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Fig. 13.6 Modied cell membrane with GAGs
R. Guizzardi et al.
A different lipid-based strategy was proposed by Woods et al. [91]: a glyco-engineered lipid anchor based on cholesterylamine was synthe­sized, in order to display synthetic glycans on plasma membrane with improved residence times, emulating natural mucins, to shed light on the role of mucins overexpression within cancer. Glyco-engineered cholesterylamine was durably displayed on cell surface after internalization, exposing linked glycopolymers for up to 10days. The presence of the engineered glycans on cell surface mimicked the effect of mucins in focal adhesion and cell survival, both in vitro and in vivo. More strikingly, the engineered glyco­lipid was expressed also in daughter cells derived from a labeled mother cell. The same group proposed an additional strategy to expose mucin mimetics on cell surfaces. Synthetically challenging mucin-like domains were chemically synthesized and conjugated through a site-specic bioorthogonal strategy to a genetically engineered membrane protein on live mammalian cells [92]. These strategies could be adapted to replace native glycodomains in mucin proteins with syn­thetic analogs, systematically varying their chem-
ical and biological properties potentially delivering great benets to tissue engineering.
One of the greatest challenges in cell therapy for tissue engineering is the efcient delivery of viable cells to the tissue to be restored. Inefcient homing of systemically delivered mesenchymal stem cells (MSCs) is probably the major limita­tion of existing MSC-based cell therapies, prob­ably caused by poor expression of adhesion molecules on cell surfaces. In order to improve MSCs homing, their surface was modied with a nanopolymeric structure bearing the tetrasac­charide sialyl LeX (sLe
X
). This trisaccharide is usually overexpressed on leukocytes and is responsible for cell rolling during inammation processes. sLeX glyco-engineered MSCs exhib­ited a robust rolling response on inamed endothelium invivo and homed to inamed tis­sue with higher efciency than native MSCs. This approach highlights an effective glycoen­gineering strategy for cell homing through cir­culation [93].
Following a similar line of research, homing of human mesenchymal stem cells (hMSCs) to bone was obtained by glycan engineering of
Blood group 0
Blood group A Blood group B
HO
13 Carbohydrates inRegenerative Medicine: FromScaolds toCell Fate Modulators
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Fig. 13.7 Blood group antigenic determinants
HO
OH
AcHN
OH
HO
HO
O
O
HO
HO
OH
OH
O
O
O
OH
OH
OH
O
HO
O
O
OH
O
O
HO
HO
OH
OH
O
O
O
OH
glycoside hydrolase
HO
HO
HO
CD44 [94]. The native CD44 glycans of MSCs bear α-2,3-sialylosides, but cellular recruitment to bone occurs via E-selectin, a lectin recogniz­ing sialofucosylated determinants. As native CD44 lacks the key α-1,3-fucosyl hematopoietic cell E-selectin/L-selectin ligand, an enzymatic α-1,3-fucosylation of MSCs CD44 was per­formed under physiological conditions speci­cally designed for treating live cells. This modication conferred potent E-selectin binding and tropism to bone, without detrimental effects on cell viability or multipotency. This study unveiled a great potential to program cellular trafcking on through membrane glycoprotein engineering for directing cellular migration, a key issue for cell therapies.
An example of cell surface glyco-engineering is related to blood group antigenic determinants. The antigenicity of the ABO blood groups is dened by the structure of carbohydrates units (Fig.13.7) expressed on red blood cells surfaces (RBCs). Conversion of type A, B, and AB RBCs
to universal 0 type by enzymatic removal of galactose or N-acetylgalactosamine antigenic carbohydrate residues by specic glycosidases has emerged as a useful strategy to convert all blood donations of different types into the 0 uni­versal one. Although cell surface modication may be accomplished by suitable enzymes, in many practical applications, their poor associa­tion with cell surfaces due to the repulsion of two hydrophilic entities hamper this technology, because of the need of high concentrations of reagent that may be toxic for cells and making the process costly. Thus, the development of a general process improving enzymatic reactions on cell surfaces are particularly attractive. In this example, the enzymatic reaction on blood group determinants was performed with good efciency by a strategy based on macromolecular crowding with biocompatible and cheap neutral polymers, such as dextran and coll [95], granting over 440-fold increase of enzymatic glycoengineering activity on cells surface. This research empha-
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sized the relevance of discovering new and ef­cient methods for glycan-specic cell surface engineering that hold promise for clinical applications.
