuss recent advances in STING signaling in the brain. We summarize how molecular threats activate STING signaling in the diseased brain and how STING signaling activities in glial and neuronal cells cause neuropathology. We also review human studies of STING neurobiology and consider therapeutic challenges in targeting STING to treat neurological diseases.

Understanding and therapeutically exploiting cGAS/STING signaling in glioblastoma

Since the discovery that cGAS/STING recognizes endogenous DNA released from dying cancer cells and induces type I interferon and antitumor T cell responses, efforts to understand and therapeutically target the STING pathway in cancer have ensued. Relative to other cancer types, the glioma immune microenvironment harbors few infiltrating T cells, but abundant tumor-associated myeloid cells, possibly explaining disappointing responses to immune checkpoint blockade therapies in cohorts of patients with glioblastoma. Notably, unlike most extracranial tumors, STING expression is absent in the malignant compartment of gliomas, likely due to methylation of the STING promoter. Nonetheless, several preclinical studies suggest that inducing cGAS/STING signaling in the glioma immune microenvironment could be therapeutically beneficial, and cGAS/STING signaling has been shown to mediate inflammatory and antitumor effects of other modalities either in use or being developed for glioblastoma therapy, including radiation, tumor-treating fields, and oncolytic virotherapy. In this Review, we discuss cGAS/STING signaling in gliomas, its implications for glioma immunobiology, compartment-specific roles for STING signaling in influencing immune surveillance, and efforts to target STING signaling - either directly or indirectly - for antiglioma therapy.

The brain-bone marrow axis and its implications for chronic traumatic brain injury

Traumatic brain injury (TBI) causes acute and chronic alterations in systemic immune function which contribute to posttraumatic neuroinflammation and neurodegeneration. However, how TBI affects bone marrow (BM) hematopoietic stem/progenitor cells chronically and to what extent such changes may negatively impact innate immunity and neurological function has not been examined. To further understand the role of BM cell derivatives on TBI outcome, we generated BM chimeric mice by transplanting BM from chronically injured or sham congenic donor mice into otherwise healthy, age-matched, irradiated hosts. After 8 weeks of reconstitution, peripheral myeloid cells from TBI→WT mice showed significantly higher oxidative stress levels and reduced phagocytic activity. At eight months after reconstitution, TBI→WT chimeric mice were leukopenic, with continued alterations in phagocytosis and oxidative stress responses, as well as persistent neurological deficits. Gene expression analysis revealed BM-driven changes in neuroinflammation and neuropathology after 8 weeks and 8 months of reconstitution, respectively. Chimeric mice subjected to TBI showed that longer reconstitution periods were associated with increased microgliosis and leukocyte infiltration. Thus, TBI causes chronic activation and progressive dysfunction of the BM stem/progenitor cell pool, which drives long-term deficits in innate immunity and neurological function, as well as altered sensitivity to subsequent brain injury.

Perturbation of 3D nuclear architecture, epigenomic aging and dysregulation, and cannabinoid synaptopathy reconfigures conceptualization of cannabinoid pathophysiology: part 2-Metabolome, immunome, synaptome

The second part of this paper builds upon and expands the epigenomic-aging perspective presented in Part 1 to describe the metabolomic and immunomic bases of the epigenomic-aging changes and then considers in some detail the application of these insights to neurotoxicity, neuronal epigenotoxicity, and synaptopathy. Cannabinoids are well-known to have bidirectional immunomodulatory activities on numerous parts of the immune system. Immune perturbations are well-known to impact the aging process, the epigenome, and intermediate metabolism. Cannabinoids also impact metabolism via many pathways. Metabolism directly impacts immune, genetic, and epigenetic processes. Synaptic activity, synaptic pruning, and, thus, the sculpting of neural circuits are based upon metabolic, immune, and epigenomic networks at the synapse, around the synapse, and in the cell body. Many neuropsychiatric disorders including depression, anxiety, schizophrenia, bipolar affective disorder, and autistic spectrum disorder have been linked with cannabis. Therefore, it is important to consider these features and their complex interrelationships in reaching a comprehensive understanding of cannabinoid dependence. Together these findings indicate that cannabinoid perturbations of the immunome and metabolome are important to consider alongside the well-recognized genomic and epigenomic perturbations and it is important to understand their interdependence and interconnectedness in reaching a comprehensive appreciation of the true nature of cannabinoid pathophysiology. For these reasons, a comprehensive appreciation of cannabinoid pathophysiology necessitates a coordinated multiomics investigation of cannabinoid genome-epigenome-transcriptome-metabolome-immunome, chromatin conformation, and 3D nuclear architecture which therefore form the proper mechanistic underpinning for major new and concerning epidemiological findings relating to cannabis exposure.

