IP 6 Got Rhythm?

Abstract

Motile cilia are plasma membrane projections that contain a central core of microtubules whose movement relative to each other contributes to ciliary beating. The left-right body patterning of developing embryos is thought to result from cilia-dependent differential distribution of morphogens. Knockout of inositol 1,3,4,5,6-pentakisphosphate (IP ₅ ) 2-kinase (Ipk1), the enzyme that synthesizes IP ₆ , in zebrafish embryos results in defective left-right positioning of organs, although the mechanism involved is unknown. Sarmah et al . extended this work and knocked down expression of ipk1 in zebrafish by injecting embryos with an antisense morpholino oligonucleotide (MO) targeted to ipk1 mRNA ( ipk1 ^MO1 embryos). The Kupffer’s vesicle (KV) is the organ that initiates left-right organ placement in the zebrafish. The authors found that whereas KV cilia from wild-type embryos beat with a regular frequency in a counterclockwise direction, those from ipk1 ^MO1 embryos vibrated without a defined direction of motion. Furthermore, immunocytochemical analyses revealed that cilia in ipk1 ^MO1 embryos were shorter than those in wild-type embryos. Although the ectopic expression in ipk1 ^MO1 embryos of Ipk1 resulted in the rescue of ciliary length and motion, that of a catalytically inactive mutant of Ipk1 did not. Fluorescence microscopy showed that Ipk1 was localized to centrosomes in the basal bodies of cilia in wild-type embryos. Transmission electron microscopy demonstrated that the organization of microtubules in the cilia from ipk1 ^MO1 embryos was similar to that in wild-type embryos. However, the anterograde microtubule-dependent movement of melanosomes (pigmented vesicles), which is mediated by kinesin proteins, was much slower in ipk1 ^MO1 cilia than in wild-type cilia. Together these data implicate Ipk1, and potentially its product IP ₆ , in the maintenance of cilia length and the regulation of ciliary motion. B. Sarmah, V. P. Winfrey, G. E. Olson, B. Appel, S. R. Wente, A role for the inositol kinase Ipk1 in ciliary beating and length maintenance. Proc. Natl. Acad. Sci. U.S.A. 104 , 19843-19848 (2007). [Abstract] [Full Text]