A Titin mutation defines roles for circulation in endothelial morphogenesis.

Morphogenesis of the developing vascular network requires coordinated regulation of an extensive array of endothelial cell behaviors. Precisely regulated signaling molecules such as vascular endothelial growth factor (VEGF) direct some of these endothelial behaviors. Newly forming blood vessels also become subjected to novel biomechanical forces upon initiation of cardiac contractions. We report here the identification of a recessive mouse mutation termed shrunken-head (shru) that disrupts function of the Titin gene. Titin was found to be required for the initiation of proper heart contractions as well as for maintaining the correct overall shape and orientation of individual cardiomyocytes. Cardiac dysfunction in shrunken-head mutant embryos provided an opportunity to study the effects of lack of blood circulation on the morphogenesis of endothelial cells. Without blood flow, differentiating endothelial cells display defects in their shapes and patterns of cell-cell contact. These endothelial cells, without exposure to blood circulation, have an abnormal distribution within vasculogenic vessels. Further effects of absent blood flow include abnormal spatial regulation of angiogenesis and elevated VEGF signaling. The shrunken-head mutation has provided an in vivo model to precisely define the roles of circulation on cellular and network aspects of vascular morphogenesis.

Atrophin 2 recruits histone deacetylase and is required for the function of multiple signaling centers during mouse embryogenesis.

Atrophins are evolutionarily conserved proteins that are thought to act as transcriptional co-repressors. Mammalian genomes contain two atrophin genes. Dominant polyglutamine-expanded alleles of atrophin 1 have been identified as the cause of dentatorubralpallidoluysian atrophy, an adult-onset human neurodegenerative disease with similarity to Huntington's. In a screen for recessive mutations that disrupt patterning of the early mouse embryo, we identified a line named openmind carrying a mutation in atrophin 2. openmind homozygous embryos exhibit a variety of patterning defects that first appear at E8.0. Defects include a specific failure in ventralization of the anterior neural plate, loss of heart looping and irregular partitioning of somites. In mutant embryos, Shh expression fails to initiate along the anterior midline at E8.0, and Fgf8 is delocalized from the anterior neural ridge at E8.5, revealing a crucial role for atrophin 2 in the formation and function of these two signaling centers. Atrophin 2 is also required for normal organization of the apical ectodermal ridge, a signaling center that directs limb pattern. Elevated expression of atrophin 2 in neurons suggests it may interact with atrophin 1 in neuronal development or function. We further show that atrophin 2 associates with histone deacetylase 1 in mouse embryos, providing a biochemical link between Atr2 and a chromatin-modifying enzyme. Based on our results, and on those of others, we propose that atrophin proteins act as transcriptional co-repressors during embryonic development.

Predator-induced behavioral and morphological plasticity in the tropical marine Gastropod Strombus gigas.

Florida queen conch stocks once supported a significant fishery, but overfishing prompted the state of Florida to institute a harvest moratorium in 1985. Despite the closure of the fishery, the queen conch population has been slow to recover. One method used in the efforts to restore the Florida conch population has been to release hatchery-reared juvenile conch into the wild; however, suboptimal predator avoidance responses and lighter shell weights relative to their wild counterparts have been implicated in the high mortality rates of released hatchery juveniles. We conducted a series of experiments in which hatchery-reared juvenile conch were exposed to a predator, the spiny lobster (Panulirus argus), to determine whether they could develop behavioral and morphological characteristics that would improve survival. Experiments were conducted in tanks with a calcareous sand substrate to simulate a natural environment. Conditioned conch were exposed to caged lobsters while conch in the control tanks were exposed to empty cages. Conditioned conch moved significantly less and buried themselves more frequently than the naive control conch. Morphometric data indicated that the conditioned conch grew at a significantly slower rate than the naive conch, but the shell weights of the two groups were not significantly different. This implies that the conditioned conch had thicker or denser shells than the control group. As a result, the conditioned conch had significantly higher survival than naive conch in a subsequent predation experiment in which a lobster was allowed to roam free in each tank for 24 hours. In the future, the conditioning protocols documented in this study will be used to increase the survival of hatchery-reared conch in the wild.