Disruption of MAM integrity in mutant FUS oligodendroglial progenitors from hiPSCs.

Amyotrophic lateral sclerosis (ALS) is a rapidly progressive and fatal neurodegenerative disorder, characterized by selective loss of motor neurons (MNs). A number of causative genetic mutations underlie the disease, including mutations in the fused in sarcoma (FUS) gene, which can lead to both juvenile and late-onset ALS. Although ALS results from MN death, there is evidence that dysfunctional glial cells, including oligodendroglia, contribute to neurodegeneration. Here, we used human induced pluripotent stem cells (hiPSCs) with a R521H or a P525L mutation in FUS and their isogenic controls to generate oligodendrocyte progenitor cells (OPCs) by inducing SOX10 expression from a TET-On SOX10 cassette. Mutant and control iPSCs differentiated efficiently into OPCs. RNA sequencing identified a myelin sheath-related phenotype in mutant OPCs. Lipidomic studies demonstrated defects in myelin-related lipids, with a reduction of glycerophospholipids in mutant OPCs. Interestingly, FUSR521H OPCs displayed a decrease in the phosphatidylcholine/phosphatidylethanolamine ratio, known to be associated with maintaining membrane integrity. A proximity ligation assay further indicated that mitochondria-associated endoplasmic reticulum membranes (MAM) were diminished in both mutant FUS OPCs. Moreover, both mutant FUS OPCs displayed increased susceptibility to ER stress when exposed to thapsigargin, and exhibited impaired mitochondrial respiration and reduced Ca2+ signaling from ER Ca2+ stores. Taken together, these results demonstrate a pathological role of mutant FUS in OPCs, causing defects in lipid metabolism associated with MAM disruption manifested by impaired mitochondrial metabolism with increased susceptibility to ER stress and with suppressed physiological Ca2+ signaling. As such, further exploration of the role of oligodendrocyte dysfunction in the demise of MNs is crucial and will provide new insights into the complex cellular mechanisms underlying ALS.

Fast and Efficient Generation of Isogenic Induced Pluripotent Stem Cell Lines Using Adenine Base Editing.

Isogenic induced pluripotent stem cell (iPSC) lines are currently mostly created by homology directed repair evoked by a double-strand break (DSB) generated by CRISPR-Cas9. However, this process is in general lengthy and inefficient. This problem can be overcome, specifically for correction or insertion of transition mutations, by using base editing (BE). BE does not require DSB formation, hence avoiding creation of genomic off-target breaks and insertions and deletions, and as it is highly efficient, it also does not require integration of selection cassettes in the genome to enrich for edited cells. BE has been successfully used in many cell types as well as in some in vivo settings to correct or insert mutations, but very few studies have reported generation of isogenic iPSC lines using BE. Here, we describe a simple and fast workflow to generate isogenic iPSCs efficiently with a compound heterozygous or a homozygous Wolfram syndrome 1 (WFS1) mutation using adenine BE, without the need to include a genomic selection cassette and without off-target modifications. We demonstrated that correctly base-edited clones can be generated by screening only five cell clones in less than a month, provided that the mutation is positioned in a correct place with regards to the protospacer adjacent motif sequence and no putative bystander bases exist.

HiPSC-Derived Hepatocyte-like Cells Can Be Used as a Model for Transcriptomics-Based Study of Chemical Toxicity.

Traditional toxicity risk assessment approaches have until recently focussed mainly on histochemical readouts for cell death. Modern toxicology methods attempt to deduce a mechanistic understanding of pathways involved in the development of toxicity, by using transcriptomics and other big data-driven methods such as high-content screening. Here, we used a recently described optimised method to differentiate human induced pluripotent stem cells (hiPSCs) to hepatocyte-like cells (HLCs), to assess their potential to classify hepatotoxic and non-hepatotoxic chemicals and their use in mechanistic toxicity studies. The iPSC-HLCs could accurately classify chemicals causing acute hepatocellular injury, and the transcriptomics data on treated HLCs obtained by TempO-Seq technology linked the cytotoxicity to cellular stress pathways, including oxidative stress and unfolded protein response (UPR). Induction of these stress pathways in response to amiodarone, diclofenac, and ibuprofen, was demonstrated to be concentration and time dependent. The transcriptomics data on diclofenac-treated HLCs were found to be more sensitive in detecting differentially expressed genes in response to treatment, as compared to existing datasets of other diclofenac-treated in vitro hepatocyte models. Hence iPSC-HLCs generated by transcription factor overexpression and in metabolically optimised medium appear suitable for chemical toxicity detection as well as mechanistic toxicity studies.

