Frazzled/DCC facilitates cardiac cell outgrowth and attachment during dorsal vessel formation.

Frazzled/DCC facilitates cardiac cell outgrowth and attachment during dorsal vessel formation. Frank D. Macabenta^1,2, Amber G. Jensen^1,2, Yi-Shan Cheng¹, Joseph J. Kramer¹, Sunita G. Kramer^1,2. 1) Pathology Department, UMDNJ/RWJMS, Piscataway, NJ; 2) Cell and Developmental Biology, Rutgers University, Piscataway, NJ.

Embryonic dorsal vessel formation is a highly stereotyped process that involves the migration and morphogenesis of 52 pairs of cardioblasts (CBs) in order to form a linear tube. This process requires spatiotemporally-regulated localization of signaling and adhesive proteins in order to coordinate the formation of a central lumen while maintaining simultaneous adhesion between CBs. Previous studies have shown that the Slit/Roundabout and Netrin/Unc5 signaling pathways facilitate site-specific de-adhesion between contralateral CBs in order to form a luminal space. However, the concomitant mechanism by which dorsoventrally-polarized attraction initiates cell shape changes and discrete localization of adhesive proteins remains poorly understood. Our findings support the idea that the axon guidance receptor Frazzled/DCC (Fra) plays an attractive role in lumen formation. fra mRNA is expressed in the dorsal vessel prior to and during lumen formation. Loss-of-fra-function results in cell shape change delays and alignment defects between contralateral CB rows; additionally, diminished or absent junctional domains are observed between CB pairs. Deletion mutants of both Netrin genes (NetA and NetB) exhibit phenotypes similar to that observed in fra mutants. Furthermore, overexpression of fra at high levels in the dorsal vessel results in delayed CB outgrowth. To localize Fra, we expressed a Myc-tagged fra transgene in CBs. In a wild type background, Fra-Myc accumulates at dorsal and ventral leading edges of paired CBs, corresponding to future sites of attachment. However, we observe mislocalization of Fra-Myc in a NetAB background, suggesting a role for Netrin in mediating discrete Fra localization. Taken together, our data supports the idea that while Slit/Roundabout and Netrin/Unc5 signaling contribute to proper lumen formation by facilitating de-adhesion, Netrin/Frazzled signaling conversely allows for attraction and subsequent membrane outgrowth between CBs.