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- Herausgeber: Bentham Science Publishers
- Kategorie: Wissenschaft und neue Technologien
- Serie: Frontiers in Stem Cell and Regenerative Medicine Research
- Sprache: Englisch
Frontiers in Stem Cell and Regenerative Medicine Research Volume 10
[Edited for Volume 10]
Stem cell and regenerative medicine research is an important area of clinical research which promises to change the face of medicine as it will be practiced in the years to come. Challenges in the 21st century to combat diseases such as cancer, Alzheimer’s disease and retinal disorders, among others, may well be addressed employing stem cell therapies and tissue regeneration techniques. Frontiers in Stem Cell and Regenerative Medicine Research brings updates on multidisciplinary topics relevant to stem cell research and their application in regenerative medicine. The series is essential reading for researchers seeking updates in stem cell therapeutics and regenerative medicine.
Volume 10 includes 5 chapters on these topics:
-Novel drugs and their stem cell-based targets for osteoporosis: challenges and proceedings
-The role of cancer stem cells in disease progression and therapy resistance
-Stem cells from human exfoliated deciduous teeth in tissue regeneration
-The fate of toxicological studies: from animal models to stem cell-based methods
-Effect of material properties on differentiation of mesenchymal stem cells
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Seitenzahl: 311
Veröffentlichungsjahr: 2022
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PREFACE
The tenth volume of ‘Frontiers in Stem Cell and Regenerative Medicine Research’ presents important recent developments in this fast-growing field.
Edda et al. in their chapter focus on the differentiation and signaling pathways of osteoblasts and osteoclasts. Pillai et al. discuss the role of cancer stem cells (CSCs) in therapy resistance with detailed molecular mechanisms. Ponnuraj et al. in the third chapter of the book present the potential use of dental stem cells, particularly stem cells from human exfoliated deciduous teeth (SHED) for tissue regeneration. Masoud et al. give an overview of toxicological studies from animal models to stem cell-based methods. In the last chapter of the book, Feng et al. discuss the possible mechanisms proposed to explain how certain factors affect the differentiation of MSCs.
We owe our special thanks to all the contributors for their valuable contribution in bringing together the tenth volume of this book series. We are thankful to the efficient team of Bentham Science Publishers for the timely efforts made by the editorial personnel, especially Mr. Mahmood Alam (Editorial Director), Mr. Obaid Sadiq (in-charge Books Department) and Ms. Asma Ahmed (Manager Publications).
List of Contributors
Novel Drugs and Their Stem Cell-based Targets for Osteoporosis: Challenges and Proceedings
Basma El Khaldi-Hansen1,Markus Witzler1,Margit Schulze1,Patrick F. Ottensmeyer1,Juliana Baranova2,Tobiasch Edda1,*Abstract
The aging of the population goes along with age-related diseases, such as osteoporosis, a disorder of bone remodeling. Bone homeostasis is maintained by bone-building osteoblasts and bone-resorbing osteoclasts. During osteoporosis, this balance is disturbed by augmented bone resorption, which leads to an increased risk of bone fractures, with potentially lethal consequences. To battle this, various drugs with different target sites are used. Currently, the gold standard osteoporosis medications are the bisphosphonates, which induce apoptosis of the osteoclasts. However, bisphosphonates may cause adverse effects, such as osteonecrosis of the jawbone. Other available drugs for bone metabolism disorders also exhibit undesired side- and off-target effects of varying severity. Thus, new potential drug candidates are being developed, some already reached phase II or phase III clinical trials. The modes of action of these drug candidates range from anti-resorptive to osteoanabolic therapies. Osteoanabolic therapies stimulate the formation of bone, while anti-resorptive therapies decrease the bone resorption. Most anti-resorptive therapies induce apoptosis of the osteoclasts, which negatively affects the osteoblasts as well since there is a feedback loop between these two cell types. A better understanding of bone homeostasis, beginning with the differentiation pathways of mesenchymal stem cells towards osteoblasts and hematopoietic stem cells towards osteoclasts and their interactions during these differentiation processes are of increasing interest for future osteoporosis treatments with minimal side effects. This chapter focuses on the differentiation and signaling pathways of osteoblasts and osteoclasts. In addition, new osteoporosis drugs are illuminated from the biological and the chemical point of view. Their progress from bench to bedside is presented.
