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Ad ce isof l ligame ction)Os genomof mechby cytoskGe ut becall roteinsy ce oad blastne mbranini new bidentification of regulatory molecules, the genetic mechanism of orchestrated synthesis between differentcells, tissues, and systems remains largely unknown. Interpatient variation in mechanobiological response ismost likely due to differences in periodontal ligament and bone cell populations, genomes, and proteinexthomifunefftrech20

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45pression patterns. Discovery of mutations in OTM-associated genes of orthodontic patients, includingse regulating osteoclast bone-matrix acidification, chloride channel function, and osteoblast-derivedneral and protein matrices, will permit gene therapy to restore normal matrix and protein synthesis andction. Achieving selectivity in targeting abnormal genes, cells, and tissues is a major obstacle to safe andective clinical application of gene engineering and stem-cell mediated tissue growth. Orthodonticatment is likely to evolve into a combination of mechanics and molecular-genetic-cellular interventions: aange from shotgun to tightly focused communication with OTM cells. (Am J Orthod Dentofacial Orthop06;129:458-68)

ifes complexity and organization are illustratedin the biological phenomena underlying orth-odontic tooth movement (OTM). A daunting

ay of coordinated biochemical reactions occur in andund cells, leading to end points of protein synthesis,tosis (cell division), and cell differentiation. Mechan-lly induced, cell-mediated time and space changes inne and soft tissue return the craniomandibular systemhomeostasis.Capability of adaptive response to applied orth-

ontic force rests in the DNA of periodontal ligamentDL) and alveolar bone cells. Cell vitality and num-

bers determine the molecular genetic responses makingtooth movement possible. In the dramatic words ofKiberstis et al,1 the robust and unceasing activities ofosteoblasts and osteoclasts imbue humans with themechanical prowess to climb mountains or run mara-thons and, we add, to undergo orthodontic treatment.

PURPOSE

This article reviews and synthesizes current bio-medical literature on processes in OTM. It seeks to linkclinical orthodontics with mainstream molecular-ge-netic research. It does not propose a complete picturebut orients the reader to bases for the bioadaptability oforthodontic force application and areas where progressin mechanobiological diagnosis and treatment is likely.

The demands of professionalism require orthodon-tists to be conversant with biological principles under-lying treatment. Numerous instances link such knowl-edge to better patient care.2,3 Roberts and Hartsfield4even suggested that the importance of bone pathophys-

m the Department of Orthodontics, College of Dental Medicine, Novatheastern University, Fort Lauderdale, Fla.sociate professor.ofessor and chair.rint requests to: Richard S. Masella, Department of Orthodontics, Collegeental Medicine, Nova Southeastern University, 3200 S University Dr, Fort

derdale, FL 33328; e-mail, [email protected], April 2005; revised and accepted, July 2005.9-5406/$32.00yright 2006 by the American Association of Orthodontists.

:10.1016/j.ajodo.2005.12.013

8urrent concepts in theoth movementhard S. Masellaa and Malcolm Meisterb

rt Lauderdale, Fla

aptive biochemical response to applied orthodontic fornetworked reactions occur in and around periodontachanical force into molecular events (signal transduteoblasts and osteoclasts are sensitive environment-to-restoring system homeostasis disturbed by orthodonticorthodontic force: extracellular matrix, cell membrane,ne activation (or suppression) is the point at which inp5 environments. Hundreds of genes and thousands of pnthesis, modification, and integration form the essenaptation to orthodontic force depends on normal osteoeded proteins at the right times and places. Cell metiator of signal transduction and a discovery target forology of orthodontic

a highly sophisticated process. Many layersment and alveolar bone cells that changeand orthodontic tooth movement (OTM).e-to-environment communicators, capableanics. Five micro-environments are alteredeleton, nuclear protein matrix, and genome.omes output, and further changes occur ins participate in OTM. Gene-directed proteinf all life processes, including OTM. Boneand osteoclast genes that correctly expresse receptor-ligand docking is an importantone-enhancing drugs. Despite progress in

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American Journal of Orthodontics and Dentofacial OrthopedicsVolume 129, Number 4

