Glucose-dependent insulinotropic polypeptide (GIP) communicates nutrient intake from the gut to islets, enabling optimal levels of insulin secretion via the GIP receptor (GIPR) on β cells. The GIPR is also expressed in α cells, and GIP stimulates glucagon secretion; however, the role of this action in the postprandial state is unknown. Here, we demonstrate that GIP potentiates amino acid–stimulated glucagon secretion, documenting a similar nutrient-dependent action to that described in β cells. Moreover, we demonstrate that GIP activity in α cells contributes to insulin secretion by invoking paracrine α to β cell communication. Last, specific loss of GIPR activity in α cells prevents glucagon secretion in response to a meal stimulus, limiting insulin secretion and driving glucose intolerance. Together, these data uncover an important axis by which GIPR activity in α cells is necessary to coordinate the optimal level of both glucagon and insulin secretion to maintain postprandial homeostasis.

BackgroundThe Ras/Raf/MEK/ERK signaling pathway is involved in essential cell processes and it is abnormally activated in ~30 % of cancers and cognitive disorders. Two ERK isoforms have been described, ERK1 and ERK2; ERK2 being regarded by many as essential due to the embryonic lethality of ERK2 knock-out mice, whereas mice lacking ERK1 are viable and fertile. The controversial question of why we have two ERKs and whether they have differential functions or display functional redundancy has not yet been resolved.ResultsTo investigate this question we used a novel approach based on comparing the evolution of ERK isoforms’ sequences and protein expression across vertebrates. We gathered and cloned erk1 and erk2 coding sequences and we examined protein expression of isoforms in brain extracts in all major clades of vertebrate evolution. For the first time, we measured each isoforms’ relative protein level in phylogenetically distant animals using anti-phospho antibodies targeting active ERKs. We demonstrate that squamates (lizards, snakes and geckos), despite having both genes, do not express ERK2 protein whereas other tetrapods either do not express ERK1 protein or have lost the erk1 gene. To demonstrate the unexpected squamates’ lack of ERK2 expression, we targeted each ERK isoform in lizard primary fibroblasts by specific siRNA-mediated knockdown. We also found that undetectable expression of ERK2 in lizard is compensated by a greater strength of lizard’s erk1 promoter. Finally, phylogenetic analysis revealed that ERK1 amino acids sequences evolve faster than ERK2’s likely due to genomic factors, including a large difference in gene size, rather than from functional differences since amino acids essential for function are kept invariant.ConclusionsERK isoforms appeared by a single gene duplication at the onset of vertebrate evolution at least 400 Mya. Our results demonstrate that tetrapods can live by expressing either one or both ERK isoforms, supporting the notion that ERK1/2 act interchangeably. Substrate recognition sites and catalytic cleft are nearly invariant in all vertebrate ERKs further suggesting functional redundancy. We suggest that future ERK research should shift towards understanding the role and regulation of total ERK quantity, especially in light of newly described erk2 gene amplification identified in tumors.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0450-x) contains supplementary material, which is available to authorized users.

Hagfishes defend themselves from fish predators via the rapid deployment of a fibrous slime that adheres to and clogs gills. The slime transforms from a thick glandular exudate to a fully hydrated product in a fraction of a second through a process that involves the swelling and rupture of numerous mucin vesicles. Here we demonstrate that the vesicle membrane plays an important role in regulating the swelling of mucin granules, and provide evidence that the membrane contains proteins that facilitate the movement of ions and water molecules. By exposing isolated mucin vesicles to varying combinations of inorganic ions, organic compounds and membrane channel inhibitors, we found that the majority of hagfish mucin vesicles require Ca 2+ to rupture. We also show that Ca 2+-dependent rupture can be pharmacologically inhibited, which suggests a role for Ca 2+ -activated membrane transporters. We demonstrate that the aquaporin inhibitor mercuric chloride reduces the rate of vesicle swelling by an order of magnitude, which suggests that aquaporins facilitate the influx of water during vesicle deployment. Molecular evidence of two aquaporin homologues expressed in the slime glands further supports this idea. We propose a model of hagfish slime mucin vesicle rupture that involves Ca 2+ -activated transporters and aquaporins, and suggest that the presence of these proteins is an adaptation for increasing the speed of vesicle rupture and, consequently, the speed of the sliming response of hagfishes.

The acid-base relevant molecules carbon dioxide (CO2), protons (H + ), and bicarbonate (HCO3 -) are substrates and end products of some of the most essential physiological functions including aerobic and anaerobic respiration, ATP hydrolysis, photosynthesis, and calcification. The structure and function of many enzymes and other macromolecules are highly sensitive to changes in pH, and thus maintaining acidbase homeostasis in the face of metabolic and environmental disturbances is essential for proper cellular function. On the other hand, CO2, H + and HCO3have regulatory effects on various proteins and processes, both directly through allosteric modulation and indirectly through signal transduction pathways. Life in aquatic environments 2 presents organisms with distinct acid-base challenges that are not found in terrestrial environments. These include a relatively high CO2 relative to O2 solubility that prevents internal CO2/HCO3accumulation to buffer pH, a lower O2 content that may favor anaerobic metabolism, and variable environmental CO2, pH and O2 levels that require dynamic adjustments in acid-base homeostatic mechanisms. Additionally, some aquatic animals purposely create acidic or alkaline microenvironments that drive specialized physiological functions. For example, acidifying mechanisms can enhance O2 delivery by red blood cells, lead to ammonia trapping for excretion or buoyancy purposes, or lead to CO2 accumulation to promote photosynthesis by endosymbiotic algae. On the other hand, alkalinizing mechanisms can serve to promote calcium carbonate skeletal formation. This non-exhaustive review summarizes some of the distinct acid-base homeostatic mechanisms that have evolved in aquatic organisms to meet the particular challenges of this environment.

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Aim: To determine whether Na + uptake in adult zebrafish (Danio rerio) exposed to acidic water adheres to traditional models reliant on Na + /H + Exchangers (NHEs), Na + channels and Na + /Cl − Cotransporters (NCCs) or if it occurs through a novel mechanism.Methods: Zebrafish were exposed to control (pH 8.0) or acidic (pH 4.0) water for 0-12 hours during which 22 Na + uptake (J Na in ), ammonia excretion, net acidic equivalent flux and net K + flux (J H net ) were measured. The involvement of NHEs, Na + channels, NCCs, K + -channels and K + -dependent Na + /Ca 2+ exchangers (NCKXs) was evaluated by exposure to Cl − -free or elevated [K + ] water, or to pharmacological inhibitors. The presence of NCKXs in gill was examined using RT-PCR. Results: J Nain was strongly attenuated by acid exposure, but gradually recovered to control rates. The systematic elimination of each of the traditional models led us to consider K + as a counter substrate for Na + uptake during acid exposure. Indeed, elevated environmental [K + ] inhibited J Na in during acid exposure in a concentration-dependent manner, with near-complete inhibition at 10 mM. Moreover, J H net loss increased approximately fourfold at 8-10 hours of acid exposure which correlated with recovered J Na in in 1:1 fashion, and both J Na in and J H net were sensitive to tetraethylammonium (TEA) during acid exposure. Zebrafish gills expressed mRNA coding for six NCKX isoforms.Conclusions: During acid exposure, zebrafish engage a novel Na + uptake mechanism that utilizes the outwardly directed K + gradient as a counter-substrate for Na + and is sensitive to TEA. NKCXs are promising candidates to mediate this K + -dependent Na + uptake, opening new research avenues about Na + uptake in zebrafish and other acid-tolerant aquatic species.

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