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KEYWORDS platelet-rich plasma, sport injuries, tissue healing

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104doihysical activity and sport are fundamental for a healthylifestyle. However, too much exercise can result in over-

e injuries. Many of these occur after repetitivemicrotraumaare nagging injuries following an acute event that is notowed to heal completely. The biological basis appears to beiled healing response, whether scarce or idle, which seemsbe mediated by failure in multiple healing processes. Bothte and overuse musculoskeletal injuries are frequent in

orts, but especially the latter are disabling and challengingmanage. They also represent a substantial economic bur-n to society.1

In the past 10 years, the use of platelet-rich plasma (PRP)rapies has become widespread throughout various medi-fields and has positively impacted orthopedic sports sci-

ence with the rationale that such agents have healing prop-erties in addition to an excellent safety profile.2 Initial studiesinvolving athletes with overuse lesions showed promisingresults, but more recent level I studies have shown contro-versial results of PRP injection among patients with tendi-nopathies.3-5 Hence, following the originally high expecta-tions by the public, the enthusiasm surrounding PRPtherapies may evolve into scepticism if the timeline to opti-mize PRP therapies will be longer than expected. Evolution ofour understanding of platelet biology and reinterpretation ofsome of their more traditional roles in hemostasis and tissuerepair should provide new insights on PRP therapys suc-cesses and failures. This article will highlight some of theserecent observations and evolving concepts and paradigms,and will build on and amplify previously published works inthe field of PRP therapies.6-9

Understanding Tissue HealingKnowledge of the basic biological mechanisms involved intissue response to injury is critical in therapeutic manage-ment. Indeed, the most effective way to improve tissue repair

stituto de Investigacin BIOCRUCES, Osakidetza, Basque Health Ser-vice, Pza Cruces s/n, 48903 Barakaldo, Vizcaya, Spain.nidad de Ciruga Artroscpica, UCA Mikel Snchez, Clnica USP-LaEsperanza, Vitoria-Gasteiz, Spain.entre for Sports and Exercise Medicine, Mile End Hospital, Barts and TheLondon School of Medicine and Dentistry, Queen Mary University ofLondon, London, England.dress reprint requests to Isabel Andia, PhD, Research Department, Osaki-asic Science: Molecular aspects of Platelet-Rich Pl

abel Andia, PhD,* Mikel Snchez, MD, and

Knowledge of the basic biological mechanisminform management of healing. Approachesmultiple cell types and large signaling necommunication between cells. Platelet-richgrowth factors and cytokines to the injuredplatelet biology and reinterpretation of someand tissue repair have revealed much aboutnew insights on PRP therapies successes anmechanisms acting simultaneously in tissueof critical mechanisms behind PRP therapiesin basic research to clinical differences inundermine current efforts to set effective Pwhich molecular mechanisms are more or leclarifying the molecular basis for differencescontinue to be the priority to tailor PRP therOper Tech Orthop 22:3-9 2012 Elsevier Inisba

detza, Basque Health Service, B Arteaga 107, 48170 Zamudio, Spain.E-mail: iandia2010@hotmail.com

8-6666/12/$-see front matter 2012 Elsevier Inc. All rights reserved.:10.1053/j.oto.2011.09.005Biologicalma Therapiesla Maffulli, MD, MS, PhD, FRCS(Orth)

lved in tissue response to injury shouldfluence healing may need to integrate

that are necessary for the dynamica (PRP) therapies deliver a myriad ofes. Evolution of our understanding of

eir more traditional roles in hemostasismplexity of PRP therapies and provideres. However, many potential molecularpresent a challenge to the identificationt array of barriers, ranging from deficitslations and administration procedures,otocols to manage healing. Identifyingortant during the course of healing ande healing response across patients willfor particular sports injuries.rights reserved.to understand normal healing mechanisms after a pertur-tion arising from trauma or disease. As healing mecha-

