# Gap Junctional Communication Required for the Establishment of Long-Term Robust Ca2+ Oscillations Across Human Neuronal Spheroids and Extended 2D Cultures

## TL;DR

This study shows that synchronized calcium oscillations in human neuronal cultures are driven by gap junctions, not synapses, offering a new model for studying network coordination and neurodevelopment.

## Contribution

The study introduces a human neuronal model where gap junctions, not synapses, drive synchronized calcium oscillations.

## Key findings

- Gap junction blockers disrupt synchronization but not individual cell activity.
- Calcium oscillations occur at ~0.2 Hz with high synchronization across large cultures.
- Functional dye-transfer and connexin-43 expression confirm electrical coupling via gap junctions.

## Abstract

Synchronized oscillatory fluctuations in intracellular calcium concentration across extended neuronal networks represent a functional indicator of connectivity and signal coordination. In this study, a model of human immature neurons (differentiated from LUHMES precursors) has been used to establish a robust protocol for generating reproducible intracellular Ca2+ oscillations in both two-dimensional monolayers and three-dimensional spheroids. Oscillatory activity was induced by defined ionic conditions in combination with potassium channel blockade. It was characterized by stable frequencies of approximately 0.2 Hz and high synchronization indices across millimeter-scale cultures. These properties were consistently reproduced in independent experiments and across laboratories. Single-cell imaging confirmed that oscillations were coordinated throughout large cell populations. Pharmacological interventions demonstrated that neither excitatory nor inhibitory chemical synaptic transmission influenced oscillatory dynamics. Gap junction blockers completely disrupted synchronization, while leaving individual cell activity unaffected. Functional dye-transfer assays provided additional evidence for electrical coupling. This was further supported by connexin-43 expression profiles and immunostaining. Collectively, these findings indicate that synchronized Ca2+ oscillations in LUHMES cultures are mediated by gap junctional communication rather than by conventional synaptic mechanisms. This system offers a practical platform for studying fundamental principles of network coordination and for evaluating pharmacological or toxicological modulators of intercellular coupling. Moreover, it may provide a relevant human-based model to explore aspects of neuronal maturation and to assess compounds with potential neurodevelopmental toxicity.