Bone marrow-derived cells play a major role in kidney fibrosis via proliferation and differentiation in the infiltrated site

Glucosylceramide synthase modulation ameliorates murine renal pathologies and promotes macrophage effector function in vitro

While significant advances have been made in understanding renal pathophysiology, less is known about the role of glycosphingolipid (GSL) metabolism in driving organ dysfunction. Here, we used a small molecule inhibitor of glucosylceramide synthase to modulate GSL levels in three mouse models of distinct renal pathologies: Alport syndrome (Col4a3 KO), polycystic kidney disease (Nek8jck), and steroid-resistant nephrotic syndrome (Nphs2 cKO). At the tissue level, we identified a core immune-enriched transcriptional signature that was shared across models and enriched in human polycystic kidney disease. Single nuclei analysis identified robust transcriptional changes across multiple kidney cell types, including epithelial and immune lineages. To further explore the role of GSL modulation in macrophage biology, we performed in vitro studies with homeostatic and inflammatory bone marrow-derived macrophages. Cumulatively, this study provides a comprehensive overview of renal dysfunction and the effect of GSL modulation on kidney-derived cells in the setting of renal dysfunction.

Macrophage Ontogeny, Phenotype, and Function in Ischemia Reperfusion-Induced Injury and Repair

AKI is characterized by a sudden, and usually reversible, decline in kidney function. In mice, ischemia-reperfusion injury (IRI) is commonly used to model the pathophysiologic features of clinical AKI. Macrophages are a unifying feature of IRI as they regulate both the initial injury response as well as the long-term outcome following resolution of injury. Initially, macrophages in the kidney take on a proinflammatory phenotype characterized by the production of inflammatory cytokines, such as CCL2 (monocyte chemoattractant protein 1), IL-6, IL-1 β , and TNF- α . Release of these proinflammatory cytokines leads to tissue damage. After resolution of the initial injury, macrophages take on a reparative role, aiding in tissue repair and restoration of kidney function. By contrast, failure to resolve the initial injury results in prolonged inflammatory macrophage accumulation and increased kidney damage, fibrosis, and the eventual development of CKD. Despite the extensive amount of literature that has ascribed these functions to M1/M2 macrophages, a recent paradigm shift in the macrophage field now defines macrophages on the basis of their ontological origin, namely monocyte-derived and tissue-resident macrophages. In this review, we focus on macrophage phenotype and function during IRI-induced injury, repair, and transition to CKD using both the classic (M1/M2) and novel (ontological origin) definition of kidney macrophages.

Development of Cell Therapies for Renal Disease and Regenerative Medicine

The incidence of renal disease is gradually increasing worldwide, and this condition has become a major public health problem because it is a trigger for many other chronic diseases. Cell therapies using multipotent mesenchymal stromal cells, hematopoietic stem cells, macrophages, and other cell types have been used to induce regeneration and provide a cure for acute and chronic kidney disease in experimental models. This review describes the advances in cell therapy protocols applied to acute and chronic kidney injuries and the attempts to apply these treatments in a clinical setting.

A Systematic Review of the Evidence of Hematopoietic Stem Cell Differentiation to Fibroblasts

Fibroblasts have an important role in the maintenance of the extracellular matrix of connective tissues by producing and remodelling extracellular matrix proteins. They are indispensable for physiological processes, and as such also associate with many pathological conditions. In recent years, a number of studies have identified donor-derived fibroblasts in various tissues of bone marrow transplant recipients, while others could not replicate these findings. In this systematic review, we provide an overview of the current literature regarding the differentiation of hematopoietic stem cells into fibroblasts in various tissues. PubMed, Embase, and Web of Science (Core Collection) were systematically searched for original articles concerning fibroblast origin after hematopoietic stem cell transplantation in collaboration with a medical information specialist. Our search found 5421 studies, of which 151 were analysed for full-text analysis by two authors independently, resulting in the inclusion of 104 studies. Only studies in animals and humans, in which at least one marker was used for fibroblast identification, were included. The results were described per organ of fibroblast engraftment. We show that nearly all mouse and human organs show evidence of fibroblasts of hematopoietic stem cell transfer origin. Despite significant heterogeneity in the included studies, most demonstrate a significant presence of fibroblasts of hematopoietic lineage in non-hematopoietic tissues. This presence appears to increase after the occurrence of tissue damage.

Spontaneous Polycystic Kidneys with Chronic Renal Failure in an Aged House Musk Shrew (Suncus murinus)

Polycystic kidney disease is one of the most common inheritable renal diseases, characterized by the formation of multiple fluid-filled renal cysts. This disease is a progressive and unfortunately incurable condition. A case of polycystic kidney with chronic renal failure in house musk shrew (Suncus murinus) is described. At clinical presentation, a 16-month-old Suncus murinus showed weight loss and coarse fur. Regarding the biochemical profile, total protein concentrations increased, resulting in a declined albumin: globulin ratio. Blood urea nitrogen and creatinine concentrations were markedly elevated, indicating the end stage of chronic renal failure. Serum amyloid A levels increased and revealed inflammatory reaction during the cyst formation. Histopathologically, multiple cysts were lined by a single layer of epithelial cells or low cuboidal epithelium. The contents were homogenous eosinophilic materials (mucopolysaccharides or mucoproteins) and these cysts contained abundant macrophages. There were also regeneration and dilatation of renal tubes and interstitial fibrosis. The atrophic glomeruli and glomerular capsules were thickened and hyalinized by dense amorphous mucopolysaccharides. These histopathological findings suggested that the pathogenesis of polycystic kidney disease shared a common mechanistic feature across species.

