Multisite protein kinase A and glycogen synthase kinase 3beta phosphorylation leads to Gli3 ubiquitination by SCFbetaTrCP

Beyond cyclopamine: Targeting Hedgehog signaling for cancer intervention

Hedgehog (Hh) signaling plays a significant role in embryogenesis and several physiological processes, such as wound healing and organ homeostasis. In a pathological setting, it is associated with oncogenesis and is responsible for disease progression and poor clinical outcomes. Hedgehog signaling mediates downstream actions via Glioma Associated Oncogene Homolog (GLI) transcription factors. Inhibiting Hh signaling is an important oncological strategy in which inhibitors of the ligands SMO or GLI have been looked at. This review briefly narrates the Hh ligands, signal transduction, the target genes involved and comprehensively describes the numerous inhibitors that have been evaluated for use in various neoplastic settings.

SALL4 is a CRL3REN/KCTD11 substrate that drives Sonic Hedgehog-dependent medulloblastoma

The Sonic Hedgehog (SHH) pathway is crucial regulator of embryonic development and stemness. Its alteration leads to medulloblastoma (MB), the most common malignant pediatric brain tumor. The SHH-MB subgroup is the best genetically characterized, however the molecular mechanisms responsible for its pathogenesis are not fully understood and therapeutic benefits are still limited. Here, we show that the pro-oncogenic stemness regulator Spalt-like transcriptional factor 4 (SALL4) is re-expressed in mouse SHH-MB models, and its high levels correlate with worse overall survival in SHH-MB patients. Proteomic analysis revealed that SALL4 interacts with REN/KCTD11 (here REN), a substrate receptor subunit of the Cullin3-RING ubiquitin ligase complex (CRL3REN) and a tumor suppressor lost in ~30% of human SHH-MBs. We demonstrate that CRL3REN induces polyubiquitylation and degradation of wild type SALL4, but not of a SALL4 mutant lacking zinc finger cluster 1 domain (ΔZFC1). Interestingly, SALL4 binds GLI1 and cooperates with HDAC1 to potentiate GLI1 deacetylation and transcriptional activity. Notably, inhibition of SALL4 suppresses SHH-MB growth both in murine and patient-derived xenograft models. Our findings identify SALL4 as a CRL3REN substrate and a promising therapeutic target in SHH-dependent cancers.

Non-canonical non-genomic morphogen signaling in anucleate platelets: a critical determinant of prothrombotic function in circulation

Circulating platelets derived from bone marrow megakaryocytes play a central role in thrombosis and hemostasis. Despite being anucleate, platelets express several proteins known to have nuclear niche. These include transcription factors and steroid receptors whose non-genomic functions are being elucidated in platelets. Quite remarkably, components of some of the best-studied morphogen pathways, namely Notch, Sonic Hedgehog (Shh), and Wnt have also been described in recent years in platelets, which regulate platelet function in the context of thrombosis as well as influence their survival. Shh and Notch pathways in stimulated platelets establish feed-forward loops of autocrine/juxtacrine/paracrine non-canonical signaling that helps perpetuate thrombosis. On the other hand, non-canonical Wnt signaling is part of a negative feedback loop for restricting platelet activation and possibly limiting thrombus growth. The present review will provide an overview of these signaling pathways in general. We will then briefly discuss the non-genomic roles of transcription factors and steroid receptors in platelet activation. This will be followed by an elaborate description of morphogen signaling in platelets with a focus on their bearing on platelet activation leading to hemostasis and thrombosis as well as their potential for therapeutic targeting in thrombotic disorders.

Ulk4 promotes Shh signaling by regulating Stk36 ciliary localization and Gli2 phosphorylation

The Hedgehog (Hh) family of secreted proteins governs embryonic development and adult tissue homeostasis through the Gli family of transcription factors. Gli is thought to be activated at the tip of primary cilium, but the underlying mechanism has remained poorly understood. Here, we show that Unc-51-like kinase 4 (Ulk4), a pseudokinase and a member of the Ulk kinase family, acts in conjunction with another Ulk family member Stk36 to promote Gli2 phosphorylation and Hh pathway activation. Ulk4 interacts with Stk36 through its N-terminal region containing the pseudokinase domain and with Gli2 via its regulatory domain to bridge the kinase and substrate. Although dispensable for Hh-induced Stk36 kinase activation, Ulk4 is essential for Stk36 ciliary tip localization, Gli2 phosphorylation, and activation. In response to Hh, both Ulk4 and Stk36 colocalize with Gli2 at ciliary tip, and Ulk4 and Stk36 depend on each other for their ciliary tip accumulation. We further show that ciliary localization of Ulk4 depends on Stk36 kinase activity and phosphorylation of Ulk4 on Thr1023, and that ciliary tip accumulation of Ulk4 is essential for its function in the Hh pathway. Taken together, our results suggest that Ulk4 regulates Hh signaling by promoting Stk36-mediated Gli2 phosphorylation and activation at ciliary tip.

Context-dependent ciliary regulation of hedgehog pathway repression in tissue morphogenesis

A fundamental problem in tissue morphogenesis is identifying how subcellular signaling regulates mesoscale organization of tissues. The primary cilium is a paradigmatic organelle for compartmentalized subcellular signaling. How signaling emanating from cilia orchestrates tissue organization-especially, the role of cilia-generated effectors in mediating diverse morpho-phenotypic outcomes-is not well understood. In the hedgehog pathway, bifunctional GLI transcription factors generate both GLI-activators (GLI-A) and GLI-repressors (GLI-R). The formation of GLI-A/GLI-R requires cilia. However, how these counterregulatory effectors coordinate cilia-regulated morphogenetic pathways is unclear. Here we determined GLI-A/GLI-R requirements in phenotypes arising from lack of hedgehog pathway repression (derepression) during mouse neural tube and skeletal development. We studied hedgehog pathway repression by the GPCR GPR161, and the ankyrin repeat protein ANKMY2 that direct cAMP/protein kinase-A signaling by cilia in GLI-R generation. We performed genetic epistasis between Gpr161 or Ankmy2 mutants, and Gli2/Gli3 knockouts, Gli3R knock-in and knockout of Smoothened, the hedgehog pathway transducer. We also tested the role of cilia-generated signaling using a Gpr161 ciliary localization knock-in mutant that is cAMP signaling competent. We found that the cilia-dependent derepression phenotypes arose in three modes: lack of GLI-R only, excess GLI-A formation only, or dual regulation of either lack of GLI-R or excess GLI-A formation. These modes were mostly independent of Smoothened. The cAMP signaling-competent non-ciliary Gpr161 knock-in recapitulated Gpr161 loss-of-function tissue phenotypes solely from lack of GLI-R only. Our results show complex tissue-specific GLI-effector requirements in morphogenesis and point to tissue-specific GLI-R thresholds generated by cilia in hedgehog pathway repression. Broadly, our study sets up a conceptual framework for rationalization of different modes of signaling generated by the primary cilium in mediating morphogenesis in diverse tissues.

