Renal tubular epithelium has the capacity to regenerate, repair, and reepithelialize in response to a variety of insults. Previous studies with several kidney injury models demonstrated that various growth factors, transcription factors, and extracellular matrices are involved in this process. Surviving tubular cells actively proliferate, migrate, and differentiate in the kidney regeneration process after injury, and some cells express putative stem cell markers or possess stem cell properties. Using fate mapping techniques, bone marrow-derived cells and endothelial progenitor cells have been shown to transdifferentiate into tubular components in vivo or ex vivo. Similarly, it has been demonstrated that, during tubular cell regeneration, several inflammatory cell populations migrate, assemble around tubular cells, and interact with tubular cells during the repair of tubular epithelium. In this review, we describe recent advances in understanding the regeneration mechanisms of renal tubules, particularly the characteristics of various cell populations contributing to tubular regeneration, and highlight the targets for the development of regenerative medicine for treating kidney diseases in humans.
Renal tubules express several types of transporter that are involved in renal reabsorption and secretion, as well as ion channels for the maintenance of body fluid balance. These cells comprise polarized mature epithelial cells with the capacity to regenerate following acute kidney injury. After the insult occurs, surviving tubular cells rapidly lose epithelial cell properties and acquire a more mesenchymal phenotype. The dedifferentiated cells migrate into the regions where cell necrosis, apoptosis, or detachment has resulted in denudation of the tubular basement membrane. They proliferate and eventually differentiate into mature epithelial cells with polarized lumen, completing the repair process [
The process of restoration and maturation of damaged epithelium after renal injury has many parallels with the developmental process during kidney organogenesis. Soluble factors involved in kidney development have been identified by gene targeting techniques, in vitro tubulogenesis models, and organ culture systems, and most of these also have been demonstrated to regulate kidney recovery as potential renotrophic factors [
Diverse cell populations involved in tubular regeneration after injury.
In this review, we highlight recent advances regarding the regeneration mechanisms of renal tubules after injury, particularly focusing on possible cell populations and their interactions, which contribute to the repair process of renal tubules after injury.
Renal tubular epithelium has a huge capacity for regeneration after injury. During the repair process, surviving tubular cells actively proliferate and differentiate into mature tubular cells to reconstruct their functional structures. Sophisticated lineage tracing studies have demonstrated that it is unlikely that extrarenal cells enter the tubule and differentiate into epithelial cells in mice. It is more likely that tubule recovery is controlled by a number of intratubular cells with a substantial regenerative capacity [
Despite the structural complexity of the adult kidney, attempts to identify cell populations contributing to the regenerative process have been based on the broad principles of stem cell biology. To conserve growth potential and prevent genetic injury during mitosis, stem cells cycle slowly and are recruited only as demanded by tissue turnover. To identify slow-cycling stem cells, a pulse label of 5-bromo-2-deoxyuridine (BrdU), followed by a chase period, is commonly used, allowing the detection of slow-cycling label-retaining cells (LRCs). LRCs have been identified in renal tubules of normal rat kidneys, and regenerating cells during tubular repair are essentially derived from LRCs [
In human, a resident population with progenitor characteristics was identified along the nephron [
Tubular epithelial cells play an important role in the inflammatory process after acute kidney injury. Recent studies have highlighted the importance of several tubular cell surface proteins in mediating the interaction between tubular cells and inflammation during tubular repair.
Renal tubular epithelium constitutively expresses toll-like receptors (TLRs), a family of pattern recognition receptors that detect motifs of pathogens and host material released during injury. TLRs mediate signal transduction pathways that control the expression of proinflammatory cytokines and chemokines. TLR4 is upregulated in tubular epithelial cells after renal ischemia, while TLR4 deficiency decreases the renal ischemic injury-induced production of proinflammatory cytokines and chemokines and inhibits macrophage and neutrophil accumulation [
The complement system has been shown to mediate renal ischemia-reperfusion injury. Normally, proximal tubular epithelial cells express Crry, a complement inhibitor, on the basolateral membrane. After renal ischemia, Crry production decreases, thus allowing the deposition of C3 on the tubular epithelium and stimulation of proinflammatory CXC chemokine production. These chemokines attract neutrophils and macrophages to the injured kidney [
Suppressor of cytokine signaling 3 (SOCS-3), a key intracellular negative regulator of several signaling pathways, has been found to be strongly expressed in renal proximal tubules after acute kidney injury. Conditional proximal tubular knockout mice showed accelerated tubule recovery after renal injury, which is associated with increased numbers of infiltrating anti-inflammatory M2 macrophages during the repair phase. These results suggest that upregulated expression of SOCS-3 in damaged proximal tubules inhibits the regeneration process and modulates macrophage phenotype [
Repeated or persistent injury results in cell death and limited tubule regeneration. Recruitment of leukocytes is recognized as a major mediator of tubular cell injury. The initial stages of inflammation are characterized by migration of leukocytes to the activated vascular endothelium, followed by transmigration into the interstitium. During the injury phase, tubular epithelial cells express proinflammatory cytokines and chemokines, which recruit neutrophils, M1 macrophages, and various proinflammatory lymphocyte subsets [
Neutrophils rapidly respond to injury, and adherence of neutrophils to the vascular endothelium is a crucial process in the initiation of damage to ischemic tissues [
Lymphocytes are also important for tubular repair after ischemic injury. B cells migrate to the ischemic kidney and differentiate into plasma cells, which limit tubule regeneration after acute kidney injury. Consistent with these data, B cell-deficient mice are protected from renal ischemic injury [