Monocytes and Macrophages in Kidney Transplantation and Insights from Single Cell RNA-Seq Studies

Single-cell RNA sequencing (scRNA-seq) is a powerful technology that allows for the identification of minority cell types in complex tissues, such as immune cells in the kidney. Previously, gene expression from infrequent cell types was missed using bulk RNA-sequencing methods due to an averaging effect. Additionally, scRNA-seq facilitates assignment of cell origin in a sample, a shortcoming of previous bulk sequencing technologies. Thus, scRNA-seq is ideal to study the immune cell landscape and the alloimmune response in the human kidney transplant. However, there are few studies published to date. Macrophages are known to play an important role in health and disease in the kidney. Furthermore, it is known that macrophages play key roles in rejection of the kidney transplant. The definition, ontogeny, and function of these cells is complex and nomenclature has evolved as new technologies have become available. In this review, an overview is provided of monocyte and macrophage nomenclature, ontogeny, and function, with a specific focus on kidney transplantation, and including novel scRNA-seq findings. scRNA-seq offers an unbiased transcriptional approach to defining macrophages and provides insights into macrophage ontogeny and function not possible with contemporary methods.

Proximal tubule cyclophilin D mediates kidney fibrogenesis in obstructive nephropathy

The proximal tubule (PT) is highly vulnerable to acute injury, including ischemic insult and nephrotoxins, and chronic kidney injury. It has been established that PT injury is a primary cause of the development of chronic kidney disease, but the underlying molecular mechanism remains to be defined. Here, we tested whether PT cyclophilin D (CypD), a mitochondrial matrix protein, is a critical factor to cause kidney fibrosis progression. To define the role of CypD in kidney fibrosis, we used an established mouse model for kidney fibrosis: the unilateral ureteral obstruction (UUO) model in global and PT-specific CypD knockout (KO). Global CypD KO blunted kidney fibrosis progression with inhibition of myofibroblast activation and fibrosis. UUO-induced tubular atrophy was suppressed in kidneys of global CypD KO but not tubular dilation or apoptotic cell death. PT cell cycle arrest was highly increased in wild-type UUO kidneys but was markedly attenuated in global CypD KO UUO kidneys. The number of macrophages and neutrophils was less in UUO kidneys of global CypD KO than those of wild-type kidneys. Proinflammatory and profibrotic factors were all inhibited in global CypD KO. In line with those of global CypD KO, PT-specific CypD KO also blunted kidney fibrosis progression, along with less tubular atrophy, renal parenchymal loss, cell cycle arrest in PT, and inflammation, indicating a critical role for PT CypD in fibrogenesis. Collectively, our data demonstrate that CypD in the PT is a critical factor contributing to kidney fibrosis in UUO, providing a new paradigm for mitochondria-targeted therapeutics of fibrotic diseases.NEW & NOTEWORTHY It has been established that renal proximal tubule (PT) injury is a primary cause of the development of chronic kidney disease, but the underlying molecular mechanism remains to be defined. Here, we show that cyclophilin D, a mitochondrial matrix protein, in the PT causes kidney fibrogenesis in obstructive nephropathy. Our data suggest that targeting PT cyclophilin D could be beneficial to prevent fibrosis progression.

Pharmacological Inhibition of STAT6 Ameliorates Myeloid Fibroblast Activation and Alternative Macrophage Polarization in Renal Fibrosis

A hallmark of chronic kidney disease is renal fibrosis, which can result in progressive loss of kidney function. Currently, there is no effective therapy for renal fibrosis. Therefore, there is an urgent need to identify potential drug targets for renal fibrosis. In this study, we examined the effect of a selective STAT6 inhibitor, AS1517499, on myeloid fibroblast activation, macrophage polarization, and development of renal fibrosis in two experimental murine models. To investigate the effect of STAT6 inhibition on myeloid fibroblast activation, macrophage polarization, and kidney fibrosis, wild-type mice were subjected to unilateral ureteral obstruction or folic acid administration and treated with AS1517499. Mice treated with vehicle were used as control. At the end of experiments, kidneys were harvested for analysis of myeloid fibroblast activation, macrophage polarization, and renal fibrosis and function. Unilateral ureteral obstruction or folic acid administration induced STAT6 activation in interstitial cells of the kidney, which was significantly abolished by AS1517499 treatment. Mice treated with AS1517499 accumulated fewer myeloid fibroblasts and myofibroblasts in the kidney with ureteral obstruction or folic acid nephropathy compared with vehicle-treated mice. Moreover, AS1517499 significantly suppressed M2 macrophage polarization in the injured kidney. Furthermore, AS1517499 markedly reduced the expression levels of extracellular matrix proteins, and development of kidney fibrosis and dysfunction. These findings suggest that AS1517499 inhibits STAT6 activation, suppresses myeloid fibroblast activation, reduces M2 macrophage polarization, attenuates extracellular matrix protein production, and preserves kidney function. Therefore, targeting STAT6 with AS1517499 is a novel therapeutic approach for chronic kidney disease.

The varying roles of macrophages in kidney injury and repair

PURPOSE OF REVIEW

Macrophages play an important role in regulating homeostasis, kidney injury, repair, and tissue fibrogenesis. The present review will discuss recent advances that explore the novel subsets and functions of macrophage in the pathogenesis of kidney damage and hypertension.

RECENT FINDINGS

Macrophages differentiate into a variety of subsets in microenvironment-dependent manner. Although the M1/M2 nomenclature is still applied in considering the pro-inflammatory versus anti-inflammatory effects of macrophages in kidney injury, novel, and accurate macrophage phenotypes are defined by flow cytometric markers and single-cell RNA signatures. Studies exploring the crosstalk between macrophages and other cells are rapidly advancing with the additional recognition of exosome trafficking between cells. Using murine conditional mutants, actions of macrophage can be defined more precisely than in bone marrow transfer models. Some studies revealed the opposing effects of the same protein in renal parenchymal cells and macrophages, highlighting a need for the development of cell-specific immune therapies for translation.

