The magic behind stem cells

This review article summarizes historical development of stem cell research, presents current knowledge on the plasticity potential of both embryonic and adult stem cells and discusses on the future of stem cell based therapies.

New Insights into the Pathogenesis and Treatment of Idiopathic Pulmonary Fibrosis: A Potential Role for Stem Cells in the Lung Parenchyma and Implications for Therapy

Idiopathic Pulmonary Fibrosis (IPF) is a chronic, progressive, and often fatal form of interstitial lung disease. It is characterized by injury with loss of lung epithelial cells and abnormal tissue repair, resulting in replacement of normal functional tissue, abnormal accumulation of fibroblasts and myofibroblasts, deposition of extracellular matrix, and distortion of lung architecture which results in respiratory failure. Despite improvements in the diagnostic approach to IPF and active research in recent years, the molecular mechanisms of the disease remain poorly understood. This highly lethal lung disorder continues to pose major clinical challenges since an effective therapeutic regimen has yet to be identified and developed. For example, a treatment modality has been based on the assumption that IPF is a chronic inflammatory disease, yet most available anti-inflammatory drugs are not effective in treating it. Hence researchers are now focusing on understanding alternative underlying mechanisms involved in the pathogenesis of IPF in the hope of discovering potentially new pharmaceutical targets. This paper will focus on lung tissue repair, regeneration, remodeling, and cell types that may be important to consider in therapeutic interventions and includes a more detailed discussion of the potential targets of current therapeutic attack in pulmonary fibrosis. The discovery that adult bone marrow stem cells can contribute to the formation of differentiated cell types in other tissues, especially after injury, implies that they have the potential to participate in tissue remodeling, and perhaps regeneration. The current promise of the use of adult stem cells for tissue regeneration, and the belief that once irreversibly damaged tissue could be restored to a normal functional capacity using stem cell-based therapy, suggests a novel approach for treatment of diverse chronic diseases. However this optimism is tempered by current evidence that the pathogenesis of pulmonary fibrosis may involve the recruitment of bone marrow-derived fibroblasts, which are the key contributors to the pathogenesis of this chronic progressive disorder. Nevertheless, stem cell-related therapies are widely viewed as promising treatment options for patients suffering from various types of pulmonary diseases. Gender mismatched bone marrow or lung transplant recipients serve as natural populations in which to study the role of bone marrow-derived stem cells in recovery from pulmonary diseases. Understanding the mechanism of recruitment of stem cells to sites of injury, and their involvement in tissue repair, regeneration, and remodeling may offer a novel therapeutic target for developing more effective treatments against this fatal disorder. This article reviews the new concepts in the pathogenesis, current and future treatment options of pulmonary fibrosis, and the recent advances regarding the roles of stem cells in lung tissue repair, regeneration, and remodeling.

Regeneration of the amphibian intestinal epithelium under the control of stem cell niche

The epithelium of the mammalian digestive tract originates from stem cells and undergoes rapid cell-renewal throughout adulthood. It has been proposed that the microenvironment around the stem cells, called 'niche', plays an important role in epithelial cell-renewal through cell-cell and cell-extracellular matrix interactions. The amphibian intestine, which establishes epithelial cell-renewal during metamorphosis, serves as a unique and good model for studying molecular mechanisms of the stem cell niche. By using the organ culture of the Xenopus laevis intestine, we have previously shown that larval-to-adult epithelial remodeling can be organ-autonomously induced by thyroid hormone (TH) and needs interactions with the surrounding connective tissue. Thus, in this animal model, the functional analysis of TH response genes is useful for clarifying the epithelial-connective tissue interactions essential for intestinal remodeling at the molecular level. Recent progress in culture and transgenic technology now enables us to investigate functions of such TH response genes in the X. laevis intestine and sheds light on molecular aspects of the stem cell niche that are common to the mammalian intestine.

