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Reelin Glycoprotein in Autism and Schizophrenia
REELIN GLYCOPROTEIN IN AUTISM AND SCHIZOPHRENIA
S. Hossein Fatemi Department of Psychiatry, University of Minnesota Medical School Minneapolis, Minnesota 55455, USA
I. II. III. IV. V. VI.
Reelin Gene and Protein Reelin Signaling Cascade The Reeler Mouse Reelin in Autism Reelin in Schizophrenia Conclusions References
Reelin glycoprotein is a serine protease with important roles in embryogenesis and in adult life. Reelin mutations or deficiency of the protein product could cause abnormal cortical development and/or Reelin-induced signal transduction impairment in brain. Reelin abnormalities in several neuropsychiatric disorders, such as autism and schizophrenia, may provide mechanistic explanations for etiologies of these disorders.
I. Reelin Gene and Protein
Reelin glycoprotein is a serine protease (Quattrocchi et al., 2002) with dual roles in mammalian brain: embryologically, it guides neurons and radial glial cells to their final positions in brain (Forster et al., 2002; GoYnet, 1992); during adult life, it participates in a signaling cascade which may serve synaptic plasticity, memory processing, and cognition (Fatemi, 2005; Weeber et al., 2002). Reelin gene (Reln) is localized to chromosome 7 in man (DeSilva et al., 1997). The Reln gene codes for protein products which on SDS-PAGE range from 410 to 330, 180kDa and several smaller fragments in man (Fatemi et al., 2002a, 2004, 2005a; Ignatova et al., 2004; Smallheiser et al., 2000) and in rodents (D’Arcangelo et al., 1995; Ogawa et al., 1995).
INTERNATIONAL REVIEW OF NEUROBIOLOGY, VOL. 71 DOI: 10.1016/S0074-7742(05)71008-4
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II. Reelin Signaling Cascade
Reelin protein binds several receptors including apolipoprotein E receptor 2 (ApoER2), very-low-density lipoprotein receptor (VLDLR), and /31 integrin to initiate a signaling cascade which underlies downstream biochemical events leading to synaptic plasticity (see Fig. 1). Binding of Reelin to its receptors, specifically ApoER2 and VLDLR, induces clustering of these receptors and oligomerization of the adaptor protein, disabled-1 (Dab-1) (D’Arcangelo et al., 1999; Dulabon et al., 2000; Hiesberger et al., 1999; Strasser et al., 2004). This is then followed by tyrosine phosphorylation of Dab-1 which causes actin polymerization and final ubiquitination of Dab-1 protein, steps involved in mechanisms behind cell migration and synaptic plasticity (Beffert et al., 2005; Suetsugu et al., 2004). Phosphorylation of Dab-1 can also act as a substrate for inhibition of the level of glycogen synthase-kinase 3 (GSK-3) and modulation of pathways for cell survival and growth (BeVert et al., 2002).
III. The Reeler Mouse
The significance of Reelin’s role in embryogenesis of brain became evident following the discovery of a Reln gene mutation nearly half a century ago (Falconer, 1951). The Reelin mutant mouse, which carries an autosomal recessive mutation in Reln, exhibited ataxia and a reeling gait. Examination of the brain in these animals showed inverted cortical lamination, abnormal positioning of neurons, and aberrant orientation of neuronal cell bodies and nerve fibers (Falconer, 1951; GoYnet, 1979). The mutant mice also exhibited cerebellar hypoplasia with associated lack of foliation (Caviness and Sidman, 1973). Ectopic expression of Reelin in the Reeler mouse rescues cerebellar development and corrects ataxia in the mutant mouse (Magdaleno et al., 2002). The homozygous mutant mouse does not produce Reelin. The heterozygous Reeler mouse has a 50% reduction in Reelin protein and mRNA with decreases in dendritic spine density, neuropil hypoplasticity, and decreased GABA turnover (Carboni et al., 2004). Behaviorally, the heterozygous Reeler mouse exhibits decreased prepulse inhibition, a phenomen observed in subjects with autism and schizophrenia (McAlonan et al., 2002; Meincke et al., 2004; Tueting et al., 1999). Several recent reports using various prenatal insults, e.g., viral infection in midterm pregnant mice (Fatemi et al., 1999, 2002b), and 5 methoxytryptamine exposure in E17 pregnant rats ( Janusonis et al., 2004) cause reductions in levels of brain/blood Reelin levels and result in abnormal corticogenesis in the oVspring.
