The spatially resolved transcriptome signatures of glomeruli in chronic kidney disease

Here, we used digital spatial profiling (DSP) to describe the glomerular transcriptomic signatures that may characterize the complex molecular mechanisms underlying progressive kidney disease in Alport syndrome, focal segmental glomerulosclerosis, and membranous nephropathy. Our results revealed significant transcriptional heterogeneity among diseased glomeruli, and this analysis showed that histologically similar glomeruli manifested different transcriptional profiles. Using glomerular pathology scores to establish an axis of progression, we identified molecular pathways with progressively decreased expression in response to increasing pathology scores, including signal recognition particle-dependent cotranslational protein targeting to membrane and selenocysteine synthesis pathways. We also identified a distinct signature of upregulated and downregulated genes common to all the diseases investigated when compared with nondiseased tissue from nephrectomies. These analyses using DSP at the single-glomerulus level could help to increase insight into the pathophysiology of kidney disease and possibly the identification of biomarkers of disease progression in glomerulopathies.

Urine-derived podocytes from steroid resistant nephrotic syndrome patients as a model for renal-progenitor derived extracellular vesicles effect and drug screening

Background Personalized disease models are crucial for assessing the specific response of diseased cells to drugs, particularly novel biological therapeutics. Extracellular vesicles (EVs), nanosized vesicles released by cells for intercellular communication, have gained therapeutic interest due to their ability to reprogram target cells. We here utilized urinary podocytes obtained from children affected by steroid-resistant nephrotic syndrome with characterized genetic mutations as a model to test the therapeutic potential of EVs derived from kidney progenitor cells. Methods EVs were isolated from kidney progenitor cells (nKPCs) derived from the urine of a preterm neonate. Three lines of urinary podocytes obtained from nephrotic patients' urine and a line of Alport patient podocytes were characterized and used to assess albumin permeability in response to various drugs or to nKPC-EVs. RNA sequencing was conducted to identify commonly modulated pathways. Results Podocytes appeared unresponsive to pharmacological treatments, except for a podocyte line demonstrating responsiveness, in alignment with the patient's clinical response at 48 months. At variance, treatment with the nKPC-EVs was able to significantly reduce permeability in all the steroid-resistant patients-derived podocytes as well as in the line of Alport-derived podocytes. RNA sequencing of nKPC-EV-treated podocytes revealed the common upregulation of two genes (small ubiquitin-related modifier 1 (SUMO1) and Sentrin-specific protease 2 (SENP2)) involved in the SUMOylation pathway, a process recently demonstrated to play a role in slit diaphragm stabilization. Gene ontology analysis on podocyte expression profile highlighted cell-to-cell adhesion as the primary upregulated biological activity in treated podocytes. Conclusions nKPCs emerge as a promising non-invasive source of EVs with potential therapeutic effects on podocyte dysfunction. Furthermore, our findings suggest the possibility of establishing a non-invasive in vitro model for screening regenerative compounds on patient-derived podocytes.

C3aR-initiated signaling is a critical mechanism of podocyte injury in membranous nephropathy

The deposition of antipodocyte autoantibodies in the glomerular subepithelial space induces primary membranous nephropathy (MN), the leading cause of nephrotic syndrome worldwide. Taking advantage of the glomerulus-on-a-chip system, we modeled human primary MN induced by anti-PLA2R antibodies. Here we show that exposure of primary human podocytes expressing PLA2R to MN serum results in IgG deposition and complement activation on their surface, leading to loss of the chip permselectivity to albumin. C3a receptor (C3aR) antagonists as well as C3AR gene silencing in podocytes reduced oxidative stress induced by MN serum and prevented albumin leakage. In contrast, inhibition of the formation of the membrane-attack-complex (MAC), previously thought to play a major role in MN pathogenesis, did not affect permselectivity to albumin. In addition, treatment with a C3aR antagonist effectively prevented proteinuria in a mouse model of MN, substantiating the chip findings. In conclusion, using a combination of pathophysiologically relevant in vitro and in vivo models, we established that C3a/C3aR signaling plays a critical role in complement-mediated MN pathogenesis, indicating an alternative therapeutic target for MN.

Differential detection workflows for multi-sample single-cell RNA-seq data

In single-cell transcriptomics, differential gene expression (DE) analyses typically focus on testing differences in the average expression of genes between cell types or conditions of interest. Single-cell transcriptomics, however, also has the promise to prioritise genes for which the expression differ in other aspects of the distribution. Here we develop a workflow for assessing differential detection (DD), which tests for differences in the average fraction of samples or cells in which a gene is detected. After benchmarking eight different DD data analysis strategies, we provide a unified workflow for jointly assessing DE and DD. Using simulations and two case studies, we show that DE and DD analysis provide complementary information, both in terms of the individual genes they report and in the functional interpretation of those genes.

Mitochondria Transplantation Mitigates Damage in an In Vitro Model of Renal Tubular Injury and in an Ex Vivo Model of DCD Renal Transplantation

OBJECTIVES

To test whether mitochondrial transplantation (MITO) mitigates damage in 2 models of acute kidney injury (AKI).

BACKGROUND

MITO is a process where exogenous isolated mitochondria are taken up by cells. As virtually any morbid clinical condition is characterized by mitochondrial distress, MITO may find a role as a treatment modality in numerous clinical scenarios including AKI.

METHODS

For the in vitro experiments, human proximal tubular cells were damaged and then treated with mitochondria or placebo. For the ex vivo experiments, we developed a non-survival ex vivo porcine model mimicking the donation after cardiac death renal transplantation scenario. One kidney was treated with mitochondria, although the mate organ received placebo, before being perfused at room temperature for 24 hours. Perfusate samples were collected at different time points and analyzed with Raman spectroscopy. Biopsies taken at baseline and 24 hours were analyzed with standard pathology, immunohistochemistry, and RNA sequencing analysis.

RESULTS

In vitro, cells treated with MITO showed higher proliferative capacity and adenosine 5'-triphosphate production, preservation of physiological polarization of the organelles and lower toxicity and reactive oxygen species production. Ex vivo, kidneys treated with MITO shed fewer molecular species, indicating stability. In these kidneys, pathology showed less damage whereas RNAseq analysis showed modulation of genes and pathways most consistent with mitochondrial biogenesis and energy metabolism and downregulation of genes involved in neutrophil recruitment, including IL1A, CXCL8, and PIK3R1.

CONCLUSIONS

MITO mitigates AKI both in vitro and ex vivo.

Extracellular vesicles in kidney development and pediatric kidney diseases

Extracellular vesicles (EVs) are membranous cargo particles that mediate intercellular communication. They are heterogeneous in size and mechanism of release, and found in all biological fluids. Since EV content is in relation to the originating cell type and to its physiopathological conditions, EVs are under study to understand organ physiology and pathology. In addition, EV surface cargo, or corona, can be influenced by the microenvironment, leading to the concept that EV-associated molecules can represent useful biomarkers for diseases. Recent studies also focus on the use of natural, engineered, or synthetic EVs for therapeutic purposes. This review highlights the role of EVs in kidney development, pediatric kidney diseases, including inherited disorders, and kidney transplantation. Although few studies exist, they have promising results and may guide researchers in this field. Main limitations, including the influence of age on EV analyses, are also discussed.

