Is chronic kidney disease-mineral and bone disorder associated with the presence of endothelial progenitor cells with a calcifying phenotype?

Role of endothelial cells in vascular calcification

Vascular calcification (VC) is active and regulates extraosseous ossification progress, which is an independent predictor of cardiovascular disease (CVD) morbidity and mortality. Endothelial cells (ECs) line the innermost layer of blood vessels and directly respond to changes in flow shear stress and blood composition. Together with vascular smooth muscle cells, ECs maintain vascular homeostasis. Increased evidence shows that ECs have irreplaceable roles in VC due to their high plasticity. Endothelial progenitor cells, oxidative stress, inflammation, autocrine and paracrine functions, mechanotransduction, endothelial-to-mesenchymal transition (EndMT), and other factors prompt ECs to participate in VC. EndMT is a dedifferentiation process by which ECs lose their cell lineage and acquire other cell lineages; this progress coexists in both embryonic development and CVD. EndMT is regulated by several signaling molecules and transcription factors and ultimately mediates VC via osteogenic differentiation. The specific molecular mechanism of EndMT remains unclear. Can EndMT be reversed to treat VC? To address this and other questions, this study reviews the pathogenesis and research progress of VC, expounds the role of ECs in VC, and focuses on the regulatory factors underlying EndMT, with a view to providing new concepts for VC prevention and treatment.

Endoplasmic Reticulum Stress and Pathogenesis of Vascular Calcification

Vascular calcification (VC) is characterized by calcium phosphate deposition in blood vessel walls and is associated with many diseases, as well as increased cardiovascular morbidity and mortality. However, the molecular mechanisms underlying of VC development and pathogenesis are not fully understood, thus impeding the design of molecular-targeted therapy for VC. Recently, several studies have shown that endoplasmic reticulum (ER) stress can exacerbate VC. The ER is an intracellular membranous organelle involved in the synthesis, folding, maturation, and post-translational modification of secretory and transmembrane proteins. ER stress (ERS) occurs when unfolded/misfolded proteins accumulate after a disturbance in the ER environment. Therefore, downregulation of pathological ERS may attenuate VC. This review summarizes the relationship between ERS and VC, focusing on how ERS regulates the development of VC by promoting osteogenic transformation, inflammation, autophagy, and apoptosis, with particular interest in the molecular mechanisms occurring in various vascular cells. We also discuss, the therapeutic effects of ERS inhibition on the progress of diseases associated with VC are detailed.

Delineation of the healthy rabbit kidney by immunohistochemistry - A technical note

Pre-clinical animal models are needed to investigate and study kidney injuries and diseases. The rabbit kidney model is frequently used because various important parameters can be assessed with it. For example, histology and immunohistochemistry are indispensable as tissue morphology and composition can be investigated qualitatively as well as quantitatively. Here, different histological and immunohistochemical stainings were performed in the rabbit healthy naïve kidney tissue. First, overnight formalin fixation followed by paraffin embedding and cryopreservation with a subsequent 10-minute formalin fixation prior to staining were compared. Cryosections showed a more pronounced staining pattern, with clear borders at low magnifications, but blurred borders at higher magnifications. Then, antigen retrieval (AR) for paraffin embedded sections resulted in more prominent corresponding signals compared to stainings without AR. Moreover, several advantages and disadvantages of chromogenic versus immunofluorescence stainings were considered. Chromogenic staining was advantageous compared to immunofluorescence for collagen I and III, and to a minor degree for fibronectin. Finally, distinct structures, such as the pelvis, the calices, the glomeruli and tubuli, were stained in serial sections with diverse immunohistochemical stainings in order to delineate their composition. The following stainings were performed: standard Haematoxylin&Eosin and Elastica van Gieson staining, collagen I, collagen III, fibronectin, α-SMA, ki-67 and protease-activated receptor-2 (PAR-2). While chromogenic stainings of collagen I and collagen III were particularly useful to depict kidney structures in paraffin sections compared with cryosections, cryosections immunofluorescently stained for α-SMA were superior to paraffin sections, particularly at higher magnifications. With regard to specific structures, we found renal vessel walls positive for fibronectin and α-SMA, while the Bowman's capsule was only positive for fibronectin and α-SMA showed only tiny spots. The mesangial cells of the glomeruli and the distal tubuli were PAR-2 positive, while the proximal tubuli were PAR-2 negative.

