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Prediabetes and Alzheimer’s disease both increase in prevalence with age. The former is a risk factor for the latter, but a mechanistic linkage between them remains elusive. We show that prediabetic serum hyperinsulinemia is reflected in the cerebrospinal fluid and that this chronically elevated insulin renders neurons resistant to insulin. This leads to abnormal electrophysiological activity and other defects. In addition, neuronal insulin resistance reduces hexokinase 2, thus impairing glycolysis. This hampers the ubiquitination and degradation of p35, favoring its cleavage to p25, which hyperactivates CDK5 and interferes with the GSK3β-induced degradation of β-catenin. CDK5 contributes to neuronal cell death while β-catenin enters the neuronal nucleus and re-activates the cell cycle machinery. Unable to successfully divide, the neuron instead enters a senescent-like state. These findings offer a direct connection between peripheral hyperinsulinemia, as found in prediabetes, age-related neurodegeneration and cognitive decline. The implications for neurodegenerative conditions such as Alzheimer’s disease are described.

Increasing evidence indicates that terminally differentiated neurons in the brain may recommit to a cell cycle-like process during neuronal aging and under disease conditions. Because of the rare existence and random localization of these cells in the brain, their molecular profiles and disease-specific heterogeneities remain unclear. Through a bioinformatics approach that allows integrated analyses of multiple single-nucleus transcriptome datasets from human brain samples, these rare cell populations were identified and selected for further characterization. Our analyses indicated that these cell cycle-related events occur predominantly in excitatory neurons and that cellular senescence is likely their immediate terminal fate. Quantitatively, the number of cell cycle re-engaging and senescent neurons decreased during the normal brain aging process, but in the context of late-onset Alzheimer’s disease (AD), these cells accumulate instead. Transcriptomic profiling of these cells suggested that disease-specific differences were predominantly tied to the early stage of the senescence process, revealing that these cells presented more proinflammatory, metabolically deregulated, and pathology-associated signatures in disease-affected brains. Similarly, these general features of cell cycle re-engaging neurons were also observed in a subpopulation of dopaminergic neurons identified in the Parkinson’s disease (PD)-Lewy body dementia (LBD) model. An extended analysis conducted in a mouse model of brain aging further validated the ability of this bioinformatics approach to determine the robust relationship between the cell cycle and senescence processes in neurons in this cross-species setting.

Key Points Although ageing is a difficult concept to define biologically, the accumulation of unrepaired DNA damage and the accompanying loss of genomic integrity are now regarded as signatures of ageing. DNA damage in neurons probably begins accumulating even during the developmental process, and is also a by-product of normal physiological functions such as gene transcription and enhanced neuronal activity. Neurons are permanently postmitotic and therefore cannot perform double-stranded break repair through the more accurate homologous recombination pathway. Consequently, neurons adopt a selective repair approach in which genes that are actively transcribed are repaired more than elements in the rest of the genome. Neither the transcriptional programme nor the activity pattern of any two neurons is likely to be identical. Therefore, the regions of aggressive DNA repair, as well as those that are poorly maintained, probably vary from cell to cell. The clear implication of this genetic heterogeneity is that with age, the genome of each and every neuron in the brain evolves a unique configuration of 'stress marks'. These differences may lead to regionally variable genetic patterns that are somatic in origin and may underlie the different molecular phenotypes observed in different neurodegenerative diseases. Many researchers have commented on the relationship between mutations in components of the DNA repair machinery and the occurrence of major developmental abnormalities and syndromes that include premature ageing. However, little is known about the DNA repair capacity of the cells of the ageing brain. As DNA repair is a lifelong process and because an early developmental arrest would mask the consequences of a somatically acquired loss of DNA repair fidelity, virtually nothing is known about how non-genomic factors, such as a gradual change in cellular signalling, epigenetic landscape, or even the expression pattern of non-protein coding miRNAs, may better explain the cause of various ageing-related neurodegenerative disorders. Ageing leads to the gradual loss of brain function and is a key risk factor for most late-onset neurological disease. In this Review, Chow and Herrup explore how the loss of DNA integrity drives brain ageing and contributes to the pathogenesis of many seemingly unrelated conditions. DNA damage is correlated with and may drive the ageing process. Neurons in the brain are postmitotic and are excluded from many forms of DNA repair; therefore, neurons are vulnerable to various neurodegenerative diseases. The challenges facing the field are to understand how and when neuronal DNA damage accumulates, how this loss of genomic integrity might serve as a 'time keeper' of nerve cell ageing and why this process manifests itself as different diseases in different individuals.

