Protein S-nitrosylation: Nitric oxide signalling during anuran tail regression.

Tail regression is a remarkable process where a complex organ like the tail is completely resorbed by cell death during anuran metamorphosis. Nitric oxide is a signalling molecule involved in various physiological processes and along with reactive nitrogen species induces apoptosis. The present study describes the contribution of nitric oxide and reactive nitrogen species (nitrosative stress) during tail regression in the tadpoles of Indian tree frog, Polypedates maculatus. Spectrophotometric estimation revealed significantly higher levels of nitrite, nitrate and peroxynitrite in the regressing tails of the late climactic stages as compared to the early climactic stages and pre-regressing tails. S-nitrosylated proteins were detected in the apoptotic cells of epidermis and muscle, denervated and fragmented myofibres, outer notochordal sheath of the degenerating notochord, endothelium of blood vessels, blood cells and spinal cord of the regressing tail of the late climactic stages using fluorescent detection methods. Thus, a higher level of nitrosative stress in the late climactic stages is suggested to cause S-nitrosylation of proteins and subsequent apoptosis in the tail tissues. Macrophages were found engulfing the apoptotic cells and cell debris at the distal end of the regressing tail. Interestingly, macrophages were always found to be associated with melanocytes suggesting a close association for clearing cell debris by phagocytosis.