Physical Therapists As a Force in Mechanotherapy and Musculoskeletal Regenerative Rehabilitation W.R

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Understanding Mechanobiology: Physical Therapists as a Force in Mechanotherapy and Musculoskeletal Regenerative Rehabilitation W.R. Thompson, PT, DPT, PhD, William R. Thompson, Alexander Scott, M. Terry Loghmani, Samuel R. Ward, Department of Physical Therapy, Stuart J. Warden School of Health and Rehabilita- tion Sciences, Indiana University, Indianapolis, Indiana. Achieving functional restoration of diseased or injured tissues is the ultimate goal of both A. Scott, PT, PhD, Department of regenerative medicine approaches and physical therapy interventions. Proper integration and Physical Therapy, Faculty of Med- healing of the surrogate cells, tissues, or organs introduced using regenerative medicine icine, University of British Colum- techniques are often dependent on the co-introduction of therapeutic physical stimuli. Thus, bia, Vancouver, British Columbia, regenerative rehabilitation represents a collaborative approach whereby rehabilitation special- Canada. ists, basic scientists, physicians, and surgeons work closely to enhance tissue restoration by M.T. Loghmani, PT, PhD, Depart- creating tailored rehabilitation treatments. One of the primary treatment regimens that phys- ment of Physical Therapy, School ical therapists use to promote tissue healing is the introduction of mechanical forces, or of Health and Rehabilitation Sci- mechanotherapies. These mechanotherapies in regenerative rehabilitation activate specific ences, Indiana University. biological responses in musculoskeletal tissues to enhance the integration, healing, and restor- ative capacity of implanted cells, tissues, or synthetic scaffolds. To become future leaders in the S.R. Ward, PT, PhD, Departments field of regenerative rehabilitation, physical therapists must understand the principles of of Radiology, Orthopaedic Sur- gery, and Bioengineering, Univer- mechanobiology and how mechanotherapies augment tissue responses. This perspective sity of California–San Diego, La article provides an overview of mechanotherapy and discusses how mechanical signals are Jolla, California. transmitted at the tissue, cellular, and molecular levels. The synergistic effects of physical interventions and pharmacological agents also are discussed. The goals are to highlight the S.J. Warden, PT, PhD, FACSM, critical importance of mechanical signals on biological tissue healing and to emphasize the Department of Physical Therapy need for collaboration within the field of regenerative rehabilitation. As this field continues to and Center for Translational Mus- culoskeletal Research, School of emerge, physical therapists are poised to provide a critical contribution by integrating mech- Health and Rehabilitation Sci- anotherapies with regenerative medicine to restore musculoskeletal function. ences, Indiana University, 1140 W Michigan St, CF-326. Indianapo- lis, IN 46202 (USA). Address all correspondence to Dr Warden at: [email protected]. [Thompson WR, Scott A, Logh- mani MT, et al. Understanding mechanobiology: physical therapists as a force in mechanotherapy and musculoskeletal regenerative rehabilitation. Phys Ther. 2016;96:560–569.] © 2016 American Physical Therapy Association Published Ahead of Print: December 4, 2015 Accepted: November 22, 2015 Submitted: April 12, 2015

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egenerative medicine is an emerging field that combines advances in tissue engineering and molecular Rbiology to replace or regenerate human cells, tissues, or organs with the goal of restoring or establishing normal function following loss due to injury, disease, or aging.1 Regeneration in response to injury requires the recapitulation of spe- Figure 1. cific events that occur during embryonic Mechanical forces direct cellular activities to induce tissue adaptation. Extrinsically and and fetal development, as well as a con- intrinsically generated mechanical forces load musculoskeletal tissues, with the characteristics ducive cellular milieu, so that damaged of the resultant tissue forces being dependent on the ability of the tissue to resist those forces. Tissue forces are transmitted to the micromechanical environment of resident cells, with regions are replaced with healthy tissue cellular mechanical properties influencing the characteristics of the cellular forces. Cells can that has exactly the same composition, modify their micromechanical environment via cytoskeletal rearrangement, which feeds back structure, and functional abilities as to alter cellular sensitivity to incoming forces. When cellular forces are sufficient, the cell undamaged native tissue. Unfortunately, initiates a molecular response, which ultimately alters synthesis or degradation of the extra- most musculoskeletal tissues in adults cellular matrix. The