Neural stem cells, tumour stem cells and brain tumours: dangerous relationships?

Neural stem cells (NSC) have been implicated not only in brain development and neurogenesis but also in tumourigenesis. Brain tumour stem cells (BTSC) have been isolated from several paediatric or adult human brain tumours, however their origin is still disputed. This review discusses the normal role of NSC in the adult mammalian brain and their anatomical location. It compares the molecular characteristics and the biological behaviour of NSC/BTSC, and describes the molecular pathways involved in controlling self-renewal and maintenance of adult NSC/BTSC and brain tumour development. It also assesses the current hypotheses about the origin of BTSC and the clinical consequences.     

Human non-CG methylation: Are human stem cells plant-like?

Non-CG methylation is well characterized in plants where it appears to play a role in gene silencing and genomic imprinting. Although strong evidence for the presence of non-CG methylation in mammals has been available for some time, both its origin and function remain elusive. In this review we discuss available evidence on non-CG methylation in mammals in light of evidence suggesting that the human stem cell methylome contains significant levels of methylation outside the CG site.Key words: non-CG methylation, stem cells, Dnmt1, Dnmt3a, human methylomeIn plant cells non-CG sites are methylated de novo by Chromomethylase 3, DRM1 and DRM2. Chromomethylase 3, along with DRM1 and DRM2 combine in the maintenance of methylation at symmetric CpHpG as well as asymmetric DNA sites where they appear to prevent reactivation of transposons.¹ DRM1 and DRM2 modify DNA de novo primarily at asymmetric CpH and CpHpH sequences targeted by siRNA.²Much less information is available on non-CG methylation in mammals. In fact, studies on mammalian non-CG methylation form a tiny fraction of those on CG methylation, even though data for cytosine methylation in other dinucleotides, CA, CT and CC, have been available since the late 1980s.³ Strong evidence for non-CG methylation was found by examining either exogenous DNA sequences, such as plasmid and viral integrants in mouse and human cell lines,^4,5 or transposons and repetitive sequences such as the human L1 retrotransposon⁶ in a human embryonic fibroblast cell line. In the latter study, non-CG methylation observed in L1 was found to be consistent with the capacity of Dnmt1 to methylate slippage intermediates de novo.⁶Non-CG methylation has also been reported at origins of replication^7,8 and a region of the human myogenic gene Myf3.⁹ The Myf3 gene is silenced in non-muscle cell lines but it is not methylated at CGs. Instead, it carries several methylated cytosines within the sequence CCTGG. Gene-specific non-CG methylation was also reported in a study of lymphoma and myeloma cell lines not expressing many B lineage-specific genes.¹⁰ The study focused on one specific gene, B29 and found heavy CG promoter methylation of that gene in most cell lines not expressing it. However, in two other cell lines where the gene was silenced, cytosine methylation was found almost exclusively at CCWGG sites. The authors provided evidence suggesting that CCWGG methylation was sufficient for silencing the B29 promoter and that methylated probes based on B29 sequences had unique gel shift patterns compared to non-methylated but otherwise identical sequences.¹⁰ The latter finding suggests that the presence of the non-CG methylation causes changes in the proteins able to bind the promoter, which could be mechanistically related to the silencing seen with this alternate methylation.Non-CG methylation is rarely seen in DNA isolated from cancer patients. However, the p16 promoter region was reported to contain both CG and non-CG methylation in breast tumor specimens but lacked methylation at these sites in normal breast tissue obtained at mammoplasty.¹¹ Moreover, CWG methylation at the CCWGG sites in the calcitonin gene is not found in normal or leukemic lymphocyte DNA obtained from patients.¹² Further, in DNA obtained from breast cancer patients, MspI sites that are refractory to digestion by MspI and thus candidates for CHG methylation were found to carry CpG methylation.¹³ Their resistance to MspI restriction was found to be caused by an unusual secondary structure in the DNA spanning the MspI site that prevents restriction.¹³ This latter observation suggests caution in interpreting EcoRII/BstNI or EcoRII/BstOI restriction differences as due to CWG methylation, since in contrast to the 37°C incubation temperature required for full EcoRII activity, BstNI and BstOI require incubation at 60°C for full activity where many secondary structures are unstable.The recent report by Lister et al.