Proteoglycans

In connective tissue, the material filling the spaces between cells—the extracellular matrix—takes up far more volume than the cells themselves. Collagen and elastic fibers form the framework of this matrix, while ground substance is the gel-like material filling the spaces between them. Proteoglycans are key molecules that determine the physical properties of this ground substance.

Their shape is often compared to a bottle brush. An elongated core protein forms the shaft, with sugar chains called glycosaminoglycans (GAGs) densely attached like bristles. The first part of the name means protein (proteo-), and the second means sugar (glycan), directly reflecting their structure.

These sugar chains carry a strong negative charge. Like charges repel one another and attract water, keeping the tissue full of water and creating an outward force from within, called swelling pressure. The collagen network tightly restrains this expansion, so the tissue does not easily collapse under compression.

Aggrecan — resisting compression. This large proteoglycan is abundant in articular cartilage and the inner core of the intervertebral disc. It carries more than 100 chondroitin sulfate and keratan sulfate chains, allowing it to retain a great deal of water. When body weight compresses the tissue, some water is pushed out; when the force is removed, the tissue takes up water again. This is the basic principle behind cartilage's ability to absorb impact.

Small proteoglycans — organizing collagen. These include decorin, biglycan, and fibromodulin. They attach to the surfaces of collagen fibers, regulate how thick the fibers become, and help transmit force by linking fibers together. Decorin is a representative example in tissues that primarily experience pulling forces, such as tendons and fascia.

Those associated with membranes and basement membranes. Perlecan is found in the thin membrane that supports cells, called the basement membrane, while syndecan is embedded in the cell membrane and transmits growth factor signals. These types clearly illustrate that proteoglycans do more than simply fill spaces: they also act as reservoirs, holding growth factors and releasing them when needed.

The two terms are often used together, but they refer to different things.

• Glycosaminoglycan (GAG) — The sugar chain itself, consisting of a long sequence of repeating units. Examples include chondroitin sulfate, keratan sulfate, heparan sulfate, and hyaluronic acid.
• Proteoglycan — A complex formed when these sugar chains attach to a core protein.

Hyaluronic acid is a GAG, but because it does not attach to a core protein, it is not, strictly speaking, a proteoglycan. Instead, it forms a long strand that acts as a backbone, with proteoglycans such as aggrecan attached along its length.

• Movement helps protect cartilage. Articular cartilage has almost no blood vessels, so it receives nutrients from joint fluid. As cartilage is compressed and released during walking or running, water moves out and in, carrying nutrients with it. This is why moving your joints from time to time becomes more important the longer you spend sitting.
• Your height changes slightly over the course of a day. Water is understood to leave the intervertebral discs as body weight compresses them during the day, then return when you lie down to rest at night. The difference between your morning and evening height is related to these changes.
• Water content decreases with age. As aggrecan decreases, cartilage and discs retain less water and become less resilient. This is connected to imaging findings such as age-related disc desiccation.
• Composition varies depending on the forces a tissue experiences. Large, water-retaining proteoglycans are said to increase in areas under compression, while smaller types that organize collagen are more abundant in areas under tension. You can read more about how the body uses forces as signals to shape its tissues under mechanotransduction.
• You may also see related names on supplements. Chondroitin sulfate and glucosamine are substances related to the components of proteoglycans and are available as health functional foods in Korea.

• Get up and walk several times a day, and bend and straighten your joints. This helps water and nutrients move in and out of cartilage and discs.
• Try regular exercise that places rhythmic loads on the joints, such as walking, cycling, or swimming, without overdoing it.
• Increase intensity gradually rather than suddenly making a big jump, giving your tissues time to adapt.
• Drinking enough water and maintaining a regular sleep schedule help discs compressed during the day get some rest.