Neurogenins are a family of
bHLH transcription factors
In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The func ...
involved in specifying neuronal differentiation. It is one of many
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
families related to the ''atonal'' gene in
Drosophila
''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species ...
. Other positive regulators of neuronal differentiation also expressed during early neural development include
NeuroD
NeuroD, also called Beta2, is a basic helix-loop-helix transcription factor expressed in certain parts of brain, beta pancreatic cells and enteroendocrine cells. It is involved in the differentiation of nervous system and development of pancreas. ...
and
ASCL1
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 scu ...
.
In
neural crest
Neural crest cells are a temporary group of cells unique to vertebrates that arise from the embryonic ectoderm germ layer, and in turn give rise to a diverse cell lineage—including melanocytes, craniofacial cartilage and bone, smooth muscle, per ...
cells, the neurogenin family is essential for
neurogenesis
Neurogenesis is the process by which nervous system cells, the neurons, are produced by neural stem cells (NSCs). It occurs in all species of animals except the porifera (sponges) and placozoans. Types of NSCs include neuroepithelial cells (NECs) ...
in the developing
dorsal root ganglia
A dorsal root ganglion (or spinal ganglion; also known as a posterior root ganglion) is a cluster of neurons (a ganglion) in a dorsal root of a spinal nerve. The cell bodies of sensory neurons known as first-order neurons are located in the do ...
and development of the sensory lineage.
Neurogenin-1
Neurogenin 1 (Ngn1) is a Class-A
basic-helix-loop-helix (bHLH)
transcription factor
In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The fu ...
that acts as a regulator for neuronal
differentiation, and acts by binding to
enhancer regulatory elements
A regulatory sequence is a segment of a nucleic acid molecule which is capable of increasing or decreasing the expression of specific genes within an organism. Regulation of gene expression is an essential feature of all living organisms and vi ...
on
genes
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
that encode transcriptional regulators of
neurogenesis
Neurogenesis is the process by which nervous system cells, the neurons, are produced by neural stem cells (NSCs). It occurs in all species of animals except the porifera (sponges) and placozoans. Types of NSCs include neuroepithelial cells (NECs) ...
. In order for Ngn1 to bind with high fidelity with genomic
DNA, it must
dimerize
A dimer () ('' di-'', "two" + ''-mer'', "parts") is an oligomer consisting of two monomers joined by bonds that can be either strong or weak, covalent or intermolecular. Dimers also have significant implications in polymer chemistry, inorganic ch ...
with another bHLH
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
. Ngn1 is a proneural gene because its expression is seen prior to neural lineage
determination
Determination is a positive emotional feeling that involves persevering towards a difficult goal in spite of obstacles.Kirby, L.D., Morrow, J., & Yih, J. (2014). The challenge of challenge: Pursuing determination as an emotion. In M. M. Tugade, ...
, indicating it plays a role in neuronal differentiation.
Neuronal differentiation
In E14 rats, when Ngn1 is present in the
cerebral cortex
The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consisting of ...
, it binds to the
CBP CBP may refer to:
Business parks
* Cebu Business Park, a central business district in Cebu City, Philippines
* Changi Business Park, an eco-friendly industrial park in Singapore
* Chiswick Business Park, a business park in Gunnersbury, West London
...
/
p300/
Smad1
Mothers against decapentaplegic homolog 1 also known as SMAD family member 1 or SMAD1 is a protein that in humans is encoded by the ''SMAD1'' gene.
Nomenclature
SMAD1 belongs to the SMAD, a family of proteins similar to the gene products of the ...
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
NeuroD, also called Beta2, is a basic helix-loop-helix transcription factor expressed in certain parts of brain, beta pancreatic cells and enteroendocrine cells. It is involved in the differentiation of nervous system and development of pancreas. ...
to bind to its own enhancer boxes, inducing the genes involved in neuronal differentiation.
Regulation by BMP
Bone-morphogenetic-protein (BMP) signaling is responsible for the expression of the
transcriptional co-activators CBP, p300, and Smad1.
In the presence of Ngn1, BMPs promote neuronal differentiation in
stem cells
In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...
through binding of all
endogenous
Endogenous substances and processes are those that originate from within a living system such as an organism, tissue, or cell.
In contrast, exogenous substances and processes are those that originate from outside of an organism.
For example, es ...
CBP/p300/Smad1 to Ngn1, and being recruited toward the neuronal promoters, causing neuronal differentiation.
In the
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
nic
forebrain
In the anatomy of the brain of vertebrates, the forebrain or prosencephalon is the Anatomical terms of location#Directional terms, rostral (forward-most) portion of the brain. The forebrain (prosencephalon), the midbrain (mesencephalon), and hin ...
, 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
A homeobox is a DNA sequence, around 180 base pairs long, that regulates large-scale anatomical features in the early stages of embryonic development. For instance, mutations in a homeobox may change large-scale anatomical features of the full-g ...
protein expression. This is critical for the initiation of neurogenesis.
Regulation by LIF
In the presence of Ngn1, the
leukemia inhibitory factor
Leukemia inhibitory factor, or LIF, is an interleukin 6 class cytokine that affects cell growth by inhibiting differentiation. When LIF levels drop, the cells differentiate.
Function
LIF derives its name from its ability to induce the terminal ...
(LIF) pathway is inhibited by Ngn1 blocking
STAT
STAT, Stat. , or stat may refer to:
* Stat (system call), a Unix system call that returns file attributes of an inode
* ''Stat'' (TV series), an American sitcom that aired in 1991
* Stat (website), a health-oriented news website
* STAT protein, a ...
activation. Normally, the STAT binding site promotes
GFAP transcription
Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including:
Genetics
* Transcription (biology), the copying of DNA into RNA, the fir ...
through binding the STAT1/3 complex, which is activated through the LIF pathway.
