Epithelial-mesenchymal Communication Research Articles

Exosomes Derived from Human Palatal Mesenchymal Cells Mediate Intercellular Communication During Palatal Fusion by Promoting Oral Epithelial Cell Migration.

Exosomes are important "messengers" in cell-cell interactions, but their potential effects on palatal fusion are still unknown. This study aimed to explore the role and mechanism of exosomes derived from palatal mesenchymal cells in epithelial-mesenchymal communication during palatogenesis. The expression of exosome marker CD63 and CD81 in palatal cells during palatogenesis was detected by immunofluorescence staining. After being purified from the supernatant of human embryonic palatal mesenchymal (HEPM) cells, exosomes (HEPM-EXO) were characterized by nanoparticle tracking analysis (NTA), transmission electron microscopy (TEM) and Western blot. HEPM-EXO were co-cultured with human immortalized oral epithelial cells (HIOEC). The effects of HEPM-EXO on the cell proliferation, migration, apoptosis and epithelial-mesenchymal transition (EMT) of HIOEC were evaluated. The proteins encapsulated in HEPM-EXO were analyzed by proteomic analysis. The extensive expression of CD63 and CD81 in palatal epithelial and mesenchymal cells were continuously detected during E12.5~E14.5, suggesting that exosomes were involved in the process of palatal fusion. The expression of CD63 was also observed in the acellular basement membrane between the palatal epithelium and the mesenchyme in vivo, and HEPM-EXO could be internalized by HIOEC in vitro, suggesting that exosomes are potent to diffuse through the cellular tissue boundary to mediate palatal cell-cell communication. Exposure of HEPM-EXO to HIOEC substantially inhibited the proliferation and stimulated the migration of HIOEC, but had no significant effect on cell apoptosis and EMT. Proteomic analysis revealed the basic characteristics of the proteins in HEPM-EXO and that exosomal THBS1 may potentially regulate the cell behaviors of HIOEC, which needs further verification. Gene ontology (GO) analysis uncovered that the proteins highly expressed in HEPM-EXO are closely related to wound healing, implying a promising therapeutic opportunity of HEPM-EXO in tissue injury treatment with future studies. HEPM-EXO mediated cell-cell communication by regulating cell proliferation and migration of oral epithelial cells during palatogenesis.

Roles of cutaneous cell-cell communication in wound healing outcome: An emphasis on keratinocyte-fibroblast crosstalk.

Tissue repair is a very complex event and involves a continuously orchestrated sequence of signals and responses from platelets, fibroblasts, epithelial, endothelial and immune cells. The details of interaction between these signals, which are mainly growth factors and cytokines, have been widely discussed. However, it is still not clear how activated cells at wound sites lessen their activities after epithelialization is completed. Termination of the wound healing process requires a fine balance between extracellular matrix (ECM) deposition and degradation. Maintaining this balance requires highly accurate epithelial-mesenchymal communication and correct information exchange between keratinocytes and fibroblasts. As it has been reported in the literature, a disruption in epithelialization during the process of wound healing increases the frequency of developing chronic wounds or fibrotic conditions, as seen in a variety of clinical cases. Conversely, the potential stop signal for wound healing should have a regulatory role on both ECM synthesis and degradation to reach a successful wound healing outcome. This review briefly describes the potential roles of growth factors and cytokines in controlling the early phase of wound healing and predominantly explores the role of releasable factors from epithelial-mesenchymal interaction in controlling during and the late stage of the healing process. Emphasis will be given on the crosstalk between keratinocytes and fibroblasts in ECM modulation and the healing outcome following a brief discussion of the wound healing initiation mechanism. In particular, we will review the termination of acute dermal wound healing, which frequently leads to the development of hypertrophic scarring.

