Perspective Chapter: Endocannabinoids in Renal Physiology – From Tissue Homeostasis to Precision Medicine

Abstract

Body homeostasis is fully dependent on the different physiological systems working together in an orchestrated way. Different hormones, autacoids, and other bioactive molecules are known to play a role in the modulation of such events, either during a normal response to different stimuli or upon any harmful condition that will impact tissue or organ. The kidneys are very important for whole body homeostasis as they are responsible for the control of blood pressure, maintenance of the water compartments volume and composition, detoxification, reabsorption, pH regulation, and even some hormone production. Here we will discuss the ability of cannabinoids (phyto- or endocannabinoids) as modulators of renal physiology, which may open new perspectives for the development of new therapeutic drugs or the discovery of new patterns of endocannabinoids that may be explored as biomarkers for nephropathies or kidney repair toward precision medicine initiatives.

1. Introduction

Despite their relatively small volume compared with other organs, the kidneys receive up to 20–25% of cardiac output, being this significant blood supply, the basis of most of the organ functions. Kidneys are not only “filters” that remove useless metabolites and other undesirable substances from the plasma, directing them for excretion resulting in urine production. The kidneys participate in key events for the body’s homeostasis such as regulating the volume and tonicity of the body’s fluid compartments, maintaining acid-base balance, controlling blood pressure, reabsorption of key solutes (glucose, amino acids, and bicarbonate), and regulating the body’s water balance and production of hormones such as erythropoietin and calcitriol. The anatomical functional organization of the kidneys establishes a perfect harmony between structure and function that makes these organs quite complex and precise in their physiology.

In the last century, the research on the potential medical use of plant cannabinoids emerges with some important hallmarks for the field: (i) the identification and isolation of cannabidiol (CBD), Δ⁹ tetrahydrocannabinol (THC), and many other cannabidiol derivatives; (ii) the identification, isolation, and pharmacological characterization of the cannabinoid receptors (mainly CB1, CB2, and TRPV1¹); and also (iii) as receptors should be present in the cells to trigger distinct cell signaling pathways through the action of endogenous bioactive molecules, the studies in the area allowed the identification and isolation of the so-called “endocannabinoids,” an emerging class of lipidic molecules with a broad spectrum of action considering the widespread distribution of the cannabinoid receptors within the different tissues and physiological systems (Figure 1). Arachinodonoyl ethanolamide (anandamide) and 2-arachinodoyl glycerol (2-AG) are by far, the most studied and explored endogenous cannabinoids [1]. Endocannabinoid system (ECS) is known to participate in a variety of physiological and pathophysiological processes, as referred to in the literature where we can find reviews considering the pathways and involved enzymes for endocannabinoid synthesis and metabolism [2, 3]. Here, we will try to gather aspects of the experimental evidence involving the ECS and how it can interact with kidney tissue and function. We will try to bring to the scene the emerging potential of endocannabinoids as valuable biomarkers for different diseases, focusing on their relevance in renal physiology. This will add new candidates to the different growing lists of molecules (some are still not identified), which are being considered as potential biomarkers, a striking point in the precision medicine initiative [4]. It is worth mentioning that we will emphasize those endogenous molecules, which are naturally synthesized and metabolized by our body resulting in the already known different endocannabinoids. Once produced, these bioactive molecules are able to trigger different cell signaling cascades through the activation of their specific receptors that together with the different endocannabinoids, and the enzymes for synthesis and degradation constitute the ECS, which is very well studied for its actions on the nervous system, being more recently explored in peripheral tissues. Nowadays, the importance of phyto- and endocannabinoid as regulators of physiological processes in practically all organs besides the central nervous system is a reality [5]. Among the peripheral organs, the renal ECS is growing in interest due to the importance in physiology and pathophysiology events that are known to impact the whole body. Endocannabinoids are known to share many of their actions with the components of different plant extracts that are currently being clinically administrated to different patients in order to minimize different pathologies, from a common headache to the enigmatic progression of different cancers. This new avenue opened by the medical use of cannabis allows us to reinforce that everything will be developed in this chapter regarding endocannabinoids can, to a large extent, be considered for aspects of the recreative or medical use of cannabis and their active principles (cannabidiol and THC). Thus, phytocannabinoids can be either complementary or disruptive to the function of the renal ECS with functional consequences that are not yet fully understood.

