Reelin

Reelin is a large secreted is_associated_with::extracellular matrix is_associated_with::glycoprotein that helps regulate processes of is_associated_with::neuronal migration and positioning in the developing brain by controlling is_associated_with::cell–cell interactions. Besides this important role in early development, reelin continues to work in the adult brain. It modulates is_associated_with::synaptic plasticity by enhancing the induction and maintenance of is_associated_with::long-term potentiation. It also stimulates dendrite and is_associated_with::dendritic spine development and regulates the continuing migration of is_associated_with::neuroblasts generated in is_associated_with::adult neurogenesis sites like subventricular and is_associated_with::subgranular zones. It is found not only in the is_associated_with::brain, but also in the is_associated_with::spinal cord, is_associated_with::blood, and other body organs and tissues.

Reelin has been suggested to be implicated in pathogenesis of several brain diseases. The expression of the protein has been found to be significantly lower in is_associated_with::schizophrenia and psychotic is_associated_with::bipolar disorder, but the cause of this observation remains uncertain as studies show that psychotropic medication itself affects reelin expression. Moreover, epigenetic hypotheses aimed at explaining the changed levels of reelin expression are controversial. Total lack of reelin causes a form of is_associated_with::lissencephaly. Reelin may also play a role in is_associated_with::Alzheimer's disease, is_associated_with::temporal lobe epilepsy and is_associated_with::autism.

Reelin's name comes from the abnormal reeling is_associated_with::gait of is_associated_with::reeler mice, which were later found to have a deficiency of this brain is_associated_with::protein and were homozygous for mutation of the RELN gene. The primary phenotype associated with loss of reelin function is a failure of neuronal positioning throughout the developing is_associated_with::central nervous system (CNS). The mice heterozygous for the reelin gene, while having little neuroanatomical defects, display the endophenotypic traits linked to psychotic disorders.

Discovery


Mutant mice have provided insight into the underlying molecular mechanisms of the development of the is_associated_with::central nervous system. Useful spontaneous mutations were first identified by scientists who were interested in is_associated_with::motor behavior, and it proved relatively easy to screen is_associated_with::littermates for mice that showed difficulties moving around the cage. A number of such mice were found and given descriptive names such as reeler, weaver, lurcher, nervous, and staggerer.

The "is_associated_with::reeler" mouse was described for the first time in 1951 by D.S.Falconer in is_associated_with::Edinburgh University as a spontaneous variant arising in a colony of mice maintained by geneticist is_associated_with::Charlotte Auerbach. Histopathological studies in the 1960s revealed that the is_associated_with::cerebellum of reeler mice is dramatically decreased in size while the normal laminar organization found in several brain regions is disrupted. The 1970s brought the discovery of cellular layers inversion in the mice neocortex, which attracted more attention to the reeler mutation.

In 1994, a new is_associated_with::allele of reeler was obtained by means of insertional is_associated_with::mutagenesis. This provided the first is_associated_with::molecular marker of the locus, permitting the RELN gene to be mapped to chromosome 7q22 and subsequently cloned and identified. Japanese scientists at is_associated_with::Kochi Medical School successfully raised antibodies against normal brain extracts in reeler mice, later these antibodies were found to be specific is_associated_with::monoclonal antibodies for reelin, and were termed CR-50 (Cajal-Retzius marker 50). They noted that CR-50 reacted specifically with Cajal-Retzius neurons, whose functional role was unknown until then.

The Reelin receptors, apolipoprotein E receptor 2 (ApoER2) and very-low-density lipoprotein receptor (VLDLR), were discovered by Trommsdorff, Herz and colleagues, who initially found that the cytosolic adaptor protein Dab1 interacts with the cytoplasmic domain of LDL receptor family members. They then went on to show that the double knockout mice for ApoER2 and VLDLR, which both interact with Dab1, had cortical layering defects similar to those in reeler.

The downstream pathway of reelin was further clarified with the help of other mutant mice, including is_associated_with::yotari and scrambler. These mutants have phenotypes similar to that of reeler mice, but without mutation in reelin. It was then demonstrated that the mouse disabled homologue 1 (Dab1) gene is responsible for the phenotypes of these mutant mice, as Dab1 protein was absent (yotari) or only barely detectable (scrambler) in these mutants. Targeted disruption of Dab1 also caused a phenotype similar to that of reeler. Pinpointing the is_associated_with::DAB1 as a pivotal regulator of the reelin signaling cascade started the tedious process of deciphering its complex interactions.