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4 I. Andia, M. Snchez, and N. Maffullims are capable to restore nearly every tissue in the body,viations from the normal healing pattern can contribute toious pathologic conditions. For example, disordered orufficient healing has been hypothesized to lead to osteo-hritis or tendinopathy.10 Although debated, the repair oferused tendons and joint pathologies can be aided by therrect administration of PRP therapies. However, the correctair of tissues is extremely complex, and the spatially andporally dynamic nature of healing mechanisms presents a

allenge to the identification of critical mechanisms behindP therapies. The healing response involves 3 broad over-ping phases: inflammation, a trophic or anabolic phasegiogenesis, proliferation, synthesis of extracellular ma-x), and remodeling (Fig. 1). These phases can widely over-, and the synthesis of signaling molecules in 1 phase actsstimulants for the following phase.

flammationammation and blood coagulation are intimately linked.e coagulation system and innate inflammatory responseare common ancestry and are coupled via common activa-n pathways and feedback regulation systems.11 The role oftelets is illustrative for this 2-way relationship. Within theod clot, activated platelets and leukocytes release growthtors and numerous cytokines, establishing the onset of theammatory response. Endothelial cells are not only actively

Figure 1 Dynamic phases of tissue healing: The healing respresponse that lasts until a few days after injury, and is charainjured site and monocyte/macrophage activation; the trendothelial cells are activated to initiate angiogenesis suchto support the high metabolic activity of the new tissue. Cedivide, and differentiate producing collagen, proteoglycansduring the remodeling stage, there is a decrease in cell dendecreases. In each of the described healing phases, the specendogenous signals that serve to limit the duration and toolved in hemostasis, limiting clot formation to the sites ofury, but also in localizing inflammatory processes to areas

of1damage, in part via common pathways.12 Accordingly,al regulatory mechanisms adjust the magnitude of the in-te immune response so that the amount and duration ofmune cell infiltration are adequate to phagocyte apoptotic/crotic cells.13 In the first few hours after injury, neutrophilsrecruited to tissues by chemotactic factors, which aresented in a temporarily and spatially defined manner.ithin tissues, neutrophils orchestrate many cell activitiesd progressively unleash an arsenal of diverse compounds,luding radical oxygen species, antimicrobial peptides, andine proteases, many with a definite biological potential toict further tissue injury.14 Neutrophils are short-lived;ir life spans are generally measured in terms of hours. Theertoire of signals engendered by the neutrophils engagesmonocyte/macrophage lineage. These, commanded by

nals resulting from neutrophil death or activation, largelyove the recruited neutrophils in situ and clear the dam-d local cells.The severity of tissue injuryneutrophils fate and theoptotic or necrotic condition of resident fibroblastsmaytermine the different states of macrophage activation. Int, macrophages exhibit transition from proinflamma-y to prohealing phenotypes. For example, bacterialoducts (lipopolysaccharide) and proinflammatory cyto-es (interferon [IFN-]) induce the classical proinflam-tory phenotype associated with production of high levels

volves 3 broad overlapping phases: the inflammatoryd by leukocyte extravasation and accumulation in theor anabolic phase, including angiogenesis in whichew blood vessels are initiated to promote blood flowrate into the site using the fibrin matrix as a scaffold,ther components of the extracellular matrix. Finally,nd the overall metabolic activity of the injured tissuealing activity is silenced or counterbalanced by otherte progression to a new stage.oflocnaimnearepreWanincserinfltherepthesigremage

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Molecular and biological aspects of PRP therapies 5d nitrogen species. Instead, alternative activationough IL-4/IL-23 is associated with the synthesis of healingtors, including transforming growth factors (TGF- andF-), basic fibroblastic growth factor (bFGF), platelet-rived growth factor (PDGF), and vascular endothelialwth factor (VEGF).15 These growth factors directly pro-te healing, contributing to cell proliferation, angiogenesis,d collagen deposition.Recent research suggests that the switch from proinflam-tory to prohealing activities may determine the differencetween efficient repair and the failure to repair. Actually, ammon characteristic of impaired healing is a persistentammatory response, with prolonged accumulation ofnocytes/macrophages and elevated levels of proinflamma-y cytokines.

