Gage DeKoeyer Crump Assistant Professor Center for Stem Cell and Regenerative Medicine Zilkha Neurogenetic Institute Keck School of Medicine

Education:
BA Hispanic Literature -
ScB 1994 Biochemistry and Molecular Biology - Brown University, Providence, RI
PhD 2000 Cell Biology - University of California, San Francisco

Postdoctoral Research Fellowship:
2001 - 2006 University of Oregon

Started at USC: 2006

Research Topics: Developmental Biology, Cell Structure & Organization, Human/Mammalian Genetics, Stem Cell Biology

Research Description

How does the face of each animal acquire its characteristic shape? How is facial development altered in human birth defects? Can we use the rules of skeletal development to regenerate the skeleton following disease and catastrophic injury? We use the zebrafish larva to understand the genetics and cell biology by which precursor cells are specified and then arranged into the precise three-dimensional skeletal elements of the face. Zebrafish is an excellent model for vertebrate facial development, as many of the same genes involved in human development are conserved in sequence and function in the fish. We have exploited the strengths of zebrafish – forward genetics, embryonic manipulations, and in vivo imaging – to address the mechanism of craniofacial patterning. In the long term, by understanding how dynamic tissue interactions in the embryo guide facial skeleton development, we hope to be able to direct human embryonic stem cells to rebuild damaged faces. 

Specification of Facial Skeletal Precursors

Whereas the skeleton of most of our body is of mesodermal origin, the majority of our head skeleton derives from a vertebrate-specific cell population, the cranial neural crest. Cranial neural crest cells are a multipotent stem-cell-like population that can generate a wide variety of cell types, including skeleton, neurons, glia, and smooth muscle. However, the mechanism by which specific cell types are selected from multipotent cranial neural crest cells is not well understood. In order to investigate how skeletal precursors are specified from the neural crest, we have generated several zebrafish mutants that specifically lack the crest-derived facial skeleton. By analyzing the roles of the mutated genes in early crest development, we hope to gain insights into the molecular pathways that specify skeletal precursors in the face. Next, we will use the knowledge gained in zebrafish studies to direct human embryonic stem cells to adopt first a cranial neural crest fate, and then specifically a facial skeletal fate. By combining zebrafish and human stem cell studies, we hope to generate large amounts of skeletal precursors that can be used to repair severe injuries of the head skeleton.

Role of the Endoderm in Guiding Facial Skeleton Development

We have previously found that the facial endoderm plays an instructive role in promoting head skeleton development. The endoderm forms a complex three-dimensional structure, and in particular includes a segmental series of outpocketings called “pouches”. When skeletogenic neural crest cells migrate from the brain to ventral regions, they condense on the facial endoderm to form a series of structures called pharyngeal arches. It is from these arches that the major skeletal elements of the face form. As the structure of the facial endoderm is a major determinant of later skeletal structure, we are interested in the molecular and cellular pathways that control endodermal pouch formation. We are combining transgenic, mutant, and time-lapse imaging techniques to identify the major signaling pathways that shape the facial endoderm. In addition, we are using transgenic approaches to ablate the endoderm and ectopically express signaling factors in the endoderm at different times of development. By so doing, we hope to understand how the endoderm dynamically interacts with skeletal precursors to guide their patterning.

Regional Patterning of the Facial Skeleton

Proper function of the face requires that each skeletal element acquire a distinct shape appropriate for its location in the head. For example, dorsal-ventral patterning of skeletal precursors is essential for establishing upper versus lower jaw morphology. We are investigating how skeletal precursors acquire distinct regional identities along the anterior-posterior and dorsal-ventral axes. By screening for mutant zebrafish with specific patterning defects, we have identified two major classes of mutations. The first class controls patterning by regulating the expression of Hox transcription factors in posterior versus anterior facial skeletal precursors – these mutations result in the loss of Hox expression and the acquisition of a second jaw. The second class controls patterning of the dorsal upper jaw, likely by regulating the expression of Dlx transcription factors. In particular, we have found that loss of the zebrafish jag1b gene results in specific transformations of the upper jaw and jaw support. In humans, loss of one copy of Jag1 leads to Alagille Syndrome, characterized by defects of multiple organs, including the heart, liver, and face. Thus, our work in zebrafish may help to explain the specific facial anomalies seen in Alagille Syndrome patients. We are currently using transgenic and in vivo imaging approaches to understand the precise manner in which Jagged-Notch signaling controls regional patterning of the vertebrate face.

Regeneration of the Jaw Skeleton

Whereas simple fractures of the skeleton repair naturally in humans, more severe injuries of the skeleton require surgical intervention, typically involving grafts of skeleton from other regions of the body. Unfortunately, the amount of bone and cartilage available for grafts is in short supply. Possible solutions are to differentiate stem cells in vitro to make replacement bone and cartilage, or to augment the regenerative capacity of resident stem cells adjacent to the injury site in vivo. Unlike humans, fish and some amphibians have a remarkable capacity to regenerate organs and even entire body parts. We are investigating the extent to which zebrafish can regenerate their jaws following amputation. In addition, recent evidence suggests that neural crest stem cells may persist in the adult and contribute to the regeneration of multiple tissue types. Using the zebrafish jaw regeneration assay, we will use in vivo imaging to identify neural crest stem cells in the adult head. Next, we will use transgenic and mutant approaches to test the functional and molecular requirements of these neural crest stem cells in the regeneration of the jaw skeleton. By understanding how lower vertebrates are able to regenerate the head skeleton, we hope to develop strategies for augmenting fracture repair and skeletal regeneration in human patients.