Plasticity of epidermal adult stem cells derived from adult goat ear skin

Here we report the isolation and characterization of pluripotent stem cells from adult goat skin. We found that these primary cells have the properties of embryonic stem cells (ESC), including the expression of appropriate immunological markers and the capability of forming embryoid bodies. The subcultured cells also show the characteristics of stem cells, such as the expression of CK19, beta(1-)integrin, P63, and formation of holo-clones in culture. Therefore, we termed these cells epidermal adult stem cells (EpiASC), although their origin was not identified. We have shown that clones of individual EpiASC proliferate and differentiate in culture to produce neurons, cardiomyocytes, osteoblasts, and occytes. Further, we cultivated EpiASC on bioengineered dermis and denuded human amniotic membrane (HAM), to reconstruct artificial skin and corneal epithelium. We successfully transplanted those artificial tissues in goats with acute full-thickness skin defect (AFTSD) and limbal stem cell deficiency (LSCD), respectively. Our results showed that indeed EpiASC reconstructed the skin (hair was observed in restored areas), and repaired the damaged cornea of goats with total LSCD. These data confirm that EpiASC can differentiate into different functional cell types in vivo or in vitro. Due to their high degree of inherent plasticity, and to their easy accessibility for collection from the skin, EpiASC are excellent candidate sources for diverse cell therapies.

Regenerative endodontics: a review of current status and a call for action

Millions of teeth are saved each year by root canal therapy. Although current treatment modalities offer high levels of success for many conditions, an ideal form of therapy might consist of regenerative approaches in which diseased or necrotic pulp tissues are removed and replaced with healthy pulp tissue to revitalize teeth. Researchers are working toward this objective. Regenerative endodontics is the creation and delivery of tissues to replace diseased, missing, and traumatized pulp. This review provides an overview of regenerative endodontics and its goals, and describes possible techniques that will allow regenerative endodontics to become a reality. These potential approaches include root-canal revascularization, postnatal (adult) stem cell therapy, pulp implant, scaffold implant, three-dimensional cell printing, injectable scaffolds, and gene therapy. These regenerative endodontic techniques will possibly involve some combination of disinfection or debridement of infected root canal systems with apical enlargement to permit revascularization and use of adult stem cells, scaffolds, and growth factors. Although the challenges of introducing endodontic tissue engineering therapies are substantial, the potential benefits to patients and the profession are equally ground breaking. Patient demand is staggering both in scope and cost, because tissue engineering therapy offers the possibility of restoring natural function instead of surgical placement of an artificial prosthesis. By providing an overview of the methodological issues required to develop potential regenerative endodontic therapies, we hope to present a call for action to develop these therapies for clinical use.

Positional identity of adult stem cells in salamander limb regeneration

Limb regeneration in larval and adult salamanders proceeds from a mound of mesenchymal stem cells called the limb blastema. The blastema gives rise just to those structures distal to its level of origin, and this property of positional identity is reset to more proximal values by treatment with retinoic acid. We have identified a cell surface protein, called Prod1/CD59, which appears to be a determinant of proximodistal identity. Prod1 is expressed in an exponential gradient in an adult limb as determined by detection of both mRNA and immunoreactive protein. Prod1 protein is up-regulated after treatment of distal blastemas with RA and this is particularly marked in cells of the dermis. These cells have previously been implicated in pattern formation during limb regeneration.

Regenerative potential of cardiosphere-derived cells expanded from percutaneous endomyocardial biopsy specimens

BACKGROUND

Ex vivo expansion of resident cardiac stem cells, followed by delivery to the heart, may favor regeneration and functional improvement.

METHODS AND RESULTS

Percutaneous endomyocardial biopsy specimens grown in primary culture developed multicellular clusters known as cardiospheres, which were plated to yield cardiosphere-derived cells (CDCs). CDCs from human biopsy specimens and from comparable porcine samples were examined in vitro for biophysical and cytochemical evidence of cardiogenic differentiation. In addition, human CDCs were injected into the border zone of acute myocardial infarcts in immunodeficient mice. Biopsy specimens from 69 of 70 patients yielded cardiosphere-forming cells. Cardiospheres and CDCs expressed antigenic characteristics of stem cells at each stage of processing, as well as proteins vital for cardiac contractile and electrical function. Human and porcine CDCs cocultured with neonatal rat ventricular myocytes exhibited biophysical signatures characteristic of myocytes, including calcium transients synchronous with those of neighboring myocytes. Human CDCs injected into the border zone of myocardial infarcts engrafted and migrated into the infarct zone. After 20 days, the percentage of viable myocardium within the infarct zone was greater in the CDC-treated group than in the fibroblast-treated control group; likewise, left ventricular ejection fraction was higher in the CDC-treated group.

