page_title: Reelin
section_title: Evolutionary significance
hierarchical_section_title: Reelin / Evolutionary significance
caption_attribution_description: English: Schematic illustration of differences in neuronal specification and migration patterns between the mammalian and avian pallium. In the developing mammalian telencephalon, Reelin-positive neurons are derived from several origins including ventral pallium, and Er81 and Brn2-positive neurons are isotopically generated from entire pallial regions in a temporal manner. In contrast, in the developing avian telencephalon, Reelin-positive neurons are not derived from the ventral pallium, and Er81 and Brn2-positive neurons are generated from distinct pallial regions in a spatially regulated manner. Русский: Схематическая иллюстрация отличий процессов нейрональной спецификации и паттернов миграции у млекопитающих и птиц. В развивающемся конечном мозге млекопитающих, рилин-позитивные нейроны берут начало в нескольких областях включая вентральный паллиум, а Er81-и Brn2-позитивные нейроны изотопически генерируются по всему вентральному паллиуму, с разнесением по времени. Напротив, в развивающемся конечном мозге птиц, рилин-позитивные нейроны не генерируются вентральным паллиумом, а Er81- и Brn2-позитивные нейроны возникают в специфических участках паллиума, и этот процесс регулируется не временем, а пространственным положением.
context_page_description: Reelin 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 and regulates the continuing migration of neuroblasts generated in adult neurogenesis sites like the subventricular and subgranular zones. It is found not only in the brain but also in the liver, thyroid gland, 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.
context_section_description: 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 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, 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 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.
caption: Neuronal development: mammals (left) and avians (right) have different patterns of reelin expression (pink). Nomura T. et al., 2008.[54]
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