John Archbold Klingensmith

My laboratory uses experimental research to determine how the mammalian body plan is generated during early pregnancy. We seek to understand the mechanisms that establish and pattern the body axes and organ rudiments of the embryo. Our major interest is head formation and development, from induction of brain tissues from the early ectoderm through to the elaboration of craniofacial structures later in gestation. We are also studying the development of organs that arise in concert with the head, such as the spinal cord and heart. We are using the unique genetic technologies available in the mouse to study induction, pattern formation, and morphogenesis in these contexts. One approach underway is the targeted mutation or misexpression of candidate genes likely to control these events to determine their roles. Many of these genes function analogously in the development of simpler organisms, allowing us to exploit the experimental strengths of other model systems to devise better mouse experiments. We also use existing mutant and transgenic mice to probe the roles of cellular and molecular interactions in tissue development. Our studies bear on normal mammalian embryogenesis and on its anomalies, such as human birth defects of the head, heart or neural tube. The vast majority of medically adverse birth defects involve one or more of these structures. Because birth defects are the leading cause of infant death in the United States, we believe our work has immediate as well as long-term social relevance.

Projects underway focus primarily on the role of molecules initially identified as important embryonic "organizer" genes. These are genes thought to encode the activities of Spemann's organizer, which is a small group of cells believed to be the source of the signals which induce and pattern many of the primary tissues of the vertebrate embryo, including the head primordium. For example, the organizer is presumed to induce neural tissue from naïve ectoderm and to promote a rostral (brain) fate within it. Surprisingly, we have found that mutant mouse embryos lacking the organizer nevertheless develop a neural tube with correct head-to-tail patterning. Moreover, when we delete the gene encoding a presumed organizer neural-inducing signal, we see no effect on neural development, but rather severe abnormalities in the head, neck and heart of newborns. Deletion of two known neural-inducing signals simultaneously still results in embryos with neural tissue, though some are headless. Our results show that head formation in mammals is more complex than models based on lower vertebrates suggest, and imply the existence of other means of neural induction and patterning. We are pursuing several complementary strategies to reveal the molecular and cellular bases for these phenomena.

Meanwhile, we find that these organizer genes have essential roles in development of specific organs and structures arising later in embryogenesis, including the forebrain, the spinal cord, the craniofacial skeleton, the heart, and several other critical organs. We have obtained phenotypes in our various mutant combinations which very closely resemble two common, severe human malformation syndromes. Because of the excellent embryological tools available in the mouse, we are now able to address the cellular and molecular defects underlying these birth defects. Our ongoing work on early head development has thus led us into clinically-relevant research on the etiology of important congenital malformations.