ophic Phasell Migration and Proliferationthe injured site, platelets and leukocytes release VEGF, alsomed permeability factor, which increases the permeability ofendothelial cell layer, causing plasma proteins to extravasate

d to lay down a provisional matrix scaffold that is initiallypulated with leukocytes and platelets. In response to the var-s cytokines secreted by immune cells, stromal cells migrateo this scaffold. Progenitors of differentiated cell types, such asne, cartilage, muscle, nerve sheath, and connective tissuels, are thought to contribute to the collection of proliferatingls. The progenitor cells differentiate in response to growthtors and cytokines and become the predominant tissue-ecific cell type by the third to fifth day after injury. More-er, regulated by PDGF, insulin-like growth factor (IGF),d TGF-, these cells produce collagen, proteoglycans, ander components of the extracellular matrix. Fibroblastso secrete extracellular zinc-dependent endopeptidasesled metalloproteinases (MMPs), which facilitate theirvement through the matrix and help with the removal ofmaged matrix components.16

giogenesisstinct proteases, such as MMP-2 and MMP-9, modulategiogenesis-promoting endothelial cell migration and tubemation by proteolytically remodeling the basement mem-ne. Moreover, proteases liberate angiogenic moleculesred in the extracellular matrix, producing an angiocompe-t microenvironment. VEGF-A predominantly regulatesgiogenesis in health and disease by signaling throughGF receptor 2 (VEGFR-2, also known as fetal liverase 1). Soluble VEGF isoforms promote vessel enlarge-nt, whereas matrix-bound isoforms stimulate branch-. When a quiescent cell senses an angiogenic signal, suchVEGF-A, angiopoietin 2 (ANG-2), FGF, or chemokineseased by innate inflammatory or local injured cells, peri-es first detach from the vessel wall in response to ANG-2d liberate themselves from the basement membrane by

teolytic degradationmediated byMMPs (collagenases, ge-inases, and stromelysins).

gracepTo stabilize endothelial cell channels, angiogenic endothe-l cells release PDGF-B to chemoattract pericytes. Hence,ricyte deficiency in the absence of PDGF-B causes vesselkage. Healthy vessels must be equipped with mechanismsmaintain quiescence, while remaining able to respond togiogenic stimuli. The ANG (ANG-1, -2, -3, and -4 ligands)d tyrosine kinase with immunoglobulin-like and EGF-likemains (TIE) (TIE-1 and -2 receptors) are responsible fors switch.17

ssue Remodelingllagen accumulation reaches a maximum at 2-3 weekser injury, and the transition to remodeling begins. There isalance between synthesis, deposition, and degradationring this phase. Small capillaries aggregate into largerod vessels, and there is an overall decrease in the waterntent of the wound. Similarly, cell density and the overalltabolic activity of the wound decrease. The most dramaticange occurs in the overall type, amount, and organizationthe collagen fibers, resulting in an increased tensileength of the tissue. Initially, there is increased depositioncollagen type III, a reticular collagen, which is graduallylaced by collagen type I. Collagen fibers are cross-linkedthe enzyme lysyl oxidase, which is secreted by fibroblaststhe extracellular matrix. The normal adult 4:1 ratio of typetype III collagen is restored during remodeling. Equilib-

m is established as new collagen is formed and collagene III is degraded.Most of the clinical differences between chronic and acutealing tissues can be at least partially explained by altera-ns in the local biochemical environment. Local obstacles tosue healing include tissue viability, seroma and/or hema-a, infection, insufficient blood supply, and/ormechanicaltors. For example, adequate blood supply must exist tovide nourishment and oxygenation to healing tissues. Ak of blood supplymay lead to tissue ischemia, an increasedk of infection, and delayed healing.Therefore, therapeutic approaches to manipulate healingy need to integrate multiple cell types and large signalingtworks that are necessary for the dynamic communicationtween cells. The need to target various signaling pathwaysultaneously demands the administration of a balanced

mbination of mediators. In this context, PRP technologiesll draw inspiration for the development of multimolecularrapies (Fig. 2).

