The Hedgehog signal transduction network

Abstract

Hedgehog (Hh) proteins regulate the development of a wide range of metazoan embryonic and adult structures, and disruption of Hh signaling pathways results in various human diseases. Here, we provide a comprehensive review of the signaling pathways regulated by Hh, consolidating data from a diverse array of organisms in a variety of scientific disciplines. Similar to the elucidation of many other signaling pathways, our knowledge of Hh signaling developed in a sequential manner centered on its earliest discoveries. Thus, our knowledge of Hh signaling has for the most part focused on elucidating the mechanism by which Hh regulates the Gli family of transcription factors, the so-called "canonical" Hh signaling pathway. However, in the past few years, numerous studies have shown that Hh proteins can also signal through Gli-independent mechanisms collectively referred to as "noncanonical" signaling pathways. Noncanonical Hh signaling is itself subdivided into two distinct signaling modules: (i) those not requiring Smoothened (Smo) and (ii) those downstream of Smo that do not require Gli transcription factors. Thus, Hh signaling is now proposed to occur through a variety of distinct context-dependent signaling modules that have the ability to crosstalk with one another to form an interacting, dynamic Hh signaling network.

Fig. 1. The canonical Hh signaling pathway

(A) Hh signaling in Drosophila. In the absence of Hh (left), Ptc prevents Smo membrane localization and activation so that Smo is retained on intracellular vesicles. In this context, full-length Ci (yellow, indicating partial activity) is held in a microtubule-associated complex containing the kinesin-like protein Costal2 (Cos2), the kinase Fused (Fu), and Suppressor of Fused (Sufu), which promotes phosphorylation of Ci by PKA, GSK3, and CK1 and its partial proteasomal processing into a transcriptional repressor form (Ci_R, red). Binding of Hh to the receptor complex composed of Ptc and an Ihog co-receptor (Ihog or Boi, right) results in internalization of the ligand-receptor complex and phosphorylation and translocation of Smo to the plasma membrane, where it interacts with Cos2 to partially disrupt the microtubule-associated complex, leading to release of Ci and activation of the heterotrimeric G_i protein. Ci is subsequently converted into a fully active labile transcriptional activator (Ci_A, blue) by an unknown mechanism. (B) Hh signaling in vertebrates. Hh signaling in vertebrates is similar to Hh signaling in Drosophila, with the important distinction that signaling takes place on primary cilia. When a Hh ligand binds to the receptor complex formed by Ptc and an Ihog co-receptor (Cdo, Boc, or Gas1), Smo translocates to both the plasma membrane and to the primary cilium, where it regulates Gli processing and activation.

Fig. 2. Type I noncanonical Hh signaling

In the absence of a Hh ligand (left) Ptc interacts with cyclin B1 and a proapoptotic complex that includes caspase-9, the CARD (caspase-associated recruitment domain)-domain containing protein Tucan-1, and the adaptor protein Dral. The interaction with cyclin B1 inhibits proliferation by sequestering cyclin B1 outside the nucleus. In some cell types, Ptc recruitment of this proapoptotic complex depends on a preceding cleavage event in the C-terminal domain of Ptc by caspase-3 and results in nucleation and activation of caspase-9. Active caspase-9 speeds the formation of this complex by promoting activation of caspase-3, leading to apoptosis. Hh binding (right) disrupts the interaction of Ptc with cyclin B1 and the proapoptotic complex, likely through a conformational change in Ptc, leading to increased proliferation and survival. Yellow highlighted and shaded caspase shapes indicate active and inactive caspases, respectively.

Fig. 3. Type II noncanonical Hh signaling

Smo regulates the actin cytoskeleton through the small GTPases RhoA and Rac1. This regulation occurs in a context-specifi c manner, through G_i proteins and PI3K in fi broblasts, and through Tiam1 or through the Src kinase family (SFK) members Src and Fyn in neurons. In addition, Smo stimulates calcium release from the endoplasmic reticulum (ER) in spinal neurons through G_i and PLC-γ–catalyzed generation of IP₃ and opening of IP₃-dependent channels.

Fig. 4. The Hh signaling network

In vertebrates, Hh binding to Ptc modulates one or more of the following signaling modules: non-canonical type I Hh signaling exclusively through Ptc; canonical Hh signaling through Ptc, Smo, and Glis (mediated in some instances by G_i proteins); and a noncanonical type II signaling through Smo and G_i proteins. The cellular responses to each signaling module are illustrated at the bottom. The red crosses indicate inhibition of the pathway in the presence of Hh proteins.