Cyclin-dependent kinase 5 is a protein, and more specifically an enzyme, that is encoded by the Cdk5 gene. It was discovered 15 years ago, and it is saliently expressed in post-mitotic central nervous system neurons (CNS).
The molecule belongs to the cyclin-dependent kinase family. Kinases are enzymes that catalyze reactions of phosphorylation. This process allows the substrate to gain a phosphate group donated by an organic compound known as ATP. Phosphorylations are of vital importance during glycolysis, therefore, making kinases an essential part of the cell due to their role in the metabolism, cell signaling, and many other processes.
Cdk5 is a proline-directed serine/threonine kinase, which was first identified as a CDK family member due to its similar structure to CDC2/CDK1 in humans, a protein that plays a crucial role in the regulation of the cell cycle.
The gene Cdk5 contains 12 exons in a region that contains around 5000 nucleotides (5kb), as it was determined by Ohshima after cloning the Cdk5 gene that belonged to a mouse.
Cdk5 has 292 amino acids and presents both α-helix and β strand structures. [5]
Even though Cdk5 has a similar structure to other cyclin-dependent kinases, its activators are highly specific (CDK5R1 and CDK5R2).
Some investigations [6] have reported that the active states of protein kinases structurally differ from each other in order to preserve the geometry of its machinery so that catalytic output works properly. The Cdk5 kinase has an original design as well.
Cdk5 belongs to the eukaryotic protein kinases (ePKs). A crystal structure of the catalytic domain of cAMP-dependent protein kinase showed that it holds 2 lobes; on the one hand, it has a small lobe, an N-terminal arranged as an antiparallel β-sheet structure. Furthermore, it contains nucleotide motifs as a way to orient the nucleotide for phospho-transfer. On the other hand, the large lobe, a C-terminal, is helical shaped, which helps to identify the substrate and includes crucial residues for the phospho-transfer.
Recently Cdk5 has emerged as an essential kinase in sensory pathways. Recent reports by Pareek et al. suggest its necessity in pain signaling. CDK5 is required for proper development of the brain, and to be activated, it must associate with CDK5R1 or CDK5R2. [7] [8] Unlike other cyclin-dependent kinases, CDK5 does not also require phosphorylation on the T loop. Therefore, binding with the activator is sufficient to activate the kinase. [9]
Cdk5 is abundant and mainly expressed in neurons, where it phosphorylates protein polymers with a high molecular weight called neurofilaments, and microtubule-associated protein tau, which are abundant in the CNS (Central Nervous System). [10] The enzyme is involved in many aspects of neuronal development and functions.
The main role of Cdk5 when it comes to neurons is to assure proper neuronal migration. Neurons will send out both dendrites and axons to form connections with other neurons in order to transmit information, and Cdk5 regulates this process. In order to perform, Cdk5 needs to be activated by p35 (these 3 amino acids, Asp-259, Asn-266, and Ser-270, are involved in the formation of hydrogen bonds with Cdk5 [11] ) or p39 (the isoform of p35), which are two of its neuron-specific regulatory subunits. This means that the level of expression of p35 and p39 is going to be related to the activity of the enzyme. If there is a high activity of Cdk5 during brain development, its activators will have a high expression. As a matter of fact, when studies were conducted on mice without p35 and p39, the results were the same as the ones observed on mice without Cdk5: there were clear disruptions of the laminar structures in the cerebral cortex, the olfactory bulb, the hippocampus, and the cerebellum. These areas' proper development and functionality depend on Cdk5, which relies on the correct expression of p35 and p39. Also, Cdk5 collaborates with Reelin signaling in order to assure the proper neuronal migration in the developing brain.
Cdk5 is not only implicated in neuronal migration. The enzyme will also help manage neurite extension, synapse formation, and synaptic transmission. It is also worth noting that Cdk5 also regulates the process of apoptosis, which is necessary in order to assure that the neural connections that are formed are correct. Moreover, due to the fact that Cdk5 also intervenes in the regulation of synaptic plasticity, it is implicated in the processes of learning and memory formation, as well as the creation of drug addiction.
