Ahsa1 and Hsp90 activity confers more severe craniofacial phenotypes in a zebrafish model of hypoparathyroidism, sensorineural deafness and renal dysplasia (HDR)

The severity of most human birth defects is highly variable. Our ability to diagnose, treat and prevent defects relies on our understanding of this variability. Mutation of the transcription factor GATA3 in humans causes the highly variable hypoparathyroidism, sensorineural deafness and renal dysplasia (HDR) syndrome. Although named for a triad of defects, individuals with HDR can also exhibit craniofacial defects. Through a forward genetic screen for craniofacial mutants, we isolated a zebrafish mutant in which the first cysteine of the second zinc finger of Gata3 is mutated. Because mutation of the homologous cysteine causes HDR in humans, these zebrafish mutants could be a quick and effective animal model for understanding the role of gata3 in the HDR disease spectrum. We demonstrate that, unexpectedly, the chaperone proteins Ahsa1 and Hsp90 promote severe craniofacial phenotypes in our zebrafish model of HDR syndrome. The strengths of the zebrafish system, including rapid development, genetic tractability and live imaging, make this an important model for variability.

Genome-wide, whole mount in situ analysis of transcriptional regulators in zebrafish embryos

Transcription is the primary step in the retrieval of genetic information. A substantial proportion of the protein repertoire of each organism consists of transcriptional regulators (TRs). It is believed that the differential expression and combinatorial action of these TRs is essential for vertebrate development and body homeostasis. We mined the zebrafish genome exhaustively for genes encoding TRs and determined their expression in the zebrafish embryo by sequencing to saturation and in situ hybridisation. At the evolutionary conserved phylotypic stage, 75% of the 3302 TR genes encoded in the genome are already expressed. The number of expressed TR genes increases only marginally in subsequent stages and is maintained during adulthood suggesting important roles of the TR genes in body homeostasis. Fewer than half of the TR genes (45%, n=1711 genes) are expressed in a tissue-restricted manner in the embryo. Transcripts of 207 genes were detected in a single tissue in the 24h embryo, potentially acting as regulators of specific processes. Other TR genes were expressed in multiple tissues. However, with the exception of certain territories in the nervous system, we did not find significant synexpression suggesting that most tissue-restricted TRs act in a freely combinatorial fashion. Our data indicate that elaboration of body pattern and function from the phylotypic stage onward relies mostly on redeployment of TRs and post-transcriptional processes.

Regenerative neurogenesis from neural progenitor cells requires injury-induced expression of Gata3

The adult zebrafish brain, unlike mammalian counterparts, can regenerate after injury owing to the neurogenic capacity of stem cells with radial glial character. We hypothesized that injury-induced regenerative programs might be turned on after injury in zebrafish brain and enable regenerative neurogenesis. Here we identify one such gene-the transcription factor gata3-which is expressed only after injury in different zebrafish organs. Gata3 is required for reactive proliferation of radial glia cells, subsequent regenerative neurogenesis, and migration of the newborn neurons. We found that these regeneration-specific roles of Gata3 are dependent on the injury because Gata3 overexpression in the unlesioned adult zebrafish brain is not sufficient to induce neurogenesis. Thus, gata3 acts as a specific injury-induced proregenerative factor that is essential for the regenerative capacity in vertebrates.

olig2-Expressing hindbrain cells are required for migrating facial motor neurons

The complicated trajectory of facial motor neuron migration requires coordination of intrinsic signals and cues from the surrounding environment. Migration begins in rhombomere (r) 4 where facial motor neurons are born and proceeds in a caudal direction. Once facial motor neurons reach their target rhombomeres, they migrate laterally and radially from the ventral neural tube. In zebrafish, as facial motor neurons migrate through r5/r6, they pass near cells that express olig2, which encodes a bHLH transcription factor. In this study, we found that olig2 function is required for facial motor neurons to complete their caudal migration into r6 and r7 and form stereotypical clusters. Additionally, embryos that lack mafba function, in which facial motor neurons also fail to complete caudal migration, lack olig2 expression in r5 and r6. Our data raise the possibility that cells expressing olig2 are intermediate targets that help guide facial motor neuron migration.

