Protein kinase A is a common negative regulator of Hedgehog signaling in the vertebrate embryo.

Abstract

Midline signaling by Hedgehog (Hh) family members has been implicated in patterning the vertebrate embryo. We have explored the potential regulatory role of cAMP-dependent protein kinase A (PKA) in these events. Zebrafish embryos injected with RNAs encoding Sonic hedgehog (Shh), Indian hedgehog (Ihh), or a dominant-negative regulatory subunit of PKA, PKI, have equivalent phenotypes. These include the expansion of proximal fates in the eye, ventral fates in the brain, and adaxial fates in somites and head mesenchyme. Moreover, ectopic expression of PKI partially rescues somite and optic stalk defects in no tail and cyclops mutants that lack midline structures that normally synthesize Shh. Conversely, all cell types promoted by ectopic expression of hhs and PKI are suppressed in embryos injected with RNA encoding a constitutively active catalytic subunit of PKA (PKA*). These results, together with epistasis studies on the block of ectopic Hh signaling by PKA*, indicate that PKA acts in target cells as a common negative regulator of Hedgehog signaling.[Key Words: Sonic hedgehog; Indian hedgehog; protein kinase A; midline signaling; zebrafish] Received December 20, 1995; revised version accepted February 2, 1996.Members of the hedgehog (hh) gene family encode signaling proteins involved in induction and patterning processes in vertebrate and invertebrate embryos. In vertebrates, hhs constitute a multigene family. Sonic hedgehog (Shh), which has received most of the experimental attention, has been isolated from mouse (Echelard et al. 1993; Chang et al. 1994}, chick (Riddle et al. 1993, rat (Roelink et al. 1994), Xenopus (Ekker et al. 1995), and zebrafish (Krauss et al. 1993). The early expression is limited to the notochord, the floor plate and its anterior extension in the brain, and the zone of polarizing activity (ZPA) in the posterior mesenchyme of the limb bud. All of these sites have been shown to act as organizing centers patterning neighboring tissues by the secretion of morphogenetic signals (e.g., Saunders and Gasseling 1983;Brand-Saberi et al. 1993; Pourquie et al. 1993;Yamada et al. 1993). Recent evidence indicates that Shh is involved in these interactions.For example, ectopically expressed Shh leads to a ventralization of large regions of the mid-and hindbrain in mouse and zebrafish, as revealed by the dorsally expanded expression pattern of HNF-3~ and its zebrafish homolog axial (Echelard et al. 1993;Krauss et al. 1993).In explants of intermediate neuroectoderm at spinal cord levels, Shh protein induces, in a dose-dependent fashion, floor plate, and motor neuron development (Marti et al. 1Corresponding author.1995a; Roelink et al. 1995;Tanabe et al. 1995). In explants taken at midbrain and forebrain levels, Shh also induces the appropriate ventrolateral neuronal cell types, dopaminergic (Heynes et al. 1995;Wang et al. 1995} and cholinergic (Ericson et al. 1995) precursors, respectively. In chick somites (Johnson et al. 1994) and mouse presomitic explants (Fan et al. 1995), Shh promotes the expression...

Inversion of Sonic hedgehog action on its canonical pathway by electrical activity

“…5). The mechanisms underlying this multilayered inhibition triggered by Shh involve PKA activation, which is recognized as an inhibitor of the Shh canonical pathway (30)(31)(32)(33). On the other hand, Smo activation recruits heterotrimeric Gα i βγ proteins (28,45), which inhibits adenylate cyclase, eventually leading to decreased PKA activity.…”

“…Additionally, PKA is a major inhibitor of the canonical Shh pathway in vertebrates (30)(31)(32)(33), and expressing a dominant-negative PKA mimics an ectopic Shh-induced ventralized spinal cord phenotype in mice (32). We find that stimulating PKA with the adenylate cyclase activator forskolin inhibits Gli activity (Fig.…”

Sonic Hedgehog Promotes Rod Photoreceptor Differentiation in Mammalian Retinal CellsIn Vitro

“…A current model holds that hedgehog activates decapentaplegic and wingless expression in wing and leg disks by reversing a tonic PK A-mediated inhibition of these genes (Felsenfeld and Kennison, 1995;Jiang and Struhl, 1995;Li et al, 1995;Strutt et al, 1995;Hammerschmidt et al, 1996). Data from vertebrate studies also support this model: the SHH-mediated induction of dopaminergic neurons in the ventral mesencephalon was blocked by increasing the level of PK A activity with forskolin or 8-bromo-cyclic AM P (cAM P) (Hynes et al, 1995).…”

Sonic hedgehog-Gli1 pathway in colorectal adenocarcinomas

“…Single hedgehog gene in Drosophila and three hedgehog genes, Sonic (Shh), Indian (Ihh) and Desert (Dhh) in vertebrates have been identified. All three Hh proteins in vertebrates appear to be processed by the same mechanism, use the same receptors and can elicit similar biological responses, but their relative potencies differ in an assay-dependent manner [1][2][3] . Hhs bind to receptor Patched (Ptch), a 12-transmembrane protein.…”