Although aberrant microRNA (miRNA) expression is linked to human diseases including cancer, the mechanisms that regulate the expression of each individual miRNA remain largely unknown. TAR DNA-binding protein-43 (TDP-43) is homologous to the heterogeneous nuclear ribonucleoproteins (hnRNPs), which are involved in RNA processing, and its abnormal cellular distribution is a key feature of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD), two neurodegenerative diseases. Here, we show that TDP-43 facilitates the production of a subset of precursor miRNAs (pre-miRNAs) by both interacting with the nuclear Drosha complex and binding directly to the relevant primary miRNAs (primiRNAs). Furthermore, cytoplasmic TDP-43, which interacts with the Dicer complex, promotes the processing of some of these premiRNAs via binding to their terminal loops. Finally, we show that involvement of TDP-43 in miRNA biogenesis is indispensable for neuronal outgrowth. These results support a previously uncharacterized role for TDP-43 in posttranscriptional regulation of miRNA expression in both the nucleus and the cytoplasm.M icroRNAs (miRNAs), small noncoding RNAs of ∼20-22 nt, have emerged as novel regulatory factors of gene expression (1). The expression of each individual miRNA is tightly regulated in a development-and cell-specific manner through transcriptional or posttranscriptional control. As a result, miRNAs can act as regulatory switches for development, organogenesis, and cellular differentiation and control distinct functions that are required for the maintenance of different cell subtypes. Moreover, the altered expression of certain miRNAs is involved in the pathogenesis of developmental abnormalities and human diseases such as cancer and Parkinson's disease (2, 3). As a consequence, understanding the mechanisms that regulate the expression of each individual miRNA is essential to elucidate the molecular pathogenesis of human diseases.MiRNAs are generated from long primary transcripts, termed pri-miRNAs, which consist of a short dsRNA region and a loop. The nuclear Drosha complex cleaves pri-miRNAs to release intermediate precursors that are termed pre-miRNAs. Pre-miRNAs are then transported by Exportin-5 into the cytoplasm where they are cleaved further by the Dicer complex to generate mature miRNAs. Finally, mature miRNAs are incorporated into the RNA-induced silencing complex (RISC), after which they can hybridize to the 3′-untranslated region (UTR) of their target mRNAs to repress translation or degrade these mRNAs (4). DGCR8 is a partner protein that is indispensable for the processing of pri-miRNAs by Drosha (5). In addition, recent work has identified multiple proteins that modulate the processing of specific miRNAs by interacting with the Drosha complex or by binding directly to pri-miRNAs or both (6, 7). Compared with the number of regulatory proteins that are involved in Drosha cleavage, only a few proteins have been identified that regulate the processing of pre-miRNAs by Dicer (6, 7)....