Corticospinal tract fibers cross the ephrin-B3-negative part of the midline of the spinal cord after brain injury☆
Research highlights
▶ Sprouting axons of the CST can cross the midline at ephrin-B3-negative areas. ▶ Ephrin-B3 is not expressed along the entire midline of the spinal cord. ▶ Ephrin-B3 expression in the midline disappears after postnatal day 9.
Introduction
The corticospinal tract (CST) is a bundle of fibers that originate from neurons in the sensorimotor cortex and predominantly descend in the contralateral side of the spinal cord. This tract controls fine motor function, and damage to this tract causes permanent deficits in it. In mice, most CST fibers cross to the opposite side at the pyramidal decussation, descend in the ventral portion of the dorsal funiculus, and project to the interneurons of the contralateral spinal cord, and do not recross the midline in the spinal cord to innervate the ipsilateral side. It was revealed that the expression of ephrin-B3 in the midline of the spinal cord and that of its receptor, EphA4, in the cortical neurons are crucial for preventing the CST fibers from recrossing the midline in the spinal cord during postnatal development (Dottori et al., 1998, Coonan et al., 2001, Kullander et al., 2001, Yokoyama et al., 2001). Genetic ablation of ephrin-B3 in mice results in bilateral innervation by the CST fibers (Kullander et al., 2001, Yokoyama et al., 2001) and EphA4-knockout mice show an identical phenotype (Dottori et al., 1998, Coonan et al., 2001).
There is considerable evidence that under certain conditions, CST fibers can recross the midline in the spinal cord. For example, middle cerebral artery occlusion (MCAO) induces axonal sprouting across the midline of the spinal cord in rodents (Liu et al., 2007), and treatment with a Nogo receptor antagonist after MCAO increases axonal sprouting and enhances functional recovery (Lee et al., 2004). Further, the sprouting is more pronounced if the lesion is induced during the early postnatal period (Reinoso and Castro, 1989, Kuang and Kalil, 1990, Ono et al., 1990, Takahashi et al., 2009). Axonal sprouting among intact CST fibers on the denervated side of the spinal cord might lead to the formation of an alternative network of motor fibers and contribute to functional recovery after the development of lesions involving the cerebral cortex or CST (Murphy and Corbett, 2009, Benowitz and Carmichael, 2010). However, it remains unknown why CST fibers can cross the midline despite the presence of the ephrin-B3 midline barrier.
In this study, we sought to determine (1) the reason CST fibers can cross the midline of the spinal cord despite ephrin-B3 expression and (2) the effects of a unilateral cortical lesion on ephrin-B3–EphA4 signaling. Although postnatal expression of ephrin-B3 and EphA4 has been investigated (Kullander et al., 2001, Liebl et al., 2003, Benson et al., 2005), the changes in this expression and the relation between this expression and the ability of CST fibers in the spinal cord to cross the midline after a cortical lesion have not yet been evaluated. In the current study, we first confirmed that CST fibers cross the midline of the spinal cord after unilateral cortical ablation in postnatal and adult mice. We next assessed the expression of ephrin-B3 and EphA4 in the mice. We found that ephrin-B3 was not expressed along the entire midline of the spinal cord, and sprouting axons crossed the midline at ephrin-B3-negative areas.
Section snippets
Mice
In this study, we used C57BL/6J mice at postnatal day 9 (Charles River; P9, n = 20) and adulthood (8 weeks, n = 20). The mice were divided into the following categories in each age group: (1) unilateral cortical ablation followed by anterograde tracing of CST (n = 5 for each age group), (2) unilateral cortical ablation followed by behavioral tests (n = 5 for each age group). Sham-operated mice (n = 5 for all the groups) underwent the same surgical procedures, except for the cortical ablation. All the
CST axons crossed the midline of the spinal cord after cortical ablation
We compared the potential for axonal sprouting of the CST following unilateral ablation of the sensorimotor cortex between P9 and adult mice (Fig. 1A). We firstly determined whether the cortical damage led to a complete destruction of the CST. The cervical spinal cord was stained with PKCγ, a marker of the CST, at 1 week after the injury (Bradbury et al., 2002). PKCγ immunoreactivity was present bilaterally in the dorsal CST of the cervical spinal cord in sham-operated mice (data not shown).
Discussion
In this study, we report that ephrin-B3 is expressed only in the dorsal part of the midline in the adult spinal cord, and after the development of a unilateral cortical lesion, the axons originating from intact CST neurons cross to the denervated side of the cervical cord through the ephrin-B3-negative ventral part of the midline. The ephrin-B3 receptor, EphA4, is expressed in the CST axons in neonatal and adult mice. This expression is maintained after the development of a unilateral cortical
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2014, Trends in NeurosciencesCitation Excerpt :Therefore, unlike in development, the midline of the adult CNS seems molecularly virtual and not enriched in specific axon guidance molecules. Nevertheless, all ‘midline’ cues are still highly expressed in the adult brain either by neurons or by glia cells and their expression can be upregulated in sites of injury, in the cortex or spinal cord [106–110]. Likewise, most commissural axon guidance receptors are still expressed by adult neurons.
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2014, Experimental NeurologyCitation Excerpt :Notably, peak GAP-43 expression in CSC 7 and 14 days after FL-SMC lesion occurs during periods of spontaneous recovery of forelimb function (as indicated by improvements in Montoya Staircase performance between 3 and 14 days post-stroke) and returns to baseline as recovery plateaus. Based on data showing sprouting of axon collaterals from uninjured CST into stroke-denervated gray matter in the spinal cord (DeVetten et al., 2010; Jayaram et al., 2012; LaPash Daniels et al., 2009; Liu et al., 2007, 2008, 2009, 2012; Omoto et al., 2011; Puig et al., 2010; Reitmeir et al., 2011; Starkey et al., 2012; Thomalla et al., 2004; Wang et al., 2012; Yu et al., 2009; Zhang et al., 2010), we postulate that the increases in spinal cord levels of GAP-43 after FL-SMC would primarily reflect sprouting of descending axons from the CST into denervated spinal gray matter. However, given that axonal rewiring likely includes ipsilateral (CST fibers originating in uninjured motor cortex sprouting into stroke affected gray matter) and contralateral (sprouting of spared axons originating in peri-infarct cortex) components, our measures of spinal plasticity provide a summation of structural plasticity at the level of the cervical or lumbar cord incorporating sprouting from CST originating in both cortical hemispheres, as well as neurite outgrowth from interneurons in the spinal cord (as has been reported after spinal cord injury (Bareyre et al., 2004; Vavrek et al., 2006)).
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This work was supported by a research grant from the Grant-in-Aid for Young Scientists (S) from JSPS.