Individuals with Alzheimer's disease (AD) are at an increased risk of bone fracture, while osteoporosis in women is one of the earliest predictors of AD. Yet the mechanisms linking cognitive decline and skeletal deterioration remain poorly defined. Proteomic analysis of cortical bone from aged 21-month-old mice revealed strong enrichment of neurodegeneration-associated proteins, including apolipoprotein E (Apoe) and amyloid precursor protein. Apoe localized specifically to osteocytes, with expression in aged female bone nearly twice that of young 4-month-old male bone. Because human APOE alleles confer different age-related AD risks, we examined their roles in bone using humanized APOE2, APOE3, and APOE4 knock-in mice and analyzed bone and hippocampus from the same animals. APOE4 produced marked sex-specific effects on the bone transcriptome and proteome compared with APOE2 or APOE3. Strikingly, APOE4-associated proteomic disruptions were stronger in female bone than in the hippocampus. Functionally, APOE4 caused bone fragility in females without altering cortical structure. These deficits stemmed from impaired osteocyte perilacunocanalicular remodeling. Our findings identify APOE4 as a molecular driver of early osteocyte dysfunction and reduced bone quality, disproportionately affecting females. These findings highlight osteocytes as potential targets for early diagnosis of age-related cognitive impairment and treatment for bone fragility, in females.

Efforts to rejuvenate age-related cognitive decline have predominantly targeted neurons, often overlooking non-neuronal cell types in the aging brain. Here, we show that countering alterations in oligodendrocyte-derived extracellular matrix (ECM) in the aging hippocampus restores cognition. We identify broad age-associated transcriptional and proteomic changes in oligodendrocytes, including dysregulation of the matrisome, with marked upregulation of ECM components and associated regulators with age. Among these, we detect an increase in Hyaluronan and proteoglycan link protein 2 (HAPLN2), an oligodendrocyte-derived core matrisome protein that locates specifically at the nodes of Ranvier, in the hippocampus of aged mice and older humans. Hapln2 overexpression in oligodendrocytes of young mice recapitulated age-related memory impairments. Conversely, abrogating the age-related increase in Hapln2 induced synaptic plasticity-related hippocampal transcriptional signatures and improved memory in aged mice. Together, these data define oligodendrocyte-derived ECM remodeling as a hallmark of brain aging that can be targeted to rescue cognitive decline.

Changes in peripheral CD8+ T cells are a hallmark of immune aging. However, the role of aged non-infiltrating CD8+ T cells in brain aging remains to be fully defined. Here, we showed that aged circulating CD8+ T cells and their secreted factors drove hippocampal-dependent cognitive decline. Using heterochronic parabiosis and transcriptomics analysis, we observed that peripheral CD8+ T cells maintained properties intrinsic to their age. Systemic exposure of young mice to aged CD8+ T cells elicited synaptic-related hippocampal changes and impaired cognition, and inhibiting activation, but not infiltration, mitigated their pro-aging effects. Conversely, targeting aged circulating CD8+ T cells restored youthful signatures and rescued cognition. Mechanistically, we identified granzyme K (GZMK) as a secreted pro-aging CD8+ T cell-derived factor in plasma, and GZMK inhibition rescued cognition in aged animals. Together, our data identified activated aged CD8+ T cell-derived circulating factors as potential therapeutic targets to rescue cognition in old age.

Blood factors transfer the benefits of exercise to the aged brain independent of physical activity. Here, we show that the liver-derived exercise factor (exerkine) glycosylphosphatidylinositol (GPI)-specific phospholipase D1 (GPLD1), a GPI-degrading enzyme, reverses aging- and Alzheimer's-related memory loss by targeting the brain vasculature. GPLD1 has the potential to cleave over 100 putative GPI-anchored proteins, necessitating the identification of downstream targets that mediate cognitive rejuvenation for translational application. We identified GPI-anchored tissue-nonspecific alkaline phosphatase (TNAP) on the brain vasculature as a GPLD1 substrate. Mimicking age-related increases in cerebrovascular TNAP impaired blood-brain transport and cognition in young mice and mitigated GPLD1-induced cognitive benefits in aged mice. Inhibiting TNAP recapitulated the benefits of GPLD1 in old age, restoring youthful hippocampal transcriptional signatures and rescuing cognition. In an Alzheimer's disease model, increasing GPLD1 or inhibiting TNAP ameliorated Aβ pathology and improved cognitive deficits. We thus identify brain vasculature as a mediator of the cognitive benefits of a liver-to-brain exercise axis.

