Dual function of suppressor of fused in Hh pathway activation and mouse spinal cord patterning

Hedgehog Signaling in Cortical Development

The Hedgehog (Hh) pathway plays a crucial role in embryonic development, acting both as a morphogenic signal that organizes tissue formation and a potent mitogenic signal driving cell proliferation. Dysregulated Hh signaling leads to various developmental defects in the brain. This article aims to review the roles of Hh signaling in the development of the neocortex in the mammalian brain, focusing on its regulation of neural progenitor proliferation and neuronal production. The review will summarize studies on genetic mouse models that have targeted different components of the Hh pathway, such as the ligand Shh, the receptor Ptch1, the GPCR-like transducer Smo, the intracellular transducer Sufu, and the three Gli transcription factors. As key insights into the Hh signaling transduction mechanism were obtained from mouse models displaying neural tube defects, this review will also cover some studies on Hh signaling in neural tube development. The results from these genetic mouse models suggest an intriguing hypothesis that elevated Hh signaling may play a role in the gyrification of the brain in certain species. Additionally, the distinctive production of GABAergic interneurons in the dorsal cortex in the human brain may also be linked to the extension of Hh signaling from the ventral to the dorsal brain region. Overall, these results suggest key roles of Hh signaling as both a morphogenic and mitogenic signal during the forebrain development and imply the potential involvement of Hh signaling in the evolutionary expansion of the neocortex.

TMEM132A regulates mouse hindgut morphogenesis and caudal development

Caudal developmental defects, including caudal regression, caudal dysgenesis and sirenomelia, are devastating conditions affecting the skeletal, nervous, digestive, reproductive and excretory systems. Defects in mesodermal migration and blood supply to the caudal region have been identified as possible causes of caudal developmental defects, but neither satisfactorily explains the structural malformations in all three germ layers. Here, we describe caudal developmental defects in transmembrane protein 132a (Tmem132a) mutant mice, including skeletal, posterior neural tube closure, genitourinary tract and hindgut defects. We show that, in Tmem132a mutant embryos, visceral endoderm fails to be excluded from the medial region of early hindgut, leading directly to the loss or malformation of cloaca-derived genitourinary and gastrointestinal structures, and indirectly to the neural tube and kidney/ureter defects. We find that TMEM132A mediates intercellular interaction, and physically interacts with planar cell polarity (PCP) regulators CELSR1 and FZD6. Genetically, Tmem132a regulates neural tube closure synergistically with another PCP regulator Vangl2. In summary, we have identified Tmem132a as a new regulator of PCP, and hindgut malformation as the underlying cause of developmental defects in multiple caudal structures.

WNT5A, β‑catenin and SUFU expression patterns, and the significance of microRNA deregulation in placentas with intrauterine growth restriction

Placental insufficiency is a common cause of intrauterine growth restriction (IUGR). It affects ~10% of pregnancies and increases fetal and neonatal morbidity and mortality. Although Wnt and Hh pathways are crucial for embryonic development and placentation, their role in the pathology of IUGR is still not sufficiently explored. The present study analyzed the expression of positive regulators of the Wnt pathway, WNT5A and β‑catenin, and the expression of the Hh pathway negative regulator suppressor of fused (SUFU). Immunohistochemical and reverse transcription‑quantitative PCR (RT‑qPCR) assays were performed on 34 IUGR and 18 placental tissue samples from physiologic singleton‑term pregnancies. Epigenetic mechanisms of SUFU gene regulation were also investigated by methylation‑specific PCR analysis of its promoter and RT‑qPCR analysis of miR‑214‑3p and miR‑378a‑5p expression. WNT5A protein expression was higher in endothelial cells of placental villi from IUGR compared with control tissues. That was also the case for β‑catenin protein expression in trophoblasts and endothelial cells and SUFU protein expression in trophoblasts from IUGR placentas. The SUFU gene promoter remained unmethylated in all tissue samples, while miR‑214‑3p and miR‑378a‑5p were downregulated in IUGR. The present results suggested altered Wnt and Hh signaling in IUGR. DNA methylation did not appear to be a mechanism of SUFU regulation in the pathogenesis of IUGR, but its expression could be regulated by miRNA targeting.

