Mariano Stornaiuolo

Assembly of a π–π stack of ligands in the binding site of an acetylcholine-binding protein

Acetylcholine-binding protein is a water-soluble homologue of the extracellular ligand-binding domain of cys-loop receptors. It is used as a structurally accessible prototype for studying ligand binding to these pharmaceutically important pentameric ion channels, in particular to nicotinic acetylcholine receptors, due to conserved binding site residues present at the interface between two subunits. Here we report that an aromatic conjugated small molecule binds acetylcholine-binding protein in an ordered π–π stack of three identical molecules per binding site, two parallel and one antiparallel. Acetylcholine-binding protein stabilizes the assembly of the stack by aromatic contacts. Thanks to the plasticity of its ligand-binding site, acetylcholine-binding protein can accommodate the formation of aromatic stacks of different size by simple loop repositioning and minimal adjustment of the interactions. This type of supramolecular binding provides a novel paradigm in drug design. AChBP is used as a structurally accessible prototype for studying ligand binding to nicotinic acetylcholine receptors. Stornaiuolo et al. report that a small molecule binds AChBP in an ordered p–p stack of three molecules per binding site, which may lead to new approaches in drug design.

Pharmacological folding chaperones act as allosteric ligands of Frizzled4

Upon binding, ligands can chaperone their protein targets by preventing them from misfolding and aggregating. Thus, an organic molecule that works as folding chaperone for a protein might be its specific ligand, and, similarly, the chaperone potential could represent an alternative readout in a molecular screening campaign toward the identification of new hits. Here we show that small molecules selected for acting as pharmacological chaperones on a misfolded mutant of the Frizzled4 (Fz4) receptor bind and modulate wild-type Fz4, representing what are to our knowledge the first organic ligands of this until-now-undruggable GPCR. The novelty and the advantages of the screening platform, the allosteric binding site addressed by these new ligands and the mechanism they use to modulate Fz4 suggest new avenues for development of inhibitors of the Wnt-β-catenin pathway and for drug discovery.

The cytosolic chaperone α-crystallin B rescues folding and compartmentalization of misfolded multispan transmembrane proteins.

Summary The &agr;-crystallin B chain (CRYAB or HspB5) is a cytosolic chaperone belonging to the small heat shock protein family, which is known to help in the folding of cytosolic proteins. Here we show that CRYAB binds the mutant form of at least two multispan transmembrane proteins (TMPs), exerting an anti-aggregation activity. It rescues the folding of mutant Frizzled4, which is responsible for a rare autosomal dominant form of familial exudative vitreoretinopathy (Fz4-FEVR), and the mutant ATP7B Cu transporter (ATP7B-H1069Q) associated with a common form of Wilsons disease. In the case of Fz4-FEVR, CRYAB prevents the formation of inter-chain disulfide bridges between the lumenal ectodomains of the aggregated mutant chains, which enables correct folding and promotes appropriate compartmentalization on the plasma membrane. ATP7B-H1069Q, with help from CRYAB, folds into the proper conformation, moves to the Golgi complex, and responds to copper overload in the same manner as wild-type ATP7B. These findings strongly suggest that CRYAB plays a pivotal role, previously undetected, in the folding of multispan TMPs and, from the cytosol, is able to orchestrate folding events that take place in the lumen of the ER. Our results contribute to the explanation of the complex scenario behind multispan TMP folding; additionally, they serve to expose interesting avenues for novel therapeutic approaches.

Transient transfection coupled to baculovirus infection for rapid protein expression screening in insect cells.

Baculovirus infected insect cells are widely used for heterologous protein expression. Despite the power of this system, the use of baculovirus techniques for protein expression screening is hampered by the time and resources needed to generate each recombinant baculovirus. Here, we show that a transfection/infection based expression system is suitable for screening of expression constructs in insect cells and represents a valid alternative to other traditional screening methodologies using recombinant baculovirus. The described method is based on gene delivery by transfection coupled to the induction of protein expression by non-recombinant baculovirus infection. Vectors that control expression by a combination of the baculovirus promoters ie1 and p10 and the enhancer element hr5 are among the ones suitable for this method. Infection with non-recombinant baculovirus drastically increases the basal activity of these elements, leading to protein over-expression. Multiple vectors can be simultaneously co-transfected/infected, making transfection/infection amenable for screening of multiple co-expressed proteins and protein complexes. Taken together, our results prove that the transfection/infection protocol is a valid and innovative approach for increasing speed and reducing costs of protein expression screening for structural and functional studies.

Reduced Frizzled Receptor 4 Expression Prevents WNT/β-Catenin–driven Alveolar Lung Repair in Chronic Obstructive Pulmonary Disease

