Altered phenotype and reduced function of muscle-derived hematopoietic stem cells

Stem Cell-Based Disease Models for Inborn Errors of Immunity

The intrinsic capacity of human hematopoietic stem cells (hHSCs) to reconstitute myeloid and lymphoid lineages combined with their self-renewal capacity hold enormous promises for gene therapy as a viable treatment option for a number of immune-mediated diseases, most prominently for inborn errors of immunity (IEI). The current development of such therapies relies on disease models, both in vitro and in vivo, which allow the study of human pathophysiology in great detail. Here, we discuss the current challenges with regards to developmental origin, heterogeneity and the subsequent implications for disease modeling. We review models based on induced pluripotent stem cell technology and those relaying on use of adult hHSCs. We critically review the advantages and limitations of current models for IEI both in vitro and in vivo. We conclude that existing and future stem cell-based models are necessary tools for developing next generation therapies for IEI.

Hematopoietic stem and progenitor cells outside the bone marrow: where, when, and why

Bone marrow (BM) is the primary site of adult blood production, hosting the majority of all hematopoietic stem and progenitor cells (HSPCs). Rare HSPCs are also found outside of the BM at steady state. In times of large hematopoietic demand or BM failure, substantial production of mature blood cells from HSPCs can occur in a number of tissues, in a process termed extramedullary hematopoiesis (EMH). Over the past decades, our understanding of BM hematopoiesis has advanced drastically. In contrast there has been very little focus on the study of extramedullary HSPC pools and their contributions to blood production. Here we summarize what is currently known about extramedullary HSPCs and EMH in mice and humans. We describe the evidence of existing extramedullary HSPC pools at steady state, then discuss their role in the hematopoietic stress response. We highlight that although EMH in humans is much less pronounced and likely physiologically distinct to that in mice, it can be informative about premalignant and malignant changes. Finally, we reflect on the open questions in the field and on whether a better understanding of EMH, particularly in humans, may have relevant clinical implications for hematological and nonhematological disorders.

Deregulated angiogenesis in chronic lung diseases: a possible role for lung mesenchymal progenitor cells (2017 Grover Conference Series)

Chronic lung disease (CLD), including pulmonary fibrosis (PF) and chronic obstructive pulmonary disease (COPD), is the fourth leading cause of mortality worldwide. Both are debilitating pathologies that impede overall tissue function. A common co-morbidity in CLD is vasculopathy, characterized by deregulated angiogenesis, remodeling, and loss of microvessels. This substantially worsens prognosis and limits survival, with most current therapeutic strategies being largely palliative. The relevance of angiogenesis, both capillary and lymph, to the pathophysiology of CLD has not been resolved as conflicting evidence depicts angiogenesis as both reparative or pathologic. Therefore, we must begin to understand and model the underlying pathobiology of pulmonary vascular deregulation, alone and in response to injury induced disease, to define cell interactions necessary to maintain normal function and promote repair. Capillary and lymphangiogenesis are deregulated in both PF and COPD, although the mechanisms by which they co-regulate and underlie early pathogenesis of disease are unknown. The cell-specific mechanisms that regulate lung vascular homeostasis, repair, and remodeling represent a significant gap in knowledge, which presents an opportunity to develop targeted therapies. We have shown that that ABCG2^pos multipotent adult mesenchymal stem or progenitor cells (MPC) influence the function of the capillary microvasculature as well as lymphangiogenesis. A balance of both is required for normal tissue homeostasis and repair. Our current models suggest that when lymph and capillary angiogenesis are out of balance, the non-equivalence appears to support the progression of disease and tissue remodeling. The angiogenic regulatory mechanisms underlying CLD likely impact other interstitial lung diseases, tuberous sclerosis, and lymphangioleiomyomatosis.

Growth of bone marrow and skeletal muscle side population stem cells in suspension culture

The ability to efficiently isolate and expand various stem cell populations in vitro is crucial for successful translation of cell-based therapies to the clinical setting. One such heterogeneous population that possesses a remarkable potential for the development of cell-based treatments for a variety of degenerative diseases and disorders is called the Side Population (SP). For many years, investigators have isolated these primitive cells based upon their ability to efflux the fluorophore Hoechst 33342. This attribute enabled separation of SP cells derived from multiple tissue sources from other endogenous cell populations using fluorescence-activated cell sorting (FACS). While all tissue-specific SP fractions appear to contain cells with multi-potent stem cell activity, the therapeutic utility of these cells has yet to be fully realized because of the scarcity of this fraction in vivo. In view of that, we developed a method to expand adult murine bone marrow and skeletal muscle-derived SP cells in vitro. Here, we describe a spinner-flask culture system that supports the growth of SP cells in suspension when they are combined with feeder cells cultured on spherical microcarriers. In this way, their distinguishing biological characteristics can be maintained, attachment-stimulated differentiation is avoided, and therapeutically relevant quantities of SP cells are generated. Modification of the described procedure may permit expansion of the SP from other relevant tissue sources and our method is amenable to establishing compliance with current good manufacturing practices.