Figure 1.

Chemical structures of the principal cannabinoids: Highlighted in green, the phytocannabinoids: CBD, Cannabidiol; THC, Δ⁹-tetrahydrocannabinol. Highlighted in blue, the most studied endocannabinoids: AEA, arachidonoyl ethanolamine (anandamide); 2-AG, 2-arachidonoyl glycerol. Not highlighted, some other identified endocannabinoids with physiological relevance in different tissues and organs.

In the following items, we will seek to present the identity of these signaling molecules, and what is known about their action in renal physiology or renal cells in culture, a fundamental point of any scientific research that underpins advances and applications for the clinic and the well-being of human beings. We will also consider here the importance of the development of efficient analytical methods that would help to analyze the pattern of endocannabinoids either in health or disease models, a broadened attempt to identify potential biomarkers for different nephropathies adding support to initiatives in precision medicine.

2. The endocannabinoid system (ECS) and renal physiology

There is substantial literature showing that ECS is present and active in the kidneys, where it plays an important modulatory role in tissue physiology, and therefore, whole body homeostasis. Figure 2 shows that the occurrence of the key enzymes involved in their metabolism (MAGL and FAAH) and the endocannabinoid receptors, CB1, CB2, GPCR55, and TRPV1, is expressed in the renal tissue, arterioles, and the glomeruli, which allow us to affirm that the ECS is present along the entire nephron structure [6]. The occurrence of these ECS elements shows that the renal tissue is not only capable of local synthesizing and remodeling the renal endocannabinoids pattern, but also processes different endocannabinoids and intermediates from other origins when they reach the kidneys through the blood supply. To illustrate this premise, the detection of anandamide and 2-arachidonoyl glycerol (2-AG) are well documented within the kidney tissue [7, 8].

Figure 2.

Endocannabinoid receptors and metabolizing enzymes within the nephron: renal system (A) is responsible for different key functions in body’s homeostasis, such as the control of blood pressure, maintenance of the acid/basic balance and control the volume and composition of the liquid compartments. Each kidney possesses more than a million of functional units called nephrons (B), which are divided in specific segments: glomerulus, proximal tubule, Henle’s Loop, distal tubule and collecting duct. The different components of the ECS were identified in the different nephron segments or arterioles as depicted in the table, according to the color code used. Abbreviations: CB1, CB2 and GPCR55, Cannabinoid receptors; TRPV1, transient receptor potential vanilloid type 1; FAAH, fatty acid amide hydrolase; MAGL, monoacylgliceryl lipase.

In renal tissue, to date, there are few studies showing the actions of phyto- and/or endocannabinoids, and the most complete studies available were carried out in animal models. These studies were able to describe important events, such as the effect caused by anandamide on the renal vascular endothelium [9] and, the reduction of glomerular filtration rate by promoting vasorelaxation in afferent and efferent arterioles [10]. In addition, studies show that the administration of anandamide in the renal medulla of rats is able to promote a decrease in blood pressure through interaction with the CB1 cannabinoid receptor and an increase in urine volume due to interaction with TRPV1 receptors [11].

Activation of the CB2 receptor protected the kidney against the harmful effects caused by the administration of the anticancer drug cisplatin (also used in an animal model of nephrotoxicity), attenuating the characteristic inflammation [12]. CBD was also effective in minimizing the kidney injury induced by the ischemia/reperfusion model in mice/rats, by diminishing the harmful effects of either oxidative and nitrosative stress, and also reducing pro-inflammatory signals, as previously referred to the cisplatin injury [13, 14]. The importance of the renal ECS was also explored in diabetic renal disease, where it had been shown that endocannabinoid receptors can play an important role in either the worsening or the recovery of renal function, since CB1 activation is associated with the progression of the lesion, while CB1 inhibition together with CB2 activation would have an important protective role in this disease [15, 16].

Despite its potential benefits for different diseases, little is known about the use of phytocannabinoids and the impact of the plant-derived cannabinoids (mainly CBD and THC) on modulating renal function and treating renal pathologies. There are few clinical studies using cannabinoids and/or medical use of cannabis that take into account the repercussion on kidney function of human patients.

