Neurophysiological process - Receptor-mediated axon growth repulsion

Receptor-mediated axon growth repulsion

Ephrin-A proteins which are anchored in the plasma membrane through attachment of glycosylphosphatidylinositol (GPI) [1], are the ligands for Ephrin-A receptors, which belong to the membrane family of receptor tyrosine kinases [2].

In the absence of Ephrin-A stimulation, Ephrin-A receptors are shown to target Ephexin exchange factor to the plasma membrane. Ephrin-A stimulation of Ephrin-A receptors activates exchange factors Ephexin [3], VAV-2 [4] and Tiam 1 [5]. Src-family tyrosine kinases c-Src and Fyn are recruited to Ephrin-A receptors after Ephrin-A stimulation [6]. In response to Ephrin-A signaling Ephexin becomes phosphorylated by c-Src [6] and this phosphorylation enhances its activity toward Ras homolog gene family, member A (RhoA) [7]. VAV-2 is rapidly phosphorylated by c-Src upon stimulation by Ephrin-A [4] and activates RhoA [8].

Ephrin-A receptors have also been shown to signal through the Ras-related C3 botulinum toxin substrate 1 (Rac1) exchange factors Tiam1 [5] and VAV-2 [9] to promote neurite outgrowth.

In response to Ephrin-A1 stimulation, Ras-related protein Rap-1A is activated [10] and can regulate MAPK signaling cascade by reducing c-Raf-1 activation [11] or by stimulation of B-Raf kinase [10], [12].

When Ephrin-A receptors are activated, phosphorylation of Ephexin promotes its GTPase activity toward RhoA. RhoA downstream effector Rho-associated kinase ROCK directly phosphorylates LIM-kinases LIMK1 and LIMK2, which in turn phosphorylates actin-depolymerizing factor destrin and actin-associated protein cofilin. Activity of LIMK1 is also regulated by p21-activated kinase 1 (PAK1) [13]. Cofilin and destrin both exhibit actin-depolymerizing activity followed by reorganization of the actin cytoskeleton [14], [15].

The F-actin-binding protein cortactin is an important regulator of cytoskeletal dynamics, and a prominent target of various tyrosine kinases (c-Src, Fyn, Fer) [6], [16]. Tyrosine phosphorylation of cortactin has been suggested to reduce its F-actin cross-linking capability [16].

The semaphorins family of secreted or membrane-bound proteins was identified originally as axonal guidance factors functioning during neuronal development. The class 4 semaphorin Semaphorin 4D utilizes Plexin B1 (transmembrane protein) as receptor. [17] Plexin B1 directly interacts with exchange factors PDZ-RhoGEF and LARG to regulate RhoA and the growth cone morphology [18].

Rho6 is a member of Rho family GTPases. It is activated by adaptor protein Grb7 and directly interacts with the cytoplasmic domain of Plexin B1 in response to Semaphorin 4D. Rho6 promotes the interaction between Plexin B1 and PDZ-RhoGEF and thereby potentiates the PDZ-RhoGEF-induced RhoA activation [19].

PAK1 promotes activation of actin polymerization by phosphorylation of Arp2/3 (complex of actin-related proteins) [20]. c-Raf-1 kinase, a member of the MAPK pathway, is also phosphorylated and activated by PAK1 [21]. Inhibition of Pak1 by Plexin B1 is believed to cause suppression of membrane protrusions, thus supporting the cell repulsion response. Furthermore, active Rac1 was shown to promote cell surface localization of Plexin B1 thus enhancing Semaphorin 4D binding to the receptor. Thus, Rac1 and Plexin B1 signaling appears to be bidirectional: Rac-1 modulates Plexin B1 activity, and Plexin B1 modulates Rac-1 function [22].

Another semaphorin, Semaphorin 3A, binds to Neuropilin-1/Plexin A1 complex and induces repulsive responses [23]. The active form of Rac1 directly binds to Plexin-A1. Activated Rac1 mediates endocytosis of the growth cone plasma membranes and reorganization of F-actin in neurons [24]. Endocytosis of plasma membranes is supposed to be an important step for growth cone collapse.

c-Fes tyrosine kinase also is implicated in Semaphorin 3A-induced collapse [25]. c-Fes directly binds to the cytoplasmic region of Plexin A1. In the resting state, neuropilin-1 associates with Plexin-A1 and blocks the binding of c-Fes to Plexin A1. Semaphorin 3A binding to Neuropilin-1 permits c-Fes to associate with and phosphorylate Plexin A1. This tyrosine phosphorylation stimulates repulsive action in the receptor.

c-Fes also phosphorylates collapsin response mediator protein CRMP2 [26].

Fyn, a member of src-family of tyrosine kinases, associates with Plexin A2 in response to Semaphorin 3A and phosphorylates serine/threonine kinase CDK5. [27] Activated CDK5 phosphorylates CRMP2 [28]. ROCK2 kinase also has been shown to phosphorylate CRMP2 [29]. CRMP2 binds to tubulin heterodimers to promote microtubule assembly that is important for axonal growth and branching [30]. Phosphorylation of CRMP2 reduces its tubulin-heterodimer binding and the promotion of microtubule assembly.

CDK5 also phosphorylates the microtubule-associated protein Tau, thereby reduces its ability to induce tubulin microtubule formation [31].

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