The fine-tuning of transcriptional regulation by gene x environment interaction is central to maladaptive processes associated with major depressive disorder (MDD). Research over the past decade has provided strong support for the importance of epigenetic mechanisms in MDD pathogenesis. microRNAs (miRNAs), a class of small noncoding RNAs, are generating enormous interest not only as epigenetic mega-regulators of gene expression, but also for their role in disease pathophysiology and treatment targets. We and other investigators have shown differential regulation of miRNAs in the brain of MDD individuals. It is generally agreed that miRNAs mediate post-transcriptional gene silencing in the cytoplasm through seed sequence of miRNAs and complementary sequences in the 3′-untranslated regions (UTR) of target mRNAs via Argonaute (Ago)- based-RNA-induced silencing complex (miRISC). Recently, a paradigm-shifting phenomenon has been put forth with the concept of “nuclear localization” of select mature miRNAs. These miRNAs, containing unique set(s) of nuclear signals in the 3’ terminus, can shuttle back to nucleus from cytoplasm where they can regulate the expression of select nuclear pool of coding and non-coding RNA transcripts post-transcriptionally, but more remarkably, transcriptionally. At the transcription level, there is evidence of putative binding sites of mature miRNAs in the gene promoter regions with partial or perfect sequence complementarity, which enables nuclear miRNAs to regulate gene transcription, including primary(pri-)-miRNAs. Post-transcriptionally, nuclear miRNAs in conjunction with endonuclease Drosha, can target pri-miRNAs or can bind to 3’UTR region of nuclear coding transcripts via miRISC. The newly discovered mechanism poses an interesting possibility that within the nucleus, miRNAs may have the distinct capability of repatterning the gene transcription dynamics dramatically where they can not only regulate their own expression at pri- and precursor(pre)-miRNA levels, but also at the nuclear coding transcript level. This could be highly relevant in MDD-associated maladaptation processes. We propose an overarching hypothesis that a dynamic shift and nuclear enrichment of mature miRNAs driven by specific nuclear signals and their regulation of key pri-, pre-, and mature miRNAs, and coding genes within the nucleus, and consequent functional attributes, will be central to MDD pathogenesis. Using highly innovative approaches and well characterized and matched brain samples from MDD and non-psychiatric control subjects, we aim to determine: 1) the nuclear enrichment of miRNAs, cytosolic to nuclear shift, and their functional relevance; 2) possible mechanism(s) of miRNA translocation; 3) the unique transcriptional regulatory role of nuclear miRNAs in changing the promoter dynamics of target genes as a function of Ago1 complex; and 4) the unique role of nuclear miRNAs in modifying the processing of pri-miRNAs as a function of Drosha mi...