Regulation of microRNA Expression by Induction of Bidirectional Synaptic Plasticity

MicroRNAs as effectors of brain function with roles in ischemia and injury, neuroprotection, and neurodegeneration

MicroRNAs are small RNAs that function as regulators of posttranscriptional gene expression. MicroRNAs are encoded by genes, and processed to form ribonucleoprotein complexes that bind to messenger RNA (mRNA) targets to repress translation or degrade mRNA transcripts. The microRNAs are particularly abundant in the brain where they serve as effectors of neuronal development and maintenance of the neuronal phenotype. They are also expressed in dendrites where they regulate spine structure and function as effectors in synaptic plasticity. MicroRNAs have been evaluated for their roles in brain ischemia, traumatic brain injury, and spinal cord injury, and in functional recovery after ischemia. They also serve as mediators in the brain's response to ischemic preconditioning that leads to endogenous neuroprotection. In addition, microRNAs are implicated in neurodegenerative disorders, including Alzheimer's, Huntington, Parkinson, and Prion disease. The discovery of microRNAs has expanded the potential for human diseases to arise from genetic mutations in microRNA genes or sequences within their target mRNAs. This review discusses microRNA discovery, biogenesis, mechanisms of gene regulation, their expression and function in the brain, and their roles in brain ischemia and injury, neuroprotection, and neurodegeneration.

Olfactory discrimination training up-regulates and reorganizes expression of microRNAs in adult mouse hippocampus

Adult male mice (strain C57Bl/6J) were trained to execute nose-poke responses for water reinforcement; then they were randomly assigned to either of two groups: olfactory discrimination training (exposed to two odours with reward contingent upon correctly responding to one odour) or pseudo-training (exposed to two odours with reward not contingent upon response). These were run in yoked fashion and killed when the discrimination-trained mouse reached a learning criterion of 70% correct responses in 20 trials, occurring after three sessions (a total of ∼40 min of training). The hippocampus was dissected bilaterally from each mouse (N = 7 in each group) and profiling of 585 miRNAs (microRNAs) was carried out using multiplex RT–PCR (reverse transcription–PCR) plates. A significant global up-regulation of miRNA expression was observed in the discrimination training versus pseudo-training comparison; when tested individually, 29 miRNAs achieved significance at P = 0.05. miR-10a showed a 2.7-fold increase with training, and is predicted to target several learning-related mRNAs including BDNF (brain-derived neurotrophic factor), CAMK2b (calcium/calmodulin-dependent protein kinase IIβ), CREB1 (cAMP-response-element-binding protein 1) and ELAVL2 [ELAV (embryonic lethal, abnormal vision, Drosophila)-like; Hu B]. Analysis of miRNA pairwise correlations revealed the existence of several miRNA co-expression modules that were specific to the training group. These in vivo results indicate that significant, dynamic and co-ordinated changes in miRNA expression accompany early stages of learning.

The role of microRNAs in drug addiction: a big lesson from tiny molecules

Alcoholism is a multifactorial disease of unclear molecular underpinnings. Currently, we are witnessing a major shift in our understanding of the functional elements of the genome, which could help us to discover novel insights into the nature of alcoholism. In humans, the vast majority of the genome encodes non-protein-coding DNA with unclear function. Recent research has started to unveil this mystery by describing the functional relevance of microRNAs, and examining which genes are regulated by non-protein-coding DNA. Here, I describe alcohol regulation of microRNAs and provide examples of microRNAs that control the expression of alcohol-relevant genes. Emphasis is put on the potential of microRNAs in explaining the polygenic nature of alcoholism and prospects of microRNA research and future directions of this burgeoning field.

microRNA regulation of synaptic plasticity

microRNAs play an important role in regulating synaptic plasticity. For example, microRNAs target (and are targeted by) plasticity mediators such as CREB, MECP2, and FMRP. As well, specific microRNAs have been shown to be expressed within dendrites, where they regulate protein translation of targets mediating dendritic growth. Components of the RISC machinery have been implicated in long-term memory in Drosophila. Here, we review evidence from studies of adult mouse forebrain supporting a model wherein synaptic stimulation (above a threshold value) increases calcium within dendritic spines, activates calpain, and activates and releases dicer from the postsynaptic density. Dicer processes local pre-miRs into mature miRNAs that are incorporated into RISC complexes within or near the dendritic spine, and that bind available target mRNAs in the vicinity. These may repress protein translation under resting conditions, yet permit a phasic burst of translation to occur transiently following subsequent synaptic activity. Loaded RISC complexes that are not bound to local mRNAs may serve to bind and trap mRNAs that are being transported down dendrites. Thus, locally formed microRNAs may mark the location of previously activated synapses and perform a type of synaptic tagging and capture.