Mechanisms of Hedgehog gradient formation and interpretation

Suppression of Hedgehog signaling by Cul3 ligases in proliferation control of retinal precursors

Cullin-RING ubiquitin ligases ubiquitinate protein substrates and control their levels through degradation. Here we show that cullin3 (Cul3) suppresses Hedgehog (Hh) signaling through downregulating the level of the signaling pathway effector cubitus interruptus (Ci). High-level Hh signaling promotes Cul3-dependent Ci degradation, leading to the downregulation of Hh signaling. This process is manifested in controlling cell proliferation during Drosophila retinal development. In Cul3 mutants, the population of interommatidial cells is increased, which can be mimicked by overexpression of Ci and suppressed by depleting endogenous Ci. Hh also regulates the population of interommatidial cells in the pupal stage. Alterations in the interommatidial cell population correlate with alterations in precursor proliferation in the second mitotic wave of larval eye discs. Taken together, these results suggest that Cul3 downregulates Ci levels to modulate Hh signaling activity, thus ensuring proper cell proliferation during retinal development.

The cell biology of Smo signalling and its relationships with GPCRs

The Smoothened (Smo) signalling pathway participates in many developmental processes, contributing to the regulation of gene expression by controlling the activity of transcription factors belonging to the Gli family. The key elements of the pathway were identified by means of genetic screens carried out in Drosophila, and subsequent analysis in other model organisms revealed a high degree of conservation in both the proteins involved and in their molecular interactions. Recent analysis of the pathway, using a combination of biochemical and cell biological approaches, is uncovering the intricacies of Smo signalling, placing its elements in particular cellular compartments and qualifying the molecular processes involved. These include the synthesis, secretion and diffusion of the ligand, the activation of the receptor and the modifications in the activity of nuclear effectors. In this review we discuss recent advances in understanding biochemical and cellular aspects of Smo signalling, with particular focus in the similarities in the mechanism of signal transduction between Smo and other transmembrane proteins belonging to the G-Protein coupled receptors superfamily (GPCR).

Hedgehog signaling: a biophysical or biomechanical modulator in embryonic development?

Although embryonic development is inevitably affected by biophysical or biomechanical processes, it has yet to be elucidated to what extent molecular mechanisms of development are modulated by such physical factors. The hedgehog family, including Sonic hedgehog (Shh), is the most well-known morphogens involved in the developmental pattern formation of various organs, such as the nervous system, face, limbs, and skin appendages. There are several unique features in hedgehog signaling including long-range diffusion or positive and negative feedback loops, suggesting the possible modification of hedgehog signaling by biophysical or biomechanical factors. Especially, the period of embryonic day 8-10 is characterized by various biomechanically regulated processes in mouse development, such as axial rotation and vasculoangiogenesis. We executed a series of experiments using a mouse whole embryo culture system to investigate the biomechanical roles of hedgehog signaling during this period. In this review, we examine various examples in which biophysical and biomechanical aspects of hedgehog signaling in development are revealed, including our own data using the mouse whole embryo culture system.

A large complex containing Patched and Smoothened initiates Hedgehog signaling in Drosophila

Hedgehog acts as an organizer during development. Its signaling involves the receptor Patched, signal transducer Smoothened and a cytoplasmic complex containing the transcription factor Cubitus interruptus tethered to the Smoothened carboxyl tail. Without Hedgehog, Patched represses Smoothened resulting in proteolysis of Cubitus interruptus to its repressor form. With Hedgehog, Patched repression of Smoothened is relieved and Cubitus interruptus is activated. Sex-lethal, the master switch for sex determination in Drosophila, has been shown to associate with Cubitus interruptus and the cytoplasmic components of the Hedgehog signaling pathway. Additionally, Sex-lethal responds to the presence of Hedgehog in a Patched-dependent manner. The latter prompted us to examine the role of Patched in signaling. We find that Cubitus interruptus, Sex-lethal, Patched and Smoothened co-immunoprecipitate and co-fractionate, suggesting a large complex of both membrane and cytoplasmic components of the Hedgehog pathway. The entire complex is present at the plasma membrane and the association of Patched changes depending on the activation state of the pathway; it also is not female specific. Colocalization analyses suggest that Sex-lethal alters the endocytic cycling of the Hedgehog components and may augment the Hedgehog signal in females by decreasing the proteolytic cleavage of Cubitus interruptus, availing more of it for activation.

