Characteristics of Small Leucine-rich Proteoglycans in the Intervertebral Disc Degeneration
Qingshen Wei1 and Yuntao Zhang*2
1Department of Orthopedics Surgery, Rizhao Traditiopnal Chinese Medicine Hospital, China
2School of Pharmaceutical Science, Jining Medical University, China
Submission: February 22, 2018; Published: March 07, 2018
*Corresponding author: Yuntao Zhang, School of Pharmaceutical Science, Jining Medical University, Shandong, China, Tel: +86 633 2983688; Email: zhangyt16@gmail.com
How to cite this article: Qingshen Wei, Yuntao Zhang. Characteristics of Small Leucine-rich Proteoglycans in the Intervertebral Disc Degeneration. Anatomy Physiol Biochem Int J: 2018; 4(3): 555640. DOI: 10.19080/APBIJ.2018.04.555640.
Abstract
The intervertebral disc (IVD) is important in the normal functioning of the spine. It is a cushion of fibrocartilage and the principal joint between two vertebrae in the spinal column and is responsible for spinal motion and load distribution. Small leucin e-rich proteoglycans (SLRPs) are the major bioactive components of the extracellular matrix (ECM) of intervertebral disc and associated with fibrillogenesis, cellular growth and apoptosis and tissue remodelling. The most significant biochemical change to occur in disc degeneration is loss of proteoglycans (PGs). A more in-depth understanding of molecular basis of disc degeneration is essential to the design of therapeutic solutions to treat degenerative disc. This review focuses on the SLRP biochemical characteristics in the intervertebral disc degeneration.
Keywords: SLRPs; Proteoglycans; Glycosaminoglycans; Intervertebral discs; Degeneration
Abbrevations: SLRPs: Small Leucine-Rich Proteoglycans; PGs: Proteoglycans; GAGs: Glycosaminoglycans; KS: Keratan Sulfate; CS/DS: Chondroitin Sulfate/ Dermatan Sulfate; HS: Heparan Sulfate; IVD: Intervertebral Disc; ECM: Extracellular Matrix; AF: Annulus Fibrosus; NP: Nucleus Pulposus; LBP: Low Back Pain; PRELP: Proline/arginine-rich end Leucine-rich End Leucinerich Repeat Protein; LRPs: Leucine-Rich Repeats; MMPs: Matrix Metalloproteinases; TIMPs: Tissue Inhibitors of Metalloproteinases
Introduction
The intervertebral discs (IVD) are partially movable joints that connect each of the vertebral bodies in the spine, functioning both to transfer loads and impart mobility [1]. Intervertebral discs are composed of an annulus fibrosus (AF) and a nucleus pulposus (NP). The AF is a strong radial tire–like structure made up of lamellae; concentric sheets of collagen fibers connected to the vertebral end plates. The extracellular matrix (ECM) of the central NP contains large quantities of the proteoglycans (PGs). Degeneration of the IVD is strongly implicated as a major cause of low back pain (LBP) [2,3]. Disc degeneration has been found to be associated with the loss of PGs function [4]. The etiology of disc degeneration has proven challenging to characterize because it is poorly defined and its progression is closely linked to aging [5]. Current knowledge of the principal pathogenesis resulting in this condition is limited.
Proteoglycans are macromolecules consisting of a protein core and glycosaminoglycans (GAGs) side-chains [6]. GAGs are unbranched carbohydrate chains of repeating disaccharide units. Since GAGs are negatively charged, they bind to other matrix molecules, cell adhesion molecules, and growth factors [7]. PGs can be divided into two classes: one class is the small leucine-rich proteoglycans (SLRPs) such as decorin, biglycan, fibromodulin, lumican, and mimican; another family consists of aggrecan, versican, brevican, and neurocan [6]. In this review, we discuss the biochemical characteristics of SLRPs in the intervertebral disc degeneration. Given the recent study that implicates SLRPs as the key components for IVD degeneration progression.
