α2-Chimaerin Is Required for Eph Receptor-Class-Specific Spinal Motor Axon Guidance and Coordinate Activation of Antagonistic Muscles

Abstract

Axonal guidance involves extrinsic molecular cues that bind growth cone receptors and signal to the cytoskeleton through divergent pathways. Some signaling intermediates are deployed downstream of molecularly distinct axon guidance receptor families, but the scope of this overlap is unclear, as is the impact of embryonic axon guidance fidelity on adult nervous system function. Here, we demonstrate that the Rho-GTPase-activating protein ␣2-chimaerin is specifically required for EphA and not EphB receptor signaling in mouse and chick spinal motor axons. Reflecting this specificity, the loss of ␣2-chimaerin function disrupts the limb trajectory of extensor-muscleinnervating motor axons the guidance of which depends on EphA signaling. These embryonic defects affect coordinated contraction of antagonistic flexor-extensor muscles in the adult, indicating that accurate embryonic motor axon guidance is critical for optimal neuromuscular function. Together, our observations provide the first functional evidence of an Eph receptor-class-specific intracellular signaling protein that is required for appropriate neuromuscular connectivity.

Mutant α2-chimaerin signals via bidirectional ephrin pathways in Duane retraction syndrome

“…Ephrin forward and reverse signaling has been characterized as guiding the axonal projections of limb-innervating motor neurons (32,38,39), and α2-chimaerin is known to act downstream of ephrin forward signaling in several neuronal types (12)(13)(14)(15), including limb motor neurons (29). We expand on these findings to define the involvement of EphA4-mediated ephrin forward and reverse signaling in abducens nerve development and in a human developmental disorder.…”

“…While the involvement of α2-chimaerin in ephrin forward signaling has been previously characterized (12)(13)(14)(15)29), to our knowledge the role of mutant α2-chimaerin in ephrin reverse signaling is novel. Ephrin-A lacks an intracellular signaling domain and requires a co-receptor for reverse signaling.…”

“…However, trochlear neurons/axons expressed Chn1, Epha4, and ephrin-A5 (Supplemental Figure 2A and Figure 9, E-G), and ephrin-A5 was expressed in a strong graded pattern in the midbrain tectum near where trochle- (12,14,15,29). Thus, an increase or decrease in α2-chimaerin activity has unique consequences on different neuronal cell types, which may lend insight into its specific functions in different motor neuron populations.…”

“…We focused specifically on the involvement of EphA4 in ephrin-mediated binding and repulsion because α2-chimaerin is reported to act downstream of EphA4 and not EphB isoforms (13,29). In situ hybridization (ISH) in E11.5 embryos revealed Epha4 expression in abducens motor neurons ( Figure 3G).…”

Spinal RacGAP α-Chimaerin Is Required to Establish the Midline Barrier for Proper Corticospinal Axon Guidance

“…Both spontaneous and targeted mutations of the ␣-chimaerin gene (Chn1) in mice led to aberrant midline crossing of CST axons, abnormal spinal central pattern generators, and a rabbit-like hopping gait (Beg et al, 2007;Wegmeyer et al, 2007). These phenotypes of ␣-chimaerin-deficient mice are similar to those of EphA4-deficient (Dottori et al, 1998;Coonan et al, 2001;Kullander et al, 2001aKullander et al, , 2003 and ephrinB3-deficient (Kullander et al, 2001b;Yokoyama et al, 2001;Kullander et al, 2003) mice.…”