Page 138 - Physico-Chemical Niche Conditions for Bone Cells

RGD-functionalized supported lipid bilayers modulate pre-osteoblasts
[12,14,15]. This indicates that the functionalization of biomaterial surfaces may be improved by immobilizing RGD for optimal adhesion of osteoprogenitors.
Osteoprogenitor adhesion to RGD-functionalized biomaterials is not yet optimal possibly due to the immobile ligand presentation preventing the cells from rearranging the ECM to optimize cell–ECM interaction as occurs in vivo [16]. Ligand immobilization likely inhibits integrin clustering [17] and thereby decreases cell adhesion strength and signaling resulting in differentiation [6]. Supported lipid bilayers (SLBs) provide a platform for functionalizing biomaterials with mobile ligands, including RGD [16–18]. SLBs are made of phospholipids and comparable to natural cell membranes. They are nonfouling in nature [18]. SLBs are extensively applied into modern clinical use owing to their biophysical and chemical versatility [17,19,20]. They are applied with micro- and nano-array format, which has opened new avenues to create biochip strategies, for example, sensing strategy for diagnostics, carrier role for vaccines, theranostics, and labeling capability for imaging [19], immunoassays [20], and tissue engineering approaches for multiple cellular processes [17].
SLBs can be functionalized with peptides derived from natural proteins, for example, ECM proteins, growth factors, cytokines, antibacterial agents, which influence cellular function. One of the intrinsic properties of SLBs is that they are fluid, that is, the phospholipids laterally diffuse through the layers [17,18]. Since the ligands are anchored to the phospholipids, they diffuse through the lipid layers as well, facilitating the clustering of integrins and their ligands [17]. The fluidity of SLBs and thereby the mobility of the ligands can be adjusted by changing the fatty acid composition and ligand density[16–18]. In vitro studies with mesenchymal stem cells (MSCs) [16] and C2C12 myoblasts [21] on RGD-functionalized SLBs with variable fluidity have shown contradictory results. Increased numbers of MSCs are attached to more fluid SLBs consisting of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) than to more solid SLBs consisting of 1,2-dipalmitoyl-sn-glycero- 3-phosphocholine (DPPC), with cells on DOPC exhibiting a larger cell area than on DPPC [16]. Osteogenic differentiation is enhanced in MSCs cultured on more fluid SLBs [16]. On the other hand, C2C12 myoblasts show a larger cell area and higher expression of myogenic markers on the more solid DPPC compared to the more fluid DOPC [21]. These studies indicate that the degree of ligand mobility modulates progenitor adhesion as well as osteogenic and myogenic differentiation. Whether ligand mobility also modulates osteoprogenitor adhesion and differentiation is unknown.
ECM stiffness is a critical factor determining lineage commitment of MSCs [22]. Culture of MSCs on a stiff substrate (elastic modulus [E] of 25–40 kPa) results in osteogenic differentiation, whereas culture on more compliant substrates results in myogenic (E = 8–17 kPa) or neurogenic (E = 0.1–1 kPa) differentiation [22]. The elastic moduli of the lipid bilayers used in the study by Koçer and Jonkheijm [16] were determined by Picas et al. as 19.3 and 28.1 MPa [23], which is much higher than the estimated elastic modulus of osteoid (27 ± 10
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