Decay-Accelerating Factor Restrains Complement Activation and Delays Progression of Murine cBSA-Induced Membranous Nephropathy

Key Points

In a murine model of cationic bovine serum albumin (cBSA)–induced membranous nephropathy (MN), complement regulator decay-accelerating factor is upregulated and restrains complement activation. Studies using genetic deletion or pharmacological antagonism of C3aR indicate that the main effector mechanism of complement activation in cBSA-induced MN is C3a/C3aR signaling. C3a formation and/or C3aR-mediated signaling represent promising targets for hypothesis-driven therapies for MN.

Background

Complement activation is believed to play a major pathogenic role in membranous nephropathy (MN), but its effector mechanisms are still unclear. Even less investigated is the role of podocyte-expressed complement regulators, including decay-accelerating factor (DAF) in disease pathophysiology.

Methods

We induced MN by serial injections of cationic bovine serum albumin (cBSA) in WT, DAF−/−, and C3aR−/− BALB/c mice and measured disease severity (by albuminuria, BUN, serum albumin, and glomerular histologic changes) and signs of complement activation in the glomeruli (immunofluorescence for C1q, C3b, and membrane attack complex). We also treated DAF−/− mice with cBSA-induced MN with a selective C3aR antagonist and measured the same readouts.

Results

cBSA-induced MN was associated with increased glomerular expression of DAF. Genetic deletion of DAF resulted in increased complement activation and higher disease severity than in WT animals. Treating cBSA-injected DAF−/− mice with a C3aR antagonist reduced disease severity. Similarly, C3aR−/− animals were protected from cBSA-induced MN, despite IgG deposition in the glomeruli and complement activation. Evidence of C1q and C3b deposition in the glomeruli of these mice suggest that IgG-cBSA immune complex formation in the glomeruli activates complement through the classical pathway.

Conclusions

On cBSA-induced injury, podocytes upregulate DAF expression, which restrains complement activation. However, after prolonged injury, complement activation overcomes DAF regulatory effects leading to the formation of soluble anaphylatoxin C3a that, by signaling through C3aR, promotes glomerular injury and cBSA-induced MN disease progression. Considering the growing number of complement targeting therapies, our findings may have major translational effect on the treatment of patients with MN.

Identification and Characterization of the Wilms Tumor Cancer Stem Cell

A nephrogenic progenitor cell (NP) with cancer stem cell characteristics driving Wilms tumor (WT) using spatial transcriptomics, bulk and single cell RNA sequencing, and complementary in vitro and transplantation experiments is identified and characterized. NP from WT samples with NP from the developing human kidney is compared. Cells expressing SIX2 and CITED1 fulfill cancer stem cell criteria by reliably recapitulating WT in transplantation studies. It is shown that self-renewal versus differentiation in SIX2+CITED1+ cells is regulated by the interplay between integrins ITGβ1 and ITGβ4. The spatial transcriptomic analysis defines gene expression maps of SIX2+CITED1+ cells in WT samples and identifies the interactive gene networks involved in WT development. These studies define SIX2+CITED1+ cells as the nephrogenic-like cancer stem cells of WT and points to the renal developmental transcriptome changes as a possible driver in regulating WT formation and progression.

Regenerative medicine technologies applied to transplant medicine.an update

Regenerative medicine (RM) is changing how we think and practice transplant medicine. In regenerative medicine, the aim is to develop and employ methods to regenerate, restore or replace damaged/diseased tissues or organs. Regenerative medicine investigates using tools such as novel technologies or techniques, extracellular vesicles, cell-based therapies, and tissue-engineered constructs to design effective patient-specific treatments. This review illustrates current advancements in regenerative medicine that may pertain to transplant medicine. We highlight progress made and various tools designed and employed specifically for each tissue or organ, such as the kidney, heart, liver, lung, vasculature, gastrointestinal tract, and pancreas. By combing both fields of transplant and regenerative medicine, we can harbor a successful collaboration that would be beneficial and efficacious for the repair and design of de novo engineered whole organs for transplantations.

Immune responses to SARS-CoV-2 in dialysis and kidney transplantation

Despite progressive improvements in the management of patients with coronavirus disease 2019 (COVID-19), individuals with end-stage kidney disease (ESKD) are still at high risk of infection-related complications. Although the risk of infection in these patients is comparable to that of the general population, their lower rate of response to vaccination is a matter of concern. When prevention strategies fail, infection is often severe. Comorbidities affecting patients on maintenance dialysis and kidney transplant recipients clearly account for the increased risk of severe COVID-19, while the role of uremia and chronic immunosuppression is less clear. Immune monitoring studies have identified differences in the innate and adaptive immune response against the virus that could contribute to the increased disease severity. In particular, individuals on dialysis show signs of T cell exhaustion that may impair antiviral response. Similar to kidney transplant recipients, antibody production in these patients occurs, but with delayed kinetics compared with the general population, leaving them more exposed to viral expansion during the early phases of infection. Overall, unique features of the immune response during COVID-19 in individuals with ESKD may occur with severe comorbidities affecting these individuals in explaining their poor outcomes.

The spatially resolved transcriptional profile of acute T cell-mediated rejection in a kidney allograft

Analysis of the transcriptional profile of graft biopsies represents a promising strategy to study T cell-mediated-rejection (TCMR), also known as acute cellular rejection. However, bulk RNA sequencing of graft biopsies may not capture the focal nature of acute rejection. Herein, we used the whole exome GeoMX Digital Space Profiling platform to study five tubular and three glomerular regions of interest in the kidney graft biopsy from a patient with a chronic-active TCMR episode and in analogous areas from two different normal kidney control biopsies. All kidney sections were from paraffin blocks. Overall, inflammatory genes were significantly upregulated in the tubular areas of the TCMR biopsy and showed an enrichment for gene-ontology terms associated with T-cell activation, differentiation, and proliferation. Enrichment analysis of the 100 genes with the highest coefficient of variation across the TCMR tubular regions of interest revealed that these highly variable genes are involved in kidney development and injury and interestingly do not associate with the 2019 Banff classification pathology scores within the individual regions of interest. Spatial transcriptomics allowed us to unravel a previously unappreciated variability across different areas of the TCMR biopsy related to the graft response to the alloimmune attack, rather than to the immune cells. Thus, our approach has the potential to decipher clinically relevant, new pathogenic mechanisms, and therapeutic targets in acute cellular rejection and other kidney diseases with a focal nature.

Generation of a Glomerular Filtration Barrier on a Glomerulus-on-a-Chip Platform

Despite an enormous investment of clinical and financial resources, chronic kidney disease (CKD) remains a global health threat. The lack of reliable in vitro systems that can efficiently mimic the renal and glomerular environment has hampered our ability to successfully develop novel and more renal specific drugs. Even though some success in generating in vitro tubule analogues and kidney organoids has been described, a major challenge remains for the in vitro assembly of the filtration unit of the kidney, the glomerulus. We have recently developed a novel glomerulus-on-a-chip system that mimics the characteristic and functionality of the glomerular filtration barrier, including its response to injury. This system recapitulates the functions and structure of the in vivo glomerulus, including permselectivity; indeed, we have confirmed free diffusion of insulin as well as impermeability to physiological concentrations of albumin. Exposure to nephrotoxic agents like puromycin aminonucleoside leads to a significant increase in albumin leakage. When exposed to sera from patients with anti-podocyte autoantibodies, the chip shows albumin leakage to an extent proportional to in vivo clinical data, phenomenon not observed with sera from either healthy controls, confirming functional response to injury. We describe here the detailed procedure to obtain a glomerulus-on-a-chip system that replicates both phenotypically and functionally the in vivo glomerular microenvironment.