There followed a series of speculative reports linking reelin's genetic variation and interactions to schizophrenia, Alzheimer's disease, autism and other highly complex dysfunctions. These and other discoveries, coupled with the perspective of unraveling the evolutionary changes that allowed for the creation of human brain, highly intensified the research. As of 2008, some 13 years after the gene coding the protein was discovered, hundreds of scientific articles address the multiple aspects of its structure and functioning.

Tissue distribution and secretion
Studies show that reelin is absent from is_associated_with::synaptic vesicles and is secreted via constitutive secretory pathway, being stored in Golgi secretory vesicles. Reelin's release rate is not regulated by is_associated_with::depolarization, but strictly depends on its synthesis rate. This relationship is similar to that reported for the secretion of other is_associated_with::extracellular matrix proteins.

During the brain development, reelin is secreted in the cortex and hippocampus by the so-called is_associated_with::Cajal-Retzius cells, Cajal cells, and Retzius cells. Reelin-expressing cells in the prenatal and early postnatal brain are predominantly found in the marginal zone (MZ) of the cortex and in the temporary is_associated_with::subpial granular layer (SGL), which is manifested to the highest extent in human, and in the hippocampal is_associated_with::stratum lacunosum-moleculare and the upper marginal layer of the is_associated_with::dentate gyrus.

In the developing is_associated_with::cerebellum, reelin is expressed first in the external is_associated_with::granule cell layer (EGL), before the granule cell migration to the internal granule cell layer (IGL) takes place.

Having peaked just after the birth, the synthesis of reelin subsequently goes down sharply, becoming more diffuse compared with the distinctly laminar expression in the developing brain. In the adult brain, reelin is expressed by is_associated_with::GABA-ergic is_associated_with::interneurons of the cortex and glutamatergic cerebellar neurons, and by the few extant Cajal-Retzius cells. Among GABAergic interneurons, reelin seems to be detected predominantly in those expressing is_associated_with::calretinin and is_associated_with::calbindin, like bitufted, horizontal, and is_associated_with::Martinotti cells, but not is_associated_with::parvalbumin-expressing cells, like chandelier or is_associated_with::basket neurons. In the white matter, a minute proportion of is_associated_with::interstitial neurons has also been found to stain positive for reelin expression.

Outside the brain, reelin is found in adult mammalian blood, is_associated_with::liver, pituitary is_associated_with::pars intermedia, and adrenal is_associated_with::chromaffin cells. In the liver, reelin is localized in is_associated_with::hepatic stellate cells. The expression of reelin increases when the liver is damaged, and returns to normal following its repair. In the eyes, reelin is secreted by is_associated_with::retinal ganglion cells and is also found in the endothelial layer of the cornea. Just as in the liver, its expression increases after an injury has taken place.

The protein is also produced by the is_associated_with::odontoblasts, which are cells at the margins of the dental pulp. Reelin is found here both during odontogenesis and in the mature tooth. Some authors suggest that odontoblasts play an additional role as sensory cells able to transduce pain signals to the nerve endings. According to the hypothesis, reelin participates in the process by enhancing the contact between odontoblasts and the nerve terminals.

Structure
Reelin is composed of 3461 amino acids with a relative molecular mass of 388 kDa. It also has is_associated_with::serine protease activity. Murine RELN gene consists of 65 is_associated_with::exons spanning approximately 450 kb. One exon, coding for only two amino acids near the protein's is_associated_with::C-terminus, undergoes is_associated_with::alternative splicing, but the exact functional impact of this is unknown. Two transcription initiation sites and two polyadenylation sites are identified in the gene structure.

The reelin protein starts with a signaling peptide 27 amino acids in length, followed by a region bearing similarity to F-spondin (the is_associated_with::reeler domain), marked as "SP" on the scheme, and by a region unique to reelin, marked as "H". Next comes 8 repeats of 300–350 amino acids. These are called reelin repeats and have an is_associated_with::epidermal growth factor motif at their center, dividing each repeat into two subrepeats, A (the is_associated_with::BNR/Asp-box repeat) and B (the is_associated_with::EGF-like domain). Despite this interruption, the two subdomains make direct contact, resulting in a compact overall structure.

The final reelin domain contains a highly basic and short C-terminal region (CTR, marked "+") with a length of 32 amino acids. This region is highly conserved, being 100% identical in all investigated mammals. It was thought that CTR is necessary for reelin secretion, because the Orleans is_associated_with::reeler mutation, which lacks a part of 8th repeat and the whole CTR, is unable to secrete the misshaped protein, leading to its concentration in cytoplasm. However, other studies have shown that the CTR is not essential for secretion itself, but mutants lacking the CTR were much less efficient in activating downstream signaling events.

