Presymptomatic change in microRNAs modulates Tau pathology

MicroRNA Networks in Cognition and Dementia

The change from viewing noncoding RNA as “junk” in the genome to seeing it as a critical epigenetic regulator in almost every human condition or disease has forced a paradigm shift in biomedical and clinical research. Small and long noncoding RNA transcripts are now routinely evaluated as putative diagnostic or therapeutic agents. A prominent role for noncoding microRNAs in the central nervous system has uncovered promising new clinical candidates for dementia-related disorders, treatments for which currently remain elusive even as the percentage of diagnosed patients increases significantly. Cognitive decline is a core neurodegenerative process in Alzheimer’s Disease, Frontotemporal Dementia, Lewy body dementia, vascular dementia, Huntington’s Disease, Creutzfeldt–Jakob disease, and a significant portion of Parkinson’s Disease patients. This review will discuss the microRNA-associated networks which influence these pathologies, including inflammatory and viral-mediated pathways (such as the novel SARS-CoV-2 virus implicated in COVID-19), and their current status in clinical trials.

The Role of Oxidative Stress and the Importance of miRNAs as Potential Biomarkers in the Development of Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is the primary cause of blindness in developed countries. With the progressive aging of the population, AMD is a significant ophthalmological problem in the population over 50 years of age. The etiology of AMD is known to be based on various biochemical, immunological and molecular pathways and to be influenced by a range of genetic and environmental elements. This review provides an overview of the pathophysiological role of oxidative stress and free radicals in the retina with a special focus on the DNA repair efficiency and enzymatic antioxidant defense. It also presents a correlation between miRNA profile and AMD, and indicates their involvement in inflammation, angiogenesis, increased oxidation of cellular components, enzymatic antioxidant capacity and DNA repair efficiency, which play particularly important roles in AMD pathogenesis. Gene silencing by miRNAs can induce changes in antioxidant enzymes, leading to a complex interplay between redox imbalance by free radicals and miRNAs in modulating cellular redox homeostasis.

Onset of hippocampal network aberration and memory deficits in P301S tau mice are associated with an early gene signature

Hyperphosphorylation and deposition of tau in the brain characterizes frontotemporal dementia and Alzheimer's disease. Disease-associated mutations in the tau-encoding MAPT gene have enabled the generation of transgenic mouse models that recapitulate aspects of human neurodegenerative diseases, including tau hyperphosphorylation and neurofibrillary tangle formation. Here, we characterized the effects of transgenic P301S mutant human tau expression on neuronal network function in the murine hippocampus. Onset of progressive spatial learning deficits in P301S tau transgenic TAU58/2 mice were paralleled by long-term potentiation deficits and neuronal network aberrations during electrophysiological and EEG recordings. Gene-expression profiling just prior to onset of apparent deficits in TAU58/2 mice revealed a signature of immediate early genes that is consistent with neuronal network hypersynchronicity. We found that the increased immediate early gene activity was confined to neurons harbouring tau pathology, providing a cellular link between aberrant tau and network dysfunction. Taken together, our data suggest that tau pathology drives neuronal network dysfunction through hyperexcitation of individual, pathology-harbouring neurons, thereby contributing to memory deficits.

Advances and Challenges in Understanding MicroRNA Function in Tauopathies: A Case Study of miR-132/212

In the past decade, several groups have reported that microRNAs (miRNAs) can participate in the regulation of tau protein at different levels, including its expression, alternative splicing, phosphorylation, and aggregation. These observations are significant, since the abnormal regulation and deposition of tau is associated with nearly 30 neurodegenerative disorders. Interestingly, miRNA profiles go awry in tauopathies such as Alzheimer's disease, progressive supranuclear palsy, and frontotemporal dementia. Understanding the role and impact of miRNAs on tau biology could therefore provide important insights into disease risk, diagnostics, and perhaps therapeutics. In this Perspective article, we discuss recent advances in miRNA research related to tau. While proof-of-principle studies hold promise, physiological validation remains limited. To help fill this gap, we describe herein a pure tauopathy mouse model deficient for the miR-132/212 cluster. This miRNA family is strongly downregulated in human tauopathies and shown to regulate tau in vitro and in vivo. No significant differences in survival, motor deficits or body weight were observed in PS19 mice lacking miR-132/212. Age-specific effects were seen on tau expression and phosphorylation but not aggregation. Moreover, various miR-132/212 targets previously implicated in tau modulation were unaffected (GSK-3β, Foxo3a, Mapk1, p300) or, unexpectedly, reduced (Mapk3, Foxo1, p300, Calpain 2) in miR-132/212-deficient PS19 mice. These observations highlight the challenges of miRNA research in living models, and current limitations of transgenic tau mouse models lacking functional miRNA binding sites. Based on these findings, we finally recommend different strategies to better understand the role of miRNAs in tau physiology and pathology.

Regulatory function of microRNAs in microglia

Microglia are CNS-resident cells involved in immune surveillance and maintenance of intercellular homeostasis, while also contributing to neurologic pathologies. MicroRNAs (miRNAs) are a class of small (~22 nucleotides) single-stranded noncoding RNAs that participate in gene regulation at the post-transcriptional level. miRNAs typically bind to the untranslated region (3' UTR) of RNAs. It has been shown that miRNAs are important players in controlling inflammation and that their abnormal expression is linked to cancer and ageing, and to the onset and progression of neurodegenerative disease. Furthermore, miRNAs participate in intercellular trafficking. Thus, miRNAs are released from cells in a free form, bound to proteins or packaged within extracellular vesicles (EVs), exerting paracrine and long distance signaling. In this review, recent findings on the role of miRNAs as drivers of microglia phenotypic changes and their cotribution in neurological disease are addressed. MAIN POINTS: miRNAs have a key role in microglia function/dysfunction, polarization, and restoration. Microglia are both a source and recipient of extracellular vesicles (EVs) containing miRNAs. Extracellular miRNAs may be found as soluble (free and EV cargo) and protein complexes.

Microglial priming of antigen presentation and adaptive stimulation in Alzheimer's disease

The prominent pathological consequences of Alzheimer's disease (AD) are the misfolding and mis-sorting of two cellular proteins, amyloid-β and microtubule-associated protein Tau. The accumulation of toxic phosphorylated Tau inside the neurons induces the increased processing of amyloid-β-associated signaling cascade and vice versa. Neuroinflammation-driven synaptic depletion and cognitive decline are substantiated by the cross talk of activated microglia and astroglia, leading to neuron degeneration. Microglia are the brain-resident immune effectors that prove their diverse functions in maintaining CNS homeostasis via collaboration with astrocytes and T lymphocytes. Age-related senescence and chronic inflammation activate microglia with increased pro-inflammatory markers, oxidative damage and phagocytosis. But the improper processing of misfolded protein via lysosomal pathway destines the spreading of 'seed' constituents to the nearby healthy neurons. Primed microglia process and present self-antigen such as amyloid-β and modified Tau to the infiltrated T lymphocytes through MHC I/II molecules. After an effective conversation with CD4+ T cells, microglial phenotype can be altered from pro-active M1 to neuro-protective M2 type, which corresponds to the tissue remodeling and homeostasis. In this review, we are focusing on the change in functionality of microglia from innate to adaptive immune response in the context of neuroprotection, which may help in the search of novel immune therapy in AD.