E3 ubiquitin ligase Wwp1 regulates ciliary dynamics of the Hedgehog receptor Smoothened

The intraflagellar transport cycle

Primary and motile cilia are eukaryotic organelles that perform crucial roles in cellular signalling and motility. Intraflagellar transport (IFT) contributes to the formation of the highly specialized ciliary proteome by active and selective transport of soluble and membrane proteins into and out of cilia. IFT is performed by the IFT-A and IFT-B protein complexes, which together link cargoes to the microtubule motors kinesin and dynein. In this Review, we discuss recent structural and mechanistic insights on how the IFT complexes are first recruited to the base of the cilium, how they polymerize into an anterograde IFT train, and how this complex imports cargoes from the cytoplasm. We will describe insights into how kinesin-driven anterograde trains are carried to the ciliary tip, where they are remodelled into dynein-driven retrograde trains for cargo export. We will also present how the interplay between IFT-A and IFT-B complexes, motor proteins and cargo adaptors is regulated for bidirectional ciliary transport.

A TBK1-independent primordial function of STING in lysosomal biogenesis

Stimulator of interferon genes (STING) is activated in many pathophysiological conditions, leading to TBK1-dependent interferon production in higher organisms. However, primordial functions of STING independent of TBK1 are poorly understood. Here, through proteomics and bioinformatics approaches, we identify lysosomal biogenesis as an unexpected function of STING. Transcription factor EB (TFEB), an evolutionarily conserved regulator of lysosomal biogenesis and host defense, is activated by STING from multiple species, including humans, mice, and frogs. STING-mediated TFEB activation is independent of TBK1, but it requires STING trafficking and its conserved proton channel. GABARAP lipidation, stimulated by the channel of STING, is key for STING-dependent TFEB activation. STING stimulates global upregulation of TFEB-target genes, mediating lysosomal biogenesis and autophagy. TFEB supports cell survival during chronic sterile STING activation, a common condition in aging and age-related diseases. These results reveal a primordial function of STING in the biogenesis of lysosomes, essential organelles in immunity and cellular stress resistance.

Two functional forms of the Meckel-Gruber syndrome protein TMEM67 generated by proteolytic cleavage by ADAMTS9 mediate Wnt signaling and ciliogenesis

TMEM67 mutations are the major cause of Meckel-Gruber syndrome. TMEM67 is involved in both ciliary transition zone assembly, and non-canonical Wnt signaling mediated by its extracellular domain. How TMEM67 performs these two separate functions is not known. We identify a novel cleavage motif in the extracellular domain of TMEM67 cleaved by the extracellular matrix metalloproteinase ADAMTS9. This cleavage regulates the abundance of two functional forms: A C-terminal portion which localizes to the ciliary transition zone regulating ciliogenesis, and a non-cleaved form which regulates Wnt signaling. By characterizing three TMEM67 ciliopathy patient variants within the cleavage motif utilizing mammalian cell culture and C. elegans, we show the cleavage motif is essential for cilia structure and function, highlighting its clinical significance. We generated a novel non-cleavable TMEM67 mouse model which develop severe ciliopathies phenocopying Tmem67 -/- mice, but in contrast, undergo normal Wnt signaling, substantiating the existence of two functional forms of TMEM67.

Emerging mechanistic understanding of cilia function in cellular signalling

Primary cilia are solitary, immotile sensory organelles present on most cells in the body that participate broadly in human health, physiology and disease. Cilia generate a unique environment for signal transduction with tight control of protein, lipid and second messenger concentrations within a relatively small compartment, enabling reception, transmission and integration of biological information. In this Review, we discuss how cilia function as signalling hubs in cell-cell communication using three signalling pathways as examples: ciliary G-protein-coupled receptors (GPCRs), the Hedgehog (Hh) pathway and polycystin ion channels. We review how defects in these ciliary signalling pathways lead to a heterogeneous group of conditions known as 'ciliopathies', including metabolic syndromes, birth defects and polycystic kidney disease. Emerging understanding of these pathways' transduction mechanisms reveals common themes between these cilia-based signalling pathways that may apply to other pathways as well. These mechanistic insights reveal how cilia orchestrate normal and pathophysiological signalling outputs broadly throughout human biology.

