Presynaptic NMDA receptors: dynamics and distribution in developing axons in vitro and in vivo

Abstract

BSTRACTDuring cortical development, N-methyl-D-aspartate (NMDA) receptors (NMDARs) facilitate presynaptic terminal formation, enhance neurotransmitter release and are required in presynaptic neurons for spike-timing-dependent long-term depression (tLTD). However, the extent to which NMDARs are found within cortical presynaptic terminals has remained controversial, and the subsynaptic localization and dynamics of axonal NMDARs are unknown. Here, using live confocal imaging and biochemical purification of presynaptic membranes, we provide strong evidence that NMDARs localize to presynaptic terminals in vitro and in vivo in a developmentally regulated manner. The NR1 and NR2B subunits (also known as GRIN1 and GRIN2B, respectively) were found within the active zone membrane, where they could respond to synaptic glutamate release. Surprisingly, NR1 also appeared in glutamatergic and GABAergic synaptic vesicles. During synaptogenesis, NR1 was mobile throughout axons -including growth cones and filopodia, structures that are involved in synaptogenesis. Upon synaptogenic contact, NMDA receptors were quickly recruited to terminals by neuroligin-1 signaling. Unlike dendrites, the trafficking and distribution of axonal NR1 were insensitive to activity changes, including NMDA exposure, local glutamate uncaging or action potential blockade. These results support the idea that presynaptic NMDARs play an early role in presynaptic development.

Influence of Folic Acid on Neural Connectivity during Dorsal Root Ganglion Neurogenesis

“…The role of neurotransmitter release in developing neural connections is well known, and the presence and function of the NMDA receptor on young rat hippocampal growth cones have been reported [Wang et al, 2011]. The NMDA receptor may have an important role in axonal guidance and synapse formation [Wang et al, 2011;Gill et al, 2015].…”

Roles of Presynaptic NMDA Receptors in Neurotransmission and Plasticity

“…In a variety of brain areas, NMDARs subunits have been localized to axons using both electron microscopy and fluorescence-based microscopy [16, 25, 41, 68, 69] (Box 1). In early cortical development, functional NMDARs are localized to axon growth cones and are important for regulating early synapse formation [68, 70].…”

“…In early cortical development, functional NMDARs are localized to axon growth cones and are important for regulating early synapse formation [68, 70]. Given these findings and the observation that NMDAR agonists and antagonists influence neurotransmission in isolated synaptosomes [71, 72] and in a manner consistent with presynaptic changes in vesicular release [4, 11], it has long been assumed that preNMDARs are located at axonal membranes (Figure 2).…”

“…( c ) In the absence of the GluN2B-selective antagonist ifenprodil, electrical stimulation of astrocytes (AST) increases spontaneous glutamate release onto hippocampal dentate granule cells (washout, right panel). Ifenprodil blocks this increase, suggesting astrocytes influence excitatory neurotransmission at these synapses through interaction with GluN2B subunit-containing preNMDARs (adapted from [68]). ( d ) In recordings from L5 neurons in the entorhinal cortex, application of the NMDAR co-agonist d -serine does not increase the mEPSC frequency, suggesting preNMDARs in this brain area are saturated with this co-agonist.…”

Ionotropic glutamate receptors: Which ones, when, and where in the mammalian neocortex

“…A recent study with dissociated cortical neurons shows that GluN1 and GluN2B are highly mobile in developing axons and act in the presynaptic active zone to mediate glutamate-driven presynaptic differentiation (Gill et al, 2015). Intriguingly, the receptor protein is associated with glutamatergic and also with GABA-ergic synaptic transmitter vesicles (Gill et al, 2015). NMDAR localize to presynapses (Johnson et al, 1996;Conti et al, 1997; for a recent review see Bouvier et al, 2015).…”