The human capacity for memory has its material basis in the brain area known as the hippocampus. Damage to this region’s substance caused by environmental influences can have serious consequences. A research group led by Prof. Dr. Bernd Heimrich at the University of Freiburg is investigating how the Borna disease virus can alter characteristic neuronal circuits and destroy the hippocampal nerve cells. The scientists have developed an extremely practical petri dish test system. Their results show which cell types sustain the most damage following infection and also give indications as to how apoptosis (cell death) can be prevented.
The hippocampal circuits are well known. Since plastic processes at the hippocampal synapses are associated with learning and memory processes, the hippocampus is one of the best-explored regions of the brain. Numerous neurotropic viruses can lead to the destruction of nerve cells in this area, thus causing damage to the hippocampus. But what exactly happens during infection? How can hippocampus damage be prevented? The research group led by Prof. Dr. Bernd Heimrich at the Centre of Neuroscience and a group led by virologist Prof. Dr. Martin Schwemmle at the Institute of Microbiology and Hygiene, both at the University of Freiburg, have spent several years searching for answers to these questions. The researchers, who also work on the development of the hippocampus, have chosen Borna disease virus infection as their model system. The virus infects warm-blooded animals such as horses, sheep and rats. Borna disease virus infection causes progressive motor failure and abnormal behaviour. It is hoped that clarification of the aforementioned questions might help in the development of therapeutic strategies.
"It is difficult to investigate the pathological mechanisms in living animals (e.g. rats) once they have been infected," said Heimrich. "The animals develop severe neurological symptoms and die." The scientists have therefore chosen to use thin slices of rat hippocampus. When kept in suitable media, the slices are able to survive for up to 10 weeks. The researchers are able to manipulate the slices in whatever way they chose, for example by adding substances that alter the growth of nerve fibres or by infecting the slices with a virus. Using this system, Heimrich and his group were able to show that Borna disease virus triggers cell death in the hippocampus. They also found that the infection was similar to the infection in living animals and that certain cell populations, i.e. the so-called granule cells, died earlier than other cells. "Why are these cells so much more susceptible to viral infection than other cell types?" asks Heimrich.
In order to find an answer to this question, the researchers initially looked for rat strains that displayed a different course of infection. They eventually found a rat strain where the virus-infected granule cells survived in the hippocampus. Why are the granule cells of these animals resistant to cell death? This is a difficult question to answer. Initially, the researchers relied on guesswork.
In order to find evidence for their observation, the researchers tested susceptible hippocampi and compared them with normal ones. They placed tissue slices of resistant animals and non-resistant animals in the same petri dish. The slices were spatially separated in order to prevent the cells from establishing cell-cell contacts, at the same time as being immersed in the same medium, which enabled them to exchange soluble substances. The scientists then infected the two brain slices with Borna disease virus, which led to a clear result: the susceptible hippocampus slice suddenly became resistant to cell death and the granule cells survived the infection.
“We can safely assume that the tissue of the resistant rat strain produced a soluble agent that protects the granule cells from degeneration,” said Heimrich. “Nothing else can affect the other hippocampal slice in the chosen experimental set-up.” But what is this protective agent? Is there a genetic difference that might be of importance? Several crossing experiments eventually showed that inheritance plays a major role in this process. In addition, the researchers used filters with different pore sizes to assess the size of the sought-after molecule. The researchers will now use expression analyses to find the genes and gene products that play a role in this process. Will it be possible in future to administer the animals with a substance that protects them against paralysis? And can the findings of the Freiburg researchers be transferred to the situation in humans?The experiments have at least provided further insights into what happens in the brain after viral infection. Another example provides further insights: Heimrich and his team have used a live rat model to investigate how Borna disease virus infection alters the circuit pattern in the hippocampus. They found that infection leads to the reorganisation of several connections between the entrance from other brain regions and the processing cells in the hippocampus. Infection gradually leads to a massive reduction in granule cells. The nerve processes from other brain regions thus lose their communication partners and, maybe as a kind of compensation, these nerve fibres then form a larger number of synapses at the dendrites of other hippocampal cells. Synapses are the contact sites between nerve cells and are key in the transfer of information in the nervous system. The number of synapses varies depending on the duration of infection. This is clear evidence for the finding that viral infection in the brain leads to the reorganisation of neuronal circuits.
“We are unable to assess the consequences of these changes on the circuit pattern,” said Heimrich. “When the balance changes between the entrances from other brain regions and the hippocampal cells, this might also alter the balance in the excitability of the hippocampus. But this is all pure speculation.” If the speculation turns out to be true, it may well be that a Borna disease virus infection has an effect on plasticity in the hippocampus, and hence on the function of this brain structure with regard to memory formation. It is a matter of controversial debate among scientists as to whether the Borna disease virus also leads to human diseases. In any case, the work of the Freiburg researchers shows that virus-induced nerve cell death is associated with dynamic reorganisation processes of otherwise stable neuronal circuits.