Reduced cholesterol levels impair Smoothened activation in Smith-Lemli-Opitz syndrome

Multiple modes of cholesterol translocation in the human Smoothened receptor

Smoothened (SMO), a member of the G Protein-Coupled Receptor superfamily, mediates Hedgehog signaling and is linked to cancer and birth defects. SMO responds to accessible cholesterol in the ciliary membrane, translocating it via a longitudinal tunnel to its extracellular domain. Reaching a complete mechanistic understanding of the cholesterol translocation process would help in the development of cancer therapies. Experimental data suggests two modes of translocation to support entry of cholesterol from outer and inner membrane leaflets, but the exact mechanism of translocation remains unclear. Using atomistic molecular dynamics simulations (~2 millisecond simulations) and biochemical assays of SMO mutants, we assess the energetic feasibilities of the two modes. We show that the highest energetic barrier for cholesterol translocation from the outer leaflet is lower than that from the inner leaflet. Mutagenesis experiments and complementary simulations of SMO mutants validate the role of critical amino acid residues along the translocation pathways. Our data suggests that cholesterol can take either pathway to enter SMO, thus explaining experimental observations in literature. Thus, our results illuminate the energetics and provide a first molecular description of cholesterol translocation in SMO.

Effect of DHCR7 on adipocyte differentiation in goats

Cholesterol is regarded as a signaling molecule in regulating the metabolism and function of fat cells, in which 7-Dehydrocholesterol reductase (DHCR7) is a key enzyme that catalyzes the conversion of 7-dehydrocholesterol to cholesterol, however, the exact function of DHCR7 in goat adipocytes remains unknown. Here, the effect of DHCR7 on the formation of subcutaneous and intramuscular fat in goats was investigated in vitro, and the result indicated that the mRNA level of DHCR7 showed a gradual downward trend in subcutaneous adipogenesis, but an opposite trend in intramuscular adipogenesis. In the process of subcutaneous preadipocytes differentiation, overexpression of DHCR7 inhibited the expression of adipocytes differentiation marker genes (CEBP/α, CEBP/β, SREBP1 and AP2), lipid metabolism-related genes (AGPAT6, FASN, SCD1 and LPL), and the lipid accumulation. However, in intramuscular preadipocyte differentiation, DHCR7 overexpression showed a promoting effect on adipocyte differentiation marker genes (CEBP/α, CEBP/β, PPARγ and SREBP1) and lipid metabolism-related genes (GPAM, AGPAT6, DGAT1 and SCD1) expression, and on lipid accumulation. In summary, our work demonstrated that DHCR7 played an important role in regulating adipogenic differentiation and lipid metabolism in preadipocytes in goats, which is of great significance for uncovering the underlying molecular mechanism of adipocyte differentiation and improving goat meat quality.

The changing metabolic landscape of bile acids - keys to metabolism and immune regulation

Bile acids regulate nutrient absorption and mitochondrial function, they establish and maintain gut microbial community composition and mediate inflammation, and they serve as signalling molecules that regulate appetite and energy homeostasis. The observation that there are hundreds of bile acids, especially many amidated bile acids, necessitates a revision of many of the classical descriptions of bile acids and bile acid enzyme functions. For example, bile salt hydrolases also have transferase activity. There are now hundreds of known modifications to bile acids and thousands of bile acid-associated genes, especially when including the microbiome, distributed throughout the human body (for example, there are >2,400 bile salt hydrolases alone). The fact that so much of our genetic and small-molecule repertoire, in both amount and diversity, is dedicated to bile acid function highlights the centrality of bile acids as key regulators of metabolism and immune homeostasis, which is, in large part, communicated via the gut microbiome.

Vitamin D3 suppresses the cholesterol homeostasis pathway in patient-derived glioma cell lines

Glioblastoma is one of the most common malignant brain tumors. Vitamin D, primarily its hormonally active form calcitriol, has been reported to have anti-cancer activity. In the present study, we used patient-derived glioma cell lines to examine the effect of vitamin D3 and calcitriol on glioblastoma. Surprisingly, vitamin D3 showed a more significant inhibitory effect than calcitriol on cell viability and proliferation. Vitamin D receptor (VDR) mediates most of the cellular effects of vitamin D, and thus we examined the expression level and function of VDR via gene silencing and gene knockout experiments. We observed that VDR does not affect the sensitivity of patient-derived glioma cell lines to vitamin D3, and the gene encoding VDR is not essential for growth of patient-derived glioma cell lines. RNA sequencing data analysis and sterolomics analysis revealed that vitamin D3 inhibits cholesterol synthesis and cholesterol homeostasis by inhibiting the expression level of 7-dehydrocholesterol reductase, which leads to the accumulation of 7-dehydrocholesterol and other sterol intermediates. In conclusion, our results suggest that vitamin D3, rather than calcitriol, inhibits growth of patient-derived glioma cell lines via inhibition of the cholesterol homeostasis pathway.

Sonic hedgehog signaling in craniofacial development

Mutations in SHH and several other genes encoding components of the Hedgehog signaling pathway have been associated with holoprosencephaly syndromes, with craniofacial anomalies ranging in severity from cyclopia to facial cleft to midfacial and mandibular hypoplasia. Studies in animal models have revealed that SHH signaling plays crucial roles at multiple stages of craniofacial morphogenesis, from cranial neural crest cell survival to growth and patterning of the facial primordia to organogenesis of the palate, mandible, tongue, tooth, and taste bud formation and homeostasis. This article provides a summary of the major findings in studies of the roles of SHH signaling in craniofacial development, with emphasis on recent advances in the understanding of the molecular and cellular mechanisms regulating the SHH signaling pathway activity and those involving SHH signaling in the formation and patterning of craniofacial structures.

The Inseparable Relationship Between Cholesterol and Hedgehog Signaling

Ligands of the Hedgehog (HH) pathway are paracrine signaling molecules that coordinate tissue development in metazoans. A remarkable feature of HH signaling is the repeated use of cholesterol in steps spanning ligand biogenesis, secretion, dispersal, and reception on target cells. A cholesterol molecule covalently attached to HH ligands is used as a molecular baton by transfer proteins to guide their secretion, spread, and reception. On target cells, a signaling circuit composed of a cholesterol transporter and sensor regulates transmission of HH signals across the plasma membrane to the cytoplasm. The repeated use of cholesterol in signaling supports the view that the HH pathway likely evolved by coopting ancient systems to regulate the abundance or organization of sterol-like lipids in membranes.

