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      Vascular Calcification in Animal Models of CKD: A Review

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          Abstract

          Vascular calcification is a significant contributor to the cardiovascular mortality observed in chronic kidney disease (CKD). This review discusses the animal models (5/6 nephrectomy, mouse electrocautery model and dietary adenine) that have been employed in the study of vascular calcification outcomes in CKD. Rodent models of CKD generate a range of severity in the vascular calcification phenotype. Major limitations of the 5/6th nephrectomy model include the requirement for surgery and the need to use either excessive dietary phosphorus or vitamin D. Major limitations of the mouse electrocautery model include the requirement for surgery, the mortality rate when very advanced CKD develops, and resistance to vascular calcification without the use of transgenic animals. This is balanced against the major advantage of the ability to study transgenic animals to further understand the mechanisms associated with either the acceleration or inhibition of calcification. Dietary adenine generates severe CKD and does not require surgery. The major disadvantage is the weight loss that ensues when rats receive a diet containing 0.75% adenine. In summary, animal models are useful to study CKD-associated vascular calcification and the results obtained in these pre-clinical animal studies appear to translate to the evidence, however limited, which exists in humans with CKD.

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          Most cited references42

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          Human vascular smooth muscle cells undergo vesicle-mediated calcification in response to changes in extracellular calcium and phosphate concentrations: a potential mechanism for accelerated vascular calcification in ESRD.

          Patients with ESRD have a high circulating calcium (Ca) x phosphate (P) product and develop extensive vascular calcification that may contribute to their high cardiovascular morbidity. However, the cellular mechanisms underlying vascular calcification in this context are poorly understood. In an in vitro model, elevated Ca or P induced human vascular smooth muscle cell (VSMC) calcification independently and synergistically, a process that was potently inhibited by serum. Calcification was initiated by release from living VSMC of membrane-bound matrix vesicles (MV) and also by apoptotic bodies from dying cells. Vesicles released by VSMC after prolonged exposure to Ca and P contained preformed basic calcium phosphate and calcified extensively. However, vesicles released in the presence of serum did not contain basic calcium phosphate, co-purified with the mineralization inhibitor fetuin-A and calcified minimally. Importantly, MV released under normal physiologic conditions did not calcify, and VSMC were also able to inhibit the spontaneous precipitation of Ca and P in solution. The potent mineralization inhibitor matrix Gla protein was found to be present in MV, and pretreatment of VSMC with warfarin markedly enhanced vesicle calcification. These data suggest that in the context of raised Ca and P, vascular calcification is a modifiable, cell-mediated process regulated by vesicle release. These vesicles contain mineralization inhibitors derived from VSMC and serum, and perturbation of the production or function of these inhibitors would lead to accelerated vascular calcification.
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            A role for smad6 in development and homeostasis of the cardiovascular system.

            Smad proteins are intracellular mediators of signalling initiated by Tgf-betasuperfamily ligands (Tgf-betas, activins and bone morphogenetic proteins (Bmps)). Smads 1, 2, 3, 5 and 8 are activated upon phosphorylation by specific type I receptors, and associate with the common partner Smad4 to trigger transcriptional responses. The inhibitory Smads (6 and 7) are transcriptionally induced in cultured cells treated with Tgf-beta superfamily ligands, and downregulate signalling in in vitro assays. Gene disruption in mice has begun to reveal specific developmental and physiological functions of the signal-transducing Smads. Here we explore the role of an inhibitory Smad in vivo by targeted mutation of Madh6 (which encodes the Smad6 protein). Targeted insertion of a LacZ reporter demonstrated that Smad6 expression is largely restricted to the heart and blood vessels, and that Madh6 mutants have multiple cardiovascular abnormalities. Hyperplasia of the cardiac valves and outflow tract septation defects indicate a function for Smad6 in the regulation of endocardial cushion transformation. The role of Smad6 in the homeostasis of the adult cardiovascular system is indicated by the development of aortic ossification and elevated blood pressure in viable mutants. These defects highlight the importance of Smad6 in the tissue-specific modulation of Tgf-beta superfamily signalling pathways in vivo.
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              The vascular biology of calcification.

              Vascular calcification is an active, cell-mediated process that results from an imbalance between the promoters and inhibitors of mineralization. The process of vascular calcification shares many similarities with that of skeletal mineralization. However, while skeletal mineralization is a regulated process induced by complex, well-timed developmental cues, vascular calcification is a pathological process, occurring in response to dysregulated/inappropriate environmental cues. Damage inducing agents present in the uremic milieu such as a mineral imbalance, induce vascular smooth muscle cell (VSMC) apoptosis, and vesicle release resulting in mineral nucleation and the deposition of hydroxyapatite. Under normal conditions, inhibitors of soft-tissue mineralization such as matrix gamma-carboxyglutamic acid protein are expressed locally within the vessel wall while others such as fetuin-A are present in the circulation. Down-regulation or perturbation of these proteins leads to a phenotypic transformation of VSMC into osteo/chondrocytic-like cells that have the capacity to modulate the mineralization process. Many aspects of the mechanisms underlying vascular calcification have been defined through in vitro studies and molecular biological techniques; however, there are still unanswered questions, particularly with respect to the relationship between bone and vascular calcification, processes that appear to be inversely related. A better understanding of the complex mechanisms regulating tissue calcification may have therapeutic potential in reducing the cardiovascular disease-associated morbidity and mortality in patients with renal disease.
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                Author and article information

                Journal
                AJN
                Am J Nephrol
                10.1159/issn.0250-8095
                American Journal of Nephrology
                S. Karger AG
                0250-8095
                1421-9670
                2010
                June 2010
                22 April 2010
                : 31
                : 6
                : 471-481
                Affiliations
                Departments of aPharmacology and Toxicology and bMedicine, Queen’s University, Kingston, Ont., Canada
                Author notes
                *Dr. Rachel M. Holden, 3048C Etherington Hall, Queen’s University, Kingston, Ont. K7L 3V6 (Canada), Tel. +1 613 533 3134, Fax +1 613 533 3292, E-Mail holdenr@kgh.kari.net
                Article
                299794 Am J Nephrol 2010;31:471–481
                10.1159/000299794
                20413965
                e78b7a99-486f-437b-be16-62f3be96323b
                © 2010 S. Karger AG, Basel

                Copyright: All rights reserved. No part of this publication may be translated into other languages, reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, microcopying, or by any information storage and retrieval system, without permission in writing from the publisher. Drug Dosage: The authors and the publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accord with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any changes in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new and/or infrequently employed drug. Disclaimer: The statements, opinions and data contained in this publication are solely those of the individual authors and contributors and not of the publishers and the editor(s). The appearance of advertisements or/and product references in the publication is not a warranty, endorsement, or approval of the products or services advertised or of their effectiveness, quality or safety. The publisher and the editor(s) disclaim responsibility for any injury to persons or property resulting from any ideas, methods, instructions or products referred to in the content or advertisements.

                History
                : 11 January 2010
                : 15 March 2010
                Page count
                Tables: 4, References: 58, Pages: 11
                Categories
                In-Depth Topic Review

                Cardiovascular Medicine,Nephrology
                Vascular calcification,Chronic kidney disease,Cardiovascular mortality

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