OSTEOPOROSIS AND ITS’ CHALLENGES
Osteoporosis is a mainly age-related disease, characterized by a dysregulation of bone resorption and formation, which increases the risk of fractures. The World Health Organization (WHO) classifies osteoporosis in the ten most often occurring diseases worldwide, that affected about 200 million people and caused nearly nine million fractures in 2000 [1]. The risk of suffering a fracture of the wrist, hip, or vertebra within the lifetime is about 30-50% for women and 15-30% for men in developed countries [2]. In 2017, the costs for the treatment of osteoporotic patients were estimated at 37.5 billion € in EU [3] and 22 billion $, in 2018, in the USA [4] and are expected to increase.
The major setback in osteoporosis management is its silent nature with no obvious symptoms during early phases of the disease progression, which makes it difficult to diagnose before the first fracture occurs. A closer look at the healthcare cost distribution in the EU, where only 5% is spent on prevention and 95% on fracture repair and long-term treatment, confirms the severity of the problem. Moreover, the International Osteoporosis Foundation estimated that only 25% of all osteoporosis cases are reported [3]. One possible way to improve early osteoporosis diagnosis is to implement the screening of the bone mass by means of dual-X-ray absorptiometry in the risk groups such as post-menopausal women and the elderly. The bone mass of a patient with osteoporosis is equal or less than -2.5 standard deviations of the average bone mass of young and healthy adults between the age of 20 and 29 [5]. Alterations in the bone mass are also indicative of other bone remodeling disorders.
Osteoporosis can be divided into primary and secondary osteoporosis. Both types are not curable nowadays and the only available therapeutic approach is to slow down the loss of the bone mass. Primary osteoporosis is defined by no direct or singular known cause to the disease [6] and is further classified as the idiopathic juvenile osteoporosis, which affects children; postmenopausal and senile osteoporosis, that occur mainly in elderly people. The latter case is associated with the loss of estrogens and androgens, among other contributing factors [7]. These hormonal changes alter several processes within the body and lead to a decreased defense against oxidative stress (OS).
In order to protect cells against OS, mitochondria activate the expression of members from the transcription factor sub-class FoxO. For example, FoxO3 was proven to have a positive effect on osteoblast survival during OS [8]. In addition, the FoxO transcription factors bind β-catenin, which is a co-activator of FoxO transcription, thus enhancing the process in a fast-forward reaction [9]. Furthermore, it is an important transcription factor in the differentiation of multipotent mesenchymal stem cells (MSCs) towards osteoblasts [10]. This results in a competition between osteoblast survival and the generation of new osteoblasts under OS. Hence, the early phase of postmenopausal osteoporosis is marked by a loss of calcium of up to 200 mg/day in the first 3-4 years, which decreases to 45 mg/day after 5-10 years of osteoporosis [11].
Reviews by Fitzpatrick or Brown outline that secondary osteoporosis can occur due to nutritional or lifestyle factors, inflammatory causes, genetic disorders, or be induced by medical treatments [6, 12]. The relationship between prolonged or continuous medical treatments with proton pump inhibitors, selective serotonin receptor inhibitors, and other medications and secondary osteoporosis have been reviewed by Panday and colleagues [13]. Another class of drugs associated with secondary osteoporosis is the glucocorticoids and other corticosteroids. These drugs are used to suppress inflammations during chemotherapy, asthma, or allergic reactions. Notably, glucocorticoids can regulate the differentiation of MSCs towards osteoblasts under normal circumstances, but can also cause apoptosis of osteoblasts by inducing OS [14, 15]. When applied in high concentrations, glucocorticoids increase adipogenic differentiation to the disadvantage of osteogenic differentiation [16]. This effect is also mediated by an inhibition of Wnt signaling by the upregulation of Dickkopf-1 (DKK-1) [17].