Ciliary control of adipocyte progenitor cell fate regulates energy storage

The primary cilium is a cellular sensory organelle found in most cells in our body. This includes adipocyte progenitor cells in our adipose tissue, a complex organ involved in energy storage, endocrine signaling, and thermogenesis. Numerous studies have shown that the primary cilium plays a critical role in directing the cell fate of adipocyte progenitor cells in multiple adipose tissue types. Accordingly, diseases with dysfunctional cilia called ciliopathies have a broad range of clinical manifestations, including obesity and diabetes. This review summarizes our current understanding of how the primary cilium regulates adipocyte progenitor cell fate in multiple contexts and illustrates the importance of the primary cilium in regulating energy storage and adipose tissue function.

Differential expression of the Tmem132 family genes in the developing mouse nervous system

The family of novel transmembrane proteins (TMEM) 132 have been associated with multiple neurological disorders and cancers in humans, but have hardly been studied in vivo. Here we report the expression patterns of the five Tmem132 genes (a, b, c, d and e) in developing mouse nervous system with RNA in situ hybridization in wholemount embryos and tissue sections. Our results reveal differential and partially overlapping expression of multiple Tmem132 family members in both the central and peripheral nervous system, suggesting potential partial redundancy among them.

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.

Suppressor of fused controls cerebellum granule cell proliferation by suppressing Fgf8 and spatially regulating Gli proteins

Cerebellar granule cell (GC) development relies on precise regulation of sonic hedgehog (Shh)-Gli signalling activity, failure of which is associated with motor disorders and medulloblastoma. Mutations in the pathway regulator suppressor of fused (Sufu), which modulates Gli activators and repressors, are linked to cerebellar dysfunction and tumourigenesis. The mechanism by which Sufu calibrates Shh signalling in GCs is unknown. Math1-Cre-mediated deletion of Sufu in mouse GC progenitors (GCPs) demonstrated that Sufu restricts GCP proliferation and promotes cell cycle exit, by promoting expression of Gli3R and suppressing Gli2 levels. Sufu is also required to promote a high threshold of pathway activity in GCPs. Remarkably, central cerebellar lobules are more deleteriously impacted by Sufu deletion, but are less sensitive to downstream genetic manipulations to reduce Gli2 expression or overexpress a Gli3R mimic, compared with anterior lobules. Transcriptome sequencing uncovered new Sufu targets, especially Fgf8, which is upregulated in Sufu-mutant GCPs. We demonstrate that Fgf8 is necessary and sufficient to drive Sufu-mutant GCP proliferation. This study reveals new insights into the spatial and temporal regulation of cerebellar Shh-Gli signalling, while uncovering new targets, such as Fgf8.

Distinct Activities of Gli1 and Gli2 in the Absence of Ift88 and the Primary Cilia

The primary cilia play essential roles in Hh-dependent Gli2 activation and Gli3 proteolytic processing in mammals. However, the roles of the cilia in Gli1 activation remain unresolved due to the loss of Gli1 transcription in cilia mutant embryos, and the inability to address this question by overexpression in cultured cells. Here, we address the roles of the cilia in Gli1 activation by expressing Gli1 from the Gli2 locus in mouse embryos. We find that the maximal activation of Gli1 depends on the cilia, but partial activation of Gli1 by Smo-mediated Hh signaling exists in the absence of the cilia. Combined with reduced Gli3 repressors, this partial activation of Gli1 leads to dorsal expansion of V3 interneuron and motor neuron domains in the absence of the cilia. Moreover, expressing Gli1 from the Gli2 locus in the presence of reduced Sufu has no recognizable impact on neural tube patterning, suggesting an imbalance between the dosages of Gli and Sufu does not explain the extra Gli1 activity. Finally, a non-ciliary Gli2 variant present at a higher level than Gli1 when expressed from the Gli2 locus fails to activate Hh pathway ectopically in the absence of the cilia, suggesting that increased protein level is unlikely the major factor underlying the ectopic activation of Hh signaling by Gli1 in the absence of the cilia.

Proteostasis in the Hedgehog signaling pathway

The Hedgehog (Hh) signaling pathway is crucial for the development of vertebrate and invertebrate animals alike. Hh ligand binds its receptor Patched (Ptc), allowing the activation of the obligate signal transducer Smoothened (Smo). The levels and localizations of both Ptc and Smo are regulated by ubiquitination, and Smo is under additional regulation by phosphorylation and SUMOylation. Downstream of Smo, the Ci/Gli family of transcription factors regulates the transcriptional responses to Hh. Phosphorylation, ubiquitination and SUMOylation are important for the stability and localization of Ci/Gli proteins and Hh signaling output. Finally, Suppressor of Fused directly regulates Ci/Gli proteins and itself is under proteolytic regulation that is critical for normal Hh signaling.