¹⁴ confirmed a much earlier report by Ramsahoye et al.¹⁵ suggesting that non-CG methylation is prevalent in mammalian stem cell lines. Nearest neighbor analysis was used to detect non-CG methylation in the earlier study on the mouse embryonic stem (ES) cell line,¹⁵ thus global methylation patterning was assessed. Lister et al.¹⁴ extend these findings to human stem cell lines at single-base resolution with whole-genome bisulfite sequencing. They report¹⁴ that the methylome of the human H1 stem cell line and the methylome of the induced pluripotent IMR90 (iPS) cell line are stippled with non-CG methylation while that of the human IMR90 fetal fibroblast cell line is not. While the results of the two studies are complementary, the human methylome study addresses locus specific non-CG methylation. Based on that data,¹⁴ one must conclude that non-CG methylation is not carefully maintained at a given site in the human H1 cell line. The average non-CG site is picked up as methylated in about 25% of the reads whereas the average CG methylation site is picked up in 92% of the reads. Moreover, non-CG methylation is not generally present on both strands and is concentrated in the body of actively transcribed genes.¹⁴Even so, the consistent finding that non-CG methylation appears to be confined to stem cell lines,^14,15 raises the possibility that cancer stem cells¹⁶ carry non-CG methylation while their nonstem progeny in the tumor carry only CG methylation. Given the expected paucity of cancer stem cells in a tumor cell population, it is unlikely that bisulfite sequencing would detect non-CG methylation in DNA isolated from tumor cells since the stem cell population is expected to be only a very minor component of tumor DNA. Published sequences obtained by bisulfite sequencing generally report only CG methylation, and to the best of our knowledge bisulfite sequenced tumor DNA specimens have not reported non-CG methylation. On the other hand, when sequences from cell lines have been reported, bisulfite-mediated genomic sequencing⁸ or ligation mediated PCR¹⁷ methylcytosine signals outside the CG site have been observed. In a more recent study plasmid DNAs carrying the Bcl2-major breakpoint cluster¹⁸ or human breast cancer DNA¹³ treated with bisulfite under non-denaturing conditions, cytosines outside the CG side were only partially converted on only one strand¹⁸ or at a symmetrical CWG site.¹³ In the breast cancer DNA study the apparent CWG methylation was not detected when the DNA was fully denatured before bisulfite treatment.¹³In both stem cell studies, non-CG methylation was attributed to the Dnmt3a,^14,15 a DNA methyltransferase with similarities to the plant DRM methyltransferase family¹⁹ and having the capacity to methylate non-CG sites when expressed in Drosophila melanogaster.¹⁵ DRM proteins however, possess a unique permuted domain structure found exclusively in plants¹⁹ and the associated RNA-directed non-CG DNA methylation has not been reproducibly observed in mammals despite considerable published^20–23 and unpublished efforts in that area. Moreover, reports where methylation was studied often infer methylation changes from 5AzaC reactivation studies²⁴ or find that CG methylation seen in plants but not non-CG methylation is detected.^21,22,25,26 In this regard, it is of interest that the level of non-CG methylation reported in stem cells corresponds to background non-CG methylation observed in vitro with human DNA methyltransferase I,²⁷ and is consistent with the recent report that cultured stem cells are epigenetically unstable.²⁸The function of non-CG methylation remains elusive. A role in gene expression has not been ruled out, as the studies above on Myf3 and B29 suggest.^9,10 However, transgene expression of the bacterial methyltransferase M.EcoRII in a human cell line (HK293), did not affect the CG methylation state at the APC and SerpinB5 genes²⁹ even though the promoters were symmetrically de novo methylated at ^mCWGs within each CCWGG sequence in each promoter. This demonstrated that CG and non-CG methylation are not mutually exclusive as had been suggested by earlier reports.^9,10 That observation is now extended to the human stem cell line methylome where CG and non-CG methylation co-exist.