Nuclear-localized HDAC4 promotes neuroprotection of retinal ganglion cells after optic nerve injury
Background: As the sole output neurons for the retina, retinal ganglion cells (RGCs) transmit all visual information from the retina via the optic nerve to the brain. RGCs do not regenerate when injured, and effective therapies for promoting RGC survival and axon regeneration in optic neuropathies comprise an unmet clinical need, including in the common disease glaucoma. Histone deacetylases (HDACs) are epigenetic modifiers that repress gene transcription and play a key role in retinal development and disease.  In this study, we identify the role of HDAC4 in RGC neurodegeneration and axon regeneration. Methods: The role of HDAC4 in the regulation of RGC neuroprotection and axon regeneration was studied in the mouse optic nerve crush (ONC) model for optic neuropathy by transduction of RGCs in vivo with adeno-associated virus (AAV) vectors. HDAC4- and ONC-dependent gene expression in vivo were studied by single cell RNA sequencing (scRNA-seq) of isolated RGCs.  Results: A loss-of-function screen for the role of HDACs in RGC survival after optic nerve crush (ONC) injury identified HDAC4 as essential element for RGC survival. Accordingly, expression of a constitutively nuclear HDAC4 S246/467/632A missense mutant (3SA) increased RGC survival and axon regeneration after ONC injury. Similar beneficial effects were conferred by expression of an N-terminal fragment of HDAC4 (HDAC4 NT) that constitutively represses gene expression. scRNA-seq showed that one day after ONC injury, transcriptomic profiles of RGCs were altered such that HDAC4 NT and to a lesser degree the HDAC4 3SA mutant attenuated the gene expression changes associated with injury.  Conclusions:  This study demonstrates that enhancement of nuclear HDAC4 activity will promote RGC survival and axon regeneration in the ONC model of RGC injury, identifying HDAC4 as a novel target in the development of therapeutics for RGC protection and restoration of visual function. Overall design: The role of HDAC4 in the regulation of RGC neuroprotection and axon regeneration was studied in the mouse optic nerve crush (ONC) model for optic neuropathy by transduction of RGCs in vivo with adeno-associated virus (AAV) vectors. HDAC4- and ONC-dependent gene expression in vivo were studied by single cell RNA sequencing (scRNA-seq) of isolated RGCs.