Masella and Meister 459ogy in treatment outcomes requires orthodontists tocraniofacial bone specialists.If orthodontists and other dental specialists neglectbiology of craniofacial bone and attendant thera-

utic implications, they risk a status closer to techni-ns than to front-rank health professionals.Discoveries in the molecular biology and genetics

bone and connective tissue physiology permit appre-tion of the complexity and regulatory sophisticationOTM.4,5 Yet aspects of biomedical science can

imidate clinicians. Knowledge of organic chemistry,chemistry, and physiology from predental and den-education helps orthodontists understand the behav-of OTM components. Basic chemistry includes

lecular-ionic covalent (electron) bonding, the rela-nship between molecular structural change and func-nal change, and the readiness of cell surface andracellular (cytoskeletal) proteins to communicateth the extracellular matrix (ECM) through receptor-and (binding protein) docking and signaling proteinfusion or active transport.

NOMIC REGULATION

A healthy skeleton is a feedback-controlled systemt continuously integrates signals and responsesich sustain its functions of delivering [systemic]

lcium while maintaining strength.6 Abundant scien-c literature on control of gene expression proves thats is the era of genomic regulation.6,7 Three yearser the discovery of the human genome structure, therch is on for controllers of gene expression and

ordination, including those of the skeleton.8 Thesecritical issues in adaptive responses provoked by

hodontic forces.Michael Eisen of the Lawrence Berkeley National

boratory stated that buried in the DNA sequence isegulatory code akin to the genetic code but infinitelyre complicated.8 Hood et al9 add that gene regu-

ory networks integrate dynamically changing inputsm signal transduction pathways and provide dynam-lly changing outputs to the batteries of genes medi-ng physiological and developmental responses, in-ding OTM.

A, GENES, BASE PAIRING

The DNA in PDL and alveolar bone cell chromo-mes holds the keys to life and OTM. DNA is dividedo 3 parts: sugar and phosphate backbone mole-les, and nitrogen-containing cyclic bases.10,11 Thembination makes an extremely reactive molecule.gulatory regions of nucleotide (DNA building block)

uences are scattered within long DNA molecules. A

ene is a specific base sequence containing mamyallKnase codes for long, ordered amino acid chainslypeptides or proteins). Genes are made of codinguences, exons, separated by noncoding sequences,introns. Knowing the functional significance of

,000 to 25,000 human genes and regulatory DNA inchromosome pairs, and the possible 100,000

teins they encode will no doubt provide accuratefinitions of health and disease, and open roads toproved orthodontic treatment.9,11The challenge in researching the human genomes 6

lion complementary base pairs is overwhelming.atson et al12 reminded us that the DNA alphabet isited to 4 letters: A, T, C, and G, denoting nucleotide

ses adenine, thymine, cytosine, and guanine. Inuble-stranded DNA, adenine from 1 strand typicallyirs with thymine from the other; likewise, cytosineirs with guanine, a bonding pattern called basemplementarity. If a single letter represents eachcleotide base, the human genome would make allion pages of text. A mere 1.1% to 1.5%, or 11,00015,000 pages, are represented by genes.11Adding complexity is the genomes lifetime ten-

ncy for base sequences to change (mutate) in re-onse to genetic-environmental input, defects inromosome replication, and unknown reasons. For ex-ple, cystic fibrosis is characterized by defective cellular

loride channels, made of mutant proteins encoded bylymorphic (differently arranged) nucleotide sequencesthe ClCN7 gene.11 Normal chloride channels play a keye in osteoclastic bone resorption in OTM.13,14 Ausand polymorphisms discovered in the chloride

annel gene can cause cystic fibrosis. How many totallymorphisms exist in this and hundreds of othernes expressed in OTM, and what are their clinicalplications for osteoclastic bone resorption and alltabolic processes of tooth movement?

