Temporal Analysis of PP2A Phosphatase Activity During Insulin Stimulation Using a Direct Activity Probe

Numerous Trigger-like Interactions of Kinases/Protein Phosphatases in Human Skeletal Muscles Can Underlie Transient Processes in Activation of Signaling Pathways during Exercise

Optimizing physical training regimens to increase muscle aerobic capacity requires an understanding of the internal processes that occur during exercise that initiate subsequent adaptation. During exercise, muscle cells undergo a series of metabolic events that trigger downstream signaling pathways and induce the expression of many genes in working muscle fibers. There are a number of studies that show the dependence of changes in the activity of AMP-activated protein kinase (AMPK), one of the mediators of cellular signaling pathways, on the duration and intensity of single exercises. The activity of various AMPK isoforms can change in different directions, increasing for some isoforms and decreasing for others, depending on the intensity and duration of the load. This review summarizes research data on changes in the activity of AMPK, Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), and other components of the signaling pathways in skeletal muscles during exercise. Based on these data, we hypothesize that the observed changes in AMPK activity may be largely related to metabolic and signaling transients rather than exercise intensity per se. Probably, the main events associated with these transients occur at the beginning of the exercise in a time window of about 1-10 min. We hypothesize that these transients may be partly due to putative trigger-like kinase/protein phosphatase interactions regulated by feedback loops. In addition, numerous dynamically changing factors, such as [Ca²⁺], metabolite concentration, and reactive oxygen and nitrogen species (RONS), can shift the switching thresholds and change the states of these triggers, thereby affecting the activity of kinases (in particular, AMPK and CaMKII) and phosphatases. The review considers the putative molecular mechanisms underlying trigger-like interactions. The proposed hypothesis allows for a reinterpretation of the experimental data available in the literature as well as the generation of ideas to optimize future training regimens.

Advances in Activity-Based Sensing Probes for Isoform-Selective Imaging of Enzymatic Activity

Until recently, there were no generalizable methods for assessing the effects of post-translational regulation on enzymatic activity. Activity-based sensing (ABS) has emerged as a powerful approach for monitoring small-molecule and enzyme activities within living systems. Initial examples of ABS were applied for measuring general enzymatic activity; however, a recent focus has been placed on increasing the selectivity to monitor a single enzyme or isoform. The highest degree of selectivity is required for differentiating between isoforms, where the targets display significant structural similarities as a result of a gene duplication or alternative splicing. This Minireview highlights key examples of small-molecule isoform-selective probes with a focus on the relevance of isoform differentiation, design strategies to achieve selectivity, and applications in basic biology or in the clinic.

Characterization and effects of DDG on the intake and digestibility of finishing bulls in feedlots

The aim of this study was to characterize four corn and sorghum co-products (DDG) in terms of their protein and carbohydrate fractions; we also evaluated the effects of substituting the protein source of the conventional supplement by DDG on consumption and nutrient digestibility in confined finishing cattle. Thirty-six male Nellore cattle with a mean age of 24 months were used. The treatments were: FA: concentrate with corn as an energy source and cottonseed meal as a protein source; DDG50: concentrate with a 50% substitution of the FA protein source by DDG; DDG100: concentrate with 100% substitution of the FA protein source by DDG. The experimental design was completely randomized with three treatments and three replicates (pens) containing four animals per pen. We found that the use of DDG in the finishing phase did not interfere with the animals’ food intake or the digestibility of the nutrients (p > 0.05). Nutrients were used by the animals; therefore, DDGs may be viable substitutes of cottonseed meal. We conclude that the bromatological composition of this co-product is influenced a lot during processing; therefore, the nutritional values of this co-product present in the composition tables may not be true.

A fluorescent probe for monitoring PTP-PEST enzymatic activity

Phosphatase non-receptor type 12 (PTPN12 or PTP-PEST) is a critical regulator of cell migration, acting as a tumor suppressor in cancer. Decreases in PTP-PEST expression correlate with aggressive phenotypes in hepatocellular carcinoma (HCC). Despite the importance of PTP-PEST in cellular signaling, methods to directly monitor its enzymatic activity are lacking. Herein, we report the design, synthesis, and optimization of a probe to directly monitor PTP-PEST enzymatic activity via a fluorescent readout. This activity sensor, termed pPEST1tide, is capable of detecting as little as 0.2 nM recombinant PTP-PEST. In addition, we demonstrate that this probe can selectively report on PTP-PEST activity using a panel of potential off-target enzymes. In the long-term, this activity probe could be utilized to identify small molecule modulators of PTP-PEST activity as well as provide a prognostic readout for HCC.

