Neuroanatomical characterization of the cell adhesion molecule IgSF9b reveals localization to inhibitory and excitatory synapses in the mouse limbic system

Immunoglobulin superfamily member 9b (IgSF9b) is a cell adhesion protein that has been linked to the etiology of several neuropsychiatric disorders, most notably schizophrenia and major depression. Based on previous studies in cultures, IgSF9b was proposed to specifically regulate the structure and function of inhibitory synapses in the brain through an indirect interaction with the synaptic adhesion protein Neuroligin-2 (Nlgn2). However, very little is known about the protein expression pattern of IgSF9b in the intact brain, and the synaptic localization of IgSF9b in different brain regions has never been investigated. To address this question, we conducted an immunohistochemical characterization of IgSF9b expression across the mouse brain and investigated its colocalization with gephyrin, Nlgn2 and VIAAT as markers of GABAergic inhibitory synapses, as well as with PSD-95 and VGLUT1 as markers of glutamatergic excitatory synapses. Unexpectedly, we observed that in the brain regions assessed, only a small fraction of IgSF9b puncta colocalized with inhibitory marker puncta, with a similarly small fraction colocalizing with excitatory synapses. The majority of IgSF9b puncta were not associated with any of the investigated synaptic markers, indicating that IgSF9b may have additional functions beyond those at GABAergic and glutamatergic synapses. Moreover, deletion of IgSF9b resulted in alterations in inhibitory synapse markers in the stratum lacunosum moleculare of hippocampal area CA1 as well as in the lateral and medial habenula, which play key roles in the regulation of cognitive and affective behaviors, respectively. Together, our findings provide an important context for the assessment of the role of IgSF9b in neuropsychiatric disorders. KEY MESSAGES: IgSF9b is expressed in psychiatrically relevant brain regions in the mouse brain. Loss of IgSF9b leads to a small but significant reduction in inhibitory synapses. Only a subset of inhibitory synapses in the regions assessed contain IgSF9b. IgSF9b is additionally present at a subset of excitatory synapses in these regions. Only 20% of IgSF9b is localized to synapses, while the majority is non-synaptic.