ELK1
Encyclopedia
E twenty-six-like transcription factor 1, also known as Elk1, functions as a transcription activator. It is classified as a ternary complex factor (TCF), a subclass of the ETS family, which is characterized by a common protein domain
that regulates DNA
binding to target sequences. Elk1 plays important roles in various contexts, including long-term memory
formation, drug addiction, Alzheimer’s disease, Down syndrome
, breast cancer
, and depression.
(NLS) and a nuclear export signal
(NES), which are responsible for nuclear import and export, respectively. The B domain allows Elk1 to bind to a dimer of its cofactor, serum response factor
(SRF). Located adjacent to the B domain, the R domain is involved in suppressing Elk1 transcriptional activity. This domain harbors the lysine residues that are likely to undergo SUMOylation, a post-translational event that strengthens the inhibition function of the R domain. The D domain plays the key role of binding to active Mitogen-activated protein kinases (MAPKs). Located in the C-terminal region of Elk1, the C domain includes the amino acids that actually become phosphorylated by MAPKs. In this region, Serine 383 and 389 are key sites that need to be phosphorylated for Elk1-mediated transcription to occur. Finally, the DEF domain is specific for the interaction of activated extracellular signal-regulated kinase (Erk), a type of MAPK, with Elk1.
, Elk1 is expressed in the nuclei of non-neuronal cells. The protein is present in the cytoplasm as well as in the nucleus of mature neurons. In post-mitotic neurons, a variant of Elk1, sElk1, is expressed solely in the nucleus because it lacks the NES site present in the full-length protein. Moreover, while Elk1 is broadly expressed, actual levels vary among tissues. The rat brain, for example, is extremely rich in Elk1, but the protein is exclusively expressed in neurons.
produces ∆Elk1. This variant lacks part of the DNA-binding domain that allows interaction with SRF. On the other hand, sElk1 has an intact region that binds to SRF, but it lacks the first 54 amino acids that contain the NES. Found only in neurons, sElk1 is created by employing an internal translation start site. Both ∆Elk1 and sElk1, truncated versions of full-length protein, are capable of binding to DNA and inducing various cellular signaling. In fact, sElk1 counteracts Elk1 in neuronal differentiation and the regulation of nerve growth factor
/ERK
signaling.
acts upstream of MAPKs to activate them by phosphorylating and, thereby activating, MEKs, or MAPK or ERK kinases. Raf itself is activated by Ras, which is linked to growth factor receptors with tyrosine kinase
activity via Grb2
and Sos. Grb2 and Sos can stimulate Ras only after the binding of growth factors to their corresponding receptors. However, Raf activation does not exclusively depend on Ras. Protein kinase C, which is activated by phorbol esters, can fulfill the same function as Ras. MEK kinase (MEKK) can also activate MEKs, which then activate MAPKs, making Raf unnecessary at times. Various signal transduction pathways, therefore, funnel through MEKs and MAPKs and lead to the activation of Elk1. After stimulation of Elk1, SRF, which allows Elk1 to bind to the c-fos promoter, must be recruited. The binding of Elk1 to SRF happens due to protein-protein interaction between the B domain of Elk1 and SRF and the protein-DNA interaction via the A domain.
The aforementioned proteins are like recipes for a certain signaling output. If one of these ingredients, such as SRF, is missing, then a different output occurs. In this case, lack of SRF leads to Elk1’s activation of another gene. Elk1 can, thus, independently interact with an ETS binding site, as in the case of the lck proto-oncogene in Figure 2. Moreover, the spacing and relative orientation of the Elk1 binding site to the SRE is rather flexible, suggesting that the SRE-regulated early genes other than c-fos could be targets of Elk1. egr-1 is an example of an Elk1 target that depends on SRE interaction. Ultimately, phosphorylation of Elk1 can result in the production of many proteins, depending on the other factors involved and their specific interactions with each other.