Different effects of dexmedetomidine and midazolam on the expression of NR2B and GABAA-α1 following peripheral nerve injury in rats

Neuropathic pain is a complex, chronic pain condition and the treatment is a major clinical challenge. Recent studies have shown that two FDA approved drugs dexmedetomidine (DEX) and midazolam (MZL), may be useful in treating neuropathic pain, but the mechanism is not fully dementated. Here, we investigated the effects and mechanisms of DEX and MZL treatment in the peripheral nerve injury model. Intramuscular injection with DEX and MZL attenuated the development of mechanical allodynia and thermal hyperalgesia in rats with chronic constriction injury (CCI). Concurrently, the expression of NMDA receptor subunit 2B (NR2B), GABA (A) receptor subunit alpha1 (GABAA-α1), and Sonic Hedgehog (SHH) displayed different temporal patterns in the thalamus and the ipsilateral dorsal horn of the spinal cord after CCI. Such that (1) NR2B expression was decreased on day 1 and 14, whereas GABAA-α1 expression was increased on day 1 in the thalamus, and NR2B expression was decreased on day 1, whereas GABAA-α1 expression was increased on day 1 and day 30 in the ipsilateral spinal cord dorsal horn after DEX treatment. (2) NR2B expression was increased on day 1, then decreased on day 14 and returned to baseline on day30, whereas GABAA-α1 expression was no significant changes on day 1, 14, 30 in the thalamus, and NR2B expression was decreased on day 14 and 30, whereas GABAA-α1 expression was no changes on day 1 and 14 but increased on day 30 after MZL treatment. Furthermore, the mechanical allodynia was significantly attenuated after PUR administration. Meanwhile the expression of NR2B was significantly decreased, and the expression of GABAA-α1 was significantly increased, in the thalamus and in the ipsilateral spinal cord dorsal horn when detected on postoperative day 1, 7, and 14. Our findings indicate that DEX and MZL have different mechanisms in CCI rats, suggesting different strategies could be considered in managing neuropathic pain in different individuals. © 2018 IUBMB Life, 70(2):143-152, 2018.

Non-melanoma skin cancer: new and future synthetic drug treatments

INTRODUCTION

Non-melanoma skin cancers (NMSC) mainly comprise two different entities: basal cell carcinoma (BCC) and squamous cell carcinoma (SCC); beneath these two entities, Merkel cell carcinoma, adnexal tumors, dermatofibrosarcoma protuberans, angiosarcoma, and cutaneous lymphoma belong to NMSC. These rare skin tumors are not the topic of this review. BCC and SCC are the most common cancers diagnosed in humans. The preferred treatment is surgery, which in most cases is curative. Although a high recurrence rate is seen, these cancers rarely metastasize. Therefore, systemic treatments were not a priority for these patients. It is long known that the abnormal activation of Hedgehog and epidermal growth factor receptor pathways were involved in BCC and SCC. In the last decade, metastatic disease became an important area of research, mostly because new therapies that targeted components of these two pathways became available. Areas covered: Here we cover the available therapeutic options for patients diagnosed with BCC and SCC, focus on systemic and targeted therapies. Expert opinion: BCC and SCC are common cancers, with good prognosis. More than the metastatic disease, advanced local disease and recurrent disease pose clinicians a great challenge. Albeit there are promising results with targeted therapies, resistance development has already been described.

Spop regulates Gli3 activity and Shh signaling in dorsoventral patterning of the mouse spinal cord

Sonic Hedgehog (Shh) signaling regulates the patterning of ventral spinal cord through the effector Gli family of transcription factors. Previous in vitro studies showed that an E3 ubiquitin ligase containing Speckle-type POZ protein (Spop) targets Gli2 and Gli3 for ubiquitination and degradation, but the role of Spop in Shh signaling and mammalian spinal cord patterning remains unknown. Here, we show that loss of Spop does not alter spinal cord patterning, but it suppresses the loss of floor plate and V3 interneuron phenotype of Gli2 mutants, suggesting a negative role of Spop in Gli3 activator activity, Shh signaling and the specification of ventral cell fates in the spinal cord. This correlates with a moderate but significant increase in the level of Gli3 protein in the Spop mutant spinal cords. Furthermore, loss of Spop restores the maximal Shh pathway activation and ventral cell fate specification in the Gli1;Sufu double mutant spinal cord. Finally, we show that loss of Spop-like does not change the spinal cord patterning in either wild type or Spop mutants, suggesting that it does not compensate for the loss of Spop in Shh signaling and spinal cord patterning. Therefore, our results demonstrate a negative role of Spop in the level and activity of Gli3, Shh signaling and ventral spinal cord patterning.