Numb positively regulates Hedgehog signaling at the ciliary pocket

Hedgehog (Hh) signaling relies on the primary cilium, a cell surface organelle that serves as a signaling hub for the cell. Using proximity labeling and quantitative proteomics, we identify Numb as a ciliary protein that positively regulates Hh signaling. Numb localizes to the ciliary pocket and acts as an endocytic adaptor to incorporate Ptch1 into clathrin-coated vesicles, thereby promoting Ptch1 exit from the cilium, a key step in Hh signaling activation. Numb loss impedes Sonic hedgehog (Shh)-induced Ptch1 exit from the cilium, resulting in reduced Hh signaling. Numb loss in spinal neural progenitors reduces Shh-induced differentiation into cell fates reliant on high Hh activity. Genetic ablation of Numb in the developing cerebellum impairs the proliferation of granule cell precursors, a Hh-dependent process, resulting in reduced cerebellar size. This study highlights Numb as a regulator of ciliary Ptch1 levels during Hh signal activation and demonstrates the key role of ciliary pocket-mediated endocytosis in cell signaling.

1-L Transcription of SARS-CoV-2 Spike Protein S1 Subunit

The COVID-19 pandemic prompted rapid research on SARS-CoV-2 pathogenicity. Consequently, new data can be used to advance the molecular understanding of SARS-CoV-2 infection. The present bioinformatics study discusses the "spikeopathy" at the molecular level and focuses on the possible post-transcriptional regulation of the SARS-CoV-2 spike protein S1 subunit in the host cell/tissue. A theoretical protein-RNA recognition code was used to check the compatibility of the SARS-CoV-2 spike protein S1 subunit with mRNAs in the human transcriptome (1-L transcription). The principle for this method is elucidated on the defined RNA binding protein GEMIN5 (gem nuclear organelle-associated protein 5) and RNU2-1 (U2 spliceosomal RNA). Using the method described here, it was shown that 45% of the genes/proteins identified by 1-L transcription of the SARS-CoV-2 spike protein S1 subunit are directly linked to COVID-19, 39% are indirectly linked to COVID-19, and 16% cannot currently be associated with COVID-19. The identified genes/proteins are associated with stroke, diabetes, and cardiac injury.

GRK2 Kinases in the Primary Cilium Initiate SMOOTHENED-PKA Signaling in the Hedgehog Cascade

During Hedgehog (Hh) signal transduction in development and disease, the atypical G protein-coupled receptor (GPCR) SMOOTHENED (SMO) communicates with GLI transcription factors by binding the protein kinase A catalytic subunit (PKA-C) and physically blocking its enzymatic activity. Here we show that GPCR kinase 2 (GRK2) orchestrates this process during endogenous Hh pathway activation in the vertebrate primary cilium. Upon SMO activation, GRK2 rapidly relocalizes from the ciliary base to the shaft, triggering SMO phosphorylation and PKA-C interaction. Reconstitution studies reveal that GRK2 phosphorylation enables active SMO to bind PKA-C directly. Lastly, the SMO-GRK2-PKA pathway underlies Hh signal transduction in a range of cellular and in vivo models. Thus, GRK2 phosphorylation of ciliary SMO, and the ensuing PKA-C binding and inactivation, are critical initiating events for the intracellular steps in Hh signaling. More broadly, our study suggests an expanded role for GRKs in enabling direct GPCR interactions with diverse intracellular effectors.

WWP1 E3 ligase at the crossroads of health and disease

The E3 ubiquitin ligase WWP1 (WW Domain-containing E3 Ubiquitin Protein Ligase 1) is a member of the HECT (Homologous to the E6-associated protein Carboxyl Terminus) E3 ligase family. It is conserved across several species and plays crucial roles in various physiological processes, including development, cell growth and proliferation, apoptosis, and differentiation. It exerts its functions through ubiquitination or protein-protein interaction with PPXY-containing proteins. WWP1 plays a role in several human diseases, including cardiac conditions, neurodevelopmental, age-associated osteogenic disorders, infectious diseases, and cancers. In solid tumors, WWP1 plays a dual role as both an oncogene and a tumor suppressor, whereas in hematological malignancies such as AML, it is identified as a dedicated oncogene. Importantly, WWP1 inhibition using small molecule inhibitors such as Indole-3-Carbinol (I3C) and Bortezomib or siRNAs leads to significant suppression of cancer growth and healing of bone fractures, suggesting that WWP1 might serve as a potential therapeutic target for several diseases. In this review, we discuss the evolutionary perspective, structure, and functions of WWP1 and its multilevel regulation by various regulators. We also examine its emerging roles in cancer progression and its therapeutic potential. Finally, we highlight WWP1's role in normal physiology, contribution to pathological conditions, and therapeutic potential for cancer and other diseases.