Emerging Technologies to Study the Glomerular Filtration Barrier

Kidney disease is characterized by loss of glomerular function with clinical manifestation of proteinuria. Identifying the cellular and molecular changes that lead to loss of protein in the urine is challenging due to the complexity of the filtration barrier, constituted by podocytes, glomerular endothelial cells, and glomerular basement membrane. In this review, we will discuss how technologies like single cell RNA sequencing and bioinformatics-based spatial transcriptomics, as well as in vitro systems like kidney organoids and the glomerulus-on-a-chip, have contributed to our understanding of glomerular pathophysiology. Knowledge gained from these studies will contribute toward the development of personalized therapeutic approaches for patients affected by proteinuric diseases.

Placental extracellular vesicles-associated microRNA-519c mediates endotoxin adaptation in pregnancy

BACKGROUND

Pregnancy represents a unique challenge for the maternal-fetal immune interface, requiring a balance between immunosuppression, which is essential for the maintenance of a semiallogeneic fetus, and proinflammatory host defense to protect the maternal-fetal interface from invading organisms. Adaptation to repeated inflammatory stimuli (endotoxin tolerance) may be critical in preventing inflammation-induced preterm birth caused by exaggerated maternal inflammatory responses to mild or moderate infections that are common during pregnancy. However, the exact mechanisms contributing to the maintenance of tolerance to repeated infections are not completely understood. MicroRNAs play important roles in pregnancy with several microRNAs implicated in gestational tissue function and in pathologic pregnancy conditions. MicroRNA-519c, a member of the chromosome 19 microRNA cluster, is a human-specific microRNA mainly expressed in the placenta. However, its role in pregnancy is largely unknown.

OBJECTIVE

This study aimed to explore the role of "endotoxin tolerance" failure in the pathogenesis of an exaggerated inflammatory response often seen in inflammation-mediated preterm birth. In this study, we investigated the role of microRNA-519c, a placenta-specific microRNA, as a key regulator of endotoxin tolerance at the maternal-fetal interface.

STUDY DESIGN

Using a placental explant culture system, samples from term and second-trimester placentas were treated with lipopolysaccharide. After 24 hours, the conditioned media were collected for analysis, and the placental explants were re-exposed to repeated doses of lipopolysaccharide for 3 days. The supernatant was analyzed for inflammatory markers, the presence of extracellular vesicles, and microRNAs. To study the possible mechanism of action of the microRNAs, we evaluated the phosphodiesterase 3B pathway involved in tumor necrosis factor alpha production using a microRNA mimic and phosphodiesterase 3B small interfering RNA transfection. Finally, we analyzed human placental samples from different gestational ages and from women affected by inflammation-associated pregnancies.

RESULTS

Our data showed that repeated exposure of the human placenta to endotoxin challenges induced a tolerant phenotype characterized by decreased tumor necrosis factor alpha and up-regulated interleukin-10 levels. This reaction was mediated by the placenta-specific microRNA-519c packaged within placental extracellular vesicles. Lipopolysaccharide treatment increased the extracellular vesicles that were positive for the exosome tetraspanin markers, namely CD9, CD63, and CD81, and secreted primarily by trophoblasts. Primary human trophoblast cells transfected with a microRNA-519c mimic decreased phosphodiesterase 3B, whereas a lack of phosphodiesterase 3B, achieved by small interfering RNA transfection, led to decreased tumor necrosis factor alpha production. These data support the hypothesis that the anti-inflammatory action of microRNA-519c was mediated by a down-regulation of the phosphodiesterase 3B pathway, leading to inhibition of tumor necrosis factor alpha production. Furthermore, human placentas from normal and inflammation-associated pregnancies demonstrated that a decreased placental microRNA-519c level was linked to infection-induced inflammatory pathologies during pregnancy.

CONCLUSION

We identified microRNA-519c, a human placenta-specific microRNA, as a novel regulator of immune adaptation associated with infection-induced preterm birth at the maternal-fetal interface. Our study serves as a basis for future experiments to explore the potential use of microRNA-519c as a biomarker for infection-induced preterm birth.

Intravital imaging reveals glomerular capillary distension and endothelial and immune cell activation early in Alport syndrome

Alport syndrome (AS) is a genetic disorder caused by mutations in type IV collagen that lead to defective glomerular basement membrane, glomerular filtration barrier (GFB) damage, and progressive chronic kidney disease. While the genetic basis of AS is well known, the molecular and cellular mechanistic details of disease pathogenesis have been elusive, hindering the development of mechanism-based therapies. Here, we performed intravital multiphoton imaging of the local kidney tissue microenvironment in a X-linked AS mouse model to directly visualize the major drivers of AS pathology. Severely distended glomerular capillaries and aneurysms were found accompanied by numerous microthrombi, increased glomerular endothelial surface layer (glycocalyx) and immune cell homing, GFB albumin leakage, glomerulosclerosis, and interstitial fibrosis by 5 months of age, with an intermediate phenotype at 2 months. Renal histology in mouse or patient tissues largely failed to detect capillary aberrations. Treatment of AS mice with hyaluronidase or the ACE inhibitor enalapril reduced the excess glomerular endothelial glycocalyx and blocked immune cell homing and GFB albumin leakage. This study identified central roles of glomerular mechanical forces and endothelial and immune cell activation early in AS, which could be therapeutically targeted to reduce mechanical strain and local tissue inflammation and improve kidney function.

CyTOF-Enabled Analysis Identifies Class-Switched B Cells as the Main Lymphocyte Subset Associated With Disease Relapse in Children With Idiopathic Nephrotic Syndrome

B cell depleting therapies permit immunosuppressive drug withdrawal and maintain remission in patients with frequently relapsing nephrotic syndrome (FRNS) or steroid–dependent nephrotic syndrome (SDNS), but lack of biomarkers for treatment failure. Post-depletion immune cell reconstitution may identify relapsing patients, but previous characterizations suffered from methodological limitations of flow cytometry. Time-of-flight mass cytometry (CyTOF) is a comprehensive analytic modality that simultaneously quantifies over 40 cellular markers. Herein, we report CyTOF-enabled immune cell comparisons over a 12-month period from 30 children with SDNS receiving B cell depleting therapy who either relapsed (n = 17) or remained stable (n = 13). Anti-CD20 treatment depleted all B cells subsets and CD20 depleting agent choice (rituximab vs ofatumumab) did not affect B cell subset recovery. Despite equal total numbers of B cells, 5 subsets of B cells were significantly higher in relapsing individuals; all identified subsets of B cells were class-switched. T cell subsets (including T follicular helper cells and regulatory T cells) and other major immune compartments were largely unaffected by B cell depletion, and similar between relapsing and stable children. In conclusion, CyTOF analysis of immune cells from anti-CD20 antibody treated patients identifies class-switched B cells as the main subset whose expansion associates with disease relapse. Our findings set the basis for future studies exploring how identified subsets can be used to monitor treatment response and improve our understanding of the pathogenesis of the disease.