Reelin is cleaved in vivo at two sites located after domains 2 and 6 – approximately between repeats 2 and 3 and between repeats 6 and 7, resulting in the production of three fragments. This splitting does not decrease the protein's activity, as constructs made of the predicted central fragments (repeats 3–6) bind to lipoprotein receptors, trigger Dab1 is_associated_with::phosphorylation and mimic functions of reelin during is_associated_with::cortical plate development. Moreover, the processing of reelin by embryonic neurons may be necessary for proper corticogenesis.

Function
The primary functions of Reelin are the regulation of corticogenesis and neuronal cell positioning in the prenatal period, but the protein also continues to play a role in adults. Reelin is found in numerous tissues and organs, and one could roughly subdivide its functional roles by the time of expression and by localisation of its action.

During development
A number of non-nervous tissues and organs express reelin during development, with the expression sharply going down after organs have been formed. The role of the protein here is largely unexplored, because the knockout mice show no major pathology in these organs. Reelin's role in the growing central nervous system has been extensively characterized. It promotes the differentiation of progenitor cells into is_associated_with::radial glia and affects the orientation of its fibers, which serve as the guides for the migrating neuroblasts. The position of reelin-secreting cell layer is important, because the fibers orient themselves in the direction of its higher concentration. For example, reelin regulates the development of layer-specific connections in hippocampus and entorhinal cortex.



Mammalian is_associated_with::corticogenesis is another process where reelin plays a major role. In this process the temporary layer called preplate is split into the marginal zone on the top and subplate below, and the space between them is populated by neuronal layers in the inside-out pattern. Such an arrangement, where the newly created neurons pass through the settled layers and position themselves one step above, is a distinguishing feature of mammalian brain, in contrast to the evolutionary older reptile cortex, in which layers are positioned in an "outside-in" fashion. When reelin is absent, like in the mutant is_associated_with::reeler mouse, the order of cortical layering becomes roughly inverted, with younger neurons finding themselves to be unable to pass the settled layers. Subplate neurons fail to stop and invade the upper most layer, creating the so-called superplate in which they mix with is_associated_with::Cajal-Retzius cells and some cells normally destined for the second layer.



There is no agreement concerning the role of reelin in the proper positioning of cortical layers. The original hypothesis, that the protein is a stop signal for the migrating cells, is supported by its ability to induce the dissociation, its role in asserting the compact granule cell layer in the hippocampus, and by the fact  that migrating neuroblasts evade the reelin-rich areas. But an experiment in which murine corticogenesis went normally despite the malpositioned reelin secreting layer, and lack of evidence that reelin affects the growth cones and leading edges of neurons, caused some additional hypotheses to be proposed. According to one of them, reelin makes the cells more susceptible to some yet undescribed positional signaling cascade.

Reelin may also ensure correct neuronal positioning in the is_associated_with::spinal cord: according to one study, location and level of its expression affects the movement of sympathetic preganglionic neurons.

The protein is thought to act on migrating neuronal precursors and thus controls correct cell positioning in the cortex and other brain structures. The proposed role is one of a dissociation signal for neuronal groups, allowing them to separate and go from tangential chain-migration to radial individual migration. Dissociation detaches migrating neurons from the is_associated_with::glial cells that are acting as their guides, converting them into individual cells that can strike out alone to find their final position.



Reelin takes part in the developmental change of is_associated_with::NMDA receptor configuration, increasing mobility of is_associated_with::NR2B-containing receptors and thus decreasing the time they spend at the is_associated_with::synapse. It has been hypothesized that this may be a part of the mechanism behind the "NR2B-NR2A switch" that is observed in the brain during its postnatal development. Ongoing reelin secretion by GABAergic hippocampal neurons is necessary to keep NR2B-containing NMDA receptors at a low level.

In adults
In the adult nervous system, reelin plays an eminent role at the two most active neurogenesis sites, the subventricular zone and the dentate gyrus. In some species, the neuroblasts from the subventricular zone migrate in chains in the is_associated_with::rostral migratory stream (RMS) to reach the olfactory bulb, where reelin dissociates them into individual cells that are able to migrate further individually. They change their mode of migration from tangential to radial, and begin using the radial glia fibers as their guides. There are studies showing that along the RMS itself the two receptors, is_associated_with::ApoER2 and is_associated_with::VLDLR, and their intracellular adapter is_associated_with::DAB1 function independently of Reelin, most likely by the influence of a newly proposed ligand, is_associated_with::thrombospondin-1. In the adult dentate gyrus, reelin provides guidance cues for new neurons that are constantly arriving to the granule cell layer from subgranular zone, keeping the layer compact.