STEMS ANALYSIS

The challenge in OTM is understanding systemsher than components.9 System comprehension, how-er, is easier said than done. Although we can see thetire system, the information contained in . . . thou-ds of data points is beyond our ability to interpret

uitively.15The first step in understanding OTM systems is the

llection of a comprehensive expressed messengeronucleic acid (mRNA: DNA gene sequences con-rted to complementary RNA sequences) transcripttabase for osteoblasts, osteoclasts, osteocytes, fibro-sts, mesenchymal stem cells (MSCs), cementoblasts,

mentoclasts, and macrophages. Present technology

ows detection of single mRNA transcripts per cell.9owing all mRNA transcripts of pertinent cells means

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American Journal of Orthodontics and Dentofacial OrthopedicsApril 2006

460 Masella and Meisterowing all proteins that the system can synthesize.otein function is inferred by comparisons with exist-

proteomics databases or via experimentation.For example, osteoblasts and fibroblasts are almost

netically identical. All genes expressed in fibroblasts expressed in osteoblasts.16 Only 2 osteoblast-spe-c mRNA transcripts are known: one for the tran-iption factor (TF, a protein enhancing or suppressingne expression) Cbfa1, the other for the TF osteocal-, an inhibitor of osteoblast function. Thus, present

owledge is that osteoblasts can synthesize only 2 baseteins that fibroblasts cannot.16Currently, 96 genes are identified in human osteo-

nesis.17 Functionally, 44 are grouped as growthtors (GFs), 30 as ECM proteins, and 8 as cell

hesion molecules. Simultaneous gene expression isdied in commercially available microarrays thatrmit complementary binding and analysis of experi-ntal DNA samples with known DNA sequences.Despite advances, development of a complete OTM

arts list with parts function is many yearsay.7,18 Linear depictions of signaling pathways failportray 3-dimensional complexity. Even molecularthways and networks with relatively few componentsre configured into systems that display complexhaviors.5,18 A complete parts catalogue would in-de protein-coding and nonprotein-coding genes,s, and mediators of cell-cycle progression and chro-some structure and function. It will also includeny undiscovered functional human DNA sequences.The magnitude of challenge in delineating genomes

d proteomes is shown in the 20-million-strongNA database generated for normal prostate tissue

d 1 prostate-cancer cell line. Some 300 prostate-ecific genes were identified, along with 554 ex-ssed TFs.9

ESSURE-TENSION PERSPECTIVE

Most orthodontists became aware of the role oflls in OTM in the pressure-tension theory, linkinghysiologic force application with PDL compres-nal and tensional changes and subsequent activationMSCs. The theory proposes that force-subjectedL progenitor cells differentiate into compression-ociated osteoclasts and tension-associated osteo-sts, causing bone resorption and apposition, respec-ely.19 Direct resorption is associated with lightce application ( 50-100 g per tooth), tissue and cellservation, and vascular patency.20 Indirect (under-

ning) resorption and hyalinization are associated with-intolerant heavy or necrotizing forces causing crush-

injury to PDL tissues, cell death, hemostasis, and

l-free PDL and adjacent alveolar bone zones.19,20hereclecular genetics of osteoblast differentiationd function

Although many genes control the complex processosteogenesis, the TF Cbfa1 is the earliest expressedd most specific marker of bone formation.16 Otherne-forming genes encode proteins for GFs, bonerphogenetic proteins (BMPs), transforming growthtor-beta (TGF-), and GF-associated internal signal- molecules.21-23Importantly, osteoblast differentia-n and proliferation are separate processes controlled different genes.24

Expanding the pressure-tension theory, multi-po-tial MSCs begin differentiating within hours ofhodontic force application as specialized molecules synthesized in the PDL and alveolar bone.25,26cal osteoblasts and osteocytes express early-responsennective-tissue GF, whereas osteoclasts and osteo-tes produce the ECM protein osteopontin.27 Connec-e-tissue GF promotes osteoblast precursor prolifera-n, mineralization of new bone by mature osteoblasts,d vasculogenesis.1,27,28

