SMO

Detailed Information

SMO (Smoothened) gene is located on human chromosome 7q32.3 and encodes a G protein-coupled receptor (GPCR) belonging to the Frizzled receptor family. This gene is highly conserved in vertebrates, and the SMO protein it encodes consists of 787 amino acids, with a molecular weight of approximately 87.6 kDa. Structurally, SMO has a typical seven-transmembrane domain, with its N-terminal located extracellularly, containing a cysteine-rich domain (CRD), and the C-terminal located intracellularly, containing a relatively long cytoplasmic tail. These structural features allow SMO to effectively transduce extracellular signals to intracellular effectors. Notably, the crystal structure of SMO reveals a large pocket-like structure within its transmembrane region, providing a critical target for the design of small-molecule inhibitors. As a core component of the Hedgehog (Hh) signaling pathway, SMO plays a crucial role in embryonic development, tissue homeostasis, and the onset of various diseases. In the absence of Hh ligands, SMO is inhibited by the transmembrane protein Patched (PTCH1). Upon the binding of Hh ligands to PTCH1, this inhibition is relieved, activating SMO and initiating downstream signaling cascades.

Precise Regulation of Signal Transduction Pathways

As a key transducer in the Hedgehog signaling pathway, the activation of SMO involves complex molecular interactions. Under basal conditions, PTCH1 inhibits SMO activity, potentially through the regulation of local concentrations of cholesterol or oxysterol molecules in the cell membrane. When Sonic Hedgehog (SHH) ligands bind to PTCH1, the internalization and degradation of PTCH1 relieve the inhibition of SMO, leading to its accumulation at the base of the cilium and conformational change. Activated SMO transmits signals through both G protein-dependent and independent pathways: on one hand, it recruits and activates Gαi proteins to inhibit adenylyl cyclase, reducing intracellular cAMP levels, thereby modulating protein kinase A (PKA) activity; on the other hand, it promotes the dissociation of the SUFU-GLI complex, allowing GLI transcription factors to translocate into the nucleus and initiate the expression of downstream target genes. Studies show that SMO interacts with key effector molecules such as KIF7, GLI2, and GLI3 through its cytoplasmic tail, guiding their localization and transport within the cilium, which is essential for the precise spatiotemporal regulation of the pathway. Moreover, the activity of SMO is finely regulated by post-translational modifications, such as phosphorylation and ubiquitination, which affect its stability, subcellular localization, and interaction with other signaling molecules.

Figure 1. Regulation of the Smoothened GPCR. (Arensdorf AM, et al., 2016)

Role in Embryonic Development

During embryonic development, SMO-mediated Hh signaling plays an irreplaceable role in body axis formation, limb development, and organogenesis. For example, in limb development, SMO regulates the processing of Gli3 in posterior mesenchymal cells, influencing the formation of the anterior-posterior axis. Mouse models show that Smo gene knockout results in severe limb development defects, including shortened limbs and absent phalanges. In neural tube development, SMO is involved in establishing a ventral neuronal differentiation gradient, and abnormal activity of SMO can lead to forebrain malformation or neural tube closure defects. Additionally, SMO plays a crucial role in heart development, lung branching morphogenesis, and tooth development. Notably, the activity of SMO is strictly regulated in both space and time, and even slight deviations in its expression or activity can lead to significant developmental abnormalities, reflecting the pathway's precise dose dependency in morphogenesis.

Developmental Abnormalities and Genetic Disorders

Acquired or loss-of-function mutations in the SMO gene are associated with several human developmental disorders. Specific missense mutations in SMO (e.g., c.1792G>A; p.Gly598Arg) have been found to cause isolated post-axial polydactyly (extra fingers or toes on the side of the little finger/toe), an autosomal recessive inherited disorder. These mutations occur in the cytoplasmic tail of the SMO protein and may disrupt SMO's interaction with downstream effectors or alter its subcellular localization, resulting in aberrant activation of the SHH pathway. Homology modeling analysis suggests that such mutations interfere with proper protein folding and its interaction with other molecules, ultimately affecting patterning during limb development. In addition to limb malformations, SMO mutations have been linked to Curry-Jones Syndrome (characterized by cranial suture synostosis, skin abnormalities, and intellectual disabilities) and Pallister-Hall-like Syndrome (characterized by polydactyly, cardiac defects, and urogenital abnormalities). These disease phenotypes reflect the multifunctionality of SMO at different developmental stages and in various tissues.

