Product Code Database
Reelin, encoded by the RELN gene, is a large secreted extracellular matrix glycoprotein that helps regulate processes of neuronal migration and positioning in the developing brain by controlling cell–cell interactions. Besides this important role in early development, reelin continues to work in the adult brain. It modulates synaptic plasticity by enhancing the induction and maintenance of long-term potentiation. It also stimulates dendrite and dendritic spine development in the hippocampus, and regulates the continuing migration of generated in adult neurogenesis sites of the subventricular and . It is found not only in the brain but also in the liver, Thyroid, adrenal gland, fallopian tube, breast and in comparatively lower levels across a range of anatomical regions.
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 schizophrenia and psychotic bipolar disorder, but the cause of this observation remains uncertain, as studies show that psychotropic medication itself affects reelin expression. Moreover, Epigenetics hypotheses aimed at explaining the changed levels of reelin expression are controversial. Total lack of reelin causes a form of lissencephaly. Reelin may also play a role in Alzheimer's disease, temporal lobe epilepsy and autism.
Reelin's name comes from the abnormal reeling gait of reeler mice, which were later found to have a deficiency of this brain protein and were Zygosity for mutation of the RELN gene. The primary phenotype associated with loss of reelin function is a failure of neuronal positioning throughout the developing central nervous system (CNS). The mice Zygosity for the reelin gene, while having little neuroanatomical defects, display the endophenotype traits linked to psychotic disorders.
The "reeler" mouse was described for the first time in 1951 by D.S.Falconer in Edinburgh University as a spontaneous variant arising in a colony of at least mildly inbred snowy-white bellied mice stock in 1948. Histopathology studies in the 1960s revealed that the cerebellum of reeler mice is dramatically decreased in size while the normal laminar organization found in several brain regions is disrupted. The 1970s brought about the discovery of cellular layer inversion in the mouse neocortex, which attracted more attention to the reeler mutation.
In 1994, a new allele of reeler was obtained by means of insertional mutagenesis. This provided the first molecular marker of the locus, permitting the RELN gene to be mapped to chromosome 7q22 and subsequently cloned and identified. Japanese scientists at Kochi Medical School successfully raised antibodies against normal brain extracts in reeler mice, later these antibodies were found to be specific 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 VLDLR (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 Gene knockout mice for ApoER2 and VLDLR, which both interact with Dab1, had cortical layering defects similar to those in reeler.
The downstream neural pathway of reelin was further clarified with the help of other mutant mice, including yotari and Scrambler mouse. 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 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.
During the brain development, reelin is secreted in the cortex and hippocampus by the so-called 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 subpial granular layer (SGL), which is manifested to the highest extent in human, and in the hippocampal stratum lacunosum-moleculare and the upper marginal layer of the dentate gyrus.
In the developing cerebellum, reelin is expressed first in the external 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 GABA-ergic of the cortex and glutamatergic cerebellar neurons, the glutamatergic stellate cells and fan cells in the superficial entorhinal cortex that are supposed to carry a role in encoding new episodic memories, and by the few extant Cajal-Retzius cells. Among GABAergic interneurons, reelin seems to be detected predominantly in those expressing calretinin and calbindin, like bitufted neuron, horizontal, and , but not parvalbumin-expressing cells, like chandelier cell or . In the white matter, a minute proportion of interstitial neurons has also been found to stain positive for reelin expression.
Outside the brain, reelin is found in adult mammalian blood, liver, pituitary pars intermedia, and adrenal . In the liver, reelin is localized in 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 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 , 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.
The reelin protein starts with a signaling peptide 27 amino acids in length, followed by a region bearing similarity to F-spondin (the 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 epidermal growth factor motif at their center, dividing each repeat into two subrepeats, A (the BNR/Asp-box repeat) and B (the 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 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 phosphorylation and mimic functions of reelin during cortical plate development. Moreover, the processing of reelin by embryonic neurons may be necessary for proper corticogenesis.
Mammalian 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 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 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 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 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 NMDA receptor configuration, increasing mobility of NR2B-containing receptors and thus decreasing the time they spend at the 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.