When studying signaling pathways, mutations can further highlight the importance of each component used to activate the downstream target. For instance, disruption of the C-terminal domain of Elk1 that MAPK phosphorylates triggers inhibition of c-fos activation. Similarly, dysfunctional SRF, which normally tethers Elk1 to the SRE, leads to Fos not being transcribed. At the same time, without Elk1, SRF cannot induce c-fos transcription after MAPK stimulation. For these reasons, Elk1 represents an essential link between signal transduction pathways and the initiation of gene transcription.
may be dependent on Elk1. MEK inhibitors block Elk1 phosphorylation and, thus, impair acquired conditioned taste aversion. Moreover, avoidance learning, which involves the subject learning that a particular response leads to prevention of an aversive stimulus, is correlated with a definite increase in activation of Erk, Elk1, and c-fos in the hippocampus
. This area of the brain is involved in short-term and long-term information storage. Interestingly, when Elk1 or SRF binding to DNA is blocked in the rat hippocampus, only sequestration of SRF interferes with long-term spatial memory
. While the interaction of Elk1 with DNA may not be essential for memory formation, its specific role still needs to be explored. This is because activation of Elk1 can trigger other molecular events that do not require Elk1 to bind DNA. For example, Elk1 is involved in the phosphorylation of histones, increased interaction with SRF, and recruitment of the basal transcriptional machinery, all of which do not require direct binding of Elk1 to DNA.
, a strong and momentary hyperphosphorylation
of Erk and Elk1 is observed in the striatum. When these mice are then given MEK inhibitors, Elk1 phosphorylation is absent. Without active Elk1, c-fos production and cocaine-induced conditioned place preference
are shown to be blocked. Moreover, acute ethanol
ingestion leads to excessive phosphorylation of Elk1 in the amygdala
. Silencing of Elk1 activity has also been found to decrease cellular responses to withdrawal signals and lingering treatment of opioids, one of the world’s oldest known drugs. Altogether, these results highlight that Elk1 is an important component of drug addiction.
(APP). APP makes variants of Aβ (Aβ42/43 polypeptide). Moroever, PS1 is genetically associated with most early-onset cases of familial Alzheimer’s disease. These data emphasize the intriguing link between Aβ, Elk1, and PS1.
Another condition associated with Elk1 is Down syndrome
. Fetal and aged mice with this pathophysiological condition have shown a decrease in the activity of calcineurin
, the major phosphatase for Elk1. These mice also have age-dependent changes in ERK activation. Moreover, expression of SUMO3
, which represses Elk1 activity, increases in the adult Down syndrome patient. Therefore, Down syndrome is correlated with changes in ERK, calcineurin, and SUMO pathways, all of which act antagonistically on Elk1 activity.
Elk1 also interacts with BRCA1
splice variants, namely BRCA1a and BRCA1b. This interaction enhances BRCA1-mediated growth suppression in breast cancer
cells. Elk1 may be a downstream target of BRCA1 in its growth control pathway. Recent literature reveals that c-fos promoter activity is inhibited, while overexpression of BRCA1a/1b reduces MEK-induced activation of the SRE. These results show that one mechanism of growth and tumor suppression by BRCA1a/1b proteins acts through repression of the expression of Elk1 downstream target genes like Fos.
Depression has been linked with Elk1. Decreased Erk-mediated Elk1 phosphorylation is observed in the hippocampus and prefrontal cortex
of post-mortem brains of depressed suicide individuals. As of now, imbalanced Erk signaling is correlated with depression and suicidal behavior. Future research will reveal the exact role of Elk1 in the pathophysiology of depression.
Protein domain
A protein domain is a part of protein sequence and structure that can evolve, function, and exist independently of the rest of the protein chain. Each domain forms a compact three-dimensional structure and often can be independently stable and folded. Many proteins consist of several structural...
that regulates DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
binding to target sequences. Elk1 plays important roles in various contexts, including long-term memory
Long-term memory
Long-term memory is memory in which associations among items are stored, as part of the theory of a dual-store memory model. According to the theory, long term memory differs structurally and functionally from working memory or short-term memory, which ostensibly stores items for only around 20–30...
formation, drug addiction, Alzheimer’s disease, Down syndrome
Down syndrome
Down syndrome, or Down's syndrome, trisomy 21, is a chromosomal condition caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British physician who described the syndrome in 1866. The condition was clinically described earlier in the 19th...
, breast cancer
Breast cancer
Breast cancer is cancer originating from breast tissue, most commonly from the inner lining of milk ducts or the lobules that supply the ducts with milk. Cancers originating from ducts are known as ductal carcinomas; those originating from lobules are known as lobular carcinomas...
, and depression.
Elk1 Structure
As depicted in Figure 1, the Elk1 protein is composed of several domains. Localized in the N-terminal region, the A domain is required for the binding of Elk1 to DNA. This region also contains a nuclear localization signalNuclear localization signal
A nuclear localization signal or sequence is an amino acid sequence which 'tags' a protein for import into the cell nucleus by nuclear transport. Typically, this signal consists of one or more short sequences of positively charged lysines or arginines exposed on the protein surface. Different...
