neurogenin 1 | |||||||
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Identifiers | |||||||
Symbol | NEUROG1 | ||||||
Alt. symbols | NEUROD3 | ||||||
NCBI gene | 4762 | ||||||
HGNC | 7764 | ||||||
OMIM | 601726 | ||||||
RefSeq | NM_006161 | ||||||
UniProt | Q92886 | ||||||
Other data | |||||||
Locus | Chr. 5 q23-q31 | ||||||
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neurogenin 2 | |||||||
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Identifiers | |||||||
Symbol | NEUROG2 | ||||||
NCBI gene | 63973 | ||||||
HGNC | 13805 | ||||||
OMIM | 606624 | ||||||
RefSeq | NM_024019 | ||||||
UniProt | Q9H2A3 | ||||||
Other data | |||||||
Locus | Chr. 4 q25 | ||||||
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neurogenin 3 | |||||||
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Identifiers | |||||||
Symbol | NEUROG3 | ||||||
NCBI gene | 50674 | ||||||
HGNC | 13806 | ||||||
OMIM | 604882 | ||||||
RefSeq | NM_020999 | ||||||
UniProt | Q9Y4Z2 | ||||||
Other data | |||||||
Locus | Chr. 10 q21.3 | ||||||
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Neurogenins, often abbreviated as Ngn, are a family of bHLH transcription factors involved in specifying neuronal differentiation. The family consisting of Neurogenin-1, Neurogenin-2, and Neurogenin-3, plays a fundamental role in specifying neural precursor cells and regulating the differentiation of neurons during embryonic development. It is one of many gene families related to the atonal gene in Drosophila. Other positive regulators of neuronal differentiation also expressed during early neural development include NeuroD and ASCL1. [1]
Neurogenins primarily govern the transition of neural progenitor cells to neurons by activating specific downstream genes associated with neuronal differentiation. Their involvement spans various stages of neurogenesis, including the determination of neural progenitor identity, cell cycle exit, and the acquisition of neuronal characteristics. Notably, Neurogenins influence the specification of different neuronal subtypes, contributing to the diverse array of neurons within the central and peripheral nervous systems. [2]
In neural crest cells, the neurogenin family is essential for neurogenesis in the developing dorsal root ganglia and development of the sensory lineage. [3] [4]
The activity of Neurogenins is intricately regulated by molecular pathways and environmental cues. Interaction with other transcription factors, such as proneural factors and Notch signaling, further refines the neurogenic cascades. Their spatiotemporal expression patterns and cross-regulation contribute to the exquisite precision required for proper neural development. [5] [6]
Neurogenins holds significant implications for clinical research, particularly in the context of neurodevelopmental disorders and neurological regeneration. [7] Aberrations in the expression or regulation of Neurogenins have been linked to conditions such as autism spectrum disorders and neurodegenerative diseases. [8] Ongoing research continues to explore the therapeutic potential of manipulating Neurogenin activity for neural repair and regeneration.
Neurogenin 1 (Ngn1) is a Class-A basic-helix-loop-helix (bHLH) transcription factor that acts as a regulator for neuronal differentiation, and acts by binding to enhancer regulatory elements on genes that encode transcriptional regulators of neurogenesis. In order for Ngn1 to bind with high fidelity with genomic DNA, it must dimerize with another bHLH protein. [9] Ngn1 is a proneural gene because its expression is seen prior to neural lineage determination, indicating it plays a role in neuronal differentiation. [1]
In E14 rats, when Ngn1 is present in the cerebral cortex, it binds to the CBP/p300/Smad1 transcriptional co-activator complex, which recruits it to the enhancer box upstream of the gene in the promoter for neuronal genes. Binding of Ngn1, to the enhancer box, induces the transcription factor NeuroD to bind to its own enhancer boxes, inducing the genes involved in neuronal differentiation. [10]
Bone-morphogenetic-protein (BMP) signaling is responsible for the expression of the transcriptional co-activators CBP, p300, and Smad1. [10] In the presence of Ngn1, BMPs promote neuronal differentiation in stem cells through binding of all endogenous CBP/p300/Smad1 to Ngn1, and being recruited toward the neuronal promoters, causing neuronal differentiation. [10] In the embryonic forebrain, Ngn1 is associated with dorsal patterning and cell fate specification, with the patterning molecules and proneural proteins establishing the spatial domains of both proneural and homeodomain protein expression. This is critical for the initiation of neurogenesis. [11]
In the presence of Ngn1, the leukemia inhibitory factor (LIF) pathway is inhibited by Ngn1 blocking STAT activation. Normally, the STAT binding site promotes GFAP transcription through binding the STAT1/3 complex, which is activated through the LIF pathway. [10]
Along with supporting neuronal differentiation, when expressed in embryonic neural tissue, Ngn1 also acts to inhibit glial differentiation. [12] In the absence of Ngn1, the CBP/p300/Smad1 transcriptional co-activator complex is recruited to and binds to activated STAT1/3, which in turn causes the expression of GFAP, causing glial differentiation. In the presence of Ngn1, inhibition of gliogenesis occurs through Ngn1 binding to the CBP/p300/Smad1 transcriptional co-activator complex, recruiting it away from STAT1/3. [10]
