Pericytes and perivascular fibroblasts are the primary source of collagen-producing cells in obstructive fibrosis of the kidney

Mesenchymal Regulation of the Microvascular Niche in Chronic Lung Diseases

The adult lung is comprised of diverse vascular, epithelial, and mesenchymal progenitor cell populations that reside in distinct niches. Mesenchymal progenitor cells (MPCs) are intimately associated with both the epithelium and the vasculature, and new evidence is emerging to describe their functional roles in these niches. Also emerging, following lineage analysis and single cell sequencing, is a new understanding of the diversity of mesenchymal cell subpopulations in the lung. However, several gaps in knowledge remain, including how newly defined MPC lineages interact with cells in the vascular niche and the role of adult lung MPCs during lung repair and regeneration following injury, especially in chronic lung diseases. Here we summarize how the current evidence on MPC regulation of the microvasculature during tissue homeostasis and injury may inform studies on understanding their role in chronic lung disease pathogenesis or repair. © 2019 American Physiological Society. Compr Physiol 9:1431-1441, 2019.

Defective angiogenesis in CXCL12 mutant mice impairs skeletal muscle regeneration

BACKGROUND

During muscle regeneration, the chemokine CXCL12 (SDF-1) and the synthesis of some specific heparan sulfates (HS) have been shown to be critical. CXCL12 activity has been shown to be heavily influenced by its binding to extracellular glycosaminoglycans (GAG) by modulating its presentation to its receptors and by generating haptotactic gradients. Although CXCL12 has been implicated in several phases of tissue repair, the influence of GAG binding under HS influencing conditions such as acute tissue destruction remains understudied.

METHODS

To investigate the role of the CXCL12/HS proteoglycan interactions in the pathophysiology of muscle regeneration, we performed two models of muscle injuries (notexin and freeze injury) in mutant CXCL12^Gagtm/Gagtm mice, where the CXCL12 gene having been selectively mutated in critical binding sites of CXCL12 to interact with HS. Histological, cytometric, functional transcriptomic, and ultrastructure analysis focusing on the satellite cell behavior and the vessels were conducted on muscles before and after injuries. Unless specified, statistical analysis was performed with the Mann-Whitney test.

RESULTS

We showed that despite normal histology of the resting muscle and normal muscle stem cell behavior in the mutant mice, endothelial cells displayed an increase in the angiogenic response in resting muscle despite the downregulated transcriptomic changes induced by the CXCL12 mutation. The regenerative capacity of the CXCL12-mutated mice was only delayed after a notexin injury, but a severe damage by freeze injury revealed a persistent defect in the muscle regeneration of CXCL12 mutant mice associated with vascular defect and fibroadipose deposition with persistent immune cell infiltration.

CONCLUSION

The present study shows that CXCL12 is crucial for proper muscle regeneration. We highlight that this homing molecule could play an important role in drastic muscle injuries and that the regeneration defect could be due to an impairment of angiogenesis, associated with a long-lasting fibro-adipogenic scar.

Blockade of the Adenosine A3 Receptor Attenuates Caspase 1 Activation in Renal Tubule Epithelial Cells and Decreases Interleukins IL-1β and IL-18 in Diabetic Rats

Diabetic nephropathy (DN) is the main cause of end-stage renal disease, which remains incurable. The progression of DN is associated with progressive and irreversible renal fibrosis and also high levels of adenosine. Our aim was to evaluate the effects of ADORA3 antagonism on renal injury in streptozotocin-induced diabetic rats. An ADORA3 antagonist that was administered in diabetic rats greatly inhibited the levels of inflammatory interleukins IL-1β and IL-18, meanwhile when adenosine deaminase was administered, there was a non-selective attenuation of the inflammatory mediators IL-1β, IL-18, IL-6, and induction of IL-10. The ADORA3 antagonist attenuated the high glucose-induced activation of caspase 1 in HK2 cells in vitro. Additionally, ADORA3 antagonisms blocked the increase in caspase 1 and the nuclear localization of NFκB in the renal tubular epithelium of diabetic rats, both events that are involved in regulating the production and activation of IL-1β and IL-18. The effects of the A3 receptor antagonist resulted in the attenuation of kidney injury, as evidenced by decreased levels of the pro-fibrotic marker α-SMA at histological levels and the restoration of proteinuria in diabetic rats. We conclude that ADORA3 antagonism represents a potential therapeutic target that mechanistically works through the selective blockade of the NLRP3 inflammasome.

Twist1 in Infiltrating Macrophages Attenuates Kidney Fibrosis via Matrix Metallopeptidase 13-Mediated Matrix Degradation

BACKGROUND

Following an acute insult, macrophages regulate renal fibrogenesis through the release of various factors that either encourage the synthesis of extracellular matrix synthesis or the degradation of matrix via endocytosis, proteolysis, or both. However, the roles of infiltrating versus resident myeloid cells in these opposing processes require elucidation. The transcription factor Twist1 controls diverse essential cellular functions through induction of several downstream targets, including matrix metalloproteinases (MMPs). In macrophages, Twist1 can influence patterns of cytokine generation, but the role of macrophage Twist1 in renal fibrogenesis remains undefined.

METHODS

To study Twist1 functions in different macrophage subsets during kidney scar formation, we used two conditional mutant mouse models in which Twist1 was selectively ablated either in infiltrating, inflammatory macrophages or in resident tissue macrophages. We assessed fibrosis-related parameters, matrix metallopeptidase 13 (MMP13, or collagen 3, which catalyzes collagen degradation), inflammatory cytokines, and other factors in these Twist1-deficient mice compared with wild-type controls after subjecting the animals to unilateral ureteral obstruction. We also treated wild-type and Twist1-deficient mice with an MMP13 inhibitor after unilateral ureteral obstruction.

RESULTS

Twist1 in infiltrating inflammatory macrophages but not in resident macrophages limited kidney fibrosis after ureteral obstruction by driving extracellular matrix degradation. Moreover, deletion of Twist1 in infiltrating macrophages attenuated the expression of MMP13 in CD11b+Ly6Clo myeloid cells. Inhibition of MMP13 abrogated the protection from renal fibrosis afforded by macrophage Twist1.

CONCLUSIONS

Twist1 in infiltrating myeloid cells mitigates interstitial matrix accumulation in the injured kidney by promoting MMP13 production, which drives extracellular matrix degradation. These data highlight the complex cell-specific actions of Twist1 in the pathogenesis of kidney fibrosis.

Renal Inflammation and Fibrosis: A Double-edged Sword

Renal tissue injury initiates inflammatory and fibrotic processes that occur to promote regeneration and repair. After renal injury, damaged tissue releases cytokines and chemokines, which stimulate activation and infiltration of inflammatory cells to the kidney. Normal tissue repair processes occur simultaneously with activation of myofibroblasts, collagen deposition, and wound healing responses; however, prolonged activation of pro-inflammatory and pro-fibrotic cell types causes excess extracellular matrix deposition. This review focuses on the physiological and pathophysiological roles of specialized cell types, cytokines/chemokines, and growth factors, and their implications in recovery or exacerbation of acute kidney injury.