A paravascular osteogenic response is noted indened (tensional) zones of the PDL and expandeddpalatal and facial sutures.27 Osteoblast differentia-n is a 5-generation process starting with stem-cellgration from blood vessel walls, or MSC precursortivation, and preosteoblast formation at about 10urs postforce. TF genes Cbfa1 (also called Runx-2)d Osterix help control this process (Fig 1).29,30bfa1 is also expressed in the osteoblast-homologous,ntin-synthesizing odontoblast.16,31) The late-acting-coding osteocalcin gene also controls osteoblastferentiation through an inhibitory effect.32 Other TFserting positive or negative control over osteoblastferentiation and proliferation will be discovered.All major GF families help control osteoblast dif-

entiation during embryonic development, includingF-, fibroblast GF-1, and Indian hedgehog.16,28,32Bone formation begins 40 to 48 hours postforce.27

gulatory sequences of most genes involved in secret-bone ECM contain 5 to 10-base Cbfa1 binding

es, which help control bone matrix secretion byture osteoblasts.16 The pressure-tension perspective

enlarged in noting that bone resorption and deposi-n can be present in any tension site, as well as anympression site.27,33,34

Other genes participate in mechanically inducedne modeling, expressing proteins for enzymes nitricide synthetase, prostaglandin G/H synthetase, andtamate/aspartate transporter. Parathyroid hormonelps induce expression of insulin-like GF and estrogeneptor-beta.27 The recently discovered low-density

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Masella and Meister 461oprotein receptor-related protein 5 (LRP5) gene con-ls bone formation through modifying osteoblastliferation and increasing bone mass.6,26 LRP5 mu-

ion in both alleles (gene forms) causes loss ofteoblast function and osteoporosis-pseudogliomandrome, characterized by very low bone mass.6,24gle-gene mutations in LRP5 might cause gain ofction resulting in osteoblast hyperactivity and in-ased bone mass. LRP5 might mediate other molec-r genetic processes, including cancer.6Mutations in any OTM-associated gene inducetant, missing, insufficient, or excess proteins and,

thout genetic redundancy, will alter clinical response.e more extensive the single gene mutation or numbermutant genes, the greater the clinical deficit.Osteoblasts contain a rich array of functional cell

rface receptors open to protein docking. BMPs bindsuch receptors, triggering a signaling pathway thatmotes osteoprogenitor cell differentiation and up-ulation (increase) of osteoblast function.35-37 BMPs

n induce Cbfa1 expression. In turn, BMP expressiond signaling might be controlled by signaling mole-les and Hh genes and proteins.32,36,38 Growth hor-ne promotes bone formation through ligation with

Fig 1. Transcriptional control of osteoblastic dPreosteoblasts are derived from 2 sources: MSCis early promoter of osteoblast differentiation. Ososteoblasts capable of expressing osteocalcin, asialoprotein; Col-1, type 1 collagen; OC, osteocareceptors on osteoblast surfaces and stimulation ofulin-like GF-1.39

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indOsteoblast receptor-ligand docking not only changesll form and function and initiates signaling, but alsoves as a potential point of therapeutic modification.ugs that enhance or block activity of osteoblasteptors are under investigation.Finally, a family of molecules known as ho-

obox proteins (specialized DNA sequences in ex-s of many regulatory genes) also help control osteo-st differentiation. Msx1 protein is a key modulatorbone development and modeling, participating inbryologic body patterning and skeletal adaptation in

ulthood.16 Msx2 could be another regulator of Cbfa1pression. The homeobox protein Hoxa-2 controlsond branchial arch patterning and might suppress

th Cbfa1 expression and bone formation.

UROTRANSMITTERS

Somatosensory neurons transmit signals from pe-heral tissues to the central nervous system (CNS).

example of component multi-functionality is thecovery of the efferent (output) function of afferentput) neurons: release of biologically active proteinst contribute to neurogenic inflammation. Increased

tiation and progeny of pleuripotent MSCs.ericytes from blood-vessel walls. TF Cbfa1s late-differentiation TF that induces matureibiting osteoblast differentiation. BSP, bonesx, osterix; Msx2, homeobox gene.ce

