Detergent Lysis of Tissue Culture Cells for Immunoprecipitation

Small GTPase control of pituitary hormone secretion: Evidence from studies in the goldfish (Carassius auratus) neuroendocrine model

The secretion of vertebrate pituitary hormones is regulated by multiple hypothalamic factors, which, while generally activating unique receptor systems, ultimately propagate signals through interacting intracellular regulatory elements to modulate hormone exocytosis. One important family of intracellular regulators is the monomeric small GTPases, a subset of which (Arf1/6, Rac, RhoA, and Ras) is highly conserved across vertebrates and regulates secretory vesicle exocytosis in many cell types. In this study, we investigated the roles of these small GTPases in basal and agonist-dependent hormone release from dispersed goldfish (Carassius auratus) pituitary cells in perifusion experiments. Inhibition of these small GTPases elevated basal LH and GH secretion, except for Ras inhibition which only increased basal LH release. However, variable responses were observed with regard to LH and GH responses to the two goldfish native gonadotropin-releasing hormones (GnRH2 and GnRH3). GnRH-dependent LH release, but not GH secretion, was mediated by Arf1/6 GTPases. In contrast, inhibition of Rac and RhoA GTPases selectively enhanced GnRH3- and GnRH2-dependent GH release, respectively, while Ras inhibition only enhanced GnRH3-evoked LH secretion. Together, our results reveal novel divergent cell-type- and ligand-specific roles for small GTPases in the control of goldfish pituitary hormone exocytosis in unstimulated and GnRH-evoked release.

Co-immunoprecipitation and semi-quantitative immunoblotting for the analysis of protein-protein interactions

Co-immunoprecipitation (co-IP) of protein complexes from cell lysates is widely used to study protein-protein interactions. However, establishing robust co-IP assays often involves considerable optimization. Moreover, co-IP results are frequently presented in non-quantitative ways. This protocol presents an optimized co-IP workflow with an analysis based on semi-quantitative immunoblot densitometry to increase reliability and reproducibility. For complete details on the use and execution of this protocol, please refer to Burckhardt et al. (2021).

Immunoprecipitation

Immunoprecipitation, commonly referred to as IP, involves the binding of proteinaceous antigen in solution by an antigen-specific antibody followed by purification of the antigen-antibody complex via attachment to a solid-phase matrix such as Protein A or G agarose. This rather simplistic and rapid technique yields highly purified immune complexes from multifactorial solutions, including cell lysates or homogenized tissues, and is most commonly used to identify and determine the relative abundance of interacting proteins, referred to as coimmunoprecipitation or co-IP. Although methods encompassing immunoblotting or western blotting of cell lysate preparations can also be applied to determine the presence and quantity of a specific antigen, its relative molecular weight, rate of synthesis or degradation, and state of target-specific posttranslational modification, immunoprecipitation can significantly increase the sensitivity for these methodologies.

Freezing Cell Pellets for Large-Scale Immunoprecipitation

The advance, large-scale preparation of 10⁸-10⁹ adherent or suspension cells before the performance of immunoprecipitation can be advantageous given the time commitment required. The freezing of cells before lysis can preserve protein-protein interactions and posttranslational modifications that may otherwise become denatured and/or degraded upon initiation of cell lysis. This method can also be applied to the preparation of adherent or suspension cells on a smaller scale and is especially useful when multiple time points are being investigated over the course of several days or weeks. Cells are grown under optimal culturing conditions to promote a high degree of viability before being rinsed twice in phosphate-buffered saline (PBS), scraped into a polypropylene tube, and pelleted by centrifugation. The resulting cell pellet is frozen and can be stored for several months at -80°C.

