The small GTPase Cdc42 modulates the number of exocytosis-competent dense-core vesicles in PC12 cells

A Systematic Compilation of Human SH3 Domains: A Versatile Superfamily in Cellular Signaling

SRC homology 3 (SH3) domains are fundamental modules that enable the assembly of protein complexes through physical interactions with a pool of proline-rich/noncanonical motifs from partner proteins. They are widely studied modular building blocks across all five kingdoms of life and viruses, mediating various biological processes. The SH3 domains are also implicated in the development of human diseases, such as cancer, leukemia, osteoporosis, Alzheimer's disease, and various infections. A database search of the human proteome reveals the existence of 298 SH3 domains in 221 SH3 domain-containing proteins (SH3DCPs), ranging from 13 to 720 kilodaltons. A phylogenetic analysis of human SH3DCPs based on their multi-domain architecture seems to be the most practical way to classify them functionally, with regard to various physiological pathways. This review further summarizes the achievements made in the classification of SH3 domain functions, their binding specificity, and their significance for various diseases when exploiting SH3 protein modular interactions as drug targets.

Cdc42: A Novel Regulator of Insulin Secretion and Diabetes-Associated Diseases

Cdc42, a member of the Rho GTPases family, is involved in the regulation of several cellular functions including cell cycle progression, survival, transcription, actin cytoskeleton organization and membrane trafficking. Diabetes is a chronic and metabolic disease, characterized as glycometabolism disorder induced by insulin deficiency related to β cell dysfunction and peripheral insulin resistance (IR). Diabetes could cause many complications including diabetic nephropathy (DN), diabetic retinopathy and diabetic foot. Furthermore, hyperglycemia can promote tumor progression and increase the risk of malignant cancers. In this review, we summarized the regulation of Cdc42 in insulin secretion and diabetes-associated diseases. Organized researches indicate that Cdc42 is a crucial member during the progression of diabetes, and Cdc42 not only participates in the process of insulin synthesis but also regulates the insulin granule mobilization and cell membrane exocytosis via activating a series of downstream factors. Besides, several studies have demonstrated Cdc42 as participating in the pathogenesis of IR and DN and even contributing to promote cancer cell proliferation, survival, invasion, migration, and metastasis under hyperglycemia. Through the current review, we hope to cast light on the mechanism of Cdc42 in diabetes and associated diseases and provide new ideas for clinical diagnosis, treatment, and prevention.

Low glucose-induced ghrelin secretion is mediated by an ATP-sensitive potassium channel

Ghrelin is synthesized in X/A-like cells of the gastric mucosa, which plays an important role in the regulation of energy homeostasis. Although ghrelin secretion is known to be induced by neurotransmitters or hormones or by nutrient sensing in the ghrelin-secreting cells themselves, the mechanism of ghrelin secretion is not clearly understood. In the present study, we found that changing the extracellular glucose concentration from elevated (25  mM) to optimal (10 mM) caused an increase in the intracellular Ca2+ concentration ([Ca2+]i) in ghrelin-secreting mouse ghrelinoma 3-1 (MGN3-1) cells (n=32, P<0.01), whereas changing the glucose concentration from elevated to lowered (5 or 1 mM) had little effect on [Ca2+]i increase. Overexpression of a closed form of an ATP-sensitive K+ (KATP) channel mutant suppressed the 10  mM glucose-induced [Ca2+]i increase (n=8, P<0.01) and exocytotic events (n=6, P<0.01). We also found that a low concentration of a KATP channel opener, diazoxide, with 25  mM glucose induced [Ca2+]i increase (n=23, P<0.01) and ghrelin secretion (n≥3, P<0.05). In contrast, the application of a low concentration of a KATP channel blocker, tolbutamide, significantly induced [Ca2+]i increase (n=15, P<0.01) and ghrelin secretion (n≥3, P<0.05) under 5 mM glucose. Furthermore, the application of voltage-dependent Ca2+ channel inhibitors suppressed the 10 mM glucose-induced [Ca2+]i increase (n≥26, P<0.01) and ghrelin secretion (n≥5, P<0.05). These findings suggest that KATP and voltage-dependent Ca2+ channels are involved in glucose-dependent ghrelin secretion in MGN3-1 cells.

