Biochemical, Genetic, and Molecular Interactions in Development - part 2 potx

BMP REGULATION OF LIMB CARTILAGE DEVELOPMENT AND APOPTOSIS BMPs are instrumental to the formation of the limb and are intimately involved in multiple stages of limb development, includin

box gene 9; SOX9, autosomal dominant anterior fontanelle, macrocephaly, all bones derived from endochondral ossification, include 17q24.3-q25.1 (114290) micrognathia, cleft palate, hypo- cleft secondary palate, hypoplasia and bending Skeletal

twelfth pair of ribs, hypoplastic, patients Skeletal patterning was not affected Premature poorly ossified cervical vertebrae, mineralization of skeletal elements, including craniofacial small iliac wings, short phalanges region and vertebral column Hypertrophic zone of growth for both hands and feet, anterior plate was thicker (208).

bowing of tibia, short fibula, mildly bowed femur, absent ossification of proximal tibial, and distal femoral epiphysis T-box 5; Holt-Oram syndrome, Vertebral anomalies, thoracic Conditional knockout Embryonic lethality because of mal- TBX5, 12q24.1 autosomal dominant scoliosis, absent or bifid thumb, formed heart tube Elongated phalangeal segments of first

bone anomalies, upper extremity wrist were present in multiple heterozygous mutant phocomelia, radial-ulnar mice (209).

anomalies Transforming growth Camurati-Engelmann Sclerosis of skull base, mandible Targeted disruption Lethality around weaning due to

factor, beta-1; disease autosomal involvement, sclerosis of pos- massive inflammation lesions and tissue necrosis in

TGFB1, 19q13.1 dominant (131300) terior part of vertebrae, scoliosis, many organs (210,211).

thickened cortices, narrowing of medullary canal

Vitamin D3 receptor; Vitamin D-resistant rickets, Rickets Targeted disruption Animals normal until after weaning.

retarda-tion and 40% loss of bone density Early lethality around

15 wk (212).

aHuman gene description includes gene name, symbol, corresponding OMIM number, and locus.

bHuman disease description includes disease name and corresponding OMIM number.

Shum et al.

Table 2

Human Genetic Disorders with As-Yet No Known Genetic Associations

Acrocallosal syndrome; 200990 12p13.3-p11.2 Macrocephaly, large anterior fontanel, prominent occiput and forehead, hypoplastic

postaxial polydactyly, bifid terminal phalanges of thumbs, toe syndactyly, duplicated halluces

Chondrocalcinosis 1; CCAL1 600668 8q (CCAL1) CCAL1; chondrocalcinosis, severe degenerative osteoarthritis

Chondrocalcinosis 2; CCAL2 118600 5p15(CCAL2) CCAL2; chondrocalcinosis, arthropathy, acute intermittent arthritis, ankylosis

Cohen syndrome; COH1 216550 8q22-q23 Microcephaly, maxillary hypoplasia, micrognathia, joint hyperextensibility,

narrow hands and feet, mild shortening of metacarpals and metatarsals Craniometaphyseal dysplasia; 218400 6q21-q22 Cranial hyperostosis, facial palsy, prominent supraorbital ridges and mandible,

Otopalatodigital syndrome, 304120 Xq28 Prominent forehead, severe micrognathia, midface hypoplasia, cleft palate, sclerotic

and knee, flexed, overlapping fingers, short, broad thumbs, postaxial polydactyly, syndactyly, second finger clinodactyly, hypoplastic, irregular metacarpals Craniosynostosis, 600593 4p16 Craniosynostosis, coned epiphyses of hands and feet, distal and middle phalangeal

calcaneo-navicular foot fusions

FG syndrome; FGS1 305450 Xq12-q21.31 Macrocephaly, large anterior fontanel caused by delayed closure, plagiocephaly,

micrognathia, cleft palate, joint contractures, broad thumbs, clinodactyly, syndactyly, broad halluces

Fibrodysplasia ossificans 135100 4q27-q31 Heterotopic ossification, especially of the neck, spine, and shoulder girdle,

monophalangic big toes, short thumbs, fifth finger clinodactyly, severely restricted arm mobility

Larsen syndrome; LRS1 150250 3p21.1-p14.1 Cleft palate, flattened frontal bone, small skull base, shallow orbits, dysplastic

epiphyseal centers, cervical vertebrae hypoplasia, scoliosis, spondylolysis, short metacarpals and metatarsals, multiple carpal and calcaneal ossification centers with delayed coalescence

steep clivus, dense middle-ear ossicles, short, broad distal phalanges, especially thumbs, short third, fourth, fifth metacarpals, supernumerary carpal bones, fusion

of hamate and capitate, toe syndactyly, anomalous fifth metatarsal, extracalcaneal ossification center

Pituitary dwarfism II 262500 5p13-p12 Acrohypoplasia, short limbs, delayed bone age, markedly advanced osseous

maturation for height and age Russell-Silver syndrome; 180860 7p11.2 Micrognathia, skeletal maturation retardation, craniofacial disproportion, delayed

digit middle or distal phalangeal hypoplasia, syndactyly of second and third toes Shwachman-diamond 260400 Costochondral thickening, irregular ossification at anterior rib ends, delayed skeletal

long bones Sotos syndrome 117550 5q35 Macrocephaly, frontal bossing, prognathism, advanced bone age, large hands and

feet, disharmonic maturation of phalanges and carpal bones Spastic paraplegia 9; SPG9 601162 10q23.3-q24.1 Skeletal abnormalities, short fifth finger, clinodactyly, delayed bone age, shallow

acetabulum, small carpal bones, dysplastic skull base Syndactyly, type I 185900 2q34-q36 Syndactyly, complete or partial webbing between third and fourth fingers, fusion

of third and fourth finger distal phalanges, complete or partial webbing between the second and third toes