MicroRNAs (miRNAs) are small non-coding RNAs that play crucial roles in post-transcriptional gene regulation. Poly(A) RNA polymerase D5 (PAPD5) catalyzes the addition of adenosine to the 3′ end of miRNAs. In this study, we demonstrate that the Yin Yang 1 protein, a transcriptional repressor of PAPD5, is recruited to both RNA foci and protein aggregates, resulting in an upregulation of PAPD5 expression in Huntington’s disease (HD). Additionally, we identify a subset of PAPD5-regulated miRNAs with increased adenylation and reduced expression in our disease model. We focus on miR-7-5p and find that its reduction causes the activation of the TAB2-mediated TAK1–MKK4–JNK pro-apoptotic pathway. This pathway is also activated in induced pluripotent stem cell-derived striatal neurons and post-mortem striatal tissues isolated from HD patients. In addition, we discover that a small molecule PAPD5 inhibitor, BCH001, can mitigate cell death and neurodegeneration in our disease models. This study highlights the importance of PAPD5-mediated miRNA dysfunction in HD pathogenesis and suggests a potential therapeutic direction for the disease. PAPD5 is responsible for adenylation of microRNAs. Here, the authors show that elevated level of PAPD5 enhances the adenylation and reduced expression of miR-7-5p . As a result, expression of TAB2, a target of miR-7-5p , is induced triggering neuronal apoptosis in Huntington’s disease.

Glycogen synthase kinase 3β (GSK3β) and cyclin-dependent kinase 5 (CDK5) are tau kinases and have been proposed to contribute to the pathogenesis of Alzheimer’s disease. The 3D structures of these kinases are remarkably similar, which led us to hypothesize that both might be capable of binding cyclin proteins—the activating cofactors of all CDKs. CDK5 is normally activated by the cyclin-like proteins p35 and p39. By contrast, we show that GSK3β does not bind to p35 but unexpectedly binds to p25, the calpain cleavage product of p35. Indeed, overexpressed GSK3β outcompetes CDK5 for p25, whereas CDK5 is the preferred p35 partner. FRET analysis reveals nanometer apposition of GSK3β:p25 in cell soma as well as in synaptic regions. Interaction with p25 also alters GSK3β substrate specificity. The GSK3β:p25 interaction leads to enhanced phosphorylation of tau, but decreased phosphorylation of β-catenin. A partial explanation for this situation comes from in silico modeling, which predicts that the docking site for p25 on GSK3β is the AXIN-binding domain; because of this, p25 inhibits the formation of the GSK3β/AXIN/APC destruction complex, thus preventing GSK3β from binding to and phosphorylating β-catenin. Coexpression of GSK3β and p25 in cultured neurons results in a neurodegeneration phenotype that exceeds that observed with CDK5 and p25. When p25 is transfected alone, the resulting neuronal damage is blocked more effectively with a specific siRNA against Gsk3β than with one against Cdk5 . We propose that the effects of p25, although normally attributed to activate CDK5, may be mediated in part by elevated GSK3β activity. Significance CDK5 and GSK3β are recognized as interrelated kinases; they share a strong structural resemblance, and both are known tau kinases that contribute to the etiology of Alzheimer’s disease. We report here that p25 but not p35, the normal cyclin-like activator of CDK5, unexpectedly binds to GSK3β in the AXIN-binding region. The binding of p25 increases GSK3β activity and alters its substrate specificity. Results, both in vivo and in vitro, suggest that many of the effects of p25 previously assumed to be due to hyperactivation of CDK5 must now be reexamined for the potential role of altered GSK3β activity. This result carries important implications for how we approach disease-modifying strategies for the treatment of Alzheimer’s and other neurodegenerative diseases.