In cases of low levels of Ngn1, BMPs promote glial differentiation. Since Ngn1 is the limiting factor, CBP/p300/Smad1 is able to interact with STAT1/3 and induce gliogenesis. [10]
Activation of the notch pathway, causes the inhibition of proneural bHLH genes, such as Ngn1, which allows for the CBP/p300/Smad1 to interact with STAT1/3 and induce gliogenesis. [10] Along with the embryonic rat, it was also seen in zebrafish that the repression of Ngn1 by Notch, promotes glial lineage in neural crest and central nervous system formation through the inhibition of neuronal differentiation. [1] [13] In addition to the Notch pathway activating the transcriptional factors involved in the promotion of gliogenesis, it is possible that these same factors are involved in the inhibition of other fates.
In the absence of Ngn1, the LIF pathway is able to activate STAT1/3, which allows for the promotion of GFAP transcription via the STAT binding site. The promotion of GFAP transcription induced glial differentiation. [10]
Neurogenin 2 (Ngn2) is a bHLH transcription factor involved in both neurogenesis and neural specification. This protein binds to enhancer-box regulatory elements on the promoters of many genes related to neurogenesis and neural specification. For sufficient DNA binding, Ngn2 must form a dimer with an enhancer protein. [14]
Ngn2 is a transcription factor that both increases expression of proneural genes and drives neural fate by inhibiting expression of glial genes in neural progenitor cells (NPCs). This was observed in mice lacking Ngn2 and mash-1 (another proneural bHLH transcription factor), which have more glia in the cortex and decreased capacity to generate neurons. Olig2 expression in what will become NPCs precedes Ngn2 and promotes its expression. [10] During the switch from neural progenitor fate to glial fate, Ngn2 is downregulated and Nkx2.2, which inhibits proneural genes, is upregulated. [15] Glial fate switch was reduced by inhibiting Nkx2.2 and Olig2 in neural progenitors while allowing the expression of Ngn2. The ability of Olig2 to induce expression of Ngn2 is reduced when Nkx2.2 is expressed. [16]
Mice lacking Ngn2 have fewer motor neurons and ventral interneurons, indicating that Ngn2 plays a role in specification of these neurons. [17]
Heterodimerized Ngn2/enhancer protein complex can bind to enhancer boxes to promote transcription of genes related to a non-specified neuronal fate. [17]
When an enhancer box of a promoter that has been bound by the Ngn-2/enhancer protein complex is also bound by a dimer of the adaptor nuclear LIM interactor (NLI) bound to two LIM homeobox protein 3 (Lhx3), genes related to V2 interneuron identity are expressed. [17]
A dimer of the adaptor NLI bound to two islet 1 (Isl1) proteins and each Isl1 is bound by Lhx3 is called the LIM-homeodomain (LIM-HD) transcription complex. When an enhancer box of a promoter that has been bound by the heterodimerized Ngn2/E-protein complex, the LIM-HD transcription complex is able to bind to drive expression of genes related to motor neuron fate, but only if Ngn2 has been properly phosphorylated. [17]
Ngn2 has two serines, S231 and S234, which can be phosphorylated by glycogen synthase kinase 3 (Gsk3). Phosphorylation of Ngn2 enables interaction with LIM-homeodomain proteins, leading to ventral neural fate and motor neuron specification. [18] The importance of this phosphorylation was determined by using mice that express a mutated form of Ngn2 protein which has the serines from the previously mentioned phosphorylation sites mutated into alanines, which cannot be phosphorylated. These mutant mice have a decreased number of motor neurons and an increased number of V2 interneurons, suggesting that phosphorylation is necessary for driving expression of genes related to motor neuron fate but not V2 interneuron fate and non-specified neural fate. [17]
Neurogenin 3 (Ngn3) is another member of the bHLH family of transcription factors. Ngn3 functions in the differentiation of endocrine pancreas cells. Although its key function is in the pancreas, intestinal cells and neural cells express Ngn3 as well. Several studies have highlighted the importance of Ngn3 for differentiation of endocrine cells. In mice, Ngn3 is present in cells as the pancreas begins to bud and glucagon cells are formed. There are several pathways that Ngn3 works through. [19] [20] [21] [22]
Ngn3 is a crucial component in pancreatic development and plays a supporting role in intestinal as well as neuronal cell development. Studies have demonstrated that knockout of Ngn3 in mice leads to death shortly after birth possibly due to after effects of severe diabetes. [19] Further studies are taking place to investigate Ngn3's possible role as a treatment for diabetes and regeneration of cells in the pancreas. [19] [21]
Neurogenin 3 (NGN3) is expressed by 2-10% of acinar and duct cells in the histologically normal adult human pancreas. NGN3+ cells isolated from cultured exocrine tissue by coexpressed cell surface glycoprotein CD133 have a transcriptome consistent with exocrine dedifferentiation, a phenotype that resembles endocrine progenitor cells during development, and a capacity for endocrine differentiation in vitro. [23] Human [24] and rodent [25] [26] [27] [28] [29] [30] [31] [32] exocrine cells have been reprogrammed into cells with an islet cell-like phenotype following direct expression of NGN3 or manipulation that leads to its expression.
