Serine/threonine protein kinase WNK4 also known as With No lysine (K) protein kinase 4(WNK4), is an enzyme that in humans is encoded by the WNK4 gene. [5] Missense mutations cause a genetic form of pseudohypoaldosteronism type 2, also called Gordon syndrome or Familial Hyperkalemic Hypertension.
WNK4 is a member of a serine/threonine kinase family that comprises four members. The family is so named because unlike other serine/threonine kinases, WNKs are characterized by the lack of the lysine in the subdomain II of the catalytic domain. [6] Instead, a lysine in the β2 strand of subdomain I of the catalytic domain is responsible for the kinase activity. [6]
In humans, the WNK4 gene is located on chromosome 17q21-q22. It produces a 1,243-amino acid protein encoded by a 3,732-nucleotide open reading frame within a 4 kb cDNA transcript. [7] WNK4 protein is highly expressed in the distal convoluted tubule (DCT) and the cortical collecting duct (CCD) of the kidney. [7] WNK4 is also present in the brain, lungs, liver, heart, and colon of various mammalian species. [8] [9] [10]
Gene mutations in WNK4 has been found in patients with pseudohypoaldosteronism type II (PHAII), [7] also known as familial hyperkalemic hypertension (FHHt) [11] or Gordon syndrome. [12] PHAII is an autosomal dominant hereditary disease characterized by hyperkalemia, hypertension, and metabolic acidosis. WNK4 plays a critical role in the regulation of various ion transporters and channels in the kidney. PHAII-causing mutations in WNK4 result in the dysregulation of renal sodium and potassium transporters and channels, leading to defects in sodium and potassium retention by the kidney, and in turn, elevated blood pressure and potassium concentration in the blood (hyperkalemia).
The tertiary structure of WNK4 has not been elucidated to date. Nevertheless, several individual domain structures of the protein are identified. These include a kinase domain near the amino terminus followed by an autoinhibitory domain, an acidic motif, two coiled-coil domains, and a calmodulin-binding domain in the C-terminal segment (Fig. 1). The structures for the kinase and autoinhibitory domains of WNK1 have been revealed. The high level of structural similarity between WNK4 and WNK1 allows us to deduce key structural details of WNK4 based on the insights gained from the corresponding regions within WNK1. The kinase domain of WNK4 has an 83% sequence identity with that of WNK1. The overall fold of the kinase domain of WNK1 resembles those of other protein kinases that have a typical dual-domain architecture. [13] The C-terminal domain of WNK4 bears a high degree of similarity to other kinases within the family. On the other hand, the N-terminal domain is unique in having a six-stranded instead of a five-stranded β sheet to form a complete β barrel. [13] A chloride ion binding site has been identified in the region 320DLG323 of the kinase domain in WNK4. [14] The binding of chloride Cl− in this region inhibits the activation of WNK4. The autoinhibitory domain is a homolog of the RFXV-binding PASK/FRAY homology 2 (PF2) domain. [15] Structural studies have revealed that the autoinhibitory domain consists of three β-strands and two α-helices. [16] Notably, the RFXV‐binding groove is formed by the β3-αA interface of WNK proteins where RFXV peptide ligand interacts directly with residues Phe524, Asp531, and Glu539 of WNK1. [16] The interaction between the RFXV motif and the autoinhibitory domain makes it possible for the C-terminal region of WNK4 to be in close proximity of the kinases domain and subsequently regulate its activity.
As a typical kinase, WNK4 accomplishes the phosphorylation of its substrate proteins by adding a phosphate moiety in an ATP-dependent manner. This structural modification usually results in functional alterations of downstream substrates. Some currently known substrates of WNK4 includes kinases STE20-serine-proline alanine-rich kinase (SPAK) and oxidative stress response 1 kinase (OSR1), which in turn can phosphorylate and activate the thiazide-sensitive sodium-chloride cotransporter (NCC) [17] [18] (Fig. 2). Similarly, WNK4 activates NKCC1 and deactivate KCC2 through a SPAK-dependent mechanism. [19] The kinase activity of WNK4 has been demonstrated in vitro using the WNK4 kinase domain purified from E. coli . [20] This phosphorylation cascade is critical in regulating sodium and potassium homeostasis dysregulation which is tied to the pathogenesis of PHAII.
