Project description:Well-balanced mitochondrial fission and fusion processes are essential for nervous system development. Loss of function of the main mitochondrial fission mediator, dynamin-related protein 1 (Drp1), is lethal early during embryonic development or around birth, but the role of mitochondrial fission in adult neurons remains unclear. Here we show that inducible Drp1 ablation in neurons of the adult mouse forebrain results in progressive, neuronal subtype-specific alterations of mitochondrial morphology in the hippocampus that are marginally responsive to antioxidant treatment. Furthermore, DRP1 loss affects synaptic transmission and memory function. Although these changes culminate in hippocampal atrophy, they are not sufficient to cause neuronal cell death within 10 weeks of genetic Drp1 ablation. Collectively, our in vivo observations clarify the role of mitochondrial fission in neurons, demonstrating that Drp1 ablation in adult forebrain neurons compromises critical neuronal functions without causing overt neurodegeneration.
Project description:The type III RNase Dicer is responsible for the maturation and function of microRNA (miRNA) molecules in the cell. It is now well documented that Dicer and the fine-tuning of the miRNA gene network are important for neuronal integrity. However, the underlying mechanisms involved in neuronal death, particularly in the adult brain, remain poorly defined. Here, we show that absence of Dicer in the adult forebrain is accompanied by a mixed neurodegenerative phenotype. While neuronal loss is observed in the hippocampus, cellular shrinkage is predominant in the cortex. Interestingly, neuronal degeneration coincides with the hyperphosphorylation of endogenous tau at several epitopes previously associated with neurofibrillary pathology. Transcriptome analysis of enzymes involved in tau phosphorylation identified ERK1 as one of the candidate kinases responsible for this event in vivo. We further demonstrate that miRNAs belonging to the miR-15 family are potent regulators of ERK1 expression in mouse neuronal cells and co-expressed with ERK1/2 in vivo. Last, we show that miR-15a is specifically downregulated in Alzheimer’s disease brain. In sum, these results support the hypothesis that changes in the miRNA network may contribute to a neurodegenerative phenotype by affecting tau phosphorylation. Dicer KO vs. control
Project description:The type III RNase Dicer is responsible for the maturation and function of microRNA (miRNA) molecules in the cell. It is now well documented that Dicer and the fine-tuning of the miRNA gene network are important for neuronal integrity. However, the underlying mechanisms involved in neuronal death, particularly in the adult brain, remain poorly defined. Here, we show that absence of Dicer in the adult forebrain is accompanied by a mixed neurodegenerative phenotype. While neuronal loss is observed in the hippocampus, cellular shrinkage is predominant in the cortex. Interestingly, neuronal degeneration coincides with the hyperphosphorylation of endogenous tau at several epitopes previously associated with neurofibrillary pathology. Transcriptome analysis of enzymes involved in tau phosphorylation identified ERK1 as one of the candidate kinases responsible for this event in vivo. We further demonstrate that miRNAs belonging to the miR-15 family are potent regulators of ERK1 expression in mouse neuronal cells and co-expressed with ERK1/2 in vivo. Last, we show that miR-15a is specifically downregulated in Alzheimer’s disease brain. In sum, these results support the hypothesis that changes in the miRNA network may contribute to a neurodegenerative phenotype by affecting tau phosphorylation.
Project description:MicroRNAs (miRNAs) are important regulators of gene expression; however, their contribution to protein homeostasis remains unclear. Impaired protein homeostasis could contribute to brain aging and neurodegeneration; however, the underlying mechanisms are not well understood. Here, we show that conditional inactivation of Dicer in the adult mouse brain postnatal forebrain causes age-dependent accumulation of lipofuscin and polyubiquitinated protein aggregates. Impaired protein turnover in the cerebral cortex of Dicer conditional knockout (cKO) mice is associated with protein misfolding and endoplasmic reticulum stress. Conditional Drosha inactivation using the same Cre driver results in similar phenotypes, suggesting that the phenotypes may be due to the loss of canonical miRNAs. RNA-sequencing analysis revealed increased expression of target genes of neocortical neuron-enriched miRNAs in the Dicer cKO neocortex and dysregulated expression of genes involved in protein homeostasis maintenance. Further analysis revealed a similar gene expression profile in the hippocampal CA1 region of Dicer cKO mice as in the aging brain. Moreover, translational inhibition using anisomycin or rapamycin ameliorated the protein aggregation and neurodegeneration in Dicer-deficient brains. Thus, protein translational control by miRNAs is an essential component of the protein homeostasis network controlling neuronal survival in the adult brain. Our results show that excessive translation can produce key features of human neurodegenerative diseases, and suggest that translational suppression may be a therapeutic strategy to restore protein homeostasis and combat neurodegeneration.