latter alters tissue mechanical properties, which feeds back to influence lack the ability to regenerate, with injury tissue forces. resulting in a repair response whereby fibrous connective tissue is laid down, forming a scar with inferior mechanical, physiologic, and functional properties. individuals who receive regenerative The success of therapies in regenerative therapies will require rehabilitation to medicine at repairing or regenerating Regenerative medicine has opened the make best use of their restored anatomy musculoskeletal tissues ultimately possibility for full healing of injured or and newly regained abilities. Physical depends on the therapies being accepted degenerated musculoskeletal tissues, therapists are specifically trained to and incorporated into the native tissue thereby offering hope for people who assess and manage musculoskeletal (eg, in the case of ex vivo grown tissues have conditions that traditionally have pathologies and thus are well positioned or bioengineered scaffolds) and creating had limited recovery potential. Examples to be important allies in musculoskeletal a musculoskeletal tissue with optimized of musculoskeletal conditions that may regenerative medicine. However, the mechanical characteristics (eg, in the benefit from regenerative medicine role of physical therapists extends case of biologic or pharmaceutical approaches include: (1) injury-related beyond the serial approach of simply agents). One group of therapies that conditions that use repair processes to reestablishing function at the organism physical therapists have in their reper- heal, such as muscle strains, ligament level following tissue healing.2,3 In par- toire that have great potential of having sprains, tendon ruptures, and integu- ticular, physical therapists have the additive, or even synergistic, effects ment wounds; (2) injury-related condi- potential to become the leaders in mus- when introduced in conjunction with tions that exhibit compromised healing, culoskeletal regenerative rehabilitation. regenerative medicine treatments is such as osteochondral defects and non- mechanotherapies. united bone fractures; (3) injury-related Musculoskeletal regenerative rehabili- conditions that have little prospect of tation can be defined as the integration Mechanotherapy healing, such as volumetric muscle loss of principles and approaches from reha- Musculoskeletal tissues are critical for and segmental bone defects; and (4) bilitation and regenerative medicine, load bearing but also generate, absorb, disease-related conditions, such as sar- with the ultimate goal of promoting the and transmit force, thereby enabling copenia, osteoporosis, and osteoarthro- restoration of function through musculo- functional movement. Given their sis. Examples of some regenerative ther- skeletal tissue regeneration and repair.4 mechanical role, it follows teleologically apies currently being used or developed This definition does not confine the role that musculoskeletal tissues are capable for these conditions include the intro- of physical therapists to restoring func- of responding and adapting to their duction of stem cells, progenitor cells, or tion after tissue regeneration or repair mechanical environment. Mechanical biologically active molecules and the but also enables therapists to play an forces direct cellular activities influenc- implantation of bioengineered scaffolds active role by facilitating regeneration ing the tissue-level processes of growth, or ex vivo grown tissues. In this article, and repair at the tissue level during heal- modeling, remodeling, and repair, with we provide a perspective of how mech- ing. In addition, the definition encour- the ultimate outcomes being altered tis- anotherapies influence the development ages therapists to contribute to the con- sue mass, structure, and quality (Fig. 1). and healing of various tissues, with a par- ception and development of novel Nearly every physical therapy interven- ticular emphasis on bone. regenerative therapies by working col- tion in musculoskeletal rehabilitation laboratively with other disciplines introduces mechanical forces, regardless As the goal of regenerative medicine is to involved in regenerative medicine in a of whether the forces are generated restore or establish normal function, team-based approach to optimize func- extrinsically via therapist intervention tional outcomes.2,3 (eg, during joint or tissue mobilization or

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apy on the tissue-level processes respon- sible for the development, maintenance, healing, and regeneration of tissues. Additionally, although musculoskeletal tissues are the primary focus of the current article, it is important to acknowl- edge that essentially every cell type within the body is responsive to mechanical signals, extending the principles of mechanotherapy to nonmusculoskeletal tissues.

