Transglutaminases: the missing link in nephrogenic systemic fibrosis

The safety of magnetic resonance imaging contrast agents

Gadolinium-based contrast agents are increasingly used in clinical practice. While these pharmaceuticals are verified causal agents in nephrogenic systemic fibrosis, there is a growing body of literature supporting their role as causal agents in symptoms associated with gadolinium exposure after intravenous use and encephalopathy following intrathecal administration. Gadolinium-based contrast agents are multidentate organic ligands that strongly bind the metal ion to reduce the toxicity of the metal. The notion that cationic gadolinium dissociates from these chelates and causes the disease is prevalent among patients and providers. We hypothesize that non-ligand-bound (soluble) gadolinium will be exceedingly low in patients. Soluble, ionic gadolinium is not likely to be the initial step in mediating any disease. The Kidney Institute of New Mexico was the first to identify gadolinium-rich nanoparticles in skin and kidney tissues from magnetic resonance imaging contrast agents in rodents. In 2023, they found similar nanoparticles in the kidney cells of humans with normal renal function, likely from contrast agents. We suspect these nanoparticles are the mediators of chronic toxicity from magnetic resonance imaging contrast agents. This article explores associations between gadolinium contrast and adverse health outcomes supported by clinical reports and rodent models.

Implications of enigmatic transglutaminase 2 (TG2) in cardiac diseases and therapeutic developments

Cardiac diseases are the leading cause of mortality and morbidity worldwide. Mounting evidence suggests that transglutaminases (TGs), tissue TG (TG2) in particular, are involved in numerous molecular responses underlying the pathogenesis of cardiac diseases. The TG family has several intra- and extracellular functions in the human body, including collagen cross-linking, angiogenesis, cell growth, differentiation, migration, adhesion as well as survival. TGs are thiol- and calcium-dependent acyl transferases that catalyze the formation of a covalent bond between the γ-carboxamide group of a glutamine residue and an amine group, thus increasing the stability, rigidity, and stiffness of the myocardial extracellular matrix (ECM). Excessive accumulation of cross-linked collagen leads to increase myocardial stiffness and fibrosis. Beyond TG2 extracellular protein cross-linking action, mounting evidence suggests that this pleiotropic TG isozyme may also promote fibrotic diseases through cell survival and profibrotic pathway activation at the signaling, transcriptional and translational levels. Due to its multiple functions and localizations, TG2 fulfils critical yet incompletely understood roles in myocardial fibrosis and associated heart diseases, such as cardiac hypertrophy, heart failure, and age-related myocardial stiffness under several conditions. This review summarizes current knowledge and existing gaps regarding the ECM-dependent and ECM-independent roles of TG2 and highlights the therapeutic prospects of targeting TG2 to treat cardiac diseases.

Epithelial mesenchymal transition (EMT): a universal process in lung diseases with implications for cystic fibrosis pathophysiology

Cystic Fibrosis (CF) is a genetic disorder that arises due to mutations in the Cystic Fibrosis Transmembrane Conductance Regulator gene, which encodes for a protein responsible for ion transport out of epithelial cells. This leads to a disruption in transepithelial Cl-, Na + and HCO₃− ion transport and the subsequent dehydration of the airway epithelium, resulting in infection, inflammation and development of fibrotic tissue. Unlike in CF, fibrosis in other lung diseases including asthma, chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis has been well characterised. One of the driving forces behind fibrosis is Epithelial Mesenchymal Transition (EMT), a process where epithelial cells lose epithelial proteins including E-Cadherin, which is responsible for tight junctions. The cell moves to a more mesenchymal phenotype as it gains mesenchymal markers such as N-Cadherin (providing the cells with migration potential), Vimentin and Fibronectin (proteins excreted to help form the extracellular matrix), and the fibroblast proliferation transcription factors Snail, Slug and Twist. This review paper explores the EMT process in a range of lung diseases, details the common links that these have to cystic fibrosis, and explores how understanding EMT in cystic fibrosis may open up novel methods of treating patients with cystic fibrosis.

