The central role of RNA in human development and cognition

Dark Matter RNA: Existence, Function, and Controversy

The mysteries surrounding the ∼97–98% of the human genome that does not encode proteins have long captivated imagination of scientists. Does the protein-coding, 2–3% of the genome carry the 97–98% as a mere passenger and neutral “cargo” on the evolutionary path, or does the latter have biological function? On one side of the debate, many commentaries have referred to the non-coding portion of the genome as “selfish” or “junk” DNA (Orgel and Crick, 1980), while on the other side, authors have argued that it contains the real blueprint for organismal development (Penman, 1995; Mattick, 2003), and the mechanisms of developmental complexity. Thus, this question could be referred to without much exaggeration as the most important issue in genetics today.

Transcription factors are targeted by differentially expressed miRNAs in primates

MicroRNAs (miRNAs) are small RNA molecules involved in the regulation of mammalian gene expression. Together with other transcription regulators, miRNAs modulate the expression of genes and thereby potentially contribute to tissue and species diversity. To identify miRNAs that are differentially expressed between tissues and/or species, and the genes regulated by these, we have quantified expression of miRNAs and messenger RNAs in five tissues from multiple human, chimpanzee, and rhesus macaque individuals using high-throughput sequencing. The breadth of this tissue and species data allows us to show that downregulation of target genes by miRNAs is more pronounced between tissues than between species and that downregulation is more pronounced for genes with fewer binding sites for expressed miRNAs. Intriguingly, we find that tissue- and species-specific miRNAs target transcription factor genes (TFs) significantly more often than expected. Through their regulatory effect on transcription factors, miRNAs may therefore exert an indirect influence on a larger proportion of genes than previously thought.

Roles of histone deacetylases in epigenetic regulation: emerging paradigms from studies with inhibitors

The zinc-dependent mammalian histone deacetylase (HDAC) family comprises 11 enzymes, which have specific and critical functions in development and tissue homeostasis. Mounting evidence points to a link between misregulated HDAC activity and many oncologic and nononcologic diseases. Thus the development of HDAC inhibitors for therapeutic treatment garners a lot of interest from academic researchers and biotechnology entrepreneurs. Numerous studies of HDAC inhibitor specificities and molecular mechanisms of action are ongoing. In one of these studies, mass spectrometry was used to characterize the affinities and selectivities of HDAC inhibitors toward native HDAC multiprotein complexes in cell extracts. Such a novel approach reproduces in vivo molecular interactions more accurately than standard studies using purified proteins or protein domains as targets and could be very useful in the isolation of inhibitors with superior clinical efficacy and decreased toxicity compared to the ones presently tested or approved. HDAC inhibitor induced-transcriptional reprogramming, believed to contribute largely to their therapeutic benefits, is achieved through various and complex mechanisms not fully understood, including histone deacetylation, transcription factor or regulator (including HDAC1) deacetylation followed by chromatin remodeling and positive or negative outcome regarding transcription initiation. Although only a very low percentage of protein-coding genes are affected by the action of HDAC inhibitors, about 40% of noncoding microRNAs are upregulated or downregulated. Moreover, a whole new world of long noncoding RNAs is emerging, revealing a new class of potential targets for HDAC inhibition. HDAC inhibitors might also regulate transcription elongation and have been shown to impinge on alternative splicing.

Non-coding RNAs with essential roles in neurodegenerative disorders

The importance of various classes of regulatory non-protein-coding RNA molecules (ncRNAs) in the normal functioning of the CNS is becoming increasingly evident. ncRNAs are involved in neuronal cell specification and patterning during development, but also in higher cognitive processes, such as structural plasticity and memory formation in the adult brain. We discuss advances in understanding of the function of ncRNAs in the CNS, with a focus on the potential involvement of specific species, such as microRNAs, endogenous small interfering RNAs, long intergenic non-coding RNAs, and natural antisense transcripts, in various neurodegenerative disorders. This emerging field is anticipated to profoundly affect clinical research, diagnosis, and therapy in neurology.

Molecular targets of omega 3 and conjugated linoleic Fatty acids - "micromanaging" cellular response

Essential fatty acids cannot be synthesized de novo by mammals and need to be ingested either with the diet or through the use of supplements/functional foods to ameliorate cardiovascular prognosis. This review focus on the molecular targets of omega 3 fatty acids and conjugated linoleic acid, as paradigmatic molecules that can be exploited both as nutrients and as pharmacological agents, especially as related to cardioprotection. In addition, we indicate novel molecular targets, namely microRNAs that might contribute to the observed biological activities of such essential fatty acids.

