Glucagon signalling in the dorsal vagal complex is sufficient and necessary for high‐protein feeding to regulate glucose homeostasis in vivo

High‐protein feeding acutely lowers postprandial glucose concentration compared to low‐protein feeding, despite a dichotomous rise of circulating glucagon levels. The physiological role of this glucagon rise has been largely overlooked. We here first report that glucagon signalling in the dorsal vagal complex (DVC) of the brain is sufficient to lower glucose production by activating a Gcgr–PKA–ERK–K_ATP channel signalling cascade in the DVC of rats in vivo. We further demonstrate that direct blockade of DVC Gcgr signalling negates the acute ability of high‐ vs. low‐protein feeding to reduce plasma glucose concentration, indicating that the elevated circulating glucagon during high‐protein feeding acts in the brain to lower plasma glucose levels. These data revise the physiological role of glucagon and argue that brain glucagon signalling contributes to glucose homeostasis during dietary protein intake.     

SIRT1 shows no substrate specificity in vitro

SIR2 is a key regulator of the aging process in many model organisms. The human ortholog SIRT1 plays a pivotal role in the regulation of cellular differentiation, metabolism, cell cycle, and apoptosis. SIRT1 is an NAD(+)-dependent deacetylase, and its enzymatic activity may be regulated by cellular energy. There is a growing number of known SIRT1 substrates that contain epsilon-acetyl lysine but for which no obvious consensus sequence has been defined. In this study, we developed a novel unbiased method to identify deacetylase sequence specificity using oriented peptide libraries containing acetylated lysine. Following incubation with SIRT1, the subset of deacetylated peptides was selectively captured using a photocleavable N-hydroxysuccinimide (NHS)-biotin linker and streptavidin beads and analyzed using mass spectrometry and Edman degradation. These studies revealed that substrate recognition by SIRT1 does not depend on the amino acid sequence proximate to the acetylated lysine. This result brings us one step closer to understanding how SIRT1 and possibly other protein deacetylases chose their substrate.     

MT1-MMP-dependent neovessel formation within the confines of the three-dimensional extracellular matrix

During angiogenesis, endothelial cells initiate a tissue-invasive program within an interstitial matrix comprised largely of type I collagen. Extracellular matrix-degradative enzymes, including the matrix metalloproteinases (MMPs) MMP-2 and MMP-9, are thought to play key roles in angiogenesis by binding to docking sites on the cell surface after activation by plasmin- and/or membrane-type (MT) 1-MMP-dependent processes. To identify proteinases critical to neovessel formation, an ex vivo model of angiogenesis has been established wherein tissue explants from gene-targeted mice are embedded within a three-dimensional, type I collagen matrix. Unexpectedly, neither MMP-2, MMP-9, their cognate cell-surface receptors (i.e., beta3 integrin and CD44), nor plasminogen are essential for collagenolytic activity, endothelial cell invasion, or neovessel formation. Instead, the membrane-anchored MMP, MT1-MMP, confers endothelial cells with the ability to express invasive and tubulogenic activity in a collagen-rich milieu, in vitro or in vivo, where it plays an indispensable role in driving neovessel formation.     

Complement inhibition reduces injury in the type 2 diabetic heart following ischemia and reperfusion

Chronic inflammation exacerbates the cardiovascular complications of diabetes. Complement activation plays an important role in the inflammatory response and is known to be involved in ischemia-reperfusion (I/R) injury in the nondiabetic heart. The purpose of this study was to determine if increased complement deposition explains, in part, the increased severity of neutrophil-mediated I/R injury in the type 2 diabetic heart. Nondiabetic Zucker lean control (ZLC) and Zucker diabetic fatty (ZDF) rats underwent 30 min of coronary artery occlusion followed by 120 min of reperfusion. Another group of ZDF rats was treated with the complement inhibitor FUT-175 before reperfusion. Left ventricular (LV) tissue samples were stained for complement deposition and neutrophil accumulation following reperfusion. We found significantly more complement deposition in the ZDF LV compared with the ZLC (P < 0.05), and complement deposition was associated with significantly greater neutrophil accumulation. In whole blood samples taken preischemia and at 120 min reperfusion, neutrophils exhibited significantly more CD11b expression in the ZDF group compared with the ZLC group (P < 0.05). Furthermore, intracellular adhesion molecule (ICAM)-1 expression following I/R was increased significantly in ZDF hearts compared with ZLC hearts (P < 0.001). These results indicate that, in the ZDF heart, increased ICAM-1 and polymorphonuclear neutrophil (PMN) CD11b expression play a role in increasing PMN accumulation following I/R. The infarct size of the ZDF was significantly greater than ZLC (P < 0.05), and treatment with FUT-175 significantly decreased infarct size, complement deposition, and PMN accumulation in the diabetic heart. These findings indicate an exacerbated inflammatory response in the type 2 diabetic heart that contributes to the increased tissue injury observed following ischemia and reperfusion.     

Mechanical properties of muscles from Xenopus borealis following maintenance in organ culture

The mechanical properties of sartorii muscles from the toad Xenopus borealis were studied in organ cultures lasting up to 26 days. Isometric twitch and tetanic tensions diminished in force reaching half their initial values after 15 days in culture. In contrast, twitch time to peak time to half relaxation, twitch-tetanus ratio and muscle weight-body weight ratio, were unchanged. Maximum isotonic shortening velocity (Vmax) declined, with a half time similar to that of the isometric force. The fall in isometric tension is probably due to a breakdown of activation in some fibres. The change in Vmax could be due to a loss of functional sarcomeres in series with the tendons.