Cholesterol biosynthesis modulates differentiation in murine cranial neural crest cells

Cranial neural crest cells (cNCC) are a multipotent embryonic cell population that give rise to a diverse set of cell types. These cells are particularly vulnerable to external metabolic stressors, as exemplified by the association between maternal hyperglycemia and congenital malformations. We were interested in studying the effect of various concentrations of glucose and pyruvate on cNCC metabolism, migration, and differentiation using an established murine neural crest cell model (O9-1). We unexpectedly observed a pattern of gene expression suggestive of cholesterol biosynthesis induction under glucose depletion conditions in O9-1 cells. We further showed that treatment with two different cholesterol synthesis inhibitors interfered with cell migration and differentiation, inhibiting chondrogenesis while enhancing smooth muscle cell differentiation. As congenital arhinia (absent external nose), a malformation caused by mutations in SMCHD1, appears to represent, in part, a defect in cNCC, we were also interested in investigating the effects of glucose and cholesterol availability on Smchd1 expression in O9-1 cells. Smchd1 expression was induced under high glucose conditions whereas cholesterol synthesis inhibitors decreased Smchd1 expression during chondrogenesis. These data highlight a novel role for cholesterol biosynthesis in cNCC physiology and demonstrate that human phenotypic variability in SMCHD1 mutation carriers may be related, in part, to SMCHD1's sensitivity to glucose or cholesterol dosage during development.

Smith-Lemli-Opitz syndrome: A pathophysiological manifestation of the Bloch hypothesis

The biosynthesis of cholesterol, an essential component of higher eukaryotic membranes, was worked out by Konrad Bloch (and Feodor Lynen) in the 1960s and they received the Nobel Prize around that time in recognition of their pioneering contributions. An elegant consequence of this was a hypothesis proposed by Konrad Bloch (the Bloch hypothesis) which suggests that each subsequent intermediate in the cholesterol biosynthesis pathway is superior in supporting membrane function in higher eukaryotes relative to its precursor. In this review, we discuss an autosomal recessive metabolic disorder, known as Smith-Lemli-Opitz syndrome (SLOS), associated with a defect in the Kandutsch-Russell pathway of cholesterol biosynthesis that results in accumulation of the immediate precursor of cholesterol in its biosynthetic pathway (7-dehydrocholesterol) and an altered cholesterol to total sterol ratio. Patients suffering from SLOS have several developmental, behavioral and cognitive abnormalities for which no drug is available yet. We characterize SLOS as a manifestation of the Bloch hypothesis and review its molecular etiology and current treatment. We further discuss defective Hedgehog signaling in SLOS and focus on the role of the serotonin_1A receptor, a representative neurotransmitter receptor belonging to the GPCR family, in SLOS. Notably, ligand binding activity and cellular signaling of serotonin_1A receptors are impaired in SLOS-like condition. Importantly, cellular localization and intracellular trafficking of the serotonin_1A receptor (which constitute an important determinant of a GPCR cellular function) are compromised in SLOS. We highlight some of the recent developments and emerging concepts in SLOS pathobiology and suggest that novel therapies based on trafficking defects of target receptors could provide new insight into treatment of SLOS.

The emerging roles of Hedgehog signaling in tumor immune microenvironment

The Hedgehog (Hh) signaling pathway is pervasively involved in human malignancies, making it an effective target for cancer treatment for decades. In addition to its direct role in regulating cancer cell attributes, recent work indicates that it has an immunoregulatory effect on tumor microenvironments. An integrated understanding of these actions of Hh signaling pathway in tumor cells and tumor microenvironments will pave the way for novel tumor treatments and further advances in anti-tumor immunotherapy. In this review, we discuss the most recent research about Hh signaling pathway transduction, with a particular emphasis on its role in modulating tumor immune/stroma cell phenotype and function, such as macrophage polarity, T cell response, and fibroblast activation, as well as their mutual interactions between tumor cells and nonneoplastic cells. We also summarize the recent advances in the development of Hh pathway inhibitors and nanoparticle formulation for Hh pathway modulation. We suggest that targeting Hh signaling effects on both tumor cells and tumor immune microenvironments could be more synergistic for cancer treatment.

Statins for Smith-Lemli-Opitz syndrome

BACKGROUND

Smith-Lemli-Opitz syndrome (SLOS) is a multiple congenital malformations syndrome caused by defective cholesterol biosynthesis. Affected individuals show cholesterol deficiency and accumulation of various precursor molecules, mainly 7-dehydrocholesterol and 8-dehydrocholesterol. There is currently no cure for SLOS, with cholesterol supplementation being primarily a biochemical therapy of limited evidence. However, several anecdotal reports and preclinical studies have highlighted statins as a potential therapy for SLOS.

OBJECTIVES

To evaluate the effects of statins, either alone or in combination with other non-statin therapies (e.g. cholesterol, bile acid, or vitamin co-supplementation), compared to cholesterol supplementation alone or in combination with other non-statin therapies (e.g. bile acid or vitamin supplementation) on several important outcomes including overall survival, neurobehavioral features, and adverse effects in individuals with SLOS.

SEARCH METHODS

We searched CENTRAL, MEDLINE, Embase, five other databases and three trials registers on 15 February 2022, together with reference checking, citation searching and contact with study authors to identify additional studies.

SELECTION CRITERIA

Randomized controlled trials (RCTs) and quasi-RCTs with parallel or cross-over designs, and non-randomized studies of interventions (NRSIs) including non-randomized trials, cohort studies, and controlled before-and-after studies, were eligible for inclusion in this review if they met our prespecified inclusion criteria, i.e. involved human participants with biochemically or genetically diagnosed SLOS receiving statin therapy or cholesterol supplementation, or both.

DATA COLLECTION AND ANALYSIS

Two authors screened titles and abstracts and subsequently full-texts for all potentially-relevant references. Both authors independently extracted relevant data from included studies and assessed the risks of bias. We analyzed the data extracted from the included NRSIs and cohort studies separately from the data extracted from the single included RCT. We used a random-effects model to account for the inherent heterogeneity and methodological variation between these different study designs. We used GRADE to assess the certainty of evidence.

MAIN RESULTS

We included six studies (61 participants with SLOS); one RCT (N = 18), three prospective NRSIs (N = 20), and two retrospective NRSIs (N = 22). Five studies included only children, and two limited their participant inclusion by disease severity. Overall, there were nearly twice as many males as females. All six studies compared add-on statin therapy to cholesterol supplementation alone. However, the dosages, formulations, and durations of treatment were highly variable across studies. We judged the RCT as having a high risk of bias due to missing data and selective reporting. All included NRSIs had a serious or critical overall risk of bias assessed by the Risk Of Bias In Non-randomized Studies of Interventions tool (ROBINS-I). None of the included studies evaluated survival or reported quality of life (QoL). Only the included RCT formally assessed changes in the neurobehavioral manifestations of SLOS, and we are uncertain whether statin therapy improves this outcome (very low-certainty evidence). We are also uncertain whether the adverse events reported in the RCT were statin-related (very low-certainty evidence). In contrast, the adverse events reported in the NRSIs seem to be possibly due to statin therapy (risk ratio 13.00, 95% confidence interval 1.85 to 91.49; P = 0.01; low-certainty evidence), with only one of the NRSIs retrospectively mentioning changes in the irritability of two of their participants. We are uncertain whether statins affect growth based on the RCT or NRSI results (very low-certainty evidence). The RCT showed that statins may make little or no difference to plasma biomarker levels (low-certainty evidence), while we are uncertain of their effects on such parameters in the NRSIs (very low-certainty evidence).

