Transcriptomics Analysis Identifies the Decline in the AT2 Stem Cell Niche in Aged Human Lungs.

Aging poses a global public health challenge, which is linked to the rise of age-related lung diseases. The precise understanding of the molecular and genetic changes in the aging lung that elevate the risk of acute and chronic lung diseases remains incomplete. Alveolar type II (AT2) cells are stem cells that maintain epithelial homeostasis and repair the lung after injury. AT2 progenitor function decreases with aging. The maintenance of AT2 function requires niche support from other cell types, but little has been done to characterize alveolar alterations with aging in the AT2 niche. To systematically profile the genetic changes associated with age, we present a single-cell transcriptional atlas comprising nearly half a million cells from the healthy lungs of human subjects spanning various ages, sexes, and smoking statuses. Most annotated cell lineages in aged lungs exhibit dysregulated genetic programs. Specifically, the aged alveolar epithelial (AT2) cells demonstrate loss of epithelial identities, heightened inflammaging characterized by increased expression of AP-1 transcription factor and chemokine genes, and significantly increased cellular senescence. Furthermore, the aged mesenchymal cells display a remarkable decrease in Collagen and Elastin transcription and a loss of support to epithelial cell stemness. The decline of the AT2 niche is further exacerbated by a dysregulated genetic program in macrophages and dysregulated communications between AT2 and macrophages in aged human lungs. These findings highlight the dysregulations observed in both AT2 stem cells and their supportive niche cells, potentially contributing to the increased susceptibility of aged populations to lung diseases. This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/). For commercial usage and reprints please contact Diane Gern (dgern@thoracic.org).

Lipid Deficiency Contributes to Impaired Alveolar Progenitor Cell Function in Aging and Idiopathic Pulmonary Fibrosis.

Idiopathic pulmonary fibrosis (IPF) is an aging-associated interstitial lung disease resulting from repeated epithelial injury and inadequate epithelial repair. Alveolar type II cells (AEC2) are progenitor cells that maintain epithelial homeostasis and repair the lung after injury. In the current study, we assessed lipid metabolism in AEC2s from human lungs of IPF patients and healthy donors, as well as AEC2s from bleomycin-injured young and old mice. Through single cell RNA sequencing (scRNA-seq), we observed that lipid metabolism-related genes were downregulated in IPF AEC2s and bleomycin-injured mouse AEC2s. Aging aggravated this decrease and hindered recovery of lipid metabolism gene expression in AEC2s after bleomycin injury. Pathway analyses revealed down-regulation of genes related to lipid biosynthesis and fatty acid -oxidation in AEC2s from IPF lungs and bleomycin-injured, aged mouse lungs compared to the respective controls. We confirmed decreased cellular lipid content in AEC2s from IPF lungs and bleomycin-injured, aged mouse lungs using immunofluorescence staining and flow cytometry. We further show that lipid metabolism was associated with AEC2 progenitor function. Lipid supplementation and peroxisome proliferator activated receptor gamma (PPARγ) activation promoted progenitor renewal capacity of both human and mouse AEC2s in 3D organoid cultures. Lipid supplementation also increased AEC2 proliferation and expression of SFTPC in AEC2s. In summary, we identified a lipid metabolism deficiency in AEC2s from lungs of patients with IPF and bleomycin-injured aged mice. Restoration of lipid metabolism homeostasis in AEC2s might promote AEC2 progenitor function and offer new opportunities for therapeutic approaches to IPF. This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/).

CXCR3-Independent Role of CXCL10 in Alveolar Epithelial Repair.

The alveolar Type II epithelial (AEC2) cells act as stem cells in the lung for alveolar epithelial maintenance and repair. Chemokine CXCL10 is expressed in injured tissues, modulating multiple cellular functions. AEC2s, previously reported to release chemokines to recruit leukocytes, were found in our study to secrete CXCL10 after bleomycin injury. We found that Sftpc-Cxcl10 transgenic mice were protected from bleomycin injury. The transgenic mice showed an increase in the AEC2 population in the lung by flow cytometry analysis. Both endogenous and exogenous CXCL10 promoted the colony formation efficiency of AEC2s in a 3D organoid growth assay. We identified that the regenerative effect of CXCL10 was CXCR3 independent using Cxcr3-deficient mice, but it was related to the TrkA pathway. Binding experiments showed that CXCL10 interacted with TrkA directly and reversibly. This study demonstrates a previously unidentified AEC2 autocrine signaling of CXCL10 to promote their regeneration and proliferation, probably involving a CXCR3-independent TrkA pathway.