Reelin also plays an important role in the adult brain by modulating cortical pyramidal neuron dendritic spine expression density, the branching of , and the expression of long-term potentiation as its secretion is continued diffusely by the GABAergic cortical interneurons those origin is traced to the medial 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.
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 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 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.
As members of lipoprotein receptor superfamily, both VLDLR and ApoER2 have in their structure an internalization domain called NPxY Structural motif. After binding to the receptors reelin is internalized by 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 and . Furthermore, radial glia express the same amount of ApoER2 but being ten times less rich in VLDL receptor. 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 long-term potentiation is caused by ApoER2 interaction 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 open up the RELN gene. The activation of dendrite growth by reelin is apparently conducted through Src family 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 reeler phenotype, indicating that Crk/CrkL lie between DAB1 and Akt in the reelin signaling chain.
The intracellular adaptor DAB1 binds to the VLDLR and ApoER2 through an 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 actin cytoskeleton to change its shape, affecting the proportion of integrin receptors on the cell surface, which leads to the change in Cell adhesion. Phosphorylation of DAB1 leads to its ubiquitination and subsequent degradation, and this explains the heightened levels of DAB1 in the absence of reelin. Such 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 reeler phenotype, and this may indicate that a part of the signal is conducted independently of DAB1.
A protein having an important role in lissencephaly and accordingly called LIS1 (PAFAH1B1), was shown to interact with the intracellular segment of VLDLR, thus reacting to the activation of reelin pathway.
Epigenetics 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 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; Erminio Costa). A postmortem study comparing a 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. Methylation inhibitors and histone deacetylase inhibitors, such as 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 histones 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, 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.
Chromosome region 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 rs7341475 to increase the risk of the disease in women, but not in men. The women that have the 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 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, VLDLR, in the peripheral . After six months of 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 NPAS3, disruption of which is linked to schizophrenia and learning disability. Knockout mice lacking NPAS3 or the similar protein NPAS1 have significantly lower levels of reelin; the precise mechanism behind this is unknown. Another example is the schizophrenia-linked gene 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 Candidate gene. Another shared aspect between NR2B and RELN is that they both can be regulated by the TBR1 transcription factor.
The Zygosity reeler mouse, which is haploinsufficient for the RELN gene, shares several neurochemical and behavioral abnormalities with schizophrenia and bipolar disorder, but the exact relevance of these murine behavioral changes to the pathophysiology of schizophrenia remains debatable.
As previously described, reelin plays a crucial role in modulating early neuroblast migration during brain development. Evidences of altered neural cell positioning in post-mortem schizophrenia patient brains and changes to gene regulatory networks that control cell migration suggests a potential link between altered reelin expression in patient brain tissue to disrupted cell migration during brain development. To model the role of reelin in the context of schizophrenia at a cellular level, olfactory neurosphere-derived cells were generated from the Nose Biopsy of schizophrenia patients, and compared to cells from healthy controls. Schizophrenia patient-derived cells have reduced levels of reelin mRNA and protein when compared to healthy control cells, but expresses the key reelin receptors and DAB1 accessory protein. When grown in vitro, schizophrenia patient-derived cells were unable to respond to reelin coated onto tissue culture surfaces; In contrast, cells derived from healthy controls were able to alter their cell migration when exposed to reelin. This work went on to show that the lack of cell migration response in patient-derived cells were caused by the cell's inability to produce enough of the appropriate size when in contact with extracellular reelin. More research into schizophrenia cell-based models are needed to look at the function of reelin, or lack of, in the pathophysiology of schizophrenia.
In a genetic study conducted in 2009, preliminary evidence requiring further DNA replication suggested that variation of the RELN gene (SNP rs362719) may be associated with susceptibility to bipolar disorder in women.
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 rs362780 RELN polymorphism.
Studies of transgenic mice have been suggestive of an association, but not definitive.
According to the epigenetic hypothesis, drugs that shift the balance in favour of 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 intraperitoneal injections of the following drugs:
| ↔ |
| ↓ |
target="_blank" rel="nofollow"> Human RELN at WikiGenes
|
|
