Plasticity, nuclear diapause, and a requiem for the terminal differentiation of epithelia

Origin and functional heterogeneity of fibroblasts

The inherent plasticity and resiliency of fibroblasts make this cell type a conventional tool for basic research. But where do they come from, are all fibroblasts the same, and how do they function in disease? The first fibroblast lineages in mammalian development emerge from the ooze of primary mesenchyme during gastrulation. They are cells that efficiently create and negotiate the extracellular matrix of the mesoderm in order to migrate and meet their developmental fate. Mature fibroblasts in epithelial tissues live in the interstitial spaces between basement membranes that spatially delimit complex organ structures. While the function of resident fibroblasts in healthy tissues is largely conjecture, the accumulation of fibroblasts in pathologic lesions offers insight into biologic mechanisms that control their function; fibroblasts are poised to coordinate fibrogenesis in tissue injury, neoplasia, and aging. Here, we examine the developmental origin and plasticity of fibroblasts, their molecular and functional definitions, the epigenetic control underlying their identity and activation, and the evolution of their immune regulatory functions. These topics are reviewed through the lens of fate mapping using genetically engineered mouse models and from the perspective of single-cell RNA sequencing. Recent observations suggest dynamic and heterogeneous functions for fibroblasts that underscore their complex molecular signatures and utility in injured tissues.

Epithelial to Mesenchymal Transition (EMT) and Endothelial to Mesenchymal Transition (EndMT): Role and Implications in Kidney Fibrosis

Tubulointerstitial injury is one of the hallmarks of renal disease. In particular, interstitial fibrosis has a prominent role in the development and progression of kidney injury. Collagen-producing fibroblasts are responsible for the ECM deposition. However, the origin of those activated fibroblasts is not clear. This chapter will discuss in detail the concept of epithelial to mesenchymal transition (EMT) and endothelial to mesenchymal transition (EndMT) in the context of fibrosis and kidney disease. In short, EMT and EndMT involve a change in cell shape, loss of polarity and increased motility associated with increased collagen production. Thus, providing a new source of fibroblasts. However, many controversies exist regarding the existence of EMT and EndMT in kidney disease, as well as its burden and role in disease development. The aim of this chapter is to provide an overview of the concepts and profibrotic pathways and to present the evidence that has been published in favor and against EMT and EndMT.

On the language and physiology of dormancy and quiescence in plants

The language of dormancy is rich and poetic, as researchers spanning disciplines and decades have attempted to understand the spell that entranced 'Sleeping Beauty', and how she was gently awoken. The misleading use of 'dormancy', applied to annual axillary buds, for example, has confounded progress. Language is increasingly important as genetic and genomic approaches become more accessible to species of agricultural and ecological importance. Here we examine how terminology has been applied to different eco-physiological states in plants, and with pertinent reference to quiescent states described in other domains of life, in order to place plant quiescence and dormancy in a more complete context than previously described. The physiological consensus defines latency or quiescence as opportunistic avoidance states, where growth resumes in favourable conditions. In contrast, the dormant state in higher plants is entrained in the life history of the organism. Competence to resume growth requires quantitative and specific conditioning. This definition applies only to the embryo of seeds and specialized meristems in higher plants; however, mechanistic control of dormancy extends to mobile signals from peripheral tissues and organs, such as the endosperm of seed or subtending leaf of buds. The distinction between dormancy, quiescence, and stress-hardiness remains poorly delineated, most particularly in buds of winter perennials, which comprise multiple meristems of differing organogenic states. Studies in seeds have shown that dormancy is not a monogenic trait, and limited study has thus far failed to canalize dormancy as seen in seeds and buds. We argue that a common language, based on physiology, is central to enable further dissection of the quiescent and dormant states in plants. We direct the topic largely to woody species showing a single cycle of growth and reproduction per year, as these bear the majority of global timber, fruit, and nut production, as well being of great ecological value. However, for context and hypotheses, we draw on knowledge from annuals and other specialized plant conditions, from a perspective of the major physical, metabolic, and molecular cues that regulate cellular activity.