Blocking AURKA with MK-5108 attenuates renal fibrosis in chronic kidney disease

Renal fibrosis, a common feature of chronic kidney disease (CKD), is characterized by excessive deposition of extracellular matrix (ECM) leading to scar formation in the renal parenchyma. Active epithelial-mesenchymal communication (EMC), and the proliferation and activation of fibroblasts are implicated in the causation of renal fibrosis. Aurora-A kinase (AURKA) is a serine/threonine kinase required for the process of mitosis. Dysregulation of AURKA has been demonstrated in the context of various cancers. However, the role of AURKA in CKD-associated fibrosis has not been elucidated. MK-5108, a potent and highly selective AURKA inhibitor, was shown to exhibit anti-cancer activity in recent preclinical and clinical studies. In the present study, we investigated the role of MK-5108 in renal fibrosis employing animal and cell models. In vivo, AURKA was highly expressed in fibrotic kidneys of CKD patients and in mouse kidneys with unilateral ureteral obstruction (UUO). Post treatment with MK-5108 at the 3rd day after UUO remarkably alleviated renal fibrosis, possibly by inhibiting the proliferation and activation of fibroblasts and suppressing the phenotypic transition of renal cells. Moreover, the enhanced inflammatory factors in obstructive kidneys were also repressed. In vitro, MK-5108 treatment inhibited the pro-fibrotic response in renal cells induced by transforming growth factor-β1. Finally, overexpression of AURKA in renal fibroblasts promoted fibrotic response, while silencing AURKA showed anti-fibrotic effect, further confirming the pro-fibrotic role of AURKA. In this study, inhibition of AURKA by MK-5108 markedly attenuated renal fibrosis. MK-5108 is a potential therapeutic agent for treatment of renal fibrosis in CKD.

New Insights Into the Role and Mechanism of Partial Epithelial-Mesenchymal Transition in Kidney Fibrosis.

Epithelial-mesenchymal transition (EMT) is described as the process in which injured renal tubular epithelial cells undergo a phenotype change, acquiring mesenchymal characteristics and morphing into fibroblasts. Initially, it was widely thought of as a critical mechanism of fibrogenesis underlying chronic kidney disease. However, evidence that renal tubular epithelial cells can cross the basement membrane and become fibroblasts in the renal interstitium is rare, leading to debate about the existence of EMT. Recent research has demonstrated that after injury, renal tubular epithelial cells acquire mesenchymal characteristics and the ability to produce a variety of profibrotic factors and cytokines, but remain attached to the basement membrane. On this basis, a new concept of “partial epithelial-mesenchymal transition (pEMT)” was proposed to explain the contribution of renal epithelial cells to renal fibrogenesis. In this review, we discuss the concept of pEMT and the most recent findings related to this process, including cell cycle arrest, metabolic alternation of epithelial cells, infiltration of immune cells, epigenetic regulation as well as the novel signaling pathways that mediate this disturbed epithelial-mesenchymal communication. A deeper understanding of the role and the mechanism of pEMT may help in developing novel therapies to prevent and halt fibrosis in kidney disease.

What Have In Vitro Co-Culture Models Taught Us about the Contribution of Epithelial-Mesenchymal Interactions to Airway Inflammation and Remodeling in Asthma?

As the lung develops, epithelial-mesenchymal crosstalk is essential for the developmental processes that drive cell proliferation, differentiation, and extracellular matrix (ECM) production within the lung epithelial-mesenchymal trophic unit (EMTU). In asthma, a number of the lung EMTU developmental signals have been associated with airway inflammation and remodeling, which has led to the hypothesis that aberrant activation of the asthmatic EMTU may lead to disease pathogenesis. Monoculture studies have aided in the understanding of the altered phenotype of airway epithelial and mesenchymal cells and their contribution to the pathogenesis of asthma. However, 3-dimensional (3D) co-culture models are needed to enable the study of epithelial-mesenchymal crosstalk in the setting of the in vivo environment. In this review, we summarize studies using 3D co-culture models to assess how defective epithelial-mesenchymal communication contributes to chronic airway inflammation and remodeling within the asthmatic EMTU.

Deficiency in fibroblast PPARβ/δ reduces nonmelanoma skin cancers in mice.