Emerging principles of primary cilia dynamics in controlling tissue organization and function

Primary cilia project from the surface of most vertebrate cells and are key in sensing extracellular signals and locally transducing this information into a cellular response. Recent findings show that primary cilia are not merely static organelles with a distinct lipid and protein composition. Instead, the function of primary cilia relies on the dynamic composition of molecules within the cilium, the context-dependent sensing and processing of extracellular stimuli, and cycles of assembly and disassembly in a cell- and tissue-specific manner. Thereby, primary cilia dynamically integrate different cellular inputs and control cell fate and function during tissue development. Here, we review the recently emerging concept of primary cilia dynamics in tissue development, organization, remodeling, and function.

Primary cilia in skeletal development and disease

Primary cilia are non-motile, microtubule-based sensory organelle present in most vertebrate cells with a fundamental role in the modulation of organismal development, morphogenesis, and repair. Here we focus on the role of primary cilia in embryonic and postnatal skeletal development. We examine evidence supporting its involvement in physiochemical and developmental signaling that regulates proliferation, patterning, differentiation and homeostasis of osteoblasts, chondrocytes, and their progenitor cells in the skeleton. We discuss how signaling effectors in mechanotransduction and bone development, such as Hedgehog, Wnt, Fibroblast growth factor and second messenger pathways operate at least in part at the primary cilium. The relevance of primary cilia in bone formation and maintenance is underscored by a growing list of rare genetic skeletal ciliopathies. We collate these findings and summarize the current understanding of molecular factors and mechanisms governing primary ciliogenesis and ciliary function in skeletal development and disease.

The tectonic complex regulates membrane protein composition in the photoreceptor cilium

The primary cilium is a signaling organelle with a unique membrane composition maintained by a diffusional barrier residing at the transition zone. Many transition zone proteins, such as the tectonic complex, are linked to preserving ciliary composition but the mechanism remains unknown. To understand tectonic's role, we generate a photoreceptor-specific Tctn1 knockout mouse. Loss of Tctn1 results in the absence of the entire tectonic complex and associated MKS proteins yet has minimal effects on the transition zone structure of rod photoreceptors. We find that the protein composition of the photoreceptor cilium is disrupted as non-resident membrane proteins accumulate in the cilium over time, ultimately resulting in photoreceptor degeneration. We further show that fluorescent rhodopsin moves faster through the transition zone in photoreceptors lacking tectonic, which suggests that the tectonic complex acts as a physical barrier to slow down membrane protein diffusion in the photoreceptor transition zone to ensure proper removal of non-resident membrane proteins.

Primary cilia as dynamic and diverse signalling hubs in development and disease

Primary cilia, antenna-like sensory organelles protruding from the surface of most vertebrate cell types, are essential for regulating signalling pathways during development and adult homeostasis. Mutations in genes affecting cilia cause an overlapping spectrum of >30 human diseases and syndromes, the ciliopathies. Given the immense structural and functional diversity of the mammalian cilia repertoire, there is a growing disconnect between patient genotype and associated phenotypes, with variable severity and expressivity characteristic of the ciliopathies as a group. Recent technological developments are rapidly advancing our understanding of the complex mechanisms that control biogenesis and function of primary cilia across a range of cell types and are starting to tackle this diversity. Here, we examine the structural and functional diversity of primary cilia, their dynamic regulation in different cellular and developmental contexts and their disruption in disease.