Roles of klotho and stem cells in mediating vascular calcification (Review)

Vascular calcification, characterized by the active deposition of calcium phosphate in the vascular walls, is commonly observed in aging, diabetes mellitus and chronic kidney disease. This process is mediated by different cell types, including vascular stem/progenitor cells. The anti-aging protein klotho may act as an inhibitor of vascular calcification through direct effects on vascular stem/progenitor cells with osteogenic differentiation potential. A better understanding of the possible effects of klotho on vascular stem/progenitor cells may provide novel insight into the cellular and molecular mechanisms of klotho deficiency-related vascular calcification and disease. The klotho protein may be considered as a promising therapeutic agent for treating vascular calcification and disease and calcification-related vascular diseases.

Vascular Calcification: An Important Understanding in Nephrology

Vascular calcification (VC) is a life-threatening state in chronic kidney disease (CKD). High cardiovascular mortality and morbidity of CKD cases may root from medial VC promoted by hyperphosphatemia. Vascular calcification is an active, highly regulated, and complex biological process that is mediated by genetics, epigenetics, dysregulated form of matrix mineral metabolism, hormones, and the activation of cellular signaling pathways. Moreover, gut microbiome as a source of uremic toxins (eg, phosphate, advanced glycation end products and indoxyl-sulfate) can be regarded as a potential contributor to VC in CKD. Here, an update on different cellular and molecular processes involved in VC in CKD is discussed to elucidate the probable therapeutic pathways in the future.

The Role of Vitamin D in Modulating Mesenchymal Stem Cells and Endothelial Progenitor Cells for Vascular Calcification

Vascular calcification, which involves the deposition of calcifying particles within the arterial wall, is mediated by atherosclerosis, vascular smooth muscle cell osteoblastic changes, adventitial mesenchymal stem cell osteoblastic differentiation, and insufficiency of the calcification inhibitors. Recent observations implied a role for mesenchymal stem cells and endothelial progenitor cells in vascular calcification. Mesenchymal stem cells reside in the bone marrow and the adventitial layer of arteries. Endothelial progenitor cells that originate from the bone marrow are an important mechanism for repairing injured endothelial cells. Mesenchymal stem cells may differentiate osteogenically by inflammation or by specific stimuli, which can activate calcification. However, the bioactive substances secreted from mesenchymal stem cells have been shown to mitigate vascular calcification by suppressing inflammation, bone morphogenetic protein 2, and the Wingless-INT signal. Vitamin D deficiency may contribute to vascular calcification. Vitamin D supplement has been used to modulate the osteoblastic differentiation of mesenchymal stem cells and to lessen vascular injury by stimulating adhesion and migration of endothelial progenitor cells. This narrative review clarifies the role of mesenchymal stem cells and the possible role of vitamin D in the mechanisms of vascular calcification.

An Investigation Into the Role of Osteocalcin in Human Arterial Smooth Muscle Cell Calcification

Osteocalcin (OCN) is a bone-derived protein that is detected within human calcified vascular tissue. Calcification is particularly prevalent in chronic kidney disease (CKD) patients but the role of OCN in calcification, whether active or passive, has not been elucidated. Part 1: The relationship between OCN, CKD and vascular calcification was assessed in CKD patients (n = 28) and age-matched controls (n = 19). Part 2: in vitro, we analyzed whether addition of uncarboxylated osteocalcin (ucOCN) influenced the rate or extent of vascular smooth muscle cell (VSMC) calcification. Human aortic VSMCs were cultured in control media or mineralisation inducing media (MM) containing increased phosphate with or without ucOCN (10 or 30 ng/mL) for up to 21 days. Markers of osteogenic differentiation and calcification were determined [alkaline phosphatase (ALP) activity, total intracellular OCN, Runx2 expression, α-SMA expression, alizarin red calcium staining, and calcium quantification]. Part 1 results: In our human population, calcification was present (mean age 76 years), but no differences were detected between CKD patients and controls. Plasma total OCN was increased in CKD patients compared to controls (14 vs. 9 ng/mL; p < 0.05) and correlated to estimated glomerular filtration rate (p < 0.05), however no relationship was detected between total OCN and calcification. Part 2 results: in vitro, ALP activity, α-SMA expression and calcium concentrations were significantly increased in MM treated VSMCs at day 21, but no effect of ucOCN was observed. Cells treated with control media+ucOCN for 21 days did not show increases in ALP activity nor calcification. In summary, although plasma total OCN was increased in CKD patients, this study did not find a relationship between OCN and calcification in CKD and non-CKD patients, and found no in vitro evidence of an active role of ucOCN in vascular calcification as assessed over 21 days. ucOCN appears not to be a mediator of vascular calcification, but further investigation is warranted.

