Skeletal muscle of torpid Richardson's ground squirrels (Urocitellus richardsonii) exhibits a less active form of citrate synthase associated with lowered lysine succinylation

Dysregulation of protein succinylation and disease development

As a novel post-translational modification of proteins, succinylation is widely present in both prokaryotes and eukaryotes. By regulating protein translocation and activity, particularly involved in regulation of gene expression, succinylation actively participates in diverse biological processes such as cell proliferation, differentiation and metabolism. Dysregulation of succinylation is closely related to many diseases. Consequently, it has increasingly attracted attention from basic and clinical researchers. For a thorough understanding of succinylation dysregulation and its implications for disease development, such as inflammation, tumors, cardiovascular and neurological diseases, this paper provides a comprehensive review of the research progress on abnormal succinylation. This understanding of association of dysregulation of succinylation with pathological processes will provide valuable directions for disease prevention/treatment strategies as well as drug development.

Protein succinylation: regulating metabolism and beyond

Modifications of protein post-translation are critical modulatory processes, which alters target protein biological activity，function and/or location, even involved in pathogenesis of some diseases. So far, there are at least 16 types of post-translation modifications identified, particularly through recent mass spectrometry analysis. Among them, succinylation (Ksuc) on protein lysine residues causes a variety of biological changes. Succinylation of proteins contributes to many cellular processes such as proliferation, growth, differentiation, metabolism and even tumorigenesis. Mechanically, Succinylation leads to conformation alteration of chromatin or remodeling. As a result, transcription/expression of target genes is changed accordingly. Recent research indicated that succinylation mainly contributes to metabolism modulations, from gene expression of metabolic enzymes to their activity modulation. In this review, we will conclude roles of succinylation in metabolic regulation of glucose, fat, amino acids and related metabolic disease launched by aberrant succinylation. Our goal is to stimulate extra attention to these still not well researched perhaps important succinylation modification on proteins and cell processes.

Hepatic citrate synthase suppression in the freeze-tolerant wood frog (Rana sylvatica)

The wood frog, Rana sylvatica endures whole body freezing for weeks/months while overwintering at subzero temperatures. Survival of long-term freezing requires not only cryoprotectants but also strong metabolic rate depression (MRD) and reorganization of essential processes in order to maintain a balance between ATP-producing and ATP-consuming processes. Citrate synthase (CS) (E.C. 2.3.3.1) is an important irreversible enzyme of the tricarboxylic acid (TCA) cycle and forms a crucial checkpoint for many metabolic processes. Present study investigated the regulation of CS from wood frog liver during freezing. CS was purified to homogeneity by a two-step chromatographic process. Kinetic and regulatory parameters of the enzyme were investigated and, notably, demonstrated a significant decrease in the V_max of the purified form of CS from frozen frogs as compared to controls when assayed at both 22 °C and 5 °C. This was further supported by a decrease in the maximum activity of CS from liver of frozen frogs. Immunoblotting also showed changes in posttranslational modifications with a significant decrease in threonine phosphorylation (by 49 %) for CS from frozen frogs. Taken together, these results suggest that CS is suppressed and TCA flux is inhibited during freezing, likely to support MRD survival of harsh winters.

Role of NADP+-dependent isocitrate dehydrogenase from muscle tissue of Rana sylvatica in ROS defense during freeze-tolerance

The wood frog, Rana sylvatica, employs freeze tolerance as a winter survival strategy in seasonally cold environments. At subzero temperatures, up to 65-70% of total body water can freeze in extracellular spaces, halting vital functions (breathing, heartbeat) and causing ischemia that, in turn, can have numerous consequences including the generation of damaging reactive oxygen species (ROS). NADPH serves as a key donor of reductive power for most ROS detoxifying enzymes and can be generated by several metabolic pathways. One source of NADPH reducing power is the NADP-dependent isocitrate dehydrogenase (IDH) reaction. The present study evaluated the properties and regulation of IDH from skeletal muscle of R. sylvatica when frogs were exposed to stress conditions: freezing, dehydration or anoxia. Purified IDH exhibited higher affinity for isocitrate under all stress conditions as compared to controls, suggesting that the enzyme is primed to synthesize NADPH relative to the control state. Immunoblotting showed reduced serine and threonine phosphorylation of muscle IDH from frozen frogs and decreased serine phosphorylation on IDH from dehydrated frogs relative to control and anoxic states, demonstrating a reversible phosphorylation regulatory mechanism for IDH activity during freezing stress. Taken together, these results suggest activation and maintenance of IDH activity despite hypometabolic conditions. This initiation in activity of IDH during freezing may play a role in antioxidant defense by contributing to maintenance of the NADPH pool under stress conditions.

Purification and characterization of NADP-isocitrate dehydrogenase from skeletal muscle of Urocitellus richardsonii

NADP-dependent isocitrate dehydrogenase (NADP-IDH, EC 1.1.1.42) catalyzes the oxidative decarboxylation of isocitrate to α-ketoglutarate with the concomitant production of NADPH. NADPH plays important roles in many biosynthesis pathways, maintenance of proper oxidation-reduction balance, and protection against oxidative damage. This present study investigated the dynamic nature of NADP-IDH during hibernation by purifying it from the skeletal muscle of Richardson's ground squirrel (Urocitellus richardsonii) and analyzing its structural and functional changes in response to hibernation. Kinetic parameters of purified NADP-IDH from euthermic and hibernating ground squirrel skeletal muscle were characterized at 22 °C and 5 °C. Relative to euthermic muscle, -NADP-IDH in hibernating muscle had a higher affinity for its substrate, isocitrate at 22 °C, whereas at 5 °C, there was a significant decrease in isocitrate affinity. Western blot analysis revealed greater serine and threonine phosphorylation in hibernator NADP-IDH as compared to euthermic NADP-IDH. In addition, Bioinformatic analysis predicted the presence of 18 threonine and 21 serine phosphorylation sites on squirrel NADP-IDH. The structural and functional changes in NADP-IDH indicate the ability of the organism to reduce energy consumption during hibernation, while emphasizing increased NADPH production, and thus antioxidant activity, during torpor arousal cycles.