Motile cilia are evolutionarily highly conserved hair-like organelles. The core cytoskeletal axoneme of motile cilia is a rotationally asymmetric structure composed of nine doublet microtubules and a pair of central microtubules, with a definite axonemal orientation (AO).
In a study published in Nature Communications, the research team led by Prof. ZHU Xueliang from the Center for Excellence in Molecular Cell Science (Shanghai Institute of Biochemistry and Cell Biology) of the Chinese Academy of Sciences, collaborated with Prof. YAN Xiumin from Xinhua Hospital affiliated with Shanghai Jiao Tong University School of Medicine, “Directional ciliary beats across epithelia require Ccdc57-mediated coupling between axonemal orientation and basal body polarity”, revealed a new regulatory mechanism for the coordinated beating of motile cilia on a tissue-wide scale.
The doublet microtubules of the axoneme are connected to the basal body anchored on the cell membrane, and various protein complexes attached to the axoneme, including molecular motors, enable the cilia to perform periodic back-and-forth beating along the direction of the axoneme.
In mammals, motile cilia are widely distributed on the surfaces of epithelial tissues such as the brain ventricles, trachea, and fallopian tubes in a multiciliated manner (with tens to hundreds of cilia per cell). The initial beating directions of these cilia are chaotic, and they need to gradually establish planar cell polarity (PCP) during development, that is, to form a coordinated and unified beating direction on a tissue-wide scale.
For example, the multiciliation of ependymal epithelial cells in the mouse brain ventricles occurs around 3 days after birth, and PCP can only be established after 21 days of age. Such a slow process indicates the complexity of the underlying mechanisms. The coordinated beating of cilia drives the directional flow of extracellular fluids such as cerebrospinal fluid and sputum, thereby promoting the maintenance of homeostasis in the central nervous system, respiratory tract cleaning, and the movement of fertilized eggs towards the uterus. Therefore, defects in the structure and function of motile cilia can lead to primary ciliary dyskinesia, with main symptoms including chronic respiratory diseases, infertility, situs inversus, and hydrocephalus.
Previous studies have found that the beating direction of cilia is also coupled with the polarity of the basal body, that is, the direction of the basal foot (BF), an accessory structure of the basal body. Therefore, it is usually believed that the cytoskeleton pulls the tip of the BF to rotate the entire cilium and change its beating direction. When the polarities of the vast majority of basal bodies in the same tissue are adjusted to be consistent, PCP is established. However, how AO is coupled with the polarity of the basal body has always been a gap in this field. Moreover, if the coupling does not occur from the very beginning of cilium formation, the existing dogma may need to be revised.
The research team found that the evolutionary conservation of Ccdc57 is closely related to the motile cilia of epithelial tissues, suggesting that it may be an important regulator of cilia PCP. Interestingly, in the ependymal epithelial cells of the mouse brain ventricles, it is located in a rotationally asymmetric punctate pattern within the lumen at the distal end of the basal body, and with the establishment of cilia PCP, this localization changes from random to polarized to the opposite side of the BF.
Ccdc57 knockout mice exhibit severe hydrocephalus and high mortality. In-depth analysis revealed that different cilia within the ependymal epithelial cells of knockout mice can form coordinated and consistent beating, but there is a lack of uniformity among different cells, which leads to abnormal cerebrospinal fluid flow, causing hydrocephalus and death. Moreover, the AO of Ccdc57-knockout cilia is decoupled from BF, so the beating direction is not correlated with the polarity of the basal body.
Subsequently, the researchers analyzed the cilia with chaotic beating directions in the cells of 4-day-old wild-type mice and found that the majority of their AO and BF were uncoupled, while in the cilia with consistent beating directions, the majority were AO-BF coupled. The researchers also analyzed the multiciliated tracheal epithelium of Ccdc57-knockout mice and found that there were also defects in AO-BF coupling. These research results reveal that AO and BF are not coupled from the very beginning of the formation of motile cilia, and indicate that the cilia clusters within a cell can form a consistent beating without depending on the polarity of their basal bodies, but AO and BF need to be coupled to establish PCP.
Moreover, the first key factor mediating AO-BF coupling has been identified, and it is suggested that there is a new structure containing Ccdc57 at the distal end of the basal body, which is responsible for locking the polarity relationship between the axoneme and the basal body during the process of establishing PCP in the motile cilia of epithelial tissues. These findings have revised the previous understanding in the field and provided a new perspective for understanding the regulatory mechanism of motile cilia PCP and related pathological mechanisms.
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