Barragan Eden V, Anisimova Margarita, Vijayakumar Vishnu, Coblentz Azariah C, Park Deborah K, Salaka Raghava Jagadeesh, Nisan Atheer F K, Petshow Samuel, Dore Kim, Zito Karen, Gray John A
bioRxiv. 2024 Jun 1:2024.05.29.596266. doi: 10.1101/2024.05.29.596266.
NMDA-type glutamate receptors (NMDARs) are widely recognized as master regulators of synaptic plasticity, most notably for driving long-term changes in synapse size and strength that support learning. NMDARs are unique among neurotransmitter receptors in that they require binding of both neurotransmitter (glutamate) and co-agonist (e.g. d -serine) to open the receptor channel, which leads to the influx of calcium ions that drive synaptic plasticity. Over the past decade, evidence has accumulated that NMDARs also support synaptic plasticity via ion flux-independent (non-ionotropic) signaling upon the binding of glutamate in the absence of co-agonist, although conflicting results have led to significant controversy. Here, we hypothesized that a major source of contradictory results can be attributed to variable occupancy of the co-agonist binding site under different experimental conditions. To test this hypothesis, we manipulated co-agonist availability in acute hippocampal slices from mice of both sexes. We found that enzymatic scavenging of endogenous co-agonists enhanced the magnitude of LTD induced by non-ionotropic NMDAR signaling in the presence of the NMDAR pore blocker, MK801. Conversely, a saturating concentration of d -serine completely inhibited both LTD and spine shrinkage induced by glutamate binding in the presence of MK801. Using a FRET-based assay in cultured neurons, we further found that d -serine completely blocked NMDA-induced conformational movements of the GluN1 cytoplasmic domains in the presence of MK801. Our results support a model in which d -serine inhibits ion flux-independent NMDAR signaling and plasticity, and thus d -serine availability could serve to modulate NMDAR signaling even when the NMDAR is blocked by magnesium.
NMDARs are glutamate-gated cation channels that are key regulators of neurodevelopment and synaptic plasticity and unique in their requirement for binding of a co-agonist (e.g. d -serine) in order for the channel to open. NMDARs have been found to drive synaptic plasticity via non-ionotropic (ion flux-independent) signaling upon the binding of glutamate in the absence of co-agonist, though conflicting results have led to controversy. Here, we found that d -serine inhibits non-ionotropic NMDAR-mediated LTD and LTD-associated spine shrinkage. Thus, a major source of the contradictory findings might be attributed to experimental variability in d -serine availability. In addition, the developmental regulation of d -serine levels suggests a role for non-ionotropic NMDAR plasticity during critical periods of plasticity.
N-甲基-D-天冬氨酸（NMDA）型谷氨酸受体（NMDARs）被广泛认为是突触可塑性的主要调节因子，尤其在驱动支持学习的突触大小和强度的长期变化方面。NMDARs在神经递质受体中独具特色，因为它们需要神经递质（谷氨酸）和共激动剂（如D-丝氨酸）同时结合才能打开受体通道，从而导致钙离子内流，驱动突触可塑性。在过去十年中，有证据表明，在没有共激动剂的情况下，谷氨酸结合后，NMDARs也通过离子流非依赖（非离子型）信号传导来支持突触可塑性，尽管相互矛盾的结果引发了重大争议。在这里，我们假设矛盾结果的一个主要来源可归因于不同实验条件下共激动剂结合位点的可变占有率。为了验证这一假设，我们在来自雌雄小鼠的急性海马切片中操纵了共激动剂的可用性。我们发现，在内源性共激动剂的酶促清除增强了在存在NMDAR孔道阻滞剂MK801的情况下由非离子型NMDAR信号传导诱导的长时程抑制（LTD）的幅度。相反，在存在MK801的情况下，饱和浓度的D-丝氨酸完全抑制了由谷氨酸结合诱导的LTD和树突棘收缩。在培养的神经元中使用基于荧光共振能量转移（FRET）的检测方法，我们进一步发现，在存在MK801的情况下，D-丝氨酸完全阻断了NMDA诱导的GluN1胞质结构域的构象运动。我们的结果支持了一个模型，即D-丝氨酸抑制离子流非依赖的NMDAR信号传导和可塑性，因此即使NMDAR被镁阻断，D-丝氨酸的可用性也可以调节NMDAR信号传导。
NMDARs是谷氨酸门控阳离子通道，是神经发育和突触可塑性的关键调节因子，其独特之处在于需要共激动剂（如D-丝氨酸）结合才能使通道打开。已发现NMDARs在没有共激动剂的情况下，谷氨酸结合后通过非离子型（离子流非依赖）信号传导来驱动突触可塑性，尽管相互矛盾的结果引发了争议。在这里，我们发现D-丝氨酸抑制非离子型NMDAR介导的LTD和与LTD相关的树突棘收缩。因此，矛盾结果的一个主要来源可能归因于D-丝氨酸可用性的实验变异性。此外，D-丝氨酸水平的发育调节表明非离子型NMDAR可塑性在可塑性关键期发挥作用。
