The human brain consists of a dense network of neurons that can modify the efficacy and strength of their synaptic connections in response to experience-dependent (ED) external cues (Ebert and Greenberg, 2013). At the molecular level, patterns of post-synaptic excitation trigger calcium-mediated activation of gene expression profiles that encode for proteins that induce synapse-specific morphological changes such as dendritic growth and branching and synaptogenesis (Crepaldi et al., 2013). Such activity-dependent gene regulation has emerged as a powerful mechanism underlying how environmental changes are molecularly translated into functional synaptic outputs for establishing memory formation and cognitive function and can influence the severity of neurodegenerative disorders like Alzheimer's disease (AD). ED neural plasticity is driven by histone acetylation (HA) mediated epigenetic mechanisms that regulate dynamic activity-dependent gene transcription profiles in response to neuronal stimulation to influence synaptic connections (Karnay et al., 2019). Yet, how histone acetyltransferases (HATs) respond to extracellular cues in the in vivo brain to drive HA mediated activity-dependent gene control remains unclear.

Nucleocytoplasmic transport (NCT) in neurons is essential for controlling protein transport from the cytoplasm to the nucleus to regulate plasticity genes in response to environmental cues. NCT is a highly regulated process that involves regulation of karyopherin import and export proteins, nuclear pore proteins, and regulation of the Ran gradient (Wing et al., 2022). Several of these pathways have been found to be misregulated in neurodegenerative diseases, including mislocalization and aggregation of karyopherins and nuclear transport factors (Hutten and Dormann, 2020). Most recent work has focused on NCT of transcription factors (Lee and Fields, 2021), however more recent studies have elucidated the importance of NCT of epigenetic modulators, specifically histone deacetylases (HDACs; Alisafaei et al., 2019, Eldridge et al., 2020, Liu et al., 2012, McKinsey et al., 2000, Taplick et al., 2001, Wang and Yang, 2001, Wang et al., 2019, Luo et al., 2019, Fitzsimons et al., 2013, Kao et al., 2001). We previously reported Tip60 as the first histone acyltransferase (HAT) found to undergo NCT in primary hippocampal neurons, in vitro (Karnay et al., 2019). In response to extracellular cues, Tip60 translocates into the nucleus, specifically to transcriptionally active regions known as transcription factories, to activate expression of activity-dependent neuroplasticity genes (Karnay et al., 2019). Further, we demonstrated that Tip60 is largely excluded from the nucleus in the human Alzheimer's disease (AD) hippocampus (Panikker et al., 2018), suggesting Tip60 NCT could be disrupted in neurodegenerative diseases such as AD.

Here, we elucidate Tip60 HAT subcellular localization and NCT specifically in neuronal activity-dependent gene control by using the learning and memory mushroom body (MB) region of the Drosophila brain as a powerful in vivo cognitive model system. We found that inducing neural activity either genetically or by exposure to natural positive ED conditions mediated neural induction triggered Tip60 nuclear import with concomitant induction of previously identified Tip60 target genes and that this localization pattern was altered in the brains of a well characterized Drosophila AD model. Mutagenesis of a putative nuclear localization signal (NLS) sequence and nuclear export signal (NES) sequence that we identified in the Drosophila Tip60 protein revealed that both are functionally required for appropriate Tip60 subcellular localization. Our results support a model by which neuronal stimulation triggers Tip60 NCT via its NLS and NES sequences to promote induction of activity-dependent neuroplasticity gene transcription and that this process may be disrupted in AD.