EphB receptor forward signaling regulates area-specific reciprocal thalamic and cortical axon pathfinding

Abstract

In early brain development, ascending thalamocortical axons (TCAs) navigate through the ventral telencephalon (VTel) to reach their target regions in the young cerebral cortex. Descending, deep-layer cortical axons subsequently target appropriate thalamic and subcortical target regions. However, precisely how and when corticothalamic axons (CTAs) identify their appropriate, reciprocal thalamic targets remains unclear. We show here that EphB1 and EphB2 receptors control proper navigation of a subset of TCA and CTA projections through the VTel. We show in vivo that EphB receptor forward signaling and the ephrinB1 ligand are required during the early navigation of L1-CAM + thalamic fibers in the VTel, and that the misguided thalamic fibers in EphB1/2 KO mice appear to interact with cortical subregion-specific axon populations during reciprocal cortical axon guidance. As such, our findings suggest that descending cortical axons identify specific TCA subpopulations in the dorsal VTel to coordinate reciprocal corticalthalamic connectivity in the early developing brain.

Autonomous and non-autonomous roles for ephrin-B in interneuron migration

“…To address this, we utilized GAD65-GFP (Lopez-Bendito et al, 2004) and GAD67-GFP (Tamamaki et al, 2003) reporter mice which brightly label distinct GABAergic neuron populations with GFP as they are born in the subpallium and migrate tangentially into the neocortex. The two GAD-GFP reporters were each crossed with mice that either carried a doxycycline-inducible (dox) Tg-BAC- EfnB1 rtTA transgene (EB1-rtTA) (Robichaux et al, 2014) and a TetO-tdTomato indicator ( TRE-Bi-SG-T ) (Li et al, 2010) to report EB1 expression, or to the EfnB2 lz ( EB2 lz ) (Dravis et al, 2004) and EfnB3 lz ( EB3 lz ) (Yokoyama et al, 2001) lacZ knockin mice that express intracellular truncated ephrin-B-βgal fusion proteins (EB2-βgal and EB3-βgal) that traffic normally to the plasma membrane and can be used to report either EB2 or EB3 expression, respectively. These EB1-rtTA, EB2 lz , and EB3 lz genetic tools are excellent reporters of the expression of the respective gene and exhibit exceptionally high sign-to-noise ratios, much higher than possible using traditional mRNA in situ hybridization or protein antibody approaches.…”

“…All mice used in this analysis have been previously described; GAD65-GFP (Lopez-Bendito et al, 2004) and GAD67-GFP (Tamamaki et al, 2003) reporters, doxycycline-inducible (dox) Tg-BAC-ephrin-B1 rtTA transgene (Robichaux et al, 2014), TetO-tdTomato indicator (Li et al, 2010), loxP-flanked alleles in ephrin- B1 ( EB1 loxP ) (Davy et al, 2004) and ephrin- B2 ( EB2 loxP ) (Gerety and Anderson, 2002), ephrin-B2 lz ( EB2 lz ) and ephrin-B2 T ( EB2 T ) (Dravis et al, 2004), ephrin-B2 6YFΔV ( EB2 6YFΔV ) (Thakar et al, 2011), ephrin-B3 lz ( EB3 lz ) (Yokoyama et al, 2001), Dlx1/2-Cre (Potter et al, 2009), Nex-Cre (Goebbels et al, 2006), Emx1-Cre (Iwasato et al, 2000), and Rosa26-STOP-tdTomato Cre indicator ( Ai9 )(Madisen et al, 2010). Mice were maintained in a mixed CD1/129 genetic background.…”

Gene co-regulation by Fezf2 selects neurotransmitter identity and connectivity of corticospinal neurons

“…It is striking that the phenotypic abnormalities observed in the absence of EphB1 mimic the axon guidance defects observed in the Fezf2 −/− mutants, without affecting the overall fate specification of CSMN. While the possibility that expression of EphB1 in thalamus or striatum contributes to this phenotype 49 cannot be ruled out, the fact that Fezf2 is not expressed in these regions and yet Fezf2 −/− mice present similar axon guidance defects to Ephb1 mutants supports a cell-autonomous role of EphB1 in CSMN. Our data point to a direct regulation of key CSMN axon guidance receptors by the same selector gene that governs global programs of fate specification of this neuron subtype.…”

Axon guidance in the auditory system: Multiple functions of Eph receptors

“…Nonetheless, Eph proteins have multiple roles in every stage of cortical development (North et al, 2013), and thus an understanding of their integrated function will be of value in understanding assembly of auditory pathways. Eph proteins regulate areal specification of cortex and guidance of thalamocortical axons to the appropriate cortical areas (Dufour et al, 2003, Robichaux et al, 2014). They influence cortical cell migration (Steinecke et al, 2014), laminar specification (Mann et al, 2002), topography (Cang et al, 2008a), and columnar organization (Torii et al, 2009, Dimidschstein et al, 2013).…”