Vascular Dysfunction in Polycystic Kidney Disease: A Mini-Review Free

Chronic kidney disease (CKD) is a growing health problem that affects between 8% and 16% of the population worldwide [1]. It is characterized as a persistent abnormality in the kidney that results in the remodeling of renal structure and a decrease in renal function. CKD is a progressive disease with many adverse outcomes such as cardiovascular disease, end-stage renal disease (ESRD), and increased mortality. CKD is mostly attributed to diabetes or hypertension (HTN), but genetic factors like polycystic kidney disease (PKD) may also increase a patient’s risk. PKD currently accounts for 15% of all CKD cases in the USA [1].

PKD is the most common hereditary, progressive disease and is characterized by large, fluid-filled cysts that impair renal function. Complications commonly associated with PKD are HTN, renal dysfunction, vascular abnormalities, and cardiovascular complications. Inheritance occurs in two forms: autosomal dominant polycystic kidney disease (ADPKD) and autosomal recessive polycystic kidney disease (ARPKD). ADPKD is the most common form of the disease and is characterized as an adult-onset multisystem, progressive disorder that is the result of a genetic mutation in either polycystin-1 (PKD1) or polycystin-2 (PKD2) [2]. The prevalence of ADPKD is reported to be between 1 in 400 and 1 in 1,000 births, with ESRD in 50% of patients by 60 years of age [2]. In comparison, ARPKD is a perinatal, childhood-onset disease with cystogenesis that results from a mutation in PKHD1 gene that encodes for fibrocystin. Although ARPKD is less pervasive (∼1 in 10,000 births), it is the more aggressive form of the disease, with ESRD in 60% of patients by 20 years of age [2]. Despite the differences in underlying genetic determinants, both forms of PKD are associated with cyst development and renal dysfunction.

Besides cystogenesis, patients with a PKD prognosis present with extrarenal symptoms such as secondary HTN and cardiovascular complications, which include, but are not limited to, atherosclerosis, left ventricular hypertrophy (LVH), and cardiac valvular abnormalities [3]. HTN is an early and common symptom throughout PKD progression and is influenced by many pathogenic mechanisms that also contribute to increased cardiovascular morbidity and mortality. Interestingly, HTN may present before a decline in renal function is measured, which points to an extrarenal mechanism. Several mechanisms likely contribute to the progression of HTN in PKD, including activation of the renin-angiotensin-aldosterone system (RAAS), elevated peripheral sympathetic tone, insufficient levels of polycystin or fibrocystin expression, and altered endothelial response to shear stress [4]. The aims of this review were to highlight the common vascular complications and dysfunction observed in preclinical and clinical reports in both cerebral and peripheral aspects of PKD and to understand whether these changes in vascular function are purported to be primary or secondary to the underlying HTN and renal dysfunction commonly presented in PKD.

In ADPKD, HTN is an early and frequent finding that affects over 60% of diagnosed patients. Importantly, this is often prior to a decline in renal function [4, 5]. The pathogenesis of HTN is multifactorial and dependent on many interrelated factors. Specifically, the relationship between HTN and cyst growth is thought to contribute to the early development of HTN in ADPKD by activating both the systemic and intrarenal RAASs. Intrarenal RAAS refers to the local autocrine/paracrine system of tissue capable of generating angiotensin II (AngII) [4], while systemic RAAS is the well-known system that requires the interaction of multiple organs to produce AngII. In PKD, the progression of cyst development and expansion results in renal structure abnormalities that induces structural remodeling, increasing local renal ischemia and elevating both renin release and chronic stimulation of the RAAS (5), as illustrated in Figure 1. Urinary angiotensinogen and renin are two markers that can be used to measure intrarenal RAAS. Torres and colleagues have shown elevated levels of renin in cyst fluid obtained from ADPKD patients, supporting the hypothesis that intrarenal renin and the RAAS are elevated in ADPKD along with arterial pressure [6]. Previously, Torres and colleagues had demonstrated that angiotensin-converting enzyme inhibition with enalapril resulted in significant reductions in MAP, renal vascular resistance, and filtration fraction in hypertensive patients with ADPKD [7], highlighting a central role for elevated RAAS activation in ADPKD in addition to its regulation of renal hemodynamics. A clinical study done by Salih and colleagues demonstrated that while plasma levels of angiotensinogen, renin, and aldosterone were similar between ADPKD and CKD groups, their equivalent urinary markers were significantly higher in ADPKD patients [8]. Accounting for the use of RAAS inhibitors, which can increase plasma renin, the group compared both plasma and urinary renin markers in patients with and without RAAS inhibitor use. They found that, while there were differences in plasma renin, urinary renin excretion was consistently higher in ADPKD patients regardless of RAAS inhibitor use [8]. These increases in urinary output markers can be indicative of several possibilities including renal ischemia by cysts, differences in tubular reabsorption, and potential damage markers. Nevertheless, this study revealed increased activation of RAAS through the consistent increase in urinary angiotensinogen and renin excretion unique to ADPKD patients. While the underlying cause of RAAS activation remains unknown, it is speculated to be a mixture of intrarenal mechanisms involving an increase in sympathetic stimulation, changes in intrarenal pressures exerted by expanding cysts, and/or changes in perfusion experienced at the juxtamedullary cells which are responsible for renin release [9, 10, 11].