Isotope labeling and segmental labeling of larger RNAs for NMR structural studies

NMR spectroscopy has become substantial in the elucidation of RNA structures and their complexes with other nucleic acids, proteins or small molecules. Almost half of the RNA structures deposited in the Protein Data Bank were determined by NMR spectroscopy, whereas NMR accounts for only 11% for proteins. Recent improvements in isotope labeling of RNA have strongly contributed to the high impact of NMR in RNA structure determination. In this book chapter, we review the advances in isotope labeling of RNA focusing on larger RNAs. We start by discussing several ways for the production and purification of large quantities of pure isotope labeled RNA. We continue by reviewing different strategies for selective deuteration of nucleotides. Finally, we present a comparison of several approaches for segmental isotope labeling of RNA. Selective deuteration of nucleotides in combination with segmental isotope labeling is paving the path for studying RNAs of ever increasing size.

Exploring long non-coding RNAs through sequencing

Long non-coding RNAs (lncRNAs) are emerging as an important class of regulatory transcripts that are implicated in a variety of biological functions. RNA-sequencing, along with other next-generation sequencing-based approaches, enables their study on a genome-wide scale, at maximal resolution, and across multiple conditions. This review discusses how sequencing-based studies are providing global insights into lncRNA transcription, post-transcriptional processing, expression regulation and sites of function. The next few years will deepen our insight into the overall contribution of lncRNAs to genome function and to the information flow from genotype to phenotype.

Long non-coding RNAs in Huntington's disease neurodegeneration

Neurodegeneration in the brains of Huntington's disease patients is accompanied by widespread changes in gene regulatory networks. Recent studies have found that these changes are not restricted to protein-coding genes, but also include non-coding RNAs (ncRNAs). One particularly abundant but poorly understood class of ncRNAs is the long non-coding RNAs (lncRNAs), of which at least ten thousand have been identified in the human genome. Although we presently know little about their function, lncRNAs are widely expressed in the mammalian nervous system, and many are likely to play critical roles in neuronal development and activity. LncRNAs are now being implicated in neurodegenerative processes, including Alzheimer's (AD) and Huntington's disease (HD). In the present study, I discuss the potential significance of lncRNAs in HD. To support this, I have mined existing microarray data to discover seven new lncRNAs that are dysregulated in HD brains. Interestingly, several of these contain genomic binding sites for the transcriptional repressor REST, a key mediator of transcriptional changes in HD, including the known REST target lncRNA, DGCR5. Previously described lncRNAs TUG1 (necessary for retinal development) and NEAT1 (a structural component of nuclear paraspeckles) are upregulated in HD caudate, while the brain-specific tumour-suppressor MEG3 is downregulated. Three other lncRNAs of unknown function are also significantly changed in HD brains. Many lncRNAs regulate gene expression through formation of epigenetic ribonucleoprotein complexes, including TUG1 and MEG3. These findings lead me to propose that lncRNA expression changes in HD are widespread, that many of these result in altered epigenetic gene regulation in diseased neurons, and that contributes to neurodegeneration. Therefore, elucidating lncRNA network changes in HD may be important in understanding and treating this and other neurodegenerative processes.

A long noncoding RNA controls muscle differentiation by functioning as a competing endogenous RNA

Recently, a new regulatory circuitry has been identified in which RNAs can crosstalk with each other by competing for shared microRNAs. Such competing endogenous RNAs (ceRNAs) regulate the distribution of miRNA molecules on their targets and thereby impose an additional level of post-transcriptional regulation. Here we identify a muscle-specific long noncoding RNA, linc-MD1, which governs the time of muscle differentiation by acting as a ceRNA in mouse and human myoblasts. Downregulation or overexpression of linc-MD1 correlate with retardation or anticipation of the muscle differentiation program, respectively. We show that linc-MD1 “sponges” miR-133 and miR-135 to regulate the expression of MAML1 and MEF2C, transcription factors that activate muscle-specific gene expression. Finally, we demonstrate that linc-MD1 exerts the same control over differentiation timing in human myoblasts, and that its levels are strongly reduced in Duchenne muscle cells. We conclude that the ceRNA network plays an important role in muscle differentiation.

The search for endogenous siRNAs in the mammalian brain

A decade ago, RNA interference was proposed to serve as a physiologic means of regulating long-term gene expression in the mammalian brain. However, during the intervening years, this hypothesis appeared to be contradicted by both experimental data and theoretical considerations. More recently, the advent of deep sequencing technology has permitted a re-assessment of this issue. As reviewed here, a large population of small RNAs having features characteristic of endogenous siRNAs are detected within adult mouse hippocampus, which derive from genes involved in synaptic structure and signaling, and which show a significant, though modest (16-22%) up-regulation during olfactory discrimination training. Small RNAs derived from abundant cellular noncoding RNAs are also detected; in particular, a subpopulation of RNAs 25-30 nt. in length shows very large (>100 fold) up-regulation during olfactory discrimination training. Preliminary data suggest that the 25-30 nt. RNAs may associate with MIWI rather than Argonaute 1-4 homologues. I conclude that, despite their apparent low abundance, endogenous siRNAs and noncoding RNA-derived small RNAs are likely to play an important role in regulating synaptic plasticity.