Porocytosis: Fusion Pore Array Secretion of Neurotransmitter

Abstract

We believe that there is sufficient experimental evidence to support the premise that transmitter is secreted by the simultaneous activation of arrays of fusion pores at docked vesicles. This process is initiated by the action potential that activates calcium channels to increase the number of cytoplasmic calcium ions. Calcium ions trigger fusion pores to flicker open causing transmitter to diffuse from vesicular stores. We define the term porocytosis to identify this process and use the term synaptomere to indicate the anatomical and physiological functional unit of the synapse or junction. Our model shows that the simultaneous flicker of fusion pores in an array can generate unitary-end plate potentials (u-EPPs) and miniature end plate potentials (MEPPs) and that activation of all fusion pores produces EPPs. U-EPPs and EPPs generated with the model show mean values and coefficients of variation similar to experimental observations. The model is robust in that the number of docked vesicles can vary and these can be full to empty depending on nerve frequencies and vesicular traffic. The model shows that the overall process of excitation-secretion coupling is highly deterministic. At the neuromuscular junction, secretion from arrays of fusion pores ensures that a muscle fiber action potential is always produced over a range of frequencies because all transmitter release sites are activated. Our model shows that transmission at the synaptomere guarantees fidelity of information transfer at different frequencies. This characteristic shows a dynamic relationship of the secretory process to memory and learning.

Porocytosis: A transient pore array secretes the neurotransmitter packet

“…In this experimental condition, Poisson statistics do fit EPP amplitude data when the numbers of secreted packets are reduced to fewer that 10 (del Castillo and Katz, 1954). The normal small observed coefficient of variation (< 3%) (Kriebel and Keller, 2000) of end‐plate potentials is consistent with each of the 200 release sites secreting one packet as originally proposed by del Castillo and Katz (1954) and is the basis for our array model. In contrast, release of neurotransmitter via the QVE mechanism where 200 vesicles are selected for exocytosis from 10,000 docked vesicles would result in an end‐plate potential coefficient of variation of 14–30%, which would require Poisson statistics (Kriebel and Keller, 1999).…”

“…In our hypothesis, the postsynaptic quantal response results from transmitter pulsed from an array of transient pores rather than from a single presynaptic vesicular exocytotic event (Fig. 1) (Kriebel et al, 2000, 2001). In the mechanism we propose, presynaptic vesicles are anchored to the active zone of the plasma membrane and juxtaposed to calcium ion‐selective channels by proteins such as SNARE (Sudhof, 2000).…”

“…We propose that the arrayed nature of components composing a secretory site provides an ultrastructural basis for the extensive electrophysiological and statistical data that is incompatible with the QVE hypothesis. We view the “secretory organelle” described by Harlow et al, (2001), which we have called the synaptomere (Kriebel et al, 2000, 2001), to be the unit of secretion. The scaffold of the synaptomere positions vesicles in juxtaposition to the neurolemma so that transient pores connect vesicular contents to the synaptic space, permitting a pulse of transmitter.…”

Secretion without membrane fusion: Porocytosis

“…Recently, we hypothesized an alternative mechanism, named porocytosis, to describe secretion from vesicular compartments without fusion of the vesicular and plasma membranes (Kriebel et al, 2000, 2001; Silver et al, 2001, 2003). The dimensions of the transient pore would be about 1 nm in diameter, or twice the radius of a lipid molecule, the change in spacing expected for the freezing of lipid with calcium in a bilayer.…”

“…The porocytosis mechanism is consistent with all observations in synapses, including the so‐called quantal vesicular release, with some morphological evidence (Kriebel, et al, 2001; Silver et al, 2001, 2003). While our earlier reports focused on the porocytosis hypothesis as applied to release of neurotransmitter from the presynaptic terminal of the neuromuscular junction (NMJ) (Kriebel et al, 2000, 2001; Silver et al, 2001, 2003), the hypothesis is applicable to other forms of vesicular secretion, which accounts for the often observed secretions in which only part of the vesicular contents (at presynaptic endings) are released, or by granular inclusions such as chromaffin cells, pancreatic islet cells, mast cells, basophils, and eosinophils, and a wide variety of cells central to cancer and inflammatory diseases (Sagen and Pappas, 1987; Pappas and Kriho, 1988; Dvorak, 1991, 2000; Plattner et al, 1997).…”