Before the parasite\u00a0Toxoplasma gondii\u00a0can invade a cell, it has to know it has reached the right place. New research from the Whitehead Institute reveals that the parasite doesn’t simply recognize a ready-made doorway into the cell. Instead, it helps create the conditions for its own entry.<\/p>\n
Researchers in the lab of Whitehead Institute Member Sebastian Lourido have discovered that\u00a0Toxoplasma\u00a0reorganizes sugar-coated proteins on the surface of a host cell, which creates specialized membrane patches in a cholesterol-dependent manner that help trigger the parasite’s invasion machinery.<\/p>\n
The host cell, far from being a kind of passive, static entity in this process, is actually having its surface mixed around and remodeled by the parasite in order to achieve the right conditions for entry.”<\/p>\n
Sebastian Lourido, associate professor of biology at MIT<\/p>\n
The study, led by first author Dylan Valleau, appears in\u00a0The EMBO Journal\u00a0on September 25.<\/p>\n
Knowing when to invade<\/p>\n
Toxoplasma gondii\u00a0is the single-celled parasite that causes toxoplasmosis. It belongs to a larger group of parasites called apicomplexans, which includes\u00a0Plasmodium, the parasite responsible for malaria, and\u00a0Cryptosporidium, which causes cryptosporidiosis. These parasites must invade host cells to survive and reproduce.\u00a0Toxoplasma\u00a0completes the entire invasion process in less than a minute.<\/p>\n
Lourido’s lab has long been interested in a fundamental mystery underlying that process: How does a parasite know when it has reached a host cell and should commit to entering?<\/p>\n
A key step in that transition from contact to invasion is discharging specialized organelles called rhoptries. At precisely the right moment,\u00a0Toxoplasma\u00a0releases its contents into the host cell. Among their cargo are proteins that manipulate the host cell and help establish receptors the parasite uses to enter – in effect, allowing\u00a0Toxoplasma\u00a0to make its own doorway.<\/p>\n
Scientists had identified several components of the parasite machinery involved in rhoptry discharge, but what tells the parasite when to trigger that machinery remained unclear. The new study provides part of the answer.<\/p>\n
Looking at invasion from the host’s side<\/p>\n
Rather than searching for additional parasite genes involved in invasion, the researchers approached the question from the other side. They systematically disrupted genes in mammalian host cells and asked which were necessary for\u00a0Toxoplasma\u00a0to successfully discharge its rhoptries.<\/p>\n
Importantly, the screen focused specifically on rhoptry discharge rather than the broader processes of invasion and replication. That allowed the researchers to home in on host factors involved at the moment the parasite commits to entering.<\/p>\n
Two pathways stood out: N-glycosylation, through which complex sugars called glycans are attached to proteins on the cell surface, and production of cholesterol.<\/p>\n
At first, cholesterol and glycosylation seemed like two separate leads. As the researchers followed them, however, the pathways converged.<\/p>\n
The team identified three\u00a0Toxoplasma\u00a0proteins, collectively known as the MIC1\/4\/6 complex, that recognize particular glycans on the host-cell surface. The parasite proteins appear to gather glycosylated host proteins together, while cholesterol helps make that reorganization possible.<\/p>\n
The result, the researchers propose, is a small, specialized region, or microdomain, in the host membrane containing the molecular features needed for rhoptry discharge. In essence, the parasite gathers host-cell components into a spot that creates the right conditions for entry.<\/p>\n
When the researchers interfered with formation of that microdomain – by removing host-cell cholesterol, disrupting relevant glycosylation pathways, or interfering with the parasite’s ability to recognize the sugars – rhoptry discharge declined and invasion was impaired.<\/p>\n
Creating the right signal<\/p>\n
The findings may also explain why\u00a0Toxoplasma\u00a0doesn’t prematurely fire its invasion machinery whenever it encounters these common sugars.<\/p>\n
Glycans are abundant in and around cells. Responding to a single sugar could therefore trigger rhoptry discharge at the wrong time. Requiring the parasite to first cluster glycosylated proteins may provide a more reliable signal that it has made the close contact with a host-cell membrane necessary for successful invasion.<\/p>\n
That strategy could also help explain\u00a0Toxoplasma’s broad range of potential host cells. The sugars recognized by the MIC1\/4\/6 complex are widely distributed across mammalian tissues, and the complex can recognize more than one glycan type.<\/p>\n
The researchers additionally demonstrated that this newly identified interaction can be disrupted. Adding free sugars that competed with glycans on the host-cell surface inhibited rhoptry discharge. Although the sugars used in the experiments are not potential drugs, the results provide proof of principle that interfering with this host-parasite interface can disrupt invasion.<\/p>\n
The findings may extend beyond\u00a0Toxoplasma. The study suggests the parasite exploits some of the same fundamental features of host-cell membranes used by certain viruses, pointing toward common cellular vulnerabilities that different pathogens may exploit.<\/p>\n
For now, the work changes the picture of the host cell’s role in\u00a0Toxoplasma\u00a0invasion. Rather than serving as a passive surface waiting for the parasite to find the right receptor, the host membrane is actively reorganized and remodeled by the invader.<\/p>\n
By gathering host molecules into just the right arrangement,\u00a0Toxoplasma\u00a0appears to create the signal it needs to commit to invasion – effectively preparing its own doorway before stepping through.<\/p>\n
Source:<\/p>\n
Whitehead Institute for Biomedical Research<\/a><\/p>\n Journal reference:<\/p>\n