Distinct Roles of Kif6 and Kif9 in Mammalian Ciliary Trafficking and Motility

Abstract

Motile cilia composed of 9+2 axonemes generate coordinated beats to propel directional fluid flows for various physiological functions. This requires multiciliated cells to establish translational, rotational and planar polarities. However, the mechanism underlying the establishment of these polarities remain elusive. Here, we delve into the functions of mammalian kinesin-9 family and uncover that Kif6, but not Kif9, is critical for establishing both rotational and planar polarities. Kif6 associates with intraflagellar transport (IFT) pariticles and participates in IFT within motile cilia. Knockout of Kif6 leads to severe hydrocephalus due to disruptions in both rotational and tissue-level polarity across ependyma. Conversely, while Kif9 is proposed as a central pair kinesin, its depletion does not significantly affect ependymal cilia motility, although ventricular expansion and defective sperm motility are observed. These findings reveal the unique roles of Kif6 and Kif9 in regulating cilia/flagella beating, providing valuable insights into the complex regulatory mechanisms governing the polarity of multiciliated tissues and the motility of cilia/flagella.

Characterizing the in vitro motor properties of two kinesin-9 family members from Tetrahymena

“…3d, k) [14] . Kinesin-9B localization resembles that of the intra agellar transport (IFT) protein [16] ; a recent study reported that the mouse Kif6 moved bidirectionally with or without IFT-B particles along axonemes in ependymal cells at a velocity comparable to that of IFT [15] . However, a previous study [14] and our current study demonstrated that kinesin-9B moved at much slower velocities in vitro than IFT, indicating that kinesin-9B may not be a motor like kinesin-2 that drives IFT but may be a component of IFT or a cargo carried by IFT.…”

“…Kinesin-9B knockdown in Trypanosoma disrupts the asymmetrical arrangement of the para agellar rod (PFR) along the axonemes, suggesting that the trypanosome kinesin-9B serves as a linker protein for PFR in Trypanosoma [22] . Mammalian cell cilia do not have a PFR, but a kinesin-9B de ciency or mutation disrupts the basal foot alignment of mouse multiciliate brain ependymal cells [14,15] , suggesting that mammalian kinesin-9B may also enable the asymmetrical arrangement of the basal foot. Tetrahymena do not have a PFR, but possess several basal body accessory structures: post-ciliary microtubules, transverse microtubules, and kinetodesmal bers [41] .…”

“…Of these, the kinesin-9 family members are expressed exclusively in ciliated organisms such as protists, and in tissues with motile cilia, such as the testis, trachea, and brain. The kinesin-9 family consists of two subfamilies, kinesin-9A (Klp1 in Chlamydomonas and Kif9 in vertebrates) and kinesin-9B (Kif6 in vertebrates), each with distinct functions, both of which have been implicated in hydrocephalus [14][15][16] . Kinesin-9A is an axonemal component protein localized along one of the central pair of microtubules (C2) [17][18][19] .…”

“…Structural studies have revealed that kinesin-9A adopts two states on the central microtubule, positioned 8 nm apart [17] , suggesting that kinesin-9A movement may regulate dynein-induced ciliary bending [17,18,24] . Loss of kinesin-9B function disrupts the directionality of multiciliated ciliary motility in ependymal cells of the mouse brain, causing severe hydrocephalus [14][15][16] .…”