cells Review TGFβ/BMP Signaling Pathway in Cartilage Homeostasis Nathalie G.M. Thielen , Peter M. van der Kraan and Arjan P.M. van Caam * Experimental Rheumatology, Radboud University Medical Center, Geert Grooteplein 28, 6525 GA Nijmegen, The Netherlands * Correspondence: [email protected]; Tel.: +31-24-10513; Fax: +31-24-3540403 Received: 2 July 2019; Accepted: 19 August 2019; Published: 24 August 2019 Abstract: Cartilage homeostasis is governed by articular chondrocytes via their ability to modulate extracellular matrix production and degradation. In turn, chondrocyte activity is regulated by growth factors such as those of the transforming growth factor β (TGFβ) family. Members of this family include the TGFβs, bone morphogenetic proteins (BMPs), and growth and differentiation factors (GDFs). Signaling by this protein family uniquely activates SMAD-dependent signaling and transcription but also activates SMAD-independent signaling via MAPKs such as ERK and TAK1. This review will address the pivotal role of the TGFβ family in cartilage biology by listing several TGFβ family members and describing their signaling and importance for cartilage maintenance. In addition, it is discussed how (pathological) processes such as aging, mechanical stress, and inflammation contribute to altered TGFβ family signaling, leading to disturbed cartilage metabolism and disease. Keywords: transforming growth factor β; bone morphogenetic proteins; osteoarthritis; cartilage; SMADs; aging; joint loading; inflammation; linker modifications 1. Introduction The transforming growth factor β (TGFβ) family of polypeptide growth factors controls development and homeostasis of many tissues, including articular cartilage. Articular cartilage is the connective tissue covering joint surfaces and is a type of hyaline cartilage. This tissue is key in facilitating movement with its smooth lubricated surface, and it functions as a shock absorber to disperse forces acting upon movement with its physical properties. Articular cartilage is of mesodermal origin and, remarkably for a tissue, is neither innervated nor does it contain blood vessels to supply oxygen or nutrients [1,2]. The only cell type present in this tissue is the chondrocyte, which is surrounded by a large amount (up to 98% of cartilage volume) of extracellular matrix (ECM) [1,3]. The ECM is a highly organized network of hyaluronan, proteoglycans, and collagens. These collagens provide (tensile) strength and shape to the tissue [4], and they mainly are (up to 90%) collagen type II (COL2A1) fibrils [5]. Hyaluronan is a glycosaminoglycan (GAG) (i.e., a long, unbranched polysaccharide), which forms spontaneously aggregating networks to which other biomolecules such as proteoglycans can bind [6]. Proteoglycans are proteins glycosylated with sulfated GAGs. The most abundant proteoglycan (in mass) in articular cartilage is aggrecan (ACAN), and this proteoglycan forms large aggregates with hyaluronan. Because ACAN is heavily glycosylated with negatively charged sulfated GAGs, these aggregates generate a (static) charge density. This charge density generates the force required to counteract compressive forces during movement via attraction of solutes, which generate osmotic swelling pressure, and via electrostatic repulsion if aggrecan molecules come in close proximity [7]. Articular cartilage is a highly structured tissue in which form fits function. Five distinct zones can be observed: the lamina splendens, the superficial zone, the middle zone, the deep zone, and Cells 2019, 8, 969; doi:10.3390/cells8090969 www.mdpi.com/journal/cells Cells 2019, 8, 969 2 of 30 Cells 2019, 8, x FOR PEER REVIEW 2 of 29 thecalcified calcified zone zone (Figure (Figure 1). 1The). The lamina lamina splendens splendens consists consists of collagen of collagen fibrils fibrils oriented oriented in parallel in parallel to the to thesurface surface and and high high levels levels of ofhyaluronan hyaluronan and and the the lubr lubricatingicating proteoglycan proteoglycan lubricin lubricin (PRG4), (PRG4), but itit containscontains nono cells cells [8 –[8,9,10]10]. These. These characteristics characteristics provide provide a smooth a smooth surface withsurface a low with friction a low coe frictionfficient. Belowcoefficient. the laminaBelow splendensthe lamina liessplendens the superficial lies the zone,superficial which zone, contains which a relativelycontains a high relatively number high of cellsnumber with of flattenedcells with morphology flattened morphology oriented in oriented parallel in to parallel the surface, to the which surface produce, which highproduce amounts high ofamounts hyaluronan of hyaluronan and PRG4 and [8 PRG4,10]. The[8,10] collagen. 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This resistance compression to further is dueto compression an increase inis electrostaticdue to an increase repulsion in betweenelectrostatic proteoglycans repulsion between and also toproteoglycans an increase inand hyaluronan also to an increase viscosity in because hyaluronan of an viscosity increase because in concentration of an increase of this in moleculeconcentration [11]. Underneathof this molecule the middle [11]. Under zone liesneath the the deep middle zone ( zone30%). lies In the deepdeep zone,zone chondrocytes(±30%). In the are deep organized zone, ± inchondrocytes columns placed are organized between in collagen columns fibrils placed oriented between perpendicularcollagen fibrils tooriented the cartilage perpendicular surface to [6 the,9]. Furthermore,cartilage surface this [6,9] zone. Furthermore, contains relatively this zone the highest contains levels relatively of aggrecan the highest of all zones levels [1 of]. Theaggrecan highlevels of all ofzones aggrecan [1]. The restrict high water levels flow of andaggrecan thereby restrict increase water the compressive flow and thereby stiffness increase of cartilage. the Thecompressive collagen fibersstiffness are of oriented cartilage. to The withstand collagen the fibers swelling are oriented pressure to withstand generated the by swe theselling proteoglycans pressure generated and to withstandby these proteoglycans sheer stresses and generated to withstand by compression sheer stresses of thegenerated layers above.by compression Part of the of deep the layers zone, above. below thePart tidemark, of the deep contains zone, calciumbelow the salts tidemark, in the ECM contains and is,calcium thus, called salts in the the calcified ECM and zone is [,2 thus]. The, called calcified the zonecalcified forms zone the [2] interface. The calc betweenified zone bone forms and cartilagethe interface and between anchors cartilagebone and to cartilage the bone. and This anchors zone containscartilage hypertrophic-liketo the bone. This chondrocytes zone contains which hypertrophic resemble chondrocytes-like chondrocytes undergoing which endochondral resemble ossificationchondrocytes [12 undergoing]. endochondral ossification [12]. Figure 1. Appearance and structure of adult (bovine) articular cartilage. Note that the orientation of collagenFigure 1. fibrils Appearance differs perand zone, structure from of parallel adult in(bovine) the superficial articular zone cartilage. to perpendicular Note that the in theorientation deep zone. of Scalecollagen bar fibrils= 100 µdiffersm. per zone, from parallel in the superficial zone to perpendicular in the deep zone. Scale bar = 100 µm. The importance of articular cartilage for joint function is illustrated by the effects of its loss, which leadsThe to disabilityimportance and of pain,articular for example,cartilage infor the joint word’s function most is commonillustrated joint by diseasethe effects osteoarthritis of its loss, (OA).which Degenerationleads to disability and loss and of articularpain, for cartilage example, are in the the consequence word’s most of an common imbalance joint inanabolic disease (e.g.,osteoarthritis ECM production) (OA). Degeneration and catabolic and (e.g., loss of ECM articular degradation) cartilage processes. are the consequence Chondrocytes, of an as imbalance the only cellin anabolic type present, (e.g., ECM are essential product forion) balancing and catabolic these ( processes.e.g., ECM degradation) Therefore, chondrocyte processes.