Muscle side population cells from dystrophic or injured muscle adopt a fibro-adipogenic fate

Muscle side population (SP) cells are rare multipotent stem cells that can participate in myogenesis and muscle regeneration upon transplantation. While they have been primarily studied for the development of cell-based therapies for Duchenne muscular dystrophy, little is known regarding their non-muscle lineage choices or whether the dystrophic muscle environment affects their ability to repair muscle. Unfortunately, the study of muscle SP cells has been challenged by their low abundance and the absence of specific SP cell markers. To address these issues, we developed culture conditions for the propagation and spontaneous multi-lineage differentiation of muscle SP cells. Using this approach, we show that SP cells from wild type muscle robustly differentiate into satellite cells and form myotubes without requiring co-culture with myogenic cells. Furthermore, this myogenic activity is associated with SP cells negative for immune (CD45) and vascular (CD31) markers but positive for Pax7, Sca1, and the mesenchymal progenitor marker PDGFRα. Additionally, our studies revealed that SP cells isolated from dystrophic or cardiotoxin-injured muscle fail to undergo myogenesis. Instead, these SP cells rapidly expand giving rise to fibroblast and adipocyte progenitors (FAPs) and to their differentiated progeny, fibroblasts and adipocytes. Our findings indicate that muscle damage affects the lineage choices of muscle SP cells, promoting their differentiation along fibro-adipogenic lineages while inhibiting myogenesis. These results have implications for a possible role of muscle SP cells in fibrosis and fat deposition in muscular dystrophy. In addition, our studies provide a useful in vitro system to analyze SP cell biology in both normal and pathological conditions.

Muscle development, regeneration and laminopathies: how lamins or lamina-associated proteins can contribute to muscle development, regeneration and disease

The aim of this review article is to evaluate the current knowledge on associations between muscle formation and regeneration and components of the nuclear lamina. Lamins and their partners have become particularly intriguing objects of scientific interest since it has been observed that mutations in genes coding for these proteins lead to a wide range of diseases called laminopathies. For over the last 10 years, various laboratories worldwide have tried to explain the pathogenesis of these rare disorders. Analyses of the distinct aspects of laminopathies resulted in formulation of different hypotheses regarding the mechanisms of the development of these diseases. In the light of recent discoveries, A-type lamins—the main building blocks of the nuclear lamina—together with other key elements, such as emerin, LAP2α and nesprins, seem to be of great importance in the modulation of various signaling pathways responsible for cellular differentiation and proliferation.

Skeletal Muscle-derived Hematopoietic Stem Cells: Muscular Dystrophy Therapy by Bone Marrow Transplantation

For postnatal growth and regeneration of skeletal muscle, satellite cells, a self-renewing pool of muscle stem cells, give rise to daughter myogenic precursor cells that contribute to the formation of new muscle fibers. In addition to this key myogenic cell class, adult skeletal muscle also contains hematopoietic stem cell and progenitor cell populations which can be purified as a side population (SP) fraction or as a hematopoietic marker CD45-positive cell population. These muscle-derived hematopoietic stem/progenitor cell populations are surprisingly capable of differentiation into hematopoietic cells both after transplantation into irradiated mice and during in vitro colony formation assay. Therefore, these muscle-derived hematopoietic stem/progenitor cells appear to have characteristics similar to classical hematopoietic stem/progenitor cells found in bone marrow. This review outlines recent findings regarding hematopoietic stem/progenitor cell populations residing in adult skeletal muscle and discusses their myogenic potential along with their role in the stem cell niche and related cell therapies for approaching treatment of Duchenne muscular dystrophy.

Defining the heterogeneity of skeletal muscle-derived side and main population cells isolated immediately ex vivo

Myoblast transfer therapy for Duchenne muscular dystrophy (DMD) largely fails due to cell death and inability of transplanted cells to engraft in diseased muscles. One method attempting to enrich for cell subpopulations is the Hoechst 33342 dye exclusion assay, yielding a side population (SP) thought to be progenitor enriched and a main population (MP). However, in vitro and transplant studies yielded inconsistent results relative to downstream progeny. Cell surface markers expressed by skeletal muscle-derived MP and SP cells have not been fully characterized directly ex vivo. Using flow cytometry, MP and SP cells were characterized based on their expression of several well-accepted progenitor cell antigens. Both the MP and SP populations are heterogeneous and overlapping in the cells they contain. The percentages of cells in each population vary with species and specific muscle examined. MP and SP populations contain both satellite and multipotent progenitor cells, based on expression of CD34, Sca-1, Pax7, and M-cadherin. Thus, isolation using this procedure cannot be used to predict downstream differentiation outcomes, and explains the conflicting literature on these cells. Hoechst dye also results in significant mortality of sorted cells. As defined subpopulations are easily obtained using flow cytometry, sorting immediately ex vivo based on accepted myogenic precursor cell markers will yield superior results in terms of cell homogeneity for transplantation therapy.