Hedgehog/GLI and gastric cancer: Research progress and current status

Hedgehog/GLI (Hh/GLI) is an important signaling pathway. It has been confirmed in various cancer studies that mutated or dysregulated Hh signals may be the behavioral phenotype of tumors, leading to the occurrence of various cancers. The abnormally activated Hh pathway endows tumor cells with a tendency to occur, proliferate, and migrate. In recent years, studies have found that the Hh signaling pathway induces gastric cancer (GC) invasion and epithelial mesenchymal transition. This article reviews the research progress and current status of Hh/GLI related to GC. Unveiling the new veil of GC occurrence will open a new approach for targeted therapy of this malignancy.

Enhancer hijacking at the ARHGAP36 locus is associated with connective tissue to bone transformation

Heterotopic ossification is a disorder caused by abnormal mineralization of soft tissues in which signaling pathways such as BMP, TGFβ and WNT are known key players in driving ectopic bone formation. Identifying novel genes and pathways related to the mineralization process are important steps for future gene therapy in bone disorders. In this study, we detect an inter-chromosomal insertional duplication in a female proband disrupting a topologically associating domain and causing an ultra-rare progressive form of heterotopic ossification. This structural variant lead to enhancer hijacking and misexpression of ARHGAP36 in fibroblasts, validated here by orthogonal in vitro studies. In addition, ARHGAP36 overexpression inhibits TGFβ, and activates hedgehog signaling and genes/proteins related to extracellular matrix production. Our work on the genetic cause of this heterotopic ossification case has revealed that ARHGAP36 plays a role in bone formation and metabolism, outlining first details of this gene contributing to bone-formation and -disease.

Suppressor of Fused Regulation of Hedgehog Signaling is Required for Proper Astrocyte Differentiation

Hedgehog signaling is essential for vertebrate development; however, less is known about the negative regulators that influence this pathway. Using the mouse P19 embryonal carcinoma cell model, suppressor of fused (SUFU), a negative regulator of the Hedgehog (Hh) pathway, was investigated during retinoic acid (RA)-induced neural differentiation. We found Hh signaling increased activity in the early phase of differentiation, but was reduced during terminal differentiation of neurons and astrocytes. This early increase in pathway activity was required for neural differentiation; however, it alone was not sufficient to induce neural lineages. SUFU, which regulates signaling at the level of Gli, remained relatively unchanged during differentiation, but its loss through CRISPR-Cas9 gene editing resulted in ectopic expression of Hh target genes. Interestingly, these SUFU-deficient cells were unable to differentiate toward neural lineages without RA, and when directed toward these lineages, they showed delayed and decreased astrocyte differentiation; neuron differentiation was unaffected. Ectopic activation of Hh target genes in SUFU-deficient cells remained throughout RA-induced differentiation and this was accompanied by the loss of Gli3, despite the presence of the Gli3 message. Thus, the study indicates the proper timing and proportion of astrocyte differentiation requires SUFU, likely acting through Gli3, to reduce Hh signaling during late-stage differentiation.

Dose-dependent phosphorylation and activation of Hh pathway transcription factors

Graded Hedgehog (Hh) signaling is mediated by graded Cubitus interruptus (Ci)/Gli transcriptional activity, but how the Hh gradient is converted into the Ci/Gli activity gradient remains poorly understood. Here, we show that graded Hh induces a progressive increase in Ci phosphorylation at multiple Fused (Fu)/CK1 sites including a cluster located in the C-terminal Sufu-binding domain. We demonstrated that Fu directly phosphorylated Ci on S1382, priming CK1 phosphorylation on adjacent sites, and that Fu/CK1-mediated phosphorylation of the C-terminal sites interfered with Sufu binding and facilitated Ci activation. Phosphorylation at the N-terminal, middle, and C-terminal Fu/CK1 sites occurred independently of one another and each increased progressively in response to increasing levels of Hh or increasing amounts of Hh exposure time. Increasing the number of phospho-mimetic mutations of Fu/CK1 sites resulted in progressively increased Ci activation by alleviating Sufu-mediated inhibition. We found that the C-terminal Fu/CK1 phosphorylation cluster is conserved in Gli2 and contributes to its dose-dependent activation. Our study suggests that the Hh signaling gradient is translated into a Ci/Gli phosphorylation gradient that activates Ci/Gli by gradually releasing Sufu-mediated inhibition.

Hedgehog signaling mechanism and role in cancer

Cell-cell communication through evolutionarily conserved signaling pathways governs embryonic development and adult tissue homeostasis. Deregulation of these signaling pathways has been implicated in a wide range of human diseases including cancer. One such pathway is the Hedgehog (Hh) pathway, which was originally discovered in Drosophila and later found to play a fundamental role in human development and diseases. Abnormal Hh pathway activation is a major driver of basal cell carcinomas (BCC) and medulloblastoma. Hh exerts it biological influence through a largely conserved signal transduction pathway from the activation of the GPCR family transmembrane protein Smoothened (Smo) to the conversion of latent Zn-finger transcription factors Gli/Ci proteins from their repressor (GliR/CiR) to activator (GliA/CiA) forms. Studies from model organisms and human patients have provided deep insight into the Hh signal transduction mechanisms, revealed roles of Hh signaling in a wide range of human cancers, and suggested multiple strategies for targeting this pathway in cancer treatment.

Counting Degrons: Lessons From Multivalent Substrates for Targeted Protein Degradation

E3s comprise a structurally diverse group of at least 800 members, most of which target multiple substrates through specific and regulated protein-protein interactions. These interactions typically rely on short linear motifs (SLiMs), called "degrons", in an intrinsically disordered region (IDR) of the substrate, with variable rules of engagement governing different E3-docking events. These rules of engagement are of importance to the field of targeted protein degradation (TPD), where substrate ubiquitination and destruction require tools to effectively harness ubiquitin ligases (E3s). Substrates are often found to contain multiple degrons, or multiple copies of a degron, contributing to the affinity and selectivity of the substrate for its E3. One important paradigm for E3-substrate docking is presented by the Anaphase-Promoting Complex/Cyclosome (APC/C), a multi-subunit E3 ligase that targets hundreds of proteins for destruction during mitotic exit. APC/C substrate targeting takes place in an ordered manner thought to depend on tightly regulated interactions of substrates, with docking sites provided by the substoichiometric APC/C substrate adaptors and coactivators, Cdc20 or Cdh1/FZR1. Both structural and functional studies of individual APC/C substrates indicate that productive ubiquitination usually requires more than one degron, and that degrons are of different types docking to distinct sites on the coactivators. However, the dynamic nature of APC/C substrate recruitment, and the influence of multiple degrons, remains poorly understood. Here we review the significance of multiple degrons in a number of E3-substrate interactions that have been studied in detail, illustrating distinct kinetic effects of multivalency and allovalency, before addressing the role of multiple degrons in APC/C substrates, key to understanding ordered substrate destruction by APC/C. Lastly, we consider how lessons learnt from these studies can be applied in the design of TPD tools.