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462 Masella and MeisterWith application of physiologic orthodontic force,L peripheral nerve fibers release calcitonin gene-ated peptide (CGRP) and substance P.40,41 Otherdiscovered neurotransmitters might mediate the in-mmatory process. Besides acting as neurotransmit-s, CGRP and substance P serve as vasodilators,ucers of increased vascular flow and permeabilityapedesis), and stimulators of plasma extravasationd leukocyte migration into tissues (transmigration).work-horse molecule, CGRP, induces bone forma-n through osteoblast proliferation and osteoclastibition.42-44Receptors for CGRP are found on osteoblasts,nocytes, lymphocytes, and mast cells. Receptor

tivation (docking) results in amplified 2-way inter-llular communication, promoting cytokine (inflam-tory mediator molecule) synthesis and release.41,42tokine receptors on neuropeptide molecules facili-e cellular cross-talk and synthesis of other moleculest change cell behavior. Signaling pathways changechanical force into molecular events (signal trans-ction) and tooth movement.45 At least 10 networkednal transduction pathways participate in OTM. Dam-e to any signaling pathway can cause dysfunction andease.Normal PDL and alveolar bone innervation is

ential to OTM-associated periodontal remodeling.althy innervation promotes maximum blood flowring OTM, whereas denervation reduces blood flowd bone formation.46,47

Systemic factors such as age, nutritional status,g history, and metabolic bone diseases might alter

y molecules that mediate mechanically induced lo-lized inflammation, bone modeling, and homeostaticne remodeling.27

teoclast differentiation and function

Osteoclasts are specialized multinucleated giant cellst develop from monocyte-hematopoietic cells.13,14eir unique properties include adherence to bonetrix and secretion of acid and lytic enzymes that

stroy mineral and protein structures.13 At least 24nes and 60 proteins are implicated in positive andgative regulation of osteoclastogenesis and osteoclastction. A series of TFs controls osteoclast differen-

tion.14,26Roberts et al27 considered bone resorption at theL surface the rate-limiting step in OTM. Harada anddan6 specified osteoprotegerin (OPG), cathepsin K,d chloride channel 7 (ClCN7) as rate-limiting agents osteoclast differentiation and function (Fig 2). OPG

cks the TF receptor activator of nuclear factor

ppa B (RANK) and RANK ligand (RANKL) dock-ablip, cathepsin K destroys bone matrix proteins, whereasloride channel 7 maintains osteoclast neutrality byuffling chloride ions through the cell membrane.ese molecules are also targets of drug discovery.A patients bone resorption potential and timelyM outcome then depend on recruitment of mature

teoclasts and precursors, osteoclast differentiation,d numbers of functional osteoclasts at the bone-PDLerface. Clinical success also hinges on normal oste-last and osteoblast genes that correctly expresseded proteins in adequate amounts at the right timesd places, including regulatory molecules such as

or necrosis factor and its receptor, colony stimulat- factor-1, OPG, and RANK and RANKL.27The earliest marker of bone resorption could be the

tokine interleukin-1beta (IL-1). A mutant gene of-1 might be associated with down-regulation of thisportant cytokine.27 Osteoclastic bone resorption iso facilitated by PGE2, nitric oxide, IL-6, and otherammatory cytokines.48RANK and RANKL are key proteins regulating

teoclast function.13,14,24,27 Synthesis of RANKL byteoblasts and its lifetime role in promoting osteoclastferentiation supports the idea that osteoblasts controlly osteoclast differentiation, not function.24 How-er, an osteoclast-originating messenger molecule wasently discovered that appears to talk with osteo-sts. This underscores the biologic concept of contin-

l, 2-way cell-to-cell communication.2Bone resorption is a much faster process than bone

position; it can take 3 months to replace boneorbed in only 2 to 3 weeks.27 The molecular geneticsosteoclast differentiation and function might bepler than that of osteoblasts.