(NLS) and a nuclear export signal
Nuclear export signal
A nuclear export signal is a short amino acid sequence of 4 hydrophobic residues in a protein that targets it for export from the cell nucleus to the cytoplasm through the nuclear pore complex using nuclear transport. It has the opposite effect of a nuclear localization signal, which targets a...
(NES), which are responsible for nuclear import and export, respectively. The B domain allows Elk1 to bind to a dimer of its cofactor, serum response factor
Serum response factor
Serum response factor , also known as SRF, is a transcription factor.It is a member of the MADS box superfamily of transcription factors. This protein binds to the serum response element in the promoter region of target genes...
(SRF). Located adjacent to the B domain, the R domain is involved in suppressing Elk1 transcriptional activity. This domain harbors the lysine residues that are likely to undergo SUMOylation, a post-translational event that strengthens the inhibition function of the R domain. The D domain plays the key role of binding to active Mitogen-activated protein kinases (MAPKs). Located in the C-terminal region of Elk1, the C domain includes the amino acids that actually become phosphorylated by MAPKs. In this region, Serine 383 and 389 are key sites that need to be phosphorylated for Elk1-mediated transcription to occur. Finally, the DEF domain is specific for the interaction of activated extracellular signal-regulated kinase (Erk), a type of MAPK, with Elk1.
Expression of Elk1
Given its role as a transcription factorTranscription factor
In molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...
, Elk1 is expressed in the nuclei of non-neuronal cells. The protein is present in the cytoplasm as well as in the nucleus of mature neurons. In post-mitotic neurons, a variant of Elk1, sElk1, is expressed solely in the nucleus because it lacks the NES site present in the full-length protein. Moreover, while Elk1 is broadly expressed, actual levels vary among tissues. The rat brain, for example, is extremely rich in Elk1, but the protein is exclusively expressed in neurons.
Splice Variants
Aside from the full-length protein, the Elk1 gene can yield two shortened versions of Elk1: ∆Elk1 and sElk1. Alternative splicingAlternative splicing
Alternative splicing is a process by which the exons of the RNA produced by transcription of a gene are reconnected in multiple ways during RNA splicing...
produces ∆Elk1. This variant lacks part of the DNA-binding domain that allows interaction with SRF. On the other hand, sElk1 has an intact region that binds to SRF, but it lacks the first 54 amino acids that contain the NES. Found only in neurons, sElk1 is created by employing an internal translation start site. Both ∆Elk1 and sElk1, truncated versions of full-length protein, are capable of binding to DNA and inducing various cellular signaling. In fact, sElk1 counteracts Elk1 in neuronal differentiation and the regulation of nerve growth factor
Nerve growth factor
Nerve growth factor is a small secreted protein that is important for the growth, maintenance, and survival of certain target neurons . It also functions as a signaling molecule. It is perhaps the prototypical growth factor, in that it is one of the first to be described...
/ERK
Extracellular signal-regulated kinases
In molecular biology, extracellular-signal-regulated kinases or classical MAP kinases are widely expressed protein kinase intracellular signalling molecules that are involved in functions including the regulation of meiosis, mitosis, and postmitotic functions in differentiated cells...
signaling.
Elk1 Signaling
The downstream target of Elk1 is the serum response element (SRE) of the c-fos proto-oncogene. To produce c-fos, a protein encoded by the Fos gene, Elk1 needs to be phosphorylated by MAPKs at its C-terminus. MAPKs are the final effectors of signal transduction pathways that begin at the plasma membrane. Phosphorylation by MAPKs results in a conformational change of Elk1. As seen in Figure 2, Raf kinaseRaf kinase
Raf kinases are a family of three serine/threonine-specific protein kinases that are related to retroviral oncogenes. The mouse sarcoma virus 3611 contains a Raf kinase-related oncogene that enhances fibrosarcoma induction...
acts upstream of MAPKs to activate them by phosphorylating and, thereby activating, MEKs, or MAPK or ERK kinases. Raf itself is activated by Ras, which is linked to growth factor receptors with tyrosine kinase
Tyrosine kinase
A tyrosine kinase is an enzyme that can transfer a phosphate group from ATP to a protein in a cell. It functions as an "on" or "off" switch in many cellular functions....