13.6 Conclusions
We showed several examples highlighting the involvement of carbohydrates in the modulation of cell adhesion, function, homing, and differen­tiation. The ability to exploit and mimic glycan roles in a tissue-specic manner will afford new opportunities in regenerative medicine, which in the next future will rely on the development of innovative and efcient glycoengineering tech­nologies. However, research is still needed, requiring continued crossing of disciplines, heav­ily including chemistry, along with complemen­tary contributions of medicine, biotechnology, biology, engineering, and material science. In addition, a better understanding of glycobiology of stem cells and signaling within complex extra­cellular matrix interactions is still needed in order to fully exploit the potentialities given by the glycocode.
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Biomimetics: ANew Abstraction
https://t.me/medicina_free
forBone Implant Design
ElizabethA.Brett, MatthiasA.Sauter, andDominikDuscher
14
14.1 Introduction
Biomimetics and biomimicry are thought pro­cesses applied to biomaterial design, where bio­logical implants have properties which mirror closely those of natural material. Designing implants with this mindset may elegantly circum­vent some of the roadblocks in synthetic bioma­terial design and function [1]. Bone regeneration is a particularly attractive frame for this work, given the tissue heterogeneity and mechanical role of bone. However, in recent years, problems with physical shape, surface chemistry, and mechanical properties have been identied in implants. Implant failure through peri- implantitis, peri-mucositis, peri-implant disease, or infection can lead to pain, mechanical loosening, and even­tual need for extraction [2]. Similarly, some lit­erature has outlined concerns with implants, specically showing unfavorable physical remodeling over time [3]. Eliminating the vari­ability associated with implants would mean a
E. A. Brett · M. A. Sauter Division for Experimental Plastic Surgery, Department of Plastic and Hand Surgery, Klinikum rechts der Isar, Technical University of Munich, Munich, Germany e-mail: Eliza.Brett@tum.de; matthias.sauter@tum.de
D. Duscher (*) Department for Plastic Surgery and Hand Surgery, Division of Experimental Plastic Surgery, Technical University of Munich, Munich, Germany
huge decrease in patient morbidity and cost bur­den. These concerns have ushered biomimetics and new material design techniques to the forefront.
14.2 The Recipient Site
Types of bony reconstruction range from struc­tural nonload-bearing bone (e.g., calvarium) to larger compact load-bearing bone (e.g., femur). Host bone can be challenging, for instance if it is osteoporotic [4] or osteomyelotic [5, 6]. To ensure a t of the implant in the host bone, under­reaming is performed; a drilling process designed to facilitate the implant, sometimes known to fracture the host bone further [7]. It is especially problematic in the context of osteoporotic bone, which can exhibit low “pull out strength” of an implant once in situ [8]. Another quite common issue is poor nutrient diffusion, borne of compro­mised blood supply. Diffusion distance of oxy­gen invivo is 150–200 μm [9]. As such, certain materials may stimulate vascular ingrowth into the host bone. Inactivators of prolyl hydroxylase (e.g., cobalt ions) have been shown to stabilize HIF-1α, a potent proangiogenic factor, resulting in upregulated expression of genes such as GLUT1, erythropoietin, VEGF, and PDGF [10,
11]. Bone that has undergone irradiation falls in
the same category of hostile recipient sites. Macroscopically, a threaded implant (akin to a
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Compact bone composition
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screw) gives immediate stability and provides close, xed contact between the graft and host bone.
14.3 Mechanical Stability ofImplant
Issues with bone implants chiey lie in materi­als destined for load-bearing bone healing. The Young’s modulus of trabecular bone is 10.4–
14.8 GPa, and cortical bone measures 18.4–
20.7 GPa [12]. Naturally, stainless steel has a
much higher modulus of 180 GPa, cobalt chro­mium (Co-Cr) at 210 GPa, and titanium at 110 GPa [13]. “Stress shielding” is the physio­logical response resulting from implanting a harder material into a softer host tissue. The result is brous encapsulation of implant, allow­ing for micro-movement of the implant unit. Mobility of an implant creates a specic wear, called fretting [14]. Fretting implants can gradu­ally loosen and eventually fail within the host bone [15]. Moreover, fragments of the implant are frequently known to break off the body of the implant, causing local abrasion of the sur­rounding, softer bone tissue [16]. Debris of implants can sometimes be found in alternate locations in the body, such as the spleen, liver, and abdominal lymph nodes of arthroplasty patients [17].
