Recovering antibody secretion using a hapten ligand as a chemical chaperone

Investigating Protein Binding with the Isothermal Ligand-induced Resolubilization Assay

Target engagement assays typically detect and quantify the direct physical interaction of a protein of interest and its ligand through stability changes upon ligand binding. Commonly used target engagement methods detect ligand-induced stability by subjecting samples to thermal or proteolytic stress. Here we describe a new variation to these approaches called Isothermal Ligand-induced Resolubilization Assay (ILIRA), which utilizes lyotropic solubility stress to measure ligand binding through changes in target protein solubility. We identified distinct buffer systems and salt concentrations that compromised protein solubility for four diverse proteins: dihydrofolate reductase (DHFR), nucleoside diphosphate-linked moiety X motif 5 (NUDT5), poly [ADP-ribose] polymerase 1 (PARP1), and protein arginine N-methyltransferase 1 (PRMT1). Ligand-induced solubility rescue was demonstrated for these proteins, suggesting that ILIRA can be used as an additional target engagement technique. Differences in ligand-induced protein solubility were assessed by Coomassie blue staining for SDS-PAGE and dot blot, as well as by NanoOrange, Thioflavin T, and Proteostat fluorescence, thus offering flexibility for readout and assay throughput.

Thiamine diphosphate binds to intermediates in the assembly of adenovirus fiber knob trimers in Escherichia coli

Assembly of the adenovirus (Ad) homotrimeric fiber protein is nucleated by its C-terminal knob domain, which itself can trimerize when expressed as a recombinant protein fragment. The non-interlocked, globular structure of subunits in the knob trimer implies that trimers assemble from prefolded monomers through a dimer intermediate, but these intermediates have not been observed and the mechanism of assembly therefore remains uncharacterized. Here we report that expression of the Ad serotype 2 (Ad2) knob was toxic for thi- strains of Escherichia coli, which are defective in de novo synthesis of thiamine (vitamin B1). Ad2 knob trimers isolated from a thi+ strain copurified through multiple chromatography steps with a small molecule of mass equivalent to that of thiamine diphosphate (ThDP). Mutant analysis did not implicate any specific site for ThDP binding. Our results suggest that ThDP may associate with assembly intermediates and become trapped in assembled trimers, possibly within one of several large cavities that are partially solvent-accessible or buried completely within the trimer interior.

Chemical and pharmacological chaperones as new therapeutic agents

Proteins that are exported from the cell, or targeted to the cell surface or other organelles, are synthesised and assembled in the endoplasmic reticulum and then delivered to their destinations. Point mutations – the most common cause of human genetic diseases – can inhibit folding and assembly of the protein in the endoplasmic reticulum. The unstable or partially folded mutant protein does not undergo trafficking and is usually rapidly degraded. A potential therapy for protein misfolding is to correct defective protein folding and trafficking using pharmacological chaperones. Pharmacological chaperones are substrates or modulators that appear to function by directly binding to the partially folded biosynthetic intermediate to stabilise the protein and allow it to complete the folding process to yield a functional protein. Initial clinical studies with pharmacological chaperones have successfully reduced clinical symptoms of disease. Therefore, pharmacological chaperones show great promise as a new class of therapeutic agents that can be specifically tailored for a particular genetic disease.

Chemical chaperones: mechanisms of action and potential use

An increasing number of studies indicate that low-molecular-weight compounds can help correct conformational diseases by inhibiting the aggregation or enable the mutant proteins to escape the quality control systems, and thus their function can be rescued. The small molecules were named chemical chaperones and it is thought that they nonselectively stabilize the mutant proteins and facilitate their folding. Chemical chaperones are usually osmotically active, such as DMSO, glycerol, or deuterated water, but other compounds, such as 4-phenylbutiric acid, are also members of the chemical chaperone group. More recently, compounds such as receptor ligands or enzyme inhibitors, which selectively recognize the mutant proteins, were also found to rescue conformational mutants and were termed pharmacological chaperones. An increasing amount of evidence suggests that the action of pharmacological chaperones could be generalized to a large number of misfolded proteins, representing new therapeutic possibilities for the treatment of conformational diseases. A new and exciting strategy has recently been developed, leading to the new chemical group called folding agonist. These small molecules are designed to bind proteins and thus restore their native conformation.

Rescue of Folding Defects in ABC Transporters Using Pharmacological Chaperones

The ATP-binding cassette (ABC) family of membrane transport proteins is the largest class of transporters in humans (48 members). The majority of ABC transporters function at the cell surface. Therefore, defective folding and trafficking of the protein to the cell surface can lead to serious health problems. The classic example is cystic fibrosis (CF). In most CF patients, there is a deletion of Phe508 in the CFTR protein (DeltaF508 CFTR) that results in defective folding and intracellular retention of the protein (processing mutant). A potential treatment for most patients with CF would be to use a ligand(s) of CFTR that acts a pharmacological chaperone to correct the folding defect. The feasibility of such an approach was first demonstrated with the multidrug transporter P-glycoprotein (P-gp), an ABC transporter, and a sister protein of CFTR. It was found that P-gps with mutations at sites equivalent to those found in CFTR processing mutants were rescued when they were expressed in the presence of drug substrates or modulators of P-gp. These compounds acted as pharmacological chaperones and functioned by promoting interactions among the various domains in the protein during the folding process. Several groups have attempted to identify compounds that could rescue the folding defect in DeltaF508 CFTR. The best compound identified through high-throughout screening is a quinazoline derivative (CFcor-325). Expression of DeltaF508 CFTR as well as other CFTR processing mutants in the presence of 1 muM CFcor-325 promoted folding and trafficking of the mutant proteins to the cell surface in an active conformation. Therefore, CFcor-325 and other quinazoline derivates could be important therapeutic compounds for the treatment of CF.