SUMMARY

Macrophage-targeted therapies hold potential for limiting kidney injury and hypertension. To realize this potential, future studies will be required to understand precise mechanisms in macrophage polarization, crosstalk, proliferation, and maturation in the setting of renal disease.

Resident Macrophages in Cystic Kidney Disease

Interstitial inflammation is an important feature of cystic kidney disease. Renal macrophages are the most well-studied inflammatory cell in the kidney, and their involvement in cyst formation has been reported in different animal models and patients with cystic kidney disease. Originally, it was believed that renal macrophages were maintained from a constant supply of bone marrow-derived circulating monocytes, and could be recruited to the kidney in response to local inflammation. However, this idea has been challenged using fate-mapping methods, by showing that at least two distinct developmental origins of macrophages are present in the adult mouse kidney. The first type, infiltrating macrophages, are recruited from circulating monocytes and gradually develop macrophage properties on entering the kidney. The second, resident macrophages, predominantly originate from embryonic precursors, colonize the kidney during its development, and proliferate in situ to maintain their population throughout adulthood. Infiltrating and resident macrophages work together to maintain homeostasis and properly respond to pathologic conditions, such as AKI, cystic kidney disease, or infection. This review will briefly summarize current knowledge of resident macrophages in cystic kidney disease.

Single-cell RNA sequencing uncovers heterogenous transcriptional signatures in macrophages during efferocytosis

Efferocytosis triggers cellular reprogramming, including the induction of mRNA transcripts which encode anti-inflammatory cytokines that promote inflammation resolution. Our current understanding of this transcriptional response is largely informed from analysis of bulk phagocyte populations; however, this precludes the resolution of heterogeneity between individual macrophages and macrophage subsets. Moreover, phagocytes may contain so called "passenger" transcripts that originate from engulfed apoptotic bodies, thus obscuring the true transcriptional reprogramming of the phagocyte. To define the transcriptional diversity during efferocytosis, we utilized single-cell mRNA sequencing after co-cultivating macrophages with apoptotic cells. Importantly, transcriptomic analyses were performed after validating the disappearance of apoptotic cell-derived RNA sequences. Our findings reveal new heterogeneity of the efferocytic response at a single-cell resolution, particularly evident between F4/80+ MHCIILO and F4/80- MHCIIHI macrophage sub-populations. After exposure to apoptotic cells, the F4/80+ MHCIILO subset significantly induced pathways associated with tissue and cellular homeostasis, while the F4/80- MHCIIHI subset downregulated these putative signaling axes. Ablation of a canonical efferocytosis receptor, MerTK, blunted efferocytic signatures and led to the escalation of cell death-associated transcriptional signatures in F4/80+ MHCIILO macrophages. Taken together, our results newly elucidate the heterogenous transcriptional response of single-cell peritoneal macrophages after exposure to apoptotic cells.

Macrophages: versatile players in renal inflammation and fibrosis

Macrophages have important roles in immune surveillance and in the maintenance of kidney homeostasis; their response to renal injury varies enormously depending on the nature and duration of the insult. Macrophages can adopt a variety of phenotypes: at one extreme, M1 pro-inflammatory cells contribute to infection clearance but can also promote renal injury; at the other extreme, M2 anti-inflammatory cells have a reparative phenotype and can contribute to the resolution phase of the response to injury. In addition, bone marrow monocytes can differentiate into myeloid-derived suppressor cells that can regulate T cell immunity in the kidney. However, macrophages can also promote renal fibrosis, a major driver of progression to end-stage renal disease, and the CD206⁺ subset of M2 macrophages is strongly associated with renal fibrosis in both human and experimental diseases. Myofibroblasts are important contributors to renal fibrosis and recent studies provide evidence that macrophages recruited from the bone marrow can transition directly into myofibroblasts within the injured kidney. This process is termed macrophage-to-myofibroblast transition (MMT) and is driven by transforming growth factor-β1 (TGFβ1)-Smad3 signalling via a Src-centric regulatory network. MMT may serve as a key checkpoint for the progression of chronic inflammation into pathogenic fibrosis.

Phenotype and influx kinetics of leukocytes and inflammatory cytokine production in kidney ischemia/reperfusion injury

BACKGROUND AND AIM

Kidney ischemia/reperfusion (IR) injury is characterized by tubular epithelial cell (TEC) death and an inflammatory response involving cytokine production and immune cell infiltration. In various kidney diseases, increased macrophage numbers correlate with injury severity and poor prognosis. However, macrophage plasticity enables a diverse range of functions, including wound healing, making them a key target for novel therapies. This study aimed to comprehensively characterize the changes in myeloid and epithelial cells and the production of cytokines throughout the experimental IR model of acute kidney injury to aid in the identification of targets to promote and enhance kidney regeneration and repair.

METHODS

Flow cytometric analysis of murine unilateral IR injury was used to assess TEC and myeloid cell subpopulations in conjunction with histological analysis and cytokine production at 6 h, 1, 3, 5 and 7 days post IR injury, spanning the initial inflammatory phase and the following reparative phase.

RESULTS

IR injury resulted in a rapid infiltration of Ly6C^high monocytes and neutrophils with a steady rise in F4/80^high MHCII^high macrophages over the injury time. The production of the inflammatory cytokines IL-6, MCP-1 and TNF coincided with an increase in IL-10 production.

CONCLUSION

This characterization will provide a reference point for future studies designed to manipulate immune cell phenotype and function in order to promote endogenous repair of damaged kidneys.

