RNA has emerged as one of the most fascinating molecules in biology. As the Human Genome Project revealed that the number of genes in humans, lower than predicted, is comparable to that from Arabidopsis thaliana and Caenorhabditis elegans, and alternative splicing together with complex combinatorial transcriptional regulation took center stage as fundamental mechanisms to diversify the proteome, significant resources started to focus on the previously so-called “junk DNA”, which encompasses the majority of the human genome and is transcribed into noncoding RNA.
One class of these noncoding RNA molecules, microRNAs, recently emerged as key post-transcriptional regulators in physiological and pathological contexts.
“The work on this topic has been exciting and encouraging, and the field as a whole will be shaped as people understand the relationship between microRNAs and their targets,” says Andrew Z. Fire, Ph.D., professor of pathology and genetics at Stanford University School of Medicine and co-recipient of the 2006 Nobel Prize in Physiology or Medicine.
One of the perceived challenges in studying microRNAs is that each microRNA can apparently regulate up to hundreds of target protein-coding genes, with each target gene potentially regulated by multiple different microRNAs. In this context, the almost 2,000 microRNAs described to date in humans, some of which are present at cellular concentrations that vary by four orders of magnitude, have been viewed as part of an extremely complex and dynamic network.
“The question about the relationship between microRNA molecules and their targets is one that frustrates everyone, as for certain microRNAs it is hard to identify and study their definitive targets, but for others these have been relatively well characterized, in terms of either a broad set of targets that are regulated at modest levels or a few targets that are regulated at substantial levels,” explains Dr. Fire.
Either way, miRNAs are a key family of regulators, and understanding the source of diverse miRNA populations has become a critical part of understanding gene regulation. A recent project in Dr. Fire’s lab tested whether microRNA molecules with no direct genome match could be produced by RNA splicing.
The investigators generated intron-interrupted variants of the Caenorhabditis elegans lin-4 gene, encoding the first microRNA molecule that was discovered and, almost two decades ago, shown to be essential for the temporal control of postembryonic development.
“The intron that we used is a typical intron that a typical mRNA would use,” says Huibin Zhang, a recent Ph.D. graduate in genetics at Stanford University School of Medicine and lead author of the study.
The possibility of processing functional metazoan microRNAs by splicing an intron-interrupted precursor has multiple implications. One of them, the potential requirement for splicing for the in vivo biogenesis of certain microRNAs, would point toward an additional layer in the cellular gene expression regulatory networks.
“This also provides the ability to engineer a microRNA gene and track its expression a little more carefully, as it has to be spliced,” says Dr. Fire. The involvement of splicing in microRNA biogenesis could also help better understand factors that shape splicing in various species.
“The $64,000 question remains the one of target specificity. Once people identify the targets and learn how specific microRNAs interact with them, a lot of things will move forward, and this includes learning why a microRNA could be synthesized with or without introns in terms of the information content involved,” explains Dr. Fire.
Exploring Noncoding Genes
Central features that distinguish the biology of RNA from that of DNA are the decreased stability and increased fragility and structural dynamics that characterize RNA. Proteins that bind RNA to form ribonucleoprotein complexes are indispensable for its stability, structure, and function, and play central roles in its remodeling, an aspect that historically has made RNA biology a more challenging field experimentally.
“Only 10 years ago very little was known about the interactions between proteins and RNA in live cells. Most biochemistry was done out of the cellular context using assembled complexes and reporter constructs, and we could only guess what is going on in live cells,” says Jernej Ule, Ph.D., group leader at the Medical Research Council Laboratory of Molecular Biology.
Dr. Ule and colleagues originally developed a method known as CLIP (ultraviolet cross-linking and immunoprecipitation), which maps the RNA sites that directly contact proteins in vivo, on a genome-wide scale. Recently, Dr. Ule’s research group developed a new approach, iCLIP (individual nucleotide-resolution CLIP). iCLIP is based on the fact that reverse transcription most often stops at the crosslink site, and therefore sequencing of the truncated cDNAs provides information at nucleotide resolution.
The investigators used iCLIP to examine the in vivo binding of heterogeneous nuclear ribonucleoprotein (hnRNP) particles to the nascent transcripts. Powered to provide information at the nucleotide-level resolution, iCLIP revealed that hnRNP C binds uridine tracts, but shows a decreased binding at splice sites, pointing toward its importance in maintaining splicing fidelity.
Understanding the biology of RNA-protein interactions is an area of clinical interest, as many disease-causing mutations interrupt the function of ribonucleoprotein particles and, in this context, a focus on coding as well as noncoding RNA will be crucial.
“It is now clear that a very large proportion of mutations occur in the noncoding regions of the genome, and on the other hand most studies that tried to find disease-causing mutations have focused on the coding parts of the genome,” says Dr. Ule.
The structural and functional characterization of these regions will help define their involvement in disease. “In addition to the noncoding RNA molecules, even mRNAs contain noncoding regions, and all these harbor important regulatory sequences with relevance for disease,” explains Dr. Ule. Focusing on noncoding RNA and its involvement in disease pathogenesis promises to fill an important gap in the field.
Alternative Splicing in Alzheimer’s
According to recent estimates, 35 million individuals globally have Alzheimer’s disease, and over 4 million are newly diagnosed annually. Relatively little is known about the etiopathogenesis of neurodegenerative conditions in general, but over 90% of the patients do not appear to harbor inherited disease-causing mutations.
At the same time, an increasing body of data implicates the involvement of alternative splicing, which is implicated in many diseases. Estimates that at least 30% of disease-causing mutations affect splicing, together with the recent finding that over 95% of the multiexon human genes undergo alternative splicing, point toward the clinical importance of this process.
“The question that we wanted to ask is whether alternative splicing could be involved, and this was scarcely asked before, simply because the technology was not available,” says Hermona Soreq, Ph.D., professor of molecular neurobiology at the Hebrew University of Jerusalem.
To gain insights into molecular changes that could shape the etiopathogenesis of Alzheimer’s disease, Dr. Soreq and colleagues used a technology that specifically interrogates alternative splicing, and comparatively examined transcripts from the cortical region of the brain in individuals with this condition and in matched control adults. In addition to finding genes whose expression was upregulated or downregulated, the investigators made an additional discovery.
“We found that the RNAs for approximately 400 genes were neither up- nor downregulated, but that their composition was changed by alternative splicing,” reveals Dr. Soreq. Approaches that only measure gene expression levels would not have captured this set of genes. In most instances, the changes did not occur randomly but reflected the inclusion of a gene fragment that was not expressed in the normal brain, as a result of modifications in a family of heteronuclear ribonucleoproteins that function as splicing regulators.
By using biochemical approaches in brain sections, Dr. Soreq and colleagues revealed that these proteins were missing. This was accompanied by corresponding changes in the recently discovered family of regulator microRNAs, and the changes were very specific for Alzheimer’s disease, as they were not detected in patients with other conditions such as Parkinson’s disease or epilepsy.
Mimicking this modification in cultured neurons led to a loss of synapses, and the same changes introduced into the brains of live mice led to impairments in their learning capacities. “Altogether, these findings are pointing toward a new candidate for therapeutics that nobody knew about before,” emphasizes Dr. Soreq.