Metasurface-Enhanced Mid-Infrared Spectrochemical Imaging of Tissues

Label-free and nondestructive mid-infrared vibrational hyperspectral imaging is an essential tissue analysis tool, providing spatially resolved biochemical information critical to understanding physiological and pathological processes. However, the chemically complex and spatially heterogeneous composition of tissue specimens and the inherently weak interaction of infrared light with biomolecules limit the analytical performance of infrared absorption spectroscopy. Here, an advanced mid-infrared spectrochemical tissue imaging modality is introduced using metasurfaces that support strong surface-localized electromagnetic fields to capture quantitative molecular maps of large-area murine brain tissue sections. The approach leverages polarization-multiplexed multi-resonance plasmonic metasurfaces to simultaneously detect various functional biomolecules. The surface-enhanced mid-infrared spectral imaging method eliminates the non-specific effects of bulk tissue morphology on quantitative spectral analysis and improves chemical selectivity. This study shows that metasurface enhancement increases the retrieval of amide I and II bands associated with protein secondary structures. Moreover, it is demonstrated that plasmonic metasurfaces enhance the chemical contrast in infrared images and enable detection of ultrathin tissue regions that are not otherwise visible to conventional mid-infrared spectral imaging. While this work uses murine brain tissue sections, the chemical imaging method is well-suited for other tissue types, which broadens its potential impact for translational research and clinical histopathology.

Impact of tissue preservation on collagen fiber architecture

Microarchitectural features of collagen-rich extracellular matrices provide the mechanical foundation for tissue function and exhibit topographical cues that influence cellular behavior including proliferation, migration and protein expression. Preservation of tissue microarchitecture is required for accurate evaluation of tissue characteristics and pathology. It is unclear whether common tissue preservation methods possess equal ability to preserve microarchitecture. We investigated collagen microarchitecture in samples that had been flash frozen, fixed in formalin or preserved in RNAlater®, and which contained both collagen-rich and collagen-sparse regions. Fibrillar collagen organization was characterized using picrosirius red staining and second harmonic generation (SHG) microscopy. Maintenance of collagen fiber characteristics compared to the gold standard of flash freezing depended on both the method of preservation and the local collagen content of the tissue. Both formalin fixation and RNAlater® preserved collagen fiber characteristics similar to flash freezing in collagen-rich areas of the tissue, but not in collagen-sparse regions. Analysis using picrosirius red staining indicated preservation-dependent changes in overall tissue architecture and suprafibrillar organization. Together with considerations of cost, ease of use, storage conditions and ability to use the preserved tissue for RNA or protein analysis, our quantitative characterization of the effects of preservation method on collagen microarchitecture may help investigators select the most appropriate preservation approach for their needs.

Abstract P1-06-02: Inflammatory stromal cell response induced by collagen dense stroma is regulated by cyclooxygenase-2 in a mouse breast cancer model

Breast cancer is the most common invasive cancer in women, causing 40,000 deaths yearly in the United States. Increased mammographic density correlates with over four-fold increase risk for breast cancer, making it one of the greatest risk factors known for this disease. High breast density is mainly attributable to elevated collagen matrix deposition in the breast tissue, and we have found that increased collagen in the Col1a1tm1jae mouse model promotes mammary tumor formation and progression. Increased collagen density in vitro increases expression of PTGS2 (prostaglandin-endoperoxide synthase 2), the gene for cyclooxygenase-2 (COX-2), by over four fold. COX-2 over-expression is observed in 40% of invasive breast carcinoma cases and correlates with poor prognosis. Based on these findings, we hypothesized that inhibition of COX-2 may be an effective therapeutic in the context of mammary tumors arising in dense tissue. Celecoxib is a selective non-steroidal anti-inflammatory drug (NSAID) that specifically inhibits COX-2. To understand how COX-2 affects response to collagen matrix density, we utilized our previously characterized mouse model of MMTV-PyVT tumors in a wild type (wt) or Col1a1tm1jae background (HD, High Density). Col1a1tm1jae heterozygote or wt littermates were randomly assigned at 11 weeks of age to treatment with vehicle or celecoxib at 0.2mg per mouse per day. Oral treatment was given daily for 21 days. We found that MMTV-PyVT tumors responded to celecoxib in a manner that is regulated by matrix density. Tumors that arose on the dense Col1a1tm1jae background (HD) were larger and more invasive (p < .0001) and expressed higher levels of COX-2 and PGE2 than their wt littermates. Both COX-2 (wt: p = 0.0025, HD: p < 0.0001) and PGE2 (wt: p = 0.0196, HD: p = 0.0002) levels were decreased in animals treated with celecoxib. However, Ki-67 and syndecan 1 levels decreased only in HD mice that received treatment with celecoxib (p = 0.0003, p = 0.0007, respectively). In addition, total number of macrophages (wt: p = 0.0052, HD: p < 0.0001) and fibroblasts (wt: p = 0.0133, HD: p = 0.0003) were diminished in animals treated with celecoxib independently of collagen density. On the other hand, cancer associated fibroblast (CAF) population was diminished only in HD mice treated with celecoxib (p = 0.0002) which, along with the synedan-1 result suggests that celecoxib selectively abrogates the inflammatory response from reactive stroma only. Consequently, we find that celecoxib treatment remodels collagen fiber organization, such that it more closely resembles wt assembly. Ongoing studies will identify different macrophage populations, and which stromal cell populations express COX-2 and PGE2. These findings suggest that COX-2 has a direct role in modulating tumor progression in dense matrices which promote invasion. Moreover, these findings suggest that COX-2 may be an effective therapeutic target for women with dense breast tissue.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P1-06-02.