Dr, Dominic Norris, January the 23rd Title Cilia, flow and polcystins: telling left from right in the mouse Abstract While externally mirror symmetrical between left and right, vertebrates show clear leftright (L-R) asymmetry in the positioning and patterning of their internal organs and vasculature. This L-R asymmetry is evolutionarily conserved, reflecting both its ancient origin and physiological significance. In early embryos, while left and right are mathematically evident, L-R symmetry must be broken reliably in every embryo resulting in cells having a left or right sided identity. The initial breaking of symmetry occurs when motile cilia in the embryonic node (a transient pit shaped structure) rotate so as to drive a leftward fluid flow. This biophysical force, known as nodal flow, is itself ‘perceived’ in a cilia dependent manner. The mechanisms involved remains controversial. In this talk I will discuss our work on the role of cilia and in particular two polycystin proteins, Pkd1l1 and Pkd2; all are required for normal L-R patterning. I will provide evidence that Pkd1l1 is a strong candidate to be the elusive sensor of flow, showing that it acts downstream of flow but upstream of early gene expression asymmetries. Furthermore, our genetic experiments have revealed a novel pathway linking the generation and sensation of flow to the establishment of asymmetric gene expression. Finally, I discuss a small extracellular domain of Pkd1l1 that is essential for function and might be central to the mechanism of flow sensation. Bio During his PhD Dr, Dominic Norris (with Sohaila Rastan and Neil Brockdorff) studied mouse genetics and I worked on X-chromosome inactivation, demonstrating that CpG methylation was a generic characteristic of the inactive X. Later on he continued on to study Xist, revealing a methylation based genetic imprint; the fourth gene for which such an imprint was demonstrated. He did his postdoc in Harvard with Liz Robertson, where he trained in developmental biology and making targeted genetic modifications. There he studied the Nodal locus, mapping then systematically deleting the enhancers that control its expression. Nodal shows left-right asymmetric expression and it was here that he first became interested in left-right (L-R) determination. He then returned to the UK to take up an MRC career development award at MRC Harwell, establishing a genetic screen to look for novel L-R mutants as well as screening for novel asymmetric gene expression. So far he has identified new cilia motility mutants, novel (and informative) alleles of known loci such as Pkd2 and new loci. These mutants have formed the basis for his subsequent work. Out of these screens they identified Pkd1l1 as a novel L-R determinant and he has been analyzing this in increasing depth over the past few years.