Hedgehog signaling and its molecular perspective with cholesterol: a comprehensive review

Hedgehog (Hh) signaling is evolutionarily conserved and plays an instructional role in embryonic morphogenesis, organogenesis in various animals, and the central nervous system organization. Multiple feedback mechanisms dynamically regulate this pathway in a spatiotemporal and context-dependent manner to confer differential patterns in cell fate determination. Hh signaling is complex due to canonical and non-canonical mechanisms coordinating cell-cell communication. In addition, studies have demonstrated a regulatory framework of Hh signaling and shown that cholesterol is vital for Hh ligand biogenesis, signal generation, and transduction from the cell surface to intracellular space. Studies have shown the importance of a specific cholesterol pool, termed accessible cholesterol, which serves as a second messenger, conveying signals between smoothened (Smo) and patched 1 (Ptch1) across the plasma and ciliary membranes. Remarkably, recent high-resolution structural and molecular studies shed new light on the interplay between Hh signaling and cholesterol in membrane biology. These studies elucidated novel mechanistic insight into the release and dispersal of cholesterol-anchored Hh and the basis of Hh recognition by Ptch1. Additionally, the putative model of Smo activation by cholesterol binding and/or modification and Ptch1 antagonization of Smo has been explicated. However, the coupling mechanism of Hh signaling and cholesterol offered a new regulatory principle in cell biology: how effector molecules of the Hh signal network react to and remodel cholesterol accessibility in the membrane and selectively activate Hh signaling proteins thereof. Recognizing the biological importance of cholesterol in Hh signaling activation and transduction opens the door for translational research to develop novel therapeutic strategies. This review looks in-depth at canonical and non-canonical Hh signaling and the distinct proposed model of cholesterol-mediated regulation of Hh signaling components, facilitating a more sophisticated understanding of the Hh signal network and cholesterol biology.

Suppressive GLI2 fragment enhances liver metastasis in colorectal cancer

Linoleic acid (LA) has been shown to cause inflammation and promote development of colorectal cancer (CRC). Moreover, many literatures show that LA is associated with cancer metastasis. Metastatic cancer cells have high stemness, suggesting that LA might affect the stemness of cancer cells. In this study, we examined the effect of LA on the hedgehog system, which affects cancer stemness. In CT26 cells, LA treatment induced the expression of sonic hedgehog (Shh); the signal transduction factor, and glioma-associated oncogene homolog (Gli)2, whereas the expression of SRY-box transcription factor (Sox)17 was suppressed. Furthermore, LA reduced GLI2 ubiquitination, resulting in an increase in the N-terminal fragment of GLI2, known as suppressive GLI2, produced by cleavage of GLI2. LA-induced cleaved GLI2 was also detected in Colo320 and HT29 human CRC cells. Knocking down Gli2 abrogated the LA-mediated suppression of Sox17 expression. These results suggest that LA promotes tumor cell stemness by increasing of suppressive GLI2 fragments via GLI2 modification. In mouse liver metastasis models, LA enhanced metastasis with production of the suppressive GLI2 fragments in CT26 and HT29 cells, whereas knockdown of GLI2 abrogated LA-induced metastatic activity. In human CRCs, the cases with liver metastasis showed the suppressive GLI2 fragments. This study provides mechanistic insights into LA-induced stemness in colon cancer cells. This finding suggests that dietary intake of LA might increase the stemness of cancer cells and enhance metastatic activity of the cancer.

Gli Phosphorylation Code in Hedgehog Signal Transduction

Hedgehog (Hh) family of secreted proteins governs many key processes in embryonic development and adult tissue homeostasis in species ranging from insects to human. Deregulation of Hh signaling has been implicated in a wide range of human diseases including birth defect and cancer. Hh signaling pathway culminates in the conversion of the latent transcription factor Cubitus interruptus (Ci)/Gli from a repressor form (Ci^R/Gli^R) into an activator form (Ci^A/Gli^A). Both the production of Ci^R/Gli^R in the absence of Hh and the formation of Ci^A/Gli^A in response to Hh are regulated by phosphorylation. Whereas previous studies demonstrated that sequential phosphorylation by protein kinase A (PKA), glycogen synthase kinase 3 (GSK3), and casein kinase 1 (CK1) at multiple Ser/Thr clusters in the C-terminal region of Ci/Gli targets it for proteolytic processing to generate Ci^R/Gli^R, recent studies revealed that phosphorylation of Ci/Gli by the Fused (Fu)/Unc-51 like kinase (Ulk) family kinases Fu/Ulk3/Stk36 and other kinases contributes to Ci/Gli activation. Fu/Ulk3/Stk36-mediated phosphorylation of Ci/Gli is stimulated by Hh, leading to altered interaction between Ci/Gli and the Hh pathway repressor Sufu. Here we review our current understanding of how various Ci/Gli phosphorylation events are regulated and how they influence Hh signal transduction.

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.

Cilia kinases in skeletal development and homeostasis

Primary cilia are dynamic compartments that regulate multiple aspects of cellular signaling. The production, maintenance, and function of cilia involve more than 1000 genes in mammals, and their mutations disrupt the ciliary signaling which manifests in a plethora of pathological conditions-the ciliopathies. Skeletal ciliopathies are genetic disorders affecting the development and homeostasis of the skeleton, and encompass a broad spectrum of pathologies ranging from isolated polydactyly to lethal syndromic dysplasias. The recent advances in forward genetics allowed for the identification of novel regulators of skeletogenesis, and revealed a growing list of ciliary proteins that are critical for signaling pathways implicated in bone physiology. Among these, a group of protein kinases involved in cilia assembly, maintenance, signaling, and disassembly has emerged. In this review, we summarize the functions of cilia kinases in skeletal development and disease, and discuss the available and upcoming treatment options.

Control of the Hedgehog pathway by compartmentalized PKA in the primary cilium

The Hedgehog (Hh) signaling is one of the essential signaling pathways during embryogenesis and in adults. Hh signal transduction relies on primary cilium, a specialized cell surface organelle viewed as the hub of cell signaling. Protein kinase A (PKA) has been recognized as a potent negative regulator of the Hh pathway, raising the question of how such a ubiquitous kinase specifically regulates one signaling pathway. We reviewed recent genetic, molecular and biochemical studies that have advanced our mechanistic understanding of PKA's role in Hh signaling in vertebrates, focusing on the compartmentalized PKA at the centrosome and in the primary cilium. We outlined the recently developed genetic and optical tools that can be harvested to study PKA activities during the course of Hh signal transduction.

Ciliary and extraciliary Gpr161 pools repress hedgehog signaling in a tissue-specific manner

The role of compartmentalized signaling in primary cilia during tissue morphogenesis is not well understood. The cilia localized G protein-coupled receptor, Gpr161, represses hedgehog pathway via cAMP signaling. We engineered a knock-in at the Gpr161 locus in mice to generate a variant (Gpr161^mut1), which was ciliary localization defective but cAMP signaling competent. Tissue phenotypes from hedgehog signaling depend on downstream bifunctional Gli transcriptional factors functioning as activators or repressors. Compared to knockout (ko), Gpr161^mut1/ko had delayed embryonic lethality, moderately increased hedgehog targets, and partially down-regulated Gli3 repressor. Unlike ko, the Gpr161^mut1/ko neural tube did not show Gli2 activator-dependent expansion of ventral-most progenitors. Instead, the intermediate neural tube showed progenitor expansion that depends on loss of Gli3 repressor. Increased extraciliary receptor levels in Gpr161^mut1/mut1 prevented ventralization. Morphogenesis in limb buds and midface requires Gli repressor; these tissues in Gpr161^mut1/mut1 manifested hedgehog hyperactivation phenotypes—polydactyly and midfacial widening. Thus, ciliary and extraciliary Gpr161 pools likely establish tissue-specific Gli repressor thresholds in determining morpho-phenotypic outcomes.

