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Background & aims Methionine adenosyltransferase 1A (MAT1A) is responsible for S-adenosylmethionine (SAMe) biosynthesis in the liver. Mice lacking Mat1a have hepatic SAMe depletion, develop non-alcoholic steatohepatitis (NASH) and hepatocellular carcinoma (HCC) spontaneously. Several kinases are activated in Mat1a knockout (KO) mice livers. However, the phosphos-proteome has not been characterized and whether they contribute to liver pathology is largely unknown. Our study aimed to fill this gap. Approach & results We performed phospho-proteomics in Mat1a KO mice livers with and without SAMe treatment to identify SAMe-dependent changes that may contribute to liver pathology. Our studies used Mat1a KO mice at different ages treated with and without SAMe, cell lines, in vitro translation and kinase assays, and human liver specimens. We found the most striking change was hyperphosphorylation and increased content of La-Related Protein 1 (LARP1), which in the unphosphorylated form negatively regulates translation of 5'-terminal oligopyrimidine (TOP)-containing mRNAs. Consistently, multiple TOP proteins are induced in the KO livers. The translation of TOP mRNAs RPS3 and RPL18 was enhanced by LARP1 overexpression in liver cancer cells. We identified LARP1-T449 as a novel, SAMe-sensitive phospho-site of cyclin-dependent kinase 2 (CDK2). Knocking down CDK2 lowered LARP1 phosphorylation and prevented LARP1 overexpression mediated increase in translation. LARP1-T449 phosphorylation induced global translation, cell growth, migration, invasion, and expression of oncogenic TOP-ribosomal proteins in HCC cells. LARP1 expression is increased in human NASH and HCC. Conclusion Our results reveal a novel SAMe-sensitive mechanism of LARP1 phosphorylation that may be involved in the progression of NASH to HCC.

SUMMARYFrontotemporal dementia (FTD) due to MAPT mutation causes pathological accumulation of tau and glutamatergic cortical neuronal death by unknown mechanisms. We used human induced pluripotent stem cell (iPSC)-derived cerebral organoids expressing tau-V337M and isogenic corrected controls to discover early alterations due to the mutation that precede neurodegeneration. At 2 months, mutant organoids show upregulated expression of MAPT, and glutamatergic signaling pathways and regulators including the RNA-binding protein ELAVL4. Over the following 4 months, mutant organoids accumulate splicing changes, disruption of autophagy function and build-up of tau and P-tau S396. By 6 months, tau-V337M organoids show specific loss of glutamatergic neurons of layers affected in patients. Mutant neurons are susceptible to glutamate toxicity which was rescued pharmacologically by treatment with the PIKFYVE kinase inhibitor apilimod. Our results demonstrate a sequence of events that precede cell death, revealing molecular pathways associated with glutamate signaling as potential targets for therapeutic intervention in FTD.

Summary Frontotemporal dementia (FTD) because of MAPT mutation causes pathological accumulation of tau and glutamatergic cortical neuronal death by unknown mechanisms. We used human induced pluripotent stem cell (iPSC)-derived cerebral organoids expressing tau-V337M and isogenic corrected controls to discover early alterations because of the mutation that precede neurodegeneration. At 2 months, mutant organoids show upregulated expression of MAPT, glutamatergic signaling pathways, and regulators, including the RNA-binding protein ELAVL4, and increased stress granules. Over the following 4 months, mutant organoids accumulate splicing changes, disruption of autophagy function, and build-up of tau and P-tau-S396. By 6 months, tau-V337M organoids show specific loss of glutamatergic neurons as seen in individuals with FTD. Mutant neurons are susceptible to glutamate toxicity, which can be rescued pharmacologically by the PIKFYVE kinase inhibitor apilimod. Our results demonstrate a sequence of events that precede neurodegeneration, revealing molecular pathways associated with glutamate signaling as potential targets for therapeutic intervention in FTD.

Traumatic brain injury confers a significant and growing public health burden and represents a major environmental risk factor for dementia. Previous efforts to model traumatic brain injury and elucidate pathologic mechanisms have been hindered by complex interactions between multiple cell types, biophysical, and degenerative properties of the human brain. Here, we use high-intensity focused ultrasound to induce mechanical injury in 3D human pluripotent stem cell-derived cortical organoids to mimic traumatic brain injury in vitro. Our results show that mechanically injured organoids recapitulate key hallmarks of traumatic brain injury, phosphorylation of tau and TDP-43, neurodegeneration, and transcriptional programs indicative of energy deficits. We present high-intensity focused ultrasound as a novel, reproducible model of traumatic brain injury in cortical organoids with potential for scalable and temporally-defined mechanistic studies.