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FIG. 1. The Reelin signaling system and cognition. Extracellular Reelin glycoprotein is secreted by Cajal-Retzius cells and certain cortical and hippocampal GABAergic cells and cerebellar granule cells. Reelin can bind its receptors ApoER2, VLDLR, and 31 integrin directly, initiating the signaling system in the eVector cells (i.e., cortical pyramidal cells). Reelin induction of the cascade leads to clustering of the receptors causing dimerization/oligomerization of Dab-1 protein and activation of Src-tyrosine kinase family/Fyn-kinase leading to tyrosine phosphorylation of Dab-1 protein in a positive-feedback loop. Interaction between Dab-1, N-WASP, and ARP 2/3 complex causes formation of microspikes or filopodia which are important in processes of cell migration and synaptic plasticity. Finally, phosphorylation of a subpopulation of Dab-1 molecules causes degradation of Dab-1 via ubiquitination, resulting in termination of Reelin signaling cascade. Downstream eVector proteins involved in Reelin signaling path include phosphatidylinositol-3-kinase (PI3K) and protein kinase B (PKB/Akt), which further impact on three other important molecules, glycogen synthase kinase (GSK-3), -catenin, and tau. The latter proteins can modulate pathways, aVecting cell proliferation, apoptosis, and neurodegeneration respectively. Finally, Reelin has a direct eVect on enhancement of long term potentiation (LTP), via direct involvement of its receptors VLDLR and ApOER2. Alternately, tyrosine phosphorylation of NR2B subunit of NMDA receptor by Fyn kinase is essential for induction of LTP and modulation of synaptic plasticity, potentially converging on Reelin’s role in cognition and memory processing (Fatemi et al., 2001, 2005b).
IV. Reelin in Autism
Analogous defects involving the Reelin signaling system appear to be present in several neuropsychiatric disorders (e.g., schizophrenia (Fatemi et al., 2000, 2005a; Guidotti et al., 2000; Impagnatiello et al., 1998) and autism (Fatemi et al., 2001, 2005b)). The findings of Reelin defects are more robust in autism and are supported by two positive genetic linkage studies (Persico et al., 2001;
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S. HOSSEIN FATEMI
MRNA
Area Area 9
Cerebellum
TABLE I LEVELS FOR REELIN, VLDLR, DAB-1,
AND
GSK3*
Gene of interest (GOI)
GOI relative to age matched control
Reelin VLDLR DAB1 GSK3 Reelin VLDLR DAB1 GSK3
4.7 þ14.2 5.4 þ1.9 3.9 þ2.8 3.4 þ1
P Value p< p< p< NS p< p< p< NS
0.035 0.01 0.01 0.01 0.04 0.001
*Fatemi, S. H., et al. (2005b).
Zhang et al., 2002). Autism, which is a severe childhood disorder of the brain, is characterized by impairments in communication, social skills, and repetitive behavior (Kanner, 1943). Despite a large body of evidence supporting a genetic cause for autism (Folstein and Rosen-Sheidley, 2002), environmental causes are potentially responsible for some cases of autism (Rodier, 2000). A controversial issue is the involvement of Reln in causation of autism. Of the six genetic linkage studies, four deny a link (Bonora et al., 2003; Devlin et al., 2004; Krebs et al., 2002; Li et al., 2004) while two are supportive (Persico et al., 2001; Zhang et al., 2002). Additionally, four recent biochemical reports, however, show reductions in brain (Fatemi et al., 2001, 2005b) and blood (Fatemi et al., 2002a; Lugli et al., 2003). Reelin levels in subjects with autism (Fatemi et al., 2005b) showed that Reelin protein and mRNA species were reduced significantly in area 9 and cerebellum of autistic subjects vs. age and postmortem intervaled-matched controls (see Table I). The reductions in Reelin levels accompanied significant increases in mRNA levels of Reelin receptor VLDL-R in frontal and cerebellar cortices of autistic subjects (Table I). Surprisingly, levels of Dab-1 mRNA were also reduced significantly in the same brain sites in autistic subjects (Table I) implying involvement of the Reelin signaling cascade in the autistic pathology (see Fig. 2).
