# Spatiotemporal Remodeling of Presynaptic Terminals in Human Neuromuscular Junctions

## TL;DR

This study explores how presynaptic terminals at human neuromuscular junctions change with age and in different metabolic conditions using a muscle tissue model.

## Contribution

The study introduces a human skeletal muscle tissue model to investigate age-related and metabolic effects on presynaptic terminal remodeling.

## Key findings

- Presynaptic terminal features like number and size increase with age and correlate with inflammatory and neurotrophic proteins.
- Prolonged in vitro maintenance reproduces age-related presynaptic terminal changes and increases fragmentation.
- Increased presynaptic terminal fragmentation correlates with higher BMI, and fatty acid supplementation partially restores terminal formation.

## Abstract

Neuromuscular junctions (NMJs) connect motor neurons to muscle fibers, enabling electrical-to-chemical signal transmission and sensing of mechanical forces. We employed a previously introduced human skeletal muscle tissue model to study the NMJ presynaptic terminal (PT). Using immunohistochemical approaches, we analyzed PT formation in samples from 12 participants, either immediately after surgery or following 11 days of in vitro maintenance. In immediate postsurgical samples, the number, size, and integrated immunofluorescence intensity of detectable PTs increased with participant age, accompanied by elevated expression of neurotrophic and inflammatory proteins. Moreover, CD80+ and CD206+ macrophages were detected near PTs, suggesting a potential regulatory role in PT formation. Prolonged in vitro maintenance of muscle samples in a quiescent state for 11 days reproduced age-associated PT features and further increased PT spatial expansion and fragmentation. Notably, increased PT fragmentation positively correlated with participant body mass index (BMI), suggesting a possible link between metabolic status and PT remodeling. Consistently, the activation of skeletal muscle metabolism through long-chain fatty acid supplementation partially restored PT formation in vitro. Together, these findings highlight the utility of human skeletal muscle tissue models for investigating NMJ dynamics in pathological contexts and for identifying therapeutic strategies aimed at delaying or preventing neuromuscular decline.