Dana M. Pirone

Cells lying on a bed of microneedles: An approach to isolate mechanical force

We describe an approach to manipulate and measure mechanical interactions between cells and their underlying substrates by using microfabricated arrays of elastomeric, microneedle-like posts. By controlling the geometry of the posts, we varied the compliance of the substrate while holding other surface properties constant. Cells attached to, spread across, and deflected multiple posts. The deflections of the posts occurred independently of neighboring posts and, therefore, directly reported the subcellular distribution of traction forces. We report two classes of force-supporting adhesions that exhibit distinct force–size relationships. Force increased with size of adhesions for adhesions larger than 1 μm2, whereas no such correlation existed for smaller adhesions. By controlling cell adhesion on these micromechanical sensors, we showed that cell morphology regulates the magnitude of traction force generated by cells. Cells that were prevented from spreading and flattening against the substrate did not contract in response to stimulation by serum or lysophosphatidic acid, whereas spread cells did. Contractility in the unspread cells was rescued by expression of constitutively active RhoA. Together, these findings demonstrate a coordination of biochemical and mechanical signals to regulate cell adhesion and mechanics, and they introduce the use of arrays of mechanically isolated sensors to manipulate and measure the mechanical interactions of cells.

E-cadherin engagement stimulates proliferation via Rac1

E-cadherin has been linked to the suppression of tumor growth and the inhibition of cell proliferation in culture. We observed that progressively decreasing the seeding density of normal rat kidney-52E (NRK-52E) or MCF-10A epithelial cells from confluence, indeed, released cells from growth arrest. Unexpectedly, a further decrease in seeding density so that cells were isolated from neighboring cells decreased proliferation. Experiments using microengineered substrates showed that E-cadherin engagement stimulated the peak in proliferation at intermediate seeding densities, and that the proliferation arrest at high densities did not involve E-cadherin, but rather resulted from a crowding-dependent decrease in cell spreading against the underlying substrate. Rac1 activity, which was induced by E-cadherin engagement specifically at intermediate seeding densities, was required for the cadherin-stimulated proliferation, and the control of Rac1 activation by E-cadherin was mediated by p120-catenin. Together, these findings demonstrate a stimulatory role for E-cadherin in proliferative regulation, and identify a simple mechanism by which cell–cell contact may trigger or inhibit epithelial cell proliferation in different settings.

Evolutionary expansion of CRIB-containing Cdc42 effector proteins.

Cdc42, a small GTPase, regulates actin polymerization and other signaling pathways through interaction with many different downstream effector proteins. Most of these effector proteins contain a Cdc42-binding domain, called a CRIB domain. Here, we describe the evolutionary analysis of these CRIB-containing proteins in yeast, worms, flies and humans. The number of CRIB-containing effector proteins increases from yeast to humans, involving both an increase within families and the emergence of new families. These evolutionary changes correlate with the development of the more complex signaling pathways present in higher organisms.

SPECs, Small Binding Proteins for Cdc42

The Rho GTPase, Cdc42, regulates a wide variety of cellular activities including actin polymerization, focal complex assembly, and kinase signaling. We have identified a new family of very small Cdc42-binding proteins, designated SPECs (for SmallProtein Effector of Cdc42), that modulates these regulatory activities. The two human members, SPEC1 and SPEC2, encode proteins of 79 and 84 amino acids, respectively. Both contain a conserved N-terminal region and a centrally located CRIB (Cdc42/Rac InteractiveBinding) domain. Using a yeast two-hybrid system, we found that both SPECs interact strongly with Cdc42, weakly with Rac1, and not at all with RhoA. Transfection analysis revealed that SPEC1 inhibited Cdc42-induced c-Jun N-terminal kinase (JNK) activation in COS1 cells in a manner that required an intact CRIB domain. Immunofluorescence experiments in NIH-3T3 fibroblasts demonstrated that both SPEC1 and SPEC2 showed a cortical localization and induced the formation of cell surface membrane blebs, which was not dependent on Cdc42 activity. Cotransfection experiments demonstrated that SPEC1 altered Cdc42-induced cell shape changes both in COS1 cells and in NIH-3T3 fibroblasts and that this alteration required an intact CRIB domain. These results suggest that SPECs act as novel scaffold molecules to coordinate and/or mediate Cdc42 signaling activities.