Sympathetic hyperactivity is another potential contributor to the pathogenesis of PKD. Elevated renal sympathetic activity at the juxtaglomerular apparatus directly promotes the release of renin, driving the activation of the RAAS cascade [12]. In addition to modulating renin release, renal sympathetic nerves play a tonic role in the regulation of renal vascular resistance, and in turn, renal blood flow. Acute activation is largely considered to be a compensatory mechanism to retain sodium and/or water; however, chronic activation has been shown to produce adverse effects that contribute to cardiac and vascular structural alterations that may advance the disease [13]. Both renal sympathetic and afferent nerves contribute to the development of various models of primary HTN and renal dysfunction; however, their role in the pathogenesis of PKD and the secondary HTN remains unclear. Evidence shows that RDN reduces sympathetic hyperactivity in addition to reducing cystogenesis [14], which is a known contributor to both intrarenal RAAS activation and HTN [12]. In addition, RDN significantly increased the diameter of renal arteries up to 3 months post-procedure [15]. This direct influence over neural control is an appealing therapeutic because it would not only contribute to managing blood pressure but also evades the specificity of the ciliary protein dysfunction that defines the difference between ARPKD and ADPKD and may lead to improved renal hemodynamics.

Ciliary dysfunction is one important factor in the pathogenesis of PKD, yet it is unknown whether cilia dysfunction is involved in intrarenal RAAS activation. Primary cilia play an important role in the regulation of intracellular calcium homeostasis [16]. An abnormal sensory cilia function reported in ADPKD patients with a PKD2 mutation [17] disrupts nitric oxide (NO) biosynthesis which primary cilia generate in response to fluid-shear stress across the tubule epithelium. Saigusa et al. have demonstrated that mice without renal cilia and polycystin-1 resulted in increased kidney cyst formation, systolic blood pressure, prorenin, and kidney and urinary angiotensinogen levels. Their findings propose that in the absence of cilia or polycystin-1, there is an upregulation of intrarenal RAAS [11]. Given these data, the assumption can be made that loss of function of polycystin-1 in ADPKD may contribute to elevated blood pressure in PKD [11]. There is a lack of evidence, however, on the specific mechanisms that contribute to the activation of RAAS in ARPKD given that this diseased model is centered on a mutation found in the fibrocystin protein rather than polycystin.

The role of the vasculature in blood pressure control is necessary for maintaining hemodynamic regulation and perfusion to local tissue beds. Intrinsic properties within the vasculature allow it to autoregulate microvascular lumen diameter in response to changes in intraluminal pressure, while vascular smooth muscle cells (VSMCs) normalize the circumferential stress of the resistance vessel exposed to changes in blood pressure [18]. The endothelium is the major interface between the vascular wall and circulatory system and therefore is an important contributor to the local regulation of vascular tone through the release of different vasodilatory and vasoconstricting agents [18]. In a diseased state where peripheral resistance increases shear stress on the vessel wall, the secretory, barrier, and metabolic function of the endothelium becomes ineffective at maintaining the necessary degree of vascular homeostasis and promotes the development and maintenance of HTN [18]. A previous study that utilized high-resolution vascular ultrasound reported an impaired endothelium-dependent flow-mediated vasodilation in hypertensive patients with ADPKD when compared to both normotensive subjects with ADPKD as well as non-PKD controls, highlighting the secondary effects of elevated arterial pressure that may mediate endothelial dysfunction in patients with ADPKD [19]. A more recent study found that despite having normal blood pressure, patients with early stage ADPKD showed early manifestations of vascular abnormalities through increased arterial stiffness and pulse wave velocity, which positively correlated with factors such as IL-6, TNF-α, and hs-CRP [20].

