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

Sky（也被称为Tyro3和Rse） （Ohashi, et al.1994）是Axl/UFO受体酪氨酸激酶家族的成员，该家族在胞外区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型（FN III）样重复序列。近年来，Gas6已被确定为Axl受体酪氨酸激酶家族所有成员的配体；Axl， Sky和c-Mer具有不同的亲和力（按此顺序）。然而，尽管Sky和Gas6在大脑中的结构特征和主要表达表明其在神经系统中的重要性，但其在神经系统中的生理功能尚不清楚。本研究通过RNAse保护实验（RPA）和/或原位杂交分析表明，sky和gas6 mrna在成人海马从CA1区到CA3区和齿状回中表达。此外，这种表达在发育过程中也以类似的方式受到调节；在胚胎阶段含量较低，在发育过程中逐渐增加，并在成年时达到最高水平。在成年海马锥体层观察到Sky-但未观察到axl -免疫反应，提示Sky和Gas6海马在发育和成年过程中的作用。重组大鼠Gas6可减轻体外血清饥饿引起的海马神经元细胞死亡，提示Gas6是一种新的体外海马神经元神经营养因子。最后，kainic酸刺激后海马和神经横断后坐骨神经中gas6的表达均有调节。综上所述，这些发现表明Gas6和Sky受体酪氨酸激酶在中枢和周围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.