CLONING, CENTRAL NERVOUS SYSTEM EXPRESSION AND CHROMOSOMAL MAPPING OF THE MOUSE PAK-1 AND PAK-3 GENES

Two cDNAs encoding PAK kinases were isolated from a mouse embryo library by screening with a PCR-generated probe derived from the kinase domain of a rat PAK kinase. These cDNAs, designated PAK-1 and PAK-3, encode mouse PAK kinases of 545 and 544 amino acids, respectively. Both proteins possess an N-terminal Cdc42/Rac interacting binding domain (CRIB) and a C-terminal serine/threonine kinase domain. Comparison of the two mouse PAK kinases revealed that the proteins show 87% amino acid identity. Northern analysis of a multiple mouse tissue blot with a PAK-1 probe detected a 3.0kb transcript that was almost exclusively expressed in the brain and spinal cord compared to other tissues such as lung, liver and kidney. A similar pattern of central nervous system tissue expression of PAK-3 transcripts of 3.6 and 8kb was also observed. Analysis of two multilocus genetic crosses localized Pak1 and Pak3 to a position on chromosome 7 and X, respectively. The high level of PAK-1 and PAK-3 kinase expression in the mouse brain and spinal cord suggests a potentially important role for these kinases in the control of the cellular architecture and/or signaling in the central nervous system.

Three-dimensional biomimetic vascular model reveals a RhoA, Rac1, and N-cadherin balance in mural cell–endothelial cell-regulated barrier function

Significance Organ homeostasis requires integrity of blood vessels; alterations or disruption of the vascular barrier between blood and tissue contribute to numerous diseases. Endothelial cells and mural cells are two key cell types, which play significant roles for the maintenance of barrier function. Here, we present a 3D bicellular vascular model to mimic this barrier function and study the role of mural cells in vascular inflammation. Importantly, by using this 3D model we identified RhoA, Rac1, and N-cadherin as important regulators in mural–endothelial cell-mediated vascular barrier function. Given the recognized fundamental importance of this barrier in numerous disease settings, this in vitro microphysiological system presented herein could provide a tool for studying vascular barrier function in 3D microenvironments. The integrity of the endothelial barrier between circulating blood and tissue is important for blood vessel function and, ultimately, for organ homeostasis. Here, we developed a vessel-on-a-chip with perfused endothelialized channels lined with human bone marrow stromal cells, which adopt a mural cell-like phenotype that recapitulates barrier function of the vasculature. In this model, barrier function is compromised upon exposure to inflammatory factors such as LPS, thrombin, and TNFα, as has been observed in vivo. Interestingly, we observed a rapid physical withdrawal of mural cells from the endothelium that was accompanied by an inhibition of endogenous Rac1 activity and increase in RhoA activity in the mural cells themselves upon inflammation. Using a system to chemically induce activity in exogenously expressed Rac1 or RhoA within minutes of stimulation, we demonstrated RhoA activation induced loss of mural cell coverage on the endothelium and reduced endothelial barrier function, and this effect was abrogated when Rac1 was simultaneously activated. We further showed that N-cadherin expression in mural cells plays a key role in barrier function, as CRISPR-mediated knockout of N-cadherin in the mural cells led to loss of barrier function, and overexpression of N-cadherin in CHO cells promoted barrier function. In summary, this bicellular model demonstrates the continuous and rapid modulation of adhesive interactions between endothelial and mural cells and its impact on vascular barrier function and highlights an in vitro platform to study the biology of perivascular–endothelial interactions.