FROM OSTEOGENIC LINEAGES TO BONE FORMATION AND RESORPTION
The Wnt signaling, which is negatively affected by glucocorticoids during secondary osteoporosis, is thought to be a key pathway of osteogenesis. In the following section, the significance of Wnt, BMP, Notch, and Hedgehog signaling pathways in osteogenesis, as well as the differentiation of hematopoietic stem cells (HSCs) towards osteoclasts (osteoclastogenesis), is presented.
Development of Osteoblasts from Mesenchymal Stem Cells
Mesenchymal stem cells (MSCs) are of high interest for tissues and organ bioengineering approaches due to their accessibility and broad differentiation potential, including the osteogenic lineage [18-20]. According to the Internatio-nal Society for Cellular Therapy, MSCs are defined by their adherence to plastic under standard culture condition, the expression of at least three markers CD105, CD90, and CD73 and the lack of expression of several surface molecules, namely CD45, CD34, CD79α or CD19, CD14 or CD11b, and HLA-DR. In addition, the cells must be able to differentiate towards the osteogenic, adipogenic, and chondrogenic lineage in vitro, as demonstrated by specific stainings [21]. MSCs can be isolated from various tissues, the major ones being adipose tissue, bone marrow, and umbilical cord. The site of the isolation has a prominent effect on the differentiation preference of the cells. For example, MSCs derived from adipose tissue show a lower osteogenic differentiation capability than those isolated from jawbone chips, wisdom teeth, or bone marrow [22-24]. Unfortunately, the isolation of MSCs from bone marrow, where they are present in high abundance, is related to higher donor site morbidity and increased patient discomfort as compared to the isolation from adipose tissue. Adipose tissue-derives MSCs also show a better proliferation rate and can be obtained in large quantities [25, 26]. Since the use of stem cells with the highest osteogenesis potential is beneficial for bone reconstruction therapies, it is wise to characterize the cells from each acquisition site in order to determine the location of the cells with the best proliferative and osteogenic characteristics [27, 28].
The differentiation of MSCs towards osteoblasts is orchestrated by numerous signaling cascades [10] with Wnt signaling being among the most influential pathways of bone development. This pathway is divided into the β-catenin dependent/canonical and the β-catenin independent/non-canonical branches. In the canonical Wnt signaling Fig. (1), which is crucial for osteogenesis, the Wnt ligand binds to the frizzled-receptor and simultaneously, to the LPR5/6 co-receptor. Upon binding, Dishellved, a cytoplasmic protein downstream Fizzled is phosphorylated. The activated Dishellved then inhibits the complex formation of glycogen synthase kinase 3-beta (GSK3-β), axin, adenomatous polyposis coli protein, and casein kinase 1 [29]. This complex phosphorylates β-catenin ultimately leading to its degradation. The unphosphorylated β-catenin translocates into the nucleus and regulates the expression of proteins, such as RUNX2 and alkaline phosphatase (ALP) that induce osteogenesis [30, 31]. The canonical Wnt signaling can be blocked via inhibition of the co-receptors LRP5 and 6 by DKK-1 or sclerostin that are released by osteocytes to control bone homeostasis [32].
The non-canonical Wnt pathway is less significant during osteogenesis. Nevertheless, upon the binding of Wnt ligand to Frizzled and receptor tyrosine kinases as co-receptors, the non-canonical pathway can stimulate the canonical pathway. When the Wnt5a binds to Frizzled and tyrosine-protein kinase transmembrane receptor ROR2, the expression of LPR5/6 is upregulated, thus increasing canonical Wnt signaling [33].
Another pathway involved in the differentiation of MSCs towards osteoblasts is the transforming growth factor-beta (TGF-β)/bone morphogenic protein (BMP) pathway Fig. (1). The BMPs belong to the subtype TGF-β1 and bind to members of the family of bone morphogenic protein receptors (BMPR) 1 and 2. Upon ligand binding, BMPR1 and/or BMPR2, get(s) phosphorylated and activated. SMAD-dependent and SMAD-independent BMP-induced downstream signaling routes exist, resulting in distinct transcription factors activation Fig. (1).