Transglutaminase 2 in pulmonary and cardiac tissue remodeling in experimental pulmonary hypertension

Tissue matrix remodeling and fibrosis leading to loss of pulmonary arterial and right ventricular compliance are important features of both experimental and clinical pulmonary hypertension (PH). We have previously reported that transglutaminase 2 (TG2) is involved in PH development while others have shown it to be a cross-linking enzyme that participates in remodeling of extracellular matrix in fibrotic diseases in general. In the present studies, we used a mouse model of experimental PH (Sugen 5416 and hypoxia; SuHypoxia) and cultured primary human cardiac and pulmonary artery adventitial fibroblasts to evaluate the relationship of TG2 to the processes of fibrosis, protein cross-linking, extracellular matrix collagen accumulation, and fibroblast-to-myofibroblast transformation. We report here that TG2 expression and activity as measured by serotonylated fibronectin and protein cross-linking activity along with fibrogenic markers are significantly elevated in lungs and right ventricles of SuHypoxic mice with PH. Similarly, TG2 expression and activity, protein cross-linking activity, and fibrogenic markers are significantly increased in cultured cardiac and pulmonary artery adventitial fibroblasts in response to hypoxia exposure. Pharmacological inhibition of TG2 activity with ERW1041E significantly reduced hypoxia-induced cross-linking activity and synthesis of collagen 1 and α-smooth muscle actin in both the in vivo and in vitro studies. TG2 short interfering RNA had a similar effect in vitro. Our results suggest that TG2 plays an important role in hypoxia-induced pulmonary and right ventricular tissue matrix remodeling in the development of PH.

Understanding nephrogenic systemic fibrosis

Nephrogenic systemic fibrosis (NSF) is a rare and a debilitating disease noted uncommonly in patients with impaired renal function when exposed to low-stability gadolinium-based contrast agents (Gd-CAs). According to experimental studies, cytokines released by the stimulation of effector cells such as skin macrophages and peripheral blood monocytes activate circulating fibroblasts which play a major role in the development of NSF lesions. The presence of permissive factors, presumably, provides an environment conducive to facilitate the process of fibrosis. Multiple treatment modalities have been tried with variable success rates. More research is necessary to elucidate the underlying pathophysiological mechanisms which could potentially target the initial steps of fibrosis in these patients. This paper attempts to collate the inferences from the in vivo and in vitro experiments to the clinical observations to understand the pathogenesis of NSF. Schematic representations of receptor-mediated molecular pathways of activation of macrophages and fibroblasts by gadolinium and the final pathway to fibrosis are incorporated in the discussion.

Gadolinium-induced fibrosis testing by protein targeting assay and superparamagnetic iron oxide nanoparticle-based magnetic resonance microscopy of skin and kidneys

Gadolinium-based magnetic resonance imaging (MRI) contrast agents cause undefined fibrosis in kidneys and skin damage. Magnetic resonance microimaging of rat skin and kidney was used first time to identify the physical factors modulating the gadolinium Omniscan®-induced fibrosis by protein targeting. A 500-MHz MR imaging was done to visualize fibrosis in gadolinium-treated animals. Cationic superparamagnetic iron oxide magnetoferritin (SPIOM) was injected in rat to target basement membrane (in rat kidney and different skin structures including epidermis glycolipids and dermis proteins. After MR imaging, excised rat skin and kidneys tissues were imaged by ex vivo 900 MHz MR microimaging to confirm renal fibrosis and skin epidermis thickening. The proton density-weighted images visualized micro details of skin structures and nephron territories while T2-weighted images showed better contrast of tissue structures in both skin and kidney. The gadolinium further enhanced the image contrast and targeted the proteins in renal basement membrane and viable proteins in epidermis. SPIOM enhanced the tissue contrast due to dephasing effect caused by SPIOM on structural changes in nephron and epidermis. Conclusion: Tissue membrane protein and chelate ligand group binding with gadolinium biophysical interaction at molecular level may develop fibrosis. SPIOM injection improved the dephased image contrast of different structures in both skin and nephrons. The epidermis thickening and nephrofibrosis changes may be associated with nephrogenic systemic fibrosis or fibrosing dermatopathy.

