Mycoplasma synoviae induces upregulation of apoptotic genes, secretion of nitric oxide and appearance of an apoptotic phenotype in infected chicken chondrocytes

Species from the genus Mycoplasma have long been recognized as important pathogens that cause diseases of the respiratory tract, urogenital tract and joints in a variety of animal species, including humans. Mycoplasma synoviae (M. synoviae) is a common pathogen of chickens and turkeys. It usually colonizes the respiratory tract, although this condition can lead to the development of systemic infection and/or infectious synovitis [1-4]. Experimental infectious synovitis has been associated with the hemagglutination-positive phenotype of M. synoviae [5]. The condition is presented as an acute joint swelling accompanied by increased volume of synovial fluid, infiltration with macrophages, B and T cells, lining cell hyperplasia and fibroblastic proliferation. In the acute phase of infection, live M. synoviae have been detected in synovial fluids of infected birds, suggesting that M. synoviae can come into direct contact with chondrocytes as is the case with M. pulmonis [6] and M. artritidis [7]. M. synoviae has been reported capable of invasion into non-phagocytic chicken cells including chondrocytes in vitro [8].Mycoplasmas have been reported in many studies as active players in host-pathogen interactions leading to alterations in cell death patterns (Table 1) [9-22]. To our knowledge, there has been no report of any Mycoplasma species having apoptosis modulating effects on infected chondrocytes. Therefore, we conducted this study to elucidate the impact of Mycoplasma synoviae infection on chicken chondrocytes.Apoptosis is a highly regulated process of cell demise and is generally induced through activation of different components of two overlapping signaling pathways. The extrinsic pathway includes the activation of death receptors leading to activation of a family of cysteine proteases known as caspases [23]. Proteolytic cleavage of around 400 caspase substrates that have been identified so far [24] leads to phenotypic features of apoptosis. These include rounded mor