activity via Grb2
Grb2
Growth factor receptor-bound protein 2 also known as Grb2 is an adaptor protein involved in signal transduction/cell communication. In humans, the GRB2 protein is encoded by the GRB2 gene....
and Sos. Grb2 and Sos can stimulate Ras only after the binding of growth factors to their corresponding receptors. However, Raf activation does not exclusively depend on Ras. Protein kinase C, which is activated by phorbol esters, can fulfill the same function as Ras. MEK kinase (MEKK) can also activate MEKs, which then activate MAPKs, making Raf unnecessary at times. Various signal transduction pathways, therefore, funnel through MEKs and MAPKs and lead to the activation of Elk1. After stimulation of Elk1, SRF, which allows Elk1 to bind to the c-fos promoter, must be recruited. The binding of Elk1 to SRF happens due to protein-protein interaction between the B domain of Elk1 and SRF and the protein-DNA interaction via the A domain.
The aforementioned proteins are like recipes for a certain signaling output. If one of these ingredients, such as SRF, is missing, then a different output occurs. In this case, lack of SRF leads to Elk1’s activation of another gene. Elk1 can, thus, independently interact with an ETS binding site, as in the case of the lck proto-oncogene in Figure 2. Moreover, the spacing and relative orientation of the Elk1 binding site to the SRE is rather flexible, suggesting that the SRE-regulated early genes other than c-fos could be targets of Elk1. egr-1 is an example of an Elk1 target that depends on SRE interaction. Ultimately, phosphorylation of Elk1 can result in the production of many proteins, depending on the other factors involved and their specific interactions with each other.
When studying signaling pathways, mutations can further highlight the importance of each component used to activate the downstream target. For instance, disruption of the C-terminal domain of Elk1 that MAPK phosphorylates triggers inhibition of c-fos activation. Similarly, dysfunctional SRF, which normally tethers Elk1 to the SRE, leads to Fos not being transcribed. At the same time, without Elk1, SRF cannot induce c-fos transcription after MAPK stimulation. For these reasons, Elk1 represents an essential link between signal transduction pathways and the initiation of gene transcription.
Role in Long-term Memory
Formation of long-term memoryLong-term memory
Long-term memory is memory in which associations among items are stored, as part of the theory of a dual-store memory model. According to the theory, long term memory differs structurally and functionally from working memory or short-term memory, which ostensibly stores items for only around 20–30...
may be dependent on Elk1. MEK inhibitors block Elk1 phosphorylation and, thus, impair acquired conditioned taste aversion. Moreover, avoidance learning, which involves the subject learning that a particular response leads to prevention of an aversive stimulus, is correlated with a definite increase in activation of Erk, Elk1, and c-fos in the hippocampus
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
. This area of the brain is involved in short-term and long-term information storage. Interestingly, when Elk1 or SRF binding to DNA is blocked in the rat hippocampus, only sequestration of SRF interferes with long-term spatial memory
Spatial memory
In cognitive psychology and neuroscience, spatial memory is the part of memory responsible for recording information about one's environment and its spatial orientation. For example, a person's spatial memory is required in order to navigate around a familiar city, just as a rat's spatial memory is...
. While the interaction of Elk1 with DNA may not be essential for memory formation, its specific role still needs to be explored. This is because activation of Elk1 can trigger other molecular events that do not require Elk1 to bind DNA. For example, Elk1 is involved in the phosphorylation of histones, increased interaction with SRF, and recruitment of the basal transcriptional machinery, all of which do not require direct binding of Elk1 to DNA.
Role in Drug Addiction
Elk1 activation plays a central role in drug addiction. After mice are given cocaineCocaine
Cocaine is a crystalline tropane alkaloid that is obtained from the leaves of the coca plant. The name comes from "coca" in addition to the alkaloid suffix -ine, forming cocaine. It is a stimulant of the central nervous system, an appetite suppressant, and a topical anesthetic...
, a strong and momentary hyperphosphorylation
Hyperphosphorylation
Hyperphosphorylation occurs when a biochemical with multiple phosphorylation sites is fully saturated. Hyperphosphorylation is one of the signalling mechanisms used by the cell to regulate mitosis. When these mechanisms fail, developmental problems or cancer are a likely outcome...
of Erk and Elk1 is observed in the striatum. When these mice are then given MEK inhibitors, Elk1 phosphorylation is absent. Without active Elk1, c-fos production and cocaine-induced conditioned place preference
Conditioned place preference
Conditioned place preference is a technique commonly used in animal studies to evaluate preferences for environmental stimuli that have been associated with a positive or negative reward...
are shown to be blocked. Moreover, acute ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
ingestion leads to excessive phosphorylation of Elk1 in the amygdala
Amygdala
The ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotional reactions, the amygdalae are considered part of the limbic system.-...