Activation of pancreatic stellate cells involves an EMT-like process

Pancreatic adenocarcinoma (PDAC) and chronic pancreatitis (CP) are characterized by a desmoplastic reaction involving activated pancreatic stellate cells (PSCs). However, the mechanisms of PSC activation remain poorly understood. We examined whether the epithelial-mesenchymal transition (EMT) process might play a role in PSC activation. PSCs were isolated from a rat pancreas and characterized using immunofluorescence and immunocytochemistry. We evaluated changes in cell motility and in the expression levels of a panel of EMT-related genes during the PSC activation process. Activation of PSCs occurred after 48 h of in vitro culture, as indicated by a morphological change to a myofibroblastic shape and a decrease in the number of cytoplasmic lipid droplets. After activation, PSCs showed enhanced cell migration ability compared to quiescent cells. In addition, the expression of epithelial markers (E-cadherin, BMP7 and desmoplakin) decreased, while expression of mesenchymal markers (N-cadherin, vimentin, fibronectin1, collagen1α1 and S100A4) increased in activated PSCs. EMT-related transcription factors (Snail and Slug) were also upregulated after PSC activation. The concurrent increase in cell migration ability and alterations in EMT-related gene expression suggests that the activation of PSCs involves an EMT-like process. The knowledge that PSC activation involves an EMT‑like process may help to identify potential new therapeutic targets to alleviate pancreatic fibrosis in diseases like CP and PDAC.

How tubular epithelial cells dictate the rate of renal fibrogenesis?

The main threat to a kidney injury, whatever its cause and regardless of whether it is acute or chronic, is the initiation of a process of renal fibrogenesis, since fibrosis can auto-perpetuate and is of high prognostic significance in individual patients. In the clinic, a decrease in glomerular filtration rate correlates better with tubulointerstitial damage than with glomerular injury. Accumulation of the extracellular matrix should not be isolated from other significant cellular changes occurring in the kidney, such as infiltration by inflammatory cells, proliferation of myofibroblasts, obliteration of peritubular capillaries and atrophy of tubules. The aim of this review is to focus on tubular epithelial cells (TEC), which, necessarily involved in the repair process, eventually contribute to accelerating fibrogenesis. In the context of injury, TEC rapidly exhibit phenotypic and functional changes that recall their mesenchymal origin, and produce several growth factors known to activate myofibroblasts. Because they are high-demanding energy cells, TEC will subsequently suffer from the local hypoxia that progressively arises in a microenvironment where the matrix increases and capillaries become rarified. The combination of hypoxia and metabolic acidosis may induce a vicious cycle of sustained inflammation, at the center of which TEC dictate the rate of renal fibrogenesis.

Phenotypic plasticity in normal breast derived epithelial cells

BACKGROUND

Normal, healthy human breast tissue from a variety of volunteer donors has become available for research thanks to the establishment of the Susan G. Komen for the Cure® Tissue Bank at the IU Simon Cancer Center (KTB). Multiple epithelial (K-HME) and stromal cells (K-HMS) were established from the donated tissue. Explant culture was utilized to isolate the cells from pieces of breast tissue. Selective media and trypsinization were employed to select either epithelial cells or stromal cells. The primary, non-transformed epithelial cells, the focus of this study, were characterized by immunohistochemistry, flow cytometry, and in vitro cell culture.

RESULTS

All of the primary, non-transformed epithelial cells tested have the ability to differentiate in vitro into a variety of cell types when plated in or on biologic matrices. Cells identified include stratified squamous epithelial, osteoclasts, chondrocytes, adipocytes, neural progenitors/neurons, immature muscle and melanocytes. The cells also express markers of embryonic stem cells.

