Viral interference with neuronal integrity: what can we learn from the Borna disease virus?

The neurotropic Borna disease virus (BDV) is unusual in that it can persistently infect neurons of the central nervous system (CNS) without causing general cell death, reflecting its favourable adaptation to the brain. The activity-dependent enhancement of neuronal network activity is however disturbed after BDV infection, possibly by its effect on the protein kinase C signalling pathway. The best model for studying BDV, which has a non-cytolytic replication strategy in primary neurons, is the rat. Infection of adult rats results in a fatal immune-mediated disease, whereas BDV establishes persistent infection of the brain in newborn rats resulting in progressive neuronal cell loss in defined regions of the CNS. Our recently developed system of BDV-infected hippocampal slice cultures has clearly shown that the onset of granule cell loss begins after the formation of the mossy fibre projection. Quantitative analysis has revealed a significant increase in synaptic density on identified remaining granule cell dendrites at 6 weeks after infection, followed by a decline. Granule cells are the major target of entorhinal afferents. However, despite an almost complete loss of dentate granule cells during BDV infection, entorhinal axons persist in their correct layer, both in vivo and in slice cultures, possibly exploiting rewiring capabilities and thereby allowing new synapse formation with available targets. These morphological observations, together with electrophysiological and biochemical data, indicate that BDV is a suitable model virus for studying virus-induced morphological and functional changes of neurons and connectivity patterns.