Mechanotransduction of keratinocytes in culture and in the epidermis

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Abstract

The epidermis, like many other tissues, reacts to mechanical stress by increasing cell proliferation. Mechanically stressed skin regions often develop thicker skin and hyperkeratosis. Interestingly, a large number of skin diseases are accompanied by epidermal proliferation and hyperkeratosis even under normal mechanical stress conditions. Although, some of the molecular pathways of mechanical signaling involving integrins, the epidermal growth factor receptor and mitogen-activated protein kinases are known it is still unclear, how mechanical force is sensed and transformed into the molecular signals that induce cell proliferation. This review focuses on the molecules and pathways known to play a role in mechanotransduction in epidermal keratinocytes and discusses the pathways identified in other well-studied cell types.

Introduction

From the study of different tissues and cell types it has become evident that most if not all cells are sensitive to mechanical stress and that there are common ways of response to mechanical stimuli (Orr et al., 2006). In order to maintain the functional integrity of the epidermis, an on-going adaptation of the tissue to changing mechanical needs is essential.

In vivo there are distinct types of mechanical load: cyclic stress (rapid, heart and vasculature) or shear stress (fluid flow, endothelial cells), stretch or tensile strain, distension (slow extension, bladder), and compressional forces (bone and cartilage). In vivo, keratinocytes are probably mostly affected by stretch or tensile strain. Under physiological conditions, in a variety of tissues, mechanical stimuli may affect proliferation, differentiation, cell morphology and migration. Bone maturation has been shown to rely on mechanical forces (Turner et al., 1995). Epidermal keratinocytes – like fibroblasts (Grinnell, 2003; Wang et al., 2007), endothelial cells (Lehoux and Tedgui, 2003), smooth muscle cells (Li and Xu, 2007) and osteoblasts (Rubin et al., 2006) which represent well established model systems in the study of cellular mechanical stress response – react with an activation of mitogen-activated protein kinases (MAPK) and an increase in proliferation upon experimental mechanical stimulation in cell culture. Probably keratinocytes, as other cells, integrate several signaling cascades implicating the extracellular matrix, growth factor receptors residing in the cell membrane, protein kinases, and the cytoskeleton (Fig. 1). The distal responses utilize similar well-established signal transduction pathways. However, the complete path of force transmission resulting in the conversion of a mechanical stimulus into a biochemical signal is not yet understood. Mechanical signals might directly be transmitted via the cytoskeleton to the nucleus and regulate transcription. For example, mutations in the nuclear lamina protein lamin C cause nuclear deformation and alter stretch-induced gene expression (Lammerding et al., 2004).

In the epidermis as in other tissues, mechanical stimuli may be exogenous or may originate from neighboring cells which divide, differentiate, or migrate (Matsubayashi et al., 2004). Wound contraction generated by myofibroblasts, a specialized type of fibroblasts which express α-smooth muscle actin and are located in the dermis, as an essential step in the healing process is another cause for pressure and stress in the skin (Hinz and Gabbiani, 2003; Tomasek et al., 2002). It is not yet known, how distinct cell types in the skin sense mechanical force, translate it within the cell and how they communicate the information to close or distant neighbor cells.

How do cells in the epidermis sense mechanical forces from their environment? And how do they transform and finally transduce the physical stimulus into a biochemical response that results in proliferation increase and subsequently in skin thickening? The physiological response of tissues to mechanical stress is highlighted not only through examples such as bone maturation but also the waist skin extension during pregnancies, or in the hyperkeratotic finger tips of a guitar player.

The term mechanotransduction refers to the mechanism by which physical forces are transduced into biomolecular responses in cells. The first step in mechanotransduction is the sensing of a mechanical stimulus.

It has been shown, that mechano-sensitive ion channels are able to transform a physical signal into an ion flux (Martinac, 2004). In keratinocytes (Yano et al., 2006), and various other cell types, the function of stretch-induced Ca2+ channels has been shown to be indispensable for a response to mechanical stress (Fig. 1). An applied force may directly induce conformational changes and cause the ion channel to open, thereby allow a Ca2+ flux which mediates phospholipase C (PLC) and protein kinase C (PKC) activation and translocation of PKCα and δ from the cytoplasm to the cell membrane (Takei et al., 1997a). Inhibition of PLC abolished Ca2+ responses induced by mechanical stress (Diamond et al., 1994).

