Hapln4/Bral2 is a selective regulator for formation and transmission of GABAergic synapses between Purkinje and deep cerebellar nuclei neurons.

Purkinje cells (PCs) convey the sole output of the cerebellar cortex to the deep cerebellar nuclei (DCN). DCN neurons are enwrapped in densely organized extracellular matrix structures, known as perineuronal nets (PNNs). PNNs are typically found around fast-spiking GABAergic interneurons expressing parvalbumin but interestingly also exist surrounding other neurons, such as the neurons in the DCN and medial nucleus of the trapezoid body, which are the post-synaptic neurons of large axo-somatic synapses adapted for fast signaling. This characteristic localization prompted the hypothesis that PNNs might play a role in the maintenance and formation of large fast-signaling synapses. To elucidate the role of the PNN at these synapses, we investigated the electrophysiological and morphological properties of DCN synapses in hyaluronan and proteoglycan binding link protein 4 (Hapln4/Bral2) knockout (KO) mice around postnatal day (P)14. Hapln4/Bral2 is important for PNN structure, as it stabilizes the interaction between hyaluronan and proteoglycan. Here, using immunohistochemistry we show that Hapln4/Bral2 localized closely with GABAergic terminals. In DCN neurons of Hapln4/Bral2 KO mice, inhibitory synaptic strengths were reduced as compared to those in wild-type mice, whereas the properties of excitatory synapses were unaffected. The reduced IPSC amplitudes were mainly because of reduced numbers of releasable vesicles. Moreover, Hapln4/Bral2 deficiency reduced the number of PC GABAergic terminals in the DCN. These results demonstrate that Hapln4/Bral2 is a PNN component that selectively contributes to formation and transmission of PC-DCN synapses in the cerebellum. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.