Lowered levels of nicotinic acetylcholine receptors and elevated apoptosis in the hippocampus of brains from patients with type 2 diabetes mellitus and db/db mice

Alterations in Neuronal Nicotinic Acetylcholine Receptors in the Pathogenesis of Various Cognitive Impairments

Cognitive impairment is a typical symptom of both neurodegenerative and certain other diseases. In connection with these different pathologies, the etiology and neurological and metabolic changes associated with cognitive impairment must differ. Until these characteristics and differences are understood in greater detail, pharmacological treatment of the different forms of cognitive impairment remains suboptimal. Neurotransmitter receptors, including neuronal nicotinic acetylcholine receptors (nAChRs), dopamine receptors, and glutamine receptors, play key roles in the functions and metabolisms of the brain. Among these, the role of nAChRs in the development of cognitive impairment has attracted more and more attention. The present review summarizes what is presently known concerning the structure, distribution, metabolism, and function of nAChRs, as well as their involvement in major cognitive disorders such as Alzheimer's disease, Parkinson's disease, vascular dementia, schizophrenia, and diabetes mellitus. As will be discussed, the relevant scientific literature reveals clearly that the α4β2 and α7 nAChR subtypes and/or subunits of the receptors play major roles in maintaining cognitive function and in neuroprotection of the brain. Accordingly, focusing on these as targets of drug therapy can be expected to lead to breakthroughs in the treatment of cognitive disorders such as AD and schizophrenia.

Dynamic changes in key factors of the blood-brain barrier in early diabetic mice

Chronic hyperglycemia can result in damage to the hippocampus and dysfunction of the blood-brain barrier (BBB), potentially leading to neurological disorders. This study examined the histological structure of the hippocampus and the expression of critical genes associated with the BBB at 2 early stage time points in a streptozotocin-induced diabetes mellitus (DM) mouse model. Routine histology revealed vascular congestion and dilation of Virchow-Robin spaces in the hippocampal CA1 region of the DM group. Neuronal alterations included rounding and swelling and reduction in Nissl bodies and increased apoptosis. Compared to the control group, TJP1 mRNA expression in the DM group was significantly lower (P < .05 or P < .01), while mRNA levels of JAM3, TJP3, CLDN5, CLDN3, and OCLN initially increased and then decreased. At 7, 14, and 21 days, mRNA levels of the receptor for advanced glycation end products (AGER) were greater in the DM group than in the control group (P < .05 or P < .01). These findings indicate that early-stage diabetes may cause structural and functional impairments in hippocampal CA1 in mice. These abnormalities may parallel alterations in the expression of key BBB tight junction molecules and elevated AGER expression in early DM patients.

Are Hyperglycemia-Induced Changes in the Retina Associated with Diabetes-Correlated Changes in the Brain? A Review from Zebrafish and Rodent Type 2 Diabetes Models

Diabetes is prevalent worldwide, with >90% of the cases identified as Type 2 diabetes. High blood sugar (hyperglycemia) is the hallmark symptom of diabetes, with prolonged and uncontrolled levels contributing to subsequent complications. Animal models have been used to study these complications, which include retinopathy, nephropathy, and peripheral neuropathy. More recent studies have focused on cognitive behaviors due to the increased risk of dementia/cognitive deficits that are reported to occur in older Type 2 diabetic patients. In this review, we collate the data reported from specific animal models (i.e., mouse, rat, zebrafish) that have been examined for changes in both retina/vision (retinopathy) and brain/cognition, including db/db mice, Goto-Kakizaki rats, Zucker Diabetic Fatty rats, high-fat diet-fed rodents and zebrafish, and hyperglycemic zebrafish induced by glucose immersion. These models were selected because rodents are widely recognized as established models for studying diabetic complications, while zebrafish represent a newer model in this field. Our goal is to (1) summarize the published findings relevant to these models, (2) identify similarities in cellular mechanisms underlying the disease progression that occur in both tissues, and (3) address the hypothesis that hyperglycemic-induced changes in retina precede or predict later complications in brain.

A genome-wide association study identifies 25(OH)D3-associated genetic variants in the prediabetic Chinese population

OBJECTIVES

Diabetes mellitus has been a significant public health problem, associated with high rates of morbidity, disability, and mortality. Prediabetes is a crucial period for preventing and managing diabetes. 25(OH)D3 is an important risk factor for prediabetes. However, there is limited genetic knowledge of 25(OH)D3 in the Chinese population. This study was designed to identify genetic variants associated with 25(OH)D3 and explore the potential pathogenesis of prediabetes.

METHODS

In this study, 451 individuals with prediabetes were recruited to determine the genetic variants associated with 25(OH)D3 through a genome-wide association study (GWAS). Gene mapping and overrepresentation analysis (ORA) were further performed to explore the candidate genes and their biological mechanisms.