Myogenic stem cells

Both skeletal muscle and bone marrow tissue contain myogenic stem cells. The population residing in muscles is heterogenic. Predominant in number are "typical" satellite cells - muscle progenitors migrating from somites during embryonic life. Another population is group of multipotent muscle stem cells which, at least in part, are derived from bone marrow. These cells are tracked by gradient of growth factors releasing from muscle during injury or exercise. Recruited bone marrow-derived cells gradually change their phenotype becoming muscle stem cells and eventually can attain satellite cell position and express Pax7 protein. Mesenchymal stem cells (MSC) isolated directly from bone marrow also display myogenic potential, although methods of induction of myogenic differentiation in vitro have not been optimized yet. Concerning efforts of exploiting myogenic stem cells in cell-mediated therapies it is important to understand the cause of impaired regenerative potential of aged muscle. Up to now, most of research data suggest that majority of age related changes in skeletal muscles are reversible, thus depending on extrinsic factors. However, irreversible intrinsic features of muscle stem cells are also taken into consideration.

Efficient delivery of human single fiber-derived muscle precursor cells via biocompatible scaffold

The success of cell therapy for skeletal muscle disorders depends upon two main factors: the cell source and the method of delivery. In this work we have explored the therapeutic potential of human muscle precursor cells (hMPCs), obtained from single human muscle fibers, implanted in vivo via micropatterned scaffolds. hMPCs were initially expanded and characterized in vitro by immunostaining and flow cytometric analysis. For in vivo studies, hMPCs were seeded onto micropatterned poly-lactic-glycolic acid 3D-scaffolds fabricated using soft-lithography and thermal membrane lamination. Seeded scaffolds were then implanted in predamaged tibialis anterior muscles of CD1 nude mice; hMPCs were also directly injected in contralateral limbs as controls. Similarly to what we previously described with mouse precursors cells, we found that hMPCs were able to participate in muscle regeneration and scaffold-implanted muscles contained a greater number of human nuclei, as revealed by immunostaining and Western blot analyses. These results indicate that hMPCs derived from single fibers could be a good and reliable cell source for the design of therapeutic protocols and that implantation of cellularized scaffolds is superior to direct injection for the delivery of myogenic cells into regenerating skeletal muscle.

FGF2-dependent neovascularization of subcutaneous Matrigel plugs is initiated by bone marrow-derived pericytes and macrophages

Vessel-like networks are quickly formed in subcutaneous FGF2-supplemented Matrigel plugs by two cell types: NG2(+) pericytes and F4/80(+) macrophages. Although not detected in these networks until 7 days after plug implantation, the appearance of CD31(+) endothelial cells marks the onset of vessel perfusion and the establishment of mature vessel morphology, with endothelial cells invested tightly by pericytes and more loosely by macrophages. Evidence that mature vessels develop from pericyte/macrophage networks comes from experiments in which 5-day plugs are transplanted into EGFP(+) recipients and allowed to mature. Fewer than 5% of pericytes in mature vessels are EGFP(+) in this paradigm, demonstrating their presence in the networks prior to plug transplantation. Endothelial cells represent the major vascular cell type recruited during later stages of vessel maturation. Bone marrow transplantation using EGFP(+) donors establishes that almost all macrophages and more than half of the pericytes in Matrigel vessels are derived from the bone marrow. By contrast, only 10% of endothelial cells exhibit a bone marrow origin. The vasculogenic, rather than angiogenic, nature of this neovascularization process is unique in that it is initiated by pericyte and macrophage progenitors, with endothelial cell recruitment occurring as a later step in the maturation process.

Murine Muscle Precursor Cells Survived and Integrated in a Cryoinjured Gastroesophageal Junction

BACKGROUND

Mini-invasive techniques for gastroesophageal reflux disease (GERD), such as endoscopic injections of inert materials, have been introduced in recent years. However, results are still preliminary. Cell injection has emerged as an alternative strategy in both vesicoureteral reflux and incontinence. Here we report, for the first time, the injection of muscle precursor cells (MPCs) in the gastroesophageal junction (GEJ).

MATERIALS AND METHODS

MPCs were derived from expanded satellite cells isolated from skeletal muscle fibers of green fluorescent protein (GFP) positive mice. Via laparotomy, GFP-negative mice were subjected to cryoinjury of GEJ followed by injection of MPCs (experimental animals), bone marrow derived cells, or saline (controls).

RESULTS

Immunofluorescence analyses of experimental GEJs demonstrated coexpression of GFP and desmin in grafted cells. GFP+ muscle neofibers were evident at 4 wk after injection. Coexpression of GFP and smooth muscle actin was also observed at 2 wk.

CONCLUSIONS

Satellite cells could be easily harvested, expanded in culture, and used as injectable substance in the GEJ. These results could be the background for the development of a new injection technique for GERD treatment, which might combine bulging and functional actions.

Quantitative and qualitative in vitro analysis of the stem cell potential of hematopoietic cells purified from murine skeletal muscle

The murine skeletal muscle contains hematopoietic stem cells, but this potential has so far not been studied quantitatively or qualitatively in vitro. To quantify the hematopoietic stem cell potential, we have used highly purified SP/CD45(+) cells in long-term culture initiating cell (LTC-IC) assays. The SP/CD45(+) cell population purified from murine muscle was found to have significant stem cell activity with an LTC-IC frequency of 1/640. Single-cell-sorted SP/CD45(+) cells from muscle exhibited robust proliferative activity in vitro at day 16 (380-fold amplification), especially after culture with OP-9 layers that also support embryonic stem cells. Amplified cell populations originating from single cells exhibited multilineage differentiation ability with evidence of myeloid, lymphoid and NK cell markers. Thus, our results demonstrate that hematopoietic stem cells that can be quantified by LTC-IC assays exist in the murine skeletal muscle and show also for the first time, at the single-cell level, that these cells exhibit multilineage differentiation ability and major proliferative potential.

