**4.2 BMP-Smad pathway**

Bone morphogenetic protein (BMP) holds a well-known and fundamental role in the bone development [36]. BMP signaling is initiated through the interaction of BMPs with the BMP-receptors (type I and type II). This binding stimulates the

#### **Figure 1.**

*Wnt/β-catenin (canonical) signaling pathway in the osteoblasts. [A] In the absence of Wnt protein, the degradation complex Axin, GSK-3β and APC protein phosphorylates the β-catenin and results in the ubiquitin-mediated degradation of β-catenin. This turns off the transcription machinery for the osteogenic genes and hence transcription for the genes involved in osteogenic differentiation is hampered resulting in defective bone formation. [B] Wnt ligand interacts with the receptor complex of Fz and LRP that restricted the action of Axin, GSK-3β, and APC protein and hence permitted the transport of β-catenin to the nucleus. β-catenin combines with LEF/TCF transcription factors in the nucleus, thereby potentiating the transcription for the osteogenic genes.*

**213**

**Figure 2.**

*the osteogenic lineage.*

*MicroRNAs as Next Generation Therapeutics in Osteoporosis*

process of phosphorylation in the receptors that leads to recruitment and activation of Smad proteins, i.e., Smad 1, 5, 8 and Smad4. Smad proteins act as transcriptional regulators in the nucleus and ultimately induce the expression of the genes responsible for osteoblastogenesis (**Figure 2**) [37, 38]. There are a total of 14 members in the BMP family, out of which BMP-2,4,5,6,7 and 9 are reported to have high bone

RANKL-based pathway is an essential signaling cascade for the osteoclast differentiation. RANKL binds to its receptor RANK (present on the osteoclast precursors) and recruits TNF receptor-associated factor (TRAF) adaptor proteins to the conserved TRAF domain present at the cytoplasmic domain of the RANK [39, 40]. TRAF transduces the signal to downstream proteins viz. nuclear factor kappa B (NF-κB), extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and Nuclear Factor Of Activated T Cells 1 (NFATc1) (**Figure 3**) [41, 42]. NF-κB is an important regulator for the osteoclast differentiation. It is mainly responsible for the inflammation-based osteolysis and bone

*Bmp-Smad signaling pathway in the osteoblasts. BMP ligand binds to receptor complex viz. type I (BMPRI) and type II receptor (BMPRII). Type II receptor which is a serine/threonine kinase in nature phosphorylates and stimulates type I receptor. Upon activation, type I receptor causes phosphorylation of the downstream proteins—receptor activated Smads (R-Smads), Smad 1/5/8. Further, R-Smads complexes with co-Smad, Smad4 and hence the complex transports to the nucleus. In the nucleus, Smad complex interacts with coactivators and finally results in the transcription of osteogenic gene viz. Runx2. Runx2 is a master* 

*transcription factor for the bone development. It aids in the differentiation of mesenchymal stem cells (MSC) to* 

*DOI: http://dx.doi.org/10.5772/intechopen.91223*

formation ability [37].

resorption [43].

**4.3 RANKL mediated signaling**

*MicroRNAs as Next Generation Therapeutics in Osteoporosis DOI: http://dx.doi.org/10.5772/intechopen.91223*

process of phosphorylation in the receptors that leads to recruitment and activation of Smad proteins, i.e., Smad 1, 5, 8 and Smad4. Smad proteins act as transcriptional regulators in the nucleus and ultimately induce the expression of the genes responsible for osteoblastogenesis (**Figure 2**) [37, 38]. There are a total of 14 members in the BMP family, out of which BMP-2,4,5,6,7 and 9 are reported to have high bone formation ability [37].

