neural development in humans
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Neural development in humansFrom Wikipedia, the free encyclopedia
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The study of neural development draws on both neuroscience and developmental
biology to describe the cellular and molecular mechanisms by which complex nervous
systems emerge during embryonic development and throughout life.
Some landmarks of embryonic neural development include the birth and differentiationof neurons from stem cell precursors, the migration of immature neurons from their
birthplaces in the embryo to their final positions, outgrowth of axons from neurons and
guidance of the motile growth cone through the embryo towards postsynaptic partners,
the generation of synapses between these axons and their postsynaptic partners, andfinally the lifelong changes in synapses which are thought to underlie learning and
memory.
Typically, these neurodevelopmental processes can be broadly divided into two classes:activity-independent mechanisms and activity-dependent mechanisms. Activity-
independent mechanisms are generally believed to occur as hardwired processes
determined by genetic programs played out within individual neurons. These include
differentiation, migration and axon guidance to their initial target areas. These processesare thought of as being independent of neural activity and sensory experience. Once
axons reach their target areas, activity-dependent mechanisms come into play. Neural
activity and sensory experience will mediate formation of new synapses, as well assynaptic plasticity, which will be responsible for refinement of the nascent neural circuits.
The embryonic and fetal brains of all mammals develop in similar ways. Theembryonic spinal cord develops along common sequences and patterns. The nervous
system emerges from a simple elongated tube of cells, called the notochord. The
head (cranial) end of the embryonic tube expands and differentiates more robustly(than does the spinal end) into several clusters of cells which emerge as theforebrain (telencephalon and diencephalon), midbrain (mesencephalon) and
hindbrain (metencephalon and myelencephalon) portions.
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Cell signaling is part of a complex system of communication that governs basic cellular
activities and coordinates cell actions.[1] The ability of cells to perceive and correctly
respond to their microenvironment is the basis of development, tissue repair, andimmunity as well as normal tissue homeostasis. Errors in cellular information processing
are responsible for diseases such as cancer , autoimmunity, and diabetes. By
understanding cell signaling, diseases may be treated effectively and, theoretically,artificial tissues may be yielded.
Traditional work in biology has focused on studying individual parts of cell signaling
pathways. Systems biology research helps us to understand the underlying structure of
cell signaling networks and how changes in these networks may affect the transmissionand flow of information. Such networks are complex systems in their organization and
may exhibit a number of emergent properties including bistability and ultrasensitivity.
Analysis of cell signaling networks requires a combination of experimental andtheoretical approaches including the development and analysis of simulations and
modelling.
In animals, the brain is the control center of the central nervous system, responsible for
behavior . In mammals, the brain is located in the head, protected by the skull and close tothe primary sensory apparatus of vision, hearing, equilibrioception (balance), sense of
taste, and olfaction (smell).
While all vertebrates have a brain, most invertebrates have either a centralized brain or
collections of individual ganglia. Some animals such as cnidarians and echinoderms donot have a centralized brain, and instead have a decentralized nervous system, while
animals such as sponges lack both a brain and nervous system entirely.
Brains can be extremely complex. For example, the human brain contains roughly100 billion neurons, linked with up to 10,000 connections each.