The incidence of nonmelanoma skin cancer (NMSC) has been increasing worldwide. Most studies have highlighted the importance of cancer-associated fibroblasts (CAFs) in NMSC progression. However much less is known about the communication between normal fibroblasts and epithelia; disruption of this communication affects tumor initiation and the latency period in the emergence of tumors. Delineating the mechanism that mediates this epithelial-mesenchymal communication in NMSC could identify more effective targeted therapies. The nuclear receptor PPARβ/δ in fibroblasts has been shown to modulate adjacent epithelial cell behavior, however, its role in skin tumorigenesis remains unknown. Using chemically induced skin carcinogenesis, we showed that FSPCre-Pparb/dex4 mice, whose Pparb/d gene was selectively deleted in fibroblasts, had delayed emergence and reduced tumor burden compared with control mice (Pparb/dfl/fl). However, FSPCre-Pparb/dex4-derived tumors showed increased proliferation, with no difference in differentiation, suggesting delayed tumor initiation. Network analysis revealed a link between dermal Pparb/d and TGF-β1 with epidermal NRF2 and Nox4. In vitro investigations showed that PPARβ/δ deficiency in fibroblasts increased epidermal Nox4-derived H2O2 production, which triggered an NRF2-mediated antioxidant response. We further showed that H2O2 upregulated NRF2 mRNA via the B-Raf-MEK1/2 pathway. The enhanced NRF2 response altered the activities of PTEN, Src, and AKT. In vivo, we detected the differential phosphorylation profiles of B-Raf, MEK1/2, PTEN, Src, and AKT in the vehicle-treated and chemicallytreated epidermis of FSPCre-Pparb/dex4 mice compared with that in Pparb/dfl/fl mice, prior to the first appearance of tumors in Pparb/dfl/fl. Our study revealed a role for fibroblast PPARβ/δ in the epithelial-mesenchymal communication involved in cellular redox homeostasis.

Tubule-Derived Wnts Are Required for Fibroblast Activation and Kidney Fibrosis

Cell-cell communication via Wnt ligands is necessary in regulating embryonic development and has been implicated in CKD. Because Wnt ligands are ubiquitously expressed, the exact cellular source of the Wnts involved in CKD remains undefined. To address this issue, we generated two conditional knockout mouse lines in which Wntless (Wls), a dedicated cargo receptor that is obligatory for Wnt secretion, was selectively ablated in tubular epithelial cells or interstitial fibroblasts. Blockade of Wnt secretion by genetic deletion of Wls in renal tubules markedly inhibited myofibroblast activation and reduced renal fibrosis after unilateral ureteral obstruction. This effect associated with decreased activation of β-catenin and downstream gene expression and preserved tubular epithelial integrity. In contrast, fibroblast-specific deletion of Wls exhibited little effect on the severity of renal fibrosis after obstructive or ischemia-reperfusion injury. In vitro, incubation of normal rat kidney fibroblasts with tubule-derived Wnts promoted fibroblast proliferation and activation. Furthermore, compared with kidney specimens from patients without CKD, biopsy specimens from patients with CKD also displayed increased expression of multiple Wnt proteins, predominantly in renal tubular epithelium. These results illustrate that tubule-derived Wnts have an essential role in promoting fibroblast activation and kidney fibrosis via epithelial-mesenchymal communication.

TGF-β Family Signaling in Ductal Differentiation and Branching Morphogenesis.

Epithelial cells contribute to the development of various vital organs by generating tubular and/or glandular architectures. The fully developed forms of ductal organs depend on processes of branching morphogenesis, whereby frequency, total number, and complexity of the branching tissue define the final architecture in the organ. Some ductal tissues, like the mammary gland during pregnancy and lactation, disintegrate and regenerate through periodic cycles. Differentiation of branched epithelia is driven by antagonistic actions of parallel growth factor systems that mediate epithelial-mesenchymal communication. Transforming growth factor-β (TGF-β) family members and their extracellular antagonists are prominently involved in both normal and disease-associated (e.g., malignant or fibrotic) ductal tissue patterning. Here, we discuss collective knowledge that permeates the roles of TGF-β family members in the control of the ductal tissues in the vertebrate body.

