Cerebral Cortex, Vol. 13, No. 6, 627-633,
June 2003
© 2003 Oxford University Press
The Reelin Signaling Pathway: Some Recent Developments
Developmental Genetics Unit, UCL GEDE 7382, 73 Avenue E. Mounier, B1200 Brussels and , 1 Neurobiology Unit 2853, FUNDP Medical School, 61 Rue de Bruxelles, B5000 Namur, Belgium
Address correspondence to André M. Goffinet, Developmental Genetics Unit, UCL GEDE 7382, 73 Avenue E. Mounier, B1200 Brussels, Belgium. Email: andre.goffinet{at}gede.ucl.ac.be.
| Abstract |
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The Reelin signaling pathway plays a key role in the architectonic development of the central nervous system. Extracellular Reelin binds to receptors of the lipoprotein receptor family and induces tyrosine phosphorylation of the adaptor Dab1. In this paper, we discuss three recent developments. First, we show that the central part of Reelin is involved in receptor binding and signal activation as reflected in Dab1 phosphorylation. Second, we examine the genomic organization, alternative splicing and promoter use of the Dab1 gene, which hint at a particularly complex regulation. Third, we present preliminary studies by in situ hybridization that demonstrate regulated expression of Reelin receptors and Dab1 by radial precursors in the ventricular zone.
| Introduction |
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The mammalian cerebral cortex develops by following a finely tuned sequence of events that regulate cell proliferation, neuronal migration and architectonic pattern formation, differentiation, synaptogenesis and myelination. Neuronal cell precursor proliferation takes place in germinative zones located along the cerebral ventricles, from which neurons migrate over variable distances to settle in the cortical plate. Like other developmental events, neuronal migration and architectonic pattern formation are under genetic control. Among the control mechanisms that have been identified, the Reelin signaling pathway plays a key role in cortical development in man, mouse and presumably all mammals.
Reelin is a large (>400 kDa) extracellular glycoprotein that is secreted by several neurons, particularly, in the embryonic cortex, by CajalRetzius cells. Defective Reelin is the cause of the reeler brain malformation in mice (Lambert de Rouvroit and Goffinet, 1998
; Rice and Curran, 2001
) and of a peculiar type of lissencephaly in man (Hong et al., 2000
). In reeler mice, neurons are generated in ventricular zones (VZ) and migrate initially as in normal animals. However, unlike normal cells, reeler neurons form defective architectonic patterns. Whereas normal cortical neurons form a dense, radially and laminarly organized cortical plate (CP) in which maturation proceeds from inside to outside, reeler neurons form a loose CP in which neurons are oriented obliquely and the gradient of maturation is almost inverted.
The organization of the Reelin protein is as follows (DArcangelo et al., 1995
). The sequence begins with a signal peptide of 2527 residues, followed by a region with similarity to F-spondin (amino acids 27190). A unique region between amino acids 191 and 500 is followed by a succession of eight repeats (18) of 300350 amino acids (1, residues 501860; 2, 8611220; 3, 12211596; 4, 15971947; 5, 19482314; 6, 23152661; 7, 26623051; 8, 30523428). Each Reelin repeat contains an EGF motif at its center, which divides the repeats into two sub-repeats, A and B, that show weak similarity to each other. The protein terminates with a basic stretch of 33 amino acids (34293461). Furthermore, Reelin is cleaved in vivo at two sites approximately located between repeats 2 and 3 and between repeats 6 and 7, as schematized in Figure 1
(Lambert de Rouvroit et al., 1999
).
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Reelin is not expressed in CP cells, but may act via the extracellular milieu, instructing them to assume their position and orientation. This view is supported by studies of three other genes, Disabled-1 (Dab1), VLDLR and ApoER2, mutations of which generate a reeler-like phenotype. The response of CP neurons to Reelin requires expression of at least one of two receptors, the Very Low Density Lipoprotein Receptor (VLDLR) and the Apolipoprotein-E Receptor Type 2 (ApoER2). Reelin does not bind to the closely related Low Density Lipoprotein Receptor (LDLR) nor to more distantly related members of the family such as LRP (Hiesberger et al., 1999
In the present paper, we focus on some recent results from our laboratories that concern: (i) the interaction between Reelin and its receptors and its relation to Dab1 phosphorylation; (ii) the presence and putative functional implications of alternative splicing and promoter utilization events in the Dab1 gene; (iii) mRNA expression data that point to a possible role of Reelin signaling in cortical ventricular zones.
