Mechanotransduction

When I have mentioned the term tensegrity I will source from the following article:

The FASEB Journal • Review October 2010 Vol 24 No 10  3625-3632
Mechanotransduction in bone repair and regeneration
Chenyu Huang*,†,1 and Rei Ogawa*
*Department of Plastic, Reconstructive and Aesthetic Surgery, Nippon Medical School, Tokyo, Japan;
and †Department of Plastic Surgery, Meitan General Hospital, Beijing, China

So what exactly is, mechanotransduction?

The present cited authors respond with, “Mechanotransduction is the process by which physical forces are converted into biochemical signals that are then integrated into cellular responses (1). (Their numbers refer to their bibliography, go to PubMed with this title to up load the entire article if you wish)
Consequently, mechanotransduction in the bone includes 4 phases: mechanocoupling, biochemical coupling, transmission of the signal from the sensor cell to the effector cell, and the effector cell response (2).”

Physiological bone adaptation

“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). Mechanostat theory is a refinement of Wolff’s law and proposes that bone adapts so that it can function mechanically as needed by detecting and responding to mechanical loads (3). 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. The other side of the coin is that immobilization can lead to the loss of bone. For example, 120 d of bed rest can induce bone loss by accelerating bone resorption and retarding bone formation (4). This is also true for spaceflight, where bone loss can be induced because of increased bone resorption and decreased calcium absorption (5).”

So we see from the last descriptions that astronauts suffer bone loss as a price to pay for being in microgravity while on space missions. Same goes for long bed rest, the bone profile changes getting more fragile with a higher risk toward fracturing. Where will the fracture likely occur? Now we have the amazing 1925 observation from Benninghoff with his bone drilled holes from my blog yesterday changing shape with time to conform to the underlying stresses, what I have termed the Benninghoff tensegrity image of bone stress profiles. Of course we know from current clinical studies that the highest susceptible areas are typically at wrist, shoulder ankle and hip. It would appear due to inherent bone strength weakness not contact during a fall.  As of 2011 there are no known bone anatomical studies using Benninghoff’ India ink awl holed bone tracings, which means we can start a publication on exactly just this, isn’t that cool! But the authors are also stressing a balance of bone creation versus bone absorption, this is crucial to grasp.

“Factors such as age and gender also affect the mechanotransduction process in bone. Aging can inhibit mechanotransduction, as shown by the fact that 19-mo old rats exhibit over 16-fold less mechanically induced bone formation than 9-mo-old rats (9). There also appears to be a “window of opportunity” during the development of peak bone mass in which the bone is especially responsive to weight-bearing physical activity (10). Furthermore, 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 (11). In fact, 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 (12, 13). In addition, gender affects bone mechanotransduction because males rather than females exhibit significantly reduced mechanoresponsiveness compared with controls (14).”

So what is interesting here in this good review article is the authors emphasis on local detail at the fracture zone. But the authors have not mentioned central regulation of bone changes, which they cover by stating, “…additional research is needed to clarify this.” so that is exactly the thirst of our efforts to reveal some hard held secrets of Nature for central control of bone metabolism, in addition to the healing right at the fracture.

“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 (21). Loads applied to a whole bone are related to the flow past the osteocytic processes in their canaliculi (22). 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 (23). 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 (24).”

Did you notice the sentence above, “..sense the flow of fluid ” ? Here again is the neat conundrum that although you are dealing with bone,  a solid substance yet the integration of its mass and volume in terms of its equilibrium is a  sensing system for fluid inside the bone. I find this fascinating to think that liquid movement sensing is used to form and/or change a bone’s density.

“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 (25, 26), or shear strain and fluid flow, have been identified as the key stimulating mechanical forces (27, 28).
However, to date, far less is understood about the signal
mechanotransductive pathways that lead to fracture repair (29). One study has shown that receptor activator of nuclear factor B (RANK) signaling is not required for the early phase of fracture healing, as evidenced by the repair (callus formation) observed in RANK/ mice with bone fractures; this is supported by the fact that administration of RANK signaling inhibitors sufficient to reduce bone resorption is not detrimental to fracture healing (30). 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 (31).”

Notice those critical key words, ‘shear strain and fluid flow, have been identified as the key stimulating mechanical forces ‘ Just like I said in the priority of listening in on Mother Nature’s signaling, you have to pay attention to the real ancient networks, like shape sensing, which is mechanotransduction (my goodness they like to create complicated medical lingo terms!) Also notice the immune system is involved big time with the bone remodeling.

Tensegrity theory
“Drawing on control theory and architecture, 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 (37). 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 (38).
The key determinants in tensegrity are architecture (the 3-dimensional arrangement of the elements) and the level of prestress (isometric tension) in the cytoskeleton (37). 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 (39, 40).
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) (38). 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 (41, 42), 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 (42).”