Functional characterization of a novel 3D model of the epithelial-mesenchymal trophic unit

ABSTRACTBackground/Aim: Epithelial-mesenchymal communication plays a key role in tissue homeostasis and abnormal signaling contributes to chronic airways disease such as COPD. Most in vitro models are limited in complexity and poorly represent this epithelial-mesenchymal trophic unit. We postulated that cellular outgrowth from bronchial tissue would enable development of a mucosal structure that recapitulates better in vivo tissue architecture. Materials and Methods: Bronchial tissue was embedded in Matrigel and outgrowth cultures monitored using time-lapse microscopy, electrical resistance, light and electron microscopy. Cultures were challenged repetitively with cigarette smoke extract (CSE). Results: The outgrowths formed as a multicellular sheet with motile cilia becoming evident as the Matrigel was remodeled to provide an air interface; cultures were viable for more than one year. Immunofluorescence and electron microscopy (EM) identified an upper layer of mucociliary epithelium and a lower layer of highly organized extracellular matrix (ECM) interspersed with fibroblastic cells separated by a basement membrane. EM analysis of the mucosal construct after repetitive exposure to CSE revealed epithelial damage, loss of cilia, and ECM remodeling, as occurs in vivo. Conclusions: We have developed a robust bronchial mucosal model. The structural changes observed following CSE exposure suggest the model should have utility for drug discovery and preclinical testing, especially those targeting airway remodeling.

Sonic hedgehog signaling in kidney fibrosis: a master communicator.

The hedgehog signaling cascade is an evolutionarily conserved pathway that regulates multiple aspects of embryonic development and plays a decisive role in tissue homeostasis. As the best studied member of three hedgehog ligands, sonic hedgehog (Shh) is known to be associated with kidney development and tissue repair after various insults. Recent studies uncover an intrinsic link between dysregulated Shh signaling and renal fibrogenesis. In various types of chronic kidney disease (CKD), Shh is upregulated specifically in renal tubular epithelium but targets interstitial fibroblasts, thereby mediating a dynamic epithelial-mesenchymal communication (EMC). Tubule-derived Shh acts as a growth factor for interstitial fibroblasts and controls a hierarchy of fibrosis-related genes, which lead to the excessive deposition of extracellular matrix in renal interstitium. In this review, we recapitulate the principle of Shh signaling, its activation and regulation in a variety of kidney diseases. We also discuss the potential mechanisms by which Shh promotes renal fibrosis and assess the efficacy of blocking this signaling in preclinical settings. Continuing these lines of investigations will provide novel opportunities for designing effective therapies to improve CKD prognosis in patients.

Structural Defects of Laminin β3 N-terminus Underlie Junctional Epidermolysis Bullosa with Altered Granulation Tissue Response

Mutations in the laminin-332 (α3Aβ3γ2) genes cause junctional epidermolysis bullosa (JEB), a recessively inherited disease characterized by blistering and altered wound repair. In addition, specific mutations that affect the N-terminus of the α3A chain cause a JEB-related non-blistering condition characterized by chronic production of granulation tissue, suggesting a critical role of this region in epithelial-mesenchymal communication. We report here a 9-year-old patient with JEB with a few long-standing skin ulcers with prominent granulation tissue in the absence of active blistering. He bears a homozygous missense mutation, p.Gly254Asp, within the first laminin epidermal growth factor-like (LE) repeat of the β3 short arm. We show that p.Gly254Asp causes mis-folding of the LE motif, leading to reduced secretion of laminin-332 and structural alterations of the cutaneous basement membrane zone. These findings demonstrate, in a patient in vivo, that the β3 short arm is also involved in the outcome of the granulation tissue response.