Nek2A/SuFu feedback loop regulates Gli-mediated Hedgehog signaling pathway

Suppressor of Fused (SuFu), one of the most conserved components of the Hedgehog (Hh) signaling, binds Gli transcription factors and impedes activation of target gene expression in mammalian cells. Despite the central importance of SuFu in the Hh pathway, little is known about SuFu regulation. In a previous study, we identified NIMA-related expressed kinase 2A (Nek2A) as a SuFu-interacting protein. Here, we show that Nek2A stabilizes SuFu through impairing ubiquitin/proteasome degradation of SuFu. In addition, Nek2A negatively regulates target genes of Hh signaling as well as Gli2 transcriptional activity. In turn, inhibition of Hh signaling by GANT61 diminishes mRNA and protein levels of Nek2A, and Hh agonist promotes transcription of NEK2A gene. Chromatin immunoprecipitation assays revealed that Gli1 and Gli2 directly bind to the promoter regions of NEK2A gene and induced its transcription. Thus, we uncovered one of the mechanisms by which Nek2A acts as a modulator of the Hh signaling pathway in the context of a novel negative-feedback loop, which may offer new insights into Gli-mediated Hh signaling regulation in development and human diseases.

Loss of Suppressor of Fused in Mid-Corticogenesis Leads to the Expansion of Intermediate Progenitors

Neural progenitors in the embryonic neocortex must be tightly regulated in order to generate the correct number and projection neuron subtypes necessary for the formation of functional neocortical circuits. In this study, we show that the intracellular protein Suppressor of Fused (Sufu) regulates the proliferation of intermediate progenitor (IP) cells at later stages of corticogenesis to affect the number of Cux1+ upper layer neurons in the postnatal neocortex. This correlates with abnormal levels of the repressor form of Gli3 (Gli3R) and the ectopic expression of Patched 1 (Ptch1), a Sonic Hedgehog (Shh) target gene. These studies reveal that the canonical role of Sufu as an inhibitor of Shh signaling is conserved at later stages of corticogenesis and that Sufu plays a crucial role in regulating neuronal number by controlling the cell cycle dynamics of IP cells in the embryonic neocortex.

Members of the Rusc protein family interact with Sufu and inhibit vertebrate Hedgehog signaling

Hedgehog (Hh) signaling is fundamentally important for development and adult tissue homeostasis. It is well established that in vertebrates Sufu directly binds and inhibits Gli proteins, the downstream mediators of Hh signaling. However, it is unclear how the inhibitory function of Sufu towards Gli is regulated. Here we report that the Rusc family of proteins, the biological functions of which are poorly understood, form a heterotrimeric complex with Sufu and Gli. Upon Hh signaling, Rusc is displaced from this complex, followed by dissociation of Gli from Sufu. In mammalian fibroblast cells, knockdown of Rusc2 potentiates Hh signaling by accelerating signaling-induced dissociation of the Sufu-Gli protein complexes. In Xenopus embryos, knockdown of Rusc1 or overexpression of a dominant-negative Rusc enhances Hh signaling during eye development, leading to severe eye defects. Our study thus uncovers a novel regulatory mechanism controlling the response of cells to Hh signaling in vertebrates.

Nek2A phosphorylates and stabilizes SuFu: A new strategy of Gli2/Hedgehog signaling regulatory mechanism

Suppressor of Fused (SuFu) plays a conservative role in the regulation of the Gli transcription factors within the Hedgehog (Hh) signaling pathway. Despite the central importance of SuFu in the Hh pathway, little is known about its regulation. Here, we performed a GAL4-based yeast two-hybrid screen using human SuFu as bait, and identified NIMA-related expressed kinase 2A (Nek2A) as a new SuFu-interacting protein, which was also confirmed by glutathione-S-transferase pull-down and co-immunoprecipitation assays. Intriguingly, Nek2A is found to stabilize SuFu at least partly depending on its kinase activity, thereby triggering phosphorylation of the SuFu protein. Moreover, the phosphorylated SuFu inhibits the nuclear localization and transcriptional activity of Gli2/Hh signaling. These findings reveal a new mechanism of mammalian SuFu regulation, and offers novel insights into Hh signaling regulation in development and human disease.

