Bassam G. Abu Jawdeh

Regulation of cell survival by Na+/H+ exchanger-1

Na(+)/H(+) exchanger-1 (NHE1) is a ubiquitous plasma membrane Na(+)/H(+) exchanger typically associated with maintenance of intracellular volume and pH. In addition to the NHE1 role in electroneutral Na(+)/H(+) transport, in renal tubular epithelial cells in vitro the polybasic, juxtamembrane NHE1 cytosolic tail domain acts as a scaffold, by binding with ezrin/radixin/moesin (ERM) proteins and phosphatidylinositol 4,5-bisphosphate, which initiates formation of a signaling complex that culminates in Akt activation and opposition to initial apoptotic stress. With robust apoptotic stimuli renal tubular epithelial cell NHE1 is a caspase substrate, and proteolytic cleavage may permit progression to apoptotic cell death. In vivo, genetic or pharmacological NHE1 loss of function causes renal tubule epithelial cell apoptosis and renal dysfunction following streptozotocin-induced diabetes, ureteral obstruction, and adriamycin-induced podocyte toxicity. Taken together, substantial in vivo and in vitro data demonstrate that NHE1 regulates tubular epithelial cell survival. In contrast to connotations of NHE1 as an unimportant housekeeping protein, this review highlights that NHE1 activity is critical for countering tubular atrophy and chronic renal disease progression.

Late antibody-mediated rejection in renal allografts: outcome after conventional and novel therapies.

Background Although several strategies for treating early antibody-mediated rejection (AMR) in kidney transplants have been investigated, evidence on treatment of late AMR manifesting after 6 months is sparse. In this single-center series, we present data on 23 consecutive patients treated for late AMR. Methods Late AMR was diagnosed using Banff 2007 criteria along with presence of donor-specific antibodies (DSA) and acute rise in serum creatinine (SCr). Response to therapy was assessed by improvement in SCr, histologic improvement, and decline in DSA strength. Results Overall, 17% (4/23) had documented nonadherence while 69% (16/23) had physician-recommended reduction in immunosuppression before AMR. Eighteen patients (78%) were treated with plasmapheresis or low-dose IVIg+rituximab; 11 (49%) with refractory AMR also received one to three cycles of bortezomib. While there was an improvement (P=0.02) in mean SCr (2.4 mg/dL) at the end of therapy compared with SCr at the time of diagnosis (2.9 mg/dL), this improvement was not sustained at most recent follow-up. Eleven (48%) patients had no histologic resolution on follow-up biopsy. Lack of histologic response was associated with older patients (odds ratio [OR]=3.17; P=0.04), presence of cytotoxic DSA at time of diagnosis (OR=200; P=0.04), and severe chronic vasculopathy (cv≥2) on index biopsy (OR=50; P=0.06). Conclusions A major setting in which late AMR occurred in our cohort was reduction or change in immunosuppression. Our data demonstrate an inadequate response of late AMR to current and novel (bortezomib) therapies. The benefits of therapy need to be counterweighed with potential adverse effects especially in older patients, large antibody loads, and chronic allograft vasculopathy.

Desensitization in kidney transplantation: review and future perspectives.

More than half of the kidney transplant candidates awaiting transplantation are sensitized to human leukocyte antigens (HLAs). Desensitization to HLAs involves treatment with immunomodulating therapies designed to reduce levels of anti‐HLA antibodies in order to make kidney transplantation possible. Over the last two decades, desensitization therapies have been limited to plasmapheresis (PP), immunoadsorption (IA), intravenous immunoglobulins (IVIg), and rituximab. Review of reported experiences with desensitization in kidney transplant candidates revealed that PP or IA alone is inadequate to achieve durable reductions in HLA antibodies. Increasing evidence has accumulated indicating that high‐dose IVIg has limited ability to reduce HLA antibodies, but a few centers have reported success with high‐dose IVIg+rituximab in non‐randomized trials. Overall experience in multiple centers, however, has shown high antibody‐mediated rejection (AMR) rates, particularly in patients with the highest degrees of HLA sensitization. Low‐dose IVIg combined with alternate day PP in living donor transplant candidates has been shown to provide enhanced survival over dialysis. However, low‐dose IVIg/PP regimens also continue to be associated with unacceptable AMR rates. Recent experiences with plasma cell‐targeted therapies based on the proteasome inhibitor bortezomib are relatively small but may represent an important alternative to non‐deletional strategies with IVIg.

Phosphoinositide binding differentially regulates NHE1 Na+/H+ exchanger-dependent proximal tubule cell survival.