Hedgehog (Hh), Notch, and Wingless (Wnt) signaling control normaldevelopment of the cerebellum, and dysregulation of these signalingpathways are associated with medulloblastoma (MB). As an initial step in thestudy of the role of interacting signaling pathways in MB pathogenesis, we
demonstrate the expression of several components of the Notch and Wntsignaling pathways, and activation of Notch signaling in MB from Ptch +/− mice
that have elevated Hh signaling. We also show a marked downregulation in
the expression of Notch2, Jagged1, Hes1, mSfrp1, and mFrz7 in cerebella of developing mice with reduced Hh signaling, suggesting that Hh signalingregulates the expression of these genes. Together with recent publisheddata, these findings indicate that Hh signaling might synergizesimultaneously with Notch and Wnt signaling in MB development by
controlling Notch and Wnt pathway ligand, receptor and/or target geneexpression
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Disabled-1 (Dab1) is a cytoplasmic adaptor protein that regulates neuronal migrations
during mammalian brain development. Dab1 function in vivo depends on tyrosine
phosphorylation, which is stimulated by extracellular Reelin and requires Src familykinases. Reelin signaling also negatively regulates Dab1 protein levels in vivo, and
reduced Dab1 levels may be part of the mechanism that regulates neuronal migration. We
have made use of mouse
embryo cortical neuron cultures in which Reelin induces Dab1tyrosine phosphorylation and Src family kinase activation. We have found that Dab1 is
normally stable, but in response to Reelin it becomes polyubiquitinated and degraded via
the proteasome pathway. We have established that tyrosine phosphorylation of Dab1 isrequired for its degradation. Dab1 molecules lacking phosphotyrosine are not degraded in
neurons in which the Dab1 degradation pathway is active. The requirements for Reelin-
induced degradation of Dab1 in vitro correctly predict Dab1 protein levels in vivo in
different mutant mice. We also provide evidence that Dab1 serine/threoninephosphorylation may be important for Dab1 tyrosine phosphorylation. Our data provide
the first evidence for how Reelin down-regulates Dab1 protein expression in vivo. Dab1
degradation may be important for ensuring a transient Reelin response and may play a
role in normal brain developmentThe extracellular protein Reln controls neuronal migrations in parts of the cortex,
hippocampus and cerebellum. In vivo, absence of Reln correlates with up-regulation of the docking protein Dab1 and decreased Dab1 tyrosine phosphorylation. Loss of the Reln
receptor proteins, apolipoprotein receptor 2 and very low density lipoprotein receptor,
results in a Reln-like phenotype accompanied by increased Dab1 protein expression.
Complete loss of Dab1, however, recapitulates the Reln phenotype.
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Vertebrates[edit] Me The hedgehog signaling pathway gives cells this information that they need to
make the embryo develop properly. Different parts of the embryo have differentconcentrations of hedgehog signaling proteins. The pathway also has roles in the adult.
When the pathway malfunctions, it can result in diseases like basal cell carcinoma. [1]
The hedgehog signaling pathway is one of the key regulators of animal developmentconserved from flies to humans. The pathway takes its name from its polypeptide ligand,
an intercellular signaling molecule called Hedgehog (Hh) found in fruit flies of the genus
Drosophila. Hh is one of Drosophila's segment polarity gene products, involved inestablishing the basis of the fly body plan. The molecule remains important during later
stages of embryogenesis and metamorphosis.
Mammals have three Hedgehog homologues, of which Sonic hedgehog is the beststudied. The pathway is equally important during vertebrate embryonic development. Inknockout mice lacking components of the pathway, the brain, skeleton, musculature,
gastrointestinal tract and lungs fail to develop correctly
chanism
Figure 5. Overview of Sonic hedgehog signaling. Click here for a more detailed diagram
Sonic hedgehog (SHH) is the best studied ligand of the vertebrate pathway. Most of what
is known about hedgehog signaling has been established by studying SHH. It is translatedas a ~45kDa precursor and undergoes autocatalytic processing to produce an ~20kDa N-
terminal signaling domain (referred to as SHH-N) and a ~25kDa C-terminal domain with
no known signaling role (1 on figure 5). During the cleavage, a cholesterol molecule isadded to the carboxyl end of the N-terminal domain, which is involved in trafficking,
secretion and receptor interaction of the ligand. SHH can signal in an autocrine fashion,
affecting the cells in which it is produced. Secretion and consequent paracrine hedgehog
signaling require the participation of Dispatched protein(2).
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When SHH reaches its target cell, it binds to the Patched-1 (PTCH1) receptor(3). In the
absence of ligand, PTCH1 inhibits Smoothened (SMO), a downstream protein in the
pathway(4). It has been suggested that SMO is regulated by a small molecule, the cellular localisation of which is controlled by PTCH[13]. PTCH1 has homology to Niemann-Pick
disease, type C1 (NPC1) that is known to transport lipophilic molecules across a
membrane.[14]
PTCH1 has a sterol sensing domain (SSD), which has been shown to beessential for suppression of Smo activity. [15]A current theory of how PTCH regulates
SMO is by removing oxysterols from SMO. PTCH acts like a sterol pump and remove
oxysterols that have been created by 7-dehydrocholesterol reductase. [16]Upon binding of a Hh protein or a mutation in the SSD of PTCH the pump is turned off allowing
oxysterols to accumulate around SMO.