Amelioration of High Fructose-Induced Cardiac Hypertrophy by Naringin

Heart failure is a frequent unfavorable outcome of pathological cardiac hypertrophy. Recent increase in dietary fructose consumption mirrors the rise in prevalence of cardiovascular diseases such as cardiac hypertrophy leading to concerns raised by public health experts. Mitochondria, comprising 30% of cardiomyocyte volume, play a central role in modulating redox-dependent cellular processes such as metabolism and apoptosis. Furthermore, mitochondrial dysfunction is a key cause of pathogenesis of fructose-induced cardiac hypertrophy. Naringin, a major flavanone glycoside in citrus species, has displayed strong antioxidant potential in models of oxidative stress. In this study, we evaluated protective effects of naringin against fructose-induced cardiac hypertrophy and associated mechanisms of action, using in vitro and in vivo models. We found that naringin suppressed mitochondrial ROS production and mitochondrial dysfunction in cardiomyocytes exposed to fructose and consequently reduced cardiomyocyte hypertrophy by regulating AMPK-mTOR signaling axis. Furthermore, naringin counteracted fructose-induced cardiomyocyte apoptosis, and this function of naringin was linked to its ability to inhibit ROS-dependent ATM-mediated p53 signaling. This result was supported by observations in in vivo mouse model of cardiac hypertrophy. These findings indicate a novel role for naringin in protecting against fructose-induced cardiac hypertrophy and suggest unique therapeutic strategies for prevention of cardiovascular diseases.

IDH2 deficiency accelerates skin pigmentation in mice via enhancing melanogenesis

Melanogenesis is a complex biosynthetic pathway regulated by multiple agents, which are involved in the production, transport, and release of melanin. Melanin has diverse roles, including determination of visible skin color and photoprotection. Studies indicate that melanin synthesis is tightly linked to the interaction between melanocytes and keratinocytes. α-melanocyte-stimulating hormone (α-MSH) is known as a trigger that enhances melanin biosynthesis in melanocytes through paracrine effects. Accumulated reactive oxygen species (ROS) in skin affects both keratinocytes and melanocytes by causing DNA damage, which eventually leads to the stimulation of α-MSH production. Mitochondria are one of the main sources of ROS in the skin and play a central role in modulating redox-dependent cellular processes such as metabolism and apoptosis. Therefore, mitochondrial dysfunction may serve as a key for the pathogenesis of skin melanogenesis. Mitochondrial NADP⁺-dependent isocitrate dehydrogenase (IDH2) is a key enzyme that regulates mitochondrial redox balance and reduces oxidative stress-induced cell injury through the generation of NADPH. Downregulation of IDH2 expression resulted in an increase in oxidative DNA damage in mice skin through ROS-dependent ATM-mediated p53 signaling. IDH2 deficiency also promoted pigmentation on the dorsal skin of mice, as evident from the elevated levels of melanin synthesis markers. Furthermore, pretreatment with mitochondria-targeted antioxidant mito-TEMPO alleviated oxidative DNA damage and melanogenesis induced by IDH2 deficiency both in vitro and in vivo. Together, our findings highlight the role of IDH2 in skin melanogenesis in association with mitochondrial ROS and suggest unique therapeutic strategies for the prevention of skin pigmentation.

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Melanogenesis is associated with the production of ROS.

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IDH2 is an essential enzyme in the mitochondrial antioxidant system.

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Downregulation of IDH2 induces ROS-dependent ATM-mediated p53 signaling.

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IDH2 deficiency promotes skin pigmentation.

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mito-TEMPO alleviates melanogenesis caused by IDH2 deficiency.

Cell-intrinsic sphingosine kinase 2 promotes macrophage polarization and renal inflammation in response to unilateral ureteral obstruction

Sphingosine Kinase-2 (Sphk2) is responsible for the production of the bioactive lipid Sphingosine-1 Phosphate, a key regulator of tissue repair. Here we address the in vivo significance of Sphingosine Kinase -2 in renal inflammation/fibrosis in response to unilateral ureteral obstruction using both genetic and pharmacological strategies. Obstructed kidneys of Sphk2^-/- mice showed reduced renal damage and diminished levels of the renal injury markers TGFβ₁ and αSMA when compared to wild type controls. We found a consistently significant increase in anti-inflammatory (M2) macrophages in obstructed Sphk2^-/- kidneys by flow cytometry and a decrease in mRNA levels of the inflammatory cytokines, MCP1, TNFα, CXCL1 and ILβ₁, suggesting an anti-inflammatory bias in the absence of Sphk2. Indeed, metabolic profiling showed that the pro-inflammatory glycolytic pathway is largely inactive in Sphk2^-/- bone marrow-derived macrophages. Furthermore, treatment with the M2-promoting cytokines IL-4 or IL-13 demonstrated that macrophages lacking Sphk2 polarized more efficiently to the M2 phenotype than wild type cells. Bone marrow transplant studies indicated that expression of Sphk2^-/- on either the hematopoietic or parenchymal cells did not fully rescue the pro-healing phenotype, confirming that both infiltrating M2-macrophages and the kidney microenvironment contribute to the damaging Sphk2 effects. Importantly, obstructed kidneys from mice treated with an Sphk2 inhibitor recapitulated findings in the genetic model. These results demonstrate that reducing Sphk2 activity by genetic or pharmacological manipulation markedly decreases inflammatory and fibrotic responses to obstruction, resulting in diminished renal injury and supporting Sphk2 as a novel driver of the pro-inflammatory macrophage phenotype.

