PRESENTED BY
KLAUS OLGAARD

Presentation Summary

Written by: Milica Maksimovic
Reviewed by: Jasna Trbojevic-Stankovic and Klaus Olgaard

There has been no significant progress in the therapeutic management of chronic kidney disease (CKD)-mineral bone disorders (MBD) since the introduction of KDIGO CKD-MBD Guidelines in 2009, hence, an urgent need exists to better understand the pathophysiology of this condition[1]. The new paradigm on the parathyroid-kidney–vascular-bone axis has revealed that CKD-MBD consists of a fascinating number of complex disturbances in the tissue cross-talk between these different organs in uraemia.

A new paradigm of Kidney-Vascular-Bone Axis in CKD-MBD

Recent mechanistic studies suggest that in CKD, damaged kidney tissue, in an attempt to repair, may cause induction of circulating factors (e.g. Dkk1, sclerostin and activin-A) with systemic effects on the skeleton and the cardiovascular system [2]. Increased fibroblast growth factor (FGF)-23 levels and decreased Klotho expression, hypocalcemia and hyperphosphatemia all contribute to secondary hyperparathyroidism and vitamin D deficiency, and collectively have been associated with cardiovascular risk in CKD. In addition, there is an inhibition of osteoblast function and stimulation of osteoclast activity that produce abnormal skeletal remodelling and high-turnover renal osteodystrophy (Figure 1).

Figure 1. A new paradigm on Kidney-Vascular-Bone Axis in CKD-MBD as proposed by Keith Hruska (slide 4[3]).

 

This concept was examined in the animal model of unilateral ureter obstruction (UUO) and the results showed very early vascular effects of unilateral kidney fibrosis supporting existence of a new kidney-vasculature axis [4]. The UUO in rats caused an upregulation of activin-A in obstructed kidneys as well as a doubling of activin-A in plasma after ten days, suggesting a secretion of activin-A from the obstructed kidney with potentially systemic effects on CKD-MBD. As such, increased aortic sclerostin was observed in UUO rats, that have normal kidney function, due to the untouched contralateral kidney. In mice with CKD, a ligand trap for the activin type IIA receptor is protective against vascular disease and renal fibrosis [5]. Compounds such as those interfering with activin type IIA receptor signalling are therefore of particular therapeutic interest as potential treatments to break this bone-vascular cross-talk in CKD [6].

The bone-kidney endocrine axis

A defect in the expression of Klotho gene in the mouse results in a syndrome resembling human ageing, that include a short lifespan, infertility, arteriosclerosis, skin atrophy, osteoporosis and emphysema. Since it was discovered in 1997 the Klotho gene product was suggested as a functional part of a signalling pathway that regulates ageing in vivo and morbidity in age-related diseases, including uraemia [7]. However, its functions remained somewhat unclear until the discovery of FGF-23, which is mainly produced in the bone. The experimental studies suggested FGF-23 as an important in vivo regulator of phosphate homeostasis and vitamin D metabolism in the kidney. Strikingly similar physical and biochemical phenotypes of mice that lack either Fgf-23 or Klotho gene were then demonstrated [8], and a functional relationship between these molecules postulated. This has led to the identification of Klotho as a cofactor in FGF-23 and FGF receptor interactions [9]. FGF-23-Klotho complex counter regulates plasma vitamin D levels, by modulating the renal 1α-hydroxylase, and phosphate levels, by suppressing sodium–phosphate co-transporter activities.

Klotho is a membrane-bound protein primarily expressed in the kidney, mostly in the distal tubule and, at a low level, in the proximal tubule. The kidney is essential in Klotho homeostasis, both producing Klotho and releasing it into the circulation, as well as clearing it from blood into the urine [10-12]. Obstructed kidneys showed early Klotho gene depletion and induction of pro-fibrotic TGF- and periostin. Contralateral kidneys exhibited no compensatory upregulation of Klotho, suggesting importance of kidney mass for Klotho production [4].

Both FGF-23 and Klotho deficiencies result in phosphate retention and premature ageing syndrome. The animal studies on FGF-23 and Klotho-deficient mice have been pivotal in linking phosphate levels and longevity in mammals [11]. The transgenic overexpression of Klotho in mice extended their life span through inhibition of insulin and IGF1 signalling [13]. In addition, soluble Klotho restored phosphate retention, suppressed accelerated ageing and significantly reduced both renal and aorta calcium deposits in Klotho mutant mice [14]. These results created a hope for a therapeutic potential of soluble Klotho protein in treatment of age-related disorders  bearing in mind the relationship between Klotho deficiency and accelerated ageing in uraemia.

Despite the evidence that Klotho is neither expressed in normal nor uraemic aorta [15], there are several vascular effects such as protection of vascular smooth muscle cells against calcification, resistance to oxidative stress, maintenance of endothelial function and integrity, that are presumably mediated via its soluble form [16]. However, strategies to upregulate endogenous Klotho might prove challenging – a blockade of FGF receptor upregulated normal kidney Klotho gene and protein expression but had no effect on the kidney in long term uraemic rats [17].