Kidney tubular β-catenin signaling controls interstitial fibroblast fate via epithelial-mesenchymal communication

Activation of β-catenin, the principal mediator of canonical Wnt signaling, is a common pathologic finding in a wide variety of chronic kidney diseases (CKD). While β-catenin is induced predominantly in renal tubular epithelium in CKD, surprisingly, depletion of tubular β-catenin had little effect on the severity of renal fibrosis. Interestingly, less apoptosis was detected in interstitial fibroblasts in knockout mice, which was accompanied by a decreased expression of Bax and Fas ligand (FasL). Tubule-specific knockout of β-catenin diminished renal induction of matrix metalloproteinase (MMP-7), which induced FasL expression in interstitial fibroblasts and potentiated fibroblast apoptosis in vitro. These results demonstrate that loss of tubular β-catenin resulted in enhanced interstitial fibroblast survival due to decreased MMP-7 expression. Our studies uncover a novel role of the tubular β-catenin/MMP-7 axis in controlling the fate of interstitial fibroblasts via epithelial-mesenchymal communication.

Endometrial stromal beta-catenin is required for steroid-dependent mesenchymal-epithelial cross talk and decidualization

BackgroundBeta-catenin is part of a protein complex associated with adherens junctions. When allowed to accumulate to sufficient levels in its dephosphorylated form, beta-catenin serves as a transcriptional co-activator associated with a number of signaling pathways, including steroid hormone signaling pathways.MethodsTo investigate the role of beta-catenin in progesterone (P4) signaling and female reproductive physiology, conditional ablation of Ctnnb1 from the endometrial mesenchymal (i.e. stromal and myometrial), but not epithelial, compartment was accomplished using the Amhr2-Cre mice. Experiments were conducted to assess the ability of mutant female mice to undergo pregnancy and pseudopregnancy by or through oil-induced decidualization. The ability of uteri from mutant female mice to respond to estrogen (E2) and P4 was also determined.ResultsConditional deletion of Ctnnb1 from the mesenchymal compartment of the uterus resulted in infertility stemming, in part, from complete failure of the uterus to decidualize. E2-stimulated epithelial cell mitosis and edematization were not altered in mutant uteri indicating that the mesenchyme is capable of responding to E2. However, exposure of ovariectomized mutant female mice to a combined E2 and P4 hormone regimen consistent with early pregnancy revealed that mesenchymal beta-catenin is essential for indirectly opposing E2-induced epithelial proliferation by P4 and in some mice resulted in development of endometrial metaplasia. Lastly, beta-catenin is also required for the induced expression of genes that are known to play a fundamental role in decidualization such as Ihh, Ptch1, Gli1 and Muc1ConclusionsThree salient points derive from these studies. First, the findings demonstrate a mechanistic linkage between the P4 and beta-catenin signaling pathways. Second, they highlight an under appreciated role for the mesenchymal compartment in indirectly mediating P4 signaling to the epithelium, a process that intimately involves mesenchymal beta-catenin. Third, the technical feasibility of deleting genes in the mesenchymal compartment of the uterus in an effort to understand decidualization and post-natal interactions with the overlying epithelium has been demonstrated. It is concluded that beta-catenin plays an integral role in selective P4-directed epithelial-mesenchymal communication in both the estrous cycling and gravid uterus.

Sonic Hedgehog Signaling Mediates Epithelial–Mesenchymal Communication and Promotes Renal Fibrosis

Sonic hedgehog (Shh) signaling is a developmental signal cascade that plays an essential role in regulating embryogenesis and tissue homeostasis. Here, we investigated the potential role of Shh signaling in renal interstitial fibrogenesis. Ureteral obstruction induced Shh, predominantly in the renal tubular epithelium of the fibrotic kidneys. Using Gli1(lacZ) knock-in mice, we identified renal interstitial fibroblasts as Shh-responding cells. In cultured renal fibroblasts, recombinant Shh protein activated Gli1 and induced α-smooth muscle actin (α-SMA), desmin, fibronectin, and collagen I expression, suggesting that Shh signaling promotes myofibroblast activation and matrix production. Blockade of Shh signaling with cyclopamine abolished the Shh-mediated induction of Gli1, Snail1, α-SMA, fibronectin, and collagen I. In vivo, the kidneys of Gli1-deficient mice were protected against the development of interstitial fibrosis after obstructive injury. In wild-type mice, cyclopamine did not affect renal Shh expression but did inhibit induction of Gli1, Snail1, and α-SMA. In addition, cyclopamine reduced matrix expression and mitigated fibrotic lesions. These results suggest that tubule-derived Shh mediates epithelial-mesenchymal communication by targeting interstitial fibroblasts after kidney injury. We conclude that Shh/Gli1 signaling plays a critical role in promoting fibroblast activation, production of extracellular matrix, and development of renal interstitial fibrosis.