Mechanotransduction In order for physical therapists to fully contribute to regenerative medicine and be viewed as the leaders in regenerative rehabilitation, there is a need to under- Figure 2. stand how mechanotherapies work at Common micromechanical stimuli to which musculoskeletal cells are exposed: (A) tension— the cellular and molecular levels. pulling force that increases cell dimensions in the direction of pull; (B) compression—pushing Although the adaptive ability of tissues in force that decreases cell dimensions in the direction of push; (C) shear—parallel forces pushing or pulling in opposite directions to distort the cell; (D) hydrostatic pressure— response to mechanical stimuli has long pressure exerted by surrounding fluid that changes cell volume; (E) vibration—oscillating, been established, the precise mecha- reciprocal back-and-forth shaking of a cell; and (F) fluid shear—force created by the flow of nisms underlying the response at the cel- fluid parallel to a cell membrane. lular and molecular levels have only recently begun to be unraveled and remain to be fully elucidated. Neverthe- less, it is accepted that the mechanism via the introduction of external thera- remained ambiguous for decades, and involves some form of mechanotransduc- peutic modalities) or intrinsically within limited empirical evidence brought tion, which refers to the conversion of a the individual themselves via the pre- broad interpretations and clinical biophysical force into a cellular and scription of exercise therapy. As an implementation. molecular response. exhaustive review of the numerous forms of mechanotherapies used by The first formal definition of mechano- Mechanotransduction at the physical therapists is outside the scope therapy was published in 1890 as “the of the current perspective article, read- employment of mechanical means for Cellular Level Mechanotransduction requires a ers are referred to the following reviews the cure of disease.”12(p181) The term that detail various forms of mechanical remained relatively unchanged until it mechanical signal to be transmitted to interventions, including: joint mobiliza- was updated in 2009 to “the employ- the microenvironment of a cell and tions,5 muscle or tendon stretching,6 ment of mechanotransduction for the for the cell to possess machinery to sense resistance exercises,7 vibration plat- stimulation of tissue repair and the signal. Cells can be exposed to a variety of micromechanical stimuli, with forms,8 interventional ultrasound,9 and remodeling.”13(p248) The revised descrip- massage.10 tion highlighted the cellular basis of tis- the precise nature of the stimulus sue responses and the distinction depending on the mechanical properties Remedy through mechanical interven- between healthy and injured tissues. of the cells themselves and the interac- tion has been sought for thousands of More recently, the definition was again tion between the incoming mechanical years; however, physical medicine was updated to reflect the influence of mech- signal and the extracellular matrix (ECM) not recognized as a medical specialty anotherapy on tissues outside of the mus- (Fig. 1). Common stimuli include tension, compression, and shear; however, until the early-to-mid 19th century.11 culoskeletal system.14 In keeping with During that period, as medicine became those revisions, we propose a definition cells also can be exposed to other more specialized, terms such as “physi- of mechanotherapy as “any intervention mechanical stimuli, such as hydrostatic cal medicine,” “physical therapy,” “phys- that introduces mechanical forces with pressure, vibration, and fluid shear iotherapy,” and others used to describe the goal of altering molecular pathways (Fig. 2). the use of exercise and physical manip- and inducing a cellular response that ulation were collectively known as enhances tissue growth, modeling, The tissue in which a cell resides and the “mechanotherapy.” Forming organized remodeling, or repair.” As such, we seek location of the cell within that tissue groups of physicians and therapists to highlight the multisystem hierarchy influence the forces to which the cell is helped establish mechanotherapy as a (molecules – cells – tissues), which is exposed; yet, the exact nature of the recognized medical intervention; how- responsive to mechanical signals, and to forces may not always be evident. For ever, the definition of mechanotherapy recognize the influence of mechanother- example, it may be assumed that mecha-

562 f Physical Therapy Volume 96 Number 4 April 2016 Physical Therapists and Mechanotherapy nosensitive cells in bone are predomi- cells are attached are equally important. nantly exposed to compression, whereas Recent work has shown that altering the those in tendon are exposed to tension stiffness of the ECM directs stem cell due to the function of the tissues in differentiation, where increased stiffness which they reside. However, long bones directs differentiation to more mechani- (eg, tibia and femur) are curved and bend cally competent tissues, including carti- when axially loaded to generate com- lage and bone, but away from commit- pressive forces within the tissue on the ment to adipose and neuronal tissues.19 side the bone is bending toward and ten- Studies also have shown that controlling sile stresses within the contralateral the area in which stem cells can adhere side.15 Thus, bone cells can be exposed to the matrix regulates lineage fate. Stem to either compressive or tensile forces cells forced to attach on small fibronec- (although fluid shear appears to be tin island-like posts assumed