Two-photon phosphorescence lifetime imaging of cells and tissues using a long-lived cyclometallated (NpyridylCphenylNpyridyl)-C-boolean AND-N-boolean AND Pt(II) complex

Using a combination of multiphoton excitation, confocal scanning, TCSPC and beam blanking in conjunction with a cyclometallated Npyridyl^Cphenyl^Npyridyl Pt(II) complex (1) with a long luminescence lifetime, we demonstrate lifetime mapping of living cells and histological tissue sections over a time-frame of 50 microseconds, using a laser on/off “beam blanking” approach. This method of performing phosphorescence lifetime imaging microscopy (PLIM) represents an order of magnitude enhancement of the two-photon time-resolved emission imaging microscopy (TP-TREM) method, where in order to achieve a longer imaging window, the excitation laser repetition rate was reduced by cavity dumping [Chem. Sci., 2014, 5, 879]. The method complements and expands other existing imaging methodologies by enabling simultaneous PLIM and FLIM (fluorescence lifetime imaging microscopy – recorded between beam blanking), whilst maintaining essential sub-micron spatial resolution. We demonstrate how the Pt(II) complex can be used to distinguish between cell nuclei and matrix proteins on the basis of emission lifetime, in both structured and homogeneous tissue sections; whilst also revealing how the Pt(II) emission lifetime varies with tissue matrix composition. The proposed imaging approach can be used in conjunction with any biocompatible emissive probe with a long emission lifetime – exemplified here by (1) – and for an array of fluorescent/phosphorescent labels, where discrimination is lifetime-based

Direct measurement of local oxygen concentration in the bone marrow of live animals

Characterizing how the microenvironment, or niche, regulates stem cell activity is central to understanding stem cell biology and to developing strategies for therapeutic manipulation of stem cells1. Low oxygen tension (hypoxia) is commonly thought to be a shared niche characteristic in maintaining quiescence in multiple stem cell types2–4. However, support for the existence of a hypoxic niche has largely come from indirect evidence such as proteomic analysis5, expression of HIF-1 and related genes6, and staining with surrogate hypoxic markers (e.g. pimonidazole)6–8. Here we perform direct in vivo measurements of local oxygen tension (pO2) in the bone marrow (BM) of live mice. Using two-photon phosphorescence lifetime microscopy (2PLM), we determined the absolute pO2 of the BM to be quite low (<32 mmHg) despite very high vascular density. We further uncovered heterogeneities in local pO2, with the lowest pO2 (~9.9 mmHg, or 1.3%) found in deeper peri-sinusoidal regions. The endosteal region, by contrast, is less hypoxic as it is perfused with small arteries that are often positive for the marker nestin. These pO2 values change dramatically after radiation and chemotherapy, pointing to the role of stress in altering the stem cell metabolic microenvironment

Microcirculation of the small intestine at various phases of its motor activity (Russian)

[No abstract available