Emx2 defines bidirectional polarity of neuromasts by changing hair-bundle orientation and not hair-cell positions
The bidirectional sensitivity of neuromasts to water flow in the zebrafish lateral line system is attributed to the opposite orientation of the hair bundles on top of sensory hair cells (HC) within a neuromast. After each HC precursor divides to form two nascent HCs, HCs of the same bundle orientation are positioned on one side of the neuromast, across from HCs with opposite bundle orientation. The transcription factor emx2 is expressed in only one of the sibling HCs. Loss or gain of function of emx2 in HCs causes unidirectional hair bundle orientation in neuromasts. It is not clear whether Emx2 is required specifically in establishing hair bundle orientation after HC formation or Emx2 has an earlier role in HC fate and/or positioning, which indirectly changes hair bundle orientation. A phenomenon, in which nascent sibling HCs exchange positions with each other, has been postulated to be the mechanism for HCs to acquire their designated positions within the neuromast. We asked whether Emx2 is involved in positional acquisition of HCs within the neuromast. Using live-imaging and our emx2 reporter we find that the HC rearrangement, redefined as two processes named Rock & Roll are required for HCs to acquire their positions. Although Emx2 regulates the duration of the Rock and the frequency of Roll of nascent HCs, it is not required for their positional acquisition. Instead, Emx2 regulates the morphology of nascent HCs, which facilitates the rearrangement process. Overall design: Single-cell RNA sequencing on FACS sorted Hair-Cells from zebrafish neuromast in 4dpf wild type larvae.