| Reelin Binding to Lipoprotein Receptors and Dab1 Phosphorylation |
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Previous work showed that Reelin binding to the VLDLR and ApoER2 receptors does not require the N-terminal region of Reelin from the N-terminus up to the end of the second repeat (Hiesberger et al., 1999
In order to better understand the interaction of Reelin with VLDLR and ApoER2, we studied binding of different partial Reelin recombinant proteins to extracellular regions of receptors, fused to an Fc immunoglobulin fragment (provided by J. Herz). The reelin cDNA construct pCrl (provided by T. Curran) was used to express Reelin and as a template to derive different parts of the reelin cDNA, by nuclease restriction or PCR amplification. In the terminology of the Reelin constructs, R is used for repeat, N for N-terminus and Del for deletion of a given region. The constructs containing R3R8, R3R6, R3R5, R3R4, R4R5, R5R6, R7R8, R4 and R6 were cloned in the pSecTag2B vector (Invitrogen), which encodes the polypeptide in phase with a signal peptide and a Myc epitope. Constructs NR6, DelR3R5A, NR5A and NR2 were obtained from pCrl by nuclease restriction followed by ligation. Partial and full-length recombinant Reelin were produced by transient transfection of HEK293 cells and fresh supernatant was used without further purification. LDLR, VLDLR and ApoER2 receptor constructs were produced in the same manner. Receptor binding was estimated by co-immunoprecipitation. For estimation of Dab1 tyrosine phosphorylation, primary cortical neuronal cultures were incubated with full-length or recombinant proteins, the Dab1 protein was immunoprecipitated using a rabbit polyclonal antibody directed against the C-terminal peptide of Dab1 and the precipitates were analyzed on duplicate PAGE gels for estimation of Dab1 protein content using a monoclonal antibody against the PTB region of Dab1 and, for estimation of Dab1 tyrosine phosphorylation, using a commercial anti-phosphotyrosine antibody (clone 4G10; Upstate) (Howell et al., 1999a
).
Binding to Lipoprotein Receptors
Binding to LDLR was very low or negative for all secreted proteins (Fig. 2A
). In accord with published evidence, construct NR2, composed of the N-terminal spondin similarity region, the unique segment and repeats 1 and 2, did not bind lipoprotein receptors. Even construct NR5A, which includes the first four reelin repeats and part of repeat 5, did not bind significantly. This confirmed that the N-terminal moiety of Reelin is not directly involved in receptor binding (Hiesberger et al., 1999
). Similarly, all secreted constructs that contain one, two or three Reelin repeats failed to bind detectably to VLDLR and ApoER2. In contrast, constructs R3R6 and R3R8 and NR6 bound to VLDLR and ApoER2 similarly to full-length Reelin (Fig. 2A
). Such an increase in binding capacity with increased repeat number may indicate cooperativity between repeats. Repeat number is not the sole factor involved, however. For example, construct DelR3R5A, in which repeats 3, 4 and the N-terminal part of repeat 5 are deleted, did not bind detectably to either receptor, even though it contains five Reelin repeats. Altogether, these experiments suggest that at least four repeats are necessary to detect significant binding of Reelin to VLDLR and ApoER2. Furthermore, repeats contained in the central region of Reelin are particularly important for receptor binding.
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Dab1 Phosphorylation
As a first approach to assess whether receptor binding is sufficient to trigger the Reelin signal or whether additional features are required, the ability of the different Reelin constructs to induce tyrosine phosphorylation of the Dab1 adapter in neuronal cells was studied. Full-length Reelin and constructs R3R6, R3R8 and NR6 were equally able to induce Dab1 phosphorylation, whereas all other Reelin polypeptide constructs were inactive (Fig. 2B
).
The central part of Reelin defined in these experiments is large in comparison with the extracellular domains of VLDLR and ApoER2, which leaves ample room for interaction with several proteins in addition to lipoprotein receptors. However, the correlation between binding to receptors and stimulation of Dab1 tyrosine phosphorylation indicates that the central region of Reelin is sufficient to initiate the Dab1-dependent part of the signal. This central region of Reelin corresponds approximately to the fragment generated by cleavage; whether this is functionally relevant remains to be demonstrated.