Velocardiofacial syndrome 192430 22q11 Microcephaly, Pierre Robin syndrome, cleft palate

the many functions of BMPs is to induce cartilage, bone, and connective tissue formation in

verte-brates (24,25) This osteochondro-inductive capacity of BMPs is highly promising for orthopedic

applications, such as skeletal repair and regeneration, and in dental applications, such as the

treat-ment of periodontal diseases (26–30) Since the discovery of BMPs over three decades ago, their

abil-ity to induce ectopic bone and cartilage formation remains a topic of intense investigation In particular,the characterization of the molecular mechanisms of BMP functions was reignited after the cloning

of the activin receptor, the first TGF-` type receptor, in 1991 (31) Thereafter, the molecular pathways

to differentiation have been meticulously dissected and exposed

BMP signals through heterodimeric serine–threonine kinase receptor complexes, containing type

I and type II receptors, each class having a number of subtypes (32,33) Both type I and type II

recep-tors are capable of low-affinity interaction with BMP but only when the ligand binds to both receprecep-torscan result in high-affinity heteromeric ligand–receptor complex formation capable of BMP-depen-

dent signaling (34,35) Therefore, it is likely that the presence and number of different BMP receptors

determine the cellular responses to the many ligands Evidence suggests that the subtype BMPR-IB

is essential for chondrogenesis for the entire developing skeletal system (36–39) However, target deletion studies of the BMPR-IB receptor suggest otherwise (39,40) In these animals, the BMPR-IB

does appear to have an essential role to play during limb bud morphogenesis because the ties are located in the appendicular skeletal elements and not in the axial skeletal structures Moreover,

abnormali-in vitro studies show that BMPR-IB does not possess exclusive chondrogenic potential, suggestabnormali-ing

that other BMP type I receptors may exert redundant functions during chondrogenesis (41–43) Taken

together, the response to BMP signal is not solely defined by the identity of the type I receptor butadditionally by elements in the signal transduction pathways that lie downstream of the receptor Theseare the various cytoplasmic and nuclear transducers, both positive and negative

Downstream from the receptors, Smads are the predominant effectors of TGF-`/BMP signaling (44,

45) An important issue for BMP-dependent signaling is the type of Smad proteins involved in

chondro-genic differentiation and whether the Smads alone are sufficient to direct differentiation Smads tion as dimeric complexes and belong to three classes: regulatory, inhibitory, and common The receptor-regulated Smads (R-Smads) are further subdivided into two groups Smad1, Smad5, and Smad8 aredirectly phosphorylated and activated by BMP type I receptors Smad2 and Smad3 are mediators ofactivin or TGF-` type I receptor signaling A series of in vitro studies have shown that Smad1, Smad5,

func-and Smad8 may be involved in osteochondrogenic differentiation (46–49) These findings suggest that

different Smads or Smad combinations are engaged at different stages of mesenchymal cell tion into osteoblasts and chondrocytes However, in vivo manipulations of Smads have not resulted

differentia-in conclusive evidence because genetically engdifferentia-ineered animal models targeted agadifferentia-inst Smads

pro-duce embryonic lethality (50) Nevertheless, a glimpse of in vivo Smad function can be observed in Smad3 knockout animals, which manifest osteopenia and early onset osteoarthritis (51) The class of

inhibitory Smads (I-Smads) includes Smad6 and Smad7 They have been shown to inhibit the effect of

R-Smads by competing for binding to activated type I receptors (52–56) Indeed, I-Smads are potent inhibitors of skeletogenic differentiation (48,57,58) The common Smad4 (Co-Smad) associates with

activated R-Smad complex, which translocates into the nucleus and participates in the regulation of

target genes (59) Smad4 functions as a tumor suppressor gene, and mutations of the human SMAD4

lead to pancreatic carcinoma and juvenile intestinal polyposis, further illustrating the significance ofTGF-` superfamily signaling and its regulation of cellular physiology (60)

BMP signaling can be channeled through Smad-independent pathways, such as the extracellularsignal-regulated kinase, Jun N-terminal kinase, Wnt, and p38 mitogen-activated protein kinase path-

ways (61–65) Therefore, crosstalk between the signaling pathways during chondrogenic

differentia-tion is inevitable However, a detailed recount of these interacdifferentia-tions is beyond the scope of this review.Finally, BMP and other growth factor signaling can coactivate chondrogenic differentiation For exam-ple, fibroblast growth factor (FGF) signaling through mitogen-activated protein kinase promotes

chondrogenesis by increasing the level of Sox9 expression as well as increases its binding affinity on

the type II collagen promoter (66) It is obvious that BMPs control of chondrogenesis is a highly

regu-lated developmental process that involves multiple pathways and checkpoints This combinatorialmode of signaling ensures fidelity in the patterning and timing of the cartilaginous template onto whichmost of the bony skeleton is produced

CRANIOFACIAL MORPHOGENESIS

AND CRANIAL NEURAL CREST CELLS (CNCCS)

CNCCs give rise to most of the craniofacial tissues (67–69) Interestingly, this cell population is

derived from the dorsal cephalic neural tube During embryogenesis, the ectoderm at the midline lying the notochord thickens to form the neural plate Progressively, the flattened neural plate begins

over-to bend, creating elevations, called the neural folds, with a central depression the neural groove Asneurulation proceeds, the bilateral neural folds oppose each other and fuse at the midline to form theclosed neural tube At the time of neural tube closure and at the junction of where the thickened neuro-ectoderm meets the non-thickened surface ectoderm, epithelial cells delaminate and emerge as mes-

enchymal cells into the underlying space These are the neural crest cells (70) Neural crest cells are

formed along the entire length of the primary neural tube CNCCs are formed from the neural tube atthe level of the forebrain, midbrain, and hindbrain

Neural crest cells are multipotential, and they give rise to a number of cell lineages (71,72) Those

arising from the cranial region have different sets of potentials when compared with those arising inthe trunk For example, trunk neural crest cells do not normally produce cartilage However, recentevidence from lineage tracing and transplantation strategies suggest that some trunk crest cells are

capable of differentiating into cranial cartilages when transplanted into the cranial region (73,74).