Reelin also plays an important role in the adult brain by modulating cortical pyramidal neuron is_associated_with::dendritic spine expression density, the branching of is_associated_with::dendrites, and the expression of is_associated_with::long-term potentiation as its secretion is continued diffusely by the GABAergic cortical interneurons those origin is traced to the medial is_associated_with::ganglionic eminence.

In the adult organism the non-neural expression is much less widespread, but goes up sharply when some organs are injured. The exact function of reelin upregulation following an injury is still being researched.

Evolutionary significance
Reelin-DAB1 interactions could have played a key role in the structural evolution of the cortex that evolved from a single layer in the common predecessor of the is_associated_with::amniotes into multiple-layered cortex of contemporary mammals. Research shows that reelin expression goes up as the cortex becomes more complex, reaching the maximum in the human brain in which the reelin-secreting Cajal-Retzius cells have significantly more complex axonal arbour. Reelin is present in the telencephalon of all the vertebrates studied so far, but the pattern of expression differs widely. For example, is_associated_with::zebrafish have no Cajal-Retzius cells at all; instead, the protein is being secreted by other neurons. These cells do not form a dedicated layer in amphibians, and radial migration in their brains is very weak.

As the cortex becomes more complex and convoluted, migration along the radial glia fibers becomes more important for the proper lamination. The emergence of a distinct reelin-secreting layer is thought to play an important role in this evolution. There are conflicting data concerning the importance of this layer, and these are explained in the literature either by the existence of an additional signaling positional mechanism that interacts with the reelin cascade, or by the assumption that mice that are used in such experiments have redundant secretion of reelin compared with more localized synthesis in the human brain.

Cajal-Retzius cells, most of which disappear around the time of birth, coexpress reelin with the is_associated_with::HAR1 gene that is thought to have undergone the most significant evolutionary change in humans compared with chimpanzee, being the most "evolutionary accelerated" of the genes from the is_associated_with::human accelerated regions. There is also evidence of that variants in the DAB1 gene have been included in a recent selective sweep in Chinese populations.

Receptors
Reelin's control of cell–cell interactions is thought to be mediated by binding of reelin to the two members of is_associated_with::low density lipoprotein receptor gene family: VLDLR and the ApoER2. The two main reelin receptors seem to have slightly different roles: VLDLR conducts the stop signal, while ApoER2 is essential for the migration of late-born neocortical neurons. It also has been shown that the N-terminal region of reelin, a site distinct from the region of reelin shown to associate with VLDLR/ApoER2 binds to the alpha-3-beta-1 is_associated_with::integrin receptor. The proposal that the protois_associated_with::cadherin CNR1 behaves as a Reelin receptor has been disproven.

As members of lipoprotein receptor superfamily, both VLDLR and ApoER2 have in their structure an internalization domain called is_associated_with::NPxY motif. After binding to the receptors reelin is internalized by is_associated_with::endocytosis, and the N-terminal fragment of the protein is re-secreted. This fragment may serve postnatally to prevent apical dendrites of cortical layer II/III pyramidal neurons from overgrowth, acting via a pathway independent of canonical reelin receptors.

Reelin receptors are present on both is_associated_with::neurons and is_associated_with::glial cells. Furthermore is_associated_with::radial glia express the same amount of ApoER2 but being ten times less rich in VLDLR. beta-1 integrin receptors on glial cells play more important role in neuronal layering than the same receptors on the migrating neuroblasts.

Reelin-dependent strengthening of is_associated_with::long-term potentiation is caused by is_associated_with::ApoER2 interaction with is_associated_with::NMDA receptor. This interaction happens when ApoER2 has a region coded by exon 19. ApoER2 gene is alternatively spliced, with the exon 19-containing variant more actively produced during periods of activity. According to one study, the hippocampal reelin expression rapidly goes up when there is need to store a memory, as is_associated_with::demethylases open up the RELN gene. The activation of dendrite growth by reelin is apparently conducted through Src family is_associated_with::kinases and is dependent upon the expression of Crk family proteins, consistent with the interaction of Crk and CrkL with tyrosine-phosphorylated Dab1. Moreover, a Cre-loxP recombination mouse model that lacks Crk and CrkL in most neurons was reported to have the is_associated_with::reeler phenotype, indicating that Crk/CrkL lie between is_associated_with::DAB1 and Akt in the reelin signaling chain.

