Current Molecular Medicine - Volume 14, Issue 2, 2014

The low molecular weight GTP-binding protein RhoA regulates many cellular events, including cell migration, organization of the cytoskeleton, cell adhesion, progress through the cell cycle and gene expression. Physical forces influence these cellular processes in part by regulating RhoA activity through mechanotransduction of cell adhesion molecules (e.g. integrins, cadherins, Ig superfamily molecules). RhoA activity is regulated by guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs) that are themselves regulated by many different signaling pathways. Significantly, the engagement of many cell adhesion molecules can affect RhoA activity in both positive and negative ways. In this brief review, we consider how RhoA activity is regulated downstream from cell adhesion molecules and mechanical force. Finally, we highlight the importance of mechanotransduction signaling to RhoA in normal cell biology as well as in certain pathological states.

Three decades after the beginning of the study of the Wnt signaling pathway, major contributions have been made to elucidate the molecular mechanisms that regulate this signaling pathway and its role in development, homeostasis and disease. However, there is still a lack of understanding about the relationships between Wnt signaling and cell-extracellular matrix (ECM) adhesion. Data gathered in the last years is helping to uncover these relationships. Several ECM proteins are able to regulate components of the Wnt pathway during development and disease, and their misregulation leads to changes in Wnt signaling. Fibronectin, a major ECM protein, regulates non-canonical Wnt signaling during embryogenesis in Xenopus and in muscle regeneration in mouse, whereas it modulates canonical Wnt signaling through modulation of β -catenin. Integrins, which act as Fibronectin receptors, also modulate Wnt activity, and Syndecan-4, a heparan sulphate proteoglycan, is able to regulate canonical and non-canonical Wnt pathways, notably during embryogenesis. Other secreted ECM proteins have been recently associated to the regulation of Wnt signaling, albeit molecular mechanisms are still unclear. The non-canonical Wnt pathway plays a role in the regulation of the ECM assembly, and modulates focal adhesion dynamics through the involvement of Wnt components, whereas Wnt/β-catenin signaling regulates the expression of genes encoding ECM proteins. This evidence indicates that Wnt signaling and cell-ECM adhesion are two closely related processes, and alterations in this cross-talk might be involved in disease.

Rho guanine exchange factors (GEFs) are a large, diverse family of proteins defined by their ability to catalyze the exchange of GDP for GTP on small GTPase proteins such as Rho family members. GEFs act as integrators from varied intra- and extracellular sources to promote spatiotemporal activity of Rho GTPases that control signaling pathways regulating cell proliferation and movement. Here we review recent studies elucidating roles of RhoGEF proteins in cell motility. Emphasis is placed on Dbl-family GEFs and connections to development, integrin signaling to Rho GTPases regulating cell adhesion and movement, and how these signals may enhance tumor progression. Moreover, RhoGEFs have additional domains that confer distinctive functions or specificity. We will focus on a unique interaction between Rgnef (also termed Arhgef28 or p190RhoGEF) and focal adhesion kinase (FAK), a non-receptor tyrosine kinase that controls migration properties of normal and tumor cells. This Rgnef-FAK interaction activates canonical GEF-dependent RhoA GTPase activity to govern contractility and also functions as a scaffold in a GEF-independent manner to enhance FAK activation. Recent studies have also brought to light the importance of specific regions within the Rgnef pleckstrin homology (PH) domain for targeting the membrane. As revealed by ongoing Rgnef-FAK investigations, exploring GEF roles in cancer will yield fundamental new information on the molecular mechanisms promoting tumor spread and metastasis.

Uncontrolled endosome trafficking is a common feature of certain cancer cells, which has been acknowledged during the last decade. Migration and invasiveness of metastatic tumor cells are both regulated by components of the endocytic machinery, including Rab proteins. Rab GTPases are essential in processes of endosome fusion, as well as targeting, tethering and transport along the cytoskeleton. In addition to this canonical role, some Rabs depict other functions, such as controlling cell proliferation, apoptosis, adhesion and motility. Here, we review our current knowledge on the role of Rab5, a key regulator of early endosome dynamics, in migration of normal and tumor cells. Rab5 promotes cell migration in vitro and in vivo by mechanisms described at different levels. One such mechanism is by controlling the rates of integrin internalization and recycling, thereby affecting its activation and availability at the cell surface. On the other hand, Rab5 promotes focal adhesion disassembly and modulates downstream pathways of integrin signaling, involving proteins such as Ras and Rho family GTPases. In this context, identification of upstream regulators and downstream effectors of Rab5, and their study represents a big challenge in order to understand how cancer cells depend on endosome control, in order to acquire more aggressive traits that lead to metastatic disease.

