Biomaterials Science Blog

By precisely controlling the number of various adhesive ligands in integrin-sized nanodomains, this study provides important insights about the impact of local ligand redundancy on mesenchymal stem cell adhesion and phenotype.

In regenerative medicine, one of the limiting factors has been the number, complex isolation and limited lifespan of patients’ differentiated cell types for seeding scaffolds and subsequent cultivation of a functional tissue. The use of patient-specific stem cell populations, which have a prolonged life-span and can potentially be differentiated into any cell type desired, has emerged as the prevailing tissue engineering paradigm in the past decade. Despite its tremendous potential, steering stem cell differentiation towards a specific phenotypical outcome has been challenging. Polymer surfaces have been used extensively to elucidate the permissive cues required to drive selective differentiation, such as surface functionalities and adhesive molecules. Especially the impact of surface distribution and concentration of adhesive ligands remains a question mark. Yet, it is challenging to tether multiple distinct functionalities in a controllable manner and assess their respective effects on stem cell adhesion and differentiation.

Based at the University of Queensland, Justin Cooper-White and Haiqing Li sought to increase the understanding of surface ligand-dependent stem cell fate by precisely controlling the concentration and spatial organization of various ligands. To this end, they analyzed the effects of two well-known cell adhesion ligands, IKVAV and RGD, on mesenchymal stem cell adhesion and morphology.

Polystyrene-polyethyleneoxide (PS-PEO) block copolymers were shown to self-assemble into polymer films with a uniform distribution of cylindrical nanodomains of PEO, approximately the size of adhesion-controlling cell integrin pairs. By adding varying percentages of azide- or aminooxy-terminated PEO, orthogonal click chemistry was used to sequentially functionalize those cylindrical nanodomains with a controllable local density of grafted adhesion sequences IKVAV and RGD. Then, the researchers assessed the effects of varying local IKVAV and RGD densities in the nanodomains on human mesenchymal stem cell adhesion, spreading morphology and focal adhesion complex formation. They found that with increasing IKVAV or RGD ligand density, leading to ligand ‘redundancy’ for integrin pairs, stem cells showed increased attachment, spreading and stress fiber formation. Moreover, an increase in ratio between RGD and IKVAV densities increased stem cell adhesion.

Together, the researchers found that PS-PEO block co-polymers with functional end-groups, permitting orthogonal chemistry, allowed tight control of ligand decoration within cylindrical nanodomains. Furthermore, using IKVAV and RGD as exemplars, they showed the effect of varying ligand redundancy within nanodomains on stem cell fate. The reported self-assembling substrates offer a highly flexible platform technology to investigate and elucidate the impact of various ligands and their density on integrin binding, which also determines cell phenotype.

Changing ligand number and type within nanocylindrical domains through kinetically constrained self-assembly – impacts of ligand ‘redundancy’ on human mesenchymal stem cell adhesion and morphology
H. Li and J. J. Cooper-White
Biomater. Sci., 2014, Advance Article DOI: 10.1039/C4BM00109E

Robert van Lith is currently a Ph.D. Candidate in the Biomedical Engineering department at Northwestern University, working on novel biomaterials to modulate oxidative stress in tissues. He received an American Heart Association Fellowship and Society for Biomaterials award for his work. He holds B.S. and M.S. degrees in Biomedical Engineering from Eindhoven University of Technology, the Netherlands. Read more about Robert’s research publications here.

The cover story for the July 2014 issue of Biomaterials Science highlights the use of nanovehicle particles for the targeted delivery of chemotherapy drugs to tumor cells in bone.

Bone metastasis, or the spreading of tumor cells from the initial tumor site to bone tissue, marks the stage of cancer progression where the diseased is deemed incurable. Currently, the main method to treat bone metastasis is to utilize aggressive chemotherapy drugs and destroy metastatic tumor cells.  Unfortunately, untargeted chemotherapy treatments result in death of both cancerous and healthy tissue. Treatments could be significantly improved with the technology to specifically target the destruction of tumor cells without affecting the surrounding bone tissue.

Researchers at the University of Utah, University of Iowa, and Wuhan University in China are making this desired technology a reality. Wang et al. have developed a nanovehicle capable of targeting tumor cells within bone tissue.  Nanovehicles (NVs) are essentially nano-sized cargo containers capable of delivering drugs to a desired destination. The cleverly designed NVs are designed to target diseased bone tissue to deliver chemotherapy drugs at a specific site.

