TUNING CELL-CELL COHESION
Single-stranded DNA's connected to lipids can be inserted into membranes to induce a form of programmable cell-cell adhesion, or cohesion, between complementarily-modified cells. Unlike most natural cohesion modes, these molecules are not connected directly to the internal cytoskeleton. This means that we can exert independent changes in either the concentration of adhesive molecules or in the elastic mechanical properties of the cell to evoke changes in the geometry of cohered cell doublets. We demonstrate precision tuning of cohesion that is unparalleled by any other method we are aware of. This is thanks to the decoupled nature of membrane inserts, which only indirectly feedback with the cytoskeleton.
I.T. Hoffecker*, Y. Arima, H. Iwata Mechanically-orthogonal tuning of intercellular cohesion in cell doublets prepared by oligonucleotide-lipid hybridization Submitted to Biophysical Journal. Preprint on ArXiv. 2015
PROGRAMMABLE CELL ATTACHMENT AND SEVERING
In this paper, we explore methods to cleave DNA mediating artificial attachment of DNA-lipid-modified cells to flat substrates or other cells. We used BamHI to sever the DNA tethers of cells bearing the correct BamHI recognition site while leaving other tethered cells alone. We also used Benzonase, a powerful nonspecific nuclease, to digest all the tethers and detach cells irrespective of their anchor sequences. Now we have a way to programmably attach and detach cells!
I.T. Hoffecker, N. Takemoto, Y. Arima, H. Iwata* Sequence-specific nuclease-mediated release of cells tethered by oligonucleotide phospholipids Biomaterials. 53 , 318-329 , 2015
PATTERNING SOFT GELS WITH STIFF ISLANDS To examine the rigidity sensing mechanisms of cells, I previously designed a method for producing micropatterned arrays of rigid photoresist islands grafted to an underlying hydrogel of tunable stiffness. The islands can be made adhesive to cells while the exposed hydrogel is left inert. This is the detailed protocol for producing such substrates.
S. Wong, W.H. Guo, I.T. Hoffecker, Y.L. Wang* Preparation of a micropatterned rigid-soft composite substrate for probing cellular rigidity sensing Methods Cell Biol. 121 , 3-15 , 2014
INDUCING MAJOR CHANGES IN CELL SORTING PATTERNS Cell sorting is a process where cells of different types spontaneously undergo relative migration, leading to distinct multicellular domains of differing cell type composition. The range of cell-cell communication is limited, and global patterns emerge due to the iteration of interactions between neighbors. We observed major changes in global sorting patterns after pharmacologically inducing changes in the individual cell-cell cohesive interactions.
I.T. Hoffecker, H. Iwata* Manipulation of cell sorting in mesenchymal stromal cell-islet cell co-aggregate spheroids Tissue Engineering Part A. 20 , 1643-1653 , 2014
DNA-LIPID-MEDIATED ADHESION vs INTEGRIN-MEDIATED ADHESION We induced an artificial form of adhesion between cells modified with ssDNA-lipids and substrates inkjet-printed with complementary ssDNA. When incubated without serum or adhesive proteins in the media, cells took on a bizarre dendrite-like morphology. When in the presence of serum proteins, cells slowly pulled away from their DNA anchors leaving trails of lipid matter behind. This study demonstrates the clear and interesting distinction between natural and artificially induced forms of adhesion.
K. Sakurai, I.T. Hoffecker, H. Iwata* Long term culture of cells patterned on glass via membrane-tethered oligonucleotides Biomaterials. 34 , 361-370 , 2013
CELL RIGIDITY SENSING IS BLIND TO FINE DETAILS Do cell focal adhesion contain molecular mechanical sensors to measure the intrinsic stiffness of their underlying substrates? Perhaps not. We built micropatterned arrays of rigid islands of sub-cellular size grafted to a soft underlying hydrogel. Though their adhesive contact was restricted to the islands, the cells on these patterns behaved as they would on uniformly soft substrates, apparently ignoring the intrinsic stiffness of the islands. You could say that cells 'pull' their substrates rather than 'pinching' them.
I.T. Hoffecker, W.H. Guo, Y.L. Wang* Assessing the spatial resolution of cellular rigidity sensing using a micropatterned hydrogel-photoresist composite Lab Chip. 11 , 3538-3544 , 2011
MECHANICS OF POROUS HYDROGEL SCAFFOLDS Preparing hydrogel scaffolds with pores via sphere-templating reduced the elastic modulus and ultimate tensile stress but interestingly increased ultimate tensile strain. We looked at several parameters such as polymer type, pore diameter, and gel crosslinking density and examined their effect on scaffold mechanical properties. Our results will help guide better hydrogel scaffold designs for tissue engineering.
S.M. LaNasa, I.T. Hoffecker, S.J. Bryant* The role of pore size on the mechanical properties of porous poly(ethylene glycol) and poly(2-hydroxyethyl methacrylate) hydrogels Journal of Biomedical Materials Research - Part B: Applied Biomaterials. 96B , 294-302 , 2011