Hybrid liquid chromatography high resolution and accuracy mass spectrometry approach for quantification of antibody-drug conjugates at the intact protein level in biological samples

Preclinical in vivo and in vitro characterization of Antibody-Drug Conjugates (ADCs) involves the development of several bioanalytical methods to address many drug exposure questions. The current pharma industry approach requires at least three different assays that must be run, i.e., total antibody (mAb), conjugated payload or conjugated mAb, and free payload assays. Herein we present analytical performances of a quantitative hybrid Ligand Binding/Liquid Chromatography High Resolution and Accuracy Mass Spectrometry (LB/LCHRAM) method that can condense much of the necessary bioanalytical information in one method. The method includes an immuno-capture step, and it detects whole ADC molecules. It was applied to plasma mouse samples and showed reliable bioanalytical performance according to full method validation standards. Quantitation using extracted ion chromatograms and deconvoluted mass peaks was evaluated. The limit of quantitation resulted in 0.5ng of protein on column with a linear dynamic range spanning from 0.5 to 10μg/mL. Moreover, lower drug-to-antibody ratio (DAR) ADC species can be simultaneously detected, also enabling qualitative characterization of in vivo ADC conjugation.

R399E, A Mutated Form of Growth and Differentiation Factor 5, for Disease Modification of Osteoarthritis

OBJECTIVE

To preclinically characterize a mutant form of growth and differentiation factor 5, R399E, with reduced osteogenic properties as a potential disease-modifying osteoarthritis (OA) drug.

METHODS

Cartilage, synovium, and meniscus samples from patients with OA were used to evaluate anabolic and antiinflammatory properties of R399E. In the rabbit joint instability model, 65 rabbits underwent transection of the anterior cruciate ligament plus partial meniscectomy. Three intraarticular (IA) R399E doses were administered biweekly 6 times, and static incapacitance was determined to assess joint pain. OA was evaluated 13 weeks after surgery. In sheep, medial meniscus transection was performed to induce OA, dynamic weight bearing was measured in-life, and OA was assessed after 13 weeks.

RESULTS

Intermittent exposure to R399E (1 week per month) was sufficient to induce cell proliferation and release of anabolic markers in 3-dimensional chondrocyte cultures. R399E also inhibited the release of interleukin-1β (IL-1β), IL-6, and prostaglandin E₂ from cartilage with synovium, meniscal cell, and synoviocyte cultures. In rabbits, the mean difference (95% confidence interval [95% CI]) in weight bearing for R399E compared to vehicle was -5.8 (95% confidence interval [95% CI] -9.54, -2.15), -7.2 (95% CI -10.93, -3.54), and -7.7 (95% CI -11.49, -3.84) for the 0.6, 6, and 60 μg doses, respectively, 6 hours after the first IA injection, and was statistically significant through the entire study for all doses. Cartilage surface structure improved with the 6-μg dose. Structural and symptomatic improvement with the same dose was confirmed in the sheep model of OA.

CONCLUSION

R399E influences several pathologic processes contributing to OA, highlighting its potential as a disease-modifying therapy.

Anti-drug Antibody Sample Testing and Reporting Harmonization

A clear scientific and operational need exists for harmonized bioanalytical immunogenicity study reporting to facilitate communication of immunogenicity findings and expedient review by industry and health authorities. To address these key bioanalytical reporting gaps and provide a report structure for documenting immunogenicity results, this cross-industry group was formed to establish harmonized recommendations and a develop a submission template to facilitate agency filings. Provided here are recommendations for reporting clinical anti-drug antibody (ADA) assay results using ligand-binding assay technologies. This publication describes the essential bioanalytical report (BAR) elements such as the method, critical reagents and equipment, study samples, results, and data analysis, and provides a template for a suggested structure for the ADA BAR. This publication focuses on the content and presentation of the bioanalytical ADA sample analysis report. The interpretation of immunogenicity data, including the evaluation of the impact of ADA on safety, exposure, and efficacy, is out of scope of this publication.

Biomarker context-of-use: how organizational design can impact the implementation of the appropriate biomarker assay strategy

Since 2011, the European Bioanalysis Forum has been discussing the topic of context-of-use for biomarker assays, in support of a cross-industry implementation of its principles. The discussions have led to the acknowledgement of the challenges that we face as an industry in implementing these principles. In addition to scientific recommendations, the European Bioanalysis Forum has addressed these challenges by providing recommendations on organizational design, and what works in both sponsor and contract research organizations, to support and enable context-of-use across biomarker strategies. Here, we highlight the key considerations for organizational design to help ensure that biomarker assays are characterized and validated according to the right context-of-use, to ensure that the right decisions based on the biomarker data can be made during drug development.

