Regulatory pathways patterning the vertebrate brain and body plan in development, disease and evolution
Our laboratory is interested in understanding the molecular and cellular pathways that govern patterning of the nervous system and body plan of vertebrate embryos during development; how they are altered or affected in human diseases; and how these pathways are conserved in evolution. One biological focus of the group is the hindbrain and its relationship to head development. This is a good model system for addressing fundamental patterning problems in neurobiology related to cell signaling, proliferation, migration, commitment, identity and differentiation. The hindbrain is a complex co-ordination center in the vertebrate CNS and an important source of patterning information that influences the generation of head and facial structures. It serves as a higher order relay center that controls respiration, blood pressure, arousal and wakefulness and it contains the nuclei and fibers of the cranial nerves, which innervate the muscles of the head and neck, transmit sensory information on hearing, balance and taste and control the cardiovascular and gastrointestinal systems.
The formation of regional diversity in the hindbrain is achieved through a process of segmentation, whereby neural tissue is transiently divided into seven segmental units, termed rhombomeres. Each rhombomere defines a lineage-restricted cellular compartment that creates a distinct microenvironment. This allows each segment to adopt a unique set of molecular and cellular properties distinct from its immediate neighbors, and ultimately give rise to well-defined regions of the adult brain. This segmental organization is critical for patterning of the cranial neural crest and establishing tissue interactions essential for proper head development. Our long-term goal is to understand the coordinated mechanisms that control the process of segmentation.
A molecular focus of our interest in development has centered on the Hox homeobox gene network. We have demonstrated that there is extended homology between the vertebrate and Drosophila Hox/HOM homeotic complexes and that these transcription factors have a conserved role in the molecular mechanisms that specify regional identity and morphogenesis in many embryonic tissues. Based on highly ordered and segment-restricted patterns of gene expression and loss and gain-of-function analyses in several vertebrate systems, we have demonstrated that Hox genes play multiple roles in diverse aspects of segmentation. Using evolutionary comparisons between the Hox complexes of different species, combined with experimental embryology and transgenic analyses, we have begun to build a picture of the tissue interactions, signals and transcriptional regulatory components upstream of the Hox cascade that modulate their expression and function. Furthermore, by characterizing cofactors (MEIS and PBX) that work with HOX proteins to regulate their DNA binding properties, we have facilitated our ability to identify downstream target sites and genes in the Hox cascade. By developing techniques that allow genetic marking, lineage tracing, and tissue transplantation in cultured embryos we are able to simultaneously analyze changes in gene expression and cell behaviors in the developing head of wild type and mutant mouse embryos. This provides new insight into plasticity of cellular patterning and tissue interactions required to control head morphogenesis. We have devised embryonic assays to screen for the signals involved in this process and those that influence anterior-posterior patterning in the CNS. Through this process we have isolated novel factors that modulate the Wnt and other known signaling pathways.
We are interested in studying how the Hox genetic pathways that control brain development are related to the roles of these genes in patterning other tissues, such as the skeleton, limbs, digestive systems and organs. Hox genes are not only important for normal development, but they appear to be critical targets associated with human diseases. Knowledge of the genes and pathways that control Hox expression and brain patterning provides candidates for investigating diseases and genetic syndromes in the nervous systems and other tissues. Abnormal Hox expression occurs in many cancers and leukemias and some of the genetic lesions arise in factors that may contribute to regulation of Hox pathways. A future direction of our research will be to investigate the direct and indirect roles of Hox genes in disease to provide insight into ways of developing strategies for prevention, diagnosis and treatment of genetic diseases.
To approach our research problems we exploit the basic conservation of patterning processes by using a variety of experimental model systems (mouse, chick, frog, fish and fly) each with their own advantages. Furthermore, we are developing methods for in vivo gene transfer, cell grafting and embryo culture and combining them with techniques in embryology, molecular biology, imaging, genetics, cell biology genomics and bioinformatics.