Fig. (1)) BMP and Wnt signaling pathways during osteogenesis. Upon binding of BMP-2 to its receptors, both SMAD-dependent and SMAD-independent pathways can be activated. The SMAD-dependent pathway activates SMADs by phosphorylation. These SMADs can act as or build complexes with transcription factors. The SMAD-independent pathway uses MAPK signaling, resulting in the activation of p38 and subsequently, phosphorylation of RUNX2, which promotes the expression of osteogenesis-related genes. The most prominent pathway for the differentiation of MSCs towards osteoblasts is the Wnt signaling pathway. Upon binding of the Wnt ligand, the phosphorylation and thus, degradation of β-catenin is blocked, resulting in translocation of β-catenin to the nucleus, where it stimulates the expression of osteogenic genes.In the SMAD dependent pathway, the binding of BMP to the receptors activates SMADs by the phosphorylation of the cytoplasmic SMAD1, SMAD5, and SMAD8, while the binding of TGF-β phosphorylates SMAD2 and SMAD3. The phosphorylation rescues the SMADs from ubiquitination and thus, degradation [34]. The phosphorylated SMADs then form complexes with SMAD4 and transcription factors (RUNX2) or transcriptional inhibitors (histone deacetylases) to, respectively, induce or halt the expression of the osteogenic marker genes such as RUNX2, Dlx5, and Osterix [34, 35].
During SMAD-independent BMP/TGF-β signaling, TGF-β1 activated kinase 1 (TAK1) and recruits TAK1 binding protein 1 Table 1. The resulting TAK1/Table 1 complex initiates the mitogen-activated protein kinase (MAPK) pathway, triggering in the activation of p38-MAPK. Phosphorylation of transcription factors, such as RUNX2 or Dlx5 via p38-MAPK, induces the expression of osteogenic genes [36]. In addition to the beneficial effect of TGF-β/BMP signaling in osteogenesis, it also inhibits canonical Wnt signaling by upregulating the expression of DKK-1 and sclerostin [37].
The Notch pathway can regulate osteogenesis in a bidirected manner. It can rather promote or inhibit the differentiation of MSCs towards osteoblasts. Notch is a transmembrane receptor, which contains an extracellular domain and a cleavable intracellular domain Fig. (2).
Notch intracellular domain is cleaved and translocated to the nucleus upon ligand binding. In the nucleus, the Notch intracellular domain binds to transcription factors of the CSL family and activates the expression of genes that are part of TGF-β/BMP signaling. BMP signaling is increased by the upregulation of Activin A receptor type I (ACVR1 or ALK2), a member of the BMPR1 family [38]. Following the upregulation of ALK2, Notch activation leads to an expression of HES and HEY proteins that inhibit RUNX2 and thus, suppression of osteogenesis [39].
Hedgehog (Hh) signaling plays an important role in skeletal development and digital patterning during embryogenesis and controls bone homeostasis during adulthood. Canonical Hedgehog signaling is activated, when the Hedgehog ligand binds to the transmembrane receptor Patched. Ligand-bound Patched undergoes internalization, which allows the downstream receptor Smoothened to translocate to the membrane and become phosphorylated. This promotes the dissociation and transformation of Glioma-associated transcriptional factors (Gli1/2) into transcriptional activators (GliA) and their nuclear dislocation. GliA binds to the osteogenesis-related gene promoters inducing their expression Fig. (2). For example, BMP2 and osteopontin are upregulated via GliA [40, 41]. Gli3 is kept in the cytoplasm and does not undergo transformation into a functional transcriptional repressor form (GliR). Canonical hedgehog signaling is inactive in the absence of Hh ligands due to the suppression of Smoothened by Patched and target gene expression blocked by GliR [42].
