Abbreviations
PARs- Protease activated receptors
I/R- Ischemia reperfusion
AKI- Acute kidney injury
PASD- Periodic acid- Schiff diastase
WT- Wild type
KIM-1- Kidney injury molecule-1
MCP-1- Monocyte chemoattractant protein-1
UUO- Unilateral ureteral obstruction
TEM- Transmission electron microscopy
LC3-II- Light chain 3-II
HE- Hematoxylin and eosin staining
PAN- Puromycin aminonucleoside
NS- Nephrotic syndrome
DIN-Drug induced nephropathy
ESRD-End stage renal disease
TF-Tissue factor
PLA- Proximity ligation assays
ERK- Extracellular signal-regulated kinase
RT-PCR- Real-Time polymerase chain reaction
IHC- Immunohistochemistry
eNOS- Endothelial NO synthase
uPA- Urokinase plasminogen activator
uPAR- Urokinase plasminogen activator receptor
PTC-Proximal tubule cells
Introduction
PARs are cell surface receptors, of the family G-protein- coupled receptors (GPCRs) with extracellular amino terminus domain, and consisting of four identified subtypes namely; PAR1, PAR2, PAR3, and PAR4 [1-3]. Although the pathological role of PAR1 and PAR2 is well described in certain experimental kidney injury models, less information is available on PAR3 and PAR4. Notably, some endogenous proteases such as trypsin, tryptase, cathepsin, urokinase, and kallikreins are activators of PARs that regulate renal homeostasis, inflammation, and tissue remodeling as both are interlinked with the pathogenesis of acute kidney injury (AKI) [4-8]. Similarly, during disease conditions such as sepsis-induced glomerulonephritis or septic shock, PARs can also be activated by clotting factors namely thrombin, FVIIa, and FXa. These pathological conditions might involve hyper-coagulability, and inflammation which can lead to clotting factor-dependent up-regulation of PAR receptor expressions and this further induce(s) glomerular fibrin deposition and macrophage infiltration [9-10]. Moreover, during the diseased conditions, activation of PARs by proteolytic cleavage at the extracellular amino terminus domain results in the formation of tethered ligand. Later, this newly formed tethered ligand binds to the receptor body and can activate the transmembrane signaling molecules involving MAPKs, NF-kB and PI3K/AKT/mTOR, which are responsible for producing various pathophysiological responses such as mesangial cell proliferation, extracellular matrix production, renal fibrin deposition, podocyte apoptosis and also necrosis in kidney disease [1-3, 11-14]. Both PAR1 and PAR2 subtypes are expressed in the kidney and known to promote fibro-proliferative disorders and exacerbate diabetic nephropathy (DN) [6, 15-18]. Additionally, PAR1 and PAR2 are also responsible for initiating podocyte cell injury, tubular epithelial cell inflammation and mesangial cell expansion in the kidney [5, 19, 20]. Moreover, few studies have reported that the PAR2 receptor subtype contributes to the progression of kidney damage in cisplatin and also IgA-induced nephropathy, which suggests the disease’s specific involvement of particular subtypes [21-23]. This review summarizes the various findings which have tested the role of PAR’s in the pathophysiology of kidney diseases, provides in-depth insights into the mechanisms and discusses the standing challenges in therapeutic testing of these receptor subtypes in kidney diseases.
A PubMed based literature survey was performed using the following phrases in all possible combinations mainly: “protease activating receptor subtypes in kidney diseases” (43 results), “role of PAR-1 in renal injuries” (09 results), “role of PAR-2 in kidney disease” (12 results), “role of PAR-3 in renal disease” (5 results), “PAR in glomerular and tubular injury” (10 results), “PAR in drug-induced nephrotoxicities” (27 results), “PAR in kidneys inflammation and fibrosis” (27 results), “effects of PAR in diabetic kidney disease” (25 results), “role of PAR 1 and PAR2 in ischemia injury in rats” (4 results) This survey resulted into a total of 162 published articles and after scrutinization only 13 original research evidence focused on the involvement of PARs in renal diseases, published between 1995-2021. The selected studies tested the role of different PAR subtypes in diverse renal injuries including ischemia-reperfusion injury, type I/II diabetic nephropathy, sepsis-induced acute kidney injury, nephrotic syndrome, biologicals based renal injury, obstructive injury, drug-induced nephrotoxicities, podocytopenia, in-vitro and ex-vivo findings, glomerular as well as tubular diseases. Evidence pertaining to the impact of PARs subtypes in co-morbidities, other organ injuries, and clinical manifestations were excluded from the present review.