Ciliary Dyneins and Dynein Related Ciliopathies

Although ubiquitously present, the relevance of cilia for vertebrate development and health has long been underrated. However, the aberration or dysfunction of ciliary structures or components results in a large heterogeneous group of disorders in mammals, termed ciliopathies. The majority of human ciliopathy cases are caused by malfunction of the ciliary dynein motor activity, powering retrograde intraflagellar transport (enabled by the cytoplasmic dynein-2 complex) or axonemal movement (axonemal dynein complexes). Despite a partially shared evolutionary developmental path and shared ciliary localization, the cytoplasmic dynein-2 and axonemal dynein functions are markedly different: while cytoplasmic dynein-2 complex dysfunction results in an ultra-rare syndromal skeleto-renal phenotype with a high lethality, axonemal dynein dysfunction is associated with a motile cilia dysfunction disorder, primary ciliary dyskinesia (PCD) or Kartagener syndrome, causing recurrent airway infection, degenerative lung disease, laterality defects, and infertility. In this review, we provide an overview of ciliary dynein complex compositions, their functions, clinical disease hallmarks of ciliary dynein disorders, presumed underlying pathomechanisms, and novel developments in the field.

Hedgehog/GLI Signaling Pathway: Transduction, Regulation, and Implications for Disease

Simple Summary

The Hedgehog/GLI (Hh/GLI) pathway plays a major role during development and it is commonly dysregulated in many diseases, including cancer. This highly concerted series of ligands, receptors, cytoplasmic signaling molecules, transcription factors, and co-regulators is involved in regulating the biological functions controlled by this pathway. Activation of Hh/GLI in cancer is most often through a non-canonical method of activation, independent of ligand binding. This review is intended to summarize our current understanding of the Hh/GLI signaling, non-canonical mechanisms of pathway activation, its implication in disease, and the current therapeutic strategies targeting this cascade.

Abstract

The Hh/GLI signaling pathway was originally discovered in Drosophila as a major regulator of segment patterning in development. This pathway consists of a series of ligands (Shh, Ihh, and Dhh), transmembrane receptors (Ptch1 and Ptch2), transcription factors (GLI1–3), and signaling regulators (SMO, HHIP, SUFU, PKA, CK1, GSK3β, etc.) that work in concert to repress (Ptch1, Ptch2, SUFU, PKA, CK1, GSK3β) or activate (Shh, Ihh, Dhh, SMO, GLI1–3) the signaling cascade. Not long after the initial discovery, dysregulation of the Hh/GLI signaling pathway was implicated in human disease. Activation of this signaling pathway is observed in many types of cancer, including basal cell carcinoma, medulloblastoma, colorectal, prostate, pancreatic, and many more. Most often, the activation of the Hh/GLI pathway in cancer occurs through a ligand-independent mechanism. However, in benign disease, this activation is mostly ligand-dependent. The upstream signaling component of the receptor complex, SMO, is bypassed, and the GLI family of transcription factors can be activated regardless of ligand binding. Additional mechanisms of pathway activation exist whereby the entirety of the downstream signaling pathway is bypassed, and PTCH1 promotes cell cycle progression and prevents caspase-mediated apoptosis. Throughout this review, we summarize each component of the signaling cascade, non-canonical modes of pathway activation, and the implications in human disease, including cancer.

Structure-activity mapping of ARHGAP36 reveals regulatory roles for its GAP homology and C-terminal domains

ARHGAP36 is an atypical Rho GTPase-activating protein (GAP) family member that drives both spinal cord development and tumorigenesis, acting in part through an N-terminal motif that suppresses protein kinase A and activates Gli transcription factors. ARHGAP36 also contains isoform-specific N-terminal sequences, a central GAP-like module, and a unique C-terminal domain, and the functions of these regions remain unknown. Here we have mapped the ARHGAP36 structure-activity landscape using a deep sequencing-based mutagenesis screen and truncation mutant analyses. Using this approach, we have discovered several residues in the GAP homology domain that are essential for Gli activation and a role for the C-terminal domain in counteracting an N-terminal autoinhibitory motif that is present in certain ARHGAP36 isoforms. In addition, each of these sites modulates ARHGAP36 recruitment to the plasma membrane or primary cilium. Through comparative proteomics, we also have identified proteins that preferentially interact with active ARHGAP36, and we demonstrate that one binding partner, prolyl oligopeptidase-like protein, is a novel ARHGAP36 antagonist. Our work reveals multiple modes of ARHGAP36 regulation and establishes an experimental framework that can be applied towards other signaling proteins.

Developmental and regenerative paradigms of cilia regulated hedgehog signaling

Primary cilia are immotile appendages that have evolved to receive and interpret a variety of different extracellular cues. Cilia play crucial roles in intercellular communication during development and defects in cilia affect multiple tissues accounting for a heterogeneous group of human diseases called ciliopathies. The Hedgehog (Hh) signaling pathway is one of these cues and displays a unique and symbiotic relationship with cilia. Not only does Hh signaling require cilia for its function but the majority of the Hh signaling machinery is physically located within the cilium-centrosome complex. More specifically, cilia are required for both repressing and activating Hh signaling by modifying bifunctional Gli transcription factors into repressors or activators. Defects in balancing, interpreting or establishing these repressor/activator gradients in Hh signaling either require cilia or phenocopy disruption of cilia. Here, we will summarize the current knowledge on how spatiotemporal control of the molecular machinery of the cilium allows for a tight control of basal repression and activation states of the Hh pathway. We will then discuss several paradigms on how cilia influence Hh pathway activity in tissue morphogenesis during development. Last, we will touch on how cilia and Hh signaling are being reactivated and repurposed during adult tissue regeneration. More specifically, we will focus on mesenchymal stem cells within the connective tissue and discuss the similarities and differences of how cilia and ciliary Hh signaling control the formation of fibrotic scar and adipose tissue during fatty fibrosis of several tissues.

Remodeling without destruction: non-proteolytic ubiquitin chains in neural function and brain disorders

Ubiquitination is a fundamental posttranslational protein modification that regulates diverse biological processes, including those in the CNS. Several topologically and functionally distinct polyubiquitin chains can be assembled on protein substrates, modifying their fates. The classical and most prevalent polyubiquitin chains are those that tag a substrate to the proteasome for degradation, which has been established as a major mechanism driving neural circuit deconstruction and remodeling. In contrast, proteasome-independent non-proteolytic polyubiquitin chains regulate protein scaffolding, signaling complex formation, and kinase activation, and play essential roles in an array of signal transduction processes. Despite being a cornerstone in immune signaling and abundant in the mammalian brain, these non-proteolytic chains are underappreciated in neurons and synapses in the brain. Emerging studies have begun to generate exciting insights about some fundamental roles played by these non-degradative chains in neuronal function and plasticity. In addition, their roles in a number of brain diseases are being recognized. In this article, we discuss recent advances on these nonconventional ubiquitin chains in neural development, function, plasticity, and related pathologies.