The NVs developed in this study have three design features:

(1) The NVs core is made of hydrophilic and hydrophobic block co-polymers able to form a shell-like structure known as a micelle. The micelle serves as the cargo vessel that contains the chemotherapy drug doxorubicin.

(2) The outermost segments of the NVs are negatively charged bone-targeting peptides. The peptide is responsible for the accumulation of NVs in skeletal tissue, and the negative charge prevents cellular uptake of the NVs into healthy tissue.

(3) For uptake into cancer cells, the particle must be positively charged. Therefore, a positively charged peptide linker was added in between the micelle core and the negatively charged bone-targeting region. The peptide linker is cleavable by cathepsin K enzyme (CTSK), which is overexpressed in bone metastatic lesions. When the NV enters a metastatic lesion, the CTSK cleaves off the negatively charged peptides and exposes the positively charged peptides. This charge reversal allows for tumor cells present in the metastatic lesion to uptake the NV.

After extensively characterizing the nanoparticles with NMR and zeta potential measurements, the NVs were used to observe decreased tumor cell viability under CTSK-rich conditions in vitro. More strikingly, in vivo tests of the NVs in a mouse model of bone metastasis showed significant increases in mouse survival and decreases in overall tumor burden.

Collectively, these ingeniously designed nanovehicles show great promise in providing an effective targeted therapy for bone metastasis.

Peptide decoration of nanovehicles to achieve active targeting and pathology-responsive cellular uptake for bone metastasis chemotherapy
X. Wang, Y. Yang, H. Jia, W. Jia, S. Miller, B. Bowman, J. Feng, and F. Zhan
Biomater. Sci., 2014, Advance Article DOI: 10.1039/C4BM00020J

Brian Aguado is currently a Ph.D. Candidate and NSF Fellow in the Biomedical Engineering department at Northwestern University. He holds a B.S. degree in Biomechanical Engineering from Stanford University and a M.S. degree in Biomedical Engineering from Northwestern University. Read more about Brian’s research publications here.

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This study introduces the concept of the “personalized protein corona” as a determinant factor in nano-biomedicine, specifically regarding the biological identity and fate of nanoparticles.

The concept of personalized medicine has become increasingly relevant in healthcare over the past decade. Ever since the human genome was unraveled, the idea of tailoring therapies specific to the individual patient has been at the forefront of medicine. Diagnosis, prognosis and most importantly treatment are thought to eventually be adapted to each individual’s genetic and pathophysiological makeup. Nonetheless, biomaterials for invasive use (e.g. implantation or injection) are still largely being designed for general use. This study argues for a much more specific approach of biomaterials design, looking at the nanoparticles (NPs) in particular.

A collaborative effort headed by Dr. Mahmoudi from Stanford University, US, and the University of Medical Sciences in Tehran, Iran, Hajipour et al. evaluated effects of the altered protein composition of human plasma in different disease states on NP corona formation. Two NP materials were chosen for their analysis, hydrophobic polystyrene and hydrophilic silica, which are widely investigated for biomedical material approaches and currently evaluated for safety purposes. 

 NPs were exposed to plasma solutions of different patient populations to form hard protein coronas, after which the composition of NP protein coronas was analyzed using SDS-page, densitometry and zeta-potential measurements. Interestingly, the authors found that changes in plasma protein composition due to underlying conditions alter the protein decoration on NPs, resulting in an unpredictable biologic identity of the NPs and thus their biological fate. The severity of the condition was also found to impact protein deposition. Intriguingly, even among healthy subjects there was considerable variation in corona composition, creating even more of an impetus for personalized NP design.

Collectively, the researchers found that NP protein corona distinctly depends on particular condition, severity of condition and combination of conditions, with relatively low variation between subjects with similar health profiles. Moving forward, identifying the personalized protein corona for individual patients or at least the expected protein corona in specific patient populations may become an integral aspect of nanoparticle-based biomaterial therapies.

Personalized protein coronas: a “key” factor at the nanobiointerface 
M. J. Hajipour, S. Laurent, A. Aghaie, F. Rezaee and M. Mahmoudi
Biomater. Sci., 2014, Advance Article DOI: 10.1039/c4bm00131a

Robert van Lith is currently a Ph.D. Candidate in the Biomedical Engineering department at Northwestern University, working on novel biomaterials to modulate oxidative stress in tissues. He received an American Heart Association Fellowship and Society for Biomaterials award for his work. He holds B.S. and M.S. degrees in Biomedical Engineering from Eindhoven University of Technology, the Netherlands. Read more about Robert’s research publications here.