"Integrins are well known as molecules that many different viruses use as doorknobs to gain entry into human cells," says Rana. "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."

Rana's group also used a mouse model to demonstrate that αvβ5 integrin could be blocked by daily treatment with two experimental anticancer drugs - Cilengitide or SB273005 - both of which target αvβ5 integrin. The mice were treated with the drugs, followed by Zika virus infection. Six days after virus infection, the mice brains were harvested. The brains of the drug-treated mice showed half the number of virus particles, compared to the brains of untreated mice.

"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," says Rana.

Rana's group is currently designing a mouse model in which αvβ5 integrin is absent in the brain. This mouse model will help them to conclusively prove that αvβ5 integrin is absolutely vital for entry of Zika virus into the brain.

Study 2: Inhibition of Integrin - Blocks the Entry of Zika Virus into the Brain

The second study was led by Dr. Jeremy N. Rich, MD, MHS, who is a Clinical Professor of Medicine and Director of Neuro-Oncology at UC San Diego School of Medicine. He also holds joint appointments as Faculty Member in the Sanford Consortium for Regenerative Medicine and Sanford Stem Cell Clinical Center at UC San Diego. He is also Director of the Brain Tumor Institute, San Diego, California, USA.

Based on the fact that viruses exploit integrins to gain entry into cells, Rich's group inhibited each integrin molecule with a different antibody to establish, which produced the most pronounced effect.

"When we blocked other integrins, there was no difference. You might as well be putting water on a cell," says Rich. "But with αvβ5, blocking it with an antibody almost completely blocked the ability of the virus to infect brain cancer stem cells and normal brain stem cells."

They also used a mouse model of glioblastoma to demonstrate the inhibition of αvβ5 integrin with an antibody or by deactivating the gene encoding it. Both these approaches blocked Zika virus entry, thereby prolonging the life of the treated mice, compared to untreated mice. Importantly, inhibition of αvβ5 integrin in human glioblastoma tissue samples obtained from patients during surgery also blocked Zika virus entry into brain cells.

Treatment of Brain Cancer by Leveraging Zika Virus

The idea of leveraging Zika virus for treating brain cancer came to Rich, from the observation that Zika infection causes shrinking of the brain, which is seen in microcephalic babies born to Zika virus-infected mothers. He subsequently established that Zika virus selectively targets and kills glioblastoma stem cells, which couldn't be treated with conventional anticancer drugs.

Rich's present study added to his previous findings by establishing why Zika virus preferentially kills glioblastoma stem cells while sparing healthy cells. His group elucidated the underlying reason by analyzing the structure of αvβ5 integrin. They found that the αv subunit was associated with the stem cells, while the β5 subunit was associated with cancer cells. High levels of these two subunits collectively (in the form of αvβ5 integrin) play a crucial role in glioblastoma stem cell survival, which resulted in a higher rate of killing of glioblastoma stem cells compared to healthy stem cells or other types of brain cells.

Rich indicates: "It turns out that the very thing that helps cancer cells become aggressive cancer stem cells is the same thing Zika virus uses to infect our cells."

Proof-of-Principle in a Human Organoid Model

The applicability of the cell culture studies of Zika virus infection of glioblastoma cells was tested in a human organoid model. The human organoid model was developed in collaboration with Professor Alysson Muotri, PhD, who is the Director of the UC San Diego Stem Cell Program and a Member of the Sanford Consortium for Regenerative Medicine.

The human organoid model was established by transplanting human glioblastoma tumors into human brain organoids that essentially acted as "mini-brains", which could be utilized for drug discovery studies. It was established that Zika virus exposure of human organoids selectively destroyed the transplanted glioblastoma cells, sparing other cells within the organoids. Importantly, when αvβ5 integrin was inhibited, the anticancer effect of Zika virus was reversed, which proved beyond doubt the crucial role played by αvβ5 integrin in modulating Zika virus infection.

Future Plans

The research team is planning to identify novel drug molecules that are capable of blocking Zika virus infection of brain cells more effectively. They are also interested in genetically modifying the virus so that it can better target and specifically destroy the cancerous brain cells, leaving behind the healthy ones.

Concluding Remarks

Rich concludes: "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 adds: "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."

Funding Source

The studies were funded by the National Institutes of Health, California Institute for Regenerative Medicine, and the International Rett Syndrome Foundation.

References :

1. Study 1: Integrin αvβ5 Internalizes Zika Virus during Neural Stem Cells Infection and Provides a Promising Target for Antiviral Therapy - (https://doi.org/10.1016/j.celrep.2019.11.020)
2. Study 2: Zika Virus Targets Glioblastoma Stem Cells through a SOX2-Integrin αvβ5 Axis - (https://doi.org/10.1016/j.stem.2019.11.016)

Source: Medindia