Attenuation of Notch and Hedgehog signaling is required for fate specification in the spinal cord

During the development of the spinal cord, proliferative neural progenitors differentiate into postmitotic neurons with distinct fates. How cells switch from progenitor states to differentiated fates is poorly understood. To address this question, we studied the differentiation of progenitors in the zebrafish spinal cord, focusing on the differentiation of Kolmer-Agduhr″ (KA″) interneurons from lateral floor plate (LFP) progenitors. In vivo cell tracking demonstrates that KA″ cells are generated from LFP progenitors by both symmetric and asymmetric cell divisions. A photoconvertible reporter of signaling history (PHRESH) reveals distinct temporal profiles of Hh response: LFP progenitors continuously respond to Hh, while KA″ cells lose Hh response upon differentiation. Hh signaling is required in LFP progenitors for KA″ fate specification, but prolonged Hh signaling interferes with KA″ differentiation. Notch signaling acts permissively to maintain LFP progenitor cells: activation of Notch signaling prevents differentiation, whereas inhibition of Notch signaling results in differentiation of ectopic KA″ cells. These results indicate that neural progenitors depend on Notch signaling to maintain Hh responsiveness and rely on Hh signaling to induce fate identity, whereas proper differentiation depends on the attenuation of both Notch and Hh signaling.

During tissue formation, progenitor cells generate both differentiated cells and progenitor cells. It is poorly understood how this balance between self-renewal and differentiation generates the correct number of different cell types. Here, we use zebrafish spinal cord development as a model system to investigate how neural progenitor cells switch from progenitor states to differentiated fates. Combining genetic manipulation and a novel method to study cell signaling in live embryos, our data show that this process requires the dynamic regulation of two signaling pathways: the Notch signaling pathway and the Hedgehog (Hh) signaling pathway. In neural progenitors, Notch signaling maintains the competence of neural progenitors to respond to Hh signaling. In parallel, Hedgehog signaling functions to induce cell fate identity. As cells switch from progenitor states to differentiated states, both Notch and Hh signaling become attenuated. Thus, the dynamic deployment of Notch and Hh signaling controls the renewal and differentiation of progenitor cells.

Occurrence of two distinct urotensin II-related peptides in zebrafish provides new insight into the evolutionary history of the urotensin II gene family

The urotensin II (UII) family is currently known to consist of two paralogous peptides, namely UII and UII-related peptide (URP). In contrast to UII, which has been identified in all vertebrate classes so far, URP has only been characterized in tetrapods. We report here the occurrence of two distinct URP genes in teleosts, which we have named URP1 and URP2. Synteny analysis revealed that teleost URP1 and URP2 genes and tetrapod URP genes represent three distinct paralog genes that, together with the UII gene, probably arose from the two rounds of tetraploidization, which took place early in vertebrate evolution. The absence of URP in fish indicates that the corresponding gene has been lost in the teleost lineage, whereas it is likely that both the URP1 and URP2 genes have been lost in the tetrapod lineage. Quantitative RT-PCR analysis revealed that the URP2 gene is mainly expressed in the spinal cord and the brain in adult zebrafish. In situ hybridization experiments showed that in zebrafish embryos, URP2 mRNA-containing cells are located in the floor plate of the neural tube. In adult, URP2-expressing cells occur in close contact with the ventral side of the ependymal canal along the whole spinal cord, whereas in the brain, they are located below the fourth ventricle. These URP-expressing cells may correspond to cerebrospinal fluid-contacting neurons. In conclusion, our study reveals the occurrence of four distinct UII paralogous systems in vertebrates that may exert distinct functions, both in tetrapods and teleosts.

Neurogenesis

For more than a decade, the zebrafish has proven to be an excellent model organism to investigate the mechanisms of neurogenesis during development. The often cited advantages, namely external development, genetic, and optical accessibility, have permitted direct examination and experimental manipulations of neurogenesis during development. Recent studies have begun to investigate adult neurogenesis, taking advantage of its widespread occurrence in the mature zebrafish brain to investigate the mechanisms underlying neural stem cell maintenance and recruitment. Here we provide a comprehensive overview of the tools and techniques available to study neurogenesis in zebrafish both during development and in adulthood. As useful resources, we provide tables of available molecular markers, transgenic, and mutant lines. We further provide optimized protocols for studying neurogenesis in the adult zebrafish brain, including in situ hybridization, immunohistochemistry, in vivo lipofection and electroporation methods to deliver expression constructs, administration of bromodeoxyuridine (BrdU), and finally slice cultures. These currently available tools have put zebrafish on par with other model organisms used to investigate neurogenesis.