A moderate response to plasmapheresis in nephrogenic systemic fibrosis

Nephrogenic systemic fibrosis (NSF) is a recently identified idiopathic cutaneous fibrosing disorder that occurs in the setting of renal failure. The disease initially called nephrogenic fibrosing dermopathy is closely linked to exposure to gadolinium-based contrast media used during magnetic resonance imaging in patients with renal insufficiency. Although little is known about the pathogenesis of this disease, the increased expression of transforming growth factor-beta has been demonstrated recently. Herein, we present a case of NSF was partially treated due to a moderate and temporary response to plasmapheresis with no recurrence for 6 months, but returned at the end of 6^th month.

Nephrogenic systemic fibrosis: current concepts

Nephrogenic systemic fibrosis (NSF) was first described in 2000 as a scleromyxedema-like illness in patients on chronic hemodialysis. The relationship between NSF and gadolinium contrast during magnetic resonance imaging was postulated in 2006, and subsequently, virtually all published cases of NSF have had documented prior exposure to gadolinium-containing contrast agents. NSF has been reported in patients from a variety of ethnic backgrounds from America, Europe, Asia and Australia. Skin lesions may evolve into poorly demarcated thickened plaques that range from erythematous to hyperpigmented. With time, the skin becomes markedly indurated and tethered to the underlying fascia. Extracutaneous manifestations also occur. The diagnosis of NSF is based on the presence of characteristic clinical features in the setting of chronic kidney disease, and substantiated by skin histology. Differential diagnosis is with scleroderma, scleredema, scleromyxedema, graft-versus-host disease, etc. NSF has a relentlessly progressive course. While there is no consistently successful treatment for NSF, improving renal function seems to slow or arrest the progression of this condition. Because essentially all cases of NSF have developed following exposure to a gadolinium-containing contrast agent, prevention of this devastating condition involves the careful avoidance of administering these agents to individuals at risk.

Nephrogenic systemic fibrosis: a brief review

Nephrogenic systemic fibrosis is a fibrosing disorder of the skin that develops in patients with advanced renal failure. It mostly presents with progressive hardening or induration of the skin of the extremities. Systemic involvement is also known to occur in this entity. Exposure to gadolinium contrast for radiological evaluation has been identified as the offending agent. The condition is progressive and can be seriously disabling. Therapeutic options are limited and not rewarding in majority of the cases. Awareness of this entity is important so that proper precautionary measures can be taken at the earliest to ameliorate the condition.

Immunohistochemical aspects of the fibrogenic pathway in nephrogenic systemic fibrosis

Nephrogenic systemic fibrosis (NSF) is a rare gadolinium-dependent disorder of the skin and viscera. The aim of this study was to revisit some immunopathologic clues of NSF, including the characterization of glycosaminoglycans, cell tensegrity, and cell proliferation in the dermis. Immunohistochemistry was done using antibodies directed to vimentin, CD34, Factor XIIIa, calprotectin, α-smooth muscle actin, Ulex europaeus agglutinin-1 (UEA-1), and MIB1/Ki67 and to glycosaminoglycans, including CD44 var3, versican, and perlecan. The vimentin+ cell density was markedly increased. The vast majority of them corresponded to CD34+ or Factor XIIIa+ dermal dendrocytes (DD) showing distinct cell tensegrity. CD34+DD were slender, elongated, and usually scattered in the dermis but focally clustered in nodular collections. By contrast, Factor XIIIa+ was plump with squat dendrites showing no evidence for being under mechanical stress. Cells in the vicinity of the microvasculature were rounded and exhibited calprotectin immunoreactivity typical for monocyte/macrophages. The microvasculature highlighted by UEA-1 and α-smooth muscle actin looked unremarkable. The cell proliferation highlighted by the MIB/Ki67 immunoreactivity was unusually high (>20%) in the interstitial stromal cells. Stromal cells enriched in versican were plump, abundant, and seemed interconnected each other by a dense network of dendrites. By contrast, the immunolabeling for perlecan and CD44 var 3 was unremarkable. In conclusion, the cell population involved in NSF seemed phenotypically heterogeneous, and its growth fraction was clearly boosted in the skin. The intracellular load in versican was prominent. The aspect of cell tensegrity did not suggest the influence of mechanical stress putting stromal cells under tension in the dermis.