Fig. (2)) Notch and Hedgehog signaling pathways during osteogenesis. Upon binding of the Notch ligand, the intracellular domain of Notch is cleaved from the extracellular membrane and translocated to the nucleus. Within the nucleus, it binds to transcription factors to facilitate gene expression. The Hedgehog signaling pathway activation by ligand binding (Hh) results in the activation of Smoothened, dissociation of Gli1/2 from the Suppressor of Fused (SuFu) and their processing into GliA transcriptional activator, which then dislocates into the nucleus and stimulates the target gene expression. Transcriptional repressor precursor Gli3 is kept in the cytosol and cannot enter the nucleus during pathway activation but is functioning in the absence of the ligand.The application of Hh pathway agonist purmorphamine leads to osteogenic phenotype acquisition via the upregulation of the expression of RUNX2, BMPs, and SMAD transcription factors in hMSCs [43]. Other Hh pathway agonists induce alkaline phosphatase activity and elevate Osterix expression in mesenchymal cell line C3H10T1/2 [44]. Reduction of the Hh signaling by the knockdown of Sonic hedgehog (Shh) and Gli2 or inhibition Smoothened by cyclopamine, downregulated BMP2, SP7, and col1a1α expression and resulted in decreased bone mineralization and collagen deposition, while Patched knockdown resulted in increased osteogenesis through the expression of SP7 and col10a1 in zebrafish larvae [45]. Hh pathway activity, which is high during embryogenesis, decreases with age and is only moderately active in adult bone. Prolonged elevated Hh signaling activity in osteoblasts leads to a high parathyroid hormone-related peptide and RANKL expression, which induces excessive osteoclast formation and thus, bone resorption, as demonstrated in co-culture experiments and in vivo [46]. Overactive Hh signaling due to excessive Shh ligand concentration contributes to tumor-associated osteolysis in the oral squamous cell carcinoma environment [47].
Development of Osteoclasts from Hematopoietic Stem Cells
The differentiation of HSCs towards osteoclasts is a multi-step process Fig. (3). First, HSCs develop to myeloid progenitor cells dependent on the expression of GATA1 and PU.1. Once GATA1 is downregulated and PU.1 is upregulated, the cells differentiate towards myelolymphoid progenitor cells (LMPCs) [48]. Next, LMPCs differentiate towards osteoclast precursors (OCPs), which will later form mature osteoclasts. The development of OCPs is influenced by the binding of macrophage colony-stimulating factor (M-CSF) to its receptor c-Fms and the expression of Microphthalmia transcription factors (MITFs) [49]. M-CSF positively impacts the proliferation and survival of the OCPs [50, 51]. The progression of differentiation of OCPs is maintained by RANKL produced by osteoblasts. By binding to the receptor activator of NF-κB (RANK), RANKL can influence several signaling components downstream Fig. (3). This includes the tumor necrosis factor receptor-associated factor 6 (TRAF6), which binds to the intracellular residue of RANK [52]. Markedly, TRAF6 can build complexes with a variety of co-factors from different signaling cascades.
Fig. (3)) Development of osteoclasts from hematopoietic precursor cells. (A) The osteoclast differentiation begins with the lineage HSCs' commitment towards osteoclast progenitors (OCPs) regulated by PU.1 and GATA1 expression. During OCP maturation M-CSF, MITFs, RANK, NFATc, and DC-STAMP are upregulated. The mature osteoclast expresses genes, indispensable for its function, such as CatK or TRAP. (B) Major pathways that control osteoclastogenesis. The pathways are activated by the binding of RANKL to its receptor that is associated with TRAF6. TRAF6 is able to bind different signaling molecules downstream, resulting in the expression of osteoclastogenic genes.When TRAF6 is bound to MAPK-related TAK1 or TRAF-binding adapter protein Table 2, the activation of RANK leads to phosphorylation of IκB kinase (IKK). IKK then further phosphorylates inhibitory kappa B protein (IκB), which, under normal conditions, builds an inactive complex with nuclear factor-kappa B (NF-κB) [49]. The phosphorylated IκB is degraded, which enables NF-κB migration to the nucleus and the upregulation of the transcription of the nuclear factor of activated T cells cytoplasmic 1 (NFATc1) and c-Fos. The latter is mandatory for the activator protein 1 (AP-1) heterodimer complex, which induces an auto amplification of NFATc1 [53]. The transcription factor NFATc1 further regulates the expression of osteoclast specific proteins like TRAP, cathepsin K (CatK), calcitonin receptor, and β3-integrin, though their contribution to osteoclastogenesis is yet unknown [54, 55].
