Depletion of bleomycin hydrolase (Blmh) downregulates histone demethylase Phf8, impairs mTOR signaling/autophagy, accelerates amyloid beta accumulation, and induces neurological deficits in mice

Bleomycin hydrolase (BLMH), a homocysteine (Hcy)-thiolactone detoxifying enzyme, is attenuated in brains of Alzheimer’s disease patients. In mice, Blmh depletion causes astrogliosis and behavioral changes. Depletion of histone demethylase PHF8, which controls mTOR signaling by demethylating H4K20me1, causes neuropathy in humans and mice. Here we examined how Blmh depletion affects the Phf8/H4K20me1/mTOR signaling/autophagy pathway and amyloid beta (Aβ) accumulation and cognitive/neuromotor performance in mice. We found that Phf8 was significantly downregulated in brains of Blmh -/- mice vs . Blmh +/+ sibling controls. H4K20me1, mTOR, phospho-mTOR, and App were upregulated while autophagy markers Bcln1, Atg5, and Atg7 were downregulated in Blmh -/- brains. Blmh depletion caused similar biochemical changes and significantly elevated Aβ in Blmh -/-5xFAD vs . Blmh +/+5xFAD brains. Behavioral testing identified cognitive/neuromotor deficits in Blmh -/- and Blmh -/-5xFAD mice. In Blmh-depleted N2a-APPswe cells, Phf8 was downregulated, while APP, total H4K20me1, and H4K20me1- mTOR promoter binding were elevated. This led to mTOR upregulation, autophagy downregulation, and significantly increased APP and Aβ levels. Phf8 depletion or treatments with Hcy-thiolactone or N -Hcy-protein, metabolites that are elevated in Blmh-depleted mice, induced similar biochemical changes in N2a-APPswe cells, akin to those in induced by Blmh depletion. Taken together, our findings indicate that Blmh interacts with APP and the Phf8/H4K20me1/mTOR/autophagy pathway and show that disruption of these interactions lead to Aβ accumulation and cognitive and neuromotor deficits. ### Competing Interest Statement The authors have declared no competing interest.