The common hepatic branch of the vagus is not required to mediate the glycemic and food intake suppressive effects of glucagon-like-peptide-1

The incretin and food intake suppressive effects of intraperitoneally administered glucagon-like peptide-1 (GLP-1) involve activation of GLP-1 receptors (GLP-1R) expressed on vagal afferent fiber terminals. Central nervous system processing of GLP-1R-driven vagal afferents results in satiation signaling and enhanced insulin secretion from pancreatic-projecting vagal efferents. As the vast majority of endogenous GLP-1 is released from intestinal l-cells following ingestion, it stands to reason that paracrine GLP-1 signaling, activating adjacent GLP-1R expressed on vagal afferent fibers of gastrointestinal origin, contributes to glycemic and food intake control. However, systemic GLP-1R-mediated control of glycemia is currently attributed to endocrine action involving GLP-1R expressed in the hepatoportal bed on terminals of the common hepatic branch of the vagus (CHB). Here, we examine the hypothesis that activation of GLP-1R expressed on the CHB is not required for GLP-1's glycemic and intake suppressive effects, but rather paracrine signaling on non-CHB vagal afferents is required to mediate GLP-1's effects. Selective CHB ablation (CHBX), complete subdiaphragmatic vagal deafferentation (SDA), and surgical control rats received an oral glucose tolerance test (2.0 g glucose/kg) 10 min after an intraperitoneal injection of the GLP-1R antagonist, exendin-(9-39) (Ex-9; 0.5 mg/kg) or vehicle. CHBX and control rats showed comparable increases in blood glucose following blockade of GLP-1R by Ex-9, whereas SDA rats failed to show a GLP-1R-mediated incretin response. Furthermore, GLP-1(7-36) (0.5 mg/kg ip) produced a comparable suppression of 1-h 25% glucose intake in both CHBX and control rats, whereas intake suppression in SDA rats was blunted. These findings support the hypothesis that systemic GLP-1R mediation of glycemic control and food intake suppression involves paracrine-like signaling on GLP-1R expressed on vagal afferent fibers of gastrointestinal origin but does not require the CHB.     

The temporal structure of feeding behavior

Meals have long been considered relevant units of feeding behavior. Large data sets of feeding behavior of cattle, pigs, chickens, ducks, turkeys, dolphins, and rats were analyzed with the aims of 1) describing the temporal structure of feeding behavior and 2) developing appropriate methods for estimating meal criteria. Longer (between-meal) intervals were never distributed as the negative exponential assumed by traditional methods, such as log-survivorship analysis, but as a skewed Gaussian, which can be (almost) normalized by log-transformation of interval lengths. Log-transformation can also normalize frequency distributions of within-meal intervals. Meal criteria, i.e., the longest interval considered to occur within meals, can be estimated after fitting models consisting of Gaussian functions alone or of one Weibull and one or more Gaussian functions to the distribution of log-transformed interval lengths. Nonuniform data sets may require disaggregation before this can be achieved. Observations from all species were in conflict with assumptions of random behavior that underlie traditional methods for criteria estimation. Instead, the observed structure of feeding behavior is consistent with 1) a decrease in satiety associated with an increase in the probability of animals starting a meal with time since the last meal and 2) an increase in satiation associated with an increase in the probability of animals ending a meal with the amount of food already consumed. The novel methodology proposed here will avoid biased conclusions from analyses of feeding behavior associated with previous methods and, as demonstrated, can be applied across a range of species to address questions relevant to the control of food intake.     

PIN polarity maintenance by the cell wall in Arabidopsis

A central question in developmental biology concerns the mechanism of generation and maintenance of cell polarity, because these processes are essential for many cellular functions and multicellular development. In plants, cell polarity has an additional role in mediating directional transport of the plant hormone auxin that is crucial for multiple developmental processes. In addition, plant cells have a complex extracellular matrix, the cell wall, whose role in regulating cellular processes, including cell polarity, is unexplored. We have found that polar distribution of PIN auxin transporters in plant cells is maintained by connections between polar domains at the plasma membrane and the cell wall. Genetic and pharmacological interference with cellulose, the major component of the cell wall, or mechanical interference with the cell wall disrupts these connections and leads to increased lateral diffusion and loss of polar distribution of PIN transporters for the phytohormone auxin. Our results reveal a plant-specific mechanism for cell polarity maintenance and provide a conceptual framework for modulating cell polarity and plant development via endogenous and environmental manipulations of the cellulose-based extracellular matrix.     

