SIGNIFICANCE OF THE MULTIFUNCTIONAL INSULIN-LIKE GROWTH FACTOR-II RECEPTOR IN REGULATING NORMAL BRAIN FUNCTION

胰岛素样生长因子-II(IGF-II)受体是一种单程多功能糖蛋白，含有一个大的胞外结构域和一个小的胞浆尾巴。该受体通过两个不同的位点与IGF-II和甘露糖6-磷酸(M6P)配体相互作用，如溶酶体酶。大多数受体表达在跨高尔基体/内体室，并参与新合成的溶酶体酶的分离，以分选到内体/溶酶体。位于质膜上的一组受体调节含有IGF-II和M6P的配体的内化以清除或激活它们。受体二聚化被认为在M6P配体的最佳结合和内化中起作用。也有证据表明，IGF-II受体通过激活特定的细胞内信号通路参与介导IGF-II的某些生物学作用。目前，虽然受体的转运作用已经确定，但它在响应IGF-II结合的跨膜信号转导中的意义仍然存在争议。使用各种方法，我们已经报道：i)单通道IGF-II受体与G蛋白偶联，并被Leu27IGF-II激活，Leu27IGF-II类似物优先与受体结合，通过百日咳毒素(PTX)敏感的途径促进乙酰胆碱的释放，但抑制成年大鼠海马区GABA的释放；ii)该受体的激活导致其从洗涤剂抵抗的膜组分转移到洗涤剂可溶的膜组分，并通过PTX依赖的途径刺激细胞外信号相关激酶1/2。我们最近的初步数据进一步表明：1)GTP类似物γS抑制[125I]IGF-II而不是M6P配体[125I]PMB-BSA与IGF-II受体的结合；ii)PTX治疗减弱了海马神经元[125I]IGF-II的内化；iii)Leu27IGF-II促进了海马谷氨酸的释放并增强了成人ART的空间学习能力。因此，我们假设单个跨膜结构域的IGF-II受体可以通过与G蛋白的相互作用直接调节成年大鼠脑内IGF-II的生物学功能。为了解决这个问题，我们建议：i)确定IGF-II受体介导的G-蛋白信号是否可以在M6P配体存在的情况下发生，ii)确定IGF-II受体的二聚化是否对其信号效应是必需的，iii)确定G-蛋白介导的信号级联诱导后IGF-II受体的命运，以及iv)确定谷氨酸和乙酰胆碱在IGF-II介导的认知功能增强中的作用。我们认为，这项研究应该产生关于IGF-II受体和G蛋白之间相互作用的特异性的原始数据，并有助于确定其在调节正常大脑功能方面的意义。

The insulin-like growth factor-II (IGF-II) receptor is a single-pass multifunctional glycoprotein containing a large extracellular domain and a small cytoplasmic tail. The receptor interacts with IGF-II and mannose 6-phosphate (M6P)-bearing ligands, such as lysosomal enzymes via two distinct sites. Majority of the receptors are expressed within trans-Golgi/endosomal compartments and are involved in the segregation of newly synthesized lysosomal enzymes for sorting to endosomes/lysosomes. A subset of receptors located on the plasma membrane regulates internalization of IGF-II and M6P-containing ligands for their clearance or activation. Receptor dimerization has been suggested to play a role in optimal binding and internalization of M6P-bearing ligands. There is also evidence that the IGF-II receptor participates in mediating certain biological actions of IGF-II by activating specific intracellular signaling pathways. At present, while the trafficking role of the receptor is well established, its significance in transmembrane signaling in response to IGF-II binding remains controversial. Using a variety of approaches, we have reported that i) the single-pass IGF-II receptor is coupled to a G-protein and its activation by Leu27IGF-II, an IGF-II analog that binds preferentially to the receptor, potentiates acetylcholine release but attenuates GABA release from adult rat hippocampus by a pertussis toxin (PTX)-sensitive pathway and ii) activation of the receptor leads to its translocation from detergent-resistant to detergent-soluble membrane fractions and stimulation of extracellular signal-related kinase 1/2 via a PTX-dependent pathway. Our recent preliminary data further indicate that i) binding of [125I]IGF-II but not the M6P ligand [125I]PMB-BSA to the IGF-II receptor is inhibited by GTP analogue GTPγS, ii) PTX treatment attenuates internalization of [125I]IGF-II in hippocampal cultured neurons, iii) Leu27IGF-II potentiates hippocampal glutamate release and enhances spatial learning in adult arts. Thus, we hypothesize that single transmembrane domain IGF-II receptor, by interaction with a G-protein, can directly regulate biological functions of IGF-II in the adult rat brain. To address this issue we propose: i) to determine whether IGF-II receptor-mediated G-protein signaling can occur in the presence of M6P-ligands, ii) to establish if dimerization of the IGF-II receptor is necessary for its signaling effects, iii) to determine the fate of the IGF-II receptor following induction of G-protein-mediated signaling cascade and iv) to establish the role of glutamate and acetylcholine in IGF-II-mediated enhancement of cognitive function. We believe that this study should generate original data regarding specificity of interactions between the IGF-II receptor and G protein and help to define its significance in regulating normal brain functions.