On top of that, Cdk5 modulates actin-cytoskeleton dynamics by phosphorylating Pak1 and filamin 1 and regulates the microtubules by also phosphorylating tau, MAP1B, doublecortin, Nudel, and CRMPs, which are all microtubule-associated proteins. A non-proper expression of Cdk5 will generate defects in these substrates that can lead to multiple illnesses. For example, a defect on filamin 1 in humans provokes periventricular heterotopia; and a defect on Lis1 and doublecortin will cause lissencephaly type 1. As a matter of fact, four members of a consanguineous Israeli Muslim family that suffered from lissencephaly-7 with cerebellar hypoplasia had a splice site mutation in the Cdk5 gene. [12] [13]
Cdk5 has been proven to be directly linked with drug abuse. It is established that drugs act on the reward system by disturbing intracellular signal transduction pathways, with Cdk5 being involved. [14] Upon repetitive administration, several components of dopamine signalling are modified, including changes in gene expression and the circuitry of dopaminoceptive neurons.
In the example of cocaine, CREB (cAMP Response Element Binding) causes a transient burst in immediate-early gene expression in the striatum, as well as the expression of ΔFosB isoforms, which accumulate and persist in striatal neurons with an extremely long half-life. Many studies have revealed that the overexpression of ΔFosB due to drug abuse is the cause of an upregulation of Cdk5, it being downstream of ΔFosB expression in the striatum, including the nucleus accumbens.
It has been established that with repeated exposure to drugs such as cocaine and overexpression of ΔFosB isoforms, Cdk5 is upregulated, mediated by the upregulation of p35.
It has also been demonstrated that this enzyme has an important place in dopamine neurotransmission regulation. Indeed, Cdk5 can act on the dopamine system by phosphorylating DARPP-32. As a consequence of tof Cdk5 upregulation, there is also a rise in the number of dendritic branch points and spines, both in medium spiny neurons in the nucleus accumbens and pyramidal neurons in the medial prefrontal cortex. Hence, its involvement in the reward system, and by extension addiction.
Further analysis of the relationship between Cdk5 proportion and drug effects has shown that there is a strong dependence on the dose and frequency of administration. [15] For instance, if the frequency of the cocaine dose is low, or the dose is continuously administered over a period, the cocaine effects will be present even though the production of Cdk5 in the nucleus accumbens, in the ventral tegmental area, and prefrontal cortex activity will not increase. However, when it comes to significantly frequent doses, the effects of cocaine are not displayed despite the enhanced proportion of Cdk5. Those differences can be explained by the fact that Cdk5 is a transitional state to overexposure to drugs like cocaine.
Cdk5 has been suggested as a therapeutic target in addiction management. For example, it has been proved that sustained administration of Cdk5 antagonists inhibits the growth of spiny dendrites in the nucleus accumbens, which could be an avenue for addiction management. Further, Cdk5 could be used as a diagnostic marker for addiction.
Even though the main role of Cdk5 is related to neuronal migration, its impact on the human body is not limited to the nervous system. Indeed, Cdk5 plays an important part in the control of insulin secretion in the pancreas.
Actually, this enzyme has been found in pancreatic β cells and has been proven to reduce insulin exocytosis by phosphorylating L-VDCC (L-type voltage-dependent Ca2+ channel). [16]
During T-cell activation, Cdk5 phosphorylates coronin 1a, a protein that contributes to the process of phagocytosis and regulates actin polarization. Therefore, this kinase promotes T-cell survival and motility. [17]
Cdk5 also takes part in the production of interleukin 2 (IL-2), a cytokine involved in cell signaling, by T-cells. To do so, it disrupts the repression of interleukin 2 transcription by the Histone deacetylase 1 (HDAC1) through mSin3a protein phosphorylation. This reduces the ability of the HDAC1/mSin3a complex to bind to the IL-2 promoter, which leads to an increased interleukin 2 production. [18]
Synaptic vesicle exocytosis is also regulated by CdK5, with the phosphorylation of the munc-18-a protein, which is indispensable for secretion, as it has a great affinity with a derivative of SNAP receptor (SNARE protein). This phosphorylation was demonstrated with the simulation of secretion from neuroendocrine cells, since the Cdk5 activity increased. When Cdk5 was removed, the norepinephrine secretion decreased. [19]
Thanks to an experiment with mice, a relation between memory and Cdk5 was demonstrated. On one hand, mice did not show fear integrated by a previous activity when Cdk5 was inactivated. On the other hand, when the enzyme activity was increased in the hippocampus -where memories are stored- the fear reappeared.
