‘Full fusion’ is not ineluctable during vesicular exocytosis of neurotransmitters by endocrine cells

Abstract

Vesicular exocytosis is an essential and ubiquitous process in neurons and endocrine cells by which neurotransmitters are released in synaptic clefts or extracellular fluids. It involves the fusion of a vesicle loaded with chemical messengers with the cell membrane through a nanometric fusion pore. In endocrine cells, unless it closes after some flickering (‘Kiss-and-Run’ events), this initial pore is supposed to expand exponentially, leading to a full integration of the vesicle membrane into the cell membrane—a stage called ‘full fusion’. We report here a compact analytical formulation that allows precise measurements of the fusion pore expansion extent and rate to be extracted from individual amperometric spike time courses. These data definitively establish that, during release of catecholamines, fusion pores enlarge at most to approximately one-fifth of the radius of their parent vesicle, hence ruling out the ineluctability of ‘full fusion’.

Electrochemical Monitoring of ROS/RNS Homeostasis Within Individual Phagolysosomes Inside Single Macrophages

“…The two quantities are proportional through the Faraday law. Herein, the time‐dependent quantity of ROS/RNS, Q in ( t ), present at time t inside the phagolysosome is then formulated as: …”

“…where Q prod ( t ) is a time‐dependent kinetic term accounting for the ROS/RNS production inside the phagolysosome during release; is a pseudo‐rate constant featuring the diffusion‐controlled release out of the phagolysosome pore of time‐dependent radius ρ ( t ) . The current, i ( t ), featuring the ROS/RNS oxidation at the Pt‐NWE surface is then: …”

“…By analogy to the release of neurotransmitters [11,12] it is expected that in the absence of any ROS/RNS homeostasis all recorded spikes display three main features:1 )a small prespike feature evidencing the initial opening of the pore followed by 2) arapid increase of the current due to the rapid expansion of the pore up to its maximum size,a nd 3) finally an exponentially decaying branch ( Figure 1D)r epresenting the diffusion-limited emptying of the initial ROS/RNS content. However,o nly 25 %o ft he events (N = 1040) presented this expected pattern [10][11][12] (Figures 1D,2 C). The large majority (75 %) of spikes displayed the anticipated behavior only up to ag iven point (t < t exp in Figure 2A; Supporting Information, Figure S2A,C) located on their descending branch.…”

Surface Heterogeneities Matter in Fast Scan Cyclic Voltammetry Investigations of Catecholamines in Brain with Carbon Microelectrodes of High-Aspect Ratio: Dopamine Oxidation at Conical Carbon Microelectrodes

“…This amounts to assume that the diffusional equivalent of the Newton and Kelvin law of cooling applies, i.e., that diffusional equilibration within each type of nanodomains is much faster than exchanges between adjacent ones. 64,65 This leads at each instant to the fast setting of a quasi-steady state inside each nanodomain, so that f (θ) = [1 − exp(−k f θ)]. 64,65 It follows that to describe the current peak intensity at a given scan rate one may consider that the electrochemical system behaves as if it initially consisted of a single domain containing h A h {1 + γ[1 − exp(−k f RT /Fv)]} oxidizable adsorbed molecules irrespective of the fact that only h A h of them were initially adsorbed onto electroactive nanopatches before the voltammetric scan was initiated.…”

“…64,65 This leads at each instant to the fast setting of a quasi-steady state inside each nanodomain, so that f (θ) = [1 − exp(−k f θ)]. 64,65 It follows that to describe the current peak intensity at a given scan rate one may consider that the electrochemical system behaves as if it initially consisted of a single domain containing h A h {1 + γ[1 − exp(−k f RT /Fv)]} oxidizable adsorbed molecules irrespective of the fact that only h A h of them were initially adsorbed onto electroactive nanopatches before the voltammetric scan was initiated. In practice the kinetics of DA non−el ads molecules migration may be more intricate, e.g., involving radial or cylindrical diffusion according to the exact nanodomain shapes and their imbrication onto the electrode surface, 41,66,67 thus leading to more complex formulations of f (θ).…”