From a number of studies using various lineage tracing approaches, we have learned that neural crestcells from the forebrain and midbrain contribute to the frontonasal mesenchyme for the formation of

the upper and midface structures, including part of the cranial base, nasal, and otic capsules (75–77).

CNCCs in the branchial arches are destined for skeletal, odontogenic, myogenic, neuronal, and nective tissue lineages of the lower face and neck regions Following the cartilage lineage in particular,CNCCs in the first branchial arch contribute to form Meckel’s cartilage and the temporomandibularjoint cartilage The hyoid is derived from CNCCs in both the second and third arches, and the fourthand sixth arches in combination give rise to the thyroid, cricoid, arytenoid, corniculate, and cunei-

con-form cartilage (68,72,75,78–80).

BMP REGULATION OF CRANIOFACIAL

CARTILAGE DEVELOPMENT AND APOPTOSIS

The hindbrain is a segmented structure, each segment called a rhombomere (Fig 1) In the brate head, there are eight pairs of rhombomeres and each gives rise to segment-specific CNCCs.During the migratory phase of CNCC development, CNCCs converge into three major streams directed

verte-toward the branchial arches in an orderly and patterned manner (81,82) Therefore, an early step in

the regulation of craniofacial cartilage differentiation is CNCC production and patterning within thehindbrain Similar to setting up the overall body plan, the hindbrain is patterned by a series of homeo-

box (Hox)- and homeobox-containing genes (83) The production of CNCCs from these rhombomeres

is in part regulated by their Hox genes In addition, cell fate determination in the CNCCs is an

orches-trated process (84–86) CNCCs exert a “community effect” among themselves and cell–cell and/or cell–matrix signaling in the group can maintain their segmental identity (87,88) In addition to this

“community” effect, it is also discovered that the isthmus, a region between the midbrain and brain, serves as a patterning center for the rhombomeres and the CNCC derivatives The isthmusexpresses high levels of FGF8 that regulates the expression of the Hox genes in the rhombomeres

hind-Transplantation experiments that include or exclude the isthmus yield different outcomes The sion of the isthmus during grafting allows the rhombomeres and CNCCs to maintain their original iden-

inclu-tity, whereas the exclusion of the isthmus renders CNCCs responsive to environmental cues (89) In

addition to the isthmus, CNCCs can be patterned by signals from the endoderm to give rise to distinctpieces of craniofacial cartilages Interestingly, this is only limited to CNCCs above the level of the

second rhombomere, the so-called Hox-negative cells CNCCs expressing Hox genes are not sive to endodermal induction (90).

respon-Although each rhombomere can give rise to CNCCs, it is observed that those of rhombomeres 3and 5 contribute to a minority of the population A large number of CNCCs within the rhombomereundergo apoptosis, and only a small population migrate out These cells join the major streams and,

thus, lateral to rhombomeres 3 and 5, the area appears relatively free of CNCCs (91–98) This may

serve to gauge the number of CNCCs being produced and to better delimit the migratory streams andtheir eventual destination Evidence suggests that CNCC apoptosis is regulated by BMP and Wntsignaling BMP4 is expressed coincidentally within rhombomeres 3 and 5 BMP4 induces the expres-sion of Msx2 in these rhombomeres, and ectopic expression of Msx2 increases the number of apopto-

tic CNCCs (5,13,93,99) The lack of BMP signaling in even-numbered rhombomeres may be attributed

to the presence of the BMP antagonist, noggin (100) Taken together, these experiments suggest that

CNCC apoptosis is regulated by signals from BMP4 and is mediated by Msx2 Wnt signaling is icant in this cascade because of the expression of cSFRP2 in rhombomeres that have limited apoptosis.cSFRP2 is an antagonist of the Wnt signaling and overexpression of cSFRP2 inhibits BMP4 expressionand rescues CNCC from apoptotic elimination Consistently, inhibition of cSFRP2 or overexpression

signif-of Wnt1 results in ectopic CNCC apoptosis (101) However, another Wnt family member; Wnt6, has been recently shown to be necessary and sufficient for the induction of neural crest formation (102).

The use of different Wnt genes in combination that regulate CNCC formation is an elegant example

of the complexity of the system

As CNCCs migrate from the neural tube towards the forming face, they converge into major streams,migrating toward the respective branchial arches Migration is largely governed by adhesive proper-ties between cells and substrate, and a number of factors have defining roles in this developmental

event (103) During migration, the cells remain in an undifferentiated state such that they are allowed

to reach their destination before they expand further and undergo overt differentiation Localizationstudies reveal that premigratory CNCCs and a subpopulation of migrating CNCCs may already be par-tially committed to the cartilage lineage by virtue of their expression of the key cartilage transcription

factor, Sox9 (104,105) However, these cells do not differentiate yet Differentiation of these cells may

be suppressed by the coexpression of Msx2 in the Sox9-expressing cells Msx2 may serve to tain these cells in an undifferentiated state until migration is completed Overexpression of dominant-negative forms of Msx2 in these migratory cells inhibits normal Msx2 functions and leads to preco-

main-cious cartilage differentiation (105).

The mandible and maxilla arise from the anterior and posterior processes of the first branchial arch,respectively These structures receive extensive contributions of CNCCs from the posterior midbrainand rhombomeres 1 and 2 of the anterior hindbrain In addition to the lineages found in the otherbranchial arches, CNCCs in the first arch also differentiate into tooth structures that are unique to this

arch (106,107).