During embryogenesis, Cdk5 is essential for brain development as it is crucial for the regulation of the cytoskeleton that in turn is important for remodelling in the brain. [20] Several neuronal processes: pain signalling, drug addiction, behavioural changes, the formation of memories and learning, related to the development of the brain, derive from rapid modifications in cytoskeleton. A negative remodelling of neuronal cytoskeleton will be associated with a loss of synapses and neurodegeneration in brain diseases, where the Cdk5 activity is deregulated. Therefore, most part of Cdk5 substrates are related to the actin skeleton; both, the physiological and the pathological ones. Some of them have been identified in the recent decades: ephexin1, p27, Mst3, CaMKv, kalirin-7, RasGRF2, Pak1, WAVE1, neurabin-1, TrkB, 5-HT6R, talin, drebrin, synapsin I, synapsin III, CRMP1, GKAP, SPAR, PSD-95, and LRRK2. [20]
The mammalian circadian clock is controlled by Cdk5 with the phosphorylation of PER2. [21] [22] In the laboratory, Cdk5 was blocked in the SCN (suprachiasmatic nuclei, a master oscillator of the circadian system), consequently the free-running period in mice was reduced. During the diurnal period, the PER2 [23] (at serine residue 394) was phosphorylated by the Cdk5, thus, the Cryptochrome 1 (CRY1 [24] ) could easily interact with it and the PER2-CRY1 complex went into the nucleus. The molecular circadian cycle and period are properly established thanks to the task of the Cdk5 as a nuclear driver of these proteins.
In addition to all the roles previously mentioned, the Cdk5 is involved in numerous cellular functions such as cell mobility survival, apoptosis, and gene regulation. [16] [25]
The plasma membrane, cytosol and perinuclear region are the locations where Cdk5/p35 activators are found. Nevertheless, Cdk5 can also be activated by cyclin I, this regulator causes an increase in the expression of BCl-2 family proteins, which are associated with anti-apoptotic functions.
The chemical explanation of a wide variety of neurological disorders lead to the Cdk5; the abnormal phosphorylation of tau is a pathological action carried out by this kinase and the neurofibrillary tangles are the consequences.
Cdk5 plays an essential role in the central nervous system. During the process of embryogenesis, this kinase is necessary for the development of the brain; and in adult brains, Cdk5 is needed for many neuronal processes; for instance, learning and the formation of memories. Nevertheless, if Cdk5 activity is deregulated, it can lead to really severe neurological diseases, including Alzheimer's, Parkinson, Multiple sclerosis and Huntington's disease. [26]
Cdk5 is involved in invasive cancers, apparently by reducing the activity of the actin regulatory protein caldesmon. [32]
Although Cdk5 is not mutated in cancer tissues, its activity and expression are deregulated. The kinase phosphorylates tumor suppressors and transcription factors, which are involved in cell cycle progression. Cdk5 is involved in tumor proliferation, migration, angiogenesis and also chemotherapy resistance and anti-tumor immunity. It also participates in signalling pathways that lead to metastasis, and it regulates the cytoskeleton and focal adhesions. [33]
Possible angiogenesis mechanisms mediated by Cdk5 | |
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Cdk5 promotes the expression of vascular endothelial growth factor (VEGF), a protein that regulates vasculogenesis and angiogenesis, according to a study on pituitary adenomas. VEGF stimulates the division and migration of endothelial cells, as well as vascular permeability. [34] | |
Cdk5 promotes angiogenesis by remodelling the actin cytoskeleton via Rac1, a signaling GTPase. It may also regulate the formation of lamellipodia, which are membrane protrusions involved in cell migration. [35] | |
Cdk5 phosphorylation and activation of presenilin stimulates NICD (Notch intracellular domain). As a consequence, Notch-dependent signalling, a key angiogenesis-promoting pathway, is activated. [36] |
A possible cancer treatment could consist in targeting Cdk5 and avoiding its binding to its activators and substrates.