Human and mouse tissue-engineered small intestine both demonstrate digestive and absorptive function

Short bowel syndrome (SBS) is a devastating condition in which insufficient small intestinal surface area results in malnutrition and dependence on intravenous parenteral nutrition. There is an increasing incidence of SBS, particularly in premature babies and newborns with congenital intestinal anomalies. Tissue-engineered small intestine (TESI) offers a therapeutic alternative to the current standard treatment, intestinal transplantation, and has the potential to solve its biggest challenges, namely donor shortage and life-long immunosuppression. We have previously demonstrated that TESI can be generated from mouse and human small intestine and histologically replicates key components of native intestine. We hypothesized that TESI also recapitulates native small intestine function. Organoid units were generated from mouse or human donor intestine and implanted into genetically identical or immunodeficient host mice. After 4 wk, TESI was harvested and either fixed and paraffin embedded or immediately subjected to assays to illustrate function. We demonstrated that both mouse and human tissue-engineered small intestine grew into an appropriately polarized sphere of intact epithelium facing a lumen, contiguous with supporting mesenchyme, muscle, and stem/progenitor cells. The epithelium demonstrated major ultrastructural components, including tight junctions and microvilli, transporters, and functional brush-border and digestive enzymes. This study demonstrates that tissue-engineered small intestine possesses a well-differentiated epithelium with intact ion transporters/channels, functional brush-border enzymes, and similar ultrastructural components to native tissue, including progenitor cells, whether derived from mouse or human cells.

Integrative function of adrenaline receptors for glucagon-like peptide-1 exocytosis in enteroendocrine L cell line GLUTag

Adrenaline reacts with three types of adrenergic receptors, α1, α2 and β-adrenergic receptors (ARs), inducing many physiological events including exocytosis. Although adrenaline has been shown to induce glucagon-like peptide-1 (GLP-1) secretion from intestinal L cells, the precise molecular mechanism by which adrenaline regulates GLP-1 secretion remains unknown. Here we show by live cell imaging that all types of adrenergic receptors are stimulated by adrenaline in enteroendocrine L cell line GLUTag cells and are involved in GLP-1 exocytosis. We performed RT-PCR analysis and found that α1B-, α2A-, α2B-, and β1-ARs were expressed in GLUTag cells. Application of adrenaline induced a significant increase of intracellular Ca(2+) and cAMP concentration ([Ca(2+)]i and [cAMP]i, respectively), and GLP-1 exocytosis in GLUTag cells. Blockade of α1-AR inhibited adrenaline-induced [Ca(2+)]i increase and exocytosis but not [cAMP]i increase, while blockade of β1-AR inhibited adrenaline-induced [cAMP]i increase and exocytosis but not [Ca(2+)]i increase. Furthermore, overexpression of α2A-AR suppressed the adrenaline-induced [cAMP]i increase and exocytosis. These results suggest that the fine-turning of GLP-1 secretion from enteroendocrine L cells is established by the balance between α1-, α2-, and β-ARs activation.

Host endoplasmic reticulum COPII proteins control cell-to-cell spread of the bacterial pathogen Listeria monocytogenes

Listeria monocytogenes is a food-borne pathogen that uses actin-dependent motility to spread between human cells. Cell-to-cell spread involves the formation by motile bacteria of plasma membrane-derived structures termed 'protrusions'. In cultured enterocytes, the secreted Listeria protein InlC promotes protrusion formation by binding and inhibiting the human scaffolding protein Tuba. Here we demonstrate that protrusions are controlled by human COPII components that direct trafficking from the endoplasmic reticulum. Co-precipitation experiments indicated that the COPII proteins Sec31A and Sec13 interact directly with a Src homology 3 domain in Tuba. This interaction was antagonized by InlC. Depletion of Sec31A or Sec13 restored normal protrusion formation to a Listeria mutant lacking inlC, without affecting spread of wild-type bacteria. Genetic impairment of the COPII component Sar1 or treatment of cells with brefeldin A affected protrusions similarly to Sec31A or Sec13 depletion. These findings indicated that InlC relieves a host-mediated restriction of Listeria spread otherwise imposed by COPII. Inhibition of Sec31A, Sec13 or Sar1 or brefeldin A treatment also perturbed the structure of cell-cell junctions. Collectively, these findings demonstrate an important role for COPII in controlling Listeria spread. We propose that COPII may act by delivering host proteins that generate tension at cell junctions.