Meckel’s cartilage formed within the mandibular process has a unique pattern It consists of ananterior, triangular piece at the midline, bilateral rod-shaped pieces that regress to form the sphenom-andibular ligament, and posterior pieces that give rise to the malleus, incus, and temporomandibularjoint cartilage The formation of Meckel’s cartilage is regulated by the mandibular epithelium through

epithelial-mesenchymal interactions (108,109) The instructive signal from the epithelium can be

substituted by epidermal growth factor (EGF), which sustains mesenchymal proliferation and delays

chondrocyte differentiation (110,111) Removal of the epithelium results in increased but dysmorphic cartilage formation (112,113) Indeed, EGF and EGF receptors are endogenous to the mandibular

process (114,115) Antisense oligonucleotide inhibition of EGF in the mandibular process results in

ectopic cartilage formation In contrast, exogenous EGF reduces and disrupts cartilage formation

(46,115) Furthermore, targeted disruption of EGF receptor in the mouse results in Meckel’s

carti-lage deficiency as well (116) These defects are attributed to changes in matrix metalloproteinases

expression and its regulation of cartilage morphogenesis Expression of matrix metalloproteinases isregulated by EGF, and they function in multiple tissue morphogenesis, including that of the anterior

segment of the developing Meckel’s cartilage (117).

Within the mesenchyme, cartilage formation is further delimited by the expression of the

tran-scription factor Msx2, which is excluded from regions with chondrogenic potential (118) Antisense

oligonucleotide inhibition of Msx2 expression in the mandible results in disruption of Meckel’s

car-tilage formation (119) Furthermore, adenoviral expression of ectopic Msx2 also abrogates carcar-tilage formation (120) Interestingly, endogenous Msx2 expression is regulated by BMP expression and

that ectopic BMP signaling can alter Msx2 expression domain, leading to cartilage dysmorphogenesis

(120–122) Msx2 can also inhibit ectopic cartilage formation that is induced by BMP4 as a feedback

reaction However, the competence of the mesenchyme to respond to BMP4 is dependent on localsignals and the key cartilage transcription factor Sox9, functions in antagonistic combination with

Msx2 to regulate cartilage formation (120).

LIMB MORPHOGENESIS AND LIMB MESENCHYME

The limb cartilage develops from paired primordial buds that appear on the embryo’s lateral face at specific levels along its anterior posterior body axis At the early stages of limb development,the buds exhibit a paddle shape and consist of undifferentiated mesenchymal cells derived from thelateral plate and somitic mesoderm, and overlying ectoderm At the distal tip of the bud, the ectodermforms a specialized thickened epithelial structure, known as the apical ectodermal ridge (AER) Pat-

sur-terning along the proximal–distal axis depends in part on signaling molecules from the AER (123,

124) Instrumental to this process is the family of FGFs (125–130) The classic model of limb

pat-terning involves the determination of positional values along the proximal–distal axis specified by

instructive signaling from the AER to the subridge mesenchyme, known as the progress zone (131).

However, recent revolutionary interpretation of limb patterning describes the specification of distinctproximal–distal segments of the limb early in development, with subsequent development involving

expansion of these mesenchymal progenitor before differentiation (128,132) The anterior–posterior

axis of the limb is patterned by the zone of polarizing activity (ZPA), which is located at the posterior

margin of the limb (124,133,134) The major morphogen from this organizing center is the sonic hog (Shh) gene (135), which maintains anterior–posterior patterning in conjunction with other gene products, such as the HoxD gene (136), and participate in regulatory feedback signaling with the AER

hedge-(137) Dorsal–ventral patterning is governed by ectodermally expressed Wnt7a and engrailed-1

pro-teins and their coregulation of Lmx1b gene expression at the dorsal mesenchyme (138,139) Therefore,

patterning along the three axes is interlinked with each other

The limb cartilage elements form in a temporal proximal-to-distal sequence but are initially

con-tiguous (36) Through the gradual recruitment of cells, the primary condensation of the stylopod

(humorous/femur) forms first, the zeugopod (radius-ulna/tibia-fibula) forms second, and the autopod(carpals/tarsals and phalanges) forms last There is considerable mixing of cells along the proximal–distal axis within each future segment but not between segments Positional information is expressed

by determinants of the Hox family of genes The first part of the limb in which a subset of Hoxa and

Hoxd genes are activated is the posterior limb (140,141) Subsequently, the expression domains extend

anteriorly, in the distal part In the final stage of limb morphogenesis, the mesenchyme in the distalregion of the limb bud (autopod) can have two different fates, chondrogenesis or apoptosis, depend-ing on whether they are incorporated into the digital ray or into the interdigital regions There is nowconsiderable evidence to indicate that BMPs are essential mediators in specifying mesenchymal cells

undergoing either apoptosis or chondrogenesis and in the determination of digit identity (3,6,142–144).

This point will be elaborated further in the next section Finally, in regions of the mesenchymal sation where joints from, condensed chondroprogenitors do not differentiate into chondrocytes but

conden-instead become tightly packed and adopt a fate of apoptosis as part of the normal program (25,145,146).

Therefore, the orchestration of the apoptotic and chondrogenic response results in the formation anddelineation of the limb cartilaginous template Failure of either process results in limb malformationssuch as syndactyly or polydactyly of soft or hard tissues

BMP REGULATION OF LIMB

CARTILAGE DEVELOPMENT AND APOPTOSIS

BMPs are instrumental to the formation of the limb and are intimately involved in multiple stages

of limb development, including patterning, outgrowth, AER regression, digit formation, digit identity,and interdigital apoptosis To function in multiple developmental events, BMPs engage in signalingnetworks during limb morphogenesis and operate in concert with other key morphoregulatory factors,such as FGFs and Shh Furthermore, several BMPs are already present during early development.BMP2, BMP4, and BMP7 are expressed in the limb mesenchyme in overlapping patterns before the

formation of precartilagenous condensation (20) The specificity of BMPs for multistep action during

limb morphogenesis is also reflected by different expression profile of the receptor subtypes

trans-ducing the BMP signal (38) BMPs at an early stage regulate mesenchymal condensation into lage nodules, as well as the induction of the AER (147) At later stages, BMPs are responsible for the maturation of limb cartilage and the regression of the AER (148) In vitro evidence supports that exog- enous BMP enhances chondrogenesis in limb mesenchyme after the condensation step (149) Through

carti-their function in the maintenance of the AER and consequential regulation of limb outgrowth alongthe proximal–distal axis, BMPs also relay information and participate in interdependent developmen-

tal processes, such as patterning along the dorsal–ventral axis (150) There is little genetic evidence to

support the role of BMPs in limb development because target mutation in animal studies of BMPsand their receptors result in early lethality or lack of phenotype directly related to cartilage formation

(14,20,151) However, experiments using retroviral-mediated misexpression to simulate loss of

func-tion result in limbs that show a lack of Alcian blue stain cartilage elements (37,42) However,

infec-tion of the chick limb with retrovirus encoding BMP2, BMP4, or constitutively active receptor type

I to simulate BMP gain of function results in fusion and hyperplasia of the cartilage elements (37,152).

