The secretome of myocardial telocytes modulates the activity of cardiac stem cells

Stromal cells/telocytes and endothelial progenitors in the perivascular niches of the trigeminal ganglion

Stromal cells/telocytes (SCs/TCs) were recently described in the human adult trigeminal ganglion (TG). As some markers are equally expressed in SCs/TCs and endothelial cells, we hypothesized that a subset of the TG SCs/TCs is in fact represented by endothelial progenitor cells of a myelomonocytic origin. This study aimed to evaluate whether the interstitial cells of the human adult TG correlate with the myelomonocytic lineage. We used primary antibodies for c-erbB2/HER-2, CD31, nestin, CD10, CD117/c-kit, von Willebrand factor (vWF), CD34, Stro-1, CD146, α-smooth muscle actin (α-SMA), CD68, VEGFR-2 and cytokeratin 7 (CK7). The TG pial mesothelium and subpial vascular microstroma expressed c-erbB2/HER-2, CK7 and VEGFR-2. SCs/TCs neighbouring the neuronoglial units (NGUs) also expressed HER-2, which suggests a pial origin. These cells were also positive for CD10, CD31, CD34, CD68 and nestin. Endothelial cells expressed CD10, CD31, CD34, CD146, nestin and vWF. We also found vasculogenic networks with spindle-shaped and stellate endothelial progenitors expressing CD10, CD31, CD34, CD68, CD146 and VEGFR-2. Isolated mesenchymal stromal cells expressed Stro-1, CD146, CK7, c-kit and nestin. Pericytes expressed α-SMA and CD146. Using transmission electron microscopy (TEM), we found endothelial-specific Weibel-Palade bodies in spindle-shaped stromal progenitors. Our study supports the hypothesis that an intrinsic vasculogenic niche potentially involved in microvascular maintenance and repair might be present in the human adult trigeminal ganglion and that it might be supplied by either the pial mesothelium or the bone marrow niche.

Cardiac telocytes. From basic science to cardiac diseases. II. Acute myocardial infarction

INTRODUCTION

The purpose of this study was to evaluate the scientific evidence regarding a potential role of telocytes in myocardial infarction.

MATERIALS AND METHODS

To this purpose, we performed a systematic review of relevant scientific literature, indexed in PubMed, Web of Science, and Scopus.

RESULTS AND DISCUSSIONS

We found six articles containing relevant studies aimed at liking myocardial infarction and telocytes. The studies that were analysed in this review failed to show, beyond a reasonable doubt, that telocytes do actually have significant roles in myocardial regeneration after myocardial infarction. The main issues to be addressed in future studies are a correct characterization of telocytes, and a differentiation from other cell types that either have similar morphologies (using electron microscopy) or similar immunophenotypes, with emphasis on endothelial progenitors, which were previously shown to have similar morphology, and functions in cardiac regeneration after myocardial infarction.

Morphological evidence of telocytes in human synovium

A new cell type named telocyte (i.e. cell with distinctive prolongations called telopodes) has recently been identified in the stroma of various organs in humans. However, no study has yet reported the existence of telocytes in the synovial membrane of diarthrodial joints. This work was therefore undertaken to search for telocytes in the normal human synovium using transmission electron microscopy, immunohistochemistry and immunofluorescence. Ultrastructural analyses demonstrated the presence of numerous spindle-shaped telocytes in the whole synovial sublining layer. Synovial telocytes exhibited very long and thin moniliform telopodes and were particularly concentrated at the boundary between the lining and sublining layers and around blood vessels. Light microscopy confirmed the presence of CD34-positive telocytes in the aforementioned locations. Moreover, synovial telocytes coexpressed CD34 and platelet-derived growth factor receptor α. Double immunostaining further allowed to unequivocally differentiate synovial telocytes (CD34-positive/CD31-negative) from vascular endothelial cells (CD34-positive/CD31-positive). The in vitro examination of fibroblast-like synoviocyte primary cultures revealed the coexistence of different cell types, including CD34-positive telocytes projecting typical moniliform telopodes. In conclusion, our work provides the first evidence that telocytes do exist in the human synovium and lays the groundwork for future studies on synovial telocytes in a variety of degenerative and destructive joint diseases.

Cardiac Telocyte-Derived Exosomes and Their Possible Implications in Cardiovascular Pathophysiology

Among cardiac interstitial cells, the recently described telocytes (TCs) display the unique ability to build a supportive three-dimensional network formed by their very long and thin prolongations named telopodes. Cardiac TCs are increasingly regarded as pivotal regulators in intercellular signaling with multiple cell types, such as cardiomyocytes, stem/progenitor cells, microvessels, nerve endings, fibroblasts and immune cells, thus converting the cardiac stromal compartment into an integrated system that may drive either heart development or maintenance of cardiac homeostasis in post-natal life. Besides direct intercellular communications between TCs and neighboring cells, different types of TC-released extracellular vesicles (EVs), namely exosomes, ectosomes and multivesicular cargos, may act as shuttles for paracrine molecular signal exchange between cardiac TCs and cardiomyocytes or putative cardiomyocyte progenitors. In this review, we summarize the recent research findings on cardiac TCs and their EVs. We first provide an overview of the general features of TCs, including their peculiar morphological traits and immunophenotypes, intercellular signaling mechanisms and possible functional roles. Thereafter, we describe the distribution of TCs in normal and diseased hearts, as well as their role as intercellular communicators via the release of exosomes and other types of EVs. Finally, the involvement of cardiac TCs in cardiovascular diseases and the potential utility of TC transplantation and TC-derived exosomes in cardiac regeneration and repair are discussed.