Cellular rejuvenation through transcriptional reprogramming is an exciting approach to counter aging. Using a fibroblast-based model of human cell aging and Perturb-seq screening, we developed a systematic approach to identify single transcription factor (TF) perturbations that promote rejuvenation without dedifferentiation. Overexpressing E2F3 or EZH2, and repressing STAT3 or ZFX, reversed cellular hallmarks of aging-increasing proliferation, proteostasis, and mitochondrial activity, while decreasing senescence. EZH2 overexpression in vivo rejuvenated livers in aged mice, reversing aging-associated gene expression profiles, decreasing steatosis and fibrosis, and improving glucose tolerance. Mechanistically, single TF perturbations led to convergent downstream transcriptional programs conserved in different aging and rejuvenation models. These results suggest a shared set of molecular requirements for cellular and tissue rejuvenation across species.

Understanding cellular and molecular drivers of age-related cognitive decline is necessary to identify targets to restore cognition at old age. Here we identify ferritin light chain 1 (FTL1), an iron-associated protein, as a pro-aging neuronal factor that impairs cognition. Using transcriptomic and mass spectrometry approaches, we detect an increase in neuronal FTL1 in the hippocampus of aged mice, the levels of which correlate with cognitive decline. Mimicking an age-related increase in neuronal FTL1 in young mice alters labile iron oxidation states and promotes synaptic and cognitive features of hippocampal aging. Targeting neuronal FTL1 in the hippocampi of aged mice improves synaptic-related molecular changes and cognitive impairments. Using neuronal nuclei RNA sequencing, we detect changes in metabolic processes, such as ATP synthesis, and boosting these metabolic functions through NADH supplementation mitigated pro-aging effects of neuronal FTL1 on cognition. Our data identify neuronal FTL1 as a key molecular mediator of cognitive rejuvenation.

Individuals diagnosed with Alzheimer's disease (AD) are at an increased risk of bone fractures. Conversely, a diagnosis of osteoporosis in women is the earliest known predictor for AD. However, mechanisms responsible for the coupled decline in cognitive and skeletal health remain unclear. Proteomic analysis of cortical bone from aged mice revealed neurological disease-associated proteins that are highly enriched in aged mouse bones, including apolipoprotein E (Apoe) and amyloid precursor protein. Further, Apoe localized specifically to bone-embedded osteocytes with expression twice as high in aged female bone as in young or male counterparts. In humans, APOE allele variants carry differing AD risk with age. To investigate APOE allelic roles in bone, we utilized a humanized APOE knock-in mouse model that expresses either the protective APOE2, the neutral APOE3, or the AD risk factor APOE4, and analyzed bone and hippocampus from the same mice. APOE4 exerted strong sex-specific effects on the bone transcriptome and proteome, relative to APOE2 or APOE3. Interestingly, the APOE4-associated perturbation in the female bone proteome was more pronounced than the corresponding alterations observed in the hippocampus. APOE4 protein causes bone fragility in females, but not males, even without changes in cortical bone structure. These bone quality deficits arose from suppression of osteocyte perilacunocanalicular remodeling. We find that APOE4 is a new molecular culprit capable of disrupting osteocyte maintenance of bone quality as early as midlife in a manner that disproportionately affects females. These findings highlight osteocytes as potential targets for early diagnosis of age-related cognitive impairment, and treatment for bone fragility, in females.

BACKGROUND:Many neurodegenerative disease treatments, such as deep brain stimulation for Parkinson's Disease, can alleviate symptoms by primarily compensating for circuit dysfunctions. However, the stimulation's effect on the underlying disease progression remains relatively unknown. Here, we report that neuromodulation can not only modulate circuit function but also modulate the in vivo spreading dynamics of α-synuclein pathology, the primary pathological hallmark observed in Parkinson's Disease. METHODS:In a mouse model, pre-formed fibrils were injected into the striatum to induce widespread α-synuclein pathology. Two days after fibril injection, mice were treated for two weeks with daily optogenetic stimulation of the Secondary Motor Area, Layer V. Whole brains were then extracted, immunolabeled, cleared, and imaged with light-sheet fluorescent microscopy. RESULTS:Repeated optogenetic stimulation led to a decrease in pathology at the site of stimulation and at various cortical and subcortical regions, while the contralateral cortex saw a consistent increase. Aligning the pathology changes with optogenetic-fMRI measured brain activity, we found that the changes in pathology and brain function had similar spatial locations but opposite polarity. CONCLUSION:These results demonstrate the ability to modulate and predict whole brain pathology changes using neuromodulation, opening a new horizon for investigating optimized neuromodulation therapies.

