Collective cell invasion (CCI) through interstitial collagenous extracellular matrix (ECM) is crucial to the initial stages of branching morphogenesis, and a hallmark of tissue repair and dissemination of certain tumors. The collagenous ECM acts as a mechanical barrier against CCI. However, the physical nature of this barrier and how it is overcome by cells remains incompletely understood. To address these questions, we performed theoretical and experimental analysis of mammary epithelial branching morphogenesis in 3D type I collagen (collagen-I) gels. We found that the mechanical resistance of collagen-I is largely due to its elastic rather than its viscous properties. We also identified two strategies utilized by mammary epithelial cells that can independently minimize ECM mechanical resistance during CCI. First, cells adopt a narrow tube-like geometry during invasion, which minimizes the elastic opposition from the ECM as revealed by theoretical modeling of the most frequent invasive shapes and sizes. Second, the stiffness of the collagenous ECM is reduced at invasive fronts due to its degradation by matrix metalloproteinases (MMPs), as indicated by direct measurements of collagen-I microelasticity by atomic force microscopy. Molecular techniques further specified that the membrane-bound MMP14 mediates degradation of collagen-I at invasive fronts. Thus, our findings reveal that MMP14 is necessary to efficiently reduce the physical restraints imposed by collagen-I during branching morphogenesis, and help our overall understanding of how forces are balanced between cells and their surrounding ECM to maintain collective geometry and mechanical stability during CCI.

The crucial role of tumor-associated fibroblasts (TAF) in cancer progression is now clear in non-small cell lung cancer (NSCLC). However, therapies against TAFs are limited due to a lack of understanding in the subtype-specific mechanisms underlying their accumulation. Here, the mechanical (i.e., matrix rigidity) and soluble mitogenic cues that drive the accumulation of TAFs from major NSCLC subtypes: adenocarcinoma (ADC) and squamous cell carcinoma (SCC) were dissected. Fibroblasts were cultured on substrata engineered to exhibit normal-or tumor-like stiffnesses at different serum concentrations, and critical regulatory processes were elucidated. In control fibroblasts from nonmalignant tissue, matrix stiffening alone increased fibroblast accumulation, and this mechanical effect was dominant or comparable with that of soluble growth factors up to 0.5% serum. The stimulatory cues of matrix rigidity were driven by b1 integrin mechanosensing through FAK (pY397), and were associated with a posttranscriptionally driven rise in b1 integrin expression. The latter mechano-regulatory circuit was also observed in TAFs but in a subtype-specific fashion, because SCC-TAFs exhibited higher FAK (pY397), b1 expression, and ERK1/2 (pT202/Y204) than ADCTAFs. Moreover, matrix stiffening induced a larger TAF accumulation in SCC-TAFs (>50%) compared with ADC-TAFs (10%-20%). In contrast, SCC-TAFs were largely serum desensitized, whereas ADC-TAFs responded to high serum concentration only. These findings provide the first evidence of subtype-specific regulation of NSCLC-TAF accumulation. Furthermore, these data support that therapies aiming to restore normal lung elasticity and/or b1 integrin-dependent mechano regulation may be effective against SCC-TAFs, whereas inhibiting stromal growth factor signaling may be effective against ADC-TAFs.Implications: This study reveals distinct mechanisms underlying the abnormal accumulation of tumor-supporting fibroblasts in two major subtypes of lung cancer, which will assist the development of personalized therapies against these cells.

Three-dimensional (3D) cultures are increasingly used as tissue surrogates to study many physiopathological processes. However, to what extent current 3D culture protocols provide physiologic oxygen tension conditions remains ill defined. To address this limitation, oxygen tension was measured in a panel of acellular or cellularized extracellular matrix (ECM) gels with A549 cells, and analyzed in terms of oxygen diffusion and consumption. Gels included reconstituted basement membrane, fibrin and collagen. Oxygen diffusivity in acellular gels was up to 40% smaller than that of water, and the lower values were observed in the denser gels. In 3D cultures, physiologic oxygen tension was achieved after 2 days in dense (≥3 mg/mL) but not sparse gels, revealing that the latter gels are not suitable tissue surrogates in terms of oxygen distribution. In dense gels, we observed a dominant effect of ECM composition over density in oxygen consumption. All diffusion and consumption data were used in a simple model to estimate ranges for gel thickness, seeding density and time-window that may support physiologic oxygen tension. Thus, we identified critical variables for oxygen tension in ECM gels, and introduced a model to assess initial values of these variables, which may short-cut the optimization step of 3D culture studies.

