PubMed search for Dr. Tamara Caspary

Areas of Specialization/Research Interests:
Mouse embryogenesis especially patterning of the neural system and left-right axis specification

Education:
B.S. The University of North Carolina, 1992
Ph.D. Princeton University, 1999

Research Description:
Although we now have the sequence of the mouse genome, we still don’t know what most of the genes do. We are interested in identifying novel genes that are important for mouse embryogenesis, specifically those genes that specify cell fate in the embryonic nervous system. In order to do this in an unbiased way, we use phenotype driven screens in the mouse. We induce random point mutations with the chemical mutagen, N-ethylnitrosourea (ENU). Through a three generation breeding scheme, we identify recessive mutations of interest by looking for those which cause morphological defects midway through embryogenesis. Now that we have the complete sequence of the mouse genome, it is straightforward to clone the genes responsible for the phenotypes we find. In the course of our screens, we have identified seven mutations that change cell fate in the nervous system.

The nervous system is patterned along the anterior-posterior axis and the dorsal-ventral axis. While some of the pathways that specify the various cell fates in the developing nervous system are known, our current knowledge has clear gaps: we don’t understand the mechanism of how all the proteins function in specific steps of the known pathways and we know other signaling pathways are involved but do not yet know their identity. The new mutant lines we have identified in the screen should help to fill in some of these gaps.

One such mutant is called hennin (hnn). hnn embryos display a novel change in dorsal-ventral patterning of the caudal neural tube: too many motor neurons (MN) are specified, but there is no floor plate. The simplest model for ventral neural tube patterning invokes a concentration gradient of Sonic Hedgehog (Shh) that that mediates the specification of various cell types from the most ventral to the most dorsal. However, it is difficult to reconcile this model with the hnn phenotype as it would predict that hnn mutants should have fewer, not more, motor neurons. We cloned hnn and found it encodes a novel gene with an ADP ribosylation factor-related like (ARL) domain. While there are other vertebrate homologues of hnn, it has no homologues in invertebrates, reinforcing the need to identify genes important for mammalian development in mammals. We found HNN is predominantly localized to cilia in many cell types including cells in the node and the ventricular zone of the neural tube where cilia have been shown to be essential for Shh signaling. In addition, hnn mutants display random heart looping indicative of defects in setting up a proper left-right body axis. We found Shh signaling in the node of hnn mutants is intact but the nodal cilia were half the length of wild type. Furthermore, our examination of double mutant embryos for hnn and components of the Shh signaling pathway found that hnn is most likely to act in a Shh-independent pathway. Taken together, these data suggest cilia are necessary for multiple signaling pathways. We are using biochemical and cellular approaches to understand how HNN functions in patterning the nervous system and in setting up the left-right axis.

The other lines await our analysis. Three new lines have defects in the midbrain-hindbrain junction where the isthmus, a signaling center that patterns the brain is located. One line displays exencephaly, polydactaly and a kinked neural tube, phenotypes that are reminiscent of many other mutants with defects in dorsal ventral patterning of the neural tube. lilR3 is a mutant line which does not properly specify the anterior parts of the brain. The diencephelon is missing from another mutant line. By pinpointing the biological defects in these mutants and identifying the genes that underlie them we will better understand how specific molecules function in the complex environment of the developing mammalian embryo.

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