ALTERNATIVE SPLICING AND PHOSPHORYLATION PROVIDE A MECHANISTIC BASIS FOR FUNCTIONAL SPECIALIZATION OF LSD1/KDM1 HISTONE DEMETHYLASE IN THE CENTRAL NERVOUS SYSTEM

Chromatin represents the physiological substrate of epigenetic regulation, underlying several biological processes, from replication and transcriptional activity to cell lineage commitment and adaptation in response to specific cues. In the central nervous system, chromatin integrates a plethora of converging signaling pathways, leading to short- and long-term changes in gene expression that are crucial for neuronal commitment, terminal differentiation and neuroplasticity throughout life. Although a variety of ubiquitously expressed chromatin-remodeling complexes assist tissue-specific transcription factors in mediating histotype-restricted transcriptional regulation, neuro restricted chromatin-remodeling factors have just been recently described. Here, I report the identification of four mammal-specific variants of the histone demethylase LSD1 / KDM1 arising from combinatorial retention of two alternatively spliced exons, resulting in either ubiquitous isoforms or neuro-restricted ones, which are dynamically regulated during cortical development. The neuro-specific variants, whose functional diversification partly relies on the phosphorylation status, contribute to the acquisition of neurite morphology in an exclusive way. The expression of LSD1 splice variants is particularly regulated during perinatal stages, with a progressive increase of LSD1 neuro-specific isoforms over the ubiquitous ones and the same LSD1 splice dynamics can be fairly recapitulated in cultured cortical neurons reflecting the early synaptic establishment. While LSD1 isoforms exhibit a comparable demethylase activity in vitro, the sole neuro-specific isoforms display an altered repressor activity on a reporter gene in cortical neurons, where their overexpression enhances neurite morphogenesis. Conversely, the imbalance of LSD1 isoforms that are devoid of the neurospecific exon elicits no morphogenic effect, indicating that the arousal of neuronal LSD1 isoforms pace-makes early neurite morphogenesis. The genome wide-location of LSD1 in cortical neurons by chromatin immunoprecipitation sequencing reveals its extensive role in the regulation of promoters related to developmental processes, pattern specification and forebrain organogenesis, as well as the control of cell fate decisions and calcium induced neuronal signaling, confirming its neurogenic implication. Furthermore, for those genes where multiple binding locations can be found at short and long distance from the same transcriptional start site, a wide-range LSD1 regulatory role can be hypothesized, possibly related to enhancers function.