## **4.3 RANKL mediated signaling**

*Clinical Implementation of Bone Regeneration and Maintenance*

binds to a receptor complex of Frizzled (Fz) and low-density lipoprotein receptorrelated proteins (LRP). There are two modes of functioning for the Wnt proteins, i.e., canonical and non-canonical pathways, wherein the canonical pathway has a more specific role to play in the bone development. In the canonical pathway (**Figure 1**), the interaction of Wnt to the receptor complex hinders the functioning of axin, glycogen synthase kinase 3β (GSK-3β) and adenomatous polyposis coli (APC) protein. This primes the accumulation of β-catenin in the cytoplasm, further β-catenin travels down to the nucleus and ultimately stimulates lymphoid-enhancer-binding factor/Tcell-specific transcription factors (LEF/TCF). This results in the transcriptional activation for the genes that participate in bone formation and regeneration. While the absence of Wnt signal leads to phosphorylation of the cytosolic β-catenin and its subsequent ubiquitin-mediated degradation [34]. The degradation of the β-catenin finally turns off the downstream activation of the osteogenic genes. Accurate Wnt signaling is a pre-requisite for adequate bone mass in the body while mutation of the

Wnt signaling components results in fractures and bone injuries [35].

Bone morphogenetic protein (BMP) holds a well-known and fundamental role in the bone development [36]. BMP signaling is initiated through the interaction of BMPs with the BMP-receptors (type I and type II). This binding stimulates the

*Wnt/β-catenin (canonical) signaling pathway in the osteoblasts. [A] In the absence of Wnt protein, the degradation complex Axin, GSK-3β and APC protein phosphorylates the β-catenin and results in the ubiquitin-mediated degradation of β-catenin. This turns off the transcription machinery for the osteogenic genes and hence transcription for the genes involved in osteogenic differentiation is hampered resulting in defective bone formation. [B] Wnt ligand interacts with the receptor complex of Fz and LRP that restricted the action of Axin, GSK-3β, and APC protein and hence permitted the transport of β-catenin to the nucleus. β-catenin combines with LEF/TCF transcription factors in the nucleus, thereby potentiating the transcription* 

**4.2 BMP-Smad pathway**

**212**

**Figure 1.**

*for the osteogenic genes.*

RANKL-based pathway is an essential signaling cascade for the osteoclast differentiation. RANKL binds to its receptor RANK (present on the osteoclast precursors) and recruits TNF receptor-associated factor (TRAF) adaptor proteins to the conserved TRAF domain present at the cytoplasmic domain of the RANK [39, 40]. TRAF transduces the signal to downstream proteins viz. nuclear factor kappa B (NF-κB), extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and Nuclear Factor Of Activated T Cells 1 (NFATc1) (**Figure 3**) [41, 42]. NF-κB is an important regulator for the osteoclast differentiation. It is mainly responsible for the inflammation-based osteolysis and bone resorption [43].

#### **Figure 2.**

*Bmp-Smad signaling pathway in the osteoblasts. BMP ligand binds to receptor complex viz. type I (BMPRI) and type II receptor (BMPRII). Type II receptor which is a serine/threonine kinase in nature phosphorylates and stimulates type I receptor. Upon activation, type I receptor causes phosphorylation of the downstream proteins—receptor activated Smads (R-Smads), Smad 1/5/8. Further, R-Smads complexes with co-Smad, Smad4 and hence the complex transports to the nucleus. In the nucleus, Smad complex interacts with coactivators and finally results in the transcription of osteogenic gene viz. Runx2. Runx2 is a master transcription factor for the bone development. It aids in the differentiation of mesenchymal stem cells (MSC) to the osteogenic lineage.*

#### **Figure 3.**

*RANKL-RANK signaling pathway in the osteoclasts. RANKL (present on the surface of osteoblasts) interacts with RANK receptor (on the surface of osteoclasts). RANK does not possess any kinase activity and hence recruits TRAF proteins to the cytoplasmic region of the receptor. This further transduces the signal to downstream components and activates. (1) TAK1 (member of mitogen activated kinase family, MAPK), promotes the ERK dependent activation of NFATc1. (2) NFK-β, after the phosphorylation based degradation of inhibitor of NFK-β (IKβ). (3) Other downstream proteins viz. JNK that leads to transcriptional activation of NFATc1 and additional factors resulting in the osteoclastogenesis.*