CONCLUSIONS

A method is presented for the isolation of adult human stem cells from endomyocardial biopsy specimens. CDCs are cardiogenic in vitro; they promote cardiac regeneration and improve heart function in a mouse infarct model, which provides motivation for further development for therapeutic applications in patients.

Neurotrophin-directed differentiation of human adult marrow stromal cells to dopaminergic-like neurons

Marrow-isolated adult multilineage inducible (MIAMI) cells were differentiated in vitro to neuronal cells in a neurotrophin-dependent fashion. After induction, the cells revealed electrophysiological features similar to those observed in mature neurons. Primary early passage human MIAMI cells without any type of co-cultures with other cell types were used. The developmental program involved a multi-step process requiring the concerted action of brain-derived neurotrophic factor, nerve growth factor and depended on neurotrophin-3, after basic fibroblast growth factor withdrawal. MIAMI-derived neuron-like cells sequentially expressed the neuronal markers, developed a complex neurite outgrowth and arborization, and acquired electrophysiological characteristics similar to those observed in mature neurons. The young and old MIAMI-derived neuronal cells developed both inward and outward currents upon depolarization, similar to those observed in normal neurons. These results represent the earliest evidence that neurotrophin-3 can direct the differentiation of non-neural stem cells from human adult bone marrow stroma to neuron-like cells in vitro. Supplementing the aforementioned multi-step process with sonic hedgehog, fibroblast growth factor 8, and retinoic acid increased the expression of molecules involved in dopaminergic differentiation and of tyrosine hydroxylase, the rate limiting enzyme of dopamine synthesis. MIAMI cells from young and old individuals represent autologous human cell populations for the treatment of disorders of the skeletal and nervous systems and for applications in cell therapy and reparative medicine approaches.

Therapeutic targeting of a stem cell niche

The specialized microenvironment or niche where stem cells reside provides regulatory input governing stem cell function. We tested the hypothesis that targeting the niche might improve stem cell-based therapies using three mouse models that are relevant to clinical uses of hematopoietic stem (HS) cells. We and others previously identified the osteoblast as a component of the adult HS cell niche and established that activation of the parathyroid hormone (PTH) receptor on osteoblasts increases stem cell number. Here we show that pharmacologic use of PTH increases the number of HS cells mobilized into the peripheral blood for stem cell harvests, protects stem cells from repeated exposure to cytotoxic chemotherapy and expands stem cells in transplant recipients. These data provide evidence that the niche may be an attractive target for drug-based stem cell therapeutics.

The potential role of endogenous stem cells in regeneration of the inner ear

Stem cells in various mammalian organs retain the capacity to renew themselves and may be able to restore damaged tissue. Their existence has been proven by genetic tracer studies that demonstrate their differentiation into multiple tissue types and by their ability to self-renew through proliferation. Stem cells from the adult nervous system proliferate to form clonal floating colonies called spheres in vitro, and recent studies have demonstrated sphere formation by cells in the cochlea in addition to the vestibular system and the auditory ganglia, indicating that these tissues contain cells with stem cell properties. The presence of stem cells in the inner ear raises the hope of regeneration of mammalian inner ear cells but is difficult to correlate with the lack of spontaneous regeneration seen in the inner ear after tissue damage. Loss of stem cells postnatally in the cochlea may correlate with the loss of regenerative capacity and may limit our ability to stimulate regeneration. Retention of sphere forming ability in adult vestibular tissues suggests that the limited capacity for repair may be attributed to the continued presence of progenitor cells. Future strategies for regeneration must consider the distribution of endogenous stem cells in the inner ear and whether the tissue retains cells with the capacity for regeneration.

Stem cells in dental practice: perspectives in conservative pulp therapies

Stem cells are undifferentiated cells that have the capacity to self-renew. They have been discovered in many adult tissues, including teeth. The Dental Pulp Stem Cells are involved in dentinal repair by activation of growth factors, released after caries process and have the ability to regenerate the dentin-pulp-like complex. The molecular/cellular research raises the possibilities to grow new tissues and biological structures for clinical application, providing cells for therapies including cell transplantation and tissue engineering.