RP Therapiestelets are anucleate myeloid cells produced bymegakaryo-es in the bone marrow, comprise up to 1.4-4 1011

ls/L of blood, and circulate for about 10 days. Plateletshere avidly at sites of vascular injurya critical first step inmostasis. Adhesion and activation, along with fibrin for-tion, then cause the release of intracellular storespre-minantly granules (50-80 granules per platelet), dense

nules (3-5 granules per platelet), and lysosomes. The con-t that platelets are essential for hemostasis and vascular

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6 I. Andia, M. Snchez, and N. Maffulliegrity is a fundamental tenet in physiology and medicinet has progressively evolved to the concept that platelets areief effector cells in the various mechanisms involved insue repair with specialized roles in chemotaxis, inflamma-n, and angiogenesis.6,8 Substantial progress in understand-the function of platelets has revealed much about the

mplexity of PRP therapies.To date, the effectors of the beneficial function of PRPrapies were growth factors, such as PDGF, TGF, FGF,dothelial growth factor (EGF), hepatocyte growth factorGF), connective tissue growth factor, and VEGF, amongers. However, evolution of our understanding and recentteomic analyses indicate that granules contain moren 300 proteins.18 Thus, reinterpretation of PRP therapiesuires us to consider new classes of molecules. As illustra-e examples of the different biological mechanisms modu-ed by PRPs, we quote molecules seldom mentioned indies of PRPs, such as the chemokine (C-X-C motif)and (CXCL)-7 and platelet factor 4 (PF-4 or CXCL-4) inate immune response, thrombospondin-1 (TSP-1) ingiogenesis, and urokinase plasminogen activator (uPA)cell migration.

flammationtelets modulate inflammation largely because of their abil-to secrete high levels of chemokines (a subset of small,fusible cytokines), required to control trafficking and thether accumulation of leukocytes and monocytes in the

Figure 2 Complexity of PRP therapies: PRP therapies providofmolecules involved in the healingmechanisms. Thereforbiological processes, including inflammation, angiogenesisolism (synthesis and remodeling) of extracellular matrix. Gof the healing response will depend on the relative local bincluding TGF- 1, PDGF, BDNF, bFGF, and IGF-I, functiodifferent outcomes depending on the conditions of the hostfor fibroblasts, also induces the synthesis of collagen type IIGF BP, IGFBP-2, IGFBP-3, and IGFBP-4, which are alsoured tissue. Supporting this hypothesis, the inhibition oftelet activation with antiplatelet glycoprotein Ib decreased

Placytlymorphonuclear leukocyte influx by 50%.19 Platelets arensidered the major source of -thromboglobulin (CXCL-7neutrophil-activating peptide-2 [NAP-2]), a strong che-attractant and an activator for neutrophils. Actually, plate-s secrete 2 CXCL-7 precursors (ie, platelet basic protein)d connective tissue-activating peptide III, but proteolyticcessing by neutrophils is essential to make chemotacti-ly active CXCL-7.20 Hence, leukocyte-PRP fibrin is likelyattract more neutrophils from the bloodstream than PRPrin alone. This issue, although needing experimental con-ation, may be clinically relevant when deciding to use

re PRPs or leukocyte-platelet concentrates. A priori, weclude neutrophils because theymay exacerbate tissue dam-via several different mechanisms (ie, secreting proinflam-tory cytokines, such as tumor necrosis factor, interferon N-], and interleukins [IL-6 or IL-1]) that cause matrixstruction through the production of metalloprotease-1MP-1), -3, and -13. In addition, the interaction of neutro-ils with platelets may induce a hyperactive leukotactic re-onse of circulating neutrophils toward the injury site.us, a large influx of these cells to the site of injury andbsequent activation of oxidative and enzymatic processesintensify host tissue damage. Unfortunately, there are few

blished studies on fixed PRPs, and there is a lack of com-lling evidence for the preferential use of either pure PRP orkocyte PRP.Another crucial event in inflammation is dying cell re-val, which depends on monocyte/macrophage lineage.

ltifunctional microenvironment by releasing amyriadtarget multiple cell phenotypes andmodulate variousigration and proliferation, and the anabolism/catab-, the role of platelet secretome at the different stagesof pro- and antimolecular activities. Growth factors,rious stages during the healing response and produceFor instance, PDGF, a chemotactic andmitotic factoranabolic and antiapoptotic activities are regulated byin the early healing response.ormoletanprocaltofibfirmpuexagema[IFde(MphspThsucanpupeleu

