Single-Cell RNA Sequencing Identifies Response of Renal Lymphatic Endothelial Cells to Acute Kidney Injury

SIGNIFICANCE STATEMENT

The renal lymphatic vasculature and the lymphatic endothelial cells that make up this network play important immunomodulatory roles during inflammation. How lymphatics respond to AKI may affect AKI outcomes. The authors used single-cell RNA sequencing to characterize mouse renal lymphatic endothelial cells in quiescent and cisplatin-injured kidneys. Lymphatic endothelial cell gene expression changes were confirmed in ischemia-reperfusion injury and in cultured lymphatic endothelial cells, validating renal lymphatic endothelial cells single-cell RNA sequencing data. This study is the first to describe renal lymphatic endothelial cell heterogeneity and uncovers molecular pathways demonstrating lymphatic endothelial cells regulate the local immune response to AKI. These findings provide insights into previously unidentified molecular pathways for lymphatic endothelial cells and roles that may serve as potential therapeutic targets in limiting the progression of AKI.

BACKGROUND

The inflammatory response to AKI likely dictates future kidney health. Lymphatic vessels are responsible for maintaining tissue homeostasis through transport and immunomodulatory roles. Owing to the relative sparsity of lymphatic endothelial cells in the kidney, past sequencing efforts have not characterized these cells and their response to AKI.

METHODS

Here, we characterized murine renal lymphatic endothelial cell subpopulations by single-cell RNA sequencing and investigated their changes in cisplatin AKI 72 hours postinjury. Data were processed using the Seurat package. We validated our findings by quantitative PCR in lymphatic endothelial cells isolated from both cisplatin-injured and ischemia-reperfusion injury, by immunofluorescence, and confirmation in in vitro human lymphatic endothelial cells.

RESULTS

We have identified renal lymphatic endothelial cells and their lymphatic vascular roles that have yet to be characterized in previous studies. We report unique gene changes mapped across control and cisplatin-injured conditions. After AKI, renal lymphatic endothelial cells alter genes involved in endothelial cell apoptosis and vasculogenic processes as well as immunoregulatory signaling and metabolism. Differences between injury models were also identified with renal lymphatic endothelial cells further demonstrating changed gene expression between cisplatin and ischemia-reperfusion injury models, indicating the renal lymphatic endothelial cell response is both specific to where they lie in the lymphatic vasculature and the kidney injury type.

CONCLUSIONS

In this study, we uncover lymphatic vessel structural features of captured populations and injury-induced genetic changes. We further determine that lymphatic endothelial cell gene expression is altered between injury models. How lymphatic endothelial cells respond to AKI may therefore be key in regulating future kidney disease progression.

Single-cell RNA sequencing identifies response of renal lymphatic endothelial cells to acute kidney injury

The inflammatory response to acute kidney injury (AKI) likely dictates future renal health. Lymphatic vessels are responsible for maintaining tissue homeostasis through transport and immunomodulatory roles. Due to the relative sparsity of lymphatic endothelial cells (LECs) in the kidney, past sequencing efforts have not characterized these cells and their response to AKI. Here we characterized murine renal LEC subpopulations by single-cell RNA sequencing and investigated their changes in cisplatin AKI. We validated our findings by qPCR in LECs isolated from both cisplatin-injured and ischemia reperfusion injury, by immunofluorescence, and confirmation in in vitro human LECs. We have identified renal LECs and their lymphatic vascular roles that have yet to be characterized in previous studies. We report unique gene changes mapped across control and cisplatin injured conditions. Following AKI, renal LECs alter genes involved endothelial cell apoptosis and vasculogenic processes as well as immunoregulatory signaling and metabolism. Differences between injury models are also identified with renal LECs further demonstrating changed gene expression between cisplatin and ischemia reperfusion injury models, indicating the renal LEC response is both specific to where they lie in the lymphatic vasculature and the renal injury type. How LECs respond to AKI may therefore be key in regulating future kidney disease progression.