Telocytes in normal and keratoconic human cornea: an immunohistochemical and transmission electron microscopy study

Telocytes (TC) are typically defined as cells with telopodes by their ultrastructural features. Their presence was reported in the interstitium of various organs in vertebrates, including humans. However, no study has yet described the presence of TC in the human eye and in particular, within the stromal compartment of the cornea. To address this issue, samples of normal and pathologic (keratoconic) human corneas were tested by immunohistochemistry for CD34, platelet‐derived growth factor receptor α (PDGFRα) and c‐kit/CD117 or examined by transmission electron microscopy. We found that TC coexpressing CD34 and PDGFRα were distributed throughout the whole normal corneal stroma with different TC subtypes being distinguishable on the basis of the expression of the stemness marker c‐kit (i.e. c‐kit‐positive and c‐kit‐negative TC subpopulations). Transmission electron microscopy examination confirmed the existence of spindle‐shaped and bipolar TC typically displaying two long and thin moniliform telopodes establishing intercellular contacts formed by gap junctions. Keratoconic corneas were characterized by ultrastructural damages and patchy loss of TC with an almost complete depletion of the c‐kit‐positive TC subpopulation. We propose that TC may contribute to the maintenance of corneal stromal homoeostasis and that, in particular, the c‐kit‐positive TC subtype might have stemness capacity participating in corneal regeneration and repair processes. Further studies are needed to clarify the differential roles of corneal TC subtypes as well as the possible therapeutic applications of TC in degenerative corneal disorders such as keratoconus.

The Telocyte Subtypes

Several cells are endowed in the interstitial space of the connective tissue; among them, a peculiar type has been recently described and named telocyte (TC). The increasing interest on this cell type has allowed identifying it in almost all the organs. All TCs have a proper ultrastructural feature that makes them undoubtedly recognizable under the transmission electron microscope (TEM). On the contrary, a complex often confusing picture comes out from the immunohistochemical investigations either due to the technical procedures used or, intriguingly, to the possibility that diverse subtypes of TC might exist.Among the several markers used to label the TC, the most common are the CD34 and the PDGFRalpha, and, in many organs, the TC expresses both these markers. An exception is represented by the human urinary bladder where none of the TC, as recognized under the TEM, was double labelled. All the data indicate that TCs show immunohistochemical differences depending on the organ where they are located and/or the animal species.On the basis of their ubiquitous distribution, TCs are unanimously considered organizers of the connective tissue because of their ability to form 3-D networks. Close to this common role, numerous other roles have been attributed to the TC. Indeed, each of the TC subtype likely plays an own organ-/tissue-specific role contributing to different aspects of physiological regulation in the various anatomical niches they occupy.

Telocytes in Chronic Inflammatory and Fibrotic Diseases

Telocytes are a peculiar stromal (interstitial) cell type implicated in tissue homeostasis and development, as well as in the pathophysiology of several disorders. Severe damage and reduction of telocytes have been reported during fibrotic remodeling of multiple organs in various diseases, including scleroderma, Crohn's disease, ulcerative colitis, and liver fibrosis, as well as in chronic inflammatory lesions like those of primary Sjögren's syndrome and psoriasis. Owing to their close relationship with stem cells, telocytes are also supposed to contribute to tissue repair/regeneration. Indeed, telocytes are universally considered as "connecting cells" mostly oriented to intercellular signaling. On the basis of recent promising experimental findings, in the near future, telocyte transplantation might represent a novel therapeutic opportunity to control the evolution of chronic inflammatory and fibrotic diseases. Notably, there is evidence to support that telocytes could help in preventing abnormal activation of immune cells and fibroblasts, as well as in attenuating the altered matrix organization during the fibrotic process. By targeting telocytes alone or in tandem with stem cells, we might be able to promote regeneration and prevent the evolution to irreversible tissue injury. Besides exogenous transplantation, exploring pharmacological or non-pharmacological methods to enhance the growth and/or survival of telocytes could be an additional therapeutic strategy for many disorders.

Calcium Signaling in Interstitial Cells: Focus on Telocytes

In this review, we describe the current knowledge on calcium signaling pathways in interstitial cells with a special focus on interstitial cells of Cajal (ICCs), interstitial Cajal-like cells (ICLCs), and telocytes. In detail, we present the generation of Ca²⁺ oscillations, the inositol triphosphate (IP₃)/Ca²⁺ signaling pathway and modulation exerted by cytokines and vasoactive agents on calcium signaling in interstitial cells. We discuss the physiology and alterations of calcium signaling in interstitial cells, and in particular in telocytes. We describe the physiological contribution of calcium signaling in interstitial cells to the pacemaking activity (e.g., intestinal, urinary, uterine or vascular pacemaking activity) and to the reproductive function. We also present the pathological contribution of calcium signaling in interstitial cells to the aortic valve calcification or intestinal inflammation. Moreover, we summarize the current knowledge of the role played by calcium signaling in telocytes in the uterine, cardiac and urinary physiology, and also in various pathologies, including immune response, uterine and cardiac pathologies.