Bone marrow-derived osteoblast progenitor cells in circulating blood contribute to ectopic bone formation in mice

Recent studies have suggested the existence of osteoblastic cells in the circulation, but the origin and role of these cells in vivo are not clear. Here, we examined how these cells contribute to osteogenesis in a bone morphogenetic protein (BMP)-induced model of ectopic bone formation. Following lethal dose-irradiation and subsequent green fluorescent protein-transgenic bone marrow cell-transplantation (GFP-BMT) in mice, a BMP-2-containing collagen pellet was implanted into muscle. Three weeks later, a significant number of GFP-positive osteoblastic cells were present in the newly generated ectopic bone. Moreover, peripheral blood mononuclear cells (PBMNCs) from the BMP-2-implanted mouse were then shown to include osteoblast progenitor cells (OPCs) in culture. Passive transfer of the PBMNCs isolated from the BMP-2-implanted GFP-mouse to the BMP-2-implanted nude mouse led to GFP-positive osteoblast accumulation in the ectopic bone. These data provide new insight into the mechanism of ectopic bone formation involving bone marrow-derived OPCs in circulating blood.

Cardiac regeneration by resident stem and progenitor cells in the adult heart

Two main pieces of data have created a new field in cardiac research. First, the traditional view on the heart as a postmitotic organ has been challenged by the finding of small dividing cells in the heart expressing cardiac contractile proteins with stem cell properties and, second, cellular therapy of the diseased heart using a variety of different cells has shown encouraging effects on cardiac function. These findings immediately raise questions like "what is the identity and origin of the cardiac progenitor cells?","which molecular factors are involved in their mobilization and differentiation?", and "can these cells repair the damaged heart?" This review will address the state of current answers to these questions. Emerging evidence suggests that several subpopulations of cardiac stem or progenitor cells (CPCs) reside within the adult heart. CPCs with the ability to differentiate into all the constituent cells in the adult heart including cardiac myocytes, vascular smooth muscle and endothelial cells have been identified. Valuable knowledge has been obtained from the large number of animal studies and a number of small clinical trials that have utilized a variety of adult stem cells for regenerating infarcted hearts. However, contradictory reports on the regenerative potential of the CPCs exist, and the mechanisms behind the reported hemodynamic effects are intensely debated. Besides directly replenishing cardiac tissue, CPCs could also function by stimulating angiogenesis and improving survival of existing cells by secretion of paracrine factors. With this review we suggest that a better understanding of CPC biology will be pivotal for progressing therapeutic cardiac regeneration. This includes an extended knowledge of the molecular mechanisms behind their mobilization, differentiation, survival and integration in the myocardium.

Mathematical model for NF-kappaB-driven proliferation of adult neural stem cells

Neural stem cells (NSCs) are early precursors of neuronal and glial cells. NSCs are capable of generating identical progeny through virtually unlimited numbers of cell divisions (cell proliferation), producing daughter cells committed to differentiation. Nuclear factor kappa B (NF-kappaB) is an inducible, ubiquitous transcription factor also expressed in neurones, glia and neural stem cells. Recently, several pieces of evidence have been provided for a central role of NF-kappaB in NSC proliferation control. Here, we propose a novel mathematical model for NF-kappaB-driven proliferation of NSCs. We have been able to reconstruct the molecular pathway of activation and inactivation of NF-kappaB and its influence on cell proliferation by a system of nonlinear ordinary differential equations. Then we use a combination of analytical and numerical techniques to study the model dynamics. The results obtained are illustrated by computer simulations and are, in general, in accordance with biological findings reported by several independent laboratories. The model is able to both explain and predict experimental data. Understanding of proliferation mechanisms in NSCs may provide a novel outlook in both potential use in therapeutic approaches, and basic research as well.

Tooth development: 2. Regenerating Teeth in the Laboratory

Tooth loss can occur for a number of reasons and a variety of prosthetic tooth replacement solutions are available to the dental practitioner. This article discusses current approaches in the use of tissue engineering to replace teeth or repair dental tissues. These strategies will depend upon the manipulation of stem cells in the laboratory and, whilst much progress has recently been made, it is likely that successful human tooth regeneration is still some years ahead.

Adult stem cells: the real root into the embryo?

