Hedgehog (Hh) signaling is important for patterning and cell differentiation within many embryonic tissues. Hh is expressed in the notochord and the floor plate and acts as a morphogen in patterning of the ventral central nervous system (CNS), pituitary gland, somites, pancreas, and many other tissues. Gli transcription factors act as main mediators of Hh signaling in vertebrates. Their unique and overlapping functions lead to differential outcome of Hh signaling in different cells and tissues. The goal of my dissertation is to determine how zebrafish Gli proteins work together to transduce Hh signals and to activate or repress the transcription of Hh target genes. I have shown that Gli1 acts only as an activator of Hh signaling similar to what is known from other species, while Gli2 and Gli3 act as both activators and repressors. Gli2 functions as a co-activator of Gli1 in subset of ventral cells in the posterior diencephalon and in the adaxial cells in the embryonic trunk. In contrast, activator role of Gli3 overlaps with Gli1 throughout the ventral CNS during early development of the embryo. Later, GO represses Hh targets in the dorsal spinal cord, but not in the forebrain. In contrast, Gli2 represses genes in both, the dorsal telencephalon and the spinal cord. I also demonstrate that Gli3 repressor function is temporally regulated by active Hh signaling, unlike Hh independent Gli2 repressor function. Thus, my detailed analysis of zebrafish Gli functions reveals complex interactions between Gli proteins in embryonic patterning. I also studied a novel zebrafish mutation umleitung (uml) that was identified because of defects in axon guidance and neural patterning in the ventral forebrain. My detailed phenotypic analysis showed that the uml mutation disrupts Hh signaling and forebrain patterning. I have genetically mapped uml on zebrafish chromosome 24 near the Zmarker z10372. My linkage analysis indicates there are no known components of the Hh signaling cascade in the uml genetic region. This leads to the exciting possibility that uml may encode a previously undefined regulator of Hh signaling. I have initiated a genomic walk toward finding a gene that encodes uml.

Synaptic transmission between a neuron and its target is crucially dependent upon the precise spatial arrangement of proteins in the pre- and postsynaptic apparatus. PDZ domain-containing proteins such as the Drosophila tumor suppressor Discs-Large (DLG) play critical roles in synapse maturation by regulating the assembly of synaptic protein complexes. DLG is composed of a number of modular domains, including three PDZ-domains, an SH3 domain and an enzymatically inactive Guanylate Kinase-like (GUK) domain. Previous studies have shown that the PDZ domains of DLG mediate clustering of Shaker K+ channels and of the cell adhesion molecule Fasciclin II. However, the function of the GUK domain has been unclear. To understand the role of the GUK domain, we carried out a yeast-two hybrid screen for interacting partners of the DLG GUK domain. This screen lead to the identification of a novel synapse-associated protein, GUKHolder (GUKH). GUKH is a 1044 amino acid protein with a molecular weight of 110 kDa. Its sequence includes a GUK-holding domain, a region homologous to the C-terminal of the long isoform of Kelch, a WH1-like domain, and a PDZ-domain binding motif. These latter two features suggest that GUKH may interact not only with DLG, but also with other proteins, including proteins containing PDZ domains. GUKH is expressed at the larval neuromuscular junction and at epithelial cell borders in partial colocalization with DLG. Further, DLG can be co-immunoprecipitated with GUKH from Drosophila extracts, indicating an in vivo interaction between the two proteins. GUKH has also been shown to interact directly with SCRIBBLE (SCRIB), another synaptically expressed PDZ-domain protein known to have a genetic interaction with dlg in epithelial tissues. Synaptic SCRIB immunoreactivity is mislocalized in both gukh and dlg mutants. gukh, scrib, and dlg mutants all exhibit synaptic bouton defects at the ultrastructural level. These data indicate that all three proteins are required for proper synapse maturation, and support a model that the three proteins exist in a tripartite complex, with GUKH forming a link between the other two proteins, and further, between the protein scaffolds organized by the two proteins.