Telocytes - a Hope for Cardiac Repair after Myocardial Infarction

Cardiovascular diseases, particularly myocardial infarction, remain the leading cause of morbidity and mortality worldwide, even though pharmacological and interventional therapies improved significantly in the last years. Moreover, despite encouraging results of cell - based therapies in experimental myocardial infarction models, clinical trials showed inconsistent and modest efficiency. Therefore the next step should be the revealing of a new cell type, capable of regenerating the damaged myocardium. Telocytes (TCs), a relatively new type of interstitial cells, were described few years ago and are credited with important roles in regenerative therapies. In this paper we review their most important characteristics and functions, showing the evidences of their potential role in cardiac repair and regeneration. Our research leads to the conclusion that TCs might be a novel target for therapeutic strategies in myocardial infarction.

Telocytes in gastric lamina propria of the Chinese giant salamander, Andrias davidianus

In this study, we attempt to identify gastric telocytes (TCs) of the Chinese giant salamander Andrias davidianus, by light microscopy, immunohistochemistry and transmission electron microscopy (TEM) methods. Toluidine blue staining showed TCs with one to two very thin and long telopodes (Tps) that were located in gastric lamina propria. Tps had characteristic structures, including podoms, podomers and dichotomous branching. Immunohistochemistry showed the existence of CD34⁺/PDGFRα⁺ TCs with moniliform Tps in stroma and were close to gastric glands and blood vessels. TEM micrographs also demonstrated the presence of TCs in interstitium between gastric glands. TCs/Tps were located in close proximity to gastric glands, blood vessels, endocrine cells and stem cells. In particular, Tps frequently surrounded stem cells. TCs and Tps, Tps and stem cells established close contacts. Moreover, the exosomes were also found near TCs/Tps. Our data confirmed the presence of TCs in gastric lamina propria of the amphibian, and suggested that TCs cooperate with resident stem cells to regulate endocrine cells and gastric glands regeneration and homeostasis.

Telocytes heterogeneity: From cellular morphology to functional evidence

Telocytes (TCs), located ubiquitously in the internal organs of vertebrates, are a heterogeneous, recently described, cell population of the stromal space. Characterized by lengthy cytoplasmic extensions that can reach tens of microns and are called telopodes (Tps), TCs are difficult to see using conventional microscopes. It was the electron microscopy which led to their first identification and Popescu's team the first responsible for the reconstructions indicating TCs 'organization' in a three-dimensional (3D) network that is believed to be accountable for the complex roles of TCs. Gradually, it became increasingly evident that TCs are difficult to characterize in terms of immunophenotype and that their phenotype is different depending on the location and needs of the tissue at one time. This review discusses the growing body of evidence accumulated since TCs were discovered and highlights how the complex interplay between TCs and stem cells might be of importance for tissue engineering and regenerative medicine.

Telocytes and Their Extracellular Vesicles-Evidence and Hypotheses

Entering the new millennium, nobody believed that there was the possibility of discovering a new cellular type. Nevertheless, telocytes (TCs) were described as a novel kind of interstitial cell. Ubiquitously distributed in the extracellular matrix of any tissue, TCs are regarded as cells with telopodes involved in intercellular communication by direct homo- and heterocellular junctions or by extracellular vesicle (EVs) release. Their discovery has aroused the interest of many research groups worldwide, and many researchers regard them as potentially regenerative cells. Given the experience of our laboratory, where these cells were first described, we review the evidence supporting the fact that TCs release EVs, and discuss alternative hypotheses about their future implications.

Ultrastructural and immunohistochemical characteristics of telocytes in the skin and skeletal muscle of newborn rats

Telocytes (TCs) are newly described interstitial cells that might play a role in normal and pathological conditions. The aim of this study was designed to investigate its existence in the skin and skeletal muscle of one day old newborn rats and to study their ultrastructure and immunohistochemical characteristics. Ten rats (one day old newborn) were used in this study. Dorsal skin and femoral skeletal muscle samples were obtained and examined by CD117, CD34, semi-thin and ultrathin sections examination. Semi-thin sections examination revealed multiple spindle shape cells with cytoplasmic extension in the skin and in between muscle fibers. Telocytes showed positive reaction for both CD117 and CD34 immunostains. By electron microscopy these cells were spindle shaped with small cell bodies and long processes. Telocytes showed homo-cellular junctions between two adjacent telocytes and hetero-cellular junctions between telocytes and other cellular and non-cellular structures. Multiple vesicles were seen either intra-cellular or budding from the cell membrane or detached from the telocytes leaving caveolae. It could be concluded that telocytes are present in the skin and skeletal muscle of one day old newborn rats. They might play a role in pathologies and regenerative medicine due to their ability to release vesicles.

