Genetic analysis of Shudderer, the lithium-responsive neurological mutant of Drosophila melanogaster

Lithium has been used for more than 50 years as a primary therapy for bipolar affective disorder (BPD) and has proven highly effective for both acute and long-term phases of the disease. Unfortunately, the molecular and cellular mechanisms underlying the mood-stabilizing action of lithium for the treatment of BPD remains largely unknown. In an effort towards understanding the complexities of lithium's action in the nervous system, I have utilized the Drosophila neurological mutant Shudderer (Shu). Previous findings have suggested that the adult Shu phenotypes may be improved by providing a diet containing millimolar concentrations of lithium.

Using well-established genetic techniques and behavioral paradigms I thoroughly characterized the Shu mutant phenotypes. I found that the mutant displays morphological and behavioral abnormalities indicative of dysregulated neuronal excitability that include: down-turning wings and indented dorsal thorax, defects in negative geotaxis, deficits in locomotion, abnormal sleep architecture and unusual patterns of leg-shaking behaviors upon recovery of ether anesthetics. Furthermore, I confirmed that lithium was able to significantly improve many aspects of Shu behaviors.

Recombination-based mutation mapping in Shu revealed that the genetic lesion lies somewhere within the gene CG9907, which encodes the voltage-gated sodium channel á-subunit paralytic (para). Subsequent genetic experiments using para hypomorphic mutant alleles as well as a UAS-RNAi/GAL4 system showed that a reduction in sodium channel levels resulted in a drastic improvement of the mutant defects. Together, these data suggest that the lithium-responsive Shu mutant is likely a gain-of-function allele of para. Sequencing of the entire para coding region identified a missense mutation in a highly conserved region of the para coding sequence, in transmembrane segment S2 of homology domain III ((M1350I). To date, this is the first known discovery of a sodium channel mutant allele in Drosophila which causes hyperactivity. These data suggest that the Shu phenotypes are somehow caused by an increase in sodium channel activation.

Lastly, I identified a number of genes likely to functionally interact with the Shu mutation. Of note, the Ca2+/calmodulin-activated Ser/Thr protein phosphatase alpha subunit gene CanA-14F is up-regulated in Shu and reduction of its activity suppresses the mutant phenotypes. Furthermore, a large percentage of genes encoding anti-microbial peptides (AMP) were also significantly up-regulated in Shu, possibly acting downstream of CanA-14F. A genetic deficiency screen looking for genes that alter the Shu phenotypes has identified that the gene Glutathione S-transferase S1 (Gsts1) suppresses the morphological and behavioral defects in the lithium-responsive mutant. Overall, these genes will help decipher how the gain-of-function sodium channel Shu mutation alters nervous system function. In addition, they will shed light on those mechanisms responsible for lithium's mood-stabilizing effects in the brain.