doublesex/mab3 related-1 (dmrt1) is essential for development of anterior neural plate derivatives in Ciona.

Ascidian larvae have a hollow, dorsal central nervous system that shares many morphological features with vertebrate nervous systems yet is composed of very few cells. We show here that a null mutation in the gene dmrt1 in the ascidian Ciona savignyi results in profound abnormalities in the development of the sensory vesicle (brain), as well as other anterior ectodermal derivatives, including the palps and oral siphon primordium (OSP). Although the phenotype of the mutant embryos is variable, the majority have a complete loss of the most anterior structures (palps and OSP) and extensive disruption of sensory structures, such as the light-sensitive ocellus, in the sensory vesicle. dmrt1 is expressed early in the blastula embryo in a small group of presumptive ectodermal cells as they become restricted to anterior neural, OSP and palp fates. Despite the early and restricted expression of dmrt1, we were unable, using several independent criteria, to observe a defect in the mutant embryos until the early tailbud stage. We speculate that the variability and late onset in the phenotype may be due to partially overlapping activities of other gene products.

Pigmentation in the sensory organs of the ascidian larva is essential for normal behavior.

Free-living animals and their larvae utilize light and gravity as cues to navigate in open space. Detection and response to these environmental stimuli are important for the dispersal and settlement of ascidian larvae. Two pigmented structures in the brain of the ascidian larva, the ocellus and the otolith, have been shown to function as the photoreceptive and gravity sensitive organs, respectively. Here, we show that pigmentation is essential for proper phototactic and geotactic behavior in larvae of the ascidian species Ciona savignyi. Two recessive and complementing mutant lines of C. savignyi, immaculate and spotless, that specifically disrupt the pigmentation of the sensory organs during larval development are described. Homozygous mutant larvae are unable to respond properly to gravity and illumination cues while settling. Genetic analysis shows that spotless is caused by a point mutation within the tyrosinase gene that creates a premature stop codon, while the molecular nature of immaculate is unknown. Although the role of pigmentation in the ocellus of C. savignyi is similar to that in vertebrate visual systems, our results demonstrate a novel use of melanin in geotactic behavior.

Alpha 4 integrins and vascular cell adhesion molecule-1 play a role in sympathetic innervation of the heart.

Sympathetic neurons innervate the heart early in postnatal development, an event that is crucial for proper modulation of blood pressure and cardiac function. However, the axon guidance cues that direct sympathetic neurons to the heart, and the neuronal receptors that recognize those cues, are poorly understood. Here we present evidence that interactions between the alpha4beta1 integrin on sympathetic neurons and vascular cell adhesion molecule-1 (VCAM-1) in the heart plays a role in cardiac innervation. The alpha4 subunit was detected on postnatal rat superior cervical ganglion (SCG) neurons in culture and in cryosections of SCG and heart. VCAM-1 immunoreactivity was detected on cardiac myocytes that associate with invading sympathetic neurons. Purified recombinant soluble VCAM-1 (rsVCAM-1) stimulated SCG neurite outgrowth at levels comparable with laminin 2/4 and fibronectin (Fn), and outgrowth on rs-VCAM-1 and Fn was blocked by antibodies specific for the alpha4 and beta1 integrin subunits. Intrathoracic injection of function-blocking antibodies to alpha4 and VCAM-1, as well as a small molecule inhibitor of alpha4 integrins, significantly reduced sympathetic innervation of the heart. These results indicate that the interaction between alpha4 integrin and VCAM-1 is important for sympathetic innervation of the heart.

Structural studies of Salmonella typhimurium ArnB (PmrH) aminotransferase: a 4-amino-4-deoxy-L-arabinose lipopolysaccharide-modifying enzyme.

Lipid A modification with 4-amino-4-deoxy-L-arabinose confers on certain pathogenic bacteria, such as Salmonella, resistance to cationic antimicrobial peptides, including those derived from the innate immune system. ArnB catalysis of amino group transfer from glutamic acid to the 4"-position of a UDP-linked ketopyranose molecule to form UDP-4-amino-4-deoxy-L-arabinose represents a key step in the lipid A modification pathway. Structural and functional studies of the ArnB aminotransferase were undertaken by combining X-ray crystallography with biochemical analyses. High-resolution crystal structures were solved for two native forms and one covalently inhibited form of S. typhimurium ArnB. These structures permitted identification of key residues involved in substrate binding and catalysis, including a rarely observed nonprolyl cis peptide bond in the active site.