Multiple Fibroblast Subtypes Contribute to Matrix Deposition in Pulmonary Fibrosis.

Progressive pulmonary fibrosis results from a dysfunctional tissue repair response and is characterized by fibroblast proliferation, activation, and invasion and extracellular matrix accumulation. Lung fibroblast heterogeneity is well recognized. With single-cell RNA sequencing, fibroblast subtypes have been reported by recent studies. However, the roles of fibroblast subtypes in effector functions in lung fibrosis are not well understood. In this study, we incorporated the recently published single-cell RNA-sequencing datasets on murine lung samples of fibrosis models and human lung samples of fibrotic diseases and analyzed fibroblast gene signatures. We identified and confirmed the novel fibroblast subtypes we reported recently across all samples of both mouse models and human lung fibrotic diseases, including idiopathic pulmonary fibrosis, systemic sclerosis-associated interstitial lung disease, and coronavirus disease (COVID-19). Furthermore, we identified specific cell surface proteins for each fibroblast subtype through differential gene expression analysis, which enabled us to isolate primary cells representing distinct fibroblast subtypes by flow cytometry sorting. We compared matrix production, including fibronectin, collagen, and hyaluronan, after profibrotic factor stimulation and assessed the invasive capacity of each fibroblast subtype. Our results suggest that in addition to myofibroblasts, lipofibroblasts and Ebf1+ (Ebf transcription factor 1+) fibroblasts are two important fibroblast subtypes that contribute to matrix deposition and also have enhanced invasive, proliferative, and contraction phenotypes. The histological locations of fibroblast subtypes are identified in healthy and fibrotic lungs by these cell surface proteins. This study provides new insights to inform approaches to targeting lung fibroblast subtypes to promote the development of therapeutics for lung fibrosis.

Basal Cell-derived WNT7A Promotes Fibrogenesis at the Fibrotic Niche in Idiopathic Pulmonary Fibrosis.

Loss of epithelial integrity, bronchiolarization, and fibroblast activation are key characteristics of idiopathic pulmonary fibrosis (IPF). Prolonged accumulation of basal-like cells in IPF may impact the fibrotic niche to promote fibrogenesis. To investigate their role in IPF, basal cells were isolated from IPF explant and healthy donor lung tissues. Single-cell RNA sequencing was used to assess differentially expressed genes in basal cells. Basal cell and niche interaction was demonstrated with the sLP-mCherry niche labeling system. Luminex assays were used to assess cytokines secreted by basal cells. The role of basal cells in fibroblast activation was studied. Three-dimensional organoid culture assays were used to interrogate basal cell effects on AEC2 (type 2 alveolar epithelial cell) renewal capacity. Perturbation was used to investigate WNT7A function in vitro and in a repetitive bleomycin model in vivo. We found that WNT7A is highly and specifically expressed in basal-like cells. Proteins secreted by basal cells can be captured by neighboring fibroblasts and AEC2s. Basal cells or basal cell-conditioned media activate fibroblasts through WNT7A. Basal cell-derived WNT7A inhibits AEC2 progenitor cell renewal in three-dimensional organoid cultures. Neutralizing antibodies against WNT7A or a small molecule inhibitor of Frizzled signaling abolished basal cell-induced fibroblast activation and attenuated lung fibrosis in mice. In summary, basal cells and basal cell-derived WNT7A are key components of the fibrotic niche, providing a unique non-stem cell function of basal cells in IPF progression and a novel targeting strategy for IPF.

Senescence of alveolar epithelial progenitor cells: a critical driver of lung fibrosis.