Angiotensin II type 1 receptor blockade prevents decrease in adult stem-like cells in kidney after ureteral obstruction

Infusion of renal side population (SP) cells, enriched with adult stem-like cells, can ameliorate acute renal failure. We investigated the effects of an angiotensin II type 1 (AT(1)) receptor antagonist, valsartan on SP cell changes in renal injury by ureteral obstruction. Renal SP fraction was reduced by 38%, and the number of cells expressing CD45, a marker of hematopoietic system, in renal SP cells was increased in obstructed kidneys. Valsartan attenuated renal injury and the associated SP profile changes. Angiotensin AT(1) receptor blockade may exert regenerative effect by preserving adult stem-like cells such as SP cells in the kidney.

Towards hematopoietic reconstitution from embryonic stem cells: a sanguine future

PURPOSE OF REVIEW

To review recent progress towards the derivation of hematopoietic stem cells (HSCs) and blood lineages from embryonic stem cells (ESCs), and to highlight the hurdles that must be overcome in order to move the field closer to a clinical application.

RECENT FINDINGS

Hematopoietic repopulating cells, red blood cells, and T cells have recently been derived from both murine and human ESCs. Although these results are encouraging, several outstanding issues remain to be addressed by the field before realizing clinical applicability: the phenotype of the ESC-derived HSC must be characterized, methods to purge residual teratoma-forming cells from differentiated populations must be established, and in-vivo models of human HSC function must be optimized to better assess the functionality of putative human ESC-derived HSCs. In addition, embryonic stem-cell derived progeny often represent primitive embryonic hematopoietic cells, rather than their definitive adult counterparts; this critical issue must also be addressed.

SUMMARY

The literature firmly establishes that it is possible to isolate HSCs and certain mature blood lineages from both mouse and human ESCs. Although several issues remain to be addressed, these data demonstrate the value of ESCs as a potential source of transplantable HSCs.

Satellite cells delivered by micro-patterned scaffolds: a new strategy for cell transplantation in muscle diseases

Myoblast transplantation is a potentially useful therapeutic tool in muscle diseases, but the lack of an efficient delivery system has hampered its application. Here we have combined cell biology and polymer processing to create an appropriate microenvironment for in vivo transplantation of murine satellite cells (mSCs). Cells were prepared from single muscle fibers derived from C57BL/6-Tgn enhanced green fluorescent protein (GFP) transgenic mice. mSCs were expanded and seeded within micro-patterned polyglycolic acid 3-dimensional scaffolds fabricated using soft lithography and thermal membrane lamination. Myogenicity was then evaluated in vitro using immunostaining, flow cytometry, and reverse transcription polymerase chain reaction analyses. Scaffolds containing mSCs were implanted in pre-damaged tibialis anterior muscles of GFP-negative syngenic mice. Cells detached from culture dishes were directly injected into contra-lateral limbs as controls. In both cases, delivered cells participated in muscle regeneration, although scaffold-implanted muscles showed a much higher number of GFP-positive fibers in CD57 mice. These findings suggest that implantation of cellularized scaffolds is better than direct injection for delivering myogenic cells into regenerating skeletal muscle.

Stem and progenitor cells in skeletal muscle development, maintenance, and therapy

Satellite cells are dormant progenitors located at the periphery of skeletal myofibers that can be triggered to proliferate for both self-renewal and differentiation into myogenic cells. In addition to anatomic location, satellite cells are typified by markers such as M-cadherin, Pax7, Myf5, and neural cell adhesion molecule-1. The Pax3 and Pax7 transcription factors play essential roles in the early specification, migration, and myogenic differentiation of satellite cells. In addition to muscle-committed satellite cells, multi-lineage stem cells encountered in embryonic, as well as adult, tissues exhibit myogenic potential in experimental conditions. These multi-lineage stem cells include side-population cells, muscle-derived stem cells (MDSCs), and mesoangioblasts. Although the ontogenic derivation, identity, and localization of these non-conventional myogenic cells remain elusive, recent results suggest their ultimate origin in blood vessel walls. Indeed, purified pericytes and endothelium-related cells demonstrate high myogenic potential in culture and in vivo. Allogeneic myoblasts transplanted into Duchenne muscular dystrophy (DMD) patients have been, in early trials, largely inefficient owing to immune rejection, rapid death, and limited intramuscular migration--all obstacles that are now being alleviated, at least in part, by more efficient immunosuppression and escalated cell doses. As an alternative to myoblast transplantation, stem cells such as mesoangioblasts and CD133+ progenitors administered through blood circulation have recently shown great potential to regenerate dystrophic muscle.