Influenza virus reduces ubiquitin E3 ligase MARCH10 expression to decrease ciliary beat frequency

Respiratory viruses, such as influenza, decrease airway cilia function and expression, which leads to reduced mucociliary clearance and inhibited overall immune defense. Ubiquitination is a posttranslational modification using E3 ligases, which plays a role in the assembly and disassembly of cilia. We examined the role of membrane-associated RING-CH (MARCH) family of E3 ligases during influenza infection and determined that MARCH10, specifically expressed in ciliated epithelial cells, is significantly decreased during influenza infection in mice, human lung epithelial cells, and human lung tissue. Cellular depletion of MARCH10 in differentiated human bronchial epithelial cells (HBECs) using CRISPR/Cas9 showed a decrease in ciliary beat frequency. Furthermore, MARCH10 cellular knockdown in combination with influenza infection selectively decreased immunoreactive levels of the ciliary component, dynein axonemal intermediate chain 1. Cellular overexpression of MARCH10 significantly decreased influenza hemagglutinin protein levels in the differentiated HBECs and knockdown of MARCH10 increased IL-1β cytokine expression, whereas overexpression had the reciprocal effect. These findings suggest that MARCH10 may have a protective role in airway pulmonary host defense and innate immunity during influenza infection.

The ancestral ESCRT protein TOM1L2 selects ubiquitinated cargoes for retrieval from cilia

Many G protein-coupled receptors (GPCRs) reside within cilia of mammalian cells and must undergo regulated exit from cilia for the appropriate transduction of signals such as hedgehog morphogens. Lysine 63-linked ubiquitin (UbK63) chains mark GPCRs for regulated removal from cilia, but the molecular basis of UbK63 recognition inside cilia remains elusive. Here, we show that the BBSome-the trafficking complex in charge of retrieving GPCRs from cilia-engages the ancestral endosomal sorting factor target of Myb1-like 2 (TOM1L2) to recognize UbK63 chains within cilia of human and mouse cells. TOM1L2 directly binds to UbK63 chains and the BBSome, and targeted disruption of the TOM1L2/BBSome interaction results in the accumulation of TOM1L2, ubiquitin, and the GPCRs SSTR3, Smoothened, and GPR161 inside cilia. Furthermore, the single-cell alga Chlamydomonas also requires its TOM1L2 ortholog in order to clear ubiquitinated proteins from cilia. We conclude that TOM1L2 broadly enables the retrieval of UbK63-tagged proteins by the ciliary trafficking machinery.

A targeted multi-proteomics approach generates a blueprint of the ciliary ubiquitinome

Establishment and maintenance of the primary cilium as a signaling-competent organelle requires a high degree of fine tuning, which is at least in part achieved by a variety of post-translational modifications. One such modification is ubiquitination. The small and highly conserved ubiquitin protein possesses a unique versatility in regulating protein function via its ability to build mono and polyubiquitin chains onto target proteins. We aimed to take an unbiased approach to generate a comprehensive blueprint of the ciliary ubiquitinome by deploying a multi-proteomics approach using both ciliary-targeted ubiquitin affinity proteomics, as well as ubiquitin-binding domain-based proximity labelling in two different mammalian cell lines. This resulted in the identification of several key proteins involved in signaling, cytoskeletal remodeling and membrane and protein trafficking. Interestingly, using two different approaches in IMCD3 and RPE1 cells, respectively, we uncovered several novel mechanisms that regulate cilia function. In our IMCD3 proximity labeling cell line model, we found a highly enriched group of ESCRT-dependent clathrin-mediated endocytosis-related proteins, suggesting an important and novel role for this pathway in the regulation of ciliary homeostasis and function. In contrast, in RPE1 cells we found that several structural components of caveolae (CAV1, CAVIN1, and EHD2) were highly enriched in our cilia affinity proteomics screen. Consistently, the presence of caveolae at the ciliary pocket and ubiquitination of CAV1 specifically, were found likely to play a role in the regulation of ciliary length in these cells. Cilia length measurements demonstrated increased ciliary length in RPE1 cells stably expressing a ubiquitination impaired CAV1 mutant protein. Furthermore, live cell imaging in the same cells revealed decreased CAV1 protein turnover at the cilium as the possible cause for this phenotype. In conclusion, we have generated a comprehensive list of cilia-specific proteins that are subject to regulation via ubiquitination which can serve to further our understanding of cilia biology in health and disease.