Suppressor of Fused Is Critical for Maintenance of Neuronal Progenitor Identity during Corticogenesis

Proper lineage progression and diversification of neural progenitor cells (NPCs) ensures the generation of projection neuron (PN) subtypes in the mammalian neocortex. Here, we show that Suppressor of Fused (Sufu) controls PN specification by maintaining the identity of NPCs in the embryonic neocortex. Deletion of Sufu in NPCs of the E10.5 mouse neocortex led to improper specification of progenitors and a reduction in intermediate progenitors (IPs) during corticogenesis. We found that Sufu deletion resulted in unstable Gli2 and Gli3 activity, leading to the ectopic activation of Sonic hedgehog (Shh) signaling. The role of Sufu in maintaining progenitor identity is critical at early stages of corticogenesis, since deletion of Sufu at E13.5 did not cause similar abnormalities. Our studies revealed that Sufu critically modulates Shh signaling at early stages of neurogenesis for proper specification and maintenance of cortical NPCs to ensure the appropriate generation of cortical PN lineages.

Suppressor of Fused Is Required for Determining Digit Number and Identity via Gli3/Fgfs/Gremlin

The anterior-posterior patterning of the vertebrate limb bud requires closely coordinated signaling interactions, including Sonic Hedgehog (Shh)-mediated counteraction of the Gli3 transcription factor in the distal and posterior mesenchyme of the limb bud. Suppressor of Fused (Sufu), an intracellular negative regulator of Shh signaling via Gli2 and Gli3, is implicated in early development of the mouse limb bud. However, how Sufu is involved in the genetic regulation of limb bud patterning still remains elusive. In this study, we show that the conditional deletion of Sufu in the mesenchyme of the early limb bud results in polydactyly with loss of digit identity and supernumerary bones in the wrist and the ankle. These pattern alterations are associated with anterior expansion of HoxD genes located at the 5’ end of the cluster. By focusing on gene expression analysis of Shh/Gremlin1/Fgf signaling critical for the establishment and maintenance of anterior-posterior patterning, we show that early response to loss of Sufu involves anterior prolongation of Fgf4 and Fgf8 expression in the apical ectodermal ridge at E10.5. We also reveal the anterior activation of Shh-dependent posterior markers Ptc1, Gli1 and Gremlin in limb buds lacking Sufu. Furthermore, we find that loss of Sufu leads to attenuated levels of repressor Gli2 and repressor Gli3 in the early limb bud. Moreover, expression of Hand2 is activated in the entire limb bud at the early outgrowth stage in the mutant lacking Sufu. Thus, we provide evidence that Sufu is involved in the genetic network that restricts the posterior expression of Gli2/3/Hand2 and Gremlin/Fgf in limb bud patterning.

Cell-autonomous activation of Hedgehog signaling inhibits brown adipose tissue development

Although recent studies have shown that brown adipose tissue (BAT) arises from progenitor cells that also give rise to skeletal muscle, the developmental signals that control the formation of BAT remain largely unknown. Here, we show that brown preadipocytes possess primary cilia and can respond to Hedgehog (Hh) signaling. Furthermore, cell-autonomous activation of Hh signaling blocks early brown-preadipocyte differentiation, inhibits BAT formation in vivo, and results in replacement of neck BAT with poorly differentiated skeletal muscle. Finally, we show that Hh signaling inhibits BAT formation partially through up-regulation of chicken ovalbumin upstream promoter transcription factor II (COUP-TFII). Taken together, our studies uncover a previously unidentified role for Hh as an inhibitor of BAT development.