In addition to NCC, WNK4 also regulates multiple ions channels and cotransporters in the kidney through various mechanisms. These include epithelial Na+ channel (ENaC), renal outer medullary potassium channel (ROMK), transient receptor potential vanilloid member 4 and 5 (TRPV4/5, calcium channels), Na-K-2Cl cotransporter 1 and 2 (NKCC1/2), K+-Cl− cotransporter type 2 (KCC2), and other channels/transporters. [21] WNK4 inhibits the functions of ENaC, ROMK, and TRPV4 by reducing the total and cell surface expression of these channels. [22] [23] [24] WNK4 enhances TRPV5 by increasing its forward trafficking to the plasma membrane in a kinase-dependent manner. [25] The inhibitory effect of WNK4 on ROMK is reversed by serum and glucocorticoid kinase 1 (SGK1) or by a corresponding phosphomimetic S1169D mutation on WNK4. [23] The N-terminal segment of WNK4 containing the kinase domain and acidic motif is required for the WNK4-mediated inhibition of ROMK. [26] The second coiled-coil domain of WNK4 mediates the downregulation of TRPV4. WNK4 and calcium-binding protein 39 (Cab39) act together to activate transporters NKCC1 and NKCC2. [27]
In 2001, four missense mutations in the WNK4 gene were identified in patients with pseudohypoaldosteronism type 2 (PHAII) (Fig. 1). [7] Three of these mutations (E562K, D564A, and Q565E) are charge-changing substitutions in the acidic motif of WNK4, which are conserved among all members of the WNK family in human and rodent species. The fourth substitution (R1185C) is located in the calmodulin-binding domain near the second coiled-coil domain. Few other PHAII mutations in WNK4 have also been reported. Examples of these mutations include E560G, [28] P561L, [29] and D564H, [30] all of which are located close to or in the acidic motif; and K1169E [31] which is located between the coiled-coil 2 and the calmodulin-binding domain. The PHAII mutations appear to disrupt the mechanisms underlying Ca2+-sensitivity of WNK4 kinase. Two mechanisms are important in this regard. First, the PHAII-causing mutations in the acidic motif make the kinase domain insensitive to Ca2+ concentration. The acidic motif of WNK4 potentially acts as a Ca2+ sensor, and WNK4 kinase activity rises when Ca2+ concentration is elevated. This has been demonstrated using isolated WNK4 kinase domain truncated to contain the acidic motif. [20] The kinase activity is elevated when a PHAII-causing mutation is present in the acidic motif, similar to what is observed in a Ca2+-binding state (Fig. 3). Second, the WNK4 C-terminal region containing the calmodulin-binding domain and multiple SGK1 phosphorylation sites inhibits the WNK4 activity at the resting state. [32] However, when Ca2+ levels are elevated, Ca2+/calmodulin complex binds to the C-terminal region, derepressing WNK4 kinase activity. Additionally, the RFXV motif is believed to interact with the autoinhibitory domain and subsequently triggers a conformational change that brings the C-terminal and kinase domain close for the inhibitory effect to take place. Angiotensin II increases the SPAK phosphorylation and activates NCC through a WNK-dependent mechanism. [33] The activation of SPAK and NCC by angiotensin II is abrogated by WNK4 knockdown. [34] Activation of angiotensin II receptor AT1 couples to Gq/11 to activate phospholipase C and to increase the intracellular Ca2+ concentration. An increase in Ca2+ concentration then elevates WNK4 activity through mechanisms described above (Fig. 3, left panel). The PHAII-causing mutations in the acidic motif and the R1185C mutation in the calmodulin-binding domain constitutively activate the WNK4 kinase domain allowing it to function despite the absence of angiotensin II (Fig. 3, right panel).
