Ataxia telangiectasia patient-derived neuronal and brain organoid models reveal mitochondrial dysfunction and oxidative stress.

Ataxia Telangiectasia (AT) is a rare disorder caused by mutations in the ATM gene and results in progressive neurodegeneration for reasons that remain poorly understood. In addition to its central role in nuclear DNA repair, ATM operates outside the nucleus to regulate metabolism, redox homeostasis and mitochondrial function. However, a systematic investigation into how and when loss of ATM affects these parameters in relevant human neuronal models of AT was lacking. We therefore used cortical neurons and brain organoids from AT-patient iPSC and gene corrected isogenic controls to reveal levels of mitochondrial dysfunction, oxidative stress, and senescence that vary with developmental maturity. Transcriptome analyses identified disruptions in regulatory networks related to mitochondrial function and maintenance, including alterations in the PARP/SIRT signalling axis and dysregulation of key mitophagy and mitochondrial fission-fusion processes. We further show that antioxidants reduce ROS and restore neurite branching in AT neuronal cultures, and ameliorate impaired neuronal activity in AT brain organoids. We conclude that progressive mitochondrial dysfunction and aberrant ROS production are important contributors to neurodegeneration in AT and are strongly linked to ATM's role in mitochondrial homeostasis regulation.

Senolytic therapy alleviates physiological human brain aging and COVID-19 neuropathology.

Aging is a major risk factor for neurodegenerative diseases, and coronavirus disease 2019 (COVID-19) is linked to severe neurological manifestations. Senescent cells contribute to brain aging, but the impact of virus-induced senescence on neuropathologies is unknown. Here we show that senescent cells accumulate in aged human brain organoids and that senolytics reduce age-related inflammation and rejuvenate transcriptomic aging clocks. In postmortem brains of patients with severe COVID-19 we observed increased senescent cell accumulation compared with age-matched controls. Exposure of human brain organoids to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) induced cellular senescence, and transcriptomic analysis revealed a unique SARS-CoV-2 inflammatory signature. Senolytic treatment of infected brain organoids blocked viral replication and prevented senescence in distinct neuronal populations. In human-ACE2-overexpressing mice, senolytics improved COVID-19 clinical outcomes, promoted dopaminergic neuron survival and alleviated viral and proinflammatory gene expression. Collectively our results demonstrate an important role for cellular senescence in driving brain aging and SARS-CoV-2-induced neuropathology, and a therapeutic benefit of senolytic treatments.

Noncoding RNA of Zika Virus Affects Interplay between Wnt-Signaling and Pro-Apoptotic Pathways in the Developing Brain Tissue.

Zika virus (ZIKV) has a unique ability among flaviviruses to cross the placental barrier and infect the fetal brain causing severe abnormalities of neurodevelopment known collectively as congenital Zika syndrome. In our recent study, we demonstrated that the viral noncoding RNA (subgenomic flaviviral RNA, sfRNA) of the Zika virus induces apoptosis of neural progenitors and is required for ZIKV pathogenesis in the developing brain. Herein, we expanded on our initial findings and identified biological processes and signaling pathways affected by the production of ZIKV sfRNA in the developing brain tissue. We employed 3D brain organoids generated from induced human pluripotent stem cells (ihPSC) as an ex vivo model of viral infection in the developing brain and utilized wild type (WT) ZIKV (producing sfRNA) and mutant ZIKV (deficient in the production of sfRNA). Global transcriptome profiling by RNA-Seq revealed that the production of sfRNA affects the expression of >1000 genes. We uncovered that in addition to the activation of pro-apoptotic pathways, organoids infected with sfRNA-producing WT, but not sfRNA-deficient mutant ZIKV, which exhibited a strong down-regulation of genes involved in signaling pathways that control neuron differentiation and brain development, indicating the requirement of sfRNA for the suppression of neurodevelopment associated with the ZIKV infection. Using gene set enrichment analysis and gene network reconstruction, we demonstrated that the effect of sfRNA on pathways that control brain development occurs via crosstalk between Wnt-signaling and proapoptotic pathways.

Zika virus noncoding RNA cooperates with the viral protein NS5 to inhibit STAT1 phosphorylation and facilitate viral pathogenesis.

