Wnt signaling and neurite outgrowth: insights and questions

Abstract

Wnt signaling consists of a highly conserved set of biochemical pathways that have a multitude of functions during embryonic development and in the adult. The Wnt proteins are extracellular agents that often act as gradient morphogens, indicating that their distribution in tissues is tightly controlled. This attribute is also characteristic of factors that regulate neurite outgrowth and guide axons precisely to their specific destinations. Several studies in various species now have established that Wnts and their receptors have an important role in axonal guidance. Different ligand/receptor combinations have been identified that mediate this activity in many of the experimental models. Clues about downstream effector molecules have come from in vitro systems. In this article, the authors review the results from many of these models, evaluate what is known about the associated signaling pathways and speculate about the direction of future research.

Figure 1

Schematic model of the most extensively studied Wnt signaling pathways. (A) β‐Catenin pathway. Wnt‐Fzd‐LRP5/6 binding induces Dvl‐ and Gα‐dependent disruption of Axin/APC/GSK‐3β/CKI‐α degradation complex, resulting in β‐catenin stabilization. β‐Catenin enters nucleus and stimulates TCF/LEF‐dependent transcription. (B) PCP pathway. Fzd‐mediated recruitment of Dvl to the membrane results in stimulation of Rho/ROCK and/or Rac/JNK activation, thereby reorganizing the actin cytoskeleton. Vangl2/stbm (Vang‐like 2/Strabismus)‐Pk (Prickle) complex negatively regulates Fzd‐mediated recruitment of Dvl. Celsr, a cadherin, is a positive mediator of PCP signaling. (C) Calcium pathway. Wnt‐Fzd binding, through a Gα‐dependent mechanism, activates PLC‐β, which catalyzes the production of IP3 and DAG. IP3 stimulates the release of intracellular calcium, while DAG activates PKC. Wnt‐Fzd binding also activates cGMP‐PDE (phosphodiesterase), causing a decrease in intracellular cGMP, which controls the activity of downstream targets such as protein kinase G (PKG), guanylyl cyclases, and cyclic nucleotide‐gated ion‐channels. Inhibition of PKG also promotes calcium release inside the cell, enhancing the activity of enzymes such as CaMKII (Ca²⁺/calmodulin‐dependent kinase II) and the protein phosphatase, calcineurin. The latter activates NF‐AT transcription factors.

Figure 2

Ryk/Drl and Ror: additional Wnt receptors, and associated downstream signaling. (A) Ryk. Ryk binds Wnt through its WIF domain; it has a single pass TM (transmembrane) domain and an inactive TK (tyrosine kinase) domain. Ryk has been alternatively reported to function as a Wnt coreceptor with Fzd, or an independent Wnt receptor. It can stimulate β‐catenin transcriptional activity; little is known about other signaling mechanisms. (B) Ror1/2. Ror1/2 contain an immunoglobulin (Ig)‐like domain, a Fzd‐like CRD, kringle domain, TK domain and proline‐rich domain (PRD). Ror2 negatively regulates the β‐catenin pathway by an unknown mechanism, and activates JNK. Filamin A associates with Ror2 via its PRD, and contributes to filopodia formation.^{( 21 )}

Figure 3

Potential key mediators of Wnt‐associated neurite outgrowth. (A) GSK‐3. GSK‐3 activity is negatively regulated by Wnt in the β‐catenin pathway (see Fig. 1A), and also by ILK (integrin‐linked kinase), Akt, and lithium. Axin and Dvl may control GSK‐3 activity in proximity of microtubules. GSK‐3 has many substrates that are involved in microtubule stability, including MAP1B, CRMP2, Tau and APC. (B) JNK. Wnt/PCP signaling pathway activates JNK via small GTPase Rac and MKK7. JNK has many substrates directly associated with microtubule stability, such as MAP1B, MAP2, Tau, DCX (doublecortin) and kinesin.^{( 49 )} (C) Rho family of small GTPases. Rho stimulates ROCK and mDia (mammalian Diaphanous), but inhibits myosin phosphatase, resulting in reorganization of actin cytoskeleton.^{( 78 )} Smurf‐1, an E3 ubiquitin ligase, mediates RhoA degradation. Rac1/Cdc42 induce JNK activation, contributing to regulation of microtubule stability. (D) Par3/Par6/aPKC. Interaction between Wnt/Fzd signaling and Par polarity complex is hypothetical. Par‐6‐Par‐3 mediates Cdc42‐induced Rac activation through Rac‐specific guanine nucleotide‐exchange factors (GEF), contributing to neurite growth and axon specification.^{( 62 )} Par‐6‐aPKC (aPKC‐ζ) inhibits GSK‐3.^{( 38 )} aPKC also activates Par‐1 kinase, resulting in regulation of multiple molecules involved in microtubule stability.