¹⁴ Gene expression at the APC locus was likewise unaffected by transgene expression of M.EcoRII. In those experiments genome wide methylation of the CCWGG site was detected by restriction analysis and bisulfite sequencing,²⁹ however stem cell characteristics were not studied.Many alternative functions can be envisioned for non-CG methylation, but the existing data now constrains them to functions that involve low levels of methylation that are primarily asymmetric. Moreover, inheritance of such methylation patterns requires low fidelity methylation. If methylation were maintained with high fidelity at particular CHG sites one would expect that the spontaneous deamination of 5-methylcytosine would diminish the number of such sites, so as to confine the remaining sites to those positions performing an essential function, as is seen in CG methylation.^30–33 However, depletion of CWG sites is not observed in the human genome.³⁴ Since CWG sites account for only about 50% of the non-CG methylation observed in the stem cell methylome¹⁴ where methylated non-CG sites carry only about 25% methylation, the probability of deamination would be about 13% of that for CWG sites that are subject to maintenance methylation in the germ line. Since mutational depletion of methylated cytosines has to have its primary effect on the germ line, if the maintenance of non-CG methylation were more accurate and more widespread, one would have had to argue that stem cells in the human germ lines lack CWG methylation. As it is the data suggests that whatever function non-CG methylation may have in stem cells, it does not involve accurate somatic inheritance in the germ line.The extensive detail on non-CG methylation in the H1 methylome¹⁴ raises interesting questions about the nature of this form of methylation in human cell lines. A key finding in this report is the contrast between the presence of non-CG methylation in the H1 stem cell line and its absence in the IMR90 human fetal lung fibroblast cell line.¹⁴ This suggests that it may have a role in the origin and maintenance of the pluripotent lineage.¹⁴By analogy with the well known methylated DNA binding proteins specific for CG methylation,³⁵ methylated DNA binding proteins that selectively bind sites of non-CG methylation are expected to exist in stem cells. Currently the only protein reported to have this binding specificity is human Dnmt1.^36–38 While Dnmt1 has been proposed to function stoichiometrically³⁹ and could serve a non-CG binding role in stem cells, this possibility and the possibility that other stem-cell specific non-CG binding proteins might exist remain to be been explored.Finally, the nature of the non-CG methylation patterns in human stem cell lines present potentially difficult technical problems in methylation analysis. First, based on the data in the H1 stem cell methylome,⁴⁰ a standard MS-qPCR for non-CG methylation would be impractical because non-CG sites are infrequent, rarely clustered and are generally characterized by partial asymmetric methylation. This means that a PCR primer that senses the 3 adjacent methylation sites usually recommended for MS-qPCR primer design^41,42 cannot be reliably found. For example in the region near Oct4 (Chr6:31,246,431), a potential MS-qPCR site exists with a suboptimal set of two adjacent CHG sites both methylated on the + strand at Chr6:31,252,225 and 31,252,237.^14,40 However these sites were methylated only in 13/45 and 30/52 reads. Thus the probability that they would both be methylated on the same strand is about 17%. Moreover, reverse primer locations containing non-CG methylation sites are generally too far away for practical bisulfite mediated PCR. Considering the losses associated with bisulfite mediated PCR⁴³ the likelihood that such an MS-qPCR system would detect non-CG methylation in the H1 cell line or stem cells present in a cancer stem cell niche^44,45 is very low.The second difficulty is that methods based on the specificity of MeCP2 and similar methylated DNA binding proteins for enriching methylated DNA (e.g., MIRA,⁴⁶ COMPARE-MS⁴⁷) will discard sequences containing non-CG methylation since they require cooperative binding afforded by runs of adjacent methylated CG sites for DNA capture. This latter property of the methylated cytosine capture techniques makes it also unlikely that methods based on 5-methylcytosine antibodies (e.g., meDIP⁴⁸) will capture non-CG methylation patterns accurately since the stem cell methylome shows that adjacent methylated non-CG sites are rare in comparison to methylated CG sites.¹⁴In summary, whether or not mammalian stem cells in general or human stem cells in particular possess functional plant-like methylation patterns is likely to continue to be an interesting and challenging question. At this point we can conclude that the non-CG patterns reported in human cells appear to differ significantly from the non-CG patterns seen in plants, suggesting that they do not have a common origin or function.     

