NGL-3 in the regulation of brain development, Akt/GSK3b signaling, long-term depression, and locomotive and cognitive behaviors

Netrin-G ligand-3 (NGL-3) is a postsynaptic adhesion molecule known to directly interact with the excitatory postsynaptic scaffolding protein postsynaptic density-95 (PSD-95) and trans-synaptically with leukocyte common antigen-related (LAR) family receptor tyrosine phosphatases to regulate presynaptic differentiation. Although NGL-3 has been implicated in the regulation of excitatory synapse development by in vitro studies, whether it regulates synapse development or function, or any other features of brain development and function, is not known. Here, we report that mice lacking NGL-3 (Ngl3−/− mice) show markedly suppressed normal brain development and postnatal survival and growth. A change of the genetic background of mice from pure to hybrid minimized these developmental effects but modestly suppressed N-methyl-D-aspartate (NMDA) receptor (NMDAR)-mediated synaptic transmission in the hippocampus without affecting synapse development, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor (AMPAR)-mediated basal transmission, and presynaptic release. Intriguingly, long-term depression (LTD) was near-completely abolished in Ngl3−/− mice, and the Akt/glycogen synthase kinase 3β (GSK3β) signaling pathway, known to suppress LTD, was abnormally enhanced. In addition, pharmacological inhibition of Akt, but not activation of NMDARs, normalized the suppressed LTD in Ngl3−/− mice, suggesting that Akt hyperactivity suppresses LTD. Ngl3−/− mice displayed several behavioral abnormalities, including hyperactivity, anxiolytic-like behavior, impaired spatial memory, and enhanced seizure susceptibility. Among them, the hyperactivity was rapidly improved by pharmacological NMDAR activation. These results suggest that NGL-3 regulates brain development, Akt/GSK3β signaling, LTD, and locomotive and cognitive behaviors.Netrin-G配体3（Netrin-G ligand-3, NGL-3）是一种突触后黏附分子，已知可直接与兴奋性突触后脚手架蛋白突触后致密物95（postsynaptic density-95, PSD-95）发生相互作用，并通过跨突触方式与白细胞共同抗原相关（leukocyte common antigen-related, LAR）家族受体型酪氨酸磷酸酶结合，从而调控突触前分化。尽管体外研究已证实NGL-3参与兴奋性突触发育的调控，但其是否可调控突触发育与功能，抑或大脑发育与功能的其他特征，目前仍不明确。本研究发现，NGL-3敲除小鼠（Ngl3⁻/⁻小鼠）表现出显著的正常大脑发育受阻，以及出生后存活与生长受限。将小鼠的遗传背景从纯系转换为杂交系，可部分缓解上述发育表型，但会轻度抑制海马体中N-甲基-D-天冬氨酸（N-methyl-D-aspartate, NMDA）受体（NMDAR）介导的突触传递，且不影响突触发育、α-氨基-3-羟基-5-甲基-4-异恶唑丙酸（α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, AMPA）受体（AMPAR）介导的基础突触传递，以及突触前释放。值得注意的是，Ngl3⁻/⁻小鼠的长时程抑制（long-term depression, LTD）几乎完全消失；而已知可抑制LTD的Akt/糖原合成酶激酶3β（glycogen synthase kinase 3β, GSK3β）信号通路，在此类小鼠中出现了异常活化。此外，对Akt进行药物抑制可恢复Ngl3⁻/⁻小鼠受损的LTD，但激活NMDAR则无此效果，这表明Akt过度活化可抑制LTD。Ngl3⁻/⁻小鼠还表现出多种行为异常，包括活动过度、抗焦虑样行为、空间记忆受损，以及癫痫易感性增高。其中，活动过度可通过药物性NMDAR激活得到快速改善。上述结果表明，NGL-3可调控大脑发育、Akt/GSK3β信号通路、LTD，以及运动与认知行为。