Background Late-onset Alzheimer’s disease (LOAD) is the most common form of dementia; it disproportionally affects women in terms of both incidence rates and severity of progression. The cellular and molecular mechanisms underlying this clinical phenomenon remain elusive and ill-defined. Methods In-depth analyses were performed with multiple human LOAD single-nucleus transcriptome datasets to thoroughly characterize cell populations in the cerebral cortex. ROSMAP bulk human brain tissue transcriptome and DNA methylome datasets were also included for validation. Detailed assessments of microglial cell subpopulations and their relevance to sex-biased changes at the tissue level were performed. Clinical trait associations, cell evolutionary trajectories, and transcription regulon analyses were conducted. Results The relative numbers of functionally defective microglia were aberrantly increased uniquely among affected females. Substratification of the microglia into different subtypes according to their transcriptomic signatures identified a group of female-enriched and disease-associated microglia (FDAMic), the numbers of which were positively associated with disease severity. Phenotypically, these cells exhibit transcriptomic signatures that support active proliferation, MHC class II autoantigen presentation and amyloid-β binding, but they are also likely defective in phagocytosis. FDAMic are likely evolved from female activated response microglia (ARMic) with an APOE4 background and compromised estrogen receptor (ER) signaling that is deemed to be active among most subtypes of microglia. Conclusion This study offered important insights at both the cellular and molecular levels into how ER signaling affects microglial heterogeneity and function. FDAMic are associated with more advanced pathologies and severe trends of cognitive decline. Their emergence could, at least in part, explain the phenomenon of greater penetrance of the APOE4 genotype found in females. The biases of FDAMic emergence toward female sex and APOE4 s tatus may also explain why hormone replacement therapy is more effective in APOE4 carriers. The pathologic nature of FDAMic suggests that selective modulations of these cells may help to regain brain neuroimmune homeostasis, serving as a new target for future drug development.

Sex-biased differences in Alzheimer’s disease (AD) are well documented, but the mechanisms underlying increased vulnerability in postmenopausal women remain unclear. This study aimed to model the effects of perimenopausal hormonal fluctuations on AD pathophysiology. Using a VCD-induced accelerated ovarian failure model in young female C57BL/6 J and 3xTg mice, we simulated a perimenopausal state with hormonal changes characterised by elevated oestradiol levels and reduced progesterone levels. Supporting human brain transcriptomic and metabolomic data from the ROSMAP study revealed that impaired oestrogen-related receptor alpha (ERRα) function was a key driver of female sex-biased vulnerability. In female mice, progesterone-guided oestrogen receptor signalling maintained ERRα activity by regulating neuronal cholesterol homoeostasis and the TCA cycle. Hormonal imbalances disrupted this mechanism, triggering an aspartate-driven “minicycle,” which increased glutamate release, neuronal excitability, ATP depletion, and energy crisis susceptibility. This study demonstrates how perimenopausal hormonal imbalances exacerbate AD risk via ERRα dysfunction, linking neuronal cholesterol and energy homeostasis to disease vulnerability. Low progesterone-to-oestradiol ratio during perimenopause increases Alzheimer’s disease (AD) risk by disrupting ERRα, hence impairing neuronal cholesterol and energy balance. ERRα is a key regulator linking peripheral hormonal changes to female-biased AD susceptibility.

