Au nanostar nanoparticle as a bio-imaging agent and its detection and visualization in biosystems

In the present work, we report the imaging of Au nanostars nanoparticles (AuNSt) and their multifunctional applications in biomedical research and theranostics applications. Their optical and spectroscopic properties are considered for the multimodal imaging purpose. The AuNSt are prepared by the seed-meditated method and characterized for use as an agent for bio-imaging. To demonstrate imaging with AuNSt, penetration and localization in different biological models such as cancer cell culture (A549 lung carcinoma cell), 3D tissue model (multicellular tumor spheroid on the base of human oral squamous carcinoma cell, SAS) and murine skin tissue are studied. AuNSt were visualized using fluorescence lifetime imaging (FLIM) at two-photon excitation with a pulse duration 140 fs, repetition rate 80 MHz and 780 nm wavelength femtosecond laser. Strong emission of AuNSt at two-photon excitation in the near infrared range and fluorescence lifetime less than 0.5 ns were observed. It allows using AuNSt as a fluorescent marker at two-photon fluorescence microscopy and lifetime imaging (FLIM). It was shown that AuNSt can be observed inside a thick sample (tissue and its model). This is the first demonstration using AuNSt as an imaging agent for FLIM at two-photon excitation in biosystems. Increased scattering of near-infrared light upon excitation of AuNSt surface plasmon oscillation was also observed and rendered using a possible contrast agent for optical coherence tomography (OCT). AuNSt detection in a biological system using FLIM is compared with OCT on the model of AuNSt penetrating into animal skin. The AuNSt application for multimodal imaging is discussed.

A road to kidney tubules via the Wnt pathway

Classical in vitro studies indicate that tubule induction in the kidney mesenchyme is mediated by cell-cell contacts between the inducer tissue and the metanephric mesenchyme. Induction is completed within the first 24 h, after which tubules will form because of stimulated cell proliferation, migration, and cell adhesion. Recent evidence has revealed an essential role for the secreted signals from the Wnt gene family. Of these, Wnt-4 is expressed in developing tubules and knocking out its function perturbed kidney development. More detailed studies demonstrated normal condensation, but tubules were missing. Subsequent experiments indicated that Wnt-4 is also a sufficient signal to trigger tubulogenesis. Cells that were engineered to express Wnt-4 not only induced tubulogenesis in the kidney mesenchyme of Wnt-4 mutant embryos, but also induced tubules in the wild type mesenchyme. With the transfilter induction assay, Wnt-4-mediated induction was completed within the first 24 h, depending on the presence of proteoglycans and cell-cell contacts between the interactants. In addition, Wnt-4 autoinduced expression of its own gene and a panel of other components of the Wnt signalling pathway, such as frizzleds and a candidate Wnt antagonist from the secreted frizzled-related protein family. Taken together, the data provide evidence of an essential role for Wnt signal transmission and transduction pathways in the induction of kidney tubules, and the findings have paved the way for detailed molecular studies.

Wnts as kidney tubule inducing factors

Since the discovery that inductive tissue interactions regulate nephrogenesis, one of the aims has been to identify the molecules that mediate this induction. The small size of embryonic tissue has limited the possibilities to identify the inducers biochemically, even though such efforts were directed to study, e.g. neural induction (for a comprehensive review, Saxén and Toivonen, Primary embryonic induction, Academic Press, London, 1962). The rapid progress in molecular biology made it possible to identify genes from minute amounts of tissue and provided techniques to generate recombinant proteins to assay their action in classic experimental systems. This led to the identification of some signals that are involved in primary and secondary inductive interactions during embryogenesis. Here, we will review evidence suggesting that secreted signaling molecules from the Wnt gene family mediate kidney tubule induction.