Figure 6. Overview of PTCH/SMO signaling.
Figure 7. Overview of PTCH/SMO signaling.This accumulation of sterols allows SMO to become active or stay on the membrane for a
longer period of time. This hypothesis is supported by the existence of a number of smallmolecule agonists and antagonists of the pathway that act on SMO. The binding of SHH
relieves SMO inhibition, leading to activation of the GLI transcription factors(5): the
activators Gli1 and Gli2 and the repressor Gli3. The sequence of molecular events that
connect SMO to GLIs is poorly understood. Activated GLI accumulates in the nucleus(6)and controls the transcription of hedgehog target genes(7). PTCH1 has recently been
reported to repress transcription of hedgehog target genes through a mechanism
independent of Smoothened.[17]
In addition to PTCH1, mammals have another hedgehog receptor PTCH2 whose
sequence identity with PTCH1 is 54%.[18] All three mammalian hedgehogs bind both
receptors with similar affinity, so PTCH1 and PTCH2 cannot discriminate between theligands. They do, however, differ in their expression patterns. PTCH2 is expressed at
much higher levels in the testis and mediates desert hedgehog signaling there.[18] It
appears to have a distinct downstream signaling role from PTCH1. In the absence of ligand binding PTCH2 has a decreased ability to inhibit the activity of SMO. [19]
Furthermore, overexpression of PTCH2 does not replace mutated PTCH1 in basal cell
carcinoma.[20]
In invertebrates, just as in Drosophila, the binding of hedgehog to PTCH leads tointernalisation and sequestration of the ligand.[21] Consequently in vivo the passage of
hedgehog over a receptive field that expresses the receptor leads to attenuation of the
signal, an effect called ligand-dependent antagonism (LDA). In contrast to Drosophila,vertebrates possess another level of hedgehog regulation through LDA mediated by Hh-
interacting protein 1 (HHIP1). HHIP1 also sequesters hedgehog ligands, but unlike
PTCH, it has no effect on the activity of SMO.[22]
[edit] Role
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Members of the hedgehog family play key roles in a wide variety of developmental
processes.[12] One of the best studied examples is the action of Sonic hedgehog during
development of the vertebrate limb. The classic experiments of Saunders and Gasselingin 1968 on the development of the chick limb bud formed the basis of the morphogen
concept. They showed that identity of the digits in the chick limb was determined by a
diffusible factor produced by the zone of polarizing activity (ZPA), a small region of tissue at the posterior margin of the limb. Mammalian development appeared to follow
the same pattern. This diffusible factor was later shown to be Sonic hedgehog. However,
precisely how SHH determines digit identity remained elusive until recently. The currentmodel, proposed by Harfe et al [23], states that both the concentration and the time of
exposure to SHH determines, which digit the tissue will develop into in the mouse
embryo (figure 6).
Figure 6. Sonic hedgehog specifies digit identity in mammalian development.
Digits V, IV and part of III arise directly from cells that express SHH during
embryogenesis. In these cells SHH signals in an autocrine fashion and these digitsdevelop correctly in the absence of DISP, which is required for extracellular diffusion of
the ligand. These digits differ in the length of time that SHH continues to be expressed.The most posterior digit V develops from cells that express the ligand for the longest
period of time. Digit IV cells express SHH for a shorter time, and digit III cells shorter
still. Digit II develops from cells that are exposed to moderate concentrations of
extracellular SHH. Finally, digit I development does not require SHH. It is, in a sense, thedefault program of limb bud cells.
Hedgehog signaling remains important in the adult. Sonic hedgehog has been shown to
promote the proliferation of adult stem cells from various tissues, including primitivehematopoietic cells[24], mammary[25] and neural[26] stem cells. Activation of the hedgehog
pathway is required for transition of the hair follicle from the resting to the growth phase.[27] Curis Inc. together with Procter & Gamble are developing a hedgehog agonist to beused as a drug for treatment of hair growth disorders.[28] This failed due to toxicities
found in animal models.[29]
Neuronal Differentiation of Precursors in the Neocortical Ventricular Zone Is Triggered by BMP
Weiwei Li, Catherine A. Cogswell, and Joseph J. LoTurco
Department of Physiology and Neurobiology, University of Connecticut, Storrs,
Connecticut 06269-4156
Neocortical neurons begin to differentiate soon after they are generated by mitoses at thesurface of the ventricular zone (VZ). We provide evidence here that bone morphogenetic
protein (BMP) triggers neuronal differentiation of neocortical precursors within the VZ.