SLRPs Classification
The class of SLRPs is a family of homologous proteoglycans harboring relatively small (36–42 kDa) protein cores harboring tandem leucine-rich repeats and undergoing post-translational modifications, including substitution with glycosaminoglycans (GAGs) side chains of various types [8, 9]. Originally, the SLRPs were grouped into three distinct classes based on nucleotide and protein sequence conservation, the organization of disulfide bonds at their N and C termini, and their genomic organization. More recently, the SLRPs gene family has expanded to encompass 18 genes classified into five distinct subfamilies by common structural and functional properties [10] (Figure 1). SLRPs are proteoglycans that have both protein cores and GAGs chains, although non-canonical class IV and V SLRPs that do not contain any GAGs are also included in this family. The first class has a unique N-terminal Cys sequence. This includes decorin, biglycan and asporin, which are encoded by genes composed of eight exons with intron junctions in highly conserved positions [11]. The second class is comprised of five sub-members, including fibromodulin, lumican, keratocan, proline/arginine-rich end leucinerich repeat protein (PRELP) and osteoadherin, which have an identical cysteine-rich region before the leucine-rich repeats (LRRs) [12, 13, 14]. This class of SLRPs is characterized by clusters of Tyr sulfate residues at their N-termini and contains primarily keratan sulfate chains and polylactosamine [15].
The GAGs of SLRPs are differentially processed in development and aging, and are variable with regard to size, number, sulfation and epimerization in different tissues [16]. Through O-linked oligosaccharide, chondroitin sulfate/dermatan sulfate (CS/ DS) chains are attached to core protein decorin [17,18]. In the case of decorin, a single CS/DS linkage site is present near the amino terminus of the core protein [19, 20], whereas lumican and keratocan possess four or five potential keratan sulfate (KS) attachment sites in the central leucine-rich repeat region of each core protein molecule [21,22,23], and mimecan has two potential KS attachment sites [24, 25]. Current molecular models of the corneal stroma suggest that these proteoglycan core proteins wrap themselves laterally around the collagen fibrils in a manner that folds their hydrophobic domains inside, against the collagen fibrils [26]. In contrast, the highly sulfated GAG chains (together with their associated water molecules of hydration) are thought to stick out laterally away from the sides of the collagen fibrils, forming an exterior hydrophilic shell. The thickness of that shell matches the thickness of the shell surrounding adjoining fibrils, producing a very precise centerto- center spacing between the collagen fibrils characteristic of the corneal stroma and necessary for its transparency [27]. Through this interaction with collagen (mostly with type I), PGs play important biological roles in collagen fibrillogenesis and matrix assembly.
Structure of the Intervertebral Disc
The intervertebral discs lie between the vertebral bodies, linking them together (Figure 2). They are the main joints of the spinal column and occupy one-third of its height. Their major role is mechanical, as they constantly transmit loads arising from body weight and muscle activity through the spinal column. They provide flexibility to this, allowing bending, flexion and torsion. They are approximately 7–10mm thick and 4cm in diameter (anterior–posterior plane) in the lumbar region of the spine [2,5]. Intervertebral discs consist of an outer fibrous ring, the AF disci intervertebralis, which surrounds an inner gel-like center, the AP [5]. The AF is a strong radial tire–like structure made up of lamellae; concentric sheets of collagen fibers connected to the vertebral end plates. The central NP contains large quantities of the SLRPs and aggrecan, which aggregates along chains of hyaluronan [28].
The GAGs side chains of these PGs carry a fixed negative charge and generate an osmotic swelling pressure within an irregular meshwork of collagen II fibrils. Two thin endplates of hyaline cartilage extend superiorly and inferiorly over the inner AF and NP to interface with the vertebral bodies, and function to regulate nutrient diffusion between the disc and the vertebral bodies [29,30]. In the outer regions of the AF, collagen fibers anchor directly into the vertebral bone.
Biological Roles of SLRPs in the Intervertebral Disc
It is now firmly established that specific SLRPs are functionally involved in intervertebral disc development and homeostasis. The SLRPs play important roles in the control of collagen fibrillogenesis, growth factor binding and sequestration or presentation and they can interact with signaling molecules controlling proliferation, differentiation and ECM synthesis and turnover [31]. Decorin regulates collagen fibrillogenesis, collagen degradation, cell growth and extracellular signaling in the ECM and connective tissue formation in skeletal muscle [32- 34]. Fibromodulin and lumican are close homologues and play a role in the regulation of the assembly of collagen monomers into fibrils, which is important to the structural and mechanical integrity of connective tissues [35,36]. It has been reported that fibromodulin and lumican can influence collagen fibrillogenesis and hence fibril thickness [37].