Take a step back and breathe, 1….2…..3……relax this isn’t that complicated, OK ?

Remember our burden of thinking of cells as M&M’s lying in  bowl as if they were cells attached together? What a massive wrong picture here. From the tensegrity window cells have to attach to things these are what are called the integrins. The cell attaches also to the surrounding framework, what is called the scaffold outside of the cell, the fancy term is extracellular scaffold. So things happen at the actual attachment like to make it sticky, that’s what is called a glycoprotein. Think of the Fuller Biodome again, OK? It’s the struts between the central collectors that attach, these are doing the same thing only at a different scale, remember tensegrity is a scaleable function. The same principles apply for a cell attachment as to a Biodome attachment to a continental tectonic plate- it’s all scalable interactions.

Now to be fair, there is another hypothesis called the mechanosome theory. In science you must be rigorous so I can’t dismiss it, yet. here it is.

“Mechanosomes theory is an appealing hypothesis that was proposed by Pavalko (43). 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 load induced 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” (43). 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 (43).”

Role of osteocyte in mechanosignal transduction
“Osteocytes orchestrate different cell populations in bone in response to mechanical loading (44). 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 (45).
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 (46). Osteocytes are more responsive to pulsating fluid flow than osteoblasts with respect to the production of soluble factors of nitric oxide, which affect both osteoblast proliferation and differentiation (44), as well as osteoclast formation and bone resorption (47). Pulsating fluid flow for 15 min stimulates by 3-fold prostaglandin G/H synthase-2 (PGHS-2, COX-2) mRNA expression in osteocytes but does not have the same effect in osteoblasts (48).
Besides, targeted ablation of osteocytes in mice induces osteoporosis with defective mechanotransduction (24). In fact, the main source of signaling induced by oscillatory fluid flow in osteoblasts occurs through the focal adhesion complex and the focal adhesion kinase (49). Formation of focal adhesions on fibronectin promotes fluid shear stress induction of cyclooxygenase (COX)-2 and PGE2 release in osteoblasts (50). The linkage of actin stress fibers to integrins in focal adhesions via -actinin is a critical link in osteoblasts, because blocking this linkage prevents fluid shear-induced COX-2 and c-Fos expression (51). Excitation of osteocytes may also be related to interaction of pericellular matrix and cell process cytoskeleton (52).”

Tomorrow I’m going to write more about pulsating fluid flow, with the water- circulation approach so keep an eye out for that. Notice too all the descriptive talk about shear linkage, what is linkage? It’s pushing the open fridge door closed with a hockey stick -it’s a mechanical contact that does something, its part of shape sensing. The fridge knows the hockey stick is closing the door, it knows, that’s the analogy, OK ?

“Compared to the bone lining cells that cover the bone surface, osteocytes located within the bone matrix would be more efficient sensors. With osteocytes as sensors, remodeling would be more sensitive to external loads; such remodeling would lead to adaptation of the architecture rather than that of strut thickness, and the remodeling process drifts more easily and effectively toward a significantly different morphology. As for changes in load magnitudes only, surface bone lining cells should be equally capable of regulating adaptation as osteocytes. However, adding surface cells to the osteocyte model makes no difference to the output of the model (53).”

Please notice again these authors gaze through a window of observation quite fixated at the fracture zone with all the various cellular process happening there. But there is the additional central traffic happening as well too, which is hopefully our best focus effort to influence metabolic understanding at the fracture zone. Finally the authors describe mechanotransducers, the molecular changes of transduction.

Molecular skeletal mechanotransducers
“Bone, as a mechanosensitive organ, reacts to mechanical stimuli through a series of molecules and crosstalk between them. Research into mechanotransduction has shown that gaps between physical forces and biochemical signals are bridged by intracellular ion channels such as K (54) and Ca2 (55) channels, intracellular
signaling, such as that of Wingless-type/-catenin and inositol 1,4,5-triphosphate, mechanically induced signaling molecules such as that of prostaglandin and nitric oxide (56), transmembrane molecules such as integrin (57), growth factors such as insulin-like growth factor and bone morphogenetic protein, and systemic hormones such as parathyroid and estrogen.”