AUTHORS' CONCLUSIONS

Currently, there is no evidence on the potential effects of statin therapy in people with SLOS regarding survival or QoL, and very limited evidence on the effects on neurobehavioral manifestations. Likewise, current evidence is insufficient and of very low certainty regarding the effects of statins on growth parameters in children with SLOS and plasma or cerebrospinal fluid (CSF) levels of various disease biomarkers. Despite these limitations, current evidence seemingly suggests that statins may increase the risk of adverse reactions in individuals with SLOS receiving statins compared to those who are not. Given the insufficient evidence on potential benefits of statins in individuals with SLOS, and their potential for causing adverse reactions, anyone considering this therapy should take these findings into consideration. Future studies should address the highlighted gaps in evidence on the use of statins in individuals with SLOS by collecting prospective data on survival and performing serial standardized assessments of neurobehavioral features, QoL, anthropometric measures, and plasma and CSF biomarker levels after statin introduction. Future studies should also attempt to use consistent dosages, formulations and durations of cholesterol and statin therapy.

Structural dynamics of Smoothened (SMO) in the ciliary membrane and its interaction with membrane lipids

The Smoothened receptor (SMO, a 7 pass transmembrane domain, Class F GPCR family protein) plays a crucial role in the Hedgehog (HH) signaling pathway, which is involved in embryonic development and is implicated in various types of cancer throughout the animal kingdom. In the absence of HH signaling, SMO is inhibited by Patched 1 (PTC1; a 12 pass transmembrane domain protein), which is localized in the primary cilia. HH binding leads to the dislocation of PTC1 from the cilia, thus making way for SMO to localize in the primary cilia, as an essential prerequisite for its activation. We have carried out MARTINI coarse-grained molecular dynamics simulations of SMO in POPC and in ciliary membrane models, respectively, to study the interactions of SMO with cholesterol and other lipid molecules in the ciliary membrane, and to gain molecular-level insights into the role of the primary cilia in shaping the functional dynamics of SMO. We are able to identify the interaction of membrane cholesterols with definite sites and domains within SMO and relate them with known cholesterol-binding sequence and structure motifs. We show that cholesterol interactions with the transmembrane domain TMD, unlike those with the cysteine-rich domain (CRD) and the intracellular domain (ICD), are through residues belonging to known cholesterol-binding motifs. Notably, a few persistent interactions of cholesterol with lower TM cholesterol-binding domains are governed by the presence of multiple cholesterol-binding motifs. These analyses have also helped to identify and define a strict cholesterol consensus motif (CCM), which may well steer cholesterol into the hitherto identified binding sites within the TMD of SMO. We have also reported the interaction of phosphatidylinositol 4-phosphate with the intracellular region of transmembrane (TM) helices (TM1, TM3, TM4, and TM5), intracellular loop1, helix8, and Arg/Lys clusters of the ICD. Structural analysis of SMO domains shows significant changes in the CRD and ICD, during the course of the simulation. Further detailed analysis of the dynamics of the TMD reveals the movements of TM5, TM6, and TM7, linked with the helix8, which are possibly involved in shaping the conformational disposition of the ICD. The movement of these TM helices could possibly be a consequence of interactions involving the extracellular domain and extracellular loops. In addition, our analysis also shows that phosphatidylinositol-4-phosphate (PI4P), along with some ICD cholesterols, are implicated in anchoring SMO in the membrane.

Ciliary membrane, localised lipid modification and cilia function

Cilium, a tiny microtubule-based cellular appendage critical for cell signalling and physiology, displays a large variety of receptors. The composition and turnover of ciliary lipids and receptors determine cell behaviour. Due to the exclusion of ribosomal machinery and limited membrane area, a cilium needs adaptive logistics to actively reconstitute the lipid and receptor compositions during development and differentiation. How is this dynamicity generated? Here, we examine whether, along with the Intraflagellar-Transport, targeted changes in sector-wise lipid composition could control the receptor localisation and functions in the cilia. We discuss how an interplay between ciliary lipid composition, localised lipid modification, and receptor function could contribute to cilia growth and signalling. We argue that lipid modification at the cell-cilium interface could generate an added thrust for a selective exchange of membrane lipids and the transmembrane and membrane-associated proteins.

Pregnane X receptor (PXR) represses osteoblast differentiation through repression of the Hedgehog signaling pathway

The pregnane X receptor (PXR, NR1I2) belongs to the nuclear receptor family and functions as a xenobiotic and endobiotic sensor by binding to various molecules through its relatively flexible ligand-binding domain. In addition to these well-known canonical roles, we previously reported that PXR represses osteoblast differentiation. However, the mechanisms underlying the PXR-mediated repression of osteoblast differentiation remains unknown. In this study, we analyzed the changes in global gene expression profiles induced by PXR in calvarial osteoblasts cultured in standard fetal bovine serum (in which PXR induces repression of differentiation), and in those cultured in charcoal-stripped fetal bovine serum (in which PXR does not induce repression of differentiation). The comparison revealed that PXR attenuated the Hedgehog-mediated signaling in culture conditions that induced PXR-mediated repression of differentiation. Real-time PCR analysis showed that PXR repressed the Hedgehog signaling-induced genes such as Gli1 and Hhip, and conversely induced the Hedgehog signaling-repressed genes such as Cdon, Boc, and Gas1. Activation of Smo-mediated signaling in osteoblasts following treatment with a Smo agonist (SAG) significantly restored Gli-mediated transcriptional activity and osteoblast differentiation. Our results demonstrate the osteoblast-autonomous effects of PXR and identify a novel regulation of Hedgehog signaling by nuclear receptors.

Sterol regulation of developmental and oncogenic Hedgehog signaling

The Hedgehog (Hh) family of lipid-modified signaling proteins directs embryonic tissue patterning and postembryonic tissue homeostasis, and dysregulated Hh signaling drives familial and sporadic cancers. Hh ligands bind to and inhibit the tumor suppressor Patched and allow the oncoprotein Smoothened (SMO) to accumulate in cilia, which in turn activates the GLI family of transcription factors. Recent work has demonstrated that endogenous cholesterol and oxidized cholesterol derivatives (oxysterols) bind and modulate SMO activity. Here we discuss the myriad sterols that activate or inhibit the Hh pathway, with emphasis on endogenous 24(S),25-epoxycholesterol and 3β,5α-dihydroxycholest-7-en-6-one, and propose models of sterol regulation of SMO. Synthetic inhibitors of SMO have long been the focus of drug development efforts. Here, we discuss the possible utility of steroidal SMO ligands or inhibitors of enzymes involved in sterol metabolism as cancer therapeutics.