Some of the highly conserved transient receptor potential (TRP) channels have been shown to respond to multiple kinds of stresses, e.g. TRPA1 upon mechanical stress in the hair cells of the ear (Corey et al., 2004), although their mechanism of activation is not yet clear. TRPV3 and TRPV4, which are also expressed in keratinocytes, are sensitive to heat and to hypoosmotic swelling and shear stress, respectively (Moqrich et al., 2005; O’Neil and Heller, 2005). Connexin hemichannels represent yet another form of mechano-sensitive channel in the plasma membrane which may determine cytosolic Ca2+ concentrations (Evans et al., 2006; Jiang et al., 2007). It is well established that keratinocyte proliferation and differentiation in the epidermis, as well as in cell culture, is regulated by the extracellular Ca2+ concentration (Hennings et al., 1980). Mechanical gating of Ca2+ channels could therefore directly modulate the keratinocyte proliferation and differentiation program. The mobilization of Ca2+ may trigger Ras and MAPK pathways which are central in mechanical signaling in various cell types and will be further discussed below.

Receptors embedded in the plasma membrane are ideally positioned to fulfill the sensing of physical forces and allow the transduction of the stimuli to signaling pathways within the cytoplasm. Among the growth factors shown to play a role in keratinocyte mechanotransduction is platelet-derived growth factor (PDGF) (Wilson et al., 1993). Growth factor receptor activation upon physical force can also occur by transactivation without the involvement of ligands as has been shown for the angiotensin II type 1 receptor (AT1-R) in cardiomyocytes (Zou et al., 2004) and for the epidermal growth factor receptor (EGFR) in mesangial cells (Zhang et al., 2007) and in keratinocytes (Knies et al., 2006) (Fig. 1).

In the past, multiple mechano-sensory structural motifs, often repeating, have been identified in various proteins which respond to mechanical force with conformational changes. Among them are components of the extracellular matrix and intracellular proteins like components of the focal adhesion complex, and proteins that link transmembrane proteins, like integrins, to the cytoskeleton. Protein deformations may regulate enzymatic activity, enable new molecular interactions or liberate bound factors which may then activate signaling pathways (Kung, 2005; Vogel, 2006; Vogel and Sheetz, 2006). Some of these stress-sensitive proteins expose binding domains upon mechanical stretch and become phosphorylated (Katz et al., 2000; Tamada et al., 2004). Recently, Sawada et al. (2006), using the human embryonic kidney cell line HEK 293, showed that tyrosine phosphorylation of p130Cas by Src family kinases was increased upon stretch. Traction forces acting on the focal adhesion complex-anchored p130Cas, probably generated by the actin cytoskeleton, extended the central substrate domain of the molecule which exposed the tyrosine-containing motifs. Phosphorylated p130Cas in turn is involved in force-dependent activation of the small GTPase Rap1 (Sawada et al., 2001). In their recent paper, the authors proposed that this mechanism is a simple way for sensing the level of force as well as its cellular location and suggest that it might be a general mechanism in the transduction of cell forces and termed it “substrate priming”.

It has been shown that, e.g. ankyrin repeat motifs may directly respond to stretch by protein distortion (Lee et al., 2006). TRP channels contain C-terminal ankyrin repeat motifs, however it is not known whether these motifs play a role in channel activation. Ortiz et al. (2005) published a detailed analysis of the unfolding of spectrin repeat domains under stress. Spectrin repeat domains are found in a large family of proteins among them α-actinin and the cytoskeletal linker molecules of the spectraplakin family. To date it is not known, whether mechano-perception in keratinocytes involves the deformation of any of these motifs.

Interestingly, lengthening of the extracellular matrix protein fibronectin due to tension exposes cryptic binding sites (Craig et al., 2001; Gao et al., 2002; Geiger and Bershadsky, 2002), and mechanical stretching of immobilized fibronectin induced matrix assembly by exposing self-assembly sites (Zhong et al., 1998). The increase in matrix stiffness might be perceived and transduced to the cytoplasm by specific integrins which are the cellular receptors for extracellular matrix proteins like fibronectin.

Atomic force microscopy and optical tweezers (Felsenfeld et al., 1999) can be used to stretch molecules and measure deformations and adhesive forces of single molecules. Recently, fluorescence resonance energy transfer in combination with optical tweezers was applied to follow the mechanical activation of c-Src upon integrin clustering in umbilical vein endothelial cells (Wang et al., 2005).