CONCLUSIONS

The cell culture conditions employed select an epithelial cell that is pluri/multipotent. The plasticity of the epithelial cells developed mimics that seen in metaplastic carcinoma of the breast (MCB), a subtype of triple negative breast cancer; and may provide clues to the origin of this particularly aggressive type of breast cancer. The KTB is a unique biorepository, and the normal breast epithelial cells isolated from donated tissue have significant potential as new research tools.

EMT-MET in renal disease: should we curb our enthusiasm?

Renal epithelial cells arise during embryogenesis by mesenchymal to epithelial transition (MET). In the context of renal diseases, these cells can switch back to a mesenchymal phenotype, in a process thus reminiscent of an epithelial-to-mesenchymal transition (EMT) in which we referred to as "Epithelial Phenotypic Changes" (EPC). The pathophysiological consequence of EPC is controversial: in particular, to what extent EPC contribute to the pool of disease-associated renal fibroblasts is very uncertain. However, there is strong evidence that EPC correlate with a poor renal outcome. EPC indeed reflect an exposure to a profibrotic environment, at an early and potentially reversible stage. Detecting EPC has potential therapeutic implications for patients prone to renal fibrosis, both as a marker of efficacy or more directly as a target. In opposition to the EMT occurring during embryogenesis, EMT in fibrosis as well as in cancer is an anarchic cellular process actually developing at the expense of the whole organ(ism).

The role of EMT in renal fibrosis

It is clear that the well-described phenomenon of epithelial-mesenchymal transition (EMT) plays a pivotal role in embryonic development, wound healing, tissue regeneration, organ fibrosis and cancer progression. EMTs have been classified into three subtypes based on the functional consequences and biomarker context in which they are encountered. This review will highlight findings on type II EMT as a direct contributor to the kidney myofibroblast population in the development of renal fibrosis, specifically in diabetic nephropathy, the signalling molecules and the pathways involved in type II EMT and changes in the expression of specific miRNA with the EMT process. These findings have provided new insights into the activation and development of EMT during disease processes and may lead to possible therapeutic interventions to suppress EMTs and potentially reverse organ fibrosis.

Chromatin-based mechanisms of renal epithelial differentiation

Successful regenerative renal medicine depends on understanding the molecular mechanisms by which diverse phenotypes of epithelial cells differentiate from metanephric mesenchyme to populate nephrons. Whereas many genes are maintained in a poised state within the population of pluripotent progenitors, specialized epithelial functions reflect the selective expression of a subset of genes and the repression of all others. Here we highlight some common mechanisms of cell differentiation and epigenetic regulation to discuss their implications for renal epithelial development, repair, and disease.

Contribution of epithelial plasticity to renal transplantation-associated fibrosis

Every year in the United States, 5000 renal transplant recipients start or restart dialysis because of the unusual propensity of these allografts to develop interstitial fibrosis and tubular atrophy (IF/TA). Although IF/TA often follows one or more identifiable events, our capacity to specifically treat, prevent, or even detect IF/TA at an early stage is poor. These limitations are largely related to our lack of adequate tools to assess graft failure over time. Data accumulated over the past 5 years have demonstrated that tubular epithelial cells may react to certain fibrogenic stimuli to engage in the process of epithelial-to-mesenchymal transition (EMT). In this review, we highlight the current view of EMT with a focus on both its role in the context of renal transplantation and the potential for utilizing markers of EMT to identify patients undergoing early IF/TA.