Fluorogenic probes for imaging cellular phosphatase activity

The ability to visualize enzyme activity in a cell, tissue, or living organism can greatly enhance our understanding of the biological roles of that enzyme. While many aspects of cellular signaling are controlled by reversible protein phosphorylation, our understanding of the biological roles of the protein phosphatases involved is limited. Here, we provide an overview of progress toward the development of fluorescent probes that can be used to visualize the activity of protein phosphatases. Significant advances include the development of probes with visible and near-infrared (near-IR) excitation and emission profiles, which provides greater tissue and whole-animal imaging capabilities. In addition, the development of peptide-based probes has provided some selectivity for a phosphatase of interest. Key challenges involve the difficulty of achieving sufficient selectivity for an individual member of a phosphatase enzyme family and the necessity of fully validating the best probes before they can be adopted widely.

A Panel of Protein Kinase Chemosensors Distinguishes Different Types of Fatty Liver Disease

The worldwide incidence of fatty liver disease continues to rise, which may account for concurrent increases in the frequencies of more aggressive liver ailments. Given the existence of histologically identical fatty liver disease subtypes, there is a critical need for the identification of methods that can classify disease and potentially predict progression. Herein, we show that a panel of protein kinase chemosensors can distinguish fatty liver disease subtypes. These direct activity measurements highlight distinct differences between histologically identical fatty liver diseases arising from diets rich in fat versus alcohol and identify a previously unreported decrease in p38α activity associated with a high-fat diet. In addition, we have profiled kinase activities in both benign (diet-induced) and progressive (STAM) disease models. These experiments provide temporal insights into kinase activity during disease development and progression. Altogether, this work provides the basis for the future development of clinical diagnostics and potential treatment strategies.

Distinct phosphorylation and dephosphorylation dynamics of protein arginine kinases revealed by fluorescent activity probes

Protein arginine (Arg) phosphorylation regulates stress responses and virulence in bacteria. With fluorescent activity probes, we show that McsB, a protein Arg kinase, can dephosphorylate phosphoarginine (pArg) residues to produce ATP from ADP, implicating the dynamic control of protein pArg levels by the kinase even without a phosphatase.

Specific Fluorescent Probe for Protein Histidine Phosphatase Activity

Protein histidine phosphorylation plays a vital role in cell signaling and metabolic processes, and phosphohistidine (pHis) phosphatases such as protein histidine phosphatase 1 (PHPT1) and LHPP have been linked to cancer and diabetes, making them novel drug targets and biomarkers. Unlike the case for other classes of phosphatases, further studies of PHPT1 and other pHis phosphatases have been hampered by the lack of specific activity assays in complex biological mixtures. Previous methods relying on radiolabeling are hazardous and technically laborious, and small-molecule phosphatase probes are not selective toward pHis phosphatases. To address these issues, we herein report a fluorescent probe based on chelation-enhanced fluorescence (CHEF) to continuously measure the pHis phosphatase activity of PHPT1. Our probe exhibited excellent sensitivity and specificity toward PHPT1, enabling the first specific measurement of PHPT1 activity in cell lysates. Using this probe, we also obtained more physiologically relevant kinetic parameters of PHPT1, overcoming the limitations of previously used methods.

Design and synthesis of fluorescent activity probes for protein phosphatases

Protein phosphatases act in concert with protein kinases to regulate and maintain the phosphoproteome. However, the catalog of chemical tools to directly monitor the enzymatic activity of phosphatases has lagged behind their kinase counterparts. In this chapter, we provide protocols for repurposing the phosphorylation-sensitive sulfonamido-oxine fluorophore known as Sox to afford direct activity probes for phosphatases. With validated activity probes in-hand, inhibitor screens can be conducted with recombinant enzyme and the role of phosphatases in cell signaling can be investigated in unfractionated cell lysates.

Interrogating Protein Phosphatases with Chemical Activity Probes

Protein phosphatases, while long overlooked, have recently become appreciated as drivers of both normal- and disease-associated signaling events. As a result, the spotlight is now turning torwards this enzyme family and efforts geared towards the development of modern chemical tools for studying these enzymes are well underway. This Minireview focuses on the evolution of chemical activity probes, both optical and covalent, for the study of protein phosphatases. Small-molecule probes, global monitoring of phosphatase activity through the use of covalent modifiers, and targeted fluorescence-based activity probes are discussed. We conclude with an overview of open questions in the field and highlight the potential impact of chemical tools for studying protein phosphatases.