The skeletal muscle satellite cell: the stem cell that came in from the cold

The muscle satellite cell was first described and actually named on the basis of its anatomic location under the basement membrane surrounding each myofiber. For many years following its discovery, electron microscopy provided the only definitive method of identification. More recently, several molecular markers have been described that can be used to detect satellite cells, making them more accessible for study at the light microscope level. Satellite cells supply myonuclei to growing myofibers before becoming mitotically quiescent in muscle as it matures. They are then activated from this quiescent state to fulfill their roles in routine maintenance, hypertrophy, and repair of adult muscle. Because muscle is able to efficiently regenerate after repeated bouts of damage, systems must be in place to maintain a viable satellite cell pool, and it was proposed over 30 years ago that self-renewal was the primary mechanism. Self-renewal entails either a stochastic event or an asymmetrical cell division, where one daughter cell is committed to differentiation whereas the second continues to proliferate or becomes quiescent. This classic model of satellite cell self-renewal and the importance of satellite cells in muscle maintenance and repair have been challenged during the past few years as bone marrow-derived cells and various intramuscular populations were shown to be able to contribute myonuclei and occupy the satellite cell niche. This is a fast-moving and dynamic field, however, and in this review we discuss the evidence that we think puts this enigmatic cell firmly back at the center of adult myogenesis.

Clonal diversity of the stem cell compartment

PURPOSE OF REVIEW

Hematopoietic stem cells are functionally heterogeneous even when isolated as phenotypically homogenous populations. How this heterogeneity is generated is incompletely understood. Several models have been formulated to explain the generation of diversity. All of these assume the existence of a single type of hematopoietic stem cell that generates heterogeneous daughter stem cells in response to extrinsic or intrinsic (stochastic) signals. This view has encouraged the idea that stem cells can be instructed to adapt their function. Newer data, however, challenge this concept. Here, we summarize these findings and discuss their implication for applications of stem cells.

RECENT FINDINGS

Hematopoietic stem cells that differ in function have been documented during development and within the adult stem cell compartment. The differences in function are stably inherited to daughter stem cells when these cells proliferate to self-renew. Collectively, the data show that the adult stem cell compartment consists of a limited number of distinct classes of stem cells.

SUMMARY

The most important stem cell functions, including self-renewal and differentiation capacity, are preprogrammed through epigenetic or genetic mechanisms. Thus, stem cells are much more predictable than previously thought. Changes in the stem cell compartment through disease or aging can be interpreted as shifts in its clonal composition, rather than a modification of individual hematopoietic stem cells.

Functional heterogeneity of side population cells in skeletal muscle

Skeletal muscle regeneration has been exclusively attributed to myogenic precursors, satellite cells. A stem cell-rich fraction referred to as side population (SP) cells also resides in skeletal muscle, but its roles in muscle regeneration remain unclear. We found that muscle SP cells could be subdivided into three sub-fractions using CD31 and CD45 markers. The majority of SP cells in normal non-regenerating muscle expressed CD31 and had endothelial characteristics. However, CD31(-)CD45(-) SP cells, which are a minor subpopulation in normal muscle, actively proliferated upon muscle injury and expressed not only several regulatory genes for muscle regeneration but also some mesenchymal lineage markers. CD31(-)CD45(-) SP cells showed the greatest myogenic potential among three SP sub-fractions, but indeed revealed mesenchymal potentials in vitro. These SP cells preferentially differentiated into myofibers after intramuscular transplantation in vivo. Our results revealed the heterogeneity of muscle SP cells and suggest that CD31(-)CD45(-) SP cells participate in muscle regeneration.

Somitic origin of limb muscle satellite and side population cells

Repair of mature skeletal muscle is mediated by adult muscle progenitors. Satellite cells have long been recognized as playing a major role in muscle repair, whereas side population (SP) cells have more recently been identified as contributing to this process. The developmental source of these two progenitor populations has been considerably debated. We explicitly tested and quantified the contribution of embryonic somitic cells to these progenitor populations. Chick somitic cells were labeled by using replication-defective retroviruses or quail/chick chimeras, and mouse cells were labeled by crossing somite-specific, Pax3-derived Cre driver lines with a Cre-dependent reporter line. We show that the majority of, if not all, limb muscle satellite cells arise from cells expressing Pax3 specifically in the hypaxial somite and their migratory derivatives. We also find that a significant number of, but not all, limb muscle SP cells are derived from the hypaxial somite. Notably, the heterogeneity in the developmental origin of SP cells is reflected in their functional heterogeneity; somitically derived SP cells are intrinsically more myogenic than nonsomitically derived ones. Thus, we show that the somites, which supply embryonic and fetal myoblasts, are also an important source of highly myogenic adult muscle progenitors.

Identification of novel resident pulmonary stem cells: form and function of the lung side population

Resident lung stem cells function to replace all lineages of pulmonary tissue, including mesenchyme, epithelium, and vasculature. The phenotype of the lung side population (SP) cells is currently under investigation; their function is currently unknown. Recent data suggest lung SP cells are an enriched tissue-specific source of organ-specific pulmonary precursors and, therefore, a source of adult stem cells. The adult lung SP cell population has been isolated and characterized for expression of markers indicative of stem cell, epithelial, and mesenchymal lineages. These studies determined that the adult mouse lung SP has epithelial and mesenchymal potential that resides within a CD45- mesenchymal subpopulation, as well as limited hematopoietic ability, which resides in the bone marrow-derived CD45+ subpopulation. The ability to identify these adult lung precursor cells allows us to further study the potential of these cells and their role in the regulation of tissue homeostasis and response to injury. The identification of this target population will potentially allow earlier treatment and, long term, a functional restoration of injured pulmonary tissue and lung health.