NFATc1 can be modulated by GSK3-β via phosphorylation resulting in an increased nuclear export of NFATc1 and therefore, a reduction in its transcriptional activity [56]. This process is inhibited upon the binding of RANKL to RANK. For this pathway, the tyrosine kinase c-Src builds a complex with TRAF6 when RANK is activated. The resulting complex activates PI3K that catalyzes the formation of phosphatidylinositol-(3,4,5)-phosphate (PIP3) on the inner cell membrane. PIP3 recruits Akt via an interaction with the pleckstrin homology domain of Akt [57]. Akt then phosphorylates GSK3-β, which inactivates its ability to phosphorylate NFATc1 [56].
Another mechanism of regulation of NFATc1 expression is through TAK1- TRAF6 complex formation, which initiates the MAP kinase kinases (MKK) 6 and 7 cascades, where MKK6 and MKK7 activate p38-MAPK via phosphorylation. The transcription factors MITF, NFATc1, NF-κB, and STAT1 become phosphorylated by p38 and induce the expression of genes promoting osteoclastic differentiation, including the NFATc1 [58]. According to Shimo and colleagues, Hedgehog pathway activation also facilitates the NFATc1 expression via activation of ERK and p38 and contributes to RANKL-induced osteoclastogenesis [47].
An important part of OCP differentiation towards multinucleated osteoclasts is cell fusion. Recent studies highlighted the influence of the dendritic cell-specific transmembrane protein (DC-STAMP) on cell fusion. OCPs expressing DC-STAMP on their surface attract each other and build up multicellular units [59]. The cells within these units fuse into multinucleated osteoclasts upon activation by a yet unidentified ligand [60, 61]. Alongside the DC-STAMP, E-cadherin seems to be pivotal for OCP cell fusion. A decrease of E-cadherin in mice leads to a lower number of multinucleated osteoclasts [49]. Other essential molecules for the OCP fusion are CD47 and its receptor, the macrophage fusion receptor that mediates cleavage of the extracellular domain enable the convergence of OCPs [60].
DEPENDENCE OF OSTEOCLASTS, OSTEOBLASTS, AND OSTEOCYTES
The bone is mainly composed of collagen type I fibers, calcium phosphate, non-collagenous proteins, and water. Within and surrounding this matrix, multiple cell types are found, including osteoclasts, osteoblasts, osteocytes, and bone lining cells. Bone lining cells are latent osteoblasts with the proliferation potential, while mature osteoblasts (or osteoclasts), which reside inside the bone, do not proliferate. Despite being terminally differentiated and not proliferating, osteoblasts orchestrate bone homeostasis comprised of bone resorption and formation [62, 63]. For the treatment of osteoporosis, modulators of the intercellular signaling Fig. (4), which tune the bone metabolism into the required direction, are of great interest.
Fig. (4)) Interactions between osteoblast- and osteoclast precursors. Differentiation towards both cell types is balanced by the expression of additional signaling molecules, such as RANKL, Wnt, and BMP-2. The Indian hedgehog ligand (Ihh), on one hand, is beneficial for osteogenic differentiation, while on the other hand, it accelerates bone resorption by stimulating the expression of CatK. Sclerostin released by osteocytes inhibits osteogenic differentiation and is adding another cell type to the relationship.The Pathways of Bone Remodeling
The process of bone remodeling happens continuously throughout the life of the individual. Bone strength and architecture must adapt to organism growth and mechanical load. Occurring micro damages to the bone tissue must be repaired to ensure high bone fitness through life [64]. Within the bone matrix, the interconnected network of osteocytes steers the processes on bone metabolism, including bone remodeling [65]. The initiation of bone remodeling by osteocytes can be prompted by mechanical loading and unloading. The apoptosis of osteocytes, induced programmed death process, may occur, which will promote the recruitment of osteoclasts to the site [66]. Furthermore, the impact of osteocytes on the differentiation from MSCs to osteoblasts and OCPs to osteoclasts can be endorsed by the expression of cell fate-regulating proteins.
