Cx36, Cx43 and Cx45 in mouse and rat cerebellar cortex: species‐specific expression, compensation in Cx36 null mice and co‐localization in neurons vs. glia

Abstract

Electrical synapses formed by connexin36 (Cx36)-containing gap junctions between interneurons in the cerebellar cortex have been well characterized, including those formed between basket cells and between Golgi cells, and there is gene reporter-based evidence for the expression of connexin45 (Cx45) in the cerebellar molecular layer. Here, we used immunofluorescence approaches to further investigate expression patterns of Cx36 and Cx45 in this layer and to examine localization relationships of these connexins with each other and with glial connexin43 (Cx43). In mice, strain differences were found, such that punctate labelling for Cx36 was differentially distributed in the molecular layer of C57BL/6 vs. CD1 mice. In mice with EGFP reporter representing Cx36 expression, Cx36-puncta were localized to processes of stellate cells and other cerebellar interneurons. Punctate labelling of Cx45 was faint in the molecular layer of wild-type mice and was increased in intensity in mice with Cx36 gene ablation. The vast majority of Cx36-puncta co-localized with Cx45-puncta, which in turn was associated with the scaffolding protein zonula occludens-1. In rats, Cx45-puncta were also co-localized with Cx36-puncta and additionally occurred along Bergmann glial processes adjacent to Cx43-puncta. The results indicate strain and species differences in Cx36 as well as Cx45 expression, possible compensatory processes after loss of Cx36 expression and localization of Cx45 to both neuronal and Bergmann glial gap junctions. Further, expression of both Cx43 and Cx45 in Bergmann glia of rat may contribute to the complex properties of junctional coupling between these cells and perhaps to their reported coupling with Purkinje cells.

Gjd2b-mediated gap junctions promote glutamatergic synapse formation and dendritic elaboration in Purkinje neurons

“…Gap junction proteins are widely expressed in the cerebellum of developing and adult vertebrates. Cx36, the mammalian homolog of Gjd2b has been shown to be expressed in the rodent cerebellar cortex in the molecular layer and granule cell layer (Belluardo et al, 2000;Degen et al, 2004;Nagy and Rash, 2017), and Cx36 puncta were observed localized to Purkinje neurons (Alcami and Marty, 2013), although the mRNA was not found in Purkinje neurons (Belluardo et al, 2000). In larval zebrafish, cell type specific transcriptome analysis in larval zebrafish revealed gjd2b expression in PNs, granule cells, eurydendroid cells and Bergmann glial cells (Takeuchi et al, 2016).…”

Ephaptic Coupling Promotes Synchronous Firing of Cerebellar Purkinje Cells

“…It remains possible that gap junction coupling between PC axons is present (Middleton et al, 2008;Traub et al, 2008). However, electrical coupling between pairs of PCs has never been observed, and PCs do not express Connexin 36 (Belluardo et De Zeeuw et al, 2008;Nagy and Rash, 2017), which mediates most electrical coupling between neurons. Gap junction coupling between PCs would therefore have to be mediated by an unidentified mechanism that couples their axons in a manner that cannot be detected with somatic recordings and does not lead to dye coupling between cells (De Zeeuw et al, 2008).…”

Gjd2b-mediated gap junctions promote glutamatergic synapse formation and dendritic elaboration in Purkinje neurons

“…In rodents, few studies have reported dye-coupling of PNs to other PNs or to other cell types in the cerebellar cortex [42–45]. Cx36, the mammalian homolog of Gjd2b has been shown to be expressed in the cerebellar cortex in the molecular layer and granule cell layer [46–48], although the mRNA was not found in Purkinje neurons [46]. From these studies, it appears that PNs in fish and mammals are electrically coupled to other neurons, while there may be species-specific differences in the molecular components of these electrical synapses.…”