The Origins and Functions of Tissue-Resident Macrophages in Kidney Development

The adult kidney hosts tissue-resident macrophages that can cause, prevent, and/or repair renal damage. Most of these macrophages derive from embryonic progenitors that colonize the kidney during its development and proliferate in situ throughout adulthood. Although the precise origins of kidney macrophages remain controversial, recent studies have revealed that embryonic macrophage progenitors initially migrate from the yolk sac, and later from the fetal liver, into the developing kidney. Once in the kidney, tissue-specific transcriptional regulators specify macrophage progenitors into dedicated kidney macrophages. Studies suggest that kidney macrophages facilitate many processes during renal organogenesis, such as branching morphogenesis and the clearance of cellular debris; however, little is known about how the origins and specification of kidney macrophages dictate their function. Here, we review significant new findings about the origins, specification, and developmental functions of kidney macrophages.

Toll-like Receptor 4 Deficiency Reduces Oxidative Stress and Macrophage Mediated Inflammation in Hypertensive Kidney

Oxidative stress and inflammation are integral to hypertension-induced renal injury. A unifying feature for the two components is Toll-like receptors (TLR), which are key regulators of the innate immune system. Recent studies implicate TLR4 activation and oxidative stress in cardiovascular diseases and also as a link between inflammation and hypertension. However, its role in hypertension induced renal injury remains unexplored. In the present study, we investigated whether TLR-4 deficiency reduces Ang-II-induced renal injury and fibrosis by attenuating reactive oxygen species (ROS) production and inflammation. C3H/HeOuJ mice with normal TLR-4 and C3H/HeJ Lps-d with dysfunctional TLR4 (TLR4 deficiency) were treated without or with Ang-II. In response to Ang-II, TLR4 deficient mice had reduced renal resistive index and increased renal cortical blood flow compared to mice with normal TLR4. Further, TLR4 deficiency reduced oxidative stress and increased antioxidant capacity (MnSOD, CuSOD and Catalase activity). TLR4 deficiency was also associated with reduced inflammation (MCP-1, MIP-2, TNF-α, IL-6 and CD68), decreased accumulation of bone marrow-derived fibroblasts and TGF-β expression. Our data suggests that in C3H/HeJ Lps-d mice, deficiency of functional TLR4 reduces oxidative stress and macrophage activation to decrease TGF-β-induced extracellular matrix protein deposition in the kidney in Ang-II induced hypertension.

The Tubulointerstitial Pathophysiology of Progressive Kidney Disease

Accumulating evidence suggests that the central locus for the progression of CKD is the renal proximal tubule. As injured tubular epithelial cells dedifferentiate in attempted repair, they stimulate inflammation and recruit myofibroblasts. At the same time, tissue loss stimulates remnant nephron hypertrophy. Increased tubular transport workload eventually exceeds the energy-generating capacity of the hypertrophied nephrons, leading to anerobic metabolism, acidosis, hypoxia, endoplasmic reticulum stress, and the induction of additional inflammatory and fibrogenic responses. The result is a vicious cycle of injury, misdirected repair, maladaptive responses, and more nephron loss. Therapy that might be advantageous at one phase of this progression pathway could be deleterious during other phases. Thus, interrupting this downward spiral requires narrowly targeted approaches that promote healing and adequate function without generating further entry into the progression cycle.

TGF-β/Smad3 signalling regulates the transition of bone marrow-derived macrophages into myofibroblasts during tissue fibrosis

Myofibroblasts are a main cell-type of collagen-producing cells during tissue fibrosis, but their origins remains controversial. While bone marrow-derived myofibroblasts in renal fibrosis has been reported, the cell origin and mechanisms regulating their transition into myofibroblasts remain undefined. In the present study, cell lineage tracing studies by adoptive transfer of GFP+ or dye-labelled macrophages identified that monocyte/macrophages from bone marrow can give rise to myofibroblasts via the process of macrophage-myofibroblast transition (MMT) in a mouse model of unilateral ureteric obstruction. The MMT cells were a major source of collagen-producing fibroblasts in the fibrosing kidney, accounting for more than 60% of α-SMA+ myofibroblasts. The MMT process occurred predominantly within M2-type macrophages and was regulated by TGF-β/Smad3 signalling as deletion of Smad3 in the bone marrow compartment of GFP+ chimeric mice prevented the M2 macrophage transition into the MMT cells and progressive renal fibrosis. In vitro studies in Smad3 null bone marrow macrophages also showed that Smad3 was required for TGF-β1-induced MMT and collagen production. In conclusion, we have demonstrated that bone marrow-derived fibroblasts originate from the monocyte/macrophage population via a process of MMT. This process contributes to progressive renal tissue fibrosis and is regulated by TGF-β/Smad3 signalling.

Inhibition of microtubule dynamics impedes repair of kidney ischemia/reperfusion injury and increases fibrosis

The microtubule cytoskeleton is composed of α-tubulin and β-tubulin heterodimers, and it serves to regulate the shape, motility, and division of a cell. Post-translational modifications including acetylation are closely associated with the functional aspects of the microtubule, involving in a number of pathological diseases. However, the role of microtubule acetylation in acute kidney injury (AKI) and progression of AKI to chronic kidney disease have yet to be understood. In this study, ischemia/reperfusion (I/R), a major cause of AKI, resulted in deacetylation of the microtubules with a decrease in α-tubulin acetyltransferase 1 (α-TAT1). Paclitaxel (taxol), an agent that stabilizes microtubules by tubulin acetylation, treatment during the recovery phase following I/R injury inhibited tubular cell proliferation, impaired renal functional recovery, and worsened fibrosis. Taxol induced α-tubulin acetylation and post-I/R cell cycle arrest. Taxol aggregated the microtubule in the cytoplasm, resulting in suppression of microtubule dynamics. Our studies have demonstrated for the first time that I/R induced deacetylation of the microtubules, and that inhibition of microtubule dynamics retarded repair of injured tubular epithelial cells leading to an acceleration of fibrosis. This suggests that microtubule dynamics plays an important role in the processes of repair and fibrosis after AKI.

