Kay, R. | Search bioRxiv

bioRxiv2022

The formation and closure of macropinocytic cups in a model system

Macropinocytosis is a conserved endocytic process where cells take up medium into micron-sized vesicles. In Dictyostelium, macropinocytic cups form around domains of PIP3 in the plasma membrane and extend by actin polymerization. Using lattice light-sheet microscopy, we describe how cups originate, are supported by an F-actin scaffold and shaped by a ring of actin polymerization, created around PIP3 domains. How cups close is unknown. We find two ways: lip closure, where actin polymerization at the lip is re-directed inwards; and basal closure, where it stretches the cup, eventually causing membrane delamination and vesicle sealing. Cups grow as expanding waves of actin polymerization that travel across the cell surface, capturing new membrane. We propose that cups close when these waves stall. This stalled wave hypothesis is tested through a conceptual model, where the interplay of forces from actin polymerization and membrane tension recreates many of our observations.

cell biology↗

bioRxiv2019

Secretin release after Roux-en-Y Gastric Bypass reveals a population of glucose-sensitive S-cells in distal small intestine

ObjectiveGastrointestinal hormones contribute to the beneficial effects of Roux-en-Y gastric bypass surgery (RYGB) on glycemic control. Secretin is secreted from duodenal S-cells in response to low luminal pH, but it is unknown whether its secretion is altered after RYGB and if secretin contributes to the post-operative improvement in glycemic control. We hypothesized that secretin secretion increases after RYGB as a result of the diversion of nutrients to more distal parts of the small intestine, and thereby affects islet hormone release.\n\nMethodsA specific secretin radioimmunoassay was developed, evaluated biochemically, and used to quantify plasma concentrations of secretin in 13 obese individuals before, 1 week after and 3 months after RYGB. Distribution of secretin and its receptor was assessed by RNA-sequencing, mass-spectrometry and in situ hybridization in human and rat tissues. Isolated, perfused rat intestine and pancreas were used to explore the molecular mechanism underlying glucose-induced secretin secretion and to study direct effects of secretin on glucagon, insulin and somatostatin secretion. Secretin was administered alone or in combination with GLP-1 to non-sedated rats to evaluate effects on glucose regulation.\n\nResultsPlasma postprandial secretin was more than doubled in humans after RYGB (P<0.001). The distal small intestine harbored secretin expressing cells in both rats and humans. Glucose increased secretion of secretin in a sodium-glucose co-transporter dependent manner when administered to the distal part but not into the proximal part of the rat small intestine. Secretin stimulated somatostatin secretion (fold change: 1.59, P<0.05) from the perfused rat pancreas but affected neither insulin (P=0.2) nor glucagon (P=0.97) secretion. When administered to rats in vivo, insulin secretion was attenuated and glucagon secretion increased (P=0.04), while blood glucose peak time was delayed (from 15 min to 45 min) and gastric emptying time prolonged (P=0.004).\n\nConclusionGlucose-sensing secretin cells located in the distal part of the small intestine may contribute to increased plasma concentrations observed after RYGB. The metabolic role of the distal S-cells warrants further studies.

physiology↗

bioRxiv2019

The Piezo channel is used by migrating cells to sense compressive load

Migrating cells face varied mechanical and physical barriers in physiological environments, but how they sense and respond to them remains to be fully understood. We used a custom-built cell squasher to apply uniaxial pressure to Dictyostelium cells migrating under soft agarose. Within 10 seconds of application, loads of as little as 100 Pa cause cells to move using blebs instead of pseudopods. Cells lose volume and surface area under pressure and their actin dynamics are perturbed. Myosin-II is recruited to the cortex, potentially increasing contractility and so driving blebbing. The blebbing response depends on extra-cellular calcium, is accompanied by increased cytosolic calcium and largely abrogated in null mutants of the Piezo stretch-operated channel. We propose that migrating cells sense mechanical force through mechano-sensitive channels, leading to an influx of calcium and cortical recruitment of myosin, thus re-directing the motile apparatus to produce blebs rather than pseudopods.

biophysics↗