Abstracts

Rationale: Mutations of voltage-gated sodium channels (Nav) are implicated in neurological disorders like epilepsy. It is not well known how much post-transcriptional and translational modifications of Nav genes contribute to these channelopathies. In the slamdance epileptic mutant of the fruit fly, Drosophila melanogaster, an increase in the Nav alternative splicing of one exon L results in an increased persistent sodium (NaP) current (Lin, Günay, Marley, Prinz, Baines, 2012. J Neurosci. 32(21):7267-7277). The change in splicing and increase in the NaP current can be partially reversed using the anti-convulsive drug phenytoin and other activity-modulation therapies in the fly. However, measuring the effect of splicing changes on neuronal activity is more difficult in genetic mutants because of compensatory regulation. Because of this, the biological experiments can be complemented by using a computational model for testing isolated manipulations of Nav parameters. Methods: Individual Nav splice variants can be heterologously expressed and their biophysical properties measured in Xenopus oocytes (Lin, Wright, Muraro, Baines, 2009. J Neurophysiol 102:1994-2006). We previously constructed a single-compartment computational model of an identified larval Drosophila abdominal dorsomedial motoneuron called aCC or MN1-Ib that innervates the dorsal muscles (Gunay C et al., 2011. Program No. 708.01. Society for Neuroscience abstract). This model reproduces the firing rate and membrane potential response characteristics recorded from aCC motoneurons and allows simulating splicing changes in Nav properties. However, the smallness of the fly motoneurons causes filtering and offset responses observed in recordings, which cannot be replicated using a single-compartment model. By adding a second compartment in the model motoneuron to separate the active channels, we replicated these recorded responses. Results: Using the two-compartment model motoneuron, we manipulated model parameters to mimic splicing changes observed in epileptic fly mutants. The increase in the NaP magnitude was confirmed to have an excitatory effect (Lin et al 2012, see above). The decrease in Nav activation voltage also had excitatory effect that we quantified. Furthermore, the modulatory effect of the altered presynaptic activity properties on the model motoneuron activity was characterized. Conclusions: Although similar splicing patterns have not yet been observed in the mammalian Nav, the extensive splicing in the fly Nav promises a tractable testbed for examining the effects of genetic changes in Nav on neuronal activity. Here, we present a computational motoneuron model that provides mechanistic explanations and predictions of increased excitability in epileptic fly mutants.