Tumorigenesis and Progression

SMO is abnormally activated in various types of cancer, with activation driven by both genetic mutations and upstream signaling abnormalities. In sporadic basal cell carcinoma (BCC), approximately 10% of cases carry SMO activating mutations, often located in the transmembrane domain, such as p.L303F and p.W535L, leading to sustained activation of SMO in the absence of SHH ligands. In medulloblastoma (SHH subtype), the mutation frequency of SMO can be as high as 20%, with these mutations promoting tumorigenesis by stabilizing the active conformation of SMO. In addition to mutations, SMO overexpression is observed in various epithelial cancers, such as pancreatic ductal adenocarcinoma (PDAC), where its expression levels correlate with tumor invasiveness and poor prognosis. Clinical studies show that SMO mRNA expression levels in pancreatic cancer tissues are significantly higher than in adjacent normal tissues and are positively correlated with higher histological grade and lymph node metastasis (p<0.05). At the molecular mechanism level, SMO contributes to tumor progression by promoting cell proliferation, survival, and epithelial-mesenchymal transition (EMT). Furthermore, SMO modulates angiogenesis and immune cell infiltration in the tumor microenvironment, influencing tumor growth. It is noteworthy that SMO's abnormal activation often coexists with PTCH1 loss-of-function mutations, leading to constitutive activation of the SHH pathway.

Tissue Injury and Repair

In tissue injury repair, SMO-mediated SHH signaling regulates stem cell activation, tissue regeneration, and fibrosis. For example, in a high-oxygen-induced lung injury model, the expression of SMO and its downstream effector molecule GLI1 is significantly upregulated over time, with SMO protein expression increasing by about 7-fold on day 7 (0.423±0.056 vs 0.061±0.008), reaching a peak on day 14 (0.612±0.082 vs 0.132±0.011). These changes in expression correlate with alveolar structural damage, inflammatory cell infiltration, and fibrosis, suggesting the dynamic role of the SMO pathway in lung injury and repair. In liver fibrosis models, the SHH pathway promotes extracellular matrix deposition by activating hepatic stellate cells. Additionally, SMO plays a positive role in angiogenesis following myocardial infarction and axon regeneration following neuronal damage. These studies suggest that the SMO pathway has a "double-edged sword" effect in tissue repair: moderate activation promotes regeneration and healing, while sustained activation may lead to pathological fibrosis.

Targeted Therapeutic Strategies

Given the central role of SMO in various diseases, particularly cancers, small molecule inhibitors targeting SMO have become a hot area in drug development. Several SMO inhibitors are currently in clinical use or in trial phases. Vismodegib (GDC-0449), as the first FDA-approved SMO inhibitor (2012), is used to treat locally advanced or metastatic BCC. It works by binding to the transmembrane pocket of SMO, stabilizing its inactive conformation and inhibiting downstream signaling. Clinical studies show that Vismodegib induces objective responses in approximately 30-45% of advanced BCC patients and is also effective in treating multiple BCC and odontogenic keratocyst (OKC) associated with Nevoid Basal Cell Carcinoma Syndrome (NBCCS). Other SMO inhibitors, such as Sonidegib (LDE225) and Glasdegib, have also been approved for treating BCC or acute myeloid leukemia (AML). Notably, it has been reported that GDC-0449 therapy in NBCCS-associated BCC patients also reduces the range of multiple mandibular OKC lesions, providing a new avenue for the use of SMO inhibitors in non-tumor diseases.