Systems-Based Analysis of the Sarcocystis neurona Genome Identifies Pathways That Contribute to a Heteroxenous Life Cycle

Blazejewski, Tomasz; Nursimulu, Nirvana; Pszenny, Viviana; Dangoudoubiyam, Sriveny; Namasivayam, Sivaranjani; Chiasson, Melissa A.; Chessman, Kyle; Tonkin, Michelle; Swapna, Lakshmipuram S.; Hung, Stacy S.; Bridgers, Joshua; Ricklefs, Stacy M.; Boulanger, Martin J.; Dubey, Jitender P.; Porcella, Stephen F.; Kissinger, Jessica C.; Howe, Daniel K.; Grigg, Michael E.; Parkinson, John

The S. neurona attachment and invasion machinery is broadly conserved with T. gondii. The process of host cell inva-sion by apicomplexan parasites is a rapid and complex process that relies on a coordinated cascade of interactions between the invading parasite and the host cell. To orchestrate these processes, apicomplexans have evolved families of invasion proteins that are broadly conserved but nevertheless exhibit unique lineagespecific innovations (25) . To identify S. neurona gene models involved in invasion relative to T. gondii, we constructed an invasion protein coexpression network (Fig. 3B ) in which pairs of T. gondii proteins are linked if they exhibit significant coexpression with S. neurona (Pearson correlation coefficient, Ͼ0.8), as has been done for other organisms (26) (27) (28) . This network provides a scaffold onto which conservation and expression data from S. neurona are mapped to yield insights into evolutionary and functional relationships. Consistent with previous studies, we found that conserved proteins (those that have an ortholog in common with S. neurona) tend to have more correlated expression (high Pearson correlation coefficients) and more connections (high node degree) and are better connected within the network (shorter average path lengths and higher betweenness) than their nonconserved counterparts (Fig. 3C) . These findings highlight the potential importance of conserved proteins to the function of the invasion machinery. Within our T. gondii invasion network, we identified two main clusters of highly correlated genes associated with key invasion events. The first involves proteins associated with the micronemes (MIC proteins) which strengthen host cell attachment and play a major role in the formation of the moving junction that forms a specific interface, facilitating invasion. The second involves proteins associated with the rhoptries (RON and ROP proteins), an organelle that is absent from the merozoite stage of all Sarcocystis species, including S. neurona (29) .