Are folliculo-stellate cells in the anterior pituitary gland supportive cells or organ-specific stem cells?

Folliculo-stellate cells (FS-cells) in the anterior pituitary gland are star-shaped cells and form tiny follicles. FS-cells are positive for S-100 protein and produce many cytokines or growth factors, such as interleukin-6 (IL-6), leukemia inhibitory factor (LIF), basic fibroblastic growth factor (bFGF) and vascular endothelial cell growth factor (VEGF). Therefore, it is generally accepted that FS-cells regulate endocrine cells through these growth factors. FS-cells also exhibit a phagocytotic activity and are known to work as scavenger cells. In addition to these functions, FS-cells are considered to have some unknown functions. In order to reveal the biological significance of FS-cells in the anterior pituitary gland, we performed a morphological study and obtained some new findings. First, we were interested in the colloid formation in the senescent porcine pituitary gland. We analyzed the colloids and found that clusterin is a major protein in them. We also found that the accumulation of clusterin in the colloids is related to the phagocytotic activity of FS-cells. In our next study, we found that FS-cells have the potential to differentiate into striated muscle cells. From FS-cells show multi-potent cell character and other cytological evidence, we propose that FS-cells are candidate of organ-specific stem cells in the anterior pituitary gland.     

Insulin producing cells derived from embryonic stem cells: Are we there yet?

Derivation of insulin producing cells (IPCs) from embryonic stem (ES) cells provides a potentially innovative form of treatment for type 1 diabetes. Here, we discuss the current state of the art, unique challenges, and future directions on generating IPCs.     

A polarized anchor for hematopoietic stem cells: Synapse between stem cells and their niche?

Multipotent hematopoietic stem cells are maintained by the bone marrow niche, but how niche-derived membrane-bound stem cell factor (mSCF) regulates HSCs remains unclear. In this issue, Hao et al. (2021. J. Cell Biol. https://doi.org/10.1083/jcb.202010118) describe that mSCF, synergistically with VCAM-1, induces large, polarized protrusions that serve as anchors for HSCs to their niche.

