Mairi Brittan

CD133: molecule of the moment

CD133 (prominin‐1) was the first in a class of novel pentaspan membrane proteins to be identified in both humans and mice, and was originally classified as a marker of primitive haematopoietic and neural stem cells. Due to the highly restricted expression of CD133 family molecules on plasma membrane protrusions of epithelial and other cell types, in association with membrane cholesterol, a role in the organization of plasma membrane topology has also recently been assigned to this family. Studies have now confirmed the utility of CD133 as a marker of haematopoietic stem cells for human allogeneic transplantation. In addition, CD133 represents a marker of tumour‐initiating cells in a number of human cancers, and therefore it may be possible to develop future therapies towards targeting cancer stem cells via this marker. The development of such therapies will be aided by a clearer understanding of the molecular mechanisms and signalling pathways that regulate the behaviour of CD133‐expressing cells, and new data outlining the role of Wnt, Notch, and bone morphogenetic protein (BMP) signalling in CD133+ cancer stem cell regulation are discussed within. Copyright

Gastrointestinal stem cells

Turnover of the epithelial cell lineages within the gastrointestinal tract is a constant process, occurring every 2–7 days under normal homeostasis and increasing after damage. This process is regulated by multipotent stem cells, which give rise to all gastrointestinal epithelial cell lineages and can regenerate whole intestinal crypts and gastric glands. The stem cells of the gastrointestinal tract are as yet undefined, although it is generally agreed that they are located within a ‘niche’ in the intestinal crypts and gastric glands. Studies of allophenic tetraparental chimeric mice and targeted stem cell mutations suggest that a single stem cell undergoes asymmetrical division to produce an identical daughter cell, and thus replicate itself, and a committed progenitor cell which further differentiates into an adult epithelial cell type. The discovery of stem cell plasticity in many tissues, including the ability of transplanted bone marrow to transdifferentiate into intestinal subepithelial myofibroblasts, provides a potential use of bone marrow cells to deliver therapeutic genes to damaged tissues, for example, in treatment of mesenchymal diseases in the gastrointestinal tract, such as fibrosis and Crohns disease. Studies are beginning to identify the molecular pathways that regulate stem cell proliferation and differentiation into adult gastrointestinal cell lineages, such as the Wnt and Notch/Delta signalling pathways, and the importance of mesenchymal–epithelial interactions in normal gastrointestinal epithelium and in development and disease. Copyright

Bone marrow derivation of pericryptal myofibroblasts in the mouse and human small intestine and colon

Background and aims: In order to establish whether extraintestinal cells contribute to the turnover and repair of gastrointestinal tissues, we studied the colons and small intestines of female mice that had received a male bone marrow transplant, together with gastrointestinal biopsies from female patients that had developed graft versus host disease after receiving a bone marrow transplant from male donors. Methods: Using in situ hybridisation to detect Y chromosomes and immunohistochemistry, we demonstrated that cells derived from injected bone marrow frequently engrafted into the intestine and differentiated into pericryptal myofibroblasts. Results: In the human intestine, we confirmed by combining in situ hybridisation with immunostaining for smooth muscle actin that the bone marrow derived cells within the intestine exhibited a myofibroblast phenotype. In female mouse recipients of male bone marrow grafts, we observed colocalisation of Y chromosomes and clusters of newly formed marrow derived myofibroblasts. While few of these were present at seven days after bone marrow transplantation, they were numerous at 14 days, and by six weeks entire columns of pericryptal myofibroblasts could be seen running up the sides of crypts in both the small intestine and colon. These columns appeared to extend into the villi in the small intestine. Within the intestinal lamina propria, these Y chromosome positive cells were negative for the mouse macrophage marker F4/80 antigen and CD34. Conclusions: Bone marrow derived pericryptal myofibroblasts were present in the mouse intestine following irradiation and bone marrow transplant, and in the intestines of human patients suffering graft versus host disease following a bone marrow transplant. Our data indicate that bone marrow cells contribute to the regeneration of intestinal myofibroblasts and epithelium after damage, and we suggest that this could be exploited therapeutically.

