Receptor Clustering Is Involved in Reelin Signaling

Abstract

The Reelin signaling cascade plays a crucial role in the correct positioning of neurons during embryonic brain development. Reelin binding to apolipoprotein E receptor 2 (ApoER2) and very-low-density-lipoprotein receptor (VLDLR) leads to phosphorylation of disabled 1 (Dab1), an adaptor protein which associates with the intracellular domains of both receptors. Coreceptors for Reelin have been postulated to be necessary for Dab1 phosphorylation. We show that bivalent agents specifically binding to ApoER2 or VLDLR are sufficient to mimic the Reelin signal. These agents induce Dab1 phosphorylation, activate members of the Src family of nonreceptor tyrosine kinases, modulate protein kinase B/Akt phosphorylation, and increase long-term potentiation in hippocampal slices. Induced dimerization of Dab1 in HEK293 cells leads to its phosphorylation even in the absence of Reelin receptors. The mechanism for and the sites of these phosphorylations are identical to those effected by Reelin in primary neurons. These results suggest that binding of Reelin, which exists as a homodimer in vivo, to ApoER2 and VLDLR induces clustering of ApoER2 and VLDLR. As a consequence, Dab1 becomes dimerized or oligomerized on the cytosolic side of the plasma membrane, constituting the active substrate for the kinase; this process seems to be sufficient to transmit the signal and does not appear to require any coreceptor.Correct positioning of neurons of the cortical plate depends on Reelin, an extracellular matrix protein produced by CajalRetzius cells (10), on the Reelin receptors apolipoprotein E receptor 2 (ApoER2) and very-low-density-lipoprotein receptor (VLDLR) (35), and on the intracellular adaptor protein disabled 1 (Dab1) (30). Mutations in the corresponding genes, i.e., the Reelin gene (as in the reeler mouse) (12) and the Dab1 gene (as in the scrambler and yotari mice) (16,32,37), and deletions of the genes for both ApoER2 and VLDLR (35) result in identical cortical layering defects, suggesting that the gene products are part of the same signaling pathway. The current working model proposes that Reelin binds to ApoER2 and VLDLR (11,14). Subsequent phosphorylation of Dab1 is a key event leading to the ultimate cell responses required for correct positioning of newly generated neurons (17, 18). Dab1 was originally identified as an interaction partner of Src (15) and contains a phosphotyrosine binding domain which interacts with the unphosphorylated NPXY motif present in the cytoplasmic domains of ApoER2 and VLDLR (19,34). Phosphorylation of Dab1 induced by Reelin is dependent on the presence of ApoER2 and VLDLR (5) and occurs on Tyr198 and Tyr220 (20). Recent studies demonstrated that members of the Src family of nonreceptor tyrosine kinases (SFKs) are involved in Dab1 phosphorylation in neurons (2, 6). Coreceptors, such as members of the family of cadherin-related neuronal receptors (CNRs), have been proposed to be involved in this pathway (31). Neuronal migration is also regulated by cyclin-dependent kinase 5 (27, 28), but whether t...

LRP8 mediates Wnt/β-catenin signaling and controls osteoblast differentiation

“…(34,35) Reelin signals through LRP8 or through very-low-density lipoprotein receptor (VLDLR), induces the interaction between LRP8 and adaptor protein Disabled-1 (Dab1) and tyrosine phosphorylation of Dab1, and further, results in the recruitment of phosphatidylinositol-3-kinase (PI3K) and GSK3b. (34,(36)(37)(38) Interaction between LRP8 and Reelin also initiates internalization of Reelin in the clathrin-mediated endocytosis pathway. (39) LRP8 single-knockout mice were grossly normal, but showed reduced male fertility and suffered accelerated cell death during normal aging 4,35 ; LRP8 and VLDLR double-knockout mice displayed disorganized neurons, and cortical lamination was affected in brain development.…”

Heterozygous Reelin Mutations Cause Autosomal-Dominant Lateral Temporal Epilepsy

“…39 Reelin serves a dual purpose in the mammalian brain: it is crucial to the correct cytoarchitecture of laminated structures during embryonic development 18,40 and modulates dendritic growth and synaptic plasticity at post-natal and adult stages. 41,42 These activities are, at least in part, mediated by binding to apolipoprotein E receptor 2 (ApoER2) and very low-density lipoprotein receptor (VLDLR), [43][44][45] resulting in phosphorylation of the intracellular adaptor protein protein disabled homolog 1 (DAB1). [46][47][48] Phosphorylated DAB1 activates several different signaling pathways involved in formation and plasticity of neuronal networks (for review, see ref.…”

Reelin glycoprotein: structure, biology and roles in health and disease

“…11 Reelin binds several proteins as likely receptors, including apolipoprotein E receptor 2 (ApoER2), very-low-density lipoprotein receptor (VLDL-R) and a3b1 integrin protein. [12][13][14] Reelin binding to ApoER2 and VLDLR receptors induces clustering of the latter receptors, causing dimerization/oligomerization of the adaptor protein, disabled-1 (Dab-1), on the cytosolic aspect of the plasma membrane 15 with eventual tyrosine phosphorylation of Dab-1 adapter protein, 16 facilitating the transduction of signaling pathway from the Reelin-producing cells (GABAergic neurons 17 or Cajal-Retzius cells of layer I) 18 to downstream receptor sites on cortical pyramidal cells. 19 In vivo, Reelin is processed by cleavage at two locations, that is, between repeats 2 and 3 and repeats 6 and 7, 20 resulting in three final fragments.…”