Anandamide (AEA), the first endocannabinoid described as showing a modulatory action in central and peripheral tissues, was the first element of the ECS that was characterized in the kidneys [9]. Actually, it is almost established that the renal production of anandamide appears to occur in different cell types, and most interestingly, it was observed that the medullary region produces higher levels of this endocannabinoid when compared with the renal cortex. It was also further described that other endocannabinoids, including 2-AG, are also synthesized in renal tissue [17, 18].

Physiology and pharmacodynamics study techniques, such as the binding assay, were explored to first show that anandamide was a ligand in renal endothelial microvascular cells with an efficiency comparable to that of specific agonists for CB1/CB2 cannabinoid receptors, as the synthetic cannabinoid CP55940, thus including anandamide in the hall of cannabinoid receptor agonists within the renal tissue. Indeed, the administration of exogenous anandamide has been shown to modulate the levels of released norepinephrine from renal sympathetic nerves [9]. From this milestone work, it became suggestive that the kidney tissue of mammals could present an active ECS that would be an important part of the maintenance and control of renal physiology, which highlighted a brand-new avenue for research focused on the effects of endocannabinoids in renal physiology and pathophysiology.

Regarding the expression, location, and functionality of cannabinoid receptors, the presence of CB1 receptor occurs in several renal regions, in the tubular segments of the nephrons, specifically in the proximal tubules, distal tubules, and intercalated cells of the collecting duct. In addition, CB1 receptor expression was also found in afferent and efferent arterioles and glomeruli, as well as in various renal cell subtypes, such as mesangial cells and podocytes (Figure 2). Likewise, the expression of CB2 receptors, although thought to be predominantly related to immune cells, has also been demonstrated in kidney tissue to be localized in podocytes, proximal tubule cells, and mesangial cells [7]. The CB2 receptor also appears to play an important role in the regulation of renal hemodynamics, as the work by Pressly and colleagues showed that the administration of a selective synthetic agonist for CB2 was able to increase renal cortical blood flow and also promote direct vasodilation of afferent arterioles when they were perfused alone [19].

The effects of anandamide and other cannabinoids on renal hemodynamics suggest that the renal ECS would be able to play a role in the regulation of blood pressure. This should involve a mechanism dependent on CB1 receptor activation, resulting in diuretic effects (increase in urinary flow without change in sodium excretion) probably acting on renal innervation [11]. It was also demonstrated that TRPV1 receptor is an important target for modulating renal hemodynamics, sodium, and water excretion, being precisely expressed in the sympathetic fibers of the renal innervation [20]. Indeed, activation of TRPV1 in the kidney by its exogenous agonist capsaicin produces diuresis and natriuresis mediated in part by a marked increase in glomerular filtration rate and activation of renal innervation. Activation of the TRPV1 receptor provides a protective mechanism against the elevation of blood pressure induced by high salt intake; however, the assessment of the specific contribution of anandamide-induced TRPV1 receptor modulation is complicated by the various other sensory stimuli capable of activating this receptor [8]. The diuretic effect of endogenous and exogenous cannabinoids was most extensively investigated in the work by Paronis and colleagues [21]. Using Sprague-Dawley rats as a model, the work showed that cannabinoids, including anandamide itself (meta-anandamide, a more stable synthetic analog, often used also), the phytocannabinoid THC, and the synthetic cannabinoids WIN55,212-2, AM2389, and AM4054, thus showing that cannabinoids of different classes are able to increase diuresis in a dose-dependent manner. Strikingly, the diuretic effects produced by THC, WIN55,212-2, AM2389, and AM4054 were comparable to that of the loop diuretic furosemide, known to be the chosen drug of the most potent class of diuretics.

There is substantial information in the literature ascribing anandamide an important role in the regulation of renal hemodynamics, either through its direct action or through the generation of active metabolites, such as prostamide E2, with equal diuretic and natriuretic effects [17]. The diuretic effect implies the possible modulation of different ion transporters that are present in the different nephron segments, being the Na⁺/K⁺-ATPase, the main pump involved in Na⁺ reabsorption and in the maintenance of the Na⁺ gradient that is crucial for other solutes reabsorption or secretion. Thus, a direct correlation between augmented diuresis and Na⁺ excretion is a clue that would include the endocannabinoids in the hall of the bioactive molecules with potential action on the different ion transporters.