Translational strategies in drug development for knee osteoarthritis

Osteoarthritis (OA) is a common disease worldwide with large unmet medical needs. To bring innovative treatments to OA patients, we at Merck have implemented a comprehensive strategy for drug candidate evaluation. We have a clear framework for decision-making in our preclinical pipeline, to design our clinical proof-of-concept trials for OA patients. We have qualified our strategy to define and refine dose and dosing regimen, for treatments administered either systemically or intra-articularly (IA). We do this through preclinical in vitro and in vivo studies, and by back-translating results from clinical studies in OA patients.

Effects of RG7652, a Monoclonal Antibody Against PCSK9, on LDL-C, LDL-C Subfractions, and Inflammatory Biomarkers in Patients at High Risk of or With Established Coronary Heart Disease (from the Phase 2 EQUATOR Study)

RG7652 (MPSK3169A), a fully human immunoglobulin G1 (IgG1) monoclonal antibody directed against proprotein convertase subtilisin/kexin type 9 (PCSK9), blocks the interaction between PCSK9 and low-density lipoprotein (LDL) receptor. EQUATOR (ClinicalTrials.govNCT01609140), a randomized, double-blind, and dose-ranging phase 2 study, evaluated RG7652 in patients (1) at high risk for or (2) with coronary heart disease (CHD). The primary end point was change in LDL cholesterol (LDL-C) from baseline to day 169. Patients (n = 248; median age, 64 years; 57% men; 52% with established CHD; 82% on statins) with baseline LDL-C levels of 90 to 250 mg/dl (mean, 126 mg/dl) continuing on standard-of-care therapy were randomized to receive 1 of 5 RG7652 doses or placebo, subcutaneously every 4, 8, or 12 weeks for 24 weeks. Significant dose-dependent reductions in LDL-C levels from baseline to nadir (56 to 74 mg/dl [48% to 60%]) were observed in all RG7652-dosed patients; effects persisted to day 169 with the highest doses (reduction vs placebo up to 62 mg/dl [51%]) with no unexpected safety signals. RG7652 reduced apolipoprotein B and lipoprotein(a) levels. LDL-C subfraction analysis by nuclear magnetic resonance spectroscopy revealed a prominent decrease in large LDL-C and some decrease in small LDL particles. There was significant reduction in mean particle size of LDL-C on day 169 but no significant reductions in systemic markers of inflammation (high-sensitivity C-reactive protein, interleukin-6, and tumor necrosis factor-alpha). RG7652 reduced LDL-C levels and was well tolerated in patients at high risk for or with CHD on standard-of-care therapy. In conclusion, RG7562 treatment affected large LDL-C and, to a lesser extent, small LDL-C particles; RG7562 did not affect systemic circulating pro-inflammatory cytokines or high-sensitivity C-reactive protein.

A phase 1 study to evaluate the safety and LDL cholesterol-lowering effects of RG7652, a fully human monoclonal antibody against proprotein convertase subtilisin/kexin type 9

BACKGROUND

Proprotein convertase subtilisin/kexin type 9 (PCSK9) downregulates low-density lipoprotein (LDL) receptors, thereby leading to a rise in circulating LDL cholesterol (LDL-C). RG7652 is a fully human monoclonal antibody against PCSK9. This placebo-controlled, phase 1 ascending-dose study in healthy subjects evaluated the safety of RG7652 and its efficacy as a potential LDL-C-lowering drug.

HYPOTHESIS

Anti-PCSK9 antibody therapy safely and effectively reduces LDL-C.

METHODS

Subjects (N = 80) were randomized into 10 cohorts. Six sequential single-dose cohorts received 10, 40, 150, 300, 600, or 800 mg of RG7652 via subcutaneous injection. Four multiple-dose cohorts received 40 or 150 mg of RG7652 once weekly for 4 weeks, either with or without statin therapy (atorvastatin).

RESULTS

Adverse events (AEs) were generally mild; the most common AEs were temporary injection-site reactions. No serious AEs, severe AEs, AEs leading to study-drug discontinuation, or dose-limiting toxicities were reported. RG7652 monotherapy reduced mean LDL-C levels by up to 64% and as much as 100 mg/dL at week 2; the effect magnitude and duration increased with dose (≥57 days following a single RG7652 dose ≥300 mg). Exploratory analyses showed reduced oxidized LDL, lipoprotein(a), and lipoprotein-associated phospholipase A2 with RG7652. Antidrug antibody against RG7652 tested positive in 2 of 60 (3.3%) RG7652-treated and in 4 of 20 (20.0%) placebo-treated subjects. Simultaneous atorvastatin administration did not appear to impact the pharmacokinetic profile or lipid-lowering effects of RG7652.

