Osteogenic lineage restriction by osteoprogenitors cultured on nanometric grooved surfaces – the role of focal adhesion maturation

The differentiation of progenitor cells is dependent on more than biochemical signalling. Topographical cues in natural bone extracellular matrix guide cellular differentiation through the formation of focal adhesions, contact guidance, cytoskeletal rearrangement and ultimately gene expression. Osteoarthritis and a number of bone disorders present as growing challenges for our society. Hence, there is a need for next generation implantable devices to substitute for, or guide, bone repair in vivo. Cellular responses to nanometric topographical cues need to be better understood in vitro in order to ensure the effective and efficient integration and performance of these orthopaedic devices. In this study, the FDA approved plastic polycaprolactone, was embossed with nanometric grooves and the response of primary and immortalised osteoprogenitor cells observed. Nanometric groove dimensions were 240 nm or 540 nm deep and 12.5 μm wide. Cells cultured on test surfaces followed contact guidance along the length of groove edges, elongated along their major axis and showed nuclear distortion, they formed more focal complexes and a lower proportions of mature adhesions relative to planar controls. Down-regulation of the osteoblast marker genes RUNX2 and BMPR2 in primary and immortalised cells was observed on grooved substrates. Down-regulation appeared to directly correlate with focal adhesion maturation, indicating the involvement of ERK 1/2 negative feedback pathways following integrin mediated FAK activation

Capillary-induced contact guidance

Topographical features are known to impose capillary forces on liquid droplets, and this phenomenon is exploited in applications such as printing, coatings, textiles and microfluidics. Surface topographies also influence the behavior of biological cells (i.e., contact guidance), with implications ranging from medicine to agriculture. An accurate physical description of how cells detect and respond to surface topographies is necessary in order to move beyond a purely heuristic approach to optimizing the topographies of biomaterial interfaces. Here, we have used a combination of Langmuir−Blodgett lithography and nanoimprinting to generate a range of synthetic microstructured surfaces with grooves of subcellular dimensions in order to investigate the influence of capillary forces on the biological process of contact guidance. The physical−chemical properties of these surfaces were assessed by measuring the anisotropic spreading of sessile water droplets. Having established the physical properties of each surface, we then investigated the influence of capillary forces on the processes of cellular contact guidance in biological organisms, using mammalian osteoblasts and germinating fungal spores as tester organisms. Our results demonstrate that capillary effects are present in topographical contact guidance and should therefore be considered in any physical model that seeks to predict how cells will respond to a particular surface topography