LMK235, a small molecule inhibitor of HDAC4/5, protects dopaminergic neurons against neurotoxin- and α-synuclein-induced degeneration in cellular models of Parkinson’s disease

Epigenetic modifications are increasingly studied for their roles in the progression of neurodegenerative diseases. Changes in the acetylation rates of specific Parkinson’s disease (PD)-linked genes have been associated with the pathological advancement of this disorder. Given this context—and the absence of disease-modifying therapies for PD—histone deacetylase inhibitors (HDIs) have emerged as potential pharmacological candidates. Previous studies have evaluated the neuroprotective effects of pan-HDAC inhibitors and certain class-specific inhibitors in both in vivo and in vitro models of PD, with mixed results.

In this study, we employed gene co-expression analysis to identify HDACs associated with human dopaminergic (DA) neuron development. We found that HDAC3, HDAC5, HDAC6, and HDAC9 were highly correlated with the DA markers SLC6A3 and NR4A2. RT-qPCR analysis further revealed that the mRNA expression of these HDACs followed similar temporal expression patterns during embryonic mouse midbrain DA neuron development.

To explore their neuroprotective potential, we tested several class-specific small-molecule HDIs on human SH-SY5Y cells, using neurite outgrowth as an indicator of neurotrophic activity. Neither the class I-specific HDIs RGFP109 and RGFP966, nor the HDAC6-selective inhibitor ACY1215, significantly affected neurite extension. In contrast, the class IIa HDI LMK235, which selectively inhibits HDAC4 and HDAC5, markedly increased histone acetylation and promoted neurite outgrowth.

Further investigation showed that LMK235 enhanced BMP-Smad-dependent transcription in SH-SY5Y cells, and this signaling was essential for its neurite growth-promoting effects, both in SH-SY5Y cells and in primary cultures of embryonic day 14 rat ventral mesencephalon (VM) DA neurons. Similar effects were observed in SH-SY5Y cells following HDAC5 knockdown with siRNA. Additionally, LMK235 conferred neuroprotection against degeneration induced by the dopaminergic neurotoxin 1-methyl-4-phenylpyridinium (MPP+) in both SH-SY5Y cells and cultured DA neurons.

Notably, LMK235 treatment also protected against axonal degeneration triggered by overexpression of either wild-type or A53T mutant α-synuclein in both SH-SY5Y cells and primary DA neuron cultures.

In summary, these findings highlight the neuroprotective potential of the class IIa HDI LMK235 in cellular models relevant to Parkinson’s disease, supporting its RG2833 further exploration as a therapeutic candidate.