Journal of Stem Cells Research Development & Therapy Category: Medical Type: Research Article
Perlecan Delineates Stem Cell Niches in Human Foetal Hip, Knee and Elbow Cartilage Rudiments and Has Potential Roles in the Regulation of Stem Cell Differentiation
- Susan M Smith1, James Melrose2*
- 1 Raymond Purves Bone And Joint Research Laboratory, Kolling Institute Northern Sydney Local Health District, St Leonards, Australia
- 2 Raymond Purves Bone And Joint Research Laboratory, Kolling Institute Northern Sydney Local Health District, Sydney Medical School, University Of New South Wales, St Leonards, Australia
*Corresponding Author:James Melrose
Raymond Purves Bone And Joint Research Laboratory, Kolling Institute Northern Sydney Local Health District, Sydney Medical School, University Of New South Wales, St Leonards, Australia
Received Date: Sep 26, 2016 Accepted Date: Nov 07, 2016 Published Date: Nov 21, 2016
The stem cell is a specialized cell type that undergoes self-renewal to maintain a self-sustaining undifferentiated cell population within the stem cell niche .The specialized microenvironment of the stem cell niche provides chemical and physical feedback cues which maintain the stem cells in a slowly recycling state of self renewal . The stem cell niche acts as a reserve of cells for the replenishment of damaged or dead cells with ageing or in disease or after traumatic damage to the tissue maintaining viable cell numbers and tissue homeostasis.
Perlecan (HSPG2) is a modular proteoglycan interactive with diverse Extracellular Matrix (ECM) and cellular components [4-6]. Perlecan is a major HS substituted proteoglycan component of basement membranes and vascular tissues  and is also present in a vascular tissues such as articular cartilage, intervertebral disc, meniscus and tendon where it is present as a hybrid HS-CS proteoglycan [8-11]. Interactions between perlecan and structural and cell adhesive glycoproteins such as laminin, fibronectin, WARP (von Willebrand A domain-Related Protein), PRELP (Proline/Arginine-Rich End Leucine-Rich repeat protein), type IV collagen, type VI collagen, fibrillin-1, Latent Transforming growth factor Β1 Binding Protein-2 (LTBP2), α2β1 and α5β1 integrins stabilize the ECM [12-14]. The HS chains of perlecan domain-1 bind Fibroblast Growth Factors (FGF), Vascular Endothelial Cell Growth Factor (VEGF), Platelet Derived Growth Factor (PDGF), BMP family members, Wnt (wingless-type MMTV [Mouse Mammary Tumor Virus]) integration site related proteins, hedgehog proteins regulating cellular adhesion, proliferation, differentiation, ECM synthesis, chondrogenesis and tissue morphogenesis during skeletal development [8,14-17].
Perlecan is a component of a number of stem cell niches located in the allantois, intestine, bone marrow, limbal epithelium, nervous system and elbow perichondrium [1,18-23]. Perlecan has a role in the isolation of the stem cells from ECM components outside the niche which could result in activation, migration and development of differentiated stem cell lineages . FGF-2 is a major ligand for perlecan with roles in the long term viability of the niche stem cells [22,23]. Perlecan displays anti-adhesive properties to stem cells in bone marrow contributing to their isolation from extrinsic influences . ECM components extrinsic to the niche are known to exert directive cues on cellular proliferation, adhesion and differentiation of stem cells . Perlecan colocalises with type VI collagen pericellularly in chondrocytes and intervertebral disc cells [25,26]. Atomic Force Microscopy has demonstrated that perlecan provides compliancy to the type VI collagen pericellular matrix [27,28] and modulates tensile stresses otherwise transmitted to cells within tensional and weight bearing connective tissues through type VI collagen . Perlecan also provides an adhesive interface between the chondrocyte and its surrounding ECM . The cartilage rudiments act as a transient developmental scaffold transformed by chondroprogenitor cell populations during endochondral ossification resulting in elongation of the long bones and extension of the axial skeleton .
