This work leads to three major conclusions. First, we demonstrate a role for centrin2 in cilia disorders and cyst formation in a vertebrate organism. Given the fundamental role of centrosome proteins during the cell cycle, all known attempts to deplete centrosomal proteins, such as centrin in mice, have led to difficulties due to early embryonic lethality. Using an alternative strategy in zebrafish, we demonstrate a key role for centrin2 in vertebrate development. Second, taking advantage of zebrafish embryos as a platform for cell biology in vivo our data define a cellular role for centrin2 in both cilia formation and mitosis in vivo in a vertebrate organism. Finally, given that both cilia and mitotic defects are observed in defective zebrafish organs, this work suggests that like cilia, mitotic dysfunction may contribute to global ciliopathy phenotypes, including cyst formation. Along these lines, our data highlight the potential of zebrafish as a model organism to study cellular mechanisms contributing to developmental abnormalities, particularly embryonic defects.
Centrin2 is well-characterized for its function at the basal body11,24,34,35
and in dividing cells.32,43
In this context, our work confirms previously described roles of centrin in cilia formation and mitosis in vivo in a vertebrate and suggests that both defects can contribute to the disease phenotype. Further work will be required to address wether noncentrosomal roles of centrins37–40
can also contribute to the disease phenotype in vivo in vertebrates.
The phenotype of centrin2-depleted zebrafish embryos described here is essentially identical to that of the extensively characterized cilia protein depletion model of ciliopathy, from the gross anatomical features (e.g., cysts, curly tail or trunk) to the cell biological details (e.g., cilia and mitotic defects).3,41,42
In this context, our work suggests a centrosome-mediated model for ciliopathies involving both cilia and mitotic dysfunctions. In non-cycling cells, loss of centrosome integrity would lead to defects in cilia formation. In mitosis, centrosome/spindle pole defects would lead to mitotic dysfunction. In agreement with this model, recent studies suggest mitotic roles for well-characterized cilia proteins,3,4
although the mechanism of function remains to be fully addressed. Moreover, the fact that cell division is required for cyst formation supports a role for mitotic defects in cystogenesis/ciliopathy. Indeed, the fact that high proliferation rates during early development44–46
and during tissue regeneration following injury47
exacerbate cyst formation indicates that cell division is an important parameter in the induction of cysts and suggests that defects occurring during mitosis could contribute to ciliopathy-related phenotypes.
Depletion of a diversity of centrosome proteins disrupts primary cilia formation in cell culture.11–13
Centrosome proteins such as pericentrin, when mutated, affect cilia organization in vivo.48,49
It will be interesting to determine if centrin2 is in a subclass of centrosome proteins involved in ciliopathies or if centrosome proteins of diverse mitotic functions and locations within the centrosome (e.g., centrioles, pericentriolar material, centriole linkers, subdistal and distal appendages) contribute to these disorders in vertebrates. This work and future studies will provide insight into the precise contributions of centrosome proteins to cilia related disorders.
Finally, it is important to note that the term “ciliopathy,” accurately describes the disorganization/dysfunction of cilia commonly observed in these disorders. However, this study, together with other work showing that processes other than cilia formation and function are disrupted when cilia proteins are depleted (i.e., spindle function, cell division, cell polarity),3–5
suggests that cilia dysfunction may explain only part of the mechanistic underpinnings of these disorders. Further work will be required to clearly define the primary cause(s) of these disorders.