Amino Acid Uptake Limitations during Human Mesenchymal Stem Cell-Based Chondrogenesis

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Abstract
Amino acids are the essential building block for collagen and proteoglycan, which are the main constituents for cartilage extracellular matrix (ECM). Synthesis of ECM proteins require uptake of various essential/non-essential amino acids. Analyzing amino acids metabolism during chondrogenesis can help to relate tissue quality to amino acids metabolism under different conditions. In our study, we studied amino acids uptake/secretion using hMSC-based aggregate chondrogenesis in a serum-free induction medium with a defined chemical formulation. The initial glucose level and medium-change frequency were varied. Our results showed that essential amino acids uptake increased with time during hMSCs chondrogenesis for all initial glucose levels and medium-change frequencies. Essential amino acid uptake rates were initial glucose-level independent. The DNA-normalized GAG and HYP content of chondrogenic aggregates correlated with cumulative uptake of leucine, valine, and tryptophan regardless of initial glucose levels and medium-change frequencies. Collectively, our results showed that amino acids uptake rates during in vitro chondrogenesis were insufficient to produce a tissue with an ECM content similar to that of neonatal cartilage or adult cartilage. Further, this deficiency is likely related to the downregulation of some key amino acid transporters on the cells. Such deficiency could be partially improved by increasing the amino acid availability in chondrogenic medium by changing culture conditions.

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