Bone, inflammation and the bone marrow niche in chronic kidney disease: what do we know?

Recent improvements in our understanding of physiology have altered the way in which bone is perceived: no longer is it considered as simply the repository of divalent ions, but rather as a sophisticated endocrine organ with potential extraskeletal effects. Indeed, a number of pathologic conditions involving bone in different ways can now be reconsidered from a bone-centred perspective. For example, in metabolic bone diseases like osteoporosis (OP) and renal osteodystrophy (ROD), the association with a worse cardiovascular outcome can be tentatively explained by the possible derangements of three recently discovered bone hormones (osteocalcin, fibroblast growth factor 23 and sclerostin) and a bone-specific enzyme (alkaline phosphatase). Further, in recent years the close link between bone and inflammation has been better appreciated and a wide range of chronic inflammatory states (from rheumatoid arthritis to ageing) are being explored to discover the biochemical changes that ultimately lead to bone loss and OP. Also, it has been acknowledged that the concept of the bone-vascular axis may explain, for example, the relationship between bone metabolism and vessel wall diseases like atherosclerosis and arteriosclerosis, with potential involvement of a number of cytokines and metabolic pathways. A very important discovery in bone physiology is the bone marrow (BM) niche, the functional unit where stem cells interact, exchanging signals that impact on their fate as bone-forming cells or immune-competent haematopoietic elements. This new element of bone physiology has been recognized to be dysfunctional in diabetes (so-called diabetic mobilopathy), with possible clinical implications. In our opinion, ROD, the metabolic bone disease of renal patients, will in the future probably be identified as a cause of BM niche dysfunction. An integrated view of bone, which includes the BM niche, now seems necessary in order to understand the complex clinical entity of chronic kidney disease-mineral and bone disorders and its cardiovascular burden. Bone is thus becoming a recurrently considered paradigm for different inter-organ communications that needs to be considered in patients with complex diseases.

The Key Role of Phosphate on Vascular Calcification

Vascular calcification (VC) is common in dialysis and non-dialysis chronic kidney disease (CKD) patients, even in the early stage of the disease. For this reason, it can be considered a CKD hallmark. VC contributes to cardiovascular disease (CVD) and increased mortality among CKD patients, although it has not been proven. There are more than one type of VC and every form represents a marker of systemic vascular disease and is associated with a higher prevalence of CVD in CKD patients, as shown by several clinical studies. Major risk factors for VC in CKD include: Increasing age, dialysis vintage, hyperphosphatemia (particularly in the setting of intermittent or persistent hypercalcemia), and a positive net calcium and phosphate balance. Excessive oral calcium intake, including calcium-containing phosphate binders, increases the risk for VC. Moreover, it has been demonstrated that there is less VC progression with non-calcium-containing phosphate binders. Unfortunately, until now, a specific therapy to prevent progression or to facilitate regression of VC has been found, beyond careful attention to calcium and phosphate balance.

Diabetic endothelial colony forming cells have the potential for restoration with glycomimetics

Endothelial colony forming progenitor cell (ECFC) function is compromised in diabetes, leading to poor vascular endothelial repair, which contributes to impaired diabetic foot ulcer healing. We have generated novel glycomimetic drugs with protective effects against endothelial dysfunction. We investigated the effect of glycomimetic C3 on the functional capacity of diabetic ECFCs. ECFCs were isolated from healthy controls and patients with diabetes with neuroischaemic (NI) or neuropathic (NP) foot ulcers. Functionally, diabetic ECFCs demonstrated delayed colony formation (p < 0.02), differential proliferative capacity (p < 0.001) and reduced NO bioavailability (NI ECFCs; p < 0.05). Chemokinetic migration and angiogenesis were also reduced in diabetic ECFCs (p < 0.01 and p < 0.001), and defects in wound closure and tube formation were apparent in NP ECFCs (p < 0.01). Differential patterns in mitochondrial activity were pronounced, with raised activity in NI and depressed activity in NP cells (p < 0.05). The application of glycomimetic improved scratch wound closure in vitro in patient ECFCs (p < 0.01), most significantly in NI cells (p < 0.001), where tube formation (p < 0.05) was also improved. We demonstrate restoration of the deficits in NI cells but not NP cells, using a novel glycomimetic agent, which may be advantageous for therapeutic cell transplantation or as a localised treatment for NI but not NP patients.