RESULTS

In this study, we identified two independent significant loci (rs9457733 and rs11243373, p < 5 × 10-6 and r2 < 0.6) and 37 candidate genes associated with 25(OH)D3 in prediabetes. Furthermore, the ORA analysis revealed that two genes in the gene sets, SLC22A1 and SLC22A3, were found to be significantly enriched in monoamine transmembrane transporter activity and quaternary ammonium group transmembrane transporter activity, as determined by WebGestalt and g:Profiler (padj < 0.05).

CONCLUSION

The identification of potential genes associated with 25(OH)D3 provides a foundation for a better understanding of the pathogenesis, diagnosis, and treatment of prediabetes.

The interaction between insulin resistance and Alzheimer's disease: a review article

Insulin serves multiple functions as a growth-promoting hormone in peripheral tissues. It manages glucose metabolism by promoting glucose uptake into cells and curbing the production of glucose in the liver. Beyond this, insulin fosters cell growth, drives differentiation, aids protein synthesis, and deters degradative processes like glycolysis, lipolysis, and proteolysis. Receptors for insulin and insulin-like growth factor-1 are widely expressed in the central nervous system. Their widespread presence in the brain underscores the varied and critical functions of insulin signaling there. Insulin aids in bolstering cognition, promoting neuron extension, adjusting the release and absorption of catecholamines, and controlling the expression and positioning of gamma-aminobutyric acid (GABA). Importantly, insulin can effortlessly traverse the blood-brain barrier. Furthermore, insulin resistance (IR)-induced alterations in insulin signaling might hasten brain aging, impacting its plasticity and potentially leading to neurodegeneration. Two primary pathways are responsible for insulin signal transmission: the phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) pathway, which oversees metabolic responses, and the mitogen-activated protein kinase (MAPK) pathway, which guides cell growth, survival, and gene transcription. This review aimed to explore the potential shared metabolic traits between Alzheimer's disease (AD) and IR disorders. It delves into the relationship between AD and IR disorders, their overlapping genetic markers, and shared metabolic indicators. Additionally, it addresses existing therapeutic interventions targeting these intersecting pathways.

Targeting ERS-mitophagy in hippocampal neurons to explore the improvement of memory by tea polyphenols in aged type 2 diabetic rats

Increasing evidence demonstrated that mitophagy and endoplasmic reticulum stress (ERS) was closely associated with memory decline in elderly type 2 diabetes mellitus (T2DM). Tea polyphenols (TP), an excellent natural antioxidant, has been reported to have neuroprotective properties in aging and diabetes, but the underlying mechanisms are still not fully understood. This study targets ERS-mitophagy in hippocampal neurons to investigate the improvement effect of memory in aged T2DM rats by TP. Rats were randomly divided into the control group, the aged group, the aged T2DM model group, the TP 75, 150, 300 mg/kg groups. TP 300 mg/kg ameliorated mitophagy by decreasing the levels of p-mTOR (S2448), P62 and HSP60 and increasing the levels of PINK1 and Parkin, the ratio of LC3Ⅱ/LC3Ⅰ, co-localization of LC3 and HSP60 and the number of autophagosomes and autolysosomes. TP 300 mg/kg attenuated ERS by downregulating the levels of p-PERK, p-eIF2α, ATF4, GRP78 and restoring the ER structure. To further verify epigallocatechin gallate (EGCG), which is the main active component of TP, enhanced mitophagy by inhibiting ERS, PC12 cells were pretreated with ERS activator tunicamycin (TM) or ERS inhibitor 4-phenylbutyric acid (4-PBA). The results showed that the improvement of mitophagy by EGCG was inhibited by TM and promoted by 4-PBA. Collectively, ERS-mitophagy in hippocampal neurons plays a key role in the improvement of memory by TP in aged T2DM rats. This study will provide a new perspective and strategy for the prevention of memory decline in elderly with T2DM.

Insulin resistance induces earlier initiation of cognitive dysfunction mediated by cholinergic deregulation in a mouse model of Alzheimer's disease

Although insulin resistance increases the risk of Alzheimer's disease (AD), the mechanisms remain unclear, partly because no animal model exhibits the insulin-resistant phenotype without persistent hyperglycemia. Here we established an AD model with whole-body insulin resistance without persistent hyperglycemia (APP/IR-dKI mice) by crossbreeding constitutive knock-in mice with P1195L-mutated insulin receptor (IR-KI mice) and those with mutated amyloid precursor protein (AppNL-G-F mice: APP-KI mice). APP/IR-dKI mice exhibited cognitive impairment at an earlier age than APP-KI mice. Since cholinergic dysfunction is a major characteristic of AD, pharmacological interventions on the cholinergic system were performed to investigate the mechanism. Antagonism to a nicotinic acetylcholine receptor α7 (nAChRα7) suppressed cognitive function and cortical blood flow (CBF) response to cholinergic-regulated peripheral stimulation in APP-KI mice but not APP/IR-dKI mice. Cortical expression of Chrna7, encoding nAChRα7, was downregulated in APP/IR-dKI mice compared with APP-KI. Amyloid β burden did not differ between APP-KI and APP/IR-dKI mice. Therefore, insulin resistance, not persistent hyperglycemia, induces the earlier onset of cognitive dysfunction and CBF deregulation mediated by nAChRα7 downregulation. Our mouse model will help clarify the association between type 2 diabetes mellitus and AD.