a rounded the most likely signal involved in shape, whereas cells attached to larger skeletal mechanotransduction [dis- islands had an elongated morphology cussed later]).16 Similarly, although ten- with increased Ras homolog gene family dons are exposed to large tensile forces member A (RhoA) and Rho-associated Figure 3. in their role of transmitting muscle protein kinase (ROCK) activity, resulting Transducing mechanical signals into bio- 20 forces, the tensile loading of collagen can in enhanced osteogenic commitment. chemical responses requires unique machin- cause cell-occupying spaces to narrow, As RhoA/ROCK signaling increases actin ery. Forces are transmitted at the matrix/cell resulting in the generation of compres- stress fiber formation, the spacing of the membrane interface where specialized com- sive forces, whereas differential elonga- substrate to which the cells attach plexes called focal adhesions form. Integrins tion of adjacent collagen fibers can gen- informs the physical structure of the span the plasma membrane, uniting the erate microscopic shearing forces.17 cells, and thus their differentiation extracellular matrix with the internal actin Also, cells located in tendon near bony potential. This knowledge is critical cytoskeleton. Linker proteins, such as vincu- prominences (eg, within the supraspina- when designing engineered substrates lin and talin, reinforce the structural integrity of the adhesion complex, and associated sig- tus tendon as it passes through the sub- for tissue regeneration, which should not naling effectors, including focal adhesion acromial space or the Achilles tendon only incorporate bioactive anabolic mol- kinase (FAK) and Src, activate biochemical near its calcaneal insertion) can be ecules but also take into account the spa- signaling pathways in response to force. exposed principally to compressive, tial and physical properties of the matrix rather than tensile, forces.17 to which they are attached. Work is continuing toward generating synthetic By understanding the forces to which extracellular environments capable of force within the ECM to which a cell is cells are exposed and respond, it may be directing stem cells to regenerate spe- attached results in disruption of cellular possible to develop novel means of intro- cific tissues.21 Introducing appropriate mechanical homeostasis. The resultant ducing those forces to induce a desired physical stimuli to these engineered conformational changes within the cyto- cellular response and resultant tissue components may enhance their regener- skeleton directly alter chromatin struc- adaptation. In particular, it may be pos- ative capacity. ture and thus modulate gene transcrip- sible to encourage the commitment of tional activity via direct connections endogenous adult stem and progenitor In terms of cellular force–sensing between cytoskeletal elements and the cells to a particular lineage to enhance machinery, most attention has focused DNA itself,24 or by activating intermedi- regenerative potential. There is a recip- on the ECM-integrin-cytoskeletal signal- ate molecular signals by interactions rocal relationship between cells and tis- ing axis. Cells of the musculoskeletal sys- between integrins and intracellular sig- sue during development wherein the tis- tem possess transmembrane receptors naling molecules (eg, focal adhesion sue type influences the forces to which called integrins, some of which connect kinase [FAK] and Src tyrosine kinase).25 cells are exposed, while forces deter- extracellularly to ECM proteins and intra- Also, as cells are attached to one another mine cellular differentiation and subse- cellularly to the cytoskeleton, which via cadherin-containing adhesion com- quently what tissue type is produced.18 consists of actin filaments, nonmuscle plexes, disruption of cellular mechanical By introducing specific forces at specific myosin, and associated proteins homeostasis of one cell may be mechan- 26 times, resident regenerative cells can be (Fig. 3).22 The cytoskeleton is pre- ically transferred to neighboring cells. encouraged to commit to a specific lin- stressed and has a tensegrity architec- eage and produce a particular tissue ture, a design achieving structural cohe- Although it is possible that the ECM- type.19 sion by creating a dynamic balance integrin-cytoskeleton axis principally between the counteracting forces of acts to alter the mechanosensitivity of a Although the nature and timing of the compression and tension on the individ- cell by changing the cell’s internal stiff- applied forces are critical for cellular ual struts within the cell.23 The self- ness and how much it pulls on the sur- responses, the biochemical and physical equilibrated mechanical environment rounding ECM,27 it also is possible that properties of the matrix to which the within a cell means that any change in the actual conversion of a mechanical stimulus into a molecular response (ie,

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Mechanotransduction at the Molecular Level Once a cell has detected a local mechanical stimulus, the signal needs to be con- verted into a biochemical response. This process is commonly referred to as biochemical coupling. As there are multiple potential mechanosensory mechanisms, there also are multiple signaling pathways that a cell may use to create a biochemical response. Although a detailed description of all of the constantly evolv- ing biochemical pathways underlying mechanotransduction within the musculoskeletal system is beyond the scope of the current article, several common pathways are detailed in Figure 4.