The caspases are a family of ubiquitously expressed cysteine proteases best known for their roles in programmed cell death. However, caspases play a number of other roles in vertebrates. In the case of caspase-8, loss of expression is an embryonic lethal phenotype, and caspase-8 plays roles in suppressing cellular necrosis, promoting differentiation and immune signaling, regulating autophagy, and promoting cellular migration. Apoptosis and migration require localization of caspase-8 in the periphery of the cells, where caspase-8 acts as part of distinct biosensory complexes that either promote migration in appropriate cellular microenvironments, or cell death in inappropriate settings. In the cellular periphery, caspase-8 interacts with components of the focal adhesion complex in a tyrosine-kinase dependent manner, promoting both cell migration in vitro and metastasis in vivo. Mechanistically, caspase-8 interacts with components of both focal adhesions and early endosomes, enhancing focal adhesion turnover and promoting rapid integrin recycling to the cell surface. Clinically, this suggests that the expression of caspase-8 may not always be a positive prognostic sign, and that the role of caspase-8 in cancer progression is likely context-dependent.

Caveolin-1 is a member of the caveolin family that has been ascribed a dual role in cancer. In early stages of disease the protein functions predominantly as a tumor suppressor, whereas at later stages, caveolin-1 expression is associated with tumor progression and metastasis. Here, some mechanisms associated with caveolin-1-dependent tumor suppression will be briefly discussed before focusing on the role of this protein and particularly phosphorylation of tyrosine-14 in promoting cell migration, invasion and metastasis. Models are provided summarizing possible explanations for these dramatic changes in function, as well as mechanisms by which this may be achieved.

Astrocytes in the normal brain possess a stellate shape reflecting their non-migratory properties. Alternatively, in neurodegenerative diseases or after injury, astrocytes become “reactive” in a process known as astrocytosis or reactive gliosis, retract their processes, become polarized and acquire front-to-rear asymmetry typical of migratory cells. On the other hand, neuronal migration is a common process during embryonic development, but only few types of neurons can migrate and differentiate during adult life in the central nervous system. Those that do migrate follow tracks made by glial cells and mainly give rise to interneurons. In vitro, molecular mechanisms involved in adhesion of cells to and migration on extracellular matrix proteins have been widely studied; however, signal transduction pathways explaining how particularly neurons and astrocytes, mutually modulate adhesion and migration are less well known. In this review, we describe and discuss how ligand/receptor interactions in astrocytes and neurons trigger signaling events leading to actin and microtubule reorganization, changes in cell morphology, as well as cell adhesion and migration. The biological significance these cell-cell interactions and signaling events might have in the brain are discussed.

Cell migration is a complex biological process that involves changes in shape and organization at the sub-cellular, cellular, and supra-cellular levels. Individual and collective cell migration can be assessed in vitro and in vivo starting from the flagellar driven movement of single sperm cells or bacteria, bacterial gliding and swarming, and amoeboid movement to the orchestrated movement of collective cell migration. One key technology to access migration phenomena is the combination of optical microscopy with image processing algorithms. This approach resolves simple motion estimation (e.g. preferred direction of migrating cells or path characteristics), but can also reveal more complex descriptors (e.g. protrusions or cellular deformations). In order to ensure an accurate quantification, the phenomena under study, their complexity, and the required level of description need to be addressed by an adequate experimental setup and processing pipeline. Here, we review typical workflows for processing starting with image acquisition, restoration (noise and artifact removal, signal enhancement), registration, analysis (object detection, segmentation and characterization) and interpretation (high level understanding). Image processing approaches for quantitative description of cell migration in 2- and 3-dimensional image series, including registration, segmentation, shape and topology description, tracking and motion fields are presented. We discuss advantages, limitations and suitability for different approaches and levels of description.