Ptch1 and Gli regulate Shh signalling dynamics via multiple mechanisms

In the vertebrate neural tube, the morphogen Sonic Hedgehog (Shh) establishes a characteristic pattern of gene expression. Here we quantify the Shh gradient in the developing mouse neural tube and show that while the amplitude of the gradient increases over time, the activity of the pathway transcriptional effectors, Gli proteins, initially increases but later decreases. Computational analysis of the pathway suggests three mechanisms that could contribute to this adaptation: transcriptional upregulation of the inhibitory receptor Ptch1, transcriptional downregulation of Gli and the differential stability of active and inactive Gli isoforms. Consistent with this, Gli2 protein expression is downregulated during neural tube patterning and adaptation continues when the pathway is stimulated downstream of Ptch1. Moreover, the Shh-induced upregulation of Gli2 transcription prevents Gli activity levels from adapting in a different cell type, NIH3T3 fibroblasts, despite the upregulation of Ptch1. Multiple mechanisms therefore contribute to the intracellular dynamics of Shh signalling, resulting in different signalling dynamics in different cell types.

The loss of Hh responsiveness by a non-ciliary Gli2 variant

Hedgehog signaling is crucial for vertebrate development and physiology. Gli2, the primary effector of Hedgehog signaling, localizes to the tip of the primary cilium, but the importance of its ciliary localization remains unclear. We address the roles of Gli2 ciliary localization by replacing endogenous Gli2 with Gli2(ΔCLR), a Gli2 variant not localizing to the cilium. The resulting Gli2(ΔCLRKI) and Gli2(ΔCLRKI);Gli3 double mutants resemble Gli2-null and Gli2;Gli3 double mutants, respectively, suggesting the lack of Gli2(ΔCLR) activation in development. Significantly, Gli2(ΔCLR) cannot be activated either by pharmacochemical activation of Smo in vitro or by loss of Ptch1 in vivo. Finally, Gli2(ΔCLR) exhibits strong transcriptional activator activity in the absence of Sufu, suggesting that the lack of its activation in vivo results from a specific failure in relieving the inhibitory function of Sufu. Our results provide strong evidence that the ciliary localization of Gli2 is crucial for cilium-dependent activation of Hedgehog signaling.

T396I mutation of mouse Sufu reduces the stability and activity of Gli3 repressor

Hedgehog signaling is primarily transduced by two transcription factors: Gli2, which mainly acts as a full-length activator, and Gli3, which tends to be proteolytically processed from a full-length form (Gli3^FL) to an N-terminal repressor (Gli3^REP). Recent studies using a Sufu knockout mouse have indicated that Sufu is involved in regulating Gli2 and Gli3 activator and repressor activity at multiple steps of the signaling cascade; however, the mechanism of specific Gli2 and Gli3 regulation remains to be elucidated. In this study, we established an allelic series of ENU-induced mouse strains. Analysis of one of the missense alleles, Sufu^T396I, showed that Thr³⁹⁶ residue of Sufu played a key role in regulation of Gli3 activity. Sufu^T396I/T396I embryos exhibited severe polydactyly, which is indicative of compromised Gli3 activity. Concomitantly, significant quantitative reductions of unprocessed Gli3 (Gli3^FL) and processed Gli3 (Gli3^REP) were observed in vivo as well as in vitro. Genetic experiments showed that patterning defects in the limb buds of Sufu^T396I/T396I were rescued by a constitutive Gli3^REP allele (Gli3^∆699), strongly suggesting that Sufu^T396I reduced the truncated Gli3 repressor. In contrast, Sufu^T396I qualitatively exhibited no mutational effects on Gli2 regulation. Taken together, the results of this study show that the Thr³⁹⁶ residue of Sufu is specifically required for regulation of Gli3 but not Gli2. This implies a novel Sufu-mediated mechanism in which Gli2 activator and Gli3 repressor are differentially regulated.

Hedgehog signaling: From basic research to clinical applications

Studies of the major signaling pathways have revealed a connection between development, regeneration, and cancer, highlighting common signaling networks in these processes. The Hedgehog (Hh) pathway plays a central role in the development of most tissues and organs in mammals. Hh signaling is also required for tissue homeostasis and regeneration in adults, while perturbed Hh signaling is associated with human cancers. A fundamental understanding of Hh signaling will not only enhance our knowledge of how the embryos are patterned but also provide tools to treat diseases related to aberrant Hh signaling. Studies have yielded a basic framework of Hh signaling, which establishes the foundation for addressing unresolved issues of Hh signaling. A detailed characterization of the biochemical interactions between Hh components will help explain the production of graded Hh responses required for tissue patterning. Additional cell biological and genetic studies will offer new insight into the role of Hh signaling in homeostasis and regeneration. Finally, drugs that are capable of manipulating the Hh pathway can be used to treat human diseases caused by disrupted Hh signaling. These investigations will serve as a paradigm for studying signal transduction/integration in homeostasis and disease, and for translating discovery from bench to bedside.