Pulmonary fibrosis comprises a range of chronic interstitial lung diseases (ILDs) that impose a significant burden on patients and public health. Among these, idiopathic pulmonary fibrosis (IPF), a disease of aging, is the most common and most severe form of ILD and is treated largely by lung transplantation. The lack of effective treatments to stop or reverse lung fibrosis-in fact, fibrosis in most organs-has sparked the need to understand causative mechanisms with the goal of identifying critical points for potential therapeutic intervention. Findings from many groups have indicated that repeated injury to the alveolar epithelium-where gas exchange occurs-leads to stem cell exhaustion and impaired alveolar repair that, in turn, triggers the onset and progression of fibrosis. Cellular senescence of alveolar epithelial progenitors is a critical cause of stemness failure. Hence, senescence impairs repair and thus contributes significantly to fibrosis. In this review, we discuss recent evidence indicating that senescence of epithelial progenitor cells impairs alveolar homeostasis and repair creating a profibrotic environment. Moreover, we discuss the impact of senescent alveolar epithelial progenitors, alveolar type 2 (AT2) cells, and AT2-derived transitional epithelial cells in fibrosis. Emerging evidence indicates that transitional epithelial cells are prone to senescence and, hence, are a new player involved in senescence-associated lung fibrosis. Understanding the complex interplay of cell types and cellular regulatory factors contributing to alveolar epithelial progenitor senescence will be crucial to developing targeted therapies to mitigate their downstream profibrotic sequelae and to promote normal alveolar repair.NEW & NOTEWORTHY With an aging population, lung fibrotic diseases are becoming a global health burden. Dysfunctional repair of the alveolar epithelium is a key causative process that initiates lung fibrosis. Normal alveolar regeneration relies on functional progenitor cells; however, the senescence of these cells, which increases with age, hinders their ability to contribute to repair. Here, we discuss studies on the control and consequence of progenitor cell senescence in fibrosis and opportunities for research.

Targeting FSTL1 for Multiple Fibrotic and Systemic Autoimmune Diseases.

Follistatin-like 1 (FSTL1) is a matricellular protein that is upregulated during development and disease, including idiopathic pulmonary fibrosis (IPF), keloid, and arthritis. The profibrotic and pro-inflammatory roles of FSTL1 have been intensively studied during the last several years, as well as in this report. We screened and identified epitope-specific monoclonal neutralizing antibodies (nAbs) to functionally block FSTL1. FSTL1 nAbs attenuated bleomycin-induced pulmonary and dermal fibrosis in vivo and transforming growth factor (TGF)-ß1-induced dermal fibrosis ex vivo in human skin. In addition, FSTL1 nAbs significantly reduced existing lung fibrosis and skin fibrosis in experimental models. FSTL1 nAbs exerted their potent antifibrotic effects via reduced TGF-ß1 responsiveness and subsequent myofibroblast activation and extracellular matrix production. We also observed that FSTL1 nAbs attenuated the severity of collagen-induced arthritis in mice, which was accompanied by reduced inflammatory responses in vitro. Our findings suggest that FSTL1 nAbs are a promising new therapeutic strategy for the treatment of multiple organ fibrosis and systemic autoimmune diseases.

Mitogen-activated Protein Kinase-activated Protein Kinase 2 Inhibition Attenuates Fibroblast Invasion and Severe Lung Fibrosis.

Severe pulmonary fibrosis such as idiopathic pulmonary fibrosis (IPF) is characterized by the accumulation of extracellular matrix and fibroblast activation. Targeting fibroblast activation has contributed to the development of antifibrotic therapeutics for patients with IPF. Mitogen-activated protein kinase-activated protein kinase 2 (MK2), downstream in the transforming growth factor-ß/p38 mitogen-activated protein kinase pathway, has been implicated in inflammatory and fibrosing diseases. Increased concentrations of activated MK2 were expressed in IPF lung and in the mouse bleomycin model of lung fibrosis. The aim of the present study was to determine the role and the mechanisms of MK2 in fibroblast invasion and lung fibrosis. Our results showed that an MK2 inhibitor (MMI-0100) was able to inhibit the invasive capacity of lung fibroblasts isolated from patients with IPF, as well as fibroblasts isolated from both wild-type mice and mice with overexpressing hyaluronan synthase 2 (HAS2) in the myofibroblast compartment. We previously showed that hyaluronan and HAS2 regulate fibroblast invasion and lung fibrosis in vivo. The results of the present study showed that MMI-0100 reduced transforming growth factor-ß-induced hyaluronan production in human and mouse fibroblasts in vitro and that HAS2 mediated MK2 activation, suggesting a feed-forward loop in fibroblast activation. More importantly, MK2 inhibition attenuated hyaluronan accumulation and reduced collagen content in bleomycin-injured mouse lungs in vivo. Conditional deletion of MK2 in fibroblasts attenuated bleomycin-induced lung fibrosis. These data provide evidence that MK2 has a role in fibroblast invasion and fibrosis and may be a novel therapeutic target in pulmonary fibrosis.