Vascular function can be heavily influenced by local inflammation and oxidative stress, and in a diseased state such as HTN, vascular endothelial function becomes progressively impaired. In particular, the bioavailability of NO, a vital vasodilator produced mainly by the endothelium, is severely compromised in an oxidative environment. The superoxide anion promptly reacts with NO, generating the highly reactive and toxic peroxynitrite (ONOO•^-), thus chelating much of the beneficial NO, leading to further loss of endothelium-dependent dilation and hypercontractility [21]. Overproduction of ROS reduces vascular NO bioavailability and promotes cellular damage, hence, an increase in these metabolites is considered to be a major mechanism involved in the pathogenesis of endothelial dysfunction. In addition, NO reduces the adhesion of leukocytes to the endothelium and limits the expression of adhesion molecules and chemotactic factors. Vascular inflammation, in turn, is known to reduce endothelium-dependent dilation of resistance-sized arteries in hypertensive rats [22‒24]. Thus, endothelial dysfunction in HTN represents a likely trigger for an increased inflammatory response in the vascular wall [22]. Inflammation and oxidative stress are evident in subjects at different stages of ADPKD, and a potential mechanism is illustrated in Figure 1. Inflammation is pronounced with progressive kidney disease whereas oxidative stress appears to precede clinical HTN and is apparent in subjects with preserved kidney function [25]. Interestingly, NADPH-oxidase complex 4 (NOX4), which is a major source of endogenous ROS, is reported to be associated with early PKD, and predominantly induces oxidative stress in endothelial cells (ECs), contributes to endothelial dysfunction, and precedes capillary loss [26]. This study highlights another potential contributor to vascular pathology and disease progression in PKD. Furthermore, the importance of oxidative stress as both a cause and biomarker in PKD strengthens the development of HTN as excess vascular ROS decreases NO bioavailability, increasing endothelial dysfunction, and increasing arterial stiffness [27]. The consequences of HTN on the endothelium are not limited to changes in endothelial regulation but also determined by smooth muscle sensitivity and responsiveness, as well as circulating inflammatory and oxidative markers, intracellular adhesion molecules, and the expression of other chemotactic factors.

EC Pkd2 is particularly important for regulating vascular responses to changes in intravascular flow and, consequently, shear stress. EC-specific deletion of Pkd2 significantly reduces flow-mediated dilation of mesenteric arteries without affecting cholinergic-induced increases in vascular diameter or smooth muscle cell contractility [28]. In an elegant study, MacKay et al. identified that flow induces Ca²⁺ influx in EC, which in turn activates NO production, as well as EC hyperpolarization caused by the opening of small and medium conductance Ca²⁺-activated K⁺ channels. Both NO and hyperpolarization are then transmitted to smooth muscle cells, leading to their relaxation and, consequently, vasodilation. However, in the absence of Pkd2, these responses are significantly blunted, leading to a purported systemic vascular impairment and increase in resistance, which may explain the hypertensive phenotype observed in EC-specific Pkd2 knockout mice [28]. Interestingly, all these vascular alterations observed in the model were independent of changes in cardiac or renal function, and no renal cysts developed in these mice up to 16 weeks of age.

Intracellular cAMP concentrations via AVP activation of vasopressin type 2 receptor (V2), which is known to contribute to cystogenesis which can then contribute to local and distant mechanical effects of vascular remodeling, followed by atrophy, apoptosis, inflammation, and fibrosis [29]. The pressor activity of AVP is accompanied by activation of RAAS, allowing both systems to operate as reciprocal mechanisms in the control of both resistant vessels and arterial pressure. Preclinical evidence indicates that Pkd2^+/− mice have an increased susceptibility to vascular injury and develop prominent irregular thickening in the tunica media layer of intracranial vessels [30]. Areas of irregular vessel wall thickening correlate with an abnormal expression of VSMCs, indicating an imbalance in smooth muscle cellular proliferation. Recent studies indicate that reduced intracellular Ca²⁺ concentration along with increased cAMP levels plays a role in this phenotype [30]. The relationship between intracellular Ca²⁺ and cAMP levels indicates how their roles influence the VSMC phenotype; evidence shows that the PKD mutant VSMC contained higher levels of cAMP compared to the wildtype, and mild disturbance in Ca²⁺ concentrations had a strong effect on their VSMC phenotype, which further supports the relevance of the AVP-cAMP axis in the pathogenesis of PKD [30].

