The successful incorporation of active proteins into synthetic polymers could lead to a new class of materials with functions found only in living systems. However, proteins rarely function under the conditions suitable for polymer processing. On the basis of an analysis of trends in protein sequences and characteristic chemical patterns on protein surfaces, we designed four-monomer random heteropolymers to mimic intrinsically disordered proteins for protein solubilization and stabilization in non-native environments. The heteropolymers, with optimized composition and statistical monomer distribution, enable cell-free synthesis of membrane proteins with proper protein folding for transport and enzyme-containing plastics for toxin bioremediation. Controlling the statistical monomer distribution in a heteropolymer, rather than the specific monomer sequence, affords a new strategy to interface with biological systems for protein-based biomaterials.

Aging reduces the number of mesenchymal stem cells (MSCs) in the bone marrow which leads to impairment of osteogenesis. However, if MSCs could be directed toward osteogenic differentiation, they could be a viable therapeutic option for bone regeneration. We have developed a method to direct the MSCs to the bone surface by attaching a synthetic high affinity and specific peptidomimetic ligand (LLP2A) against integrin α4β1 on the MSC surface, to a bisphosphonate (alendronate, Ale) that has high affinity for bone. LLP2A-Ale increased MSCs migration and osteogenic differentiation in vitro. A single intravenous injection of LLP2A-Ale increased trabecular bone formation and bone mass in both xenotransplantation and immune competent mice. Additionally, LLP2A-Ale prevented trabecular bone loss after peak bone acquisition was achieved or following estrogen deficiency. These results provide a proof of principle that LLP2A-Ale can direct MSCs to the bone to form new bone and increase bone strength.

Peptide/protein-polymer conjugates make up a new class of soft matter comprising natural and synthetic building blocks. They have the potential to combine the advantages of proteins and synthetic polymers (i.e., the precise chemical structure and diverse functionalities of biomolecules and the stability and processability of synthetic polymers) to generate hybrid materials with properties yet to be realized with either component alone. Here we briefly discuss recent developments in the design, fundamental understanding, and self-assembly of various peptide-polymer conjugates, as well as emerging biological and nonbiological applications that range from nanomedicine, to separation, and beyond.

Vitamin D deficiency is a widespread medical condition that plays a major role in human bone health. Fracture susceptibility in the context of low vitamin D has been primarily associated with defective mineralization of collagenous matrix (osteoid). However, bone's fracture resistance is due to toughening mechanisms at various hierarchical levels ranging from the nano- to the microstructure. Thus, we hypothesize that the increase in fracture risk with vitamin D deficiency may be triggered by numerous pathological changes and may not solely derive from the absence of mineralized bone. We found that the characteristic increase in osteoid-covered surfaces in vitamin D-deficient bone hampers remodeling of the remaining mineralized bone tissue. Using spatially resolved synchrotron bone mineral density distribution analyses and spectroscopic techniques, we observed that the bone tissue within the osteoid frame has a higher mineral content with mature collagen and mineral constituents, which are characteristic of aged tissue. In situ fracture mechanics measurements and synchrotron radiation micro-computed tomography of the crack path indicated that vitamin D deficiency increases both the initiation and propagation of cracks by 22 to 31%. Thus, vitamin D deficiency is not simply associated with diminished bone mass. Our analyses reveal the aged nature of the remaining mineralized bone and its greatly decreased fracture resistance. Through a combination of characterization techniques spanning multiple size scales, our study expands the current clinical understanding of the pathophysiology of vitamin D deficiency and helps explain why well-balanced vitamin D levels are essential to maintain bone's structural integrity.

Paget's disease of bone (PDB) is the second most common bone disease mostly developing after 50 years of age at one or more localized skeletal sites; it is associated with severely high bone turnover, bone enlargement, bowing/deformity, cracking, and pain. Here, to specifically address the origins of the deteriorated mechanical integrity, we use a cohort of control and PDB human biopsies to investigate multiscale architectural and compositional modifications to the bone structure (ie, bone quality) and relate these changes to mechanical property measurements to provide further insight into the clinical manifestations (ie, deformities and bowing) and fracture risk caused by PDB. Here, at the level of the collagen and mineral (ie, nanometer-length scale), we find a 19% lower mineral content and lower carbonate-to-phosphate ratio in PDB, which accounts for the 14% lower stiffness and 19% lower hardness promoting plastic deformation in pathological bone. At the microstructural scale, trabecular regions are known to become densified, whereas cortical bone loses its characteristic parallel-aligned osteonal pattern, which is replaced with a mosaic of lamellar and woven bone. Although we find this loss of anisotropic alignment produces a straighter crack path in mechanically-loaded PDB cases, cortical fracture toughness appears to be maintained due to increased plastic deformation. Clearly, the altered quality of the bone structure in PDB affects the mechanical integrity leading to complications such as bowing, deformities, and stable cracks called fissure fractures associated with this disease. Although the lower mineralization and loss of aligned Haversian structures do produce a lower modulus tissue, which is susceptible to deformities, our results indicate that the higher levels of plasticity may compensate for the lost microstructural features and maintain the resistance to crack growth.