CONCLUSIONS

Overall, RG7652 elicited substantial and sustained dose-related LDL-C reductions with an acceptable safety profile and minimal immunogenicity.

Impact of SPR biosensor assay configuration on antibody: Neonatal Fc receptor binding data

Binding interactions with the neonatal Fc receptor (FcRn) are one determinant of pharmacokinetic properties of recombinant human monoclonal antibody (rhumAb) therapeutics, and a conserved binding motif in the crystallizable fragment (Fc) region of IgG molecules interacts with FcRn. Surface plasmon resonance (SPR) biosensor assays are often used to characterize interactions between FcRn and rhumAb therapeutics. In such assays, generally either the rhumAb (format 1) or the FcRn protein (format 2) is immobilized on a biosensor chip. However, because evidence suggests that, in some cases, the variable domains of a rhumAb may also affect FcRn binding, we evaluated the effect of SPR assay configuration on binding data. We sought to assess FcRn binding properties of 2 rhumAbs (rhumAb1 and rhumAb2) to FcRn proteins using these 2 biosensor assay formats. The two rhumAbs have greater than 99% sequence identity in the Fc domain but differ in their Fab regions. rhumAb2 contains a positively charged patch in the variable domain that is absent in rhumAb1. Our results showed that binding of rhumAb1 to FcRn was independent of biosensor assay configuration, while binding of rhumAb2 to FcRn was highly SPR assay configuration dependent. Further investigations revealed that the format dependency of rhumAb2-FcRn binding is linked to the basic residues that form a positively charged patch in the variable domain of rhumAb2. Our work highlights the importance of analyzing rhumAb-FcRn binding interactions using 2 alternate SPR biosensor assay configurations. This approach may also provide a simple way to identify the potential for non-Fc-driven FcRn binding interactions in otherwise typical IgGs.

Preclinical pharmacokinetics, pharmacodynamics, tissue distribution, and tumor penetration of anti-PD-L1 monoclonal antibody, an immune checkpoint inhibitor

MPDL3280A is a human monoclonal antibody that targets programmed cell death-1 ligand 1 (PD-L1), and exerts anti-tumor activity mainly by blocking PD-L1 interaction with programmed cell death-1 (PD-1) and B7.1. It is being investigated as a potential therapy for locally advanced or metastatic malignancies. The purpose of the study reported here was to characterize the pharmacokinetics, pharmacodynamics, tissue distribution and tumor penetration of MPDL3280A and/or a chimeric anti-PD-L1 antibody PRO304397 to help further clinical development. The pharmacokinetics of MPDL3280A in monkeys at 0.5, 5 and 20 mg · kg(-1) and the pharmacokinetics / pharmacodynamics of PRO304397 in mice at 1, 3 10 mg · kg(-1) were determined after a single intravenous dose. Tissue distribution and tumor penetration for radiolabeled PRO304397 in tumor-bearing mouse models were determined. The pharmacokinetics of MPDL3280A and PRO304397 were nonlinear in monkeys and mice, respectively. Complete saturation of PD-L1 in blood in mice was achieved at serum concentrations of PRO304397 above ∼ 0.5 µg · mL(-1). Tissue distribution and tumor penetration studies of PRO304397 in tumor-bearing mice indicated that the minimum tumor interstitial to plasma radioactivity ratio was ∼ 0.3; saturation of target-mediated uptake in non-tumor tissues and desirable exposure in tumors were achieved at higher serum concentrations, and the distribution into tumors was dose-and time-dependent. The biodistribution data indicated that the efficacious dose is mostly likely higher than that estimated based on simple pharmacokinetics/pharmacodynamics in blood. These data also allowed for estimation of the target clinical dose for further development of MPDL3280A.