Methionine sulfoxide reductase A deficiency exacerbates progression of kidney fibrosis induced by unilateral ureteral obstruction

Methionine sulfoxide reductase A (MsrA), which stereospecifically catalyzes the reduction of methionine-S-sulfoxide, is an important reactive oxygen species (ROS) scavenger. Tissue fibrosis is a maladaptive repair process following injury, associated with oxidative stress. In this study, we investigated the role of MsrA in unilateral ureteral obstruction (UUO)-induced kidney fibrosis and its underlying mechanisms by using MsrA gene-deleted mice (MsrA(-/-)). MsrA deletion increased collagen deposition in the interstitium and the expression of collagen III and α-smooth muscle actin in the UUO kidneys, indicating that MsrA deficiency exacerbated the progression of UUO-induced kidney fibrosis. UUO reduced the kidney expression of MsrA, MsrB1, and MsrB2, thereby decreasing MsrA and MsrB activity. UUO increased hydrogen peroxide and lipid peroxidation levels and the ratio of oxidized glutathione (GSSG) to total glutathione (GSH) in the kidneys. The UUO-induced elevations in the levels of these oxidative stress markers and leukocyte markers were much higher in the MsrA(-/-) than in the MsrA(+/+) kidneys, the latter suggesting that the exacerbated kidney fibrosis in MsrA(-/-) mice was associated with enhanced inflammatory responses. Collectively, our data suggest that MsrA plays a protective role in the progression of UUO-induced kidney fibrosis via suppression of fibrotic responses caused by oxidative stress and inflammation.

Nox2 is a mediator of ischemia reperfusion injury

Delayed graft function (DGF) results from ischemia-reperfusion injury (IRI) and the generation of reactive oxygen species. We hypothesized that NADPH oxidase 2 (Nox2) plays an important role in pathways leading to DGF. We tested this hypothesis in vitro, in an animal model of IRI using wild type and Nox2(-/-) mice, and in patients with DGF. Under hypoxic conditions, primary tubular epithelial cells from Nox2(-/-) mice had reduced expression of MMP2, vimentin, and HSP27. BUN and creatinine levels were significantly increased in both Nox2(-/-) and WT mice at 4 weeks and 6 months after IRI, suggesting the development of acute and chronic kidney injury. At 4 weeks, kidney fibrosis (α-SMA, picrosirius) and oxidative stress (dihydroethidine, HNE) were significantly reduced in Nox2(-/-) mice, confirming the oxidative and pro-fibrotic effects of Nox2. The molecular signature of IRI using genomic analyses demonstrated a significant decline in hypoxia reponse, oxidative stress, fibrosis, and inflammation in Nox2(-/-) mice. Immunohistochemical analyses of pre-implanatation kidney allograft biopsies from patients with subsequent DGF showed significantly greater Nox2 levels and vascular injury compared with patients without DGF. These studies demonstrate that Nox2 is a modulator of IRI and its absence is associated with reduced inflammation, OS, and fibrosis.

Inhibitory effects of fasudil on renal interstitial fibrosis induced by unilateral ureteral obstruction

Renal fibrosis is the major cause of chronic kidney disease, and the Rho/Rho-associated coiled-coil kinase (ROCK) signaling cascade is involved in the renal fibrotic processes. Several studies have reported that ROCK inhibitors attenuate renal fibrosis. However, the mechanism of this process remains to be fully elucidated. The present study assessed the inhibitory effect of fasudil, a ROCK inhibitor using immunohistochemistry, reverse transcription-quantitative polymerase chain reaction and western blot analyses, in vivo and in vitro, to elucidate the mechanisms underlying renal interstitial fibrosis. In mice induced with unilateral ureteral obstruction (UUO), collagen accumulation, the expression of fibrosis-associated genes and the content of hydroxyproline in the kidney increased 3, 7, and 14 days following UUO. Fasudil attenuated the histological changes, and the production of collagen and extracellular matrix in the UUO kidney. The expression of α-smooth muscle actin (α-SMA) and the transforming growth factor-β (TGFβ)-Smad signaling pathway, and macrophage infiltration were suppressed by fasudil in the kidneys of the UUO mice. The present study also evaluated the role of intrinsic renal cells and infiltrated macrophages using NRK-52E, NRK-49F and RAW264.7 cells. The mRNA and protein expression levels of collagen I and α-SMA increased in the NRK-52E and NRK-49F cells stimulated by TGF-β1. Hydroxyfasudil, a bioactive metabolite of fasudil, attenuated the increase in the mRNA and protein expression levles of α-SMA in the two cell types. However, the reduction in the mRNA expression of collagen I was observed in the NRK-49F cells only. Hydroxyfasudil decreased the mRNA expression of monocyte chemoattractant protein-1 (MCP-1) induced by TGF-β1 in the NRK-52E cells, but not in the NRK-49F cells. In the RAW264.7 cells, the mRNA expression levels of MCP-1, interleukin (IL)-1β, IL-6 and tumor necrosis factor α were increased significantly following lipopolysaccharide stimulation, and were not suppressed by hydroxyfasudil. These data suggested that the inhibition of ROCK activity by fasudil suppressed the transformation of renal intrinsic cells into the myofibroblast cells, and attenuated the infiltration of macrophages, without inhibiting the expression or the activation of cytokine/chemokines, in the progression of renal interstitial fibrosis.