Long non-coding RNA HOTAIR induces GLI2 expression through Notch signalling in systemic sclerosis dermal fibroblasts

OBJECTIVES

Systemic sclerosis (SSc) is characterised by tissue fibrosis of the major organs of the body including the skin, lungs and heart. We have previously reported that the lncRNA HOTAIR plays a central role in the activation of SSc myofibroblasts, the key cellular elements of fibrosis. HOTAIR induces fibroblast activation through H3K27me3-mediated activation of the Notch signalling pathway. Here we aimed to identify the signalling events downstream of Notch that drive SSc myofibroblast activation.

METHODS

Patient fibroblasts were obtained from full-thickness forearm skin biopsies of 3 adult patients with SSc of recent onset. The lncRNA HOTAIR was expressed in healthy dermal fibroblasts by lentiviral transduction. Hedgehog signalling pathway was inhibited with GANT61 and GLI2 siRNA. Gamma secretase inhibitors RO4929097 and DAPT were used to block Notch signalling. GSK126 was used to inhibit Enhancer of Zeste 2 (EZH2).

RESULTS

Overexpression of HOTAIR in dermal fibroblasts induced the expression of the Hedgehog pathway transcription factor GLI2. This is mediated by activation of Notch signalling following epigenetic downregulation of miRNA-34a expression. Inhibition of H3K27 methylation and Notch signalling reduced expression of GLI2 in HOTAIR-expressing fibroblasts as well as in SSc dermal fibroblasts. Importantly, the inhibition of GLI2 function using GANT61 or siRNA mitigates the pro-fibrotic phenotype induced by HOTAIR.

CONCLUSIONS

Our data indicates that GLI2 expression is stably upregulated in SSc myofibroblasts through HOTAIR and that GLI2 mediates the expression of pro-fibrotic markers downstream of Notch.

Involvement of Wnt signaling in primary cilia assembly and disassembly

The primary cilium is a nonmotile microtubule-based structure, which functions as an antenna-like cellular sensing organelle. The primary cilium is assembled from the basal body, a mother centriole-based structure, during interphase or a quiescent cell stage, and rapidly disassembles before entering mitosis in a dynamic cycle. Defects in this ciliogenesis dynamics are associated with human diseases such as ciliopathy and cancer, but the molecular mechanisms of the ciliogenesis dynamics are still largely unknown. To date, various cellular signaling pathways associated with primary cilia have been proposed, but the main signaling pathways regulating primary cilia assembly/disassembly remain enigmatic. This review describes recent findings in Wnt-induced primary cilia assembly/disassembly and potential future directions for the study of the cellular signaling related to the primary ciliogenesis dynamics.

Crosstalk of Hedgehog and mTORC1 Pathways

Hedgehog (Hh) signaling and mTOR signaling, essential for embryonic development and cellular metabolism, are both coordinated by the primary cilium. Observations from cancer cells strongly indicate crosstalk between Hh and mTOR signaling. This hypothesis is supported by several studies: Evidence points to a TGFβ-mediated crosstalk; Increased PI3K/AKT/mTOR activity leads to increased Hh signaling through regulation of the GLI transcription factors; increased Hh signaling regulates mTORC1 activity positively by upregulating NKX2.2, leading to downregulation of negative mTOR regulators; GSK3 and AMPK are, as members of both signaling pathways, potentially important links between Hh and mTORC1 signaling; The kinase DYRK2 regulates Hh positively and mTORC1 signaling negatively. In contrast, both positive and negative regulation of Hh has been observed for DYRK1A and DYRK1B, which both regulate mTORC1 signaling positively. Based on crosstalk observed between cilia, Hh, and mTORC1, we suggest that the interaction between Hh and mTORC1 is more widespread than it appears from our current knowledge. Although many studies focusing on crosstalk have been carried out, contradictory observations appear and the interplay involving multiple partners is far from solved.

Ankmy2 Prevents Smoothened-Independent Hyperactivation of the Hedgehog Pathway via Cilia-Regulated Adenylyl Cyclase Signaling

The mechanisms underlying subcellular targeting of cAMP-generating adenylyl cyclases and processes regulated by their compartmentalization are poorly understood. Here, we identify Ankmy2 as a repressor of the Hedgehog pathway via adenylyl cyclase targeting. Ankmy2 binds to multiple adenylyl cyclases, determining their maturation and trafficking to primary cilia. Mice lacking Ankmy2 are mid-embryonic lethal. Knockout embryos have increased Hedgehog signaling and completely open neural tubes showing co-expansion of all ventral neuroprogenitor markers, comparable to the loss of the Hedgehog receptor Patched1. Ventralization in Ankmy2 knockout is completely independent of the Hedgehog pathway transducer Smoothened. Instead, ventralization results from the reduced formation of Gli2 and Gli3 repressors and early depletion of adenylyl cyclase III in neuroepithelial cilia, implicating deficient pathway repression. Ventralization in Ankmy2 knockout requires both cilia and Gli2 activation. These findings indicate that cilia-dependent adenylyl cyclase signaling represses the Hedgehog pathway and promotes morphogenetic patterning.

Merkel Cell Polyomavirus Large T Antigen Unique Domain Regulates Its Own Protein Stability and Cell Growth

Merkel cell polyomavirus (MCV) is the only known human oncogenic virus in the polyomaviridae family and the etiological agent of most Merkel cell carcinomas (MCC). MCC is an aggressive and highly metastatic skin cancer with a propensity for recurrence and poor prognosis. Large tumor antigen (LT), is an essential oncoprotein for MCV transcription, viral replication, and cancer cell proliferation. MCV LT is a short-lived protein that encodes a unique domain: MCV LT unique regions (MURs). These domains consist of phosphorylation sites that interact with multiple E3 ligases, thus limiting LT expression and consequently, viral replication. In this study, we show that MURs are necessary for regulating LT stability via multiple E3 ligase interactions, resulting in cell growth arrest. While expression of wild-type MCV LT induced a decrease in cellular proliferation, deletion of the MUR domains resulted in increased LT stability and cell proliferation. Conversely, addition of MURs to SV40 LT propagated E3 ligase interactions, which in turn, reduced SV40 LT stability and decreased cell growth activity. Our results demonstrate that compared to other human polyomaviruses (HPyVs), MCV LT has evolved to acquire the MUR domains that are essential for MCV LT autoregulation, potentially leading to viral latency and MCC.

Protein phosphatase 4 promotes Hedgehog signaling through dephosphorylation of Suppressor of fused

Reversible phosphorylation of Suppressor of fused (Sufu) is essential for Sonic Hedgehog (Shh) signal transduction. Sufu is stabilized under dual phosphorylation of protein kinase A (PKA) and glycogen synthase kinase 3β (GSK3β). Its phosphorylation is reduced with the activation of Shh signaling. However, the phosphatase in this reversible phosphorylation has not been found. Taking advantage of a proteomic approach, we identified Protein phosphatase 4 regulatory subunit 2 (Ppp4r2), an interacting protein of Sufu. Shh signaling promotes the interaction of these two proteins in the nucleus, and Ppp4 also promotes dephosphorylation of Sufu, leading to its degradation and enhancing the Gli1 transcriptional activity. Finally, Ppp4-mediated dephosphorylation of Sufu promotes proliferation of medulloblastoma tumor cells, and expression of Ppp4 is positively correlated with up-regulation of Shh pathway target genes in the Shh-subtype medulloblastoma, underscoring the important role of this regulation in Shh signaling.