Increased telocytes involved in the proliferation of vascular smooth muscle cells in rat carotid artery balloon injury

Telocytes (TCs) are a novel type of interstitial cells that are thought to be involved in tissue regeneration and repair. However, the possible roles of TCs in vascular diseases remain unclear. In this study, we used a rat model of carotid artery balloon injury (CABI) to study the changes and potential roles of vascular TCs after vascular injury. Transmission electron microscopy (TEM) and CD34/vimentin immunolabeling were used to identify and quantify TCs in normal and injured carotid arteries. Quantitative immunofluorescence analysis revealed that, compared with the sham group, the number of TCs in the CABI group increased from 7.2±1.0 to an average of 20.4±1.8 per 1-mm(2) vascular area. The expression level of miR-24 in TCs was three times higher than in vascular smooth muscle cells (VSMCs). The percentage of VSMCs in S phase and G2/M phase increased by approximately 5% when VSMCs were incubated with the supernatant of TCs. The antagomir of miR-24 in TCs reduced the ratio of VSMCs in S phase and G2/M phase. This study illuminates the function of TCs in the proliferation of VSMCs.

Telocytes of the human adult trigeminal ganglion

Telocytes (TCs) are typically defined as cells with telopodes by their ultrastructural features. Their presence was reported in various organs, however little is known about their presence in human trigeminal ganglion. To address this issue, samples of trigeminal ganglia were tested by immunocytochemistry for CD34 and examined by transmission electron microscopy (TEM). We found that TCs are CD34 positive and form networks within the ganglion in close vicinity to microvessels and nerve fibers around the neuronal-glial units (NGUs). TEM examination confirmed the existence of spindle-shaped and bipolar TCs with one or two telopodes measuring between 15 to 53 μm. We propose that TCs are cells with stemness capacity which might contribute in regeneration and repair processes by: modulation of the stem cell activity or by acting as progenitors of other cells present in the normal tissue. In addition, further studies are needed to establish if they might influence the neuronal circuits.

Telocytes in exercise-induced cardiac growth

Exercise can induce physiological cardiac growth, which is featured by enlarged cardiomyocyte cell size and formation of new cardiomyocytes. Telocytes (TCs) are a recently identified distinct interstitial cell type, existing in many tissues and organs including heart. TCs have been shown to form a tandem with cardiac stem/progenitor cells in cardiac stem cell niches, participating in cardiac regeneration and repair. Although exercise‐induced cardiac growth has been confirmed as an important way to promote cardiac regeneration and repair, the response of cardiac TCs to exercise is still unclear. In this study, 4 weeks of swimming training was used to induce robust healthy cardiac growth. Exercise can induce an increase in cardiomyocyte cell size and formation of new cardiomyocytes as determined by Wheat Germ Lectin and EdU staining respectively. TCs were identified by three immunofluorescence stainings including double labelling for CD34/vimentin, CD34/platelet‐derived growth factor (PDGF) receptor‐α and CD34/PDGF receptor‐β. We found that cardiac TCs were significantly increased in exercised heart, suggesting that TCs might help control the activity of cardiac stem/progenitor cells, cardiomyocytes or endothelial cells. Adding cardiac TCs might help promote cardiac regeneration and renewal.

Horizontal transfer of RNA and proteins between cells by extracellular microvesicles: 14 years later

Extracellular microvesicles (ExMVs) are part of the cell secretome, and evidence has accumulated for their involvement in several biological processes. Fourteen years ago our team demonstrated for the first time that ExMVs carry functional RNA species and proteins from one cell to another, an observation that opened up the new research field of horizontal transfer of bioactive molecules in cell-to-cell communication. Moreover, the presence of mRNA, noncoding RNA, and miRNA in ExMVs in blood and other biological fluids opened up the possibility of employing ExMVs as new detection markers for pathological processes, and ExMVs became a target for "liquid biopsy" approaches. While ExMV-derived mRNAs may be translated in target cells into appropriate proteins, miRNAs regulate expression of corresponding mRNA species, and both RNA-depended ExMV-mediated mechanisms lead to functional changes in the target cells. Following from this observation, several excellent papers have been published that confirm the existence of the horizontal transfer of RNA. Moreover, in addition to RNA, proteins, bioactive lipids, infectious particles and intact organelles such as mitochondria may follow a similar mechanism. In this review we will summarize the impressive progress in this field-14 years after initial report.

Telocytes in cardiac regeneration and repair

Telocytes (TCs) are a novel type of stromal cells reported by Popescu's group in 2010. The unique feature that distinguishes TCs from other "classical" stromal cells is their extremely long and thin telopodes (Tps). As evidenced by electron microscopy, TCs are widely distributed in almost all tissues and organs. TCs contribute to form a three-dimensional interstitial network and play as active regulators in intercellular communication via homocellular/heterocellular junctions or shed vesicles. Interestingly, increasing evidence suggests the potential role of TCs in regenerative medicine. Although the heart retains some limited endogenous regenerative capacity, cardiac regenerative and repair response is however insufficient to make up the loss of cardiomyocytes upon injury. Developing novel strategies to increase cardiomyocyte renewal and repair is of great importance for the treatment of cardiac diseases. In this review, we focus on the role of TCs in cardiac regeneration and repair. We particularly describe the intercellular communication between TCs and cardiomyocytes, stem/progenitor cells, endothelial cells, and fibroblasts. Also, we discuss the current knowledge about TCs in cardiac repair after myocardial injury, as well as their potential roles in cardiac development and aging. TC-based therapy or TC-derived exosome delivery might be used as novel therapeutic strategies to promote cardiac regeneration and repair.

