**2. microRNA**

#### **2.1 miRNA biogenesis and mechanism of action**

miRNAs are transcribed by RNA polymerase II/III from either independent miRNA genes (monocistronic), as clusters of up to a few hundred miRNA (polycistronic) or from the introns of protein-coding genes (intronic). Approximately half of miRNAs are considered intronic, however a functional relationship between miR-NAs and host genes is rarely found. Long primary miRNA (pri-miRNA) transcripts are processed in the nucleus by a microprocessor complex containing ribonuclease III, Drosha, and RNA-binding protein subunit DGCR8 (DiGeorge syndrome critical region 8). Cleavage of the pri-miRNA by Drosha results in a 2 nt 3′ overhang and the characteristic 'hairpin' structure of the 65 nt precursor miRNA (pre-miRNA). The pre-miRNA is then exported to the cytosol by the exportin-5 (XPO5)/RanGTP complex, where it is further processed by the endonuclease Dicer, removing the terminal loop resulting in a double stranded miRNA containing the mature miRNA guide strand and passenger strand, typically 21 – 23 nts in length (**Figure 1A**).

The RNA-induced silencing complex (RISC) is a heterogeneous multi-protein complex that uses one miRNA strand as a template to target complimentary mRNAs *Emerging Roles of Non-Coding RNA in Neuronal Function and Dysfunction DOI: http://dx.doi.org/10.5772/intechopen.101327*

**Figure 1.**

*Illustration of the biogenesis of (A) miRNA and (B) tsRNA.*

for degradation or translational repression, post-transcriptionally regulating gene expression. The double-stranded miRNA duplex is loaded into a binding pocket within an Argonaute family (Ago1-4) protein, which constitutes the principal component of RISC, mediated by Hsc70/Hsp90. The miRNA is unwound to single-stranded miRNAs and one 'guide strand' is anchored into the Ago protein, determining the specificity of the RISC, while the passenger strand is subject to degradation. The directionality of the mature miRNA guide strand originating from the 5′ or 3′ arm of the pre-miRNA duplex determines the miRNA-5p and -3p species. While typically one strand is preferentially loaded, for some miRNA duplexes both arms can give rise to functional mature miRNAs that can be loaded into Ago proteins and used to guide the RISC to mRNA transcripts.

Recognition of target mRNA occurs by complementary base pairing between the miRNA seed region (2–8 nt) of the 5′ end of the guide strand and the mRNA transcript, typically within the 3′ UTR; however miRNA can also bind within mRNA promoter regions, the coding sequence, and 5′ UTR. The Ago protein present and the degree of complementarity between the guide and target strand determines the mechanism of gene silencing, triggering target degradation or translational repression. Importantly, the short seed sequence requirement for mRNA targeting confers ability for individual miRNAs to target multiple genes across several different pathways. Similarly, an individual mRNA may contain target sites for multiple miRNAs, placing miRNAs in a powerful position in the regulation and modulation of the transcriptomic landscape. Dysregulation of miRNAs, therefore, has significant implications and consequences for biological functions in physiological and pathological conditions.

### **2.2 miRNA functions**

#### *2.2.1 Neuronal development and function*

An extensive catalog of work has demonstrated the involvement of miRNAs across the development, function and maintenance of the CNS. The cell-specific deletion of Dicer inhibits the maturation of miRNA and has been shown to delay embryonic CNS development, alter dendritic and spine morphology and lead to early postnatal death [5–7]. Specific miRNAs have been identified with central roles in regulation of adult neural stem cell proliferation [8–10] and the differentiation of cells into specific neuronal sub-types [11–14]. Post-transcriptional regulation of N-cadherin expression by the miR-379-410 cluster mediates neuronal migration [15] and miR-132 is involved in the activity-dependent integration of neurons into the adult dentate gyrus [16].

The controlled extension of neuronal processes as well as the generation of adaptable synapses are key in the development of functional neural networks in the CNS. A number of miRNAs have been closely associated with the regulation of axonal and dendritic morphology, and synaptic plasticity. Neurite outgrowth is highly dependent on extracellular trophic cues that stimulate cAMP response element binding protein (CREB) transcription factor, a target of which is miR-132. In axons, miR-132 downregulates the activity of the GTPase-activating protein p250 GAP resulting in axonal sprouting [17]. A number of counteracting miRNAs tightly regulate axonal length. The miR-17/92 cluster downregulates *PTEN* resulting in activation of the mTOR pathway and axonal extension [18], whereas miR-9 has been shown to locally repress *Map1b* expression and inhibit axonal growth [19]. Conversely, miR-9 promotes dendritic development and its loss results in reduced dendritic length and complexity [20]. Similar to axonal extension, miR-132 has been shown to positively regulate dendritic length, arborization and spine density in dendritic extensions in an activity-dependent manner [21, 22]. miR-132-mediated regulation of spine density has been attributed to its direct association with matrix metalloproteinase-9 [23] and miR-132-medited repression of p250GAP in dendritic spines has been associated with Leptin-induced synaptogenesis [24]. In *Drosophila melanogaster* miR-284 has been shown to affect the expression of the glutamate receptors GluRIIA and GluRIIB indicating a role in the regulation of synaptic strength [25] and in higher order animals the inhibition of miR-132 and miR-219 have been associated with disturbed circadian rhythm and the impairment of memory acquisition [26].

## *2.2.2 Inflammation*

Inflammation in the CNS is an important process for the alleviation of infection or the resolution of cerebral damage; however, aberrant or chronic inflammation has been implicated in a number of neurological disorders [27]. Microglial cells, the resident immune cells of the CNS, are enriched in a number of miRNAs [28] and expression of these is altered in response to inflammatory stimuli [29]. Specific miRNAs have been associated with the development of a pro- or anti-inflammatory phenotype. miR-155 is a well-studied pro-inflammatory mediator in macrophages and microglia, targeting a number of anti-inflammatory regulators for degradation induced in response to NF-κB dependent TLR signaling. Furthermore, p53-mediated induction of miR-155 is known to target anti-inflammatory transcription factor *c-Maf,* resulting in a pro-inflammatory reaction [30]. miR-124 and miR-146a are both widely reported negative regulators of CNS inflammation, down-regulating inflammatory mediators. miR-146a is also induced through TLR/NF-κB-dependent signaling in response to various immune mediators, and subsequently reduces NF-κB transcriptional activity. miR-146a expression is inversely correlated with inflammatory-related proteins [31]. Similarly miR-124, a highly abundant neuronal and immune cell miRNA, has been reported to negatively regulate TLR signaling [32] promote microglial quiescence, and reduce microglial MHC-II, TNF-α and ROS production [33].

#### *2.2.3 Apoptosis*

Neuronal cell death is a key feature in neurodegenerative diseases and has been shown to involve a number of miRNAs. In models of spinal cord injury, activation of miR-21-5p and miR-494 as well as the inhibition of miR-29b, reduced apoptosis through stimulation of the AKT/mTOR signaling pathway [34–36]. Specific miR-NAs have been shown to have a more direct effect on the apoptotic cascade. Indeed, the inhibition of miR-24, miR-497, miR-15a/16-1, miR-181a and miR-106b-5p

increases expression of anti-apoptotic proteins Bcl-w, Bcl-2 and Bcl-xl resulting in attenuation of neuronal apoptosis [37–41].
