Regulation of the apical extension morphogenesis tunes the mechanosensory response of microvilliated neurons

Abstract

Multiple types of microvilliated sensory cells exhibit an apical extension thought to be instrumental in the detection of sensory cues. The investigation of the mechanisms underlying morphogenesis of sensory apparatus is critical to understand the biology of sensation. Most of what we currently know comes from the study of the hair bundle of the inner ear sensory cells, but morphogenesis and function of other sensory microvilliated apical extensions remain poorly understood. We focused on spinal sensory neurons that contact the cerebrospinal fluid (CSF) through the projection of a microvilliated apical process in the central canal, referred to as cerebrospinal fluid-contacting neurons (CSF-cNs). CSF-cNs respond to pH and osmolarity changes as well as mechanical stimuli associated with changes of flow and tail bending. In vivo time-lapse imaging in zebrafish embryos revealed that CSF-cNs are atypical neurons that do not lose their apical attachment and form a ring of actin at the apical junctional complexes (AJCs) that they retain during differentiation. We show that the actin-based protrusions constituting the microvilliated apical extension arise and elongate from this ring of actin, and we identify candidate molecular factors underlying every step of CSF-cN morphogenesis. We demonstrate that Crumbs 1 (Crb1), Myosin 3b (Myo3b), and Espin orchestrate the morphogenesis of CSF-cN apical extension. Using calcium imaging in crb1 and espin mutants, we further show that the size of the apical extension modulates the amplitude of CSF-cN sensory response to bending of the spinal cord. Based on our results, we propose that the apical actin ring could be a common site of initiation of actin-based protrusions in microvilliated sensory cells. Furthermore, our work provides a set of actors underlying actin-based protrusion elongation shared by different sensory cell types and highlights the critical role of the apical extension shape in sensory detection.

Adrenergic activation modulates the signal from the Reissner fiber to cerebrospinal fluid-contacting neurons during development

“…We recently showed that the Reissner fiber is functionally coupled to the mechano-sensory function of these interoceptive neurons in the larva ( Orts-Del'Immagine et al, 2020 ). However, embryonic CSF-cNs are not fully differentiated, as they do not harbor a fully developed apical extension known to tune their mechanosensory function at larval stage ( Desban et al, 2019 ). We therefore favor the hypothesis that the Reissner fiber acts via the modulation of the CSF content, which is supported by the restoration of calcium signaling in CSF-cNs upon monoamine injections in the brain ventricles.…”

“…In the zebrafish embryo, urp1 and urp2 are expressed along the antero-posterior axis of the neural tube in a subset of spinal sensory neurons ( Quan et al, 2015 ) called CSF-contacting neurons (CSF-cNs). CSF-cNs extend into the CSF a microvilliated apical extension that starts differentiating in the embryo and becomes fully mature in the larva to tune the mechanosensory responses of CSF-cNs upon tail bending ( Desban et al, 2019 ). These conserved interoceptive neurons form two populations of different developmental origins that are located ventrally and dorso-laterally relative to the central canal of the zebrafish spinal cord ( Djenoune et al, 2017 ; Park et al, 2004 ).…”

The role of motile cilia in the development and physiology of the nervous system

“…The morphology of CSF-cNs is peculiar. They display an apical dendritic extension directed towards the central canal, protruding a tuft of microvilli [69,90,95]. CSF-cNs possess a cilium in clawed frog [96,97], lamprey [88,98,99], zebrafish [100], chick [101], and turtle [102], and the motility of this cilium was confirmed in lamprey and zebrafish [88,99,100] ( Figure 2c).…”

Somatostatin 1.1 contributes to the innate exploration of zebrafish larva

“…Previous studies both in lamprey and zebrafish have shown that somatostatin is expressed in the interoceptive neurons contacting the cerebrospinal fluid (‘CSF-cNs’ 42 , 69 , 70 ). These neurons around the central canal 71 – 73 can detect chemical 40 , 70 and mechanical cues 41 , 44 , 74 – 76 from the cerebrospinal fluid and, in turn, modulate locomotion 40 , 43 , 44 , 70 , 77 , 78 . In zebrafish, dorsal CSF-cNs that express sst1.1 have been involved in the modulation of slow locomotion by synapsing onto V0-v interneurons 43 .…”