Identifying an uptake mechanism for the antiepileptic and bipolar disorder treatment valproic acid using the simple biomedical modelDictyostelium

Abstract

SummaryValproic acid (VPA) is the most highly prescribed epilepsy treatment worldwide and is also used to prevent bipolar disorder and migraine. Surprisingly, very little is known about its mechanisms of cellular uptake. Here, we employ a range of cellular, molecular and genetic approaches to characterize VPA uptake using a simple biomedical model, Dictyostelium discoideum. We show that VPA is taken up against an electrochemical gradient in a dose-dependent manner. Transport is protein-mediated, dependent on pH and the proton gradient and shows strong substrate structure specificity. Using a genetic screen, we identified a protein homologous to a mammalian solute carrier family 4 (SLC4) bicarbonate transporter that we show is involved in VPA uptake. Pharmacological and genetic ablation of this protein reduces the uptake of VPA and partially protects against VPA-dependent developmental effects, and extracellular bicarbonate competes for VPA uptake in Dictyostelium. We further show that this uptake mechanism is likely to be conserved in both zebrafish (Danio rerio) and Xenopus laevis model systems. These results implicate, for the first time, an uptake mechanism for VPA through SLC4-catalysed activity.

Valproate-Induced Neurodevelopmental Deficits inXenopus laevisTadpoles

“…VPA blocks GABA transaminase, an enzyme that breaks down GABA (Löscher, 1999, Johannessen, 2000), resulting in elevated GABA levels in the CNS. Long-term alterations in GABA signaling during development are known to cause abnormalities in neural circuit formation and in developmental plasticity during the critical period in both mammals and Xenopus (Iwai et al, 2003;Shen et al, 2011), and these could account for some of the effects of VPA in our system. VPA is also known to reduce neuronal excitability through effects on voltage-gated Na ϩ and Ca 2ϩ channels (Löscher, 1999), and, in Xenopus tadpole tectum, long-term reductions in spike firing result in increased synaptic transmission, consistent with the findings in this study (Pratt and Aizenman, 2007).…”

An ancestral non-proteolytic role for presenilin proteins in multicellular development of the social amoeba Dictyostelium discoideum

“…However, the social amoeba Dictyostelium discoideum is the simplest model organism that possesses two presenilin proteins as well as the other three components of the c-secretase complex (Boeckeler and Williams, 2007;McMains et al, 2010). Dictyostelium has been extensively used in a range of motility (Janetopoulos and Firtel, 2008), developmental (Loomis and Shaulsky, 2011) and biomedical studies (Chang et al, 2012;Francione and Fisher, 2011;Ludtmann et al, 2011;Myre et al, 2011;Terbach et al, 2011), and has a number of experimental advantages over existing models (Williams et al, 2006). In this study, we employ the advantages of Dictyostelium to explore the role of the human presenilin 1 protein (PSEN1) in development.…”

Bestatin Inhibits Cell Growth, Cell Division, and Spore Cell Differentiation in Dictyostelium discoideum

“…More recently, it has been shown to be a useful organism for the elucidation of disease processes, including Huntington's and Alzheimer's diseases (29,31,53). It has also provided new insight into the mode of action of various drugs such as valproic acid and lithium, which are widely used mood stabilizers (50,53). The use of pharmaceuticals is especially valuable in this microorganism when gene knockouts are ineffective (18).…”