Stem cell niche

Stem cell niche is a phrase loosely used in the scientific community to describe the microenvironment in which stem cells are found, which interacts with stem cells to regulate stem cell fate. The word 'niche' can be in reference to the in vivo or in vitro stem cell microenvironment. During embryonic development, various niche factors act on embryonic stem cells to alter gene expression, and induce their proliferation or differentiation for the development of the fetus. Within the human body, stem cell niches maintain adult stem cells in a quiescent state, but after tissue injury, the surrounding micro-environment actively signals to stem cells to either promote self renewal or differentiation to form new tissues. Several factors are important to regulate stem cell characteristics within the niche: cell-cell interactions between stem cells, as well as interactions between stem cells and neighbouring differentiated cells, interactions between stem cells and adhesion molecules, extracellular matrix components, the oxygen tension, growth factors, cytokines, and physiochemical nature of the environment including the pH, ionic strength (e.g. Ca 2+ concentration) and metabolites, like ATP, are also important. The stem cells and niche may induce each other during development and reciprocally signal to maintain each other during adulthood.

Scientists are studying the various components of the niche and trying to replicate the in vivo niche conditions in vitro. This is because for regenerative therapies, cell proliferation and differentiation must be controlled in flasks or plates, so that sufficient quantity of the proper cell type are produced prior to being introduced back into the patient for therapy.

Human embryonic stem cells are often grown in fibroblastic growth factor-2 containing, fetal bovine serum supplemented media. They are grown on a feeder layer of cells, which is believed to be supportive in maintaining the pluripotent characteristics of embryonic stem cells. However, even these conditions may not truly mimic in vivo niche conditions.

Adult stem cells remain in an undifferentiated state throughout adult life. However, when they are cultured in vitro, they often undergo an 'aging' process in which their morphology is changed and their proliferative capacity is decreased. It is believed that correct culturing conditions of adult stem cells needs to be improved so that adult stem cells can maintain their stemness over time.

A Nature Insight review defines niche as follows:

""Stem-cell populations are established in 'niches' — specific anatomic locations that regulate how they participate in tissue generation, maintenance and repair. The niche saves stem cells from depletion, while protecting the host from over-exuberant stem-cell proliferation. It constitutes a basic unit of tissue physiology, integrating signals that mediate the balanced response of stem cells to the needs of organisms. Yet the niche may also induce pathologies by imposing aberrant function on stem cells or other targets. The interplay between stem cells and their niche creates the dynamic system necessary for sustaining tissues, and for the ultimate design of stem-cell therapeutics...The simple location of stem cells is not sufficient to define a niche. The niche must have both anatomic and functional dimensions""

- David T. Scadden

History
Though the concept of stem cell niche was prevailing in vertebrates, the first characterization of stem cell niche in vivo was worked out in Drosophila germinal development.

The Germline Stem Cell niche
Germline stem cells (GSCs) are found in organisms that continuously produce sperm and eggs until they are sterile. These specialized stem cells reside in the GSC niche, the initial site for gamete production, which is composed of the GSCs, somatic stem cells, and other somatic cells. In particular, the GSC niche is well studied in the genetic model organism Drosophila melanogaster and has provided an extensive understanding of the molecular basis of stem cell regulation.



GSC Niche in Drosophila ovaries
In Drosophila melanogaster, the GSC niche resides in the anterior-most region of each ovariole, known as the germarium. The GSC niche consists of necessary somatic cells-terminal filament cells, cap cells, escort cells, and other stem cells which function to maintain the GSCs. The GSC niche holds on average 2-3 GSCs, which are directly attached to somatic cap cells and Escort stem cells, which send maintenance signals directly to the GSCs. GSCs are easily identified through histological staining against vasa protein (to identify germ cells) and 1B1 protein (to outline cell structures and a germline specific fusome structure). Their physical attachment to the cap cells is necessary for their maintenance and activity. A GSC will divide asymmetrically to produce one daughter cystoblast, which then undergoes 4 rounds of incomplete mitosis as it progresses down the ovariole (through the process of oogenesis) eventually emerging as a mature egg chamber; the fusome found in the GSCs functions in cyst formation and may regulate asymmetrical cell divisions of the GSCs. Because of the abundant genetic tools available for use in Drosophila melanogaster and the ease of detecting GSCs through histological stainings, researchers have uncovered several molecular pathways controlling GSC maintenance and activity.