Methods to monitor and experimentally quantify mechanical stress in keratinocytes are rare. Some systems apply measurable uni-, bidirectional or radial stress on cells plated on flexible silicone supports. The elastic substratum allows deformation through an attached cell monolayer e.g. via computer-driven vacuum (Flexcell system). The mechanical stress may be static or cyclic with different frequencies and duration. Most experiments have been performed with a maximum stretching by up to 20%. A change in length of 0.1% is expressed as 1000 microstrain (Rubin and Lanyon, 1984).

In cell culture experiments uniaxial and radial stretch has different effects on the formation of actin stress fibers and on the orientation of cells with respect to the applied force (Shirinsky et al., 1989; Takemasa et al., 1998). Interestingly, in keratinocytes, mechanical pressure (studied by using teflon weights in cell culture experiments) seems to stimulate pathways different from those induced by mechanical stress. Mechanical pressure implicates c-Src, PKC and p38 MAPK activation, the latter peaking between 5 and 10 min after exerting the pressure (Hofmann et al., 2004). Four days of cyclic pressure (0.015 N/cm2) caused a decrease in proliferation and an increase in differentiation (Gormar et al., 1990). Whereas in most skin regions, stretch is probably the most relevant kind of mechanical stress, sole skin experiences in addition to this a considerable amount of pressure. Palmar keratinocytes express a unique set of keratins resulting in intermediate filaments with distinct properties (Swensson et al., 1998). To date, the impact of the keratin intermediate filament composition on mechanotransduction is not yet clear and will be discussed below.

Most studies on the mechanical stress response of keratinocytes to date have been performed with two-dimensional (2D) approaches using flexible silicone supports. As tissue cells in vivo rather grow in a three-dimensional (3D) environment, and as fibroblasts show different cytoskeletal properties in 2D versus 3D culture (Cukierman et al., 2001), there is a need to develop experimental settings using 3D culture on mechanotransduction in keratinocytes.

Many publications implicate integrins in mechanosensing and mechanotransduction (Katsumi et al., 2004). Integrins are the main receptors for extracellular matrix proteins and they are localized in focal contacts (Turchi et al., 2003). They link the extracellular matrix to the actin cytoskeleton and regulate several signaling pathways (Burridge and Fath, 1989). Therefore, they are in a good position to translate information on matrix rigidity as well as mechanical stimuli into a biochemical response. Their implication in mechanical signal transduction has been demonstrated in various cell types including keratinocytes. It was shown that clustering of integrins in focal adhesions, involving activation of c-Src and focal adhesion kinase (FAK), is required for MAP kinase activation upon force generation (Okuda et al., 1999; Roovers and Assoian, 2003). β1-Integrin clustering in adhesion sites depended on binding to its ligands and was apparent in keratinocytes grown on collagen IV or fibronectin substrates but not in cells grown on laminin substrate. For integrin-mediated mechanotransduction, a dynamic integrin–ligand interaction is required (Jalali et al., 2001; Katsumi et al., 2004, Katsumi et al., 2005). Integrin cluster formation has been observed in various cell types such as osteoblasts, heart muscle cells, endothelial cells, fibroblasts, and in keratinocytes (Knies et al., 2006) (Fig. 1). The role of β1-integrin in mechanical signaling via ERK1/2 in keratinocytes will be explained further below.

Focal adhesions have been discussed as mechanosensors (Shemesh et al., 2005). In endothelial cells a mechanosensory complex of adhesion molecules and a receptor tyrosine kinase (RTK) was identified in adherens junctions that mediates the response of the cells to fluid shear stress (Tzima et al., 2005). Actin stress fibers are connected to focal and fibrillar adhesions and form a network that is a generator and sensor of mechanical force (Bershadsky et al., 2003).

The kind of mechanical stress a keratinocyte senses may be diverse. It may be as strong and permanent as the body weight resting on the plantar epidermis or it might be as subtle as the neighbor cell starting to divide. Keratinocytes, as other cells, must sense all these impulses in order to react adequately. It is not known, how a cell distinguishes between distinct stimuli and how the different stresses are transformed to lead to the adequate biochemical response. However, it seems that common general signaling pathways are involved. The MAP kinase-signaling pathway seems to be central for the proliferative response of distinct cells towards mechanical stress.

