There’s been a rush of new papers out lately which are starting to explain how Zika virus causes fetal damage. Understandably, since the suspicion of a link between Zika and microcephaly (an abnormally small head, associated with neurological defects) in humans was raised, there’s been an intensive research effort directed at uncovering the causality of this process, but I’m still impressed at the speed at which scientists are gaining answers. It was only last month, after all, that the CDC declared that there was a “causal link” between Zika and microcephaly.
The initial confirmation that Zika causes neural damage came from an in vitro study showing that Zika infects human neural stem cell precursors (link here: I am giving the PubMed database links to these papers, but of course the full text of each is usually behind a paywall). These are the cells that will divide and form the neurons in the brain. There’s a reason an in vitro study was one of the earliest results: these studies are quick, and relatively cheap. Strictly speaking, however, all you can definitively conclude from this is that the virus is capable of infecting these neural stem cells in a dish. It says nothing as to whether it is capable of doing so in a developing embryo or if and how this causes damage. The first point is an important one: in a developing mammalian embryo, the virus must be able to cross the placenta, which is quite good at filtering out unwanted intruders. Moreover, the developing brain itself is also developing a blood-brain barrier: a question I wanted to know from the beginning of this story was whether, if Zika was responsible for damaging developing brains, there was a time-window in fetuses were susceptible because of the immaturity of their blood-brain barrier, and a point past which they might not be affected.
Most of the answers to these questions are now coming from animal studies – still really the only way of answering something as complicated as “How does a virus damage a developing mammalian fetus?” If we could grow a fetus in a dish – well, nobody would need to get pregnant anymore for a start! As it is, we can grow simple mammalian embryos up to the ball-of-cells stage, but no further.
However, after writing about organoids a couple of weeks ago, I was interested to see that this system has now been used to study Zika. A study in Science infected human brain organoids with either Zika, or Dengue virus (as a control). The Dengue had no effect, but Zika targeted the human brain cells, reduced their size and viability, and caused programmed cell death responses. (These are failsafe programmes that cause cells to self-destruct if they are irreparably damaged; they are notably critical in preventing cancer).
It looks, now, as if damage by Zika might be both direct and indirect. Two studies in mice (here and here), plus one using mice and organoids (here) have revealed how Zika is causing miscarriages and microcephaly. Between them, they have shown extremely high levels of Zika in the placenta – up to 1000x that in the maternal blood – and this is causing a phenomenon called intra-uterine growth restriction: simply put, the placenta is damaged and cannot function as well as it should, meaning the fetal blood supply is reduced, and the fetus doesn’t grow properly as a result. In severe cases, this will lead to miscarriage. Additionally the virus was also found in the heads of the fetuses, so it did cross the placenta. Finally, some of the results showed microcephaly in the fetal mice.
Of course, mice aren’t humans, and further caution is warranted by the fact that Zika doesn’t naturally infect mice, so the researchers had to come up with some complicated workarounds to overcome this problem. Monkeys aren’t humans either, but rhesus macaques are the go-to primate model for this type of question, and there are several studies underway to confirm the mouse results. Like humans, macaques can be infected by Zika naturally, and genetically and immunologically they are far more similar to us. It’ll take a while to get the results though, because they have much longer pregnancies than mice. (As an aside, I’m not wild about the use of primates in research, but as a general rule, no other scientist much likes the idea either: believe me, nobody would use a hugely expensive monkey system if they didn’t have to, nor would any ethics committee approve it. When it comes to developing a vaccine, however, it’s also true that nobody is going to let you test it on a human before you’ve tested it on an animal, for safety reasons if nothing else).
Notice the trend here: from simple in vitro systems to as-close-to-human-as-you-can-get, with the majority of the significant results, as in most research of this nature, still coming from mouse studies. There’s a reason mice are the most widely used laboratory animal: they’re small, so you can keep large numbers of them cheaply, there’s a whole range of research techniques available, including genetic engineering, and they don’t have that long a generation time. It’s interesting doing a search on the PubMed medical database for Zika to see what comes up, because it does provide a snapshot of how this kind of research proceeds: in addition to the work I’ve just discussed, there are huge numbers of epidemiological reports discussing outbreaks and spread of the virus in various regions of the globe, and reports of improved diagnostic tests. There are two new papers on the crystal structure of the virus, which will hopefully provide potential targets for molecular drug design.
Where does this leave our organoids? Well, in terms of tackling Zika, I can see them potentially being used to screen potential drugs before they are trialled in animals. More likely, they will be used to provide insights into the molecular mechanism of Zika action: how does it affect the neurons?
In the meantime, however, I think it’s going to be the mice that give us most of the answers.