Fujigaki et al. BMC Nephrology (2017) 18:339 DOI 10.1186/s12882-017-0756-6 RESEARCH ARTICLE Open Access Unique proximal tubular cell injury and the development of acute kidney injury in adult patients with minimal change nephrotic syndrome Yoshihide Fujigaki1*, Yoshifuru Tamura2, Michito Nagura2, Shigeyuki Arai2, Tatsuru Ota2, Shigeru Shibata2, Fukuo Kondo3, Yutaka Yamaguchi3 and Shunya Uchida2 Abstract Background: Adult patients with minimal change nephrotic syndrome (MCNS) are often associated with acute kidney injury (AKI). To assess the mechanisms of AKI, we examined whether tubular cell injuries unique to MCNS patients exist. Methods: We performed a retrospective analysis of clinical data and tubular cell changes using the immunohistochemical expression of vimentin as a marker of tubular injury and dedifferentiation at kidney biopsy in 37 adult MCNS patients. AKI was defined by the criteria of the Kidney Disease: Improving Global Outcomes (KDIGO) Clinical Practice Guidelines for AKI. Results: Thirteen patients (35.1%) were designated with AKI at kidney biopsy. No significant differences in age, history of hypertension, chronic kidney disease, diuretics use, proteinuria, and serum albumin were noted between the AKI and non- AKI groups. Urinary N-acetyl-β-D-glucosaminidase (uNAG) and urinary alpha1-microglobulin (uA1MG) as markers of tubular injury were increased in both groups, but the levels were significantly increased in the AKI group compared with the non- AKI group. The incidence of vimentin-positive tubules was comparable between AKI (84.6%) and non-AKI (58.3%) groups, but vimentin-positive tubular area per interstitial area was significantly increased in the AKI group (19.8%) compared with the non-AKI group (6.8%) (p = 0.011). Vimentin-positive injured tubules with tubular simplification (loss of brush-border of the proximal tubule/dilated tubule with flattening of tubular epithelium) were observed in the vicinity of glomeruli in both groups, suggesting that the proximal convoluted tubules were specifically injured. Two patients exhibited relatively severe tubular injuries with vimentin positivity and required dialysis within 2 weeks after kidney biopsy. The percentage of the vimentin-positive tubular area was positively correlated with uNAG but not with uA1MG in the non-AKI group. Conclusions: Proximal tubular injuries with increased uNAG exist in MCNS patients without renal dysfunction and were more severe in the AKI group than they were in the non-AKI group. The unique tubular injuries probably due to massive proteinuria might be a predisposing factor for the development of severe AKI in adult MCNS patients. Keywords: Acute kidney injury, Minimal change nephrotic syndrome, Proximal tubule, N-acetyl-β-D-glucosaminidase, Vimentin * Correspondence: [email protected] 1Department of Internal Medicine and Central Laboratory, Teikyo University School of Medicine, 2-11-1 Kaga, Itabashi-ku, Tokyo, Japan Full list of author information is available at the end of the article © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Fujigaki et al. BMC Nephrology (2017) 18:339 Page 2 of 9 Background Methods Acute kidney injury (AKI) is a well-known complication Clinical characteristics of patients and definition of clinical of minimal change nephrotic syndrome (MCNS) that oc- parameters curs in 25 to 40% of adult patients [1–4] and is typically The clinical records of adult patients (older than 18 years) reversible with a favourable response to corticosteroid who had the initial presentation of biopsy-proven MCNS treatment [1]. However, a small number of MCNS patients at Teikyo University Hospital from January 2008 to with severe AKI with or without the requirement of December 2015 were retrospectively reviewed. All patients dialysis has been reported, and these patients develop received corticosteroid treatment several days before or irreversible renal dysfunction [1, 5]. after kidney biopsy. History of hypertension was defined It was suggested that AKI in patients with MCNS is as patients who used an anti-hypertensive drug, and the result of severe plasma volume depletion due to low hypertension was defined as systolic blood pressure read- plasma oncotic pressure. However, the absolute and relative ings of ≥140 mmHg or diastolic blood pressure readings blood volumes and renal plasma flow were well preserved of ≥90 mmHg. Hypotension was defined as systolic blood in most patients with nephrotic syndrome [6]. Recent pressure readings of ≤90 