In cultures of dissociated neocortical neuroepithelial cells, BMPs increase the number of MAP-2- and TUJ1-positive cells within 24 hr of treatment. In explant cultures, BMP-4
treatment leads to an increase in the number of TUJ1-positive cells within the ventricular
zone. Furthermore, truncated, dominant-negative, BMP type I receptor, introduced intoneocortical precursors by retrovirus-mediated gene transfer, blocks neurite elaboration
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and migration out of the VZ. Finally, immunocytochemistry indicates that BMP protein is
present at the VZ surface. Together, these results indicate that BMP protein is present
within the VZ, that BMP is capable of promoting neuronal differentiation, and thatsignaling through BMP receptors triggers neuronal precursors to differentiate and migrate
out of the VZ.
Members of the bone morphogenetic protein (BMP) family have been implicated in
multiple aspects of neural development in both the CNS and peripheral nervous system.
BMP ligands and receptors, as well as the BMP antagonist noggin, are expressed in thedeveloping cerebral cortex, making the BMPs likely candidates for regulating cortical
development. To define the role of these factors in the developing cerebral cortex, we
examined the effects of BMP2 and BMP4 on cortical cells in vitro. Cells were cultured
from embryonic day 13 (E13) and E16 rat cerebral cortex in the absence or presence of different concentrations of fibroblast growth factor 2, a known regulator of cortical cell
proliferation and differentiation. At E13, the BMPs promoted cell death and inhibited
proliferation of cortical ventricular zone cells, resulting in the generation of fewer neurons
and no glia. At E16, the effects of the BMPs
were more complex. Concentrations of BMP2 in the range of 1-10 ng/ml promoted neuronal and astroglial differentiation and
inhibited oligodendroglial differentiation, whereas 100 ng/ml BMP2 promoted cell deathand inhibited proliferation. Addition of the BMP antagonist noggin promoted
oligodendrogliogenesis in vitro, demonstrating that endogenous BMP signaling
influences the differentiation of cortical cells in vitro. The distribution of BMP2 and
noggin within the developing cortex suggests that local concentrations of ligands andantagonists define gradients of BMP signaling during corticogenesis. Together, these
results support the hypothesis that the BMPs and their antagonist noggin co-regulate
cortical cell fate and morphogenesis.
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Frizzled Receptors and Wnt Signaling
Receptors
Frizzled receptors, like GPCRs, are transmembrane proteins that wind 7 times back andforth through the plasma membrane.
• Their ligand-binding site is exposed outside the surface of the cell.
• Their effector site extends into the cytosol.
Ligands
Their ligands are Wnt proteins. These get their name from two of the first to bediscovered, proteins encoded by
• wingless (wg ) in Drosophila and its homolog
• Int-1 in mice.
The roles of β-catenin
β-catenin molecules connect actin filaments to the cadherins that make up adherens
junctions that bind cells together.
Any excess β-catenin is quickly destroyed by a multiprotein degradation complex. (One
component is the protein encoded by the APC tumor suppressor gene .)The degradation complex
• phosphorylates β-catenin so it can
• have ubiquitin molecules attached to prepare it for destruction in
• proteasomes (not shown).
But undegraded β-catenin takes on a second function: it becomes a potent transcription
factor .
Mechanism
• The binding of a Wnt ligand to Frizzled (done with the aid of cofactors) activates
Frizzled. This, in turn,
• activates a cytosolic protein called Dishevelled.
• Activated Dishevelled inhibits the β-catenin degradation complex so
• β-catenin escapes destruction by proteasomes and is free to enter the nucleus
where
• it binds to the promoters and/or enhancers of its target genes.
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(Note the similarities to the strategy used by the NF-κB signaling pathway.)
Wnt-controlled gene expression plays many roles in embryonic development and
regeneration as well as regulating activities in the adult body.
