Expression of apoptosis-related factors in muscles infected with Trichinella spiralis

We found that the expression of mitochondrial apoptosis related genes (Bcl-2 associated protein X, BAX; apoptotic protease activating factor 1, Apaf-1; Caspase 9 and serine/threonine protein kinase, PKB) is elevated in Trichinella spiralis-infected muscles during encapsulation. Micro-dissection of the capsule and subsequent reverse transcription polymerase chain reaction (RT-PCR) confirmed that the expressions of these genes are restricted to the nurse cell. Immunocytochemistry revealed that pro-apoptosis factor (BAX, Apaf-1 and Caspase 9) are predominantly expressed in the basophilic cytoplasm (infected muscle cell origin) and anti-apoptosis factor (PKB) in the eosinophilic cytoplasm (satellite cell origin) of the nurse cell. Electron microscopy revealed that the pre-existing mitochondria in the muscle cells became swollen and disappeared immediately after newborn larva invasion, but new mitochondria of smaller size appeared in the cytoplasm. Nuclear fragmentation and condensation were observed in basophilic cytoplasm which is known to die. Together, the results suggest that the infected muscle cells transform but die through the process of apoptosis which is triggered by factors from the newly formed mitochondria. The anti-apoptosis factor may help the eosinophilic cytoplasm with its survival to ensure nurse cell function.