Recommendations for Use and Fit-for-Purpose Validation of Biomarker Multiplex Ligand Binding Assays in Drug Development

Multiplex ligand binding assays (LBAs) are increasingly being used to support many stages of drug development. The complexity of multiplex assays creates many unique challenges in comparison to single-plexed assays leading to various adjustments for validation and potentially during sample analysis to accommodate all of the analytes being measured. This often requires a compromise in decision making with respect to choosing final assay conditions and acceptance criteria of some key assay parameters, depending on the intended use of the assay. The critical parameters that are impacted due to the added challenges associated with multiplexing include the minimum required dilution (MRD), quality control samples that span the range of all analytes being measured, quantitative ranges which can be compromised for certain targets, achieving parallelism for all analytes of interest, cross-talk across assays, freeze-thaw stability across analytes, among many others. Thus, these challenges also increase the complexity of validating the performance of the assay for its intended use. This paper describes the challenges encountered with multiplex LBAs, discusses the underlying causes, and provides solutions to help overcome these challenges. Finally, we provide recommendations on how to perform a fit-for-purpose-based validation, emphasizing issues that are unique to multiplex kit assays.

Effects of altered FcγR binding on antibody pharmacokinetics in cynomolgus monkeys

Antibody interactions with Fcγ receptors (FcγRs), like FcγRIIIA, play a critical role in mediating antibody effector functions and thereby contribute significantly to the biologic and therapeutic activity of antibodies. Over the past decade, considerable work has been directed towards production of antibodies with altered binding affinity to FcγRs and evaluation of how the alterations modulate their therapeutic activity. This has been achieved by altering glycosylation status at N297 or by engineering modifications in the crystallizable fragment (Fc) region. While the effects of these modifications on biologic activity and efficacy have been examined, few studies have been conducted to understand their effect on antibody pharmacokinetics (PK). We present here a retrospective analysis in which we characterize the PK of three antibody variants with decreased FcγR binding affinity caused by amino acid substitutions in the Fc region (N297A, N297G, and L234A/L235A) and three antibody variants with increased FcγRIIIA binding affinity caused by afucosylation at N297, and compare their PK to corresponding wild type antibody PK in cynomolgus monkeys. For all antibodies, PK was examined at a dose that was known to be in the linear range. Since production of the N297A and N297G variants in Chinese hamster ovary cells results in aglycosylated antibodies that do not bind to FcγRs, we also examined the effect of expression of an aglycosylated antibody, without sequence change(s), in E. coli. All the variants demonstrated similar PK compared with that of the wild type antibodies, suggesting that, for the six antibodies presented here, altered FcγR binding affinity does not affect PK.

Targeting oxidized LDL improves insulin sensitivity and immune cell function in obese Rhesus macaques

Oxidation of LDL (oxLDL) is a crucial step in the development of cardiovascular disease. Treatment with antibodies directed against oxLDL can reduce atherosclerosis in rodent models through unknown mechanisms. We demonstrate that through a novel mechanism of immune complex formation and Fc-γ receptor (FcγR) engagement, antibodies targeting oxLDL (MLDL1278a) are anti-inflammatory on innate immune cells via modulation of Syk, p38 MAPK phosphorylation and NFκB activity. Subsequent administration of MLDL1278a in diet-induced obese (DIO) nonhuman primates (NHP) resulted in a significant decrease in pro-inflammatory cytokines and improved overall immune cell function. Importantly, MLDL1278a treatment improved insulin sensitivity independent of body weight change. This study demonstrates a novel mechanism by which an anti-oxLDL antibody improves immune function and insulin sensitivity independent of internalization of oxLDL. This identifies MLDL1278a as a potential therapy for reducing vascular inflammation in diabetic conditions.

HGF as a circulating biomarker of onartuzumab treatment in patients with advanced solid tumors

The objective of this study was to evaluate circulating hepatocyte growth factor (cHGF) as a pharmacodynamic biomarker of Met inhibition for onartuzumab (MetMAb, OA5D5v2) in a phase I trial in patients with advanced cancers and a phase II trial in non-small cell lung cancer (NSCLC). The phase I study was a dose escalation trial with onartuzumab administered i.v. once every three weeks. The phase II study was a randomized two-arm trial in which onartuzumab or placebo was administered in combination with erlotinib in 137 patients with second and third line (2/3L) NSCLC. cHGF levels were evaluated by ELISA at multiple time points over the treatment period. Onartuzumab administration resulted in an acute and sustained rise in cHGF in both the phase I and phase II studies. Elevation in cHGF was independent of dose or drug exposure and was restricted to onartuzumab treatment. Neither higher baseline nor elevated change in cHGF levels upon treatment could simply be attributed to tumor burden or number of liver metastasis. We have shown that elevated cHGF can consistently and reproducibly be measured as a pharmacodynamic biomarker of onartuzumab activity. The elevation in cHGF is independent of tumor type, dose administered, or dose duration. Although these studies were not powered to directly address the contribution of cHGF as a predictive, on-treatment, circulating biomarker, these data suggest that measurement of cHGF in future expanded studies is warranted.

