“Pituitary adenylate cyclase activating polypeptide (PACAP) is a neurohormone exerting protective function during various stress conditions either in mature or developing tissues. Previously we proved the presence of PACAP signaling elements in chicken limb bud-derived chondrogenic cells in micromass cell cultures. Since no data can be found if PACAP signaling is playing any role during mechanical stress in any tissues, we aimed to investigate its contribution in mechanotransduction during chondrogenesis. Expressions of the mRNAs of PACAP and its major receptor, PAC1 increased, while that of other receptors, VPAC1, VPAC2 decreased upon mechanical stimulus. Mechanical load enhanced the expression of collagen type X, a marker of hypertrophic differentiation of chondrocytes and PACAP addition attenuated this elevation{maybe if there was a PACAP or PAC1 inhibitor, longitudinal bone growth could be increased}. Moreover, exogenous PACAP also prevented the mechanical load evoked activation of hedgehog signaling: protein levels of Sonic and Indian Hedgehogs and Gli1 transcription factor were lowered while expressions of Gli2 and Gli3 were elevated by PACAP application during mechanical load. Our results suggest that mechanical load activates PACAP signaling and exogenous PACAP acts against the hypertrophy inducing effect of mechanical load.”
“Various cell surface receptors can serve for mechanosensation of chondrocytes, such as
certain purinoreceptors, integrins, stretch activated Ca2+ channels, TRPV4 and NMDA receptors. Downstream targets of these receptors can regulate the morphology, metabolism,
proliferation and Ca2+ homeostasis of chondrocytes. Our group demonstrated a PKA regulated
mechanosensitive activation of Sox9 and PP2A and the involvement of the hedgehog signaling in mechanotransduction”
“PACAP is a short peptide existing in two biological active forms; PACAP 1–38 and PACAP 1–27. Both forms have a very short life span. Three main G-protein coupled receptors of PACAP have been identified; PAC1, VPAC1 and VPAC2, the latter two have lower affinity to the neuropeptide. Activation of these receptors can regulate divers signaling pathways, from which the canonical signaling connection induces the activation of PKA and/or MAPK system”
“exogenous addition of PACAP exerts positive effects both on chondro- and osteogenesis”
“PACAP was proven to prevent apoptosis, ischemic conditions, and oxidative stress”
“PACAP 1–38 at 100 nM was applied on day two of culturing and continuously from day one . MS, mechanical stimulation applied on day two and days 2–3 for 30 min.”
“The enhanced protein levels of collagen type IX and X imply that chondrogenic cells are shifted
toward a prehypertrophic differentiation”
“the mechanical stimulation resulted in a significant elevation of SHH protein expression which was normalized by the application of exogenous PACAP. The mRNA expression of Indian hedgehog (IHH) was decreased, moreover, the protein expression reduced to an almost undetectable level after PACAP administration. In contrast with this, the mechanical load enhanced the mRNA and protein expression of IHH”
“a simultaneous significant elevation of the amount of type IX collagen[concurrent with an elevation of Type X collagen expression] may reflect on certain matrix reorganization upon mechanical load”
“The presence of collagen type IX determines the architecture of the collagen network in cartilage and its expressional changes have been demonstrated in aging. Addition of PACAP elevated the amount of collagen type II, but reduced the type IX collagen-content in micromass cultures”
“Strong mechanical load is known to induce the apoptotic program of chondrocytes or mesenchymal stem cells resulting in a shift toward the prehypertrophic phenotype. In mature joints, intense mechanical force can result in the reduction of the superficial layer of articulating cartilage and may cause a decrease of the epiphysial growth plate followed by a reduced expression of collagen type II and X”
“Since mechanical load can be regarded as a physical stress to the chondrogenic cells,
one can suppose a mechanoprotective role of PACAP during chondrogenesis.”
“Gli2 has been shown as a primary cilium specific transcription factor influenced by mechanical load and has also been demonstrated to regulate vascularization of cartilage following chondrocyte hypertrophy”