Cellular rejuvenation protects neurons from inflammation mediated cell death

In multiple sclerosis (MS), the invasion of the central nervous system by peripheral immune cells is followed by the activation of resident microglia and astrocytes. This cascade of events results in demyelination, which triggers neuronal damage and death. The molecular signals in neurons responsible for this damage are not yet fully characterized. In MS, retinal ganglion cell neurons (RGCs) of the central nervous system (CNS) undergo axonal injury and cell death. This phenomenon is mirrored in the experimental autoimmune encephalomyelitis (EAE) mouse model of MS. To understand the molecular landscape, we isolated RGCs from mice subjected to the EAE protocol. RNA-sequencing and ATAC-sequencing analyses were performed. Pathway analysis of the RNA-sequencing data revealed that RGCs displayed a molecular signature, similar to aged neurons, showcasing features of senescence. Single-nucleus RNA-sequencing analysis of neurons from human MS patients revealed a comparable senescence-like phenotype., which was supported by immunostaining RGCs in EAE mice. These changes include alterations to the nuclear envelope, modifications in chromatin marks, and accumulation of DNA damage. Transduction of RGCs with an Oct4 - Sox2 - Klf4 transgene to convert neurons in the EAE model to a more youthful epigenetic and transcriptomic state enhanced the survival of RGCs. Collectively, this research uncovers a previously unidentified senescent-like phenotype in neurons under pathological inflammation and neurons from MS patients. The rejuvenation of this aged transcriptome improved visual acuity and neuronal survival in the EAE model supporting the idea that age rejuvenation therapies and senotherapeutic agents could offer a direct means of neuroprotection in autoimmune disorders.

Identification of novel key markers that are induced during traumatic brain injury in mice

Background

Traumatic brain injury (TBI) has emerged as an increasing public health problem but has not been well studied, particularly the mechanisms of brain cellular behaviors during TBI.

Methods

In this study, we established an ischemia/reperfusion (I/R) brain injury mice model using transient middle cerebral artery occlusion (tMCAO) strategy. After then, RNA-sequencing of frontal lobes was performed to screen key inducers during TBI. To further verify the selected genes, we collected peripheral blood mononuclear cells (PBMCs) from TBI patients within 24 h who attended intensive care unit (ICU) in the Affiliated Hospital of Yangzhou University and analyzed the genes expression using RT-qPCR. Finally, the receiver operator characteristic (ROC) curves and co-expression with cellular senescence markers were applied to evaluate the predictive value of the genes.

Results

A total of six genes were screened out from the RNA-sequencing based on their novelty in TBI and implications in apoptosis and cellular senescence signaling. RT-qPCR analysis of PBMCs from patients showed the six genes were all up-regulated during TBI after comparing with healthy volunteers who attended the hospital for physical examination. The area under ROC (AUC) curves were all >0.7, and the co-expression scores of the six genes with senescence markers were all significantly positive. We thus identified TGM1, TGM2, ATF3, RCN3, ORAI1 and ITPR3 as novel key markers that are induced during TBI, and these markers may also serve as potential predictors for the progression of TBI.

BRCA1 heterozygosity promotes DNA damage-induced senescence in a sex-specific manner following repeated mild traumatic brain injury

Emerging evidence suggests cellular senescence, as a consequence of excess DNA damage and deficient repair, to be a driver of brain dysfunction following repeated mild traumatic brain injury (rmTBI). This study aimed to further investigate the role of deficient DNA repair, specifically BRCA1-related repair, on DNA damage-induced senescence. BRCA1, a repair protein involved in maintaining genomic integrity with multiple roles in the central nervous system, was previously reported to be significantly downregulated in post-mortem brains with a history of rmTBI. Here we examined the effects of impaired BRCA1-related repair on DNA damage-induced senescence and outcomes 1-week post-rmTBI using mice with a heterozygous knockout for BRCA1 in a sex-segregated manner. Altered BRCA1 repair with rmTBI resulted in altered anxiety-related behaviours in males and females using elevated zero maze and contextual fear conditioning. Evaluating molecular markers associated with DNA damage signalling and senescence-related pathways revealed sex-specific differences attributed to BRCA1, where females exhibited elevated DNA damage, impaired DNA damage signalling, and dampened senescence onset compared to males. Overall, the results from this study highlight sex-specific consequences of aberrant DNA repair on outcomes post-injury, and further support a need to develop sex-specific treatments following rmTBI.