Background: Chronic kidney disease is perpetuated by tubular epithelial cell apoptosis, and the NHE1 Na+/H+ exchanger defends against apoptosis in response to undefined regulatory mechanisms. Results: Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) and phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3) bind and differentially regulate NHE1 through weak electrostatic and pH-dependent interactions. Conclusion: NHE1-phospholipid binding regulates NHE1 activities. Significance: NHE1-dependent cell survival is mediated through toggling between interactions with PI(4,5)P2 and PI(3,4,5)P3. Tubular atrophy predicts chronic kidney disease progression, and is caused by proximal tubular epithelial cell (PTC) apoptosis. The normally quiescent Na+/H+ exchanger-1 (NHE1) defends against PTC apoptosis, and is regulated by PI(4,5)P2 binding. Because of the vast array of plasma membrane lipids, we hypothesized that NHE1-mediated cell survival is dynamically regulated by multiple anionic inner leaflet phospholipids. In membrane overlay and surface plasmon resonance assays, the NHE1 C terminus bound phospholipids with low affinity and according to valence (PIP3 > PIP2 > PIP = PA > PS). NHE1-phosphoinositide binding was enhanced by acidic pH, and abolished by NHE1 Arg/Lys to Ala mutations within two juxtamembrane domains, consistent with electrostatic interactions. PI(4,5)P2-incorporated vesicles were distributed to apical and lateral PTC domains, increased NHE1-regulated Na+/H+ exchange, and blunted apoptosis, whereas NHE1 activity was decreased in cells enriched with PI(3,4,5)P3, which localized to basolateral membranes. Divergent PI(4,5)P2 and PI(3,4,5)P3 effects on NHE1-dependent Na+/H+ exchange and apoptosis were confirmed by selective phosphoinositide sequestration with pleckstrin homology domain-containing phospholipase Cδ and Akt peptides, PI 3-kinase, and Akt inhibition in wild-type and NHE1-null PTCs. The results reveal an on-off switch model, whereby NHE1 toggles between weak interactions with PI(4,5)P2 and PI(3,4,5)P3. In response to apoptotic stress, NHE1 is stimulated by PI(4,5)P2, which leads to PI 3-kinase activation, and PI(4,5)P2 phosphorylation. The resulting PI(3,4,5)P3 dually stimulates sustained, downstream Akt survival signaling, and dampens NHE1 activity through competitive inhibition and depletion of PI(4,5)P2.

Lipotoxic disruption of NHE1 interaction with PI(4,5)P2 expedites proximal tubule apoptosis

Chronic kidney disease progression can be predicted based on the degree of tubular atrophy, which is the result of proximal tubule apoptosis. The Na+/H+ exchanger NHE1 regulates proximal tubule cell survival through interaction with phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2], but pathophysiologic triggers for NHE1 inactivation are unknown. Because glomerular injury permits proximal tubule luminal exposure and reabsorption of fatty acid/albumin complexes, we hypothesized that accumulation of amphipathic, long-chain acyl-CoA (LC-CoA) metabolites stimulates lipoapoptosis by competing with the structurally similar PI(4,5)P2 for NHE1 binding. Kidneys from mouse models of progressive, albuminuric kidney disease exhibited increased fatty acids, LC-CoAs, and caspase-2-dependent proximal tubule lipoapoptosis. LC-CoAs and the cytosolic domain of NHE1 directly interacted, with an affinity comparable to that of the PI(4,5)P2-NHE1 interaction, and competing LC-CoAs disrupted binding of the NHE1 cytosolic tail to PI(4,5)P2. Inhibition of LC-CoA catabolism reduced NHE1 activity and enhanced apoptosis, whereas inhibition of proximal tubule LC-CoA generation preserved NHE1 activity and protected against apoptosis. Our data indicate that albuminuria/lipiduria enhances lipotoxin delivery to the proximal tubule and accumulation of LC-CoAs contributes to tubular atrophy by severing the NHE1-PI(4,5)P2 interaction, thereby lowering the apoptotic threshold. Furthermore, these data suggest that NHE1 functions as a metabolic sensor for lipotoxicity.

Evidence-based approach for prevention of radiocontrast-induced nephropathy†

The frequency of radiocontrast administration is dramatically increasing, with over 80 million doses delivered annually worldwide. Although recently developed radiocontrast agents are relatively safe in most patients, contrast nephropathy (CN) is still a major source of in-hospital and long-term morbidity and mortality, particularly in patients with preexisting kidney disease. Multiple protocols for CN prevention have been studied; however, strict guidelines have not been established, in part because of conflicting efficacy data for most prevention approaches. In this work, we critically review the major trials that have addressed common CN prophylaxis strategies, including type of radiocontrast media, N-acetylcysteine administration, extracellular fluid volume expansion, and hemofiltration/hemodialysis. We conclude with evidence-based recommendations for CN prevention, which emphasize concurrent NaHCO3 infusion and N-acetylcysteine administration. These guidelines should be helpful to hospitalists, who frequently order radiocontrast studies, and could therefore have a significant impact on prevention of CN.

Delayed kidney allograft function - what does it tell us about acute kidney injury?