A novel stem-cell population in adult liver with potent hematopoietic-reconstitution activity

A number of recent reports have documented that cells possessing hematopoietic-reconstitution ability can be identified and isolated from a variety of solid organs in the adult animal. In all studies to date, however, purified organ-derived stem cells demonstrate a diminished repopulating capacity relative to that of purified bone marrow-derived hematopoietic stem cells (BM HSCs). It has therefore been unclear whether organ-derived HSCs possess functional properties distinct from those of BM HSCs, or simply have not been purified to a comparable extent. Here we report the identification of a rare subset of cells in adult murine liver that possess potent blood-repopulating potential, approaching that of BM HSCs. The cells, isolated on the basis of dye-efflux activity and CD45 expression (termed CD45(+) liver side population [SP] tip cells), exhibit a surface phenotype similar to that of freshly isolated BM HSCs derived from normal adult animals, but are phenotypically distinct in that they do not express the stem-cell marker c-kit. Single-cell transplantation studies indicate that CD45(+) liver SP tip cells can be generated from BM HSCs, suggesting a relationship between stem-cell populations in the liver and bone marrow compartments. Overall, these studies have important implications for understanding extramedullary hematopoiesis, and may be relevant to current strategies aimed at inducing tolerance to transplanted organs.

[In vivo and in vitro bone regeneration from cord blood derived mesenchymal stem cells]

BACKGROUND

Mesenchymal stem cells with an osteoblastic differentiating potency are investigated in regard of probable tissue engineering for further clinical application. The following report describes the use of cord blood derived stem cells as an alternative to other stem cell populations for bone regenerating tissue engineering.

METHODS

To demonstrate the multipotency of cord blood derived mesenchymal stem cells, unrestringated somatic stem cells (USSC) were isolated from cord blood and underwent an osteo-, chondro- and adipoblastic in vitro stimulation. To evaluate the osteoinductive potency of a porcine collagen I/III cell carrier USSC were incubated on this matrix. To investigate the in vivo effects of human USSC an athymic rat model was developed. These cells were transplanted into a femoral defect.

RESULTS

Cord blood derived mesenchymal stem cells (USSC) have an in vitro multipotency and show adipo-, chondro- and osteogenic differentiation. The porcine collagen I/III carrier promoted an osteoblastic differentiation. USSC survived after xenotransplantation in an athymic rat and differentiated into osteoblasts filling the bony defect zone.

CONCLUSION

Human USSC are a mesenchymal multipotent stem cell population that shows osteoblastic differentiation onto a collagen I/III carrier in vitro as well as in an athymic rat in vivo.

Stem cell bioengineering for regenerative medicine

Stem cells can be used to treat a variety of diseases and several recent studies in animal models demonstrate the potential of bioengineering strategies targeting adult and embryonic stem cells. In order to obtain the desired cells for transplantation, stem cell bioengineering approaches entail the manipulation of environmental signals influencing cell survival, proliferation, self-renewal and differentiation. In that regard, multivariate analytical approaches have been used with success to optimise different stem cell culture processes. The genetic or molecular enhancement of stem cells is also a powerful means to control their proliferation or differentiation or to correct genetic defects in recipients. In the future, systems-level approaches have the potential to revolutionise the field of stem cell bioengineering by improving our understanding of regulatory networks controlling cellular behaviour. This advance in basic biology will be instrumental for the implementation of many stem cell-based regenerative therapies at the clinical level, as treatment accessibility will depend on the development of robust technologies to produce sufficient cell numbers.

Side Population cells isolated from different tissues share transcriptome signatures and express tissue-specific markers

Side Population (SP) cells, isolated from murine adult bone marrow (BM) based on the exclusion of the DNA dye Hoechst 33342, exhibit potent hematopoietic stem cell (HSC) activity when compared to Main Population (MP) cells. Furthermore, SP cells derived from murine skeletal muscle exhibit both hematopoietic and myogenic potential in vivo. The multipotential capacity of SP cells isolated from variable tissues is supported by an increasing number of studies. To investigate whether the SP phenotype is associated with a unique transcriptional profile, we characterized gene expression of SP cells isolated from two biologically distinct tissues, bone marrow and muscle. Comparison of SP cells with differentiated MP cells within a tissue revealed that SP cells are in an active transcriptional and translational status and underexpress genes reflecting tissue-specific functions. Direct comparison of gene expression of SP cells isolated from different tissues identified genes common to SP cells as well as genes specific to SP cells within a particular tissue and further define a muscle and bone marrow environment. This study reports gene expression of muscle SP cells, common features and differences between SP cells isolated from muscle and bone marrow, and further identifies common signaling pathways that might regulate SP cell functions.