Multiple Organ Engraftment by Bone‐Marrow‐Derived Myofibroblasts and Fibroblasts in Bone‐Marrow‐Transplanted Mice

Myofibroblasts are ubiquitous cells with features of both fibroblasts and smooth muscle cells. We suggest that the bone marrow can contribute to myofibroblast populations in a variety of tissues and that this is exacerbated by injury. To assess this, female mice were transplanted with male bone marrow and the male cells were tracked throughout the body and identified as myofibroblasts. Skin wounding and paracetamol administration were used to assess whether myofibroblast engraftment was modulated by damage. Following radiation injury, a proportion of myofibroblasts in the lung, stomach, esophagus, skin, kidney, and adrenal capsule were bone‐marrow derived. In the lung, there was significantly greater engraftment following paracetamol administration (17% versus 41% p < 0.005). Bone‐marrow‐derived fibroblasts were also found. We suggest that bone marrow contributes to a circulating population of cells and, in the context of injury, these cells are recruited and contribute to tissue repair.

Intestinal stem cells.

The intestinal tract has a rapid epithelial cell turnover, which continues throughout life. The process is regulated and maintained by a population of stem cells, which give rise to all the intestinal epithelial cell lineages. Studies in both the mouse and the human show that these cells are capable of forming clonal crypt populations. Stem cells remain hard to identify, however it is thought that they reside in a ‘niche’ towards the base of the crypt and their activity is regulated by the paracrine secretion of growth factors and cytokines from surrounding mesenchymal cells. Stem cell division is usually asymmetric with the formation of an identical daughter stem cell and committed progenitor cells. Progenitor cells retain the ability to divide until they terminally differentiate. Occasional symmetric division produces either 2 daughter cells with stem cell loss, or 2 stem cells and eventual clone dominance. This stochastic extinction of stem cell lines with eventual dominance of one cell line is called ‘niche succession’. The discovery of plasticity, the ability of stem cells to engraft into, and in some cases replace the function of damaged host tissues has generated a large amount of scientific and clinical interest: however the concept remains controversial and is still a subject of hot debate. Studies are beginning to identify the complex molecular, genetic and cellular pathways underlying stem cell function such as Wnt signalling, bone morphogenetic protein (BMP) and Notch/Delta pathways. The derangement of these pathways within stem cells plays an integral part in the development of malignancy within the intestinal tract.

STEM CELL IN GASTROINTESTINAL STRUCTURE AND NEOPLASTIC DEVELOPMENT

Stem cells are primitive cells located in a specialised mesenchymal “niche” that lack expression of any definitive markers of lineage commitment and are therefore difficult to define and identify. Stem cells maintain their capacity for limitless self replication throughout the lifetime of their host, and can also divide to produce daughter cells, committed to the formation of every adult cell lineage within their tissue of origin. The stem cells of the gastrointestinal tract remain unidentified which has led to many conflicting hypotheses as to their precise nature and function. For example, the numbers and location of stem cells in the intestinal crypts and gastric glands have never been conclusively proven and, consequently, the clonal origins of these structures under normal circumstances and in neoplasia are clouded issues. The morphological events of gastrointestinal carcinoma formation are hotly debated, with two main conflicting hypotheses of the mechanisms of expansion of a mutated stem cell clone. However, with the emergence of the molecular pathways governing gastrointestinal stem cell function, and the identification of putative intestinal molecular stem cell markers, such as Musashi-1, comes a clearer insight into the properties of the gastrointestinal stem cell. Adult stem cells from several tissues can leave their niche and engraft into extraneous tissues, including the gastrointestinal mucosa and underlying mesenchyme, and transform to produce adult cell lineages common to these foreign environments. This process is optimal when the requirement for regeneration is enhanced (that is, in diseased or damaged tissue) and indeed, the contribution of transplanted bone marrow stem cells to intestinal myofibroblasts is significantly upregulated in colitis. However, adult stem cell plasticity has recently been disparaged by reports suggesting that stem cells spontaneously fuse with indigenous adult cells to form a diploid cell with an aberrant karyotype, and it is important to investigate if bone marrow …

Bone marrow cells engraft within the epidermis and proliferate in vivo with no evidence of cell fusion.