docrine regulation of bone physiology

Sex steroids also influence bone homeostasis (Fig 3).trogen suppresses bone resorption by reducing oste-last numbers.6 Testosterone reduces bone resorptionmales, promotes bone formation in males and fe-les, and can be converted to estrogen to inhibit boneorption. Much remains to be learned about estrogen-diated bone resorption.24A central nervous system component is anotherulator of osteoblast function. The hormone leptin is

nthesized by fat cells and acts by binding to signal-nsduction receptors on hypothalamic neurons.16rough hypothalamic relay, leptin strongly inhibitsne formation by suppressing osteoblast functionuroendocrine control).24 Peripheral signaling is me-ted by the sympathetic nervous system. When leptin issent because of adipocyte depletion, as in generalizedodystrophy, accelerated bone growth and osteosclerosis

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Masella and Meister 463sue. Similar to thyroid hormones or cortisol, leptin isught to have many target organs and functions.16

e proteome

Cellular biochemistry is carried out by proteins.oteins commonly function in concert with other

Fig 2. Determinants of skeletal homeostasis andprotein; Cbfa1, transcription factor, earliest markpeptide; ClCN7, chloride channel 7; CSF-1, cologrowth factor; ER-, estrogen receptor-beta; GHtransporter; Hoxa-2, homeobox gene; IGF, insuliLeptin, central nervous system hormone; LRP5,Msx-2, homeobox gene; NOS, nitric oxide syntscription factor; Osterix, transcription factor promglandin G/H synthetase; PTH, parathyroid hormfactor kappa-b and ligand; Smad, cytoplasmicTGF-, transforming growth factor-beta family; Tteins in complexes or networks.9 Liebler advisedt each protein, whether a transmembrane receptor, ounscription factor, [or] protein kinase, expresses action that assumes significance only in the context ofthe other functions and activities also being expressedthe same cell (italics added).15 He also references theousands of genes that may be expressed in each cell in

rying combinations (italics added).15

hanges in OTM. BMP, bone morphogeneticsteogenesis; CGRP, calcitonin gene-relatedmulating factor 1; CTGF, connective tissuewth hormone; GLAST, glutamate/aspartategrowth factor; IL-1, Il-6, Il-11, interleukins;nsity lipoprotein receptor-related protein 5;; OPG, osteoprotegerin; Osteocalcin, tran-osteoblast differentiation; PGHS-2, prosta-ANK/RANKL, receptor activator of nuclearling molecules; SOST, gene for sclerostin;, tumor necrosis factor and receptor.trafunallin thva

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464 Masella and Meisterional modification (after ribosome-based synthesis)proteins is an important point of functional changed increased protein versatility.11,15,49 PhosphatePO3-, phosphorylation) and methyl groups (-CH3,thylation), other ions, and lipids might bond to

se proteins. Protein modification results in changedimensional shape (conformation), bonding poten-

l, pattern of molecular folding, and molecular func-n.10,11,15 Gorlin et al50 referred to molecular parsi-ny in describing the ubiquitous multi-potentiality,pleiotropy, of human genes and protein products.Another rich source of human protein variability is

lternative splicing of primary mRNA. One gene canduce mRNA transcripts that differ in structurefferentially cut by splicing enzymes) and thereforeding potential.4,11,15,49,50 The fibronectin gene com-nly expressed in OTM is an example. The ECMtein fibronectin is a mediator of cell-ECM inter-

tion. Differential mRNA splicing produces manyms of fibronectin (isoforms), each with a specificction.49,51

racellular and extracellular environments

Fig 3. Hormonal control of bOrthodontic force-induced system adaptation oc-rs in the context of 5 related microstructures: PDL

can

attd alveolar bone ECM, cell membrane, cytoskeleton,trix of nuclear proteins, and genome (Fig 4).Orthodontic force causes physical distortion ofL and alveolar bone cells and the ECM, triggeringny biochemical reaction cascades that affect all 5cro-entities.52 ECM and cell distortion initiate struc-al and functional changes in extracellular, cell mem-ne, and cytoskeletal proteins. At the same time,

merous submembrane proteins associate in cellularal adhesions. These complex structural/functional

aptations transmit survival and growth signals to thetoplasm and help mediate cell adhesion via integrintivation.53 A regulator of integrin-mediated focalhesions is the enzyme focal adhesion kinase. Illus-ting molecular parsimony, focal adhesion kinaseo regulates cell growth, migration, proliferation,d survival. Protein phosphorylation mediated bytein kinase enzymes is important in all aspects oflogy.53-55