Loss of cognitive function with age is devastating. EGL-30/GNAQ and Gaq signaling pathways are highly conserved between C. elegans and mammals, and murine Gnaq is enriched in hippocampal neurons and declines with age. We found that activation of EGL-30 in aged worms triples memory span, and GNAQ gain of function significantly improved memory in aged mice: GNAQ(gf) in hippocampal neurons of 24-month-old mice (equivalent to 70-to 80-year-old humans) rescued age-related impairments in well-being and memory. Single-nucleus RNA sequencing revealed increased expression of genes regulating synaptic function, axon guidance, and memory in GNAQ-treated mice, and worm orthologs of these genes were required for longterm memory extension in worms. These experiments demonstrate that C. elegans is a powerful model to identify mammalian regulators of memory, leading to the identification of a pathway that improves memory in extremely old mice. To our knowledge, this is the oldest age at which an intervention has improved age related cognitive decline.

The beneficial effects of physical activity on brain ageing are well recognised, with exerkines, factors that are secreted into the circulation in response to exercise, emerging as likely mediators of this response. However, the source and identity of these exerkines remain unclear. Here we provide evidence that an anti-geronic exerkine is secreted by platelets. We show that platelets are activated by exercise and are required for the exercise-induced increase in hippocampal precursor cell proliferation in aged mice. We also demonstrate that increasing the systemic levels of the platelet-derived exerkine CXCL4/platelet factor 4 (PF4) ameliorates age-related regenerative and cognitive impairments in a hippocampal neurogenesis-dependent manner. Together these findings highlight the role of platelets in mediating the rejuvenating effects of exercise during physiological brain ageing.

Most reports agree that aging negatively impacts pain processing and that the prevalence of chronic pain increases significantly with age. To improve current therapies, it is critical that aged animals be included in preclinical studies. Here we compared sensitivities to pain and itch-provoking stimuli in naïve and injured young and aged mice. Surprisingly, we found that in the absence of injury, aged male and female mice are significantly less responsive to mechanical stimuli and, in females, also to noxious thermal (heat) stimuli. In both older male and female mice, compared to younger (6-month-old mice), we also recorded reduced pruritogen-evoked scratching. On the other hand, after nerve injury, aged mice nevertheless developed significant mechanical hypersensitivity. Interestingly, however, and in contrast to young mice, aged mice developed both ipsilateral and contralateral postinjury mechanical allodynia. In a parallel immunohistochemical analysis of microglial and astrocyte markers, we found that the ipsilateral to the contralateral ratio of nerve injury-induced expression decreased with age. That observation is consistent with our finding of contralateral hypersensitivity after nerve injury in the aged but not the young mice. We conclude that aging has opposite effects on baseline versus postinjury pain and itch processing. PERSPECTIVE: Aged male and female mice (22-24 months) are less sensitive to mechanical, thermal (heat), and itch-provoking stimuli than are younger mice (6 months).

Identifying therapeutics to delay, and potentially reverse, age-related cognitive decline is critical in light of the increased incidence of dementia-related disorders forecasted in the growing older population 1 . Here we show that platelet factors transfer the benefits of young blood to the ageing brain. Systemic exposure of aged male mice to a fraction of blood plasma from young mice containing platelets decreased neuroinflammation in the hippocampus at the transcriptional and cellular level and ameliorated hippocampal-dependent cognitive impairments. Circulating levels of the platelet-derived chemokine platelet factor 4 (PF4) (also known as CXCL4) were elevated in blood plasma preparations of young mice and humans relative to older individuals. Systemic administration of exogenous PF4 attenuated age-related hippocampal neuroinflammation, elicited synaptic-plasticity-related molecular changes and improved cognition in aged mice. We implicate decreased levels of circulating pro-ageing immune factors and restoration of the ageing peripheral immune system in the beneficial effects of systemic PF4 on the aged brain. Mechanistically, we identified CXCR3 as a chemokine receptor that, in part, mediates the cellular, molecular and cognitive benefits of systemic PF4 on the aged brain. Together, our data identify platelet-derived factors as potential therapeutic targets to abate inflammation and rescue cognition in old age.