It was Albrektsson etal. in 1981 [18] who rst showed the complete “osseointegration” of titanium. During a fracture xation experiment in a rabbit femur, Branemark discovered the removal of the titanium implant from bone was completely impossible. Further studies using transmission electron microscopy showed a new phenomenon of direct contact between bone and implant, without the surrounding brous capsule responsible for implant looseness and micro­movement [19]. Filamentous collagen type 1 was found to form brils at the implant–bone interface (resembling the strong Sharpey’s bers of the scalp), giving the tight coupling of metal and bone [18]. The phenomenon of the absence of brous encapsulation around titanium implants has inspired implant design for more
than just bone. Tissue expanders used for breast reconstruction can include a titanium-coated mesh to reduce the brotic content of the breast [20]. In the context of bone, titanium is now being tested in advanced models of craniofacial bone healing, using titanium granules to stimu­late maxillary sinuses [21]. However, in terms of biomimetics, titanium presents some mechanical challenges, specically in terms of difference in stiffness between it and the host tissue, for exam­ple, femur. This hurdle is called modular mis­match [22] and can be dodged by using material whose bulk mechanical properties more closely resemble bone.
14.4 Grafts Based onBone Mineral Components
The unique nature of bone is that it is largely mineralized. In its dry mass, bone is 60–70% mineral, which is non-immunogenic and ubiqui­tously found (Fig.14.1) [23]. As such, the use of natural material already in existence presents an option for creating biomimetic implant matter. Nacre, or mother-of-pearl, is pure calcium car­bonate produced by mollusks. Mixing pulverized
20%
80%
Compact Spongey
MINERAL (Hydroxyapatite)
PROTEIN (Collagen, cells, hyaluronic acid)
WATER
Fig. 14.1 Bone composition
70%
22%
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nacre with patient blood and implanting the mixture into a human mandibular defect site was largely effective in closing the defect and stimu­lating regenerative cellular activity in the loca­tion [24]. Many successful bone biomaterials incorporate calcium or hydroxyapatite to help facilitate bone formation and graft “take.” A study comparing biologic bovine-derived bone grafts (BioOss® Bone Substitute; Ed. Geistlich Soehne, Wolhusen, Switzerland), with a highly porous synthetic hydroxyapatite scaffold (IngeniOs™ HA Synthetic Bone Particles; Zimmer Dental, Inc., Carlsbad, CA), showed highly similar chemistry, morphology, and struc­ture [21]. FT-IR spectra revealed high crystallin­ity (thus low resorption) of the synthetic IngeniOs Hydroxyapatite Synthetic Bone Particles, owing to the purity of its manufacturing processes ver­sus natural variation [25]. The difference in purity between synthetic and biologic material by proxy of uniform industrial manufacture is seen throughout most implant forms.
Beta tricalcium phosphate (β-TCP) is a cal­cium salt abundant in bone, and has been shown to be highly and quickly reactive as part of a bone graft. This is because in an aqueous envi­ronment, TCP reacts to form hydroxyapatite [26]. Effects of TCP can be exaggerated with strategic addition of growth factors invivo, as was seen throughout a series of randomized con­trol trials using platelet-derived growth factor and β-TCP as agents to heal periodontal intraos­seous defects [27]. In fact, β-TCP has been shown to contribute to bone healing faster than hydroxyapatite alone, secondary to its rapid rate of resorption [28]. Meanwhile, HAPEX is an amalgamation of the biologic mineral hydroxy­apatite and synthetic high-weight polyethylene. This mixture makes for a bioactive polymer, which has been used in the reconstruction of orbital oor and middle ear defects [29]. Cerasorb is pure β-TCP, designed to be mixed with the patient’s own blood or platelet- rich plasma and added to the defect site, primarily for periodontic healing. As a different oceanic source of bone mineral, coral holds properties which mimic bone. Coral forms hydroxyapatite on its surface due to its calcium carbonate core
and also retains its native trabecular structure, having inherent biomedical value as a spongy bone substitute [30, 31].
14.5 Grafts Based onStructural Protein
An important component of bone structure is protein, both structural and supportive. Hyaluronic acid (HA) is a high-molecular-weight nonsulfated glycosaminoglycan, which is formed in the plasma membrane of cells [32]. It is a highly negative molecule capable of attracting proteoglycans, which in turn harbor water. Logically, ECM scaffolds containing HA have reached the market for dermal applications, given that hyaluronate is an antibrotic, hydrating agent in a healing wound [33]. However, in the context of bone grafts, rabbit tibias which received HA showed increased healing 20days after injury, exhibiting brocartilage formation, which later ossied, in comparison to the non­HA- treated bones, which formed purely brous unions [34]. More sophisticated, combinatorial approaches have been designed around HA, e.g., use of HA in a hydrogel with calcium sulfate hemihydrate, a bioresorbable, osteoconductive compound [35].
