Regionalized protein localization domains in the zebrafish hair cell kinocilium

Abstract

Sensory hair cells are the receptors for auditory, vestibular, and lateral line sensory organs in vertebrates. These cells are distinguished by "hair"-like projections from their apical surface collectively known as the hair bundle. Along with the staircase arrangement of actin-filled stereocilia, the hair bundle features a single, non-motile, true cilium called the kinocilium. The kinocilium plays important roles in hair bundle development and the mechanics of sensory detection. To understand more about kinocilial development and structure, we performed a transcriptomic analysis of zebrafish hair cells looking for cilia-associated genes that have yet to be characterized in hair cells. Here, we focus on three such genes - ankef1a, odf3l2a, and saxo2 - because the human or mouse orthologs are either associated with sensorineural hearing loss or are located near un-characterized deafness loci. We made transgenic fish that express fluorescently-tagged versions of their proteins, demonstrating their localization to the kinocilia of zebrafish hair cells. Furthermore, we find that Ankef1a, Odf3l2a, and Saxo2 exhibit distinct localization patterns along the length of the kinocilium. Lastly, we report a novel overexpression phenotype for Saxo2. Overall, these results suggest that the hair cell kinocilium is regionalized along its proximal-distal axis and set the groundwork to understand the role of these kinocilial proteins in hair cell structure and function.

A gene regulatory network for specification and morphogenesis of a Mauthner Cell homolog in non-vertebrate chordates

“…Saxo (ANISEED unique gene identifier Cirobu.g00001698) is the sole Ciona homolog of human SAXO1/SAXO2 (Dardaillon et al, 2020), which encode microtubule-stabilizing/bundling proteins. These are part of a larger class of microtubule-binding Saxo proteins that are conserved from unicellular eukaryotes to vertebrates (Cuveillier et al, 2020; Dacheux et al, 2012; Dacheux et al, 2015; Erickson et al, 2023; Gui et al, 2022; Leung et al, 2023). Saxo was the top most-enriched gene transcript in the ddNs in our overlap list, and modestly upregulated by Pax3/7 overexpression ( Figure 1D ).…”

“…SAXO (formerly known as FAM154) proteins belong to a larger family of STOP/MAP6-related proteins that localize to centrosomes and cilia to mediate stabilization of microtubules through their Mn domains (Dacheux et al, 2015; Erickson et al, 2023; Leung et al, 2023). Because we had previously demonstrated a role for the centrosome-localized Defcab in ddN axon outgrowth, we hypothesized that Saxo may be playing a similar role as well.…”

“…Similarly, we found that Pou4 regulates Saxo in the ddNs and in the neurons of the peripheral nervous system. Saxo proteins are known to localize to cilia in neural cells in Ciona and vertebrates (Dacheux et al, 2015; Erickson et al, 2023; Kim et al, 2020). The ciliated PNs of the papillae mechanically sense the attachment of swimming Ciona larvae to suitable substrates and this is sufficient and necessary to trigger metamorphosis into the sessile adult phase (Sakamoto et al, 2022; Wakai et al, 2021).…”