Signaling cascades
Reelin activates the signaling cascade of Notch-1, inducing the expression of is_associated_with::FABP7 and prompting progenitor cells to assume is_associated_with::radial glial phenotype. In addition, corticogenesis in vivo is highly dependent upon reelin being processed by embrionic neurons, which are thought to secrete some as yet unidentified is_associated_with::metalloproteinases that free the central signal-competent part of the protein. Some other unknown proteolytic mechanisms may also play a role. It is supposed that full-sized reelin sticks to the extracellular matrix fibers on the higher levels, and the central fragments, as they are being freed up by the breaking up of reelin, are able to permeate into the lower levels. It is possible that as is_associated_with::neuroblasts reach the higher levels they stop their migration either because of the heightened combined expression of all forms of reelin, or due to the peculiar mode of action of the full-sized reelin molecules and its homodimers.

The intracellular adaptor is_associated_with::DAB1 binds to the VLDLR and ApoER2 through an is_associated_with::NPxY motif and is involved in transmission of Reelin signals through these lipoprotein receptors. It becomes phosphorylated by Src and Fyn kinases and apparently stimulates the is_associated_with::actin cytoskeleton to change its shape, affecting the proportion of integrin receptors on the cell surface, which leads to the change in adhesion. Phosphorylation of DAB1 leads to its is_associated_with::ubiquitination and subsequent degradation, and this explains the heightened levels of DAB1 in the absence of reelin. Such is_associated_with::negative feedback is thought to be important for proper cortical lamination. Activated by two antibodies, VLDLR and ApoER2 cause DAB1 phosphorylation but seemingly without the subsequent degradation and without rescuing the is_associated_with::reeler phenotype, and this may indicate that a part of the signal is conducted independently of DAB1.



A protein having an important role in is_associated_with::lissencephaly and accordingly called is_associated_with::LIS1 (is_associated_with::PAFAH1B1), was shown to interact with the intracellular segment of VLDLR, thus reacting to the activation of reelin pathway.

Complexes
Reelin molecules have been shown to form a large protein complex, a disulfide-linked is_associated_with::homodimer. If the homodimer fails to form, efficient tyrosine is_associated_with::phosphorylation of DAB1 in vitro fails. Moreover, the two main receptors of reelin are able to form clusters that most probably play a major role in the signaling, causing the intracellular adaptor DAB1 to dimerize or oligomerize in its turn. Such clustering has been shown in the study to activate the signaling chain even in the absence of Reelin itself. In addition, reelin itself can cut the peptide bonds holding other proteins together, being a is_associated_with::serine protease, and this may affect the cellular adhesion and migration processes. Reelin signaling leads to phosphorylation of is_associated_with::actin-interacting protein is_associated_with::cofilin 1 at ser3; this may stabilize the actin cytoskeleton and anchor the leading processes of migrating neuroblasts, preventing their further growth.

Interaction with Cdk5
is_associated_with::Cyclin-dependent kinase 5 (Cdk5), a major regulator of neuronal migration and positioning, is known to phosphorylate is_associated_with::DAB1  and other cytosolic targets of reelin signaling, such as Tau, which could be activated also via reelin-induced deactivation of is_associated_with::GSK3B, and NUDEL, associated with Lis1, one of the DAB1 targets. LTP induction by reelin in hippocampal slices fails in p35 knockouts. P35 is a key Cdk5 activator, and double p35/Dab1, p35/RELN, p35/ApoER2, p35/VLDLR knockouts display increased neuronal migration deficits, indicating a synergistic action of reelin → ApoER2/VLDLR → DAB1 and p35/p39 → Cdk5 pathways in the normal corticogenesis.

Lissencephaly
Disruptions of the RELN gene are considered to be the cause of the rare form of is_associated_with::lissencephaly with is_associated_with::cerebellar hypoplasia called is_associated_with::Norman-Roberts syndrome. The mutations disrupt splicing of the RELN mRNA transcript, resulting in low or undetectable amounts of reelin protein. The is_associated_with::phenotype in these patients was characterized by is_associated_with::hypotonia, is_associated_with::ataxia, and developmental delay, with lack of unsupported sitting and profound mental retardation with little or no language development. Seizures and is_associated_with::congenital lymphedema are also present. A novel is_associated_with::chromosomal translocation causing the syndrome was described in 2007. The mutations affecting reelin in human are usually associated with consanguineous marriage.