Mac-1(low) early myeloid cells in the bone marrow-derived SP fraction migrate into injured skeletal muscle and participate in muscle regeneration

Recent studies have shown that bone marrow (BM) cells, including the BM side population (BM-SP) cells that enrich hematopoietic stem cells (HSCs), are incorporated into skeletal muscle during regeneration, but it is not clear how and what kinds of BM cells contribute to muscle fiber regeneration. We found that a large number of SP cells migrated from BM to muscles following injury in BM-transplanted mice. These BM-derived SP cells in regenerating muscles expressed different surface markers from those of HSCs and could not reconstitute the mouse blood system. BM-derived SP/Mac-1(low) cells increased in number in regenerating muscles following injury. Importantly, our co-culture studies with activated satellite cells revealed that this fraction carried significant potential for myogenic differentiation. By contrast, mature inflammatory (Mac-1(high)) cells showed negligible myogenic activities. Further, these BM-derived SP/Mac-1(low) cells gave rise to mononucleate myocytes, indicating that their myogenesis was not caused by stochastic fusion with host myogenic cells, although they required cell-to-cell contact with myogenic cells for muscle differentiation. Taken together, our data suggest that neither HSCs nor mature inflammatory cells, but Mac-1(low) early myeloid cells in the BM-derived SP fraction, play an important role in regenerating skeletal muscles.

Demystifying SP cell purification: viability, yield, and phenotype are defined by isolation parameters

Side population (SP) cells isolated from bone marrow, skeletal muscle, and skin have been shown to engraft in dystrophic muscle. However, there have been questions on the phenotypical heterogeneity, tissue of origin, and relationships among SP cell populations extracted from different tissues. Studies on bone marrow SP cells have followed a consistent protocol for their isolation and results obtained are concordant. In contrast, protocols for the isolation of muscle SP cells vary greatly, and consequently reports on their phenotype, differentiation potential and origin have been inconsistent. To address this controversy, we demonstrate that isolation parameters, such as tissue dissociation, cell counting, Hoechst concentration, and stringency in the selection of SP cells, have an effect on the yield, viability, and homogeneity of SP cells derived from bone marrow, skeletal muscle, and skin. In this paper, we demonstrate that SP cells isolated from the bone marrow are distinct from SP cells extracted from skeletal muscle and skin tissues. This study offers an explanation for the controversy surrounding muscle SP cells, provides a detailed standardized protocol for their isolation, and highlights basic guidelines for reproducible and reliable isolation of SP cells from any tissue.

Circulating hematopoietic stem cells do not efficiently home to bone marrow during homeostasis

OBJECTIVE

Hematopoietic stem cells (HSC), normally resident in bone marrow, can be detected in the murine and human circulation. It is thought that HSC move in and out of bone marrow daily and that returning HSC are generally equivalent to their bone marrow counterparts in phenotype and function. However, large numbers of mononuclear blood cells are required to rescue animals from lethal irradiation, indicating either that the prevalence of circulating HSC is low, or they are inherently deficient in their repopulating ability. Accordingly, recent data suggest that circulating HSC may be unable to stably engraft WBM under homeostatic conditions. The purpose of this study was to explore these dynamics in detail using parabiosis and bone marrow transplantation.

MATERIALS AND METHODS

The WBM and skeletal muscle HSC stem cell compartments of parabiosed CD45 congenic mice were analyzed functionally (via bone marrow transplantation) and phenotypically (via flow cytometry) for circulating stem cells at specific time points postparabiosis and after surgical separation.

RESULTS

Surprisingly, we find that stem cells trafficking out of bone marrow and into the circulation do not stably return to bone marrow, although long-lived lymphoid precursors do stably re-engraft. Circulating HSC do, however, take up residence in skeletal muscle, wherein they account for HSC activity.

CONCLUSION

Circulating HSC are not in flux with the bone marrow HSC and can persist in peripheral tissues.

Rat adult stem cells (marrow stromal cells) engraft and differentiate in chick embryos without evidence of cell fusion

Cell fusion was recently reported to account for the plasticity of adult stem cells in vivo. Adult stem cells, referred to as mesenchymal stem cells or marrow stromal cells, from rat marrow, were infused into 1.5- to 2-day-old chick embryos. After 4 days, the rat cells had expanded 1.3- to 33-fold in one-third of surviving embryos. The cells engrafted into many tissues, and no multinuclear cells were detected. The most common site of engraftment was the heart, apparently because the cells were infused just above the dorsal aorta. Some of the cells in the heart expressed cardiotin, and alpha-heavy-chain myosin. GFP(+) cells reisolated from the embryos had a rat karyotype. Therefore, the cells engrafted and partially differentiated without evidence of cell fusion.

Role of bone marrow cell trafficking in replenishing skeletal muscle SP and MP cell populations

The multipotent nature of skeletal muscle-derived side population cells is demonstrated by their myogenic and hematopoietic potential in vivo. However, whether muscle side population cells are derived from the bone marrow is unclear. To study the long-term contribution of the hematopoietic system to muscle side population, whole bone marrow cells from Ly5.1 males or from e-GFP transgenic male mice were transplanted into lethally irradiated Ly5.2 females. Long-term cell trafficking of donor bone marrow cells to muscle side population was monitored 17 times in a 34-week study. Fluorescence-activated cell sorter analyses were used to detect Ly5.1 and GFP+ donor cells, which were confirmed by fluorescence in situ hybridization of the Y-chromosome. Analyses post-transplantation indicated that whereas cells of donor origin could be found in the muscle, donor bone marrow cells had contributed little to the muscle side population. Attempts to increase cell trafficking by induced muscle damage again confirmed that more than 90% of side population cells present in the muscle were derived from the host. These results demonstrate that muscle side population cells are not replenished by the bone marrow and suggest a non-hematopoietic origin for this cell population.