Neurotransmitters in Type 2 Diabetes and the Control of Systemic and Central Energy Balance

Efficient signal transduction is important in maintaining the function of the nervous system across tissues. An intact neurotransmission process can regulate energy balance through proper communication between neurons and peripheral organs. This ensures that the right neural circuits are activated in the brain to modulate cellular energy homeostasis and systemic metabolic function. Alterations in neurotransmitters secretion can lead to imbalances in appetite, glucose metabolism, sleep, and thermogenesis. Dysregulation in dietary intake is also associated with disruption in neurotransmission and can trigger the onset of type 2 diabetes (T2D) and obesity. In this review, we highlight the various roles of neurotransmitters in regulating energy balance at the systemic level and in the central nervous system. We also address the link between neurotransmission imbalance and the development of T2D as well as perspectives across the fields of neuroscience and metabolism research.

Impaired spatial navigation and age-dependent hippocampal synaptic dysfunction are associated with chronic inflammatory response in db/db mice

Type 2 diabetes mellitus (T2DM) increases the risk of developing Alzheimer's disease (AD), which has been proposed to be driven by an abnormal neuroinflammatory response affecting cognitive function. However, the impact of T2DM on hippocampal function and synaptic integrity during aging has not been investigated. Here, we investigated the effects of aging in T2DM on AD-like pathology using the leptin receptor-deficient db/db mouse model of T2DM. Our results indicate that adult T2DM mice exhibited impaired spatial acquisition in the Morris water maze (MWM). Morphological analysis showed an age-dependent neuronal loss in the dentate gyrus. We found that astrocyte density was significantly decreased in all regions of the hippocampus in T2DM mice. Our analysis showed that microglial activation was increased in the CA3 and the dentate gyrus of the hippocampus in an age-dependent manner in T2DM mice. However, the expression of presynaptic marker protein (synaptophysin) and the postsynaptic marker protein [postsynaptic density protein 95 (PSD95)] was unchanged in the hippocampus of adult T2DM mice. Interestingly, synaptophysin and PSD95 expression significantly decreased in the hippocampus of aged T2DM mice, suggesting an impaired hippocampal synaptic integrity. Cytokine profiling analysis displayed a robust pro-inflammatory cytokine profile in the hippocampus of aged T2DM mice compared with the younger cohort, outlining the role of aging in exacerbating the neuroinflammatory profile in the diabetic state. Our results suggest that T2DM impairs cognitive function by promoting neuronal loss in the dentate gyrus and triggering an age-dependent deterioration in hippocampal synaptic integrity, associated with an aberrant neuroinflammatory response.

Glial-neuron crosstalk in health and disease: A focus on metabolism, obesity, and cognitive impairment

Dementia is a complex set of disorders affecting normal cognitive function. Recently, several clinical studies have shown that diabetes, obesity, and components of the metabolic syndrome (MetS) are associated with cognitive impairment, including dementias such as Alzheimer's disease. Maintaining normal cognitive function is an intricate process involving coordination of neuron function with multiple brain glia. Well-orchestrated bioenergetics is a central requirement of neurons, which need large amounts of energy but lack significant energy storage capacity. Thus, one of the most important glial functions is to provide metabolic support and ensure an adequate energy supply for neurons. Obesity and metabolic disease dysregulate glial function, leading to a failure to respond to neuron energy demands, which results in neuronal damage. In this review, we outline evidence for links between diabetes, obesity, and MetS components to cognitive impairment. Next, we focus on the metabolic crosstalk between the three major glial cell types, oligodendrocytes, astrocytes, and microglia, with neurons under physiological conditions. Finally, we outline how diabetes, obesity, and MetS components can disrupt glial function, and how this disruption might impair glia-neuron metabolic crosstalk and ultimately promote cognitive impairment.