This issue was explored in vitro, using porcine proximal tubule cells (LLC-PK1) that are related to the most cortical region of the kidney. The authors showed that the activation of the CB1 receptor through the addition of a synthetic cannabinoid agonist, WIN55,212-2, was able to increase sodium reabsorption as a result of increased Na⁺/K⁺-ATPase activity present in the basolateral membrane of these cells [22]. Conversely, it was also demonstrated that anandamide reduced the sodium reabsorption by the proximal tubule Na⁺/H⁺ exchanger and the Na⁺/K⁺/2Cl⁻ cotransporter present in the thick ascending limb of the Henle’s loop [23]. Unfortunately, few studies have been carried out in humans to date, so little is known about the applicability of cannabinoids and their impact on renal physiology, especially with regard to the impact on renal function over the years. Meanwhile, one of the studies in humans is an extensive one that followed 5115 volunteers over a long period (25 years) in order to assess whether recreational cannabis use had implications for adult kidney function. Overall, although a modest association was identified between greater cannabis exposure and lower glomerular filtration rate (cystatin C assay) among volunteers, the results were inconclusive and did not demonstrate any significant association between cannabis use and changes in glomerular filtration rate or prevalent albuminuria [24]. Besides, recreational use of synthetic cannabinoids appears to have a negative impact on kidney function. Some case reports in recent years have pointed to a direct correlation between the use of synthetic cannabinoids (used recreationally, often through the use of e-cigarettes) and severe acute kidney disease, but without any mechanistic understanding of this correlation [25]. In summary, the case reports only state that patients who used these synthetic substances developed significant hypertension, agitation, respiratory failure (some required intubation), pulmonary hypertension, and acute kidney injury, with high levels of creatinine and urea. The pathological mechanism of acute kidney disease remains unclear, but it was suggested that would be acute tubular necrosis and/or acute interstitial nephritis, as evidenced by biopsies performed in patients from some of the case report studies. Although the above-mentioned harmful effects attributed to the recreational use of synthetic cannabinoids, the emerging potential use of endocannabinoids as well as phytocannabinoids as modulators of the renal machinery, would allow the development of new therapeutical strategies, products, and clinical procedures for the treatment of renal and non-renal pathologies [26, 27].

2.1 Cannabinoids as a therapeutical perspective for different nephropathies

The functional relevance of ECS in renal tissue is credited by previously mentioned findings, with a direct impact mainly on the regulation of renal hemodynamics, as well as on the dysregulation of this function in pathological states, such as in renal disease associated with acute kidney disease and diabetic renal disease [28]. The endpoint of these pathological conditions is chronic kidney disease, which presents marked proteinuria, inflammation, fibrosis, and renal failure, converging to dialysis and even transplantation. There are no efficient treatments for the different nephropathies, so any initiative to develop new clinical protocols, here included those exploring the versatility of the endocannabinoids and phytocannabinoids, should be strongly encouraged.

2.1.1 Acute kidney injury

Acute kidney injury (AKI) describes a sudden loss of kidney function that is determined on the basis of increased serum creatinine levels (a marker of kidney excretory function) and reduced urinary output (oliguria; a quantitative marker of urine production) and is limited to a duration of 7 days. We will not discuss the pathophysiology of AKI as there are substantial literature detailing it [29]. AKI can occur due to a number of factors, such as the onset of a sepsis process, exposure to nephrotoxins, and organ hypoperfusion due to ischemia. In fact, ischemia-reperfusion (IR) injury is one of the most common forms of AKI and involves a complex series of cellular changes that can lead to damage and death of tubular cells and loss of renal function in the most severe cases. Regulation of renal blood flow dynamics is necessary to preserve glomerular filtration rate during IR-induced AKI critical in damage propagation and kidney recovery [19]. To understand the mechanisms by which the endocannabinoid system is correlated with the events that lead to AKI, many animal models are used; among these, cisplatin-induced nephrotoxicity and the IR lesion induction technique.

