Neurogenesis in the inner ear: the zebrafish statoacoustic ganglion provides new neurons from a Neurod/Nestin-positive progenitor pool well into adulthood

Abstract

The vertebrate inner ear employs sensory hair cells and neurons to mediate hearing and balance. In mammals, damaged hair cells and neurons are not regenerated. In contrast, hair cells in the inner ear of zebrafish are produced throughout life and regenerate after trauma. However, it is unknown whether new sensory neurons are also formed in the adult zebrafish statoacoustic ganglion (SAG), the sensory ganglion connecting the inner ear to the brain. Using transgenic lines and marker analysis, we identify distinct cell populations and anatomical landmarks in the juvenile and adult SAG. In particular, we analyze a Neurod/Nestin-positive progenitor pool that produces large amounts of new neurons at juvenile stages, which transitions to a quiescent state in the adult SAG. Moreover, BrdU pulse chase experiments reveal the existence of a proliferative but otherwise marker-negative cell population that replenishes the Neurod/Nestin-positive progenitor pool at adult stages. Taken together, our study represents the first comprehensive characterization of the adult zebrafish SAG showing that zebrafish, in sharp contrast to mammals, display continued neurogenesis in the SAG well beyond embryonic and larval stages.

Altered Morpho-Functional Features of Neurogenesis in Zebrafish Embryos Exposed to Non-Combustion-Derived Magnetite

“…To evaluate the impact of magnetite during embryonic neurogenesis, as mentioned before, we treated embryos with magnetite (MT) (100, 200, and 400 µg/mL) and then fixed them at 96 hpf. To evaluate the effects of magnetite on neural progenitor cells, we analyzed the expression levels of the nestin gene, described as the main marker of these precursors [ 35 ]. Embryos treated with magnetite showed a significant dose-dependent reduction in nestin -expressing cells in the ventricular forebrain (FV) and the post-midbrain (PM) compared to non-treated embryos ( Figure 7 a).…”

New Insights into the Identity of the DFNA58 Gene

“…For the analysis of the transcript levels, we used, as a control, genes that are known to be related to hearing or HL and whose expression pattern in the mouse IE was similar to that observed for the genes Cnrip1 , Ppp3r1 , and Plek [ 33 ]. Thus, eight genes were selected: Eya4 (DFNA10) [ 38 ] and Dfna5 ( Gsdmea and Gsdmeb ) [ 39 ] due to high evidence of association with HL ( ; accessed on 1 July 2021); Pou4f1 and Elavl4 due to high expression in sensory neurons on the statoacoustic ganglion of zebrafish, equivalent to the spiral ganglion in mice [ 31 , 40 ]; Scl1a3a because its expression is mostly in the IE of zebrafish and with a possible role in hearing in mice [ 40 , 41 , 42 , 43 ]; and Gab1 , known to be an essential structure in the tyrosine kinase/HGF receptor pathway of the MET/MET/HGF proto-oncogene and responsible for the HL associated with DFNB26 and its modifier Mettl1 [ 44 ]. The choice of these two genes had, as a parameter, the fact that the CNRIP1 and PPP3R1 genes are related to cancer in humans.…”

Vestibular physiology and function in zebrafish

“…The inner ear becomes less accessible as bone hardens and tissues become opaque with age. But research indicates that the vestibular system continues to develop and change as the fish grows in adulthood ( Ehrlich and Schoppik, 2019 ; Schwarzer et al, 2020 ). Investigation of adult vestibular hair cells, afferents, and circuits, in addition to the current neurobiological research in larval and juvenile fish, will complete our understanding of mature vestibular function.…”

“…Zebrafish are known to regenerate hair cells continuously throughout their lifespan, but do not necessarily regenerate afferent neurons. Studies have demonstrated that new neurons are developed in the SAG from 48 hpf well into late juvenile stages (28 dpf) ( Schwarzer et al, 2020 ). However, new neurons are few in the adult, indicating that new hair cells regenerated in mature fish are likely innervated by preexisting neurons ( Schwarzer et al, 2020 ).…”

“…Studies have demonstrated that new neurons are developed in the SAG from 48 hpf well into late juvenile stages (28 dpf) ( Schwarzer et al, 2020 ). However, new neurons are few in the adult, indicating that new hair cells regenerated in mature fish are likely innervated by preexisting neurons ( Schwarzer et al, 2020 ). Synapse formation and regeneration has been studied extensively in the lateral line, where hair cells and their synapses are easily accessible on the external surface of the fish ( Lush and Piotrowski, 2014 ; Pujol-Martí et al, 2014 ; Suli et al, 2016 ; Hardy et al, 2021 ; Holmgren et al, 2021 ).…”