Neuroprotective effect of alpha-pinene is mediated by suppression of the TNF-α/NF-κB pathway in Alzheimer's disease rat model

Monoterpene alpha-pinene possesses antioxidant, cardioprotective, and neuroprotective properties. We evaluated the effect of alpha-pinene on oxidative/nitrosative stress, neuroinflammation, and molecular and behavioral changes induced by beta-amyloid (Aβ)_1-42 in rats and investigated the possible mechanisms of these outcomes. Male Wistar rats received alpha-pinene (50 mg/kg intraperitoneally) for 14 consecutive days after intrahippocampal injection of Aβ_1-42 . Alpha-pinene decreased malondialdehyde and nitric oxide levels, increased glutathione content, and enhanced catalase activity in Aβ-injected rats. Also, messenger RNA expression of tumor necrosis factor-α, interleukin-1β, interleukin-6, nuclear factor κB, and N-methyl- d-aspartate receptor subunits 2A and 2B reduced in the hippocampus of these animals. Besides this, alpha-pinene repressed the Aβ_1-42 -induced reduction of nicotinic acetylcholine receptor α7 subunit and brain-derived neurotrophic factor expression. Treatment with alpha-pinene caused Aβ-receiving rats to spend more time in the target quadrant in the Morris water maze test and led to an increase in percentages of open arm entrance and time spent in the open arm in the elevated plus-maze test. We concluded that alpha-pinene strengthens the antioxidant system and prevents neuroinflammation in the hippocampus of rats receiving Aβ. It improves spatial learning and memory and reduces anxiety-like behavior in these animals. Consequently, alpha-pinene alleviates Aβ-induced oxidative/nitrosative stress, neuroinflammation, and behavioral deficits. It is probably a suitable candidate for the treatment of neurodegenerative diseases.

Single-Cell Sequencing Analysis of the db/db Mouse Hippocampus Reveals Cell-Type-Specific Insights Into the Pathobiology of Diabetes-Associated Cognitive Dysfunction

Diabetes-associated cognitive decline (DCD), is one of the complications of diabetes, which is characterized by a series of neurophysiological and pathological abnormalities. However, the exact pathogenesis of DCD is still unknown. Single-cell RNA sequencing (scRNA-seq) could discover unusual subpopulations, explore functional heterogeneity and identify signaling pathways and potential markers. The aim of this research was to provide deeper opinion into molecular and cellular changes underlying DCD, identify different cellular types of the diabetic mice hippocampus at single-cell level, and elucidate the factors mediating the pathogenesis of DCD. To elucidate cell specific gene expression changes in the hippocampus of diabetic encephalopathy. Single-cell RNA sequencing of hippocampus from db/m and db/db mice was carried out. Subclustering analysis was performed to further describe microglial cell subpopulations. Interestingly using immunohistochemistry, these findings were confirmed at the protein level. Single cell analysis yielded transcriptome data for 14621 hippocampal cells and defined 11 different cell types. Analysis of differentially expressed genes in the microglia compartments indicated that infection- and immune system process- associated terms, oxidative stress and inflammation play vital roles in the progression of DCD. Compared with db/m mouse, experiments at the protein level supported the activation of microglia, increased expression of inflammatory factors and oxidative stress damage in the hippocampus of db/db mouse. In addition, a major finding of our research was the subpopulation of microglia that express genes related to pro-inflammatory disease-associated microglia (DAM). Our research reveals pathological alterations of inflammation and oxidative stress mediated hippocampal damage in the db/db mice, and may provide potential diagnostic biomarkers and therapeutic interventions for DCD.

Functional Analysis of Estrogen Receptor 1 in Diabetic Wound Healing: A Knockdown Cell-Based and Bioinformatic Study

Background

Diabetic wound (DW) treatment is a serious challenge for clinicians, and the underlying mechanisms of DWs remain elusive. We sought to identify the critical genes in the development of DWs and provide potential targets for DW therapies.

Material/Methods

Datasets of GSE38396 from the Gene Expression Omnibus (GEO) database were reviewed. Pathway analysis was performed using the Kyoto Encyclopedia of Genes and Genomes (KEGG), Gene Ontology term analyses were carried out, and Cytoscape software (Cytoscape 3.7.2) was used to construct the protein interaction network. Serum samples from patients with diabetes and control participants were collected, and the expression of estrogen receptor 1 (ESR1) was measured by quantitative reverse-transcription polymerase chain reaction. In addition, the function of ESR1 in human skin fibroblasts was investigated in vitro.

Results

Eight samples were analyzed using the Morpheus online tool, which identified 637 upregulated and 448 downregulated differentially expressed genes. The top 5 KEGG pathways of upregulated differentially expressed genes were associated with sphingolipid metabolism, estrogen signaling, ECM-receptor interaction, MAPK signaling, and PI3K-Akt signaling. The hub genes for DWs were JUN, ESR1, CD44, SMARCA4, MMP2, BMP4, GSK3B, WDR5, PTK2, and PTGS2. JUN, MMP2, and ESR1 were the upregulated hub genes, and ESR1 was found to be consistently enriched in DW patients. Inhibition of ESR1 had a stimulative role in human skin fibroblasts.

Conclusions

ESR1 was identified as a crucial gene in the development of DWs, which suggests potential therapeutic targets for DW healing.