Baculovirus entry into the central nervous system of Spodoptera exigua caterpillars is independent of the viral protein tyrosine phosphatase

Neuroparasitism concerns the hostile take-over of a host's nervous system by a foreign invader, in order to alter the behaviour of the host in favour of the parasite. One of the most remarkable cases of parasite-induced host behavioural manipulation comprises the changes baculoviruses induce in their caterpillar hosts. Baculoviruses may manipulate caterpillar behaviour in two ways: hyperactivity (increased movement in the horizontal plane) and/or tree-top disease (movement to elevated levels in the vertical plane). Those behavioural changes are followed by liquefaction and death of the caterpillar. In Autographa californica multiple nucleopolyhedrovirus (AcMNPV)-infected Spodoptera exigua caterpillars, an enzymatic active form of the virally encoded protein tyrosine phosphatase (PTP) is needed for the expression of hyperactivity from 3 days post infection (dpi). Using eGFP-expressing recombinant AcMNPV strains, we show that infection of the caterpillar's central nervous system (CNS) can be observed primarily from 3 dpi onwards. In addition, we demonstrate that the structural and enzymatic function of PTP does not play a role in infection of the CNS. Instead we show that the virus entered the CNS via the trachea, progressing caudally to frontally through the CNS and that the infection progressed from the outermost cell layers towards the inner cell layers of the CNS, in a PTP independent manner. These findings help to further understand parasitic manipulation and the mechanisms by which neuroparasites infect the host nervous system to manipulate host behaviour.

Where the baculoviruses lead, the caterpillars follow: baculovirus-induced alterations in caterpillar behaviour

Baculoviruses are well-known for altering the behaviour of their caterpillar hosts by inducing hyperactivity (enhanced locomotion) and/or tree-top disease (climbing to elevated positions before death). These features, along with the genomic small size of baculoviruses compared to non-viral parasites and the at hand techniques for producing mutants, imply that baculoviruses are excellent tools for unravelling the molecular mechanisms underlying parasitic alteration of host behaviour. Baculoviruses can be easily mutated, allowing an optimal experimental setup in comparative studies, where for instance host gene expression can be compared between insects infected with wild-type viruses or with mutant viruses lacking genes involved in behavioural manipulation. Recent studies have revealed the first insight into the underlying molecular pathways that lead to the typical behaviour of baculovirus-infected caterpillars and into the role of light therein. Since host behaviour in general is mediated through the host's central nervous system (CNS), a promising future step will be to study how baculoviruses regulate the neuronal activity of the host.