Sufu and Kif7 in limb patterning and development

BACKGROUND

The vertebrate digit pattern is defined by the morphogen Sonic hedgehog (Shh), which controls the activity of Gli transcription factors. Gli1, 2 and 3 are dynamically expressed during patterning. Downstream of Shh, their activity is regulated by Sufu and Kif7, core components of the Shh signaling cascade. The precise roles of these regulators during limb development have not been fully described. We analyze the role of Sufu and Kif7 in the limb and demonstrate that their loss has distinct and synergistic effects on Gli activity and digit pattern.

RESULTS

Using a series of mouse mutants, we show that Sufu and Kif7 are expressed throughout limb development and their deletion has distinct effects on Gli levels and limb formation. Concomitant deletion of Sufu and Kif7 results in constitutive pathway activity and severe limb truncation. This is consistent with the recently published two-population model, which suggests that precocious activation of Shh signaling inhibits organizing center formation and limb outgrowth.

CONCLUSIONS

Together, our findings demonstrate that perturbations of Sufu and Kif7 affect Gli activity and recapitulate the full spectrum of vertebrate limb defects, ranging from severe truncation to polydactyly.

Regulation of Sufu activity by p66β and Mycbp provides new insight into vertebrate Hedgehog signaling

Control of Gli function by Sufu, a major negative regulator, is a key step in mammalian Hedgehog (Hh) signaling. Lin et al. identified several Sufu-interacting proteins, including p66β and Mycbp. Sufu recruits p66β to block Gli-mediated Hh target gene expression. Meanwhile, Mycbp forms a complex with Gli and Sufu without Hh stimulation but remains inactive. Hh pathway activation leads to dissociation of Sufu/p66β from Gli, enabling Mycbp to promote Gli protein activity and Hh target gene expression.

Control of Gli function by Suppressor of Fused (Sufu), a major negative regulator, is a key step in mammalian Hedgehog (Hh) signaling, but how this is achieved in the nucleus is unknown. We found that Hh signaling results in reduced Sufu protein levels and Sufu dissociation from Gli proteins in the nucleus, highlighting critical functions of Sufu in the nucleus. Through a proteomic approach, we identified several Sufu-interacting proteins, including p66β (a member of the NuRD [nucleosome remodeling and histone deacetylase] repressor complex) and Mycbp (a Myc-binding protein). p66β negatively and Mycbp positively regulate Hh signaling in cell-based assays and zebrafish. They function downstream from the membrane receptors, Patched and Smoothened, and the primary cilium. Sufu, p66β, Mycbp, and Gli are also detected on the promoters of Hh targets in a dynamic manner. Our results support a new model of Hh signaling in the nucleus. Sufu recruits p66β to block Gli-mediated Hh target gene expression. Meanwhile, Mycbp forms a complex with Gli and Sufu without Hh stimulation but remains inactive. Hh pathway activation leads to dissociation of Sufu/p66β from Gli, enabling Mycbp to promote Gli protein activity and Hh target gene expression. These studies provide novel insight into how Sufu controls Hh signaling in the nucleus.

Hedgehog-induced phosphorylation by CK1 sustains the activity of Ci/Gli activator

Hedgehog (Hh) signaling governs many developmental processes by regulating the balance between the repressor (Ci(R)/Gli(R)) and activator (Ci(A)/Gli(A)) forms of Cubitus interruptus (Ci)/glioma-associated oncogene homolog (Gli) transcription factors. Although much is known about how Ci(R)/Gli(R) is controlled, the regulation of Ci(A)/Gli(A) remains poorly understood. Here we demonstrate that Casein kinase 1 (CK1) sustains Hh signaling downstream of Costal2 and Suppressor of fused (Sufu) by protecting Ci(A) from premature degradation. We show that Hh stimulates Ci phosphorylation by CK1 at multiple Ser/Thr-rich degrons to inhibit its recognition by the Hh-induced MATH and BTB domain containing protein (HIB), a substrate receptor for the Cullin 3 family of E3 ubiquitin ligases. In Hh-receiving cells, reduction of CK1 activity accelerated HIB-mediated degradation of Ci(A), leading to premature loss of pathway activity. We also provide evidence that Gli(A) is regulated by CK1 in a similar fashion and that CK1 acts downstream of Sufu to promote Sonic hedgehog signaling. Taken together, our study not only reveals an unanticipated and conserved mechanism by which phosphorylation of Ci/Gli positively regulates Hh signaling but also provides the first evidence, to our knowledge, that substrate recognition by the Cullin 3 family of E3 ubiquitin ligases is negatively regulated by a kinase.

