Researchers have discovered a new way human cells defend against bacterial infection: a protein complex that slows down the growth of tuberculosis (Mtb) bacteria in infected macrophages, a type of immune cells, and enables the cells to survive infection.

The protein complex, known as GID/CTLH, was known to control the degradation of glucose in yeast. Researchers have previously targeted this complex to starve human cancer cells, which are notoriously glucose-hungry. Its role in antimicrobial defense was discovered through a screening that Dr. Nelson Simwela, a post-doc in the Russell lab did to find new biological targets to allow Mtb-infected cells to resist infection. 

“Before our study, the GID complex had never been implicated in any infection,” said Dr. David Russell, the William Kaplan Professor of Infection Biology in the Department of Microbiology and Immunology, lead author on the paper. “The GID complex is already  a focus of drug discovery in cancer, so if people are out there looking for drugs to inhibit this complex, this could new opportunities for TB.”

The results, published Oct 29 in Nature Communications, were obtained in collaboration with Dr. Craig Altier, professor in the Department of Population Medicine and Diagnostic Sciences, and Dr. Christopher Sassetti, professor of microbiology at UMass Chan Medical School. 

Simwela and the Russell lab made their discovery through a genome-wide CRISPR/Cas9 screen to identify genes that promote cell survival during bacterial infection. To perform that screening, the gene-editing technology was used to inactivate genes at random in primary macrophages. The team performed sufficient knockouts to provide 300-fold coverage of the mouse genome. 

They then infected this macrophage population with Mtb and waited to see which macrophages died and survived. “We waited until fifty percent of the macrophages had died from the infection,” said Russell. “We took the other living fifty percent and analyzed them as the output of our screen. The hope was to find those cells that survive through controlling bacterial growth.”

They found 259 knockout genes that that promoted cell survival. This list included genes previously known to help cells resist infections, and new genes that had never been associated with this function. Among these, five encoded protein subunits that form the GID complex. “With this overlap we were confident we had hit a pathway relevant to our phenotype of interest,” said Russell.

To see if their results applied to other bacterial infections in cells, the team tested their discovery on cells infected by Salmonella. Cells lacking a functional GID complex were also better at controlling Salmonella growth and had a higher survival rate, indicating that their discovery was not limited to Mtb.

In parallel with the genetic screening, Russell and his team are also screening for chemicals that could mimic the effects of the GID-knockout. “Genetic screening is informational but chemical screening is tractable in terms of drug development,” Russell said. “We have compounds that produce a comparable metabolic shift to the knockout.”

The Russell lab is interested chemicals that reprogram macrophages to be more resistant to infection, particularly compounds that help existing drugs works better or keep working. “This is particularly important for tuberculosis because these bacteria readily become less sensitive to drugs.

Currently, chemicals that specifically affect the GID complex are being sought for cancer therapy, and Russell hopes to leverage these findings for tuberculosis therapeutics. “We are monitoring the GID complex literature very closely,” said Russell. “If new compounds are identified, we want to plug them immediately in our tuberculosis drug discovery platform.” 

Written by Elodie Smith

A version of this story also appears in the Cornell Chronicle