Pharmacokinetics and biodistribution of a human monoclonal antibody to oxidized LDL in cynomolgus monkey using PET imaging

Purpose

Oxidized low-density lipoprotein (LDL) plays an essential role in the pathogenesis of atherosclerosis. The purpose of this study was to characterize the pharmacokinetics (PK) of a human recombinant IgG1 antibody to oxidized LDL (anti-oxLDL) in cynomolgus monkey. The tissue biodistribution of anti-oxLDL was also investigated using positron emission tomography (PET) imaging.

Methods

Anti-oxLDL was conjugated with the N-hydroxysuccinimide ester of DOTA (1,4,7,10-tetraazacyclododecane 1,4,7,10-tetraacetic acid) and radiolabeled by chelation of radioactive copper-64 (⁶⁴Cu) for detection by PET. Anti-oxLDL was administered as a single intravenous (IV) dose of 10 mg/kg (as a mixture of radiolabeled and non-labeled material) to two male and two female cynomolgus monkeys. Serum samples were collected over 29 days. Two ELISA methods were used to measure serum concentrations of anti-oxLDL; Assay A was a ligand binding assay that measured free anti-oxLDL (unbound and partially bound forms) and Assay B measured total anti-oxLDL. The biodistribution was observed over a 48-hour period following dose administration using PET imaging.

Results

Anti-oxLDL serum concentration-time profiles showed a biphasic elimination pattern that could be best described by a two-compartment elimination model. The serum concentrations obtained using the two ELISA methods were comparable. Clearance values ranged from 8 to 17 ml/day/kg, while beta half-life ranged from 8 to12 days. The initial volume of distribution and volume of distribution at steady state were approximately 55 mL/kg and 150 mL/kg, respectively. PET imaging showed distribution predominantly to the blood pool, visible as the heart and great vessels in the trunk and limbs, plus diffuse signals in the liver, kidney, spleen, and bone marrow.

Conclusions

The clearance of anti-oxLDL is slightly higher than typical IgG1 antibodies in cynomolgus monkeys. The biodistribution pattern appears to be consistent with an antibody that has no large, rapid antigen sink outside the blood space.

Minipig as a potential translatable model for monoclonal antibody pharmacokinetics after intravenous and subcutaneous administration

Subcutaneous (SC) delivery is a common route of administration for therapeutic monoclonal antibodies (mAbs) with pharmacokinetic (PK)/pharmacodynamic (PD) properties requiring long-term or frequent drug administration. An ideal in vivo preclinical model for predicting human PK following SC administration may be one in which the skin and overall physiological characteristics are similar to that of humans. In this study, the PK properties of a series of therapeutic mAbs following intravenous (IV) and SC administration in Göttingen minipigs were compared with data obtained previously from humans. The present studies demonstrated: (1) minipig is predictive of human linear clearance; (2) the SC bioavailabilities in minipigs are weakly correlated with those in human; (3) minipig mAb SC absorption rates are generally higher than those in human and (4) the SC bioavailability appears to correlate with systemic clearance in minipigs. Given the important role of the neonatal Fc-receptor (FcRn) in the PK of mAbs, the in vitro binding affinities of these IgGs against porcine, human and cynomolgus monkey FcRn were tested. The result showed comparable FcRn binding affinities across species. Further, mAbs with higher isoelectric point tended to have faster systemic clearance and lower SC bioavailability in both minipig and human. Taken together, these data lend increased support for the use of the minipig as an alternative predictive model for human IV and SC PK of mAbs.

Urea and KCl have differential effects on enzyme activities in liver and muscle of estivating versus nonestivating species

The effects of 300 mM urea or 300 mM KCl on the maximal activities of 25 enzymes of intermediary metabolism were assessed in extracts of liver and muscle from spadefoot toads (Scaphiopus couchii), leopard frogs (Rana pipiens), and rats to assess their sensitivity to these osmolytes. During estivation, toads can lose -50% of total body water, and urea, which is known for its action as a protein denaturant, accumulates to 200-300 mM. The data show that the maximal activities of toad liver enzymes were not affected when assayed in the presence of 300 mM urea in vitro whereas urea inhibited the activities of seven enzymes in frog and 11 enzymes in rat liver. High KCl affected 12 or 13 enzymes in liver of each species causing inhibition in eight or nine cases each, and for frog and rat enzymes, inhibition was frequently more pronounced than for urea. Both urea and KCl affected enzyme activities in muscle extracts of all three species, but whereas their effects were largely negative for frog and rat enzymes, the enzymes affected by urea or KCl in toad muscle were primarily activated by these osmolytes (six out of nine enzymes affected by urea and eight of 15 enzymes affected by KCl). Urea, KCl, and polyethylene glycol (a protein crowding agent) also had species-specific effects on the dissociation constant (Ka) for cAMP of protein kinase A. The data suggest that the accumulation of urea by water-stressed anurans not only contributes to minimizing cell volume reduction, but by doing so also limits the increase in intracellular ionic strength that occurs and thereby helps to minimize the potential inhibitory effects of high salts on metabolic enzymes.

