Identification, tissue distribution and developmental expression of tjp1/zo-1, tjp2/zo-2 and tjp3/zo-3 in the zebrafish, Danio rerio.

The tight junction (TJ) or zona occludens (ZO) proteins TJP1/ZO-1, TJP2/ZO-2 and TJP3/ZO-3 belong to the membrane associated guanylate kinase-like (MAGUK) protein family and link TJ integral membrane proteins to the actin cytoskeleton. TJPs also serve as scaffolds for signaling proteins and transcription factors that regulate vesicular traffic and cell proliferation and differentiation. Here, we report the identification of two tjp1/zo-1 (tjp1.1 and tjp1.2) and tjp2/zo-2 (tjp2.1 and tjp2.2) genes each and one tjp3/zo-3 gene, and characterize their tissue specific distribution and developmental expression in zebrafish embryos. Transcripts for all five TJPs are maternally supplied and localized expression due to embryonic transcription is observed following the midblastula transition stage of development. In addition to a widespread distribution, individual genes show tissue specific expression patterns and a dynamic regulation during the developmental stages from 2 cells to 4 dpf analyzed. The most noticeable differences in expression patterns are observed in the posterior part of the embryo during somitogenesis. While all TJPs are expressed in the pronephric ducts and epidermis by 18 hpf, tjp1.1 is highly expressed in the hypochord and blood island, tjp1.2 in the somites and the posterior part of the notochord, tjp2.1 in the somites and the ventral part of the spinal cord, and tjp2.2 in the somites only. Individual TJPs are also strongly expressed in different layers of the eye and, at later stages, in central nervous system (CNS) tissues. Interestingly, the differential tissue and developmental expression of the two tjp1 and tjp2 genes indicates that the duplicated genes have been adapted for distinct transcriptional regulations during evolution.

SHH and FGF8 play distinct roles during development of noradrenergic neurons in the locus coeruleus of the zebrafish.

Several signaling pathways have been implicated in the development of dopaminergic and serotonergic neurons. Here, we analyzed the formation of noradrenergic (NAergic) cells in the locus coeruleus (LC) of zebrafish. In the sonic hedgehog (shh) mutant, cells positive for tyrosine hydroxylase, a marker for putative NAergic cells in the LC were reduced. Similarly, the inhibition of translation of all hh genes and the perturbation of Shh signaling by forskolin resulted in a decrease in the number of cells. Conversely, when SHH was overexpressed, an increase in number was observed. Thus, Shh is involved in maintaining the appropriate number of cells in the LC. While elevated levels of bone morphogenetic protein 4 (BMP4) did not attenuate tyrosine hydroxylase-positive cells, exogenous fibroblast growth factor 8 (FGF8) rescued NAergic neurons in the acerebellar (ace) mutant, providing direct in vivo evidence that Fgf8 is required for the induction of NAergic neurons in the LC.