Fibrocytes and fibroblasts-Where are we now

Fibroblasts are considered major contributors to the process of fibrogenesis and the progression of matrix deposition and tissue distortion in fibrotic diseases such as Pulmonary Fibrosis. Recent discovery of the fibrocyte, a circulating possible precursor cell to the tissue fibroblast in fibrosis, has raised issues regarding the characterization of fibrocytes with respect to their morphology, growth characteristics in vitro, their biological role in vivo and their potential utility as a biomarker and/ or treatment target in various human diseases. Characterization studies of the fibrocyte continue as does emerging conflicting data concerning the relationship to or with the lung fibroblast. The source of signals that direct the traffic of these cells, as well as their response to therapeutic intervention with newly available drugs, bring new insights to the understanding of this cell type. The identification of exosomes from fibrocytes that can affect resident fibroblast activities suggest mechanisms of their influence on pathogenesis. Moreover, interesting comparisons with other pathologies are emerging involving the influence of circulating mesenchymal precursor cells on tissue responses.

Pericytes in Metastasis

Pericytes have long been known to contribute indirectly to tumour growth by regulating angiogenesis. Thus, remodelling tumour blood vessels to maintain blood supply is critical for continued tumour growth. A role for pericytes in restricting leakage of tumour cells through blood vessels has also become evident given that adequate pericyte coverage of these blood vessels is critical for maintaining vascular permeability. Interestingly, the relocation of pericytes from blood vessels to the tumour microenvironment results in the emergence of different properties in these cells that actively promote tumour growth and metastasis-functions not associated with their well-studied role in vascular stability and permeability. These form the focus of this review.

Pericytes in the Lung

The lung has numerous roles, including gas exchange, immune surveillance, and barrier function. Being a highly vascularized organ, the lung receives dual blood supply from both the pulmonary and bronchial circulation. Therefore, pericytes likely play a prominent role in lung physiology given their localization in the perivascular niche. New genetic approaches have increased our understanding of the origin and the diverse functions of lung pericytes. Lung pericytes are myofibroblast progenitors, contributing to development of fibrosis in mouse models. Lung pericytes are also capable of responding to danger signals and amplify the inflammatory response through elaboration of cytokines and adhesion molecules. In this chapter, we describe the molecular, anatomical, and phenotypical characterization of lung pericytes. We further highlight their potential roles in the pathogenesis of lung diseases including pulmonary fibrosis, asthma, and pulmonary hypertension. Finally, current gaps in knowledge and areas of ongoing investigation in lung pericyte biology are also discussed.

Monocyte-derived fibrocytes elimination had little contribution on liver fibrosis

BACKGROUND

Monocyte-derived fibrocytes play an important role in the progression of fibrosis in the skin, lungs, heart and kidney. However, the contribution of fibrocytes to liver fibrosis is unclear. The aim of this study was to investigate whether fibrocytes contributed to fibrosis progression in the livers of carbon tetrachloride (CCl₄)-treated mice.

METHODS

C57BL/6J mice were divided into 4 groups: normal control group, CCl₄-treated group, CCl₄ + control liposome-treated group, and CCl₄ + clodronate liposome-treated group. For the elimination of systemic monocyte and monocyte-derived fibrocyte, one group was treated with clodronate liposome, and another group with control liposome as a control. After 4 weeks of treatment, hepatic mononuclear cells were subjected to immunofluorescent (IF) staining and fluorescence-activated cell sorter (FACS) analysis to detect fibrocytes. Measurement of collagen-positive Sirius red stained area and collagen-I mRNA expression in the liver were performed to evaluate the degree of liver fibrosis quantitatively.

RESULTS

In the liver of the CCl₄-treated and CCl₄ + control liposome-treated groups, the number of fibrocytes, the area positive for Sirius red staining and collagen-I mRNA expression significantly increased compared with those in the normal control group. In the liver of the CCl₄ + clodronate liposome-treated group, few fibrocytes was observed as in the normal control group, but Sirius red staining positive area and collagen-I mRNA expression were increased and equivalent to the CCl₄-treated and CCl₄ + control liposome-treated groups.

CONCLUSION

Monocyte-derived fibrocytes play a minimal role in CCl₄-induced liver fibrosis. Cells other than fibrocytes such as hepatic stellate cells play a central role in liver fibrosis.

LPS-Binding Protein Modulates Acute Renal Fibrosis by Inducing Pericyte-to-Myofibroblast Trans-Differentiation through TLR-4 Signaling

During sepsis, the increased synthesis of circulating lipopolysaccharide (LPS)-binding protein (LBP) activates LPS/TLR4 signaling in renal resident cells, leading to acute kidney injury (AKI). Pericytes are the major source of myofibroblasts during chronic kidney disease (CKD), but their involvement in AKI is poorly understood. Here, we investigate the occurrence of pericyte-to-myofibroblast trans-differentiation (PMT) in sepsis-induced AKI. In a swine model of sepsis-induced AKI, PMT was detected within 9 h from LPS injection, as evaluated by the reduction of physiologic PDGFRβ expression and the dysfunctional α-SMA increase in peritubular pericytes. The therapeutic intervention by citrate-based coupled plasma filtration adsorption (CPFA) significantly reduced LBP, TGF-β, and endothelin-1 (ET-1) serum levels, and furthermore preserved PDGFRβ and decreased α-SMA expression in renal biopsies. In vitro, both LPS and septic sera led to PMT with a significant increase in Collagen I synthesis and α-SMA reorganization in contractile fibers by both SMAD2/3-dependent and -independent TGF-β signaling. Interestingly, the removal of LBP from septic plasma inhibited PMT. Finally, LPS-stimulated pericytes secreted LBP and TGF-β and underwent PMT also upon TGF-β receptor-blocking, indicating the crucial pro-fibrotic role of TLR4 signaling. Our data demonstrate that the selective removal of LBP may represent a therapeutic option to prevent PMT and the development of acute renal fibrosis in sepsis-induced AKI.

Biomarkers of Chronic Renal Tubulointerstitial Injury

Progression of renal parenchyma injury is characterized by increasing interstitial fibrosis and tubular atrophy, irrespective of the cause. Histopathologic assessment of renal tissue obtained by biopsy remains the gold standard for determining the presence and extent of tubulointerstitial scarring. Discovery of robust non-invasive means for capturing a snapshot and for longitudinal monitoring of parenchymal deterioration has been the focus of intense multimodal effort by investigators within the renal community and beyond. Research in this field has included the use of in vitro and in vivo experimental models and has fostered the development and evaluation of tissue and biofluid assays for novel analytes with potential translation to the diagnosis and prognosis of kidney disease. Here, we examine recent advances in the search of "biomarkers" for detection of renal tubulointerstitial scarring and prediction of renal outcome in human renal disease.

