Pkd2 Affects Cilia Length and Impacts LR Flow Dynamics and Dand5

Understanding laterality disorders and the left-right organizer: Insights from zebrafish

Vital internal organs display a left-right (LR) asymmetric arrangement that is established during embryonic development. Disruption of this LR asymmetry-or laterality-can result in congenital organ malformations. Situs inversus totalis (SIT) is a complete concordant reversal of internal organs that results in a low occurrence of clinical consequences. Situs ambiguous, which gives rise to Heterotaxy syndrome (HTX), is characterized by discordant development and arrangement of organs that is associated with a wide range of birth defects. The leading cause of health problems in HTX patients is a congenital heart malformation. Mutations identified in patients with laterality disorders implicate motile cilia in establishing LR asymmetry. However, the cellular and molecular mechanisms underlying SIT and HTX are not fully understood. In several vertebrates, including mouse, frog and zebrafish, motile cilia located in a "left-right organizer" (LRO) trigger conserved signaling pathways that guide asymmetric organ development. Perturbation of LRO formation and/or function in animal models recapitulates organ malformations observed in SIT and HTX patients. This provides an opportunity to use these models to investigate the embryological origins of laterality disorders. The zebrafish embryo has emerged as an important model for investigating the earliest steps of LRO development. Here, we discuss clinical characteristics of human laterality disorders, and highlight experimental results from zebrafish that provide insights into LRO biology and advance our understanding of human laterality disorders.

Sequential action of JNK genes establishes the embryonic left-right axis

The establishment of the left-right axis is crucial for the placement, morphogenesis and function of internal organs. Left-right specification is proposed to be dependent on cilia-driven fluid flow in the embryonic node. Planar cell polarity (PCP) signalling is crucial for patterning of nodal cilia, yet downstream effectors driving this process remain elusive. We have examined the role of the JNK gene family, a proposed downstream component of PCP signalling, in the development and function of the zebrafish node. We show jnk1 and jnk2 specify length of nodal cilia, generate flow in the node and restrict southpaw to the left lateral plate mesoderm. Moreover, loss of asymmetric southpaw expression does not result in disturbances to asymmetric organ placement, supporting a model in which nodal flow may be dispensable for organ laterality. Later, jnk3 is required to restrict pitx2c expression to the left side and permit correct endodermal organ placement. This work uncovers multiple roles for the JNK gene family acting at different points during left-right axis establishment. It highlights extensive redundancy and indicates JNK activity is distinct from the PCP signalling pathway.

Zebrafish Model as a Screen to Prevent Cyst Inflation in Autosomal Dominant Polycystic Kidney Disease

In autosomal dominant polycystic kidney disease (ADPKD), kidney cyst growth requires the recruitment of CFTR (cystic fibrosis transmembrane conductance regulator), the chloride channel that is defective in cystic fibrosis. We have been studying cyst inflation using the zebrafish Kupffer’s vesicle (KV) as model system because we previously demonstrated that knocking down polycystin 2 (PC2) induced a CFTR-mediated enlargement of the organ. We have now quantified the PC2 knockdown by showing that it causes a 73% reduction in the number of KV cilia expressing PC2. According to the literature, this is an essential event in kidney cystogenesis in ADPKD mice. Additionally, we demonstrated that the PC2 knockdown leads to a significant accumulation of CFTR-GFP at the apical region of the KV cells. Furthermore, we determined that KV enlargement is rescued by the injection of Xenopus pkd2 mRNA and by 100 µM tolvaptan treatment, the unique and approved pharmacologic approach for ADPKD management. We expected vasopressin V2 receptor antagonist to lower the cAMP levels of KV-lining cells and, thus, to inactivate CFTR. These findings further support the use of the KV as an in vivo model for screening compounds that may prevent cyst enlargement in this ciliopathy, through CFTR inhibition.