Mouse models show that BMP receptor type IB appears to be the necessary mediator of BMP-induced

chondrogenesis (39,40), although overexpression of the receptor or constitutive activation of the receptor can also cause apoptosis (37,153).

In addition to driving chondrogenesis, BMPs are key regulators of interdigital apoptosis that leads

to the delineation of the digits Among them, BMP2, BMP4, and BMP7 are expressed in the

inter-digital regions before and during the occurrence of apoptosis, suggesting a role in cell death (20).

Implantation of BMP4-soaked beads in interdigital regions accelerates interdigital cell death In

addi-tion, BMP4 can also cause ectopic cell death when applied at the tip of the developing digit pad (154) The apoptotic effect of BMP4 can be antagonized by FGF2 (155) Similarly, BMP2 and BMP7 are

potent apoptotic signals for the undifferentiated limb mesenchyme but not for the ectoderm or the

differentiating chondrogenic cells (156) Perturbations of BMP signaling through manipulation of

BMP receptors also result in aberrations in interdigital apoptosis For example, overexpressing nant-negative BMP receptors in chick leg bud via replication-competent retrovirus to block endog-enous BMP signals results in inhibition of apoptosis in the interdigital mesenchyme, which leads to

domi-webbed chick feet (37) Taken together, these results indicate that BMP signaling is necessary for the

apoptotic cascade in the interdigital mesenchyme Interestingly, in parallel with craniofacial apoptosis

as described in previous sections, Msx2 is also a mediator of BMP-induced interdigital apoptosis

(157) However, it is still unclear as to how Msx2 expression is instructive or permissive to apoptosis.

Therefore, the totality of limb development and the emergence of its intricate design are dependent inpart on BMP signaling in the larger context of many other growth and transcription factor signalingnetworks

Of particular interest are the role of retinoic acid and its interactions with BMP signaling and theircoregulation of limb development Retinoic acid is an endogenous morphogen at physiological levels

and a teratogen in excess Endogenous retinoids serve to pattern the hindbrain and the limb bud (158).

Excessive retinoids lead to retinoic acid embryopathy characterized by craniofacial abnormalities

(159) There are three distinct aspects of how retinoic acid modulates BMP signals First, retinoic acid

is well known for its ability to pattern the limb bud by virtue of its ability to substitute for the ZPA

and for its upregulation of Shh that is endogenous to the ZPA (160,161) In tandem, retinoic acid also upregulates BMP expression that is needed for anterior–posterior patterning event (162,163) Second, retinoic acid regulates interdigital apoptosis by activating BMP expression and activities (164) Third,

retinoic acid can also enhance chondrogenesis mediated by both BMP-dependent and

BMP-indepen-dent pathways (164,165) Therefore, the regulation of chondrogenesis and apoptosis by BMP may rest

on the ability of retinoic acid to divert BMP signaling to one pathway vs another, or the regulation byretinoic acid on distinct cofactors of BMP signaling for different pathways

SUMMARY AND FUTURE CHALLENGES

Studies from classical developmental models suggest that cell fate determination is a progressiveprocess that is dependent on combinatorial signaling of a repertoire of growth and differentiation fac-tor networks Signaling is modulated by restricted expression profiles of factors organized in precisetemporal and spatial arrays Signaling is gauged by checkpoints where rate-limiting factors determinethe threshold for progression Because BMPs are multifunctional factors, the challenge is to identifythe molecular basis for chondrogenic differentiation of mesenchymal cells Functional studies shouldestablish the mechanisms of lineage commitment and diversification, and provide a platform for molec-ular manipulations with predictable lineage outcomes This knowledge will provide the molecular basisfor tissue engineering and biomimetics of mesenchymal cells

ACKNOWLEDGMENTS

We are grateful to Dr Rocky Tuan for his support and encouragement We have benefited from

a long-standing scientific partnership with Dr Glen Nuckolls We have been blessed with ing visiting and postdoctoral scientists who had contributed to our knowledge base Finally, we areindebted to Dr Harold Slavkin, who continues to be an inspiration This work was supported by NIHfunding Z01AR41114

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From: The Skeleton: Biochemical, Genetic, and Molecular Interactions in Development and Homeostasis

Edited by: E J Massaro and J M Rogers © Humana Press Inc., Totowa, NJ

3

Regulation of Chondrocyte Differentiation Andreia M Ionescu, M Hicham Drissi, and Regis J O’Keefe