Potential of transglutaminase 2 as a therapeutic target

IMPORTANCE OF THE FIELD

Increased expression and activity of transglutaminase 2 - a calcium-dependent enzyme which catalyzes protein cross-linking, polyamination or deamidation at selective glutamine residues - are involved in the etiopathogenesis of several pathological conditions, such as neurodegenerative disorders, autoimmune diseases and inflammatory diseases. Inhibition of enzyme activity has potential for therapeutic management of these diseases.

AREAS COVERED IN THIS REVIEW

The major results achieved in the last twelve years of research in the field of inhibition of tranglutaminase activity using cell cultures as well as in vivo models of high-social-impact or widespread diseases, such as CNS neurodegenerative disorders, celiac sprue, cancer and fibrotic diseases.

WHAT THE READER WILL GAIN

Beneficial effects of enzyme activity inhibition have been observed in neurodegeneration and fibrosis in vivo models by delivery of the competitive inhibitor cystamine and more recently designed inhibitors, such as thiomidaziolium or norleucine derivatives, which irreversibly bind the active site cysteine residue. Transglutaminase 2 targeting with specific antibodies has also been shown to be a promising tool for celiac disease treatment.

TAKE HOME MESSAGE

New insights from transglutaminase inhibition studies dealing with side effects of in vivo administration of pan-transglutaminase inhibitors will help in design of novel therapeutic approaches to various diseases.

Importance of syndecan-4 and syndecan -2 in osteoblast cell adhesion and survival mediated by a tissue transglutaminase-fibronectin complex

Tissue transglutaminase (TG2) has been identified as an important extracellular crosslinking enzyme involved in matrix turnover and in bone differentiation. Here we report a novel cell adhesion/survival mechanism in human osteoblasts (HOB) which requires association of FN bound TG2 with the cell surface heparan sulphates in a transamidase independent manner. This novel pathway not only enhances cell adhesion on FN but also mediates cell adhesion and survival in the presence of integrin competing RGD peptides. We investigate the involvement of cell surface receptors and their intracellular signalling molecules to further explore the pathway mediated by this novel TG-FN heterocomplex. We demonstrate by siRNA silencing the crucial importance of the cell surface heparan sulphate proteoglycans syndecan-2 and syndecan-4 in regulating the compensatory effect of TG-FN on osteoblast cell adhesion and actin cytoskeletal formation in the presence of RGD peptides. By use of immunoprecipitation and inhibitory peptides we show that syndecan-4 interacts with TG2 and demonstrate that syndecan-2 and the α5β1 integrins, but not α4β1 function as downstream modulators in this pathway. Using function blocking antibodies, we show activation of α5β1 occurs by an inside out signalling mechanism involving activation and binding of protein kinase PKCα and phosphorylation of focal adhesion kinase (FAK) at Tyr(861) and activation of ERK1/2.