Calcium ion signaling
“Several studies have revealed that calcium ion signaling plays an important role in osseous mechanobiology. In vitro studies have shown that the intracellular calcium in osteoblastic cells can be increased within minutes in response to fluid flow and that this can be suppressed by gadolinium, a stretch-activated channel blocker
(58). In vivo studies have shown that mechanically induced bone formation in rats is substantially suppressed by verapamil and nifedipine, 2 blockers of L-type calcium channels (59). Furthermore, inositol 1,4,5-triphosphate (IP3) can mediate intracellular Ca2 release, which is required for modulating flow-inducedresponses (60).
Integrin signaling
Integrins, as mentioned above, are transmembrane molecules connected at one end to the ECM and at the other end to the intracellular cytoskeleton. They also play an important role in osseous mechanobiology. When focal adhesion kinase, an intracellular component of the integrin signaling pathway, was conditionally inactivated, the mechanically induced osteogenic response was abolished (61). Moreover, an integrinmediated, extracellular signal-related kinase ERK 1/2- dependent mechanotransduction pathway was found to play a significant role in distraction osteogenesis (33).
Prostaglandin (PG) and nitric oxide (NO) signaling PG is an important biochemical mediator of mechanical loading in bone. Fluid flow shear stress can promote the release of intracellular PGE2 from osteocytes (62). It induces the translocation of Cx43 to the membrane surface, and the unapposed hemichannels formed by Cx43 serve as a novel portal for the release of PGE2 in response to mechanical strain (63). Moreover, the bone formation that follows brief bouts of mechanical loading involves the release of PG from cells at the time mechanical loading is applied, rather than new PG synthesis associated with mechanically induced COX-2 expression (64). Since PGs are synthesized through the conversion of arachidonic acid by the rate-limiting COX enzymes, the role of COX in PG signaling remains unclear. One study has shown that a functional COX-2 gene is not required for skeletal mechanotransduction because mice with a null mutation in the COX-2 gene respond to loading by inducing skeletal COX-1 expression (65). In contrast, another study has shown by administering the COX-2 inhibitors NSAIDs or NS-398 that COX-2 does play an important role in the bone formation elicited by mechanical strain (66). NO signaling is also involved, as shown by the fact that application of mechanical strain onto bone stromal cells
regulates NO synthase in synergy with RANK ligand to promote positive bone remodeling; this process involves mitogen-activated protein kinase (67). Moreover, mechanically induced bone formation can be suppressed by L-NAME, a NO synthesis inhibitor (68). Wingless-type (Wnt)/-catenin signaling Wnt signaling through LDL receptor-related protein-5(LRP5) receptor plays an important and complex role.”

Nitric oxide, (NO) is a gas so in the middle of all this transduction you’ve got to pay attention to a diffusible gas too. I love it! The neat thing with nitric oxide molecules is they don’t need to dock with something like a receptor to make a receptor response they pass through, there are no key locks for a gas. Can you picture this in your head, the NO gas effect?Remeber I studied in Dr Stewarts lab for two years coming to grips with NO gas as a signalling moiety.

So this is a HUGE whack of information I have tossed at your feet. But it shows I believe we are on the right track, so we’re able to compare ourselves with other competent researchers. Like always the hard part is sticking your head into that open window of observation, then trying to pay attention to the wind blast forcing your eyes closed at the information streaming at you, trying to make sense of the scene.

So lets recapitulate, are you ready for the quiz, sorry just kidding! Bone cells are held in exquisite balance, bone cell density is the dynamic output of both the osteoblast -laying down bone with the osteoclast -taking bone away. Different metabolic conditions shift this exquisite balance. Our hypothesis is that with minor traumatic brain injury, brain concussion is both hemispheres rotating as an event inside the brain case. This central rotation is a torque pulling on very susceptible areas within the brain. We believe both the vagal nerve plus the glossopharyngeal are pulled at the medulla entry point. This strained medulla brain stem zone now reacts.Central cardiac pacing is now affected in its preferred chaotic timing profile seen as a pacing change., yet everything appears normal, normal blood pressure, normal heart rate only the control of he heart rate appears to be affected. This derangement involves a balance shift within the autonomic control systems located in the medullary region of pull. We suspect a very traceable change implicating  parasympathetic/sympathetic dynamic has now revealed itself.

This in a nutshell is the window of observation we are focusing on. Our query becomes: can we detect central bone changes after a minor traumatic injury. Looking at bone density changes a few weeks after a brain concussion appears as a sensible query to evaluate. We will pay attention to strain induced deformations a la tensegrity as our primal language of investigation, as a shape shifting event between bone cells -osteoblasts versus osteoclast, which output changes? That is our specific query.

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About cerebrovortex

Montreal Grandmother, Agnes Kent was saved by Raul Wallenberg from certain death, when he provided papers for her and her Mom to escape away from the Nazis. Today when asked what that escape meant, she replied,"Remind people, that while statesmen and whole countries remained silent and did nothing, a single individual chose to act, with ramifications that proved enormous. Similar choices confront us today. Write that simple truth she said, it can never be repeated often enough because the world keeps forgetting it."
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