. Silencing of Elk1 activity has also been found to decrease cellular responses to withdrawal signals and lingering treatment of opioids, one of the world’s oldest known drugs. Altogether, these results highlight that Elk1 is an important component of drug addiction.
Role in Pathophysiological Conditions
Buildup of beta amyloid (Aβ) peptides is shown to cause and/or trigger Alzheimer’s disease. Aβ interferes with BDNF-induced phosphorylation of Elk1. With Elk1 activation being hindered in this pathway, the SRE-driven gene regulation leads to increased vulnerability of neurons. Elk1 also inhibits transcription of presenilin 1 (PS1), which encodes a protein that is necessary for the last step of the sequential proteolytic processing of amyloid precursor proteinAmyloid precursor protein
Amyloid precursor protein is an integral membrane protein expressed in many tissues and concentrated in the synapses of neurons. Its primary function is not known, though it has been implicated as a regulator of synapse formation, neural plasticity and iron export...
(APP). APP makes variants of Aβ (Aβ42/43 polypeptide). Moroever, PS1 is genetically associated with most early-onset cases of familial Alzheimer’s disease. These data emphasize the intriguing link between Aβ, Elk1, and PS1.
Another condition associated with Elk1 is Down syndrome
Down syndrome
Down syndrome, or Down's syndrome, trisomy 21, is a chromosomal condition caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British physician who described the syndrome in 1866. The condition was clinically described earlier in the 19th...
. Fetal and aged mice with this pathophysiological condition have shown a decrease in the activity of calcineurin
Calcineurin
Calcineurin is a protein phosphatase also known as protein phosphatase 3, PPP3CA, and calcium-dependent serine-threonine phosphatase, and formerly known as protein phosphatase 2B . It activates the T cells of the immune system and can be blocked by drugs...
, the major phosphatase for Elk1. These mice also have age-dependent changes in ERK activation. Moreover, expression of SUMO3
SUMO3
Small ubiquitin-related modifier 3 is a protein that in humans is encoded by the SUMO3 gene.-Interactions:SUMO3 has been shown to interact with ARNTL and Thymine-DNA glycosylase.-Further reading:...
, which represses Elk1 activity, increases in the adult Down syndrome patient. Therefore, Down syndrome is correlated with changes in ERK, calcineurin, and SUMO pathways, all of which act antagonistically on Elk1 activity.
Elk1 also interacts with BRCA1
BRCA1
BRCA1 is a human caretaker gene that produces a protein called breast cancer type 1 susceptibility protein, responsible for repairing DNA. The first evidence for the existence of the gene was provided by the King laboratory at UC Berkeley in 1990...
splice variants, namely BRCA1a and BRCA1b. This interaction enhances BRCA1-mediated growth suppression in breast cancer
Breast cancer
Breast cancer is cancer originating from breast tissue, most commonly from the inner lining of milk ducts or the lobules that supply the ducts with milk. Cancers originating from ducts are known as ductal carcinomas; those originating from lobules are known as lobular carcinomas...
cells. Elk1 may be a downstream target of BRCA1 in its growth control pathway. Recent literature reveals that c-fos promoter activity is inhibited, while overexpression of BRCA1a/1b reduces MEK-induced activation of the SRE. These results show that one mechanism of growth and tumor suppression by BRCA1a/1b proteins acts through repression of the expression of Elk1 downstream target genes like Fos.
Depression has been linked with Elk1. Decreased Erk-mediated Elk1 phosphorylation is observed in the hippocampus and prefrontal cortex
Prefrontal cortex
The prefrontal cortex is the anterior part of the frontal lobes of the brain, lying in front of the motor and premotor areas.This brain region has been implicated in planning complex cognitive behaviors, personality expression, decision making and moderating correct social behavior...
of post-mortem brains of depressed suicide individuals. As of now, imbalanced Erk signaling is correlated with depression and suicidal behavior. Future research will reveal the exact role of Elk1 in the pathophysiology of depression.