Hyaluronan as an immune regulator in human diseases.

Accumulation and turnover of extracellular matrix components are the hallmarks of tissue injury. Fragmented hyaluronan stimulates the expression of inflammatory genes by a variety of immune cells at the injury site. Hyaluronan binds to a number of cell surface proteins on various cell types. Hyaluronan fragments signal through both Toll-like receptor (TLR) 4 and TLR2 as well as CD44 to stimulate inflammatory genes in inflammatory cells. Hyaluronan is also present on the cell surface of epithelial cells and provides protection against tissue damage from the environment by interacting with TLR2 and TLR4. Hyaluronan and hyaluronan-binding proteins regulate inflammation, tissue injury, and repair through regulating inflammatory cell recruitment, release of inflammatory cytokines, and cell migration. This review focuses on the role of hyaluronan as an immune regulator in human diseases.

MicroRNA-29c Prevents Pulmonary Fibrosis by Regulating Epithelial Cell Renewal and Apoptosis.

Successful repair and renewal of alveolar epithelial cells (AECs) are critical in prohibiting the accumulation of myofibroblasts in pulmonary fibrogenesis. MicroRNAs (miRNAs) are multifocal regulators involved in lung injury and repair. However, the contribution of miRNAs to AEC2 renewal and apoptosis is incompletely understood. We report that miRNA-29c (miR-29c) expression is lower in AEC2s of individuals with idiopathic pulmonary fibrosis than in healthy lungs. Epithelial cells overexpressing miR-29c show higher proliferative rates and viability. miR-29c protects epithelial cells from apoptosis by targeting forkhead box O3a (Foxo3a). Both overexpression of miR-29c conventionally and AEC2s specifically lead to less fibrosis and better recovery in vivo. Furthermore, deficiency of miR-29c in AEC2s results in higher apoptosis and reduced epithelial renewal. Interestingly, a gene network including a subset of apoptotic genes was coregulated by both Toll-like receptor 4 and miR-29c. Taken together, miR-29c maintains epithelial integrity and promotes recovery from lung injury, thereby attenuating lung fibrosis in mice.

Targeted HAS2 Expression Lessens Airway Responsiveness in Chronic Murine Allergic Airway Disease.

Hyaluronan (HA), a major component of the extracellular matrix, is secreted by airway structural cells. Airway fibroblasts in allergic asthma secrete elevated levels of HA in association with increased HA synthase 2 (HAS2) expression. Thus, we hypothesized that HA accumulation in the airway wall may contribute to airway remodeling and hyperresponsiveness in allergic airways disease. To examine this hypothesis, transgenic mice in which the α-smooth muscle actin (α-SMA) promoter drives HAS2 expression were generated. Mixed male and female α-SMA-HAS2 mice (HAS2+ mice, n = 16; HAS2- mice, n = 13) were sensitized via intraperitoneal injection and then chronically challenged with aerosolized ovalbumin (OVA) for 6 weeks. To test airway responsiveness, increasing doses of methacholine were delivered intravenously and airway resistance was measured using the forced oscillation technique. HA, cytokines, and cell types were analyzed in bronchoalveolar lavage fluid, serum, and whole lung homogenates. Lung sections were stained using antibodies specific for HA-binding protein (HABP) and α-SMA, as well as Masson's trichrome stain. Staining of lung tissue demonstrated significantly increased peribronchial HA, α-SMA, and collagen deposition in OVA-challenged α-SMA-HAS2+ mice compared with α-SMA-HAS2- mice. Unexpectedly, OVA-challenged α-SMA-HAS2+ mice displayed significantly reduced airway responsiveness to methacholine compared with similarly treated α-SMA-HAS2- mice. The total numbers of inflammatory cell types in the bronchoalveolar lavage fluid did not differ significantly between OVA-challenged α-SMA-HAS2+ mice and α-SMA-HAS2- mice. We conclude that allergen-challenged mice that overexpress HAS2 in myofibroblasts and smooth muscle cells develop increased airway fibrosis, which lessens airway hyperresponsiveness to bronchoconstrictors.