Angiotensin II stimulates the secretion of aldosterone, which induces SGK1. SGK1 influences both the WNK-SPAK-NCC [35] and SGK1-ENaC signaling cascades. [36] There are multiple SGK1 phosphorylation sites in the C-terminal region of WNK4 located within or close to the calmodulin-binding domain. SGK1-mediated phosphorylation of these sites is thought to disrupt the effect of the C-terminal inhibitory domain and concomitantly increase WNK4 kinase activity. [32] Additionally, it has been shown that PKA and PKC kinases are able to phosphorylate several sites of WNK4, which increases its activity. [37] The alteration of SGK1 phosphorylation by the R1185C mutation is another indication that the mutation disrupts the C-terminal inhibitory mechanism in WNK4 (Fig. 3, right panel).
Besides WNK1 and WNK4, mutations in two other genes, CUL3 (encoding Cullin 3) and KLHL3 (encoding Kelch Like Family Member 3) have been found in patients with PHAII. [38] [39] These two proteins are part of the ubiquitin E3 ligase complex involved in the ubiquitin-mediated degradation of WNK1 and WNK4. The PHAII-causing mutations in KLHL3 and cullin 3 prevent the interactions of these proteins with each other and with WNK1/4. The mutations in these proteins impair the degradation of WNK1/4. This in turn increases the protein abundance of WNK1/4 and concomitantly enhances the total kinase activity. [40] The increased WNK4 kinase activity leads to the hyperactivation of NCC through WNK4-SPAK/OSR1-NCC cascade, ultimately resulting in the retention of sodium and potassium by the kidney.
The primary effect of elevated WNK4 kinase activity is the increase of NCC-mediated sodium reabsorption in the distal convoluted tubule of the kidney. The increase in sodium reabsorption in this segment of the nephron reduces the sodium load in the collecting duct, where sodium reabsorption by the ENaC provides the driving force for potassium secretion through ROMK (Fig. 4). The sodium reabsorption by hyperactive NCC overrides the loss of reabsorption by ENaC, and the net effect is moderate sodium retention. Over time, this potentially contributes to the elevated blood pressure observed in PHAII patients. The reduction of potassium secretion by ROMK contributes to the development of hyperkalemia. The direct effects of the elevated WNK4 activity on other channels and transporters, such as ENaC, ROMK, and Ca2+-activated maxi K+ channels, may also contribute to the pathogenesis of PHAII; however, the primary features of PHAII could be explained by the gain-of-function of NCC. [41]
Calmodulin (CaM) (an abbreviation for calcium-modulated protein) is a multifunctional intermediate calcium-binding messenger protein expressed in all eukaryotic cells. It is an intracellular target of the secondary messenger Ca2+, and the binding of Ca2+ is required for the activation of calmodulin. Once bound to Ca2+, calmodulin acts as part of a calcium signal transduction pathway by modifying its interactions with various target proteins such as kinases or phosphatases.
In cell biology, Protein kinase C, commonly abbreviated to PKC (EC 2.7.11.13), is a family of protein kinase enzymes that are involved in controlling the function of other proteins through the phosphorylation of hydroxyl groups of serine and threonine amino acid residues on these proteins, or a member of this family. PKC enzymes in turn are activated by signals such as increases in the concentration of diacylglycerol (DAG) or calcium ions (Ca2+). Hence PKC enzymes play important roles in several signal transduction cascades.
CAMK, also written as CaMK or CCaMK, is an abbreviation for the Ca2+/calmodulin-dependent protein kinase class of enzymes. CAMKs are activated by increases in the concentration of intracellular calcium ions (Ca2+) and calmodulin. When activated, the enzymes transfer phosphates from ATP to defined serine or threonine residues in other proteins, so they are serine/threonine-specific protein kinases. Activated CAMK is involved in the phosphorylation of transcription factors and therefore, in the regulation of expression of responding genes. CAMK also works to regulate the cell life cycle (i.e. programmed cell death), rearrangement of the cell's cytoskeletal network, and mechanisms involved in the learning and memory of an organism.