The Hedgehog Signaling Pathway
Receptor
Patched (Ptc) — a 12-pass transmembrane protein embedded in the plasma membrane.
Ligands
Secreted hedgehog proteins (Hh) that diffuse to their targets. Mammals have threehedgehog genes encoding three different receptors. However, hedgehog was first
identified in Drosophila, and the bristly phenotype produced by mutations in the gene
gave rise to the name.
Mechanism
• In mammals, when there is no hedgehog protein present, the patched receptors
bind a second transmembrane protein called smoothened (Smo).
• However, when Hh protein binds to patched, the Smo protein separates from Ptc
• enabling Smo to activate a zinc-finger transcription factor designated GLI.
• GLI migrates into the nucleus when it activates a variety of target genes.
Hedgehog signaling plays many important developmental roles in the animal kingdom.For example,
• wing development in Drosophila
• development of the brain, GI tract, fingers and toes in mammals.
Mutations or other sorts of regulatory errors in the hedgehog pathway are associated with
a number of birth defects as well as some cancers. Basal-cell carcinoma, the mostcommon skin cancer (and, in fact, the most common of all cancers in much of the world),
usually reveals mutations causing
• extra-high hedgehog or
• suppressed patched activity (both leading to elevated GLI activity).
The Notch Signaling Pathway
This pathway is found throughout the animal kingdom. It differs from many of the other
signaling pathways discussed here in that the ligands as well as their receptors are
transmembrane proteins embedded in the plasma membrane of cells. Thus, signaling inthis pathway requires direct cell-to-cell contact.
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Receptors The Notch signaling pathway is a highly conserved cell signaling system
present in most multicellular organisms. Notch is present in all metazoans, andvertebrates possess four different notch receptors, referred to as Notch1 to
Notch4. The Notch receptor is a single-pass transmembrane receptor protein. Itis a hetero-oligomer composed of a large extracellular portion which associates
in a calcium dependent, non-covalent interaction with a smaller piece of theNotch protein composed of a short extracellular region, a single transmembrane-
pass, and a small intracellular region.[
Notch proteins are single-pass transmembrane glycoproteins. They are encoded by four
genes in vertebrates. However, the first notch gene was discovered in Drosophila whereits mutation produced notches in the wings.
Ligands
Their ligands are also single-pass transmembrane proteins. There are many of them and
often several versions within a family (such as the serrate and delta protein families).
Mechanism
When a cell bearing the ligand comes in contact with a cell displaying the notch receptor,
the external portion of notch is cleaved away from the cell surface (and engulfed by the
ligand-bearing cell by endocytosis). The internal portion of the notch receptor is cut awayfrom the interior of the plasma membrane and travels into the nucleus where it activates
transcription factors that turn the appropriate genes on (and off).
It would appear that proper development of virtually all organs (brain, pancreas, GI tract,heart, blood vessels, mammary glands — to name a few) depends on notch signaling.
Notch signaling appears to be a mechanism by which one cell tells an adjacent cell which
path of differentiation to take (or not take).Defects in notch signaling have been implicated in some cancers, e.g. melanoma.
Discovery
The Notch gene was discovered in 1917 by Thomas Hunt Morgan when it was first
noticed in a strain of the fruit fly Drosophila melanogaster with notches apparent in their
wingblades.[2][3] Its molecular analysis and sequencing was undertaken in the 1980s.[4][5]
[ edit ] ActionThe Notch protein sits like a trigger spanning the cell membrane, with part of it inside
and part outside. Ligand proteins binding to the extracellular domain induce proteolytic
cleavage and release of the intracellular domain, which enters the cell nucleus to alter gene expression.[6]
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[ edit ] Functions
The Notch signaling pathway is important for cell-cell communication, which involves
gene regulation mechanisms that control multiple cell differentiation processes duringembryonic and adult life. Notch signaling also has a role in the following processes:
• neuronal function and development[7][8] • stabilizing arterial endothelial fate and angiogenesis[9]
• regulating crucial cell communication events betweenendocardium and myocardium during both the formation of the valve
primordial and ventricular development and differentiation [10]
• cardiac valve homeostasis as well as implications in other humandisorders involving the cardiovascular system[11]
• timely cell lineage specification of both endocrine and exocrine
pancreas [12]
• influencing binary fate decisions of cells that must choose between
the secretory and absorptive lineages in the gut[13]
• expanding the HSC compartment during bone development andparticipation in commitment to the osteoblastic lineage suggesting apotential therapeutic role for Notch in bone regeneration and
osteoporosis[14]
• regulating cell-fate decision in mammary gland at several distinctdevelopment stages[15]
• possibly some non-nuclear mechanisms, such as controlling the
actin cytoskeleton through the tyrosine kinase Abl[16]
Notch signaling is dysregulated[1] in many cancers, and faulty Notch signaling isimplicated in many diseases including T-ALL (T-cell acute lymphoblastic leukemia),[17]
CADASIL (Cerebral Autosomal Dominant Arteriopathy with Sub-cortical Infarcts andLeukoencephalopathy), MS (Multiple Sclerosis), Tetralogy of Fallot, Alagille syndrome,and myriad other disease states.
