My laboratory studies the molecular mechanisms that govern mammalian development, using the mouse as a model. We use a combination of biochemical, molecular and genetic approaches to identify and characterize signaling molecules and pathways that control the development and maintenance of the musculoskeletal and hypothalamic systems.
The muskuloskeletal system:
The musculoskeletal system provides the mechanical support for our posture and movement. How it arises during embryogenesis pertains to the basic problem of embryonic induction. How the components of this system are repaired after injury and maintained throughout life is of biological and clinical significance. We study how this system is generated and maintained.
The origin and induction: The musculoskeletal system of the trunk originates from a common embryonic structure called the somite. Somites are segmented mesodermal units flanking both sides of the spinal cord. Their reiterated pattern is the basis for the repeated organization of the trunk. Under the inductive influence of adjacent tissues, cells within the somite give rise to muscles and bones. We have developed a 3-dimensional culture system that allows characterization of crucial long-range and contact-dependent cellular interactions that induce early skeletal and muscle fates. Our efforts toward designing new methods and assays to track somite development have enabled us to make novel observations.
Induction of the axial skeletal progenitors: We have identified the Hedgehog (Hh) proteins responsible for inducing the early skeletal fate. Hh largely utilizes evolutionarily conserved downstream mediators for inductive signaling. In addition, we also found a vertebrate-specific cell surface Hh binding protein Gas1. Gas1 mutants display skeletal defects related to or due to altered Hh signaling. Mechanistically, Gas1 helps transform the Hh diffusion gradient into its observed signaling activity gradient. This unexpected mechanism provides a new vision of Hh signaling pathway initiation and has direct implications for the long-range action of Hh.
Induction of the early embryonic muscle: Conversely, the Wnt family of proteins plays a key role in inducing the dermis/muscle dual potential progenitors. Combining our in vitro assay with microarrays analyses, we have uncovered previously unknown effectors and target genes of Wnt. Using an ex vivo whole embryo culture system coupled with somite-specific gene delivery, we discovered an unconventional pathway for Wnt signaling via the adenylyl cyclase/protein kinase A/Creb cascade that selectively activates myogenic transcriptional determinants Myf5 and MyoD.
Embryonic and adult muscle stem cells: Somites not only supply cells for embryonic muscles, but also contain muscle progenitors. The proliferative capacity of these progenitors depends upon the transcription factors Pax3 and Pax7. Both genes are activated by Wnt. Using inducible cell lineage tracing, we have found that early Pax7-expressing somitic cells directly give rise to adult muscle stem cells, i.e. the satellite cells. Lineage tracing of Pax7-expressing adult satellite cells indicates that they are indeed a stem cell source for muscle regeneration. Conditional inactivation of Pax7 at different developmental time points reveals that Pax7 is required for the proliferative properties of muscle progenitors up to 3 weeks after birth when they transition into quiescence. After this transition is made, however, both Pax3 and Pax7 are completely dispensable. Our finding of an age-dependent cell-intrinsic change in the genetic requirement for muscle stem cells cautions against inferring adult stem cell biology from embryonic studies, and has direct implications for the use of stem cells from hosts of different ages in transplantation-based therapies.
The neuroendocrine hypothalamus:
The hypothalamus is an essential brain center that maintains multiple physiological homeostatic processes by modulating pituitary hormone secretions. Two centers (nuclei) of the hypothalamus, the paraventricular and supraoptic nuclei (PVN and SON), contain various hormone-producing neurons. Studies of these hormones have been instrumental to our understanding of endocrine homeostasis, including the maintenance of bodily fluid osmolarity and the balance of energy metabolism. Despite the importance of PVN and SON function, molecules that control their embryonic specification, neuronal differentiation, nucleus formation, and neuronal connections are largely unknown.
During early embryogenesis, the PVN and SON have a common origin but segregate from each other to diversify their inputs from other brain regions. After their segregation, the PVN and SON project axons to other homeostasis centers, including the pituitary. Through a series of genetic analyses, we have constructed a hierarchical map composed of five transcription factors that define the key steps of proliferation, survival, differentiation, and hormone expression of the PVN and SON neurons during development. Sitting at the top of this genetic cascade is the Sim1 gene, which also controls both the segregation and axonal projections of the PVN and SON. We have therefore expanded our work to investigate neuronal migration cues. We found that a Sim1 downstream gene, PlexinC1 (a receptor for neuronal guidance signals), mediates PVN and SON segregation.
We found that heterozygosity at the Sim1 locus causes the animal to develop morbid obesity due to over-feeding. Sim1 heterozygous mice exhibit compromised PVN structure and function, supporting a role for PVN in maintaining energy homeostasis via the regulation of feeding. Interestingly, people haploid for the SIM1 gene also clinically present with childhood obesity. Thus, our approach directed toward studying embryonic development of the PVN and SON has directly led us to identify a genetic contributor to feeding regulation and obesity in rodents and humans. Therefore, our basic developmental studies of the PVN and SON suggest a new avenue for finding genetic determinants of homeostasis regulation.