Osteoporotic patients treated with antiresorptive or anabolic agents experience an increase in bone mass and a reduction in incident fractures. However, the effects of these medications on bone quality and strength after a prolonged discontinuation of treatment are not known. We evaluated these effects in an osteoporotic rat model. Six-month-old ovariectomized (OVX) rats were treated with placebo, alendronate (ALN, 2 µg/kg), parathyroid hormone [PTH(1–34); 20 µg/kg], or raloxifene (RAL, 2 mg/kg) three times a week for 4 months and withdrawn from the treatments for 8 months. Treatment with ALN, PTH, and RAL increased the vertebral trabecular bone volume (BV/TV) by 47%, 53%, and 31%, with corresponding increases in vertebral compression load by 27%, 51%, and 31%, respectively (p < .001). The resulting bone strength was similar to that of the sham-OVX control group with ALN and RAL and higher (p < .001) with PTH treatment. After 4 months of withdrawal, bone turnover (BFR/BS) remained suppressed in the ALN group versus the OVX controls (p < .001). The vertebral strength was higher than in the OVX group only in ALN-treated group (p < .05), whereas only the PTH-treated animals showed a higher maximum load in tibial bending versus the OVX controls (p < .05). The vertebral BV/TV returned to the OVX group level in both the PTH and RAL groups 4 months after withdrawal but remained 25% higher than the OVX controls up to 8 months after withdrawal of ALN (p < .05). Interestingly, cortical bone mineral density increased only with PTH treatment (p < .05) but was not different among the experimental groups after withdrawal. At 8 months after treatment withdrawal, none of the treatment groups was different from the OVX control group for cortical or cancellous bone strength. In summary, both ALN and PTH maintained bone strength (maximum load) 4 months after discontinuation of treatment despite changes in bone mass and bone turnover; however, PTH maintained cortical bone strength, whereas ALN maintained cancellous bone strength. Additional studies on the long-term effects on bone strength after discontinuation and with combination of osteoporosis medications are needed to improve our treatment of osteoporosis. © 2011 American Society for Bone and Mineral Research.

Introduction Individual agents used to treat human osteoporosis reduce fracture risk by ~50-60%. Since agents that act with complementary mechanisms are available, sequential therapies that mix anti-resorptive and anabolic agents could improve fracture risk reduction, when compared to monotherapies. Methods We evaluated bone mass, bone microarchitecture, and bone strength in adult ovariectomized (OVX), osteopenic rats, during different sequences of vehicle (Veh), parathyroid hormone (PTH), alendronate (Aln), or raloxifene (Ral) in three 90 day treatment periods, over nine months. Differences among groups were evaluated. The interrelationships of bone mass and microarchitecture endpoints, and their relationship to bone strength were studied. Results Estrogen deficiency caused bone loss. OVX rats treated with Aln monotherapy had significantly better bone mass, microarchitecture, and bone strength than untreated OVX rats. Rats treated with an Aln drug holiday had bone mass and microarchitecture similar to the Aln monotherapy group, but with significantly lower bone strength. PTH-treated rats had markedly higher bone endpoints, but all were lost after PTH withdrawal without follow-up treatment. Rats treated with PTH followed by Aln had better bone endpoints than those treated with Aln monotherapy, PTH monotherapy, or an Aln holiday. Rats treated initially with Aln or Ral, then switched to PTH, also had better bone endpoints, than monotherapy treatment. Rats treated with Aln, then PTH, and returned to Aln had the highest values for all endpoints. Conclusion Our data indicate that anti-resorptive therapy can be coupled with an anabolic agent, to produce and maintain better bone mass, microarchitecture, and strength than can be achieved with any monotherapy.

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