Is acute kidney injury in the early phase of sepsis a sign of metabolic downregulation in tubular epithelial cells?

Peritubular Capillary Oxygen Consumption in Sepsis-Induced AKI: Multi-Parametric Photoacoustic Microscopy

Understanding and measuring parameters responsible for the pathogenesis of sepsis-induced AKI (SI-AKI) is critical in developing therapies. Blood flow to the kidney is heterogeneous, partly due to the existence of dynamic networks of capillaries in various regions, responding differentially to oxygen demand in cortex versus medulla. High energy demand regions, especially the outer medulla, are susceptible to hypoxia and subject to damage during SI-AKI. Proximal tubule epithelial cells in the cortex and the outer medulla can also undergo metabolic reprogramming during SI-AKI to maintain basal physiological status and to avoid potential damage. Current data on the assessment of renal hemodynamics and oxygen metabolism during sepsis is limited. Preclinical and clinical studies show changes in renal hemodynamics associated with SI-AKI, and in clinical settings, interventions to manage renal hemodynamics seem to help improve disease outcomes in some cases. Lack of proper tools to assess temporospatial changes in peritubular blood flow and tissue oxygen metabolism is a barrier to our ability to understand microcirculatory dynamics and oxygen consumption and their role in the pathogenesis of SI-AKI. Current tools to assess renal oxygenation are limited in their usability as these cannot perform continuous simultaneous measurement of renal hemodynamics and oxygen metabolism. Multi-parametric photo-acoustic microscopy (PAM) is a new tool that can measure real-time changes in microhemodynamics and oxygen metabolism. Use of multi-parametric PAM in combination with advanced intravital imaging techniques has the potential to understand the contribution of microhemodynamic and tissue oxygenation alterations to SI-AKI.

Metabolic reprogramming and tolerance during sepsis-induced AKI

The host defence against infection is an adaptive response in which several mechanisms are deployed to decrease the pathogen load, limit tissue injury and restore homeostasis. In the past few years new evidence has suggested that the ability of the immune system to limit the microbial burden - termed resistance - might not be the only defence mechanism. In fact, the capacity of the host to decrease its own susceptibility to inflammation- induced tissue damage - termed tolerance - might be as important as resistance in determining the outcome of the infection. Metabolic adaptations are central to the function of the cellular immune response. Coordinated reprogramming of metabolic signalling enables cells to execute resistance and tolerance pathways, withstand injury, steer tissue repair and promote organ recovery. During sepsis-induced acute kidney injury, early reprogramming of metabolism can determine the extent of organ dysfunction, progression to fibrosis, and the development of chronic kidney disease. Here we discuss the mechanisms of tolerance that act in the kidney during sepsis, with particular attention to the role of metabolic responses in coordinating these adaptive strategies. We suggest a novel conceptual model of the cellular and organic response to sepsis that might lead to new avenues for targeted, organ-protective therapies.

The Role of Energy Regulation in the Tubular Epithelial Cell Response to Sepsis

Sepsis is considered today a major public health problem. Despite that mortality has been consistently associated with organ compromise, the mechanisms by which sepsis causes multiple organ dysfunction are not well understood, and hence, therapy remains reactive and non-specific. Recent studies have challenged previous paradigms by demonstrating that acute kidney injury can occur in the setting of a normal or an even increased renal blood flow, and that it is characterized by tubular injury and not by necrosis or apoptosis. This finding suggests that mechanisms other than hypoperfusion may be at play, and that adaptive responses of the tubular epithelial cell may be key to understanding the origin of organ dysfunction in the setting of sepsis. In this review, we discuss evidence suggesting that the activation of energy regulatory processes and mitochondrial quality control processes may not only be drivers of this response, but also be factors that may alter the course of organ dysfunction during sepsis in clinically relevant ways.