V. Reelin in Schizophrenia
Schizophrenia, a neurodevelopmental disorder, which in contrast to autism aVects youth in puberty and is manifested by presence of hallucinations, disorganized behavior, and fragmentation of thought (Kraeplin, 1923), may also share
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FIG. 2. The role of Reelin signaling system in autism. Normally, extracellular Reelin is secreted by Cajal-Retzius cells and certain GABAergic cells to bind its receptors VLDLR, ApoER2, and /31 integrin on eVector cells. Following binding of Reelin to its receptors, Dab-1 protein is oligomerized and phosphorylated. In autistic brain, Reelin signaling system appears to be impaired in 3 steps (marked by *); (1) Reelin ligand is not produced adequately as evident by reductions in mRNA and protein levels in superior frontal cortex and cerebellum; (2) Reelin receptor VLDLR mRNA is upregulated potentially in response to reduced levels of its ligand, Reelin; (3) Dab-1 mRNA is also reduced potentially due to reduction in levels of Reelin which normally activates Dab-1 phosphorylation via a positive-feedback loop. Alternatively, Dab-1 levels may be reduced in response to increases in levels of VLDLR acting via a negative-feedback loop. Alterations in levels of Reelin, its receptor VLDLR, and adaptor protein Dab-1 interfere with the Reelin signaling system aVecting LTP, synaptic plasticity, cognition, and memory, modalities involved in autism (Fatemi et al., 2005b).
Reelin abnormalities with autism (Fatemi, 2005). Investigation of the postmortem brains using a multitude of techniques showed downregulation of Reelin protein and mRNA in the prefrontal cortex (Guidotti et al., 2000), and decreased Reelin protein in hippocampus (Fatemi et al., 2000) and cerebellum (Fatemi et al., 2005a) of subjects with schizophrenia. Mechanistically, it appears that hypermethylation of the promoter for Reln may be partially responsible for the observed decreases in Reelin in schizophrenia (Abdolmaleky et al., 2005; Costa et al., 2003). Two genetic linkage studies have not been able to show a significant linkage between Reelin polymorphisms and schizophrenia (Akahane et al., 2002; Chen et al., 2002). The biochemical data also support involvement of Reelin abnormalities in mood disorders (Fatemi et al., 2000, 2005a; Guidotti et al., 2000) regardless of presence of psychosis.
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VI. Conclusions
In summary, increasing biochemical evidence points to involvement of Reelin glycoprotein in a number of psychiatric disorders including schizophrenia, autism, and mood disorders. The disparity seen in levels of Reelin production in man appears to be similar to the same scenario seen in various animal models which aVect Reelin production, leading to production of cognitive deficits in rodents (Fatemi, 2005). Future correlative studies of human postmortem brains in schizophrenia and autism and pertinent animal models of mental disorders may expand our knowledge of the role of Reelin in cognition.
Acknowledgments
The work of the author has been supported by Stanley Medical Research Institute, March of Dimes, National Institute of Child Health and Human Development, The Jonty Foundation, and the Kunin Fund of St. Paul Foundation. I am grateful for technical assistance by Mr. T. Folsom and Ms. T. Reutiman and secretarial assistance by Ms. Laurie Iversen.
References
