DEVELOPMENT 2605High levels of interspecies conservation characterise all signal transduction cascades and demonstrate the significance of these pathways over evolutionary time. Here, we review advances in the field of JAK/STAT signalling, focusing on recent developments in Drosophila. In particular, recent results from genetic and genome-wide RNAi screens, as well as studies into the developmental roles played by this pathway, highlight striking levels of physical and functional conservation in processes such as cellular proliferation, immune responses and stem cell maintenance. These insights underscore the value of model organisms for improving our understanding of this human disease-relevant pathway.

Summary The Drosophila tumour suppressor gene fat encodes a large cadherin that regulates growth and a form of tissue organization known as planar cell polarity (PCP). Fat regulates growth via the Hippo kinase pathway [1–4], which controls expression of genes promoting cell proliferation and inhibiting apoptosis (reviewed in [5–11]). The Hippo pathway is highly conserved and is implicated in the regulation of mammalian growth and cancer development [12–18]. Genetic studies suggest that Fat activity is regulated by binding to another large cadherin Dachsous (Ds) [19–25]. The tumour suppressor, discs overgrown (dco)/Casein Kinase I δ/ε, also regulates Hippo activity and PCP [1, 26, 27]. The biochemical nature of how Fat, Ds and Dco interact to regulate these pathways is poorly understood. Here we demonstrate that Fat is cleaved to generate 450kDa and 110kDa fragments (Fat450 and Fat110). Fat110 contains the cytoplasmic and transmembrane domain. The cytoplasmic domain of Fat binds Dco, and is phosphorylated by Dco at multiple sites. Importantly, we show Fat forms cis-dimers, and that Fat phosphorylation is regulated by Dachsous and Dco in vivo. We propose that Ds regulates Dco-dependent phosphorylation of Fat and Fat-associated proteins to control Fat signaling in growth and PCP.

A limited number of evolutionarily conserved signal transduction pathways are repeatedly reused during development to regulate a wide range of processes. Here we describe a new negative regulator of JAK/STAT signaling and identify a potential mechanism by which the pleiotropy of responses resulting from pathway activation is generated in vivo. As part of a genetic interaction screen, we have identified Ken & Barbie (Ken) , which is an ortholog of the mammalian proto-oncogene BCL6 , as a negative regulator of the JAK/STAT pathway. Ken genetically interacts with the pathway in vivo and recognizes a DNA consensus sequence overlapping that of STAT92E in vitro. Tissue culture-based assays demonstrate the existence of Ken-sensitive and Ken-insensitive STAT92E binding sites, while ectopically expressed Ken is sufficient to downregulate a subset of JAK/STAT pathway target genes in vivo. Finally, we show that endogenous Ken specifically represses JAK/STAT-dependent expression of ventral veins lacking (vvl) in the posterior spiracles. Ken therefore represents a novel regulator of JAK/STAT signaling whose dynamic spatial and temporal expression is capable of selectively modulating the transcriptional repertoire elicited by activated STAT92E in vivo.

"Tissue" or "planar" polarity is a characteristic of many epithelial tissues and is not only required for proper cell alignment, but in many instances is absolutely essential for normal function. Planar cell polarity (PCP) is the polarization of cells within the plane of an epithelium in a direction perpendicular to the axis of the apico-basal polarity. The oriented hair alignment in mammalian skin (feather in birds or scales in fish) and highly organized stereocilia bundles in the vertebrate inner ear are examples of such tissue organization. PCP was first described in Drosophila, with non-canonical Wnt signaling (also called PCP signaling, see Chapter 10, Volume 1) shown to be critical for its establishment. Two of the best characterized PCP models in Drosophila are the developing wing and the eye, where the graded activity of the Frizzled (Fz) receptor determines proximo-distal orientation of the wing hairs and mirror-imaged patterning of ommatidia, respectively. In this chapter, we describe simple methods to visualize PCP defects in the Drosophila eye and wing, in both developing and adult tissues. These methods include confocal immunofluorescent analysis of larval or pupal tissues, stained with antibodies specific to PCP components or cytoskeleton markers, and light microscopy of the adult eye and wing.