Deciphering calcineurin inhibitor nephrotoxicity: a pharmacological approach

The calcineurin inhibitors ciclosporin and tacrolimus are used to prevent acute rejection of solid organs after transplantation. Their use can lead to chronic renal damage characterized by progressive and irreversible deterioration of renal function associated with interstitial fibrosis, tubular atrophy, arteriolar hyalinosis and glomerulosclerosis. Many approaches to better understand the mechanisms of this toxicity are in use. The aim of these approaches is to find biomarkers of early kidney injury and potential therapeutic targets. Despite these efforts, the biological processes leading to calcineurin inhibitor nephrotoxicity remain poorly understood. Furthermore, the diagnosis of chronic renal damage remains inaccurate without definitive diagnostic tools, no effective prevention exists and a therapy to treat the damage has yet to be developed. In this article, theories of pharmacodynamics, pharmacokinetics, therapeutic drug monitoring and pharmacogenetics are synthesized in ways that may improve the understanding of mechanisms leading to calcineurin inhibitor toxicity. The importance of global approaches such as toxicogenomics is emphasized to characterize early cellular responses implicated in calcineurin inhibitor nephrotoxicity.

Mechanisms of tubulointerstitial fibrosis

The pathologic paradigm for renal progression is advancing tubulointerstitial fibrosis. Whereas mechanisms underlying fibrogenesis have grown in scope and understanding in recent decades, effective human treatment to directly halt or even reverse fibrosis remains elusive. Here, we examine key features mediating the molecular and cellular basis of tubulointerstitial fibrosis and highlight new insights that may lead to novel therapies. How to prevent chronic kidney disease from progressing to renal failure awaits even deeper biochemical understanding.

[Interstitial fibrosis in renal grafts: On the way to a better detection]

In renal grafts, the progression of interstitial fibrosis and tubular atrophy (IF/TA) is exponential during the first months post-transplant. Consequently, roughly 40% of the cadaveric grafts will function less than ten years. There is, however, no specific strategy to halt fibrogenesis, i.e. the progression of fibrosis with time, in kidney recipients. Epithelial to mesenchymal transition (EMT) is a biological process used to disperse cells during embryogenesis. In the setting of injury, it is also a mechanism to escape cellular death. The last five years, several studies demonstrated that EMT does occur in tubular epithelial cells, which have been shown to loose the expression of epithelial markers, and acquire the expression of mesenchymal proteins, like vimentin. The aim of this review is triple: 1) discuss the connections between EMT and the context of transplantation; 2) explain how EMT markers may be useful in clinical practice, as promising surrogate markers for fibrogenesis; 3) discuss some therapeutic perspectives.

Direct contribution of epithelium to organ fibrosis: epithelial-mesenchymal transition

Fibrosis of epithelial parenchymal organs and end-stage organ failure represent the final common pathway of many chronic diseases and are a major determinant of morbidity and mortality worldwide. Fibrosis is a complex response initiated to protect the host from an injurious event; nevertheless, it leads to serious organ damage when it becomes independent from the initiating stimulus. It involves massive deposition of matrix by an expanded pool of fibrogenic cells, disruption of the normal tissue architecture, and parenchymal destruction. Fibroblasts, the effector cells of matrix production, when engaged in fibrogenesis, display the highly activated phenotype characteristic of myofibroblasts. These cells are present in a large number in sites with ongoing inflammation, reparative reaction, and fibrosis, but their origin has not yet been definitely elucidated. Although proliferation of preexisting stromal fibroblasts and, probably, recruitment of bone marrow-derived fibrogenic cells may account for a portion of them, emerging evidence seems to indicate that an important number of matrix-producing fibroblasts/myofibroblasts arises through a mechanism of epithelial-mesenchymal transition. Through this process, epithelial cells would lose intercellular cohesion and would translocate from the epithelial compartment into the interstitium where, gaining a full mesenchymal phenotype, they could participate in the synthesis of the fibrotic matrix. Epithelial-mesenchymal transition is induced by the integrated actions of many stimuli including transforming growth factor-beta and matrix-generated signals that are also known to be implicated in inflammation, repair responses, and fibrosis. The consequences of epithelial-mesenchymal transition in chronic fibrosing diseases could be two-fold as follows: on one hand, by supplementing new mesenchymal cells, it might feed the expanding pool of interstitial fibroblasts/myofibroblasts responsible for the matrix accumulation; on the other hand, it could cause loss of epithelial cells, thus, contributing to the parenchyma destruction seen in advanced fibrosis. Markers of epithelium undergoing epithelial-mesenchymal transition include loss of E-cadherin and cytokeratin; de novo expression of fibroblast-specific protein 1/S100A4, vimentin, and alpha-smooth muscle actin; basement membrane component loss; and production of interstitial-type matrix molecules such as fibronectin and type I/III collagen. Evidence of epithelial-mesenchymal transition has been reported in the kidney, lung, liver, eye, and serosal membranes suggesting that epithelial-mesenchymal transition could be involved in the pathogenesis of fibrotic disorders in these organs. Thus, because of its fibrogenic potential, the detection of epithelial-mesenchymal transition in biopsy specimens could be useful diagnostically and represent a new biomarker of progression in chronic fibrosing diseases.

