Microelectrode studies on the acid microenvironment beneath adherent macrophages and osteoclasts

Area of Science:

• Cell Biology
• Biochemistry
• Physiology

Background:

• Osteoclasts and macrophages are critical immune cells involved in bone remodeling and resorption.
• Understanding the microenvironment these cells create is crucial for comprehending bone diseases and inflammatory processes.

Purpose of the Study:

• To investigate and quantify the acidic microenvironment generated by osteoclasts and activated macrophages.
• To determine the pH limits and rates of acidification achieved by these cells in vitro and in vivo.
• To correlate the acidic microenvironment with the potential for extracellular matrix degradation.

Main Methods:

• Utilized pH microelectrodes to measure pH changes in the attachment zone of cultured osteoclasts and macrophages.
• Employed a combination H+ and Ca2+ double-barrelled electrode for in situ measurements in rabbit ear chambers with osteoporotic bone fragments.
• Assessed acidification rates and limit pH values under various cell activation and aggregation conditions.

Main Results:

• Osteoclasts achieved a pH fall of ~1 unit/min to ≤ pH 3.0 in culture.
• Activated macrophages showed a slower acidification of 0.5-2 pH units/h to pH 3.6-3.7, enhanced in clumps.
• In vivo, osteoclasts in osteoporotic bone reached pH 4.7 with increased calcium concentration (40 mM) at erosion sites.

Conclusions:

• Both osteoclasts and activated macrophages actively generate a significantly acidic microenvironment.
• The observed acidity, particularly in vivo, supports the hypothesis that it facilitates extracellular collagen degradation by lysosomal enzymes.
• These findings provide insights into the mechanisms of bone resorption and tissue remodeling.