Using the IR injury model in mice, Feizi and co-workers showed that pretreatment with both selective CB1 and CB2 receptor agonists was able to protect kidney tissue from IR damage, suggesting an important role for ECS to protect the kidneys from possible cellular damage caused by IR [30]. In this work, it is important to emphasize that the treatment performed with synthetic cannabinoids was prior to the induction of the IR lesion, which implies a protective and not a curative effect. In another study, Sampaio and colleagues used the IR lesion in both in vitro and in vivo models in Wistar rats, demonstrating that the lesion leads a significant reduction in the anandamide levels, as well as in the expression of CB1 receptors in the renal cortex region [18]. Another study using the IR injury in mice showed that renal cell damage and characteristic biochemical changes were associated with increased levels of 2-AG in all renal tissue [31]. The relation between kidney disease and ECS was also explored by Sampaio and co-workers, who had demonstrated in rats that after IR injury there was a significative inhibition in the Na⁺/K⁺-ATPase activity present in the basolateral membrane of proximal tubule cells, which was fully reverted in the experimental group that was treated with the synthetic cannabinoid WIN55,212,-2 immediately after the IR injury [18]. The cannabinoid agonist led to the recovery of sodium transport, through a pathway dependent on the activation of the CB1 receptor. This is in agreement with previous work from the group and allowed them to suggest that the ECS plays an important role in the re-establishment of Na⁺ reabsorption and consequence, other solutes, such as glucose and amino acids in the renal proximal tubule, since the normal Na⁺/K⁺-ATPase activity in these tubular cells directly impacts all solute transport in this tubular segment as it restores or keeps the Na⁺ gradient, which is the driven-force for different secondary active transporters above mentioned [32].

In 2012, a pioneer study was carried out to evaluate the potential protective effect of CBD, in an IR model injury in rats. The intravenous administration of CBD (before and after the IR procedure) was able to protect kidney tissue from injury-associated damage [14]. Authors showed that IR promoted changes in different histological and clinical biochemical markers, such as azotemia and uremia (increased levels of toxic nitrogenous compounds in the bloodstream) associated with a significant decrease in renal glutathione levels. CBD treatment significantly attenuated the observed harmful alterations in the biochemical parameters evaluated. Furthermore, the histopathological analysis showed that CBD improved the healthy condition of the renal tissue, significantly reducing the expression of inducible nitric oxide synthase (related to macrophage infiltration and inflammation), tumor necrosis factor-alpha, cyclooxygenase-2, and caspase-3 [14]. These findings were further confirmed by the work of Baban and colleagues using a similar model of injury. These authors showed that the treatment with CBD protected the renal tissue from damage, restoring renal blood flow, and serum creatinine levels. It was also observed that the phytocannabinoid was able to reduce neutrophil infiltration and inflammatory signals [33].

One of the main pharmacological targets of CBD is the anandamide-degrading enzyme, fatty acid amino hydrolase (FAAH). This important observation allows us to postulate that along their own beneficial effects, CBD would inhibit anandamide degradation, leading to increased levels of this endocannabinoid, which plays an important role in renal tissue homeostasis. Another hypothesis would be the action of CBD on TRPV1 receptors, which, plays a role in the maintenance of renal hemodynamics.

Although rare, other studies also sought to investigate the possible role of cannabinoid receptors in the IR model of AKI. Zhou and colleagues showed that blockage of the CB2 receptor decreases the fibrosis cascade, one of the hallmarks after IR injury both in vitro and in vivo. Agonist administration and/or CB2 overexpression was directly associated with increased synthesis of extracellular matrix proteins, such as smooth muscle alpha-actin and fibronectin, early markers of fibrosis. It was also shown that treatment with transforming growth factor beta 1 (TGFβ-1), an important pro-fibrotic cytokine, was related to increased CB2 expression. In this work, the group tested a new synthetic drug, which acts as a CB2 antagonist, and they were able to demonstrate how the administration of this cannabinoid reduced inflammation and fibrosis in animals subjected to IR injury [34].