Linking myofibroblast generation and microvascular alteration: The role of CD248 from pathogenesis to therapeutic target (Review)

Fibrosis is characterized by excessive extracellular matrix (ECM) deposition, and is the pathological outcome of tissue injury in a number of disorders. Accumulation of the ECM may disrupt the structure and function of native tissues and organs, including the lungs, heart, liver and skin, resulting in significant morbidity and mortality. On this basis, multiple lines of evidence have focused on the molecular pathways and cellular mechanisms involved in fibrosis, which has led to the development of novel antifibrotic therapies. CD248 is one of several proteins identified to be localized to the stromal compartment in cancers and fibroproliferative disease, and may serve a key role in myofibroblast generation and accumulation. Numerous studies have supported the contribution of CD248 to tumour growth and fibrosis, stimulating interest in this molecule as a therapeutic target. In addition, it has been revealed that CD248 may be involved in pathological angiogenesis. The present review describes the current understanding of the structure and function of CD248 during angiogenesis and fibrosis, supporting the hypothesis that blocking CD248 signalling may prevent both myofibroblast generation and microvascular alterations during tissue fibrosis.

Pericyte loss leads to circulatory failure and pleiotrophin depletion causing neuron loss

Pericytes are positioned between brain capillary endothelial cells, astrocytes and neurons. They degenerate in multiple neurological disorders. However, their role in the pathogenesis of these disorders remains debatable. Here we generate an inducible pericyte-specific Cre line and cross pericyte-specific Cre mice with iDTR mice carrying Cre-dependent human diphtheria toxin receptor. After pericyte ablation with diphtheria toxin, mice showed acute blood-brain barrier breakdown, severe loss of blood flow, and a rapid neuron loss that was associated with loss of pericyte-derived pleiotrophin (PTN), a neurotrophic growth factor. Intracerebroventricular PTN infusions prevented neuron loss in pericyte-ablated mice despite persistent circulatory changes. Silencing of pericyte-derived Ptn rendered neurons vulnerable to ischemic and excitotoxic injury. Our data demonstrate a rapid neurodegeneration cascade that links pericyte loss to acute circulatory collapse and loss of PTN neurotrophic support. These findings may have implications for the pathogenesis and treatment of neurological disorders that are associated with pericyte loss and/or neurovascular dysfunction.

Infiltrating CCR2+ monocytes and their progenies, fibrocytes, contribute to colon fibrosis by inhibiting collagen degradation through the production of TIMP-1

Intestinal fibrosis is a serious complication in inflammatory bowel disease (IBD). Despite the remarkable success of recent anti-inflammatory therapies for IBD, incidence of intestinal fibrosis and need for bowel resection have not significantly changed. To clarify the contribution of haematopoietic-derived cells in intestinal fibrosis, we prepared bone marrow (BM) chimeric mice (chimeras), which were reconstituted with BM cells derived from enhanced green fluorescent protein (EGFP)-transgenic mice or CC chemokine receptor 2 (CCR2)-deficient mice. After 2 months of transplantation, BM chimeras were treated with azoxymethane/dextran sodium sulphate. During chronic inflammation, CCR2⁺ BM-derived monocyte and fibrocyte infiltration into the colon and CC chemokine ligand 2 production increased, leading to colon fibrosis in EGFP BM chimeras. In CCR2-deficient BM chimeras, monocyte and fibrocyte numbers in the colonic lamina propria significantly decreased, and colon fibrosis was attenuated. In colon tissue, mRNA expression of tissue inhibitor of metalloproteinase (TIMP)-1 but not of collagen I, transforming growth factor-β1 or matrix metalloproteinases was significantly different between the two chimeras. CCR2⁺ monocytes and fibrocytes showed high Timp1 mRNA expression. Our results suggest that infiltrating CCR2⁺ monocytes and their progenies, fibrocytes, promote colon fibrosis by inhibiting collagen degradation through TIMP-1 production.

Single-Cell Receptor Quantification of an In Vitro Coculture Angiogenesis Model Reveals VEGFR, NRP1, Tie2, and PDGFR Regulation and Endothelial Heterogeneity

Angiogenesis, the formation of new blood vessels from pre-existing ones, is essential for both normal development and numerous pathologies. Systems biology has offered a unique approach to study angiogenesis by profiling tyrosine kinase receptors (RTKs) that regulate angiogenic processes and computationally modeling RTK signaling pathways. Historically, this systems biology approach has been applied on ex vivo angiogenesis assays, however, these assays are difficult to quantify and limited in their potential of temporal analysis. In this study, we adopted a simple two-dimensional angiogenesis assay comprised of human umbilical vein endothelial cells (HUVECs) and human dermal fibroblasts (HDFs) and examined temporal dynamics of a panel of six RTKs and cell heterogeneity up to 17 days. We observed ~2700 VEGFR1 (vascular endothelial growth factor receptor 1) per cell on 24-h-old cocultured HDF plasma membranes, which do not express VEGFR when cultured alone. We observed 4000–8100 VEGFR2 per cell on cocultured HUVEC plasma membranes throughout endothelial tube formation. We showed steady increase of platelet-derived growth factor receptors (PDGFRs) on cocultured HDF plasma membranes, and more interestingly, 1900–2900 PDGFRβ per plasma membrane were found on HUVECs within the first six hours of coculturing. These quantitative findings will offer us insights into molecular regulation during angiogenesis and help assess in vitro tube formation models and their physiological relevance.

Vascular Dysfunction Induced by Mercury Exposure

Methylmercury (MeHg) causes severe damage to the central nervous system, and there is increasing evidence of the association between MeHg exposure and vascular dysfunction, hemorrhage, and edema in the brain, but not in other organs of patients with acute MeHg intoxication. These observations suggest that MeHg possibly causes blood-brain barrier (BBB) damage. MeHg penetrates the BBB into the brain parenchyma via active transport systems, mainly the l-type amino acid transporter 1, on endothelial cell membranes. Recently, exposure to mercury has significantly increased. Numerous reports suggest that long-term low-level MeHg exposure can impair endothelial function and increase the risks of cardiovascular disease. The most widely reported mechanism of MeHg toxicity is oxidative stress and related pathways, such as neuroinflammation. BBB dysfunction has been suggested by both in vitro and in vivo models of MeHg intoxication. Therapy targeted at both maintaining the BBB and suppressing oxidative stress may represent a promising therapeutic strategy for MeHg intoxication. This paper reviews studies on the relationship between MeHg exposure and vascular dysfunction, with a special emphasis on the BBB.

