Abstract
Through somatic exocytosis neurons liberate immense amounts of transmitter molecules that modulate the functioning of the nervous system. A stream of action potentials triggers an ATP-dependent transport of transmitter-containing vesicles to the plasma membrane, that ends with a large-scale exocytosis. It is commonly assumed that biological processes use metabolic energy with a high thermodynamic efficiency, meaning that most energy generates work with minor dissipation. However, the intricate ultrastructure underlying the pathway for the vesicle flow necessary for somatic exocytosis challenges this possibility. To study this problem here we first applied thermodynamic theory to quantify the efficiency of somatic exocytosis of the vital transmitter serotonin. Then we correlated the efficiency to the ultrastructure of the transport pathway of the vesicles. Exocytosis was evoked in cultured Retzius neurons of the leech by trains of 10 impulses delivered at 20 Hz. The kinetics of exocytosis was quantified from the gradual fluorescence increase of FM1-43 dye as it became incorporated into vesicles that underwent their exo-endocytosis cycle. By fitting a model of the vesicle transport carried by motor forces to the kinetics of exocytosis, we calculated the thermodynamic efficiency of the ATP expenses per vesicle, as the power of the transport divided by total energy ideally produced by the hydrolysis of ATP during the process. The efficiency was remarkably low (0.1–6.4%) and the values formed a W-shape distribution with the transport distances of the vesicles. Electron micrographs and fluorescent staining of the actin cortex indicated that the slopes of the W chart could be explained by the interaction of vesicles with the actin cortex and the calcium-releasing endoplasmic reticulum. We showed that the application of thermodynamic theory permitted to predict aspects of the intracellular structure. Our results suggest that the distribution of subcellular structures that are essential for somatic exocytosis abates the thermodynamic efficiency of the transport by hampering vesicle mobilization. It is remarkable that the modulation of the nervous system occurs at the expenses of an efficient use of metabolic energy.
Highlights
Extrasynaptic exocytosis, namely the release of transmitters and peptides from the neuronal soma, dendrites, and axons, is a source of modulators of the activity of the nervous system (De-Miguel and Fuxe, 2012; De-Miguel and Nicholls, 2015)
Dimensionless constant referring to the fluorescence increase per vesicle fused Transport distance of vesicle clusters measured from the center of mass of the cluster to the plasma membrane Gibb’s free energy Gibb’s free energy of ATP hydrolysis Baseline value of the fluorescence intensity Fluorescence as a function of time Force exerted by the motors Friction forces per mass unit Random forces due to thermal agitation per mass unit Elastic forces per mass unit Molecular motor force per mass unit Power of the large-scale exocytosis Current density of vesicles Elastic constant of the cytoskeleton Mass Thermodynamic efficiency Number of vesicles per cluster Rate constant of ATP hydrolysis Average velocity of vesicle cluster transport Characteristic frequency of the elastic force Work of the large-scale exocytosis
These obstacles bottlenecked the flux of vesicles and can be considered as sources of friction forces that reduce the thermodynamic efficiency of somatic exocytosis
Summary
Extrasynaptic exocytosis, namely the release of transmitters and peptides from the neuronal soma, dendrites, and axons, is a source of modulators of the activity of the nervous system (De-Miguel and Fuxe, 2012; De-Miguel and Nicholls, 2015). The actin cortex at rest prevents the vesicle transport, but after the onset of electrical activity turns into a component of the transport system by binding to myosin motors (Wang and Hatton, 2006; Tobin and Ludwig, 2007) Such observations raised the possibility that somatic exocytosis operates under low thermodynamic efficiency, opposing to the common assumption that biological processes make a high efficiency use of their metabolic energy. Activation of serotonin autoreceptors evoke an intracellular IP3dependent release of calcium from the endoplasmic reticulum adjacent to the plasma membrane This localized calcium elevation maintains exocytosis until the fusion of the last vesicles in the clusters (Leon-Pinzon et al, 2014). The thermodynamic efficiency along the traveling distance of the vesicles was correlated with the ultrastructure of the transport pathway, analyzed from electron micrographs, and fluorescent staining of the actin cortex
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