Axon guidance and injury-lessons from Wnts and Wnt signaling

Abstract

Many studies in the past decade have revealed the role and mechanisms of Wnt signaling in axon guidance during development and the reinduction of Wnt signaling in adult central nervous system axons upon traumatic injury, which has profound influences on axon regeneration. With 19 Wnts and 14 known receptors (10 Frizzleds (Fzds), Ryk, Ror1/2 and PTK7), the Wnt family signaling proteins contribute significantly to the wiring specificity of the complex brain and spinal cord circuitry. Subsequent investigation into the signaling mechanisms showed that conserved cell polarity pathways mediate growth cone steering. These cell polarity pathways may unveil general principles of growth cone guidance. The reappeared Wnt signaling system after spinal cord injury limits the regrowth of both descending and ascending motor and sensory axons. Therefore, the knowledge of Wnt signaling mechanisms learned from axon development can be applied to axon repair in adulthood.

Figure 1

Spinal commissural axon A-P guidance is dependent on Wnt-PCP signaling. a. A depiction of dorsal spinal commissural axonal tract in the developing (E11.5) spinal cord. Commissural neurons extend the axons toward the floor plate according to Netrin and Shh gradient. After midline crossing, commissural axons turn anteriorly following Wnts gradient (anterior-high to posterior-low). b. A typical phenotype of commissural axon A-P guidance defect caused by PCP gene disruption. In wild type spinal cord, dorsal commissural axons strictly turn anteriorly after midline crossing. On the other hand, Frizzled3 knockout randomizes post-crossing commissural axon guidance, meaning axons randomly turn toward anterior or posterior. Same phenotype is observed in Vangl2 mutant (looptail mutation; S464N) mice, Celsr3 straight knockout mice, and Celsr3 conditional knockout (Celsr3 fl/fl;Wnt1-Cre) mice. A, anterior; P, posterior; D, dorsal; V, ventral; RP, roof plate; FP, flor plate.

Figure 2

Axon guidance mediated by Wnts. a. A depiction of dopaminergic and serotonergic axon guidance in the developing midbrain and hindbrain. In the ventral midbrain, Wnt5a is expressed in increasing anterior to posterior gradient, and Wnt7b is expressed in opposite gradient. The dopaminergic neurons in A9 and A10 nuclei extend the axons toward anterior due to Wnt5a repulsion and Wnt7b attraction. On the other hand, the serotonergic neurons in the hindbrain (B4∼9 and B1∼3) sense attractive Wnt5a gradient. b. A depiction of corticospinal axon tract (CST) in the developing (∼P0) central nervous system. Wnt1 and Wnr5a are expressed in decreasing anterior to posterior gradient within the dorsal spinal cord. CST neurons express Ryk, and Wnt-Ryk signaling repels axons, causing descending CST axon tract in the dorsal spinal cord. c. Wnt5a-Ryk repulsive signal in corpus callosum. In the cortex, Wnt5a is expressed in indusium griseum (IG) and in the glial wedge (GW). Callosal axons express Ryk and are guided by Wnt5a repulsive signal.

Figure 3

A working hypothesis for Wnt-PCP signaling-mediated axon guidance. a. Scheme of molecular mechanism in Wnt-PCP signaling. In the absence of Wnt, Dishevelled1 inhibits Frizzled3 endocytosis by inducing Frizzled3 hyperphosphorylation. Upon Wnt binding, Frizzled3 is endocytosed and activates the Dishevelled2-aPKC/PAR6 axis. aPKC then inhibits Dishevelled1. In the meantime, Vangl2 also inhibits Dishevelled1. As a result, more Frizzled3 is endocytosed and activates downstream signaling, which then in turn further removes inhibition by Dishevelled1. Arf6 mediates Frizzled3 endocytosis and its essential roles in PCP signaling activation. b. A working hypothesis of commissural axonal growth cone turning according to Wnt gradients. In the absence of Wnts gradient, Frizzled3 is endocytosed randomly through the filopodia tips and active aPKC is distributed uniformly (left growth cone). In a Wnt gradient, Frizzled3 endocytosis occurs more frequently in the proximal side and causes more aPKC activation (middle growth cone). Those asymmetry may help to steer growth cone turning (right growth cone).

Figure 4

A model of Wnt5a-Ryk repulsive signaling. Wnt5a evokes calcium activity through Ryk and Frizzled(s), which involves calcium entry through TRP channels and calcium release from intracellular stores through IP3 receptor on the ER. Ryk and TRP channels activity are required for both Wnt5a-induced axon elongation and repulsion. On the other hand, Frizzled(s) and IP3 receptor activity are required for Wnt5a-induced axon repulsion and elongation, respectively. It remains unclear how Ryk and Frizzled regulate TRP channels, and how calcium signal from different source regulates different process.

Figure 5

Reinduced Wnts repel both motor and sensory axons in injured spinal cord.