Targeting Astrocyte Signaling Alleviates Cerebrovascular and Synaptic Function Deficits in a Diet-Based Mouse Model of Small Cerebral Vessel Disease

Role and Relationship Between Homocysteine and H2S in Ischemic Stroke

Homocysteine (Hcy), a sulfur-containing amino acid, is an important intermediate product of methionine metabolism. Hcy can be either metabolized to cysteine, a precursor for glutathione synthesis and hydrogen sulfide (H2S) production, or regenerated back to methionine. Besides, the Hcy metabolism is central to supply methyl groups, which are essential for DNA methylation. In the transsulfuration pathway of Hcy metabolism, Hcy is metabolized to form cysteine and H2S by catalytic enzymes, containing cystathionine-β-synthase (CBS) and cystathionine-γ-lyase (CSE). Hcy metabolism-related enzymes and coenzymes, such as vitamin B6, vitamin B12, and folic acid, are closely related to hyperhomocysteinemia (HHcy), which is frequently accompanied by reduced H2S content. An accumulating study has revealed that HHcy is a risk factor for ischemic stroke, while H2S, served as a gaseous mediator at the physiological level, has protective effects against ischemic stroke. This review outlined the literature data from recent research related to Hcy metabolism and H2S production and described the roles and relationship among Hcy metabolism and H2S in ischemic stroke.

Nicorandil treatment improves survival and spatial learning in aged granulin knockout mice

Mutations in the human granulin (GRN) gene are associated with multiple diseases, including dementia disorders such as frontotemporal dementia (FTD) and limbic-predominant age-related TDP-43 encephalopathy (LATE). We studied a Grn knockout (Grn-KO) mouse model in order to evaluate a potential therapeutic strategy for these diseases using nicorandil, a commercially available agonist for the ABCC9/Abcc9-encoded regulatory subunit of the "K+ATP" channel that is well-tolerated in humans. Aged (13 months) Grn-KO and wild-type (WT) mice were treated as controls or with nicorandil (15 mg/kg/day) in drinking water for 7 months, then tested for neurobehavioral performance, neuropathology, and gene expression. Mortality was significantly higher for aged Grn-KO mice (particularly females), but there was a conspicuous improvement in survival for both sexes treated with nicorandil. Grn-KO mice performed worse on some cognitive tests than WT mice, but Morris Water Maze performance was improved with nicorandil treatment. Neuropathologically, Grn-KO mice had significantly increased levels of glial fibrillary acidic protein (GFAP)-immunoreactive astrocytosis but not ionized calcium binding adaptor molecule 1 (IBA-1)-immunoreactive microgliosis, indicating cell-specific inflammation in the brain. Expression of several astrocyte-enriched genes, including Gfap, were also elevated in the Grn-KO brain. Nicorandil treatment was associated with a subtle shift in a subset of detected brain transcript levels, mostly related to attenuated inflammatory markers. Nicorandil treatment improved survival outcomes, cognition, and inflammation in aged Grn-KO mice.

Inhibition of astrocyte signaling leads to sex-specific changes in microglia phenotypes in a diet-based model of small cerebral vessel disease

Hyperhomocysteinemia (HHcy)-inducing diets recapitulate small cerebral vessel disease phenotypes in mice including cerebrovascular pathology/dysfunction, neuroinflammation, synaptic deficits, and cognitive decline. We recently showed that astrocyte signaling through calcineurin(CN)/nuclear factor of activated T cells (NFATs) plays a causative role in these phenotypes. Here, we assessed the impact of astrocytic signaling on microglia, which set inflammatory tone in brain. Seven-to-eight-week-old male and female C57BL/6J mice received intrahippocampal injections of AAV2/5-Gfa2-EGFP (control) or adeno-associated virus (AAV) expressing the NFAT inhibitor VIVIT (i.e., AAV2/5-Gfa2-VIVIT-EGFP). Mice were then fed with control chow (CT) or B-vitamin-deficient chow for 12 weeks to induce HHcy. Immunohistochemistry was used to assess the expression of the pan-microglial marker Iba1 and the homeostatic microglial marker P2ry12. Iba1 showed little sensitivity to diet, AAV treatment, or sex. Conversely, P2ry12 expression was reduced with HHcy diet in males, but not females. Treatment of males with AAV-Gfa2-VIVIT prevented the loss of P2ry12. We next conducted single-cell RNA sequencing (scRNAseq) to determine if microglial genes and/or microglial clustering patterns were sensitive to astrocyte signaling in a sex-dependent manner. In males, disease-associated microglial genes and subclusters were overrepresented in HHcy-treated mice, while VIVIT promoted the appearance of homeostatic microglial genes and clusters. In contrast, microglial genes in females were less sensitive to diet and AAV treatments, though disease-like patterns in gene expression were also observed in the HHcy condition. However, very few of the HHcy-sensitive microglial genes in females were affected by VIVIT. The results suggest a sexually dimorphic influence of astrocyte signaling on microglial phenotypes in the context of HHcy and small cerebral vessel disease.