Schizophrenia
Reduced expression of reelin and its mRNA levels in the brains of is_associated_with::schizophrenia sufferers had been reported in 1998 and 2000 and independently confirmed in the postmortem studies of hippocampus, is_associated_with::cerebellum, is_associated_with::basal ganglia, and in the cortex studies. The reduction may reach up to 50% in some brain regions and is coupled with reduced expression of is_associated_with::GAD-67 is_associated_with::enzyme, which catalyses the transition of is_associated_with::glutamate to is_associated_with::GABA. Blood levels of reelin and its is_associated_with::isoforms are also altered in schizophrenia, along with is_associated_with::mood disorders, according to one study. Reduced reelin mRNA prefrontal expression in schizophrenia was found to be the most statistically relevant disturbance found in the multicenter study conducted in 14 separate laboratories in 2001 by Stanley Foundation Neuropathology Consortium.

Epigenetic hypermethylation of DNA in schizophrenia patients is proposed as a cause of the reduction, in agreement with the observations dating from the 1960s that administration of is_associated_with::methionine to schizophrenic patients results in a profound exacerbation of schizophrenia symptoms in sixty to seventy percent of patients. The proposed mechanism is a part of the "epigenetic hypothesis for schizophrenia pathophysiology" formulated by a group of scientists in 2008 (D. Grayson; A. Guidotti; E. Costa). A postmortem study comparing a is_associated_with::DNA methyltransferase (DNMT1) and Reelin mRNA expression in cortical layers I and V of schizophrenic patients and normal controls demonstrated that in the layer V both DNMT1 and Reelin levels were normal, while in the layer I DNMT1 was threefold higher, probably leading to the twofold decrease in the Reelin expression. There is evidence that the change is selective, and DNMT1 is overexpressed in reelin-secreting GABAergic neurons but not in their glutamatergic neighbours. is_associated_with::Methylation inhibitors and is_associated_with::histone deacetylase inhibitors, such as is_associated_with::valproic acid, increase reelin mRNA levels,  while L-methionine treatment downregulates the phenotypic expression of reelin. One study indicated the upregulation of histone deacetylase HDAC1 in the hippocampi of patients. Histone deacetylases suppress gene promoters; hyperacetylation of hystones was shown in murine models to demethylate the promoters of both reelin and GAD67. DNMT1 inhibitors in animals have been shown to increase the expression of both reelin and GAD67, and both DNMT inhibitors and HDAC inhibitors shown in one study to activate both genes with comparable dose- and time-dependence. As one study shows, is_associated_with::S-adenosyl methionine (SAM) concentration in patients' prefrontal cortex is twice as high as in the cortices of non-affected people. SAM, being a methyl group donor necessary for DNMT activity, could further shift epigenetic control of gene expression.

The factors mentioned above serve to corroborate the epigenetic hypothesis. But it is worth mentioning that in contrast with initial data, two recent studies have failed to confirm the RELN hypermethylation, and psychotropic medication could in itself affect the reelin expression in the brain, as animal studies show (see below).

Other interesting findings probably linking reelin pathway to developmental hypotheses of schizophrenia are noted in the studies on mice that are either prenatally infected with is_associated_with::influenza virus or have their immune system activated artificially during pregnancy. The is_associated_with::Cajal-Retzius cells in the newborns secrete significantly less reelin despite keeping their expression of is_associated_with::calretinin and nNos within normal range. These data run in parallel with the findings of increased risk of schizophrenia in humans after a prenatal infection during the second trimester.

Chromosome region is_associated_with::7q22 that harbours the RELN gene is associated with schizophrenia, and the gene itself was associated with the disease in a large study that found the polymorphism is_associated_with::rs7341475 to increase the risk of the disease in women, but not in men. The women that have the is_associated_with::single-nucleotide polymorphism (SNP) are about 1.4 times more likely to get ill, according to the study. Allelic variations of RELN have also been correlated with working memory, memory and executive functioning in nuclear families where one of the members suffers from schizophrenia. The association with working memory was later replicated. In one small study, nonsynonymous polymorphism is_associated_with::Val997Leu of the gene was associated with left and right ventricular enlargement in patients.

One study showed that patients have decreased levels of one of reelin receptors, is_associated_with::VLDLR, in the peripheral is_associated_with::lymphocytes. After six months of is_associated_with::antipsychotic therapy the expression went up; according to authors, peripheral VLRLR levels may serve as a reliable peripheral biomarker of schizophrenia.