To highlight a few example pathways, integrin-mediated transmission of membrane strain induces activation of several kinases, including focal adhesion kinase (FAK) and Fyn at focal adhesions (con- tact points where the cell attaches to the matrix),25 as indicated in Figure 4. These signals activate Akt, resulting in down- Figure 4. stream activation of both ␤-catenin and A variety of extracellular receptors activate an overlapping network of mechanosensitive RhoA, resulting in repression of adipo- pathways. (A) Musculoskeletal cells can sense incoming mechanical signals using a diverse group of transmembrane mechanosensitive proteins (mechanosensors), including stretch- genic genes. Thus, signals emanating activated ion channels, cell-membrane spanning G-protein-coupled receptors, growth-factor from focal adhesions diverge into 2 path- receptors, and integrins. The mechanical stimulation of these proteins can lead to changes in ways, resulting in ␤-catenin nuclear their affinity to binding partners or ion conductivity. (B) Mechanical stimulation of the translocation, which alters transcrip- mechanosensors and alteration in their binding capacity or ion conductivity converts the tional control,29 and activation of RhoA, mechanical signal into a biochemical signal (biochemical coupling) triggering intracellular which increases cell stiffness.25 Here, signaling cascades. Many of the pathways overlap sharing signaling molecules. The conver- mechanical transmission through integ- gence of the pathways results in the activation of select transcription factors, including rins results in reduced formation of fat ␬␤ ␬␤ nuclear factor of activated T cells (NFAT), nuclear factor- (NF- ), activator protein 1 from mesenchymal stem cell (MSC) pre- (AP1), GATA4 (a member of the transcription factor family characterized by the ability to bind cursors. Another example is the ability of the DNA sequence “GATA”), and signal transducer and activator of transcription factors (STATs). The transcription factors translocate to the nucleus and modulate the expression of force to regulate intracellular calcium a panel of mechanosensitive genes, including early growth response 1 (Egr1), lex1, Fos, Jun, entry (Fig. 4). Pharmacological inhibition and cyclo-oxygenase-2 (Cox2). Ultimately, the net sum of gene-expression reprogramming of mechanosensitive calcium channels determines the functional response of the cell to a mechanical stimulus. Akt/PKBϭprotein results in reduced anabolic responses in kinase B; CaMKϭcalcium/calmodulin-dependent kinase; DAGϭdiacyl-glycerol; bone.30 Recent work also has implicated ERKϭextracellular signal-regulated kinase; FAKϭfocal adhesion kinase; IP3ϭinositol triphos- an auxiliary voltage sensitive calcium phate; JNKsϭc-Jun N-terminal kinases; MEKϭmitogen-activated protein kinase; channel subunit, which is partially ϭ ϭ ϭ MEKK mitogen-activated protein kinase; MLCK myosin light-chain kinase; NO nitric anchored in the cell membrane, capable ϭ ϭ ϭ oxide; NOS nitric oxide synthase; PAK p21-activated kinase; PI3K phosphoinositide of attaching to the ECM, in the mechan- 3-kinase; PKCϭprotein kinase C; PLCϭphospholipase C; Rafϭrapidly accelerated fibrosar- ical activation of osteocytes.31 Addition- coma kinase; Rasϭrat sarcoma small GTPase. ally, these channels are important in cartilage, where inhibition reduces load- induced osteoarthritis in mice.32 mechanotransduction) is primarily medi- cellular force transmission through ated by conformational changes in trans- changes in intracellular or ECM structure For a more in-depth discussion of the membrane mechanosensitive proteins. and organization may lead to altered numerous molecular pathways respon- These proteins include stretch-activated forces acting on these proteins, resulting sive to mechanical signals, we direct ion channels, cell membrane spanning in a change in their affinity to binding readers to recent reviews.17,28,33 It is G-protein-coupled receptors, and partners or ion conductivity and the ini- important to point out that some path- growth factor receptors, to name a few tiation of downstream molecular signal- ways are complementary, whereas oth- (Fig. 4A).28 Any changes in normal intra- ing pathways.