CD44high alveolar type II cells show stem cell properties during steady-state alveolar homeostasis.

The alveolar epithelium is composed of type I cells covering most of the gas-blood exchange surface and type II cells secreting surfactant that lowers surface tension of alveoli to prevent alveolar collapse. Here, we have identified a subgroup of type II cells expressing a higher level of cell surface molecule CD44 (CD44high type II cells) that composed ~3% of total type II cells in 5-10-wk-old mice. These cells were preferentially apposed to lung capillaries. They displayed a higher proliferation rate and augmented differentiation capacity into type I cells and the ability to form alveolar organoids compared with CD44low type II cells. Moreover, in aged mice, 18-24 mo old, the percentage of CD44high type II cells among all type II cells was increased, but these cells showed decreased progenitor properties. Thus CD44high type II cells likely represent a type II cell subpopulation important for constitutive regulation of alveolar homeostasis.

Down-regulation of USP13 mediates phenotype transformation of fibroblasts in idiopathic pulmonary fibrosis.

BACKGROUND: Idiopathic pulmonary fibrosis (IPF) is a fatal disease characterized by fibroblastic foci and progressive scarring of the pulmonary parenchyma. IPF fibroblasts display increased proliferation and enhanced migration and invasion, analogous to cancer cells. This transformation-like phenotype of fibroblasts plays an important role in the development of pulmonary fibrosis, but the mechanism for this is not well understood. METHODS: In this study, we compared gene expression profiles in fibrotic lung tissues from IPF patients and normal lung tissues from patients with primary spontaneous pneumothorax using a cDNA microarray to examine the mechanisms involved in the pathogenesis of IPF. In a cDNA microarray, we found that USP13 was decreased in lung tissues from patients with IPF, which was further confirmed by results from immunohistochemistry and western blot assays. Then, we used RNA interference in MRC-5 cells to inhibit USP13 and evaluated its effects by western blot, real-time RT-PCR, bromodeoxyuridine incorporation, and transwell assays. We also used co-immunoprecipitation and immunofluorescence staining to identify the correlation between USP13 and PTEN in IPF. RESULTS: USP13 expression levels were markedly reduced in fibroblastic foci and primary IPF fibroblast lines. The depletion of USP13 resulted in the transformation of fibroblasts into an aggressive phenotype with enhanced proliferative, migratory, and invasive capacities. Additionally, USP13 interacted with PTEN and mediated PTEN ubiquitination and degradation in lung fibroblasts. CONCLUSIONS: Down-regulation of USP13 mediates PTEN protein loss and fibroblast phenotypic change, and thereby plays a crucial role in IPF pathogenesis.

MicroRNA-127 inhibits lung inflammation by targeting IgG Fcγ receptor I.

The molecular mechanisms of acute lung injury are incompletely understood. MicroRNAs (miRNAs) are crucial biological regulators that act by suppressing their target genes and are involved in a variety of pathophysiologic processes. miR-127 appears to be downregulated during lung injury. We set out to investigate the role of miR-127 in lung injury and inflammation. Expression of miR-127 significantly reduced cytokine release by macrophages. Looking into the mechanisms of regulation of inflammation by miR-127, we found that IgG FcγRI (CD64) was a target of miR-127, as evidenced by reduced CD64 protein expression in macrophages overexpressing miR-127. Furthermore, miR-127 significantly reduced the luciferase activity with a reporter construct containing the native 3' untranslated region of CD64. Importantly, we demonstrated that miR-127 attenuated lung inflammation in an IgG immune complex model in vivo. Collectively, these data show that miR-127 targets macrophage CD64 expression and promotes the reduction of lung inflammation. Understanding how miRNAs regulate lung inflammation may represent an attractive way to control inflammation induced by infectious or noninfectious lung injury.

Multiple stromal populations contribute to pulmonary fibrosis without evidence for epithelial to mesenchymal transition.

