Signaling Factors in a Variety of Cells Derived from Mesenchymal Stem Cells

L-Glutamate (Glu) and γ-aminobutyric acid (GABA) has been thought to be an excitatory/inhibitory amino acid neurotransmitter in the mammalian central nervous system (CNS). Limited information is available in the literature with regard to an extracellular transmitter role of Glu and GABA in peripheral neuronal and non-neuronal tissues, whereas recent molecular biological analyses including ours give rise to a novel function for Glu and GABA as an autocrine and/or paracrine factor in a variety of cells derived from mesenchymal stem cells, in addition to other peripheral tissues including pancreas, adrenal, and pituitary glands. Emerging evidence suggests that Glu and GABA could play a dual role in mechanisms underlying maintenance of cellular homeostasis as a neurotransmitter in the CNS and as an extracellular signal mediator in peripheral autocrine and/or paracrine tissues. In this review, therefore, we summarized the possible signaling by Glu and GABA as an extracellular signal mediator in mechanisms underlying maintenance of cellular homeostasis in mesenchymal stem cells, osteoblasts and chondrocytes.