The role of T cells in the pathogenesis of renal ischemic injury has been analyzed in different mouse models. T cells have a major role in vascular permeability, potentially through production of cytokines during ischemic injury. Gene microarray analysis has shown that production of TNF and interferon-gamma increased in CD3-positive and CD4-positive T cells from the blood and kidney after renal ischemia. In mice lacking T cells expressing CD3, CD4, and CD8, the increase in renal vascular permeability was attenuated after ischemic injury [
Among several lymphocyte subsets with anti-inflammatory properties, regulatory T cells (TREG cells) have been shown to promote tubular repair, possibly through the modulation of proinflammatory cytokine production of other T-cell subsets after acute kidney injury. Increased TREG cell trafficking into the kidneys was observed after ischemic injury. Infusion of TREG cells after initial injury reduced interferon-gamma production by T cell receptor (TCR)
Natural killer T (NKT) cells, a unique subset of T lymphocytes with surface receptors and functional properties shared with conventional T cells and natural killer (NK) cells, have also been shown to be involved in tubular regeneration after injury. Invariant NKT (iNKT) cells possess a conserved invariant TCR together with the NK cell marker NK1.1. Renal ischemic injury leads to an increase in activated CD4+CD69+ cells, and the number of IFN-gamma-producing iNKT cells in the kidney increases significantly after renal ischemia. Blockade of NKT cell activation with an anti-CD1d monoclonal antibody, NKT cell depletion with an anti-NK1.1 monoclonal antibody in wild-type mice, or use of iNKT cell-deficient mice inhibited the accumulation of IFN-gamma-producing neutrophils after renal injury and reduced acute kidney injury [
Macrophages are derived from monocytes in the blood and are named for their role in phagocytosis. In addition, macrophages produce proinflammatory cytokines that can stimulate the activity of other leukocytes. Macrophages are involved in all phases of tissue injury, including regeneration [
Macrophages infiltrate the injured kidney shortly after renal ischemia and this infiltration is mediated by CX3CR1 signaling pathways [
It is thought that proinflammatory stimuli preserve M1 macrophage activity that induces tubular atrophy. Sustained secretion of TNF partially accounts for these antiregenerative effects, which potentially triggers tubular cell injury. Therefore, tubular regeneration can be observed only when M1 macrophages are deactivated [
Dendritic cells, an important link between innate and adaptive immunity, are abundant in the normal mouse kidney. Upon stimulation, dendritic cells develop into a mature cell type characterized by high levels of class II major histocompatibility complex (MCH class II) and costimulatory molecules and low phagocytic capacity. Mature dendritic cells are specialized in T cell activation. However, dendritic cells are also important in the innate immune response by releasing proinflammatory factors, interacting with NKT cells via CD40-CD40L and presenting glycolipids via the CD1d molecule to activate iNKT cells.
Dendritic cells play a role in acute kidney injury. After acute kidney injury, renal dendritic cells produce the proinflammatory cytokines TNF, IL-6, C-C motif chemokine 2, and C-C motif chemokine 5, and depletion of dendritic cells before ischemia substantially reduces the levels of TNF produced by the kidney [
The activation of intrarenal dendritic cells by factors released from dying tubular cells also promotes tubule regeneration. Necrotic tubular cells release a number of intracellular molecules that act as immunostimulatory DAMPs, specifically activate TLR4 on intrarenal dendritic cells, and induce secretion of IL-22 [
The integrity of vascular endothelium is determined by the balance between endothelial turnover and repair. In the kidney, peritubular capillary endothelium is reported to act as a source of factors required for tubular recovery after injury [
Peritubular capillary endothelium also plays an important early role in the inflammatory response to kidney damage by promoting the accumulation of leukocytes. After renal ischemic injury, capillary endothelium is activated, leading to an increase in vascular permeability [
Similar to the results observed in other organs, BMDCs appear in the kidney in response to renal injury. BMDCs significantly contribute to the regeneration of the renal tubular epithelium, differentiate into renal tubules [
Mesenchymal stem cells (MSCs) derived from bone marrow also have been reported to enhance the intrinsic tubular recovery in several acute kidney injury models in a paracrine manner. Treatment with MSCs promoted proximal tubular cell proliferation, reduced apoptosis, and preserved microvascular integrity, leading to the amelioration of renal tissue oxygenation [
Recently, many new concepts in the regeneration process of renal tubules after acute kidney injury have emerged. Growing evidence suggests that the immune system supports the regeneration process of the kidney after injury. Using gene targeting techniques or fate mapping analysis, the involvement of diverse cell populations including tubular cells, inflammatory cells, resident renal endothelial cells, endothelial progenitor cells, and bone marrow-derived cells has been clarified. Critical roles for neutrophils, lymphocytes, and macrophages have been established in mouse models of acute kidney injury. In addition, several studies have reported that complement, TLRs, and numerous cytokines and chemokines are clearly involved in amplifying the immune response to kidney injury. However, the complex interplay between tubular cells and neighboring cells in renal ischemic injury is not yet fully understood. Termination of renal inflammation will enable tubular regeneration by inhibiting tubular cell necrosis or by driving a phenotype switch from proinflammatory to anti-inflammatory immune cells. In this context, specific and selective anti-inflammatory drugs will be required to suppress systemic and local causes of renal inflammation to promote regeneration in patients with kidney diseases. Specific activation of surviving tubular epithelial cells with regenerative capacity also represents an attractive approach to enable sufficient reepithelialization and nephron survival after renal injury. These new concepts will provide important clues for identifying new targets for the development of clinically relevant treatment strategies for acute kidney injury.
The authors declare that there is no conflict of interests regarding the publication of this paper.