Cdc42 controls the dilation of the exocytotic fusion pore by regulating membrane tension

On exocytosis, membrane fusion starts with the formation of a narrow fusion pore that must expand to allow the release of secretory compounds. The GTPase Cdc42 promotes fusion pore dilation in neuroendocrine cells by controlling membrane tension.

Membrane fusion underlies multiple processes, including exocytosis of hormones and neurotransmitters. Membrane fusion starts with the formation of a narrow fusion pore. Radial expansion of this pore completes the process and allows fast release of secretory compounds, but this step remains poorly understood. Here we show that inhibiting the expression of the small GTPase Cdc42 or preventing its activation with a dominant negative Cdc42 construct in human neuroendocrine cells impaired the release process by compromising fusion pore enlargement. Consequently the mode of vesicle exocytosis was shifted from full-collapse fusion to kiss-and-run. Remarkably, Cdc42-knockdown cells showed reduced membrane tension, and the artificial increase of membrane tension restored fusion pore enlargement. Moreover, inhibiting the motor protein myosin II by blebbistatin decreased membrane tension, as well as fusion pore dilation. We conclude that membrane tension is the driving force for fusion pore dilation and that Cdc42 is a key regulator of this force.

Molecular mechanism of protrusion formation during cell-to-cell spread of Listeria

The bacterial pathogen Listeria monocytogenes spreads within human tissues using a motility process dependent on the host actin cytoskeleton. Cell-to-cell spread involves the ability of motile bacteria to remodel the host plasma membrane into protrusions, which are internalized by neighboring cells. Recent results indicate that formation of Listeria protrusions in polarized human cells involves bacterial antagonism of a host signaling pathway comprised of the scaffolding protein Tuba and its effectors N-WASP and Cdc42. These three human proteins form a complex that generates tension at apical cell junctions. Listeria relieves this tension and facilitates protrusion formation by secreting a protein called InlC. InlC interacts with a Src Homology 3 (SH3) domain in Tuba, thereby displacing N-WASP from this domain. Interaction of InlC with Tuba is needed for efficient Listeria spread in cultured human cells and infected animals. Recent structural data has elucidated the mechanistic details of InlC/Tuba interaction, revealing that InlC and N-WASP compete for partly overlapping binding surfaces in the Tuba SH3 domain. InlC binds this domain with higher affinity than N-WASP, explaining how InlC is able to disrupt Tuba/N-WASP complexes.

The G protein-coupled receptor family C group 6 subtype A (GPRC6A) receptor is involved in amino acid-induced glucagon-like peptide-1 secretion from GLUTag cells

Although amino acids are dietary nutrients that evoke the secretion of glucagon-like peptide 1 (GLP-1) from intestinal L cells, the precise molecular mechanism(s) by which amino acids regulate GLP-1 secretion from intestinal L cells remains unknown. Here, we show that the G protein-coupled receptor (GPCR), family C group 6 subtype A (GPRC6A), is involved in amino acid-induced GLP-1 secretion from the intestinal L cell line GLUTag. Application of l-ornithine caused an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) in GLUTag cells. Application of a GPRC6A receptor antagonist, a phospholipase C inhibitor, or an IP(3) receptor antagonist significantly suppressed the l-ornithine-induced [Ca(2+)](i) increase. We found that the increase in [Ca(2+)](i) stimulated by l-ornithine correlated with GLP-1 secretion and that l-ornithine stimulation increased exocytosis in a dose-dependent manner. Furthermore, depletion of endogenous GPRC6A by a specific small interfering RNA (siRNA) inhibited the l-ornithine-induced [Ca(2+)](i) increase and GLP-1 secretion. Taken together, these findings suggest that the GPRC6A receptor functions as an amino acid sensor in GLUTag cells that promotes GLP-1 secretion.