Transcriptional, epigenetic and microRNA regulation of growth plate

Endochondral ossification is a critical event in bone formation, particularly in long shaft bones. Many cellular differentiation processes work in concert to facilitate the generation of cartilage primordium to formation of trabecular structures, all of which occur within the growth plate. Previous studies have revealed that the growth plate is tightly regulated by various transcription factors, epigenetic systems, and microRNAs. Hence, understanding these mechanisms that regulate the growth plate is crucial to furthering the current understanding on skeletal diseases, and in formulating effective treatment strategies. In this review, we focus on describing the function and mechanisms of the transcription factors, epigenetic systems, and microRNAs known to regulate the growth plate.

Dual functions of Rack1 in regulating Hedgehog pathway

Hedgehog (Hh) pathway plays multiple roles in many physiological processes and its dysregulation leads to congenital disorders and cancers. Hh regulates the cellular localization of Smoothened (Smo) and the stability of Cubitus interruptus (Ci) to fine-tune the signal outputs. However, the underlying mechanisms are still unclear. Here, we show that the scaffold protein Rack1 plays dual roles in Hh signaling. In the absence of Hh, Rack1 promotes Ci and Cos2 to form a Ci–Rack1–Cos2 complex, culminating in Slimb-mediated Ci proteolysis. In the presence of Hh, Rack1 dissociates from Ci–Rack1–Cos2 complex and forms a trimeric complex with Smo and Usp8, leading to Smo deubiquitination and cell surface accumulation. Furthermore, we find the regulation of Rack1 on Hh pathway is conserved from Drosophila to mammalian cells. Our findings demonstrate that Rack1 plays dual roles during Hh signal transduction and provide Rack1 as a potential drug target for Hh-related diseases.

Emerging roles of metazoan cell cycle regulators as coordinators of the cell cycle and differentiation

In multicellular organisms, cell proliferation must be tightly coordinated with other developmental processes to form functional tissues and organs. Despite significant advances in our understanding of how the cell cycle is controlled by conserved cell-cycle regulators (CCRs), how the cell cycle is coordinated with cell differentiation in metazoan organisms and how CCRs contribute to this process remain poorly understood. Here, we review the emerging roles of metazoan CCRs as intracellular proliferation-differentiation coordinators in multicellular organisms. We illustrate how major CCRs regulate cellular events that are required for cell fate acquisition and subsequent differentiation. To this end, CCRs employ diverse mechanisms, some of which are separable from those underpinning the conventional cell-cycle-regulatory functions of CCRs. By controlling cell-type-specific specification/differentiation processes alongside the progression of the cell cycle, CCRs enable spatiotemporal coupling between differentiation and cell proliferation in various developmental contexts in vivo. We discuss the significance and implications of this underappreciated role of metazoan CCRs for development, disease and evolution.

GLI3: a mediator of genetic diseases, development and cancer

The transcription factor GLI3 is a member of the Hedgehog (Hh/HH) signaling pathway that can exist as a full length (Gli3-FL/GLI3-FL) or repressor (Gli3-R/GLI3-R) form. In response to HH activation, GLI3-FL regulates HH genes by targeting the GLI1 promoter. In the absence of HH signaling, GLI3 is phosphorylated leading to its partial degradation and the generation of GLI3-R which represses HH functions. GLI3 is also involved in tissue development, immune cell development and cancer. The absence of Gli3 in mice impaired brain and lung development and GLI3 mutations in humans are the cause of Greig cephalopolysyndactyly (GCPS) and Pallister Hall syndromes (PHS). In the immune system GLI3 regulates B, T and NK-cells and may be involved in LPS-TLR4 signaling. In addition, GLI3 was found to be upregulated in multiple cancers and was found to positively regulate cancerous behavior such as anchorage-independent growth, angiogenesis, proliferation and migration with the exception in acute myeloid leukemia (AML) and medulloblastoma where GLI plays an anti-cancerous role. Finally, GLI3 is a target of microRNA. Here, we will review the biological significance of GLI3 and discuss gaps in our understanding of this molecule. Video Abstract.

Cellular signalling by primary cilia in development, organ function and disease

Primary cilia project in a single copy from the surface of most vertebrate cell types; they detect and transmit extracellular cues to regulate diverse cellular processes during development and to maintain tissue homeostasis. The sensory capacity of primary cilia relies on the coordinated trafficking and temporal localization of specific receptors and associated signal transduction modules in the cilium. The canonical Hedgehog (HH) pathway, for example, is a bona fide ciliary signalling system that regulates cell fate and self-renewal in development and tissue homeostasis. Specific receptors and associated signal transduction proteins can also localize to primary cilia in a cell type-dependent manner; available evidence suggests that the ciliary constellation of these proteins can temporally change to allow the cell to adapt to specific developmental and homeostatic cues. Consistent with important roles for primary cilia in signalling, mutations that lead to their dysfunction underlie a pleiotropic group of diseases and syndromic disorders termed ciliopathies, which affect many different tissues and organs of the body. In this Review, we highlight central mechanisms by which primary cilia coordinate HH, G protein-coupled receptor, WNT, receptor tyrosine kinase and transforming growth factor-β (TGFβ)/bone morphogenetic protein (BMP) signalling and illustrate how defects in the balanced output of ciliary signalling events are coupled to developmental disorders and disease progression.

Hedgehog Signal and Genetic Disorders

The hedgehog (Hh) family comprises sonic hedgehog (Shh), Indian hedgehog (Ihh), and desert hedgehog (Dhh), which are versatile signaling molecules involved in a wide spectrum of biological events including cell differentiation, proliferation, and survival; establishment of the vertebrate body plan; and aging. These molecules play critical roles from embryogenesis to adult stages; therefore, alterations such as abnormal expression or mutations of the genes involved and their downstream factors cause a variety of genetic disorders at different stages. The Hh family involves many signaling mediators and functions through complex mechanisms, and achieving a comprehensive understanding of the entire signaling system is challenging. This review discusses the signaling mediators of the Hh pathway and their functions at the cellular and organismal levels. We first focus on the roles of Hh signaling mediators in signal transduction at the cellular level and the networks formed by these factors. Then, we analyze the spatiotemporal pattern of expression of Hh pathway molecules in tissues and organs, and describe the phenotypes of mutant mice. Finally, we discuss the genetic disorders caused by malfunction of Hh signaling-related molecules in humans.

β-Catenin: A Metazoan Filter for Biological Noise?