Hematopoietic stem cells (HSCs) generate all blood and immune cells throughout life via self-renewal and multilineage differentiation within the bone marrow niche. HSCs are the basis for bone marrow transplantation, saving thousands of lives yearly. The bone marrow niche often serves as a paradigm for studying stem cell biology. In addition, elucidating the underlying mechanism in the niche helps devise strategies to expand functional HSCs for clinical use. Within the niche, leptin receptor–positive perisinusoidal stromal cells and endothelial cells are the major source of essential cytokines for HSC maintenance, including vascular cell adhesion molecule 1 (VCAM-1) and stem cell factor (SCF; 1, 2). Locally produced soluble and membrane-bound cytokines preserve the unique localization and anchorage of HSCs to stromal cells within their niche. Consistent with this notion, mouse genetic data have shown that membrane-bound SCF (mSCF) is important for HSC maintenance in vivo (3). However, given that both soluble and membrane-bound forms of SCF can engage with the cognate cKIT receptors, the mechanisms by which mSCF sustains HSCs function in vivo remain elusive. Likewise, it is unclear why the expansion and maintenance of HSCs ex vivo by adding SCF to culture as an either soluble or immobilized form has only been achieved with limited success.In this issue, Hao et al. addressed this question by using a supported lipid bilayer (SLB) system to model the interaction between HSCs and membrane-bound cytokines, including SCF (4). SLBs present an advantage over conventional immobilization methods; they allow the lateral mobility of membrane-bound proteins and clustering of receptors and signaling complexes, thus resembling the lipid bilayer of plasma membrane in vivo. Focusing on HSC cytokines that may be presented as membrane-bound forms in the bone marrow niche, the authors performed an imaging screen in vitro using SLBs and found that mSCF but not soluble SCF (sSCF) induced mSCF/cKIT clustering and the formation of membrane protrusions on HSCs. While mSCF alone was sufficient to promote cell protrusions, HSCs required both mSCF and VCAM-1 for large, polarized protrusions. They followed HSCs at different time points after exposure to VCAM-1 and mSCF by scanning electron microscopy and observed that HSCs first formed diffuse mSCF clusters and multifocal thin protrusions and then proceeded to a polarized, clustered morphology with larger and thicker protrusions. Using a controlled sheer stress device, Hao et al. showed that these polarized protrusions had a functional consequence on the adhesion strength of HSCs. mSCF and VCAM-1 dramatically increased the adhesion of HSCs to SLB compared with VCAM-1 or mSCF alone. Interestingly, the effect was more prominent in HSCs compared with their immediate downstream progenies, multipotent progenitors. This phenotype was also specific to ligands presented on SLB because the effect was canceled when the cytokines were directly immobilized onto the glass surface. Then, they had a close look into the cytoskeletal organization of HSCs in the presence of both mSCF and VCAM-1 on SLB. They found that F-actin and myosin IIa concentrated at the protrusion, which led them to speculate that the cytoskeleton remodeling mediates the formation of the polarized morphology. Indeed, chemical inhibitors blocking myosin contraction, actin polymerization, or Rho-associated protein kinase disrupted the formation of the large and polarized protrusion. The authors noted that phosphatidylinositol 3-kinase (PI3K) also localized with mSCF/cKIT clusters, so they further assessed the contribution of the PI3K/Akt pathway to the polarized morphology of HSCs by using total internal reflection fluorescence microscopy and PI3K and Akt chemical inhibitors. PI3K/Akt activation contributed downstream of the mSCF–VCAM-1 synergy to regulating HSC cell adhesion and polarized mSCF/cKIT distribution. In addition, PI3K signaling enhanced the nuclear retention of FOXO3a, a crucial factor for HSC self-renewal; this enhancement was induced by mSCF but lessened by sSCF. Intriguingly, sSCF also competed with mSCF and abrogated the effect of the mSCF–VCAM-1 synergy on polarized protrusion formation. However, whether and how PI3K transmits the mSCF–VCAM-1 synergy into proliferation or quiescence cues in HSCs requires further investigation. Taken together, these data suggest that mSCF and VCAM-1 synergize to induce polarized protrusions on HSCs, which regulates their adhesion to the niche (Fig. 1). These protrusions share many features with the immunological synapse (5), which points toward the existence of a similar model for stem cells, “stem cell synapse,” where HSCs interact with and receive a variety of signals from their niche cells.Open in a separate windowFigure 1.VCAM-1 and mSCF synergistically promote the formation of polarized protrusions (stem cell synapse) on HSCs. (A and B) VCAM-1 or mSCF alone does not induce apparent polarized morphology on HSCs. The signaling and adhesion of HSCs to the niche is not at its full potential. (C) VCAM-1 and mSCF together induce robust receptor clustering on HSCs, optimal signaling, and strong adhesion. (D) sSCF can competitively disrupt the polarized protrusions on HSCs. The figure was created with BioRender.com.While the study by Hao et al. sheds light on how niche signals, particularly mSCF, regulate HSCs, several outstanding questions remain. First, even though many hematopoietic cells express cKIT (some of them even express higher levels than HSCs), HSCs respond to mSCF + VCAM-1 the strongest by recruiting the most mSCF to clusters. What is the specific mechanism in HSCs underlying this specificity? Second, SCF is produced both as mSCF and sSCF in vivo, through alternative splicing and proteolytic cleavage; if mSCF is mainly responsible for anchoring HSCs in the niche, what is the function of sSCF in vivo? Does sSCF modulate the available pool of mSCF? Third, robust maintenance of HSCs in culture has been challenging. HSCs can be maintained in a system composed of sSCF, thromopoietin (TPO), fibronectin, and polyvinyl alcohol (6). Tethering cytokines to SLB elicits more physiological response from HSCs compared with soluble cytokines or direct immobilization. Does SLB improve maintenance of HSCs in in vitro culture? Fourth, some cytokines, such as TPO, act on HSCs in a long-range manner (7). How do these systemic cytokines induce robust signaling in HSCs? Do they participate in the stem cell synapse even if they are not the initiators? Finally, do stem cells and their niche interact by forming similar synapses in other stem cell systems? Answering these questions will deepen our understanding of the stem cell niche and help integrate the niche component into potential, more successful applications in regenerative medicine.     