[edit] Details of the path and the determination of neural stem cell fate [3]. Notch signaling
remains active in the adult where it is involved in synaptic plasticity and memory [4;5].Perturbations in Notch signaling have been implicated in cancer [6;7] and a number of neurological diseases [8] including cerebral autosomal dominant arteriopathy with subcorticalinfarcts and leukoencephalopathy (CADASIL) [9], Alzheimer’s disease (AD) [10] andschizophrenia [11].
There are four mammalian Notch receptors (Notch 1-4) that are single pass, transmembrane
proteins, which undergo a series of sequential proteolytic cleavages [12;13]. The first cleavage(S1) occurs in the Golgi by a furin-like convertase generating two fragments that remain non-
covalently attached to form the mature Notch receptor at the cell surface. Ligands of the Jagged and Delta families, expressed on neighbouring cells, bind to the Notch receptor leading to the
second cleavage (S2) by tumor necrosis factor alpha converting enzyme (TACE) resulting in theshedding of the extracellular domain (Figure 1).
way
Maturation of the Notch receptor involves cleavage at the prospective extracellular side
during intracellular trafficking in the Golgi complex.[18] This results in a bipartite protein,
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composed of a large extracellular domain linked to the smaller transmembrane and
intracellular domain. Binding of ligand promotes two proteolytic processing events; as a
result of proteolysis, the intracellular domain is liberated and can enter the nucleus toengage other DNA-binding proteins and regulate gene expression.
Notch and most of its ligands are transmembrane proteins, so the cells expressing the
ligands typically need to be adjacent to the Notch expressing cell for signaling to occur.[citation needed ] The Notch ligands are also single-pass transmembrane proteins and are
members of the DSL (Delta/Serrate/LAG-2) family of proteins. In Drosophilamelanogaster (the fruit fly) there are two ligands named Delta and Serrate. In mammals,the corresponding names are Delta-like and Jagged. In mammals there are multiple Delta-
like and Jagged ligands, as well as possibly a variety of other ligands, such as
F3/contactin[16].
In the nematode Caenorhabditis elegans two genes encode homologous proteins, glp-1
and lin-12. There has been at least one report that suggests that some cells can send out
processes which allow signaling to occur between cells which are as much as four or five
cell diameters apart.[citation needed ]
The Notch extracellular domain is composed primarily of small cysteine knot motifscalled EGF-like repeats.[19] Notch 1 for example has 36 of these repeats. Each EGF-like
repeat is approximately 40 amino acids, and its structure is defined largely by sixconserved cysteine residues that form three conserved disulfide bonds. Each EGF-like
repeat can be modified by O -linked glycans at specific sites.[20] An O -glucose sugar may
be added between the first and second conserved cysteine, and an O -fucose may be added
between the second and third conserved cysteine. These sugars are added by an as yetunidentified O -glucosyltransferase , and GDP-fucose Protein O-fucosyltransferase 1
(POFUT1) respectively. The addition of O -fucose by POFUT1 is absolutely necessary for
Notch function, and without the enzyme to add O-fucose, all Notch proteins fail tofunction properly. As yet, the manner in which the glycosylation of Notch affects
function is not completely understood.