Please cite this article as: R. Galgoczy, I. Pastor, A. Colom, A. Giménez, F. Mas, J. Alcaraz, A spectrophotometer-based diffusivity assay reveals that diffusion hindrance of small molecules in extracellular matrix gels used in 3D cultures is dominated by viscous effects, Colloids and Surfaces B: Biointerfaces (2014), http://dx.doi.org/10. 1016/j.colsurfb.2014.05.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. AbstractThe design of 3D culture studies remains challenging due to the limited understanding of extracellular matrix (ECM)-dependent hindered diffusion and the lack of simple diffusivity assays. To address these limitations, we set up a cost-effective diffusivity assay based on a Transwell plate and the spectrophotometer of a Microplate Reader, which are readily accessible to cell biology groups. The spectrophotometer-based assay was used to assess the apparent diffusivity D of FITC-dextrans with molecular weight (4-70 kDa) spanning the physiological range of signaling factors in a panel of acellular ECM gels including Matrigel, fibrin and type I collagen. Despite their technical differences, D data exhibited ~15% relative difference with respect to FRAP measurements. Our results revealed that diffusion hindrance of small particles is controlled by the enhanced viscosity of the ECM gel in conformance with the Stokes-Einstein equation rather than by geometrical factors. Moreover, we provided a strong rationale that the enhanced ECM viscosity is largely contributed to by unassembled ECM macromolecules. We also reported that gels with the lowest D exhibited diffusion hindrance closest to the large physiologic hindrance of brain tissue, which has a typical pore size much smaller than ECM gels.Conversely, sparse gels (≤ 1 mg/ml), which are extensively used in 3D cultures, failed to reproduce the hindered diffusion of tissues, thereby supporting that dense (but not sparse) ECM gels are suitable tissue surrogates in terms of macromolecular transport. Finally, the consequences of reduced diffusivity in terms of optimizing the design of 3D culture experiments were addressed in detail.

Atomic force microscopy (AFM) is a local probe nanotechnique that belongs to the large family of scanning probe microscopies (SPM). It was introduced in 1986, and is the most versatile SPM, because it can be used as an imaging instrument, a force sensor and actuator and as a molecular sensor. AFM enables imaging samples with subnanometer resolution and detecting intermolecular forces as low as a single hydrogen bond. Its high spatial and force resolution, combined with its ability to probe samples in liquid make AFM a very suitable nanotechnique for the life sciences. Accordingly, AFM has been used to examine biological samples embracing all levels of complexity including biomolecules, viruses, prokaryotic and eukaryotic cells and more recently, tissue sections. Moreover, AFM biomedical applications are rising and include their use as a diagnostic tool. Key Concepts: AFM is a nanotechnique that allows visualising the surface of biological samples by directly touching them with a mechanical‐based force sensor or cantilever tip. The essential components of an AFM are a flexible cantilever provided with a sharp tip at its end and a piezoelectric scanner that controls the tip–sample distance. AFM can be operated in vacuum, air and liquid. The only sample requirements are immobilisation on a flat substrate and a surface roughness smaller than the height of the tip. The most common modes of operation of AFM as an imaging device are with the tip in either constant or intermitent contact with the sample. The topography image obtained with AFM is strongly influenced by both the shape and size of the tip and the elastic properties of the sample. AFM probes can measure intermolecular forces as low as few piconewtons, and apply user‐defined loading forces on the sample surface, thereby acting as a force sensor or actuator. Using AFM tips with a suitable functionalisation enables probing the specific intermolecular forces involved in molecular recognition events. The spatial and force range accessible with AFM enables using it to examine biological samples spanning many length scales of biological complexity, from single biomolecules to tissue sections. Arrays of cantilevers functionalised with specific recognition sites can potentially be used to detect soluble biomarkers in fluid samples of patients at concentrations lower than current diagnostic devices.