Polyglutamine protein aggregation and toxicity are linked to the cellular stress response

Chronic exposure of cells to expanded polyglutamine proteins results in eventual cell demise. We constructed mouse cell lines expressing either the full-length androgen receptor (AR), or truncated forms of AR containing 25 or 65 glutamines to study the cellular consequences of chronic low-level exposure to these proteins. Expression of the polyglutamine-expanded truncated AR protein, but not the full-length expanded protein, resulted in the formation of cytoplasmic and nuclear aggregates and eventual cell death. Nuclear aggregates preferentially stained positive for heat shock protein (hsp)72, a sensitive indicator of a cellular stress response. Biochemical studies revealed that the presence of nuclear aggregates correlated with activation of the c-jun NH2-terminal kinase (JNK). Different metabolic insults, including heat shock treatment, and exposure to sodium arsenite or menadione, proved more toxic to those cells expressing the polyglutamine-expanded truncated protein than to cells expressing the non-expanded form. Cells containing cytoplasmic polyglutamine-protein aggregates exhibited a delayed expression of hsp72 after heat shock. Once expressed, hsp72 failed to localize normally and instead was sequestered within the protein aggregates. This was accompanied by an inability of the aggregate-containing cells to cease their stress response as evidenced by the continued presence of activated JNK. Finally, activation of the cellular stress response increased the overall extent of polyglutamine protein aggregation, especially within the nucleus. Inclusion of a JNK inhibitor reduced this stress-dependent increase in nuclear aggregates. Abnormal stress responses may contribute to enhanced cell vulnerability in cells expressing polyglutamine-expanded proteins and may increase the propensity of such cells to form cytoplasmic and nuclear inclusions.

Mitogen-activated protein kinases: new signaling pathways functioning in cellular responses to environmental stress

The mitogen-activated protein kinase (MAPK) superfamily consists of three main protein kinase families: the extracellular signal-regulated protein kinases (ERKs), the c-Jun N-terminal kinases (JNKs) and the p38 family of kinases. Each is proving to have major roles in the regulation of intracellular metabolism and gene expression and integral actions in many areas including growth and development, disease, apoptosis and cellular responses to external stresses. To date, this cellular signal transduction network has received relatively little attention from comparative biochemists, despite the high probability that MAPKs have critical roles in the adaptive responses to thermal, osmotic and oxygen stresses. The present article reviews the current understanding of the roles and regulation of ERKs, JNKs and p38, summarizes what is known to date about MAPK roles in animal models of anoxia tolerance, freeze tolerance and osmoregulation, and highlights the potential that studies of MAPK pathways have for advancing our understanding of the mechanisms of biochemical adaptation.

Protein kinase and phosphatase responses to anoxia in crayfish, Orconectes virilis: purification and characterization of cAMP-dependent protein kinase

The freshwater crayfish, Orconectes virilis, shows good anoxia tolerance, enduring 20 h in N(2)-bubbled water at 15 degrees C. Metabolic responses to anoxia by tolerant species often include reversible phosphorylation control over selected enzymes. To analyze the role of serine/threonine kinases and phosphatases in signal transduction during anoxia in O. virilis, changes in the activities of cAMP-dependent protein kinase (PKA) and protein phosphatases 1, 2A, and 2C were measured in tail muscle and hepatopancreas over a time course of exposure to N(2)-bubbled water. A strong increase in the percentage of PKA present as the free catalytic subunit (% PKAc) occurred between 1 and 2 h of anoxia exposure whereas phosphatase activities were strongly reduced. This suggests that PKA-mediated events are important in the initial response by tissues to declining oxygen availability. As oxygen deprivation became severe and prolonged (5-20 h) these changes reversed; the % PKAc fell to below control values and activities of phosphatases returned to or rose above control values. Subcellular fractionation also showed a decrease in PKA associated with the plasma membrane after 20 h anoxia whereas cytosolic PKA content increased. PKAc purified from tail muscle showed a molecular weight of 43.8+/-0.4 kDa, a pH optimum of 6.8, a high affinity for Mg ATP (K(m)=131.0+/-14.4 microM) and Kemptide (K(m)=31.6+/-5.2 microM). Crayfish PKAc was sensitive to temperature change; a break in the Arrhenius plot occurred at approximately 15 degrees C with a 2.5-fold rise in activation energy at temperatures <15 degrees C. These studies demonstrate a role for serine/threonine protein kinases and phosphatases in the metabolic adjustments to oxygen depletion by crayfish organs.

