Rapid, Single-Cell Analysis and Discovery of Vectored mRNA Transfection In Vivo with a loxP-Flanked tdTomato Reporter Mouse.

mRNA therapeutics hold promise for the treatment of diseases requiring intracellular protein expression and for use in genome editing systems, but mRNA must transfect the desired tissue and cell type to be efficacious. Nanoparticle vectors that deliver the mRNA are often evaluated using mRNA encoding for reporter genes such as firefly luciferase (FLuc); however, single-cell resolution of mRNA expression cannot generally be achieved with FLuc, and, thus, the transfected cell populations cannot be determined without additional steps or experiments. To more rapidly identify which types of cells an mRNA formulation transfects in vivo, we describe a Cre recombinase (Cre)-based system that permanently expresses fluorescent tdTomato protein in transfected cells of genetically modified mice. Following in vivo application of vectored Cre mRNA, it is possible to visualize successfully transfected cells via Cre-mediated tdTomato expression in bulk tissues and with single-cell resolution. Using this system, we identify previously unknown transfected cell types of an existing mRNA delivery vehicle in vivo and also develop a new mRNA formulation capable of transfecting lung endothelial cells. Importantly, the same formulations with mRNA encoding for fluorescent protein delivered to wild-type mice did not produce sufficient signal for any visualization in vivo, demonstrating the significantly improved sensitivity of our Cre-based system. We believe that the system described here may facilitate the identification and characterization of mRNA delivery vectors to new tissues and cell types.

Tumor Cell-Driven Extracellular Matrix Remodeling Drives Haptotaxis during Metastatic Progression.

UNLABELLED: Fibronectin (FN) is a major component of the tumor microenvironment, but its role in promoting metastasis is incompletely understood. Here, we show that FN gradients elicit directional movement of breast cancer cells, in vitro and in vivo Haptotaxis on FN gradients requires direct interaction between α5ß1 integrin and MENA, an actin regulator, and involves increases in focal complex signaling and tumor cell-mediated extracellular matrix (ECM) remodeling. Compared with MENA, higher levels of the prometastatic MENA(INV) isoform associate with α5, which enables 3-D haptotaxis of tumor cells toward the high FN concentrations typically present in perivascular space and in the periphery of breast tumor tissue. MENA(INV) and FN levels were correlated in two breast cancer cohorts, and high levels of MENA(INV) were significantly associated with increased tumor recurrence as well as decreased patient survival. Our results identify a novel tumor cell-intrinsic mechanism that promotes metastasis through ECM remodeling and ECM-guided directional migration. SIGNIFICANCE: Here, we provide new insight into how tumor cell:ECM interactions generate signals and structures that promote directed tumor cell migration, a critical component of metastasis. Our results identify a tumor cell-intrinsic mechanism driven by the actin regulatory protein MENA that promotes ECM remodeling and haptotaxis along FN gradients. Cancer Discov; 6(5); 516-31. ©2016 AACR.See related commentary by Santiago-Medina and Yang, p. 474This article is highlighted in the In This Issue feature, p. 461.

PTP1B-dependent regulation of receptor tyrosine kinase signaling by the actin-binding protein Mena.

During breast cancer progression, alternative mRNA splicing produces functionally distinct isoforms of Mena, an actin regulator with roles in cell migration and metastasis. Aggressive tumor cell subpopulations express Mena(INV), which promotes tumor cell invasion by potentiating EGF responses. However, the mechanism by which this occurs is unknown. Here we report that Mena associates constitutively with the tyrosine phosphatase PTP1B and mediates a novel negative feedback mechanism that attenuates receptor tyrosine kinase signaling. On EGF stimulation, complexes containing Mena and PTP1B are recruited to the EGFR, causing receptor dephosphorylation and leading to decreased motility responses. Mena also interacts with the 5' inositol phosphatase SHIP2, which is important for the recruitment of the Mena-PTP1B complex to the EGFR. When Mena(INV) is expressed, PTP1B recruitment to the EGFR is impaired, providing a mechanism for growth factor sensitization to EGF, as well as HGF and IGF, and increased resistance to EGFR and Met inhibitors in signaling and motility assays. In sum, we demonstrate that Mena plays an important role in regulating growth factor-induced signaling. Disruption of this attenuation by Mena(INV) sensitizes tumor cells to low-growth factor concentrations, thereby increasing the migration and invasion responses that contribute to aggressive, malignant cell phenotypes.

A new chemotaxis device for cell migration studies.

This study presents the design and optimization for in vitro use of a new versatile chemotaxis device called the NANIVID (NANo IntraVital Imaging Device), developed using advanced nano/micro fabrication techniques. The device is fabricated using microphotolithographic techniques and two substrates are bonded together using a thin polymer layer creating a sealed device with one outlet. The main structure of the device consists of two Pyrex substrates: an etched chemoattractant reservoir and a top cover, with a final size of 0.2 × 2 × 3 mm. This reservoir contains a hydrogel blend with EGF which diffuses out through a small (∼9.10(3)µm(2)) outlet. This reservoir sustains a steady release of growth factor into the surrounding environment for several hours establishing a consistent concentration gradient from the device. The focus of this study was to design and optimize the new device for cell chemotaxis studies in breast cancer cells in cell culture. Our results show that we have created a flexible, cheap, miniature and autonomous chemotaxis device and demonstrate its usefulness in 2D and 3D cell culture. We also provide preliminary data for use of the device in vivo.

Macrophages promote collagen fibrillogenesis around terminal end buds of the developing mammary gland.

Development of the ductal network in the mammary gland is dependent in part on the presence of macrophages. Here we utilize multi-photon microscopy and second harmonic generation to describe terminal end bud 3-dimensional structure and the organization of the surrounding collagen matrix. We have applied this approach to analyze the effect of macrophage deficiency on terminal end bud structure and collagen organization, using mice homozygous for a null mutation in the colony stimulating factor-1 gene (Csf1op/Csf1op). Primary terminal end buds have an oblong shape, with long collagen I fibers close to the neck of the terminal end bud and radiating upwards in the direction of growth. Around the terminal end buds, the amount of total collagen I detected by antibody staining was not affected by macrophage deficiency. However the amount of collagen I organized into long fibers, detected by second harmonic generation signal, was reduced in Csf1op/Csf1op mice. Macrophage deficiency also caused terminal end buds to be rounder and shorter. These studies reveal a role for macrophages in collagen fibrillogenesis and in organization of the structure of terminal end buds.