ENDOCHONDRAL OSSIFICATION: OVERVIEW

During the last decade, great progress has been made toward a better understanding of skeletaldevelopment, cartilage, and bone formation In particular, many mechanisms underlying a variety ofcellular and molecular processes that regulate growth and differentiation of chondrocytes, osteoblasts,and osteoclasts have been elucidated This chapter will review some of the molecular and genetic path-ways known to regulate cartilage development Skeletal formation occurs through both endochondraland intramembraneous ossification Flat bones and craniofacial bones are formed through intramem-braneous ossification that relies on osteoblast differentiation directly from mesenchymal stem cells.The axial and appendicular skeleton form through endochondral ossification, which requires the forma-tion of a cartilage intermediate that forms a template for osteoid deposition and bone formation Duringendochondral bone formation, mesenchymal stem cells differentiate into both chondrocytes and osteo-blasts During development of the long bone, growth plates localize to either end of the skeletal elementand the region of cartilage is surrounded by a perichondrium that is composed of undifferentiatedmesenchymal cells In the growth plates, chondrocytes undergo several stages of differentiation One

of the important transitions is from proliferation to hypertrophy, an event that precedes tion of the cartilage matrix (Fig 1) Chondrocyte hypertrophy is characterized by profound physicaland biochemical changes, including a 5- to 10-fold increase in volume and expression of alkaline phos-

mineraliza-phatase, type X collagen, and MMP-13 (1,2) Type X collagen is a short-chain collagen found only in

the hypertrophic zone of the growth plate Although its exact function remains unclear, mutations in

the colX gene have been found to cause Schmid metaphyseal chondrodysplasia (3), and transgenic mice with disruption in the colX gene exhibit a mild alteration of the growth plate architecture (4).

Alkaline phosphatase is essential for calcification of the matrix and is present in high concentration

in matrix vesicles, which are small membrane vesicles released by budding from the surfaces of

hyper-trophic chondrocytes into the surrounding matrix (5) Matrix vesicles are the initial sites of

mineral-ization in the hypertrophic region of the growth plate and are critical components of the calcificationprocess The calcified matrix subsequently serves as the template for primary bone formation In paral-lel, the perichondrium flanking the cartilage element differentiates into osteoblast-forming periosteum.Primary bone formation is initiated at the center of the cartilage template and results in the subse-quent formation of two separate regions of endochondral bone that develop at either end of the long bone.The growth plate is responsible for longitudinal growth of bones Both chondrocyte proliferation and

hypertrophy contribute to lengthening of the limb (6,7) Because terminally differentiated hypertrophic

cartilage is continuously replaced by bone, the tight regulation of the various steps of chondrocyte ferentiation, particularly proliferation and hypertrophy, is critical for balancing the growth and ossifi-cation of the skeletal elements

dif-Both local and systemic signaling molecules regulate endochondral ossification Here we reviewsome of the factors that regulate chondrocyte maturation, including parathyroid hormone-related pep-tide (PTHrP), Indian hedgehog (Ihh), transforming growth factor-` (TGF-`), and bone morphogeneticproteins (BMPs) Although these factors are also involved in the early stages of endochondral ossifi-cation, such as chondrogenesis and differentiation of precursor mesenchymal cells in chondrocyte, wespecifically address their role in the precise transition of chondrocytes from the proliferative phase tothe hypertrophic phase (Fig 1)

IHH/PTHRP SIGNALING LOOP:

A CLASSIC MODEL FOR CHONDROCYTE DIFFERENTIATIONS

The paradigm for regulation of endochondral ossification involves the Ihh gene Ihh expression

delineates the zone of prehypertrophic chondrocytes During limb development, secreted Ihh binds

to receptors located on perichondrial cells and influences the expression of PTHrP by periarticular

chondrocytes PTHrP subsequently signals back to the growth plate and regulates the rate of

chondro-cyte differentiation (Fig 2) It is still unclear whether Ihh controls PTHrP expression directly or

in-directly through TGF-` superfamily members Finally, it is possible that the role of Ihh in skeletal

Fig 1 Regulation of chondrocyte maturation Multiple factors control cell differentiation from mesenchymal

stem cells to hypertrophic chondrocytes, including members of the TGF-` superfamily and their downstream SMAD mediators, the homeodomain proteins and AP-1/CREB/ATF/Runx2 transcription factors Phenotypic genes corresponding to each step of chondrocyte maturation are also indicated.

development is primarily during embryonic growth and is less important postnatally, as we subsequentlydescribe.

Hedgehog proteins are a conserved family of secreted molecules that provide key signals in

embry-onic patterning in many organisms In vertebrates, there are three hedgehog genes: sembry-onic hedgehog (Shh), desert hedgehog (Dhh), and the aforementioned Ihh Shh functions in embryonic development

by controlling the establishment of left–right and anterior–posterior limb axes, Dhh functions as a spermatocyte survival factor in the testes, whereas Ihh is involved in endochondral ossification (8).

Hedgehog proteins signal through a transmembrane receptor called Patched (Ptc) and a transcription

factor called Gli In vivo overexpression of Ihh in chicken limb bud through retrovirally mediated

infection leads to shorter and broader skeletal elements with a continuous cartilage core that lacks

hypertrophic chondrocytes (9) In contrast, Ihh-deficient mice exhibit premature hypertrophic entiation, reduced chondrocyte proliferation, and failure of osteoblast development (10).

differ-The phenotype of Ihh misexpression in either chicken limb bud or in transgenic mice is similarwith the phenotype of PTHrP misexpression Animals that overexpress PTHrP exhibit delay in chon-

drocyte terminal differentiation (11) Humans with an activating mutation in the PTH/PTHrP

recep-tor have Jansen’s metaphyseal chondrodysplasia, characterized by disorganization of the growth plate

and delayed chondrocyte terminal differentiation (12) In contrast, mice null for either PTHrP (13)

or its receptor (14) display accelerated chondrocyte differentiation and abnormal endochondral bone

treated with hedgehog protein in culture demonstrate that intact PTHrP signaling is required to mediate

the inhibitory effect of Ihh on chondrocyte differentiation (9) These findings established the following

Fig 2 The role of Ihh/PTHrP in endochondral ossification Chondrocyte cell differentiation is associated

with expression of specific genes involved in the regulation of chondrocyte maturation Ihh is expressed in the hypertrophic chondrocytes and signals through TGF-`2 located in the perichondrium to enhance transcription of the PTHrP gene in the periarticular region.