Disrupted Calcium Dynamics in Reactive Astrocytes Occur with Endfeet-Arteriole Decoupling in an Amyloid Mouse Model of Alzheimer's Disease

While cerebrovascular dysfunction and reactive astrocytosis are extensively characterized hallmarks of Alzheimer's disease (AD) and related dementias, the dynamic relationship between reactive astrocytes and cerebral vessels remains poorly understood. Here, we used jGCaMP8f and two photon microscopy to investigate Ca2+ signaling in multiple astrocyte subcompartments, concurrent with changes in cerebral arteriole activity, in fully awake eight-month-old male and female 5xFAD mice, a model for AD-like pathology, and wild-type (WT) littermates. In the absence of movement, spontaneous Ca2+ transients in barrel cortex occurred more frequently in astrocyte somata, processes, and perivascular regions of 5xFAD mice. However, evoked arteriole dilations (in response to air puff stimulation of contralateral whiskers) and concurrent Ca2+ transients across astrocyte compartments were reduced in 5xFAD mice relative to WTs. Synchronous activity within multi-cell astrocyte networks was also impaired in the 5xFAD group. Using a custom application to assess functional coupling between astrocyte endfeet and immediately adjacent arteriole segments, we detected deficits in Ca2+ response probability in 5xFAD mice. Moreover, endfeet Ca2+ transients following arteriole dilations exhibited a slower onset, reduced amplitude, and lacked relative proportionality to vasomotive activity compared to WTs. The results reveal nuanced alterations in 5xFAD reactive astrocytes highlighted by impaired signaling fidelity between astrocyte endfeet and cerebral arterioles. The results have important implications for the mechanistic underpinnings of brain hypometabolism and the disruption of neurophysiological communication found in AD and other neurodegenerative conditions.

Glial 'omics in ischemia: Acute stroke and chronic cerebral small vessel disease

Vascular injury and pathologies underlie common diseases including ischemic stroke and cerebral small vessel disease (CSVD). Prior work has identified a key role for glial cells, including microglia, in the multifaceted and temporally evolving neuroimmune response to both stroke and CSVD. Transcriptional profiling has led to important advances including identification of distinct gene expression signatures in ischemia-exposed, flow cytometrically sorted microglia and more recently single cell RNA sequencing-identified microglial subpopulations or clusters. There is a reassuring degree of overlap in the results from these two distinct methodologies with both identifying a proliferative and a separate type I interferon responsive microglial element. Similar patterns were later seen using multimodal and spatial transcriptomal profiling in ischemia-exposed microglia and astrocytes. Methodological advances including enrichment of specific neuroanatomic/functional regions (such as the neurovascular unit) prior to single cell RNA sequencing has led to identification of novel cellular subtypes and generation of new credible hypotheses as to cellular function based on the enhanced cell sub-type specific gene expression patterns. A ribosomal tagging strategy focusing on the cellular translatome analyses carried out in the acute phases post stroke has revealed distinct inflammation-regulating roles for microglia and astrocytes in this setting. Early spatial transcriptomics experiments using cerebral ischemia models have identified regionally distinct microglial cell clusters in ischemic core versus penumbra. There is great potential for combination of these methods for multi-omics approaches to further elucidate glial responses in the context of both acute ischemic stroke and chronic CSVD.

