Spinal motor neurons and motor function in older adults

Aging spinal cord microglia become phenotypically heterogeneous and preferentially target motor neurons and their synapses

Microglia have been found to acquire unique region-dependent deleterious features with age and diseases that contribute to neuronal dysfunction and degeneration in the brain. However, it remains unknown whether microglia exhibit similar phenotypic heterogeneity in the spinal cord. Here, we performed a regional analysis of spinal cord microglia in 3-, 16-, 23-, and 30-month-old mice. Using light and electron microscopy, we discovered that spinal cord microglia acquire an increasingly activated phenotype during the course of aging regardless of regional location. However, aging causes microglia in the ventral but not dorsal horn to lose their spatial organization. Aged ventral horn microglia also aggregate around the somata of motor neurons and increase their contacts with motor synapses, which have been shown to be lost with age. These findings suggest that microglia may affect the ability of motor neurons to receive and relay motor commands during aging. To generate additional insights about aging spinal cord microglia, we performed RNA-sequencing on FACS-isolated microglia from 3-, 18-, 22-, and 29-month-old mice. We found that spinal cord microglia acquire a similar transcriptional identity as those in the brain during aging that includes altered expression of genes with roles in microglia-neuron interactions and inflammation. By 29 months of age, spinal cord microglia exhibit additional and unique transcriptional changes known and predicted to cause senescence and to alter lysosomal and ribosomal regulation. Altogether, this work provides the foundation to target microglia to ameliorate aged-related changes in the spinal cord, and particularly on the motor circuit.

The dual role of microglia in neuropathic pain after spinal cord injury: Detrimental and protective effects

Spinal cord injury (SCI) is a debilitating condition that is frequently accompanied by neuropathic pain, resulting in significant physical and psychological harm to a vast number of individuals globally. Despite the high prevalence of neuropathic pain following SCI, the precise underlying mechanism remains incompletely understood. Microglia are a type of innate immune cell that are present in the central nervous system (CNS). They have been observed to have a significant impact on neuropathic pain following SCI. This article presents a comprehensive overview of recent advances in understanding the role of microglia in the development of neuropathic pain following SCI. Specifically, the article delves into the detrimental and protective effects of microglia on neuropathic pain following SCI, as well as the mechanisms underlying their interconversion. Furthermore, the article provides a thorough overview of potential avenues for future research in this area.

Cellular and Molecular Mechanisms of Vestibular Ageing

While age-related auditory deficits and cochlear alterations are well described, those affecting the vestibular sensory organs and more broadly the central vestibular pathways are much less documented. Although there is inter-individual heterogeneity in the phenomenon of vestibular ageing, common tissue alterations, such as losses of sensory hair cells or primary and secondary neurons during the ageing process, can be noted. In this review, we document the cellular and molecular processes that occur during ageing in the peripheral and central vestibular system and relate them to the impact of age-related vestibular deficits based on current knowledge.

Explainable deep learning approach for extracting cognitive features from hand-drawn images of intersecting pentagons

Hand drawing, which requires multiple neural systems for planning and controlling sequential movements, is a useful cognitive test for older adults. However, the conventional visual assessment of these drawings only captures limited attributes and overlooks subtle details that could help track cognitive states. Here, we utilized a deep-learning model, PentaMind, to examine cognition-related features from hand-drawn images of intersecting pentagons. PentaMind, trained on 13,777 images from 3111 participants in three aging cohorts, explained 23.3% of the variance in the global cognitive scores, 1.92 times more than the conventional rating. This accuracy improvement was due to capturing additional drawing features associated with motor impairments and cerebrovascular pathologies. By systematically modifying the input images, we discovered several important drawing attributes for cognition, including line waviness. Our results demonstrate that deep learning models can extract novel drawing metrics to improve the assessment and monitoring of cognitive decline and dementia in older adults.

Neighborhood-level social vulnerability and individual-level cognitive and motor functioning over time in older non-Latino Black and Latino adults

Introduction

Despite known health disparities in cognitive aging, a comprehensive rationale for the increased burden in older minoritized populations including non-Latino Black and Latino adults has yet to be elucidated. While most work has focused on person-specific risk, studies are increasingly assessing neighborhood-level risk. We evaluated multiple aspects of the environmental milieu that may be critical when considering vulnerability to adverse health outcomes.

