The Potential Applications and Limitations of in vitro Models for Studying the Gut-kidney Axis in Chronic Kidney Disea

Abstract

Chronic kidney disease is a detrimental condition characterized by the irreversible and progressive loss of renal function. CKD can be initiated by various triggers including but not limited to abnormal kidney development, hypertension, diabetes, or a toxic insult. Due to loss of renal function, uremic metabolites accumulate in the bloodstream, possibly affecting the intestines. CKD patients show an altered microbiome, increased intestinal permeability, and elevated inflammatory markers. If these intestinal alterations are related to CKD and if these alterations would then be a resulting or a causing factor, remains up for debate. To better understand the involvement of intestinal alterations in CKD’s pathophysiology, studies using appropriate models should be conducted. Various in vitro leaky gut models are utilized within and outside the CKD research field. This review discusses the present in vitro leaky gut models including their possible applications and limitations in studying the gut-kidney axis in CKD. The simplest models used are cell cultures, which mainly involve the Caco-2, HT-29, T84, and HIEC-6 cells in mono- or co-cultures. Besides cell lines, primary intestinal cells can be applied to better resemble the leaky gut in vivo. By using inserts, an additional compartment is established within the leaky gut model, separating luminal from basolateral sides. The inserts allow for a mono-, co- or triple culture, for example by the inclusion of immune cells in the basolateral compartment. Advanced 3D models have been established to mimic the intestinal architecture. The most frequently used 3D model is the intestinal organoid model. Other 3D systems include fluidic flow such as the Organoplate® and the gut-on-a-chip. Besides the addition of a fluidic flow, the renal cells can be introduced establishing the gut-kidney axis on-a-chip. These monolayer, transwell and 3D models can exposed to various insults to induce the leaky gut. Insults involve uremic metabolites, patient plasma or serum, and inflammatory cytokines. This review represents a comparison of currently used in vitro models and their suitability to answer specific research questions on intestinal integrity and intestinal inflammation in the context of CKD. We have identified shortcomings and propose novel approaches to improve the relevance, applicability and translatability of these in vitro models in CKD research.