ECs found in the microvasculature of the kidney can regulate blood flow to local tissue beds and modulate inflammation and vascular permeability, while ECs located in the glomerulus contribute to the glomerular filtration barrier [31]. These varied functions make ECs key drivers of inflammatory processes and allow them to contribute to endothelial dysfunction in addition to renal dysfunction. Unresolved chronic pro-inflammatory signaling in vascular ECs is a common feature in the progression of cardiovascular diseases, including atherosclerosis, HTN, congestive heart failure, and other inflammatory syndromes [32]. Inflammation to the vasculature exacerbates the permeability of the ECs, exaggerating both the extravasation of blood components and accumulation of fluid into the extravascular space, therefore, increasing the secretion of both chemokines that attract leukocytes and monocytes to the inflamed endothelium [32]. Recent studies in PKD demonstrate that disruption of ciliary proteins in the tubule epithelium cause cyst formation that is associated with an inflammatory response [33]. A global gene analysis of human PKD1 kidneys reported activation of the inflammatory pathways JAK-STAT and NF-κB, both of which are pathways that report elevated cytokines and contribution to cyst formation [33]. While PKD is not a primary inflammatory disorder, in vitro studies have shown that PC1 and PC2 are required for JAK1 and JAK2 activation [34]. This suggests that PKD1/2 mutations may promote inflammation by interrupting the control of JAK-STAT signaling. Additionally, increased activation of JAK-STAT and NF-kB signaling is activated by AngII through the stimulation of AT₁ receptors [35]. Under stressful physiological conditions, ECs undergo phenotypic changes that are characterized by downregulation of protective regulators and upregulation of inflammatory receptors and pro-adhesive molecules [31]. Taken altogether, the production of strong pro-inflammatory mediators and activation of intracellular inflammatory pathways increases vascular permeability, tissue damage, and ultimately organ injury.

Endothelial dysfunction further promotes vascular inflammation by inducing the production of vasoconstrictor agents, adhesion molecules, and growth factors including AngII and endothelin I [36]. In HTN, AngII enhances basal superoxide production in the vasculature by the reduction of NADPH, activation of NADPH oxidase, and increased oxidative stress in the vascular wall [36]. Inflammation further contributes to vascular remodeling by promoting cell growth and proliferation of VSMCs; this is supported by the increased expression in the vascular endothelium of adhesion molecules and ligands, leukocyte extravasation, increased oxidative stress, cytokine production, and activation of pro-inflammatory signaling pathways [36]. Increased AngII-induced ROS generation is involved in the process of vascular remodeling by VSMC proliferation and hypertrophy, as well as modulating cytokine release and pro-inflammatory transcription factors. In addition, ROS reduces the vascular bioavailability of NO [37], which, as mentioned before, is associated with impaired endothelium-dependent vascular relaxation. Several markers of vascular inflammation and oxidative stress have been reported to correlate with the severity of ADPKD. In particular, ADMA, which is an endogenous competitive NOS inhibitor, is an established cardiovascular risk marker that is elevated in ADPKD patients [38], suggesting that these patients have an abnormality in regulating NO biosynthesis. This same study also highlighted an increase in prostaglandin production, specifically elevated serum PGD₂ and PGE₂, in ADPKD patients [38].

Endothelin-1 (ET-1) is an endothelium-derived vasoconstrictor and pro-fibrotic peptide. Renal expression of ET-1 is substantially elevated in murine models of PKD and is associated with cyst formation and development of HTN [39]. One study reported an inverse relationship between urine ET-1 and eGFR in ADPKD, and positively associated with increased excretion of urine albumin, which is consistent with the well-documented role of ET-1 in progressive renal dysfunction and injury [39]. ROS are potent stimulators of ET-1 synthesis by ECs and VSMCs. ET-1 in turn activates NADPH oxidase in VSMCs while ET-1-induced oxidative stress promotes inflammatory responses and contributes to vascular remodeling [40].