There are currently few treatment options for pulmonary fibrosis. Innovations may come from a better understanding of the cellular origin of the characteristic fibrotic lesions. We have analyzed normal and fibrotic mouse and human lungs by confocal microscopy to define stromal cell populations with respect to several commonly used markers. In both species, we observed unexpected heterogeneity of stromal cells. These include numerous cells with molecular and morphological characteristics of pericytes, implicated as a source of myofibroblasts in other fibrotic tissues. We used mouse genetic tools to follow the fates of specific cell types in the bleomcyin-induced model of pulmonary fibrosis. Using inducible transgenic alleles to lineage trace pericyte-like cells in the alveolar interstitium, we show that this population proliferates in fibrotic regions. However, neither these cells nor their descendants express high levels of the myofibroblast marker alpha smooth muscle actin (Acta2, aSMA). We then used a Surfactant protein C-CreER(T2) knock-in allele to follow the fate of Type II alveolar cells (AEC2) in vivo. We find no evidence at the cellular or molecular level for epithelial to mesenchymal transition of labeled cells into myofibroblasts. Rather, bleomycin accelerates the previously reported conversion of AEC2 into AEC1 cells. Similarly, epithelial cells labeled with our Scgb1a1-CreER allele do not give rise to fibroblasts but generate both AEC2 and AEC1 cells in response to bleomycin-induced lung injury. Taken together, our results show a previously unappreciated heterogeneity of cell types proliferating in fibrotic lesions and exclude pericytes and two epithelial cell populations as the origin of myofibroblasts.

Arrestin beta 1 Regulates Alveolar Progenitor Renewal and Lung Fibrosis.

The molecular mechanisms that regulate progressive pulmonary fibrosis remain poorly understood. Type 2 alveolar epithelial cells (AEC2s) function as adult stem cells in the lung. We previously showed that there is a loss of AEC2s and a failure of AEC2 renewal in the lungs of idiopathic pulmonary fibrosis (IPF) patients. We also reported that beta-arrestins are the key regulators of fibroblast invasion, and beta-arrestin 1 and 2 deficient mice exhibit decreased mortality, decreased matrix deposition, and increased lung function in bleomycin-induced lung fibrosis. However, the role of beta-arrestins in AEC2 regeneration is unclear. In this study, we investigated the role and mechanism of Arrestin beta 1 (ARRB1) in AEC2 renewal and in lung fibrosis. We used conventional deletion as well as cell type-specific deletion of ARRB1 in mice and found that Arrb1 deficiency in fibroblasts protects mice from lung fibrosis, and the knockout mice exhibit enhanced AEC2 regeneration in vivo, suggesting a role of fibroblast-derived ARRB1 in AEC2 renewal. We further found that Arrb1-deficient fibroblasts promotes AEC2 renewal in 3D organoid assays. Mechanistically, we found that CCL7 is among the top downregulated cytokines in Arrb1 deficient fibroblasts and CCL7 inhibits AEC2 regeneration in 3D organoid experiments. Therefore, fibroblast ARRB1 mediates AEC2 renewal, possibly by releasing chemokine CCL7, leading to fibrosis in the lung.

Airway epithelial progenitors are region specific and show differential responses to bleomycin-induced lung injury.

Mechanisms that regulate regional epithelial cell diversity and pathologic remodeling in airways are poorly understood. We hypothesized that regional differences in cell composition and injury-related tissue remodeling result from the type and composition of local progenitors. We used surface markers and the spatial expression pattern of an SFTPC-GFP transgene to subset epithelial progenitors by airway region. Green fluorescent protein (GFP) expression ranged from undetectable to high in a proximal-to-distal gradient. GFP(hi) cells were subdivided by CD24 staining into alveolar (CD24(neg)) and conducting airway (CD24(low)) populations. This allowed for the segregation of three types of progenitors displaying distinct clonal behavior in vitro. GFP(neg) and GFP(low) progenitors both yielded lumen containing colonies but displayed transcriptomes reflective of pseudostratified and distal conducting airways, respectively. CD24(low)GFP(hi) progenitors were present in an overlapping distribution with GFP(low) progenitors in distal airways, yet expressed lower levels of Sox2 and expanded in culture to yield undifferentiated self-renewing progeny. Colony-forming ability was reduced for each progenitor cell type after in vivo bleomycin exposure, but only CD24(low) GFP(hi) progenitors showed robust expansion during tissue remodeling. These data reveal intrinsic differences in the properties of regional progenitors and suggest that their unique responses to tissue damage drive local tissue remodeling.