Injury Pathways That Lead to AKI in a Mouse Kidney Transplant Model

Remote Kidney and Liver Injury After Transplantation of Lung Allografts in an Allogeneic Mouse Model

BACKGROUND

Remote organ dysfunction is common after lung transplantation and might negatively affect the outcome. The local anesthetic ropivacaine was previously demonstrated to attenuate acute rejection after allogeneic lung transplantation in mice. We hypothesized that lung transplantation might result in detectable molecular signs of injury in kidneys and liver and that ropivacaine might attenuate this damage.

METHODS

Organs from C57BL/6 mice undergoing allogeneic orthotopic single-lung transplantation were procured at postoperative day 5 and analyzed using Western blot and real-time quantitative polymerase chain reaction probing for Src protein tyrosine kinase, STAT3, and bax/bcl-2. During cold ischemia, the allograft had either been flushed with normal saline only or in combination with ropivacaine (1 µM). A nontransplanted group of animals served as the baseline controls.

RESULTS

The allogeneic stimulus induced by transplantation led to an increase in Src-phosphorylation and STAT3-expression in the kidneys and livers of lung-transplanted mice compared to nontransplanted animals. Bax/bcl-2 as a marker of cellular apoptosis was not affected by the transplantation. In contrast to the findings in the transplanted lungs, the addition of ropivacaine did not have an effect on the examined markers of inflammation in the remote organs.

CONCLUSIONS

The observed increase in the inflammatory signaling provides first insight into a possible mechanism, by which remote organ dysfunction after lung transplantation might occur.

High RIPK3 expression is associated with a higher risk of early kidney transplant failure

Renal ischemia-reperfusion injury (IRI) is associated with reduced allograft survival, and each additional hour of cold ischemia time increases the risk of graft failure and mortality following renal transplantation. Receptor-interacting protein kinase 3 (RIPK3) is a key effector of necroptosis, a regulated form of cell death. Here, we evaluate the first-in-human RIPK3 expression dataset following IRI in kidney transplantation. The primary analysis included 374 baseline biopsy samples obtained from renal allografts 10 minutes after onset of reperfusion. RIPK3 was primarily detected in proximal tubular cells and distal tubular cells, both of which are affected by IRI. Time-to-event analysis revealed that high RIPK3 expression is associated with a significantly higher risk of one-year transplant failure and prognostic for one-year (death-censored) transplant failure independent of donor and recipient associated risk factors in multivariable analyses. The RIPK3 score also correlated with deceased donation, cold ischemia time and the extent of tubular injury.

Necroptosis in Organ Transplantation: Mechanisms and Potential Therapeutic Targets

Organ transplantation remains the only treatment option for patients with end-stage organ dysfunction. However, there are numerous limitations that challenge its clinical application, including the shortage of organ donations, the quality of donated organs, injury during organ preservation and reperfusion, primary and chronic graft dysfunction, acute and chronic rejection, infection, and carcinogenesis in post-transplantation patients. Acute and chronic inflammation and cell death are two major underlying mechanisms for graft injury. Necroptosis is a type of programmed cell death involved in many diseases and has been studied in the setting of all major solid organ transplants, including the kidney, heart, liver, and lung. It is determined by the underlying donor organ conditions (e.g., age, alcohol consumption, fatty liver, hemorrhage shock, donation after circulatory death, etc.), preservation conditions and reperfusion, and allograft rejection. The specific molecular mechanisms of necroptosis have been uncovered in the organ transplantation setting, and potential targeting drugs have been identified. We hope this review article will promote more clinical research to determine the role of necroptosis and other types of programmed cell death in solid organ transplantation to alleviate the clinical burden of ischemia-reperfusion injury and graft rejection.

External pressure dynamics promote kidney viability and perfusate filtration during ex vivo kidney perfusion

Normothermic machine perfusion (NMP) has not yet been established as a technique for preserving organs for a day. A key contributing factor to the same is that the perfusing solutions cannot circulate continuously and evenly in the organs. Here, we conceived a method of applying intermittent air pressure from outside the organ to assist its circulatory distribution during perfusion. We used a perfusion culture system while applying external pressure to culture rat kidneys and compared the circulatory distribution in the kidneys, changes in tissue morphology due to injury, and perfusate filtration. The intermittent pressurization (IMP) (-) group showed markedly poorer circulation on the upper side compared with that in the lower side, alongside histological damage. On the other hand, the IMP (+) group showed improved circulation in the upper side and had lesser histological damage. Furthermore, the IMP (+) group maintained the ability to filter perfusate for 24 h. In transplantation medicine and regenerative medicine research, this method has the potential to contribute to more efficient organ preservation and more functional tissue regeneration in the future.