The morphogenetic code and colon cancer development

The initiating genetic lesion in sporadically occurring cancers is impossible to identify. The existence of rare inherited cancer syndromes has helped to uncover some of the mutations that can initiate tumorigenesis. Most of these initiating lesions affect genes belonging to morphogenetic signaling pathways. We review the evidence that the cellular fate of individual epithelial cells in the adult is nonautonomous and depends on extrinsic information, just like cells in a developing embryo. Cancer stem cells need to disrupt these extrinsic restraints to gain an autonomous clonal proliferative advantage over neighboring stem cells.

Detecting tagged Hedgehog with intracellular and extracellular immunocytochemistry for functional analysis

In this chapter, we explain different strategies to analyze the extracellular Hedgehog (Hh) morphogen distribution and Hh intracellular trafficking by immunohistochemistry techniques. For this purpose, it has been very useful to have a transgenic fly line that expresses a Hh-green fluorescent protein (GFP) fusion protein. These flies can be used to study the way Hh spreads through the anterior compartment where it signals, and analyze in detail how Hh is internalized by its receptor Patched. In addition, this Hh-GFP fusion made without lipid modifications (cholesterol or palmitic acid) can be used to investigate the function of these lipids on Hh in terms of spreading, internalization, and signaling abilities.

A conserved mechanism of Hedgehog gradient formation by lipid modifications

Members of the Hedgehog (Hh) family of proteins are conserved morphogens that modulate cell fates in target tissues in different developmental systems. Dysregulation of Hh signaling results in a wide range of human diseases. The mature Hh is modified by lipids in two places, with palmitate at the N-terminus and cholesterol at the C-terminus. The lipid modifications are essential to the proper secretion and spreading of the morphogen throughout the extracellular matrix, interacting with heparan sulfate proteoglycans. However, the role of lipid modifications in regulating the range and activity of Hh proteins remains controversial. Here, we aim to resolve this issue by providing a model that is consistent with current and past reports. We propose that the cholesterol moiety functions to restrict the dilution and deregulated spread of the morphogen in the extracellular space.

How does cholesterol affect the way Hedgehog works?

Members of the Hedgehog (Hh) family of proteins are conserved morphogens that spread and modulate cell fates in target tissue. Mature Hh carries two lipid adducts, a palmitoyl group at the N terminus and cholesterol at the C terminus. Recent findings have addressed how these lipid modifications affect the function and transport of Hh in Drosophila. In contrast to the palmitoyl adduct, cholesterol appears not to be essential for signalling. However, the absence of the cholesterol adduct affects the spread of Hh within tissues. As we discuss here, the exact nature of this effect is controversial.

Human receptors patched and smoothened partially transduce hedgehog signal when expressed in Drosophila cells

In humans, dysfunctions of the Hedgehog receptors Patched and Smoothened are responsible for numerous pathologies. However, signaling mechanisms involving these receptors are less well characterized in mammals than in Drosophila. To obtain structure-function relationship information on human Patched and Smoothened, we expressed these human receptors in Drosophila Schneider 2 cells. We show here that, as its Drosophila counterpart, human Patched is able to repress the signaling pathway in the absence of Hedgehog ligand. In response to Hedgehog, human Patched is able to release Drosophila Smoothened inhibition, suggesting that human Patched is expressed in a functional state in Drosophila cells. We also provide experiments showing that human Smo, when expressed in Schneider cells, is able to bind the alkaloid cyclopamine, suggesting that it is expressed in a native conformational state. Furthermore, contrary to Drosophila Smoothened, human Smoothened does not interact with the kinesin Costal 2 and thus is unable to transduce the Hedgehog signal. Moreover, cell surface fluorescent labeling suggest that human Smoothened is enriched at the Schneider 2 plasma membrane in response to Hedgehog. These results suggest that human Smoothened is expressed in a functional state in Drosophila cells, where it undergoes a regulation of its localization comparable with its Drosophila homologue. Thus, we propose that the upstream part of the Hedgehog pathway involving Hedgehog interaction with Patched, regulation of Smoothened by Patched, and Smoothened enrichment at the plasma membrane is highly conserved between Drosophila and humans; in contrast, signaling downstream of Smoothened is different.

