Excitatory neurotransmission activates compartmentalized calcium transients in Muller glia without affecting lateral process motility

eLife digest When it comes to studying the nervous system, neurons often steal the limelight; yet, they can only work properly thanks to an ensemble cast of cell types whose roles are only just emerging. For example, 'glial cells' - their name derives from the Greek word for glue - were once thought to play only a passive, supporting function in nervous tissues. Now, growing evidence shows that they are, in fact, integrated into neural circuits: their activity is influenced by neurons, and, in turn, they help neurons to function properly. The role of glial cells is becoming clear in the retina, the thin, light-sensitive layer that lines the back of the eye and relays visual information to the brain. There, beautifully intricate Mu''ller glial cells display fine protrusions (or 'processes') that intermingle with synapses, the busy space between neurons where chemical messengers are exchanged. These messengers can act on Muller cells, triggering cascades of molecular events that may influence the structure and function of glia. This is of particular interest during development: as Muller cells mature, they are exposed to chemicals released by more fully formed retinal neurons. Tworig et al. explored how neuronal messengers can influence the way Muller cells grow their processes. To do so, they tracked mouse retinal glial cells 'live' during development, showing that they were growing fine, highly dynamic processes in a region rich in synapses just as neurons and glia increased their communication. However, using drugs to disrupt this messaging for a short period did not seem to impact how the processes grew. Extending the blockade over a longer timeframe also did not change the way Muller cells developed, with the cells still acquiring their characteristic elaborate process networks. Taken together, these results suggest that the structural maturation of Mu''ller glial cells is not impacted by neuronal signaling, giving a more refined understanding of how glia form in the retina and potentially in the brain. Neural activity has been implicated in the motility and outgrowth of glial cell processes throughout the central nervous system. Here, we explore this phenomenon in Muller glia, which are specialized radial astroglia that are the predominant glial type of the vertebrate retina. Muller glia extend fine filopodia-like processes into retinal synaptic layers, in similar fashion to brain astrocytes and radial glia that exhibit perisynaptic processes. Using two-photon volumetric imaging, we found that during the second postnatal week, Muller glial processes were highly dynamic, with rapid extensions and retractions that were mediated by cytoskeletal rearrangements. During this same stage of development, retinal waves led to increases in cytosolic calcium within Muller glial lateral processes and stalks. These regions comprised distinct calcium compartments, distinguished by variable participation in waves, timing, and sensitivity to an M1 muscarinic acetylcholine receptor antagonist. However, we found that motility of lateral processes was unaffected by the presence of pharmacological agents that enhanced or blocked wave-associated calcium transients. Finally, we found that mice lacking normal cholinergic waves in the first postnatal week also exhibited normal Muller glial process morphology. Hence, outgrowth of Muller glial lateral processes into synaptic layers is determined by factors that are independent of neuronal activity.

Automated summary

Glial cells, once thought to have a passive supporting function, are now known to be integrated into neural circuits and play a crucial role in neural communication. In the retina, Muller glial cells exhibit dynamic processes that interact with synapses, impacting the structure and function of glia. Despite the influence of neuronal messengers, the structural maturation of Muller glial cells seems to be independent of neuronal signaling, providing new insights into glial development in the retina and potentially the brain.