During embryonic development, a pool of cells may become a reserve of undifferentiated cells, the embryo-stolen adult stem cells (ESASC). ESASC may be responsible for adult tissue homeostasis, as well as disease development. Transdifferentiation is a sort of reprogramming of ESASC from one germ layer-derived tissue towards another. Transdifferentiation has been described to take place from mesoderm to ectodermal- or endodermal-derived tissues and viceversa but not from ectodermal- to endodermal-derived tissues. We hypothesise that two different populations of ESASC could exist, the first ecto/mesoblast-committed and the second endo/mesoblast-committed. If confirmed, this hypothesis could lead to new studies on the molecular mechanisms of cell differentiation and to a better understanding of the pathogenesis of a number of diseases.

Unmasking recurrent excitation generated by mossy fiber sprouting in the epileptic dentate gyrus: an emergent property of a complex system

Seizure-induced sprouting of the mossy fiber pathway in the dentate gyrus has been observed nearly universally in experimental models of limbic epilepsy and in the epileptic human hippocampus. The observation of progressive mossy fiber sprouting induced by kindling demonstrated that even a few repeated seizures are sufficient to alter synaptic connectivity and circuit organization. As it is now recognized that seizures induce synaptic reorganization in hippocampal and cortical pathways, the implications of seizure-induced synaptic reorganization for circuit properties and function have been subjects of intense interest. Detailed anatomical characterization of the sprouted mossy fiber pathway has revealed that the overwhelming majority of sprouted synapses in the inner molecular layer of the dentate gyrus form recurrent excitatory connections, and are thus likely to contribute to recurrent excitation and potentially to enhanced susceptibility to seizures. Nevertheless, difficulties in detecting functional abnormalities in circuits reorganized by mossy fiber sprouting and the fact that some sprouted axons appear to form synapses with inhibitory interneurons have been cited as evidence that sprouting may not contribute to seizure susceptibility, but could form recurrent inhibitory circuits and be a compensatory response to prevent seizures. Quantitative analysis of the synaptic connections of the sprouted mossy fiber pathway, assessment of the functional features of sprouted circuitry using reliable physiological measures, and the perspective of complex systems analysis of neural circuits strongly support the view that the functional effects of the recurrent excitatory circuits formed by mossy fiber sprouting after seizures or injury emerge only conditionally and intermittently, as observed with spontaneous seizures in human epilepsy. The recognition that mossy fiber sprouting is induced after hippocampal injury and seizures and contributes conditionally to emergence of recurrent excitation has provided a conceptual framework for understanding how injury and seizure-induced circuit reorganization may contribute to paroxysmal network synchronization, epileptogenesis, and the consequences of repeated seizures, and thus has had a major influence on understanding of fundamental aspects of the epilepsies.

Ischemia-Induced Neurogenesis: Role of Growth Factors

The neurogenic response in ischemic brain to growth factors is the net result of cell division and cell survival in specific regions of the brain. To increase the cell number, these physiologic processes should be active. Hence, when growth factors are infused into the brain, they might stimulate survival, cell division, or both to enhance neurogenesis. The end result is the interplay of all the endogenous factors with the infused exogenous factors. It is essential to understand the growth factors and their regulators that are expressed after ischemia if one is to pharmacologically enhance neurogenesis. It seems that a combinational therapy of factors or their inhibitors may provide powerful therapeutic potential for enhancing stroke-induced neurogenesis and restoring the damaged tissue to function.

Non-epithelial oral mucosal progenitor cell populations

This review considers the potential existence and role of stem or progenitor cell populations within the non-epithelial tissues of the oral mucosa. Currently, there is little published evidence supporting this hypothesis; however, because of the similarities in structure and function of the oral mucosa and skin, findings within the dermis of the skin may potentially reflect the situation within the oral mucosa. Over recent years, the identification of the skin as a local reservoir of adult stem cell populations and the idea that multipotent cell populations exist within the dermal tissues of skin has gained increasing credibility. Indeed, numerous multipotent progenitor cells have been identified within the dermis and resident appendages, all capable of differentiating into multiple cell lineages. Furthermore, a number of these cell populations have been implicated in the repair of these tissues following injury. There is increasing evidence suggesting that such populations of progenitor cells may also reside within the lamina propria. In this respect, the ability to isolate large numbers of multipotent progenitor cells from a tissue which when biopsied heals without a scar would be of great interest scientifically and commercially, particularly with respect to future therapeutic applications and the developing discipline of tissue engineering.

Glial cells more than support cells?