Behaviour of telocytes during physiopathological activation

We consider CD34+ stromal cells/telocytes (CD34+ SC/TCs) in normal and pathological conditions. These cells are involved in organisation and control of the extracellular matrix, structural support, creation of microenvironments, intercellular communication, neurotransmission, immunomodulation and immunosurveillance, inhibition of apoptosis, and control, regulation and source of other cell types. CD34+ SC/TCs are widely reported in the origin of interstitial cells of Cajal and in regeneration in the heart, skeletal muscle, skin, respiratory tree, liver, urinary system and the eye. In addition, we contribute CD34+ SC/TC hyperplasia associated with several processes, including neurogenous hyperplasia (neuroma of the appendix), hyperplasia of Leydig cells in undescended testes (Cryptorchidism), peripheral areas of inflammatory/repair processes (pericicatricial tissue and transitional zones between diseased segments in Crohn's disease and normal bowel), benign tumours (neurofibromas, Antoni-B zones of neurilemmomas, granular cell tumours, and melanocytic nevi) and in some lesions with myxoid, oedematous and degenerative changes (Reinke's oedema, myxomatous mitral valve degeneration, thyroid-associated ophthalmopathy and basophilic degenerative changes of the collagen in the dermis). We pay particular attention to the role of CD34+ SC/TCs during repair through granulation tissue, including morphologic changes, loss of CD34 expression and gain of αSMA expression with myofibroblast transformation, and interactions with pericytes, endothelial and inflammatory cells. Finally, we consider CD34 or αSMA expression in stromal cells of malignant epithelial tumours, and the role of CD34+ SC/TCs in the origin of carcinoma-associated fibroblasts (CAFs) and myofibroblasts. In conclusion, CD34+ SC/TCs play an important role in the maintenance and modulation of tissue homeostasis and in morphogenesis/renewal/repair.

Telocyte implications in human pathology: An overview

Telocytes are a recently described interstitial cell population widely distributed in the stromal compartment of many organs in vertebrates, including humans. Owing to their close spatial relationship with multiple cell types, telocytes are universally considered as 'connecting cells' mostly committed to intercellular signaling by converting the interstitium into an integrated system that drives organ development and contributes to the maintenance of local tissue homeostasis. Increasing evidence indicates that telocytes may cooperate with tissue-resident stem cells to foster organ repair and regeneration, and that telocyte damage and dysfunction may occur in several disorders. The goal of this review is to provide an overview of the most recent findings concerning the implication of telocytes in a variety of pathologic conditions in humans, including heart disease, chronic inflammation and multiorgan fibrosis. Based on recent promising experimental data, there is realistic hope that by targeting telocytes alone or in tandem with stem cells, we might be able to promote organ regeneration and/or prevent irreversible end-stage organ damage in different pathologies.

The potential role of telocytes in Tissue Engineering and Regenerative Medicine

Research and ideas for potential applications in the field of Tissue Engineering (TE) and Regenerative Medicine (RM) have been constantly increasing over recent years, basically driven by the fundamental human dream of repairing and regenerating lost tissue and organ functions. The basic idea of TE is to combine cells with putative stem cell properties with extracellular matrix components, growth factors and supporting matrices to achieve independently growing tissue. As a side effect, in the past years, more insights have been gained into cell-cell interaction and how to manipulate cell behavior. However, to date the ideal cell source has still to be found. Apart from commonly known various stem cell sources, telocytes (TC) have recently attracted increasing attention because they might play a potential role for TE and RM. It becomes increasingly evident that TC provide a regenerative potential and act in cellular communication through their network-forming telopodes. While TE in vitro experiments can be the first step, the key for elucidating their regenerative role will be the investigation of the interaction of TC with the surrounding tissue. For later clinical applications further steps have to include an upscaling process of vascularization of engineered tissue. Arteriovenous loop models to vascularize such constructs provide an ideal platform for preclinical testing of future therapeutic concepts in RM. The following review article should give an overview of what is known so far about the potential role of TC in TE and RM.

Extracellular Microvesicles (ExMVs) in Cell to Cell Communication: A Role of Telocytes

There are several mechanisms by which cells communicate with each other. Evidence accumulates that the evolutionary oldest mechanisms of cell-cell communication involves extracellular microvesicles (ExMVs). Generally, these circular membrane fragments enriched for mRNA, miRNA, proteins, and bioactive lipids are released by exocytosis from endosomal compartment or are directly formed by budding from cell surface membranes. ExMVs from endosomal compartment called exosomes are smaller in size ~100 nM as compared to larger ones released from cell membranes that are in size up to 1 μM. In this chapter we will present an emerging link between ExMVs and recently identified novel cell-cell communication network involving a new type of cell known as telocyte. Mounting evidence accumulates that telocytes mediate several of their biological effects in several organs by releasing ExMVs enriched in mRNA, miRNA, proteins, and several biological mediators to the target cells.

