Regulation ofα-cell glucagon secretion:The role of second messengers

Glucagon is a potent glucose-elevating hormone that is secreted by pancreaticα-cells.While well-controlled glucagon secretion plays an important role in maintaining systemic glucose homeostasis and preventing hypoglycaemia,it is increasingly apparent that defects in the regulation of glucagon secretion contribute to impaired counter-regulation and hyperglycaemia in diabetes.It has therefore been proposed that pharmacological interventions targeting glucagon secretion/signalling can have great potential in improving glycaemic control of patients with diabetes.However,despite decades of research,a consensus on the precise mechanisms of glucose regulation of glucagon secretion is yet to be reached.Second messengers are a group of small intracellular molecules that relay extracellular signals to the intracellular signalling cascade,modulating cellular functions.There is a growing body of evidence that second messengers,such as cAMP and Ca^(2+),play critical roles inα-cell glucose-sensing and glucagon secretion.In this review,we discuss the impact of second messengers onα-cell electrical activity,intracellular Ca^(2+)dynamics and cell exocytosis.We highlight the possibility that the interaction between different second messengers may play a key role in the glucose-regulation of glucagon secretion.

Calcium Signaling is Involved in Negative Phototropism of Rice Seminal Roots

Calcium ions （Ca2＋） act as an intracellular second messenger and affect nearly all aspects of cellular life. They are functioned by interacting with polar auxin transport, and the negative phototropism of plant roots is caused by the transport of auxin from the irradiated side to the shaded side of the roots. To clarify the role of calcium signaling in the modulation of rice root negative phototropism, as well as the relationship between polar auxin transport and calcium signaling, calcium signaling reagents were used to treat rice seminal roots which were cultivated in hydroculture and unilaterally illuminated at an intensity of 100-200 pmol/（m2.s） for 24 h. Negative phototropism curvature and growth rate of rice roots were both promoted by exogenous CaCI2 lower than 100 pmol/L, but inhibited by calcium channel blockers （verapamil and LaCI3）, calcineurin inhibitor （chlorpromazine, CPZ）, and polar auxin transport inhibitor （N-l-naphthylphthalamic acid, NPA）. Roots stopped growing and negative phototropism disappeared when the concentrations increased to 100 pmol/L verapamil, 12.500 ~Jmol/L LaCI3, 60 pmol/L CPZ, and 6 pmol/L NPA. Moreover, 100 pmol/L CaCI2 could relieve the inhibition of LaCI3, verapamil and NPA. The enhanced negative phototropism curvature was caused by the transportation of more auxin from the irradiated side to the shaded side in the presence of exogenous Ca2＋. Calcium signaling plays a key role as a second messenger in the process of light signal regulation of rice root growth and negative phototropism.

Structure, biochemical function, and signaling mechanism of plant NLRs

To counter pathogen invasion,plants have evolved a large number of immune receptors,including membrane-resident pattern recognition receptors(PRRs)and intracellular nucleotide-binding and leucine-rich repeat receptors(NLRs).Our knowledge about PRR and NLR signaling mechanisms has expanded significantly over the past few years.Plant NLRs form multi-protein complexes called resistosomes in response to pathogen effectors,and the signaling mediated by NLR resistosomes converges on Ca2+-permeable channels.Ca2+-permeable channels important for PRR signaling have also been identified.These findings highlight a crucial role of Ca2+in triggering plant immune signaling.In this review,we first discuss the structural and biochemical mechanisms of non-canonical NLR Ca2+channels and then summarize our knowledge about immune-related Ca2+-permeable channels and their roles in PRR and NLR signaling.We also discuss the potential role of Ca2+in the intricate interaction between PRR and NLR signaling.