Effects of anoxia exposure and aerobic recovery on metabolic enzyme activities in the freshwater turtle Trachemys scripta elegans

The effects of anoxic submergence (20 h at 7°C in nitrogen-bubbled water) and subsequent aerobic recovery (24 h at 7°C) on the maximal activities of 21 metabolic enzymes were assessed in liver, kidney, heart, brain, and red and white skeletal muscle of an anoxia-tolerant freshwater turtle, the red-eared slider, Trachemys scripta elegans. Anoxia exposure affected the activities of only a few enzymes; for example, it reduced the activity of phosphofructokinase in liver and brain, hexokinase in kidney, glycerol-3-phosphate dehydrogenase and glutamate-oxaloacetate transaminase in heart, glutamate dehydrogenase and serine dehydratase in brain, and 3-hydroxyacyl-CoA dehydrogenase in red muscle. During aerobic recovery, activities of most of these enzymes rebounded and activities of 10 others that were not affected by anoxia rose during recovery. Anoxia-induced changes in selected enzymes appear to meet very specific needs such as glycolytic-rate depression, regulation of glycolytic versus gluconeogenic flux in liver, or alterations in amino acid neurotransmitter levels in brain. Overall, the data demonstrate that the enzymatic make-up of turtle organs undergoes very few changes during anoxia exposure and recovery, which shows that the constitutive activities of enzymes are well designed to meet the metabolic demands of anoxic excursions.

Tyrosine kinases and phosphatases in the estivating spadefoot toad

To endure seasonally arid conditions, spadefoot toads (Scaphiopus couchii) spend 9-10 months underground each year in a hypometabolic state, termed estivation. Protein tyrosine kinases (PTKs) and phosphatases (PTPs) were evaluated in organs of control and estivating toads to assess their possible role in signal transduction during estivation. Total PTK activity decreased by 27-52 % in liver, lung and skeletal muscle during estivation but rose by 66% in heart. Total PTP activity changed only in liver (55 % decrease) and heart (74 %increase). Analysis of the distribution of PTKs between cytoplasmic and membrane-associated forms showed that estivation-linked changes in both fractions occurred in heart (increase) and liver (decrease) whereas in lung and skeletal muscle only the soluble fraction was affected. PTPs were assessed using both a general substrate (ENDpYINASL) and a substrate (DADEpYLIPQQG) specific for PTPs containing the SH2 binding site; both revealed estivation-associated changes in activities and subcellular distribution of PTPs in all tissues. DEAE-Sephadex chromatography showed multiple forms of skeletal muscle PTKs and PTPs in both soluble and insoluble fractions. Each fraction showed three major peaks of PTK activity, two of which shifted in elution position during estivation. The data show that PTKs and PTPs are modified in an organ-specific fashion during estivation by three mechanisms: changes in total activity, changes in subcellular distribution and possible protein covalent modification.

Freeze-thaw effects on metabolic enzymes in wood frog organs

To determine whether episodes of natural freezing and thawing altered the metabolic makeup of wood frog (Rana sylvatica) organs, the maximal activities of 28 enzymes of intermediary metabolism were assessed in six organs (brain, heart, kidney, liver, skeletal muscle, gut) of control (5 degrees C acclimated), frozen (24 h at -3 degrees C), and thawed (24 h back at 5 degrees C) frogs. The enzymes assessed represented pathways including glycolysis, gluconeo-genesis, amino acid metabolism, fatty acid metabolism, the TCA cycle, and adenylate metabolism. Organ-specific responses seen included (a) the number of enzymes affected by freeze-thaw (1 in gut ranging to 17 in heart), (b) the magnitude and direction of response (most often enzyme activities decreased during freezing and rebounded with thawing but, liver showed freeze-specific increases in several enzymes), and (c) the response to freezing versus thawing (enzyme activities in gut and kidney changed during freezing, whereas most enzymes in skeletal muscle responded to thawing). Overall, the data show that freeze-thaw implements selected changes to the maximal activities of various enzymes of intermediary metabolism and that these may aid organ-specific responses that alter fuel use during freeze-thaw, support cryoprotectant metabolism, and aid organ endurance of freeze-induced ischemia.