Cargo adapters expand the transport range of intraflagellar transport

The assembly and maintenance of most cilia and eukaryotic flagella depends on intraflagellar transport (IFT), the bidirectional movement of multi-megadalton IFT trains along the axonemal microtubules. These IFT trains function as carriers, moving ciliary proteins between the cell body and the organelle. Whereas tubulin, the principal protein of cilia, binds directly to IFT particle proteins, the transport of other ciliary proteins and complexes requires adapters that link them to the trains. Large axonemal substructures, such as radial spokes, outer dynein arms and inner dynein arms, assemble in the cell body before attaching to IFT trains, using the adapters ARMC2, ODA16 and IDA3, respectively. Ciliary import of several membrane proteins involves the putative adapter tubby-like protein 3 (TULP3), whereas membrane protein export involves the BBSome, an octameric complex that co-migrates with IFT particles. Thus, cells employ a variety of adapters, each of which is substoichiometric to the core IFT machinery, to expand the cargo range of the IFT trains. This Review summarizes the individual and shared features of the known cargo adapters and discusses their possible role in regulating the transport capacity of the IFT pathway.

CEP19-RABL2-IFT-B axis controls BBSome-mediated ciliary GPCR export

The intraflagellar transport (IFT) machinery mediates the import and export of ciliary proteins across the ciliary gate, as well as bidirectional protein trafficking within cilia. In addition to ciliary anterograde protein trafficking, the IFT-B complex participates in the export of membrane proteins together with the BBSome, which consists of eight subunits encoded by the causative genes of Bardet-Biedl syndrome (BBS). The IFT25-IFT27/BBS19 dimer in the IFT-B complex constitutes its interface with the BBSome. We show here that IFT25-IFT27 and the RABL2 GTPase bind the IFT74/BBS22-IFT81 dimer of the IFT-B complex in a mutually exclusive manner. Cells expressing GTP-locked RABL2 [RABL2(Q80L)], but not wild-type RABL2, phenocopied IFT27-knockout cells, that is, they demonstrated BBS-associated ciliary defects, including accumulation of LZTFL1/BBS17 and the BBSome within cilia and the suppression of export of the ciliary GPCRs GPR161 and Smoothened. RABL2(Q80L) enters cilia in a manner dependent on the basal body protein CEP19, but its entry into cilia is not necessary for causing BBS-associated ciliary defects. These observations suggest that GTP-bound RABL2 is likely to be required for recruitment of the IFT-B complex to the ciliary base, where it is replaced with IFT25-IFT27.

Arih2 regulates Hedgehog signaling through smoothened ubiquitylation and ER-associated degradation

During Hedgehog signaling, the ciliary levels of Ptch1 and Smo are regulated by the pathway. At the basal state, Ptch1 localizes to cilia and prevents the ciliary accumulation and activation of Smo. Upon binding a Hedgehog ligand, Ptch1 exits cilia, relieving inhibition of Smo. Smo then concentrates in cilia, becomes activated and activates downstream signaling. Loss of the ubiquitin E3 ligase Arih2 elevates basal Hedgehog signaling, elevates the cellular level of Smo and increases basal levels of ciliary Smo. Mice express two isoforms of Arih2 with Arih2α found primarily in the nucleus and Arih2β found on the cytoplasmic face of the endoplasmic reticulum (ER). Re-expression of ER-localized Arih2β but not nuclear-localized Arih2α rescues the Arih2 mutant phenotypes. When Arih2 is defective, protein aggregates accumulate in the ER and the unfolded protein response is activated. Arih2β appears to regulate the ER-associated degradation (ERAD) of Smo preventing excess and potentially misfolded Smo from reaching the cilium and interfering with pathway regulation.

Cilia-localized GID/CTLH ubiquitin ligase complex regulates protein homeostasis of sonic hedgehog signaling components

Cilia are evolutionarily conserved organelles that orchestrate a variety of signal transduction pathways, such as sonic hedgehog (SHH) signaling, during embryonic development. Our recent studies have shown that loss of GID ubiquitin ligase function results in aberrant AMP-activated protein kinase (AMPK) activation and elongated primary cilia, which suggests a functional connection to cilia. Here, we reveal that the GID complex is an integral part of the cilium required for primary cilia-dependent signal transduction and the maintenance of ciliary protein homeostasis. We show that GID complex subunits localize to cilia in both Xenopus laevis and NIH3T3 cells. Furthermore, we report SHH signaling pathway defects that are independent of AMPK and mechanistic target of rapamycin (MTOR) activation. Despite correct localization of SHH signaling components at the primary cilium and functional GLI3 processing, we find a prominent reduction of some SHH signaling components in the cilium and a significant decrease in SHH target gene expression. Since our data reveal a critical function of the GID complex at the primary cilium, and because suppression of GID function in X. laevis results in ciliopathy-like phenotypes, we suggest that GID subunits are candidate genes for human ciliopathies that coincide with defects in SHH signal transduction.