Molecular noise refers to fluctuations of biological signals that facilitate phenotypic heterogeneity in a population. While endogenous mechanisms exist to limit genetic noise in biological systems, such restrictions are sometimes removed to propel phenotypic variability as an adaptive strategy. Herein, we review evidence for the potential role of β-catenin in restricting gene expression noise by transcriptional and post-transcriptional mechanisms. We discuss mechanisms that restrict intrinsic noise subsequent to nuclear mobilization of β-catenin. Nuclear β-catenin promotes initiation of transcription but buffers against the resultant noise by restraining transcription elongation. Acceleration of cell cycle, mediated via Wnt/β-catenin downstream signals, further diminishes intrinsic noise by curtailing the efficiency of protein synthesis. Extrinsic noise, on the other hand, is restricted by β-catenin–mediated regulation of major cellular stress pathways.

GPR17 is an essential regulator for the temporal adaptation of sonic hedgehog signalling in neural tube development

Dorsal-ventral pattern formation of the neural tube is regulated by temporal and spatial activities of extracellular signalling molecules. Sonic hedgehog (Shh) assigns ventral neural subtypes via activation of the Gli transcription factors. Shh activity in the neural progenitor cells changes dynamically during differentiation, but the mechanisms regulating this dynamicity are not fully understood. Here, we show that temporal change of intracellular cAMP levels confers the temporal Shh signal, and the purinergic G-protein-coupled receptor GPR17 plays an essential role in this regulation. GPR17 is highly expressed in the ventral progenitor regions of the neural tube and acts as a negative regulator of the Shh signal in chick embryos. Although the activation of the GPR17-related signal inhibits ventral identity, perturbation of Gpr17 expression leads to aberrant expansion of ventral neural domains. Notably, perturbation of Gpr17 expression partially inhibits the negative feedback of Gli activity. Moreover, GPR17 increases cAMP activity, suggesting that it exerts its function by inhibiting the processing of Gli3 protein. GPR17 also negatively regulates Shh signalling in neural cells differentiated from mouse embryonic stem cells, suggesting that GPR17 function is conserved among different organisms. Our results demonstrate that GPR17 is a novel negative regulator of Shh signalling in a wide range of cellular contexts.

ERAP1 promotes Hedgehog-dependent tumorigenesis by controlling USP47-mediated degradation of βTrCP

The Hedgehog (Hh) pathway is essential for embryonic development and tissue homeostasis. Aberrant Hh signaling may occur in a wide range of human cancers, such as medulloblastoma, the most common brain malignancy in childhood. Here, we identify endoplasmic reticulum aminopeptidase 1 (ERAP1), a key regulator of innate and adaptive antitumor immune responses, as a previously unknown player in the Hh signaling pathway. We demonstrate that ERAP1 binds the deubiquitylase enzyme USP47, displaces the USP47-associated βTrCP, the substrate-receptor subunit of the SCF^βTrCP ubiquitin ligase, and promotes βTrCP degradation. These events result in the modulation of Gli transcription factors, the final effectors of the Hh pathway, and the enhancement of Hh activity. Remarkably, genetic or pharmacological inhibition of ERAP1 suppresses Hh-dependent tumor growth in vitro and in vivo. Our findings unveil an unexpected role for ERAP1 in cancer and indicate ERAP1 as a promising therapeutic target for Hh-driven tumors.

ERAP1 is an endoplasmic reticulum aminopeptidase that trims MHC Class-I peptides for antigen presentation. Here, the authors show that ERAP1 enhances Hedgehog signalling by sequestering USP47 from  βTrCP and promoting tumorigenesis through βTrCP degradation and increased Gli protein stability.

Impaired neurodevelopmental pathways in autism spectrum disorder: a review of signaling mechanisms and crosstalk

Background

The development of an autistic brain is a highly complex process as evident from the involvement of various genetic and non-genetic factors in the etiology of the autism spectrum disorder (ASD). Despite being a multifactorial neurodevelopmental disorder, autistic patients display a few key characteristics, such as the impaired social interactions and elevated repetitive behaviors, suggesting the perturbation of specific neuronal circuits resulted from abnormal signaling pathways during brain development in ASD. A comprehensive review for autistic signaling mechanisms and interactions may provide a better understanding of ASD etiology and treatment.

Main body

Recent studies on genetic models and ASD patients with several different mutated genes revealed the dysregulation of several key signaling pathways, such as WNT, BMP, SHH, and retinoic acid (RA) signaling. Although no direct evidence of dysfunctional FGF or TGF-β signaling in ASD has been reported so far, a few examples of indirect evidence can be found. This review article summarizes how various genetic and non-genetic factors which have been reported contributing to ASD interact with WNT, BMP/TGF-β, SHH, FGF, and RA signaling pathways. The autism-associated gene ubiquitin-protein ligase E3A (UBE3A) has been reported to influence WNT, BMP, and RA signaling pathways, suggesting crosstalk between various signaling pathways during autistic brain development. Finally, the article comments on what further studies could be performed to gain deeper insights into the understanding of perturbed signaling pathways in the etiology of ASD.

Conclusion

The understanding of mechanisms behind various signaling pathways in the etiology of ASD may help to facilitate the identification of potential therapeutic targets and design of new treatment methods.

GSK3 regulates hair cell fate in the developing mammalian cochlea

The development of asymmetric patterns along biologically relevant axes is a hallmark of many vertebrate organs or structures. One example is the sensory epithelium of the mammalian auditory system. Two distinct types of mechanosensory hair cells (inner and outer) and at least six types of associated supporting cells are precisely and asymmetrically arrayed along the radial (medial-lateral) axis of the cochlear spiral. Immunolabeling of developing cochleae indicates differential expression of Glycogen synthase kinase 3β (GSK3β) along the same axis. To determine whether GSK3β plays a role in specification of cell fates along the medial-lateral axis, GSK3 activity was blocked pharmacologically in cochlear explants. Results indicate significant changes in both the number of hair cells and in the specification of hair cell phenotypes. The overall number of inner hair cells increased as a result of both a shift in the medial boundary between sensory and non-sensory regions of the cochlea and a change in the specification of inner and outer hair cell phenotypes. Previous studies have inhibited GSK3 as a method to examine effects of canonical Wnt signaling. However, quantification of changes in Wnt pathway target genes in GSK3-inhibited cochleae, and treatment with more specific Wnt agonists, indicated that the Wnt pathway is not activated. Instead, expression of Bmp4 in a population of GSK3β-expressing cells was shown to be down-regulated. Finally, addition of BMP4 to GSK3-inhibited cochleae achieved a partial rescue of the hair cell phenotype. These results demonstrate a role for GSK3β in the specification of cellular identities along the medial-lateral axis of the cochlea and provide evidence for a positive role for GSK3β in the expression of Bmp4.

Biochemical mechanisms of vertebrate hedgehog signaling

Signaling pathways that mediate cell-cell communication are essential for collective cell behaviors in multicellular systems. The hedgehog (HH) pathway, first discovered and elucidated in Drosophila, is one of these iconic signaling systems that plays many roles during embryogenesis and in adults; abnormal HH signaling can lead to birth defects and cancer. We review recent structural and biochemical studies that have advanced our understanding of the vertebrate HH pathway, focusing on the mechanisms by which the HH signal is received by patched on target cells, transduced across the cell membrane by smoothened, and transmitted to the nucleus by GLI proteins to influence gene-expression programs.