Local signals
The Bone Morphogenetic Protein (BMP) ligands Decapentaplegic (Dpp) and Glass-bottom-boat (Gbb) ligand are directly signaled to the GSCs, and are essential for GSC maintenance and self-renewal. BMP signaling in the niche functions to directly repress expression of Bag-of-marbles(Bam) in GSCs, which is up-regulated in developing cystoblast cells. Loss of function of dpp in the niche results in de-repression of Bam in GSCs, resulting in rapid differentiation of the GSCs. Along with BMP signaling, cap cells also signal other molecules to GSCs: Yb and Piwi. Both of these molecules are required non-autonomously to the GSCs for proliferation-piwi is also required autonomously in the GSCs for proliferation. Interestingly, in the germarium, BMP signaling has a short-range effect, therefore the physical attachment of GSCs to cap cells is important for maintenance and activity.

Physical attachment of GSCs to cap cells
The GSCs are physically attached to the cap cells by Drosophila E-cadherin (DE-cadherin) adherens junctions and if this physical attachment is lost GSCs will differentiate and lose their identity as a stem cell. The gene encoding DE-cadherin, shotgun (shg), and a gene encoding Beta-catenin ortholog, armadillo, control this physical attachment. A GTPase molecule, rab11, is involved in cell trafficking of DE-cadherins. Knocking out rab11 in GSCs results in detachment of GSCs from the cap cells and premature differentiation of GSCs. Additionally, zero population growth (zpg), encoding a germline-specific gap junction is required for germ cell differentiation.

Systemic signals regulating GSCs
Both diet and insulin-like signaling directly control GSC proliferation in Drosophila melanogaster. Increasing levels of Drosophila insulin-like peptide (DILP) through diet results in increased GSC proliferation. Up-regulation of DILPs in aged GSCs and their niche results in increased maintenance and proliferation. It has also been shown that DILPs regulate cap cell quantities and regulate the physical attachment of GSCs to cap cells.

Renewal mechanisms
There are two possible mechanisms for stem cell renewal, symmetrical GSC division or de-differentiation of cystoblasts. Normally, GSCs will divide asymmetrically to produce one daughter cystoblast, but it has been proposed that symmetrical division could result in the two daughter cells remaining GSCs. If GSCs are ablated to create an empty niche and the cap cells are still present and sending maintenance signals, differentiated cystoblasts can be recruited to the niche and de-differentiate into functional GSCs.

Stem cell aging
As the Drosophila female ages, the stem cell niche undergoes age-dependent loss of GSC presence and activity. These losses are thought to be caused in part by degradation of the important signaling factors from the niche that maintains GSCs and their activity. Progressive decline in GSC activity contributes to the observed reduction in fecundity of Drosophila melanogaster at old age; this decline in GSC activity can be partially attributed to a reduction of signaling pathway activity in the GSC niche. It has been found that there is a reduction in Dpp and Gbb signaling through aging. In addition to a reduction in niche signaling pathway activity, GSCs age cell-autonomously. In addition to studying the decline of signals coming from the niche, GSCs age intrinsically; there is age-dependent reduction of adhesion of GSCs to the cap cells and there is accumulation of Reactive Oxygen species (ROS) resulting in cellular damage which contributes to GSC aging. There is an observed reduction in the number of cap cells and the physical attachment of GSCs to cap cells through aging. Shg is expressed at significantly lower levels in an old GSC niche in comparison to a young one.

GSC Niche in Drosophila testes
In the Drosophila testis the niche consists of the hub cells which support two adjacent stem cell populations: the germline stem cells and the somatic cyst progenitor cells.

A. Hematopoietic stem cell niche
Vertebrate hematopoietic stem cells niche in the bone marrow is formed by cells subendoosteal osteoblasts, sinusoidal endothelial cells and bone marrow stromal (also sometimes called reticular) cells which includes a mix of fibroblastoid, monocytic and adipocytic cells.

B. Hair follicle stem cell niche
The bulge area at the junction of arrectores pili muscle to the hair follicle sheath has been shown to host the skin stem cells with maximum span of developmental potential. There cells are maintained by signaling in concert with niche cells - signals include paracrine (e.g. sonic hedgehog), autocrine and juxtacrine signals.

C. Intestinal stem cell niche
The subepithelial fibroblast/myofibroblast network which surround the intestinal crypts constitute the niche.