Shenkang injection, a modern preparation of Chinese patent medicine, diminishes tubulointerstitial fibrosis in obstructive nephropathy via targeting pericyte-myofibroblast transition

Shenkang injection (SKI), a modern preparation of Chinese patent medicine, has been widely applied to clinical therapy in the chronic renal failure patients. However, it remains elusive whether SKI can ameliorate tubulointerstitial fibrosis (TIF) in vivo. Recently, pericyte-myofibroblast transition (PMT) plays an important role in the pathogenesis of TIF in obstructive nephropathy (ON). This report thus aims to demonstrate the therapeutic mechanisms of the dose-effects of SKI on TIF by targeting PMT and its signaling activation, compared with imatinib. All rats were divided into 5 groups, the sham-operated group, the vehicle-intervened group, the high dose of SKI-treated group, the low dose of SKI-treated group and the imatinib-treated group. The ON model rats were induced by unilateral ureteral obstruction (UUO), and administered with either the different doses of SKI or imatinib before and after modeling and for a period of 4 weeks. The changes before and after drugs intervention in TIF and PMT markers, and in platelet-derived growth factor receptor (PDGFR) and vascular endothelial growth factor receptor (VEGFR) signaling pathways activation in the kidneys were analyzed, respectively. As a result, PMT trigger was persistently accompanied with TIF exasperation in the obstructed kidneys after UUO, and that SKI definitely targeted PMT and significantly diminished TIF in vivo. In addition, the high dose of SKI, superior to imatinib, specifically blocked PMT through inhibiting the activation of PDGFR and VEGFR signaling in the kidneys of the UUO model rats. Overall, these findings may further suggest that targeting PMT can provide new strategies for ON treatment.

Microvascular bioengineering: a focus on pericytes

Capillaries within the microcirculation are essential for oxygen delivery and nutrient/waste exchange, among other critical functions. Microvascular bioengineering approaches have sought to recapitulate many key features of these capillary networks, with an increasing appreciation for the necessity of incorporating vascular pericytes. Here, we briefly review established and more recent insights into important aspects of pericyte identification and function within the microvasculature. We then consider the importance of including vascular pericytes in various bioengineered microvessel platforms including 3D culturing and microfluidic systems. We also discuss how vascular pericytes are a vital component in the construction of computational models that simulate microcirculation phenomena including angiogenesis, microvascular biomechanics, and kinetics of exchange across the vessel wall. In reviewing these topics, we highlight the notion that incorporating pericytes into microvascular bioengineering applications will increase their utility and accelerate the translation of basic discoveries to clinical solutions for vascular-related pathologies.

Kidney organoids in translational medicine: Disease modeling and regenerative medicine

The kidney is one of the most complex organs composed of multiple cell types, functioning to maintain homeostasis by means of the filtering of metabolic wastes, balancing of blood electrolytes, and adjustment of blood pressure. Recent advances in 3D culture technologies in vitro enabled the generation of "organoids" which mimic the structure and function of in vivo organs. Organoid technology has allowed for new insights into human organ development and human pathophysiology, with great potential for translational research. Increasing evidence shows that kidney organoids are a useful platform for disease modeling of genetic kidney diseases when derived from genetic patient iPSCs and/or CRISPR-mutated stem cells. Although single cell RNA-seq studies highlight the technical difficulties underlying kidney organoid generation reproducibility and variation in differentiation protocols, kidney organoids still hold great potential to understand kidney pathophysiology as applied to kidney injury and fibrosis. In this review, we summarize various studies of kidney organoids, disease modeling, genome-editing, and bioengineering, and additionally discuss the potential of and current challenges to kidney organoid research.

Putative endothelial progenitor cells do not promote vascular repair but attenuate pericyte-myofibroblast transition in UUO-induced renal fibrosis

Background

Putative endothelial progenitor cells (pEPCs) have been confirmed to participate in alleviation of renal fibrosis in several ischaemic diseases. However, their mechanistic effect on renal fibrosis, which is characterized by vascular regression and further rarefaction-related pathology, remains unknown.

Methods

To explore the effect and molecular mechanisms by which pEPCs act on unilateral ureteral obstruction (UUO)-induced renal fibrosis, we isolated pEPCs from murine bone marrow. In vivo, pEPCs (2 × 10⁵ cells/day) and pEPC-MVs (microvesicles) were injected into UUO mice via the tail vein. In vitro, pEPCs were co-cultured with renal-derived pericytes. Pericyte-myofibroblast transition was evaluated using the myofibroblast marker α-smooth muscle actin (α-SMA) and pericyte marker platelet-derived growth factor receptor β (PDGFR-β).

Results

Exogenous supply of bone marrow-derived pEPCs attenuated renal fibrosis by decreasing pericyte-myofibroblast transition without significant vascular repair in the UUO model. Our results indicated that pEPCs regulated pericytes and their transition into myofibroblasts via pEPC-MVs. Co-culture of pericytes with pEPCs in vitro suggested that pEPCs inhibit transforming growth factor-β (TGF-β)-induced pericyte–myofibroblast transition via a paracrine pathway.

Conclusion

pEPCs effectively attenuated UUO-induced renal fibrosis by inhibiting pericyte–myofibroblast transition via a paracrine pathway, without promoting vascular repair.

Electronic supplementary material

The online version of this article (10.1186/s13287-019-1201-5) contains supplementary material, which is available to authorized users.

Pericytes as a Source of Osteogenic Cells in Bone Fracture Healing

Pericytes are mesenchymal cells that surround the endothelial cells of small vessels in various organs. These cells express several markers, such as NG2, CD146, and PDGFRβ, and play an important role in the stabilization and maturation of blood vessels. It was also recently revealed that like mesenchymal stem cells (MSCs), pericytes possess multilineage differentiation capacity, especially myogenic, adipogenic, and fibrogenic differentiation capacities. Although some previous studies have reported that pericytes also have osteogenic potential, the osteogenesis of pericytes can still be further elucidated. In the present study, we established novel methods for isolating and culturing primary murine pericytes. An immortalized pericyte line was also established. Multilineage induction of the pericyte line induced osteogenesis, adipogenesis, and chondrogenesis of the cells in vitro. In addition, pericytes that were injected into the fracture site of a bone fracture mouse model contributed to callus formation. Furthermore, in vivo pericyte-lineage-tracing studies demonstrated that endogenous pericytes also differentiate into osteoblasts and osteocytes and contribute to bone fracture healing as a cellular source of osteogenic cells. Pericytes can be a promising therapeutic candidate for treating bone fractures with a delayed union or nonunion as well as bone diseases causing bone defects.

Incomplete Differentiation of Engrafted Bone Marrow Endothelial Progenitor Cells Initiates Hepatic Fibrosis in the Rat

Normal liver sinusoidal endothelial cells (LSECs) promote quiescence of hepatic stellate cells (HSCs). Prior to fibrosis, LSECs undergo capillarization, which is permissive for HSC activation, the proximate event in hepatic fibrosis. The aims of this study were to elucidate the nature of and mechanisms leading to capillarization and to determine how LSECs promote HSC quiescence and why "capillarized LSECs" lose control of HSC activation. The contribution of bone marrow (BM) endothelial progenitor cells to capillarization was identified using rats transplanted with transgenic enhanced green fluorescent protein-positive BM. Shotgun proteomics and informatics were used to identify the LSEC mediator that maintains HSC quiescence. The study shows that capillarization is due to repair of injured LSECs by BM endothelial progenitors that engraft but fail to fully mature. Lack of maturation of BM-derived LSECs is due to cell autonomous pathways that inhibit the nitric oxide pathway. We identify heparin binding epidermal growth factor-like growth factor (HB-EGF) as the signal that maintains HSC quiescence and show that immature LSECs are unable to shed HB-EGF from the cytosolic membrane. Conclusion: Chronic liver injury can recruit BM progenitors of LSECs that engraft and fail to fully differentiate, which creates an environment that is permissive for hepatic fibrosis; elucidation of these early events in the fibrotic process will provide targets for treatment of hepatic fibrosis.