While associative, these findings may explain the mechanism involved in tubule injury that often precedes a decline in GFR. This impairment in the microvasculature in the blood-urine interface may reflect signs of endothelial dysfunction at the site of the glomerulus and contribute to renal injury. Vascular endothelial growth factor stimulates endothelial nitric oxide synthase (eNOS) production of NO, which as previously mentioned, has a key role in inducing vasodilation, altering shear stress, and modulating hydrostatic pressure changes in the glomerulus. Previous studies have demonstrated that eNOS deficiency exacerbates renal injury and glomerulopathy [41]. Sun et al. [42] demonstrated that eNOS-deficient mice developed proteinuria, persistent glomerular EC and podocyte injury, and inflammation, suggesting that endothelial dysfunction may play a role in the development and progression of CKD. Morphological changes of ECs after ischemia are characterized by loss of normal endothelial function and barrier, thus, the idea of glomerular endothelial dysfunction may contribute to the understanding of a “leaky” barrier in the PKD model.

A major nonrenal complication of ADPKD is the formation and rupture of intracranial aneurysms (IAs), with reported prevalence varying between 4 and 40% [43]. The mechanisms underlying the coexistence of ADPKD and IAs may lie on the role of Pkd1, and its resulting protein polycystin-1, in the maintenance of structural integrity of the vascular wall. Polycystin-1 is expressed by both endothelial and vascular smooth muscle cells of the microvasculature, and global knockout of polycystin-1 is embryonic lethal, likely due to excessive edema and generalized hemorrhages [44]. Less severe impairment of Pkd1, caused by the generation of an aberrant splicing variant rather than deletion, results in viable offspring that shows progressive renal cyst development, vascular aberrations, and lethality at 1–2 months post-birth [45]. These mice do show reduced expression of wild-type polycystin in aortas, which is associated with dissecting aneurysms, thickening of the aortic wall and focal bleeding [46]. Given the substantial intima thickening observed, there is increased focus on the role of Pkd1 in vascular smooth muscle cell function, proliferation, and integrity.

Smooth muscle-specific deletion of Pkd1 leads to impairments in mechanosensitivity and pressure-induced myogenic contraction [47], which can affect vascular autoregulation. Interestingly, it seems that this effect is the consequence of an interaction between polycystin-1 and polycystin-2, in a model where polycystin-2 acts as an inhibitor of stretch-induced cation influx and contractility in smooth muscle cells, but polycystin-1 interacts with polycystin-2 and reduces its free availability, thus releasing mechanosensitivity [47]. In agreement with this anti-mechanosensitive effect of polycystin-2, silencing Pkd2 in cerebral artery smooth muscle cells leads to the loss of stretch-sensitive cationic currents and reduced pressure-induced constriction [48]. Interestingly, smooth muscle-specific deletion of Pkd2 in mice results in hypotension without alterations in heart or kidney function, suggesting a reduction in total peripheral resistance [49], similar to what was observed in EC-specific Pkd2 deletion [28]. As with cerebral arteries, pressure-induced constriction of resistance-sized hind limb arteries of SMC Pkd2 knockout mice was impaired [49]. The contractile effects of polycystin-2 are dependent on its interaction with actin filaments of the cytoskeleton, as its disruption prevents polycystin-2 inhibition of stretch-activated cationic currents in smooth muscle cells [47]. Importantly, in cerebral artery smooth muscle cells, the actin cytoskeleton is necessary for pressure-induced vascular contractility by maintaining structural integrity of the cell [50]. Thus, the higher incidence of aneurysms observed in ADPKD patients may be associated with reduced interactions between polycystin-2 and the actin cytoskeleton, resulting in impaired pressure-induced contraction and autoregulation, thus exposing the vasculature to excessive pressure and disruption of the vascular wall. This hypothesis, however, remains untested and is currently speculative.

Polycystins have an important role in the development and maintenance of the vascular system [51], as they are expressed in ECs and VSMCs, both of which make up the vascular wall. The association between ADPKD and vascular abnormalities is hypothesized to be attributed to the mutations found in PKD1 and PKD2 within the vascular tissue. Similar to their function in renal cilia, polycystins 1 and 2 function as fluid-shear sensors in endothelial cilia through a complex biochemical cascade involving the upregulation of intracellular calcium levels [52]. Polycystin-1 is proposed as a mechanosensor and interacts with polycystin-2 calcium channel, creating a polycystin complex localized in the cilium. An increase in blood pressure would result in fluid-shear increase, followed by activation of cilia and polycystin complex to generate NO and stimulate smooth muscle relaxation [17]. The abnormal ciliary function then, leads to compromised fluid sensing and impairs the synthesis of NO, as well as downstream signaling pathways involved in smooth muscle relaxation [52].

