Biology:Aggrecan

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A representation of the 3D structure of the protein myoglobin showing turquoise α-helices.
Generic protein structure example

Aggrecan (ACAN), also known as cartilage-specific proteoglycan core protein (CSPCP) or chondroitin sulfate proteoglycan 1, is a protein that in humans is encoded by the ACAN gene.[1] This gene is a member of the lectican (chondroitin sulfate proteoglycan) family. The encoded protein is an integral part of the extracellular matrix in cartilagenous tissue and it withstands compression in cartilage.

Aggrecan is a proteoglycan, or a protein modified with large carbohydrates; the human form of the protein is 2316 amino acids long and can be expressed in multiple isoforms due to alternative splicing.[1]

Structure

Aggrecan is a high molecular weight (1x106 < M < 3x106) proteoglycan. It exhibits a bottlebrush structure, in which chondroitin sulfate and keratan sulfate glycosaminoglycan (GAG) chains are attached to an extended protein core.[2]

Aggrecan has a molecular mass >2,500 kDa.[3] The core protein (~300 kDa[4]) has around 100 GAG chains attached to it.[5]

Aggrecan consists of two globular structural domains (G1 and G2) at the N-terminal end and one globular domain (G3) at the C-terminal end, separated by a large extended domain (CS) heavily modified with GAGs. (N-G1-G2-CS-G3-C) The two main modifier moieties are themselves arranged into distinct regions, a chondroitin sulfate and a keratan sulfate region.

The three globular domains, G1, G2, and G3 are involved in aggregation, hyaluronan binding, cell adhesion, and chondrocyte apoptosis.

Along with type-II collagen, aggrecan forms a major structural component of cartilage, particularly articular cartilage.

The aggrecan family includes other important members such as versican, also named PG-M, neurocan, brevican and the cell surface HA receptor CD44. They are modular proteoglycans containing combinations of structural motifs, such as EGF-like domains, carbohydrate recognition domains (CRD), complement binding protein (CBP)-like domains, immunoglobulin folds and proteoglycan tandem repeats.

Function

Aggrecan is a critical component for cartilage structure and the function of joints.

Functionally, the G1 domain interacts with hyaluronic acid and link protein, forming stable ternary complexes in the extracellular matrix. G2 is homologous to the tandem repeats of G1 and of link protein and is involved in product processing. G3 makes up the carboxyl terminus of the core protein. It enhances glycosaminoglycan modification and product secretion. Aggrecan plays an important role in mediating chondrocyte-chondrocyte and chondrocyte-matrix interactions through its ability to bind hyaluronan.[5]

Aggrecan provides intervertebral disc and cartilage with the ability to resist compressive loads. The localized high concentrations of aggrecan provide the osmotic properties necessary for normal tissue function with the GAGs producing the swelling pressure that counters compressive loads on the tissue. This functional ability is dependent on a high GAG/aggrecan concentration being present in the tissue extracellular matrix.[6] In the disc, aggrecan concentrations peak in a person's twenties, and decline thereafter, with aggrecan degradation products slowly accumulating over the following decades.[7] This causes discs to get stiffer and less resilient with age.

Aggrecan also plays an important role in the organization of the extracellular spaces between neurons in the brain.[8] Through interactions with link protein and tenascins, aggrecan binds to hyaluronan, forming large aggregated complexes at the cell surface.

Clinical significance

The synthesis and degradation of aggrecan are being investigated for their roles in cartilage deterioration during joint injury, disease, and aging.

The linker domain between the N-terminal globular domains, called the interglobular domain, is highly sensitive to proteolysis. Such degradation has been associated with the development of arthritis. Proteases capable of degrading aggrecans are called aggrecanases, and they are members of the ADAM (A Disintegrin And Metalloprotease) protein family.[9]

Degenerative joint disease is a leading source of morbidity resulting in significant social and economic impact. Osteoarthritis is characterized by the slow progressive deterioration of articular cartilage and fibrosis of the synovium and joint capsule. Articular cartilage contains up to 10% proteoglycan by weight, most of which is aggrecan, and its loss is an early sign of the disease.

References

  1. ↑ 1.0 1.1 "Complete coding sequence and deduced primary structure of the human cartilage large aggregating proteoglycan, aggrecan. Human-specific repeats, and additional alternatively spliced forms". J. Biol. Chem. 266 (2): 894–902. January 1991. doi:10.1016/S0021-9258(17)35257-2. PMID 1985970. 
  2. ↑ "Structure and interactions of aggrecans: statistical thermodynamic approach". Biophys. J. 95 (10): 4570–83. November 2008. doi:10.1529/biophysj.108.133801. PMID 18689463. 
  3. ↑ Hascall, VC; Sajdera, SW (10 October 1970). "Physical properties and polydispersity of proteoglycan from bovine nasal cartilage.". The Journal of Biological Chemistry 245 (19): 4920–30. doi:10.1016/S0021-9258(18)62796-6. PMID 5506265. 
  4. ↑ Chandran, PL; Horkay, F (January 2012). "Aggrecan, an unusual polyelectrolyte: review of solution behavior and physiological implications.". Acta Biomaterialia 8 (1): 3–12. doi:10.1016/j.actbio.2011.08.011. PMID 21884828. 
  5. ↑ 5.0 5.1 "Structure and function of aggrecan". Cell Res. 12 (1): 19–32. March 2002. doi:10.1038/sj.cr.7290106. PMID 11942407. 
  6. ↑ "The involvement of aggrecan polymorphism in degeneration of human intervertebral disc and articular cartilage". Eur Cell Mater 11: 1–7; discussion 7. 2006. doi:10.22203/eCM.v011a01. PMID 16425147. 
  7. ↑ "Structure, function, aging and turnover of aggrecan in the intervertebral disc". Biochim. Biophys. Acta 1840 (10): 3181–9. 2014. doi:10.1016/j.bbagen.2014.07.013. PMID 25065289. 
  8. ↑ "Aggrecan: Beyond cartilage and into the brain". The International Journal of Biochemistry & Cell Biology 44 (5): 690–3. May 2012. doi:10.1016/j.biocel.2012.01.010. PMID 22297263. 
  9. ↑ "ADAMTS-5 deficiency does not block aggrecanolysis at preferred cleavage sites in the chondroitin sulfate-rich region of aggrecan". J. Biol. Chem. 282 (12): 8632–40. 2007. doi:10.1074/jbc.M605750200. PMID 17255106. 

Further reading

External links




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