Indole Propionic Acid Increases T Regulatory Cells and Decreases T Helper 17 Cells and Blood Pressure in Mice with Salt-Sensitive Hypertension

Hypertension affects over a billion adults worldwide and is a major risk factor for cardiovascular disease. Studies have reported that the microbiota and its metabolites regulate hypertension pathophysiology. Recently, tryptophan metabolites have been identified to contribute to and inhibit the progression of metabolic disorders and cardiovascular diseases, including hypertension. Indole propionic acid (IPA) is a tryptophan metabolite with reported protective effects in neurodegenerative and cardiovascular diseases; however, its involvement in renal immunomodulation and sodium handling in hypertension is unknown. In the current study, targeted metabolomic analysis revealed decreased serum and fecal IPA levels in mice with L-arginine methyl ester hydrochloride (L-NAME)/high salt diet-induced hypertension (LSHTN) compared to normotensive control mice. Additionally, kidneys from LSHTN mice had increased T helper 17 (Th17) cells and decreased T regulatory (Treg) cells. Dietary IPA supplementation in LSHTN mice for 3 weeks resulted in decreased systolic blood pressure, along with increased total 24 h and fractional sodium excretion. Kidney immunophenotyping demonstrated decreased Th17 cells and a trend toward increased Treg cells in IPA-supplemented LSHTN mice. In vitro, naïve T cells from control mice were skewed into Th17 or Treg cells. The presence of IPA decreased Th17 cells and increased Treg cells after 3 days. These results identify a direct role for IPA in attenuating renal Th17 cells and increasing Treg cells, leading to improved sodium handling and decreased blood pressure. IPA may be a potential metabolite-based therapeutic option for hypertension.

Impact of High Fat Diet and Bolus Feeding on Chyle Accumulation in a Mouse Model of Generalized Lymphatic Anomaly

Background: Generalized lymphatic anomalies (GLA) are complex vessel malformations that can impair lymphatic function. Potential GLA complications include lipid-rich lymph in the thoracic space or peritoneal cavity, respectively chylothorax and chylous ascites. To reduce the potential for chyle accumulation, GLA patients limit dietary fats. We hypothesized that dietary fatty acid composition impacts the potential for lymphatic dysfunction and chyle accumulation in GLA. Methods and Results: Adipose-specific overexpression of lymphatic growth factors has demonstrated lethal chylothorax in mice. Here, we utilized mice with inducible adipocyte overexpression of vascular endothelial growth factor-D (VD mice) to mimic lymphatic proliferation in GLA and assessed the incidence of chyle accumulation on a mixed high fat diet (HFD), high saturated fat diet (HSFD), or high unsaturated fat diet (HUSFD). Lipid transport was assessed by uptake rates of bolus oral triglyceride load and mesenteric fat analysis. Lymphatic expansion and inflammation were determined by whole mount immunofluorescence and gene expression. Body composition was assessed by MRI. HSFD 2-month wildtype groups resulted in an increase in TNF-α, IL-6, and IL-10 expression compared with chow-fed controls. The chyle accumulation incidence was highest in HFD-fed mice compared with either HSFD or HUSFD. Strikingly, increased mortality was observed irrespective of which high fat diet was consumed after administration of a bolus lipid load. Conclusion: Chronic HFD increases risk of chyle accumulation, however increased mortality was driven particularly by a bolus lipid load in VD mice. These findings suggest that although chronic HFD increases chyle accumulation risk, a single large meal feeding may increase risk of lethal chylothorax instances for GLA patients.

Expanded renal lymphatics improve recovery following kidney injury

Acute kidney injury (AKI) is a major cause of patient mortality and a major risk multiplier for the progression to chronic kidney disease (CKD). The mechanism of the AKI to CKD transition is complex but is likely mediated by the extent and length of the inflammatory response following the initial injury. Lymphatic vessels help to maintain tissue homeostasis through fluid, macromolecule, and immune modulation. Increased lymphatic growth, or lymphangiogenesis, often occurs during inflammation and plays a role in acute and chronic disease processes. What roles renal lymphatics and lymphangiogenesis play in AKI recovery and CKD progression remains largely unknown. To determine if the increased lymphatic density is protective in the response to kidney injury, we utilized a transgenic mouse model with inducible, kidney-specific overexpression of the lymphangiogenic protein vascular endothelial growth factor-D to expand renal lymphatics. "KidVD" mouse kidneys were injured using inducible podocyte apoptosis and proteinuria (POD-ATTAC) or bilateral ischemia reperfusion. In the acute injury phase of both models, KidVD mice demonstrated a similar loss of function measured by serum creatinine and glomerular filtration rate compared to their littermates. While the initial inflammatory response was similar, KidVD mice demonstrated a shift toward more CD4+ and fewer CD8+ T cells in the kidney. Reduced collagen deposition and improved functional recovery over time was also identified in KidVD mice. In KidVD-POD-ATTAC mice, an increased number of podocytes were counted at 28 days post-injury. These data demonstrate that increased lymphatic density prior to injury alters the injury recovery response and affords protection from CKD progression.

