The Wnt signaling network has critical roles in embryonic development and is implicated in human disease. One of the outputs of the Wnt network, called the planar cell polarity (PCP) pathway, regulates tissue polarity and directs cell migration. Core PCP components (Frizzled, Dishevelled, Prickle, Vangl, Celsr) localize asymmetrically in polarized cells and establish polarity across the tissue through protein interactions between adjacent cells. The core PCP component activate tissue-specific "effectors" which translate the signal into morphological changes. PCP is related to several disease conditions, including neural tube defects, cystic kidney disease, and cance metastasis. However, mechanisms of the PCP underlying physiological and disease-related conditions are not well understood. Here, I explore functions of the core PCP component Pk, and its relationship to disease, in the zebrafish model system.
Mutations in Pk1 and Pk2 have been identified in human progressive myoclonic epilepsy patients. Pk coodinate cell movement, neuronal migration and axonal outgrowth during embryonic development. Yet, how dysfunctions of pk relates to epilepsy is unknown. Here, I show that knockdown of pk1a sensitizes the zebrafish larva to convulsant drug. To model the defects in central nervous system, I examine neurogenesis in the retina and find that both pk1a and pk2 are required for proper dendritic outgrowth in the retinal inner plexiform layer. Furthermore, I characterize the epilepsy-related mutant forms of Pk1a and Pk2. The mutant Pk1a forms show reduced ability to suppress the retinal neurogenesis defects compared to the wild-type, as well as differential ubiquitination levels. Pk2 mutant forms also show differential activities in overexpression assays and seemingly more stable proteins relative to the wild-type. Taken together, pk1a and pk2 may contribute to epilepsy by affecting neuronal patterning and thus signal processing.
Another aspect of PCP function has been implicated in cilia and cilia-related disorders, also called ciliopathy. PCP effectors have been shown to modulate ciliogenesis and core PCP proteins (Vang and Dvl) regulate cilia orientation. On the other hand, cilia are not required for PCP signaling, especially asymmetric core PCP protein localization. These findings leave open the question what is the precise relationship between PCP and cilia. The Bardet Biedl Syndrome (BBS) is a type of ciliopathy that leads to obesity, retinitis pigmentosa, polydactyly, mental retardation and other symptons. A subset of BBS genes share similar knockdown phenotype in cell migration as seen in PCP knockdown embryos. Shared pehnotypes have led some to proposethat PCP and bbs genes may interact. Yet a direct relationship has yet to be established. I examine the interaction between pk2 and a central Bbs gene, bbs7. By analyzing shared phenotypes in double knockdown embryos, I find no synergistic interaction between the two, suggesting they act in distinct pathways. Bbs regulate ciliary trafficking and in zebrafish, knockdown of bbs genes leads to delayed retrograde melanosome transport. Interestingly, I find knockdown of pk2 suppresses this retrograde transport delay. Additionally, pk2 knockdown embryos show a delay in anterograde melanosome transport. These findings highlight a new role for pk2 in intracellular transport and clarifies the relationship between PCP and BBS.
In summary, my work here strengthens the link between pk mutations and human epilepsy and identifies functions of pk in retinal neurogenesis and in intracellular transport. To what extent the role of neurogenesis and intracellular transport are related is worth future study. Yet, this new information provides insights into potential mechanisms of epilepsy and the relationship between PCP and BBS.