High-efficient CRISPR/Cas9-mediated gene targeting to establish cell models of ciliopathies

Primary cilia are antenna-like structures developed on the cell surface of mammalian cells during the quiescent G₀ phase. Primary cilia in mammalian cells receive extracellular signals for early development and cell tissue homeostasis. Ciliopathies characterized with congenital anomalies such as cerebellar hypoplasia, polycystic kidney and polydactyly are caused by germline mutations of ciliary structure- and function-related genes. Gene knock-out techniques in ciliated cultured cells with the uniformed genetic background are useful to evaluate the pathophysiological roles of ciliopathy-related gene products. Genome editing technology has been applied into the gene knock-out in many types of cultured cell lines. However, the frequency of genome editing varies according to cell species and cycle because of dependency on error-free homology-directed repair (HDR) activity. The human telomerase reverse transcriptase-immortalized retinal pigmented epithelial cell line (hTERT-RPE1) is well known for its suitability in cilia research. However, the efficacy of the HDR-mediated knock-out clone isolation was low. Here, we introduce the clustered regularly interspaced short palindromic repeats-obligate ligation-gated recombination (CRISPR-ObLiGaRe) system, which is a nonhomologous end-joining (NHEJ)-mediated gene targeting method, to generate the knock-out clones effectively even in the lower-HDR activity cell lines including hTERT-RPE1 cell. This CRISPR-ObLiGaRe system is a powerful tool for establishing ciliopathy model cell libraries and identifying each gene function in cilia-related phenotypes.

Sterols lower energetic barriers of membrane bending and fission necessary for efficient clathrin-mediated endocytosis

Clathrin-mediated endocytosis (CME) is critical for cellular signal transduction, receptor recycling, and membrane homeostasis in mammalian cells. Acute depletion of cholesterol disrupts CME, motivating analysis of CME dynamics in the context of human disorders of cholesterol metabolism. We report that inhibition of post-squalene cholesterol biosynthesis impairs CME. Imaging of membrane bending dynamics and the CME pit ultrastructure reveals prolonged clathrin pit lifetimes and shallow clathrin-coated structures, suggesting progressive impairment of curvature generation correlates with diminishing sterol abundance. Sterol structural requirements for efficient CME include 3′ polar head group and B-ring conformation, resembling the sterol structural prerequisites for tight lipid packing and polarity. Furthermore, Smith-Lemli-Opitz fibroblasts with low cholesterol abundance exhibit deficits in CME-mediated transferrin internalization. We conclude that sterols lower the energetic costs of membrane bending during pit formation and vesicular scission during CME and suggest that reduced CME activity may contribute to cellular phenotypes observed within disorders of cholesterol metabolism.

Anderson et al. demonstrate that sterol abundance and identity play a dominant role in facilitating clathrin-mediated endocytosis. Detailed analyses of clathrin-coated pits under sterol depletion support a requirement for sterol-mediated membrane bending during multiple stages of endocytosis, implicating endocytic dysfunction within the pathogenesis of disorders of cholesterol metabolism.

Possible Association of Cholesterol as a Biomarker in Suicide Behavior

Suicides and suicidal behavior are major causes of mortality and morbidity in public health and are a global problem. Various authors have proposed changes in lipid metabolism (total cholesterol decrease) as a possible biological marker for suicidal behavior. The objective of this study was to review the studies that have demonstrated a relationship between serum cholesterol levels and suicidal behavior and to describe the possible pathophysiological mechanisms that associate changes in cholesterol concentration and suicidal behavior. Relevant literature related to serum cholesterol levels and suicidal behavior was identified through various database searches. The data from the existing literature present the findings that relate low cholesterol levels and possible pathophysiological mechanisms (neuroinflammation, serotonergic neurotransmission), genes related to cholesterol synthesis, pharmacological treatments that alter lipid metabolism and the possible participation in suicidal behavior. Nevertheless, future research is required to describe how serum cholesterol affects cholesterol metabolism in the CNS to establish and understand the role of cholesterol in suicidal behavior.

Forebrain Shh overexpression improves cognitive function and locomotor hyperactivity in an aneuploid mouse model of Down syndrome and its euploid littermates

Down syndrome (DS) is the leading genetic cause of intellectual disability and causes early-onset dementia and cerebellar hypoplasia. The prevalence of attention deficit hyperactivity disorder is elevated in children with DS. The aneuploid DS mouse model "Ts65Dn" shows prominent brain phenotypes, including learning and memory deficits, cerebellar hypoplasia, and locomotor hyperactivity. Previous studies indicate that impaired Sonic hedgehog (Shh) signaling contributes to neurological phenotypes associated with DS and neurodegenerative diseases. However, because of a lack of working inducible Shh knock-in mice, brain region-specific Shh overexpression and its effects on cognitive function have not been studied in vivo. Here, with Gli1-LacZ reporter mice, we demonstrated that Ts65Dn had reduced levels of Gli1, a sensitive readout of Shh signaling, in both hippocampus and cerebellum at postnatal day 6. Through site-specific transgenesis, we generated an inducible human Shh knock-in mouse, TRE-bi-hShh-Zsgreen1 (TRE-hShh), simultaneously expressing dually-lipidated Shh-Np and Zsgreen1 marker in the presence of transactivator (tTA). Double transgenic mice "Camk2a-tTA;TRE-hShh" and "Pcp2-tTA;TRE-hShh" induced Shh overexpression and activated Shh signaling in a forebrain and cerebellum, respectively, specific manner from the perinatal period. Camk2a-tTA;TRE-hShh normalized locomotor hyperactivity and improved learning and memory in 3-month-old Ts65Dn, mitigated early-onset severe cognitive impairment in 7-month-old Ts65Dn, and enhanced spatial cognition in euploid mice. Camk2a-tTA;TRE-hShh cohort maintained until 600days old showed that chronic overexpression of Shh in forebrain from the perinatal period had no effect on longevity of euploid or Ts65Dn. Pcp2-tTA;TRE-hShh did not affect cognition but mitigated the phenotype of cerebellar hypoplasia in Ts65Dn. Our study provides the first in vivo evidence that Shh overexpression from the perinatal period protects DS brain integrity and enhances learning and memory in normal mice, indicating the broad therapeutic potential of Shh ligand for other neurological conditions. Moreover, the first inducible hShh site-specific knock-in mouse could be widely used for spatiotemporal Shh signaling regulation.

Cholesterol Contributes to Male Sex Differentiation Through Its Developmental Role in Androgen Synthesis and Hedgehog Signaling

Two specialized functions of cholesterol during fetal development include serving as a precursor to androgen synthesis and supporting hedgehog (HH) signaling activity. Androgens are produced by the testes to facilitate masculinization of the fetus. Recent evidence shows that intricate interactions between the HH and androgen signaling pathways are required for optimal male sex differentiation and defects of either can cause birth anomalies indicative of 46,XY male variations of sex development (VSD). Further, perturbations in cholesterol synthesis can cause developmental defects, including VSD, that phenocopy those caused by disrupted androgen or HH signaling, highlighting the functional role of cholesterol in promoting male sex differentiation. In this review, we focus on the role of cholesterol in systemic androgen and local HH signaling events during fetal masculinization and their collective contributions to pediatric VSD.