Evolutionary background of PAR subtypes
The major breakthrough came in the 1990s when it was first discovered that in a wide variety of tissues some membrane-spanning proteins do exist on the cell surfaces and are known as protease-activated receptors (PAR) that belong to the G protein-coupled receptor family. In the year 1991, the first member of PAR receptors was identified and cloned known as protease-activated receptor-1 (PAR-1) [24]. To date, four members of PARs have been identified, of which PAR1, PAR3, and PAR4 are known as thrombin receptors, and on the other hand PAR2 is known as serine receptor [25]. Notably, a study in PAR1 knockout mice lead to the discovery of PAR3 and PAR4 receptors which showed a similar response to thrombin receptor on platelet as that of PAR1 [26]. In the later year, PAR2 was discovered serendipitously and was found to be activated only by a serine receptor protein called trypsin [25]. Presently Vorapaxar and atopaxar are available PAR-1 antagonists that have undergone extensive clinical development and are approved as antiplatelet agents [27].
Expression of PARs in the kidneys
In the kidneys, the expression of PAR1 and PAR2 is most abundant in contrast to that of other receptor subtypes [2, 28-30]. Previous in vitro studies using the RT-PCR technique have detected the transcripts of PAR1, PAR2, and PAR4 in both isolated endothelial and epithelial cells of humans and rodent kidneys. However, mRNA for PAR3 was found to be absent in two of these studies [2, 29]. The PAR receptors are endogenously activated by serine proteases which are also part of the clotting system, where thrombin is responsible for PAR1, PAR3, PAR4 activation, and trypsin activates PAR2. Moreover, some urinary proteases including urokinase, kallikrein can also activate PAR1 and PAR2 and have been summarized in table 1 [1, 2, 28]. The Urokinase enzyme is also called a urokinase plasminogen activator (uPA). It is synthesized in the kidneys by proximal and distal tubular epithelial cells and is secreted in the urine. Other sources which also synthesize urokinase are monocytes (macrophages), fibroblast, and myofibroblasts [31,32]. Moreover, it is believed that urokinase is released during inflammation or injury as confirmed by experimental studies in mice [33]. This study showed that mRNA expression of urokinase plasminogen activator receptor (uPAR) was absent in normal kidneys whereas, it was up-regulated in the kidneys of UUO induced chronic kidney injury in mice. Another urinary protease, kallikrein is also released in the kidneys from the distal convoluted tubular cells and secreted into the urine. It is also reported that renal kallikrein plays an important role in the regulation of renal hemodynamics and in glomerular filtration [34, 35]. Moreover, the up-regulation of tissue factor upon injury or inflammatory state leads to activation of PAR1 and PAR2 by involving FXa [36, 37], which subsequently, activates various signaling pathways including PKC, MAPK, NF-kB, thus making PARs responsible for triggering several inflammation processes [4, 12, 13]. Eventually, the activation of these signaling molecules increases the release of pro-inflammatory markers such as TNF-α, IL-1β, IL-6/8; which alter the renal hemodynamics including GFR and also fluid reabsorption [4, 13, 29, 37]. Apart from endogenous proteases and FXa, there are some synthetic peptide activators of PAR1 (TRAP-6, TFLLR-NH2) and PAR2 (SLIGKV-NH2, SLIGL-NH2) [2, 38], and an endogenous protein called activated protein C (APC) which also acts as an agonist for both PAR1 and PAR2 [4, 36, 39].
Table.1: A summary of PAR activators