[Frontiers in Bioscience 12, 2631-2645, January 1, 2007] What triggers cell-mediated mineralization? Leonie F.A. Huitema1,2, Arie B. Vaandrager1 1 Department of Biochemistry and Cell Biology, Faculty of Veterinary Medicine, and Graduate School of Animal Health, Utrecht University, Utrecht, The Netherlands, 2 Department of Equine Sciences, Faculty of Veterinary Medicine, and Graduate School of Animal Health, Utrecht University, Utrecht, The Netherlands TABLE OF CONTENTS 1. Abstract 2. Introduction 3. Different types of mineralization 3.1. Intramembranous ossification 3.2. Endochondral ossification 3.3. Pathological calcification 4. Mechanism of cell-mediated mineral deposition 4.1. The role of matrix vesicles 4.2. The role of cell death 4.3. The role of nucleating proteins 4.4. The role of mineralization inhibitors 5. Perspectives 6. Acknowledgement 7. References 1. ABSTRACT 2. INTRODUCTION Mineralization is an essential requirement for Mineralization is an essential requirement for normal skeletal development, but under certain normal skeletal development, which is generally pathological conditions organs like articular cartilage and accomplished through the function of two cell types, cardiovascular tissue are prone to unwanted mineralization. osteoblasts and chondrocytes (1). In contrast, soft Recent findings suggest that the mechanisms regulating tissues do not mineralize under normal conditions. skeletal mineralization may be similar to those regulating However, under certain pathological conditions some pathological mineralization. In general, three forms of cell- tissues like articular cartilage and cardiovascular tissues mediated mineralization are recognized in an organism: are prone to mineralization (Table 1) (2;3). intramembranous ossification, endochondral ossification Mineralization of articular cartilage contributes to and pathological mineralization. This review summarizes significant morbidity because of its association with recent work that tried to elucidate how cell-mediated joint inflammation and worsening of the progression of mineralization is initiated and regulated. To explain osteoarthritis (4). Articular cartilage calcification occurs mineralization, several theories have been proposed. One in association with aging, degenerative joint disease theory proposes that mineralization is initiated within (e.g. osteoarthritis), some genetic disorders and various matrix vesicles (MVs). A second, not mutually exclusive, metabolic disorders (4-7). Similarly, arterial theory proposes that phosphate induces apoptosis, and that calcification occurs with advanced age, atherosclerosis, apoptotic bodies nucleate crystals composed of calcium and metabolic disorders, including end stage renal disease phosphate. A third theory suggests that mineralization is and diabetes mellitus, and some genetic disorders (8). mediated by certain non-collagenous proteins, which Arterial calcification contributes to hypertension and associate with the extracellular matrix. Regardless of the increased risks of cardiovascular events, leading to way mineralization is initiated, the organism also actively morbidity and mortality (9). While pathological inhibits mineralization by specific proteins and removal of mineralization has long been considered to result from a an inhibitor may also induce mineralization. Although mere physiochemical precipitation of calcium and many studies greatly contributed to a better understanding phosphate, recent studies have provided evidence that of the mechanisms regulating cell-mediated mineralization, soft tissue mineralization is a regulated process, which many questions remain about the mechanisms that trigger has many similarities with bone formation (10;11). For cell-mediated mineralization and how this process is now it is unclear why soft tissues have the tendency to regulated. Further investigation is necessary to develop in mineralize. Therefore, the purpose of this review is to the future novel therapeutic strategies to prevent discuss the components involved in cell-mediated pathological mineralization. mineralization. 