Are T cells the only HIV-1 reservoir?

Current antiretroviral therapies have improved the duration and quality of life of people living with HIV-1. However, viral reservoirs impede complete eradication of the virus. Although there are many strategies to eliminate infectious virus, the most actively pursued are latency reversing agents in conjunction with immune modulation. This strategy, known as “shock and kill”, has been tested primarily against the most widely recognized HIV-1 latent reservoir found in resting memory CD4+ T cells. This is in part because of the dearth of conclusive evidence about the existence of non-T cell reservoirs. Studies of non-T cell reservoirs have been difficult to interpret because of technical and biological issues that have hampered a better understanding. This review considers the current knowledge of non-T cell reservoirs, the challenges encountered in a better understanding of these populations, and their implications for HIV-1 cure research.     

Overexpression of protein kinase C ? improves retention and survival of transplanted mesenchymal stem cells in rat acute myocardial infarction

We assessed the effects of protein kinase C ɛ (PKCɛ) for improving stem cell therapy for acute myocardial infarction (AMI). Primary mesenchymal stem cells (MSCs) were harvested from rat bone marrow. PKCɛ-overexpressed MSCs and control MSCs were transplanted into infarct border zones in a rat AMI model. MSCs and PKCɛ distribution and expression of principal proteins involved in PKCɛ signaling through the stromal cell-derived factor 1 (SDF-1)/CXC chemokine receptor type 4 (CXCR4) axis and the phosphatidylinositol 3 kinase (PI3K)/protein kinase B (AKT) pathway were analyzed by immunofluorescence and western blot 1 day after transplantation. Echocardiographic measurements and histologic studies were performed at 4 weeks after transplantation, and MSC survival, expression of vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), transforming growth factor β (TGFβ), cardiac troponin I (cTnI), von Willebrand factor (vWF), smooth muscle actin (SMA) and factor VIII and apoptosis in infarct border zones were assessed. Rat heart muscles retained more MSCs and SDF-1, CXCR4, PI3K and phosphorylated AKT increased with PKCɛ overexpression 1 day after transplantation. MSC survival and VEGF, bFGF, TGFβ, cTnI, vWF, SMA and factor VIII expression increased in animals with PKCɛ-overexpressed MSCs at 4 weeks after transplantation and cardiac dysfunction and remodeling improved. Infarct size and apoptosis decreased as well. Inhibitory actions of CXCR4 or PI3K partly attenuated the effects of PKCɛ. Activation of PKCɛ may improve retention, survival and differentiation of transplanted MSCs in myocardia. Augmentation of PKCɛ expression may enhance the therapeutic effects of stem cell therapy for AMI.Irreversible and widespread loss of myocardial cells and subsequent ventricular remodeling induced by acute myocardial infarction (AMI) is the main cause of chronic heart failure¹ and globally >17 million people died of ischemic heart diseases in 2008.² Stem cell-based regenerative therapy for AMI is encouraging with respect to preclinical^{3, 4} and clinical data,^{5, 6, 7, 8} and this may soon be a therapeutic modality for injury resulting from coronary artery disease. Two problems – poor homing of transplanted cells to injury sites, and poor cell survival – require resolution before transplantation therapy can be broadly effective. The stromal cell-derived factor 1 (SDF-1)/CXC chemokine receptor type 4 (CXCR4) axis has an important role during migration, proliferation and survival of stem cells, but using this knowledge to improve homing and survival of therapeutic stem cells has not been successful.Previous studies^{9, 10, 11} suggest that protein kinase C ɛ (PKCɛ) is essential for signal transduction for ischemic cardioprotection, but whether it has an effect on stem cell retention and survival and what mechanism underlies this effect is uncertain. We know that SDF-1 increased significantly in mesenchymal stem cells (MSCs) after treatment with PKC activator and decreased after treatment with a PKCɛ inhibitor in preliminary experiments, and our latest work indicates that activating PKCɛ improves migration and paracrine function of MSCs in vitro.¹² Thus, we suggest that PKCɛ overexpression in transplanted bone marrow MSCs (BMMSCs) would improve retention and survival of MSC''s and improve cardiac function and remodeling in a rat AMI model.     