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Chen, M. L., Chen, S. Y., Huang, C. H., and Chen, C. H. (2002). Identification of a single nucleotide polymorhism at the 5’ promoter region of human reelin gene and association study with schizophrenia. Mol. Psychiatry 7, 447–448. Costa, E., Grayson, D. R., and Guidotti, A. (2003). Epigenetic downregulation of GABAergic function in schizophrenia; potential for pharmacological intervention? Mol. Interv. 3, 220–229. D’Arcangelo, G., Miao, G. G., Chon, S. C., Soares, H. D., Morgan, J. I., and Curran, T. (1995). A protein related to extracellular matrix proteins detected in the mouse mutant reeler. Nature 374, 719–723. D’Arcangelo, G., Homayouni, R., Keshvara, L., Rice, D. S., Sheldon, M., and Curran, T. (1999). Reelin is a ligand for lipoprotein receptors. Neuron 24, 471–479. DeSilva, U., D’ Arcangelo, G., Braden, V. V., Chen, J., Miao, G. G., Currant, T., and Green, E. D. (1997). The human reelin gene: Isolation, sequencing, and mapping on chromosome 7. Genome Res. 7, 157–164. Devlin, B., Bennett, P., Dawson, G., Figlewicz, D. A., Grigorenko, E. L., McMahon, W., Minshew, N., Pauls, D., Smith, M., Spence, M. A., Rodier, P. M., Stodgell, C., and Schellenberg, G. D. (2004). Alleles of a reelin CGG repeat do not convey liability to autism in a sample from the CPEA network. Am. J. Med. Genet. B. Neuropsychiatr. Genet. 126, 46–50. Dulabon, L., Olson, E. C., Taglienti, M. G., Eisenhuth, S., McGrath, B., Walsh, C. A., Kreidberg, J. A., and Anton, E. S. (2000). Reelin binds alpha 3 beta 1 integrin and inhibits neuronal migration. Neuron 27, 33–44. Falconer, D. S. (1951). Two new mutants, Trembler and ‘Reeler’, with neurological actions in the house mouse. J. Genetics 50, 182–201. Fatemi, S. H., Emamian, E. S., Kist, D., Sidwell, R. W., Nakajima, K., Akhter, P., Shier, A., Sheikh, S., and Bailey, K. (1999). Defective corticogenesis and reduction in Reelin immunoreactivity in cortex and hippocampus of prenatally infected neonatal mice. Mol. Psychiatry 4, 145–154. Fatemi, S. H., Earle, J. A., and McMenomy, T. (2000). Reduction in Reelin immunoreactivity in hippocampus of subjects with schizophrenia, bipolar disorder and major depression. Mol. Psychiatry 5, 654–663. Fatemi, S. H., Kroll, J. L., and Stary, J. M. (2001). Altered levels of Reelin and its isoforms in schizophrenia and mood disorders. Neuroreport 12, 3209–3215. Fatemi, S. H., Stary, J. M., and Egan, E. A. (2002a). Reduced blood levels of reelin as a vulnerability factor in pathophysiology of autistic disorder. Cell. Mol. Neurobiol. 22, 139–52. Fatemi, S. H., Earle, J. A., Kanodia, R., Kist, D. A., Emamian, E. S., Patterson, P. H., Shi, L., and Sidwell, R. W. (2002b). Prenatal viral infection leads to pyramidal cell atrophy and macrocephaly in adulthood: Implications for genesis of autism and schizophrenia. Cell. Mol. Neurobiol. 22, 25–33. Fatemi, S. H. (2005). Reelin glycoprotein: Structure, biology and roles in health and disease. Mol. Psychiatry 10, 251–257. Fatemi, S. H., Stary, J. M., Araghi-Niknam, M., and Egan, E. (2005a). GABAergic dysfunction in schizophrenia and mood disorders as reflected by decreased levels of Reelin and GAD 65 & 67 kDa proteins in cerebellum. Schizophr. Res. 72, 109–122. Fatemi, S. H., Snow, A. V., Stary, J. M., Araghi-Niknam, M., Reutiman, T. J., Lee, S., Brooks, A. I., and Pearce, D. A. (2005b). Reelin signaling is impaired in autism. Biol. Psychiatry 57, 777–787. Folstein, S. E., and Rosen-Sheidley, B. (2002). Genetics of autism: Complex etiology for a heterogeneous disorder. Nature Rev. Genetics 2, 943–955. Forster, E., Tielsch, A., Saum, B., Weiss, K. H., Johanssen, C., Graus-Porta, D., Muller, U., and Frotscher, M. (2002). Reelin, disabled 1, and beta 1 integrins are required for the of the radial glial scaffold in the hippocampus. Proc. Natl. Acad. Sci. USA 99, 13178–13183. Goffinet, A. M. (1979). An early developmental defect in the cerebral cortex of the Reelor mouse. Anat. Embryol. 157, 205–218.
