5. Non-Coding RNAs in Brain Plasticity
This section reveals the molecular alterations that may serve as biomarkers of plastic changes in the brain. These comprise non-coding RNAs, microRNAs, circular RNAs, long intergenic non-coding RNAs, Y RNAs. The recently discovered world of non-coding RNAs (ncRNAs) is continuously expanding to all areas of biomolecular interactions and variety of cellular processes including control of metabolism, gene regulation and protein turnover. Expectedly, multiple ncRNA players were found to be involved in brain plasticity (for several examples, see
Table 1). Furthermore, interactions between different types of ncRNAs create multidimensional networks that respond to a range of endogenous and exogenous stimuli.
Noncoding RNAs (ncRNAs) represent the major part of the transcriptome. Various classes of ncRNAs have emerged as critical regulators of transcription, epigenetic processes, gene silencing and play important roles in neural brain plasticity and cognitive processes [
127]. ncRNAs regulate diverse intracellular and neuronal functions: modulate chromatin structure, act as chaperones, and contribute to synaptic remodeling and behavior [
128].
Neurons are highly compartmentalized due to their morphological and functional complexity. This happens due to the transport of messenger RNA (mRNA) transcripts to specific subcellular areas, e.g. synaptic regions, for the local translation. Increasing evidence shows that highly expressed cerebral ncRNAs participate in the spatial and temporal control of the mRNA translation, therefore, in synaptic plasticity [
129].
ncRNAs may contribute to the development of a variety of neuropsychiatric disorders, including schizophrenia, addiction, and fear-related anxiety disorders [
127,
128]. Moreover, the diversity of ncRNAs and their association with neurodegenerative diseases render them particularly interesting as putative targets of brain disease [
130]. New RNA-based therapeutics can be developed due to the new knowledge on the ncRNA regulation and the downstream effects of their interactions in different pathologies.
Long non-coding RNAs (lncRNAs) act as scaffolds for biomolecule binding and mediate different RNA-protein interactions. LncRNAs are increasingly recognized for their involvement in neurodevelopmental processes including cell proliferation, neurite outgrowth, synaptogenesis, and neuroplasticity [
159]. Neuronal lncRNAs are crucial for orchestrating neurogenesis, for tuning neuronal differentiation, and for the exact calibration of neuronal excitability [
130]. Particularly, Malat1 is an lncRNA which is abundant in the nuclei of neurons. It promotes synapse formation by recruiting the serine/arginine splicing factors to the transcription sites of genes involved in synaptogenesis. Overexpression of Malat1 enhances the number of synapses in hippocampal neurons in vitro while its deficiency reduces number of synapses between dendrites and axons [
160,
161]. Gomafu is another lncRNA involved in ES cell, neuronal cell and retinal cell differentiation. The lack of Gomafu led to hyperactive phenotype and increased sensitivity to the psychostimulant MAP in Gomafu KO mice [
131]. lncRNA Gas5 promotes neuronal differentiation of hippocampal NSCs and restores learning and memory in rats with cholinergic injury [
132]. Furthermore, synapse-specific Gas5 KO led to impaired fear extinction memory [
162].
MicroRNAs (miRNAs) are small endogenous RNAs about 20–25 nucleotides in length that regulate gene expression posttranscriptionally [
163,
164]. They are commonly expressed in specific brain regions and affect nervous system development, plasticity, and function [
165]. For example, miR-9 has a critical role in hippocampal synaptic plasticity and memory [
136], miR-34 regulates synaptogenesis [
137], miR-132 participates in axon growth, neural migration, and plasticity [
138]. In the temperament-character molecular integration network (TCMIN), only three miRNAs (hsa-miR-1-3p, hsa-miR-335-5p, hsa-miR-34a-5p) are sufficient to coordinate interactions between two gene networks in brain involved in self-regulation of emotional reactivity to extracellular stimuli (e.g., self-regulation of anxiety) and interpretations of meaning (e.g., production of concepts and language) [
144].
Potential targeting or therapeutic use was demonstrated for several miRNAs [
166]. Particularly, miR-17-92 cluster shown to enhace neuroplasticity [
139] and regulate adult hippocampal neurogenesis, anxiety, and depression [
140]. miR-144-5p is currently considered as a key target in major depressive disorder [
141] and miRNA-145 was recently shown to enhance neural repair after spinal cord injury [
142]. One of the highly conserved miRNAs in mice and humans, miRNA-153 stabilizes the neurogenesis of neural stem cells and enhances cognitive ability through the Notch signaling pathway [
143].
Circular RNAs (circRNAs) are closed structural isoforms of linear mRNA. They are abundant in the brain and play a significant role in the development of the nervous system [
167]. Cerebral circRNAs are linked with neurotransmitter function, synaptic activities, and neuronal maturation. Levels of ciRS-7, circRMST, and circFAT3 increased during the differentiation of human embryonic stem cells into rostral and caudal neural progenitor cells [
145]. The level of a recently discovered circular RNAs - circIgfbp2 - is significantly increased in injured brain tissue. It is involved in neural plasticity and might be a future therapeutic target for anxiety and sleep disorders after traumatic brain injury [
146]. At least four circRNAs (cirC_0000400, cirC_0000331, cirC_0000406, cirC_0000798) are involved in postoperative neurocognitive disorders [
147]. In a rat model, a large number of circRNAs including 1167 cerebral circRNAs displayed a developmental-dependent expression pattern and may have important biological function in differentiation, development, and aging [
148]. Enhancer RNAs (eRNAs) are long non-coding RNAs, bidirectionally transcribed by RNA polymerase II from enhancer regions of the genome. Generally, eRNAs are not spliced or polyadenylated [
168,
169,
170]. Bdnf-Enhg1 and Bdnf-Enhg2 were characterised as novel enhancers that regulates Bdnf expression in developing neurons [
149]. Conserved enhancer Evf2 was shown to functionally and spatially organizes megabase distant genes in the developing forebrain [
150].
Long intergenic non-coding RNAs (lincRNAs) are biochemically identical to other lncRNAs but differ in their genomic organization as they reside in the space between genes [
171]. Knockout of linc-Brn1b showed a reduced number of intermediate progenitor cells in the subventricular zone. This suggests that linc-Brn1b can be involved in the development of cortex [
151]. Long non-coding RNA X-inactive specific transcript (XIST) was mentioned as a promising molecular target for SCI therapy [
172] and may have a significant role in AD [
152].
Piwi-interacting RNAs (piRNAs) are a class of Piwi-associated, 26–32 nucleotide small non-coding RNAs that, unlike other small RNAs, are generated from long genomic clusters [
173,
174,
175]. piRNAs are part of a gene regulatory mechanism responsible for establishing stable long-term changes in neurons and the persistence of memory in brain synaptic plasticity [
153]. The main molecular function of piRNAs is to regulate transposons. The co-existence of piRNA and retrotransposons might play important roles in the brain development and the adult brains [
154]. A number of piRNAs across brain transcriptome are associated with Alzheimer’s disease [
155].
Y RNAs (yRNAs) are a class of non-coding RNA often found abundantly expressed in brain and neuronal tissues. Y RNAs are linked to neuronal stress and very often associated with neuronal ELAV-like proteins in Alzheimer’s disease [
156] and could serve as biomarkers in glioma [
157]. Recent study suggested that the strong tendency of Y RNAs to bind nELAVL proteins to in response to stress conditions might prevent these proteins from associating with their normal messenger RNA targets [
156].