Senolytic therapy is neuroprotective and improves functional outcome long-term after traumatic brain injury in mice

Introduction

Chronic neuroinflammation can exist for months to years following traumatic brain injury (TBI), although the underlying mechanisms remain poorly understood.

Methods

In the current study, we used a controlled cortical impact mouse model of TBI to examine whether proinflammatory senescent cells are present in the brain long-term (months) after TBI and whether ablation of these cells via administration of senolytic drugs can improve long-term functional outcome after TBI. The results revealed that astrocytes and microglia in the cerebral cortex, hippocampus, corpus callosum and lateral posterior thalamus colocalized the senescent cell markers, p16Ink4a or p21Cip1/Waf1 at 5 weeks post injury (5wpi) and 4 months post injury (4mpi) in a controlled cortical impact (CCI) model. Intermittent administration of the senolytic drugs, dasatinib and quercetin (D + Q) beginning 1-month after TBI for 13 weeks significantly ablated p16Ink4a-positive- and p21Cip1/Waf1-positive-cells in the brain of TBI animals, and significantly reduced expression of the major senescence-associated secretory phenotype (SASP) pro-inflammatory factors, interleukin-1β and interleukin-6. Senolytic treatment also significantly attenuated neurodegeneration and enhanced neuron number at 18 weeks after TBI in the ipsilateral cortex, hippocampus, and lateral posterior thalamus. Behavioral testing at 18 weeks after TBI further revealed that senolytic therapy significantly rescued defects in spatial reference memory and recognition memory, as well as depression-like behavior in TBI mice.

Discussion

Taken as a whole, these findings indicate there is robust and widespread induction of senescent cells in the brain long-term after TBI, and that senolytic drug treatment begun 1-month after TBI can efficiently ablate the senescent cells, reduce expression of proinflammatory SASP factors, reduce neurodegeneration, and rescue defects in reference memory, recognition memory, and depressive behavior.

"Bone-SASP" in Skeletal Aging

Senescence is a complex cell state characterized by stable cell cycle arrest and a unique secretory pattern known as the senescence-associated secretory phenotype (SASP). The SASP factors, which are heterogeneous and tissue specific, normally include chemokines, cytokines, growth factors, adhesion molecules, and lipid components that can lead to multiple age-associated disorders by eliciting local and systemic consequences. The skeleton is a highly dynamic organ that changes constantly in shape and composition. Senescent cells in bone and bone marrow produce diverse SASP factors that induce alterations of the skeleton through paracrine effects. Herein, we refer to bone cell-associated SASP as "bone-SASP." In this review, we describe current knowledge of cellular senescence and SASP, focusing on the role of senescent cells in mediating bone pathologies during natural aging and premature aging syndromes. We also summarize the role of cellular senescence and the bone-SASP in glucocorticoids-induced bone damage. In addition, we discuss the role of bone-SASP in the development of osteoarthritis, highlighting the mechanisms by which bone-SASP drives subchondral bone changes in metabolic syndrome-associated osteoarthritis.

Prevalence of comorbidities post mild traumatic brain injuries: a traumatic brain injury model systems study

Traumatic brain injury (TBI) is associated with an increased risk of long-lasting health-related complications. Survivors of brain trauma often experience comorbidities which could further dampen functional recovery and severely interfere with their day-to-day functioning after injury. Of the three TBI severity types, mild TBI constitutes a significant proportion of total TBI cases, yet a comprehensive study on medical and psychiatric complications experienced by mild TBI subjects at a particular time point is missing in the field. In this study, we aim to quantify the prevalence of psychiatric and medical comorbidities post mild TBI and understand how these comorbidities are influenced by demographic factors (age, and sex) through secondary analysis of patient data from the TBI Model Systems (TBIMS) national database. Utilizing self-reported information from National Health and Nutrition Examination Survey (NHANES), we have performed this analysis on subjects who received inpatient rehabilitation at 5 years post mild TBI. Our analysis revealed that psychiatric comorbidities (anxiety, depression, and post-traumatic stress disorder (PTSD)), chronic pain, and cardiovascular comorbidities were common among survivors with mild TBI. Furthermore, depression exhibits an increased prevalence in the younger compared to an older cohort of subjects whereas the prevalence of rheumatologic, ophthalmological, and cardiovascular comorbidities was higher in the older cohort. Lastly, female survivors of mild TBI demonstrated increased odds of developing PTSD compared to male subjects. The findings of this study would motivate additional analysis and research in the field and could have broader implications for the management of comorbidities after mild TBI.

