Increased Excitatory Synaptic Transmission of Dentate Granule Neurons in Mice Lacking PSD-95-Interacting Adhesion Molecule Neph2/Kirrel3 during the Early Postnatal Period

Abstract

Copy number variants and point mutations of NEPH2 (also called KIRREL3) gene encoding an immunoglobulin (Ig) superfamily adhesion molecule have been linked to autism spectrum disorders, intellectual disability and neurocognitive delay associated with Jacobsen syndrome, but the physiological roles of Neph2 in the mammalian brain remain largely unknown. Neph2 is highly expressed in the dentate granule (DG) neurons of the hippocampus and is localized in both dendrites and axons. It was recently shown that Neph2 is required for the formation of mossy fiber filopodia, the axon terminal structure of DG neurons forming synapses with GABAergic neurons of CA3. In contrast, however, it is unknown whether Neph2 also has any roles in the postsynaptic compartments of DG neurons. We here report that, through its C-terminal PDZ domain-binding motif, Neph2 directly interacts with postsynaptic density (PSD)-95, an abundant excitatory postsynaptic scaffolding protein. Moreover, Neph2 protein is detected in the brain PSD fraction and interacts with PSD-95 in synaptosomal lysates. Functionally, loss of Neph2 in mice leads to age-specific defects in the synaptic connectivity of DG neurons. Specifically, Neph2−/− mice show significantly increased spontaneous excitatory synaptic events in DG neurons at postnatal week 2 when the endogenous Neph2 protein expression peaks, but show normal excitatory synaptic transmission at postnatal week 3. The evoked excitatory synaptic transmission and synaptic plasticity of medial perforant pathway (MPP)-DG synapses are also normal in Neph2−/− mice at postnatal week 3, further confirming the age-specific synaptic defects. Together, our results provide some evidence for the postsynaptic function of Neph2 in DG neurons during the early postnatal period, which might be implicated in neurodevelopmental and cognitive disorders caused by NEPH2 mutations.

Neph2/Kirrel3 regulates sensory input, motor coordination, and home‐cage activity in rodents

“…33,34 It is imaginable that discrepancies are Data from invertebrates and recent data on mammals provide strong support for the notion that Neph2 is involved in spatiotemporal synapse formation, synaptic transmission levels, and synaptic ultrastructure, particularly in the hippocampus. 12,[35][36][37] It is tempting to surmise that Neph2 exerts similar effects on synapses in the auditory, cerebellar and olfactory circuits. Ultrastructural and electrophysiological approaches are warranted.…”

Kirrel3-mediated synapse formation is attenuated by disease-associated missense variants

“…Wild-type Kirrel3 localizes at or near excitatory synapses (Martin et al, 2015; Roh et al, 2017). Thus, we tested if Kirrel3 missense variants also localize to the cell surface at synapses in neurons.…”

“…Most interestingly, the Kirrel3 intracellular domain ends in a conserved PDZ-binding domain, which is a common protein motif of synaptic organizing molecules. It was suggested that mouse Kirrel3 binds to the PDZ domain proteins CASK, PSD95, and ZO-1 (Huber et al, 2003; Gerke et al, 2006; Roh et al, 2017). Other evidence for putative Kirrel3 binding partners comes from studies using Drosophila and C. elegans.…”

“…This makes it difficult to identify the essential cell and circuit level changes driving disorders such as ASD and ID, and any future therapeutics based on widely expressed gene pathways are likely to have many adverse side effects. In contrast, Kirrel3 is expressed by a relatively limited and specific subset of cell types (Lein et al, 2007; Allen Institute for Brain Science, 2004; Allen institute for brain science, 2010; Hawrylycz et al, 2012; Martin et al, 2015; Roh et al, 2017). Thus, understanding the mechanism of Kirrel3 in synapse formation and function could point to a core set of synapse and circuit changes underlying some forms of ASD and ID.…”

“…Several other recent studies also support a general role for Kirrel3 in mammalian neurodevelopment. Specifically, Kirrel3 may also function at post-synapses in DG dendrites as DG neurons from Kirrel3 knockout mice have more frequent miniature excitatory postsynaptic events in young animals (Roh et al, 2017). Furthermore, Kirrel3 knockout mice have an array of aberrant behavioral phenotypes, most notably characterized by hyperactivity, social, and communication deficits (Choi et al, 2015; Hisaoka et al, 2018; Völker et al, 2018).…”