Effect of disease progression on the podocyte cell cycle in Alport Syndrome

Progression of glomerulosclerosis is associated with loss of podocytes with subsequent glomerular tuft instability. It is thought that a diminished number of podocytes may be able to preserve tuft stability through cell hypertrophy associated with cell cycle reentry. At the same time, reentry into the cell cycle risks podocyte detachment if podocytes cross the G1/S checkpoint and undergo abortive cytokinesis. In order to study cell cycle dynamics during chronic kidney disease (CKD) development, we used a FUCCI model (fluorescence ubiquitination-based cell cycle indicator) of mice with X-linked Alport Syndrome. This model exhibits progressive CKD and expresses fluorescent reporters of cell cycle stage exclusively in podocytes. With the development of CKD, an increasing fraction of podocytes in vivo were found to be in G1 or later cell cycle stages. Podocytes in G1 and G2 were hypertrophic. Heterozygous female mice, with milder manifestations of CKD, showed G1 fraction numbers intermediate between wild-type and male Alport mice. Proteomic analysis of podocytes in different cell cycle phases showed differences in cytoskeleton reorganization and metabolic processes between G0 and G1 in disease. Additionally, in vitro experiments confirmed that damaged podocytes reentered the cell cycle comparable to podocytes in vivo. Importantly, we confirmed the upregulation of PDlim2, a highly expressed protein in podocytes in G1, in a patient with Alport Syndrome, confirming our proteomics data in the human setting. Thus, our data showed that in the Alport model of progressive CKD, podocyte cell cycle distribution is altered, suggesting that cell cycle manipulation approaches may have a role in the treatment of various progressive glomerular diseases characterized by podocytopenia.

Persistent COUP-TFII Expression Underlies the Myopathy and Impaired Muscle Regeneration Observed in Resistance to Thyroid Hormone-Alpha

Myopathic changes, including muscular dystrophy and weakness, are commonly described in hypothyroid and hyperthyroid patients. Thyroid hormone signaling, via activation of thyroid nuclear receptor alpha (THRA), plays an essential role in maintaining muscle mass, function, and regeneration. A mouse model of resistance to thyroid hormone carrying a frameshift mutation in the THRA gene (THRA-PV) is associated with accelerated skeletal muscle loss with aging and impaired regeneration after injury^(1,2). We previously demonstrated that the expression of nuclear orphan receptor chicken ovalbumin upstream promoter-factor II (COUP-TFII, or Nr2f2) persists during myogenic differentiation in THRA-PV myoblasts and skeletal muscle of aged THRA- PV mice. COUP-TFII is known to regulate myogenesis negatively and has a role in Duchenne-like Muscular Dystrophies⁽³⁾. COUP-TFII physically and functionally interacts with THRA in primary myoblasts isolated from WT and THRA-PV mice, as demonstrated via co-immunoprecipitation and chromatin-immunoprecipitation. We observed that satellite cells from THRA-PV mice display impaired myoblast proliferation and in vitro myogenic differentiation compared to WT cells. However, the silencing of COUP-TFII expression using siRNA probes restores in vitro myogenic potential of THRA-PV myoblasts and shifts the mRNA expression profile closer to WT myoblasts, with a higher proliferation of myoblasts and a higher number of fully differentiated myotubes after 5 days of myogenic induction. Moreover, RNAseq analysis on myoblasts from THRA-PV mice after COUP-TFII knockdown shows that COUP-TFII silencing reverses the transcriptomic profile of THRA-PV myoblasts and results in reactivation of pathways involved in muscle function and extracellular matrix remodeling/deposition. These findings indicate that the persistent COUP-TFII expression in THRA-PV mice is responsible for the abnormal muscle phenotype. In conclusion, COUP-TFII and THRA cooperate during murine post-natal myogenesis, and COUP-TFII is critical for the accelerated skeletal muscle loss with aging and impaired muscle regeneration after injury in THRA-PV mice. These studies can help increase our knowledge of the mechanisms involved in thyroid hormone signaling during skeletal muscle regeneration, ultimately increasing the possibility of designing more specific treatments for patients with thyroid hormone-induced myopathies. References: 1. Milanesi, A., et al., Endocrinology 2016; 2. Kaneshige, M. et al., Proc Natl Acad Sci U S 2001; 3. Lee HJ, et al, Sci Rep. 2017.

Loss of decay-accelerating factor triggers podocyte injury and glomerulosclerosis

Kidney glomerulosclerosis commonly progresses to end-stage kidney failure, but pathogenic mechanisms are still poorly understood. Here, we show that podocyte expression of decay-accelerating factor (DAF/CD55), a complement C3 convertase regulator, crucially controls disease in murine models of adriamycin (ADR)-induced focal and segmental glomerulosclerosis (FSGS) and streptozotocin (STZ)-induced diabetic glomerulosclerosis. ADR induces enzymatic cleavage of DAF from podocyte surfaces, leading to complement activation. C3 deficiency or prevention of C3a receptor (C3aR) signaling abrogates disease despite DAF deficiency, confirming complement dependence. Mechanistic studies show that C3a/C3aR ligations on podocytes initiate an autocrine IL-1β/IL-1R1 signaling loop that reduces nephrin expression, causing actin cytoskeleton rearrangement. Uncoupling IL-1β/IL-1R1 signaling prevents disease, providing a causal link. Glomeruli of patients with FSGS lack DAF and stain positive for C3d, and urinary C3a positively correlates with the degree of proteinuria. Together, our data indicate that the development and progression of glomerulosclerosis involve loss of podocyte DAF, triggering local, complement-dependent, IL-1β–induced podocyte injury, potentially identifying new therapeutic targets.

Persistent COUP-TFII expression underlies the myopathy and impaired muscle regeneration observed in resistance to thyroid hormone-alpha

Thyroid hormone signaling plays an essential role in muscle development and function, in the maintenance of muscle mass, and in regeneration after injury, via activation of thyroid nuclear receptor alpha (THRA). A mouse model of resistance to thyroid hormone carrying a frame-shift mutation in the THRA gene (THRA-PV) is associated with accelerated skeletal muscle loss with aging and impaired regeneration after injury. The expression of nuclear orphan receptor chicken ovalbumin upstream promoter-factor II (COUP-TFII, or Nr2f2) persists during myogenic differentiation in THRA-PV myoblasts and skeletal muscle of aged THRA-PV mice and it is known to negatively regulate myogenesis. Here, we report that in murine myoblasts COUP-TFII interacts with THRA and modulates THRA binding to thyroid response elements (TREs). Silencing of COUP-TFII expression restores in vitro myogenic potential of THRA-PV myoblasts and shifts the mRNA expression profile closer to WT myoblasts. Moreover, COUP-TFII silencing reverses the transcriptomic profile of THRA-PV myoblasts and results in reactivation of pathways involved in muscle function and extracellular matrix remodeling/deposition. These findings indicate that the persistent COUP-TFII expression in THRA-PV mice is responsible for the abnormal muscle phenotype. In conclusion, COUP-TFII and THRA cooperate during post-natal myogenesis, and COUP-TFII is critical for the accelerated skeletal muscle loss with aging and impaired muscle regeneration after injury in THRA-PV mice.