In 2009, a study using cisplatin injury model of AKI, showed that CBD attenuated tissue damage and the expression of enzymes involved in oxidative processes, inflammation, necrosis, and renal apoptosis, in mice, associating this phytocannabinoid with a marked improvement in renal function [13]. The treatment with CBD was performed in mice 1 hour before the administration of cisplatin, a procedure repeated for 10 days. CBD largely attenuated the symptoms induced by cisplatin, mainly in terms of the increased expression of enzymes that generate reactive oxygen species (NOX4 and NOX1). It also decreased the cisplatin-induced inflammatory response, decreasing the levels of pro-inflammatory cytokines such as TNF-alpha and IL-1β, being such effects probably associated with the already reported antioxidant function of CBD. Further, Mukhopadhyay and colleagues using the same animal model for AKI showed that CB1 receptor activation plays a central role in the progression of kidney injury. The lesion caused by cisplatin was associated with an increase in renal anandamide levels, activation of signaling pathways involved with cell death, oxidative stress, leukocyte infiltration into the renal tissue, and inflammation, in addition to impaired renal function, with an increase in serum creatinine and urea levels [35]. Both the genetic deletion and the pharmacological inhibition of CB1 receptors with the use of the antagonists AM281 or SR141716 markedly attenuated cisplatin-induced renal dysfunction, but were not able to prevent the lesion and its characteristics, thus demonstrating that CB1 may play an important role in the progression of nephrotoxicity-induced AKI. The CB2 receptor, on the other hand, seems to behave in the opposite way, since it was shown, in the same model of cisplatin-induced renal injury that the use of a synthetic and selective CB2 agonist was able to attenuate the inflammatory response, oxidative stress, cell damage and improved renal function in animals [35], evidencing an important effect of the CB2 receptor in attenuating damage in this injury model.

2.1.2 Diabetic kidney disease

Most complications of diabetes are associated with pathological changes in the vascular endothelium wall. The most common macrovascular complication of diabetes is atherosclerosis, which increases the risk of myocardial infarction, stroke, and peripheral arterial disease; while microvascular complications underlie nephropathy, retinopathy, and peripheral neuropathy. Thus, diabetic kidney disease (DKD), initially referred to as diabetic nephropathy, is one of the main causes of kidney failure. In the diabetic patient, hyperglycemia stimulates the generation of reactive oxygen species, which ultimately leads (by several pathways) to DKD, characterized by mesangial and tubular cell hypertrophy, glomerular basement membrane thickening, and glomerular sclerosis [36]. DKD markers are increased glomerular permeability to proteins and excessive accumulation of extracellular matrix in the mesangium, eventually resulting in glomerulosclerosis and damage to the tubular epithelium due to increased filtered glucose load that results in osmotic effect, tubular cell death, increased urinary flux and progressive renal failure [15].

In studies using models of type I diabetes, such as the administration of streptozotocin or using spontaneously diabetic mice, it was demonstrated that the renal tissue of these animals showed a significant increase in the expression of the CB1 receptor, mainly in podocytes. In animals that received treatment with a synthetic CB1 antagonist, a significant reduction in albuminuria was observed, as well as a recovery in the expression of glomerular proteins associated with the proper functioning of the renal filtration barrier, such as nephrin, an important fact since in both human and experimental DKD there is a reduction in nephrin expression. Studies in patients with microalbuminuria have shown that downregulation of this protein occurs at an early stage of the disease [15, 37]. The involvement of the CB2 receptor in this model was also investigated, which shows a decrease in the expression in the glomerulus, and since diabetic mice induced by streptozotocin and treated for 14 weeks with AM1241, a synthetic selective agonist for CB2, an improvement in renal function with concomitant restoration of nephrin expression levels and reduction in the glomerular monocytes infiltration, observations that were also present in the study using animals with selective genetic deletion of CB2, showing that the absence of this receptor induces an even more severe renal damage condition in response to diabetes [16, 38]. In the aforementioned animal model, the levels of endocannabinoids were determined in the renal cortex and those of 2-AG were reduced in diabetic animals. In the same work, CB2 expression was also studied in human patients and cultured podocytes, showing that the CB2 receptor was less expressed in renal biopsies from diabetic patients, suggesting that CB2 activation is involved in both albuminuria and the loss of podocyte proteins that act on the stability of the glomerular filtration barrier. Therefore, this receptor would play a protective effect in patients with DKD [16]. Interestingly, when a combined treatment using the CB2 agonist AM1241 and the CB1 receptor antagonist AM6545, in streptozotocin-induced animal model of diabetes, resulted in a better prognostic than that observed using the usual monotherapies, abolishing albuminuria, monocyte infiltration and inflammation, tubular injury, and markedly reducing renal fibrosis [39].

The ECS modulatory action in models of type I diabetes was also confirmed in different animal models of renal disease associated with type II diabetes, in which animals showed an increase in the expression of CB1 in the renal glomerulus, an increase in albuminuria, a reduction in the glomerular filtration rate and nephrin expression, monocyte infiltration and inflammation, in addition to the activation of the renin-angiotensin system. Treatment with both CB1 antagonists and CB2 receptor agonists promotes significant improvement in these studied renal parameters [40, 41].