Are mesenchymal stem cells in rheumatoid arthritis the good or bad guys?

The advancements in our understanding of the inflammatory and immune mechanisms in rheumatoid arthritis (RA) have fuelled the development of targeted therapies that block cytokine networks and pathogenic immune cells, leading to a considerable improvement in the management of RA patients. Nonetheless, no therapy is curative and clinical remission does not necessarily correspond to non-progression of joint damage. Hence, the biomedical community has redirected scientific efforts and resources towards the investigation of other biological aspects of the disease, including the mechanisms driving tissue remodelling and repair. In this regard, stem cell research has attracted extraordinary attention, with the ultimate goal to develop interventions for the biological repair of damaged tissues in joint disorders, including RA. The recent evidence that mesenchymal stem cells (MSCs) with the ability to differentiate into cartilage are present in joint tissues raises an opportunity for therapeutic interventions via targeting intrinsic repair mechanisms. Under physiological conditions, MSCs in the joint are believed to contribute to the maintenance and repair of joint tissues. In RA, however, the repair function of MSCs appears to be repressed by the inflammatory milieu. In addition to being passive targets, MSCs could interact with the immune system and play an active role in the perpetuation of arthritis and progression of joint damage. Like MSCs, fibroblast-like synoviocytes (FLSs) are part of the stroma of the synovial membrane. During RA, FLSs undergo proliferation and contribute to the formation of the deleterious pannus, which mediates damage to articular cartilage and bone. Both FLSs and MSCs are contained within the mononuclear cell fraction in vitro, from which they can be culture expanded as plastic-adherent fibroblast-like cells. An important question to address relates to the relationship between MSCs and FLSs. MSCs and FLSs could be the same cell type with functional specialisation or represent different functional stages of the same stromal lineage. This review will discuss the roles of MSCs in RA and will address current knowledge of the relative identity between MSCs and FLSs. It will also examine the immunomodulatory properties of the MSCs and the potential to harness such properties for the treatment of RA.     

Are mesenchymal stem cells major sources of safe signals in immune system?

Numerous reports have shown that mesenchymal stem cells (MSCs) are implicated in immuno-regulation. Several factors expressed from MSCs, especially indoleamine 2,3-dioxygenase (IDO) and prostaglandin E2 (PGE2), are of importance in immuno-regulation on immune cells. In current minireview, we provided evidences to support a novel notion that MSCs may be a major source of "safe signals" in the immune system to balance "dangerous signals" based on a well accepted theory of "danger model". Furthermore, MSCs are of lifecycle characterized by age-and diseased-related changes, such as decreased growth rate, increased senescence, and altered morphology. Thus, defected and abnormal MSCs are implicated in auto-immune diseases, such as systemic lupus erythematosus (SLE). Clinically, it is important to determine clinical benefits and sides effects of cell therapies using autologous self-MSCs or healthy allogeneic MSCs in treatment of autoimmune diseases.     