A Brief History of Asthma and Its Mechanisms to Modern Concepts of Disease Pathogenesis

The original concept of asthma being primarily a disease of airways smooth muscle drove the development of bronchodilator drugs. However when it was realised that airway inflammation underpinned the disordered airway function, this gave way to the development of controller therapies such as inhaled cromones and corticosteroids. More recently the discovery of complex interconnecting cytokine and chemokine networks has stimulated the development of biologics with varying success. With the recognition that airway wall "remodelling" is present early in asthma inception and is in part driven by aberrant epithelial-mesenchymal communication both genetic and environmental factors beyond allergen exposure such as virus infection and air pollution are being seen as being increasingly important not only in asthma exacerbations but in the origins of asthma and its evolution into different sub-phenotypes. This brings us round full circle to once again considering that the origins of asthma lie in defects in the formed elements of the airway; the epithelium, smooth muscle, and vasculature. Over the last 25 years Professor You Young Kim has engaged in the exciting discovery science of allergy and asthma and has made an enormous contribution in bringing Korea to the forefront of disease management and research, a position that both he and his colleagues can justly be proud of.

Asthma: where is it going?

Asthma is characterized by reversible airway obstruction, airway hyperresponsiveness, and airway inflammation. Although our understanding of its pathophysiological mechanisms continues to evolve, the relative contributions of airway hyperresponsiveness and inflammation are still debated. The first mechanism identified as important for asthma was bronchial hyperresponsiveness. In a second step, asthma was recognized also as an inflammatory disease, with chronic inflammation inducing structural changes or remodeling. However, persistence of airway dysfunction despite inflammatory control is observed in chronic severe asthma of both adults and children. More recently, a potential role for epithelial-mesenchymal communication or transition is emerging, with epithelial injury often resulting in a self-sustaining phenotype of wound repair modulation by activation/reactivation of the epithelial-mesenchymal trophic unit, suggesting that chronic asthma can be more than an inflammatory disease. It is noteworthy that the gene-environmental interactions critical for the development of a full asthma phenotype involve processes similar to those occurring in branching morphogenesis. In addition, a central role for airway smooth muscle in the pathogenesis of the disease has been explored, highlighting its secretory function as well as different intrinsic properties compared to normal subjects. These new concepts can potentially shed light on the mechanisms underlying some asthma phenotypes and improve our understanding of the disease in terms of the therapeutic strategies to be applied. How we understand asthma and its mechanisms along time will be the focus of this overview.

TGF-β in Renal Injury and Disease

Chronic progressive kidney diseases typically are characterized by loss of differentiated epithelial cells and activation of mesenchymal cell populations leading to renal fibrosis in response to a broad range of diverse renal injuries. Recent evidence has indicated that epithelial microinjury leads to unbalanced epithelial-mesenchymal communication to initiate the fibrotic response. Transforming growth factors beta constitute a large family of cytokines that control key cellular responses in development and tissue repair. Activation of autocrine and paracrine transforming growth factor-beta signaling cascades in the context of epithelial microinjuries initiate a variety of cell type-dependent signaling and activity profiles, including epithelial apoptosis and epithelial-to-mesenchymal transition, that trigger fibrogenic foci and initiate progressive fibrogenesis in chronic renal injury.