Subsequent changes in cytoskeletal protein struc-e and function, such as activin polymerization andpolymerization, continue the signaling process, which

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nal input, genetic output

Cytoplasmic signaling proteins Hh, sonic hedge-g, the TGF- superfamily, and many TFs and ionsa, PO3-) reach the nuclear matrix and thennome, resulting in enhanced or suppressed genepression. Input becomes output as gene-expressedteins, or protein synthesis inhibition, mobilize mi-is, cell motility, secretion of other proteins, andgrammed cell death (apoptosis) that further modify

toskeleton, cell membrane, and ECM.52 The processcontinuous (Fig 4).

Changes in cell environment (development, aging,ternal conditions) might also change the patterns ofne expression.11 Epigenetic regulation of gene ex-ssion involves heritable changes not from DNA base

Fig 4. Mechanical force-induced reciprocal coOTM. By using osteoblast schematic, mechanicacells and triggers multilevel cascade of signaenvironmental componenets cause functional chaor lack therof, is turn-around point at whichsialoprotein; Ca, free calcium; CM, cell memnetwork; DC-STAMP, dendritic cell-specific transextracellular matrix; FAK, enzyme focal adhesionGFR, growth factor receptor; mRNA, messengprotein osteopontin; TF, transcription factor; TNFRNA.uence changes but from chemical modification ofses ATCG or TFs bound with DNA. Such genomic

inhgeanges can occur throughout life. Hartl and Jones11phasized that there is a great deal to be learned

out molecular mechanisms underlying epigeneticdifications.

L and alveolar bone cell death and renewal

Apoptosis is a critically important process in skel-l maturation, adult bone remodeling, and bone re-ir.25 In health, a close linkage exists between bonell proliferation, differentiation, and apoptosis, result-

in an adequate pool of osteoblasts and osteoclasts bone homeostasis.22,25,57In reducing osteoblast numbers through apoptosis

d thereby decreasing bone formation, the SOST genean important regulator of bone remodeling.21-25,27,29cated in chromosome 17q12-q21 (long arm, region

below centromere to region 2-1), SOST expressesprotein sclerostin, a BMP-antagonist and signaling

ication between 5 micro-environments ofcauses distortion of PDL and alveolar bone

sduction pathways. Structural changes inincluding signal input. Genomic expression,input becomes output. BSP, ECM bone; CS, cytoskeleton or cytoplasmic proteinbrane protein; ECM, PDL and alveolar bonee; FN, ECM protein fibronectin; G, genome,A; N, nuclear matrix proteins; OPN, ECMor necrosis factor receptor; tRNA, transferchem

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466 Masella and Meistern of sclerostin might allow restoration of osteopo-ic bone.The many proteins involved in signal transduction,control, and cell cycle regulation are rapidly de-

ded after use as a means of regulating their activi-s.11,15

ogress in diagnosis and treatment

Interpatient variability in mechanical response ismmon in orthodontic practice. Reasons can includeferences in PDL and alveolar bone cell numbers andnomes, including genes for signaling proteins; num-rs of white blood cells, GFs, and cytokines perlumetric measure of tissue-blood; alveolar bone min-l density; and PDL and alveolar bone vascularity perlumetric tissue measure.The ENCODE (ENCyclopedia Of DNA Elements)ject has started identifying all structural and func-

nal elements of the human genome.58 This andch other research will eventually permit correlationpatient genotype with clinical presentation and

oratory-derived protein profiles. This will allowhodontists to identify biological promoters and in-itors of OTM and plan molecular intervention toximize adaptive response.If gene expression in tissues subjected to mechan-

l force shows patterns of alteration in secretedteins in blood or gingival crevicular fluid, perhapsse media can serve as windows for diagnosis andgnosis, and sources of active-treatment biomarkers assessing mechanics.48,59,60On another front, embryology shows that the PDL,

eolar bone, dentin, and dental pulp are neural-crestrivatives.61 A short step is identification of undiffer-tiated adult neural-crest cells from marrow spacesd pulp as candidates for resolution of craniofacialeletal and dental defects. Discovery of molecularnals that induce stem-cell differentiation and cell-livery methods are necessary elements. A roadblockstem-cell enhancement, as with gene engineering, isbility to selectively target deficient genes, cells, and

sues. Yet, as Helms and Schneider61 state, cellulard molecular therapies, rather than [brackets, arch-res], handpieces and scalpels, may one day be used[orthodontists and] dentists to treat [craniofacial]ictions.