Considering the role of reelin in promoting dendritogenesis, suggestions were made that the localized dendritic spine deficit observed in schizophrenia could be in part connected with the downregulation of reelin.

Reelin pathway could also be linked to schizophrenia and other psychotic disorders through its interaction with risk genes. One example is the neuronal transcription factor is_associated_with::NPAS3, disruption of which is linked to schizophrenia and learning disability. Knockout mice lacking NPAS3 or the similar protein is_associated_with::NPAS1 have significantly lower levels of reelin; the precise mechanism behind this is unknown. Another example is the schizophrenia-linked gene is_associated_with::MTHFR, with murine knockouts showing decreased levels of reelin in the cerebellum. Along the same line, it is worth noting that the gene coding for the subunit NR2B that is presumably affected by reelin in the process of NR2B->NR2A developmental change of NMDA receptor composition, stands as one of the strongest risk gene candidates. Another shared aspect between NR2B and RELN is that they both can be regulated by the is_associated_with::TBR1 transcription factor.

The heterozygous reeler mouse, which is haploinsufficient for the RELN gene, shares several neurochemical and behavioral abnormalities with schizophrenia and bipolar disorder, but is considered not suitable for use as a genetic mouse model of schizophrenia.

Bipolar disorder
Decrease in RELN expression with concurrent upregulation of is_associated_with::DNMT1 is typical of is_associated_with::bipolar disorder with psychosis, but is not characteristic of patients with major depression without psychosis, which could speak of specific association of the change with psychoses. One study suggests that unlike in schizophrenia, such changes are found only in the cortex and do not affect the deeper structures in psychotic bipolar patients, as their basal ganglia were found to have the normal levels of DNMT1 and subsequently both the reelin and GAD67 levels were within the normal range.

In a genetic study conducted in 2009, preliminary evidence requiring further is_associated_with::DNA replication suggested that variation of the RELN is_associated_with::gene (SNP is_associated_with::rs362719) may be associated with susceptibility to is_associated_with::bipolar disorder in women.

Autism
Autism is a is_associated_with::neurodevelopmental disorder that is generally believed to be caused by mutations in several locations, likely triggered by environmental factors. The role of reelin in autism is not decided yet.

Reelin was originally in 2001 implicated in a study finding associations between autism and a polymorphic GGC/CGG repeat preceding the 5' ATG initiator codon of the RELN gene in an Italian population. Longer triplet repeats in the 5’ region were associated with an increase in autism susceptibility. However, another study of 125 multiple-incidence families and 68 single-incidence families from the subsequent year found no significant difference between the length of the polymorphic repeats in affected and controls. Although, using a family based association test larger reelin alleles were found to be transmitted more frequently than expected to affected children. An additional study examining 158 subjects with German lineage likewise found no evidence of triplet repeat polymorphisms associated with autism. And a larger study from 2004 consisting of 395 families found no association between autistic subjects and the CGG triplet repeat as well as the allele size when compared to age of first word. In 2010 a large study using data from 4 European cohorts would find some evidence for an association between autism and the is_associated_with::rs362780 RELN polymorphism.

Studies of transgenic mice have been suggestive of an association, but not definitive.

Temporal lobe epilepsy: granule cell dispersion
Decreased reelin expression in the hippocampal tissue samples from patients with is_associated_with::temporal lobe epilepsy was found to be directly correlated with the extent of is_associated_with::granule cell dispersion (GCD), a major feature of the disease that is noted in 45%–73% of patients. The dispersion, according to a small study, is associated with the RELN promoter hypermethylation. According to one study, prolonged seizures in a rat model of mesial temporal lobe epilepsy have led to the loss of reelin-expressing interneurons and subsequent ectopic chain migration and aberrant integration of newborn dentate granule cells. Without reelin, the chain-migrating neuroblasts failed to detach properly. Moreover, in a is_associated_with::kainate-induced mouse epilepsy model, exogenous reelin had prevented GCD, according to one study.

