New insights in mechano-transduction in bone repair and regeneration

Rezumat:

Acest articol reprezinta o trecere in revista a datelor recente din literatura de specialitate privind mecanotransducerea cu cele patru faze ale sale,: cuplarea mecanica, cuplarea biochimica, transmisia semnalului de la celula sensor la cea efectoare si raspunsul celulei efectoare, care joaca un rol decisiv in repararea si regenerarea osoasa, evidenta clinica asupra repararii si regenerarii osoase, precum si asupra teoriilor tensegritatii si mecanosomilor care postuleaza cum fortele mecanice isi manifesta efectele asupra repararii si regenerarii osoase.
Cuvinte-cheie: mecanotransducerea, stimularea mecanica, repararea osoasa, regenerarea osoasa

Abstract:

This is a review of the most recent literature data on mechanotransduction and its four phases: mechanocoupling, biochemical coupling, transmission of the signal from the sensor cell to the effector cell, and the effector cell response, which plays a crucial role in bone repair and regeneration, clinical evidence in bone repair and regeneration as well as on its tensegrity and mechanosome theories that postulate how mechanical force exhibits effects on bone repair and regeneration.
Keywords: mechanotransduction, mechanical stimulation, bone repair, bone regeneration

Introduction

The body skeleton is designed to support the body’s weight, connect tendons and muscles, provide mechanical protection, assist movement, store minerals, and produce blood cells. Bone repair and regeneration are undoubtedly influenced by intimate interactions between the physiological, biochemical, and mechanobiological environments. In particular, the effect of mechanical force on bone regeneration has been studied widely, and this has led to an increasing awareness of the importance of studying mechanotransduction.

The process by which physical forces are converted into biochemical signals that are then integrated into cellular responses is known as mechanotransduction which includes 4 phases: mechanocoupling, biochemical coupling, transmission of the signal from the sensor cell to the effector cell, and the effector cell response [1].

The dynamic balance between bone formation and bone absorption is achieved by such complex mechanisms . A better understanding of these processes may lead to the development of advanced clinical applications that can lead to predictable and reproducible improvements in bone structure. In this review of the latest achivements on bone research area is presenting the clinical evidence evidence of bone mechanotransduction and the basic research into the mechanotransduction mechanisms involved in bone repair and regeneration. Moreover, future perspectives of bone mechanotransduction research are discussed.

In order to find out how mechanical stimuli influence bone regeneration it has been explored by studying processes such as physiological bone adaptation and clinical conditions such as pathological bone fracture and distraction osteogenesis.

Physiological bone adaptation

In order to meet the functional demands of its mechanical environment, the mass and geometry of bone is physically remodeled in a dynamic fashion (Wolff’s law). According to mechanostat theory (a refinement of Wolff’s law) bone adapts so that it can function mechanically as needed by detecting and responding to mechanical loads [2]. Thus, in this dynamic and lifelong biological control system, bone formation in terms of shape, size, and density can be directed by high mechanical loads.

On the other hand, immobilization can lead to the loss of bone. For example, 120 days of bed rest can induce bone loss by accelerating bone resorption and retarding bone formation [3]. This is also true for spaceflight, where bone loss can be induced because of increased bone resorption and decreased calcium absorption [4]. Mechanical loading presents a potent osteogenic stimulus, and it is absolutely clear that the skeleton needs “time off” from mechanical loading. , but bone cells desensitize rapidly to mechanical stimulation. Resensitization must occur before the cells can transducer future mechanical signals effectively [5]. It has been suggested that cyclical or intermittent loading, which provides the skeleton with regular “time off” periods, may be more effective than continuous loading in inducing the bone formation needed to promote the growth and repair of the skeleton [6].

The osteogenic response to simulated high-impact exercise over a long period can be enhanced by dividing the exercise into brief sessions of loading that are separated by recovery periods [7].

Mecanostrasduction process in bone is also affected by factors such as age and gender. Literature datahave pointed out that aging can inhibit mechanotransduction, as experimental results on 19-months old rats exhibit over 16-fold less mechanically induced bone formation than 9-months-old rats [8]. During advancing age increased remodeling rate and worsening of basic multicellular unit (BMU) imbalance increase bone loss and structural damage, resulting in a predisposition to bone fracture following mechanical stimulation such as minimal trauma[9].