These observations are worth discussing in relation to published aspects of Reelin signaling. The protocadherin CNR1 and integrin
3ß1 have been proposed as Reelin co-receptors (Senzaki et al., 1999
; Dulabon et al., 2000
). Whereas the part of Reelin implicated in integrin binding was not defined, binding of CNR1 to the N-terminal region of Reelin was considered critical. Our observations suggest that binding of Reelin to CNR is not necessary for its Dab1-dependent function, but may serve some other independent role that remains to be defined further. Another point is the recent observations that: (i) the N-terminal region of Reelin mediates Reelin aggregation; (ii) homodimerization of Reelin and Dab1 phosphorylation are both inhibited by the function blocking antibody CR50; (iii) Reelin devoid of its N-terminal region fails to trigger Dab1 phosphorylation, even though it is still able to bind to the VLDLR and ApoER2 receptors (Utsunomiya-Tate et al., 2000
; Kubo et al., 2002
). These findings suggested that Reelin homodimerization is necessary for activation of the signal as reflected by Dab1 phosphorylation. For reasons that we do not understand, our observations that constructs containing Reelin R3R8 and R3R6 are able to induce Dab1 phosphorylation are at variance with that evidence. Our results, however, do not necessarily argue against a functional role of Reelin aggregation in vivo. Aggregation could increase the effective local concentration of Reelin, for example by anchoring the protein to the extracellular matrix. Due to the use of transfected cell supernatants and neuronal cultures, our binding and phosphorylation assays could not be quantified using pure components and provide at best an estimate of the relative affinity of the different ligands. Aggregation of full-length Reelin may increase its affinity to an extent that we could not measure.
| Alternative Forms of Dab1 |
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As Dab1 is a key component of the Reelin signaling pathway, a detailed study of the genomic organization and transcriptional regulation of the Dab1 gene was carried out in man and mouse. Here, we would like to emphasize some elements that concern the regulation of Dab1 expression and may be relevant to its function.
Alternative First Exons and Organization of the 5'-Region (Figs 3 and 4![]()
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Comparison of the different Dab1 sequences revealed extensive variation in the 5'-untranslated regions (5'-UTR), as schematized in Figure 3
. In mouse embryonic brain RNA, four different products named 1A, 1B, 1C and 1D were obtained. UTR 1B does not correspond to a single exon but is composed of 10 exons, 1B11B10. The Dab1 5'-UTR spreads over 850 kb of genomic DNA, pointing to a vast complexity. The human DAB1 gene has an almost identical organization, further suggesting that this evolutionarily conserved, complex organization may be functionally important.
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In order to assess whether the different 5' exons have different expression patterns, RTPCR reactions were carried out on mouse brain cDNA at different stages from E11 to adult. As shown in Figure 4A
Internal Alternative Splicing Events (Fig. 5
)
Four different Dab1 cDNA forms were initially described in the mouse (Howell et al., 1997a
), as schematized in Figure 5A,B
. The main form encodes a protein of 555 residues and is named Dab1-555. Another, Dab1-217, is due to recognition of an alternative polyadenylation signal in intron 7 and is predicted to encode a 217 amino acid polypeptide. A third form, Dab1-271, is due to insertion of an additional exon between exons 9 and 10 and results in the production of a 271 residue protein. Finally, another segment, named 555*, is included in some Dab1 cDNA forms between exons 9 and 10. We showed that fragment 555* corresponds to two small exons of 51 and 48 bp separated by an intron of 91 bp and that both exons are consistently co-amplified. In undifferentiated P19 cells, the Dab1 cDNA included fragment 555*. However, when differentiation of P19 cells was induced with RA, a proportion of Dab1 cDNA without fragment 555* appeared at day 2 and increased progressively to become the major form at day 9 (Fig. 5C
). In early embryonic mouse brain (E11 and E12), the Dab1 isoform with fragment 555* was predominant, but RNA from later developmental stages, E12 and later, and from primary neuronal cultures did not contain this fragment (Fig. 5D
). In non-neural tissues such as liver and kidney, the Dab1 mRNA contained fragment 555*. A similar pattern was found in chick, with inclusion of a small 57 nt exon in RNA from E6 embryo, but exclusion of that small exon from brain RNA at E20 (Fig. 4E
). In situ hybridization with a cDNA probe covering fragment 555* and adjacent segments confirmed expression in ventricular zones (Fig. 5F
). Altogether, these observations suggest strongly that the exclusion of exon 555* occurs in parallel with neural differentiation.