mmHg or diastolic blood pres- evidence has demonstrated that mechanisms intrinsic to sure readings of ≤60 mmHg. the kidney contribute to sodium retention and edema Laboratory parameters at kidney biopsy included serum formation in nephrotic syndrome, including activation of creatinine (Cr), serum albumin, urinary protein excretion, epithelial sodium channel (ENaC) by aberrantly filtered haematuria (≥ 5 red blood cells per high power field), proteases [7]. The pathogenesis of severe AKI remains urinary N-acetyl-β-D-glucosaminidase (uNAG), urinary uncertain and is suggested to include (1) ischaemic renal alpha1-microglobulin (uA1MG), and urinary Cr excretion. injury [2, 8], (2) tubular obstruction by surrounding inter- The estimated glomerular filtration rate (eGFR) was calcu- stitial edema [9], (3) redistribution of renal blood flow lated using the revised serum Cr–based Japanese equation from cortical to juxtaglomerular nephrons [10] and (4) [18]. The presence of chronic kidney disease (CKD) was decrease in capillary filtration coefficient (Kf) [3, 6, 11]. In defined by an eGFR <60 mL/min/1.73 m2 [19]. All of the addition to these functional and structural alterations in urinary proteins were expressed as the ratio-to-Cr to cor- the kidney, proteinuria has been proposed to induce tubu- rect variations in urine concentration among individuals. lar cell injury and apoptosis [12–14]. AKI generally lasts No accepted criteria are available regarding an abnormal longer in MCNS patients than it does in patients with amount of uNAG and uA1MG expressed as the ratio to ischaemic acute tubular necrosis (ATN) (average duration Cr. Thus, according to the reported values, the abnormal- of 7 weeks vs. 10 to 14 days) [8], and reduction of protein- ities were defined as uNAG >9.1 IU/gCr and uA1MG > uria is indispensable for recovery from AKI. However, it is 25.8 mg/gCr, respectively [20]. Complete remission (CR) unlikely that massive proteinuria itself is the exclusive of nephrotic syndrome was defined as a daily urinary mechanism of severe AKI because massive proteinuria protein excretion of <0.3 g/gCr. The time to CR after is a constant phenomenon in all MCNS patients. Thus, corticosteroid-based therapy and the serum Cr at CR were multiple factors may be involved in the pathophysiology recorded. of developing severe AKI in MCNS. Post-renal AKI was excluded by abdominal echogra- Clear vacuoles or hyaline droplets, which represent phy in all patients. AKI was defined, and severity was resorbed protein, are commonly observed in tubular cells staged based on the criteria for serum Cr of the Kidney in MCNS patients [15]. However, it is not clear whether Disease: Improving Global Outcomes (KDIGO) Clinical vacuoles or hyaline droplets indicate tubular injuries as Practice Guidelines for AKI [21]. At kidney biopsy, AKI pathological changes. Tubular lesions associated with was defined as any of the following: an absolute increase acute tubular injuries were also reported in MCNS pa- in serum Cr ≥ 0.3 mg/dl within 48 or increase in serum tients with severe AKI. However, given the lack of unified Cr to ≥1.5 times the baseline value, which is known or standard morphologic criteria for acute tubular injuries, it presumed to have occurred within the prior 7 days. seems difficult to evaluate tubular injuries quantitatively Baseline serum Cr was defined as the lowest serum Cr in MCNS patients by light microscopic morphology. value available prior to the date of kidney biopsy or the In the present study, we attempted to investigate the value of serum Cr when it rapidly decreased. mechanisms of AKI in MCNS patients and focused specif- ically on the expression of vimentininthekidneytubules Histological characteristics as a marker of tubular injury and dedifferentiation [16] and Kidney biopsy tissue specimens including at least 10 the expression of Ki67 as a marker of tubular cell prolifera- glomeruli were examined by light microscopy, immuno- tion or regeneration [17]. Then, the findings of vimentin ex- fluorescence and electron microscopy, and all the patients pression in tubules and clinical parameters were compared were diagnosed with MCNS [15]. ATN and tubular lesions between AKI and non-AKI groups at kidney biopsy. associated with acute tubular injuries [5, 22] were assessed Fujigaki et al. BMC Nephrology (2017) 18:339 Page 3 of 9 by the presence of tubular necrosis/detachment of tubular Results cells from the basal lamina and tubular simplification Characteristics