Causal ALS genes impact the MHC class II antigen presentation pathway.

Mutations in RNA/DNA-binding proteins cause amyotrophic lateral sclerosis (ALS), but the underlying disease mechanisms remain unclear. Here, we report that a set of ALS-associated proteins, namely FUS, EWSR1, TAF15, and MATR3, impact the expression of genes encoding the major histocompatibility complex II (MHC II) antigen presentation pathway. Both subunits of the MHC II heterodimer, HLA-DR, are down-regulated in ALS gene knockouts/knockdown in HeLa and human microglial cells, due to loss of the MHC II transcription factor CIITA. Importantly, hematopoietic progenitor cells (HPCs) derived from human embryonic stem cells bearing the FUSR495X mutation and HPCs derived from C9ORF72 ALS patient induced pluripotent stem cells also exhibit disrupted MHC II expression. Given that HPCs give rise to numerous immune cells, our data raise the possibility that loss of the MHC II pathway results in global failure of the immune system to protect motor neurons from damage that leads to ALS.

Erratum: Generation and analysis of innovative genomically humanized knockin SOD1, TARDBP (TDP-43), and FUS mouse models.

[This corrects the article DOI: 10.1016/j.isci.2021.103463.].

TIA1 potentiates tau phase separation and promotes generation of toxic oligomeric tau.

Tau protein plays an important role in the biology of stress granules and in the stress response of neurons, but the nature of these biochemical interactions is not known. Here we show that the interaction of tau with RNA and the RNA binding protein TIA1 is sufficient to drive phase separation of tau at physiological concentrations, without the requirement for artificial crowding agents such as polyethylene glycol (PEG). We further show that phase separation of tau in the presence of RNA and TIA1 generates abundant tau oligomers. Prior studies indicate that recombinant tau readily forms oligomers and fibrils in vitro in the presence of polyanionic agents, including RNA, but the resulting tau aggregates are not particularly toxic. We discover that tau oligomers generated during copartitioning with TIA1 are significantly more toxic than tau aggregates generated by incubation with RNA alone or phase-separated tau complexes generated by incubation with artificial crowding agents. This pathway identifies a potentially important source for generation of toxic tau oligomers in tau-related neurodegenerative diseases. Our results also reveal a general principle that phase-separated RBP droplets provide a vehicle for coassortment of selected proteins. Tau selectively copartitions with TIA1 under physiological conditions, emphasizing the importance of TIA1 for tau biology. Other RBPs, such as G3BP1, are able to copartition with tau, but this happens only in the presence of crowding agents. This type of selective mixing might provide a basis through which membraneless organelles bring together functionally relevant proteins to promote particular biological activities.

Generation and analysis of innovative genomically humanized knockin SOD1, TARDBP (TDP-43), and FUS mouse models.

Amyotrophic lateral sclerosis/frontotemporal dementia (ALS/FTD) is a fatal neurodegenerative disorder, and continued innovation is needed for improved understanding and for developing therapeutics. We have created next-generation genomically humanized knockin mouse models, by replacing the mouse genomic region of Sod1, Tardbp (TDP-43), and Fus, with their human orthologs, preserving human protein biochemistry and splicing with exons and introns intact. We establish a new standard of large knockin allele quality control, demonstrating the utility of indirect capture for enrichment of a genomic region of interest followed by Oxford Nanopore sequencing. Extensive analysis shows that homozygous humanized animals only express human protein at endogenous levels. Characterization of humanized FUS animals showed that they are phenotypically normal throughout their lifespan. These humanized strains are vital for preclinical assessment of interventions and serve as templates for the addition of coding or non-coding human ALS/FTD mutations to dissect disease pathomechanisms, in a physiological context.