Mechanoregulation of gene expression in fibroblasts

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James H.-C. Wanga, , , Bhavani P. Thampattya, Jeen-Shang Linb and Hee-Jeong Imc aMechanoBiology Laboratory, Departments of Orthopaedic Surgery, Bioengineering, Mechanical Engineering, and Physical Medicine and Rehabilitation, University of Pittsburgh, 210 Lothrop St., BST, E1640, Pittsburgh, PA 15213, United StatesbDepartment of Civil and Environmental Engineering, School of Engineering, University of Pittsburgh, United StatescDepartments of Biochemistry and Internal Medicine, Rush University Medical Center, Cohn Research BD 558, 1735 W. Harrison, Chicago, IL 60612, United States

Intro: Mechanical loads placed on connective tissues alter gene expression in fibroblasts through mechanotransduction mechanisms by which cells convert mechanical signals into cellular biological events, such as gene expression of extracellular matrix components (e.g., collagen). This mechanical regulation of ECM gene expression affords maintenance of connective tissue homeostasis. However, mechanical loads can also interfere with homeostatic cellular gene expression and consequently cause the pathogenesis of connective tissue diseases such as tendinopathy and osteoarthritis. Therefore, the regulation of gene expression by mechanical loads is closely related to connective tissue physiology and pathology. This paper reviews the effects of various mechanical loading conditions on gene regulation in fibroblasts and discusses several mechanotransduction mechanisms. Future research directions in mechanoregulation of gene expression are also suggested.

used: Among many mechanoresponsive cells in our body, especially fibroblasts (collagen producing ECM cells) are studies for mechanotransduction resulting in gene regulation in this paper. Fibroblasts are also important for maintenance of most of the ECM components such as proteoglycans, growth factors, and cytokines.

experimental method: To mimic in vivo environment, tensile mechanical loading on fibroblasts in vitro is introduced using a substrate stretching method. This method is versatile; the loading parameters, including magnitude, frequency, and duration, are easy to control; and the mechanical properties of substrates as well as surface chemistry for cell attachment can be readily altered. The substrate deformation induced by substrate stretching is characterized by substrate surface strains.
To determine the effects of mechanical loading on fibroblasts, static or cyclic stretching is applied to a group of cells cultured on smooth, flexible elastic membranes (e.g.,silicone elastomers). And the mechanical force can be calculated by observating fluorescent beads attached on the sillicone substrate (for those in 116, we learned about this!:) ). In fact, 3D simulation of ECM by using collagen gels is more effective and accurate in terms of mimicking what's happening in vivo. Gene expression can be observed using PCR or RT-PCR.

Results: Induced collagen production, change in ECM components, growth factor, MMP secretion, TIMP (MMP inhibitor) expression, focal adhesion change, etc. (refer to Table 1.).

Discussion:
Gene regulation in fibroblasts depends on mechanical loading conditions: type (e.g., tension vs.compression), magnitude, frequency, and duration. The mechanoregulation of gene expression in fibroblasts also depends on the tissue location from which fibroblasts are derived, as well as ECM protein with which cells interact. To better understand how tissue homeostasis is maintained and how pathological conditions initiate and develop, it is necessary to study the effects of various mechanical loading conditions on fibroblasts. One challenging task in future research is to understand how a cell “decides” its response from “crosstalks” of many mechanotransduction signals, since mechanotransduction mechanisms do not function in isolation, but rather by an integrated network of various signaling pathways. Another particularly challenging task is to identify specific “force receptors,” for which specific proteins at the membrane–cytoskeletal interface(e.g., integrins and G proteins) are good candidates. Ultimately, additional research in mechanoregulation of gene expression in fibroblasts will aid in developing new therapeutic strategies and new approaches to engineering tissue constructs for improved repair and regeneration of connective tissues.