Simultaneous deletion of Bax and Bak is required to prevent apoptosis and interstitial fibrosis in obstructive nephropathy

Proximal tubular injury and apoptosis are key mediators of the development of kidney fibrosis, a hallmark of chronic kidney disease. However, the molecular mechanism by which tubular apoptotic cell death leads to kidney fibrosis is poorly understood. In the present study, we tested the roles of Bcl-2-associated X (Bax) and Bcl-2 antagonist/killer (Bak), two crucial proteins involved in intrinsic apoptotic cell death, in the progression of kidney fibrosis. Mice with proximal tubule-specific Bax deletion, systemic deletion of Bak, and dual deletion of Bax and Bak were subjected to unilateral ureteral obstruction (UUO). Dual deficiency of Bax and Bak inhibited tubular apoptosis and atrophy. Consistent with decreased tubular injury, dual ablation of Bax and Bak suppressed UUO-induced inflammation and kidney fibrosis with decreased tubular cell cycle arrest, expression of fibrogenic and inflammatory cytokines, and oxidative stress in the kidney. Bax or Bak deficiency was insufficient to prevent apoptosis and all other aforementioned malevolent effects, suggesting compensatory mediation by each other in the respective signaling pathways. These data suggest that dual ablation of Bax and Bak in the kidney is required to prevent UUO-induced tubular apoptosis and the consequent kidney inflammation and fibrosis.

The therapeutic potential of a C-X-C chemokine receptor type 4 (CXCR-4) antagonist on hypertrophic scarring in vivo

Effective prevention and treatment of hypertrophic scars (HTSs), a dermal form of fibrosis that frequently occurs following thermal injury to deep dermis, are unsolved significant clinical problems. Previously, we have found that stromal cell-derived factor 1/CXCR4 signaling is up-regulated during wound healing in burn patients and HTS tissue after thermal injury. We hypothesize that blood-borne mononuclear cells are recruited into wound sites after burn injury through the chemokine pathway of stromal cell-derived factor 1 and its receptor CXCR4. Deep dermal injuries to the skin are often accompanied by prolonged inflammation, which leads to chemotaxis of mononuclear cells into the wounds by chemokine signaling where fibroblast activation occurs and ultimately HTS are formed. Blocking mononuclear cell recruitment and fibroblast activation, CXCR4 antagonism is expected to reduce or minimize scar formation. In this study, the inhibitory effect of CXCR4 antagonist CTCE-9908 on dermal fibrosis was determined in vivo using a human HTS-like nude mouse model, in which split-thickness human skin is transplanted into full-thickness dorsal excisional wounds in athymic mice, where these wounds subsequently develop fibrotic scars that resemble human HTS as previously described. CTCE-9908 significantly attenuated scar formation and contraction, reduced the accumulation of macrophages and myofibroblasts, enhanced the remodeling of collagen fibers, and down-regulated the gene and protein expression of fibrotic growth factors in the human skin tissues. These findings support the potential therapeutic value of CXCR4 antagonist in dermal fibrosis and possibly other fibroproliferative disorders.

Origin of myofibroblasts and cellular events triggering fibrosis

Renal fibrosis is a major hallmark of chronic kidney disease that is considered to be a common end point of various types of renal disease. To date, the biological meaning of fibrosis during the progression of chronic kidney diseases is unknown and possibly depends on the cell type contributing to extracellular matrix production. During the past decade, the origin of myofibroblasts in the kidney has been intensively investigated. Determining the origins of renal myofibroblasts is important because these might account for the heterogeneous characteristics and behaviors of myofibroblasts. Current data strongly suggest that collagen-producing myofibroblasts in the kidney can be derived from various cellular sources. Resident renal fibroblasts and cells of hematopoietic origin migrating into the kidney seem to be the most important ancestors of myofibroblasts. It is likely that both cell types communicate with each other and also with other cell types in the kidney. In this review, we will discuss the current knowledge on the origin of scar-producing myofibroblasts and cellular events triggering fibrosis.

Regulator of G protein signaling 2 (RGS2) deficiency accelerates the progression of kidney fibrosis

The regulator of G protein signaling 2 (RGS2) is a potent negative regulator of Gq protein signals including the angiotensin II (AngII)/AngII receptor signal, which plays a critical role in the progression of fibrosis. However, the role of RGS2 on the progression of kidney fibrosis has not been assessed. Here, we investigated the role of RGS2 in kidney fibrosis induced by unilateral ureteral obstruction (UUO) in mice. UUO resulted in increased expression of RGS2 mRNA and protein in the kidney along with increases of AngII and its type 1 receptor (AT1R) signaling and fibrosis. Furthermore, UUO increased the levels of F4/80, Ly6G, myeloperoxidase, and CXCR4 in the kidneys. RGS2 deficiency significantly enhanced these changes in the kidney. RGS2 deletion in the bone marrow-derived cells by transplanting the bone marrow of RGS2 knock-out mice into wild type mice enhanced UUO-induced kidney fibrosis. Overexpression of RGS2 in HEK293 cells, a human embryonic kidney cell line, and RAW264.7 cells, a monocyte/macrophage line, inhibited the AngII-induced activation of ERK and increase of CXCR4 expression. These findings provide the first evidence that RGS2 negatively regulates the progression of kidney fibrosis following UUO, likely by suppressing fibrogenic and inflammatory responses through the inhibition of AngII/AT1R signaling.

