The Inseparable Relationship Between Cholesterol and Hedgehog Signaling

International Union of Basic and Clinical Pharmacology CXV: The Class F of G Protein-Coupled Receptors

The class F of G protein-coupled receptors (GPCRs) consists of 10 Frizzleds (FZD1-10) and Smoothened (SMO). FZDs bind and are activated by secreted lipoglycoproteins of the Wingless/Int-1 (WNT) family, and SMO is indirectly activated by the Hedgehog (Hh) family of morphogens acting on the transmembrane protein Patched. The advance of our understanding of FZDs and SMO as dynamic transmembrane receptors and molecular machines, which emerged during the past 14 years since the first-class F GPCR IUPHAR nomenclature report, justifies an update. This article focuses on the advances in molecular pharmacology and structural biology providing new mechanistic insight into ligand recognition, receptor activation mechanisms, signal initiation, and signal specification. Furthermore, class F GPCRs continue to develop as drug targets, and novel technologies and tools such as genetically encoded biosensors and CRISP/Cas9 edited cell systems have contributed to refined functional analysis of these receptors. Also, advances in crystal structure analysis and cryogenic electron microscopy contribute to the rapid development of our knowledge about structure-function relationships, providing a great starting point for drug development. Despite the progress, questions and challenges remain to fully understand the complexity of the WNT/FZD and Hh/SMO signaling systems. SIGNIFICANCE STATEMENT: The recent years of research have brought about substantial functional and structural insight into mechanisms of activation of Frizzleds and Smoothened. While the advance furthers our mechanistic understanding of ligand recognition, receptor activation, signal specification, and initiation, broader opportunities emerge that allow targeting class F GPCRs for therapy and regenerative medicine employing both biologics and small molecule compounds.

Let’s Talk about Sex Hormone Receptors and Their Physical Interaction with Sonic Hedgehog Protein: A Computational Study with Emphasis on Progesterone Receptor

The mature form of the sonic hedgehog protein (SHH-N) is the main canonical activator of the Hedgehog-GLI signaling pathway whose aberrant activity can lead to the development of hormone-dependent cancers like breast or prostate cancer. In this study, we employed computational methods to explore the potential binding of SHH-N with the progesterone receptor (PR), the sole member of the nuclear sex hormone receptor (SHRs) subfamily not previously linked to SHH-N. Through a combination of molecular docking, robust molecular dynamics (MD) simulations, and free energy calculations, we predicted a stable binding between SHH-N-cholesterol and PR. To validate our findings, we extended our in silico investigation to encompass the complexes between SHH-N-cholesterol and estrogen receptor alpha (ERα) and androgen receptor (AR)—complexes that have been experimentally confirmed in our prior studies. The calculations not only confirmed the stable binding of SHH-N-cholesterol with both ERα and AR but also revealed the strongest binding occurred with ERα, followed by AR and PR, suggesting a non-canonical interaction with potential biological significance. Microsecond-long MD simulations unveiled tight cholesterol binding in the SHRs’ binding sites, and we gained insights into sub-molecular interactions contributing to protein-protein stabilization in complexes involving PR and ERα for the first time. The MM/PBSA calculations indicated comparable binding affinities of PR for progesterone and SHH-N-cholesterol, with ERα exhibiting a more favorable enthalpy of binding with SHH-N-cholesterol than with estradiol.

Estimating the Cholesterol Affinity of Integral Membrane Proteins from Experimental Data

The cholesterol affinities of many integral plasma membrane proteins have been estimated by molecular computation. However, these values lack experimental confirmation. We therefore developed a simple mathematical model to extract sterol affinity constants and stoichiometries from published isotherms for the dependence of the activity of such proteins on the membrane cholesterol concentration. The binding curves for these proteins are sigmoidal, with strongly lagged thresholds attributable to competition for the cholesterol by bilayer phospholipids. The model provided isotherms that matched the experimental data using published values for the sterol association constants and stoichiometries of the phospholipids. Three oligomeric transporters were found to bind cholesterol without cooperativity, with dimensionless association constants of 35 for Kir3.4* and 100 for both Kir2 and a GAT transporter. (The corresponding ΔG° values were -8.8, -11.4, and -11.4 kJ/mol, respectively). These association constants are significantly lower than those for the phospholipids, which range from ∼100 to 6000. The BK channel, the nicotinic acetylcholine receptor, and the M192I mutant of Kir3.4* appear to bind multiple cholesterol molecules cooperatively (n = 2 or 4), with subunit affinities of 563, 950, and 700, respectively. The model predicts that the three less avid transporters are approximately half-saturated in their native plasma membranes; hence, they are sensitive to variations in cholesterol in vivo. The more avid proteins would be nearly saturated in vivo. The method can be applied to any integral protein or other ligands in any bilayer for which there are reasonable estimates of the sterol affinities and stoichiometries of the phospholipids.