Cerebellins are differentially expressed in selective subsets of neurons throughout the brain

Abstract

Cerebellins are secreted hexameric proteins that form tripartite complexes with the presynaptic cell-adhesion molecules neurexins or ‘deleted-in-colorectal-cancer’ (DCC), and the postsynaptic glutamate-receptor-related proteins GluD1 and GluD2. These tripartite complexes are thought to regulate synapses. However, cerebellins are expressed in multiple isoforms whose relative distributions and overall functions are not understood. Three of the four cerebellins, Cbln1, Cbln2, and Cbln4, autonomously assemble into homohexamers, whereas the Cbln3 requires Cbln1 for assembly and secretion. Here, we show that Cbln1, Cbln2, and Cbln4 are abundantly expressed in nearly all brain regions, but exhibit strikingly different expression patterns and developmental dynamics. Using newly generated knockin reporter mice for Cbln2 and Cbln4, we find that Cbln2 and Cbln4 are not universally expressed in all neurons, but only in specific subsets of neurons. For example, Cbln2 and Cbln4 are broadly expressed in largely non-overlapping subpopulations of excitatory cortical neurons, but only sparse expression was observed in excitatory hippocampal neurons of the CA1- or CA3-region. Similarly, Cbln2 and Cbln4 are selectively expressed, respectively, in inhibitory interneurons and excitatory mitral projection neurons of the main olfactory bulb; here, these two classes of neurons form dendrodendritic reciprocal synapses with each other. A few brain regions, such as the nucleus of the lateral olfactory tract, exhibit astoundingly high Cbln2 expression levels. Viewed together, our data show that cerebellins are abundantly expressed in relatively small subsets of neurons, suggesting specific roles restricted to subsets of synapses.

Visualizing and Interpreting Single-Cell Gene Expression Datasets with Similarity Weighted Nonnegative Embedding

“…We selected four factors (Myelin formation, Cell Junctions, Glutamate transport and Axon projection) and ten genes (PLP1, GRIK1, SLC1A2, LHFPL3, CBLN2, NRGN, FSTL5, POSTN, DCC, DAB1, NTNG1) to embed onto the SWNE plot using cell type markers and gene loadings (Figure 4b, 4d, Table S1), adding biological context to the spatial placement of the cell types (Figure 3b). CBLN2, a gene known to be expressed in excitatory neuron types (Seigneur and Sudhof, 2017), is visually close to Layer 2/3 excitatory neurons (Ex_L2/3) and GRIK1, a key glutamate receptor (Sander, 1997), is close to inhibitory neurons (Figure 3b, Figure 4d). Additionally, the Myelin formation biological factor is near Oligodendrocytes (Oli), consistent with their function in creating the myelin sheath (Bunge, 1968) (Figure 3b).…”

Synaptic Neurexin Complexes: A Molecular Code for the Logic of Neural Circuits

“…Only Cbln3 is present exclusively in cerebellum, while the other cerebellins are broadly expressed throughout brain in discrete and characteristic patterns (Miura et al, 2006; Seigneur and Südhof, 2017). For example, the entorhinal cortex expresses high levels of both Cbln1 and Cbln4, whereas the hippocampus proper lacks detectable cerebellin expression; in the olfactory bulb, similarly, Clbn2 is detectable at high levels only in mitral cells, whereas Clbn4 is expressed only in inhibitory neurons (Seigneur and Südhof, 2017). …”

Visualizing and interpreting single-cell gene expression datasets with Similarity Weighted Nonnegative Embedding

“…We selected three factors (Myelin, Cell Junctions, and Immune Response) and 8 genes (PLP1, GRIK1, SLC1A2, LHFPL3, CBLN2, NRGN, GRM1, FSTL5) to project onto the SWNE plot using the cell type markers and gene loadings ( Figure S3d, Figure S3e, Figure S3f, Table S1), adding biological context to the spatial placement of the cell types ( Figure 3b). We can see that CBLN2, a gene known to be expressed in excitatory neuron types 31 , is expressed in the visual cortex excitatory neurons and that GRIK1, a key glutamate receptor 32 , is expressed in inhibitory neurons (Figure 3b, Figure S4b). Additionally, the Myelin biological factor is associated with Oligodendrocytes (Oli), consistent with their function in creating the myelin sheath 33 (Figure 3b).…”