A Comprehensive Phenotypic and Functional Immune Analysis Unravels Circulating Anti-Phospholipase A2 Receptor Antibody Secreting Cells in Membranous Nephropathy Patients

Introduction

Primary membranous nephropathy (MN) is characterized by the presence of antipodocyte antibodies, but studies describing phenotypic and functional abnormalities in circulating lymphocytes are limited.

Methods

We analyzed 68 different B- and T-cell subsets using flow cytometry in 30 MN patients (before initiating immunosuppression) compared with 31 patients with non–immune-mediated chronic kidney disease (CKD) and 12 healthy individuals. We also measured 19 serum cytokines in MN patients and in healthy controls. Lastly, we quantified the ex vivo production of phospholipase A2 receptor (PLA2R)-specific IgG by plasmablasts (measuring antibodies in culture supernatants and by the newly developed FluoroSpot assay [AutoImmun Diagnostika, Strasberg, Germany]) and assessed the circulating antibody repertoire by phage immunoprecipitation sequencing (PhIP-Seq).

Results

After adjusting for multiple testing, plasma cells and regulatory B cells (B_REG) were significantly higher (P < 0.05) in MN patients compared with both control groups. The percentages of circulating plasma cells correlated with serum anti-PLA2R antibody levels (P = 0.042) and were associated with disease activity. Ex vivo–expanded PLA2R-specific IgG-producing plasmablasts generated from circulating PLA2R-specific memory B cells (mBCs) correlated with serum anti-PLA2R IgG antibodies (P < 0.001) in MN patients. Tumor necrosis factor-α (TNF-α) was the only significantly increased cytokine in MN patients (P < 0.05), whereas there was no significant difference across study groups in the autoantibody and antiviral antibody repertoire.

Conclusion

This extensive phenotypic and functional immune characterization shows that autoreactive plasma cells are present in the circulation of MN patients, providing a new therapeutic target and a candidate biomarker of disease activity.

Glomerular endothelial cell heterogeneity in Alport syndrome

Glomerular endothelial cells (GEC) are a crucial component of the glomerular physiology and their damage contributes to the progression of chronic kidney diseases. How GEC affect the pathology of Alport syndrome (AS) however, is unclear. We characterized GEC from wild type (WT) and col4α5 knockout AS mice, a hereditary disorder characterized by progressive renal failure. We used endothelial-specific Tek-tdTomato reporter mice to isolate GEC by FACS and performed transcriptome analysis on them from WT and AS mice, followed by in vitro functional assays and confocal and intravital imaging studies. Biopsies from patients with chronic kidney disease, including AS were compared with our findings in mice. We identified two subpopulations of GEC (dim^tdT and bright^tdT) based on the fluorescence intensity of the Tek^tdT signal. In AS mice, the bright^tdT cell number increased and presented differential expression of endothelial markers compared to WT. RNA-seq analysis revealed differences in the immune and metabolic signaling pathways. In AS mice, dim^tdT and bright^tdT cells had different expression profiles of matrix-associated genes (Svep1, Itgβ6), metabolic activity (Apom, Pgc1α) and immune modulation (Apelin, Icam1) compared to WT mice. We confirmed a new pro-inflammatory role of Apelin in AS mice and in cultured human GEC. Gene modulations were identified comparable to the biopsies from patients with AS and focal segmental glomerulosclerosis, possibly indicating that the same mechanisms apply to humans. We report the presence of two GEC subpopulations that differ between AS and healthy mice or humans. This finding paves the way to a better understanding of the pathogenic role of GEC in AS progression and could lead to novel therapeutic targets.

OR01-03 Thyroid Receptor Alpha Interacts with COUP-TFII in the Regulation of Postnatal Skeletal Muscle Regeneration

Myopathic changes, including muscular dystrophy and weakness, are commonly described in hypothyroid and hyperthyroid patients. Thyroid hormone signaling, via activation of thyroid nuclear receptors (TRs), plays an essential role in the maintenance of muscle mass, function, and regeneration. TRs are ligand-inducible transcription factors expressed in almost all tissues, including skeletal muscle. In a mouse model of Resistance to Thyroid Hormone carrying a frame-shift mutation in the TRα gene (TRα1PV)^1,2 we observed skeletal muscle loss with aging and impaired skeletal muscle regeneration after injury. We recently described that TRα interacts with the nuclear orphan receptor Chicken Ovalbumin Upstream Promoter-factor II (COUP-TFII, or NR2F2), which is known to regulate myogenesis negatively and has a role in Duchenne-like Muscular Dystrophies³. We showed that COUP-TFII expression declines with age in WT mice, while the skeletal muscle of TRα1PV mice shows a sustained significantly higher expression of COUP-TFII. Our findings suggest that the TRα/COUP-TFII interaction might mediate the impaired skeletal muscle phenotype observed in TRα1PV mice. To better characterize this interaction, we isolated SC from 10 months old WT and TRα1PV mice and cultured them in vitro using novel methods established within our lab. Using siRNA probes, we next silenced COUP-TFII and characterized the cells via RNA-seq analysis. In vitro, we assessed myoblast differentiation and proliferation using differentiation assays and EdU incorporation. We observed that satellite cells from TRα1PV mice display impaired myoblast proliferation and in vitro myogenic differentiation compared to WT SCs. However, when COUP-TFII was silenced, the myogenic potential of TRα1PV satellite cells was restored, with a higher proliferation of myoblasts and a higher number of fully differentiated myotubes after 4 days of myogenic induction. RNAseq analysis on satellite cells from TRα1PV mice after COUP-TFII knockdown showed upregulation of genes involved in the myogenic pathway, such as Myod1 and Pax7, and of genes in the thyroid hormone signaling, such as Dio2. Ingenuity Pathway Analysis further showed activation of pathways regarding cell growth, differentiation, matrix remodeling along with muscle function, muscle contractility, and muscle contraction. These in vitro results suggest that by silencing COUP-TFII we promote the myogenic pathway and may further rescue the impaired phenotype of TRα1PV mice. These studies can help increase our knowledge of the mechanisms involved in thyroid hormone signaling in skeletal muscle regeneration, which will ultimately increase the possibility of designing more specific treatments for patients with thyroid hormone-induced myopathies. References: ¹Milanesi, A., et al, Endocrinology 2016; ²Kaneshige, M. et al, Proc Natl Acad Sci U S 2001; ³Lee HJ, et al, Sci Rep. 2017.