Muscle-derived stem cells for musculoskeletal tissue regeneration and repair

Muscle recently has been identified as a good source of adult stem cells that can differentiate into cells of different lineages. The most well-known muscle progenitor cells are satellite cells, which not only contribute to the replenishment of the myogenic cell pool but also can become osteoblasts, adipocytes and chondrocytes. Other populations of stem cells that appear to be distinct from satellite cells also have been discovered recently. Muscle-derived stem cells (MDSCs) can be divided into two major categories based on these cells' varied abilities to differentiate into myogenic lineages. Interestingly, MDSCs that can differentiate readily into myogenic cells are usually CD45-. In contrast, MDSCs with less myogenic potential are CD45+. Various lines of evidence suggest that different populations of MDSCs are closely related. Furthermore, MDSCs appear to be closely related to endothelial cells or pericytes of the capillaries surrounding myofibers. When used in tissue engineering applications, MDSCs--particularly those genetically engineered to express growth factors--have been demonstrated to possess great potential for the regeneration and repair of muscle, bone and cartilage. Further research is necessary to delineate the relationship between different populations of MDSCs and between MDSCs and other adult stem cells, to investigate their developmental origin, and to determine the regulatory pathways and factors that control stem cell self-renewal, proliferation and differentiation. This knowledge could greatly enhance the usefulness of muscle-derived stem cells, as well as other adult stem cells, for tissue repair and regeneration applications.

Stem cell plasticity: from transdifferentiation to macrophage fusion

The past 5 years have witnessed an explosion of interest in using adult-derived stem cells for cell and gene therapy. This has been driven by a number of findings, in particular, the possibility that some adult stem cells can differentiate into non-autologous cell types, and also the discovery of multipotential stem cells in adult bone marrow. These discoveries suggested a quasi-alchemical nature of cells derived from adult organs, thus raising new and exciting therapeutic possibilities. Recent data, however, argue against the whole idea of stem cell 'plasticity', and bring into question the therapeutic strategies based upon this concept. Here, we will review the current state of knowledge in the field and discuss some of the clinical implications.

Differentiation of adult bone marrow CD45-CD117- stem cells into hepatocyte in vitro

AIM: To observe whether the CD117-CD45- bone marrow cell can differentiate into the cells with characters of hepatocyte by the stimulation of hepatocyte growth factor (HGF)and fibroblast growth factor-4 (FGF4), and to explore the acting mechanism of the growth factors.

METHODS: Bone marrow cells were from the stern or ilium of 4 healthy volunteers between 4-40 years old. CD117-CD45- bone marrow cells separated by magnetic cell sorting method, cultured in DMEM medium with FGF4, HGF, FGF4+ HGF or no growth factor were divided into four groups: groups A, B, C, and D accordingly. Cells were collected on d 0, 7, 14, 21, and 28 for detecting the hepatocyte markers: AFP, CK18, albumin by immunocytochemistry, glycogen by PAS staining and c-met, FGFR2 mRNA by RT-PCR.

RESULTS: The specific markers of hepatocyte were positive in groups A, B and C but not in group D. C-met and FGFR2 mRNA expressed at a low level in the cells newly isolated or cultured in the medium without growth factor, but at increasing level after cutured with growth factor.

CONCLUSION: HGF and FGF4 can induce CD45-CD117- differentiate into hepatocyte-like cells. The effect of growth factor on cell differentiation is probably by the positive regulation between the growth factor and its receptor.

Plasticity of Adult Stem Cells

Recent years have seen much excitement over the possibility that adult mammalian stem cells may be capable of differentiating across tissue lineage boundaries, and as such may represent novel, accessible, and very versatile effectors of therapeutic tissue regeneration. Yet studies proposing such "plasticity" of adult somatic stem cells remain controversial, and in general, existing evidence suggests that in vivo such unexpected transformations are exceedingly rare and in some cases can be accounted for by equally unexpected alternative explanations.

Single hematopoietic stem cells generate skeletal muscle through myeloid intermediates

Recent studies have shown that cells from the bone marrow can give rise to differentiated skeletal muscle fibers. However, the mechanisms and identities of the cell types involved have remained unknown, and the validity of the observation has been questioned. Here, we use transplantation of single CD45+ hematopoietic stem cells (HSCs) to demonstrate that the entire circulating myogenic activity in bone marrow is derived from HSCs and their hematopoietic progeny. We also show that ongoing muscle regeneration and inflammatory cell infiltration are required for HSC-derived contribution, which does not occur through a myogenic stem cell intermediate. Using a lineage tracing strategy, we show that myofibers are derived from mature myeloid cells in response to injury. Our results indicate that circulating myeloid cells, in response to inflammatory cues, migrate to regenerating skeletal muscle and stochastically incorporate into mature myofibers.