Are mesenchymal stromal cells immune cells?

Mesenchymal stromal cells (MSCs) are considered to be promising agents for the treatment of immunological disease. Although originally identified as precursor cells for mesenchymal lineages, in vitro studies have demonstrated that MSCs possess diverse immune regulatory capacities. Pre-clinical models have shown beneficial effects of MSCs in multiple immunological diseases and a number of phase 1/2 clinical trials carried out so far have reported signs of immune modulation after MSC infusion. These data indicate that MSCs play a central role in the immune response. This raises the academic question whether MSCs are immune cells or whether they are tissue precursor cells with immunoregulatory capacity. Correct understanding of the immunological properties and origin of MSCs will aid in the appropriate and safe use of the cells for clinical therapy. In this review the whole spectrum of immunological properties of MSCs is discussed with the aim of determining the position of MSCs in the immune system.     

Hepatic stem cells: from inside and outside the liver?

The liver is normally proliferatively quiescent, but hepatocyte loss through partial hepatectomy, uncomplicated by virus infection or inflammation, invokes a rapid regenerative response from all cell types in the liver to perfectly restore liver mass. Moreover, hepatocyte transplants in animals have shown that a certain proportion of hepatocytes in foetal and adult liver can clonally expand, suggesting that hepatoblasts/hepatocytes are themselves the functional stem cells of the liver. More severe liver injury can activate a potential stem cell compartment located within the intrahepatic biliary tree, giving rise to cords of bipotential transit amplifying cells (oval cells), that can ultimately differentiate into hepatocytes and biliary epithelial cells. A third population of stem cells with hepatic potential resides in the bone marrow; these haematopoietic stem cells may contribute to the albeit low renewal rate of hepatocytes, but can make a more significant contribution to regeneration under a very strong positive selection pressure. In such instances, cell fusion rather than transdifferentiation appears to be the underlying mechanism by which the haematopoietic genome becomes reprogrammed.     

Are T cells at the origin of B cell lymphomas?

Lymphoma pathogenesis is at least in some cases related to transformed B cells (BCs) arising from germinal centre reactions (GCRs). In this article possible deregulations of GCRs are investigated using in silico simulations. It is found that the final differentiation of BCs as regulated by helper T cells (TCs) is the best candidate mechanism for such a deregulation. This shifts the paradigm of BC lymphoma pathogenesis from BC transformations to an emphasized role of TC-BC interactions.     

Treating muscular dystrophy with stem cells?

There is currently no effective treatment for the devastating muscle-wasting disease Duchenne muscular dystrophy (DMD). Cossu and colleagues report in a recent Nature paper that transplantation of mesoangioblast stem cells may hold promise for treating DMD. Further studies are required to fully evaluate the clinical potential of these blood-vessel-associated stem cells.     

Bone marrow as a source of stem cells and germ cells? Perspectives for transplantation

Recent publications have suggested the existence of germ stem cells in the mouse at postnatal stages. The mechanism of de novo oocyte formation is proposed to involve a contribution from the bone marrow to the germ cell pool, via the bloodstream. Critical examination of the data underpinning these contentious claims is under way from a reproductive biology perspective but little has been said about the nature of this elusive bone marrow population with germ cell potential. Furthermore, whereas the prospect of marrow-derived germ cells may appear propitious for fertility applications, its wider impact on transplantation medicine remains to be considered. This paper examines the evidence leading to the current debate and considers the implications of such findings for the field of bone marrow transplantation. The author is indebted to the Anne McLaren Fellowship Scheme of the University of Nottingham and to the Alzheimer’s Society for their support.