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Goffinet, A. M. (1992). The reeler gene: A clue to brain development and evolution. Int. J. Dev. Biol. 36, 101–107. Guidotti, A., Auta, J., Davis, J. M., Di-Giorgi-Gerevini, V., Dwivedi, Y., Grayson, D. R., Impagnatiello, F., Pandey, G., Pesold, C., Sharma, R., Uzunov, D., and Costa, E. (2000). Decrease in Reelin and glutamic acid decarboxylase 67 (GAD 67) expression in schizophrenia and bipolar disorder: A postmortem brain study. Arch. Gen. Psychiatry 57, 1061–1069. Hiesberger, T., Trommsdorff, M., Howell, B. W., Goffinet, A., Mumby, M. C., Cooper, J. A., and Herz, J. (1999). Direct biding of Reelin to VLDL receptor and ApoE receptor 2 induces tyroene phosphorylation of disabled-1 and modulates tau phosphorylation. Neuron 24, 481–489. Ignatova, N., Sindic, C. J. M., and Goffinet, A. M. (2004). Characterization of the various farms of the Reelin protein in the cerebrospinal fluid of normal subjects and in neurological diseases. Neurobiol. Disease 15, 326–330. Impagnatiello, F., Guidotti, A. R., Pesold, C., Dwivedi, Y., Caruncho, H., Pisu, M. G., Uzunov, D. P., Smalheiser, N. R., Davis, J. M., Pandey, G. N., Pappas, G. D., Tueting, P., Sharma, R. P., and Costa, E. (1998). A decrease of Reelin expression as a putative vulnerability factor in schizophrenia. Proc. Natl. Acad. Sci. USA 95, 15718–15723. Janusonis, S., Gluncic, V., and Rakic, P. (2004). Early serotonergic projections to Cajal-Retzius cells: Relevance for cortical development. J. Neuroscience 24, 1652–1659. Kanner, L. (1943). Autistic disturbances of affective contact. J. Nerv. Child 2, 217–250. Kraeplin, E. (1923). ‘‘Psychiatrie,’’ (8th edition). Lepizig, Barth. Krebs, M. O., Betancur, C., Leroy, S., Bourdel, M. C., Gillberg, C., and Leboyer, M. (2002). Paris Autism Research International Sibpair (PARIS) study. Absence of association between a polymorphic GGC repeat in the 50 untranslated region of the reelin gene and autism. Mol. Psychiatry 7, 801–804. Li, J., Nguyen, L., Gleason, C., Lotspeich, L., Spiker, D., Risch, N., and Myers, R. M. (2004). Lack of evidence for an association between WNT2 and RELN polymorphisms and autism. Am. J. Med. Genet. B. Neuropsychiatr. Genet. 126, 51–57. Lugli, G., Krueger, J. M., Davis, J. M., Persico, A. M., Keller, F., and Smalheiser, N. R. (2003). Methodological factors influencing measurement and processing of plasma reelin in humans. BMC Biochemistry 4, 9. Magdaleno, S., Keshvara, L., and Curran, T. (2002). Rescue of ataxia and prepalte splitting by ectopic expression of Reelin in reeler mice. Neuron 33, 573–586. McAlonan, G. M., Daly, E., Kumari, V., Critchley, H. D., van Amelsvoort, T., Suckling, J., Simmons, A., Sigmundsson, T., Greenwood, K., Russell, A., Schmitz, N., Happe, F., Howlin, P., and Murphy, D. G. (2002). Brain anatomy and sensorimotor gating in Asperger’s syndrome. Brain 125, 1594–606. Meincke, U., Light, G. A., Geyer, M. A., Braff, D. L., and Gouzoulis-Mayfrank, E. (2004). Sensitization and habituation of the acoustic startle reflex in patients with schizophrenia. Psychiatry Res. 126, 51–61. Ogawa, M., Miyata, T., Nakajima, K., Yagyu, K., Seike, M., Ikenaka, K., Yamamoto, H., and Mikoshiba, K. (1995). The reeler gene-associated antigen on Cajal–Retzius neurons is a crucial molecule for laminar organization of cortical neurons. Neuron 14, 899–912. Persico, A. M., D’Agruma, L., Maiorano, N., Totaro, A., Militerni, R., Bravaccio, C., Wassink, T. H., Schneider, C., Melmed, R., Trillo, S., Montecchi, F., Palermo, M., Pascucci, T., Puglisi-Allegra, S., Reichelt, K. L., Conciatori, M., Marino, R., Quattrocchi, C. C., Baldi, A., Zelante, L., Gasparini, P., and Keller, F. (2001). Reelin gene alleles and haplotypes as a factor predisposing to autistic disorder. Mol. Psychiatry 6, 150–159. Collaborative Linkage, Study of Autism. Quattrocchi, C. C., Wannenes, F., Persico, A. M., Ciafre, S. A., D’Arcangelo, G., Farace, M. G., and Keller, F. (2002). Reelin is a serine protease of the extracellular matrix. J. Biol. Chem. 277, 303–309.