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Molecular and biological aspects of PRP therapies 7reover, recent experiments using microarray technologyve shown that PF-4 (representing 25% of the content of nules) induces a unique macrophage transcriptome dis-ct from the known macrophage activation patterns (clas-al and inflammatory) that share diverse molecular similar-s with both the pro- and antiinflammatory activationtterns.21 Minimal research has been conducted to optimizeP formulation at this critical level, and further basic sci-ce knowledge would help in refining PRP therapies. Forample, inducing classical macrophage activation whileiding innate activation may produce an antiinflamma-y environment.PRPs may terminate inflammation by restoring cells to aninflammatory phenotype.22 This effect could bemediatedvarious GFs, including HGF, VEGF, and TGF-, whichtect the function of the endothelial barrier. For example,F treatment induces an antiinflammatory cytokine profileendothelial cells specifically by suppressing E-selectin. In adel of inflammatory activation (LPS stimulation of macro-ages), the presence of HGF induced a decrease in the pro-ammatory cytokine IL-6 and an increase in the antiin-mmatory cytokine IL-10.23 Furthermore, these changesre a result of signaling through themesenchymal epithelialnsition factor receptor. Interestingly, the molecular basisthe antiinflammatory action of PRP on human chondro-es relies on the action of HGF to inhibit NFB.24 HGF ismarily found in plasma, and very little is found in plate-s. Therefore, it is important to consider the balance be-een plasma and platelet proteins when formulating PRPs.These insights on the role of PRP in inflammationmay leadtailored and targeted formulations able to discriminatetween the beneficial and harmful effects of this relationshiptween PRP and inflammation.

giogenesise influence of platelets on angiogenesis has been well es-lished. However, because platelets contain both stimula-s and inhibitors of angiogenesis,25 the mechanism byich platelets regulate angiogenesis remains unclear.Platelet granules contain a variety of angiogenic proteins;radoxically, granules also contain established inhibitorsangiogenesis, such as TSP-1, an adhesive protein modu-ing vascular cell behavior by altering endothelial and vas-lar smooth muscle cell adhesion, proliferation, motility,d survival. TSP-1 concentration is proportional to thember of platelets. For example, PRP containing a 4-foldncentration of platelets releases 183 21 g/mL of TSP-4 By preventing VEGF and bFGF binding, TSP-1 interferesth their mitogenic effects; in addition, it inhibits nitricide signaling.26 Other antiangiogenic proteins in PRPsangiostatin, endostatin, and fibronectin and the tissueibitors of metalloproteinases (TIMPs-1 to -4).9 Pro- and an-ngiogenic proteinsmaywell be stored separately and differ-tially released because the secretion of pro- versus antian-genic stores may be agonist-specific.27 For example,

R1-activating peptide, ADP, and the glycoprotein VI-geting collagen-related peptide induced massive release of

anExGF but modest release of PF-4 or endostatin. In contrast,R4-AP triggered marked PF-4 and endostatin release.28

is suggests that different activation mechanisms and envi-ment stimuli evoke distinct secretion patterns of pro- andtiangiogenic factors. Considering these data, further re-rch is required to produce selective pro- or antiangiogenicvironment by means of PRP therapies.Furthermore, vesicles within platelets (ie, dense granules)re and deliver a pool of small molecules, such as hista-ne, noradrenaline, dopamine, and serotonin, which in-ase vascular permeability by allowing the extravasation ofsma proteins into the injury.

ll Migration and Proliferationtelets also contain fibrinolytic factors (and their inhibitors)t may regulate precisely the pericellular proteolytic envi-ment required for the control of cell migration andmatrixodeling. For example, several proteases and protease in-itors released by platelets, most notably uPA and plasmin-en activator inhibitor type 1, proceed as both targets anddifiers of pathways that impact proliferative/migratorynts and coordinately titrate the overall pericellular proteo-ic balance (directly via the generation of plasmin) as well asirectly by activating several members of the MMP family.early, the binding of plasminogen activator inhibitor type 1th its several targets, including vitronectin, uPA, and uPA/AR, has the potential to affect the motile program on mul-le levels, providing the opportunity to therapeutically ma-ulate this pathway in pathophysiological settings.29