Advances in Renal Cell Imaging

A great variety of cell imaging technologies are used routinely every day for the investigation of kidney cell types in applications ranging from basic science research to drug development and pharmacology, clinical nephrology, and pathology. Quantitative visualization of the identity, density, and fate of both resident and nonresident cells in the kidney, and imaging-based analysis of their altered function, (patho)biology, metabolism, and signaling in disease conditions, can help to better define pathomechanism-based disease subgroups, identify critical cells and structures that play a role in the pathogenesis, critically needed biomarkers of disease progression, and cell and molecular pathways as targets for novel therapies. Overall, renal cell imaging has great potential for improving the precision of diagnostic and treatment paradigms for individual acute kidney injury or chronic kidney disease patients or patient populations. This review highlights and provides examples for some of the recently developed renal cell optical imaging approaches, mainly intravital multiphoton fluorescence microscopy, and the new knowledge they provide for our better understanding of renal pathologies.

Acute kidney injury to chronic kidney disease transition: insufficient cellular stress response

PURPOSE OF REVIEW

Recent epidemiological and preclinical mechanistic studies provide strong evidence that acute kidney injury (AKI) and chronic kidney disease (CKD) form an interconnected syndrome. Injured kidneys undergo a coordinated reparative process with an engagement of multiple cell types after injury; however, maladaptation to the injury subjects kidneys to a vicious cycle of fibrogenesis and nephron loss. In this review, we will outline and discuss the pathogenesis of AKI-to-CKD transition with an emphasis on dysregulated 'cellular stress adaptation' as a potential therapeutic target.

RECENT FINDINGS

Recent studies identify the crucial role of injured tubular epithelial cells in the transition from AKI to CKD. Damaged tubular cells undergo reactivation of developmental and epithelial-mesenchymal transition signaling, metabolic alteration, and cell-cycle arrest, thereby driving inflammation and fibrogenesis. Recent work highlights that cellular stress-adaptive pathways against hypoxic and oxidative stress provide insufficient protection after severe AKI episode.

SUMMARY

Insufficient cellular stress adaptation may underpin the persistent activation of inflammatory and fibrogenic signaling in damaged kidneys. We propose that harnessing cellular stress-adaptive responses will be a promising therapeutic strategy to halt or even reverse the deleterious process of AKI-to-CKD transition.

Peroxiredoxin 3 deficiency accelerates chronic kidney injury in mice through interactions between macrophages and tubular epithelial cells

Chronic kidney disease (CKD) has become epidemic worldwide. Mitochondrial reactive oxygen species (ROS)-induced oxidative stress is an important mediator of CKD, and Prx3 plays a critical role in maintenance of mitochondrial ROS. The present study examined the role of Prx3 in the context of fibrosis, a common feature of CKD, using Prx3 KO mice under obstructive and diabetic stress. Prx3 deficiency accelerated fibrosis and inflammation accompanied by mitochondrial oxidative stress in obstructed and diabetic kidneys as well as in proximal tubular epithelial (mProx) cells. In addition, Prx3 deficiency induced Raw264.7 macrophages activation, leading to upregulation of proinflammatory cytokines. Conditioned media from LPS-stimulated Prx3 deficient macrophages accelerated proinflammatory and profibrotic cytokines in mProx cells. Interestingly, Prx3 deficiency induced most inflammatory and fibrotic cytokines at basal condition in both tissues and cells. Taken together, these results demonstrate that Prx3 deficiency can accelerate CKD through interactions between macrophages and tubular epithelial 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.

Tubulointerstitial Infiltration of M2 Macrophages in Henoch-Schönlein Purpura Nephritis Indicates the Presence of Glomerular Crescents and Bad Clinical Parameters

Henoch-Schönlein purpura (HSP) is the most common systemic vasculitis in children, and renal involvement (HSP nephritis, HSPN) is a severe manifestation. HSPN is histologically classified by the International Study of Kidney Disease in Children (ISKDC) based on mesangial hypercellularity and the extent of glomerular crescents. Macrophages, categorized as M1 or M2, frequently infiltrate in various glomerular and tubulointerstitial diseases and infiltration of specific subtypes is associated with disease progression. Therefore, to identify whether infiltration of M1 or M2 macrophages has clinical significance, we quantified the subtypes of macrophages in 49 HSPN specimens and correlated the counts with histologic features and clinical parameters. Higher tubulointerstitial M2 counts were associated with chronic renal failure (CRF), ISKDC classes III-IV, and crescents (P<0.001, 0.002, 0.001). Glomerular M2 counts were significantly related to ISKDC classes III-IV and crescents (area under curve, AUC 0.804, 0.833). Tubulointerstitial M2 counts were associated with CRF, ISKDC classes III-IV, and crescents (AUC 0.872, 0.778, 0.830). Tubulointerstitial M2 counts also revealed higher AUC than tubulointerstitial M1 counts for CRF (P=0.036) and ISKDC classes III-IV (P=0.047). Glomerular M2 counts revealed higher AUC than glomerular M1 counts for ISKDC classes III–IV (P=0.024). Tubulointerstitial M2 counts were the most powerful parameter for CRF (AUC 0.872) and revealed even higher AUC than ISKDC classification (AUC 0.716) with borderline significance (P=0.086) for CRF. In summary, tubulointerstitial M2 counts were a superior parameter to tubulointerstitial M1 counts and even to ISKDC classification indicating the presence of CRF.

Inflammatory Mediators and Renal Fibrosis

Renal inflammation is the initial, healthy response to renal injury. However, prolonged inflammation promotes the fibrosis process, which leads to chronic pathology and eventually end-stage kidney disease. There are two major sources of inflammatory cells: first, bone marrow-derived leukocytes that include neutrophils, macrophages, fibrocytes and mast cells, and second, locally activated kidney cells such as mesangial cells, podocytes, tubular epithelial cells, endothelial cells and fibroblasts. These activated cells produce many profibrotic cytokines and growth factors that cause accumulation and activation of myofibroblasts, and enhance the production of the extracellular matrix. In particular, activated macrophages are key mediators that drive acute inflammation into chronic kidney disease. They produce large amounts of profibrotic factors and modify the microenvironment via a paracrine effect, and they also transdifferentiate to myofibroblasts directly, although the origin of myofibroblasts in the fibrosing kidney remains controversial. Collectively, understanding inflammatory cell functions and mechanisms during renal fibrosis is paramount to improving diagnosis and treatment of chronic kidney disease.