Renal studies provide an insight into cardiac extracellular matrix remodeling during health and disease

The remodeling of a heart ventricle after myocardial infarction involves numerous inflammatory mediators that may trigger a long-lasting and a highly fibrogenic process. Likewise, in the kidney, acute and chronic injuries may lead to abnormal extracellular matrix deposition and eventually lead to the loss of renal function. Major breakthroughs have emerged during the last ten years with respect to the pathophysiology of matrix remodeling. Epithelial and endothelial cells are plastic, and able to engage in epithelial (or endothelial)-to-mesenchymal transition (EMT or EndMT), thus actively contributing to the fibrogenesis. Members of the fibrinolytic system were demonstrated to possess unsuspected properties and interact with receptors and integrins on endothelial and epithelial cells. Finally, a notion that stem cells could integrate into damaged tissue has recently emerged, which likely contributes to the tissue repair. In many aspects, the kidney and the heart share many common injury mechanisms. We envision that some of them will be accessible as common therapeutic targets in the future.

Biomarkers for epithelial-mesenchymal transitions

Somatic cells that change from one mature phenotype to another exhibit the property of plasticity. It is increasingly clear that epithelial and endothelial cells enjoy some of this plasticity, which is easily demonstrated by studying the process of epithelial-mesenchymal transition (EMT). Published reports from the literature typically rely on ad hoc criteria for determining EMT events; consequently, there is some uncertainty as to whether the same process occurs under different experimental conditions. As we discuss in this Personal Perspective, we believe that context and various changes in plasticity biomarkers can help identify at least three types of EMT and that using a collection of criteria for EMT increases the likelihood that everyone is studying the same phenomenon - namely, the transition of epithelial and endothelial cells to a motile phenotype.

Early epithelial phenotypic changes predict graft fibrosis

Chronic allograft nephropathy accounts for the loss of approximately 40% of allografts at 10 yr. Currently, no biomarker is available to detect interstitial fibrosis and tubular atrophy in the renal graft at an early stage, when intervention may be beneficial. Because tubular epithelial cells have been shown to exhibit phenotypic changes suggestive of epithelial-to-mesenchymal transition, we studied whether these changes predict the progression of fibrosis in the allograft. Eighty-three kidney transplant recipients who had undergone a protocol graft biopsy at both 3 and 12 mo after transplantation were enrolled. De novo vimentin expression and translocation of beta-catenin into the cytoplasm of tubular cells were detected on the first biopsy by immunohistochemistry. Patients with expression of these markers in >or=10% of tubules at 3 mo had a higher interstitial fibrosis score at 1 yr and a greater progression of this score between 3 and 12 mo. The intensity of these phenotypic changes positively and significantly correlated with the progression of fibrosis, and multivariate analysis showed that their presence was an independent risk factor for this progression. In addition, the presence of early phenotypic changes was associated with poorer graft function 18 mo after transplantation. In conclusion, early phenotypic changes indicative of epithelial-to-mesenchymal transition predict the progression toward interstitial fibrosis in human renal allografts.