A FAK/HDAC5 signaling axis controls osteocyte mechanotransduction.

Osteocytes, cells ensconced within mineralized bone matrix, are the primary skeletal mechanosensors. Osteocytes sense mechanical cues by changes in fluid flow shear stress (FFSS) across their dendritic projections. Loading-induced reductions of osteocytic Sclerostin (encoded by Sost) expression stimulates new bone formation. However, the molecular steps linking mechanotransduction and Sost suppression remain unknown. Here, we report that class IIa histone deacetylases (HDAC4 and HDAC5) are required for loading-induced Sost suppression and bone formation. FFSS signaling drives class IIa HDAC nuclear translocation through a signaling pathway involving direct HDAC5 tyrosine 642 phosphorylation by focal adhesion kinase (FAK), a HDAC5 post-translational modification that controls its subcellular localization. Osteocyte cell adhesion supports FAK tyrosine phosphorylation, and FFSS triggers FAK dephosphorylation. Pharmacologic FAK catalytic inhibition reduces Sost mRNA expression in vitro and in vivo. These studies demonstrate a role for HDAC5 as a transducer of matrix-derived cues to regulate cell type-specific gene expression.

Selectivity and Kinetic Requirements of HDAC Inhibitors as Progranulin Enhancers for Treating Frontotemporal Dementia.

Frontotemporal dementia (FTD) arises from neurodegeneration in the frontal, insular, and anterior temporal lobes. Autosomal dominant causes of FTD include heterozygous mutations in the GRN gene causing haploinsufficiency of progranulin (PGRN) protein. Recently, histone deacetylase (HDAC) inhibitors have been identified as enhancers of PGRN expression, although the mechanisms through which GRN is epigenetically regulated remain poorly understood. Using a chemogenomic toolkit, including optoepigenetic probes, we show that inhibition of class I HDACs is sufficient to upregulate PGRN in human neurons, and only inhibitors with apparent fast binding to their target HDAC complexes are capable of enhancing PGRN expression. Moreover, we identify regions in the GRN promoter in which elevated H3K27 acetylation and transcription factor EB (TFEB) occupancy correlate with HDAC-inhibitor-mediated upregulation of PGRN. These findings have implications for epigenetic and cis-regulatory mechanisms controlling human GRN expression and may advance translational efforts to develop targeted therapeutics for treating PGRN-deficient FTD.