Inorganic phosphate transport in matrix vesicles from bovine articular cartilage

Area of Science:

• Biochemistry
• Cell Biology
• Orthopedics

Background:

• Extracellular organelles called matrix vesicles (MV) in mineralizing tissues, including articular cartilage, initiate hydroxyapatite crystal formation.
• MV possess membrane transporters crucial for accumulating calcium (Ca2+) and inorganic phosphate (Pi).
• In osteoarthritis, hydroxyapatite crystals in articular cartilage are implicated in disease progression.

Purpose of the Study:

• To characterize inorganic phosphate (Pi) transport mechanisms in matrix vesicles (MV) derived from articular cartilage.
• To investigate the properties of Pi uptake, including its dependence on sodium (Na+) and pH.

Main Methods:

• Matrix vesicles (MV) were isolated from bovine articular cartilage using collagenase digestion and serial centrifugation.
• Inorganic phosphate (Pi) uptake was quantified using radiolabeled phosphate ((33)[P]HPO(4)(2-)).
• Sodium dependence, pH sensitivity, and inhibition by phosphate analogues were assessed.

Main Results:

• Pi uptake by MV was temperature-sensitive and exhibited both Na+-dependent and Na+-independent components.
• The Na+-dependent Pi transport showed saturation kinetics with a K(m) of 0.16 mM.
• Uptake was maximal at pH 6.5 and inhibited by phosphonoacetate and arsenate, with a persistent Na+-independent fraction.

Conclusions:

• The characterized Pi transport properties in articular cartilage MV closely resemble those of chondrocytes, suggesting inherited transporters.
• A Na+-independent Pi uptake mechanism in MV, previously undescribed in growth plate cartilage, warrants further investigation.
• Understanding these Pi transport pathways is critical for addressing inappropriate mineralization in articular cartilage, particularly in osteoarthritis.