One major limitation in clinical PKD research is the phenotypic variability of ADPKD. As stated previously, because polycystin-1 is expressed in both the endothelium and vascular smooth muscle, it is essential for maintaining the integrity of blood vessels, in addition to the epithelium. One study specifically observed the vascular response in the aorta after selective disruption of PKD1 in VSMCs [53]. Interestingly, PKD1 deletion did not affect structural abnormalities or vascular reactivity, indicating that PKD1 gene disruption in VSMC alone is not sufficient to induce structural changes. While the 40-week-old heterozygous Pkd1^del/+ mice, which carry one deletion allele, did show an increase in vascular contractility in response to phenylephrine, however, this gene disruption in VSMCs alone is not sufficient to elicit this response, which may indicate that other cell types are contributing to the vascular phenotype. Another study highlighted the primary role of PKD1 mutations in vascular fragility where mouse embryos homozygous for the mutant allele exhibited subcutaneous edema, vascular leaks, and rupture of blood vessels, all of which were secondary to embryonic lethality [44]. These data indicate that in addition to maintaining the integrity of vasculature, polycystin-1 also plays a direct role in the maturation of vascular endothelium.

Vascular abnormalities are well-documented in clinical and preclinical ADPKD models, given that the mutation credited for the development of parenchymal cysts in the renal tubule is polycystin-1 and polycystin-2. Whether ARPKD is also associated with vascular abnormalities, and its relation to renal dysfunction, remains to be determined. As stated previously, ARPKD is primarily attributed to a mutation in the PKHD1 gene, which is responsible for the expression of the cilia-associated protein fibrocystin. While the mechanisms by which PKHD1 causes disease phenotypes remain largely unidentified, its function in cell signaling allows it to maintain structural integrity in the primary cilia of renal epithelium. One study demonstrated that a disruption of fibrocystin expression induced malformation of the primary cilia in the kidney in vivo [54]. Peterson et al. [55] have demonstrated a decrease in vasorelaxation to endothelium-dependent vasodilators as well as a correction to abnormal vasoreactivity to l-arginine (NO precursor) in an ARPKD murine model.

An impaired relaxation response of resistance vessels to ACh has been previously demonstrated and proposed as a contributory factor to vascular disease in ADPKD patients [56]. Accordingly, we hypothesized a similar vascular dysfunction is also observed in ARPKD. To test this hypothesis, we measured vascular function in isolated mesenteric vessels to assess the time-dependent relationship with renal dysfunction in a preclinical model of ARPKD (PCK Rat). We hypothesized that male and female PCK rats would have a progressive impairment in endothelial and smooth muscle function compared to non-cystic controls.

Vascular function was assessed ex vivo by Mulvany wire myography on secondary mesenteric arterioles that had an average arteriole diameter of 200 µm. Experiments were conducted in young PCK animals (10 weeks; n = 5; 2m/3F), mature PCK animals (30 weeks; n = 16; 8m/8F) and age-matched, non-cystic Sprague-Dawley controls (young; n = 5; 5m) (mature; n = 4; 4m). All experiments were approved by the University of Arizona IACUC and in compliance with the National Institute of Health Guide for Care and Use of Laboratory Animals. Endothelial and smooth muscle vasodilatory function was assessed as described previously [57, 58] by dose responses (10⁻¹¹-10⁻⁵M) to acetylcholine (Ach) and sodium nitroprusside (SNP), respectively, following pre-constriction to thromboxane mimetic U46619 (10⁻⁴M). As shown in Figure 2a, Ach-mediated relaxation in the young PCK rat (43.9 ± 9.6 %max) were significantly impaired versus age-matched Sprague-Dawley controls (86.1 ± 1.9% max). However, there were no differences in %max relaxation to Ach in the mature PCK rats when compared to their age-matched controls. The young PCK rats had a lower vasorelaxation (43.9 ± 9.6 %max) to Ach when compared to the mature PCK rats (88.5 ± 4.7 %max) (p < 0.05), EC50 values in the young PCK rats (8.8 × 10⁻⁸ ± 2. × 10⁻⁸ M Ach) were lower (p < 0.05) compared to the mature PCK rats (1.8 × 10⁻⁸ ± 4.2 × 10⁻⁹ M Ach) (p < 0.05). Final % relaxation levels and EC50 in response to SNP did not vary between aged groups or their respective controls. No sex-dependent differences were observed in the PCK animals. These results concluded that the endothelial-dependent vasorelaxation to Ach was impaired in the PCK rats compared to their non-cystic controls while smooth muscle sensitivity to NO, assessed by incubation with the NO donor SNP, was not affected. Interestingly, endothelial function is decreased in young PCK rats, yet improves over time compared to the mature PCK cohort as indicated in the %max vasorelaxation to Ach. There is also a shift in the EC50 in the PCK animals compared to the non-cystic controls, indicating that the PCK rat is more sensitive at lower concentrations.