Structural basis of βTrCP1-associated GLI3 processing

Controlled ubiquitin-mediated protein degradation is essential for various cellular processes. GLI family regulates the transcriptional events of the sonic hedgehog pathway genes that are implicated in almost one fourth of human tumors. GLI3 phosphorylation by Ser/Thr kinases is a primary factor for their transcriptional activity that incurs the formation of both GLI3 repressor and activator forms. GLI3 processing is triggered in an ubiquitin-dependent manner via SCF^βTrCP1 complex; however, structural characterization, mode of action based on sequence of phosphorylation signatures and induced conformational readjustments remain elusive. Here, through structural analysis and molecular dynamics simulation assays, we explored comparative binding pattern of GLI3 phosphopeptides against βTrCP1. A comprehensive and thorough analysis demarcated GLI3 presence in the binding cleft shared by inter-bladed binding grooves of β-propeller. Our results revealed the involvement of all seven WD40 repeats of βTrCP1 in GLI3 interaction. Conversely, GLI3 phosphorylation pattern at primary protein kinase A (PKA) sites and secondary casein kinase 1 (CK1) or glycogen synthase kinase 3 (GSK3) sites was carefully evaluated. Our results indicated that GLI3 processing depends on the 19 phosphorylation sites (849, 852, 855, 856, 860, 861, 864, 865, 868, 872, 873, 876, 877, 880, 899, 903, 906, 907 and 910 positions) by a cascade of PKA, GSK3β and CSKI kinases. The presence of a sequential phosphorylation in the binding induction of GLI3 and βTrCP1 may be a hallmark to authenticate GLI3 processing. We speculate that mechanistic information of the individual residual contributions through structure-guided approaches may be pivotal for the rational design of specific and more potent inhibitors against activated GLI3 with a special emphasis on the anticancer activity.

Role of Protein Kinases in Hedgehog Pathway Control and Implications for Cancer Therapy

Hedgehog (HH) signaling is an evolutionarily conserved pathway that is crucial for growth and tissue patterning during embryonic development. It is mostly quiescent in the adult, where it regulates tissue homeostasis and stem cell behavior. Aberrant reactivation of HH signaling has been associated to several types of cancer, including those in the skin, brain, prostate, breast and hematological malignancies. Activation of the canonical HH signaling is triggered by binding of HH ligand to the twelve-transmembrane protein PATCHED. The binding releases the inhibition of the seven-transmembrane protein SMOOTHENED (SMO), leading to its phosphorylation and activation. Hence, SMO activates the transcriptional effectors of the HH signaling, that belong to the GLI family of transcription factors, acting through a not completely elucidated intracellular signaling cascade. Work from the last few years has shown that protein kinases phosphorylate several core components of the HH signaling, including SMO and the three GLI proteins, acting as powerful regulatory mechanisms to fine tune HH signaling activities. In this review, we will focus on the mechanistic influence of protein kinases on HH signaling transduction. We will also discuss the functional consequences of this regulation and the possible implications for cancer therapy.

A proteomic approach identifies SAFB-like transcription modulator (SLTM) as a bidirectional regulator of GLI family zinc finger transcription factors

In Sonic hedgehog (SHH) signaling, GLI family zinc finger (GLI)-mediated diverse gene transcription outcomes are strictly regulated and are important for SHH function in both development and disease. However, how the GLI factors differentially regulate transcription in response to variable SHH activities is incompletely understood. Here, using a newly generated, tagged Gli3 knock-in mouse (Gli3^TAP ), we performed proteomic analyses and identified the chromatin-associated SAFB-like transcription modulator (SLTM) as a GLI-interacting protein that context-dependently regulates GLI activities. Using immunoprecipitation and immunoblotting, RT-quantitative PCR, and ChIP assays, we show that SLTM interacts with all three GLI proteins and that its cellular levels are regulated by SHH. We also found that SLTM enhances GLI3 binding to chromatin and increases GLI3 repressor (GLI3R) form protein levels. In a GLI3-dependent manner, SLTM promoted the formation of a repressive chromatin environment and functioned as a GLI3 co-repressor. In the absence of GLI3 or in the presence of low GLI3 levels, SLTM co-activated GLI activator (GLIA)-mediated target gene activation and cell differentiation. Moreover, in vivo Sltm deletion generated through CRISPR/Cas9-mediated gene editing caused perinatal lethality and SHH-related abnormal ventral neural tube phenotypes. We conclude that SLTM regulates GLI factor binding to chromatin and contributes to the transcriptional outcomes of SHH signaling via a novel molecular mechanism.

Gli Proteins: Regulation in Development and Cancer

Gli proteins are transcriptional effectors of the Hedgehog signaling pathway. They play key roles in the development of many organs and tissues, and are deregulated in birth defects and cancer. We review the molecular mechanisms of Gli protein regulation in mammals, with special emphasis on posttranslational modifications and intracellular transport. We also discuss how Gli proteins interact with co-activators and co-repressors to fine-tune the expression of Hedgehog target genes. Finally, we provide an overview of the regulation of developmental processes and tissue regeneration by Gli proteins and discuss how these proteins are involved in cancer progression, both through canonical regulation via the Hedgehog pathway and through cross-talk with other signaling pathways.

Abnormal activity of transcription factors gli in high-grade gliomas

Malignant transformation is associated with loss of cell differentiation, anaplasia. Transcription factors gli, required for embryonic development, may be involved in this process. We studied the activity of transcription factors gli in high-grade gliomas and their role in maintenance of stem cell state and glioma cell survival. 20 glioma cell lines and a sample of a normal adult brain tissue were used in the present study. We found the expression of gli target genes, including GLI1 and FOXM1, in all tested glioma cell lines, but not in the normal tissue. Interestingly, the expression of gli target genes in some glioma cell lines was observed together with a high level of their transcriptional repressor, Gli3R. Knockdown of GLI3 in one of these lines resulted in decrease of gli target gene expression. These data suggest that Gli3R does not prevent the gli target genes transcription, and gli3 acts in glioma cells more as an activator, than a repressor of transcription. We observed that gli regulated the expression of such genes, as SOX2 or OCT4 that maintain stem cell state, and TET1, involving in DNA demethylation. Treatment with GANT61 or siRNA against GLI1, GLI2, or GLI3 could result in complete glioma cell death, while cyclopamine had a weaker and line-specific effect on glioma cell survival. Thus, the gli transcription factors are abnormally active in high-grade gliomas, regulate expression of genes, maintaining the stem cell state, and contribute to glioma cell survival.

Primary Cilia as Signaling Hubs in Health and Disease

Primary cilia detect extracellular cues and transduce these signals into cells to regulate proliferation, migration, and differentiation. Here, the function of primary cilia as signaling hubs of growth factors and morphogens is in focus. First, the molecular mechanisms regulating the assembly and disassembly of primary cilia are described. Then, the role of primary cilia in mediating growth factor and morphogen signaling to maintain human health and the potential mechanisms by which defects in these pathways contribute to human diseases, such as ciliopathy, obesity, and cancer are described. Furthermore, a novel signaling pathway by which certain growth factors stimulate cell proliferation through suppression of ciliogenesis is also described, suggesting novel therapeutic targets in cancer.