Pericytes in Breast Cancer

Breast cancer is a heterogeneous disease driven not only by evolutionally diverse cancer cell themselves but also by highly dynamic microenvironment. At the center of the tumor microenvironment, tumor vasculature plays multiple roles from supporting tumor growth to providing a route for metastasis to the distant organ sites. Blood vessels in breast cancer present with perfusion defects associated with vessel dilation, tortuosity, and poor perivascular coverage (Li et al., Ultrasound Med 32:1145-1155, 2013; Eberhard et al., Cancer Res 60:1388-1393, 2000; Cooke et al., Cancer Cell 21:66-81, 2012). Such abnormal vascular system is partly due to the morphological and molecular alteration of pericytes that is accompanied by a significant heterogeneity within the populations (Kim et al., JCI Insight 1:e90733, 2016). While pericytes are implicated for their controversial roles in breast cancer metastasis (Cooke et al., Cancer Cell 21:66-81, 2012; Gerhardt and Semb, J Mol Med (Berl) 86:135-144, 2008; Keskin et al., Cell Rep 10:1066-1081, 2015; Meng et al., Future Oncol 11:169-179, 2015; Xian et al., J Clin Invest 116:642-651, 2006), the impact of their heterogeneity on breast cancer progression, metastasis, intratumoral immunity, and response to chemotherapy are largely unknown. Due to the complexity of angiogenic programs of breast cancer, the anti-angiogenic or anti-vascular treatment has been mostly unsuccessful (Tolaney et al., Proc Natl Acad Sci U S A 112:14325-14330, 2015; Mackey et al., Cancer Treat Rev 38:673-688, 2012; Sledge, J Clin Oncol 33:133-135, 2015) and requires much in-depth knowledge on different components of tumor microenvironment and how these stromal cells are interacting and communicating to each other. Therefore, understanding pericyte heterogeneity and their differential functional contribution will shed light on new potential approaches to treat breast cancer.

Macrophages in Renal Fibrosis

Monocytes/macrophages are highly involved in the process of renal injury, repair and fibrosis in many aspects of experimental and human renal diseases. Monocyte-derived macrophages, characterized by high heterogeneity and plasticity, are recruited, activated, and polarized in the whole process of renal fibrotic diseases in response to local microenvironment. As classically activated M1 or CD11b⁺/Ly6C^high macrophages accelerate renal injury by producing pro-inflammatory factors like tumor necrosis factor-alpha (TNFα) and interleukins, alternatively activated M2 or CD11b⁺/Ly6C^intermediate macrophages may contribute to kidney repair by exerting anti-inflammation and wound healing functions. However, uncontrolled M2 macrophages or CD11b⁺/Ly6C^low macrophages promote renal fibrosis via paracrine effects or direct transition to myofibroblast-like cells via the process of macrophage-to-myofibroblast transition (MMT). In this regard, therapeutic strategies targeting monocyte/macrophage recruitment, activation, and polarization should be emphasized in the treatment of renal fibrosis.

Morphology and Evaluation of Renal Fibrosis

With continuing damage, both the indigenous cells of the cortex and medulla, and inflammatory cells are involved in the formation and development of renal fibrosis. Furthermore, interactions among the glomerular, tubular, and interstitial cells contribute to the process by excessive synthesis and decreased degradation of extracellular matrix. The morphology of kidney is different from pathological stages of diseases and changes with various causes. At the end stage of the disease, the kidneys are symmetrically contracted with diffuse granules. Most glomeruli show diffuse fibrosis and hyaline degeneration, and intervening tubules become atrophied. Renal interstitium shows obvious hyperplasia of fibrous tissues with marked infiltration of lymphocytes, mononuclear cells, and plasma cells. The renal arterioles are wall thickening frequently because of hyaline degeneration. Morphologic analysis based on Masson staining of the kidney tissues has been regarded as the golden standard to evaluate the visual fibrosis. However, the present studies have found that the evaluation system has poor repeatability. Several computer-aided image analysis techniques have been used to assess interstitial fibrosis. It is possible that the evaluation of renal fibrosis is carried out by the artificial intelligence renal biopsy pathological diagnosis system in the near future.

Pericytes in Muscular Dystrophies

The muscular dystrophies are an heterogeneous group of inherited myopathies characterised by the progressive wasting of skeletal muscle tissue. Pericytes have been shown to make muscle in vitro and to contribute to skeletal muscle regeneration in several animal models, although recent data has shown this to be controversial. In fact, some pericyte subpopulations have been shown to contribute to fibrosis and adipose deposition in muscle. In this chapter, we explore the identity and the multifaceted role of pericytes in dystrophic muscle, potential therapeutic applications and the current need to overcome the hurdles of characterisation (both to identify pericyte subpopulations and track cell fate), to prevent deleterious differentiation towards myogenic-inhibiting subpopulations, and to improve cell proliferation and engraftment efficacy.

Pericytes in Skeletal Muscle

Skeletal muscle regeneration is a highly orchestrated process and involves the activation of many cellular and molecular pathways. Although satellite cells (SCs) are the major cell type responsible for muscle regeneration, pericytes show remarkable myogenic potential and various advantages as cell therapy in muscular disorders. This chapter first introduces the structure, marker expression, origin, and category of pericytes. Next, we discuss their functions in muscular dystrophy and/or muscle injuries, focusing on their myogenic, adipogenic, fibrogenic, chondrogenic, and osteogenic activities. Understanding this knowledge will promote the development of innovative cell therapies for muscle disorders, including muscular dystrophy.

A conditionally immortalized Gli1-positive kidney mesenchymal cell line models myofibroblast transition

Glioma-associated oncogene homolog-1 (Gli1)-positive resident mesenchymal stem cell-like cells are the predominant source of kidney myofibroblasts in fibrosis, but investigating Gli1-positive myofibroblast progenitor activation is hampered by the difficulty of isolating and propagating primary cultures of these cells. Using a genetic strategy with positive and negative selection, we isolated Kidney-Gli1 (KGli1) cells that maintain expression of appropriate mesenchymal stem cell-like cell markers, respond to hedgehog pathway activation, and display robust myofibroblast differentiation upon treatment with transforming growth factor-β (TGF-β). Coculture of KGli1 cells with endothelium stabilizes capillary formation. Single-cell RNA sequencing (scRNA-seq) analysis during differentiation identified autocrine ligand-receptor pair upregulation and a strong focal adhesion pathway signal. This led us to test the serum response factor inhibitor CCG-203971 that potently inhibited TGF-β-induced pericyte-to-myofibroblast transition. scRNA-seq also identified the unexpected upregulation of nerve growth factor (NGF), which we confirmed in two mouse kidney fibrosis models. The Ngf receptor Ntrk1 is expressed in tubular epithelium in vivo, suggesting a novel interstitial-to-tubule paracrine signaling axis. Thus, KGli1 cells accurately model myofibroblast activation in vitro, and the development of this cell line provides a new tool to study resident mesenchymal stem cell-like progenitors in health and disease.

