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Human induced pluripotent stem cells (iPSCs) are a powerful tool for studying development and disease. However, different iPSC lines show considerable phenotypic variation. The lack of common well-characterized cell lines that are used widely frustrates efforts to integrate data across research groups or replicate key findings. Inspired by model organism communities who addressed this issue by establishing a limited number of widely accepted strains, we characterised candidate iPSC lines in unprecedented detail to select a well-performing line to underpin collaborative studies. Specifically, we characterised the morphology, growth rates, and single-cell transcriptomes of iPSC lines in the pluripotent state and assessed their genomic integrity using karyotyping, DNA microarrays, whole genome sequencing, and functional assays for p53 activity. We further tested their ability to be edited by CRISPR/Cas9 and used single-cell RNA sequencing to compare the efficiency with which they could be differentiated into multiple lineages. We found that there was significant variability in the performance of lines across the tested assays that enabled the rational selection of a lead line, KOLF2.1J, which is a gene-corrected derivative of a publicly available line from the Human Induced Pluripotent Stem Cells Initiative (HipSci) resource. We are now using this line in an initiative from the NIH Center for Alzheimer’s and Related Dementias to derive hundreds of gene-edited and functionalized sub-clones to be distributed widely throughout the research community along with associated datasets, with the aim of promoting the standardisation required for large-scale collaborative science in the stem cell field.SummaryThe authors of this collaborative science study describe a deep characterization of widely available induced pluripotent stem cell (iPSC) lines to rationally select a line that performs well in multiple experimental approaches. Analysis of transcriptional patterns in the pluripotent state, whole genome sequencing, genomic stability after highly efficient CRISPR-mediated gene editing, integrity of the p53 pathway, and differentiation efficiency towards multiple lineages identified KOLF2.1J as a well-performing cell line. The widespread distribution and use of this line makes it an attractive cell line for comparative and collaborative efforts in the stem cell field.HighlightsDeep genotyping and phenotyping reveals KOLF2.1J as well-performing cell line that is readily distributed and could serve as common reference lineDespite rare copy-neutral loss of heterozygosity (CN-LOH) events, iPSC lines retain genomic fidelity after CRISPR/Cas9-based gene editingOur multifactorial pipeline serves as a blueprint for future efforts to identify other lead iPSC linesGraphical abstract

Stress granules (SGs), RNA/protein condensates assembled in cells under stress, are believed to play a critical role in the pathogenesis of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). However, how SG assembly is regulated and related to pathomechanism is incompletely understood. Here, we show that ER stress activates JNK via IRE1 in fly and cellular models of C9orf72-mediated ALS/FTD (c9ALS/FTD), the most common genetic form of ALS/FTD. Furthermore, activated JNK promotes SG assembly induced by poly(GR) and poly(PR), two toxic proteins implicated in c9ALS/FTD, by promoting the transcription of G3BP1, a key SG protein. Consistent with these findings, JNK or IRE1 inhibition reduced SG formation, G3BP1 mRNA and protein levels, and neurotoxicity in cells overexpressing poly(GR) and poly(PR) or neurons derived from c9ALS/FTD patient induced pluripotent stem cells (iPSCs). Our findings connect ER stress, JNK, and SG assembly in a unified pathway contributing to c9ALS/FTD neurodegeneration.

Stress granules are the RNA/protein condensates assembled in the cells under stress. They play a critical role in the pathogenesis of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). However, how stress granule assembly is regulated and related to ALS/FTD pathomechanism is incompletely understood. Mutation in the C9orf72 gene is the most common cause of familial ALS and FTD. C9orf72 mutation causes the formation of toxic dipeptide repeats. Here we show, the two most toxic dipeptide repeats i.e. poly(GR) and poly(PR) activate JNK via the ER-stress response protein IRE1 using fly and cellular models. Further, we show activated JNK promotes stress granule assembly in cells by promoting the transcription of one of the key stress granule proteins i.e. G3BP1 by inducing histone 3 phosphorylation. Consistent with these findings, JNK or IRE1 inhibition reduced stress granule formation, histone 3 phosphorylation, G3BP1 mRNA and protein levels, and neurotoxicity in cells overexpressing poly(GR) and poly(PR) or neurons derived from male and female C9ALS/FTD patient-induced pluripotent stem cells (iPSCs). Our findings connect ER stress, JNK activation, and stress granule assembly in a unified pathway contributing to C9ALS/FTD neurodegeneration.Significance StatementJNK is a part of the mitogen-activated protein kinase pathway which is the central node for the integration of multiple stress signals. Cells are under constant stress in neurodegenerative diseases and how these cells respond to stress signals is a critical factor in determining their survival or death. Previous studies have shown JNK as a major contributor to cellular apoptosis. Here, we show the role of JNK in stress granule assembly. We identify toxic dipeptide repeats produced in ALS/FTD conditions activate JNK. The activated JNK in the nucleus can induce histone modifications which increase G3BP1 expression thus promoting stress granule assembly and neurodegeneration.