mechanism for the Ihh/PTHrP regulation of chondrocyte differentiation: Ihh is produced by the hypertrophic chondrocytes, signals through Ptc and Gli in the adjacent perichondrium, and inducesexpression of PTHrP in the periarticular region In turn, PTHrP would diffuse across the growth plate,bind to its receptor, which is expressed in the prehypertrophic chondrocyte, and subsequently delaychondrocyte maturation.

pre-One of the elusive aspects of this signaling pathway is the signals that are generated by ulated perichondrium to influence PTHrP expression in the periarticular cartilage Indeed, removal

Ihh-stim-of perichondrium causes inability Ihh-stim-of hedgehog signaling to delay chondrocyte hypertrophy (15) Other

studies have shown that removal of perichondrium results in an extended zone of cartilage expressing

colX and an extended zone of cartilage incorporating BrdU, indicating that perichondrium negatively

regulates both proliferation and hypertrophy of chondrocytes (16) Possible candidates as Ihh

secon-dary signals are the members of the TGF-` superfamily, namely the BMPs and TGF-` isoforms 1–3

ROLE IN CHONDROCYTE DIFFERENTIATIONS

TGF-` family members are considered possible mediators of Ihh signaling on PTHrP expression.TGF-` enhances PTHrP expression in both mouse metatarsal explants (17) and isolated chondrocytes

cultures (18,19) Similarly, intact PTHrP signaling appears to be required for of TGF-`-mediated effects

on chondrocyte hypertrophy (17) Furthermore, transgenic mice overexpressing a dominant-negative

TGF-` receptor exhibited a very similar phenotype to the PTHrP knockout mice Both mouse models

present an accelerated chondrocyte differentiation (17,20) Finally, inhibition of TGF-` signaling in

the perichondrium, with a dominant-negative type II receptor, also inhibits the effects of hedgehog

on chondrocyte differentiation and on induction of PTHrP expression (15) The dominant-negative

type II receptor inhibits all three TGF-` isoforms

Alvarez et al recently showed that treatment of mouse metatarsals cultures with hedgehog protein

leads to upregulation of TGF-`2 and TGF-`3 isoforms but not TGF-`1 in the perichondrium

Fur-thermore, the effects of hedgehog protein signaling are specifically dependent on TGF-`2 because tures from TGF-`3-null embryos respond to hedgehog protein signaling but cultures from TGF-`2-

cul-null embryos do not (15) Altogether, the evidence suggests that TGF-`2 acts as a signal messenger

between Ihh and PTHrP in the regulation of cartilage hypertrophic differentiation in embryonic opment Although the phenotype of the dominant-negative type II TGF-` receptor resembles the pheno-types of Ihh and PTHrP knockout mice, the skeletal malformations are still less severe, suggestingthat, TGF-` may not be the only mediator of Ihh on PTHrP expression

devel-The role of Ihh/PTHrP signaling loop after embryonic development is not clearly established Ithas been suggested that this pathway is not operational in neonatal mice because of low or absent

levels of Ihh expression in the postnatal growth plate (21) Also for postnatal development, the

physi-cal distance between the periarticular region and the growth plate increases dramatiphysi-cally, and theability of PTHrP to diffuse to these tissues is questionable A possible signaling scenario involvingTGF-` and PTHrP in postnatal development has been defined by our group (22) All of the necessaryelements for a signaling pathway involving TGF-` and PTHrP are existent in the growth plate, elimi-nating the need for diffusion The growth plate makes large amounts of TGF-` and PTHrP expres-sion is induced up to 20-fold in chondrocytes stimulated with TGF-` (18) Because TGF-` is secreted

in an inactive complex, a possible source for TGF-` could be from hypertrophic cartilage duringremodeling Matrix metalloproteinase 13 (MMP-13) is highly expressed in the hypertrophic regionand has been shown to activate latent TGF-` (23) Another source is the zone of calcification as osteo-clasts have been also been shown to activate TGF-` (24) During matrix catabolism, TGF-` is activatedfrom the latency-associated peptide (LAP) Activated TGF-` then acts in an autocrine/paracrine man-

ner to stimulate the expression of PTHrP in the growth plate (18) The elevated expression of PTHrP

then slows the rate of chondrocyte differentiation This leads to a decrease in the terminal

differentia-tion and a fall in the activadifferentia-tion of TGF-`, which results in a reacceleradifferentia-tion in the rate of chondrocytedifferentiation, with a subsequent increase in the release of active TGF-` from the matrix This positivefeedback loop between TGF-` and PTHrP in the growth plate results in a cycling of differentiationfrom an on or off state, similar to the effect of the Ihh/PTHrP signaling pathway during development.Interestingly, although animals overexpressing dominant-negative TGF-` receptors and the deletion

of Smad3, a critical transcription factor downstream of TGF-`, both develop premature maturation, the

phenotype only becomes evident postnatally (25) This suggests that unique signals might be present

during limb development and postnatal growth and that TGF-` might be particularly important in thislatter process

BMP REGULATION OF CHONDROCYTE DIFFERENTIATION

Similar to the TGF-` family, BMPs are key regulators of organogenesis in early embryonic ment because they also play an active role in regulating cartilage formation and differentiation during

develop-later stages BMP-2, BMP-3, BMP-4, BMP-5, and BMP-7 all are expressed in the perichondrial cells