Recruitment and subsequent proliferation of bone marrow-derived cells in the postischemic kidney are important to the progression of fibrosis

Acute kidney injury (AKI) is an independent risk factor of the development of chronic kidney disease. Kidney fibrosis is a typical feature of chronic kidney disease and is characterized as an expansion of the interstitium due to increases in extracellular matrix molecules and interstitial cells caused by accumulations of extrarenal cells and by the proliferation or differentiation of intrarenal cells. However, the role of bone marrow-derived cells (BMDCs) in AKI-induced kidney fibrosis remains to be defined. Here, we investigated the role of BMDCs in kidney fibrosis after ischemia-reperfusion injury (IRI)-induced AKI in green fluorescent protein (GFP)-expressing bone marrow chimeric mice. IRI resulted in severe fibrotic changes in kidney tissues and dramatically increased interstitial cell numbers. Furthermore, GFP-expressing BMDCs accounted for >80% of interstitial cells in fibrotic kidneys. Interstitial GFP-expressing cells expressed α-smooth muscle actin (a myofibroblast marker), fibroblast-specific protein-1 (a fibroblast marker), collagen type III, and F4/80 (a macrophage marker). Over 20% of interstitial cells were bromodeoxyuridine-incorporating (proliferating) cells, and of these, 80% cells were GFP-expressing BMDCs. Daily treatment of IRI mice with apocynin (a NADPH oxidase inhibitor that functions as an antioxidant) from the day after surgery until euthanization slightly inhibited these changes with a small reduction of fibrosis. Taken together, our findings show that BMDCs make a major contribution to IRI-induced fibrosis due to their infiltration, subsequent differentiation, and proliferation in injured kidneys, suggesting that BMDCs be considered an important target for the treatment of kidney fibrosis.

Dual role of acetaminophen in promoting hepatoma cell apoptosis and kidney fibroblast proliferation

Acetaminophen (APAP), is a safe analgesic and antipyretic drug at therapeutic dose, and is widely used in the clinic. However, high doses of APAP can induce hepatotoxicity and nephrotoxicity. Most studies have focused on high‑dose APAP‑induced acute liver and kidney injury. So far, few studies have investigated the effects of the therapeutic dose (1/10 of the high dose) or of the low dose (1/100 of the high dose) of APAP on the cells. The aim of this study was to investigate the cellular effects of therapeutic- or low‑dose APAP treatment on hepatoma cells and kidney fibroblasts. As expected, high‑dose APAP treatment inhibited while therapeutic and low‑dose treatment did not inhibit cell survival of kidney tubular epithelial cells. In addition, therapeutic-dose treatment induced an increase in the H2O2 level, activated the caspase‑9/‑3 cascade, and induced cell apoptosis of hepatoma cells. Notably, APAP promoted fibroblast proliferation, even at low doses. This study demonstrates that different cellular effects are exerted upon treatment with different APAP concentrations. Our results indicate that treatment with the therapeutic dose of APAP may exert an antitumor activity on hepatoma, while low‑dose treatment may be harmful for patients with fibrosis, since it may cause proliferation of fibroblasts.

Involvement of hydrogen sulfide and homocysteine transsulfuration pathway in the progression of kidney fibrosis after ureteral obstruction

Hydrogen sulfide (H2S) produced by cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE) in the transsulfuration pathway of homocysteine plays a number of pathophysiological roles. Hyperhomocysteinemia is involved in kidney fibrosis. However, the role of H2S in kidney fibrosis remains to be defined. Here, we investigated the role of H2S and its acting mechanism in unilateral ureteral obstruction (UO)-induced kidney fibrosis in mice. UO decreased expressions of CBS and CSE in the kidney with decrease of H2S concentration. Treatment with sodium hydrogen sulfide (NaHS, a H2S producer) during UO reduced UO-induced oxidative stress with preservations of catalase, copper-zinc superoxide dismutase (CuZnSOD), and manganese superoxide dismutase (MnSOD) expression, and glutathione level. In addition, NaHS mitigated decreases of CBS and CSE expressions, and H2S concentration in the kidney. NaHS treatment attenuated UO-induced increases in levels of TGF-β1, activated Smad3, and activated NF-κB. This study provided the first evidence of involvement of the transsulfuration pathway and H2S in UO-induced kidney fibrosis, suggesting that H2S and its transsulfuration pathway may be a potential target for development of therapeutics for fibrosis-related diseases.

Defect in Runx2 gene accelerates ureteral obstruction-induced kidney fibrosis via increased TGF-β signaling pathway

Runt-related transcription factor 2 (Runx2) plays an important role in bone formation and de novo synthesis of proteins, including type 1 collagen. Runx2 has a potent effect on signaling of transforming growth factor (TGF)-β and vice versa, implicating its significant role in fibrosis. Chronic renal failure comprises fibrosis, characterized as an increase in TGF-β signaling, and expression of α-smooth muscle actin (α-SMA), and extracellular matrix proteins. Here, we evaluated the role of Runx2 in ureteral obstruction (UO)-induced kidney fibrosis using mice whose Runx2 gene expression is genetically down-regulated. UO caused tubular atrophy and dilation, expansion of interstitium, and increased expression of collagens and α-SMA with a concomitant decrease in expression of Runx2. Deficiency of Runx2 gene (Runx2(+/-) mice) showed higher expression of collagens and α-SMA in the kidney following UO compared to wild type (Runx2(+/+)) mice. UO-induced activation of TGF-β signaling was higher in the Runx2(+/-) kidney than Runx2(+/+) kidney, suggesting an inhibitory effect of Runx2 on TGF-β signaling in kidney fibrosis. Besides, overexpression of the Runx2 gene using an adenoviral vector in kidney tubule cells resulted in attenuated TGF-β-induced Smad3 phosphorylation and expressions of α-SMA and collagen I. Furthermore, Runx2 gene deficient mouse embryonic fibroblasts induced greater activation of Smad3 and expression of α-SMA in response to TGF-β. Collectively, Runx2 plays a protective role in UO-induced kidney fibrosis by inhibition of TGF-β signaling, suggesting Runx2 as a novel target for protection against fibrosis-related diseases such as chronic renal failure.