SOX9+/PTF1A+ Cells Define the Tip Progenitor Cells of the Human Fetal Pancreas of the Second Trimester

Significant progress has been made in recent years in characterizing human multipotent progenitor cells (hMPCs) of the early pancreas; however, the identity and persistence of these cells during the second trimester, after the initiation of branching morphogenesis, remain elusive. Additionally, studies on hMPCs have been hindered by few isolation methods that allow for the recovery of live cells. Here, we investigated the tip progenitor domain in the branched epithelium of human fetal pancreas between 13.5 and 17.5 gestational weeks by immunohistological staining. We also used a novel RNA-based technology to isolate live cells followed by gene expression analyses. We identified cells co-expressing SOX9 and PTF1A, two transcription factors known to be important for pancreatic MPCs, within the tips of the epithelium and observed a decrease in their proportions over time. Pancreatic SOX9+/PTF1A+ cells were enriched for MPC markers, including MYC and GATA6. These cells were proliferative and appeared active in branching morphogenesis and matrix remodeling, as evidenced by gene set enrichment analysis. We identified a hub of genes pertaining to the expanding tip progenitor niche, such as FOXF1, GLI3, TBX3, FGFR1, TGFBR2, ITGAV, ITGA2, and ITGB3. YAP1 of the Hippo pathway emerged as a highly enriched component within the SOX9+/PTF1A+ cells. Single-cell RNA-sequencing further corroborated the findings by identifying a cluster of SOX9+/PTF1A+ cells with multipotent characteristics. Based on these results, we propose that the SOX9+/PTF1A+ cells in the human pancreas are uncommitted MPC-like cells that reside at the tips of the expanding pancreatic epithelium, directing self-renewal and inducing pancreatic organogenesis. Stem Cells Translational Medicine 2019;8:1249&1264.

Circulating B Cells With Memory and Antibody-Secreting Phenotypes Are Detectable in Pediatric Kidney Transplant Recipients Before the Development of Antibody-Mediated Rejection

Development of anti-human leukocyte antigen donor-specific antibodies (DSAs) is associated with antibody-mediated rejection (AMR) and reduced allograft survival in kidney transplant recipients. Whether changes in circulating lymphocytes anticipate DSA or AMR development is unclear.

METHODS

We used time-of-flight mass cytometry to analyze prospectively collected peripheral blood mononuclear cells (PBMC) from pediatric kidney transplant recipients who developed DSA (DSA-positive recipients [DSAPOS], n = 10). PBMC were obtained at 2 months posttransplant, 3 months before DSA development, and at DSA detection. PBMC collected at the same time points posttransplant from recipients who did not develop DSA (DSA-negative recipients [DSANEG], n = 11) were used as controls.

RESULTS

DSAPOS and DSANEG recipients had similar baseline characteristics and comparable frequencies of total B and T cells. Within DSAPOS recipients, there was no difference in DSA levels (mean fluorescence intensity [MFI]: 13 687 ± 4159 vs 11 375 ± 1894 in DSAPOSAMR-positive recipients (AMRPOS) vs DSAPOSAMR-negative recipients (AMRNEG), respectively; P = 0.630), C1q binding (5 DSAPOSAMRPOS [100%] vs 4 DSAPOSAMRNEG [80%]; P = 1.000), or C3d binding (3 DSAPOSAMRPOS [60%] vs 1 DSAPOSAMRNEG [20%]; P = 0.520) between patients who developed AMR and those who did not. However, DSAPOS patients who developed AMR (n = 5; 18.0 ± 3.6 mo post-DSA detection) had increased B cells with antibody-secreting (IgD-CD27+CD38+; P = 0.002) and memory (IgD-CD27+CD38-; P = 0.003) phenotypes compared with DSANEG and DSAPOSAMRNEG recipients at DSA detection.

CONCLUSIONS

Despite the small sample size, our comprehensive phenotypic analyses show that circulating B cells with memory and antibody-secreting phenotypes are present at DSA onset, >1 year before biopsy-proven AMR in pediatric kidney transplant recipients.

A glomerulus-on-a-chip to recapitulate the human glomerular filtration barrier

In this work we model the glomerular filtration barrier, the structure responsible for filtering the blood and preventing the loss of proteins, using human podocytes and glomerular endothelial cells seeded into microfluidic chips. In long-term cultures, cells maintain their morphology, form capillary-like structures and express slit diaphragm proteins. This system recapitulates functions and structure of the glomerulus, including permselectivity. When exposed to sera from patients with anti-podocyte autoantibodies, the chips show albuminuria proportional to patients' proteinuria, phenomenon not observed with sera from healthy controls or individuals with primary podocyte defects. We also show its applicability for renal disease modeling and drug testing. A total of 2000 independent chips were analyzed, supporting high reproducibility and validation of the system for high-throughput screening of therapeutic compounds. The study of the patho-physiology of the glomerulus and identification of therapeutic targets are also feasible using this chip.

SAT-560 Role of COUP-TFII and Thyroid Hormone Receptor α in Skeletal Muscle Loss

Myopathic changes including muscular dystrophy and weakness are commonly described in hypothyroid and hyperthyroid patients. The genomic actions of triiodothyronine (T3) are mediated by thyroid hormone nuclear receptors (TRs), which act as ligand-inducible transcription factors in almost all tissues, including skeletal muscle. Impaired skeletal muscle regeneration and sarcopenia with aging have been reported in a mouse model of Resistance to Thyroid Hormone (RTH) carrying a frame-shift mutation in the TRα gene (TRα1PV). One salient finding was a significantly smaller pool of PAX7-positive satellite cells (SCs) in the skeletal muscle of TRα1PV mice. Moreover, SC function during skeletal muscle injury was impaired four days after cardiotoxin (CTX)-induced skeletal muscle injury with a decreased activation of SC and a reduced proliferation of Myf5 expressing cells. TRα plays an important role in the maintenance of muscle mass and skeletal muscle regeneration after injury with aging, and loss of skeletal muscle mass with aging in TRα1PV mice was reported to be associated with loss of SC pool {1,2}. Interestingly, overexpression of the nuclear orphan receptor Chicken Ovalbumin Upstream Promoter-factor II (COUP-TFII or NR2F2) in murine satellite cells have been shown to induce a similar skeletal muscle phenotype, including skeletal muscle loss with aging, and inhibit myogenesis through modulation of Myf5 and MyoD expression {3}. We detected a higher expression of COUP-TFII in C2C12, a murine myoblast cell line, during proliferation and a decline during differentiation into myotube. Moreover, we analyzed the skeletal muscle of mice at different ages and we found a higher expression of COUP-TFII in the first 2 months of life followed by a decline with age. Thus, suggesting the important role of COUP-TFII in modulating post-natal myogenesis and onset of sarcopenia with aging. In proliferating C2C12 myoblasts and SCs from wild-type and TRα1PV mice, we demonstrated via co-immunoprecipitation that COUP-TFII and TRα interact. In addition, the skeletal muscle of TRα1PV mice showed significantly higher expression of COUP-TFII compared to their WT siblings, and in TRα1PV mice COUP-TFII expression remained higher with skeletal muscle aging. These results suggest a COUP-TFII role in skeletal muscle loss with aging and impaired skeletal muscle regeneration in TRαPV mice that may be mediated by COUP-TFII-TRα interaction. These new insights can provide a therapeutic target to prevent or treat myopathies, such as sarcopenia and Duchenne-like muscular dystrophy. 1. Milanesi A et al, Endocrinology 2015. 2. Milanesi A et al, Thyroid, 2017. 3. Lee HJ et al, Scientific Reports 2017. Sources of Research Support: NIH grants and VA MERIT GRANT.