NCLUSIONS

We communicate with PDL and alveolar bone cellsapplying orthodontic forces to teeth and bones. Inviding genomic-derived layers of networked bio-emical processes to meet homeostatic-disturbinghodontic mechanics, these cells enable bone re-onse, tooth movement, and return to homeostasis.L and alveolar bone cells are sensitive environment-genome-to-environment communicators, and thein and heart of OTM.The following biological concepts are noted:

. Gene-directed protein synthesis and modification,and integration of many proteins, form the essenceof all life processes, including OTM.

. PDL and alveolar bone cells provide hundreds ofgenes and thousands of proteins for OTM.

. Bone adaptation to orthodontic force depends onnormal osteoblast and osteoclast genes that cor-rectly express needed proteins in adequateamounts at the right times and places.

. PDL and alveolar bone cells constantly talk viamolecular messengers or signaling. Communica-tion is a 2-way process.

. Healthy and pathological bone metabolism in-volves many interactions between genetic andenvironmental (epigenetic) factors. This adds tothe challenge of understanding bone pathophysi-ology.6

. The molecular genetic processes underlying OTMfunction as a feedback system of checks andbalances. Activator molecules beget suppressormolecules and return to steady state.

. OTM biology can be envisioned in the physicalcontext of PDL-alveolar bone ECM, cell mem-brane, cytoskeleton, nuclear matrix, and genome,giving a relatively broad perspective on locationand flow of processes critical to adaptive response.

. Receptor-ligand docking is a potent and commoninitiator of signal transduction of mechanicalforces into molecular events and OTM. It is also adiscovery target for bone-enhancing drugs.

. Despite identification of many regulatory mole-cules, the genetic mechanism of orchestratedsynthesis, or how genes and molecules work inconcert through DNA command centers to in-duce and coordinate cell mobility, differentiation,proliferation, function, and apoptosis, still re-main[s] largely unknown.61

. Identification of mutations in OTM-associatedgenes of orthodontic patients, especially thosedriving osteoclast proton pumps (mineral matrixacidification) and chloride channels, and osteo-blast-derived mineral and protein matrices, willpermit gene therapy to restore normal matrix andprotein synthesis and function. Achieving cell andtissue selectivity in repairing mutant genes with

engineered DNA sequences, or in using stem cellsto grow craniofacial tissues, is a major obstacle.

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Masella and Meister 467These therapies are perhaps 10 to 15 years away ineveryday orthodontics.

. Interpatient variation in mechanobiological re-sponse is most likely due to differences in boneand PDL cell populations, genomes, and proteinexpression patterns.

. Orthodontic treatment probably will evolve into acombination of mechanics and molecular-genetic-cellular interventions: a change from shotgun tofocused communication with OTM cells.8

We thank Drs Roberto Carvalho, Zeev Dav-vitch, J. K. Hartsfield, Jr, and W. E. Roberts for theirights, inspiration, and graciousness.

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Current concepts in the biology of orthodontic tooth movementPURPOSEGENOMIC REGULATIONDNA, GENES, BASE PAIRINGSYSTEMS ANALYSISPRESSURE-TENSION PERSPECTIVEMolecular genetics of osteoblast differentiation and function

NEUROTRANSMITTERSOsteoclast differentiation and functionEndocrine regulation of bone physiologyThe proteomeIntracellular and extracellular environmentsSignal input, genetic outputPDL and alveolar bone cell death and renewalProgress in diagnosis and treatment

CONCLUSIONSAcknowledgmentsREFERENCES

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