The O-glucose on Notch can be further elongated to a trisaccharide with the addition of two xylose sugars by xylosyltransferases, and the O -fucose can be elongated to a
tetrasaccharide by the ordered addition of an N-acetylglucosamine (GlcNAc) sugar by an
N-Acetylglucosaminyltransferase called Fringe, the addition of a galactose by agalactosyltransferase, and the addition of a sialic acid by a sialyltransferase.[21]
To add another level of complexity, in mammals there are three Fringe GlcNAc-
transferases, named Lunatic Fringe, Manic Fringe, and Radical Fringe. These enzymes
are responsible for something called a "Fringe Effect" on Notch signaling. [22] If Fringeadds a GlcNAc to the O -fucose sugar, then the subsequent addition of a galactose and
sialic acid will occur. In the presence of this tetrasaccharide, Notch signals strongly when
it interacts with the Delta ligand, but has markedly inhibited signaling when interactingwith the Jagged ligand.[23] The means by which this addition of sugar inhibits signaling
through one ligand, and potentiates signaling through another is not clearly understood.
Notch signaling steps
Once the Notch extracellular domain interacts with a ligand, an ADAM-family
metalloprotease called TACE (Tumor Necrosis Factor Alpha Converting Enzyme)
cleaves the Notch protein just outside the membrane.[24] This releases the extracellular
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portion of Notch, which continues to interact with the ligand. The ligand plus the Notch
extracellular domain is then endocytosed by the ligand-expressing cell. There may be
signaling effects in the ligand-expressing cell after endocytosis; this part of Notchsignaling is a topic of active research. After this first cleavage, an enzyme called γ-
secretase (which is implicated in Alzheimer's disease) cleaves the remaining part of the
Notch protein just inside the inner leaflet of the cell membrane of the Notch-expressingcell. This releases the intracellular domain of the Notch protein, which then moves to the
nucleus where it can regulate gene expression by activating the transcription factor CSL.[16] Other proteins also participate in the intracellular portion of the Notch signalingcascade.
[ edit ] Triggering
Because most ligands are also transmembrane proteins, the receptor is normally only
triggered from direct cell-to-cell contact. In this way, groups of cells can organise
themselves, such that if one cell expresses a given trait, this may be switched off inneighbour cells by the inter-cellular Notch signal. In this way groups of cells influence
one another to make large structures.The Notch cascade consists of Notch and Notch ligands, as well as intracellular proteins
transmitting the Notch signal to the cell's nucleus. The Notch/Lin-12/Glp-1 receptor family[25] was found to be involved in the specification of cell fates during development
in Drosophila and C. elegans.[26] The Notch signaling pathway begins to inhibit new cell
growth when adolescence is reached, and keeps neural networks stable
Notch signaling: key players and mechanism
Core components of Notch signaling
Key components of Notch signaling were originally recognized genetically throughmutant animals whose phenotypes resembled those of Notch mutants. In flies, Notch was
the founding member
of a collection of `neurogenic' mutants (see Box 1), so namedbecause they produce a remarkable excess of neurons at the expense of epidermis(Poulson, 1945 ; Lehmann et al., 1983 ). Nematodes have two homologs of Notch (LIN-
12 and GLP-1), which were identified by mutations that affect cell lineages and germ-line
proliferation. The lin-12/glp-1 double mutant displays an aggregate phenotype that
constitutes the full loss of Notch activity in the worm; this phenotype is characteristic of asmall class of `LIN and GLP' or `LAG' mutants in worms (Lambie and Kimble, 1991 ).
These mutants laid the foundation for genetic, molecular and biochemical studies that
established the core Notch signaling apparatus. At its heart lies a Delta-type ligand, aNotch-type receptor and a transcription factor of the CBF1/Su(H)/LAG1 (CSL) family
(Fig. 1). All metazoan organisms studied to date contain one or more orthologs of each of
these proteins, and these are
summarized in Table 1. Delta- and Notch-related proteins areall single-pass transmembrane proteins that contain extracellular arrays of epidermal
growth factor (EGF) repeats; specific EGF repeats mediate direct contact between ligand
and receptor (Rebay et al., 1991 ). CSL proteins are sequence-specific DNA-binding
proteins (Henkel et al., 1994 ) that function downstream of Notch. Because almost alllocations of Notch signaling involve this ligand-receptor-transcription factor trio, they are
generally considered as the `core' components of Notch signaling.
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