Loss of <i>miR‐34</i> in <i>Drosophila</i> dysregulates protein translation and protein turnover in the aging brain

Aging is a risk factor for neurodegenerative disease, but precise mechanisms that influence this relationship are still under investigation. Work in Drosophila melanogaster identified the microRNA miR-34 as a modifier of aging and neurodegeneration in the brain. MiR-34 mutants present aspects of early aging, including reduced lifespan, neurodegeneration, and a buildup of the repressive histone mark H3K27me3. To better understand how miR-34 regulated pathways contribute to age-associated phenotypes in the brain, here we transcriptionally profiled the miR-34 mutant brain. This identified that genes associated with translation are dysregulated in the miR-34 mutant. The brains of these animals show increased translation activity, accumulation of protein aggregation markers, and altered autophagy(definition) activity. To determine if altered H3K27me3 was responsible for this proteostasis(definition) dysregulation, we studied the effects of increased H3K27me3 by mutating the histone demethylase Utx. Reduced Utx activity enhanced neurodegeneration and mimicked the protein accumulation seen in miR-34 mutant brains. However, unlike the miR-34 mutant, Utx mutant brains did not show similar altered autophagy or translation activity, suggesting that additional miR-34-targeted pathways are involved. Transcriptional analysis of predicted miR-34 targets identified Lst8, a subunit of Tor Complex 1 (TORC1), as a potential target. We confirmed that miR-34 regulates the 3' UTR of Lst8 and identified several additional predicted miR-34 targets that may be critical for maintaining proteostasis and brain health. Together, these results present novel understanding of the brain and the role of the conserved miRNA miR-34 in impacting proteostasis in the brain with age.