Mechanisms of Hedgehog Gradient Formation and Interpretation Carlos Torroja, * Nicole Gorfinkiel, y Isabel Guerrero Centro de Biologı ´a Molecular Severo Ochoa, CSIC, Universidad Auto ´ noma de Madrid, Cantoblanco, E-28049 Madrid, Spain Received 18 January 2005; accepted 12 February 2005 ABSTRACT: Morphogens are molecules that spread from localized sites of production, specifying distinct cell outcomes at different concentrations. Members of the Hedgehog (Hh) family of signaling molecules act as morphogens in different developmen- tal systems. If we are to understand how Hh elicits multiple responses in a temporally and spatially spe- cific manner, the molecular mechanism of Hh gra- dient formation needs to be established. Moreover, understanding the mechanisms of Hh signaling is a central issue in biology, not only because of the role of Hh in morphogenesis, but also because of its involvement in a wide range of human diseases. Here, we review the mechanisms affecting the dynamics of Hh gradient formation, mostly in the context of Dro- sophila wing development, although parallel findings in vertebrate systems are also discussed. ' 2005 Wiley Periodicals, Inc. J Neurobiol 64: 334–356, 2005 Keywords: morphogen gradient; Hedgehog; Patched; Smoothened; extracellular matrix; Drosophila INTRODUCTION A key issue in developmental biology is how cells in a developing field acquire the positional information that will determine their fate. Secreted signaling mole- cules of the TGF-, Wnt, and Hedgehog (Hh) fami- lies have been shown to play essential roles in cell fate specification during development. In many devel- opmental contexts, they act as morphogens that ema- nate from localized sources and form extracellular gradients, which differentially regulate cell fates in a concentration dependent manner (Lawrence and Struhl, 1996; Gurdon and Bourillot, 2001; Teleman et al., 2001; Cadigan, 2002; Vincent and Dubois, 2002; Tabata and Takei, 2004). Proteins of the Hh family are powerful signaling molecules that act as morphogens in certain tissues of several different organisms at different developmental stages (Ingham and McMahon, 2001). However, it is not known whether in these different situations the same mechanism of gradient formation is operative, or whether the mechanism of gradient formation depends on where and when the morphogen is functional. Sev- eral data suggest that although the core mechanisms of Hh production, secretion, spreading, reception, and sig- nal transduction are conserved in Drosophila and verte- brates, differences appear depending on the develop- mental stage and organism studied. Also, the morphoge- netic effects of this molecule vary enormously among different organisms and tissues. In recent years, the question of how morphogen gradients arise in tissues has prompted intense research although the molecular mechanisms of these processes are still unclear. *Present address: The Wellcome Trust Sanger Institute, The Wellcome Trust Genome Campus, Hinxton, Cambridge CB10 1SA, UK. y Present address: Department of Genetics, University of Cam- bridge, Cambridge CB2 3EH, UK. Contract grant sponsor: Spanish D.G.I.C.Y.T; contract grant numbers: BMC2002-03839, MCYT GEN2001-4846-C05-01. Contract grant sponsor: Comunidad Auto ´noma de Madrid; con- tract grant numbers: 08.6/0045/2001.2, GR/SAL/0185/2004. Contract grant sponsor: Fundacio ´n Areces. ' 2005 Wiley Periodicals, Inc. Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/neu.20168 Correspondence to: I. Guerrero (iguerrero@cbm.uam.es). 334