Alzheimer's disease
The Reelin receptors is_associated_with::ApoER2 and is_associated_with::VLDLR belong to the is_associated_with::LDL receptor gene family. All members of this family are receptors for is_associated_with::Apolipoprotein E (ApoE). Therefore, they are often synonymously referred to as 'ApoE receptors'. ApoE occurs in 3 common isoforms (E2, E3, E4) in the human population. is_associated_with::ApoE4 is the primary genetic risk factor for late-onset is_associated_with::Alzheimer's disease. This strong genetic association has led to the proposal that ApoE receptors play a central role in the pathogenesis of Alzheimer's Disease. According to one study, reelin expression and is_associated_with::glycosylation patterns are altered in is_associated_with::Alzheimer's disease. In the cortex of the patients, reelin levels were 40% higher compared with controls, but the cerebellar levels of the protein remain normal in the same patients. This finding is in agreement with an earlier study showing the presence of Reelin associated with amyloid plaques in a transgenic AD mouse model. A large genetic study of 2008 showed that RELN gene variation is associated with an increased risk of Alzheimer's disease in women. The number of reelin-producing Cajal-Retzius cells is significantly decreased in the first cortical layer of patients. Reelin has been shown to interact with is_associated_with::amyloid precursor protein, and, according to one in-vitro study, is able to counteract the Aβ-induced dampening of is_associated_with::NMDA-receptor activity. This is modulated by ApoE isoforms, which selectively alter the recycling of ApoER2 as well as AMPA and NMDA receptors.

Cancer
is_associated_with::DNA methylation patterns are often changed in tumours, and the RELN gene could be affected: according to one study, in the is_associated_with::pancreatic cancer the expression is suppressed, along with other reelin pathway components In the same study, cutting the reelin pathway in cancer cells that still expressed reelin resulted in increased motility and invasiveness. On the contrary, in is_associated_with::prostate cancer the RELN expression is excessive and correlates with is_associated_with::Gleason score. is_associated_with::Retinoblastoma presents another example of RELN overexpression. This gene has also been seen recurrently mutated in cases of is_associated_with::acute lymphoblastic leukaemia.

Other conditions
One is_associated_with::genome-wide association study indicates a possible role for RELN gene variation in is_associated_with::otosclerosis, an abnormal growth of bone of the is_associated_with::middle ear. In a statistical search for the genes that are differentially expressed in the brains of cerebral malaria-resistant versus cerebral malaria-susceptible mice, Delahaye et al. detected a significant upregulation of both RELN and is_associated_with::DAB1 and speculated on possible protective effects of such over-expression.

Factors affecting reelin expression
The expression of reelin is controlled by a number of factors besides the sheer number of Cajal-Retzius cells. For example, is_associated_with::TBR1 transcription factor regulates RELN along with other is_associated_with::T-element-containing genes. On a higher level, increased maternal care was found to correlate with reelin expression in rat pups; such correlation was reported in hippocampus and in the cortex. According to one report, prolonged exposure to is_associated_with::corticosterone significantly decreased reelin expression in murine hippocampi, a finding possibly pertinent to the hypothetical role of is_associated_with::corticosteroids in depression. One small postmortem study has found increased methylation of RELN gene in the neocortex of persons past their puberty compared with those that had yet to enter the period of maturation.

Psychotropic medication
As reelin is being implicated in a number of brain disorders and its expression is usually measured posthumously, assessing the possible medication effects is important.

According to the epigenetic hypothesis, drugs that shift the balance in favour of is_associated_with::demethylation have a potential to alleviate the proposed methylation-caused downregulation of RELN and GAD67. In one study, clozapine and sulpiride but not haloperidol and olanzapine were shown to increase the demethylation of both genes in mice pretreated with l-methionine. Valproic acid, a histone deacetylase inhibitor, when taken in combination with antipsychotics, is proposed to have some benefits. But there are studies conflicting the main premise of the epigenetic hypothesis, and a study by Fatemi et al. shows no increase in RELN expression by valproic acid; that indicates the need for further investigation.

Fatemi et al. conducted the study in which RELN mRNA and reelin protein levels were measured in rat prefrontal cortex following a 21-day of is_associated_with::intraperitoneal injections of the following drugs:

In 2009, Fatemi et al. published the more detailed work on rats using the same medication. Here, cortical expression of several participants (is_associated_with::VLDLR, is_associated_with::DAB1, is_associated_with::GSK3B) of the signaling chain was measured besides reelin itself, and also the expression of is_associated_with::GAD65 and is_associated_with::GAD67.

Recommended reading

 * The book:
 * A review:

Articles, publications, webpages

 * Human RELN at WikiGenes
 * Human RELN at WikiGenes

Figures and images

 * – A figure from
 * – A figure from
 * – A figure from
 * A figure from
 * – A figure from Hong et al.
 * – A figure from Hong et al.