The development of the pancreas is broken up into three phases, primary phase, secondary phase, and tertiary phase. Ngn3 is active in the primary and secondary phase. In the primary phase Ngn3 assists in α cell differentiation and in the secondary phase another wave of Ngn3 assists in differentiation of β cells, pancreatic polypeptide cells, and δ cells. Differentiation is marked as complete after the secondary phase. Ngn3 allows for the commitment of pancreatic progenitor cells to become an endocrine multipotent pro-precursor. [19]
The Notch pathway is one of the key modulators of Ngn3. The binding of Delta and Serrate, activation ligands for the Notch pathway, activates the Notch surface molecule. This allows the Notch intracellular domain to activate RBK-Jκ to translocate into the nucleus. This complex then activates hairy and enhancer of split (HES)-type proteins, which are inhibitors of Ngn3. The cells that allow the Notch/RBK-Jκ complex to enter are the ones that will not be differentiated into pancreatic cells because Ngn3 is suppressed. It is important to mention that Ngn3 has three HES1 binding sites adjacent to the TATA box sequence that allow for the regulation of this transcription factor. [19]
Ngn3 can also activate the neurogenic differentiation factor 1(NeuroD1) like most of its other family members through the enhancer boxes present in its structure. Being that NeuroD1 is expressed along with Ngn3 in differentiating cells, it is considered one of the transcription factors downstream targets. [19]
Another important target is paired box gene 4 (Pax4), which plays a major role in β cell and δ cell differentiation. Ngn3 works hand-in-hand with HNF1α to activate the Pax4 promoter to induce specific cell differentiation. [19]
Another transcription factor that may be a downstream target of Ngn3 is Nkx2.2 because it is often coexpressed with it. Studies have shown that disrupting Nkx2.2 expression results in problems with α and β cell differentiation. [20] [21]
In vertebrates, a neuroblast or primitive nerve cell is a postmitotic cell that does not divide further, and which will develop into a neuron after a migration phase. In invertebrates such as Drosophila, neuroblasts are neural progenitor cells which divide asymmetrically to produce a neuroblast, and a daughter cell of varying potency depending on the type of neuroblast. Vertebrate neuroblasts differentiate from radial glial cells and are committed to becoming neurons. Neural stem cells, which only divide symmetrically to produce more neural stem cells, transition gradually into radial glial cells. Radial glial cells, also called radial glial progenitor cells, divide asymmetrically to produce a neuroblast and another radial glial cell that will re-enter the cell cycle.
Neural stem cells (NSCs) are self-renewing, multipotent cells that firstly generate the radial glial progenitor cells that generate the neurons and glia of the nervous system of all animals during embryonic development. Some neural progenitor stem cells persist in highly restricted regions in the adult vertebrate brain and continue to produce neurons throughout life. Differences in the size of the central nervous system are among the most important distinctions between the species and thus mutations in the genes that regulate the size of the neural stem cell compartment are among the most important drivers of vertebrate evolution.
Radial glial cells, or radial glial progenitor cells (RGPs), are bipolar-shaped progenitor cells that are responsible for producing all of the neurons in the cerebral cortex. RGPs also produce certain lineages of glia, including astrocytes and oligodendrocytes. Their cell bodies (somata) reside in the embryonic ventricular zone, which lies next to the developing ventricular system.
Neuropoiesis is the process by which neural stem cells differentiate to form mature neurons, astrocytes, and oligodendrocytes in the adult mammal. This process is also referred to as adult neurogenesis.
