Transcriptional and Translational Regulation of Cardiac Progenitors in the Mouse and Zebrafish

In vertebrates, the heart is the first organ to function and cardiac progenitors are among the first cell lineages to be established. Transcriptional networks control the specification of cardiac progenitors, however, it is not fully understood how some transcription factors function in particular cardiac progenitor populations. The basic helix-loop-helix, bHLH, transcription factor, Hand2 has been discovered over a decade ago, and has a severe loss-of-function cardiac phenotype in vivo, yet its function is still not completely known. It is expressed in the early cardiac progenitors of the neural crest cells and second heart field lineages. The first part of my thesis touches on the beginnings to understand the role of Hand2 in the cardiac neural crest progenitors. Generally, expression levels in vertebrates reflect the combined transcription of both alleles of the gene being transcribed. Although there are notable exceptions (i.e., X chromosome genes), the presence of only one functional copy or more than two copies of a gene can have detrimental effects on the development of the organism. Many of the genetic examples of congenital heart disease, which affects 1% of live births, are a result of a haploinsufficient gene dose. Like Hand2, which acts in a dosage-sensitive manner to regulate ventricular formation, the precise dose of proteins can be very important in regulating cardiac development. One way to fine-tune the activity of genes is through the newly identified class of small RNAs, microRNAs (miRNAs), which translationally repress the production of proteins by binding to target sites on messenger RNA (mRNA).
MiRNAs provide a sophisticated way to adjust protein levels in a spatiotemporal manner. One miRNA may control several mRNAs, including transcription factors, which are the 'master switches' that regulate gene expression. And cooperatively, cell type-specific transcription factors can regulate the tissue-specificity of miRNA expression. Together with transcription factors, miRNAs function in cell fate determination, cell differentiation, proliferation and disease progression. Similar to transcription factors, which activate or repress a set of genes in a particular cell type, miRNAs create an environment, tailored for each cell type, allowing translation of some genes to occur, while repressing others. To date, less than a handful of miRNAs have been identified that function during heart development. The latter half of my thesis represents efforts to identify cardiac progenitor miRNAs and understand their function during development. I found that miRNA function is important in the cardiac mesodermal progenitors. In addition, I present a family of miRNAs, miR-143 and miR-145, that is specific to cardiac and smooth muscle progenitors, and I discuss their function in regulating their respective environments during cardiovascular development and disease.