In this study, we explored the concept of introducing asymmetry to cell shapes by patterned cell culture substrates, and investigated the consequence of this induced asymmetry to cell migration behaviors. Three patterns, named “Squares”, “Grating”, and “Arcs” were fabricated, representing different levels of rotational asymmetry. Using time-lapse imaging, we systematically compared the motility and directionality of mouse osteoblastic cells MC3T3-E1 cultured on these patterns. Cells were found to move progressively faster on “Arcs” than on “Grating”, and cells on “Squares” were the slowest, suggesting that cell motility correlates with the level of rotational asymmetry of the repeating units of the pattern. Among these three patterns, on the “Arcs” pattern, the least symmetrical one, cells not only moved with the highest velocity but also the strongest directional persistence. Although this enhanced motility was not associated with the detected number of focal adhesion sites in the cells, the pattern asymmetry was reflected in the asymmetrical cell spreading. Cells on the “Arcs” pattern consistently displayed larger cytoplasmic protrusion on one side of the cell. This asymmetry in cell shape determined the direction and speed of cell migration. These observations suggest that topographical patterns that enhance the imbalance between the leading and trailing fronts of adherent cells will increase cell speed and control movement directions. Our discovery shows that complex cell behaviors such as the direction of cell movement are influenced by simple geometrical principles, which can be utilized as the design foundation for platforms that guide and sort cultured cells. © 2014 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 103: 2383–2393, 2015.

We report the cellular properties of a luminescent cyclometalated iridium(III) complex, [Ir(pq)₂(phen-ITC)](PF₆) (Ir-ITC; Hpq=2-phenylquinoline, phen-ITC=5-isothiocyanate-1,10-phenanthroline), that efficiently and specifically labels mitochondria in living mammalian cells. Ir-ITC can be covalently conjugated to its protein targets, and its luminescence survived cell lysis, protein extraction, and gel electrophoresis under denaturing conditions. The conjugation of Ir-ITC with live-cell proteins is rapid and highly selective; the process requires active cellular metabolism, as the conjugation is abolished at nonphysiological temperature or in the presence of sodium azide. Based on measurements of the luminescence intensity, we have devised a biochemical fractionation procedure that allows the enrichment of the conjugated proteins, and their subsequent separation by two-dimensional gel electrophoresis (2DGE). Luminescent protein spots were picked from the gel and analyzed by mass spectrometry; this resulted in the identification of 46 proteins. Many of the strongly luminescently labeled proteins are mitochondrial proteins. One of the targets is VDAC1 (voltage-dependent anion channel 1). Consistent with known phenotypes of VDAC1 deregulation, prolonged exposure of cells to Ir-ITC led to significant mitochondrial shortening and fragmentation. As far as we know, this is the first report on the molecular characterization of the interactions of a luminescent dye with its biological targets. As many biological dyes exhibit specific intracellular staining patterns, the identification of their molecular targets can help elucidate the mechanisms behind their staining specificities and cytotoxicity. We believe our biochemical approach can be applied to identify the targets of a wide range of fluorescent and luminescent probes.

An organometallic cyclometalated platinum(II) complex, [Pt(L³)Cl][PF₆], has been synthesised from a specially designed cyclometalating ligand, HL³ (triphenyl{5-[3-(6-phenylpyridin-2-yl)-1H-pyrazol-1-yl]pentyl}phosphonium chloride), that contains a pendant carbon chain carrying a terminal cationic triphenylphosphonium moiety. Aside from its room temperature single-photon luminescent properties in solution, [Pt(L³)Cl]⁺ can also produce two-photon-induced luminescence at room temperature upon excitation at 700 nm from a mode-locked Ti:sapphire laser. Its two-photon absorption cross-section in DMF at room temperature was measured to be 28.0×10⁻⁵⁰ cm⁴ s photon⁻¹. [Pt(L³)Cl]⁺ is able to selectively stain the cell nucleolus. This has been demonstrated by two-photon confocal imaging of live and methanol-fixed HeLa (human cervical carcinoma) and 3T3 (mouse skin fibroblasts) cells. This organelle specificity is likely to be related to its special affinity for proteins within cell nucleoli. As a result of such protein affinity, [Pt(L³)Cl]⁺ is an efficient RNA transcription inhibitor and shows rather profound cytotoxicity. On the other hand, the organelle-specific labelling and two-photon-induced luminescent properties of [Pt(L³)Cl]⁺ renders it a useful nuclear dye for the 3-dimensional reconstruction of optical sections of thick tissues, for example, mouse ileum tissues, by multiphoton confocal microscopy.