Autism Spectrum Disorder: Signaling Pathways and Prospective Therapeutic Targets

The Autism Spectrum Disorder (ASD) consists of a prevalent and heterogeneous group of neurodevelopmental diseases representing a severe burden to affected individuals and their caretakers. Despite substantial improvement towards understanding of ASD etiology and pathogenesis, as well as increased social awareness and more intensive research, no effective drugs have been successfully developed to resolve the main and most cumbersome ASD symptoms. Hence, finding better treatments, which may act as "disease-modifying" agents, and novel biomarkers for earlier ASD diagnosis and disease stage determination are needed. Diverse mutations of core components and consequent malfunctions of several cell signaling pathways have already been found in ASD by a series of experimental platforms, including genetic associations analyses and studies utilizing pre-clinical animal models and patient samples. These signaling cascades govern a broad range of neurological features such as neuronal development, neurotransmission, metabolism, and homeostasis, as well as immune regulation and inflammation. Here, we review the current knowledge on signaling pathways which are commonly disrupted in ASD and autism-related conditions. As such, we further propose ways to translate these findings into the development of genetic and biochemical clinical tests for early autism detection. Moreover, we highlight some putative druggable targets along these pathways, which, upon further research efforts, may evolve into novel therapeutic interventions for certain ASD conditions. Lastly, we also refer to the crosstalk among these major signaling cascades as well as their putative implications in therapeutics. Based on this collective information, we believe that a timely and accurate modulation of these prominent pathways may shape the neurodevelopment and neuro-immune regulation of homeostatic patterns and, hopefully, rescue some (if not all) ASD phenotypes.

Bile acid biosynthesis in Smith-Lemli-Opitz syndrome bypassing cholesterol: Potential importance of pathway intermediates

Bile acids are the end products of cholesterol metabolism secreted into bile. They are essential for the absorption of lipids and lipid soluble compounds from the intestine. Here we have identified a series of unusual Δ5-unsaturated bile acids in plasma and urine of patients with Smith-Lemli-Opitz syndrome (SLOS), a defect in cholesterol biosynthesis resulting in elevated levels of 7-dehydrocholesterol (7-DHC), an immediate precursor of cholesterol. Using liquid chromatography - mass spectrometry (LC-MS) we have uncovered a pathway of bile acid biosynthesis in SLOS avoiding cholesterol starting with 7-DHC and proceeding through 7-oxo and 7β-hydroxy intermediates. This pathway also occurs to a minor extent in healthy humans, but elevated levels of pathway intermediates could be responsible for some of the features SLOS. The pathway is also active in SLOS affected pregnancies as revealed by analysis of amniotic fluid. Importantly, intermediates in the pathway, 25-hydroxy-7-oxocholesterol, (25R)26-hydroxy-7-oxocholesterol, 3β-hydroxy-7-oxocholest-5-en-(25R)26-oic acid and the analogous 7β-hydroxysterols are modulators of the activity of Smoothened (Smo), an oncoprotein that mediates Hedgehog (Hh) signalling across membranes during embryogenesis and in the regeneration of postembryonic tissue. Computational docking of the 7-oxo and 7β-hydroxy compounds to the extracellular cysteine rich domain of Smo reveals that they bind in the same groove as both 20S-hydroxycholesterol and cholesterol, known activators of the Hh pathway.

Sterol and oxysterol synthases near the ciliary base activate the Hedgehog pathway

Vertebrate Hedgehog signals are transduced through the primary cilium, a specialized lipid microdomain that is required for Smoothened activation. Cilia-associated sterol and oxysterol lipids bind to Smoothened to activate the Hedgehog pathway, but how ciliary lipids are regulated is incompletely understood. Here we identified DHCR7, an enzyme that produces cholesterol, activates the Hedgehog pathway, and localizes near the ciliary base. We found that Hedgehog stimulation negatively regulates DHCR7 activity and removes DHCR7 from the ciliary microenvironment, suggesting that DHCR7 primes cilia for Hedgehog pathway activation. In contrast, we found that Hedgehog stimulation positively regulates the oxysterol synthase CYP7A1, which accumulates near the ciliary base and produces oxysterols that promote Hedgehog signaling in response to pathway activation. Our results reveal that enzymes involved in lipid biosynthesis in the ciliary microenvironment promote Hedgehog signaling, shedding light on how ciliary lipids are established and regulated to transduce Hedgehog signals.

Sonic Hedgehog Pathway Modulation Normalizes Expression of Olig2 in Rostrally Patterned NPCs With Trisomy 21

The intellectual disability found in people with Down syndrome is associated with numerous changes in early brain development, including the proliferation and differentiation of neural progenitor cells (NPCs) and the formation and maintenance of myelin in the brain. To study how early neural precursors are affected by trisomy 21, we differentiated two isogenic lines of induced pluripotent stem cells derived from people with Down syndrome into brain-like and spinal cord-like NPCs and promoted a transition towards oligodendroglial fate by activating the Sonic hedgehog (SHH) pathway. In the spinal cord-like trisomic cells, we found no difference in expression of OLIG2 or NKX2.2, two transcription factors essential for commitment to the oligodendrocyte lineage. However, in the brain-like trisomic NPCs, OLIG2 is significantly upregulated and is associated with reduced expression of NKX2.2. We found that this gene dysregulation and block in NPC transition can be normalized by increasing the concentration of a SHH pathway agonist (SAG) during differentiation. These results underscore the importance of regional and cell type differences in gene expression in Down syndrome and demonstrate that modulation of SHH signaling in trisomic cells can rescue an early perturbed step in neural lineage specification.

Stem cell function and plasticity in the normal physiology of the adrenal cortex

The adrenal cortex functions to produce steroid hormones necessary for life. To maintain its functional capacity throughout life, the adrenal cortex must be continually replenished and rapidly repaired following injury. Moreover, the adrenal responds to endocrine-mediated organismal needs, which are highly dynamic and necessitate a precise steroidogenic response. To meet these diverse needs, the adrenal employs multiple cell populations with stem cell function. Here, we discuss the literature on adrenocortical stem cells using hematopoietic stem cells as a benchmark to examine the functional capacity of particular cell populations, including those located in the capsule and peripheral cortex. These populations are coordinately regulated by paracrine and endocrine signaling mechanisms, and display remarkable plasticity to adapt to different physiological and pathological conditions. Some populations also exhibit sex-specific activity, which contributes to highly divergent proliferation rates between sexes. Understanding mechanisms that govern adrenocortical renewal has broad implications for both regenerative medicine and cancer.

Cholesterol access in cellular membranes controls Hedgehog signaling

The Hedgehog (Hh) signaling pathway coordinates cell-cell communication in development and regeneration. Defects in this pathway underlie diseases ranging from birth defects to cancer. Hh signals are transmitted across the plasma membrane by two proteins, Patched 1 (PTCH1) and Smoothened (SMO). PTCH1, a transporter-like tumor-suppressor protein, binds to Hh ligands, but SMO, a G-protein-coupled-receptor family oncoprotein, transmits the Hh signal across the membrane. Recent structural, biochemical and cell-biological studies have converged at the surprising model that a specific pool of plasma membrane cholesterol, termed accessible cholesterol, functions as a second messenger that conveys the signal between PTCH1 and SMO. Beyond solving a central puzzle in Hh signaling, these studies are revealing new principles in membrane biology: how proteins respond to and remodel cholesterol accessibility in membranes and how the cholesterol composition of organelle membranes is used to regulate protein function.

