Pathway: Assembly and cell surface presentation of NMDA receptors
Reactions in pathway: Assembly and cell surface presentation of NMDA receptors :
Assembly and cell surface presentation of NMDA receptors
N-methyl-D-aspartate receptors (NMDARs) are tetramers that consist of two GluN1 (GRIN1) subunits and two subunits that belong to either the GluN2 (GRIN2) subfamily (GluN2A, GluN2B, GluN2C and GluN2D) or the GluN3 (GRIN3) subfamily (GluN3A and GluN3B). The GluN2/GluN3 subunits in the NMDA tetramer can either be identical, constituting an NMDA di-heteromer (di-heterotetramer), which consists of two subunit types, GluN1 and one of GluN2s/GluN3s, or they can be two different GluN2/GluN3 proteins, constituting an NMDA tri-heteromer (tri-heterotetramer), which consists of three subunit types, GluN1 and two of GluN2s/GluN3s (Monyer et al. 1992, Wafford et al. 1993, Sheng et al. 1994, Dunah et al. 1998, Perez-Otano et al. 2001, Chatterton et al. 2002, Matsuda et al. 2002, Yamakura et al. 2005, Nilsson et al. 2007, Hansen et al. 2014, Kaiser et al. 2018, Bhattacharya et al. 2018, Bhattacharya and Traynelis 2018).
NMDA tetramers assemble in the endoplasmic reticulum and traffic to the plasma membrane as part of transport vesicles (McIlhinney et al. 1998, Perez-Otano et al. 2001). NMDA receptor subunits undergo N-glycosylation, which impacts their trafficking from the endoplasmic reticulum to the plasma membrane. Trafficking efficiency may vary among different subunits of NMDARs (Lichnereva et al. 2015). Mechanistic details, such as glycosyl transferases involved and the type of sugar side chains added, are not known.
As there are eight splicing isoforms of GluN1, four different GluN2 and two different GluN3 proteins, many different combinations of NMDAR subunits are possible, but only a handful of distinct NMDAR receptors have been experimentally confirmed and functionally studied. The composition of NMDARs affects trafficking, spatial (including synaptic) localization, ligand preference, channel conductivity and downstream signal transmission. Prevalent NMDARs differ at different stages of neuronal development, in different regions of the central nervous system, and at different levels of neuronal activity. For review, please refer to Lau and Zukin 2007, Traynelis et al. 2010, Paoletti et al. 2013, Pérez-Otaño et al. 2016, Iacobucci and Popescu 2017.
NMDA tetramers assemble in the endoplasmic reticulum and traffic to the plasma membrane as part of transport vesicles (McIlhinney et al. 1998, Perez-Otano et al. 2001). NMDA receptor subunits undergo N-glycosylation, which impacts their trafficking from the endoplasmic reticulum to the plasma membrane. Trafficking efficiency may vary among different subunits of NMDARs (Lichnereva et al. 2015). Mechanistic details, such as glycosyl transferases involved and the type of sugar side chains added, are not known.
As there are eight splicing isoforms of GluN1, four different GluN2 and two different GluN3 proteins, many different combinations of NMDAR subunits are possible, but only a handful of distinct NMDAR receptors have been experimentally confirmed and functionally studied. The composition of NMDARs affects trafficking, spatial (including synaptic) localization, ligand preference, channel conductivity and downstream signal transmission. Prevalent NMDARs differ at different stages of neuronal development, in different regions of the central nervous system, and at different levels of neuronal activity. For review, please refer to Lau and Zukin 2007, Traynelis et al. 2010, Paoletti et al. 2013, Pérez-Otaño et al. 2016, Iacobucci and Popescu 2017.
Chemical synapses are specialized junctions that are used for communication between neurons, neurons and muscle or gland cells. The synapse involves a presynaptic neuron and a postsynaptic neuron, muscle cell or glad cell. The pre and the postsynaptic cell are separated by a gap (space) of 20 to 40 nm called the synaptic cleft. The signals pass in a single direction from the presynaptic to postsynaptic neuron (cell). The presynaptic neuron communicates via the release of neurotransmitter which bind the receptors on the postsynaptic cell. The process is initiated when an action potential invades the terminal membrane of the presynaptic neuron.
Action potentials occur in electrically excitable cells such as neurons and muscles and endocrine cells. They are initiated by the transient opening of voltage dependent sodium channels, causing a rapid, large depolarization of membrane potentials that spread along the axon membrane.
When action potentials arrive at the synaptic terminals, depolarization in membrane potential leads to the opening of voltage gated calcium channels located on the presynaptic membrane. The external Ca2+ concentration is approximately 10-3 M while the internal Ca2+ concentration is approximately 10-7 M. Opening of calcium channels causes a rapid influx of Ca2+ into the presynaptic terminal. The elevated presynaptic Ca2+ concentration allows synaptic vesicles to fuse with the plasma membrane of the presynaptic neuron and release their contents, neurotransmitters, into the synaptic cleft. These diffuse across the synaptic cleft and bind to specific receptors on the membrane of the postsynaptic cells. Activation of postsynaptic receptors upon neurotransmitter binding can lead to a multitude of effects in the postsynaptic cell, such as changing the membrane potential and excitability, and triggering intracellular signaling cascades.
Action potentials occur in electrically excitable cells such as neurons and muscles and endocrine cells. They are initiated by the transient opening of voltage dependent sodium channels, causing a rapid, large depolarization of membrane potentials that spread along the axon membrane.
When action potentials arrive at the synaptic terminals, depolarization in membrane potential leads to the opening of voltage gated calcium channels located on the presynaptic membrane. The external Ca2+ concentration is approximately 10-3 M while the internal Ca2+ concentration is approximately 10-7 M. Opening of calcium channels causes a rapid influx of Ca2+ into the presynaptic terminal. The elevated presynaptic Ca2+ concentration allows synaptic vesicles to fuse with the plasma membrane of the presynaptic neuron and release their contents, neurotransmitters, into the synaptic cleft. These diffuse across the synaptic cleft and bind to specific receptors on the membrane of the postsynaptic cells. Activation of postsynaptic receptors upon neurotransmitter binding can lead to a multitude of effects in the postsynaptic cell, such as changing the membrane potential and excitability, and triggering intracellular signaling cascades.
The human brain contains at least 100 billion neurons, each with the ability to influence many other cells. Clearly, highly sophisticated and efficient mechanisms are needed to enable communication among this astronomical number of elements. This communication occurs across synapses, the functional connection between neurons. Synapses can be divided into two general classes: electrical synapses and chemical synapses. Electrical synapses permit direct, passive flow of electrical current from one neuron to another. The current flows through gap junctions, specialized membrane channels that connect the two cells. Chemical synapses enable cell-to-cell communication using neurotransmitter release. Neurotransmitters are chemical agents released by presynaptic neurons that trigger a secondary current flow in postsynaptic neurons by activating specific receptor molecules. Neurotransmitter secretion is triggered by the influx of Ca2+ through voltage-gated channels, which gives rise to a transient increase in Ca2+ concentration within the presynaptic terminal. The rise in Ca2+ concentration causes synaptic vesicles (the presynaptic organelles that store neurotransmitters) to fuse with the presynaptic plasma membrane and release their contents into the space between the pre- and postsynaptic cells.