Methods

We investigated associations between a Census-tract derived Social Vulnerability Index (SVI) and level of and change in cognitive and motor functioning in 780 older adults (590 non-Latino Black adults, ∼73 years old at baseline; 190 Latinos, ∼70 years old baseline). Total SVI scores (higher = greater neighborhood-level vulnerability) were combined with annual evaluations of cognitive and motor functioning (follow-up ranged from 2 to 18 years). Demographically-adjusted mixed linear regression models tested for associations between SVI and cognitive and motor outcomes in analyses stratified by ethno-racial group.

Results

For non-Latino Black participants, higher SVI scores were associated with lower levels of global cognitive and motor functioning-specifically, episodic memory, motor dexterity and gait-as well as longitudinal change in visuospatial abilities and hand strength. For Latinos, higher SVI scores were associated with lower levels of global motor functioning only-specifically, motor dexterity; there were no significant associations between SVI and change in motor functioning.

Discussion

Neighborhood-level social vulnerability is associated with cognitive and motor functioning in non-Latino Black and Latino older adults, although associations appear to contribute to level more so than longitudinal change.

Swimming and L-arginine loaded chitosan nanoparticles ameliorates aging-induced neuron atrophy, autophagy marker LC3, GABA and BDNF-TrkB pathway in the spinal cord of rats

Aging is associated with muscle atrophy, and erosion and destruction of neuronal pathways in the spinal cord. The study aim was to assess the effect of swimming training (Sw) and L-arginine loaded chitosan nanoparticles (LA-CNPs) on the sensory and motor neuron population, autophagy marker LC3, total oxidant status/total antioxidant capacity, behavioural test, GABA and BDNF-TrkB pathway in the spinal cord of aging rats. The rats were randomized to five groups: young (8-weeks) control (n = 7), old control (n = 7), old Sw (n = 7), old LA-CNPs (n = 7) and old Sw + LA-CNPs (n = 7). Groups under LA-CNPs supplementation received 500 mg/kg/day. Sw groups performed a swimming exercise programme 5 days per week for 6 weeks. Upon the completion of the interventions the rats were euthanized and the spinal cord was fixed and frozen for histological assessment, IHC, and gene expression analysis. The old group had more atrophy in the spinal cord with higher changes in LC3 as an indicator of autophagy in the spinal cord compared to the young group (p < 0.0001). The old Sw + LA-CNPs group increased (improved) spinal cord GABA (p = 0.0187), BDNF (p = 0.0003), TrkB (p < 0.0001) gene expression, decreased autophagy marker LC3 protein (p < 0.0001), nerve atrophy and jumping/licking latency (p < 0.0001), improved sciatic functional index score and total oxidant status/total antioxidant capacity compared to the old group (p < 0.0001). In conclusion, swimming and LA-CNPs seems to ameliorate aging-induced neuron atrophy, autophagy marker LC3, oxidant-antioxidant status, functional restoration, GABA and BDNF-TrkB pathway in the spinal cord of aging rats. Our study provides experimental evidence for a possible positive role of swimming and L-arginine loaded chitosan nanoparticles to decrease complications of aging.

Piezo2 expressing nociceptors mediate mechanical sensitization in experimental osteoarthritis

Non-opioid targets are needed for addressing osteoarthritis pain, which is mechanical in nature and associated with daily activities such as walking and climbing stairs. Piezo2 has been implicated in the development of mechanical pain, but the mechanisms by which this occurs remain poorly understood, including the role of nociceptors. Here we show that nociceptor-specific Piezo2 conditional knock-out mice were protected from mechanical sensitization associated with inflammatory joint pain in female mice, joint pain associated with osteoarthritis in male mice, as well as both knee swelling and joint pain associated with repeated intra-articular injection of nerve growth factor in male mice. Single cell RNA sequencing of mouse lumbar dorsal root ganglia and in situ hybridization of mouse and human lumbar dorsal root ganglia revealed that a subset of nociceptors co-express Piezo2 and Ntrk1 (the gene that encodes the nerve growth factor receptor TrkA). These results suggest that nerve growth factor-mediated sensitization of joint nociceptors, which is critical for osteoarthritic pain, is also dependent on Piezo2, and targeting Piezo2 may represent a therapeutic option for osteoarthritis pain control.

