细胞生物学

The cell surfaceome is of vital importance across physiology, developmental biology, and disease states alike. The precise identification of proteins and their regulatory mechanisms at the cell membrane has been challenging and is typically determined using confocal microscopy, two-photon microscopy, or total internal reflection fluorescence microscopy (TIRFM). Of these, TIRFM is the most precise, as it harnesses the generation of a spatially delimited evanescent wave at the interface of two surfaces with distinct refractive indices. The limited penetration of the evanescent wave illuminates a narrow specimen field, which facilitates the localization of fluorescently tagged proteins at the cell membrane but not inside of the cell. In addition to constraining the depth of the image, TIRFM also significantly enhances the signal-to-noise ratio, which is particularly valuable in the study of live cells. Here, we detail a protocol for micromirror TIRFM analysis of optogenetically activated protein kinase C-ϵ in HEK293-T cells, as well as data analysis to demonstrate the translocation of this construct to the cell-surface following optogenetic activation.

Graphic abstract

The core planar cell polarity (PCP) protein Vang/Vangl, including Vangl1 and Vangl2 in vertebrates, is indispensable during development. Our previous studies showed that the activity of Vangl is tightly controlled by two important posttranslational modifications, ubiquitination and phosphorylation. Vangl is ubiquitinated through an endoplasmic reticulum-associated degradation (ERAD) pathway and is phosphorylated by casein kinase 1 (CK1) in response to Wnt. Here, we present step-by-step procedures to analyze Vangl ubiquitination and phosphorylation, including cell culture, transfection, sample preparation, and signal detection, as well as the use of newly available phospho-specific antibodies to detect Wnt-induced Vangl2 phosphorylation. The protocol described here can be applicable to the analysis of posttranslational modifications of other membrane proteins.

Mammalian target of rapamycin (mTOR) controls many crucial cellular functions, including protein synthesis, cell size, energy metabolism, lysosome and mitochondria biogenesis, and autophagy. Consequently, deregulation of mTOR signaling plays a role in numerous pathological conditions such as cancer, metabolic disorders and neurological diseases. Developing new tools to monitor mTOR spatiotemporal activation is crucial to better understand its roles in physiological and pathological conditions. However, the most widely used method to report mTOR activity relies on the quantification of specific mTOR-phosphorylated substrates by western blot. This approach requires cellular lysate preparation, which restricts the quantification to a single time point. Here, we present a simple protocol to study mTOR activity in living cells in real time using AIMTOR, an intramolecular BRET-based (bioluminescence resonance energy transfer) biosensor that we recently designed (Bouquier et al., 2020). We describe transfection of AIMTOR in the C2C12 cell line and procedures to monitor BRET in a cell population using a plate reader and in single cells by microscopy. Importantly, this protocol is transposable to any cell line and primary cells. In addition, several subcellular compartment-specific versions of AIMTOR have been developed, enabling compartmentalized assessment of mTOR activity. This protocol describes how to use the sensitive AIMTOR biosensor to investigate mTOR signaling dynamics in living cells.

Graphic abstract:

AIMTOR protocol overview from seeding cells to live BRET recording

In order to acquire fertilizing potential, mammalian sperm must undergo a process known as capacitation, which relies on the early activation of Protein Kinase A (PKA). Frequently, PKA activity is assessed in whole-cell experiments by analyzing the phosphorylation status of its substrates in a western-blot. This technique faces two main disadvantages: it is not a direct measure of the kinase activity and it is a time-consuming approach. However, since PKA can be readily obtained from sperm extracts, in vitro assays such as the “radioactive assay” can be performed using the native enzyme. Unlike western-blot, the radioactive assay is a straightforward technique to evaluate PKA activity by quantification of incorporated ³²P into a peptidic substrate. This approach easily allows the analysis of different agonists or antagonists of PKA. Since mouse sperm is a rich source of soluble PKA, this assay allows a simple fractionation that renders PKA usable both for in vitro testing of drugs on PKA activity and for following changes of PKA activity during the onset of capacitation.