Understanding the pathophysiology of severe asthma to generate new therapeutic opportunities

Although asthma is defined in terms of reversibility of airflow obstruction, as the disease becomes more severe and chronic, it adopts different characteristics, including a degree of fixed airflow obstruction and corticosteroid refractoriness. Underlying these phenotypes is evidence of airway wall remodeling, which should be distinguished from the increase in smooth muscle linked to airways hyperresponsiveness. Aberrant epithelial-mesenchymal communication leads to a chronic wound scenario, which is characterized by activation of the epithelial-mesenchymal trophic unit, epithelial damage, the laying down of new matrix, and greater involvement of neutrophils in the inflammatory response. In allergic asthmatic patients who remain symptomatic despite high-dose corticosteroid therapy, blockade of IgE with omalizumab confers appreciable clinical benefit. Chronic severe asthma is also accompanied by a marked increase in TNF-alpha production that might contribute to corticosteroid refractoriness. Based on this, TNF blockade with the soluble fusion protein entanercept produces improvement in asthma symptoms, lung function, and quality of life paralleled by a marked reduction in airways hyperresponsiveness. Identification of novel susceptibility genes, such as a disintegrin and metalloprotease 33 (ADAM33), will provide further targets against which to direct novel therapies for asthma, especially at the more severe end of the disease spectrum.

Mechanisms of airway epithelial damage: epithelial-mesenchymal interactions in the pathogenesis of asthma.

The aims of this article are to understand the shortfalls in thinking of asthma purely as an atopic disorder, to gain insight into the epithelial abnormalities of asthma and how these interface with the environment, and to view asthma as a disease of airway wall restructuring that engages activation of the epithelial mesenchymal trophic unit (EMTU). Furthermore, based on these developments, possible novel therapeutic targets for chronic asthma are identified. It has long been recognised that T‐helper cell (Th) type‐2 airway inflammation underpins airway dysfunction in asthma. Atopy is also a key feature of this disease but accounts for <40% of the population attributable risk. Based on a careful pathological and functional assessment of chronic asthma, the authors propose that altered epithelial-mesenchymal communication is also fundamental to disease pathogenesis. A number of factors contribute to a thickened hyperresponsive airway that provides an ideal microenvironment for the persistence of Th2‐mediated inflammation, including a more susceptible epithelium to injury, delayed epithelial repair, an altered trajectory of epithelial repair to a mucus-secreting phenotype, generation of growth factors that drive mesenchymal cell proliferation and differentiation towards increased matrix deposition and smooth muscle, and the production of neural and vascular growth factors. Cytokines and mediators derived from infiltrating inflammatory cells interact with this EMTU to augment and prolong responses. The “remodelling” changes of asthma have been observed in childhood asthma and may, indeed, precede the development of the disease. The recent recognition that atopy per se is not a key factor in the initiation of asthma (although it is important in aggravating the disease) suggests that gene-environmental interactions involving similar processes to those occurring in branching morphogenesis are critical for the full asthma phenotype to develop. A recent National Institutes for Health Workshop and a European Respiratory Society Task Force both concluded that more work …

Metabolic and cellular analysis of alopecia in vitamin D receptor knockout mice

Targeted ablation of the vitamin D receptor (VDR) results in hypocalcemia, hypophosphatemia, hyperparathyroidism, rickets, osteomalacia, and alopecia--the last a consequence of defective anagen initiation. To investigate whether the markedly elevated levels of 1,25-dihydroxyvitamin D led to the alopecia, we raised VDR-null mice in a ultraviolet light-free environment and fed them chow lacking vitamin D for five generations. Despite undetectable circulating levels of 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D, alopecia persisted in the VDR-null mice, demonstrating that the alopecia was not secondary to toxic levels of 1,25-dihydroxyvitamin D interacting with an alternative receptor. Furthermore, alopecia was not seen in control littermates, suggesting that absence of ligand and absence of receptor cause different phenotypes. To identify the cell population responsible for the alopecia, we performed hair-reconstitution assays in nude mice and observed normal hair follicle morphogenesis, regardless of the VDR status of the keratinocytes and dermal papilla cells. However, follicles reconstituted with VDR-null keratinocytes demonstrated a defective response to anagen initiation. Hence, alopecia in the VDR-null mice is due to a defect in epithelial-mesenchymal communication that is required for normal hair cycling. Our results also identify the keratinocyte as the cell of origin of the defect and suggest that this form of alopecia is due to absence of ligand-independent receptor function.