Gsx2 is required for specification of neurons in the inferior olivary nuclei from Ptf1a-expressing neural progenitors in zebrafish

Abstract

Neurons in the inferior olivary nuclei (IO neurons) send climbing fibers to Purkinje cells to elicit functions of the cerebellum. IO neurons and Purkinje cells are derived from neural progenitors expressing the proneural gene ptf1a. In this study, we found that the homeobox gene gsx2 was co-expressed with ptf1a in IO progenitors in zebrafish. Both gsx2 and ptf1a zebrafish mutants showed a strong reduction or loss of IO neurons. The expression of ptf1a was not affected in gsx2 mutants and vice versa. In IO progenitors, the ptf1a mutation increased apoptosis whereas the gsx2 mutation did not, suggesting that ptf1a and gsx2 are regulated independently of each other and have distinct roles. The fibroblast growth factors (Fgf) 3/8a and retinoic acid signals negatively and positively, respectively, regulated gsx2 expression and thereby the development of IO neurons. mafba and hox genes are at least partly involved in the Fgf- and retinoic acid-dependent regulation of IO neuronal development. Our results indicate that gsx2 mediates the rostro-caudal positional signals to specify the identity of IO neurons from ptf1a-expressing neural progenitors.

Gsx2 but not Gsx1 is necessary for early forebrain patterning and long-term survival in zebrafish

“…Across these ages gsx2 is expressed in the telencephalon, hypothalamus, pallium, olfactory bulb, and hindbrain. These patterns are largely consistent with expression of Gsx1 and Gsx2 in mouse 24,29 , medaka 26 , Xenopus 25 , and previous reports in zebrafish 10,27,30 with minor exceptions. In Xenopus, Gsx2 is first detected slightly earlier than Gsx1, however we report in zebrafish that gsx1 is expressed at 10 hpf slightly earlier than gsx2 at 12 hpf.…”

“…Known expression of gsx1 in zebrafish is limited to select CNS regions beyond 30 hpf 10,27 and expression of zebrafish gsx2 is minimally reported from 48-72 hpf 30 . Outside of a transgenic analysis documenting gsx1 and gsx2 expression together in the 36-48 hpf spinal cord 10 , expression of gsx1 and gsx2 during neurodevelopment in zebrafish has not been comprehensively analyzed.…”

“…gsx2 is first detected slightly later than gsx1 in the telencephalon and mesencephalon in mice and in the hindbrain in Xenopus 25,29 . Throughout neurodevelopment gsx2 is expressed in the telencephalon, thalamus, hypothalamus, and cerebellum in mouse 29 , Xenopus 25 , and zebrafish 30 . Like gsx1, expression of gsx2 appears similarly across species as two dorsolateral stripes in the hindbrain.…”

“…Gsx2 is expressed dorsal to Gsx1 in the hindbrain in Xenopus 25 and in the spinal cord of zebrafish 10 , consistent with their roles in DV patterning. Outside of the aforementioned reports 10,27,28,30 , expression of zebrafish gsx1 and gsx2 has not been comprehensively characterized and compared across all embryonic and early larval stages. Here we capitalize on the zebrafish model, which allows whole brain in vivo examination of expression to rigorously define the gsx1 and gsx2 expression profile.…”

“…gsx1 is implicated as a molecular marker of glutamatergic interneurons in the dorsal brainstem in zebrafish that regulate the acoustic startle response, and zebrafish with ablated gsx1-expressing neurons and mouse Gsx1 knockouts similarly exhibit disrupted responsiveness to single and paired pulse acoustic-vibrational stimuli 28,49 . In zebrafish, gsx2 is required for specification of neurons in the inferior olivary nuclei of the medulla 30 . gsx1 and gsx2 mark specific progenitor domains in the spinal cord of transgenic zebrafish similarly to mouse, with gsx1 domains specifying glutamatergic, GABAergic, and glycinergic fates, and gsx2 domains specifying glutamatergic fates only 10 .…”