More than a Marker: Arginase-1 in Kidney Repair

• acute renal failure
• cell signaling
• macrophages
• renal proximal tubule cell
• proliferation

AKI is an important risk factor for CKD. There are many unanswered questions regarding the role of specific cell types and molecular pathways in the so-called AKI-CKD transition. Too little is known about targetable factors that determine whether successful repair or a maladaptative process ensues.

To study aspects of the AKI-CKD transition, researchers often use ischemia/reperfusion injury (IRI), the most widely used murine AKI model. IRI per se leads to nephron loss and fibrosis. If IRI is restricted to one kidney and the contralateral kidney is removed, the postischemic response is more regenerative and less fibrotic.¹ In IRI, the typical chain of events starts by tubular damage/necrosis. This causes the release of danger-associated molecular patterns and other intracellular damage molecules that activate mechanisms of innate immunity, for example, via toll-like receptor stimulation on parenchymal and resident immune cells, leading to the secretion of proinflammatory cytokines and chemokines. Neutrophils, natural killer cells, macrophages, and other immune cells are subsequently recruited into the injured kidney.² Although neutrophils predominate in the first wave of infiltrating immune cells, macrophages are the most prevalent cell type during the following days. Macrophages accumulate, especially in the outer stripe of the outer medullary region, where IRI damage is most severe. They remain there for days to weeks, and play a crucial role in determining AKI outcome.³

Macrophages are fascinating, because they are extremely versatile and change their phenotype during the early and late course of IRI and repair. The early infiltrating macrophages show typical features of classically activated M1 macrophages, producing high amounts of proinflammatory cytokines and aggravating the IRI-induced tubulointerstitial destruction. In a simplified dichotomous concept, this destructive early effect of macrophages is followed by a reparative role. In the IRI model, this typically occurs between days 2 and 7, when M2 macrophages appear. In contrast to M1 macrophages, M2 cells are polarized toward an anti-inflammatory, proproliferative, and healing-supportive phenotype. The increase and functional importance of M2-type macrophages can be observed in most forms of AKI. However, the reparative role is not always as clearly visible as it is in the IRI model.⁴ M2 macrophages can develop from kidney resident M1-type cells or directly from newly infiltrating monocytes. These macrophage phenotypes have received a lot of attention in the context of AKI, but information about their actual reparative effector mechanisms is lacking.

In this issue of JASN, Shin et al. provide the first evidence for a crucial role of arginase-1 (Arg1) in M2-mediated repair after renal IRI.⁵ Arg1 is a cytosolic enzyme that is constitutively expressed in the liver, where it catalyzes the conversion of arginine to urea and ornithine. This conversion is vital for nitrogen removal, as evidenced by the fact that Arg1 knockout mice die quickly after birth with hyperammonemia.⁶ Unlike the constitutive expression in hepatocytes, Arg1 expression in macrophages depends on M2 polarization. Arg1 therefore is a robust and frequently used marker of the M2 state.

In their study, Shin et al. used Arg1^mko mice, in which they eliminated Arg1 conditionally in myelomonocytic cells. After unilateral IRI and contralateral nephrectomy, these mice showed an expected reduction of Arg1 in infiltrating macrophages of the postischemic kidney, whereas the total number of kidney macrophages did not differ from that of control mice, and Arg1-deficient macrophages continued to express identical levels of other M2 hallmark genes, including Mrc1 and Msr1. The authors observed that initial IRI damage, as judged by histology, TUNEL, creatinine, and BUN, did not differ in Arg1^mko mice. Importantly, however, the lack of Arg1 in macrophages hindered functional kidney recovery and was associated with increased mortality after IRI.

As a possible explanation, the authors found significantly decreased reparative proliferation in damaged proximal tubules of Arg1^mko mice. These findings are the first evidence to indicate that Arg1 is an active contributor to the M2-dependent renal tubular repair process and suggest that Arg1 orchestrates the activation of a proproliferative secretome in macrophages. Using cocultures and conditioned medium experiments, Shin et al. provide further evidence for crucial crosstalk between macrophages and tubular cells. Their findings suggest that exposure to debris of dead tubular cells stimulates high levels of GM-CSF expression/secretion by tubular cells, and these high GMF-CSF levels induce the expression of Arg1 in macrophages, which in turn induces the expression/secretion of proproliferative signaling molecules that drive tubular cell proliferation (Figure 1).

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Figure 1.

Schematic summary of the crosstalk between tubular cells and macrophages during acute injury. This figure was prepared using Biorender.com.

These findings in the kidney align well with suppression of fibrosis by Arg1-expressing macrophages in other organs, such as the liver.⁷ In the liver, Arg1 repressed development of profibrotic T helper 2 cells. The current work focuses on monocytic cells. Further experiments may also address the role of neutrophil Arg1, which was also ablated in the genetic model Shin et al. used.⁶ Neutrophil arginase was recently shown to suppress detrimental T cell inflammation in studies of human cells in vitro.⁸ It is possible that loss of neutrophil Arg1 contributed to impaired renal healing, and the excess early mortality observed in their model.

The current data by Shin et al. extend earlier work by the same group and other researchers, showing that stressed tubular cells are a prime source of GM-CSF and macrophage CSF, which drive M2 polarization in the surrounding macrophages.⁹ According to the proposed concept, tubular cells are at the center of a circuit, in which secretion of M2-polarizing and Arg1-inducing GM-CSF causes a secretory proproliferative feedback response, leading to more proliferation and better repair. So far, the molecular nature of this response remains unclear. It has been shown, for example, that macrophages secrete Wnt-7b as a beneficial factor of tubular repair, but the current study does not identify the Arg1-associated factor.¹⁰ Given that a better understanding of the underlying mechanism might have translational implications, further research should address the Arg1-dependent signaling pathway.

• Copyright © 2022 by the American Society of Nephrology