Reelin-Disabled-1 signaling in neuronal migration: splicing takes the stage

Abstract

Reelin-Disabled-1 (Dab1) signaling has a well-established role in regulating neuronal migration during brain development. Binding of Reelin to its receptors induces Dab1 tyrosine phosphorylation. Tyrosine-phosphorylated Dab1 recruits a wide range of SH2 domain-containing proteins and activates multiple signaling cascades, resulting in cytoskeleton remodeling and precise neuronal positioning. In this review, we summarize recent progress in the Reelin-Dab1 signaling field. We focus on Dab1 alternative splicing as a mechanism for modulating the Reelin signal in developing brain. We suggest that correct positioning of neurons in the developing brain is at least partly controlled by alternatively-spliced Dab1 isoforms that differ in the number and type of tyrosine phosphorylation motifs that they contain. We propose a model whereby different subsets of SH2 domain-containing proteins are activated by different Dab1 isoforms, resulting in coordinated migration of neurons.

Fig. 1

Migration of cortical neurons in wild-type and reeler mice. a Inside-out lamination of cortical neurons in wild-type mice. At early developmental stages, layer VI neurons split the preplate (PP) to form the subplate (SP) and pial surface (PS). Late-born neurons continue to migrate and bypass older neurons, resulting in the inside-out formation of the cortical plate, with older neurons located in the inner layers, and the younger neurons located in outer layers. b Inverted cortical neuronal layers in reeler mice. Layer VI fails to split the preplate, leading to accumulation of neurons underneath the preplate. The inability of late-born neurons to bypass the old neurons results in the inversion of neuronal layers in the cortical plate

Fig. 2

The Reelin-Disabled 1 signaling pathway. Binding of Reelin to its receptors, VLDLR and ApoER2, induces SFK activation and Dab1 tyrosine phosphorylation. Phosphorylated Dab1 acts as a hub to recruit different downstream SH2 domain-containing proteins including Crk, Nckβ, p85 (PI3K) and SOCS. Dab1–Crk interaction activates the downstream C3G-Rap1 pathway. Activated Rap1 then regulates the membrane distribution of N-cadherin. Recruitment of Nckβ to Dab1 is likely involved in actin remodeling through p130Cas. The association of PI3K p85 subunit with Dab1 activates the PI3K-Akt pathway, which in turn modulates the phosphorylation of microtubule binding proteins, tau and MAP1B, leading to microtubule remodeling. Activation of PI3K-Akt may engage LIMK1 and n-cofilin, resulting in stabilization of actin polymerization. In contrast, Dab1–SOCS association down-regulates Reelin signaling by degrading phosphorylated Dab1 through the ubiquitin proteasome system, resulting in termination of Reelin signaling. The interaction between tyrosine-phosphorylated Dab1 and Lis1 also causes microtubule remodeling. Reelin-Dab1 signaling and LKB-STRAD-Stk25 play opposing roles in regulating the rearrangement of the Golgi apparatus and neuronal polarity

Fig. 3

Mixing-and-matching tyrosine phosphorylation sites in Dab1 by alternative splicing. Schematic representation of exon–intron structure (exons 6–9) of the Dab1 gene. White boxes represent constitutive exons, whereas purple and gold boxes indicate alternative exons. The amino acid sequences of the four consensus Dab1 tyrosine phosphorylation sites are shown. Dab1 contains two YQXI motifs (Y¹⁸⁵QTI and Y¹⁹⁸QYI, framed in magenta) and two YXVP motifs (Y²²⁰QVP and Y²³²DVP, framed in green). Exclusion of exon 7 (ΔEx 7) removes Y¹⁹⁸Q but links Y¹⁸⁵Q to Y²⁰⁰I²⁰¹, thereby reconstituting a Y¹⁸⁵QYI motif identical to the original Y¹⁹⁸QYI. Exclusion of exon 8 (ΔEx 8) removes Y²²⁰Q but connects Y¹⁹⁸Q with V²²²P²²³, converting Y¹⁹⁸QXI into a Y¹⁹⁸QVP site. Exclusion of exons 7 and 8 (ΔEx 7 and 8) removes both Y¹⁹⁸Q and Y²²⁰Q and rejoins Y¹⁸⁵Q to V²²²P²²³, resulting in loss of both YQXI motifs but retention of two YXVP motifs

Fig. 4

Schematic representation of Dab1 variants. Dab1 exons are shown (exons are not drawn to scale). The start codon is located in exon 2, whereas the stop codon is located in exon 14. Exons 3–6 encode the PTB domain, while exons 6–9 encode the four critical tyrosine phosphorylation motifs. Exclusion of exons 7 and/or 8 in mice or exons 8 and 9 in zebrafish results in partial deletion of tyrosine phosphorylation motifs in Dab1. Inclusion of exons 9b/9c results in an in-frame 33 aa insertion. Inclusion of an alternative exon after exon 7 in Dab1²¹⁷ results in deletion of two tyrosine motifs and introduction of an in-frame stop codon generating a 217-aa product. Inclusion of an alternative exon after exon 9 in Dab1²⁷¹ introduces an in-frame codon generating a 271-aa product

Fig. 5

Fine-tuning Reelin signaling by Dab1 alternative splicing during development—a model. Early in development, alternative splicing produces Dab1 isoforms that exclude exons 7 and/or 8. These isoforms show absent or reduced associations with SH2 domain-containing proteins, leading to absent or attenuated activation of downstream pathways. As a result, cells expressing these isoforms may have a compromised ability to detach and translocate in response to Reelin. As development proceeds, inclusion of exons 7 and 8 in Dab1 generates the canonical Dab1 isoform that recruits the full set of SH2 domain-containing proteins, resulting in full activation of downstream signaling. Cells expressing the canonical Dab1 isoform are able to detach and translocate in response to Reelin, resulting in the inside-out lamination that is characteristic of the cortical plate. Gray indicates reduced interaction; magenta indicates normal interaction