Pericytes in the Heart

Mural cells known as pericytes envelop the endothelial layer of microvessels throughout the body and have been described to have tissue-specific functions. Cardiac pericytes are abundantly found in the heart, but they are relatively understudied. Currently, their importance is emerging in cardiovascular homeostasis and dysfunction due to their pleiotropism. They are known to play key roles in vascular tone and vascular integrity as well as angiogenesis. However, their dysfunctional presence and/or absence is critical in the mechanisms that lead to cardiac pathologies such as myocardial infarction, fibrosis, and thrombosis. Moreover, they are targeted as a therapeutic potential due to their mesenchymal properties that could allow them to repair and regenerate a damaged heart. They are also sought after as a cell-based therapy based on their healing potential in preclinical studies of animal models of myocardial infarction. Therefore, recognizing the importance of cardiac pericytes and understanding their biology will lead to new therapeutic concepts.

Heterogeneity of Adult Cardiac Stem Cells

Cardiac biology and heart regeneration have been intensively investigated and debated in the last 15 years. Nowadays, the well-established and old dogma that the adult heart lacks of any myocyte-regenerative capacity has been firmly overturned by the evidence of cardiomyocyte renewal throughout the mammalian life as part of normal organ cell homeostasis, which is increased in response to injury. Concurrently, reproducible evidences from independent laboratories have convincingly shown that the adult heart possesses a pool of multipotent cardiac stem/progenitor cells (CSCs or CPCs) capable of sustaining cardiomyocyte and vascular tissue refreshment after injury. CSC transplantation in animal models displays an effective regenerative potential and may be helpful to treat chronic heart failure (CHF), obviating at the poor/modest results using non-cardiac cells in clinical trials. Nevertheless, the degree/significance of cardiomyocyte turnover in the adult heart, which is insufficient to regenerate extensive damage from ischemic and non-ischemic origin, remains strongly disputed. Concurrently, different methodologies used to detect CSCs in situ have created the paradox of the adult heart harboring more than seven different cardiac progenitor populations. The latter was likely secondary to the intrinsic heterogeneity of any regenerative cell agent in an adult tissue but also to the confusion created by the heterogeneity of the cell population identified by a single cell marker used to detect the CSCs in situ. On the other hand, some recent studies using genetic fate mapping strategies claimed that CSCs are an irrelevant endogenous source of new cardiomyocytes in the adult. On the basis of these contradictory findings, here we critically reviewed the available data on adult CSC biology and their role in myocardial cell homeostasis and repair.

Changes in cell fate determine the regenerative and functional capacity of the developing kidney before and after release of obstruction

Congenital obstructive nephropathy is a major cause of chronic kidney disease (CKD) in children. The contribution of changes in the identity of renal cells to the pathology of obstructive nephropathy is poorly understood. Using a partial unilateral ureteral obstruction (pUUO) model in genetically modified neonatal mice, we traced the fate of cells derived from the renal stroma, cap mesenchyme, ureteric bud (UB) epithelium, and podocytes using Foxd1Cre, Six2Cre, HoxB7Cre, and Podocyte.Cre mice respectively, crossed with double fluorescent reporter (membrane-targetted tandem dimer Tomato (mT)/membrane-targetted GFP (mG)) mice. Persistent obstruction leads to a significant loss of tubular epithelium, rarefaction of the renal vasculature, and decreased renal blood flow (RBF). In addition, Forkhead Box D1 (Foxd1)-derived pericytes significantly expanded in the interstitial space, acquiring a myofibroblast phenotype. Degeneration of Sine Oculis Homeobox Homolog 2 (Six2) and HoxB7-derived cells resulted in significant loss of glomeruli, nephron tubules, and collecting ducts. Surgical release of obstruction resulted in striking regeneration of tubules, arterioles, interstitium accompanied by an increase in blood flow to the level of sham animals. Contralateral kidneys with remarkable compensatory response to kidney injury showed an increase in density of arteriolar branches. Deciphering the mechanisms involved in kidney repair and regeneration post relief of obstruction has potential therapeutic implications for infants and children and the growing number of adults suffering from CKD.

Visualization of specific collagen-producing cells by Col1-GFP transgenic mice revealed novel type I collagen-producing cells other than fibroblasts in systemic organs/tissues

Type I collagen is one of the most abundant proteins in mammals and plays important roles in maintaining the integrity of many tissues. Although fibroblasts are the main source of type I collagen, other cells also produce it; however, these cells are not well-defined owing to the lack of a specific marker. A transgenic (Tg) mouse line has been generated in which type I collagen-producing cells are labeled with enhanced green fluorescent protein (EGFP), which enables the monitoring of these cells without requiring an additional cell marker. This Tg mouse line has since been widely used to study type I collagen-producing cells and fibrosis; one study revealed that podocytes, which were not previously considered to produce type I collagen, expressed EGFP. This raises a question regarding the specificity of EGFP expression in this Tg mouse line. To exclude the possibility of non-specific EGFP expression in the existing Tg mouse line and specifically monitor type I collagen-producing cells, we generated a new Tg mouse line and histologically confirmed the specificity of EGFP expression throughout the body. Moreover, we explored type I collagen-producing cells other than fibroblasts and revealed for the first time that Leydig cells have the ability to produce type I collagen. Because of its highly specific and physiologically accurate expression, our new Tg mouse line will help to accurately elucidate not only type I collagen-producing cells in normal tissues but also the potential cells in fibrotic tissues, providing new insights into the pathology of fibrosis.

MicroRNA-493 targets STMN-1 and promotes hypoxia-induced epithelial cell cycle arrest in G2/M and renal fibrosis

Hypoxia plays an important role in the development of renal fibrosis. G₂/M arrest in renal tubular cells is an important pathway in the development of chronic kidney disease. It is unknown whether hypoxia leads to renal fibrosis via the regulation of G₂/M arrest in renal tubular epithelial cells. For the first time, to our knowledge, we showed that hypoxia induces G₂/M arrest in renal tubular cells leading to renal fibrosis, and microRNAs are involved in this regulation. We compared microRNA expression between hypoxia and normoxia in HK2 cells and found microRNA (miR)-493 to be highly expressed at 24 and 48 h after hypoxia. The overexpression of miR-493 reduced the expression of the cell cycle regulator, Stathmin (STMN)-1, and increased the percentage of G₂/M phase cells and profibrotic factors in HK2 cells. Targeting STMN-1 with short hairpin RNA produced an effect similar to that of miR-493 overexpression. On contrast, the miR-493 inhibitor reversed these effects in vitro. Consistent with these results, miR-493 sponge adeno-associated virus reduced the expression of profibrotic factors and increased STMN-1 in vivo. In summary, these results suggest that the miR-493-STMN-1 pathway contributes to hypoxia-induced tubular epithelial cell G₂/M arrest and renal fibrosis. Abrogating G₂/M arrest and blocking the miR-493-STMN-1 pathway will provide further insight for the development of antifibrosis therapy.-Liu, T., Liu, L., Liu, M., Du, R., Dang, Y., Bai, M., Zhang, L., Ma, F., Yang, X., Ning, X., Sun, S. MicroRNA-493 targets STMN-1 and promotes hypoxia-induced epithelial cell cycle arrest In G₂/M and renal fibrosis.