Dichotomous effects on lymphatic transport with loss of caveolae in mice

AIM

Fluid and macromolecule transport from the interstitium into and through lymphatic vessels is necessary for tissue homeostasis. While lymphatic capillary structure suggests that passive, paracellular transport would be the predominant route of macromolecule entry, active caveolae-mediated transcellular transport has been identified in lymphatic endothelial cells (LECs) in vitro. Caveolae also mediate a wide array of endothelial cell processes, including nitric oxide regulation. Thus, how does the lack of caveolae impact "lymphatic function"?

METHODS

Various aspects of lymphatic transport were measured in mice constitutively lacking caveolin-1 ("CavKO"), the protein required for caveolae formation in endothelial cells, and in mice with a LEC-specific Cav1 gene deletion (Lyve1-Cre x Cav1^flox/flox ; "LyCav") and ex vivo in their vessels and cells.

RESULTS

In each model, lymphatic architecture was largely unchanged. The lymphatic conductance, or initial tissue uptake, was significantly higher in both CavKO mice and LyCav mice by quantitative microlymphangiography and the permeability to 70 kDa dextran was significantly increased in monolayers of LECs isolated from CavKO mice. Conversely, transport within the lymphatic system to the sentinel node was significantly reduced in anaesthetized CavKO and LyCav mice. Isolated, cannulated collecting vessel studies identified significantly reduced phasic contractility when lymphatic endothelium lacks caveolae. Inhibition of nitric oxide synthase was able to partially restore ex vivo vessel contractility.

CONCLUSION

Macromolecule transport across lymphatics is increased with loss of caveolae, yet phasic contractility reduced, resulting in reduced overall lymphatic transport function. These studies identify lymphatic caveolar biology as a key regulator of active lymphatic transport functions.

Emerging roles for lymphatics in acute kidney injury: Beneficial or maleficent?

Acute kidney injury, a sudden decline in renal filtration, is a surprisingly common pathology resulting from ischemic events, local or systemic infection, or drug-induced toxicity in the kidney. Unchecked, acute kidney injury can progress to renal failure and even recovered acute kidney injury patients are at an increased risk for developing future chronic kidney disease. The initial extent of inflammation, the specific immune response, and how well inflammation resolves are likely determinants in acute kidney injury-to-chronic kidney disease progression. Lymphatic vessels and their roles in fluid, solute, antigen, and immune cell transport make them likely to have a role in the acute kidney injury response. Lymphatics have proven to be an attractive target in regulating inflammation and immunomodulation in other pathologies: might these strategies be employed in acute kidney injury? Acute kidney injury studies have identified elevated levels of lymphangiogenic ligands following acute kidney injury, with an expansion of the lymphatics in several models post-injury. Manipulating the lymphatics in acute kidney injury, by augmenting or inhibiting their growth or through targeting lymphatic-immune interactions, has met with a range of positive, negative, and sometimes inconclusive results. This minireview briefly summarizes the findings of lymphatic changes and lymphatic roles in the inflammatory response in the kidney following acute kidney injury to discuss whether renal lymphatics are a beneficial, maleficent, or a passive contributor to acute kidney injury recovery.

Preparation and Identification of Cardiac Myofibrils from Whole Heart Samples

Mouse models are extensively studied and well-used to study cardiomyopathies due to the genetic relevance of this model organism and increasing need to identify therapeutic targets. Cardiac myofibril preparation for whole hearts has proved to be an invaluable in vitro method for determining the fundamental molecular mechanisms in heart failure. The technique described below consistently yields intact cardiac myofibrils, which can be used in subsequent techniques such as western blotting, immunofluorescence, and mass spectrometry. Here, we describe a method to optimize the separation and yield of cardiac myofibrils from murine whole tissue samples and preparation for subsequent mass spectrometry. This method allows for quick visual identification of multiple cardiac myofibril proteins.