According to Seeman, 4 of the age-related changes in bone modeling and remodeling that compromise bone’s material properties and structural design are a reduction in bone formation at tissue level, reduction in bone formation at cellular level within each BMU, continued resorption in the BMU, and increase in the rate of bone remodeling accompanied by worsening of negative bone balance in each BMU[10,11]. Also, in addition, gender affects bone mechanotransduction because males rather than females exhibit significantly reduced mechanoresponsiveness compared with controls [12].

Conversely, males suffer larger bone mass reductions in both the cortical and cancellous compartments than females when they are subjected to hind limb unloading [13]. In fact, the position of the cortex in relationship to the long axis of the long bone differs in males and females [14].Estrogen reduces the osteogenic response induced by exercise on the periosteal surface of the bone, whereas it enhances bone formation on the endocortical surface of 8-wk-old male Sprague-Dawley rats [15]. Such age and gender-specific variations may be due to differences in the efficiency of mechanical sensors and sex hormone production and/or signaling, respectively.

Fracture healing and distraction osteogenesis

The clinical bone regeneration that occurs in fracture healing and distraction osteogenesis is also evidence of bone mechanotransduction. The healing of a bone fracture is a unique process whose result is not scarring but regeneration [16], while distraction osteogenesis is a special form of bone healing in which gradual traction on the bone leads to osteogenesis and the tandem lengthening of the skin, muscles, and nerves [17,18].

It is well known that bone cells consist mainly of osteoblasts (active osteoblasts and inactive bone-lining cells), osteoclasts, and osteocytes. Clinical osteogenesis in fracture healing and distraction osteogenesis aims to facilitate bone formation by osteoblast and bone remodeling together with osteoclast. From the perspective of mechanotransduction, bone tissue is described as an extensively connected cellular network where the osteocytes serve as sensory cells and the osteoblasts and osteoclasts are the effector cells [19]. Loads applied to a whole bone are related to the flow past the osteocytic processes in their canaliculi [20].

The osteocytes can sense the flow of fluid and then produce signaling molecules that regulate osteoclast-mediated bone resorption and osteoblast-mediated bone formation. This results in adequate bone remodeling [21]. That this process occurs has been supported by the finding that the targeted ablation of osteocytes in mice results in defective mechanotransduction and fragile bone with osteoblastic dysfunction [22].

The mechanical modulation of bone fracture healing varies depending on the type of fracture, the therapeutic fixation, and the loading that follows. A series of hypotheses have been proposed to explain mechanical influence on tissue differentiation in fracture: in these hypotheses, hydrostatic pressure and tensile strain [23, 24], or shear strain and fluid flow, have been identified as the key stimulating mechanical forces [25], [26]).

However, to date, far less is understood about the signal mechanotransductive pathways that lead to fracture repair[27]. Another study has shown that proinflammatory cytokines such as TNF-_and IL-1, which are known to regulate immune function and inflammation, participate in fracture healing because they are expressed at both the very early and late phases of the repair process. It then suggests that these cytokines are important in the initiation of the repair process and play important functional roles in intramembraneous bone formation and remodeling[28].

Distraction osteogenesis is specifically characterized by osseous regeneration that occurs primarily via intramembranous ossification, which is stimulated by several mechanotransductive pathways. In the integrin-mediated, extracellular signal-related kinase (ERK 1/2) -dependent mechanotransduction pathway, ERK 1/2 is a potential central mediator that acts as a signaling convergence point and regulates the osteogenic differentiation of mesenchymal stem cells during distraction (29,30]). In addition, increased expression of osteogenic proteins, including BMP 2/4, can be induced via the c-Src-dependent mechanotransduction pathway [31]. Mechanical forces can also stimulate the expression of the earlyresponse genes of the activator protein-1 (AP-1) family of transcription factors [32], up-regulate the expression of Runx2, and initiate the differentiation of periosteal cells into osteogenic cells[33], thus promoting osseous regeneration.

These observations show the central role mechanotransduction plays in bone regeneration, physical mechanical loading, pathological bone fracture, and the surgical intervention of distraction osteogenesis. Studies that elucidate the fundamental signaling processes that are involved are needed.
Mechanotransduction, particularly in bone, has been a focus of interest in fields ranging from molecular biophysics to clinical medicine. Research into mechanotransduction has been approached from 2 directions.
Some researchers have sought to understand it from the macro/system perspective, where it is seen as a control system that requires feedback and stability.