2631 Cell-mediated mineralization 3. DIFFERENT TYPES OF MINERALIZATION extracellular matrix of bone limits interstitial growth. To achieve rapid and directional growth in an organism, an 3.1. Intramembranous ossification intermediate structure (cartilage) that can function under Clavicles and bones in the regions of the high load, and at the same time generate space for new craniofacial skeleton develop by intramembranous bone formation, would then be very helpful (23). Generally ossification, the principle of which is shown in Figure 1, four zones are recognized during endochondral ossification panel 1. During intramembranous ossification, osteoblasts (Figure 1, panel 2A-D). The upper zone is the resting zone originating from mesenchymal cell condensations are (Figure 1, panel 2A), chondrocytes are small and dormant, responsible for bone and matrix deposition (12). During and mainly type II collagen is produced. Subsequently, in this mesenchymal cell condensation, high densities of the proliferative zone chondrocytes start to proliferate in mesenchymal cells aggregate and start to produce an vertical columns (Figure 1, panel 2B). The next zone is the extracellular matrix (13). Subsequently, mesenchymal cells hypertrophic zone (Figure 1, panel 2C), where the acquire the typical columnar shape of osteoblasts, increase chondrocytes start to enlarge, produce type X collagen, and the synthesis of alkaline phosphatase (APase), and begin to increase the synthesis of APase. Finally, hypertrophic secrete bone matrix, which consists mainly of type I chondrocytes induce mineralization (Figure 1, panel 2D), collagen (Figure 1, panel 1A). Osteoblasts than generate which is correlated with increased levels of Pi in the phosphate (Pi) by an increase of APase, which is necessary mineralization zone (24-26). Furthermore, terminally for bone formation (14;15). Numerous ossification centres differentiated chondrocytes in the growth plate are deleted then develop and eventually fuse (12). Finally, osteoblasts from the cartilage by programmed cell death (23). In this undergo apoptosis (~ 70 %) or terminally differentiate to final zone, osteoprogenitor cells and hemoatopoietic stem form osteocytes (~ 30%), which become entrapped in the cells arrive via the newly formed blood vessels, which mineralized bone matrix (14;16;17). Osteocytes are in fact penetrate through the transverse septa of mineralizing osteoblasts buried within the mineralized bone matrix hypertrophic chondrocytes. Osteoprogenitor cells beneath (Figure 1, panel 1B). Once embedded, they cease their the site of vascular invasion differentiate into osteoblasts ossification activity and communicate with each other and that aggregate on the surface of the calcified cartilage and with cells at the bone surface via a meshwork of cell deposit bone matrix (osteoid; Figure 1, panel 2E). Bone processes that run through canaliculi in the bone matrix resorbing osteoclasts start to remodel the newly formed (16). A key regulator of osteoblast differentiation and bone (27). function is the transcription factor core binding factor alpha 1 (Cbfa1) (12;18-21). Cbfa1 is targeted to the promotors of The transcription factor SOX9 is mainly several bone proteins, such as osteocalcin, bone expressed in resting and proliferating chondrocytes, while it sialoprotein, APase and type I collagen (18). No bone is switched off in hypertrophic chondrocytes (28). SOX9 is tissue is formed in Cbfa1 null mice, although a complete required for expression of cartilage specific extracellular cartilaginous skeleton is formed, indicating that Cbfa1 is matrix components, such as collagen types II, IX and XI not essential for chondrogenesis (19). (12;28). In humans, mutations in SOX9 result in a skeletal malformation syndrome called campomelic dysplasia To maintain bone structure and skeletal growth, (29;30). In addition, no chondrocyte specific markers are remodeling is necessary. The remodeling of bone that has expressed in SOX9 null cells in mouse chimeras (31). been formed by either intramembranous ossification or Interestingly, low Cbfa1 expression is detected in endochondral ossification (see below) consists of a strict hypertrophic chondrocytes, which suggests that coupling of bone resorption by osteoclasts and bone hypertrophic chondrocytes may undergo a phenotypic formation by osteoblasts that continue throughout life, with transition towards the osteoblastic phenotype (12). In a positive balance during growth and with a negative addition, in Cbfa1 null mice hypertrophic chondrocytes in balance during ageing. Osteoclasts are multinucleated giant the femur and humerus are absent (21). The hypothesis that cells formed from hemopoietic precursors of the monocyte chondrocytes transdifferentiate into osteoblasts is supported and macrophage series (22). They attach to the bone by microscopic examinations of cells present at the surface by sealing a resorbing compartment that they chondro-osseous junction in the growth plate, which have acidify by secreting H+ ions. This will than dissolve the suggested that indeed chondrocytes differentiate into bone mineral, and subsequently expose the organic matrix osteoblasts (32-35). Based on this observation, it is to proteolytic enzymes that degrade it (22). speculated that there are two subpopulations of chondrocytes, one which becomes apoptotic, while the 3.2. Endochondral ossification other population trans-differentiates