ROLE OF CD44 IN PERIPHERAL NERVE DEVELOPMENT AND INJURY

这项研究的主要目的是阐明CD44家族跨膜糖蛋白在周围神经发育和损伤后的作用。周围神经的发育和维持需要严格调控轴突和雪旺细胞之间的相互作用。雪旺细胞的存活、增殖和分化受轴突来源信号的影响。一个关键的信号是胶质生长因子(GGF)，它激活雪旺细胞中受体蛋白酪氨酸激酶erbB2和erbB3的异源二聚体。GGF及其受体也与沃勒变性有关，这是一种发生在神经损伤后的过程，包括诱导雪旺细胞增殖。GGF和相关的轴突信号是如何促进erbB2-erbB3异源二聚化和激酶活性的尚不清楚。我们的中心假设是CD44蛋白是GGF信号转导和雪旺细胞存活、增殖和分化所必需的。CD44参与了细胞与细胞、细胞与基质的相互作用，以及向高亲和力细胞表面受体递送生长因子。我们的初步数据表明，CD44是雪旺细胞中erbB2-erbB3异二聚化所必需的，抑制CD44的表达会导致雪旺细胞的凋亡。我们还发现，在发育中的周围神经中，当erbB2高表达时和在雪旺细胞活跃的增殖过程中，CD44都有高水平的表达。我们认为CD44通过促进轴突来源的GGF和雪旺细胞表面的erbB受体之间的相互作用而发挥作用。我们将通过实验测试这一概念，并达到下列特定目标：(1)确定CD44是否作为一个低亲和力的GGF受体；(2)使用表达突变CD44蛋白的细胞来确定CD44介导与ErbB2和ErbB3相互作用的结构域；(3)通过比较野生型小鼠和缺乏CD44的转基因小鼠，确定CD44是否在周围神经发育和沃勒变性期间雪旺细胞的生存、增殖和/或分化中是必需的。了解CD44是如何介导GGF-erbB2-erbB3信号复合体的，将有助于我们深入了解周围神经正常发育的分子机制，并可能有助于我们理解发生轴突变性的许多条件和疾病，包括神经创伤、脊髓损伤和周围神经病。

The principle goal of the proposed studies is to elucidate the role of the CD44 family of transmembrane glycoproteins in perpheral nerves during development and following injury. Peripheral nerve development and maintenance require tightly regulated interactions between axons and Schwann cells. Schwann cell survival, proliferation and differentiation are influenced by axon-derived signals. One key signal is glial growth factor (GGF), which activates heterodimers of the receptor protein tyrosine kinases erbB2 and erbB3 in Schwann cells. GGF and its receptors have also been implicated in Wallerian degeneration, a process that occurs following nerve injury and which includes that induction of Schwann cell proliferation. The means by which GGF and related axon-derived signals promote erbB2-erbB3 heterodimerization and kinase activity are unclear. Our central hypothesis is that CD44 proteins are required for GGF signaling and in Schwann cell survival, proliferation and differentiation. CD44 has been implicated in cell-cell and cell- matrix interactions, and in growth factor presentation to high affinity cell surface receptors. Our preliminary data indicate the CD44 is essential for erbB2-erbB3 heterodimerization in Schwann cells, and that inhibition of CD44 expression results in Schwann cell apoptosis. We also found that CD44 is expressed in developing peripheral nerve at high levels when erbB2 expression is high and during active Schwann cell proliferation. We propose that CD44 acts by facilitating the interaction between axon- derived GGF and erbB receptors on the Schwann cell surface. We will test this notion experimentally with the following specific aims: (1) To ascertain whether CD44 acts as a low affinity GGF receptor; (2) To define the structural domains of CD44 that mediate interactions with ErbB2 and ErbB3 using cells expressing mutant CD44 proteins; (3) To determine if CD44 is required for Schwann cell survival, proliferation and/or differentiation during peripheral nerve development and Wallerian degeneration by comparing wild type mice and trasngenic mice whose Scwann cells lack CD44. Understanding how CD44 mediates the GGF-erbB2-erbB3 signaling complex will provide insight into the molecular mechanisms underlying normal peripheral nerve development, and may contribute significantly to our understanding of numerous conditions and diseases where axonal degeneration occurs, including nerve trauma, spinal cord injuries, and peripheral neuropathies.