Others have viewed it from a micro/molecular perspective and have sought to understand how it works by characterizing the specialized molecular signaling pathways that are involved.

Tensegrity theory

Tensegrity theory proposes a mechanism that explains how mechanical stresses applied at the macroscopic level can influence the molecular structure and function of living cells [34]. In this theory, the whole cell, not just the single specialized mechanotransduction molecules in isolation, is believed to serve as a mechanotransducer as it integrates the local signals with other environmental inputs before eliciting a specific behavioral response [35].

The key determinants in tensegrity are architecture (the 3-dimensional arrangement of the elements) and the level of prestress (isometric tension) in the cytoskeleton (34). Thus, on the one hand, an even distribution of load protects individual cells from damage, while on the other hand, a small mechanical stimulus is allowed to have an effect on a large number of cells[36].

The role of integrins as mediators of mechanosensation

Integrins and focal adhesions are recognized as being the ubiquitous mediators of mechanosensation.Integrins are a family of ubiquitous, heterodimeric glycoproteins that mediate cell attachment to the extracellular matrix. They act as mechanoreceptors of the cell by spanning the cell membrane and being connected at one end to the cytoskeleton and at the other end to the extracellular matrix (ECM) [35]. Focal adhesions are multimolecular complexes that connect the ECM to the actin cytoskeleton and link integrins to the ends of contractile microfilament bundles. They are thought of as mechanosensory organelles[37,38] where mechanochemical signal conversion is carried out in the cell.
The forces that are channeled as described above over the ECM and to the integrins are converted into biochemical changes by producing changes in deformation of other load-bearing mechantransducer molecules, such as stress-sensitive ion channels, protein kinases, G proteins, and other signaling molecules,inside the cell [38].

Mechanosomes theory

Mechanosomes theory was proposed by Pavalko [39]. The key aspect of this model is the load-induced formation of mechanosomes,which are multiprotein complexes comprised of focal adhesion-associated or adherens junction-associated proteins. In terms of cellular structure, the loadinduced deformation of bone is converted into the deformation of the sensor cell membrane, which drives conformational changes in membrane proteins. Some of these membrane proteins are linked to a solid-state signaling scaffold that releases protein complexes called mechanosomes that are capable of carrying mechanical information into the nucleus. In this way, “bending bone ultimately bends genes” [39]. In terms of signal conversion, the solid-state and diffusion-controlled signaling pathways integrate with the tissue matrix-mediated transfer of mechanical energy to proteins of the adhesion complexes and selected proteins associated with the cyto- and nucleoskeleton. This conversion of mechanical energy into chemical energy drives changes in protein conformation, phosphorylation, and alterations in DNA geometry and mediates the formation and/or mobilization of nascent signaling complexes to the bone cell cytoplasm [39].

Role of osteocyte in mechanosignal transduction

In response to mechanical load ,osteocytes orchestrate different cell populations in bone [40]. They can be activated by strain amplification through canaliculi.The electrically coupled 3-dimensional network of osteocytes and lining cells is a communications system for the control of bone homeostasis and for structural strain adaptation, and the effector cells are osteoblasts and osteoclasts [41].

Osteocytes are the most abundant cells in bone cells and are spaced throughout the mineralized matrix. They appear to be the most mechanosensitive cells in bone and are involved in the transduction of mechanical stress into a biological response. Osteocytes, but not osteoblasts, react to a 1-h pulsating fluid flow, resulting in the sustained release of prostaglandin E2. By contrast, intermittent hydrostatic compression stimulates prostaglandin production after 6 and 24 h in osteocytes and after 6 h in osteoblasts [42].

Conclusions 

Bone mechanotransduction stands out as a highly promising research field, which will bring a greater understanding of the fundamental molecular signal pathways that are involved and collaboration with other fields of research such as investigation of angiogenesis may facilitate the development of pharmaceutical and clinical applications that can improve our knowleges about bone structures and functions.

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Gabriel Ovidiu Dinu

Floreasca Universitary Emergency Hospital