Therapeutic modalities and novel approaches in regenerative medicine for COVID-19.

The recent coronavirus disease 2019 outbreak around the world has had an enormous impact on the global health burden, threatening the lives of many individuals, and has had severe socio-economic consequences. Many pharmaceutical and biotechnology companies have commenced intensive research on different therapeutic strategies, from repurposed antiviral drugs to vaccines and monoclonal antibodies to prevent the spread of the disease and treat infected patients. Among the various strategies, advanced therapeutic approaches including cell- and gene-editing-based therapeutics are also being investigated, and initial results in in-vitro and early phase I studies have been promising. However, further assessments are required. This article reviews the underlying mechanisms for the pathogenesis of severe acute respiratory syndrome coronavirus-2, and discusses available therapeutic candidates and advanced modalities that are being evaluated in in-vitro/in-vivo models and are of note in clinical trials.

Human stem cell-derived monocytes and microglia-like cells reveal impaired amyloid plaque clearance upon heterozygous or homozygous loss of TREM2.

INTRODUCTION: Murine microglia expressing the Alzheimer's disease-linked TREM2R47H mutation display variable decrease in phagocytosis, while impaired phagocytosis is reported following loss of TREM2. However, no data exist on TREM2+/R47H human microglia. Therefore, we created human pluripotent stem cell (hPSC) monocytes and transdifferentiated microglia-like cells (tMGs) to examine the effect of the TREM2+/R47H mutation and loss of TREM2 on phagocytosis. METHODS: We generated isogenic TREM2+/R47H, TREM2+/-, and TREM2-/- hPSCs using CRISPR/Cas9. Following differentiation to monocytes and tMGs, we studied the uptake of Escherichia coli fragments and analyzed amyloid plaque clearance from cryosections of APP/PS1+/- mouse brains. RESULTS: We demonstrated that tMGs resemble cultured human microglia. TREM2+/- and TREM2-/- hPSC monocytes and tMGs phagocytosed significantly less E. coli fragments and cleared less amyloid plaques than wild-type hPSC progeny, with no difference for TREM2+/R47H progeny. DISCUSSION: In vitro phagocytosis of hPSC monocytes and tMGs was not affected by the TREM2+/R47H mutation but was significantly impaired in TREM2+/- and TREM2-/- progeny.

Generation of a human induced pluripotent stem cell-based model for tauopathies combining three microtubule-associated protein TAU mutations which displays several phenotypes linked to neurodegeneration.

INTRODUCTION: Tauopathies are neurodegenerative diseases characterized by TAU protein-related pathology, including frontotemporal dementia and Alzheimer's disease among others. Mutant TAU animal models are available, but none of them faithfully recapitulates human pathology and are not suitable for drug screening. METHODS: To create a new in vitro tauopathy model, we generated a footprint-free triple MAPT-mutant human induced pluripotent stem cell line (N279K, P301L, and E10+16 mutations) using clustered regularly interspaced short palindromic repeats-FokI and piggyBac transposase technology. RESULTS: Mutant neurons expressed pathogenic 4R and phosphorylated TAU, endogenously triggered TAU aggregation, and had increased electrophysiological activity. TAU-mutant cells presented deficiencies in neurite outgrowth, aberrant sequence of differentiation to cortical neurons, and a significant activation of stress response pathways. RNA sequencing confirmed stress activation, demonstrated a shift toward GABAergic identity, and an upregulation of neurodegenerative pathways. DISCUSSION: In summary, we generated a novel in vitro human induced pluripotent stem cell TAU-mutant model displaying neurodegenerative disease phenotypes that could be used for disease modeling and drug screening.

Strategies for In Vivo Genome Editing in Nondividing Cells.

Programmable nucleases, including zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated 9 (Cas9), have enhanced our ability to edit genomes by the sequence-specific generation of double-strand breaks (DSBs) with subsequent homology-directed repair (HDR) of the DSB. However, the efficiency of the HDR pathway is limited in nondividing cells, which encompass most of the cells in the body. Therefore, the HDR-mediated genome-editing approach has limited in vivo applicability. Here, we discuss a mutation type-oriented viewpoint of strategies devised over the past few years to circumvent this problem, along with their possible applications and limitations.