X-linked microtubule-associated protein, Mid1, regulates axon development

Abstract

Opitz syndrome (OS) is a genetic neurological disorder. The gene responsible for the X-linked form of OS, Midline-1 (MID1), encodes an E3 ubiquitin ligase that regulates the degradation of the catalytic subunit of protein phosphatase 2A (PP2Ac). However, how Mid1 functions during neural development is largely unknown. In this study, we provide data from in vitro and in vivo experiments suggesting that silencing Mid1 in developing neurons promotes axon growth and branch formation, resulting in a disruption of callosal axon projections in the contralateral cortex. In addition, a similar phenotype of axonal development was observed in the Mid1 knockout mouse. This defect was largely due to the accumulation of PP2Ac in Mid1-depleted cells as further down-regulation of PP2Ac rescued the axonal phenotype. Together, these data demonstrate that Mid1-dependent PP2Ac turnover is important for normal axonal development and that dysregulation of this process may contribute to the underlying cause of OS.

Contralateral targeting of the corpus callosum

“…Despite previous evidence showing that contralateral callosal mistargeting can be affected in isolation from midline defects in mice (Mizuno et al, 2007;Lu et al, 2013;Suárez et al, 2014b), no studies have thus far assessed the behavioural consequence of disrupted contralateral targeting in isolation. However, using diffusion magnetic resonance imaging (dMRI) and tractography, it has been demonstrated that contralateral callosal targeting can be highly variable and misregulated in humans with gross structural malformations of the corpus callosum at the midline (Tovar-Moll et al, 2007;Wahl et al, 2009;Bénézit et al, 2015).…”

“…Factors that have been shown to regulate this process include Slit2 (Shu et al, 2003a) and Wnt5a/Ryk (Keeble et al, 2006;Hutchins et al, 2011 (Mizuno et al, 2007;Lu et al, 2013;. This suggests that, while callosal malformations are diagnosed from gross midline imaging, defects arising from isolated errors in contralateral targeting may remain undiagnosed in humans.…”

“…Disruption of Mid1 expression causes exuberant axonal growth and branching in vitro, as well as accelerated callosal axon growth and branching, which results in altered patterns of contralateral innervation (Lu et al, 2013). …”

“…Indeed, the in vitro work on dissociated cells from the entire cerebral cortex in several of the above studies (e.g. Lu et al, 2013;Zhang et al, 2015) indicates that many of these molecular mechanisms may not be callosal-specific. Similarly, as discussed above, it has already been well established that reducing sensory input or intrinsic cortical activity affects contralateral callosal targeting, and thus any manipulation that has a side-effect of changing activity properties of transfected neurons may subsequently cause defects in this process, without necessarily being specific to the system.…”

“…The most specific and commonly utilised neuronal layer labelled using this method is L2/3, which can be achieved by electroporating at embryonic day (E) 15.5 in mice. This method has already been widely used to study many aspects of callosal development, including cell migration and projection (Mizuno et al, 2015;Zhang et al, 2015), axonal patterning (Sehara et al, 2010;Sehara et al, 2012), contralateral targeting (Mizuno et al, 2007;Mizuno et al, 2010;Suárez et al, 2014b), axon branching Lu et al, 2013) and functional connectivity patterns . The callosum connecting the somatosensory cortex of mice (especially the barrel cortex) is a particularly suitable model system for commissural investigations due to its point-to-point topographical arrangement from the sensory periphery to the cortex, stereotypical cytoarchitectural features and full decussation.…”