Immunohistochemistry of Telocytes in the Uterus and Fallopian Tubes

The seminal work of Popescu and colleagues first demonstrated the existence of a new cell type - the telocytes. We were among the first who reported the presence of such cells in the female genital tract and performed TEM examinations, as well as immunohistochemical staining in the attempt to find a specific marker. Telocytes from rat and from the human uterus and from human fallopian tube were extensively investigated initially by comparison with interstitial cells of Cajal. Progress in telocyte research led to the identification of different subtypes suggestive for a heterogeneous telocyte population which can even coexist in the same location. As a consequence, the functions of TCs are still elusive and can be considered a versatile phenomenon that depends on a variety of conditions, including signal reception and transmission of information via extracellular vesicles or by direct intercellular contact.

The History of Telocyte Discovery and Understanding

Telocytes (TCs) are identified as a peculiar cell type of interstitial cells in various organs. The typical features of TCs from the other cells are the extending cellular process as telopodes with alternation of podomeres and podoms. Before the year of 2010, TCs were considered as interstitial Cajal-like cells because of the similar morphology and immunohistochemical features with interstitial cells of Cajal which were found more than 100 years ago and considered to be pacemakers for gut motility. Subsequently, it demonstrated that TCs were not Cajal-like cells, and thus the new name "telocyte" was proposed in 2010. With the help of different techniques, e.g., transmission electron microscopy, immunohistochemistry, or omics science, TCs have been detected in various tissues and organs from different species. The pathological role of TCs in different diseases was also studied. According to observation in situ or in vitro, TCs played a vital role in mechanical support, signaling transduction, tissue renewal or repair, immune surveillance, and mechanical sensor via establishing homo- or heterogenous junctions with neighboring cells to form 3D network or release extracellular vesicles to form juxtacrine and paracrine. This review will introduce the origin, distribution, morphology, functions, omics science, methods, and interaction of TCs with other cells and provide a better understanding of the new cell type.

Decoding Telocytes

Telocytes (TCs) are a novel type of interstitial cells, with extremely long and thin cellular prolongations termed telopodes (Tps). TCs were first identified by Popescu et al. and described their finding as "cells with telopodes." The presence of TCs has been reported in the majority of tissues and organs (for details please visit www.telocytes.com ). TCs have been ignored or overlooked for a long time due to our inability to observe these cells via a light microscopy. TCs represent a distinct cell population, different from other types of interstitial cells, based on their distinct (ultra)structure, immunophenotype, microRNA profile, gene feature, proteome signature, and secretome features. As TCs have been suggested as new cellular targets for forthcoming therapies, developing specific methods to modulate TC numbers represents an important objective.

The Third Dimension of Telocytes Revealed by FIB-SEM Tomography

Lately, spatial three-dimensional (3D) identity of cells and their interrelations with the environment that surrounds it represent a challenging trend with the purpose to achieve a holistic view over the functions. Combining data from different imaging of cells in the third dimension can offer insight into behavior modalities making a world of difference. This chapter outlines a breakthrough in telocyte research by volume electron microscopy with the aid of focused ion beam scanning electron microscopy (FIB-SEM). Reconstructing 3D (three-dimensional) appearance of telocytes from a set of two-dimensional (2D) images by FIB-SEM tomography allowed to extract valuable data about their volume in nanoscale dimensions such as the three-dimensional morphology of telopodes and extracellular vesicles.

Telocytes in Inflammatory Gynaecologic Diseases and Infertility

Women suffered with inflammatory gynecologic diseases, such as endometriosis (EMs) and acute salpingitis (AS) often complained of sub- or infertility, even in those women without obvious macroscopic anatomical pelvic abnormalities also have unexplained infertility. Generally, besides the well-known impairment of classically described oviduct cells caused by inflammatory diseases, such as the ciliated cells, fibroblasts and myofibroblasts, the involvement of the newly identified telocytes (TCs) in disease-affected oviduct tissues and potential pathophysiological roles in fertility problems remain unknown. In this chapter, TCs was investigated in rat model of EMs- and AS-affected oviduct tissues. Results showed inflammation and ischaemia-induced extensive ultrastructural damages of TCs both in cellular body and prolongations, with obvious TCs loss and interstitial fibrotic remodelling. Such in vivo pathological alterations might contribute to structural and functional abnormalities of oviduct tissue and potentially engaged in sub- or infertility. And especially, TCs connected to various activated immunocytes in both normal and diseased tissues, thus might participate in local immunoregulation (either repression or activation) and serve a possible explanation for immune-mediated pregnancy failure. Then, in vitro cell co-culture study showed that uterine TC conditioned media (TCM) can activate mouse peritoneal macrophages and subsequently trigger its cytokine secretion, thus providepreliminary evidence that, TCs are not simply innocent bystanders, but are instead potential functional players in local immunoregulatory and immunosurveillance.