Patients with ADPKD have nearly a 12-fold higher prevalence for IAs than the general population (4.0–11.7% vs. 1.0%) [3]. An asymptomatic IA is found in about 8% of ADPKD patients, and notably the only clinical characteristic associated with IA presence is family history [59]. While the prevalence of IAs is high, the risk of rupture or expansion is quite low. IAs are among the most common vascular manifestations of ADPKD, and given the familial clustering, it may be possible that mutations in PKD1 or PKD2 may predispose patients to developing aneurysms. However, the manifestation of vascular complications that result in IAs seems to be variable. Aneurysm formation could depend on focal, somatic, and random loss of wild-type PKD allele in vascular tissue [46]. It has been noted that insufficiencies in PKD1 or PKD2 alter intracellular Ca²⁺ signaling as well as TGF-B signaling, which can contribute to aneurysm formation [51]. The risk for developing these vascular anomalies and further complications, however, requires further exploration as it remains to be fully delineated.

Although cystogenesis is the hallmark of a decline in renal function in ADPKD, the leading cause of death among patients is cardiovascular complications [3]. Besides the context of HTN, vascular dysfunction contributes to the cardiovascular abnormalities found in ADPKD through associated local inflammation, arterial remodeling, and arterial stiffness [22]. It is well accepted that endothelial dysfunction is a predictor of atherosclerosis development and future cardiovascular events; under conditions that present peripheral resistance, the endothelium can orchestrate vascular remodeling processes and inflammation, which conversely induces endothelial dysfunction. Left ventricular hypertrophy is an independent risk factor for cardiovascular morbidity. There is a significant correlation between HTN and increased left ventricular mass index in both children and adults with ADPKD, and reports indicated that 48% of hypertensive patients with ADPKD have this condition [52]. As previously mentioned, PKD1 and PKD2 encode for PC1 and PC2, which are integral membrane proteins that regulate various signaling pathways including intracellular Ca²⁺ homeostasis. Morel et al. [60] demonstrated that PKD1 deletion is associated with alterations in Ca²⁺ signaling and vascular reactivity in mouse aorta. In addition, haploinsufficiency also correlated with increased systolic blood pressure and impaired endothelium-dependent relaxation [60], which is consistent with our own observations in the young PCK vascular function measurements. In support of Figure 1, ex vivo analyses confirmed that AngII produced a large increase in renal vascular resistance in Pkd1^+/− versus Pkd1^+/+ mice [60]. These data emphasize the importance of abnormal Ca2+ signaling in ADPKD and gives further insight into the role of the PC1/PC2 complex and its contributions to the vascular phenotype observed in ADPKD.

Renal replacement therapies (RRTs) such as dialysis and renal transplantation are the currently recommended course of treatment and management for patients with PKD. While renal replacement therapy is capable of temporarily alleviating the stress that is placed on renal function, there are limited interventions for vascular function. In addition to long-term surveillance of blood pressure and renal function, secondary complications like HTN are controlled pharmacologically with angiotensin-converting enzyme inhibitors or AngII receptor blockers. Many of these pharmacological therapies are capable of mitigating the decline in renal dysfunction and cystogenesis yet are largely limited to ADPKD [3, 61]. Renal innervation in PKD is one emerging treatment target, and Gauthier et al. [14] have reported that cystogenesis is attenuated with renal nerve ablation treatment in an ARPKD rat model. This study also highlighted a novel role of afferent renal nerves, as afferent-targeted denervation mitigated the cystogenesis identically to the total nerve ablation which ablates both sympathetic and afferent renal nerves. These observations highlight the contributions of renal nerves in a PKD model but also support the overlap in etiologies between autonomic dysfunction and secondary HTN. It would be interesting to consider renal denervation as a potential therapeutic for mitigating vascular dysfunction associated with PKD.

Among the greatest challenges in researching PKD is understanding the varying pathogenesis between ARPKD and ADPKD. The age on onset, the severity of symptoms, and the rate of progression to end-stage renal failure or death are widely variable in this group of disease.

