miR155 triplicated in Down syndrome regulates hippocampal GABAergic neurogenesis in Alzheimer's disease.

MicroRNA dysregulation is implicated in neurodegenerative disorders, including Alzheimer's disease (AD). The role of neuronal microRNA155 (miR155), elevated in both AD and Down syndrome (DS), remains unknown. We found that MIR155HG (miR155 host gene) colocalizes with APP (amyloid-beta precursor protein) in a neuron-specific, topologically-associated domain (TAD) within a regulatory network, in the obligate portion of chromosome 21 triplicated in DS which causes AD neuropathology and in most cases, dementia. We investigated miR155 role during neuron development and then validated these findings in an amyloidopathy model. In human induced pluripotent stem cell (hiPSC)-derived neural stem cells (NSCs), cortical neurons and cortical organoids, MIR155 deletion enhanced NSC proliferation, ventral patterning and GABAergic interneuron generation. However, MIR155 overexpression inhibited NSC marker expression and GABAergic interneuron generation. MIR155 upregulates its mRNA targets, NR2F1 / 2 , key modulators of hippocampal GABAergic interneuron development. In an amyloidopathy mouse model, miR155 deletion induced the expansion of hippocampal NSCs and increased hippocampal GABAergic interneurons. These findings reveal previously unrecognized miR155 roles in NSC dynamics and GABAergic interneuron development which directionally diverge from extensively studied microglial miR155 in their beneficial vs negative impact on AD mouse models, suggesting that approaching miR155 therapeutically may require balancing the effects in neurons and microglia.