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In summary, Dab1 mRNA is expressed at high level in post-migratory neurons, but also at a more modest level in ventricular zones. Two main first exons and thus two main promoters are used for Dab1 translation in the brain. Dab1 mRNA in VZ includes the small exon 555*, whereas Dab1 mRNA in post-migratory neurons does not.
| Expression of VLDLR and ApoER2 in the Developing Telencephalon |
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The presence of some Dab1 mRNA expression in the telencephalic VZ prompted us to study expression of Reelin receptors in more detail, using in situ hybridization. Preliminary results in early embryonic brain revealed expression of ApoER2 and VLDLR mRNA in the VZ in addition to Dab1. At E12, the preplate stage (Fig. 6
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The co-expression of lipoprotein receptors and Dab1 in the VZ suggests that Reelin may fulfill a novel, hitherto unknown, function at this level, for example in relation to neuron and glial cell precursor proliferation or to initiation of cell migration (Meyer et al., 2002
| Conclusions |
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In conclusion, the recent data summarized above suggest that central Reelin repeats bind to VLDLR and ApoER2 receptors and this results in the organization of a supramolecular complex and in signal activation, reflected by Dab1 phosphorylation. Other components of this complex remain unknown but are likely to include tyrosine kinase(s). However, no mice defective in tyrosine kinase genes have a reeler-like phenotype, and the role of Fyn and related kinases in Dab1 phosphorylation remains open to question. Another possibility is that unidentified components of the Reelin receptor complex might not be Reelin co-receptors in that they do not bind Reelin directly, but could be recruited to the complex via cis interaction with lipoprotein receptors. The central role of Dab1 in transduction of the Reelin signal is further emphasized by the complexity of its genomic organization and transcriptional regulation. Alternatively, spliced forms of Dab1 appear specifically expressed in precursor cells in the VZ, which also express VLDLR and ApoER2, pointing to a new role of the Reelin signal in precursor proliferation and/or rostral migration in the olfactory system. Several questions remain, particularly concerning the final action of Reelin on target cells and the identification of the downstream effectors of the Reelin signal.
| Acknowledgments |
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We wish to thank T. Curran for the gift of the reelin pCrl plasmid, J. Herz for the gift of the lipoprotein receptor constructs and C. Dernoncourt for technical assistance. This work was supported by grants from FRIA (to Y.J.) and by grants FRFC 2.4504.01 and ARC 186 99/02, the Fondation Médicale Reine Elisabeth and by EU contract CONCORDE (QLG3-CT-2000-00158).
| References |
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Bernier B, Bar I, DArcangelo G, Curran T, Goffinet AM (2000) Reelin mRNA expression during embryonic brain development in the chick. J Comp Neurol 422:448463.[CrossRef][Web of Science][Medline]
DArcangelo G, Miao GG, Chen SC, Soares HD, Morgan JI, Curran T (1995) A protein related to extracellular matrix proteins deleted in the mouse mutant reeler. Nature 374:719723.[CrossRef][Medline]
DArcangelo G, Homayouni R, Keshvara L, Rice DS, Sheldon M, Curran T (1999) Reelin is a ligand for lipoprotein receptors. Neuron 24: 471479.[CrossRef][Web of Science][Medline]
de Bergeyck V, Naerhuyzen B, Goffinet AM, Lambert de Rouvroit C (1998) A panel of monoclonal antibodies against reelin, the extracellular matrix protein defective in reeler mutant mice. J Neurosci Methods 82:1724.[CrossRef][Web of Science][Medline]
Dulabon L, Olson EC, Taglienti MG, Eisenhuth S, McGrath B, Walsh CA, Kreidberg JA, Anton ES (2000) Reelin binds alpha3beta1 integrin and inhibits neuronal migration. Neuron 27:3344.[CrossRef][Web of Science][Medline]
Hack I, Loulier K, Carroll P, Cremer H (2002) A role for reelin as a detachment signal in adult neurogenesis. FENS Abstr 1:506.