In recent studies, [37] about radiation therapy in patients with large cell lung cancer, it has been found that CDK5 depletion diminishes lung cancer development and radiation resistance in vitro and in vivo. It was demonstrated that a decrease in Cdk5 reduced the expression of TAZ, [38] a component of the Hypothalamus pathway. As a result, this loss mitigates the signal activation from the Hypothalamus. Consequently, Cdk5 can be treated as a target to fight lung cancer.
CDK5 was originally named NCLK (Neuronal CDC2-Like Kinase) due to its similar phosphorylation motif. CDK5 in combination with an activator was also referred to as Tau Protein Kinase II. [39] Furthermore, Cdk5 has been reported to be involved in T cell activation and play an important role in development of autoimmune disorders, such as multiple sclerosis. [40]
Click on genes, proteins and metabolites below to link to respective articles. [§ 1]
Cyclin-dependent kinase 5 has been shown to interact with different molecules and substrates:
In biochemistry, a kinase is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule donates a phosphate group to the substrate molecule. As a result, kinase produces a phosphorylated substrate and ADP. Conversely, it is referred to as dephosphorylation when the phosphorylated substrate donates a phosphate group and ADP gains a phosphate group. These two processes, phosphorylation and dephosphorylation, occur four times during glycolysis.
Cyclin-dependent kinases (CDKs) are a predominant group of serine/threonine protein kinases involved in the regulation of the cell cycle and its progression, ensuring the integrity and functionality of cellular machinery. These regulatory enzymes play a crucial role in the regulation of eukaryotic cell cycle and transcription, as well as DNA repair, metabolism, and epigenetic regulation, in response to several extracellular and intracellular signals. They are present in all known eukaryotes, and their regulatory function in the cell cycle has been evolutionarily conserved. The catalytic activities of CDKs are regulated by interactions with CDK inhibitors (CKIs) and regulatory subunits known as cyclins. Cyclins have no enzymatic activity themselves, but they become active once they bind to CDKs. Without cyclin, CDK is less active than in the cyclin-CDK heterodimer complex. CDKs phosphorylate proteins on serine (S) or threonine (T) residues. The specificity of CDKs for their substrates is defined by the S/T-P-X-K/R sequence, where S/T is the phosphorylation site, P is proline, X is any amino acid, and the sequence ends with lysine (K) or arginine (R). This motif ensures CDKs accurately target and modify proteins, crucial for regulating cell cycle and other functions. Deregulation of the CDK activity is linked to various pathologies, including cancer, neurodegenerative diseases, and stroke.
The restriction point (R), also known as the Start or G1/S checkpoint, is a cell cycle checkpoint in the G1 phase of the animal cell cycle at which the cell becomes "committed" to the cell cycle, and after which extracellular signals are no longer required to stimulate proliferation. The defining biochemical feature of the restriction point is the activation of G1/S- and S-phase cyclin-CDK complexes, which in turn phosphorylate proteins that initiate DNA replication, centrosome duplication, and other early cell cycle events. It is one of three main cell cycle checkpoints, the other two being the G2-M DNA damage checkpoint and the spindle checkpoint.
The MAPK/ERK pathway is a chain of proteins in the cell that communicates a signal from a receptor on the surface of the cell to the DNA in the nucleus of the cell.
Protein kinase RNA-activated also known as protein kinase R (PKR), interferon-induced, double-stranded RNA-activated protein kinase, or eukaryotic translation initiation factor 2-alpha kinase 2 (EIF2AK2) is an enzyme that in humans is encoded by the EIF2AK2 gene on chromosome 2. PKR is a serine/tyrosine kinase that is 551 amino acids long.