Calcineurin inhibition may prevent Alzheimer disease in people with Down syndrome

Virtually all people with Down syndrome will develop Alzheimer disease pathology during their lifetime. As Alzheimer disease is the third leading cause of death and a significant factor in end-of-life complications for adults with Down syndrome, identifying interventions is a medical necessity. Calcineurin, a Ca2+/calmodulin-dependent protein phosphatase, has recently been investigated as a possible Alzheimer treatment. This review explores the histories behind Down syndrome and Alzheimer disease, and their intersecting pathologies. This is followed by the role that calcineurin and its U.S. Food and Drug Administration-approved pharmacological inhibitor, tacrolimus, may play in the prevention or treatment of Alzheimer disease. Finally, this review discusses the gap in the literature surrounding the role of calcineurin, its regulators, and calcineurin inhibitors in the context of Down syndrome and comorbid Alzheimer disease. Future studies investigating the role that calcineurin plays in this pathology will be essential in determining the viability of calcineurin inhibitors to treat Alzheimer disease in people with Down syndrome. HIGHLIGHTS: Calcineurin, a Ca2+/calmodulin-dependent protein phosphatase, has become prominent as a possible therapeutic target to treat Alzheimer disease. People with Down syndrome develop Alzheimer pathology as they age, requiring novel therapeutics for treatment. People with Down syndrome may exhibit contraindications to calcineurin inhibition-based therapy, as they overexpress RCAN1 and DYRK1A, regulators of calcineurin. There is a significant gap in the literature involving the expression of calcineurin, RCAN1 and DYRK1A, in people with Down syndrome and Alzheimer disease, which must be addressed to determine the efficacy and safety of newly developed therapeutics.

Cognitive disorders: Potential astrocyte-based mechanism

Cognitive disorders are a common clinical manifestation, including a deterioration in the patient's memory ability, attention, executive power, language, and other functions. The contributing factors of cognitive disorders are numerous and diverse in nature, including organic diseases and other mental disorders. Neurodegenerative diseases are a common type of organic disease related to the pathology of neuronal death and disruption of glial cell balance, ultimately accompanied with cognitive impairment. Thus, cognitive disorder frequently serves as an extremely critical indicator of neurodegenerative disorders. Cognitive impairments negatively affect patients' daily lives. However, our understanding of the precise pathogenic pathways of cognitive defects remains incomplete. The most prevalent kind of glial cells in the central nervous system are called astrocytes. They have a unique significance in cerebral function because of their wide range of functions in maintaining homeostasis in the central nervous system, regulating synaptic plasticity, and so on. Dysfunction of astrocytes is intimately linked to cognitive disorders, and we are attempting to understand this phenomenon predominantly from those perspectives: neuroinflammation, astrocytic senescence, connexin, Ca2 + signaling, mitochondrial dysfunction, and the glymphatic system.

The role of the neurovascular unit in vascular cognitive impairment: Current evidence and future perspectives

Vascular cognitive impairment (VCI) is a progressive cognitive impairment caused by cerebrovascular disease or vascular risk factors. It is the second most common type of cognitive impairment after Alzheimer's disease. The pathogenesis of VCI is complex, and neurovascular unit destruction is one of its important mechanisms. The neurovascular unit (NVU) is responsible for combining blood flow with brain activity and includes endothelial cells, pericytes, astrocytes and many regulatory nerve terminals. The concept of an NVU emphasizes that interactions between different types of cells are essential for maintaining brain homeostasis. A stable NVU is the basis of normal brain function. Therefore, understanding the structure and function of the neurovascular unit and its role in VCI development is crucial for gaining insights into its pathogenesis. This article reviews the structure and function of the neurovascular unit and its contribution to VCI, providing valuable information for early diagnosis and prevention.

Circular eight-room maze for assessing spatial learning and memory functions in rats: An example using a traumatic brain injury model

BACKGROUND AND OBJECTIVES

This study introduced an animal cognitive function assessment system using a novel circular eight-room maze (CERM). The CERM, designed for tracking path trajectories in animal models of cognitive impairment pathologies such as traumatic brain injury (TBI), comprised a 120-cm diameter disk with eight rooms (30 cm × 25 cm × 30 cm).

METHODS

These rooms have magnetic interfaces for modular assembly and disassembly. Notably, one room remained dark and contained food, while the remaining seven rooms automatically lit up when a rat entry, facilitating the assessment of the rat's learning and spatial memory. An infrared night vision camera captured the animal's search trajectory, and binary image processing techniques were employed to eliminate noise and extract the rat's position coordinates to record the rat's path trajectories. The system automatically calculated various cognitive assessment parameters, such as latency, distance traveled, time spent in each quadrant, inner and outer area exploration times, short-term and long-term memory errors, and the number of entries to all the rooms by chance/by memory.