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Rodier, P. (2000). The early origins of autism. Sci. Am. 282, 56–63. Smalheiser, N. R., Costa, E., Guidotti, A., Impagnatiello, F., Auta, J., Lacor, P., Kriho, V., and Pappas, G. D. (2000). Expression of reelin in adult mammalian blood, liver, pituitary pars intermedia, and adrenal chromaffin cells. Proc. Natl. Acad. Sci. USA 97, 1281–1286. Strasser, V., Fasching, D., Hauser, C., Mayer, H., Bock, H. H., Hiesberger, T., Herz, J., Weeber, E. J., Sweatt, J. D., Pramatarova, A., Howell, B., Schneider, W. J., and Nimpf, J. (2004). Receptor clustering is involved in reelin signaling. Mol. Cell. Biol. 24, 1378–1386. Suetsugu, S., Tezuka, T., Morimura, T., Hattori, M., Mikoshiba, K., Yamamoto, T., and Takenawa, T. (2004). Regulation of actin cytoskeleton by mDab1 through N-WASP and ubiquitination of mDab1. Biochem. J. 384, 1–8. Tueting, P., Costa, E., Dwivedi, Y., Guidotti, A., Impagnatiello, F., Manev, R., and Pesold, C. (1999). The phenotypic characteristics of heterozygous reeler mouse. NeuroReport 10, 1327–1334. Weeber, E. J., Beffert, U., Jones, C., Christian, J. M., Forster, E., Sweatt, J. D., and Herz, J. (2002). Reelin and ApoE receptors cooperate to enhance hippocampal synaptic plasticity and learning. J. Biol. Chem. 277, 39944–39952. Zhang, H., Liu, X., Zhang, C., Mundo, E., Macciardi, F., Grayson, D. R., Guidotti, A. R., and Holden, J. J. (2002). Reelin gene alleles and susceptibility for autism spectrum disorders. Mol. Psychiatry 7, 1012–1017.
S. Hossein Fatemi Department of Psychiatry, University of Minnesota Medical School Minneapolis, Minnesota 55455, USA
I. II. III. IV. V. VI.
Reelin Gene and Protein Reelin Signaling Cascade The Reeler Mouse Reelin in Autism Reelin in Schizophrenia Conclusions References
Reelin glycoprotein is a serine protease with important roles in embryogenesis and in adult life. Reelin mutations or deficiency of the protein product could cause abnormal cortical development and/or Reelin-induced signal transduction impairment in brain. Reelin abnormalities in several neuropsychiatric disorders, such as autism and schizophrenia, may provide mechanistic explanations for etiologies of these disorders.
I. Reelin Gene and Protein
Reelin glycoprotein is a serine protease (Quattrocchi et al., 2002) with dual roles in mammalian brain: embryologically, it guides neurons and radial glial cells to their final positions in brain (Forster et al., 2002; GoYnet, 1992); during adult life, it participates in a signaling cascade which may serve synaptic plasticity, memory processing, and cognition (Fatemi, 2005; Weeber et al., 2002). Reelin gene (Reln) is localized to chromosome 7 in man (DeSilva et al., 1997). The Reln gene codes for protein products which on SDS-PAGE range from 410 to 330, 180kDa and several smaller fragments in man (Fatemi et al., 2002a, 2004, 2005a; Ignatova et al., 2004; Smallheiser et al., 2000) and in rodents (D’Arcangelo et al., 1995; Ogawa et al., 1995).
INTERNATIONAL REVIEW OF NEUROBIOLOGY, VOL. 71 DOI: 10.1016/S0074-7742(05)71008-4
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Copyright 2005, Elsevier Inc. All rights reserved. 0074-7742/05 $35.00
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S. HOSSEIN FATEMI
II. Reelin Signaling Cascade
Reelin protein binds several receptors including apolipoprotein E receptor 2 (ApoER2), very-low-density lipoprotein receptor (VLDLR), and /31 integrin to initiate a signaling cascade which underlies downstream biochemical events leading to synaptic plasticity (see Fig. 1). Binding of Reelin to its receptors, specifically ApoER2 and VLDLR, induces clustering of these receptors and oligomerization of the adaptor protein, disabled-1 (Dab-1) (D’Arcangelo et al., 1999; Dulabon et al., 2000; Hiesberger et al., 1999; Strasser et al., 2004). This is then followed by tyrosine phosphorylation of Dab-1 which causes actin polymerization and final ubiquitination of Dab-1 protein, steps involved in mechanisms behind cell migration and synaptic plasticity (Beffert et al., 2005; Suetsugu et al., 2004). Phosphorylation of Dab-1 can also act as a substrate for inhibition of the level of glycogen synthase-kinase 3 (GSK-3) and modulation of pathways for cell survival and growth (BeVert et al., 2002).
