Cilia--where two Wnts collide

nlz1 is required for cilia formation in zebrafish embryogenesis

The formation of cilia is a fundamental developmental process affecting diverse functions such as cellular signaling, tissue morphogenesis and body patterning. However, the mechanisms of ciliogenesis during vertebrate development are not fully understood. In this report we describe a novel role of the Nlz1 protein in ciliogenesis. We demonstrate morpholino-mediated knockdown of nlz1 in zebrafish causes abnormal specification of the cells of Kupffer's vesicle (KV); a severe reduction of the number of cilia in KV, the pronephros, and the neural floorplate; and a spectrum of later phenotypes reminiscent of human ciliopathies. In vitro and in vivo data indicate that Nlz1 acts downstream of Foxj1a and Wnt8a/presumed canonical Wnt signaling. Furthermore, Nlz1 contributes to motile cilia formation by positively regulating Wnt11/presumed non-canonical Wnt signaling. Together, our data suggest a novel role of nlz1 in ciliogenesis and the morphogenesis of multiple tissues.

Cilia and cell cycle re-entry: more than a coincidence

With the exception of the final stages of spermatogenesis in butterfly and some unicellular ciliates and flagellates, ciliated cells undergo cell division without cilia. This reciprocal relationship between cilia formation and cell division has prompted investigators to propose that ciliogenesis and cell cycle progression are mutually exclusive processes. Early work in fibroblasts showed that deciliation occurs in two waves, as cells depart from quiescence. The first wave of deciliation occurs before entry into S, while the second wave occurs between S and mitosis. Since then, it has remained a mystery whether and how (de)ciliation is coupled to the cell cycle and further, whether ciliation can affect cell cycle progression. Several recent publications provide evidence for a causative role of ciliary resorption in influencing the duration of the G1 phase of the cell cycle impacting on several developmental processes, including left-right patterning, kidney, skeletal and brain development. This body of work argues for the existence of a molecular crosstalk between ciliary factors and regulators of the cell cycle. Here, we review the evidence connecting primary cilia and the cell cycle and evaluate the idea that the primary cilium may function as a physical checkpoint in cell cycle re-entry.