Functional analysis of the novel brain-specific receptor tyrosine kinase "Sky"

Sky(也称为Tyro3和rse)(Ohashi等人，1994)是Ax1/UFO受体酪氨酸激酶家族的成员，在细胞外域有两个免疫球蛋白(Ig)样重复序列和两个纤维连接蛋白III(FN III)样重复序列。最近，Gas6已被确定为Ax1受体酪氨酸激酶家族所有成员的配体；Ax1、Sky和c-Mer具有不同的亲和力(按顺序排列)。然而，Sky和Gas6在神经系统中的生理功能知之甚少，尽管Sky和Gas6在大脑中的结构特征和主要表达表明它在神经系统中具有重要作用。通过核糖核酸酶保护试验(RPA)和/或原位杂交分析，我们发现Sky和Gas6mRNAs在成年大鼠海马区从CA1区到CA3区和齿状回都有表达，而且在发育过程中也有类似的调节方式；在胚胎阶段表达水平很低，在发育过程中逐渐增加，在成体中达到最高水平。在成年大鼠海马锥体层观察到Sky-而不是Axl免疫反应，提示Sky和Gas6在发育和成体中的作用。重组大鼠Gas6在体外可减轻血清饥饿所致的海马神经细胞死亡，提示Gas6是一种新的海马神经元营养因子。最后，Gas6在红藻氨酸刺激后的海马区和神经切断后的坐骨神经中均有表达。综上所述，这些发现表明了Gas6和Sky受体酪氨酸激酶在中枢和外周神经系统中的生理重要性。

Sky (also known as Tyro3 and Rse) (Ohashi, et al.1994) is a member of Axl/UFO receptor tyrosine kinase family, which has two immunoglobulin (Ig)-like repeats and two fibronectin type III (FN III) -like repeats in the extracellular domain. Recently, Gas6 has been identified as ligand for all members of Axl receptor tyrosine kinase family ; Axl, Sky and c-Mer with different affinity (in this order). However physiological function of Sky and Gas6 in the nervous system are poorly understood, despite its importance in the nervous system suggested by the structural feature and predominant expression of Sky in the brain. Here we show by RNAse protection assay (RPA) and/or by in situ hybridization analysis that sky and gas6 mRNAs were expressed in adult hippocampus from CA1 region to CA3 region and dentate gyrus.Furthermore this expression was regulated in a similar manner during development ; levels were low during embryonic stages, gradually increased during development and reaching its highest levels in the adult. Sky-but not Axl-immunoreactivity was observed in the pyramidal layr of adult hippocampus, suggesting the roles of Sky and Gas6 hippocampus during development and in the adult. Recombinant rat Gas6 attenuated hippocampal neuronal cell death caused by serum starvation in vitro, indicating that Gas6 is a novel neurotrophic factor for hippocampal neurons in vitro. Finally, gas6 showed regulated expression in the hippocampus after kainic acid stimulation and in the sciativ nerve after nerve transection. Taken together, these findings indicate the physiological importance of Gas6 and Sky receptor tyrosine kinase in both central and peripheral nervous systems.