How nerve cells circulate and supply essential chemical messengers to brain synapses

What is it about?

In this study, we developed a mathematical compartmental model to investigate how neuropeptide-containing dense core vesicles (DCVs) are transported, captured, and circulated within axon terminals. Grounded in recent biological experiments demonstrating that DCVs continuously circulate along nerve fibers rather than traveling in a simple unidirectional path, our model simulates a five-compartment system comprising the main axonal pathway and four sequential release sites called en passant boutons. Using systems of ordinary differential equations based on mass conservation, we calculated how vesicle concentrations and inter-compartmental transport fluxes evolve over time from an initial state to steady-state equilibrium. Our model accounts for vesicles traveling away from the cell body via kinesin motors, slowing down to switch to dynein motors in the most distal bouton, and traveling retrogradely back toward the cell body where they can re-enter circulation. We evaluated both healthy neurons and pathological conditions where retrograde transport is impaired—a feature characteristic of several neurodegenerative diseases. The numerical solutions show that in healthy neurons, vesicle concentrations reach stable steady states within 20 to 50 hours, whereas inhibiting retrograde transport leads to excessive vesicle buildup in the terminal bouton and significantly weakens overall circulation efficiency throughout the axon.

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Photo by Jon Sailer on Unsplash

Photo by Jon Sailer on Unsplash

Why is it important?

This work provides the first mechanistic mathematical framework to explain how long-range vesicle circulation maintains a steady and uniform supply of neuropeptides across multiple synaptic release sites. While laboratory experiments revealed that vesicles move back and forth along axons, our quantitative model illuminates the precise dynamic balance between vesicle entry, local capture, motor switching, and organelle degradation. Understanding these mechanics is fundamental to neurobiology because reliable neuropeptide delivery is required for normal synaptic transmission and nervous system regulation. Additionally, our findings provide crucial theoretical insights into neurodegenerative conditions such as ALS, Alzheimer's, and Parkinson's disease, which are strongly linked to axonal transport defects. By demonstrating that a disruption in retrograde motor transport directly reduces forward anterograde delivery and causes abnormal vesicle crowding at distal terminals, our model connects molecular motor failure to broader terminal dysfunction. This mathematical model offers researchers a predictive tool to examine disease mechanisms and evaluate therapeutic approaches aimed at restoring motor protein function.

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