Protein phosphorylation is one of the most important post-translational modifications, which acts as a reversible on or off switch for the activity of a large number of proteins. Analyzing the phosphorylation status of different proteins can reveal the alterations in the state of the cells in response to cellular damage, cancer and pharmaceutical drugs. Techniques such as mass spectrometry, radiolabeling, 2D-gel electrophoresis and western blotting are used to quantify protein phosphorylation. These assays can quantify phosphorylation in the bulk population of cells, however, flow cytometry can couple cell surface marker expression data with phosphorylation data to understand differential signaling in a sub-population within a heterogeneous population of cells. Our protocol describes the use of flow-cytometry for rapid and single cell-based quantification of intracellular phospho-protein with the help of anti-phospho protein specific antibody.

This protocol describes a method to directly measure LATS activity by an in vitro kinase assay using YAP as a substrate.

While the activation of the transcription factor interferon regulatory factor 5 (IRF5) is critical for the induction of innate immune responses, it also contributes to the pathogenesis of the autoimmune disease systemic lupus erythematosus (SLE). IRF5 phosphorylation is a hallmark of its activation in the Toll-like receptor (TLR) pathway, where active IRF5 induces type I interferon and proinflammatory cytokine genes. By using the phosphate-binding molecule Phos-tag, without either radioisotopes or phospho-specific antibodies, the protocol described here enables detection of the phosphorylation of both human and murine IRF5, as well as that of other proteins.

An aquatic angiosperm Vallisneria (Alismatales: Hydrocharitaceae) has been used as an excellent experimental material over a century to study the light regulation of dynamic intracellular movements including chloroplast redistribution and cytoplasmic streaming (Senn, 1908; Seitz, 1987; Takagi, 1997). However, understanding of the molecular mechanisms lagged behind because of difficulty in applying modern techniques such as gene transformation to this plant. Especially, which kind of photoreceptors function in these intriguing responses has long been an unsolved topic. Recently, genes encoding plant-specific blue-light receptor phototropins were isolated in Vallisneria, for the first time from aquatic plants (Sakai et al., 2015). Phototropins were identified first as the photoreceptor for hypocotyl phototropism in Arabidopsis thaliana, and now known to regulate many responses including chloroplast photorelocation movements in various plant species (Christie, 2007). Phototropins are localized mainly on the plasma membrane and their auto-phosphorylation induced by blue light is the critical step of signal transduction pathway (Sakamoto and Briggs, 2002; Kong et al., 2006; Kong et al., 2013; Inoue et al., 2010). Here we describe a protocol for in vitro protein phosphorylation assay using crude-microsomal and plasma-membrane-enriched fractions of Vallisneria, which enabled us to verify the presence of phototropins and characterize their auto-phosphorylation responses. After these analyses, Sakai et al. (2015) proposed that Vallisneria phototropins mediate the high-intensity-blue-light-induced chloroplast avoidance response.

Protein kinases are enzymes that phosphorylate proteins in a cell. Determination of kinase activity in reactions of phosphorylation is a very convenient way for a biochemical characterization of this group of enzymes. Here we describe a method to determine the activity of a recombinant Ser/Thr protein kinase using as a possible substrate MBP, H1, and BSA.

TGFβ is part of a growth factor superfamily which modulates cell growth, differentiation, adhesion, migration, ECM synthesis and apoptosis (Massague, 1998; Siegel and Massague, 2003). Free TGFβ binds to its high affinity TGFβ receptor, a receptor serine/threonine kinase, inducing phosphorylation of Smad2/3 which subsequently forms a complex with Smad4 to translocate to the nucleus where it interacts with multiple co-activators and repressors generating distinct transcriptional responses.

Indeed, TGFβ signaling shows a remarkable cellular context dependency and apparent multifunctionality: e.g. TGFβ is able to inhibit cell proliferation in many epithelial cells but can also enhance proliferation in fibroblasts and cell growth in endothelial cells (Guasch et al., 2007; Xiao et al., 2012); it enhances stem cell pluripotency, but promotes differentiation in other cells (Park, 2011); in cancer development it suppresses pre-malignant cell proliferation, but at the same time promotes conversion to a metastatic phenotype (Chaudhury and Howe, 2009).

The TGFβ stimulation assay monitors the responsiveness of cells to TGFβ. Upon TGFβ stimulation short-term effects such as Smad2 phosphorylation and long-term effects such as cell proliferation can be analyzed. The assay will be described for murine keratinocytes, where TGFβ strongly inhibits cell proliferation, but both assays are applicable for other cell types as well.