The maintenance of metabolic homeostasis relies on the ability to flexibly transit between catabolic and anabolic states in response to insulin signaling. Here we show insulin-activated ATM is a critical mediator of this process, facilitating the swift transition between catabolic-and-anabolic fates of glucose by regulating the functional status of PKM2 and HIF1α. In Ataxia-Telangiectasia (A-T), these mechanisms are disrupted, resulting in intrinsic insulin resistance and glucose intolerance. Consequently, cells exhibit a compensatory dependence on glutamine as an alternative metabolite for energy metabolism. Cerebellar degeneration, a hallmark of A-T, is characterized by the pronounced vulnerability of Purkinje cells, attributed to their unexpected sensitivity to insulin. Supplementation with α-ketoglutarate, the α-keto acid backbone of glutamine, has demonstrated potentials in alleviating glutamine dependence and attenuating Purkinje cell degeneration. These findings suggest that peripheral metabolic deficiencies may contribute to sustained neurodegenerative changes in A-T, underscoring the importance of screening, monitoring and addressing these metabolic disruptions in patients. Insulin-activated ataxia-telangiectasia mutated (ATM) regulates glucose metabolism. Here the authors report that its disruption in a mouse model of ataxia-telangiectasia leads to insulin resistance, glutamine dependence, and selective Purkinje cell degeneration, while α-Ketoglutarate supplementation shows promise in mitigating neurodegeneration.

Early changes in astrocyte energy metabolism are associated with late‐onset Alzheimer's disease (LOAD), but the underlying mechanism remains elusive. A previous study suggested an association between a synonymous SNP (rs1012672, C→T) in LRP6 gene and LOAD; and that is indeed correlated with diminished LRP6 gene expression in the frontal cortex region. The authors show that LRP6 is a unique Wnt coreceptor on astrocytes, serving as a bimodal switch that modulates their metabolic landscapes. The Wnt‐LRP6 mediated mTOR‐AKT axis is essential for sustaining glucose metabolism. In its absence, Wnt switches to activate the LRP6‐independent Ca2+‐PKC‐NFAT axis, resulting in a transcription network that favors glutamine and branched chain amino acids (BCAAs) catabolism over glucose metabolism. Exhaustion of these raw materials essential for neurotransmitter biosynthesis and recycling results in compromised synaptic, cognitive, and memory functions; priming for early changes that are frequently found in LOAD. The authors also highlight that intranasal supplementation of glutamine and BCAAs is effective in preserving neuronal integrity and brain functions, proposing a nutrient‐based method for delaying cognitive and memory decline when LRP6 cell surface levels and functions are suboptimal. Astrocytes are the major brain metabolic workhorses and altered energy metabolism is associated with late‐onset Alzheimer's disease. Differential Wnt downstream signaling modulates the metabolic landscape in these cells. Intranasal supplementation of enhanced demand on glutamine and branched‐chain amino acids may help to preserve neuronal integrity and brain functions; suggesting an alternative nutrient‐based method for delaying cognitive and memory decline.

Microglial cells consume adenosine triphosphate (ATP) during phagocytosis to clear neurotoxic β-amyloid in Alzheimer’s disease (AD). However, the contribution of energy metabolism to microglial function in AD remains unclear. Here, we demonstrate that hexokinase 2 (HK2) is elevated in microglia from an AD mouse model (5xFAD) and AD patients. Genetic deletion or pharmacological inhibition of HK2 significantly promotes microglial phagocytosis, lowers the amyloid plaque burden and attenuates cognitive impairment in male AD mice. Notably, the ATP level is dramatically increased in HK2-deficient or inactive microglia, which can be attributed to a marked upregulation in lipoprotein lipase (LPL) expression and subsequent increase in lipid metabolism. We further show that two downstream metabolites of HK2, glucose-6-phosphate and fructose-6-phosphate, can reverse HK2-deficiency-induced upregulation of LPL, thus supporting ATP production and microglial phagocytosis. Our findings uncover a crucial role for HK2 in phagocytosis through regulation of microglial energy metabolism, suggesting a potential therapeutic strategy for AD by targeting HK2. Accumulation of β-amyloid plaques contributes to neuronal cell death in Alzheimer’s disease. In this study, Leng et al. describe a role for hexokinase 2 in metabolic reconfiguration in microglia that promotes phagocytosis and supports amyloid plaque clearance.