Post-transcriptional regulation of microRNA biogenesis and localization in mammalian neurons

The remarkable cognitive capabilities of our brain require a complex and dynamic network of neurons that is able to quickly and precisely react to changes. Adapting to an ever-changing environment and the storage of information requires that sensory information is transformed into long-lasting structural changes. At the molecular level, highly sophisticated and tightly regulated gene expression programs are necessary to alter synaptic connections in the brain without disrupting the existing network. MicroRNAs are important regulators in this neuronal network as they are able to precisely regulate local gene expression. These small non-coding RNAs bind complementary sequences in target mRNAs, thereby repressing their translation into protein. This plays an important role in activity-dependent synapse development, where local protein synthesis in dendrites is required to implement long-lasting changes in synaptic strength. The latter serves as the molecular basis for learning and memory processes. This cumulative dissertation presents two studies that investigate how neuronal micro- RNAs are regulated at the level of biogenesis and localization. The first publication "The DEAH-box helicase DHX36 mediates dendritic localization of the neuronal precursor-microRNA-134" investigates the transport of miRNA-134 to its final destination in the synapto-dendritic compartment. The study describes that already the precursor (pre-miRNA) is located at the synapse and identifies DHX36 as a protein that specifically binds pre-miR-134 and is important for its transport. Knockdown of DHX36 further shows that the localization of pre-miR-134 to the dendrite is of functional importance. The absence of DHX36 leads to elevated expression of known miR-134 targets, accompanied by an increase in dendritic spine volume. The second study presented in this thesis entitled "The nuclear matrix protein Matr3 regulates processing of the synaptic microRNA-138-5p" investigates the expression of microRNA-138. Two distinct precursor forms are known for miR-138, pre-miR-138-1 and pre-miR-138-2. In our study, we demonstrate that pre-miR-138-2 is the primary source for mature miR-138 in neurons. Using pulldown assays we identify the nuclear matrix protein Matrin-3 (Matr3) as a specific interactor of the hairpin structure of both the primary and precursor form of miR-138-2 (pri-/pre-miR-138-2). Knockdown of its expression demonstrates an inhibitory function of Matr3 in the nuclear processing of pri-miR-138-2, resulting in decreased mature miR-138 levels. In summary, this thesis describes novel post-transcriptional regulatory mechanisms that control the expression and sub-cellular localization of two neuronal microRNAs, miR- 134 and miR-138. Both microRNAs have important roles in synaptic plasticity and a precise regulation of their expression is crucial for maintaining a stable and functional neuronal network. A further understanding of the regulation of these microRNAs and their downstream processes is an important step to gain insight into the complex regulatory processes involved in learning and memory, as well as into malfunctions of these systems that occur in neurological diseases