Cell division protein kinase 6 (CDK6) is an enzyme encoded by the CDK6 gene. It is regulated by cyclins, more specifically by Cyclin D proteins and Cyclin-dependent kinase inhibitor proteins. The protein encoded by this gene is a member of the cyclin-dependent kinase, (CDK) family, which includes CDK4. CDK family members are highly similar to the gene products of Saccharomyces cerevisiae cdc28, and Schizosaccharomyces pombe cdc2, and are known to be important regulators of cell cycle progression in the point of regulation named R or restriction point.
Serine/threonine-protein kinase PAK 1 is an enzyme that in humans is encoded by the PAK1 gene.
Calcium/calmodulin-dependent protein kinase type II subunit alpha (CAMKIIα), a.k.a.Ca2+/calmodulin-dependent protein kinase II alpha, is one subunit of CamKII, a protein kinase (i.e., an enzyme which phosphorylates proteins) that in humans is encoded by the CAMK2A gene.
ETS Like-1 protein Elk-1 is a protein that in humans is encoded by the ELK1. Elk-1 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.
In enzymology, a tau-protein kinase is an enzyme that catalyzes the chemical reaction
Cyclin-dependent kinase 5 activator 1 is an enzyme that in humans is encoded by the CDK5R1 gene.
WNK , also known as WNK1, is an enzyme that is encoded by the WNK1 gene. WNK1 is serine-threonine protein kinase and part of the "with no lysine/K" kinase WNK family. The predominant role of WNK1 is the regulation of cation-Cl− cotransporters (CCCs) such as the sodium chloride cotransporter (NCC), basolateral Na-K-Cl symporter (NKCC1), and potassium chloride cotransporter (KCC1) located within the kidney. CCCs mediate ion homeostasis and modulate blood pressure by transporting ions in and out of the cell. WNK1 mutations as a result have been implicated in blood pressure disorders/diseases; a prime example being familial hyperkalemic hypertension (FHHt).
Protein phosphorylation is a reversible post-translational modification of proteins in which an amino acid residue is phosphorylated by a protein kinase by the addition of a covalently bound phosphate group. Phosphorylation alters the structural conformation of a protein, causing it to become activated, deactivated, or otherwise modifying its function. Approximately 13,000 human proteins have sites that are phosphorylated.
Cyclin-dependent kinase 5 activator 2 is an enzyme that in humans is encoded by the CDK5R2 gene.
Serine/threonine-protein kinase LMTK2 also known as Lemur tyrosine kinase 2 (LMTK2) is an enzyme that in humans is encoded by the LMTK2 gene.
The Akt signaling pathway or PI3K-Akt signaling pathway is a signal transduction pathway that promotes survival and growth in response to extracellular signals. Key proteins involved are PI3K and Akt.
Wee1 is a nuclear kinase belonging to the Ser/Thr family of protein kinases in the fission yeast Schizosaccharomyces pombe. Wee1 has a molecular mass of 96 kDa and is a key regulator of cell cycle progression. It influences cell size by inhibiting the entry into mitosis, through inhibiting Cdk1. Wee1 has homologues in many other organisms, including mammals.
Collapsin response mediator protein family or CRMP family consists of five intracellular phosphoproteins of similar molecular size and high (50–70%) amino acid sequence identity. CRMPs are predominantly expressed in the nervous system during development and play important roles in axon formation from neurites and in growth cone guidance and collapse through their interactions with microtubules. Cleaved forms of CRMPs have also been linked to neuron degeneration after trauma induced injury.
Li-Huei Tsai is an American neuroscientist and the director of the Picower Institute for Learning and Memory in the Department of Brain and Cognitive Sciences at the Massachusetts Institute of Technology.
Gentry Namón Patrick is an American biologist and Professor of Neurobiology at the University of California, San Diego. His research investigates the mechanisms that underpin synaptic activity in the central nervous system. He is interested in learning, the formation of memories and the processes that cause Alzheimer's disease and Parkinson's disease.