Glutamate receptor δ1 induces preferentially inhibitory presynaptic differentiation of cortical neurons by interacting with neurexins through cerebellin precursor protein subtypes

Abstract

Synapse formation is the key step in the development of neuronal networks. Precise synaptic connections between nerve cells in the brain provide the basis of perception, learning, memory, and cognition. Recently, we have shown that the trans-synaptic interaction of postsynaptic glutamate receptor (GluR) d2 and presynaptic neurexins (NRXNs) through cerebellin precursor protein (Cbln) 1 mediates excitatory synapse formation in the cerebellum (Uemura et al. 2010). This finding raises a question whether GluRd1 regulates synapse formation by trans-synaptic interaction with NRXNs through Cbln subtypes in the forebrain since GluRd1 shares 56% amino acid identity with GluRd2 (Yamazaki et al. 1992;Araki et al. 1993;Lomeli et al. 1993). Previous studies showed that GluRd1 expressed in human embryonic kidney (HEK) 293T cells induced presynaptic differentiation of cultured cerebellar granule cells AbstractGlutamate receptor (GluR) d1 is widely expressed in the developing forebrain, whereas GluRd2 is selectively expressed in cerebellar Purkinje cells. Recently, we found that trans-synaptic interaction of postsynaptic GluRd2 and presynaptic neurexins (NRXNs) through cerebellin precursor protein (Cbln) 1 mediates excitatory synapse formation in the cerebellum. Thus, a question arises whether GluRd1 regulates synapse formation in the forebrain. In this study, we showed that the N-terminal domain of GluRd1 induced inhibitory presynaptic differentiation of some populations of cultured cortical neurons. When Cbln1 or Cbln2 was added to cultures, GluRd1 expressed in HEK293T cells induced preferentially inhibitory presynaptic differentiation of cultured cortical neurons. The synaptogenic activity of GluRd1 was suppressed by the addition of the extracellular domain of NRXN1a or NRXN1b containing splice segment 4. Cbln subtypes directly bound to the N-terminal domain of GluRd1. The synaptogenic activity of GluRd1 in the presence of Cbln subtypes correlated well with their binding affinities. When transfected to cortical neurons, GluRd1 stimulated inhibitory synapse formation in the presence of Cbln1 or Cbln2. These results together with differential interactions of Cbln subtypes with NRXN variants suggest that GluRd1 induces preferentially inhibitory presynaptic differentiation of cortical neurons by interacting with NRXNs containing splice segment 4 through Cbln subtypes.

GluD1 is a common altered player in neuronal differentiation from both MECP2-mutated and CDKL5-mutated iPS cells

“…Considering the ability of GluD1 to induce the maturation of GABAergic synapses in telencephalic neurons, it can be hypothesized that its overexpression in the absence of both MeCP2 and CDKL5 might contribute to the reduction in excitatory synapses number by causing a shift toward differentiation of inhibitory synapses. Although no such alteration has been evidenced in the above mentioned studies, analyses on Mecp2 mouse models have demonstrated extensive region-specific variation in synaptic defects 58,59,60,51 . Additional studies will thus be necessary to verify whether an increased GABAergic differentiation is indeed present in our culture conditions and to possibly characterize the identity of the neuronal population involved.…”

“…However, no channel function as been presently demonstrated either alone or in combination with other receptor subunits and available data point to a role in synapse development rather than functioning 49-51 . In particular, the postsynaptic GluD1 binds to presynaptic Neurexin through Cerebellin (Cbln) to induce pre-synaptic differentiation.…”

“…In particular, the postsynaptic GluD1 binds to presynaptic Neurexin through Cerebellin (Cbln) to induce pre-synaptic differentiation. Depending on the specific brain region, this can result in the formation of glutamatergic (cerebellum and hippocampus) or GABAergic (cortex and hippocampus) synapses, although GABAergic differentiation seems to be the prevalent outcome 51-53 . Our experiments show that GluD1 is over-expressed in mature neurons derived from both MECP2 - and CDKL5 -mutated iPS.…”

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

“…Cbln1 and Cbln2, but not Cbln4 (little is known about Cbln3), bind to the N-terminal domains of GluD1 and GluD2, which are homologous to ionotropic glutamate receptors, but function as adhesion molecules instead of glutamate receptors (Matsuda et al, 2010; Wei et al, 2012; Yasamura et al, 2012). Cbln1 and Cbln2 bind to all neurexins with high affinity, but only to neurexins containing an insert in SS#4 (Uemura et al, 2010; Matsuda and Yuzaki, 2011; Joo et al, 2011).…”

Ligand-Induced Cell Adhesion in Synapse Formation

“…In a recent study it has been demonstrated that, in the presence of Cbln1 or Cbln2, GluRδ1 expressed in non-neuronal cells can induce inhibitory presynaptic differentiation on cultured cortical neurons by interacting with Nrxns containing the S4 insert [64].…”