The first cell fate regulator to be named is IGF1, which increases osteoblast differentiation by upregulation of RUNX2, collagen type I, and alkaline phosphatase expression [67, 68]. Additionally, nitric oxide (NO) can have a positive effect on bone mass. NO is generated in the osteocytes, osteoblasts, and osteoclasts by nitric oxide synthase from L-arginine. During sheer stress, NO inhibits bone resorption and increases bone formation [69].
A decreased osteogenic differentiation is mediated by sclerostin and DKK-1 expression by osteocytes. In contrast to IGF1, those proteins inhibit RUNX2 expression by blocking the LRP5/6 co-receptors from the Wnt signaling pathway, which leads to phosphorylation and degradation of β-catenin Fig. (1) [70].
Parathyroid hormone (PTH) has a binary role in bone remodeling. PTH is able to boost both processes - bone formation and resorption, depending on the duration of exposure. In one case, short term exposure of the osteocytes to PTH, an inhibition of sclerostin expression and thus, an increased osteogenic differentiation can be observed. In the other case, long-term treatment with PTH leads to an increased expression of RANKL and thus, accelerated osteoclastogenic differentiation [71].
Prior to the active phase of bone remodeling, there is a prodromal phase, where osteoclasts, osteoblasts, and their precursor cells build a basic multicellular unit (BMU). The OCP cells in the BMU further differentiate towards osteoclasts by either cell-cell contact with osteoblasts or triggered by small soluble molecules released by the osteoblasts Fig. (4) [72].
After recruitment and differentiation of functional osteoclasts, the first phase of bone remodeling, which is bone resorption, is initiated. By the release of the protease CatK, the collagen matrix is degraded at multiple cleaving sites within the collagen. The osteogenic factors trapped in the matrix are then released and trigger the differentiation of osteoblast precursor cells towards osteoblasts. Between bone resorption and bone formation, there is an intermediate stage called the reversal phase, which is branded by the apoptosis of osteoclasts and the crossover of osteoblasts into the resorption pit, resulting in reconstruction of the extracellular matrix by osteoblasts [73-75]. There are several disorders related to the mentioned processes of bone remodeling and the development of bone cells, which are described in the following paragraphs.
Disorders of Bone Remodeling: Osteoporosis and Osteopetrosis
Bone disorders related to bone remodeling are divided into bone catabolic disorders (BCDs) that lead to a decrease of bone mass and bone anabolic disorders (BADs), which are characterized by an increase in bone mass. One of the most common BCDs is osteoporosis, with over 200 million cases worldwide. Approximately, 40% of the women older than 50 years and ~15-30% of the men experienced osteoporosis-related fractures in their lifetime [76].
Other BCDs include rheumatoid arthritis, periodontitis, bone loss due to disuse and microgravity [77]. Rheumatoid arthritis is an inflammatory disease during which bone and cartilage in the joints degrade due to TGF-β-related proteins' actions. They activate osteoclastogenesis via SMAD signaling, which leads to NFATc1 expression [78]. The loss of bone due to disuse or microgravity is a consequence of osteocytes reacting to unloading by upregulating the sclerostin expression, which, in turn, leads to suppressed osteogenic differentiation of MSCs and thus, a decreased bone formation [79].
One example of BAD is osteopetrosis, which is characterized by malfunctioning of the osteoclasts and an increased bone mass [80]. During osteopetrosis trafficking of the lysosomal vesicles, containing bone-matrix degrading enzymes (such as CatK), towards the resorption lacunae is disrupted due to morphological changes in the ruffled border of the osteoclast membrane [81]. Patients with osteopetrosis suffer spontaneous fractures, anemia, compression of the cranial and facial nerves that can ultimately result in blindness and deafness [82]. Another bone resorption disorder, called pycnodysostosis, also leads to a high BMD and characteristic short stature of the affected individuals. In pycnodysostosis, a mutation in the CatK gene inhibits the growth-related bone remodeling [83]. Albers-Schonberg disease is a BAD where the development of a bone inside the bone occurs. This happens due to an osteoclast dysfunction because of a mutation in the chloride channel 7, which is necessary for acidification of the osteoblast and therefore, function and stabilization of the bone-resorbing protease CatK [84