The subgranular zone (SGZ) is a brain region in the hippocampus where adult neurogenesis occurs. The other major site of adult neurogenesis is the subventricular zone (SVZ) in the brain.
PDX1, also known as insulin promoter factor 1, is a transcription factor in the ParaHox gene cluster. In vertebrates, Pdx1 is necessary for pancreatic development, including β-cell maturation, and duodenal differentiation. In humans this protein is encoded by the PDX1 gene, which was formerly known as IPF1. The gene was originally identified in the clawed frog Xenopus laevis and is present widely across the evolutionary diversity of bilaterian animals, although it has been lost in evolution in arthropods and nematodes. Despite the gene name being Pdx1, there is no Pdx2 gene in most animals; single-copy Pdx1 orthologs have been identified in all mammals. Coelacanth and cartilaginous fish are, so far, the only vertebrates shown to have two Pdx genes, Pdx1 and Pdx2.
Protein numb homolog is a protein that in humans is encoded by the NUMB gene. The protein encoded by this gene plays a role in the determination of cell fates during development. The encoded protein, whose degradation is induced in a proteasome-dependent manner by MDM2, is a membrane-bound protein that has been shown to associate with EPS15, LNX1, and NOTCH1. Four transcript variants encoding different isoforms have been found for this gene.
Protein BTG2 also known as BTG family member 2 or NGF-inducible anti-proliferative protein PC3 or NGF-inducible protein TIS21, is a protein that in humans is encoded by the BTG2 gene and in other mammals by the homologous Btg2 gene. This protein controls cell cycle progression and proneural genes expression by acting as a transcription coregulator that enhances or inhibits the activity of transcription factors.
Transcription factor HES1 is a protein that is encoded by the Hes1 gene, and is the mammalian homolog of the hairy gene in Drosophila. HES1 is one of the seven members of the Hes gene family (HES1-7). Hes genes code nuclear proteins that suppress transcription.
Achaete-scute homolog 1 is a protein that in humans is encoded by the ASCL1 gene. Because it was discovered subsequent to studies on its homolog in Drosophila, the Achaete-scute complex, it was originally named MASH-1 for mammalian achaete scute homolog-1.
Oligodendrocyte transcription factor (OLIG2) is a basic helix-loop-helix (bHLH) transcription factor encoded by the OLIG2 gene. The protein is of 329 amino acids in length, 32 kDa in size and contains one basic helix-loop-helix DNA-binding domain. It is one of the three members of the bHLH family. The other two members are OLIG1 and OLIG3. The expression of OLIG2 is mostly restricted in central nervous system, where it acts as both an anti-neurigenic and a neurigenic factor at different stages of development. OLIG2 is well known for determining motor neuron and oligodendrocyte differentiation, as well as its role in sustaining replication in early development. It is mainly involved in diseases such as brain tumor and Down syndrome.
Protein atonal homolog 1 is a protein that in humans is encoded by the ATOH1 gene.
Gliogenesis is the generation of non-neuronal glia populations derived from multipotent neural stem cells.
Neurogenin-2 is a protein that in humans is encoded by the NEUROG2 gene.
Neurogenin-3 (NGN3) is a protein that in humans is encoded by the Neurog3 gene.
Eomesodermin also known as T-box brain protein 2 (Tbr2) is a protein that in humans is encoded by the EOMES gene.
Epigenetic regulation of neurogenesis is the role that epigenetics plays in the regulation of neurogenesis.
Proneural genes encode transcription factors of the basic helix-loop-helix (bHLH) class which are responsible for the development of neuroectodermal progenitor cells. Proneural genes have multiple functions in neural development. They integrate positional information and contribute to the specification of progenitor-cell identity. From the same ectodermal cell types, neural or epidermal cells can develop based on interactions between proneural and neurogenic genes. Neurogenic genes are so called because loss of function mutants show an increase number of developed neural precursors. On the other hand, proneural genes mutants fail to develop neural precursor cells.
Neurogenesis is the process by which nervous system cells, the neurons, are produced by neural stem cells (NSCs). This occurs in all species of animals except the porifera (sponges) and placozoans. Types of NSCs include neuroepithelial cells (NECs), radial glial cells (RGCs), basal progenitors (BPs), intermediate neuronal precursors (INPs), subventricular zone astrocytes, and subgranular zone radial astrocytes, among others.
Pancreatic progenitor cells are multipotent stem cells originating from the developing fore-gut endoderm which have the ability to differentiate into the lineage specific progenitors responsible for the developing pancreas.