Herrick TM, Cooper JA (2002) A hypomorphic allele of dab1 reveals regional differences in reelin-Dab1 signaling during brain development. Development 129:787796.
Hiesberger T, Trommsdorff M, Howell BW, Goffinet A, Mumby MC, Cooper JA, Herz J (1999) Direct binding of Reelin to VLDL receptor and ApoE receptor 2 induces tyrosine phosphorylation of disabled-1 and modulates tau phosphorylation. Neuron 24:481489.[CrossRef][Web of Science][Medline]
Hong SE, Shugart YY, Huang DT, Shahwan SA, Grant PE, Hourihane JO, Martin ND, Walsh CA (2000) Autosomal recessive lissencephaly with cerebellar hypoplasia is associated with human RELN mutations. Nature Genet 26:9396.[CrossRef][Web of Science][Medline]
Howell BW, Gertler FB, Cooper JA (1997a) Mouse disabled (mDab1): a Src binding protein implicated in neuronal development. EMBO J 16:121132.[CrossRef][Web of Science][Medline]
Howell BW, Hawkes R, Soriano P, Cooper JA (1997b) Neuronal position in the developing brain is regulated by mouse disabled-1. Nature 389:733737.[CrossRef][Medline]
Howell BW, Herrick TM, Cooper JA (1999a) Reelin-induced tryosine phosphorylation of disabled 1 during neuronal positioning. Genes Dev 13:643648.
Howell BW, Lanier LM, Frank R, Gertler FB, Cooper JA (1999b) The disabled 1 phosphotyrosine-binding domain binds to the internalization signals of transmembrane glycoproteins and to phospholipids. Mol Cell Biol 19:51795188.
Howell BW, Herrick TM, Hildebrand JD, Zhang Y, Cooper JA (2000) Dab1 tyrosine phosphorylation sites relay positional signals during mouse brain development. Curr Biol 10:877885.[CrossRef][Web of Science][Medline]
Keshvara L, Benhayon D, Magdaleno S, Curran T (2001) Identification of reelin-induced sites of tyrosyl phosphorylation on disabled 1. J Biol Chem 276:1600816014.
Keshvara L, Magdaleno S, Benhayon D, Curran T (2002) Cyclin-dependent kinase 5 phosphorylates disabled 1 independently of Reelin signaling. J Neurosci 22:48694877.
Kubo K, Mikoshiba K, Nakajima K (2002) Secreted Reelin molecules form homodimers. Neurosci Res 43:381388.[CrossRef][Web of Science][Medline]
Lambert de Rouvroit C, Goffinet AM (1998) The reeler mouse as a model of brain development. Adv Anat Embryol Cell Biol 150:1106.[Medline]
Lambert de Rouvroit C, de Bergeyck V, Cortvrindt C, Bar I, Eeckhout Y, Goffinet AM (1999) Reelin, the extracellular matrix protein deficient in reeler mutant mice, is processed by a metalloproteinase. Exp Neurol 156:214217.[CrossRef][Web of Science][Medline]
Meyer G, Lambert de Rouvroit C, Goffinet A, Wahle P (2002) DAB1 mRNA and protein expression in developing human cortex. FENS Abstr 1:506.
Rice DS, Curran T (2001) Role of the reelin signaling pathway in central nervous system development. Annu Rev Neurosci 24:10051039.[CrossRef][Web of Science][Medline]
Senzaki K, Ogawa M, Yagi T (1999) Proteins of the CNR family are multiple receptors for Reelin. Cell 99:635647.[CrossRef][Web of Science][Medline]
Trommsdorff M, Gotthardt M, Hiesberger T, Shelton J, Stockinger W, Nimpf J, Hammer RE, Richardson JA, Herz J (1999) Reeler/ Disabled-like disruption of neuronal migration in knockout mice lacking the VLDL receptor and ApoE receptor 2. Cell 97:689701.[CrossRef][Web of Science][Medline]
Utsunomiya-Tate N, Kubo K, Tate S, Kainosho M, Katayama E, Nakajima K, Mikoshiba K (2000) Reelin molecules assemble together to form a large protein complex, which is inhibited by the function-blocking CR-50 antibody. Proc Natl Acad Sci USA 97:97299734.
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300, 220, 180 and 100 kDa, as indicated in the schema. The CR50 epitope is approximately located as indicated.





