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Making gold out of lead
SAN DIEGO—It's hard to believe anything good could come from a virus, unless you count sick days as a bonus. But deeper investigation into the nature of the Zika virus may very well have offered up a boon, thanks to work by two teams from the University of California, San Diego (UCSD) School of Medicine. The teams were studying the Zika virus in hopes of discovering a way of preventing microcephaly, stunted neonatal brain development caused by infection by the Zika virus. The researchers were optimistic that by determining how the pathogen gets into brain cells, they could block that avenue of access.
Their work led them to αvβ5 integrin, the molecule responsible for Zika virus' ability to entering brain stem cells. While this obviously offers a new way to protect people from Zika infection, it could also offer an unexpected secondary bonus in the form of a new method for targeting brain cancer stem cells. The teams published their work in a pair of papers, one in Cell Press and the other in Cell Stem Cell.
Integrins are molecules found on the surface of cells that play a role in cell adherence and communication. In the Cell Press paper, titled “Integrin αvβ5 Internalizes Zika Virus during Neural Stem Cells Infection and Provides a Promising Target for Antiviral Therapy,” the authors note that “Integrins, a family of 24 heterodimers consisting of α and β subunits, are transmembrane adhesion receptors that are key components of cell signaling mechanisms involved in cancer progression and metastasis (Hynes, 2002). Specific ligands bind and cluster integrins to regulate vehicle trafficking and transduce both outside-in and inside-out signaling events (Hynes, 2002). In one of the outside-in signaling mechanisms of integrins, focal adhesion kinase (FAK) is phosphorylated and activated to recruit additional kinases and induce complex signaling cascade to regulate cell survival, proliferation, and migration (Mitra and Schlaepfer, 2006). Therefore, FAK inhibitors have been developed to control migration, invasion, and metastasis of various tumors.”
Dr. Tariq Rana, professor and chief of the Division of Genetics in the Department of Pediatrics at UC San Diego School of Medicine and Moores Cancer Center, led one of the teams, who used CRISPR to delete every gene in a 3D culture of in-vitro human glioblastoma stem cells. They labeled the Zika virus with green fluorescent protein to make its movements more visible, then exposed each variation of the cancer cells to the virus to determine which proteins' presence was necessary for Zika to invade cells. All told, 92 human brain cancer stem cell genes were identified as necessary for Zika virus to infect and replicate in the cells, but of those, the gene that encodes the αvβ5 integrin was identified as a key culprit.
“Integrins are well known as molecules that many different viruses use as doorknobs to gain entry into human cells,” Rana explained in a press release. “I was expecting to find Zika using multiple integrins, or other cell surface molecules also used by other viruses. But instead we found Zika uses αvβ5, which is unique. When we further examined αvβ5 expression in brain, it made perfect sense because αvβ5 is the only integrin member enriched in neural stem cells, which Zika preferentially infects. Therefore, we believe that αvβ5 is the key contributor to Zika's ability to infect brain cells.”
The second team, led by Dr. Jeremy Rich, professor in the Department of Medicine at UC San Diego School of Medicine and director of neuro-oncology and of the Brain Tumor Institute at UC San Diego Health, took a different approach. Since integrins also serve as cellular entry points for viruses such as adenovirus, foot-and-mouth disease virus and rotavirus, they systematically inhibited each integrin with a different antibody.
What they found was that blocking αvβ5 “almost completely blocked the ability of the virus to infect brain cancer stem cells and normal brain stem cells,” according to Rich. Next, they inhibited αvβ5 in a glioblastoma mouse model with either an antibody or by deactivating the gene that encodes the integrin; both approaches effectively blocked Zika infection and extended the mice's lifespans. In addition, blocking the integrin in glioblastoma tumor samples removed from human patients during surgery also blocked the infection. To further test ways of blocking αvβ5, they infected mice with Zika virus and then treated them daily with either cilengitide or SB273005, experimental cancer drugs that inhibit αvβ5. When they examined the mice six days after infection, the treated mice had half as much virus in their brains as the mock-treated mice.
To confirm these results, Rana's team is working a mouse model that is genetically designed to lack αvβ5 in the brain.
“The neat thing is that these findings not only help advance the Zika virus research field, but also opens the possibility that we could similarly block the entry of multiple viruses that use other integrins with antibodies or small-molecule inhibitors,” Rana remarked.
Rich's study also provided answers for why Zika virus prefers glioblastoma stem cells over healthy brain cells. Glioblastoma stem cells produce both of the subunits that comprise αvβ5: αv (which is associated with stem cells) and β5 (which is associated with cancer cells). As well as being key to Zika virus infection, αvβ5 is also pivotal to glioblastoma stem cell survival.
Moving forward, Rich's team is working with other groups on targeted drug studies to identify drugs that can block Zika virus, in addition to exploring how genetically modifying Zika virus could allow it to more effectively target brain cancer cells without damaging healthy cells.
“While we would likely need to modify the normal Zika virus to make it safer to treat brain tumors, we may also be able to take advantage of the mechanisms the virus uses to destroy cells to improve the way we treat glioblastoma,” he said. “We should pay attention to viruses. They have evolved over many years to be very good at targeting and entering specific cells in the body.”
SOURCE: UCSD press release by Dr. Heather Buschman