Cartoon of zebrafish embryo and its various cell types of interest. Artwork by Jessa Westheimer
Our group globally strives to understand how complex tissue systems are created from simple constituent components. This area, known as systems developmental biology, aims to increase fundamental knowledge of how living tissues, organs and, ultimately, organisms are constructed. To achieve this goal, we focus on how neural crest cells diversify and differentiate, with a special emphasis on nervous system tissue, using zebrafish as a research organism.
Neural Crest Cell Diversification
Neural crest cells are stem cells that migrate to various locations and give rise to diverse and fundamental cell types in the vertebrate body–including craniofacial tissues, cardiac cells and peripheral nervous system. Neural crest stem cells originate from the dorsal neural tube, a transient structure during development that will eventually give rise to the central nervous system. Some of the neural crest cells, called "Vagal" neural crest cells, eventually can give rise to cells of the outflow tract of the heart, enteric peripheral ganglia, sympathetic ganglia, thymic connective tissue, as well as pigment cells of the skin. What dictates whether vagal neural crest will give rise to ganglia or other derivatives, such as pigment cells, remains elusive.To address this lack of knowledge, we study what regulates vagal neural crest cell delineation in the zebrafish embryo.
Recently, we have expanded our studies on neural crest diversification using single-cell transcriptomics, and discovered dozens of transcriptionally-distinct neural crest-derived cellular subpopulations, greatly expanding the field's basic understanding of neural crest cell development. Ongoing studies are building upon these single-cell discoveries in the lab right now.
The ENS is composed of thousands of ganglia embedded within the walls of the gut. Each ganglia consists of diverse enteric neuron subtypes and glial cells, which together regulates gut peristalsis, water balance and hormone secretions. The number of neurons found within the ENS rivals the numbers of those found in the spinal cord, causing it to also be known as "the second brain." It is important to study development of the ENS so that we can understand not only how it forms and functions, but also to help us to understand how things go wrong in various gastrointestinal disease states (Hirschsprung disease, Achalasia), as well as neural crest stem cell defects, such as when neural crest become cancerous.
To become enteric ganglia during development, vagal neural crest migrate ventrally from the neural tube and enter the primitive foregut tissue. They then migrate along the rostrocaudal extent of the gut in response to microenvironmental signaling cues to until they eventually reach the hindgut. These enteric neural crest cells (ENC) eventually give rise to a diverse array of neurons and glia that form the enteric ganglia. During zebrafish embryogenesis, neural crest cells migrate into the primitive gut in two chains, eventually surround the gut tube and turn into neurons by the 5th day in development in order to form the ENS. Complex cellular, molecular, genetic and systems-level forces orchestrate neural crest transformation into enteric neurons that make up the ENS. Zebrafish are an excellent model system to investigate ENS development due to their rapid, transparent development, amenability to genetic manipulation and high-resolution live imaging. We currently leverage live imaging and single cell tracking experiments to understand the complex emergence of enteric ganglia in the zebrafish gut, as shown in video below.
We have expanded our ENS studies in lab to investigate how environmental factors, such as Vitamin A-derived Retinoic Acid and other signaling factors, interact with the neural crest cells and gut tissues to orchestrate early ENS patterning. We have also taken a unbiased approach by leveraging our single-cell datasets, to ask what the functional role of novel pathways are during early ENS formation. Towards this end, we are currently generating and validating various novel optogenetic and synthetic gene expression systems in zebrafish. Stay tuned!
Zebrafish larval neuropil in vivo.
Relevance to Human Health
Understanding the genetic programs and cellular interactions that drive stem cells to form the enteric nervous system, or other neural crest derivatives, is of crucial concern. Because improper neural crest development leads to developmental anomalies such as Hirschsprung disease (colonic aganglionosis), and neural crest-derived cancers, such as Melanoma and Neuroblastoma, there has been great interest in understanding the migration and differentiation of vagal neural crest cells.
Hirschsprung disease is characterized by a paucity of ganglia along variable lengths of the gut, with colonic aganglionosis being the most common form, occurring every 1 in 5000 births.. The current treatment for this pediatric developmental defect is surgical resection of the aganglionic intestinal segment--however eventual outcomes of patients varies greatly and most exhibit functional enteric defects throughout life--highlighting the need for alternative treatments and understanding the ontogeny of the disorder.
Melanoma and Neuroblastoma are neural crest-derived cancers affecting adults and children alike throughout the world. It is hypothesized that several genetic mutations in neural crest cell lineages are the basis for formation of neuroblastoma and melanoma cancers, however the signaling landscape conducive to formation of melanoma and neuroblastoma are not entirely known. Other current studies in the lab endeavor to understand how neural crest cell pathways may contribute to oncogenesis.