RESULTS

The analysis of overall path trajectories revealed increasingly erratic movement and a growing reliance on chance to enter rooms in rats with TBI over time, likely due to declining memory and the consequent inability to locate the food room. Moreover, increased trajectories in the first quadrant and inner area characterized the behavior of rats with TBI, with statistically significant differences from the sham group observed on day 7. By day 28, all cognitive parameters except short-term memory error significantly differed between the two groups.

CONCLUSION

Experimental data indicated a substantial increase in irregular search behavior in the TBI group over time, suggesting deterioration in cognitive function and an inability to accurately recall the food room. Conversely, the sham group exhibited consistent search trajectories, typically following the walls and rapidly locating the food room. Moreover, their room entries were guided by memory rather than by chance. Compared with traditional maze tests, this system's strengths lie in its ability to provide more quantitative data and vividly portray behavioral patterns. Therefore, the proposed CERM system can be used as an effective tool for cognitive assessment.

Astroglial activation: Current concepts and future directions

Astrocytes are abundantly and ubiquitously expressed cell types with diverse functions throughout the central nervous system. Astrocytes show remarkable plasticity and exhibit morphological, molecular, and functional remodeling in response to injury, disease, or infection of the central nervous system, as evident in neurodegenerative diseases. Astroglial mediated inflammation plays a prominent role in the pathogenesis of neurodegenerative diseases. This review focus on the role of astrocytes as essential players in neuroinflammation and discuss their morphological and functional heterogeneity in the normal central nervous system and explore the spatial and temporal variations in astroglial phenotypes observed under different disease conditions. This review discusses the intimate relationship of astrocytes to pathological hallmarks of neurodegenerative diseases. Finally, this review considers the putative therapeutic strategies that can be deployed to modulate the astroglial functions in neurodegenerative diseases. HIGHLIGHTS: Astroglia mediated neuroinflammation plays a key role in the pathogenesis of neurodegenerative diseases. Activated astrocytes exhibit diverse phenotypes in a region-specific manner in brain and interact with β-amyloid, tau, and α-synuclein species as well as with microglia and neuronal circuits. Activated astrocytes are likely to influence the trajectory of disease progression of neurodegenerative diseases, as determined by the stage of disease, individual susceptibility, and state of astroglial priming. Modulation of astroglial activation may be a therapeutic strategy at various stages in the trajectory of neurodegenerative diseases to modify the disease course.

Development of a monoclonal antibody specific for a calpain-generated ∆48 kDa calcineurin fragment, a marker of distressed astrocytes

BACKGROUND

Calcineurin (CN) is a Ca2+/calmodulin-dependent protein phosphatase. In healthy tissue, CN exists mainly as a full-length (∼60 kDa) highly-regulated protein phosphatase involved in essential cellular functions. However, in diseased or injured tissue, CN is proteolytically converted to a constitutively active fragment that has been causatively-linked to numerous pathophysiologic processes. These calpain-cleaved CN fragments (∆CN) appear at high levels in human brain at early stages of cognitive decline associated with Alzheimer's disease (AD).

NEW METHOD

We developed a monoclonal antibody to ∆CN, using an immunizing peptide corresponding to the C-terminal end of the ∆CN fragment.

RESULTS

We obtained a mouse monoclonal antibody, designated 26A6, that selectively detects ∆CN in Western analysis of calpain-cleaved recombinant human CN. Using this antibody, we screened both pathological and normal human brain sections provided by the University of Kentucky's Alzheimer's Disease Research Center. 26A6 showed low reactivity towards normal brain tissue, but detected astrocytes both surrounding AD amyloid plaques and throughout AD brain tissue. In brain tissue with infarcts, there was considerable concentration of 26A6-positive astrocytes within/around infarcts, suggesting a link with anoxic/ischemia pathways.

COMPARISON WITH EXISTING METHOD

The results obtained with the new monoclonal are similar to those obtained with a polyclonal we had previously developed. However, the monoclonal is an abundant tool available to the dementia research community.

CONCLUSIONS

The new monoclonal 26A6 antibody is highly selective for the ∆CN proteolytic fragment and labels a subset of astrocytes, and could be a useful tool for marking insidious brain pathology and identifying novel astrocyte phenotypes.

