**1. Introduction**

Articular cartilage can withstand compressive, tensile and shear loading and provides efficient articulation of diarthrodial joints. If left untreated, damaged articular cartilage in a joint can lead to osteoarthritis (OA) and ultimately joint failure [1,2]. Cell-based therapies have been developed to promote cartilage repair and the regeneration of complex articular structure to help patients with damaged or degenerate cartilage [3,4].

It is commonly reported that adult articular cartilage has a limited capacity for selfregeneration [5]; however, a growing body of evidence from in vitro and in vivo models suggests that in some cases, cartilage can undergo some form of natural repair [6,7]. A

**Citation:** Garcia, J.; McCarthy, H.S.; Kuiper, J.H.; Melrose, J.; Roberts, S. Perlecan in the Natural and Cell Therapy Repair of Human Adult Articular Cartilage: Can Modifications in This Proteoglycan Be a Novel Therapeutic Approach? *Biomolecules* **2021**, *11*, 92. https://doi.org/10.3390/biom11010092

Received: 24 December 2020 Accepted: 11 January 2021 Published: 13 January 2021

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bovine explant model of cartilage healing showed that both young and mature animals produced an outgrowth of tissue from the artificially damaged sites, but with young tissues generating more hyaline-like cartilage [6]. In humans, magnetic resonance imaging (MRI) observation of the knees of healthy subjects showed that some cartilage defects (tibial and patellar) reduced in size or were completely filled between a baseline scan and a two year follow up [7]. A natural healing response was also seen in some cartilage lesions of subjects with anterior cruciate ligament (ACL) injuries 6–56 months after reconstructive surgery to repair the ligament damage [8]. The mechanisms by which articular cartilage repairs itself is poorly understood, but is believed to involve an interplay between cellular, biochemical and mechanical factors [9–11].

Perlecan, also known as heparan sulphate proteoglycan-2, is a modular, multifunctional proteoglycan with an ability to promote chondrocyte proliferation, differentiation and matrix synthesis through its interactions with a large repertoire of ligands including growth factors, morphogens and extracellular matrix (ECM)-stabilising glycoproteins [12,13]. One of the glycosaminoglycans contained in perlecan, heparan sulphate (HS), is a vital extracellular component. Its cleavage causes matrix remodelling through the release of HS-bound cytokines, growth factors, morphogens, proteases and inhibitory proteins which regulate many cellular pathological and physiological processes [14,15]. Perlecan, through its HS chains, has chondrogenic properties and is able to regulate cell signalling, matrix assembly and new tissue formation [12,16,17]. These attributes make perlecan an important candidate molecule when trying to understand how cartilage repairs itself. Hence, harnessing these attributes could also be beneficial in promoting the repair of damaged articular cartilage in human joints. Heparanase is an endo-β-glucuronidase cleaving the β(1,4)-glycosidic linkages between GlcN and GlcA in heparan sulphate (HS), and is the only known mammalian enzyme displaying this glycolytic activity [18].

Interactions between perlecan and collagen type VI have been well established [19] and, like perlecan, collagen type VI is believed to be involved in chondrocyte adhesion, integrity and matrix interactions [20,21]. Collagen type III is another minor collagen found in articular cartilage and has been suggested to have a role in reinforcing the cartilage matrix as part of a healing response to matrix damage [22,23].

In the present study, we have immunolocalised perlecan and types III and VI collagen for the first time in donor-matched samples of naturally and cell therapy repaired articular cartilage of the human knee. We have also investigated whether the phenotype and proliferation of cultured human chondrocytes was affected by the removal of cell surface HS. We hypothesise that the distribution of perlecan in repaired adult cartilage mimics its distribution in embryological cartilage.