Nephrogenic systemic fibrosis in liver disease: a systematic review

Nephrogenic systemic fibrosis (NSF) may develop in patients with liver disease, a fact highlighted by Food and Drug Administration (FDA) announcements cautioning against the use of gadolinium-based contrast agents (GBCAs) in select liver disease patients. The purpose of this systematic literature review is to characterize the risk of NSF in patients with liver disease. All published articles on NSF from September 2000 through August 2008, were identified via PubMed searches and examination of articles' reference lists. Two reviewers independently read each article and identified unique patients with biopsy-proven or suspected NSF. Data on demographics, liver status, renal status, and GBCA exposure were collected. A total of 324 articles were reviewed, with 108 articles containing case descriptions of 335 unique NSF patients. After excluding the 95/335 (28%) patients in whom the presence or absence of liver disease was uncertain, liver disease was confirmed present in 41/239 (17%) patients. Renal insufficiency could be assessed in 35 of the liver disease patients; severe renal insufficiency, defined as a glomerular filtration rate (GFR) or estimated GFR (eGFR) <30 mL/min/1.73 m(2) or dialysis requirement, was present in 34/35 (97%) patients. The lone patient who developed NSF with mild/moderate renal insufficiency was atypical and received a total gadodiamide load of 0.76 mmol/kg over a 10-week period periliver transplantation. The published medical literature demonstrates that patients with liver disease who develop NSF also have severe renal insufficiency, suggesting that liver disease does not confer a risk for NSF beyond that of the underlying renal insufficiency. J. Magn. Reson. Imaging 2009;30:1313-1322. (c) 2009 Wiley-Liss, Inc.

Tissue gadolinium deposition and fibrosis mimicking nephrogenic systemic fibrosis (NSF)-subclinical nephrogenic systemic fibrosis?

Nephrogenic systemic fibrosis is a condition with significant and often debilitating cutaneous manifestations. Recent research on this disease has delineated an association between nephrogenic systemic fibrosis and exposure to magnetic resonance imaging studies using gadolinium-based contrast agents in patients with ongoing renal failure. This metal has been detected in cutaneous biopsy specimens taken from lesional skin suggesting that gadolinium provides an impetus for the deposition of circulating fibrocytes in the skin and subsequent fibrosis. We describe a hemodialysis-dependent liver transplant recipient who received a gadolinium-based contrast agent and demonstrated insoluble gadolinium deposition in a fibrotic dermis and subcutaneous septum using scanning electron microscopy/energy dispersive x-ray spectroscopy. He has yet to manifest symptoms and signs of nephrogenic systemic fibrosis 3 years after his magnetic resonance imaging study.

Involvement of gadolinium chelates in the mechanism of nephrogenic systemic fibrosis: an update

Nephrogenic systemic fibrosis (NSF) is a highly debilitating scleroderma-like disease occurring exclusively in patients with severe or end-stage renal failure. Since the recognition of a link between gadolinium chelates (GCs) used as contrast agents for MR imaging and NSF by two independent European teams in 2006, numerous studies have described the clinical issues and investigated the mechanism of this disease. So far the most commonly reported hypothesis is based on the in vivo dechelation of GCs. The physicochemical properties of GCs, especially their thermodynamic and kinetic stabilities, are described in the present article. High kinetic stability provided by the macrocyclic structure, combined with high thermodynamic stability, minimizes the amount of free gadolinium released in the body. The current hypotheses regarding the pathophysiologic mechanism are critically discussed.

Gadolinium-associated nephrogenic systemic fibrosis in a 9-year-old boy

We describe a 9-year-old boy on chronic hemodialysis who presented with hyperpigmented, tightly bound-down, indurated plaques of his bilateral lower extremities. Pertinent history included a recent series of magnetic resonance imaging tests utilizing gadolinium-based contrast media. Histopathology showed widened septae with increased fibroblasts and collagen in the subcutis consistent with nephrogenic systemic fibrosis. Nephrogenic systemic fibrosis is a unique fibrosing disorder characterized by skin thickening of the extremities and trunk, resembling that seen in systemic sclerosis. The majority of cases have been reported in dialysis or renal transplant patients. This case of nephrogenic systemic fibrosis in a young child is presented to raise awareness in the pediatric community of nephrogenic systemic fibrosis and of gadolinium contrast being a possible trigger.

