Regulation of the development and regeneration of the zebrafish posterior lateral line

Abstract

Mechanosensory hair cells located in the inner ear mediate the sensations of hearing and balance. If damaged, mammalian inner ear hair cells are unable to regenerate, resulting in permanent sensory deficits. Damage to hair cells can arise from multiple factors such as high decibel and prolonged sound exposure, genetic lesions, age, illnesses, and ototoxic medications including aminoglycoside antibiotics and some chemotherapy drugs. Aquatic vertebrates such as zebrafish (Danio rerio) have a specialized class of mechanosensory hair cells found in the lateral line (LL) system, allowing them to sense changes in water current. Unlike mammalian inner ear hair cells, zebrafish lateral line hair cells can robustly regenerate following damage throughout the life of the animal. Our work focuses on investigating the cellular and molecular mechanisms that drive development and regeneration of the zebrafish LL to make inferences about mammalian hair cells and drive medical interventions and therapies for conditions affecting hearing and balance. We used the zebrafish lateral line to research two different mutations effects on lateral line development. Foxg1 is a transcription factor in mammals that functions to promote normal development of multiple tissues including the inner ear. Murine studies demonstrate Foxg1 function is necessary for inner ear hair cell development and proper cochlear morphology. Foxg1 interacts with many critical pathways and cellular processes of hair cell development and homeostasis. YEATS domain-containing 2 (YEATS2), functions as part of an epigenetic protein complex acting as a reader of post-translational histone modifications. As a subunit of the histone acetyltransferase (HAT) Ada-two-A-containing (ATAC) protein complex, YEATS2 has been minimally investigated outside of cancer research where YEATS2 transcription affects collective cell migration and proliferation. Work investigating ATAC complex proteins in murine development models show early embryonic lethality, but YEATS2 function has not been determined. The necessity for Foxg1 in the mammalian inner ear, and function of YEATS2 during cancer collective cell migration and proliferation make both ideal targets of investigation. In foxg1aᵃ²⁶⁶ mutant zebrafish larvae, lateral line development is significantly delayed, proliferation is decreased, and fewer average hair cells form per neuromast compared to heterozygous siblings. Following regeneration, foxg1aᵃ²⁶⁶ zebrafish show reduced proliferation and differentiation into hair cells. Coinciding with the reduction in average hair cell numbers we observed a reduction in central localized cells labeled with α-Isl1. In yeats2ⁿˡ²⁴ mutant embryos we see truncated migration of the posterior lateral line primordium, but no changes in proliferation. To better characterize the function and effect YEATS2 mutations have on LL development we generated a CRISPR/Cas9 mutant line, yeats2ᴬᵀᴳ, that recapitulates the yeats2ⁿˡ²⁴ phenotype. These reduced cell numbers in the LL suggest that Foxg1a function is critical for the proper development and regeneration of support cells and hair cells. We also demonstrate a novel developmental role for YEATS2 in collective cell migration of the zebrafish posterior lateral line primordium. Our work provides novel data for Foxg1a and YEATS2 in zebrafish and shows both genes are potential therapeutic targets for development and regeneration of human inner ear hair cells.