Undisturbed climbing fiber pruning in the cerebellar cortex of CX₃CR1‐deficient mice

Abstract

Pruning, the elimination of excess synapses is a phenomenon of fundamental importance for correct wiring of the central nervous system. The establishment of the cerebellar climbing fiber (CF)‐to‐Purkinje cell (PC) synapse provides a suitable model to study pruning and pruning‐relevant processes during early postnatal development. Until now, the role of microglia in pruning remains under intense investigation. Here, we analyzed migration of microglia into the cerebellar cortex during early postnatal development and their possible contribution to the elimination of CF‐to‐PC synapses. Microglia enrich in the PC layer at pruning‐relevant time points giving rise to the possibility that microglia are actively involved in synaptic pruning. We investigated the contribution of microglial fractalkine (CX3CR1) signaling during postnatal development using genetic ablation of the CX3CR1 receptor and an in‐depth histological analysis of the cerebellar cortex. We found an aberrant migration of microglia into the granule and the molecular layer. By electrophysiological analysis, we show that defective fractalkine signaling and the associated migration deficits neither affect the pruning of excess CFs nor the development of functional parallel fiber and inhibitory synapses with PCs. These findings indicate that CX3CR1 signaling is not mandatory for correct cerebellar circuit formation. Main Points Ablation of CX3CR1 results in a transient migration defect in cerebellar microglia. CX3CR1 is not required for functional pruning of cerebellar climbing fibers. Functional inhibitory and parallel fiber synapse development with Purkinje cells is undisturbed in CX3CR1‐deficient mice.

CX3CR1 mutation alters synaptic and astrocytic protein expression, topographic gradients, and response latencies in the auditory brainstem

“…CX3CR1 −/− mice showed increased synaptic engulfment in hippocampus during the second and the third postnatal weeks (Paolicelli et al, 2011). In contrast, loss of CX3CR1 did not influence the pruning of excess climbing fibers and had no impact on the development of functional parallel fiber and inhibitory synapses with Purkinje cells in the cerebellar cortex (Kaiser et al, 2020). We found an age‐related decrease in GLYT2 expression in WT animals but not in CX3CR1 −/− mice, which may reflect loss of pruning of inhibitory synapses in MNTB in mutants.…”

Developmental profile of microglia distribution in nuclei of the superior olivary complex

“…The Iba1 antibody, generated against residues near the carboxy terminus of mouse IBA1, recognizes a single band of 16 kD on Western blot of mouse tissue and was knockout verified by immunohistochemistry in mouse brain sections (manufacturer's datasheet). This Iba1 antibody has not been tested in gerbil tissue yet, but reliably labels microglia in the cerebellum (Kaiser et al., 2020) and hippocampus (manufacturer's datasheet) in other rodents. The VGluT1 antiserum made against AA 456–560 of recombinant rat VGluT1 recognizes the appropriate bands on Western blots of rodents.…”