Nephrogenic systemic fibrosis and management of high-risk patients

The purpose of this work is to provide current information on the rapidly evolving subject of nephrogenic systemic fibrosis (NSF), to establish the radiologic approach to the management of high-risk patients for NSF, and to assess the probabilistic risk of NSF compared to contrast induced nephropathy (CIN), as encountered with iodinated contrast media used in computed tomographic (CT) imaging. NSF is a disease process of considerable concern following gadolinium-containing contrast agents (GCCA) exposure in patients with diminished renal function. To minimize the possibility of NSF development in high-risk patients, GGCAs should not be used when they are not necessary, or the GCCAs, that have not at present been associated with NSF development, should be used at the lowest possible diagnostic dose, when they are necessary. Contrast-induced nephropathy is also a great risk in this patient population following the adminstration of iodinated contrast media (CM). In patients with diminished renal function who are not on regular dialysis, the risk of CIN following the administration of iodinated CM is higher than the risk of NSF following the administration of the most stable GCCAs. Risk benefit analysis should be performed prior to the administration of all CM, and the best combination of safety and diagnostic accuracy should be sought. Concern of NSF or CIN should not prevent the use of contrast agents in magnetic resonance imaging or computed tomography when they are deemed essential.

Nephrogenic systemic fibrosis and the use of gadolinium-based contrast agents

Nephrogenic systemic fibrosis (NSF) is a disease seen exclusively in patients with decreased renal function. The use of gadolinium-based contrast agents (GBCAs) has a strong association with NSF. Linear non-ionic GBCAs that are more prone to release free gadolinium are the more likely to cause NSF. The number of reported cases has increased recently, and there are currently nine pediatric cases, the patients ranging in age from 8 years to 19 years, and the oldest adult patient is 87 years of age. The most successful treatment is improvement of renal function with renal transplantation or with recovery of acute kidney injury. NSF can be severely debilitating and even fatal. Avoidance of a GBCA in patients at risk, or limitation of the dose in the patients who need gadolinium enhancement, is recommended.

Nephrogenic systemic fibrosis is associated with transforming growth factor beta and Smad without evidence of renin-angiotensin system involvement

BACKGROUND

The mechanisms of fibrosis associated with nephrogenic systemic fibrosis (NSF) are largely unknown. Transforming growth factor beta (TGF-beta), a known profibrotic cytokine, is theorized to play a central role. The renin-angiotensin system has been linked with both TGF-beta expression and fibrosis in other organ systems.

OBJECTIVE

We sought to investigate whether these mechanisms were involved with NSF.

METHOD

Eleven biopsy specimens from 8 patients with NSF were evaluated by immunohistochemistry for the expression of TGF-beta, Smad 2/3, angiotensin-converting enzyme (ACE), and angiotensin II receptor 1 (AT1).

RESULTS

TGF-beta was detected in 8 of 11 samples of NSF. Smad 2/3 nuclear staining was seen in 8 of 11 samples. Conversely, only faint staining for ACE was seen in 2 of the 11 specimens. No AT1 staining was seen.

LIMITATIONS

We did not perform our studies on a cohort of comparable patients with renal dysfunction without NSF. Our technique may not have been sufficiently sensitive to detect renin-angiotensin system involvement.

CONCLUSIONS

TGF-beta, as well as its second messengers, Smad 2/3, appears to be associated with the fibrosis seen in NSF. No definitive evidence of renin-angiotensin system involvement could be determined.

Novel interactions of TG2 with heparan sulfate proteoglycans: reflection on physiological implications

This mini-review brings together information from publications and recent conference proceedings that have shed light on the biological interaction between transglutaminase-2 and heparan sulphate proteoglycans. We subsequently draw hypotheses of possible implications in the wound healing process. There is a substantial overlap in the action of transglutaminase-2 and the heparan sulphate proteoglycan syndecan-4 in normal and abnormal wound repair. Our latest findings have identified syndecan-4 as a possible binding and signalling partner of fibronectin-bound TG2 and support the idea that transglutaminase-2 and syndecan-4 act in synergy.