Hedgehog Signaling in Skeletal Development: Roles of Indian Hedgehog and the Mode of Its Action

Hedgehog (Hh) signaling is highly conserved among species and plays indispensable roles in various developmental processes. There are three Hh members in mammals; one of them, Indian hedgehog (Ihh), is expressed in prehypertrophic and hypertrophic chondrocytes during endochondral ossification. Based on mouse genetic studies, three major functions of Ihh have been proposed: (1) Regulation of chondrocyte differentiation via a negative feedback loop formed together with parathyroid hormone-related protein (PTHrP), (2) promotion of chondrocyte proliferation, and (3) specification of bone-forming osteoblasts. Gli transcription factors mediate the major aspect of Hh signaling in this context. Gli3 has dominant roles in the growth plate chondrocytes, whereas Gli1, Gli2, and Gli3 collectively mediate biological functions of Hh signaling in osteoblast specification. Recent studies have also highlighted postnatal roles of the signaling in maintenance and repair of skeletal tissues.

Rare hypomorphic human variation in the heptahelical domain of SMO contributes to holoprosencephaly phenotypes

Holoprosencephaly (HPE) is the most common congenital anomaly affecting the forebrain and face in humans and occurs as frequently as 1:250 conceptions or 1:10,000 livebirths. Sonic Hedgehog signaling molecule is one of the best characterized HPE genes that plays crucial roles in numerous developmental processes including midline neural patterning and craniofacial development. The Frizzled class G-protein coupled receptor Smoothened (SMO), whose signaling activity is tightly regulated, is the sole obligate transducer of Hedgehog-related signals. However, except for previous reports of somatic oncogenic driver mutations in human cancers (or mosaic tumors in rare syndromes), any potential disease-related role of SMO genetic variation in humans is largely unknown. To our knowledge, ours is the first report of a human hypomorphic variant revealed by functional testing of seven distinct nonsynonymous SMO variants derived from HPE molecular and clinical data. Here we describe several zebrafish bioassays developed and guided by a systems biology analysis. This analysis strategy, and detection of hypomorphic variation in human SMO, demonstrates the necessity of integrating the genomic variant findings in HPE probands with other components of the Hedgehog gene regulatory network in overall medical interpretations.

The role of Shh signalling pathway in central nervous system development and related diseases

Sonic hedgehog (Shh) plays important roles in developmental of vertebrate animal central nervous system (CNS), and Gli is its downstream signal molecule. Shh signalling is essential for pattern formation, cell-fate specification, axon guidance, proliferation, survival and differentiation of neurons in CNS development. The abnormal signalling pathway of Shh leads to the occurrence of many nervous system diseases. The mechanism of Shh signalling is complex and remains incompletely understood. Nevertheless, studies have revealed that Shh signalling pathway is classified into canonical and non-canonical pathways. Here we review the role of the Shh signalling pathway and its impact in CNS development and related diseases. Specifically, we discuss the role of Shh in the spinal cord and brain development, cell differentiation and proliferation in CNS and related diseases such as brain tumour, Parkinson's diseases, epilepsy, autism, depression and traumatic brain injury. We also highlight future directions of research that could help to clarify the mechanisms and consequences of Shh signalling in the process of CNS development and related diseases. SIGNIFICANCE OF THE STUDY: This review summarized the role of Shh signalling pathway in CNS development and related diseases such as brain tumour, Parkinson's diseases, epilepsy, autism, depression and traumatic brain injury. It also presented the author's opinions on the future research direction of Shh signalling pathway.

TMEM147 interacts with lamin B receptor, regulates its localization and levels, and affects cholesterol homeostasis

The structurally and functionally complex endoplasmic reticulum (ER) hosts critical processes including lipid synthesis. Here, we focus on the functional characterization of transmembrane protein TMEM147, and report that it localizes at the ER and nuclear envelope in HeLa cells. Silencing of TMEM147 drastically reduces the level of lamin B receptor (LBR) at the inner nuclear membrane and results in mistargeting of LBR to the ER. LBR possesses a modular structure and corresponding bifunctionality, acting in heterochromatin organization via its N-terminus and in cholesterol biosynthesis via its sterol-reductase C-terminal domain. We show that TMEM147 physically interacts with LBR, and that the C-terminus of LBR is essential for their functional interaction. We find that TMEM147 also physically interacts with the key sterol reductase DHCR7, which is involved in cholesterol biosynthesis. Similar to what was seen for LBR, TMEM147 downregulation results in a sharp decline of DHCR protein levels and co-ordinate transcriptional decreases of LBR and DHCR7 expression. Consistent with this, lipidomic analysis upon TMEM147 silencing identified changes in cellular cholesterol levels, cholesteryl ester levels and profile, and in cellular cholesterol uptake, raising the possibility that TMEM147 is an important new regulator of cholesterol homeostasis in cells.This article has an associated First Person interview with the first author of the paper.

Loss-of-Function Variants in TBC1D32 Underlie Syndromic Hypopituitarism

CONTEXT

Congenital pituitary hormone deficiencies with syndromic phenotypes and/or familial occurrence suggest genetic hypopituitarism; however, in many such patients the underlying molecular basis of the disease remains unknown.

OBJECTIVE

To describe patients with syndromic hypopituitarism due to biallelic loss-of-function variants in TBC1D32, a gene implicated in Sonic Hedgehog (Shh) signaling.

SETTING

Referral center.

PATIENTS

A Finnish family of 2 siblings with panhypopituitarism, absent anterior pituitary, and mild craniofacial dysmorphism, and a Pakistani family with a proband with growth hormone deficiency, anterior pituitary hypoplasia, and developmental delay.

INTERVENTIONS

The patients were investigated by whole genome sequencing. Expression profiling of TBC1D32 in human fetal brain was performed through in situ hybridization. Stable and dynamic protein-protein interaction partners of TBC1D32 were investigated in HEK cells followed by mass spectrometry analyses.

MAIN OUTCOME MEASURES

Genetic and phenotypic features of patients with biallelic loss-of-function mutations in TBC1D32.

RESULTS

The Finnish patients harboured compound heterozygous loss-of-function variants (c.1165_1166dup p.(Gln390Phefs*32) and c.2151del p.(Lys717Asnfs*29)) in TBC1D32; the Pakistani proband carried a known pathogenic homozygous TBC1D32 splice-site variant c.1372 + 1G > A p.(Arg411_Gly458del), as did a fetus with a cleft lip and partial intestinal malrotation from a terminated pregnancy within the same pedigree. TBC1D32 was expressed in the developing hypothalamus, Rathke's pouch, and areas of the hindbrain. TBC1D32 interacted with proteins implicated in cilium assembly, Shh signaling, and brain development.

CONCLUSIONS

Biallelic TBC1D32 variants underlie syndromic hypopituitarism, and the underlying mechanism may be via disrupted Shh signaling.