Regulation of Smoothened Trafficking and Abundance in Hedgehog Signaling

The GPCR-family protein Smoothened (Smo) is essential for Hedgehog (Hh) signal transduction in both insects and vertebrates. The regulation of subcellular localization and abundance of Smo is a critical step in Hh signaling. Recent studies have demonstrated that Smo is subjected to ubiquitination mediated by multiple E3 ubiquitin ligases, leading to Smo endocytosis and subsequent degradation through the proteasome- and lysosome-mediated pathways in Drosophila. Ubiquitination of Smo also promotes its ciliary exit in mammalian cells. Hh inhibits Smo ubiquitination by blocking E3 ligase recruitment and promoting Smo deubiquitination through the ubiquitin-specific protease 8 (USP8) in Drosophila. Inhibition of Smo ubiquitination by Hh promotes Smo cell surface accumulation in Drosophila and ciliary accumulation in mammalian cells. Interestingly, Hh also induces sumoylation of Smo in both Drosophila and mammalian cells, which promotes Smo cell surface/ciliary accumulation. This review focuses on how ubiquitination and sumoylation regulate Smo intracellular trafficking and abundance and how these processes are regulated by Hh.

Regulation of Hedgehog Signal Transduction by Ubiquitination and Deubiquitination

The Hedgehog (Hh) family of secreted proteins governs embryonic development and adult tissue homeostasis in species ranging from insects to mammals. Deregulation of Hh pathway activity has been implicated in a wide range of human disorders, including congenital diseases and cancer. Hh exerts its biological influence through a conserved signaling pathway. Binding of Hh to its receptor Patched (Ptc), a twelve-span transmembrane protein, leads to activation of an atypical GPCR family protein and Hh signal transducer Smoothened (Smo), which then signals downstream to activate the latent Cubitus interruptus (Ci)/Gli family of transcription factors. Hh signal transduction is regulated by ubiquitination and deubiquitination at multiple steps along the pathway including regulation of Ptc, Smo and Ci/Gli proteins. Here we review the effect of ubiquitination and deubiquitination on the function of individual Hh pathway components, the E3 ubiquitin ligases and deubiquitinases involved, how ubiquitination and deubiquitination are regulated, and whether the underlying mechanisms are conserved from Drosophila to mammals.

Impaired cooperation between IFT74/BBS22-IFT81 and IFT25-IFT27/BBS19 causes Bardet-Biedl syndrome

The IFT-B complex mediates ciliary anterograde protein trafficking and membrane protein export together with the BBSome. Bardet-Biedl syndrome (BBS) is caused by mutations in not only all BBSome subunits, but also in some IFT-B subunits, including IFT74/BBS22 and IFT27/BBS19, which form heterodimers with IFT81 and IFT25, respectively. We found that the IFT25-IFT27 dimer bind the C-terminal region of the IFT74-IFT81 dimer, and that the IFT25-IFT27-binding region encompasses the region deleted in the BBS variants of IFT74. In addition, we found that the missense BBS variants of IFT27 are impaired in IFT74-IFT81 binding, and are unable to rescue the BBS-like phenotypes of IFT27-knockout cells. Furthermore, the BBS variants of IFT74 rescued the ciliogenesis defect of IFT74-knockout cells, but the rescued cells demonstrated BBS-like abnormal phenotypes. Taken together, we conclude that the impaired interaction between IFT74-IFT81 and IFT25-IFT27 causes the BBS-associated ciliary defects.

Mechanisms of Smoothened Regulation in Hedgehog Signaling

The seven-transmembrane protein, Smoothened (SMO), has shown to be critical for the hedgehog (HH) signal transduction on the cell membrane (and the cilium in vertebrates). SMO is subjected to multiple types of post-translational regulations, including phosphorylation, ubiquitination, and sumoylation, which alter SMO intracellular trafficking and cell surface accumulation. Recently, SMO is also shown to be regulated by small molecules, such as oxysterol, cholesterol, and phospholipid. The activity of SMO must be very well balanced by these different mechanisms in vivo because the malfunction of SMO will not only cause developmental defects in early stages, but also induce cancers in late stages. Here, we discuss the activation and inactivation of SMO by different mechanisms to better understand how SMO is regulated by the graded HH signaling activity that eventually governs distinct development outcomes.