Nephrogenic systemic fibrosis: chronic imaging findings and review of the medical literature

Nephrogenic systemic fibrosis (NSF) is a systemic fibrosing disorder which has been strongly associated with exposure to gadolinium-based contrast media (GBCM) in the setting of renal insufficiency. Although this disorder primarily affects the skin, it can result in severe joint contractures, disabilities and even death. However, to date, there have been no published studies reporting chronic imaging findings of NSF. In this report we present three biopsy-proven cases of NSF with the associated chronic MRI, radiographic and bone scintigraphy findings. Two of the patients had been exposed to gadodiamide, and one had been exposed to gadopentetate dimeglumine prior to the onset of NSF. Two are newly reported cases. One patient's subacute imaging findings have previously been reported, but significant chronic images will now be presented. This patient became severely disabled from contractures and developed long bone smooth periosteal reaction, extensive intra-articular and periarticular calcifications, musculotendinous heterotopic ossification and ankylosis of several joints. One of the patients underwent renal transplantation 6 months after GBCM exposure, with near complete resolution of the skin fibrosis. The third patient had persistent MRI findings of skin thickening, with low T1 and high T2 signal intensity 5 years after exposure to gadodiamide. A review of the medical literature is provided, emphasizing the association of NSF with various GBCM. These cases broaden our understanding of the long-term imaging findings and complications of NSF and the stratified risk of NSF with various GBCM.

Possible involvement of gadolinium chelates in the pathophysiology of nephrogenic systemic fibrosis: a critical review

Nephrogenic systemic fibrosis (NSF) is a recently described, highly debilitating scleroderma-like disease occurring in patients with severe or end-stage renal failure. NSF is characterized by cutaneous papules and coalescing plaques ("peau d'orange" appearance) and a wooden consistency. It may ultimately cause disabling contractures of several joints, thus making many patients wheelchair-dependent. NSF has been associated to prior administration of gadolinium chelates (GC) used as contrast agents for magnetic resonance imaging. The best available treatment option at the present time is renal transplantation. The mechanism of NSF has not been fully elucidated. Several hypotheses have been proposed so far and are critically discussed in the present review article. Gadolinium has been found in skin biopsy samples of patients. The most widely accepted hypothesis is related to dechelation of less stable GC, progressively releasing free Gd3+ which may subsequently lead to the attraction of CD34+, CD45+, pro-collagen+ circulating fibrocytes via the release of chemokines, thereby inducing systemic fibrosing disorders. Pre-existing renal failure may facilitate the process by delaying the excretion of GC. A complex interplay between gadolinium and co-factors (pro-inflammatory status, vascular injury, high dose of erythropoietin, high levels of calcium, phosphorus, etc.) may occur in patients with impaired renal function. This and other hypotheses remain to be investigated, as well as the role and independence of co-factors.

Tissue deposition of gadolinium and development of NSF: a convergence of factors

Gadolinium-based contrast (GBC) exposure has recently been linked to the development of nephrogenic systemic fibrosis (NSF) in patients with underlying kidney disease and may in fact be the previously unrecognized trigger for the fibrosing process. As NSF is fairly rare in this patient population, a number of permissive factors are likely required for GBC exposure to initiate fibrosis. Advanced kidney disease is an absolute requirement whereas vascular injury and an inflammatory state, and a mix of co-factors including increased serum phosphate and calcium concentrations and iron overload further enhance risk. The combination of these events allows excess circulating gadolinium, which dissociates from its chelate to leak out of vessels and deposit in tissues. Free or bound tissue gadolinium, a rare earth metal of the lanthanoid series, promotes fibrosis via either direct binding to the collagen helix or, once the metal has been engulfed by macrophages, through the production of free oxygen radicals, cytokines, and other profibrotic factors that attract circulating fibrocytes to tissues. These fibrocytes then differentiate into fibroblast-like spindle cells that produce connective tissue matrix and other angiogenic and growth factors that further enhance tissue fibrosis. Direct gadolinium activation of transglutaminases on these tissue fibroblast-like cells may also promote fibrosis.