Glial differentiation
Along with supporting neuronal differentiation, when expressed in embryonic neural tissue, Ngn1 also acts to inhibit
glia
Glia, also called glial cells (gliocytes) or neuroglia, are non-neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system that do not produce electrical impulses. They maintain homeostasis, form mye ...
l differentiation. 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 Gliogenesis is the generation of non-neuronal glia populations derived from multipotent neural stem cells.
Overview
Gliogenesis results in the formation of non-neuronal glia populations from neuronal cells. In this capacity, glial cells provide m ...
occurs through Ngn1 binding to the CBP/p300/Smad1 transcriptional co-activator complex, recruiting it away from STAT1/3.
Regulation by BMP
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.
Regulation by Notch
Activation of the
notch pathway
The Notch signaling pathway is a highly conserved cell signaling system present in most animals. Mammals possess four different notch receptors, referred to as NOTCH1, NOTCH2, NOTCH3, and NOTCH4. The notch receptor is a single-pass transmembr ...
, 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.
Along with the embryonic rat, it was also seen in
zebrafish
The zebrafish (''Danio rerio'') is a freshwater fish belonging to the minnow family ( Cyprinidae) of the order Cypriniformes. Native to South Asia, it is a popular aquarium fish, frequently sold under the trade name zebra danio (and thus often ...
that the repression of Ngn1 by Notch, promotes glial lineage in
neural crest
Neural crest cells are a temporary group of cells unique to vertebrates that arise from the embryonic ectoderm germ layer, and in turn give rise to a diverse cell lineage—including melanocytes, craniofacial cartilage and bone, smooth muscle, per ...
and
central nervous system
The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all par ...
formation through the inhibition of neuronal differentiation.
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.
Regulation by LIF
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.
Neurogenin-2
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
Dimer may refer to:
* Dimer (chemistry), a chemical structure formed from two similar sub-units
** Protein dimer, a protein quaternary structure
** d-dimer
* Dimer model, an item in statistical mechanics, based on ''domino tiling''
* Julius Dimer ...
with an enhancer protein.
Neurogenesis and glial inhibition
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.
During the switch from neural progenitor fate to glial fate, Ngn2 is downregulated and
Nkx2.2, which inhibits proneural genes, is upregulated.
Glial fate switch was reduced by inhibiting Nkx2.2 and
Olig2
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 do ...
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.
Neural specification
Mice lacking Ngn2 have fewer
motor neurons
A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly ...
and ventral
interneurons
Interneurons (also called internuncial neurons, relay neurons, association neurons, connector neurons, intermediate neurons or local circuit neurons) are neurons that connect two brain regions, i.e. not direct motor neurons or sensory neurons. In ...
, indicating that Ngn2 plays a role in specification of these neurons.
Pan-neuronal fate
Heterodimerized Ngn2/enhancer protein complex can bind to enhancer boxes to promote transcription of genes related to a non-specified neuronal fate.
V2 interneuron fate
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.
[
]
Motor neuron fate
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
In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, whi ...
.[
Ngn2 has two ]serine
Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...
s, 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. 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 alanine
Alanine (symbol Ala or A), or α-alanine, is an α-amino acid that is used in the biosynthesis of proteins. It contains an amine group and a carboxylic acid group, both attached to the central carbon atom which also carries a methyl group side c ...
s, 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.[
]
Neurogenin-3
Neurogenin 3 (Ngn3) is another member of the bHLH family of transcription factors. Ngn3 functions in the differentiation of endocrine
The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In vertebrates, the hypothalamus is the neu ...
pancreas
The pancreas is an organ of the digestive system and endocrine system of vertebrates. In humans, it is located in the abdomen behind the stomach and functions as a gland. The pancreas is a mixed or heterocrine gland, i.e. it has both an end ...
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
Glucagon is a peptide hormone, produced by alpha cells of the pancreas. It raises concentration of glucose and fatty acids in the bloodstream, and is considered to be the main catabolic hormone of the body. It is also used as a Glucagon (medicati ...
cells are formed. There are several pathways that Ngn3 works through.
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.[ Further studies are taking place to investigate Ngn3's possible role as a treatment for ]diabetes
Diabetes, also known as diabetes mellitus, is a group of metabolic disorders characterized by a high blood sugar level ( hyperglycemia) over a prolonged period of time. Symptoms often include frequent urination, increased thirst and increased ap ...
and regeneration of cells in the pancreas.[
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. Human and rodent ] 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.
Phases of pancreatic development
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.[
]
Modulation via notch pathway
The Notch pathway is one of the key modulators of Ngn3. The binding of Delta
Delta commonly refers to:
* Delta (letter) (Δ or δ), a letter of the Greek alphabet
* River delta, at a river mouth
* D ( NATO phonetic alphabet: "Delta")
* Delta Air Lines, US
* Delta variant of SARS-CoV-2 that causes COVID-19
Delta may also ...
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
Transcription factor HES1 (hairy and enhancer of split-1) 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 ge ...
binding sites adjacent to the TATA box
In molecular biology, the TATA box (also called the Goldberg–Hogness box) is a sequence of DNA found in the core promoter region of genes in archaea and eukaryotes. The bacterial homolog of the TATA box is called the Pribnow box which has ...
sequence that allow for the regulation of this transcription factor.[
]
Downstream targets of Ngn3
NeuroD
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.[
]
Pax4
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.[
]
Nkx2.2
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.[
]
References
{{transcription factors, g1