&NA; Rationale: Chronic obstructive pulmonary disease (COPD), in particular emphysema, is characterized by loss of parenchymal alveolar tissue and impaired tissue repair. Wingless and INT‐1 (WNT)/&bgr;‐catenin signaling is reduced in COPD; however, the mechanisms thereof, specifically the role of the frizzled (FZD) family of WNT receptors, remain unexplored. Objectives: To identify and functionally characterize specific FZD receptors that control downstream WNT signaling in impaired lung repair in COPD. Methods: FZD expression was analyzed in lung homogenates and alveolar epithelial type II (ATII) cells of never‐smokers, smokers, patients with COPD, and two experimental COPD models by quantitative reverse transcriptase‐polymerase chain reaction, immunoblotting, and immunofluorescence. The functional effects of cigarette smoke on FZD4, WNT/&bgr;‐catenin signaling, and elastogenic components were investigated in primary ATII cells in vitro and in three‐dimensional lung tissue cultures ex vivo. Gain‐ and loss‐of‐function approaches were applied to determine the effects of FZD4 signaling on alveolar epithelial cell wound healing and repair, as well as on expression of elastogenic components. Measurements and Main Results: FZD4 expression was reduced in human and experimental COPD lung tissues as well as in primary human ATII cells from patients with COPD. Cigarette smoke exposure down‐regulated FZD4 expression in vitro and in vivo, along with reduced WNT/&bgr;‐catenin activity. Inhibition of FZD4 decreased WNT/&bgr;‐catenin‐driven epithelial cell proliferation and wound closure, and it interfered with ATII‐to‐ATI cell transdifferentiation and organoid formation, which were augmented by FZD4 overexpression. Moreover, FZD4 restoration by overexpression or pharmacological induction led to induction of WNT/&bgr;‐catenin signaling and expression of elastogenic components in three‐dimensional lung tissue cultures ex vivo. Conclusions: Reduced FZD4 expression in COPD contributes to impaired alveolar repair capacity.

Endogenous vs Exogenous Allosteric Modulators in GPCRs: A dispute for shuttling CB1 among different membrane microenvironments

A Cannabinoid Receptor 1 (CB1) binding site for the selective allosteric modulator ORG27569 is here identified through an integrate approach of consensus pocket prediction, mutagenesis studies and Mass Spectrometry. This unprecedented ORG27569 pocket presents the structural features of a Cholesterol Consensus Motif, a cholesterol interacting region already found in other GPCRs. ORG27569 and cholesterol affects oppositely CB1 affinity for orthosteric ligands. Moreover, the rise in cholesterol intracellular level results in CB1 trafficking to the axonal region of neuronal cells, while, on the contrary, ORG27568 binding induces CB1 enrichment at the soma. This control of receptor migration among functionally different membrane regions of the cell further contributes to downstream signalling and adds a previously unknown mechanism underpinning CB1 modulation by ORG27569 , that goes beyond a mere control of receptor affinity for orthosteric ligands.

A disorder-to-order structural transition in the COOH-tail of Fz4 determines misfolding of the L501fsX533-Fz4 mutant

Frizzled 4 belongs to the superfamily of G protein coupled receptors. The unstructured cytosolic tail of the receptor is essential for its activity. The mutation L501fsX533 in the fz4 gene results in a new COOH-tail of the receptor and causes a form of Familial exudative vitreoretinopathy. Here we show that the mutated tail is structured. Two amphipathic helices, displaying affinity for membranes and resembling the structure of Influenza Hemagglutinin fusion peptide, constitute the new fold. This tail induces the aggregation of the receptor in the Endoplasmic Reticulum and it is sufficient to block the export to the Golgi of a chimeric VSVG protein containing the mutated tail. Affecting the tails structure, net charge or amphipathicity relocates the mutated Fz4 receptor to the Plasma Membrane. Such disorder-to-order structural transition was never described in GPCRs and opens a new scenario on the possible effect of mutations on unstructured regions of proteins.

Beyond radio-displacement techniques for Identification of CB1 Ligands: The First Application of a Fluorescence-quenching Assay

Cannabinoid type 1 Receptor (CB1) belongs to the GPCR family and it has been targeted, so far, for the discovery of drugs aimed at the treatment of neuropathic pain, nausea, vomit, and food intake disorders. Here, we present the development of the first fluorescent assay enabling the measurement of kinetic binding constants for CB1orthosteric ligands. The assay is based on the use of T1117, a fluorescent analogue of AM251. We prove that T1117 binds endogenous and recombinant CB1 receptors with nanomolar affinity. Moreover, T1117 binding to CB1 is sensitive to the allosteric ligand ORG27569 and thus it is applicable to the discovery of new allosteric drugs. The herein presented assay constitutes a sustainable valid alternative to the expensive and environmental impacting radiodisplacement techniques and paves the way for an easy, fast and cheap high-throughput drug screening toward CB1 for identification of new orthosteric and allosteric modulators.

Structure-based lead optimization and biological evaluation of BAX direct activators as novel potential anticancer agents

The first direct activator of BAX, a pro-apoptotic member of the BCL-2 family, has been recently identified. Herein, a structure-based lead optimization turned out into a small series of analogues, where 8 is the most potent compound published so far. 8 was used as pharmacological tool to ascertain, for the first time, the anticancer potential of BAX direct activators and the obtained results would suggest that BAX direct activators are potential future anticancer drugs rather than venoms.

Screening Platform toward New Anti-HIV Aptamers Set on Molecular Docking and Fluorescence Quenching Techniques

By using a new rapid screening platform set on molecular docking simulations and fluorescence quenching techniques, three new anti-HIV aptamers targeting the viral surface glycoprotein 120 (gp120) were selected, synthesized, and assayed. The use of the short synthetic fluorescent peptide V35-Fluo mimicking the V3 loop of gp120, as the molecular target for fluorescence-quenching binding affinity studies, allowed one to measure the binding affinities of the new aptamers for the HIV-1 gp120 without the need to obtain and purify the full recombinant gp120 protein. The almost perfect correspondence between the calculated Kd and the experimental EC50 on HIV-infected cells confirmed the reliability of the platform as an alternative to the existing methods for aptamer selection and measuring of aptamer-protein equilibria.