2.1.3 Chronic kidney disease

Chronic kidney disease (CKD) is an irreversible condition that affects millions of people around the world. Regardless of its initial cause, CKD is the final stage of replacement of functional kidney tissue by altered extracellular matrix proteins, characterizing renal fibrosis that almost completely limits the functionality of kidney tissue. To date, there are no therapeutic options available to prevent progression and treat both renal fibrosis and chronic kidney disease [42].

In this context, a key role of the CB1 receptor in the development of renal fibrosis was already described, using both human patient samples and the animal model of unilateral ureteral obstruction [42]. In mice, through molecular biology assays and bioinformatics resources, the CNR1 gene, which encodes the CB1 receptor, was one of the genes with the most altered expression in fibrotic kidneys. Immunohistochemical assays revealed that CB1 receptor expression increased dramatically in the renal fibrosis model and that the receptor was highly expressed in renal tubules, parenchyma, and glomerulus. These results were accompanied by a significant increase in CB2 receptor expression, 2-AG levels, and a reduction in anandamide levels. In pharmacological trials, treatment with the specific synthetic CB2 antagonist was shown to retard the development of fibrosis, while the CB2 agonist JWH133 attenuated renal fibrogenesis. These reported results add more evidence to the view that cannabinoid receptors may have antagonistic effects on renal tissue.

CKD also becomes an evident problem in kidney transplanted patients. The progressive and inevitable impairment of renal graft function remains the primary cause of graft loss, where such impairment is due to the replacement of functional renal tissue by extracellular matrix proteins, mainly collagens, leading to both interstitial fibrosis and tubular atrophy, accompanied by glomerulosclerosis. In a study that analyzed 26 patients, CB1 receptor expression levels were investigated on the day of transplantation, 3 months and 12 months after surgery [43]. The data revealed an increase in the expression of CB1 from the 3rd month on in grafts that presented functional impairment, thus being correlated with the onset and progression of renal fibrosis. The CB1 receptor was expressed mainly in proximal and distal tubular epithelial cells, arteries, and vascular smooth muscle cells of arterioles, in infiltrated inflammatory cells and glomeruli, mainly in podocytes.

3. Endocannabinoids as emerging biomarkers for kidney diseases: a precision medicine initiative

The mechanisms that promote and lead to kidney disease are being quickly elucidated due to research progress in Nephrology. Both genetic variation and metabolic changes caused by interactions with xenobiotics and lifestyle are being understood at the level of how they can affect predisposition and disease progression.

Precision medicine is one of the objectives of this research progress in different areas including nephrology, in which the patient’s management is adapted according to the mechanisms underlying their disease aiming for maximal therapeutic success. The purpose of precision medicine is to characterize diseases based on the mechanisms involved in their pathophysiology and, thus, segregate patients and direct them to the best treatment. This is because there might be multiple pathways for the same phenotype and therapeutic strategies specific based on a single cell pathway or process may not be successful if applied to individuals differentially impacted in that specific illness. Therefore, the goal of precision medicine is to determine the right drug, in the right dose, for the right patient, at the right time.

Biomarkers are the basis of precision medicine, as they allow classifying individuals into subpopulations that differ in their susceptibility to a disease or in their response to a particular treatment. The term “biomarker,” short for “biological marker,” refers to a broad category of biological characteristics used to examine normal biological or pathological processes and responses to therapeutic or prophylactic interventions that can be accurately and reproducibly measured.

A good biomarker must link disease pathogenic mechanisms (endotypes) to visible properties (phenotypes), be reproducible, easy to measure and cost-effective, and be related to a clinical outcome. The role of biomarkers in the development of precision medicine offers an opportunity for technological developments aimed at improving human health and reducing healthcare costs. In this context, the Omics Sciences are highlighted, especially Metabolomics.

Metabolomics is the comprehensive study of the metabolome, that is, the set of biochemical compounds (or small molecules) present in cells, tissues, and body fluids. The study of metabolism at the global or “-omics” level is a rapidly growing field that has the potential to have a major impact on medical practice. The basis of metabolomics is the concept that a person’s metabolic state provides a close representation of that individual’s overall health status. This metabolic state reflects what has been encoded by the genome and modified by diet, environmental factors, and the gut microbiome, for example. The metabolic profile provides a differentiated reading of the biochemical status of normal physiology for various pathophysiology in a way that is often not seen from gene expression analysis.