Continuous stretching of human keratinocytes grown on flexible silicone dishes (e.g. for 24 h by 20%) strongly stimulated proliferation and protein synthesis (Takei et al., 1997b; Yano et al., 2004). Additionally, cell morphology was altered so that cells became aligned perpendicular to the direction of force (Takei et al., 1998). These late responses to mechanical stress are preceded by the fast activation of MAP kinases, e.g. ERK1/2 which is inhibited by the MEK1/2 inhibitor U0126 (Yano et al., 2004). ERK1/2, which is indispensable for cell proliferation and survival in various cell types, was activated within 5 min under static stretch, whereas cyclic stretch of a sine pattern activated p38 and c-Jun aminoterminal kinase (JNK) (Nguyen et al., 2000). Upstream of ERK1/2 activation, the authors identified transient and EGF-independent EGFR phosphorylation on Tyr845 as well as phosphoinositide 3-OH kinase (PI3K) and Ca2+ channel activation to be necessary for the stress-induced stimulation of proliferation (Fig. 1). The Tyr845 residue of the EGFR is normally not autophosphorylated. In keratinocytes, transactivation of the EGFR upon unidirectional mechanical stretch may also occur via the AT1-R involving G protein-coupled receptors (Kippenberger et al., 2005) (Fig. 1). It was shown before, that mechanical stress activated AT1-R and thus ERK1/2 without an involvement of angiotensin II in vascular smooth muscle cells (Hosokawa et al., 2002; Zou et al., 2004) (Fig. 1). The mechanism that activates AT1-R upon mechanical stretch remains to be determined, and the authors discussed conformational changes of the receptor itself or the activation of other sensors like integrins which may then activate AT1-R from inside the cell (Zou et al., 2004).

ERK1/2 activation was also sensitive to the function of β1-integrin. A single unidirectional extension of 10% for 5 min activated ERK1/2 and increased cell adhesion in HaCaT keratinocytes (Knies et al., 2006). These authors showed that β1-integrins clustered at the basal cell membrane and that the EGFR colocalized at these focal contacts. The dependence of ERK1/2 activation on the functionality of β1-integrin and on EGFR phosphorylation was further supported by experiments using integrin-inactivating antibodies and a specific inhibitor of EGFR tyrosine kinase (AG1478). The phosphorylation of EGFR was independent of EGF and was probably due to transactivation of the receptor by β1-integrin (Fig. 1). The general possibility of transactivation of EGFR by integrins or other factors in the absence of growth factors with a stimulating effect on MAP kinases has been demonstrated before in another context (Cabodi et al., 2004; Daub et al., 1996; Rosette and Karin, 1996) and transactivation via integrins was also observed in hyperproliferative diseases (Moro et al., 1998). Proliferative and antiapoptotic signals often occur simultaneously (Stenson, 2007). Recently, it has been shown, that mechanical stretch caused a rapid activation of Akt kinase in normal human keratinocytes and HaCaT cells which depended on EGFR activation and was attenuated by inhibitors of PI3K, MEK1/2 and Ca2+ channels (Yano et al., 2004) (Fig. 1). Kippenberger et al. (2005) presented data indicating that the activation of Akt kinase upon mechanical stress protected keratinocytes against apoptosis. Inhibition of apoptosis was also observed in endothelial cells in which Akt kinase was activated upon shear stress (Dimmeler et al., 1998; Garcia-Cardena et al., 2000).

Using a reporter assay, Yano et al. (2004) found that keratinocytes reacted upon stretching for 6–48 h with an activation, in which ERK1/2 was involved, of the transcription factor AP-1 (Fig. 1). Keratinocytes are the major source of IL-1, a key modulator in skin which also activates AP-1. Upon 10% cyclic strain IL-1α and -β are elevated in human keratinocytes (Takei et al., 1997a, Takei et al., 1998), and from 14% strain on, Il-1α secretion was shown to be promoted (Lee et al., 1997). The observed increase in proliferation could be blocked by neutralizing antibodies to IL-1, whereas the morphological changes (elongation and alignment) were not. IL-1 receptors as well as EGFR and PDGF receptors localize at focal adhesions (Morrison et al., 1989).

The cytoskeleton is well positioned to be an ideal mechanosensor and -transducer. One elaborate model, which describes how cells might integrate stress is the “tensegrity” (tensional integrity) model. It was originally developed by the architect Buckminster Fuller (1961), then applied to cells and further developed by Ingber (Ingber, 1993, Ingber, 2003a, Ingber, 2003b, Ingber, 2004, Ingber, 2006a, Ingber, 2006b). According to this concept, the cytoskeleton provides the cells with a pre-tension that is necessary for a cell in order to respond to extrinsic mechanical stress. By varying the level of isometric tension, the sensitivity of the system may be adapted. This implies that under pathological conditions where the cytoskeleton is abnormal, the “prestress” is disturbed. “Tensegrity” relies not only on the cytoskeleton but also on cell adhesion structures which link the cytoskeleton to the extracellular matrix and probably additional proteins which provide a link to the nucleus (Huang and Ingber, 1999; Lammerding et al., 2004).