abolism/Catabolismd Matrix RemodelingP-released anabolic cytokines include connective tissuewth factor (CTGF), TGF-1 and -2, IGF I and II, FGF-2,d others.8 These cytokines have different effects that aresue-context dependent. For example, TGF-1 may en-nce cartilage repair by triggering chondrocyte differentia-n. In other conditions, TGF- enhances collagen deposi-n and has been associated with fibrotic healing. Moreover,lecular changes that occur with aging could lead to growthtor dysfunction. Actually, the functionality of bothF-1 and IGF-I declines as a result of the aging-relatedanges in growth factorreceptor interactions. Assumingpaired anabolic growth factor stimulation with increased, PRP actions would differ when administered to oldertients.The biological paradox is that platelets may be, however,o implicated in matrix destruction (remodeling). Indeed,Ps are also secreted by platelets, but in an inactive form,

d their activity is regulated by themolecular microenviron-nt. Classically, MMPs are collectively viewed as capable ofgrading all components of the extracellular matrix andsement membrane, restricting their functions to tissue re-deling and maintenance. However, extracellular matrixgradation releases noncovalently bound growth factors

d cytokines, and thereby increases their bioavailability.amples include release of VEGF and TGF-1. VEGF binds

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n latent TGF--binding protein. Extracellular matrix deg-ation releases the latent complexes, and dissociation ofTGF-LAP complexes increases TGF- availability.

reover, LAP is a substrate of MMP-2, -9, -13, and -14, andent TGF--binding protein can be cleaved by MMP-7.The multitude of potential cellular mechanisms acting si-ltaneously in tissue repair helps to explain why the fieldmove toward targeting specific pathways by harnessingeffects of particular molecular subsets present in PRPparations. Moreover, PRP biotechnology can be used forch more than enhancing tissue healing. In the correctnds, PRPs can also be essential tools for discovering thecisive molecules in specific healing pathways.

itical Parameters in PRP Technologiesmposition of PRPpends on the Preparation Systeme methods of producing PRPs determine the compositiond concentration in terms of leukocytes and platelets in aen plasma volume. Accordingly, PRP preparations haveen categorized in pure PRP, in which leukocytes arerposely eliminated from the PRP, and leukocyte- andtelet-rich plasma (L-PRP), containing high concentration ofkocytes.30 Although leukocytes actually increase the concen-tion of some growth factors, such as VEGF or PDGF, theyo release radical oxygen species, catabolic cytokines, andteases. According to a recent study,31 the concentrationsMMP-9 and IL-1 in L-PRP were higher than in pure PRP.reover, both MMP-9 and IL-1 correlated with the num-r of neutrophils in the L-PRP. Moreover, neutrophils re-se various compounds with the potential for further tissuemage. Hence, the improved homogeneity of pure PRP andreduced donor-to-donor and intradonor variability wouldpport the view that pure PRPs are more reproducible anddictable than L-PRPs.

tivation Protocoltivation of the coagulation cascade is a critical step not onlyPRP-clot manipulation during surgery but also in stimu-ing growth factor and cytokine release from PRPs. In fact,PRP fibrin (gel, clot) that forms on coagulation is a useful

livery system because most growth factors and cytokinesreleased during fibrin (gel, clot) retraction or during fi-nolysis. Thus, the kinetics of fibrin formation/retraction iscial in signaling and cellular functions. For instance, PRPivated ex vivo with thrombin induces a rapid clot forma-n/retraction and a sudden burst of signals, compared withCl2 or collagen.32 Alternatively, the clotting cascade can beivated in situ by tissue factor, the initiator of the host re-onse to injury. The kinetics of cytokine release is importantcause most cellular responses are widely influenced notly by cell surface receptors but also by the method by

ich their cognate ligands (growth factors or cytokines) arereted or delivered. For example, a receptor may be acutely

resorivated on an immediate increase in ligand concentrationmimicked by most drugs), but often in physiology, a

nstitutively secreted factor needs to accumulate over timereach a threshold set by the affinity of the receptor (asmicked by slow PRP activators). So far, this aspect is underearched, and no study has addressed the hypothetical dif-ential effects of acute and gradual increases in extracellulartors in PRP-induced signaling events and cellular func-ns in vitro.