MATERIALS AND METHODS
The stem cell niche and ECM components in macroscopic views of foetal hips
Higher power views of hip stem cell niches
Stem cell niches in foetal knee rudiment cartilages
Stem cell niches have characteristic morphologies and distributions in the surface region of the cartilage rudiments of joints
Type I collagen immunolocalizes to the surface regions of newborn hip joints
The Wnt family of growth factors are ancient metazoan proteins which are conserved throughout vertebrate and invertebrate evolution. Wnt proteins (the name Wnt is a fusion of the name for the Drosophila segment polarity gene ‘wingless’ and its vertebrate homolog, ‘integrated or int-1’) regulate stem cell fate, migration, proliferation and self-renewal [40-44]. Extracellular Wnt protein induces a number of intracellular signal transduction pathways of importance in stem cell differentiation, tissue development and repair [40,41,43-45]. Perlecan regulates bi-directional Wnt signaling in Drosophila[40,46-49] and growth factor signaling in C.elegans  and also stabilizes the matrix surrounding the stem cell niche. Recent studies in the human foetal elbow showed that perlecan localized with type I collagen surrounding the stem cell niches . Type I collagen normally conveys tensile forces in tissues, AFM studies have shown that perlecan provided compliancy to the type VI collagen pericellular matrix surrounding chondrocytes where these components were co-localized as in IVD cells [25,27,28]. Thus the perlecan, which delineates the stem cell niches observed in the present study, may modulate the propagation of tensile forces to the niche stem cells from their external micro-environment. Knockdown of perlecan lowers in-situ cell and matrix stiffness in developing cartilage . The stem cell microenvironment is influenced by matrix stiffness, and mechanical forces experienced from fluid shear, compression and tension, and these regulate the proliferation and differentiation of stem cells [51-54]. MSCs sense biomechanical forces through primary cilia which are colocalised with calcium ion channels resulting in responsive changes in mechanosensitive ion channel associated proteins such as transient receptor potential melastatin 7, a mechanosensitive plasma membrane calcium channel protein, and changes in their cytoskeleton . An influx of Ca2+ ions into MSCs has attendant effects on the formation of intracellular signaling molecules such as inositol triphosphate, changes in the actin/actomyosin cytoskeleton and the activation of associated signaling pathways. The associated change in cell shape and co-ordination of focal adhesions acts as a primer for MSC migration. MSCs are also capable of secreting active heparanase-1 which degrades the HS side chains of perlecan . Heparanase released from MSCs also activates Integrin beta1/HIF-2alpha/Flk-1 signaling and stem cell migration [55,56]. Many stem cell populations utilize the SDF-1/CXCR4 axis to effect migration [57-61]. The release of progenitor cell populations from their niches by heparanase-1 is consistent with emerging roles for heparanase-1 and 2 in the promotion of wound repair [62,63]. Live cell imaging of stem cell niches where the niche is de-stressed results in a change in the stem cell quiescent state to a migratory phenotype which can home to sites of tissue damage [64,65]. This is consistent with roles proposed for biomechanical forces and ECM directive cues in the regulation of stem cell phenotype in-vivo and the promotion of wound healing [3,66-69].
The stem cell arrangements visualized in the present study were present in the rudiment margins and were clearly distinguishable from blood vessels in the stromal tissues. Like other cartilages, the rudiments contain anti-angiogenic proteins refractory to the penetration of blood vessels [14,70]. An earlier confocal study of the human foetal elbow demonstrated perlecan positive niches in the outer regions of the perichondrium along the shaft of the long bones . Activated stem cells displaying the CS sulphation motifs 4C3, 3B3(-) and 7D4 were closely associated with these perlecan positive niches . Chondroprogenitor stem cells have also been identified in the surface regions of the presumptive articular cartilage of the knee joint following joint cavitation [71,72].
With an increased awareness of the role of extrinsic forces on stem cell regulation in situ , regenerative approaches are now being developed using bio-scaffolds with defined tunable tensional properties to modulate stem cell differentiation in vitro . Decellularised tissues with their ECM, biochemical and structural cues intact in the stem cell niche are also being evaluated for their ability to modulate stem cell behaviour in culture and modify stem cell proliferation and differentiation . The stiffness of biomaterials is an important determinant which drives stem cell proliferation and differentiation . Evaluation of matrices synthesized by human fibroblasts has shown an increase in stem cell proliferation and differentiation, down-regulation of adipogenesis and osteogenesis but promotion of chondrogenesis . In the human foetal elbow the perichondrial stem cell niche is contained in a matrix layed down by fibroblastic cells in the outer regions of the perichondrium. These regions of the perichondrium are rich in type I collagen  and are similar to the areas of type I collagen deposition between the rudiment and associated stromal tissues observed in the present study and which also contain stem cell niches.
Matricryptins and matrikines have received considerable attention as prospective agents for tissue repair through their ability to modify stem cell behavior stimulating proliferation and differentiation , some matrikines stimulate stem cell migration. Chondroitin Sulphate (CS) is another Glycosaminoglycan (GAG) which is considered indispensable for stem cell pluripotency and stem cell differentiation , We are now in an exciting era in regenerative medicine and may be optimistic that armed with greater knowledge on how stem cells are regulated in their niches we may someday be able to manipulate these to improve their replicative, differentiative and migratory properties in situ to obtain the critical numbers of stem cells of specific cell lineages required at defect sites for regenerative applications. It is to be expected that a greater understanding of how stem cells target damaged tissues will also greatly improve the utility of stem cells as therapeutic agents.
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Citation:Smith SM, Melrose J (2016) Perlecan Delineates Stem Cell Niches in Human Foetal Hip, Knee and Elbow Cartilage Rudiments and Has Potential Roles in the Regulation of Stem Cell Differentiation. J Stem Cell Res Dev Ther 3: 009.
Copyright: © 2016 Susan M Smith, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.