The cell form itself is regulated by mechanosensitive pathways involving extracellular matrix–integrin–cytoskeleton complexes and tyrosine kinases, which transduce forces that originate from the environment (Giannone and Sheetz, 2006). Influenced by integrins, the actin cytoskeleton undergoes reorganization in response to mechanical signals. Integrins may initiate the formation of long parallel actin stress fibers, and this process is regulated by c-Src, which activates FAK by phosphorylation of Tyr397 (Butler et al., 2006). Using FAK−/− primary keratinocytes, Schober et al. (2007) recently showed that the function of FAK is important for the regulation of cytoskeleton and focal adhesion dynamics involved in cell migration and morphogenesis.

In most cells studied so far, the actin cytoskeleton seems to play a prominent role in mechanotransduction. In keratinocytes, however, the most prominent cytoskeletal system is the keratin intermediate filament network. Intermediate filament bundles span the cells, interact with desmosomes and connect not only neighboring cells but mechanically integrate the tissue as a whole (Herrmann et al., 2007). Upon epidermal differentiation, the keratin composition of the intermediate filaments changes. Intermediate filaments of the proliferative basal keratinocytes consist of keratins K5/K14 whereas suprabasal keratinocytes express K1/K10 heteropolymers leading to the formation of thicker filament bundles in the upper layers as observed by electron microscopy (Reichelt et al., 2001). The functional consequences of the altered quality of filaments are not known. Epidermal keratins have been implicated with specific properties. Palmoplantar epidermis, e.g. expresses an additional keratin, K9, in suprabasal layers which is supposed to enhance mechanical stability (Swensson et al., 1998). At the wound edge, K6, K16 and K17 are transiently induced whereas K1 and K10 are downregulated which has been shown to increase cell pliability (Wong and Coulombe, 2003). Interestingly, continuous stretching of keratinocytes grown on flexible silicone supports also dowregulates K10 and upregulates K6 expression (Yano et al., 2004) (Fig. 1). Depletion of K10 in mice causes – independent of additional experimental application of mechanical stress – a strong increase in proliferation in the basal epidermis, interfollicular expression of K6, and it is accompanied by an activation of the MAP kinases ERK1/2 and p38 (Reichelt et al., 2004; Reichelt and Magin, 2002). The altered suprabasal intermediate filament composition in these cells, consisting of K1/K5/K14 instead of K1/K10, certainly changes the cellular “prestress”, according to the “tensegrity” model, and might therefore alter the fine tuning of the mechanosensory sensitivity. Whereas the role of the actin cytoskeleton in strengthening integrin-mediated cell adhesion upon strain has been established (Smith et al., 1997), evidence for a similar function of intermediate filaments and desmosomes has only been gained recently (Huen et al., 2002). Comparing the behavior of the intermediate filament network of normal keratinocytes and that of keratinocytes from an epidermolysis bullosa patient bearing a “hot spot” mutation in keratin 14 upon mechanical stretch, Russell et al. (2004) provided evidence for a role for keratin filament tension in maintaining desmosomes and hemidesmosomes. Vasioukhin et al. (2001) showed that uncoupling of intermediate filaments from desmosomes, using a truncated desmoplakin molecule, decreased intercellular adhesive strength. Their data suggested a synergistic effect of intermediate filament and actin-based junctions in the strengthening of cell adhesion. A recent publication by Windoffer et al. (2006) indicated a major role of focal adhesions for keratin intermediate filament formation.

The cell–matrix relation is mutual and cells may respond to mechanical stress by actively altering the extracellular matrix composition like, e.g. smooth muscle cells which increase their collagen production upon mechanical stimulation (Sumpio et al., 1988). Depending on their respective needs, tissues certainly constantly adapt to the environmental mechanical stress by modulating their sensitivity to exogenous stimuli. In a cell culture scratch assay using human keratinocytes, Turchi et al. (2003) demonstrated that mechanical stress stimulated the expression of matrix metalloproteinase-9 (MMP-9) at the wound edge. This stimulation involved small GTPases of the Rho family and the activation of the MAP kinases p38 and JNK (Fig. 1). Among the substrates of MMP-9 are other proteinases, basement membrane components like collagen IV, RTKs, adhesion molecules, IL-1, and transforming growth factor-β (TGF-β). Via its downstream mediator Smad3, TGF-β in turn regulates the accumulation of ECM proteins and their interaction with integrins, involving MMPs and tissue inhibitors of MMPs (Arany et al., 2006). In bladder smooth muscle cells, stretch-induced proliferation and ERK1/2 activation depend on extracellular MMP activity (Aitken et al., 2006). Tschumperlin et al. (2004) demonstrated, that compressive stress shrinks the lateral intercellular space surrounding bronchial epithelial cells, which leads to an increase in the local EGFR ligand concentration. Upon the mechanical stimulus, HB-EGF was shed into the extracellular space in a metalloproteinase-dependent manner, and activated the EGFR and subsequently the MAPK ERK1/2. These findings indicate that MMPs are probably involved upstream as well as downstream of MAP kinases in mechano-signaling.