DAMPs Released From Injured Renal Tubular Epithelial Cells Activate Innate Immune Signals in Healthy Renal Tubular Epithelial Cells

BACKGROUND

Renal ischemia-reperfusion injury (IRI) predictably causes acute kidney injury after shock and major cardiovascular procedures in all kidneys procured for transplantation. The earliest events of IRI are triggered by molecules released from injured cells, damage-associated molecular patterns (DAMPs), that bind pattern recognition receptors (PRRs) constitutively expressed on many cells within the kidney. Activation of PRR signaling leads to production of proinflammatory molecules, which incite a cascade of inflammatory events leading to acute kidney injury. Renal tubular epithelial cells (RTECs) are particularly susceptible to ischemic injury, and proximal RTEC injury is pathognomonic of renal IRI. To better understand how injured RTECs contribute to the cycle of deleterious inflammation in the setting of renal IRI, this study asked whether DAMPs released from injured RTECs induced PRR signals in healthy RTECs.

METHODS

Human RTECs were necrosed ex vivo to release intracellular DAMPs and resulting necrotic supernatant used to stimulate healthy RTECs, T lymphocytes, and monocytes.

RESULTS

DAMPs released from necrosed RTECs upregulated PRRs known to be associated with renal IRI and activated mitogen-activated protein kinase signaling pathways. Proinflammatory cytokines were upregulated in response to necrotic supernatant, and this upregulation was abrogated by MEK-1 inhibition. The RTEC-derived DAMPs were also potent inducers of T-cell activation/proliferation and monocyte migration.

CONCLUSIONS

This is the first study to our knowledge to show that endogenous DAMPs released from injured RTECs directly activate PRR signaling in healthy RTECs. These findings provide new insights directed to therapeutics for renal IRI.

Deletion of TLR4 reduces apoptosis and improves histology in a murine kidney transplant model

Acute kidney injury (AKI) after transplantation of human deceased donor kidneys is associated with upregulation of tubular toll like receptor 4 (TLR4), but whether TLR4 is required for AKI is unknown. We hypothesized that TLR4 knockout mice (TLR4KO) subjected to cold ischemia followed by kidney transplant (CI + Txp) would be protected from AKI. C57Bl/6J wild type or TLR4KO kidneys were subjected to CI + Txp into wild type recipients. Tubular cell apoptosis, tubular injury and cast formation were significantly improved in recipients of TLR4KO kidneys. TLR4KO kidneys also demonstrated significantly decreased expression of the effector caspase 8. Brush border injury scores and serum creatinine were not different in recipients of TLR4KO versus wild type kidneys. Phosphorylated RIP3 and MLKL through which TLR4 signals programmed necrosis were expressed in both recipient groups. In addition, TNF-α and TNFR1 expression were significantly increased in recipient serum and TLR4KO kidneys respectively after CI + Txp, suggesting continued activation of programmed necrosis despite TLR4 deletion. Our results suggest that TLR4 deletion decreases apoptosis via inhibition of the death receptor pathway and decreases tubular injury and cast formation.

Chronic Kidney Allograft Disease: New Concepts and Opportunities

Chronic kidney disease (CKD) is increasing in most countries and kidney transplantation is the best option for those patients requiring renal replacement therapy. Therefore, there is a significant number of patients living with a functioning kidney allograft. However, progressive kidney allograft functional deterioration remains unchanged despite of major advances in the field. After the first post-transplant year, it has been estimated that this chronic allograft damage may cause a 5% graft loss per year. Most studies focused on mechanisms of kidney graft damage, especially on ischemia-reperfusion injury, alloimmunity, nephrotoxicity, infection and disease recurrence. Thus, therapeutic interventions focus on those modifiable factors associated with chronic kidney allograft disease (CKaD). There are strategies to reduce ischemia-reperfusion injury, to improve the immunologic risk stratification and monitoring, to reduce calcineurin-inhibitor exposure and to identify recurrence of primary renal disease early. On the other hand, control of risk factors for chronic disease progression are particularly relevant as kidney transplantation is inherently associated with renal mass reduction. However, despite progress in pathophysiology and interventions, clinical advances in terms of long-term kidney allograft survival have been subtle. New approaches are needed and probably a holistic view can help. Chronic kidney allograft deterioration is probably the consequence of damage from various etiologies but can be attenuated by kidney repair mechanisms. Thus, besides immunological and other mechanisms of damage, the intrinsic repair kidney graft capacity should be considered to generate new hypothesis and potential therapeutic targets. In this review, the critical risk factors that define CKaD will be discussed but also how the renal mechanisms of regeneration could contribute to a change chronic kidney allograft disease paradigm.

