Neuroblastoma is a pediatric malignancy characterized by tremendous clinical heterogeneity, in which some tumors are extremely aggressive while others spontaneously differentiate into benign forms. Because the degree of differentiation correlates with prognosis, and because differentiating agents such as retinoic acid (RA) have proven to decrease mortality, much effort has been devoted to identifying critical regulators of neuroblastoma differentiation in the cellular microenvironment, including cues encoded in the extracellular matrix (ECM). While signaling between tumor cells and the ECM is classically regarded to be based purely on biochemical recognition of ECM ligands by specific cellular receptors, a number of recent studies have made it increasingly clear that the biophysical properties of the ECM may also play an important role in this cross-talk. Given that RA-mediated neuroblastoma differentiation is accompanied by profound changes in cell morphology and neurite extension, both of which presumably rely upon mechanotransductive signaling systems, it occurred to us that mechanical cues from the ECM might also influence RA-mediated differentiation, which in turn might regulate clinically-relevant aspects of neuroblastoma biology. In this study, we tested this hypothesis by subjecting a series of neuroblastoma culture models to ECM microenvironments of varying mechanical stiffness and examined the regulatory role of ECM stiffness in proliferation, differentiation, and expression of tumor markers. We find that increasing ECM stiffness enhances neuritogenesis and suppresses cell proliferation. Remarkably, increasing ECM stiffness also reduces expression of N-Myc, a transcription factor involved in multiple aspects of oncogenic proliferation that is used for evaluating prognosis and clinical grading of neuroblastoma. Furthermore, the addition of RA enhances all of these effects for all ECM stiffnesses tested. Together, our data strongly support the notion that the mechanical signals from the cellular microenvironment influence neuroblastoma differentiation and do so synergistically with RA. These observations support further investigation of the role of microenvironmental mechanical signals in neuroblastoma proliferation and differentiation and suggest that pharmacological agents that modulate the underlying mechanotransductive signaling pathways may have a role in neuroblastoma therapy.
Fibronectin (Fn) is an extracellular matrix protein that orchestrates complex cell adhesion and signaling through cell surface integrin receptors during tissue development, remodeling, and disease, such as fibrosis. Fn is sensitive to mechanical forces in its tandem type III repeats, resulting in extensive molecular enlongation. As such, it has long been hypothesized that cell- and tissue-derived forces may activate an “integrin switch” within the critical integrin-binding ninth and 10th type III repeats—conferring differential integrin-binding specificity, leading to differential cell responses. Yet, no direct evidence exists to prove the hypothesis nor demonstrate the physiological existence of the switch. We report direct experimental evidence for the Fn integrin switch both in vitro and ex vivo using a scFv engineered to detect the transient, force-induced conformational change, representing an opportunity for detection and targeting of early molecular signatures of cell contractile forces in tissue repair and disease.
Applied forces and the biophysical nature of the cellular microenvironment play a central role in determining cellular behavior. Specifically, forces due to cell contraction are transmitted into structural ECM proteins and these forces are presumed to activate integrin "switches." The mechanism of such switches is thought to be the partial unfolding of integrin-binding domains within fibronectin (Fn). However, integrin switches remain largely hypothetical due to a dearth of evidence for their existence, and relevance, in vivo. By using phage display in combination with the controlled deposition and extension of Fn fibers, we report the discovery of peptidebased molecular probes capable of selectively discriminating Fn fibers under different strain states. Importantly, we show that the probes are functional in both in vitro and ex vivo tissue contexts. The development of such tools represents a critical step in establishing the relevance of theoretical mechanotransduction events within the cellular microenvironment.
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