The cell response to the stiffness of its matrix environment influences differentiation and development. This has been exemplified by studies on the mammary gland (Nelson and Bissell, 2006) and various other cell types including fibroblasts, myocytes and neurons (Discher et al., 2005). In tumors which are often more rigid than the normal tissue, integrins may be activated by the increased mechanical load and in turn activate MAP kinase signaling as has been shown for mouse mammary tumors (Paszek et al., 2005). Stem cells seem to respond to mechanical stress not only by proliferation, but also by differentiation. Cell lineage specification of mesenchymal stem cells can be triggered by soluble factors and by the elasticity of the microenvironment (Engler et al., 2004, Engler et al., 2006). Epidermal stem cells which rarely divide in their niche in the bulge region of the hair follicle behave differently in the environment of a culture dish and become highly proliferative. The understanding of the distinct behavior of differentiated cells, tumor cells or stem cells in response to their microenvironment might have implications on the improvement of wound healing, the treatment of tumors and on stem cell therapy. From the study of various cell types, it is well established that mechanosensitivity depends on cell adhesion and on the rigidity of the substratum (Bershadsky et al., 2003; Giannone and Sheetz, 2006). Experiments, in which cells have been grown on different substrates like collagen, fibronectin, vitronectin, elastin or laminin demonstrated the influence of the extracellular matrix composition on strain-induced cell proliferation (Wilson et al., 1995). In this context, the exposure of cryptic binding sites on fibronectin through tension should be mentioned again (Craig et al., 2001; Gao et al., 2002). Nicolas and Safran (2006) recently proposed a model based on mathematical calculations that predicts that focal adhesion formation strongly depends on the elasticity/stiffness and thickness of the extracellular matrix. However, different cells seem to react in different ways to matrix rigidity (Discher et al., 2005). In the epidermis, like in other tissues, there are probably several regions with distinct stiffness properties. In addition, the polar keratinocytes will perceive at least two microenvironments with distinct bio-mechanical properties at the same time and integrate them. The interfollicular basement membrane presumably provides a different quality of substrate to the basal keratinocytes than the microenvironment in the bulge region to the keratinocyte stem cells or than the intercellular matrix in the granular layer to the differentiated keratinocytes. Exact measurements of the microelastic properties of distinct epidermal subregions have been performed so far only on the stratum corneum (Yuan and Verma, 2006).

It is obvious, that keratinocytes like other cells constantly monitor their environment. The questions are: how they distinguish between distinct subcellular mechanical stimuli and which mechano-sensitive proteins are involved? How do keratinocytes process the complex information to orchestrate a physiological response that influences wound healing, development, morphogenesis, proliferation and differentiation? Do keratinocytes, or epithelial cells in general, use unique mechanotransduction pathways in addition to the common pathways described for other cells? Do hyperproliferation-associated skin disorders, like psoriasis or epidermolytic hyperkeratosis, involve deregulation of mechanotransduction pathways? Or is the mechanical communication between the cells of the epidermis and probably also the communication with epidermal and dermal cells affected in those diseases? And if so, are there common key proteins that might serve as targets for effective therapies of these disorders?

The mechanical stimuli in vivo and in the experimental situation in cell culture differ from each other and the 2D systems that have mostly been used in the past are artificial, partly due to the relative rigidity of the environment. Although, there are obviously common pathways activated in distinct cells and distinct experimental systems, there is certainly a need to use and develop 3D approaches to study epidermal mechanotranduction.

Section snippets

Acknowledgements

I thank Prof. Nicholas Reynolds and Monika Loeher for critically reading the manuscript and the anonymous reviewers for invaluable advice. My research in the field of mechanical signaling is funded by the German Research Foundation.

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