Insufficiency of ciliary cholesterol in hereditary Zellweger syndrome

Primary cilia are antenna‐like organelles on the surface of most mammalian cells that receive sonic hedgehog (Shh) signaling in embryogenesis and carcinogenesis. Cellular cholesterol functions as a direct activator of a seven‐transmembrane oncoprotein called Smoothened (Smo) and thereby induces Smo accumulation on the ciliary membrane where it transduces the Shh signal. However, how cholesterol is supplied to the ciliary membrane remains unclear. Here, we report that peroxisomes are essential for the transport of cholesterol into the ciliary membrane. Zellweger syndrome (ZS) is a peroxisome‐deficient hereditary disorder with several ciliopathy‐related features and cells from these patients showed a reduced cholesterol level in the ciliary membrane. Reverse genetics approaches revealed that the GTP exchange factor Rabin8, the Rab GTPase Rab10, and the microtubule minus‐end‐directed kinesin KIFC3 form a peroxisome‐associated complex to control the movement of peroxisomes along microtubules, enabling communication between peroxisomes and ciliary pocket membranes. Our findings suggest that insufficient ciliary cholesterol levels may underlie ciliopathies.

Metabolic and pathologic profiles of human LSS deficiency recapitulated in mice

Skin lesions, cataracts, and congenital anomalies have been frequently associated with inherited deficiencies in enzymes that synthesize cholesterol. Lanosterol synthase (LSS) converts (S)-2,3-epoxysqualene to lanosterol in the cholesterol biosynthesis pathway. Biallelic mutations in LSS have been reported in families with congenital cataracts and, very recently, have been reported in cases of hypotrichosis. However, it remains to be clarified whether these phenotypes are caused by LSS enzymatic deficiencies in each tissue, and disruption of LSS enzymatic activity in vivo has not yet been validated. We identified two patients with novel biallelic LSS mutations who exhibited congenital hypotrichosis and midline anomalies but did not have cataracts. We showed that the blockade of the LSS enzyme reaction occurred in the patients by measuring the (S)-2,3-epoxysqualene/lanosterol ratio in the forehead sebum, which would be a good biomarker for the diagnosis of LSS deficiency. Epidermis-specific Lss knockout mice showed neonatal lethality due to dehydration, indicating that LSS could be involved in skin barrier integrity. Tamoxifen-induced knockout of Lss in the epidermis caused hypotrichosis in adult mice. Lens-specific Lss knockout mice had cataracts. These results confirmed that LSS deficiency causes hypotrichosis and cataracts due to loss-of-function mutations in LSS in each tissue. These mouse models will lead to the elucidation of the pathophysiological mechanisms associated with disrupted LSS and to the development of therapeutic treatments for LSS deficiency.

Signaling in the primary cilium through the lens of the Hedgehog pathway

Cilia are microtubule-based, cell-surface projections whose machinery is evolutionarily conserved. In vertebrates, cilia are observed on almost every cell type and are either motile or immotile. Immotile sensory, or primary cilia, are responsive to extracellular ligands and signals. Cilia can be thought of as compartments, functionally distinct from the cell that provides an environment for signaling cascades. Hedgehog is a critical developmental signaling pathway which is functionally linked to primary cilia in vertebrates. The major components of the vertebrate Hedgehog signaling pathway dynamically localize to the ciliary compartment and ciliary membrane. Critically, G-protein coupled receptor (GPCR) Smoothened, the obligate transducer of the pathway, is enriched and activated in the cilium. While Smoothened is the most intensely studied ciliary receptor, many GPCRs localize within cilia. Understanding the link between Smoothened and cilia defines common features, and distinctions, of GPCR signaling within the primary cilium. This article is categorized under: Signaling Pathways > Global Signaling Mechanisms Signaling Pathways > Cell Fate Signaling.

Disruption of Dhcr7 and Insig1/2 in cholesterol metabolism causes defects in bone formation and homeostasis through primary cilium formation

Human linkage studies suggest that craniofacial deformities result from either genetic mutations related to cholesterol metabolism or high-cholesterol maternal diets. However, little is known about the precise roles of intracellular cholesterol metabolism in the development of craniofacial bones, the majority of which are formed through intramembranous ossification. Here, we show that an altered cholesterol metabolic status results in abnormal osteogenesis through dysregulation of primary cilium formation during bone formation. We found that cholesterol metabolic aberrations, induced through disruption of either Dhcr7 (which encodes an enzyme involved in cholesterol synthesis) or Insig1 and Insig2 (which provide a negative feedback mechanism for cholesterol biosynthesis), result in osteoblast differentiation abnormalities. Notably, the primary cilia responsible for sensing extracellular cues were altered in number and length through dysregulated ciliary vesicle fusion in Dhcr7 and Insig1/2 mutant osteoblasts. As a consequence, WNT/β-catenin and hedgehog signaling activities were altered through dysregulated primary cilium formation. Strikingly, the normalization of defective cholesterol metabolism by simvastatin, a drug used in the treatment of cholesterol metabolic aberrations, rescued the abnormalities in both ciliogenesis and osteogenesis in vitro and in vivo. Thus, our results indicate that proper intracellular cholesterol status is crucial for primary cilium formation during skull formation and homeostasis.

Statins for Smith-Lemli-Opitz syndrome

This is a protocol for a Cochrane Review (Intervention). The objectives are as follows: 1. To evaluate the efficacy of statin therapy in reducing the frequency or severity of the neurobehavioral abnormalities seen in people with SLOS (e.g. aggression, anxiety, irritability, self-mutilation, autistic behaviors, sleep disturbances, etc.) (Wassif 2017). 2. To evaluate the potential effects of statin therapy on survival.

Association of genes with phenotype in autism spectrum disorder

Autism spectrum disorder (ASD) is a genetic heterogeneous neurodevelopmental disorder that is characterized by impairments in social interaction and speech development and is accompanied by stereotypical behaviors such as body rocking, hand flapping, spinning objects, sniffing and restricted behaviors. The considerable significance of the genetics associated with autism has led to the identification of many risk genes for ASD used for the probing of ASD specificity and shared cognitive features over the past few decades. Identification of ASD risk genes helps to unravel various genetic variants and signaling pathways which are involved in ASD. This review highlights the role of ASD risk genes in gene transcription and translation regulation processes, as well as neuronal activity modulation, synaptic plasticity, disrupted key biological signaling pathways, and the novel candidate genes that play a significant role in the pathophysiology of ASD. The current emphasis on autism spectrum disorders has generated new opportunities in the field of neuroscience, and further advancements in the identification of different biomarkers, risk genes, and genetic pathways can help in the early diagnosis and development of new clinical and pharmacological treatments for ASD.

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.