564 f Physical Therapy Volume 96 Number 4 April 2016 Physical Therapists and Mechanotherapy ers are redundant, making it a challenge apy. By understanding the mechanical Based on the purported mechanical to decipher the precise contribution and stimuli to which musculoskeletal cells milieu that osteocytes are exposed to timing of activation of individual path- best respond and the mechanisms these during tissue loading (ie, fluid flow shear ways. Also, there are likely many other cells use to convert mechanical signals forces) and the observation that pressur- pathways, intermediary molecular play- into molecular responses, physical ther- ization of the intramedullary cavity ers, and pathway interactions that are yet apists may augment the response of mus- causes an outward pressure gradient that to be identified. Once the mechanical culoskeletal cells to mechanical stimuli. induces interstitial fluid flow,37,38 inves- stimulus is transduced into a biochemical The net result can be the additive or tigators have begun exploring how to signal, it is either transmitted to the synergistic facilitation of tissue regener- exogenously enhance intramedullary nucleus, where it induces expression of ation and restoration of function in indi- pressure in the absence of significant tis- mechanosensitive genes, or propagated viduals receiving regenerative therapies. sue loading. Example interventions cur- intercellularly to effector cells. The latter rently in preclinical development is facilitated by small channels, called Regenerative Rehabilitation: include oscillatory muscle stimulation, connexins, positioned in the cell mem- Mechanotransduction at the dynamic flow stimulation, and dynamic 38–43 brane, which allow small peptides to be Cellular Level joint loading. Each of these modali- passed to neighboring cells or into the Physical therapists typically use extrinsi- ties increases intramedullary pressure in 28 extracellular space. cally or intrinsically generated mechani- preclinical animal models to induce bone cal stimuli to create a tissue force with adaptation, with very negligible tissue 38–40,42–44 Decoding the biochemical pathways and the goal of evoking a cellular and molec- loading. Importantly, as the players involved in mechanotransduc- ular response (Fig. 1). However, individ- intramedullary cavity contains both tion extends beyond scientific curiosity. uals requiring regenerative therapy often hematopoietic cells and MSCs, which are Altered or reduced mechanotransduc- have limited or restricted load-bearing responsive to hydrostatic pressure and tion is considered to contribute to a num- capacity, as the introduction of such fluid flow shear forces, induction of ber of musculoskeletal disorders, ranging loads may be potentially detrimental. By altered intramedullary pressure and from osteoporosis and osteoarthritis to understanding the microscopic forces to interstitial fluid flow via exogenous muscular dystrophies and sarcopenia.34 which cells are exposed and respond, it means has the potential to contribute to 45 Identifying molecules involved in mecha- may be possible to develop novel means regenerative processes. Ultimately, the notransduction may reveal novel targets of introducing mechanical forces with- intramedullary pressure and interstitial for therapeutic intervention that not out generating excessive forces at the fluid flow modalities need to be scaled only aid in the management of tissue level. up to humans before their clinical utility mechanotransduction-related disorders can be realized, but they provide an example of how the microscopic force but also aid in stimulating musculoskele- An example of where the microscopic to which cells are exposed and respond tal tissue regeneration. Manipulation of a force that cells are exposed to has been can be developed into potential novel target to induce a biochemical signal may partly deconstructed to develop poten- mechanotherapies. independently induce a cellular response tial novel mechanotherapies is in bone. or have additive effects when combined There is general consensus that osteo- with a mechanotherapy. More interest- cytes embedded throughout the bone Osteocytes are uniquely positioned to ingly, use of pharmacological agents to matrix are the mechanosensors within sense mechanical forces; however, MSCs “target” specific molecular pathways the skeletal system.35 Physical deforma- in the bone marrow also perceive force, involved in mechanosensitive responses tion (strain) of the bone matrix is not particularly direct membrane strain, as may result in a greater response when sufficient to deform the osteocyte cell opposed to the fluid shear stress experi- 46 superimposed with an appropriate membrane and initiate a response; how- enced predominantly by osteocytes. As mechanotherapy, resulting in an overall ever, axial compression and bending bone marrow MSCs can differentiate into enhanced anabolic stimulus than with increase intramedullary pressure, induc- a variety of tissue types, including fat, pharmacological intervention or the ing the flow of interstitial fluid from areas cartilage, tendon, and bone, understand- mechanotherapy alone. of high pressure (compression) to low ing the optimal loading parameters to pressure (tension) within the lacuno- direct lineage commitment is critical for Integrating Knowledge of canalicular network housing osteocytes the incorporation of physical stimuli into and their dendritic processes.16 engineered tissue components. An ex Mechanotransduction Into vivo study has shown that direct mem- Regenerative Rehabilitation Although extravascular pressure drives a baseline flow of interstitial fluid, flow is brane strain restricts MSC adipogenesis, Acquiring knowledge in mechanotrans- heightened by the superimposition of providing a larger pool of precursor cells duction is a critical component for phys- intermittent mechanical loading and available for differentiation toward carti- ical therapists to become leaders in 47 exposes osteocytes to fluid flow shear lage, bone, or tendon. These concepts regenerative rehabilitation. Mechano- forces.36 have been carried over to the develop- transduction forms the foundation of Thus, a small level of tissue strain induces enhanced shear at the cell ment of engineered cartilage grafts, mechanotherapies, with mechanothera- where dynamic compression may pies forming one of the largest groups of membrane, enhancing the mechanical interventions prescribed in physical ther- stimulus engendered to the cells.

