Tera M. Filion

Survival Responses of Human Embryonic Stem Cells to DNA Damage

Pluripotent human embryonic stem (hES) cells require mechanisms to maintain genomic integrity in response to DNA damage that could compromise competency for lineage‐commitment, development, and tissue‐renewal. The mechanisms that protect the genome in rapidly proliferating hES cells are minimally understood. Human ES cells have an abbreviated cell cycle with a very brief G1 period suggesting that mechanisms mediating responsiveness to DNA damage may deviate from those in somatic cells. Here, we investigated how hES cells react to DNA damage induced by ionizing radiation (IR) and whether genomic insult evokes DNA repair pathways and/or cell death. We find that hES cells respond to DNA damage by rapidly inducing Caspase‐3 and ‐8, phospho‐H2AX foci, phosphorylation of p53 on Ser15 and p21 mRNA levels, as well as concomitant cell cycle arrest in G2 based on Ki67 staining and FACS analysis. Unlike normal somatic cells, hES cells and cancer cells robustly express the anti‐apoptotic protein Survivin, consistent with the immortal growth phenotype. SiRNA depletion of Survivin diminishes hES survival post‐irradiation. Thus, our findings provide insight into pathways and processes that are activated in human embryonic stem cells upon DNA insult to support development and tissue regeneration. J. Cell. Physiol. 220: 586–592, 2009.

In vivo tissue responses to thermal-responsive shape memory polymer nanocomposites.

To explore the safe use of thermal-responsive shape memory polymers (SMPs) as minimally invasive tissue scaffolds, we recently developed a class of biodegradable POSS-SMP nanocomposites exhibiting stable temporary shape fixing and facile shape recovery within a narrow window of physiological temperatures. The materials were covalently crosslinked from star-branched building blocks consisting a bioinert polyhedral oligomeric silsesquioxane (POSS) core and 8 degradable poly(D,L-lactide) (PLA) arms. Here we examine the degradation profiles and immunogenicity of POSS-SMPs as a function of the PLA arm lengths using a rat subcutaneous implantation model. We show that POSS-SMPs elicited a mild foreign body type immune response upon implantation. The degradation rates of POSS-SMPs, both in vitro and in vivo, inversely correlated with the length of the PLA chains within the crosslinked amorphous network. Upon in vivo degradation of POSS-SMPs, a second acute inflammatory response was elicited locally, and the inflammation was able to resolve over time without medical interventions. One year after the implantation of POSS-SMPs, no pathologic abnormalities were detected from the vital/scavenger organs examined. These minimally immunogenic and biodegradable SMPs are promising candidates for scaffold-assisted tissue repair where both facile surgical delivery and controlled degradation of the scaffold are desired for achieving optimal short-term and long-term clinical outcomes.

Scalable Functional Bone Substitutes: Strategic Integration of Key Structural Elements of Bone in Synthetic Biomaterials

Over 40% of the disabling medical conditions of persons aged 18 years and over are musculoskeletal related. This number is even higher within the older population (Weinstein, 2000). Surgical treatment for age-, traumaor cancer-induced critical-size bone loss is particularly challenging. Current grafting material options for scaffold-assisted surgical repair of critical-size bone loss include autogenic bone grafts (autografts), allogenic bone grafts (allografts), and synthetic bone substitutes. Still considered as a golden standard, autografts, retrieved from patients’ own skeleton, are used in approximately 50% of all orthopedic bone grafting procedures. Complications arising from possible donor-site morbidity and insufficient grafting materials are major drawbacks of autografting procedures (Bostrom & Seigerman, 2005). In addition, this option is highly limited within the aging population as the elderly are less likely to be qualified for such a procedure due to higher incidences of osteoporosis and metabolic diseases. Allografts, obtained from another human donor or animal cadaver, represent a useful alternate to autografts, and are used in approximately 40% of bone grafting surgeries. However, allografting procedures suffer from risks for rejection and disease transmission, and a significant structural failure rate due to poor tissue integration, both structurally and biochemically (Blokhuis & Lindner, 2008; Bostrom & Seigerman, 2005; Eagan & McAllister, 2009; Goldberg & Stevenson, 1994). These limitations, along with the growing aging population, has led to an increasing need for viable synthetic bone substitute alternatives (Salgado et al., 2004). Current clinically used synthetic bone grafts such as brittle ceramics and weak gel foams are used in only ~10% of all bone grafting procedures (Bostrom & Seigerman, 2005), primarily due to their unstable graft fixation and insufficient tissue-graft interactions (Carson & Bostrom, 2007; Goldberg & Stevenson, 1994; Place et al., 2009; Stevens, 2008). In the past two decades, many new synthetic bone grafts designed to mimic key structural and biochemical properties of bone to enhance osteointegration and graft healing have emerged in literature. This rapidly evolving field has been extensively reviewed by others, including broad overviews of current requirements and techniques for preparing synthetic bone grafts (Burg et al., 2000; Salgado et al., 2004), calcium phosphate–based bone substitutes (De Long et al., 2007), polymeric bone substitutes (Seal et al., 2001), and biomimetic nanocomposite orthopedic biomaterials (Chan et al., 2006; Murugan & Ramakrishna, 2005). This chapter highlights the evolvement of non-metallic orthopedic biomaterials from bioinert,