III. The Reeler Mouse
The significance of Reelin’s role in embryogenesis of brain became evident following the discovery of a Reln gene mutation nearly half a century ago (Falconer, 1951). The Reelin mutant mouse, which carries an autosomal recessive mutation in Reln, exhibited ataxia and a reeling gait. Examination of the brain in these animals showed inverted cortical lamination, abnormal positioning of neurons, and aberrant orientation of neuronal cell bodies and nerve fibers (Falconer, 1951; GoYnet, 1979). The mutant mice also exhibited cerebellar hypoplasia with associated lack of foliation (Caviness and Sidman, 1973). Ectopic expression of Reelin in the Reeler mouse rescues cerebellar development and corrects ataxia in the mutant mouse (Magdaleno et al., 2002). The homozygous mutant mouse does not produce Reelin. The heterozygous Reeler mouse has a 50% reduction in Reelin protein and mRNA with decreases in dendritic spine density, neuropil hypoplasticity, and decreased GABA turnover (Carboni et al., 2004). Behaviorally, the heterozygous Reeler mouse exhibits decreased prepulse inhibition, a phenomen observed in subjects with autism and schizophrenia (McAlonan et al., 2002; Meincke et al., 2004; Tueting et al., 1999). Several recent reports using various prenatal insults, e.g., viral infection in midterm pregnant mice (Fatemi et al., 1999, 2002b), and 5 methoxytryptamine exposure in E17 pregnant rats ( Janusonis et al., 2004) cause reductions in levels of brain/blood Reelin levels and result in abnormal corticogenesis in the oVspring.
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FIG. 1. The Reelin signaling system and cognition. Extracellular Reelin glycoprotein is secreted by Cajal-Retzius cells and certain cortical and hippocampal GABAergic cells and cerebellar granule cells. Reelin can bind its receptors ApoER2, VLDLR, and 31 integrin directly, initiating the signaling system in the eVector cells (i.e., cortical pyramidal cells). Reelin induction of the cascade leads to clustering of the receptors causing dimerization/oligomerization of Dab-1 protein and activation of Src-tyrosine kinase family/Fyn-kinase leading to tyrosine phosphorylation of Dab-1 protein in a positive-feedback loop. Interaction between Dab-1, N-WASP, and ARP 2/3 complex causes formation of microspikes or filopodia which are important in processes of cell migration and synaptic plasticity. Finally, phosphorylation of a subpopulation of Dab-1 molecules causes degradation of Dab-1 via ubiquitination, resulting in termination of Reelin signaling cascade. Downstream eVector proteins involved in Reelin signaling path include phosphatidylinositol-3-kinase (PI3K) and protein kinase B (PKB/Akt), which further impact on three other important molecules, glycogen synthase kinase (GSK-3), -catenin, and tau. The latter proteins can modulate pathways, aVecting cell proliferation, apoptosis, and neurodegeneration respectively. Finally, Reelin has a direct eVect on enhancement of long term potentiation (LTP), via direct involvement of its receptors VLDLR and ApOER2. Alternately, tyrosine phosphorylation of NR2B subunit of NMDA receptor by Fyn kinase is essential for induction of LTP and modulation of synaptic plasticity, potentially converging on Reelin’s role in cognition and memory processing (Fatemi et al., 2001, 2005b).
IV. Reelin in Autism
Analogous defects involving the Reelin signaling system appear to be present in several neuropsychiatric disorders (e.g., schizophrenia (Fatemi et al., 2000, 2005a; Guidotti et al., 2000; Impagnatiello et al., 1998) and autism (Fatemi et al., 2001, 2005b)). The findings of Reelin defects are more robust in autism and are supported by two positive genetic linkage studies (Persico et al., 2001;
182
S. HOSSEIN FATEMI
MRNA
Area Area 9
Cerebellum
TABLE I LEVELS FOR REELIN, VLDLR, DAB-1,
AND
GSK3*
Gene of interest (GOI)
GOI relative to age matched control
Reelin VLDLR DAB1 GSK3 Reelin VLDLR DAB1 GSK3
4.7 þ14.2 5.4 þ1.9 3.9 þ2.8 3.4 þ1
P Value p< p< p< NS p< p< p< NS
0.035 0.01 0.01 0.01 0.04 0.001
*Fatemi, S. H., et al. (2005b).
