Regulated exocytosis mediates the release of hormones and transmitters. The last step of this process is represented by the merger between the vesicle and the plasma membranes, and the formation of a fusion pore. Once formed, the initially stable and narrow fusion pore may reversibly widen (transient exocytosis) or fully open (full-fusion exocytosis). Exocytosis is typically triggered by an elevation in cytosolic calcium activity. However, other second messengers, such as cAMP, have been reported to modulate secretion. The way in which cAMP influences the transitions between different fusion pore states remains unclear. Here, hormone release studies show that prolactin release from isolated rat lactotrophs stimulated by forskolin, an activator of adenylyl cyclases, and by membrane-permeable cAMP analog (dbcAMP), exhibit a biphasic concentration dependency. Although at lower concentrations (2-10 m forskolin and 2.5-5 mm dbcAMP) these agents stimulate prolactin release, an inhibition is measured at higher concentrations (50 m forskolin and 10-15 mm dbcAMP). By using high-resolution capacitance (Cm) measurements, we recorded discrete increases in Cm, which represent elementary exocytic events. An elevation of cAMP leaves the frequency of full-fusion events unchanged while increasing the frequency of transient events. These exhibited a wider fusion pore as measured by increased fusion pore conductance and a prolonged fusion pore dwell time. The probability of observing rhythmic reopening of transient fusion pores was elevated by dbcAMP. In conclusion, cAMP-mediated stabilization of wide fusion pores prevents vesicles from proceeding to the full-fusion stage of exocytosis, which hinders vesicle content discharge at high cAMP concentrations.
COBISS.SI-ID: 30620889
Neuroendocrine cells contain small and large vesicles, but the functional significance of vesicle diameter is unclear. We studied unitary exocytic events of prolactin-containing vesicles in lactotrophs by monitoring discrete steps in membrane capacitance. In the presence of sphingosine, which recruits VAMP2 for SNARE complex formation, the frequency of transient and full fusion events increased. Vesicles with larger diameters proceeded to full fusion, but smaller vesicles remained entrapped in transient exocytosis. The diameter of vesicle dense cores released by full fusion exocytosis into the extracellular space was larger than the diameter of the remaining intracellular vesicles beneath the plasma membrane. Labeling with prolactin- and VAMP2-antibodies revealed a correlation between the diameters of colocalized prolactin- and VAMP2-positive structures. It is proposed that sphingosine-mediated facilitation of regulated exocytosis is not only related to the number of SNARE complexes per vesicle but also depends on the vesicle size, which may determine the transition between transient and full fusion exocytosis.
COBISS.SI-ID: 30722265
How cholesterol, a key membrane constituent, affects membrane surface area dynamics in secretory cells is unclear. Using methyl--cyclodextrin (MCD) to deplete cholesterol, we imaged melanotrophs from male Wistar rats in real-time and monitored membrane capacitance (Cm), fluctuations of which reflect exo- and endocytosis. Treatment with MCD reduced cellular cholesterol and caused a dose-dependent attenuation of the Ca2+-evoked increase in Cm (IC50 = 5.3 mM) vs. untreated cells. Cytosol dialysis of MCD enhanced the attenuation of Cm increase (IC50 = 3.3 mM), suggesting cholesterol depletion at intracellular membrane sites was involved in attenuating exocytosis. Acute extracellular application of MCD resulted in an immediate Cm decline, which correlated well with the cellular surface area decrease, indicating the involvement of cholesterol in the regulation of membrane surface area dynamics. This decline in Cm was three-fold slower than MCD-mediated fluorescent cholesterol decay, implying that the exocytosis is the likely physiological means for plasma membrane cholesterol replenishment. MCD had no effect on the specific Cm and the blockade of endocytosis by Dyngo 4a, confirmed by inhibition of dextran uptake, also had no effect on the time-course of MCD-induced Cm decline. Thus acute exposure to MCD evokes a Cm decline linked to the removal of membrane cholesterol, which cannot be compensated for by exocytosis. We propose that the primary contribution of cholesterol to surface area dynamics is via its role in regulated exocytosis.
COBISS.SI-ID: 30562009
In order to understand exocytosis and endocytosis, it is necessary to study these processes directly. An elegant way to do this is by measuring plasma membrane capacitance (Cm), a parameter proportional to cell surface area, the fluctuations of which are due to fusion and fission of secretory and other vesicles. Here we describe protocols that enable high-resolution Cm measurements in macroscopic and microscopic modes. Macroscopic mode, performed in whole-cell configuration, is used for measuring bulk Cm changes in the entire membrane area, and it enables the introduction of exocytosis stimulators or inhibitors into the cytosol through the patch pipette. Microscopic mode, performed in cell-attached configuration, enables measurements of Cm with attofarad resolution and allows characterization of fusion pore properties. Although we usually apply these protocols to primary pituitary cells and astrocytes, they can be adapted and used for other cell types. After initial hardware setup and culture preparation, several Cm measurements can be performed daily.
COBISS.SI-ID: 30613977
Oscillatory electrical activity is regarded as a hallmark of the pancreatic beta cell glucose-dependent excitability pattern. Electrophysiologically recorded membrane potential oscillations in beta cells are associated with in-phase oscillatory cytosolic calcium activity ([Ca2+]i) measured with fluorescent probes. Recent high spatial and temporal resolution confocal imaging revealed that glucose stimulation of beta cells in intact islets within acute tissue slices produces a [Ca2+]i change with initial transient phase followed by a plateau phase with highly synchronized [Ca2+]i oscillations. Here, we aimed to correlate the plateau [Ca2+]i oscillations with the oscillations of membrane potential using patch-clamp and for the first time high resolution voltage-sensitive dye based confocal imaging. Our results demonstrated that the glucose-evoked membrane potential oscillations spread over the islet in a wave-like manner, their durations and wave velocities being comparable to the ones for [Ca2+]i oscillations and waves. High temporal resolution simultaneous records of membrane potential and [Ca2+]i confirmed tight but nevertheless limited coupling of the two processes, with membrane depolarization preceding the [Ca2+]i increase. The potassium channel blocker tetraethylammonium increased the velocity at which oscillations advanced over the islet by several-fold while, at the same time, emphasized differences in kinetics of the membrane potential and the [Ca2+]i. The combination of both imaging techniques provides a powerful tool that will help us attain deeper knowledge of the beta cell network.
COBISS.SI-ID: 512362552