Targeting CaN/NFAT in Alzheimer's brain degeneration

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by a progressive loss of cognitive functions. While the exact causes of this debilitating disorder remain elusive, numerous investigations have characterized its two core pathologies: the presence of β-amyloid plaques and tau tangles. Additionally, multiple studies of postmortem brain tissue, as well as results from AD preclinical models, have consistently demonstrated the presence of a sustained inflammatory response. As the persistent immune response is associated with neurodegeneration, it became clear that it may also exacerbate other AD pathologies, providing a link between the initial deposition of β-amyloid plaques and the later development of neurofibrillary tangles. Initially discovered in T cells, the nuclear factor of activated T-cells (NFAT) is one of the main transcription factors driving the expression of inflammatory genes and thus regulating immune responses. NFAT-dependent production of inflammatory mediators is controlled by Ca2+-dependent protein phosphatase calcineurin (CaN), which dephosphorylates NFAT and promotes its transcriptional activity. A substantial body of evidence has demonstrated that aberrant CaN/NFAT signaling is linked to several pathologies observed in AD, including neuronal apoptosis, synaptic deficits, and glia activation. In view of this, the role of NFAT isoforms in AD has been linked to disease progression at different stages, some of which are paralleled to diminished cognitive status. The use of classical inhibitors of CaN/NFAT signaling, such as tacrolimus or cyclosporine, or adeno-associated viruses to specifically inhibit astrocytic NFAT activation, has alleviated some symptoms of AD by diminishing β-amyloid neurotoxicity and neuroinflammation. In this article, we discuss the recent findings related to the contribution of CaN/NFAT signaling to the progression of AD and highlight the possible benefits of targeting this pathway in AD treatment.

Hippocampal glial inflammatory markers are differentially altered in a novel mouse model of perimenopausal cerebral amyloid angiopathy

Dementia is often characterized by age-dependent cerebrovascular pathology, neuroinflammation, and cognitive deficits with notable sex differences in risk, disease onset, progression and severity. Women bear a disproportionate burden of dementia, and the onset of menopause (i.e., perimenopause) may be a critical period conferring increased susceptibility. However, the contribution of early ovarian decline to the neuroinflammatory processes associated with cerebrovascular dementia risks, particularly at the initial stages of pathology that may be more amenable to proactive intervention, is unknown. To better understand the influence of early ovarian failure on dementia-associated neuroinflammation we developed a model of perimenopausal cerebral amyloid angiopathy (CAA), an important contributor to dementia. For this, accelerated ovarian failure (AOF) was induced by 4-vinylcyclohexene diepoxide (VCD) treatment to isolate early-stage ovarian failure comparable to human perimenopause (termed "peri-AOF") in transgenic SWDI mice expressing human vasculotropic mutant amyloid beta (Aβ) precursor protein, that were also tested at an early stage of amyloidosis. We found that peri-AOF SWDI mice showed increased astrocyte activation accompanied by elevated Aβ in select regions of the hippocampus, a brain system involved in learning and memory that is severely impacted during dementia. However, although SWDI mice showed signs of increased hippocampal microglial activation and impaired cognitive function, this was not further affected by peri-AOF. In sum, these results suggest that elevated dysfunction of key elements of the neurovascular unit in select hippocampal regions characterizes the brain pathology of mice at early stages of both CAA and AOF. However, neurovascular unit pathology may not yet have passed a threshold that leads to further behavioral compromise at these early periods of cerebral amyloidosis and ovarian failure. These results are consistent with the hypothesis that the hormonal dysregulation associated with perimenopause onset represents a stage of emerging vulnerability to dementia-associated neuropathology, thus providing a selective window of opportunity for therapeutic intervention prior to the development of advanced pathology that has proven difficult to repair or reverse.

Is It Time to Repurpose Calcineurin Inhibitors for the Treatment of Cognitive Impairment and Dementia?

Numerous preclinical and human tissue studies implicate the protein phosphatase calcineurin (CN) as a pathophysiologic mechanism in Alzheimer's disease (AD) and other neurodegenerative conditions. Using public electronic records of tens of thousands of individuals across the United States, Silva et al. (2023) show that use of the FDA-approved CN inhibitor, tacrolimus (for purposes of immunosuppression) is also associated with reduced prevalence of dementia-related symptoms. Notably, the study controls for age, sex, and race as well as multiple risk factors for AD. The results suggest that tacrolimus, and possibly other immunosuppressants could be repurposed for the treatment of AD-related dementia.