April 2016 Volume 96 Number 4 Physical Therapy f 565 Physical Therapists and Mechanotherapy enhance formation and mechanical com- ing,56 other tissues,57 and stem cell fate58 it may be necessary to carefully coordi- petence of cartilaginous grafts.48 requires further consideration. For a nate the timing of mechanical load intro- more discussion on the clinical applica- duction with peak sensitization of the Low-intensity vibration (LIV) also has tions of vibration therapy, we direct the mechanotransductive pathway. been developed as a means of mechani- reader to a recent comprehensive cally stimulating musculoskeletal cells in review.8 Studies of the combined skeletal effects the absence of appreciable tissue defor- of parathyroid hormone (PTH) and mation forces. Low-intensity vibration A final example of a novel exogenously mechanical loading provide an example evolved from the observation that skele- introduced mechanical stimulus that has of where the coordinated introduction of tal muscle not only imparts force on the potential to be safely coupled with a molecule and mechanotherapy can bone during locomotion to engender other regenerative therapies is low- induce synergistic effects. Parathyroid high strain magnitudes but also generates intensity pulsed ultrasound (LIPUS). hormone is an anabolic skeletal agent low-magnitude (Ͻ100 microstrain [␮␧]), Although standard clinical ultrasound when introduced intermittently and high-frequency (30- to 90-Hz) stimuli. It therapy has fallen out of favor, LIPUS is stimulates osteocytes and osteoblasts, in induces neither strain49 nor fluid shear50 an established modality for the manage- part, through PTH type 1 receptor and thus requires a distinct mechanism ment of bone injuries. This modality (PTH1R) activation. The PTH1R in these for the vibration signals to mediate cel- refers to pulsed-wave ultrasound with cells also plays a key role in the bone lular effects. It has been proposed that a spatial-averaged, temporal-averaged anabolic response to mechanical stimuli. Յ 2 9 LIV induces acceleration of the cell intensity (ISATA)of 100 mW/cm . This As PTH and mechanical loading effects nucleus, which may activate mechano- intensity is much lower than that pro- are colocalized through the PTH1R, sensitive signaling pathways, as the duced by conventional ultrasound units simultaneous introduction of these nucleus is tethered by the internal actin utilized in physical therapy and is intro- agents may allow one modality to cytoskeleton. In support of this produced daily for 20 minutes. This modality enhance the cellular response to the posed mechanism, it was recently shown appears to work through a number of other. Indeed, a number of preclinical that LIV-induced activation of mechano- different mechanotransductive pathways studies have demonstrated synergistic sensitive pathways is reduced by physi- and has stimulatory effects on MSCs and bone adaptive responses when PTH and cally disconnecting the nucleus from the both chondrogenesis and osteogenesis.59 mechanical loading were introduced in supporting actin cytoskeleton.51 Clinical trials have shown LIPUS to combination, with PTH appearing to sen- reduce the time to both clinical and sitize the cells by enhancing the mobili- Rubin and colleagues have championed radiological healing of tibial, radial, zation of intracellular calcium.63 In order LIV as an exogenous stimulator of bone and scaphoid fractures by 30% to 38% to translate these preclinical observa- adaptation. Initially, they showed that and stimulate union in 86% of individuals tions to the clinical setting, it is impor- adult sheep exposed to LIV stimuli with with a nonunited fracture.60 Of note tant to note that PTH has a short half-life a magnitude of Ͻ0.3g (where g equals with regard to regenerative medicine, (75 minutes) and reaches maximal serum the earth’s gravitational field) and fre- LIPUS has been safely coupled with concentrations within 15 to 45 minutes quency of 30 Hz for 20 minutes per day other regenerative therapies to improve following subcutaneous injection.64 over 1 year exhibited a 34% increase in bone healing in studies involving stem Thus, coupling of a mechanotherapy proximal femur trabecular bone density cell therapies61 and to promote allograft with PTH administration should be per- compared with controls.52 In subsequent incorporation.62 formed in the period immediately follow- clinical trials, Rubin and others53,54 pro- ing PTH administration in order to opti- vided evidence suggestive of a beneficial Regenerative Rehabilitation: mize any synergistic effects between the skeletal effect of exogenously intro- Mechanotransduction at the modalities. duced LIV as an inhibitor of bone loss in: Molecular Level (1) a subset of women who were post- Understanding the biochemical path- The above example highlights the poten- menopausal, (2) young women with low ways