Identification of residues critical for topology inversion of the transmembrane protein TM4SF20 through regulated alternative translocation

Adopting a proper topology is crucial for transmembrane proteins to perform their functions. We previously reported that ceramide regulates a transmembrane protein called TM4SF20 (transmembrane 4 L six family member 20) through topological inversion by altering the direction through which the protein is translocated across membranes during translation. This regulatory mechanism, denoted regulated alternative translocation (RAT), depends on a GXXXN motif present in the first transmembrane helix of TM4SF20. Here, using site-directed mutagenesis, we show that Asn-26 in the motif is crucial for RAT of TM4SF20, as it cannot be replaced even by Gln. In contrast, Gly-22 in the motif could be substituted by other small residues such as Ala and Ser without affecting RAT of TM4SF20. We further demonstrate that the GXXXN motif alone is insufficient to induce RAT of a transmembrane protein because TM4SF4, a relative of TM4SF20 that also contains the motif in the first transmembrane helix, did not undergo RAT. Using TM4SF40-TM4SF20 chimeras, we identified Pro-29 of TM4SF20 as another important element required for RAT of the protein. Substituting Pro-29 alone did not affect RAT of TM4SF20, whereas replacing Pro-29 together with either Leu-25 or Val-17 of TM4SF20 with the corresponding residues of TM4SF4 abolished RAT of TM4SF20. Because Val-17, Gly-22, Leu-25, Asn-26, and Pro-29 are predicted to reside along the same surface of the transmembrane helix, our results suggest that interactions with other proteins mediated by this surface during translocation may be critical for RAT of TM4SF20.

GPER activation protects against epithelial barrier disruption by Staphylococcus aureus α-toxin

Sex bias in innate defense against Staphylococcus aureus skin and soft tissue infection (SSTI) is dependent on both estrogen production by the host and S. aureus secretion of the virulence factor, α-hemolysin (Hla). The impact of estrogen signaling on the immune system is most often studied in terms of the nuclear estrogen receptors ERα and ERβ. However, the potential contribution of the G protein-coupled estrogen receptor (GPER) to innate defense against infectious disease, particularly with respect to skin infection, has not been addressed. Using a murine model of SSTI, we found that GPER activation with the highly selective agonist G-1 limits S. aureus SSTI and Hla-mediated pathogenesis, effects that were absent in GPER knockout mice. Specifically, G-1 reduced Hla-mediated skin lesion formation and pro-inflammatory cytokine production, while increasing bacterial clearance. In vitro, G-1 reduced surface expression of the Hla receptor, ADAM10, in a human keratinocyte cell line and increased resistance to Hla-mediated permeability barrier disruption. This novel role for GPER activation in skin innate defense against infectious disease suggests that G-1 may have clinical utility in patients with epithelial permeability barrier dysfunction or who are otherwise at increased risk of S. aureus infection, including those with atopic dermatitis or cancer.

Tandem Immunoaffinity Purification Using Anti-FLAG and Anti-HA Antibodies

The immunoaffinity purification of target proteins followed by the identification and characterization of associated proteins by mass spectrometry is a widely used technique. An immunoaffinity purification bears resemblance to a standard immunoprecipitation; however, the end product for mass spectrometric analysis in the femtomole (10^-15) to attomole (10^-18) range is required to be of exceptional purity. This high degree of sensitivity in detection renders it of extreme importance to eliminate most if not all of the nonspecific background proteins and can be achieved by performing a tandem affinity purification (TAP). In TAP, the cDNA of the target protein is engineered to contain at least two different epitope tags, and the target protein is extracted under nondenaturing conditions upon expression using an appropriate protein expression platform (CHO cells, HEK 293 cells, or yeast). The expressed protein is initially immunoprecipitated using an antibody against one epitope tag and is eluted in the presence of excess peptide by competition for antibody-binding sites, before being reimmunoprecipitated using an antibody that specifically recognizes the second epitope. These sequential immunoprecipitations significantly reduce the presence of associated nonspecific proteins. Numerous combinations of epitope tags have been applied for tandem affinity purification, and this protocol illustrates the use of tandem hemagglutinin (HA) and FLAG epitope tags. The first immunoprecipitation uses an anti-FLAG antibody followed by the elution in the presence of a competing FLAG peptide before the reimmunoprecipitation of the protein using an anti-HA antibody. Numerous high-quality antiepitope tag antibodies are commercially available from different antibody manufacturers.

Immunoprecipitation

This immunoprecipitation protocol details individual steps for the enrichment and purification process of specific proteins from a complex cell lysate using an antibody bound to a solid matrix. Purified antigen(s) can be eluted by various methods, and the resultant protein target can be analyzed and/or identified by numerous assays, including the enzyme-linked immunosorbent assay (ELISA), western blotting, or mass spectrometry.