T-cell exhaustion correlates with improved outcomes in kidney transplant recipients

Continuous antigen stimulation during chronic infection or malignancy can promote functional T cell silencing, a phenomenon called T cell exhaustion. The prevalence and impact of T cell exhaustion following organ transplantation, another immune stimulus with persistently high antigen load, are unknown. Here, we characterized serially collected peripheral blood mononuclear cells from 26 kidney transplant recipients using time-of-flight mass cytometry (CyTOF) to define distinct subsets of circulating exhausted T cells and their relationship to induction therapy and allograft function. We observed an increase in specific subsets of CD4⁺ and CD8⁺ exhausted T cells from pre-transplant to 6-months post-transplant, with greater increases in participants given anti-thymocyte globulin induction than in participants who received no induction or non-depleting induction. The percentages of exhausted T cells at 6 months correlated inversely with adenosine triphosphate (ATP) production (a surrogate of T cell function) and with allograft interstitial fibrosis. Guided by the CyTOF data, we delineated a PD-1⁺CD57^- phenotype for CD4⁺ and CD8⁺ exhausted T cells, and confirmed that these cells have limited capacity for cytokine secretion and ATP production. In an independent cohort of 50 kidney transplant recipients, we confirmed the predicted increase of PD-1⁺CD57^- exhausted T cells after lymphocyte-depleting induction therapy and its direct correlation with better allograft function. Our findings suggest that monitoring T cell exhaustion can be useful for post-transplant risk assessment and support the need to develop and test strategies aimed at augmenting T cell exhaustion following kidney transplantation.

Concise Reviews: Stem Cells and Kidney Regeneration: An Update

Significant progress has been made to advance stem cell products as potential therapies for kidney diseases: various kinds of stem cells can restore renal function in preclinical models of acute and chronic kidney injury. Nonetheless this literature contains contradictory results, and for this reason, we focus this review on reasons for apparent discrepancies in the literature, because they contribute to difficulty in translating renal regenerative therapies. Differences in methodologies used to derive and culture stem cells, even those from the same source, in addition to the lack of standardized renal disease animal models (both acute and chronic), are important considerations underlying contradictory results in the literature. We propose that harmonized rigorous protocols for characterization, handling, and delivery of stem cells in vivo could significantly advance the field, and present details of some suggested approaches to foster translation in the field of renal regeneration. Our goal is to encourage coordination of methodologies (standardization) and long‐lasting collaborations to improve protocols and models to lead to reproducible, interpretable, high‐quality preclinical data. This approach will certainly increase our chance to 1 day offer stem cell therapeutic options for patients with all‐too‐common renal diseases. Stem Cells Translational Medicine2019;8:82–92

Impact of preformed T-cell alloreactivity by means of donor-specific and panel of reactive T cells (PRT) ELISPOT in kidney transplantation

Donor-specific (d-sp) interferon gamma enzyme-linked immunosorbent spot (d-sp ELISPOT) and Panel of reactive T-cell (PRT) ELISPOT assays have been developed to detect alloreactive memory T (Tmem) cells in order to estimate the risk of acute rejection after kidney transplantation. Adding IL15 to the PRT assay (PRT+IL15) may uncover the presence of pathogenic alloreactive CD28^-Tmem. Face-to-face comparisons of these assays have not been done yet. We performed pre-transplant d-sp ELISPOT and PRT assays (±IL15, against six B-cell lines) in 168 consecutive kidney transplant recipients and evaluated the multivariable-adjusted associations with biopsy-proven acute rejection (BPAR), de novo donor-specific antibodies (DSA), and eGFR decline over a 48-month follow-up period. D-sp ELISPOT was positive in 81 (48%) subjects, while 71 (42%) and 81 (48%) subjects displayed positive PRT and PRT+IL15, respectively. Their median [interquartile range] numerical test result was 23 [6–65], 18 [8–37], and 26 [10–45] spots/3x10⁵ PBMCs, respectively. The number of PRT spots were weakly correlated with those of d-sp ELISPOT, but highly correlated with PRT+IL15 (rho = 0.96, P<0.001). d-sp ELISPOT, but not PRT (±IL15) was independently associated with BPAR (adjusted Odds Ratio of BPAR associated with d-sp ELISPOT positivity: 4.20 [95%CI: 1.06 to 21.73; P = 0.041]). Unlike d-sp ELISPOT, median PRT and PRT+IL15 were independently associated with higher Δ3-48month eGFR decline post-transplantation (for both assays, about -3mL/min/1.73m2 per one standard deviation unit increase in the spot number). Pre-transplant T-cell immune-monitoring using d-sp ELISPOT and PRT assays identifies kidney transplant candidates at high risk of BPAR and worse kidney allograft progression.

Amniotic fluid cells: current progress and emerging challenges in renal regeneration

Amniotic fluid (AF) contains a heterogeneous population of cells that have been identified to possess pluripotent and progenitor-like characteristics. These cells have been applied in various regenerative medicine applications ranging from in vitro cell differentiation to tissue engineering to cellular therapies for different organs including the heart, the liver, the lung, and the kidneys. In this review, we examine the different methodologies used for the derivation of amniotic fluid stem cells and renal progenitors, and their application in renal repair and regeneration. Moreover, we discuss the recent achievements and newly emerging challenges in our understanding of their biology, their immunoregulatory characteristics, and their paracrine-mediated therapeutic potential for the treatment of acute and chronic kidney diseases.

The bridge between transplantation and regenerative medicine: Beginning a new Banff classification of tissue engineering pathology

The science of regenerative medicine is arguably older than transplantation-the first major textbook was published in 1901-and a major regenerative medicine meeting took place in 1988, three years before the first Banff transplant pathology meeting. However, the subject of regenerative medicine/tissue engineering pathology has never received focused attention. Defining and classifying tissue engineering pathology is long overdue. In the next decades, the field of transplantation will enlarge at least tenfold, through a hybrid of tissue engineering combined with existing approaches to lessening the organ shortage. Gradually, transplantation pathologists will become tissue-(re-) engineering pathologists with enhanced skill sets to address concerns involving the use of bioengineered organs. We outline ways of categorizing abnormalities in tissue-engineered organs through traditional light microscopy or other modalities including biomarkers. We propose creating a new Banff classification of tissue engineering pathology to standardize and assess de novo bioengineered solid organs transplantable success in vivo. We recommend constructing a framework for a classification of tissue engineering pathology now with interdisciplinary consensus discussions to further develop and finalize the classification at future Banff Transplant Pathology meetings, in collaboration with the human cell atlas project. A possible nosology of pathologic abnormalities in tissue-engineered organs is suggested.

Regenerative immunology: the immunological reaction to biomaterials

Regenerative medicine promises to meet two of the most urgent needs of modern organ transplantation, namely immunosuppression-free transplantation and an inexhaustible source of organs. Ideally, bioengineered organs would be manufactured from a patient's own biomaterials-both cells and the supporting scaffolding materials in which cells would be embedded and allowed to mature to eventually regenerate the organ in question. While some groups are focusing on the feasibility of this approach, few are focusing on the immunogenicity of the scaffolds that are being developed for organ bioengineering purposes. This review will succinctly discuss progress in the understanding of immunological characteristics and behavior of different scaffolds currently under development, with emphasis on the extracellular matrix scaffolds obtained decellularized animal or human organs which seem to provide the ideal template for bioengineering purposes.