Project description:Selective neurodegeneration is a critical causal factor in Alzheimer’s disease (AD); however, the mechanisms that lead some neurons to perish while others remain resilient are unknown. We sought potential drivers of this selective vulnerability using single-nucleus RNA sequencing and discovered that apoE expression level is a substantial driver of neuronal variability. Strikingly, neuronal expression of apoE—which has a robust genetic linkage to AD—correlated strongly, on a cell-by-cell basis, with immune response pathways in neurons in the brains of wildtype mice, human apoE knock-in mice, and humans with or without AD. Elimination or over-expression of neuronal apoE revealed a causal relationship between apoE expression, neuronal MHC-I expression, Tau pathology, and neurodegeneration. Functional reduction of MHC-I ameliorated Tau pathology in apoE4-expressing primary neurons and in mouse hippocampi expressing pathological Tau. These findings suggest a mechanism linking neuronal apoE expression to MHC-I expression and, subsequently, to Tau pathology and selective neurodegeneration.
Project description:Tau aggregates lead to progressive neurodegeneration in Alzheimer’s disease (AD) (ref). Neuron death is one of the hallmarks of neurodegeneration (ref). However, the pathological influence of neuronal death is undetermined, and the connection between Tau aggregates and neuronal death remains elusive. Here we demonstrated the essential role of neuron death in Tau-related neurodegeneration. Tau-neurons died in necroptosis, dependent on ZBP1 sensitized by Z-RNAs (an unusual left-handed conformation). Those endogenous Z-RNAs were transcripts of reactivated transposable elements (TEs) originally silenced in heterochromatin.
Project description:Tau aggregates lead to progressive neurodegeneration in Alzheimer’s disease (AD) (ref). Neuron death is one of the hallmarks of neurodegeneration (ref). However, the pathological influence of neuronal death is undetermined, and the connection between Tau aggregates and neuronal death remains elusive. Here we demonstrated the essential role of neuron death in Tau-related neurodegeneration. Tau-neurons died in necroptosis, dependent on ZBP1 sensitized by Z-RNAs (an unusual left-handed conformation). Those endogenous Z-RNAs were transcripts of reactivated transposable elements (TEs) originally silenced in heterochromatin.
Project description:Alzheimer’s disease (AD) is the most prevalent form of neurodegeneration. Despite the well-established link between tau aggregation and clinical progression, the major pathways driven by this protein to intrinsically damage neurons are incompletely understood. To model AD-relevant neurodegeneration driven by tau, we overexpressed non-mutated human tau in primary mouse neurons and observed substantial axonal degeneration and cell death, a process accompanied by activated caspase 3. Mechanistically, we detected deformation of the nuclear envelope and increased DNA damage response in tau-expressing neurons. Gene profiling analysis further revealed significant alterations in the mitogen-activated protein kinase (MAPK) pathway; moreover, inhibitors of dual leucine zipper kinase (DLK) and c-Jun N-terminal kinase (JNK) were effective in alleviating wild-type human tau-induced neurodegeneration. In contrast, mutant P301L human tau was less toxic to neurons, despite causing comparable DNA damage. Axonal DLK activation induced by wild-type tau potentiated the impact of DNA damage response, resulting in overt neurotoxicity. In summary, we have established a cellular tauopathy model highly relevant to AD and identified a functional synergy between the MAPK-DLK axis and DNA damage response in the neuronal degenerative process.
Project description:Tau is a microtubule-binding protein expressed in neurons and the equal ratio between 4-repeat (4R) and 3-repeat (3R) isoforms are maintained in normal adult brain function. Dysregulation of 3R:4R ratio causes tauopathy and human neurons that recapitulate tau isoforms in health and disease will provide a platform for elucidating pathogenic processes involving tau pathology. We carried out extensive characterizations of tau isoforms expressed in human neurons derived by microRNA-induced neuronal reprogramming of adult fibroblasts. Transcript and protein analyses showed miR-neurons expressed all six isoforms with the 3R:4R isoform ratio equivalent to that detected in human adult brains. Also, miR-neurons derived from familial tauopathypatients with a 3R:4R ratio altering mutation showed increased 4R tau and the formation of insoluble tau with seeding activity. Our results collectively demonstrate the utility of miRNA-induced neuronal reprogramming to recapitulate endogenous tau regulation comparable to the adult brain in health and disease.
Project description:The overall goal of the project is to assess the mRNA alterations in tauopathy conditions that occur at the presymptomatic stage of neurodegeneration. The animal model selected for this study is a well characterized tauopathy animal model, known as rTg4510 murine model. rTg4510 (tau) is a transgenic mouse model of human tauopathy expressing human tau containing the P301L mutation that is associated with familial frontotemporal dementia and parkinsonism linked to chromosome 17. The expression of P301L tau driven by CaMKII promoter peaks around two month-of-age and mainly in the forebrain. 2-month of age is considered the presymptomatic stage of neurodegeneration. The global expression of mRNAs was evaluated by mRNA deep sequencing in the hippocampus of tau mice at 2 months of age. The levels of several mRNAs were altered in the hippocampus of tau mice at 2 months. Many important candidates were further evaluated for their molecular and functional contributions to the process of neurodegeneration.