through which mechanical signals tial for synergistic effects when a bone density, and (3) children with neu- are transduced enables potential molec- mechanical stimulus is appropriately rologically derived disabling conditions. ular targets to be identified. Manipulation timed with the introduction of an agent Although each of these clinical studies of a target to induce a biochemical signal that sensitizes a mechanotransductive possessed important limitations (eg, a may independently induce a cellular pathway in bone. With the progressive relatively small sample size, nonblinding response or have additive effects when development of new biologically active of participants, absence of group differ- combined with a mechanotherapy. More compounds and molecules that target ences when using an intention-to-treat interestingly, molecule targeting also mechanosensitive pathways, there is a analysis) and a more recent study may sensitize a specific mechanotrans- need to explore their combined effects showed no effect of LIV in older adults,55 ductive pathway such that the superim- with mechanotherapies. For instance, the data provide the impetus to further position of mechanical loading results in monoclonal antibody therapies targeting explore LIV as an exogenous mechanical a greater response than with mechanical myostatin are progressing toward clini- intervention for bone. In addition, the loading or molecule targeting alone. In cal availability, with phase I and II trials identification of LIV effects on bone heal- order to induce such synergistic effects, being complete.65 Myostatin is ex-

566 f Physical Therapy Volume 96 Number 4 April 2016 Physical Therapists and Mechanotherapy pressed in skeletal muscle throughout controlled trial suggested that individuals All authors provided concept/idea/project embryogenesis and is a negative regula- taking NSAIDs prior to exercise exhib- design. Dr Thompson and Dr Warden pro- tor of adult skeletal muscle mass. It binds ited an impaired skeletal adaptive vided initial writing. All authors provided to activin receptor IIb to inhibit muscle response.73 These bone-related findings manuscript review and final approval. protein synthesis and myoblast prolifer- are supported by studies in muscle and This work was supported, in part, by the ation and differentiation.66 Myostatin is tendon showing that NSAID administra- National Science and Engineering Research involved in skeletal muscle mechano- tion prior to exercise may reduce adap- Council (Canada) (402108 [Dr Scott]) and transduction, with both physical activity tive responses.74 the National Institutes of Health (HD073180, and mechanical loading reducing myo- AR061303, HD050837 [Dr Ward] and statin signaling by provoking the release Conclusions AR057740 [Dr Warden]). of the myostatin inhibitor, follistatin.67 In the last 60 years, the fields of both DOI: 10.2522/ptj.20150224 Inhibition of myostatin signaling through rehabilitation science and regenerative the delivery of propeptides, neutralizing medicine have expanded exponentially. References antibodies, and other means stimulates What were once considered mutually 1 Mason C, Dunnill P. A brief definition of muscle protein synthesis, resulting in exclusive disciplines are now beginning 65,68 regenerative medicine. Regen Med. 2008; gains in muscle mass and function. to recognize their synergistic contribu- 3:1–5. Combining pharmacological inhibition tions, resulting in even greater advance- 2 Ambrosio F, Russell A. Regenerative reha- of myostatin with mechanotherapies ments in tissue engineering and rehabil- bilitation: a call to action. J Rehabil Res may lead to greater improvements in itation outcomes. Regenerative medicine Dev. 2010;47:xi–xv. functional recovery, with initial work seeks to repair and replace damaged tis- 3 Ambrosio F, Wolf SL, Delitto A, et al. The revealing myostatin inhibition to emerging relationship between regenera- sues. 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Therapeutic interventions for increasing forming myocytes.70 Thus, inhibition of Physical mobility was not always recog- ankle dorsiflexion after ankle sprain: a sys- myostatin using a biologically active tematic review. J Athl Train. 2013;48: nized as a crucial component of the heal- 696–709. compound may potentiate skeletal mus- ing process, with bed rest once being the 6 Obst SJ, Barrett RS, Newsham-West R. cle regeneration as opposed to repair, modality of choice. Although we now Immediate effect of exercise on achilles with the former potentially being further know that early physical intervention tendon properties: systematic review. Med enhanced via the cointroduction of an promotes healing and integration of Sci Sports Exerc. 2013;45:1534–1544. appropriate mechanotherapy. implanted prostheses and engineered tis- 7 Frost DM, Cronin J, Newton RU. 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