Glial cells are the most abundant cells in the human brain and have long been considered as passive supporting cells for neurons. In contrast to the extensive studies on various neuronal functions in the nervous system, we still have limited knowledge about glial cells. Recently a number of pioneering studies have provided convincing evidence that glia play active roles in development and function of the central nervous system. This review discusses recent advances in our understanding of the molecular mechanisms underlying glial cell differentiation. We then highlight some of the novel findings about glial function, i.e. the role of glia in synaptogenesis and the intricate relationship between astrocytes and adult neural stem cells. Finally, we summarize the emerging studies that implicate abnormalities in the formation or maintenance of glia leading to severe brain diseases, such as Alexander disease, glioblastoma and multiple sclerosis, and potential therapeutic strategies to tackle these diseases.

Cells from bone marrow that evolve into oral tissues and their clinical applications

There are two major well-characterized populations of post-natal (adult) stem cells in bone marrow: hematopoietic stem cells which give rise to blood cells of all lineages, and mesenchymal stem cells which give rise to osteoblasts, adipocytes, and fibroblasts. For the past 50 years, strict rules were taught governing developmental biology. However, recently, numerous studies have emerged from researchers in different fields suggesting the unthinkable--that stem cells isolated from a variety of organs are capable of ignoring their cell lineage boundaries and exhibiting more plasticity in their fates. Plasticity is defined as the ability of post-natal (tissue-specific adult) stem cells to differentiate into mature and functional cells of the same or of a different germ layer of origin. There are reports that bone marrow stem cells can evolve into cells of all dermal lineages, such as hepatocytes, skeletal myocytes, cardiomyocytes, neural, endothelial, epithelial, and even endocrine cells. These findings promise significant therapeutic implications for regenerative medicine. This article will review recent reports of bone marrow cells that have the ability to evolve or differentiate into oral and craniofacial tissues, such as the periodontal ligament, alveolar bone, condyle, tooth, bone around dental and facial implants, and oral mucosa.

The Adult Neural Stem Cell Niche: Lessons for Future Neural Cell Replacement Strategies

Transplantation of neural stem cells (NSCs) and the mobilization of endogenous neural precursors in the adult brain have been proposed as therapies for a wide range of central nervous system disorders, including neurodegenerative disease (eg, Parkinson's disease), demyelinating disorders (eg, multiple sclerosis), stroke, and trauma. Although there is great hope for the success of such therapies, the clinical development of NSC-based therapies is still in its infancy. A greater understanding of how to control the proliferation, migration, differentiation, and survival of NSCs and their progeny is critical for the development of cell replacement therapies. NSCs are partially regulated by the specialized microenvironment--or "niche"--in which these cells reside. The adult rodent brain retains NSCs in two separate niches that continually generate new neurons: the subventricular zone (SVZ) of the lateral ventricle and the dentate gyrus subgranular zone (SGZ) of the hippocampus. Similar niches may be found in the human brain. In tis article, the authors briefly review their current understanding of the SVZ and SGZ niches. Lessons learned from these niches may allow one to manipulate NSCs better in culture for therapeutic transplantation and possibly even to mobilize endogenous precursors to repair diseased or injured brain.

Adult neurogenesis in the intact and epileptic dentate gyrus

Neurogenesis persists throughout life in the adult mammalian dentate gyrus. Adult-born dentate granule cells integrate into existing hippocampal circuitry and may provide network plasticity necessary for certain forms of hippocampus-dependent learning and memory. Neural stem cells and neurogenesis in the adult dentate gyrus are regulated by a variety of environmental, physiological, and molecular factors. These include aging, stress, exercise, neurovascular components of the stem cell niche, growth factors, neurotransmitters, and hormones. Seizure activity also influences dentate granule cell neurogenesis. Production of adult-born neurons increases in rodent models of temporal lobe epilepsy, and both newborn and pre-existing granule neurons contribute to aberrant axonal reorganization in the epileptic hippocampus. Prolonged seizures also disrupt the migration of dentate granule cell progenitors and lead to hilar-ectopic granule cells. The ectopic granule neurons appear to integrate abnormally and contribute to network hyperexcitability. Similar findings of granule cell layer dispersion and ectopic granule neurons in human TLE suggest that aberrant neurogenesis contributes to epileptogenesis or learning and memory disturbances in this epilepsy syndrome.