Interpretable deep learning approach for extracting cognitive features from hand-drawn images of intersecting pentagons in older adults

Hand drawing involves multiple neural systems for planning and precise control of sequential movements, making it a valuable cognitive test for older adults. However, conventional visual assessment of drawings may not capture intricate nuances that could help track cognitive states. To address this issue, we utilized a deep-learning model, PentaMind, to examine cognition-related features from hand-drawn images of intersecting pentagons. PentaMind, trained on 13,777 images from 3,111 participants in three aging cohorts, explained 23.3% of the variance in global cognitive scores, a comprehensive hour-long cognitive battery. The model’s performance, which was 1.92 times more accurate than conventional visual assessment, significantly improved the detection of cognitive decline. The improvement in accuracy was due to capturing additional drawing features that we found to be associated with motor impairments and cerebrovascular pathologies. By systematically modifying the input images, we discovered several important drawing attributes for cognition, including line waviness. Our results demonstrate that hand-drawn images can provide rich cognitive information, enabling rapid assessment of cognitive decline and suggesting potential clinical implications in dementia.

Brain and spinal cord arteriolosclerosis and its associations with cerebrovascular disease risk factors in community-dwelling older adults

Arteriolosclerosis is common in older brains and related to cognitive and motor impairment. We compared the severity of arteriolosclerosis and its associations with cerebrovascular disease risk factors (CVD-RFs) in multiple locations in the brain and spinal cord. Participants (n = 390) were recruited in the context of a longitudinal community-based clinical-pathological study, the Rush Memory and Aging Project. CVD-RFs were assessed annually for an average of 8.7 (SD = 4.3) years before death. The annual assessments included systolic (SBP) and diastolic (DBP) blood pressure, diabetes mellitus (DM), low- and high-density lipoprotein cholesterol, triglyceride, body mass index, and smoking. Postmortem pathological assessments included assessment of arteriolosclerosis severity using the same rating scale in three brain locations (basal ganglia, frontal, and parietal white matter regions) and four spinal cord levels (cervical, thoracic, lumbar and sacral levels). A single measure was used to summarize the severity of spinal arteriolosclerosis assessments at the four levels due to their high correlations. Average age at death was 91.5 (SD = 6.2) years, and 73% were women. Half showed arteriolosclerosis in frontal white matter and spinal cord followed by parietal white matter (38%) and basal ganglia (27%). The severity of arteriolosclerosis in all three brain locations showed mild-to-moderate correlations. By contrast, spinal arteriolosclerosis was associated with brain arteriolosclerosis only in frontal white matter. Higher DBP was associated with more severe arteriolosclerosis in all three brain locations. DM was associated with more severe arteriolosclerosis only in frontal white matter. Controlling for DBP, higher SBP was inversely associated with arteriolosclerosis in parietal white matter. Blood cholesterol and triglyceride, high body mass index, or smoking were not related to the severity of arteriolosclerosis in any brain region. None of the CVD-RFs were associated with the severity of spinal arteriolosclerosis. These data indicate that severity of arteriolosclerosis and its associations with CVD-RFs may vary in different CNS locations.

Changes in neuromuscular function in elders: Novel techniques for assessment of motor unit loss and motor unit remodeling with aging

Functional muscle fiber denervation is a major contributor to the decline in physical function observed with aging and is now a recognized cause of sarcopenia, a muscle disorder characterized by progressive and generalized degenerative loss of skeletal muscle mass, quality, and strength. There is an interrelationship between muscle strength, motor unit (MU) number, and aging, which suggests that a portion of muscle weakness in seniors may be attributable to the loss of functional MUs. During normal aging, there is a time-related progression of MU loss, an adaptive sprouting followed by a maladaptive sprouting, and continuing recession of terminal Schwann cells leading to a reduced capacity for compensatory reinnervation in elders. In amyotrophic lateral sclerosis, increasing age at onset predicts worse survival ALS and it is possible that age-related depletion of the motor neuron pool may worsen motor neuron disease. MUNE methods are used to estimate the number of functional MU, data from MUNIX arguing for motor neuron loss with aging will be reviewed. Recently, a new MRI technique MU-MRI could be used to assess the MU recruitment or explore the activity of a single MU. This review presents published studies on the changes of neuromuscular function with aging, then focusing on these two novel techniques for assessment of MU loss and MU remodeling.