Comparison of Chromosome 4 gene expression profile between lung telocytes and other local cell types

Telocytes (TCs) are new cellular entities of mesenchymal origin described almost ubiquitously in human and mammalian organs (www.telocytes.com). Different subtypes of TCs were described, all forming networks in the interstitial space by homo- and heterocellular junctions. Previous studies analysed the gene expression profiles of chromosomes 1, 2, 3, 17 and 18 of murine pulmonary TCs. In this study, we analysed by bioinformatics tools the gene expression profiles of chromosome 4 for murine pulmonary TCs and compared it with mesenchymal stem cells (MSCs), fibroblasts (Fbs), alveolar type II cells (ATII), airway basal cells, proximal airway cells, CD8(+) T cells from bronchial lymph nodes (T-BL) and CD8(+) T cells from lungs (T-L). Key functional genes were identified with the aid of the reference library of the National Center for Biotechnology Information Gene Expression Omnibus database. Seventeen genes were up-regulated and 56 genes were down-regulated in chromosome 4 of TCs compared with other cells. Four genes (Akap2, Gpr153, Sdc3 and Tbc1d2) were up-regulated between one and fourfold and one gene, Svep1, was overexpressed over fourfold. The main functional networks were identified and analysed, pointing out to a TCs involvement in cellular signalling, regulation of tissue inflammation and cell expansion and movement.

In vitro morphology, viability and cytokine secretion of uterine telocyte-activated mouse peritoneal macrophages

Telocytes (TCs), a distinct interstitial cell population, have been identified in the uterus, oviduct and placenta, with multiple proposed potential biological functions. Their unique structure allows them to form intercellular junctions with various immunocytes, both in normal and diseased tissues, suggesting a potential functional relationship with the local immune response. It has been hypothesized that through direct heterocellular junctions or indirect paracrine effects, TCs influence the activity of local immunocytes that are involved in the inflammatory process and in immune-mediated reproductive abnormalities. However, no reliable cytological evidence for this hypothesis is currently available. In this study, we cultured primary murine uterine TCs and collected TC conditioned media (TCM). Mouse peritoneal macrophages (pMACs) were co-cultured for 48 hrs with TCM or with DMEM/F12 or lipopolysaccharide (LPS) as negative and positive controls, respectively. Normal uterine TCs with a typical structure and a CD-34-positive/vimentin-positive/c-kit-negative immunophenotype were observed during culture. Morphologically, TCM-treated pMACs displayed an obvious activation/immunoresponse, in contrast to over-stimulation and cell death after LPS treatment and no sign of activation in the presence of DMEM/F12. Accordingly, a cell counting kit 8 (CCK-8) assay indicated significant activation of pMACs by TCM and LPS compared to DMEM/F12, thus supporting the marked morphological differences among these groups of cells. Furthermore, within a panel of macrophage-derived cytokines/enzymes, interleukin-6 (IL-6) and inducible nitric oxide synthase were significantly elevated in TCM-treated pMACs; tumour necrosis factor α, IL1-R1, and IL-10 were slightly, but significantly, up-regulated; and no changes were observed for transforming growth factor-β1, IL-1β, IL-23α and IL-18. Our results indicate that TCs are not simply innocent bystanders but are rather functional players in the activation of pMACs; they trigger and maintain the immune response, likely through indirect paracrine effects. Thus, we provide preliminary in vitro evidence of immunoregulatory and immunosurveillance roles for TCs.

iPSC-derived human cardiac progenitor cells improve ventricular remodelling via angiogenesis and interstitial networking of infarcted myocardium

We investigate the effects of myocardial transplantation of human induced pluripotent stem cell (iPSC)‐derived progenitors and cardiomyocytes into acutely infarcted myocardium in severe combined immune deficiency mice. A total of 2 × 10⁵ progenitors, cardiomyocytes or cell‐free saline were injected into peri‐infarcted anterior free wall. Sham‐operated animals received no injection. Myocardial function was assessed at 2‐week and 4‐week post‐infarction by using echocardiography and pressure‐volume catheterization. Early myocardial remodelling was observed at 2‐week with echocardiography derived stroke volume (SV) in saline (20.45 ± 7.36 μl, P < 0.05) and cardiomyocyte (19.52 ± 3.97 μl, P < 0.05) groups, but not in progenitor group (25.65 ± 3.61 μl), significantly deteriorated as compared to sham control group (28.41 ± 4.41 μl). Consistently, pressure–volume haemodynamic measurements showed worsening chamber dilation in saline (EDV: 23.24 ± 5.01 μl, P < 0.05; ESV: 17.08 ± 5.82 μl, P < 0.05) and cardiomyocyte (EDV: 26.45 ± 5.69 μl, P < 0.05; ESV: 18.03 ± 6.58 μl, P < 0.05) groups by 4‐week post‐infarction as compared to control (EDV: 15.26 ± 2.96 μl; ESV: 8.41 ± 2.94 μl). In contrast, cardiac progenitors (EDV: 20.09 ± 7.76 μl; ESV: 13.98 ± 6.74 μl) persistently protected chamber geometry against negative cardiac remodelling. Similarly, as compared to sham control (54.64 ± 11.37%), LV ejection fraction was preserved in progenitor group from 2‐(38.68 ± 7.34%) to 4‐week (39.56 ± 13.26%) while cardiomyocyte (36.52 ± 11.39%, P < 0.05) and saline (35.34 ± 11.86%, P < 0.05) groups deteriorated early at 2‐week. Improvements of myocardial function in the progenitor group corresponded to increased vascularization (16.12 ± 1.49/mm² to 25.48 ± 2.08/mm² myocardial tissue, P < 0.05) and coincided with augmented networking of cardiac telocytes in the interstitial space of infarcted zone.