Yap/Taz mediates mTORC2-stimulated fibroblast activation and kidney fibrosis

Our previously published study demonstrated that mammalian target of rapamycin complex 2 (mTORC2) signaling mediates TGFβ1-induced fibroblast activation. However, the underlying mechanisms for mTORC2 in stimulating fibroblast activation remain poorly understood. Here, we found that TGFβ1 could stimulate mTORC2 and Yap/Taz activation in NRK-49F cells. Blocking either mTORC2 or Yap/Taz signaling diminished TGFβ1-induced fibroblast activation. In addition, blockade of mTORC2 could down-regulate the expression of Yap/Taz, connective tissue growth factor (CTGF), and ankyrin repeat domain 1 (ANKRD1). Overexpression of constitutively active Taz (Taz-S89A) could restore fibroblast activation suppressed by PP242, an mTOR kinase inhibitor in NRK-49F cells. In mouse kidneys with unilateral ureter obstructive (UUO) nephropathy, both mTORC2 and Yap/Taz were activated in the interstitial myofibroblasts. Ablation of Rictor in fibroblasts/pericytes or blockade of mTOR signaling with PP242 attenuated Yap/Taz activation and UUO nephropathy in mice. Together, this study uncovers that targeting mTORC2 retards fibroblast activation and kidney fibrosis through suppressing Yap/Taz activation.

Renal fibrosis: Primacy of the proximal tubule

Tubulointerstitial fibrosis (TIF) is the hallmark of chronic kidney disease and best predictor of renal survival. Many different cell types contribute to TIF progression including tubular epithelial cells, myofibroblasts, endothelia, and inflammatory cells. Previously, most of the attention has centered on myofibroblasts given their central importance in extracellular matrix production. However, emerging data focuses on how the response of the proximal tubule, a specialized epithelial segment vulnerable to injury, plays a central role in TIF progression. Several proximal tubular responses such as de-differentiation, cell cycle changes, autophagy, and metabolic changes may be adaptive initially, but can lead to maladaptive responses that promote TIF both through autocrine and paracrine effects. This review discusses the current paradigm of TIF progression and the increasingly important role of the proximal tubule in promoting TIF both in tubulointerstitial and glomerular injuries. A better understanding and appreciation of the role of the proximal tubule in TIF has important implications for therapeutic strategies to halt chronic kidney disease progression.

Valproic acid regulates Ang II-induced pericyte-myofibroblast trans-differentiation via MAPK/ERK pathway

Myocardial fibrosis (MF) plays an important part in cardiovascular diseases. The main cytological characteristics of MF is the increased number of myofibroblasts, which have multiple sources such as EMT, EndMT, myeloid progenitors, monocytes, and fibrocytes. Recent data showed that pericytes may represent a major source of myofibroblasts in kidney fibrosis. Valproic acid (VPA) is a kind of short-chain fatty acid. It was reported in recent studies that VPA regulates gene expression and influences various signal pathways. HDACs inhibitors can hinder the growth of tumor cells and differentiation of stem cells. And little is known about the effects of HDACs inhibitors on myofibroblasts transdiffererntiaton. This study focused on the role of HDACs in pericyte-myofibroblast trans-differentiation and how HDACs inhibitor VPA influenced proliferation, migration, viability and myofibroblast trans-differentiation of pericytes for the first time. Rat cardiac fibrosis model was induced by Ang II. Immunohistochemistry was employed to examine cardiac fibrosis and flow cytometry was used to analyze whether inflammatory cells involve VPA-induced trans-differentiation. Pericytes proliferation, migration and differentiation to myofibroblasts were performed to examine the role of VPA on pericyte trans-differentiation. Immunoblot and qPCR were applied to identify the signal transduction involving in VPA-induced trans-differentiation. In vivo study showed that HDAC inhibitor VPA blocks cardiac fibrosis, and inflammation inhibition was not involved in this process. VPA treatment inhibited Ang II pericyte proliferation, migration and transdifferentiation to myofibroblast. Furthermore, the inhibition of α-SMA expression by VPA was related to reduce phosphorylation of ERK, and a pharmacological inhibitor of MEK suppressed Ang II-induced α-SMA expression. HDAC4 knockdown resulted in inhibiting Ang II-mediated α-SMA expression as well as the phosphorylation of ERK. Moreover, the inhibitors of protein phosphatase 2A and 1 (PP2A and PP1) restored the Ang II-stimulated α-SMA expression from the inhibitory effect of VPA. Together, the current data indicate that the differentiation of pericytes to myofibroblasts is HDAC4 dependent and requires phosphorylation of ERK.

Cellular Origin and Functional Relevance of Collagen I Production in the Kidney

Background Interstitial fibrosis is associated with chronic renal failure. In addition to fibroblasts, bone marrow-derived cells and tubular epithelial cells have the capacity to produce collagen. However, the amount of collagen produced by each of these cell types and the relevance of fibrosis to renal function are unclear.Methods We generated conditional cell type-specific collagen I knockout mice and used (reversible) unilateral ureteral obstruction and adenine-induced nephropathy to study renal fibrosis and function.Results In these mouse models, hematopoietic, bone marrow-derived cells contributed to 38%-50% of the overall deposition of collagen I in the kidney. The influence of fibrosis on renal function was dependent on the type of damage. In unilateral ureteral obstruction, collagen production by resident fibroblasts was essential to preserve renal function, whereas in the chronic model of adenine-induced nephropathy, collagen production was detrimental to renal function.Conclusions Our data show that hematopoietic cells are a major source of collagen and that antifibrotic therapies need to be carefully considered depending on the type of disease and the underlying cause of fibrosis.

Organ and tissue fibrosis: Molecular signals, cellular mechanisms and translational implications

Fibrosis denotes excessive scarring, which exceeds the normal wound healing response to injury in many tissues. Although the extracellular matrix deposition appears unstructured disrupting the normal tissue architecture and subsequently impairing proper organ function, fibrogenesis is a highly orchestrated process determined by defined sequences of molecular signals and cellular response mechanisms. Persistent injury and parenchymal cell death provokes tissue inflammation, macrophage activation and immune cell infiltration. The release of biologically highly active soluble mediators (alarmins, cytokines, chemokines) lead to the local activation of collagen producing mesenchymal cells such as pericytes, myofibroblasts or Gli1 positive mesenchymal stem cell-like cells, to a transition of various cell types into myofibroblasts as well as to the recruitment of fibroblast precursors. Clinical observations and experimental models highlighted that fibrosis is not a one-way road. Specific mechanistic principles of fibrosis regression involve the resolution of chronic tissue injury, the shift of inflammatory processes towards recovery, deactivation of myofibroblasts and finally fibrolysis of excess matrix scaffold. The thorough understanding of common principles of fibrogenic molecular signals and cellular mechanisms in various organs - such as liver, kidney, lung, heart or skin - is the basis for developing improved diagnostics including biomarkers or imaging techniques and novel antifibrotic therapeutics.