All flaviviruses, including Zika virus, produce noncoding subgenomic flaviviral RNA (sfRNA), which plays an important role in viral pathogenesis. However, the exact mechanism of how sfRNA enables viral evasion of antiviral response is not well defined. Here, we show that sfRNA is required for transplacental virus dissemination in pregnant mice and subsequent fetal brain infection. We also show that sfRNA promotes apoptosis of neural progenitor cells in human brain organoids, leading to their disintegration. In infected human placental cells, sfRNA inhibits multiple antiviral pathways and promotes apoptosis, with signal transducer and activator of transcription 1 (STAT1) identified as a key shared factor. We further show that the production of sfRNA leads to reduced phosphorylation and nuclear translocation of STAT1 via a mechanism that involves sfRNA binding to and stabilizing viral protein NS5. Our results suggest the cooperation between viral noncoding RNA and a viral protein as a novel strategy for counteracting antiviral responses.

Ataxia Telangiectasia iPSC line generated from a patient olfactory biopsy identifies novel disease-causing mutations.

Ataxia Telangiectasia is a rare autosomal recessive disorder caused by a mutated ATM gene. The most debilitating symptom of Ataxia Telangiectasia is the progressive neurodegeneration of the cerebellum, though the molecular mechanisms driving this degeneration remains unclear. Here we describe the generation and validation of an induced pluripotent stem cell (iPSC) line from an olfactory biopsy from a patient with Ataxia Telangiectasia. Sequencing identified two previously unreported disease-causing mutations in the ATM gene. This line can be used to generate 2D and 3D patient-specific neuronal models enabling investigations into the mechanisms underlying neurodegeneration.

Reprogramming of human olfactory neurosphere-derived cells from olfactory mucosal biopsies of a control cohort.

Human olfactory neurosphere-derived (ONS) cells are derived from the olfactory mucosa and display some progenitor- and neuronal cell-like properties, making them useful models of neurological disorders. However, they lack several important characteristics of true neurons, which can be overcome using induced pluripotent stem cell (iPSC) -derived neurons. Here we describe, for the first time, the generation and validation of an iPSC line from an olfactory biopsy from a control cohort member. This data lays the groundwork for future reprogramming of ONS cells, which can be used to generate neuronal models and compliment current ONS cell-based investigations into numerous neurological disorders.

Inhibition of the cGAS-STING pathway ameliorates the premature senescence hallmarks of Ataxia-Telangiectasia brain organoids.

Ataxia-telangiectasia (A-T) is a genetic disorder caused by the lack of functional ATM kinase. A-T is characterized by chronic inflammation, neurodegeneration and premature ageing features that are associated with increased genome instability, nuclear shape alterations, micronuclei accumulation, neuronal defects and premature entry into cellular senescence. The causal relationship between the detrimental inflammatory signature and the neurological deficiencies of A-T remains elusive. Here, we utilize human pluripotent stem cell-derived cortical brain organoids to study A-T neuropathology. Mechanistically, we show that the cGAS-STING pathway is required for the recognition of micronuclei and induction of a senescence-associated secretory phenotype (SASP) in A-T olfactory neurosphere-derived cells and brain organoids. We further demonstrate that cGAS and STING inhibition effectively suppresses self-DNA-triggered SASP expression in A-T brain organoids, inhibits astrocyte senescence and neurodegeneration, and ameliorates A-T brain organoid neuropathology. Our study thus reveals that increased cGAS and STING activity is an important contributor to chronic inflammation and premature senescence in the central nervous system of A-T and constitutes a novel therapeutic target for treating neuropathology in A-T patients.

Klotho inhibits neuronal senescence in human brain organoids.

Aging is a major risk factor for many neurodegenerative diseases. Klotho (KL) is a glycosylated transmembrane protein that is expressed in the choroid plexus and neurons of the brain. KL exerts potent anti-aging effects on multiple cell types in the body but its role in human brain cells remains largely unclear. Here we show that human cortical neurons, derived from human pluripotent stem cells in 2D cultures or in cortical organoids, develop the typical hallmarks of senescent cells when maintained in vitro for prolonged periods of time, and that moderate upregulation or repression of endogenous KL expression in cortical organoids inhibits and accelerates senescence, respectively. We further demonstrate that KL expression alters the expression of senescence-associated genes including, extracellular matrix genes, and proteoglycans, and can act in a paracrine fashion to inhibit neuronal senescence. In summary, our results establish an important role for KL in the regulation of human neuronal senescence and offer new mechanistic insight into its role in human brain aging.