(26) Although BMP-4 and BMP-7 are expressed at low levels by chondrocytes undergoing maturation (27), BMP-6 and BMP-7 are highly expressed by hypertrophic chondrocytes (9) Finally, BMP-2, BMP-4,

and BMP-7 are expressed in the precartilaginous mesenchyme (28) and were shown to enhance genesis in vitro (29) and in vivo in chick limb buds (30) Regulation of BMP-2 and BMP-4 expression

chondro-by Ihh (31) further indicates that the interplay between these various signals in vivo increases the

intricacy by which chondrocyte growth and differentiation is regulated

The role of BMP signaling in chondrocyte differentiation has been somewhat controversial because

of disparate effects that occur in vitro in isolated chondrocyte cell cultures and in in vivo in the chicklimb bud model Cell culture studies in various models all demonstrate an induction in chondrocyte

maturation with gain of BMP signaling and a decrease in maturation with loss of function (32,33) In

contrast, in the chick limb bud, overexpression of activated type I BMP receptors inhibits

chondro-cyte maturation (26) However, our laboratory has recently demonstrated that activated BMP ing induces Ihh expression in chondrocytes, with a subsequent increase in PTHrP expression in the periarticular region (34) This activation of the Ihh/PTHrP pathway likely explains the differential

signal-effects of BMP signaling on isolated chondrocytes compared with the developing limb Whereas BMPsact to stimulate chondrocyte maturation directly, induction of Ihh/PTHrP signaling through paracrine-mediated events has the opposite effect and inhibits maturation The findings suggest that BMP sig-naling is integrated into the Ihh/PTHrP signaling loop and that the ultimate effect is caused by a finebalance of BMP signaling

INTEGRATION OF MULTIPLE SIGNALING PATHWAYS:

COMBINATORIAL REGULATION OF CARTILAGE MATURATION

The growth factors and signaling molecules that control chondrocyte differentiation are nected and center on interactions between the Ihh/PTHrP loop and the TGF-` superfamily members.Although a significant amount of information about their role in skeletal development is available,less is known about the mechanisms by which their signaling pathways affect potential targets in thegrowth plate Recently, we and others have characterized some of the transcriptional mechanisms down-

intercon-stream of these pathways (35–38) It is indeed necessary to define how interactions between these

signaling events result in progression toward the hypertrophic phenotype

PTHrP Signaling

PTHrP signaling increases cAMP and calcium levels in chondrocytes, with subsequent activation

of protein kinases A and C (PKA and PKC; refs 39–41) Potential downstream targets of PKA/PKC

sig-naling are the transcription factors cAMP response element binding protein (CREB) and activator tein 1 (AP-1) CREB is a member of the ATF/CREB family of transcription factors, which is activated

pro-in response to cAMP/PKA signals CREB bpro-inds constitutively to DNA at a consensus cAMP response

element primarily as a homodimer via a leucine zipper domain (42) Activation occurs secondary to

a phosphorylation event at Ser133 This results in the recruitment of the coactivator CBP binding protein) and activation of the transcriptional machinery The AP-1 complex is formed throughdimerization between Fos and Jun family members Subsequently, AP-1 binds DNA response elementsknown as TPA response elements

(CREB-Although PTHrP does not alter CREB protein levels or DNA binding, it stimulates kinases that

activate CREB by stimulating phosphorylation at Ser133 (35) In addition, PTHrP induces c-Fos

pro-tein production and enhances AP-1 binding to its consensus element This stimulation of CREB andAP-1 signaling is associated with an increase in gene transcription while inhibition of their signaling

leads to inhibition of PTHrP effects on both proliferation and maturation of chondrocytes (35) Thus,

the transcription factor CREB has a role in skeletal development through involvement in PTHrP naling and direct regulation of chondrocyte differentiation

sig-In addition to PTHrP signaling through cAMP, several other growth factors, including insulin-likegrowth factor, epidermal growth factor, TGF-`, fibroblast growth factor, and platelet-derived growth

factor have also been shown to promote the phosphorylation of CREB family members (42,43),

suggesting a potential role for these factors in cell proliferation Therefore, it is possible that CREBcan transduce signals of other signaling pathways and its role in skeletal development is more gener-alized than just PTHrP signaling This idea is supported by transgenic animal models exhibiting dis-

ruptions in the CREB gene CREB-null mice targeting all isoforms (44) have been generated and are

smaller than their littermates and die immediately after birth from respiratory distress This is similar

to the fate of PTHrP knockout animals, which are runted and also die in the neonatal period of tory distress Nevertheless, the skeleton of the CREB-null mice has not been analyzed and the causesfor the observed dwarfism are not yet investigated The function of CREB in skeletal formation alsowas assessed through the generation of transgenic mice in which a dominant-negative form of CREB

respira-(A-CREB; ref 45) was driven by the cartilage-specific collagen type II promoter/enhancer in the growth plate chondrocytes (46) A-CREB transgenic mice show short-limb dwarfism and a markedly reduced

rib cage that may underlie their perinatal lethality Consistent with a pronounced defect in growthplate development, tibias from transgenic embryos were bowed and exhibited asymmetric deposition

of cortical bone beneath perichondrium The proposed cause for the severe dwarfism was inhibition ofproliferation In agreement with previous studies, the expression of the dominant-negative CREBinhibits proliferation, lowering the proportion of BrdU-positive cells in the growth plate and reducingthe height of the proliferative zone in developing limbs However, contrary to the initial hypothesis,the A-CREB transgenic mouse limbs exhibit delay of maturation accompanied by delay of vascular-

ization and bone formation (46) In contrast, in isolated chick chondrocytes cultures, the same nant-negative CREB accelerates the process of maturation and inhibits PTHrP (35) Similarly, inhibition

domi-of PTHrP signaling through disruption domi-of the hormone, its receptor, or Gs_ signaling leads to premature

hypertrophy in the growth plate (13,47,48) The discrepancy between the different studies may reside

in the difference between the two systems used Studies performed in a cell culture system allow bation of CREB signaling at later stages of chondrocyte development In contrast, in the transgenic

pertur-mice, Long et al (46) used a system in which the transgene is overexpressed at very early embryonic

stages Therefore, the phenotype might reflect the effect of perturbation of CREB signaling at earlierstages of skeletal development, such as chondrogenesis, cell aggregation, and nodule formation or tran-sition from precursor (mesenchymal) cells to chondrocytes The alterations in these steps may have

an overall negative impact on the normal progression of skeletal development by causing delay in thecartilage anlage formation

TGF-````` Signaling

TGF-` receptor binding results in activation of the TGF-` type I receptor with phosphorylationevents that activate downstream signaling pathways, including the Smad family of transcription fac-