Mixed Neuropathologies, Neural Motor Resilience and Target Discovery for Therapies of Late-Life Motor Impairment

By age 85, most adults manifest some degree of motor impairment. However, in most individuals a specific etiology for motor decline and treatment to modify its inexorable progression cannot be identified. Recent clinical-pathologic studies provide evidence that mixed-brain pathologies are commonly associated with late-life motor impairment. Yet, while nearly all older adults show some degree of accumulation of Alzheimer's disease and related dementias (ADRD) pathologies, the extent to which these pathologies contribute to motor decline varies widely from person to person. Slower or faster than expected motor decline in the presence of brain injury and/or pathology has been conceptualized as more or less "resilience" relative to the average person This suggests that other factors, such as lifestyles or other neurobiologic indices may offset or exacerbate the negative effects of pathologies via other molecular pathways. The mechanisms underlying neural motor resilience are just beginning to be illuminated. Unlike its cousin, cognitive resilience which is restricted to neural mechanisms above the neck, the motor system extends the total length of the CNS and beyond the CNS to reach muscle and musculoskeletal structures, all of which are crucial for motor function. Building on prior work, we propose that by isolating motor decline unrelated to neuropathologies and degeneration, investigators can identify genes and proteins that may provide neural motor resilience. Elucidating these molecular mechanisms will advance our understanding of the heterogeneity of late-life motor impairment. This approach will also provide high value therapeutic targets for drug discovery of therapies that may offset the negative motor consequences of CNS pathologies that are currently untreatable.

Circulating Cell-Free Genomic DNA Is Associated with an Increased Risk of Dementia and with Change in Cognitive and Physical Function

BACKGROUND

Altered cell homeostasis, seen in cognitive decline and frailty, leads to cell death and turnover, releasing circulating cell-free DNA (ccf-DNA).

OBJECTIVE

The goal of this study is to determine if serum genomic cell-free DNA (ccf-gDNA) is associated with physical and cognitive decline in older adults.

METHODS

We used serum from 631 community-dwelling individuals from the Religious Orders Study or Rush Memory and Aging Project who were without cognitive impairment at baseline. ccf-gDNA fragments in serum were quantified using digital PCR. An array of cognitive and physical traits, risk of dementia, global cognition, and frailty at or nearest the time of blood draw were regressed on ccf-DNA, with adjustment for age, sex, race, and education.

RESULTS

Cross-sectionally, higher ccf-gDNA levels were associated with lower global cognition score and slower gait speed at the evaluation nearest to blood draw. Higher ccf-gDNA levels were associated with increased odds of incident dementia (OR 1.27, 95% CI 1.05, 1.54). Longitudinally, higher levels of ccf-gDNA were associated with steeper general cognitive decline and worsening frailty over eight years of follow up.

CONCLUSION

This study demonstrates that ccf-gDNA fragments have utility for identifying persons at higher risk of developing dementia and worsening cognition and frailty.

[Vestibular compensation and aging: An example of cellular and behavioral resilience over time]

The vestibular system has a remarkable capacity of self-repair. Vestibular compensation, a model of post-lesional plasticity of the central nervous system, refers to a set of endogenous neuroplasticity mechanisms in the vestibular nuclei in response to damage to the peripheral vestibular system, and underlying functional recovery. During aging, this "homeostatic" plasticity, although still present, diminishes and is accompanied by sensorimotor and cognitive disturbances. Regardless of age, vestibular compensation can be improved by pharmacological therapy but also by rehabilitation based on strengthening other sensory modalities such as visual and proprioceptive modalities, but also cognitive and motor components. In this article, we will first discuss neurobiological mechanisms of vestibular compensation, then document the impact of aging on this adaptive plasticity.

Central and Peripheral Neuromuscular Adaptations to Ageing

Ageing is accompanied by a severe muscle function decline presumably caused by structural and functional adaptations at the central and peripheral level. Although researchers have reported an extensive analysis of the alterations involving muscle intrinsic properties, only a limited number of studies have recognised the importance of the central nervous system, and its reorganisation, on neuromuscular decline. Neural changes, such as degeneration of the human cortex and function of spinal circuitry, as well as the remodelling of the neuromuscular junction and motor units, appear to play a fundamental role in muscle quality decay and culminate with considerable impairments in voluntary activation and motor performance. Modern diagnostic techniques have provided indisputable evidence of a structural and morphological rearrangement of the central nervous system during ageing. Nevertheless, there is no clear insight on how such structural reorganisation contributes to the age-related functional decline and whether it is a result of a neural malfunction or serves as a compensatory mechanism to preserve motor control and performance in the elderly population. Combining leading-edge techniques such as high-density surface electromyography (EMG) and improved diagnostic procedures such as functional magnetic resonance imaging (fMRI) or high-resolution electroencephalography (EEG) could be essential to address the unresolved controversies and achieve an extensive understanding of the relationship between neural adaptations and muscle decline.