Identification of shc as the primary protein binding to the tyrosine-phosphorylated beta 3 subunit of alpha IIbbeta 3 during outside-in integrin platelet signaling

Outside-in signaling mediated by the integrin alpha(IIb)beta(3) (GPIIbIIIa) is critical to platelet function and has been shown to involve the phosphorylation of tyrosine residues on the cytoplasmic tail of beta(3). To identify proteins that bind directly to phosphorylated beta(3), we utilized an affinity column consisting of a peptide modeled on the tyrosine-phosphorylated cytoplasmic domain of beta(3). Tandem mass spectrometric sequencing and immunoblotting demonstrated that Shc was the primary protein binding to phosphorylated beta(3). To determine the involvement of Shc in outside-in alpha(IIb)beta(3) signaling, the phosphorylation of Shc during platelet aggregation was examined; transient Shc phosphorylation was observed when thrombin-stimulated platelets were allowed to aggregate or when aggregation was induced by an LIBS (ligand-induced binding site) antibody, D3. Moreover, Shc was co-immunoprecipitated with tyrosine-phosphorylated beta(3) in detergent lysates of aggregated platelets. Using purified, recombinant protein, it was found that the binding of Shc to monophosphorylated (C-terminal tyrosine) and diphosphorylated beta(3) peptides was direct, demonstrating Shc recognition motifs on phospho-beta(3). Aggregation-induced Shc phosphorylation was also observed to be robust in platelets from wild-type mice, but not in those from mice expressing (Y747F,Y759F) beta(3), which are defective in outside-in alpha(IIb)beta(3) signaling. Thus, Shc is the primary downstream signaling partner of beta(3) in its tyrosine phosphorylation outside-in signaling pathway.

Reversible phosphorylation control of skeletal muscle pyruvate kinase and phosphofructokinase during estivation in the spadefoot toad, Scaphiopus couchii

Both pyruvate kinase (PK) and phosphofructokinase (PFK) occur in two different forms, separable by isoelectric focusing (IEF), in skeletal muscle of the spadefoot toad Scaphiopus couchii. During estivation (aerobic dormancy) the proportions of the two forms changed compared with controls; in both cases the amount of enzyme in Peak I (pI = 5.3-5.4) decreased whereas activity in Peak II (isoelectric point = 6.2-6.4) increased. In vitro incubation of crude muscle extracts with 32P-ATP under conditions that promoted the activity of cAMP-dependent protein kinase led to strong radiolabeling associated with Peak I, but not Peak II, and reverse phase HPLC confirmed that 32P was associated with the subunits of both PK and PFK found in Peak I. Specific radiolabeling of Peak I PK and PFK by protein kinase A was further confirmed using immunoprecipitation. In total, this information allowed identification of the Peaks I and II enzymes as the phosphorylated and dephosphorylated forms, respectively, and the effect of estivation was to increase the proportion of dephosphorylated PK and PFK in muscle. Analysis of the kinetic properties of partially purified PK and PFK revealed significant kinetic differences between the two forms of each enzyme. For PK, the Peak II (low phosphate) enzyme showed a 1.6-fold higher Km for phosphoenolpyruvate and a 2.4-fold higher Ka for fructose-1,6-bisphosphate than did the Peak I (high phosphate) form. These kinetic properties suggest that Peak II PK is the less active form, and coupled with the shift to predominantly the Peak II form during estivation (87% Peak II vs. 13% Peak I), are consistent with a suppression of PK activity in estivating muscle, as part of the overall metabolic rate depression of the estivating state. A similar shift to predominantly the Peak II, low phosphate, form of PFK (75% Peak II, 25% Peak I) in muscle of estivating animals is also consistent with metabolic suppression since phosphorylation of vertebrate skeletal muscle PFK is typically stimulated during exercise to enhance enzyme binding to myofibrils in active muscle. Peak II PFK also showed reduced sensitivity to inhibition by Mg:ATP (I50 50% higher) compared with the Peak I form suggesting that the enzyme in estivating muscle is less tightly regulated by cellular adenylate status than in awake toads. The data indicate that reversible phosphorylation control over the activity states of enzymes of intermediary metabolism is an important mechanism for regulating transitions between dormant and active states in estivating species.