Composite biomimetic grafts over simple hydrogel injection are required for healing of large load-bearing bones. For instance, addition of protein to a hydrogel or impregnated into an implant introduces necessary cellular osteo­genic mechanisms. In a canine femoral defect model, bone morphogenetic protein (BMP) in tandem with collagen type-1/TCP showed increased healing of the femur in the presence of bone marrow aspirate [36]. These studies suggest collagen type 1 as a logical and func­tional agent to match the modulus of the femur. Similarly, there is utility of bone marrow­derived cells stimulated by pro-osteogenic growth factors in bone healing.
A highly osteogenic component of bone is the periosteum, a stratied structure of an inner cell layer (cambium layer), and tough, brous outer layer [37]. Damaged bone which under-
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goes delayed reconstruction may show hetero­topic ossication, disorganized bony formations at the injury site due to disrupted periosteum [38]. As such, strategically placed periosteal grafts present a highly biomimetic solution of autografting onto damaged bone [39]. Due to the sheet-like structure of the periosteum, peri­osteal grafts have found their primary utility in dental and alveolar grafting, as opposed to long, load-bearing bone reconstruction [40].
14.6 Grafts Based onCellular Implants
Despite orthopedic management of fractures becoming better and better, some healed inju­ries will persist with brous nonunions [41]. To address this, “The Diamond Concept” has been developed, which encompasses four different aspects of invivo bone regeneration: an osteo­conductive scaffold, a suitable mechanical environment, osteoinductive signals, and a pro­osteogenic cell population [42].
The ability to direct stem cells to an osteo­genic pathway represents a huge regenerative role. Addition of cells to bone grafts is a concept based on isolating cell types, which will either immediately and directly add bone (osteoblasts/ osteoclasts), or have the potential to differentiate, providing angiogenic and osteogenic factors (mesenchymal stem cells (MSCs) and adipose­derived stem cells (ASCs) [43].
Cell surface markers indicative of osteo­genic behavior can vary from poorly dened CD markers to more well-known pro-osteo­genic growth factor receptors. BMP receptor type-1b (BMPR-1b) binds BMP and has an important role in directing bone formation [44]. As such, using FACS to select for BMPR-1b­positive cells from ASCs results in a population with increased osteogenic gene expression and in vitro osteogenic potential [45]. Moreover, when coupled with a porous, osteoconductive scaffold coated in osteoinductive hydroxyapa­tite, rapid bone formation is observed invivo [46]. Similarly, ASCs positive for CD90 (Thy-
1) have been shown to signicantly increase
healing of bone defects when compared with their negative and unsorted control groups [
48]. Importantly, CD90 expression has been
shown to vary dramatically between invivo and in vitro settings, making use of this marker unpredictable [49]. Similarly, other markers also have difcult expression proles to track and analyze, such as CD105 (endoglin), a bone marrow mesenchymal cell marker. However, it was discovered that isolating CD105- negative cells and waiting 36 h in culture yielded a sub­population with enhanced osteogenic potential in vivo [ although in that it acts as a co-receptor for TGF- β1, which is a known antagonist of osteo- genic differentiation [51], thus explaining a parallel decrease in bone formation with increased expression. These ndings simulta­neously highlight the promise and problems of using surface marker selection criteria in isolat­ing heterogeneous stem cell populations for bone regenerative purposes.
being researched under an autologous cell transfer lens. For instance, the efcacy of bone regeneration by autologous bone marrow har­vested from the anterior iliac crest has been shown in atrophic diaphyseal nonunion. Here, a biomaterial was created by concentrating mar­row via centrifugation, which could be loaded into a syringe and injected into the recipient site. Analysis of diseased tibias postmarrow transplant showed increased bone callus miner­alization [52]. Similarly, osteogenesis imper­fecta (OI) is a genetic disease of the mesenchymal cells, whereby a defective colla­gen type 1 is produced, giving rise to bone weakness and malformation. Unmanipulated bone marrow donations from healthy matched siblings or family members have been shown to increase trabecular bone formation in the recip­ient OI patient [53]. On the cellular level, the harvest, culture, and transplant of bone mar­row-derived stromal cells have been found to be effective in repairing large bone defects in human [54]. However, these cells are most ef­cient when placed in situ seeded on a macropo­rous scaffold [55].
50]. CD105 is especially nuanced,
There are multiple different bone diseases
47,