New "hogs" in Hedgehog transport and signal reception

A recent paper in Cell (Yao et al., 2006) and two papers in Developmental Cell (Tenzen et al., 2006; Zhang et al., 2006) identify a new receptor component for Hedgehog, a key morphogen in embryonic development. Many other proteins that bind to Hedgehog in the extracellular matrix or on the cell surface have been identified. In light of these recent discoveries, we discuss how these factors control the stability, transport, reception, and availability of Hedgehog in modulating Hedgehog-mediated responses.

Hedgehog lipid modifications are required for Hedgehog stabilization in the extracellular matrix

The Hedgehog (Hh) family of morphogenetic proteins has important instructional roles in metazoan development. Despite Hh being modified by Ct-cholesterol and Nt-palmitate adducts, Hh migrates far from its site of synthesis and programs cellular outcomes, depending on its local concentrations. We show that in the receiving cells of the Drosophila wing imaginal disc, lipid-unmodified Hh spreads across many more cell diameters than the wild type and this spreading leads to the activation of low but not high threshold responses. Unlipidated Hh forms become internalized through the apical plasma membrane, while wild-type Hh enters through the basolateral cell surface - in all cases via a dynamin-dependent mechanism. Full activation of the Hh pathway and the spread of Hh throughout the extracellular matrix depend on the ability of lipid-modified Hh to interact with heparan sulfate proteoglycans (HSPG). However, neither Hh-lipid modifications nor HSPG function are required to activate the targets that respond to low levels of Hh. All these data show that the interaction of lipid-modified Hh with HSPG is important both for precise Hh spreading through the epithelium surface and for correct Hh reception.

A highly conserved amino-terminal region of sonic hedgehog is required for the formation of its freely diffusible multimeric form

Although members of the Hedgehog (Hh) family were initially described as morphogens, many of these early conclusions were based on experiments that used non-physiologically relevant forms of Hh. Native Hh is modified by cholesterol (HhNp) and palmitate. These hydrophobic modifications are responsible for the ability of Hh to associate with cellular membranes, a property that initially appeared inconsistent with its ability to act far from its site of synthesis. Although it is now clear that Hh family members are capable of acting directly in long-range signaling, the form of Hh capable of this activity remains controversial. We have previously provided evidence for a freely diffusible multimeric form of Sonic Hedgehog (Shh) termed s-ShhNp, which is capable of accumulating in a gradient fashion through a morphogenic field. Here, we provide further evidence that s-ShhNp is the physiologically relevant form of Shh. We show that the biological activity of freely diffusible ShhNp resides in its multimeric form and that this multimeric form is exceedingly stable, even to high concentrations of salt and detergent. Furthermore, we now validate the Shh-Shh interactions previously observed in the crystal structure of human Shh, showing that a highly conserved amino-terminal domain of Shh is important for the formation of s-ShhNp. We also conclusively show that palmitoylation is required for s-ShhNp formation. Thus, our results identify both protein-protein and protein-lipid interactions that are required for s-ShhNp formation, and provide the first structural analyses supporting the existence of Shh multimers.

Mechanisms of morphogen movement

Morphogens are defined as signaling molecules that are produced locally, yet act directly at a distance to pattern the surrounding field of cells in a concentration-dependent manner. In recent years many laboratories have devoted their attention to how morphogens actually reach distant cells. Several models have been proposed, including diffusion in the extracellular space and planar transcytosis. A combination of genetic, developmental, and cell-biological approaches have been taken to tackle this issue. I will present the models and discuss the types of experiments that have been designed to test them. It stands out that most of the work has been carried out in Drosophila. Morphogens contribute to patterning of the vertebrate nervous system, and the same signaling molecules have recently been shown to play important, possibly instructive, roles in axon guidance. Little, if anything, is known about the movement of morphogens in the context of nervous system development. The long-standing tradition of biophysical studies on diffusion in the brain extracellular space, along with the sophisticated in vitro culture systems developed in neurobiology laboratories, may provide new tools and ideas to test these models in a new context.