The crystal structures of the Salmonella type III secretion system tip protein SipD in complex with deoxycholate and chenodeoxycholate

The type III secretion system (T3SS) is a protein injection nanomachinery required for virulence by many human pathogenic bacteria including Salmonella and Shigella. An essential component of the T3SS is the tip protein and the Salmonella SipD and the Shigella IpaD tip proteins interact with bile salts, which serve as environmental sensors for these enteric pathogens. SipD and IpaD have long central coiled coils and their N-terminal regions form α-helical hairpins and a short helix α3 that pack against the coiled coil. Using AutoDock, others have predicted that the bile salt deoxycholate binds IpaD in a cleft formed by the α-helical hairpin and its long central coiled coil. NMR chemical shift mapping, however, indicated that the SipD residues most affected by bile salts are located in a disordered region near helix α3. Thus, how bile salts interact with SipD and IpaD is unclear. Here, we report the crystal structures of SipD in complex with the bile salts deoxycholate and chenodeoxycholate. Bile salts bind SipD in a region different from what was predicted for IpaD. In SipD, bile salts bind part of helix α3 and the C-terminus of the long central coiled coil, towards the C-terminus of the protein. We discuss the biological implication of the differences in how bile salts interact with SipD and IpaD.     

Localization and assembly of the novel exosporium protein BetA of Bacillus anthracis

The exosporium of Bacillus anthracis is comprised of two distinct layers: a basal layer and a hair-like nap that covers the basal layer. The hair-like nap contains the glycoproteins BclA and, most likely, BclB. BclA and BclB are directed to assemble into the exosporium by motifs in their N-terminal domains. Here, we identify a previously uncharacterized putative gene encoding this motif, which we have named betA (Bacillus exosporium-targeted protein of B. anthracis). Like bclA, betA encodes a putative collagenlike repeat region. betA is present in several genomes of exosporium-producing Bacillus species but, so far, not in any others. Using fluorescence microscopic localization of a BetA-enhanced green fluorescent protein (eGFP) fusion protein and immunofluorescence microscopy with anti-BetA antibodies, we showed that BetA resides in the exosporium basal layer, likely underneath BclA. BetA assembles at the spore surface at around hour 5 of sporulation and under the control of BxpB, similar to the control of deposition of BclA. We suggest a model in which BclA and BetA are incorporated into the exosporium by a mechanism that depends on their similar N termini. These data suggest that BetA is a member of a growing family of exosporium proteins that assemble under the control of targeting sequences in their N termini.     

KefF, the regulatory subunit of the potassium efflux system KefC, shows quinone oxidoreductase activity

Escherichia coli and many other Gram-negative pathogenic bacteria protect themselves from the toxic effects of electrophilic compounds by using a potassium efflux system (Kef). Potassium efflux is coupled to the influx of protons, which lowers the internal pH and results in immediate protection. The activity of the Kef system is subject to complex regulation by glutathione and its S conjugates. Full activation of KefC requires a soluble ancillary protein, KefF. This protein has structural similarities to oxidoreductases, including human quinone reductases 1 and 2. Here, we show that KefF has enzymatic activity as an oxidoreductase, in addition to its role as the KefC activator. It accepts NADH and NADPH as electron donors and quinones and ferricyanide (in addition to other compounds) as acceptors. However, typical electrophilic activators of the Kef system, e.g., N-ethyl maleimide, are not substrates. If the enzymatic activity is disrupted by site-directed mutagenesis while retaining structural integrity, KefF is still able to activate the Kef system, showing that the role as an activator is independent of the enzyme activity. Potassium efflux assays show that electrophilic quinones are able to activate the Kef system by forming S conjugates with glutathione. Therefore, it appears that the enzymatic activity of KefF diminishes the redox toxicity of quinones, in parallel with the protection afforded by activation of the Kef system.     

Mechanism of a genetically encoded dark-to-bright reporter for caspase activity

Fluorescent proteins have revolutionized modern biology with their ability to report the presence of tagged proteins in living systems. Although several fluorescent proteins have been described in which the excitation and emission properties can be modulated by external triggers, no fluorescent proteins have been described that can be activated from a silent dark state to a bright fluorescent state directly by the activity of an enzyme. We have developed a version of GFP in which fluorescence is completely quenched by appendage of a hydrophobic quenching peptide that tetramerizes GFP and prevents maturation of the chromophore. The fluorescence can be fully restored by catalytic removal of the quenching peptide, making it a robust reporter of proteolysis. We have demonstrated the utility of this uniquely dark state of GFP as a genetically encoded apoptosis reporter that monitors the function of caspases, which catalyze the fate-determining step in programmed cell death. Caspase Activatable-GFP (CA-GFP) can be activated both in vitro and in vivo, resulting in up to a 45-fold increase in fluorescent signal in bacteria and a 3-fold increase in mammalian cells. We used CA-GFP successfully to monitor real-time apoptosis in mammalian cells. This dark state of GFP may ultimately serve as a useful platform for probes of other enzymatic processes.