Cholesterol accessibility at the ciliary membrane controls hedgehog signaling

Previously we proposed that transmission of the hedgehog signal across the plasma membrane by Smoothened is triggered by its interaction with cholesterol (Luchetti et al., 2016). But how is cholesterol, an abundant lipid, regulated tightly enough to control a signaling system that can cause birth defects and cancer? Using toxin-based sensors that distinguish between distinct pools of cholesterol, we find that Smoothened activation and Hedgehog signaling are driven by a biochemically-defined, small fraction of membrane cholesterol, termed accessible cholesterol. Increasing cholesterol accessibility by depletion of sphingomyelin, which sequesters cholesterol in complexes, amplifies Hedgehog signaling. Hedgehog ligands increase cholesterol accessibility in the membrane of the primary cilium by inactivating the transporter-like protein Patched 1. Trapping this accessible cholesterol blocks Hedgehog signal transmission across the membrane. Our work shows that the organization of cholesterol in the ciliary membrane can be modified by extracellular ligands to control the activity of cilia-localized signaling proteins.

Smoothened stimulation by membrane sterols drives Hedgehog pathway activity

Hedgehog signalling is fundamental to embryonic development and postnatal tissue regeneration1. Aberrant postnatal Hedgehog signalling leads to several malignancies, including basal cell carcinoma and paediatric medulloblastoma2. Hedgehog proteins bind to and inhibit the transmembrane cholesterol transporter Patched-1 (PTCH1), which permits activation of the seven-transmembrane transducer Smoothened (SMO) via a mechanism that is poorly understood. Here we report the crystal structure of active mouse SMO bound to both the agonist SAG21k and to an intracellular binding nanobody that stabilizes a physiologically relevant active state. Analogous to other G protein-coupled receptors, the activation of SMO is associated with subtle motions in the extracellular domain, and larger intracellular changes. In contrast to recent models3-5, a cholesterol molecule that is critical for SMO activation is bound deep within the seven-transmembrane pocket. We propose that the inactivation of PTCH1 by Hedgehog allows a transmembrane sterol to access this seven-transmembrane site (potentially through a hydrophobic tunnel), which drives the activation of SMO. These results-combined with signalling studies and molecular dynamics simulations-delineate the structural basis for PTCH1-SMO regulation, and suggest a strategy for overcoming clinical resistance to SMO inhibitors.

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.

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.

Specification of positional identity in forebrain organoids

Human brain organoids generated with current technologies recapitulate histological features of the human brain, but they lack a reproducible topographic organization. During development, spatial topography is determined by gradients of signaling molecules released from discrete signaling centers. We hypothesized that introduction of a signaling center into forebrain organoids would specify the positional identity of neural tissue in a distance-dependent manner. Here, we present a system to trigger a sonic hedgehog (SHH) protein gradient in developing forebrain organoids that enables ordered self-organization along dorso-ventral and antero-posterior positional axes. SHH-patterned forebrain organoids establish major forebrain subdivisions that are positioned with in vivo-like topography. Consistent with its behavior in vivo, SHH exhibits long-range signaling activity in organoids. Finally, we use SHH-patterned cerebral organoids as a tool to study the role of cholesterol metabolism in SHH signaling. Together, this work identifies inductive signaling as an effective organizing strategy to recapitulate in vivo-like topography in human brain organoids.

Cholesterol Interaction Sites on the Transmembrane Domain of the Hedgehog Signal Transducer and Class F G Protein-Coupled Receptor Smoothened

Transduction of Hedgehog signals across the plasma membrane is facilitated by the class F G-protein-coupled-receptor (GPCR) Smoothened (SMO). Recent studies suggest that SMO is modulated via interactions of its transmembrane (TM) domain with cholesterol. We apply molecular dynamics simulations of SMO embedded in cholesterol containing lipid bilayers, revealing a direct interaction of cholesterol with the TM domain at regions distinct from those observed in class A GPCRs. In particular the extracellular tips of helices TM2 and TM3 form a well-defined cholesterol interaction site. Potential of mean force calculations yield a free energy landscape for cholesterol binding. Alongside analysis of equilibrium cholesterol occupancy, this reveals the existence of a dynamic "greasy patch" interaction with the TM domain of SMO, which may be compared with previously identified lipid interaction sites on other membrane proteins. These predictions provide molecular-level insights into cholesterol interactions with a class F GPCR, suggesting potential druggable sites.

Pharmacological cholesterol depletion disturbs ciliogenesis and ciliary function in developing zebrafish

Patients with an inherited inability to synthesize sufficient amounts of cholesterol develop congenital malformations of the skull, toes, kidney and heart. As development of these structures depends on functional cilia we investigated whether cholesterol regulates ciliogenesis through inhibition of hydroxymethylglutaryl-Coenzyme A reductase (HMG-CoA-R), the rate-limiting enzyme in cholesterol synthesis. HMG-CoA-R is efficiently inhibited by statins, a standard medication for hyperlipidemia. When zebrafish embryos are treated with statins cilia dysfunction phenotypes including heart defects, left-right asymmetry defects and malformation of ciliated organs develop, which are ameliorated by cholesterol replenishment. HMG-CoA-R inhibition and other means of cholesterol reduction lowered ciliation frequency and cilia length in zebrafish as well as several mammalian cell types. Cholesterol depletion further triggers an inability for ciliary signalling. Because of a reduction of the transition zone component Pi(4,5)P2 we propose that cholesterol governs crucial steps of cilium extension. Taken together, we report that cholesterol abrogation provokes cilia defects.

Targeting Smoothened as a New Frontier in the Functional Recovery of Central Nervous System Demyelinating Pathologies

Myelin sheaths on vertebrate axons provide protection, vital support and increase the speed of neuronal signals. Myelin degeneration can be caused by viral, autoimmune or genetic diseases. Remyelination is a natural process that restores the myelin sheath and, consequently, neuronal function after a demyelination event, preventing neurodegeneration and thereby neuron functional loss. Pharmacological approaches to remyelination represent a promising new frontier in the therapy of human demyelination pathologies and might provide novel tools to improve adaptive myelination in aged individuals. Recent phenotypical screens have identified agonists of the atypical G protein-coupled receptor Smoothened and inhibitors of the glioma-associated oncogene 1 as being amongst the most potent stimulators of oligodendrocyte precursor cell (OPC) differentiation in vitro and remyelination in the central nervous system (CNS) of mice. Here, we discuss the current state-of-the-art of studies on the role of Sonic Hedgehog reactivation during remyelination, referring readers to other reviews for the role of Hedgehog signaling in cancer and stem cell maintenance.

Cilia-Associated Oxysterols Activate Smoothened

Primary cilia are required for Smoothened to transduce vertebrate Hedgehog signals, but how Smoothened accumulates in cilia and is activated is incompletely understood. Here, we identify cilia-associated oxysterols that promote Smoothened accumulation in cilia and activate the Hedgehog pathway. Our data reveal that cilia-associated oxysterols bind to two distinct Smoothened domains to modulate Smoothened accumulation in cilia and tune the intensity of Hedgehog pathway activation. We find that the oxysterol synthase HSD11β2 participates in the production of Smoothened-activating oxysterols and promotes Hedgehog pathway activity. Inhibiting oxysterol biosynthesis impedes oncogenic Hedgehog pathway activation and attenuates the growth of Hedgehog pathway-associated medulloblastoma, suggesting that targeted inhibition of Smoothened-activating oxysterol production may be therapeutically useful for patients with Hedgehog-associated cancers.