Fibronectin is probably the most ubiquitous and best characterized of the adhesive glycoproteins. It plays a key role in matrix organization by interacting with integrins such as a5b1 on cell surfaces, as well as ECM components such as collagen, fibrin and heparan sulfate (HS) PGs [38]. Many interactions between the cell and its surrounding ECM affecting cell adhesion, morphology and migration are modulated by glycoproteins (on cell surfaces and within the ECM). Normal disc function depends upon a balance between these activities. GAGs may also play an important role in regulating the development, growth and homeostasis of the disc through their ability to interact with soluble bioactive signaling molecules via sulfation motif sequences within their chain structure [39,40]. In general, the GAGs content of the disc is greatest within the NP, decreasing outwards towards the edges of the AF [41]. Sulfation confers a strong negative charge on the GAGs which allows them to bind water and provides viscoelastic properties to disc tissues [42]. The mature NP has the highest concentration of KS of any tissue and the KS isoform has a much larger chain length than equivalent isomers in other tissues [43,44].
Changes of SLRPs in Intervertebral Disc Degeneration
The most significant biochemical change to occur in disc degeneration is loss of proteoglycans. With increasing age and degeneration, the disc changes in morphology, becoming more and more disorganized. One of the major changes in ageing and disc degeneration is a decrease in fibromodulin in the adult NP and an increase in lumican in the AF during early juvenile development [45]. This may also involve structural changes, characterized by a loss of KS attachment [46]. Additionally, the fragmentation of fibromodulin is identified in the process of IVD degeneration and is the most extensively fragmented in the IVD [47]. Biglycan deficiency may be a possible mechanism of IVD degeneration, with disruption of the organization of collagen fibres and hence the ECM meshwork [48,49]. Fibromodulin is more abundant in the AF than in NP at all ages, and lumican is much more abundant in NP than in AF in the juvenile disc [46]. Keratocan has been identified in the IVD of patients with various disc disorders, in the forms of intact core protein and small fragments. Keratocan is either non-glycosylated or composed of monosulfated GAGs chains [50]. In addition, during maturation and ageing there is a steady increase in the ratio of KS to CS and an increase in the sulfation of the KS disaccharides. The concentration of CS/DS in the disc decreases with age and especially during the process of degeneration [51,52].
In addition, several matrix metalloproteinases (MMPs) have been identified in the IVD that appear to play a role in pathological degradation of the PGs in the ECM of the IVD [53]. Increased amounts of gelatinases (MMPs 2 and 9) [54], collagenases (MMPs 1, 8, 13) and stromelysin (MMP3) [55, 56] are found in more degenerate human IVD. Interestingly, the production of tissue inhibitors of metalloproteinases (TIMPs) and MMPs, or aggrecanases, appears to be linked; in more degenerate discs; increased MMP levels are accompanied by TIMP 1 [55] and TIMP 2 [56].
Furthermore, the loss of PGs in degenerate discs has a major effect on the disc’s load-bearing behavior [57]. With loss of PGs, the osmotic pressure of the disc falls and the disc is less able to maintain hydration under load; degenerate discs have a lower water content than do normal age-matched discs, and when loaded they lose height and fluid more rapidly, and the discs tend to bulge [58]. Loss of PGs and matrix disorganization has other important mechanical effects; because of the subsequent loss of hydration, degenerated discs no longer behave hydrostatically under load [59]. With consequent loss of elasticity, the ligament will tend to bulge into the spinal canal, leading to spinal stenosis – an increasing problem as the population ages [5]. Moreover, lumbar disc herniation is one of the most common spinal degenerative disorders which may lead to LBP, radicular leg pain and disability.
Conclusion and Perspectives
Degeneration of the intervertebral discs is a natural progression of the aging process. The most significant biochemical change to occur in disc degeneration is loss of proteoglycans. SLRPs plays a key role in mediating and keeping the normal function of intervertebral disc, which may propose a potential of SLRPs-based therapies in disc regeneration and possibly the repair of other skeletal tissues.
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