Inflammaging, cellular senescence, and cognitive aging after traumatic brain injury

Traumatic brain injury (TBI) is associated with mortality and morbidity worldwide. Accumulating pre-clinical and clinical data suggests TBI is the leading extrinsic cause of progressive neurodegeneration. Neurological deterioration after either a single moderate-severe TBI or repetitive mild TBI often resembles dementia in aged populations; however, no currently approved therapies adequately mitigate neurodegeneration. Inflammation correlates with neurodegenerative changes and cognitive dysfunction for years post-TBI, suggesting a potential association between immune activation and both age- and TBI-induced cognitive decline. Inflammaging, a chronic, low-grade sterile inflammation associated with natural aging, promotes cognitive decline. Cellular senescence and the subsequent development of a senescence associated secretory phenotype (SASP) promotes inflammaging and cognitive aging, although the functional association between senescent cells and neurodegeneration is poorly defined after TBI. In this mini-review, we provide an overview of the pre-clinical and clinical evidence linking cellular senescence with poor TBI outcomes. We also discuss the current knowledge and future potential for senotherapeutics, including senolytics and senomorphics, which kill and/or modulate senescent cells, as potential therapeutics after TBI.

Effect of peripheral cellular senescence on brain aging and cognitive decline

We examine similar and differential effects of two senolytic treatments, ABT-263 and dasatinib + quercetin (D + Q), in preserving cognition, markers of peripheral senescence, and markers of brain aging thought to underlie cognitive decline. Male F344 rats were treated from 12 to 18 months of age with D + Q, ABT-263, or vehicle, and were compared to young (6 months). Both senolytic treatments rescued memory, preserved the blood-brain barrier (BBB) integrity, and prevented the age-related decline in hippocampal N-methyl-D-aspartate receptor (NMDAR) function associated with impaired cognition. Senolytic treatments decreased senescence-associated secretory phenotype (SASP) and inflammatory cytokines/chemokines in the plasma (IL-1β, IP-10, and RANTES), with some markers more responsive to D + Q (TNFα) or ABT-263 (IFNγ, leptin, EGF). ABT-263 was more effective in decreasing senescence genes in the spleen. Both senolytic treatments decreased the expression of immune response and oxidative stress genes and increased the expression of synaptic genes in the dentate gyrus (DG). However, D + Q influenced twice as many genes as ABT-263. Relative to D + Q, the ABT-263 group exhibited increased expression of DG genes linked to cell death and negative regulation of apoptosis and microglial cell activation. Furthermore, D + Q was more effective at decreasing morphological markers of microglial activation. The results indicate that preserved cognition was associated with the removal of peripheral senescent cells, decreasing systemic inflammation that normally drives neuroinflammation, BBB breakdown, and impaired synaptic function. Dissimilarities associated with brain transcription indicate divergence in central mechanisms, possibly due to differential access.

Translating the Biology of Aging into New Therapeutics for Alzheimer's Disease: Senolytics

The recent FDA-approval for amyloid lowering therapies reflects an unwavering commitment from the Alzheimer's disease (AD) research community to identify treatments for this leading cause of dementia. The clinical benefits achieved by reducing amyloid, though modest, provide evidence that disease modification is possible. Expanding the same tenacity to interventions targeting upstream drivers of AD pathogenesis could significantly impact the disease course. Advanced age is the greatest risk factor for developing AD. Interventions targeting biological aging offer the possibility of disrupting a foundational cause of AD. Senescent cells accumulate with age and contribute to inflammation and age-related diseases like AD. Senolytic drugs that clear senescent cells improve healthy aging, halt AD disease progression in animal models and are undergoing clinical testing. This review explores the biology of aging, the role of senescent cells in AD pathology, and various senotherapeutic approaches such as senolytics, dampening the SASP (senescence associated secretory phenotype), senescence pathway inhibition, vaccines, and prodrugs. We highlight ongoing clinical trials evaluating the safety and efficacy of the most advanced senolytic approach, dasatinib and quercetin (D+Q), including an ongoing Phase II senolytic trial supported by the Alzheimer's Drug Discovery Foundation (ADDF). Challenges in the field of senotherapy for AD, including target engagement and biomarker development, are addressed. Ultimately, this research pursuit may lead to an effective treatment for AD and provide the field with another disease-modifying therapy to be used, alone or in combination, with other emerging treatment options.