Zhang et al., 2002). Autism, which is a severe childhood disorder of the brain, is characterized by impairments in communication, social skills, and repetitive behavior (Kanner, 1943). Despite a large body of evidence supporting a genetic cause for autism (Folstein and Rosen-Sheidley, 2002), environmental causes are potentially responsible for some cases of autism (Rodier, 2000). A controversial issue is the involvement of Reln in causation of autism. Of the six genetic linkage studies, four deny a link (Bonora et al., 2003; Devlin et al., 2004; Krebs et al., 2002; Li et al., 2004) while two are supportive (Persico et al., 2001; Zhang et al., 2002). Additionally, four recent biochemical reports, however, show reductions in brain (Fatemi et al., 2001, 2005b) and blood (Fatemi et al., 2002a; Lugli et al., 2003). Reelin levels in subjects with autism (Fatemi et al., 2005b) showed that Reelin protein and mRNA species were reduced significantly in area 9 and cerebellum of autistic subjects vs. age and postmortem intervaled-matched controls (see Table I). The reductions in Reelin levels accompanied significant increases in mRNA levels of Reelin receptor VLDL-R in frontal and cerebellar cortices of autistic subjects (Table I). Surprisingly, levels of Dab-1 mRNA were also reduced significantly in the same brain sites in autistic subjects (Table I) implying involvement of the Reelin signaling cascade in the autistic pathology (see Fig. 2).
V. Reelin in Schizophrenia
Schizophrenia, a neurodevelopmental disorder, which in contrast to autism aVects youth in puberty and is manifested by presence of hallucinations, disorganized behavior, and fragmentation of thought (Kraeplin, 1923), may also share
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FIG. 2. The role of Reelin signaling system in autism. Normally, extracellular Reelin is secreted by Cajal-Retzius cells and certain GABAergic cells to bind its receptors VLDLR, ApoER2, and /31 integrin on eVector cells. Following binding of Reelin to its receptors, Dab-1 protein is oligomerized and phosphorylated. In autistic brain, Reelin signaling system appears to be impaired in 3 steps (marked by *); (1) Reelin ligand is not produced adequately as evident by reductions in mRNA and protein levels in superior frontal cortex and cerebellum; (2) Reelin receptor VLDLR mRNA is upregulated potentially in response to reduced levels of its ligand, Reelin; (3) Dab-1 mRNA is also reduced potentially due to reduction in levels of Reelin which normally activates Dab-1 phosphorylation via a positive-feedback loop. Alternatively, Dab-1 levels may be reduced in response to increases in levels of VLDLR acting via a negative-feedback loop. Alterations in levels of Reelin, its receptor VLDLR, and adaptor protein Dab-1 interfere with the Reelin signaling system aVecting LTP, synaptic plasticity, cognition, and memory, modalities involved in autism (Fatemi et al., 2005b).
Reelin abnormalities with autism (Fatemi, 2005). Investigation of the postmortem brains using a multitude of techniques showed downregulation of Reelin protein and mRNA in the prefrontal cortex (Guidotti et al., 2000), and decreased Reelin protein in hippocampus (Fatemi et al., 2000) and cerebellum (Fatemi et al., 2005a) of subjects with schizophrenia. Mechanistically, it appears that hypermethylation of the promoter for Reln may be partially responsible for the observed decreases in Reelin in schizophrenia (Abdolmaleky et al., 2005; Costa et al., 2003). Two genetic linkage studies have not been able to show a significant linkage between Reelin polymorphisms and schizophrenia (Akahane et al., 2002; Chen et al., 2002). The biochemical data also support involvement of Reelin abnormalities in mood disorders (Fatemi et al., 2000, 2005a; Guidotti et al., 2000) regardless of presence of psychosis.
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VI. Conclusions
In summary, increasing biochemical evidence points to involvement of Reelin glycoprotein in a number of psychiatric disorders including schizophrenia, autism, and mood disorders. The disparity seen in levels of Reelin production in man appears to be similar to the same scenario seen in various animal models which aVect Reelin production, leading to production of cognitive deficits in rodents (Fatemi, 2005). Future correlative studies of human postmortem brains in schizophrenia and autism and pertinent animal models of mental disorders may expand our knowledge of the role of Reelin in cognition.
Acknowledgments
The work of the author has been supported by Stanley Medical Research Institute, March of Dimes, National Institute of Child Health and Human Development, The Jonty Foundation, and the Kunin Fund of St. Paul Foundation. I am grateful for technical assistance by Mr. T. Folsom and Ms. T. Reutiman and secretarial assistance by Ms. Laurie Iversen.
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