Thus, the study of the metabolome is expected to reveal biochemical changes that reflect patterns of variation in well-being states and more accurately describe specific diseases and their progression, thus helping in the differential diagnosis (Figure 3). Through metabolomics, predictive, prognostic, diagnostic, and surrogate biomarkers of various disease states can be obtained, as well as information on the underlying molecular mechanisms of diseases, which will allow their sub-classification, and the stratification of patients based on the metabolic pathways affected. It also has the potential to reveal drug response biomarkers, providing an effective means to predict the variation in a subject’s response to treatment and a means to monitor the response and recurrence of disease [44].

Figure 3.

Critical steps to stablish endocannabinoids as potential biomarkers for health and/or disease: This simply workflow allow us to identify different sequential steps that should be carried out in high quality conditions, from the pre-analytical phase to the conclusions obtained. We can assume the individual at the center, representing a healthy or a sick individual, which will be the donor (1) of any kind of biological sample, mainly urine and plasma. (2) the collected samples go further for extraction and preparation of the extracts to the different possible analysis. Here we assume that the samples will be submitted to different OMICs approaches such as Metabolomics, Lipidomics and the already mentioned in the literature, Endocannabinoidomics (3). The analysis will provide different biomolecules signatures that would be correlated to health or disease, according to the donor. This part of the analysis will allow us to identify altered patterns and specifically, altered molecular species for metabolites, lipids and also endocannabinoids (4). The results will be included in different biomolecules libraries that will help to group the different individuals, in their respective endotypes, based on the biomarkers found, which is a key point for the Precision Medicine Initiative (5).

Since Metabolomics is the study of small molecules, it encompasses the study of lipids (lipidomics) and, currently, mass spectrometry (MS) is the analytical technique that has been most used in these omics sciences. This is because MS allows accurate detection and quantification of molecules within a wide mass range. With the rapid development of MS in the detection of biomolecules, MS is emerging as an indispensable technology to accelerate research in the field of Precision Medicine [45].

In this context, the components of the ECS emerge as potential biomarkers for kidney diseases, whether for diagnostic, prognostic, or predictive applications, and also in an approach related to the discovery of new pharmacological targets. This is because the endocannabinoid system plays an important role in renal physiology, being a lipid cell signaling system that participates in different pathways. Alterations in this pathway can lead to the pathogenesis of both CKD and AKI. Recently, different anandamide-related molecules were identified in the brain, which may play a role in the normal or abnormal central nervous system physiology, according to their levels and distributions, leading to the molecular basis of human individual behavior, cognition, and temperamental differences [1]. This kind of endocannabinoid diversity may exist also in the peripheral organs, playing unknown roles in physiology and pathophysiology events. Therefore, the study of the endocannabinoid system at the level of the omics sciences is a promising area that may result in new therapies for different kidney diseases, thus contributing to the advancement of precision medicine in the field of nephrology.

4. Final remarks

In spite of the different experimental evidence shown here and elsewhere, it is inevitable to reinforce that to date, clinical studies with human patients have not yet been carried out in order to evaluate the use of cannabinoids for the treatment of chronic kidney disease, or for other kidney diseases, and even studies with the use of phytocannabinoids for these therapeutic purposes, even though preliminary tests and research in animals suggest a promising therapeutic use of phyto- and endocannabinoids for different nephropathies. It is also important to describe that the use of CBD, or even cannabis, for the treatment of other non-renal pathologies, does not seem to lead to any type of impairment of renal physiology and functioning, so, the medical use of phytocannabinoids does not lead to adverse effects on renal physiology. Obviously, specific monitoring of these aspects related to renal function and physiological events controlled by the kidneys should be better evaluated in long-term studies, since the bioavailability of exogenous cannabinoids in renal tissue can be quite significant, since kidneys are hyper perfused organs with a pleiade concentration of different receptors and other enzymes that integrate the Intra-Renal Endocannabinoid System itself.

Acknowledgments

Authors would like to acknowledge CNPq and Fundação Carlos Chagas de Amparo a Pesquisa do Estado do Rio de Janeiro for the financial support.

Conflict of interest

Authors would like to declare no conflict of interests.