While the etiologies between both forms of PKD are similar, the mechanism that drives both renal and vascular complications remains to be fully delineated. The standing hypothesis suggests that the polycystin complex functions as a mechanosensor in cilia through the contribution of ciliary proteins PC1, PC2, and fibrocystin [2]. Proliferation pathways and Ca²⁺ signaling have also been suggested to contribute to abnormal ciliary composition in some syndromic ciliopathies due to its role in downstream signaling. Because of the variability in both specific cleavage sites and localization of the ciliary proteins, clinical symptoms are also highly variable across individuals. Early stages of ADPKD progression, when renal function remains normal, many patients develop arterial HTN that contributes considerably to the increased morbidity and mortality [2]. As previously mentioned, there are various pathogenic mechanisms attributed to ADPKD progression, including activation of the RAAS system, increased sympathetic nervous activity, and disturbances in vascular tone and mechanosensitivity [2, 8, 29]. Indeed, the loss of function of the polycystin proteins in renal epithelium contributes to hyperproliferation and renal cyst formation. However, the posited role of polycystins underlying the vascular and cardiovascular defects is likely also related to the disruption of polycystin function in the ECs and vascular smooth muscle cells [2].

Substantial progress has been made in the diagnosis and treatment of vascular anomalies in PKD. In addition to peripheral resistance, endothelium affects other aspects that contribute to the development of HTN. Vascular stiffness is one parameter that can be modulated by the endothelium; it is also capable of vascular remodeling processes and inflammation, both of which impair endothelium-dependent relaxation and contribute to vascular dysfunction. The function of polycystin proteins in vasculature indicates that they are involved in mechanosensation. Specifically in ECs, they are responsible for vasodilation through fluid-shear stress sensing and regulation of NO release [52]. In vascular smooth muscle cells, the polycystins play a role in regulating pressure by modulating the myogenic contraction. Studies of haploinsufficienies in PC1 and PC2 in ADPKD have shown that vascular changes are associated with a reduction of levels of polycystin proteins [2]. In contrast, the vascular anomalies found in ARPKD are less understood given the limitation of experimental models of a mutation centered around fibrocystin (PKHD1 gene). Current studies are looking into alternative pharmacological therapeutics that can mitigate secondary complications like HTN and vascular dysfunction, in addition to directly targeting cytogenic mechanisms in both ADPKD and ARPKD [2, 9].

Given that this disease is multifaceted, and highly variable in clinical presentation, it is difficult to develop rational therapies that can retard cystic expansion, mitigate HTN, and regulate vascular anomalies that influence both cardiovascular and IA complications. Aggressive control of other cardiovascular risk factors is also very important to improve the prognosis of affected patients given that cardiovascular manifestations are the leading cause of mortality in PKD. Further research is necessary to improve both the identification of patients who require treatment, as well as the diagnosis and treatment of vascular anomalies. Although progress has been made in terms of better outcome prediction, vascular complications are rare and difficult to predict based on clinical screening or genetic analysis. To date, there is limited work reported in ARPKD, but given the related mechanisms and etiologies between the two inherited forms of PKD, there is hope that some answers can be derived and built on the data presented in Figure 1. In the meantime, further vascular function and molecular research are required to improve our understanding of the vascular contributions and responses to PKD progression.

We would like to thank Mark Morales and Alexandra Messieh at the University of Arizona for technical expertise.

All animal experiments presented were approved by the University of Arizona IACUC and in compliance with the National Institute of Health Guide for Care and Use of Laboratory Animals.

The authors have no conflicts of interest to declare.

This work was supported in part by National Institutes of Health Grants NIH R00HL141650 (CTB), R00 HL140106 (PWP), and R01 AG073230 (PWP), National Coordinating Center (NCC) for the Polycystic Kidney Disease (PKD) Research and Translation Core Centers Pilot and Feasibility Award U24DK126110 (CTB), University of Arizona College of Medicine FUTURRE Award (CTB), and Alzheimer’s Association Grant AARGD-21–850835 (PWP).

Melissa Dennis performed the experiments, analyzed and graphed the data, drafted the manuscript, edited and approved the final version. Paulo Pires assisted in data analysis, edited and approved the final version. Christopher Banek planned the experiments, assisted in data analysis, edited and approved the final version, and provided the research funding.

Correspondence and requests for data and/or materials should be addressed with the corresponding author (Christopher Banek; Email: [email protected]; Phone: 520-621–6068).