Ischemia-reperfusion (IR) injury plays a significant role in the pathogenesis of both delayed graft function (DGF) in allografts and hemodynamic mediated acute kidney injury (AKI) of native kidneys. IR injury involves the interplay of several factors, including both the innate and adaptive immune systems. Though the final effector mechanisms causing injury are likely similar in both allograft and native kidneys, DGF is confounded by various donor, recipient and graft-handling factors. DGF is becoming increasingly important because of the growing number of patients awaiting kidney transplant. Identifying mediators, new diagnostics and therapeutics for DGF is important to provide more kidneys for transplant and improve outcomes. Furthermore, given that DGF occurs in a tightly controlled scenario, studying DGF is useful for early studies of proof of principle for biomarkers and therapeutics prior to launching into less wellcontrolled studies in large numbers of native kidney AKI patients.

Contrast-Induced Nephropathy in Renal Transplant Recipients: A Single Center Experience

Background Contrast-induced nephropathy (CIN) in native kidneys is associated with a significant increase in mortality and morbidity. Data regarding CIN in renal allografts are limited, however. We retrospectively studied CIN in renal allografts at our institution: its incidence, risk factors, and effect on long-term outcomes including allograft loss and death. Methods One hundred thirty-five renal transplant recipients undergoing 161 contrast-enhanced computed tomography (CT) scans or coronary angiograms (Cath) between years 2000 and 2014 were identified. Contrast agents were iso- or low osmolar. CIN was defined as a rise in serum creatinine (SCr) by >0.3u2009mg/dl or 25% from baseline within 4u2009days of contrast exposure. After excluding 85 contrast exposures where patients had no SCr within 4u2009days of contrast administration, 76 exposures (CT: nu2009=u200945; Cath: nu2009=u200931) in 50 eligible patients were analyzed. Risk factors assessed included demographics, comorbid conditions, type/volume of contrast agent used, IV fluids, N-acetylcysteine administration, and calcineurin inhibitor use. Bivariate and multivariable analyses were used to assess the risk of CIN. Results Incidence of CIN was 13% following both, CT (6 out of 45) and Cath (4 out of 31). Significant bivariate predictors of CIN were IV fluid administration (pu2009=u20090.05), lower hemoglobin (pu2009=u20090.03), and lower albumin (pu2009=u20090.02). In a multivariable model, CIN was predicted by N-acetylcysteine (pu2009=u20090.03) and lower hemoglobin (pu2009=u20090.01). Calcineurin inhibitor use was not associated with CIN. At last follow-up, CIN did not affect allograft or patient survival. Conclusion CIN is common in kidney transplant recipients, and there is room for quality improvement with regards to careful renal function monitoring post-contrast exposure. In our study, N-acetylcysteine exposure and lower hemoglobin were associated with CIN. Calcineurin inhibitor use was not associated with CIN. Our sample size is small, however, and larger prospective studies of CIN in renal allografts are needed.

Severe BK polyomavirus-induced hemorrhagic cystitis in a kidney transplant recipient with the absence of renal allograft involvement

BK polyomavirus mostly manifests as polyomavirus‐associated nephropathy (PyVAN) in kidney transplant patients and polyoma virus‐associated hemorrhagic cystitis (PyVHC) in bone marrow transplant patients. PyVHC in kidney transplant patients is only reported in four cases in the literature. Our patient had severe hemorrhagic cystitis without renal involvement. We postulate that our patients exposure to ifosfamide and radiation 8 years prior transplantation might predispose him to this disease.

A Phase Ib, Open Label, Single Arm Study to Assess the Safety, Pharmacokinetics, and Impact on Humoral Sensitization of SANGUINATE Infusion in Patients with End-Stage Renal Disease

The endeavor to study desensitization in kidney transplantation has not been matched by an effort to investigate strategies to prevent sensitization. In this study (NCT02437422), we investigated the safety, impact on sensitization, and pharmacokinetics of SANGUINATE (SG), a hemoglobin‐based oxygen carrier, as a potential alternative to packed red blood cells (PRBC) in transplant candidates with end‐stage renal disease (ESRD). Ten ESRD subjects meeting inclusion/exclusion (I/E) criteria were planned to receive three weekly infusions of SG (320 mg/kg). The study was stopped after five subjects were enrolled, and their data were analyzed after completing a follow‐up period of 90 days. Two subjects had elevated troponin I levels in setting of SG infusion, one of which was interpreted as a non‐ST elevation myocardial infarction. All other adverse events were transient. SG pharmacokinetic analysis showed mean (SD) Cmax, Tmax, AUC, and half‐life of 4.39 (0.69) mg/mL, 2.42 (0.91) hours, 171.86 (52.35) mg h/mL, and 40.60 (11.96) hours, respectively. None of the subjects developed new anti‐HLA antibodies following SG infusion and throughout the study period. In conclusion, SG is a potential alternative to PRBCs in ESRD patients considered for kidney transplantation as it was not associated with humoral sensitization. Larger studies in highly sensitized patients are required to further evaluate for potential safety signals.