Cognitive decline is perhaps the most devastating aging loss. EGL-30/GNAQ and Gαq signaling pathways are highly conserved between C. elegans and mammals. We find that activation of EGL-30 in aged worms at least triples memory span, and we wondered if this highly conserved pathway could also improve memory in very old mice. Murine Gnaq is enriched in hippocampal excitatory neurons and declines with age. Furthermore, GNAQ gain-of-function significantly improved memory in aged mice: GNAQ(gf) in hippocampal neurons of 24-month-old mice rescued age-related impairments in health metrics and long-term memory. Single-nucleus RNAseq revealed gene expression changes related to synaptic function, axon guidance, and learning and memory pathways. Several worm orthologs of mouse genes upregulated by GNAQ(gf) overexpression are required for EGL-30(gf)-dependent memory improvement. These results demonstrate that the molecular and genetic pathways between C. elegans and mammals are highly conserved, as activation of EGL-30/GNAQ, a pathway first identified in worms, rejuvenates cognitive function in two-year old mice (the equivalent of 70-80 yo humans). To our knowledge, this is the oldest age an intervention has successfully improved age-related cognitive decline. One-Sentence Summary Neuronal activation of the Gαq protein EGL-30/GNAQ restores long-term memory at old age in worms and mice.

Aging induces molecular, cellular and functional changes in the adult brain that drive cognitive decline and increase vulnerability to dementia-related neurodegenerative diseases. Leveraging systemic and lifestyle interventions, such as heterochronic parabiosis, administration of ‘young blood’, exercise and caloric restriction, has challenged prevalent views of brain aging as a rigid process and has demonstrated that aging-associated cognitive and cellular impairments can be restored to more youthful levels. Technological advances in proteomic and transcriptomic analyses have further facilitated investigations into the functional impact of intertissue communication on brain aging and have led to the identification of a growing number of pro-aging and pro-youthful factors in blood. In this review, we discuss blood-to-brain communication from a systems physiology perspective with an emphasis on blood-derived signals as potent drivers of both age-related brain dysfunction and brain rejuvenation. In this Review, Villeda and colleagues describe blood-to-brain communication from a systems physiology perspective, with an emphasis on blood-derived signals as potent drivers of both age-related brain dysfunction and brain rejuvenation.

[This corrects the article DOI: 10.3389/fneur.2014.00012.].

DNA methylation has emerged as a critical modulator of neuronal plasticity and cognitive function. Notwithstanding, the role of enzymes that demethylate DNA remain to be fully explored. Here, we report that loss of ten-eleven translocation methylcytosine dioxygenase 2 (Tet2), which catalyzes oxidation of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC), in adult neurons enhances cognitive function. In the adult mouse hippocampus, we detected an enrichment of Tet2 in neurons. Viral-mediated neuronal overexpression and RNA interference of Tet2 altered dendritic complexity and synaptic-plasticity-related gene expression in vitro. Overexpression of neuronal Tet2 in adult hippocampus, and loss of Tet2 in adult glutamatergic neurons, resulted in differential hydroxymethylation associated with genes involved in synaptic transmission. Functionally, overexpression of neuronal Tet2 impaired hippocampal-dependent memory, while loss of neuronal Tet2 enhanced memory. Ultimately, these data identify neuronal Tet2 as amolecular target to boost cognitive function.

An emerging view regarding neurodegenerative diseases is that discreet seeding of misfolded proteins leads to widespread pathology. However, the mechanisms by which misfolded proteins seed distinct brain regions and cause differential whole-brain pathology remain elusive. We used whole-brain tissue clearing and high-resolution imaging to longitudinally map pathology in an α-synuclein pre-formed fibril injection model of Parkinson's disease. Cleared brains at different time points of disease progression were quantitatively segmented and registered to a standardized atlas, revealing distinct phases of spreading and decline. We then fit a computational model with parameters that represent α-synuclein pathology spreading, aggregation, decay, and gene expression pattern to this longitudinal dataset. Remarkably, our model can generalize to predicting α-synuclein spreading patterns from several distinct brain regions and can even estimate their origins. This model empowers mechanistic understanding and accurate prediction of disease progression, paving the way for the development and testing of therapeutic interventions.

Proteins of the major histocompatibility complex class I (MHC I), predominantly known for antigen presentation in the immune system, have recently been shown to be necessary for developmental neural refinement and adult synaptic plasticity. However, their roles in nonneuronal cell populations in the brain remain largely unexplored. Here, we identify classical MHC I molecule H2-Kb as a negative regulator of proliferation in neural stem and progenitor cells (NSPCs). Using genetic knockout mouse models and in vivo viral-mediated RNA interference (RNAi) and overexpression, we delineate a role for H2-Kb in negatively regulating NSPC proliferation and adult hippocampal neurogenesis. Transcriptomic analysis of H2-Kb knockout NSPCs, in combination with in vitro RNAi, overexpression, and pharmacological approaches, further revealed that H2-Kb inhibits cell proliferation by dampening signaling pathways downstream of fibroblast growth factor receptor 1 (Fgfr1). These findings identify H2-Kb as a critical regulator of cell proliferation through the modulation of growth factor signaling.