The renal outer medullary potassium channel (ROMK) is an ATP-dependent potassium channel (Kir1.1) that transports potassium out of cells. It plays an important role in potassium recycling in the thick ascending limb (TAL) and potassium secretion in the cortical collecting duct (CCD) of the nephron. In humans, ROMK is encoded by the KCNJ1 gene. Multiple transcript variants encoding different isoforms have been found for this gene.
Telokin is an abundant protein found in smooth-muscle. It is identical to the C-terminus of myosin light-chain kinase. Telokin may play a role in the stabilization of unphosphorylated smooth-muscle myosin filaments. Because of its origin as the C-terminal end of smooth muscle myosin light chain kinase, it is called "telokin".
Myosin light-chain kinase also known as MYLK or MLCK is a serine/threonine-specific protein kinase that phosphorylates a specific myosin light chain, namely, the regulatory light chain of myosin II.
Pseudohypoaldosteronism (PHA) is a condition that mimics hypoaldosteronism. Two major types of primary pseudohypoaldosteronism are recognized and these have major differences in etiology and presentation.
Calmodulin-binding proteins are, as their name implies, proteins which bind calmodulin. Calmodulin can bind to a variety of proteins through a two-step binding mechanism, namely "conformational and mutually induced fit", where typically two domains of calmodulin wrap around an emerging helical calmodulin binding domain from the target protein.
The sodium-chloride symporter (also known as Na+-Cl− cotransporter, NCC or NCCT, or as the thiazide-sensitive Na+-Cl− cotransporter or TSC) is a cotransporter in the kidney which has the function of reabsorbing sodium and chloride ions from the tubular fluid into the cells of the distal convoluted tubule of the nephron. It is a member of the SLC12 cotransporter family of electroneutral cation-coupled chloride cotransporters. In humans, it is encoded by the SLC12A3 gene (solute carrier family 12 member 3) located in 16q13.
Ca2+
/calmodulin-dependent protein kinase II is a serine/threonine-specific protein kinase that is regulated by the Ca2+
/calmodulin complex. CaMKII is involved in many signaling cascades and is thought to be an important mediator of learning and memory. CaMKII is also necessary for Ca2+
homeostasis and reuptake in cardiomyocytes, chloride transport in epithelia, positive T-cell selection, and CD8 T-cell activation.
Phosphorylase kinase (PhK) is a serine/threonine-specific protein kinase which activates glycogen phosphorylase to release glucose-1-phosphate from glycogen. PhK phosphorylates glycogen phosphorylase at two serine residues, triggering a conformational shift which favors the more active glycogen phosphorylase "a" form over the less active glycogen phosphorylase b.
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.
Neural precursor cell expressed developmentally downregulated gene 4-like (NEDD4L) or NEDD4-2 is an enzyme of the NEDD4 family. In human the protein is encoded by the NEDD4L gene. In mouse the protein is commonly known as NEDD4-2 and the gene Nedd4-2.
Calcium/calmodulin-dependent protein kinase type II gamma chain is an enzyme that in humans is encoded by the CAMK2G gene.
The SCNN1G gene encodes for the γ subunit of the epithelial sodium channel ENaC in vertebrates. ENaC is assembled as a heterotrimer composed of three homologous subunits α, β, and γ or δ, β, and γ. The other ENAC subunits are encoded by SCNN1A, SCNN1B, and SCNN1D.
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).
Calcium/calmodulin-dependent protein kinase type 1 is an enzyme that in humans is encoded by the CAMK1 gene.
Myosin light chain kinase 2 also known as MYLK2 is an enzyme which in humans is encoded by the MYLK2 gene.
Serine/threonine-protein kinase WNK3, also known as protein kinase lysine-deficient 3, is a protein that in humans is encoded by the WNK3 gene.
Serine/threonine-protein kinase Sgk1 also known as serum and glucocorticoid-regulated kinase 1 is an enzyme that in humans is encoded by the SGK1 gene.