Differential regulation of Gli proteins by Sufu in the lung affects PDGF signaling and myofibroblast development

Mammalian Hedgehog (Hh) signaling relies on three Gli transcription factors to mediate Hh responses. This process is controlled in part by a major negative regulator, Sufu, through its effects on Gli protein level, distribution and activity. In this report, we showed that Sufu regulates Gli1 protein levels by antagonizing Numb/Itch. Otherwise, Numb/Itch would induce Gli1 protein degradation. This is in contrast to inhibition of Spop-mediated degradation of Gli2/3 by Sufu. Thus, controlling protein levels of all three Gli genes by Sufu is a conserved mechanism to modulate Hh responses albeit via distinct pathways. These findings in cell-based assays were further validated in vivo. In analyzing how Sufu controls Gli proteins in different tissues, we discovered that loss of Sufu in the lung exerts different effects on Hh target genes. Hh targets Ptch1/Hhip are upregulated in Sufu-deficient lungs, consistent with Hh pathway activation. Surprisingly, protein levels of Hh target Gli1 are reduced. We also found that myofibroblasts are absent from many prospective alveoli of Sufu-deficient lungs. Myofibroblast development is dependent on PDGF signaling. Interestingly, analysis of the Pdgfra promoter revealed a canonical Gli-binding site where Gli1 resides. These studies support a model in which loss of Sufu contributes to compromised Pdgfra activation and disrupts myofibroblast development in the lung. Our work illustrates the unappreciated complexity of Hh responses where distinct Hh targets could respond differently depending on the availability of Gli proteins that control their expression.

Hedgehog signaling from the primary cilium to the nucleus: an emerging picture of ciliary localization, trafficking and transduction

The unexpected connection between cilia and signaling is one of the most exciting developments in cell biology in the past decade. In particular, the Hedgehog (Hh) signaling pathway relies on the primary cilium to regulate tissue patterning and homeostasis in vertebrates. A central question is how ciliary localization and trafficking of Hh pathway components lead to pathway activation and regulation. In this review, we discuss recent studies that reveal the roles of ciliary regulators, components and structures in controlling the movement and signaling of Hh players. These findings significantly increase our mechanistic understanding of how the primary cilium facilitates Hh signal transduction and form the basis for further investigations to define the function of cilia in other signaling processes.

Aberrant expression of sonic hedgehog pathway in colon cancer and melanosis coli

OBJECTIVE

To determine the hedgehog (Hh) signaling pathway correlated with the development of colon cancer and melanosis coli.

METHODS

Protein and mRNA levels of Hh signaling pathway components (sonic hedgehog [Shh], protein patched homolog 1 [Ptch 1], GLI family zinc finger 1 [Gli 1] and suppressor of fused homolog [Drosophila] [Sufu]) in 127 patients with colon cancer, 36 with melanosis coli and 20 adjacent normal mucosal tissues taken from surgical specimens were evaluated using antibody staining and quantitative real-time polymerase chain reaction.

RESULTS

In adjacent normal tissue Shh and Ptch1, but not Gli1 or Sufu, were weakly expressed and mainly in the lining epithelium of the colonic mucosa. In cancerous tissues Shh and Gli1 were uniformly strong while Ptch1 was patchy and weak, and Sufu uniformly weak, which paralleled their levels of corresponding mRNA. Elevated protein levels of Shh and Ptch were significantly associated with mucinous colonic tissues. Elevated Sufu protein levels were positively correlated with the diameter and invasion of the tumor. In patients with melanosis coli, mRNA levels of Shh, Ptch1, Gli1 and Sufu were very low, which was similar to those of adjacent normal tissues; but protein levels of Shh, Ptch1 and Gli1, but not Sufu, were high, which was similar to those of cancerous tissues.

CONCLUSIONS

The mRNA and protein levels of Hh pathway components are aberrantly elevated in colon cancer, which may be the potential molecular classification markers. Further studies are required to determine the role of melanosis coli in the colon tumorigenesis.