Pharmacologic rescue of conformationally-defective proteins: implications for the treatment of human disease

The process of quality control in the endoplasmic reticulum involves a variety of mechanisms which ensure that only correctly folded proteins enter the secretory pathway. Among these are conformation-screening mechanisms performed by molecular chaperones that assist in protein folding and prevent non-native (or misfolded) proteins from interacting with other misfolded proteins. Chaperones play a central role in the triage of newly formed proteins prior to their entry into the secretion, retention, and degradation pathways. Despite this stringent quality control mechanism, gain- or loss-of-function mutations that affect protein folding in the endoplasmic reticulum can manifest themselves as profound effects on the health of an organism. Understanding the molecular, cellular, and energetic mechanisms of protein routing could prevent or correct the structural abnormalities associated with disease-causing misfolded proteins. Rescue of misfolded, "trafficking-defective", but otherwise functional, proteins is achieved by a variety of physical, chemical, genetic, and pharmacological approaches. Pharmacologic chaperones (or "pharmacoperones") are template molecules that may potentially arrest or reverse diseases by inducing mutant proteins to adopt native-type-like conformations instead of improperly folded ones. Such restructuring leads to a normal pattern of cellular localization and function. This review focuses on protein misfolding and misrouting related to various disease states and describes promising approaches to overcoming such defects. Special attention is paid to the gonadotropin-releasing hormone receptor, since there is a great deal of information about this receptor, which has recently emerged as a particularly instructive model.

Pharmacological chaperones: potential treatment for conformational diseases

Increasing numbers of inherited diseases are found to result from mutations that lead to misfolded proteins. In many cases, the changes in conformation are relatively modest and the function of the protein would not be predicted to be affected. Yet, these proteins are recognized as "misfolded" and degraded prematurely. Recently, small molecules known as chemical and pharmacological chaperones were found to stabilize such mutant proteins and facilitate their trafficking to their site of action. Here, we review the recent published evidence suggesting that pharmacological chaperones represent promising avenues for the treatment of endocrine and metabolic diseases such as hyperinsulinemic hypoglycemia, hypogonadotropic hypogonadism and nephrogenic diabetes insipidus, and might become a general therapeutic strategy for the treatment of conformational diseases.

Disorder-associated mutations lead to functional inactivation of neuroligins

Autism is a neuro-developmental syndrome that affects 0.1-0.5% of the population. It has been proposed that alterations in neuronal circuitry and/or neuronal signaling are responsible for the behavioral and cognitive aberrations in autism patients. However, the cellular basis of such alterations is unknown. Recently, point mutations in a family of neuronal cell adhesion molecules called neuroligins have been linked to autism-spectrum disorders and mental retardation. We investigated the consequences of these disease-associated mutations on neuroligin function. We demonstrate that the point mutation at arginine 451 and a nonsense mutation at aspartate 396 of neuroligin-3 and -4 (NL3 and NL4), respectively, result in intracellular retention of the mutant proteins. Over-expression of wild-type NL3 and NL4 proteins in hippocampal neurons stimulates the formation of presynaptic terminals, whereas the disease-associated mutations result in a loss of this synaptic function. Our findings suggest that the previously identified mutations in neuroligin genes are likely to be relevant for the neuro-developmental defects in autism-spectrum disorders and mental retardation since they impair the function of a synaptic cell adhesion molecule.

Repertoire shift in the humoral response to phosphocholine-keyhole limpet hemocyanin: VH somatic mutation in germinal center B cells impairs T15 Ig function

Phosphocholine (PC) is a naturally occurring Ag common to many pathogenic microorganisms. Early in the primary response to PC conjugated to keyhole limpet hemocyanin (KLH), T15 Id(+) Abs constitute >90% of the serum Ig in BALB/c mice. During the late primary and memory response to PC-protein, a shift in the repertoire occurs and T15 Id(+) Abs lose dominance. In this study, we use immunohistochemistry and single germinal center microdissection to locate T15 Id(+) cells in the spleen in a primary response to PC-KLH. We demonstrate T15 Id(+) B cells and V(H)1-DFL16.1-JH1 and V kappa 22-J kappa 5 rearrangements in germinal centers early in the immune response; thus loss of T15 dominance is not due to lack of T15 cells within germinal centers. One-hundred thirty one V(H)1 and 57 V kappa 22 rearrangements were cloned and sequenced. Thirty four percent of the V(H)1 clones and 37% of the V kappa 22 clones contained somatic mutations indicating participation in the germinal center response. Six variant T15 H clones were expressed with wild-type T15 L chain in vitro. Two of these Abs were defective in secretion providing the first evidence that mutation occurring in vivo can disrupt Ig assembly and secretion. Of the four secretion-competent Abs, two failed to display binding to PC-protein, while the other two displayed altered carrier recognition. These results indicate that somatic mutation of T15 in vivo can result in the loss of binding and secretion, potentially leading to B cell wastage. The failure of T15 to gain affinity enhancing mutations in the face of these detrimental changes may contribute to repertoire shift.

Quality control in the endoplasmic reticulum

The endoplasmic reticulum (ER) has a quality-control system for 'proof-reading' newly synthesized proteins, so that only native conformers reach their final destinations. Non-native conformers and incompletely assembled oligomers are retained, and, if misfolded persistently, they are degraded. As a large fraction of ER-synthesized proteins fail to fold and mature properly, ER quality control is important for the fidelity of cellular functions. Here, we discuss recent progress in understanding the conformation-specific sorting of proteins at the level of ER retention and export.