With the aging of the population and the increasing prevalence of fitness disorders worldwide, an increasing number of people suffer from articular diseases. As a common symptom, joint pain can cause people to experience physical and mental distress, which can cause great inconvenience in daily work and life. Osteoarthritis (OA) is one of the most common triggers of joint pain, and a worse condition will eventually occur if scientific and timely treatment is unavailable[1]. It is estimated that 240 million people are affected by OA globally and that a medical cost of 300 billion dollars is related to OA annually[2, 3]. OA mainly features degeneration of cartilage, which is the earliest and most important pathological change[4]. As a kind of connective tissue, cartilage plays a pivotal role in physiological functions such as dispersal of pressure, reduction of friction and protection of joints[5]. Histologically, the chondrocytes and cartilage matrix constituted the cartilage tissue. The metabolism of chondrocytes is dependent on the compression and dilation of the cartilage matrix under physiological pressure[6]. The exchange of nutrients and metabolic waste will be affected when the cartilage matrix is destroyed. However, there are no available blood vessels or nerve or lymph tissue in the cartilage matrix, which puts cartilage at a slow speed of repair and regeneration when damaged[7]. In current therapeutic strategies, osteochondral allografts, which include cartilage and subchondral bone transplantation, are usually employed to repair damaged cartilage[8]. Nevertheless, cartilage transplantation is not pervasive in clinical treatment because of the limited availability and potential transmission of diseases[9].
To compensate for these shortcomings of cartilage transplantation, tissue-engineered cartilage has been fabricated and improved rapidly over the past decades[10]. Multiple types of scaffolds have been developed to mimic the cartilage matrix, which is composed of an amorphous matrix and fibers. Among these materials, hydrogels are materials with three-dimensional crossed-linked structures that are similar to those of the cartilage matrix[11]. The common materials used to fabricate hydrogels include synthetic polymers, polysaccharides, proteins and peptides[12]. While all of these materials cut both ways, on the bright side, more water could be locked into the hydrogel, and a similar natural extracellular matrix would be provided. Moreover, the porous structure of hydrogels allows free diffusion of small molecules, conveniently promoting the exchange of nutrients and metabolic waste[13]. On the dark side, the general application of hydrogels is limited by poor mechanical properties, biological inertness and low stability, among other factors[14]. Among these candidates, hydrogels based on proteins, including collagen, gelatin and silk fibroin, have attracted increasing amounts of attention because of their structural similarity to natural cartilage[15]. For instance, a collagen scaffold treated with hyaluronic acid transglutaminase has been proven to aid in cartilage regeneration in mature sheep[16]. However, most of the existing proteins are produced by animals and have several shortcomings, such as laborious fabrication processes and potential immune rejection. In recent years, natural plant proteins have attracted increasing amounts of attention because of their wide range of sources and beneficial effects on health[17]. Zein in corn, soyproteins and wheat proteins are the major proteins in plants. Plant proteins have more negative charges than collagen and can be designed to convey drugs with positive charges. Moreover, plant proteins contain more polar amino acids and are more hydrophilic, which is helpful for attracting cells[18]. Wheat protein (WP) is attracting increasing amounts of attention for its unique nutritional value. It has been proven that WP has anti-aging effects via antioxidant activities, anti-inflammatory and immune-protective activities, intestinal protection and microbial regulation, among other activities[19]. In the field of regenerative medicine, electrospun WP scaffolds exhibit desirable water stability and three-dimensional structures for tissue engineering[20]. For example, according to previous studies, nano-MgO phosphate/WP composites and mesoporous magnesium silicate/polycaprolactone/WP composite scaffolds could promote the osteogenic differentiation of mesenchymal stem cells and have good biodegradability[21, 22]. Therefore, it is possible to use WPs as substitutes for cartilage grafts.
Magnesium silicate (MS) is a synthesized silicate that consists of silicon and magnesium. The MS composite has been studied in the field of bone fracture healing and ligament grafting, and it has shown satisfactory performance[23, 24]. Silicon can promote the synthesis of collagen and proteoglycans in bone and cartilage[25]. A monolayer of phosphonates treated with silicon dioxide has been proven to promote bovine chondrocyte adhesion[26]. In addition, silicon-based bioceramic scaffolds could promote the regeneration of cartilage and subchondral bone and maintain the chondrocyte phenotype via the TGF-β signaling pathway[27]. Magnesium is a necessary element of the human body that affects several physiological functions, such as aging, the immune response and bone health[28]. Magnesium can promote the proliferation and redifferentiation of chondrocytes, and a lack of magnesium leads to fibroblast senescence[29]. Calcification of the extracellular matrix could be ameliorated by Mg via inhibition of Erk phosphorylation signaling and the induction of autophagy[30].
Glucosamine (GA) is a basic ingredient of the cartilage matrix and synovial fluid and is also a crucial nutrient for chondrocytes[31]. Many chondroitin sulfates, which are glycosaminoglycans, construct molecular sieves with abundant micropores. This structure allows the exchange of nutrients and metabolites around chondrocytes, which maintains the physiological metabolism of chondrocytes deep inside cartilage[32]. The high bioactivity of GA promotes the synthesis of proteoglycans and restores the activity of damaged chondrocytes[33]. The synthesis of collagen and hyaluronic acid can be promoted, and damaged articular cartilage can be repaired by GA[34]. In addition, GA prompts synoviocytes to produce lubricating fluid, which could relieve friction on the articular surface.
To the best of our knowledge, no studies have explored the synergistic effects of WP, MS and GA on cartilage regeneration. In the present study, WP was selected for use as the basic material, and MS and GA served as bioactive factors. The WP/MS (MSW) composite was prepared by the sol‒gel method and was loaded with GA to fabricate the MSWG composite. The objective of this study was to fabricate a new kind of artificial hydrogel composite close to the natural structure of cartilage to promote the repair of cartilage defects. The MSW and MSWG composites were characterized by observing and analyzing their morphology, mechanical strength, chemical structure and degradation. The in vitro biocompatibility and biological activities of MSW and MSWG were subsequently evaluated. The formation of new cartilage and bone tissue around defects after the implantation of MSWG was also assessed by radiology and histology methods.