The impact of Caspase-1 deletion on apoptosis and acute kidney injury in a murine transplant model

BACKGROUND

Caspase-1 knockout mice (Casp1KO) are protected from Acute Kidney Injury (AKI) after warm ischemia/reperfusion injury in non-transplant models. Since Caspase-1 plays a central role as an inflammatory response initiator, we hypothesized that Casp1KO mice would be protected from AKI following transplant.

METHODS

Renal tubular cells (RTECs) were subjected to cold storage and rewarming (CS/REW). C57Bl/6 J wild type or Casp1KO kidneys were subjected to CI for 30 min and then transplanted into wild type recipients (CI + Txp). The recipients underwent bilateral native nephrectomy at the time of transplant. Serum creatinine (sCr) was measured 24 h after native nephrectomy to assess transplant function.

RESULTS

We found that RTECs subjected to CS/REW had significantly increased expression of the Caspase-1 and inflammasome protein NLRP1. Wild type kidneys subjected to CI + Txp into wild type recipients also demonstrated significantly increased Caspase-1 and NLRP1 protein expression compared to kidneys transplanted from Casp1KO donors into wild type recipients. Caspase-1 deletion results in significantly decreased RTEC apoptosis in transplanted Casp1KO vs WT kidneys. Surprisingly, however, renal function, ATN scores including brush border injury, cast formation and tubular simplification were similar in both groups and not significantly different.

CONCLUSIONS

Our data suggest that other triggers of inflammation and programmed necrosis may need to be inhibited in addition to attenuating Caspase-1 to fully prevent AKI after kidney transplant. Importantly, requirements may be distinct for AKI induced by transplantation as opposed to other transient models such as the clamp model of AKI.

Mesenchymal stem cells and extracellular vesicles in therapy against kidney diseases

Kidney diseases pose a threat to human health due to their rising incidence and fatality rate. In preclinical and clinical studies, it has been acknowledged that mesenchymal stem cells (MSCs) are effective and safe when used to treat kidney diseases. MSCs play their role mainly by secreting trophic factors and delivering extracellular vesicles (EVs). The genetic materials and proteins contained in the MSC-derived EVs (MSC-EVs), as an important means of cellular communication, have become a research focus for targeted therapy of kidney diseases. At present, MSC-EVs have shown evident therapeutic effects on acute kidney injury (AKI), chronic kidney disease (CKD), diabetic nephropathy (DN), and atherosclerotic renovascular disease (ARVD); however, their roles in the transplanted kidney remain controversial. This review summarises the mechanisms by which MSC-EVs treat these diseases in animal models and proposes certain problems, expecting to facilitate corresponding future clinical practice.

A Standardized Warm Ischemia Time for the Induction of Injury in Murine Kidney Transplants

One of the cornerstone research models used in our laboratories is the induction of ischemic injury through cold ischemia followed by warm ischemia to donor kidneys to mimic the clinical realities of transplantation. The experimental design of the present study included bilateral nephrectomies on the day of syngeneic kidney transplant, with serum creatinine measured 24 hours postoperatively to measure acute function. Cold ischemia time in these experiments was always 30 minutes, and warm ischemia time was not standardized but always recorded. It became apparent that some transplanted kidneys that should have displayed injury were producing close to normal serum creatinine levels on postoperative day 1. In reviewing our data, we found a potential correlation between warm ischemia time and serum creatinine, in particular a significant proportion of low serum creatinine results (0.48 ± 0.26 mg/dL vs 1.99 ± 1.11 mg/dL; P < .05) was associated with warm ischemia times that were significantly shorter than our historical average (29.2 ± 2.7 min vs 35.7 ± 2.2 min; P < .05). The kidneys with lower serum creatinine also displayed lower apoptosis and brush border injury scores and fewer tubular casts. Therefore, we concluded that establishing a minimum warm ischemia time was just as important as standardized cold ischemia time to ensure consistent injury in this model.