Reaching out: junctions between cardiac telocytes and cardiac stem cells in culture

Telocytes (TCs) were previously shown by our group to form a tandem with stem/progenitor cells in cardiac stem cell (CSC) niches, fulfilling various roles in cardiac renewal. Among these, the ability to ‘nurse’ CSCs in situ, both through direct physical contact (junctions) as well as at a distance, by paracrine signalling or through extracellular vesicles containing mRNA. We employed electron microscopy to identify junctions (such as gap or adherens junctions) in a co‐culture of cardiac TCs and CSCs. Gap junctions were observed between TCs, which formed networks, however, not between TCs and CSCs. Instead, we show that TCs and CSCs interact in culture forming heterocellular adherens junctions, as well as non‐classical junctions such as puncta adherentia and stromal synapses. The stromal synapse formed between TCs and CSCs (both stromal cells) was frequently associated with the presence of electron‐dense nanostructures (on average about 15 nm in length) connecting the two opposing membranes. The average width of the synaptic cleft was 30 nm, whereas the average length of the intercellular contact was 5 μm. Recent studies have shown that stem cells fail to adequately engraft and survive in the hostile environment of the injured myocardium, possibly as a result of the absence of the pro‐regenerative components of the secretome (paracrine factors) and/or of neighbouring support cells. Herein, we emphasize the similarities between the junctions described in co‐culture and the junctions identified between TCs and CSCs in situ. Reproducing a CSC niche in culture may represent a viable alternative to mono‐cellular therapies.

Skin telocytes versus fibroblasts: two distinct dermal cell populations

It is already accepted that telocytes (TCs) represent a new type of interstitial cells in human dermis. In normal skin, TCs have particular spatial relations with different dermal structures such as blood vessels, hair follicles, arrector pili muscles or segments of sebaceous and/or eccrine sweat glands. The distribution and the density of TCs is affected in various skin pathological conditions. Previous studies mentioned the particular (ultra)structure of TCs and also their immunophenotype, miR imprint or proteome, genome or secretome features. As fibroblast is the most common intersitital cell (also in human dermis), a dedicated comparison between human skin TCs and fibroblasts (Fbs) was required to be performed. In this study, using different techniques, we document several points of difference between human dermis TCs and Fbs. By transmission electron microscopy (TEM) and scanning electron microscopy (SEM), we demonstrated TCs with their hallmark cellular prolongations - telopodes. Thus, we showed their ultrastructural distinctiveness from Fbs. By RayBio Human Cytokine Antibody Array V analyses performed on the supernatant from separately cultured TCs and Fbs, we detected the cytokine profile of both cell types, individually. Two of 79 detected cytokines - epithelial-derived neutrophil-activating peptide 78 and granulocyte chemotactic protein-2 - were 1.5 times higher in the supernatant of TCs (comparing with Fbs). On the other hand, 37 cytokines were at least 1.5 higher in Fbs supernatant (comparing with TCs), and among them six cytokines - interleukin 5, monocyte chemotactic protein-3 (MCP-3), MCP-4, macrophage inflammatory protein-3, angiogenin, thrombopoietin - being 9.5 times higher (results also confirmed by ELISA testing). In summary, using different techniques, we showed that human dermal TCs and Fbs are different in terms of ultrastructure and cytokine profile.

Telopodes of telocytes are influenced in vitro by redox conditions and ageing

Telocytes (TCs) are a novel cell type identified among interstitial cells in various organs. TCs are characterized by very long cell processes (tens to hundreds micrometres) named telopodes (Tps) with uneven calibre: dilations (podoms) and very thin segments (podomers). However, little is known about the factors which influence Tps conformation. Recently, extracellular matrix proteins were found to influence Tps extension, adherence and spreading. Here, we show that oxidative stress and ageing influence formation of new Tps of TCs cultivated from human non-pregnant myometrium. Using real-time videomicroscopy, we found that ageing the TCs to passage 21 increased the ratio of Tps/TC number with about 50 %, whereas oxidative stress hindered formation of new Tps in both aged and young TCs (passage 7). Under oxidative stress, newly formed cell processes were up to 25 % shorter. Migration pathway length was decreased by 30-40 % for both young and aged cells in an oxidative stress environment. Contrary, addition of N-acetyl cysteine in cell culture medium shifted TCs morphology to a long and slender profile. In conclusion, we showed that TCs specific morphology in vitro is influenced by oxidative status balance, as well as ageing.