Spatial patterning of gene expression using surface-immobilized recombinant adenovirus

Spatially patterned gene expression drives tissue organization and is a critical determinant of tissue function. Approaches in functional tissue engineering will require not only the spatial organization of cells but also control of their gene expression patterns. We report a method to generate patterns of gene expression within a monolayer of cells by using surface-immobilized recombinant adenovirus. This study represents a new approach to engineering tissues that relies on controlling spatial patterns of gene expression, and can be used independently or in combination with positioning of different cell types. This technique may have broad applications in biotechnology including tissue engineering and targeted gene delivery.

qPCR for second year undergraduates: A short, structured inquiry to illustrate differential gene expression

We describe a structured inquiry laboratory exercise that examines transcriptional regulation of the NOS2 gene under conditions that simulate the inflammatory response in macrophages. Using quantitative PCR and the comparative CT method, students are able determine whether transcriptional activation of NOS2 occurs and to what degree. The exercise is aimed at second year undergraduates who possess basic knowledge of gene expression events. It requires only 4–5 hr of dedicated laboratory time and focuses on use of the primary literature, data analysis, and interpretation. Importantly, this exercise provides a mechanism to introduce the concept of differential gene expression and provides a starting point for development of more complex guided or open inquiry projects for students moving into upper level molecular biology, immunology, and biochemistry course work.

Inquiry-Based Learning: Inflammation as a Model to Teach Molecular Techniques for Assessing Gene Expression

This laboratory module simulates the process used by working scientists to ask and answer a question of biological interest. Instructors facilitate acquisition of knowledge using a comprehensive, inquiry-based approach in which students learn theory, hypothesis development, experimental design, and data interpretation and presentation. Using inflammation in macrophages as a model system, students perform a series of molecular biology techniques to address the biological question: “Does stimulus ‘X’ induce inflammation?” To ask this question, macrophage cells are treated with putative inflammatory mediators and then assayed for evidence of inflammatory response. Students become familiar with their assigned mediator and the relationship between their mediator and inflammation by conducting literature searches, then using this information to generate hypotheses which address the effect of their mediator on induction of inflammation. The cellular and molecular approaches used to test their hypotheses include transfection and luciferase reporter assay, immunoblot, fluorescence microscopy, enzyme-linked immunosorbent assay, and quantitative PCR. Quantitative and qualitative reasoning skills are developed through data analysis and demonstrated by successful completion of post-lab worksheets and the generation and oral presentation of a scientific poster. Learning objective assessment relies on four instruments: pre-lab quizzes, post-lab worksheets, poster presentation, and posttest. Within three cohorts (n = 85) more than 95% of our students successfully achieved the learning objectives.

Using lab-on-a-chip Technologies to Understand Cellular Mechanotransduction

I. Introduction We are living in the age of micro and nanotechnology. The electronics industry owes its rapid expansion over the past several decades to its ability to invent new approaches to make things ever smaller. Using similar techniques, the fabrication of small structures is being applied in other disciplines, particularly in the areas of chemistry and biology, enabling scientists to ask questions in ways not previously possible. The first non-electronic microfabricated chips were used in analytical chemistry, where miniaturized assays were developed to perform gas and liquid chromatography (Manz et al., 1990a; Manz et al., 1990b). In chemistry, as in the field of electronics, miniaturization enhanced the performance of these techniques, but it also had the added benefits of smaller reagent consumption, portability, and parallel construction for high throughput applications. These same benefits have pushed micro-systems into the realm of biological chemistry, and have resulted in such developments as micro Today, variations of these lab-on-a-chip techniques are commonplace in the biological laboratory. One dramatic example is the chip-based cDNA microarray. In cDNA microarrays, DNA is immobilized onto a solid platform such that different DNA sequences are addressed to specific spots on the array. A sample is passed over the array, such that matching sequences of DNA in the sample hybridize to the immobilized cDNA array. In the process, thousands of different cDNAs can be assessed simultaneously for their relative abundance. This strategy is now widely used in gene expression profiling applications, where 20,000 to 40,000 genes can be simultaneously analyzed in a single experiment, and has immeasurable impact on biological research (Xiang and Chen, 2000). While chip-based assays have enabled enormous advances in biochemistry, many insights into the biological function of organisms and their component tissues comes from the direct observation of individual living cells. To address this need, lab-on-a-chip methods have recently been extended beyond standard biochemical assays to include direct cell culture on chips. These tools are enabling novel experiments to assess the biology of whole, living cells, such as those that assay for cell migration, polarization, or