The recent crystal structure of FGF-23-soluble Klotho-FGF receptor complex revealed that soluble and transmembrane Klotho possess a similar capacity for FGF-23 signalling and it was therefore suggested that all pleiotropic effects of Klotho are FGF-23 dependent [18]. However, it seems that in the absence of FGF-23, the receptor-binding arm of Klotho may  interact with specific carbohydrates and exert its pleiotropic FGF-23 independent activities. Local and distant anti-fibrotic and anti-tumorigenic effects are mediated at least partially by the direct inhibitory effects of soluble Klotho on TGFβ1 signaling, Wnt signaling, and FGF signalling (Figure 2) [11].

Figure 2: Pleiotropic effects of Klotho (slide 21[3]). From Mencke R et al, Adv Drug Deliv Rev. 2017

 

FGF-23 – an important driver of CKD-MBD

Elevated FGF-23 levels are strongly associated with high rates of cardiomyopathy, uraemic inflammation, impairment of host defence mechanism and increased risk of mortality in patients with CKD [19-21]. In the majority of these patients, FGF-23 levels are stable over time, but longitudinal measurements identify subpopulations with rising levels and exceptionally high risk of death [22]. FGF-23 synthesis can take place extra-skeletally in injured organs – despite not being expressed in normal kidney, it is rapidly induced in injured kidney [17]. Well-known regulators of FGF-23 expression in bone, such as parathyroid hormone, calcitriol and FGF receptor inhibitors, have no impact on kidney expression of FGF-23 nor does kidney-derived FGF-23 generate high plasma FGF-23 levels in uraemia [17]. Thus, the only direct contribution of the injured kidney to circulating FGF-23 levels in uraemia appears to be a reduced renal extraction of bone-derived FGF-23. However, the importance of kidney in regulation of FGF-23 plasma level and its metabolism is shown in acute kidney insufficiency [23].

New regulators of parathyroid hormone expression and parathyroid cell proliferation

Calcium, 1,25-dihydroxyvitamin D3 and the high phosphate of uraemia all regulate parathyroid hormone (PTH) secretion and PTH gene expression through transcriptional and posttranscriptional mechanisms [24]. In prolonged hypocalcemia and uraemia, there is also parathyroid cell proliferation. FGF-23 as well regulates the PTH plasma levels through the FGF receptor in normocalcemia, but not in hypocalcemia. FGF-23 normally has an inhibitory tonus on PTH secretion and signals through the FGF receptor – Klotho complex. In acute hypocalcemia, when increased PTH secretion is needed to restore the calcium homeostasis, this inhibitory effect of FGF-23 is abolished [25]. This mechanism may explain why a parathyroid resistance to FGF-23 and a concomitantly elevated FGF-23 and PTH levels are observed in uraemia.

Tissue cross-talk between calcified vasculature and bone

Chronic uraemia significantly changes the transcriptional profile of the calcified aorta [15]. Delivery of soluble Klotho prevents the vascular calcification in a mouse model of CKD-MBD [26]. Once established, vascular calcification persists even in the setting when hyperphosphatemia or the uraemic milieu is abolished. The administration of exogenous bone morphogenetic protein 7 in aortae of rats with chronic uraemia ameliorates expression of profibrotic genes, but does not reverse established vascular calcification [27]. These results highlight the importance of prevention of vascular calcification development in CKD.

Professor Olgaard presented the first evidence of tissue cross-talk between calcified vasculature and bone, i.e results showing that vascular calcification is inducing bone loss. In a rat model, a transplantation of uraemic calcified aorta into a normal rat led to a decreased bone mineral density and a disturbed bone metabolism (osteoblast and osteoclast function and their communication, extracellular matrix and Wnt signalling) (Figure 3, unpublished data).

Figure 3: Decreased bone mineral density and disturbed bone metabolism in normal recipient rats transplanted with calcified uraemic aorta (unpublished data, slides 39 and 40[3])

 

 

Key summary points

-The injured kidneys produce circulating factors affecting vasculature and skeleton. This early mechanism in CKD-MBD might in the future be target for new therapeutic strategies inhibiting renal fibrosis, vascular calcification and bone loss.

-The discovery of Klotho and FGF-23 has changed our understanding of mineral metabolism by identifying more complex cross-talk and endocrine feedback loops between the parathyroid gland, bone, vasculature and kidney.

-Klotho and FGF-23 likely have additional functions, not related to mineral homeostasis, that are yet to be discovered.

-The newly resolved crystal structure of Klotho adds to the understanding of its pleiotropic potential and could be used to develop drugs to treat disorders of ageing and uraemia.

-Kidney FGF-23 expression is induced in kidney injury, but does not contribute to the increased circulating levels of FGF-23 in uraemia.

-The kidney is essential for the metabolism of both Klotho and FGF-23.

-Much remains to be discovered regarding the pathological significance of FGF-23 in uraemia and whether FGF-23 itself will become a relevant therapeutic target in clinical settings.

-Vasculopathy is an early symptom in CKD-MBD, inducing de-regulation of a high number of genes in the uraemic vasculature.

-Development of vascular calcification should be prevented, as established calcification is not reversible.

-The presence of severe vascular calcification has an impact on bone metabolism, thus demonstrating true cross-talk between these tissues, where vascular calcification is inducing bone loss.

-FGF-23 has an inhibitory tonus on PTH secretion, which is overruled by hypocalcemia, and might explain the lack of an inhibitory effect of the very high plasma FGF-23 levels on PTH secretion in uraemia.

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