tifacts During Preparation (Unpredictability)aspect that has received scant attention, and can greatly

ect the biological activity of the PRP, is the length of timetween PRP activation and its application. Although all PRPparations prepared with the same commercial systemntain the samemolecular pool, the proteases (ie, thrombin,smin) ensuring plasma activation may degrade some ofgrowth factors and activate others, altering the clinical

ectiveness for specific applications. Moreover, differencesfibrin stability and in the behavior of platelet-platelet andtelet-leukocyte aggregates should be considered. Leuko-e binding to fibrin profoundly alters leukocyte function,ding to phagocytosis, NFB-mediated transcription, pro-ction of chemokines and cytokines, and degranulation.The key challenges ahead include identifying present er-s and advantages of activation techniques to improve therapeutic benefits of PRPs.

arriers to Advancing PRPherapiesast array of barriers, ranging from deficits in basic researchclinical differences in formulations and administrationcedures, undermine current efforts to set effective PRPtocols to manage healing. Inconsistencies in clinical find-s reported in the literature, even when using the same PRPmpound and application protocol, may be the result ofnor differences in PRP composition and/or host tissue con-ions.Recent proteomic studies have provided comprehensivealogs of the molecular content of platelets, which areiquely valuable to investigate PRP functions. These datas will be critical in the discovery of PRP functions andpected connections with specific molecular subsets.nce, elucidating the role of platelet proteome in tissuealing would permit tailoring PRP preparations to the dif-ent medical conditions.

pand Research on Heterogeneityher priorities include clarifying the molecular basis forferences in the healing response across patients. So far,usands of patients have benefited from the administrationplatelet-rich compounds, but limited efficacy still remainsoutstanding problem. Given the limited response rates, ap forwardwould be the discovery of predictive biomarkersidentify responders among the large patient group of non-

ponders. These biomarkers could be clinical, molecular,both. Analyses addressing candidate gene biomarker de-

terminants of healing, such as genetic polymorphisms inVEGF and/or its receptors, might improve understanding ofthe relative impact of some components of these formula-tioforAclig(techgenturCOateinfstrgening

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10. Sharma P, Maffulli N: Biology of tendon injury: Healing, modeling andremodeling. J Musculoskelet Neuronal Interact 6:181-190, 2006

11. Delvaeye M, Conway EM: Coagulation and innate immune responses:

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Molecular and biological aspects of PRP therapies 9ns. Moreover, a genetic contribution has been proposedtendon/ligament vulnerability to injuries, in particular forhilles tendon and rotator cuff pathologies and the cruciateaments in the knee. Specifically, variants within the TNCnascin), COL5A1, and MMP3 genes co-segregate withronic Achilles tendinopathy. The variant within the TNCe also appears to co-segregate with Achilles tendon rup-es, while sequence variants within the COL1A1 andL5A1 genes have been shown to be associated with cruci-ligament ruptures and/or shoulder dislocations. Thus,

ormation gained from efficient use of genomics, ie, patientatification according to functional variants in candidatees, may be crucial in refining PRP therapies and improv-efficacy.

onclusionse have focused on a few of the many molecules involved invarious biological pathways that link healing and PRP

hnologies. As the cellular and molecular contribution toaling is further elucidated, their modulation with PRPs willpact on our understanding, raising the hopes for the de-opment of better therapies. Identifying which molecularchanism(s) is more or less important during the course ofaling will continue to be a challenge, requiring excellentclinical models for different musculoskeletal conditionsd carefully designed clinical trials.

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Basic Science: Molecular and Biological Aspects of Platelet-Rich Plasma TherapiesUnderstanding Tissue HealingInflammationTrophic PhaseCell Migration and ProliferationAngiogenesis

Tissue Remodeling

PRP TherapiesInflammationAngiogenesisCell Migration and ProliferationAnabolism/Catabolism and Matrix RemodelingCritical Parameters in PRP TechnologiesComposition of PRP Depends on the Preparation SystemActivation ProtocolArtifacts During Preparation (Unpredictability)

Barriers to Advancing PRP TherapiesExpand Research on Heterogeneity

ConclusionsReferences