In adults, bone marrow‐derived cells (BMDC) can contribute to the structure of various non‐haematopoietic tissues, including skin. However, the physiological importance of these cells is unclear. This study establishes that bone marrow‐derived epidermal cells are proliferative and, moreover, demonstrates for the first time that BMDC can localize to a known stem cell niche: the CD34‐positive bulge region of mouse hair follicles. In addition, engraftment of bone marrow cells into the epidermis is significantly increased in wounded skin, bone marrow‐derived keratinocytes can form colonies in the regenerating epidermis in vivo, and the colony‐forming capacity of these cells can be recapitulated in vitro. In some tissues this apparent plasticity is attributed to differentiation, and in others to cell fusion. Evidence is also provided that bone marrow cells form epidermal keratinocytes without undergoing cell fusion. These data suggest a functional role for bone marrow cells in epidermal regeneration, entering known epidermal stem cell niches without heterokaryon formation. Copyright

Colonic subepithelial myofibroblasts in mucosal inflammation and repair : contribution of bone marrow-derived stem cells to the gut regenerative response

mesenchymal cells that exhibit the ultrastructural features of both fibroblasts and smooth muscle cells, and they are characterized by positive immunoreactivity for a-smooth muscle actin (a-SMA) and vimentin, but negative immunoreactivity for desmin (Figs. 1A and B).4–6,7 SEMFs are distinguished from smooth muscle cells, which express a-SMA but are negative for vimentin. The location of SEMFs below the basement membrane suggests that these cells may play a role in the regulation of a number of epithelial cell functions, such as epithelial proliferation and differentiation and/ or extracellular matrix (ECM) metabolism, affecting the growth of the basement membrane. Intestinal SEMFs are classified as members of a family of functionally related cells, including hepatic Ito cells, glomerular mesangial cells, and orbital and synovial fibroblasts.8 Previous literature describes the existence of SEMFs as a syncytium that extends throughout the lamina propria of the gut, merging with the pericytes surrounding the blood vessels.5,9 In the region of the crypts, SEMFs are oval and scaphoid in appearance and overlap like shingles on a roof.5,10 They are attached to each other via both gap junctions and adherens junctions. In the upper regions of the crypts and the small intestinal villi, the SEMFs display a stellate morphology.9,11

The gastrointestinal stem cell

Abstract.   The longevity of adult stem cells, and their potential for vast tissue regeneration, makes them a focal point of current research and debate, with future aspirations for the use of stem cells in the treatment of a number of human pathological conditions. Due to the rapid rate of cell turnover in the gastrointestinal tract, the stem cells of this tissue are amongst the most assiduous in the body, although they remain unidentified to this day due to their immature, undifferentiated phenotype. However, our knowledge of the mechanisms regulating gastrointestinal stem cell function is evolving, with the identification of putative cellular markers and the elucidation of signalling pathways which regulate cell behaviour in the normal and neoplastic gastrointestinal tract. This review describes the fundamental properties of the gastrointestinal stem cell including: (i) their number, location and origins, (ii) their primary function of deriving gastrointestinal cell lineages and maintaining tissue homeostasis, (iii) the acquisition of gastrointestinal cell lineages from adult stem cells of extraneous tissues and the consequences of this in a therapeutic context, and (iv) the genetic and morphological phenomena surrounding neoplastic transformation in the gastrointestinal tract.

Plastic adult stem cells: will they graduate from the school of hard knocks?

Notwithstanding the fact that adult bone marrow cell engraftment to epithelial organs seems a somewhat uncommon event, there is no doubt it does occur, and under appropriate conditions of a strong and positive selection pressure these cells will expand clonally and make a significant contribution to tissue replacement. Likewise, bone-marrow-derived cells can be amplified in vitro and differentiated into a multitude of tissues. These in essence are the goals of regenerative medicine using any source of stem cells, be it embryonic or adult. Despite such irrefutable evidence of what is possible, a veritable chorus of detractors of adult stem cell plasticity has emerged, some doubting its very existence, motivated perhaps by more than a little self-interest. The issues that have led to this state of affairs have included the inability to reproduce certain widely quoted data, one case where the apparent transdifferentiation was due to contamination of the donor tissue with haematopoietic cells and, most notoriously, extrapolating from the behaviour of embryonic stem cells to suggest that adult bone marrow cells simply fuse with other cells and adopt their phenotype. While these issues need resolving, slamming this whole new field because not everything is crystal clear is not good science. The fact that a phenomenon is quite rare in no way mitigates against its very existence: asteroid collisions with the Earth are rare, but try telling the dinosaurs they do not occur! When such events do occur (transdifferentiation or collision), they certainly can make an impact.