Thyroid Hormone Receptor Alpha is Essential to Maintain the Satellite Cell Niche During Skeletal Muscle Injury and Sarcopenia of Aging

BACKGROUND

Myopathic changes are commonly described in hypothyroid and hyperthyroid patients, including muscular atrophy and weakness. Satellite cells (SCs) play a major role in skeletal muscle maintenance and regeneration after injury. A mouse model of resistance to thyroid hormone-TRα1PV demonstrated impaired skeletal muscle regeneration after injury with significant reduction of SCs, suggesting that exhaustion of the SC pool contributes to the impaired regeneration. To test this hypothesis, SC activation and proliferation were analyzed in vivo in response to skeletal muscle injury and during aging.

METHODS

SCs of TRα1PV male mice were analyzed four days after cardiotoxin-induced muscle injury, and they were compared to wild-type (WT) male animals. TRα-knockdown C2C12 myoblasts were injected into injured skeletal muscle, and four days after transplantation, the in vivo behavior was compared to control C2C12 myoblasts. Skeletal muscle regeneration was compared in younger and older TRα1PV and WT animals.

RESULTS

The total number of SCs in skeletal muscle of TRα1PV mice was significantly lower than control, both before and shortly after muscle injury, with significant impairment of SC activation, consistent with SC pool exhaustion. TRα-knockdown myoblasts showed impaired in vivo proliferation and migration. TRα1PV mice had skeletal muscle loss and significant impairment in skeletal muscle regeneration with aging. This translated to a significant reduction of the SC pool with aging compared to WT mice.

CONCLUSION

TRα plays an important role in the maintenance of the SC pool. Impaired skeletal muscle regeneration in TRα1PV mice is associated with insufficient SC activation and proliferation, as well as the progressive loss of the SC pool with aging. Regulation of the SC pool and SC proliferation provides a therapeutic target to enhance skeletal muscle regeneration and possibly slow age-associated sarcopenia.

Advances and unmet needs in genetic, basic and clinical science in Alport syndrome: report from the 2015 International Workshop on Alport Syndrome

Alport syndrome (AS) is a genetic disease characterized by haematuric glomerulopathy variably associated with hearing loss and anterior lenticonus. It is caused by mutations in the COL4A3, COL4A4 or COL4A5 genes encoding the α3α4α5(IV) collagen heterotrimer. AS is rare, but it accounts for >1% of patients receiving renal replacement therapy. Angiotensin-converting enzyme inhibition slows, but does not stop, the progression to renal failure; therefore, there is an urgent requirement to expand and intensify research towards discovering new therapeutic targets and new therapies. The 2015 International Workshop on Alport Syndrome targeted unmet needs in basic science, genetics and diagnosis, clinical research and current clinical care. In three intensive days, more than 100 international experts including physicians, geneticists, researchers from academia and industry, and patient representatives from all over the world participated in panel discussions and breakout groups. This report summarizes the most important priority areas including (i) understanding the crucial role of podocyte protection and regeneration, (ii) targeting mutations by new molecular techniques for new animal models and potential gene therapy, (iii) creating optimal interaction between nephrologists and geneticists for early diagnosis, (iv) establishing standards for mutation screening and databases, (v) improving widespread accessibility to current standards of clinical care, (vi) improving collaboration with the pharmaceutical/biotech industry to investigate new therapies, (vii) research in hearing loss as a huge unmet need in Alport patients and (viii) the need to evaluate the risk and benefit of novel (including 'repurposing') therapies on an international basis.

Fasting-Mimicking Diet Promotes Ngn3-Driven β-Cell Regeneration to Reverse Diabetes

Stem-cell-based therapies can potentially reverse organ dysfunction and diseases, but the removal of impaired tissue and activation of a program leading to organ regeneration pose major challenges. In mice, a 4-day fasting mimicking diet (FMD) induces a stepwise expression of Sox17 and Pdx-1, followed by Ngn3-driven generation of insulin-producing β cells, resembling that observed during pancreatic development. FMD cycles restore insulin secretion and glucose homeostasis in both type 2 and type 1 diabetes mouse models. In human type 1 diabetes pancreatic islets, fasting conditions reduce PKA and mTOR activity and induce Sox2 and Ngn3 expression and insulin production. The effects of the FMD are reversed by IGF-1 treatment and recapitulated by PKA and mTOR inhibition. These results indicate that a FMD promotes the reprogramming of pancreatic cells to restore insulin generation in islets from T1D patients and reverse both T1D and T2D phenotypes in mouse models. PAPERCLIP.

Direct Isolation and Characterization of Human Nephron Progenitors

Mature nephrons originate from a small population of uninduced nephrogenic progenitor cells (NPs) within the cap mesenchyme. These cells are characterized by the coexpression of SIX2 and CITED1. Many studies on mouse models as well as on human pluripotent stem cells have advanced our knowledge of NPs, but very little is known about this population in humans, since it is exhausted before birth and strategies for its direct isolation are still limited. Here we report an efficient protocol for direct isolation of human NPs without genetic manipulation or stepwise induction procedures. With the use of RNA‐labeling probes, we isolated SIX2⁺CITED1⁺ cells from human fetal kidney for the first time. We confirmed their nephrogenic state by gene profiling and evaluated their nephrogenic capabilities in giving rise to mature renal cells. We also evaluated the ability to culture these cells without complete loss of SIX2 and CITED1 expression over time. In addition to defining the gene profile of human NPs, this in vitro system facilitates studies of human renal development and provides a novel tool for renal regeneration and bioengineering purposes. Stem Cells Translational Medicine2017;6:419–433

A step towards clinical application of acellular matrix: A clue from macrophage polarization

The outcome of tissue engineered organ transplants depends on the capacity of the biomaterial to promote a pro-healing response once implanted in vivo. Multiple studies, including ours, have demonstrated the possibility of using the extracellular matrix (ECM) of animal organs as platform for tissue engineering and more recently, discarded human organs have also been proposed as scaffold source. In contrast to artificial biomaterials, natural ECM has the advantage of undergoing continuous remodeling which allows adaptation to diverse conditions. It is known that natural matrices present diverse immune properties when compared to artificial biomaterials. However, how these properties compare between diseased and healthy ECM and artificial scaffolds has not yet been defined. To answer this question, we used decellularized renal ECM derived from WT mice and from mice affected by Alport Syndrome at different time-points of disease progression as a model of renal failure with extensive fibrosis. We characterized the morphology and composition of these ECMs and compared their in vitro effects on macrophage activation with that of synthetic scaffolds commonly used in the clinic (collagen type I and poly-L-(lactic) acid, PLLA). We showed that ECM derived from Alport kidneys differed in fibrous protein deposition and cytokine content when compared to ECM derived from WT kidneys. Yet, both WT and Alport renal ECM induced macrophage differentiation mainly towards a reparative (M2) phenotype, while artificial biomaterials towards an inflammatory (M1) phenotype. Anti-inflammatory properties of natural ECMs were lost when homogenized, hence three-dimensional structure of ECM seems crucial for generating an anti-inflammatory response. Together, these data support the notion that natural ECM, even if derived from diseased kidneys promote a M2 protolerogenic macrophage polarization, thus providing novel insights on the applicability of ECM obtained from discarded organs as ideal scaffold for tissue engineering.