Development of an invertebrate model of alternating hemiplegia of childhood

Heterozygous mutations in ATP1A3, encoding the Na+,K+-ATPase (NKA) alpha3 subunit, are identified as the cause of a phenotypic continuum of ATP1A3-related encephalopathy that includes alternating hemiplegia of childhood (AHC), a neurodevelopmental disorder that manifests as episodic hemiplegia starting in the first 18 months of life, with a spectrum of persistent motor, movement, and cognitive deficits that become progressively more apparent with age. Current studies to better understand the phenotypic effects of NKA alpha3 dysfunction rely heavily on the use of mice. The Clapcote lab and others have developed mouse models that have mutations, such as I810N, identical to those in AHC patients. Heterozygous alpha3-I810N (I810N/+) mice exhibit an unsteady, tremulous gait with occasional spontaneous bouts of hemiplegia. Other abnormalities include low body mass, CA3-CA1 hyperexcitability, increased susceptibility to seizures, locomotor hyperactivity, balance deficits, social deficits and cognitive impairment.

An interesting aspect of AHC in humans is that hemiplegic episodes are triggered by stressful events. We can reliably induce hemiplegia in I810N/+ mice with 100% penetrance by subjecting them to forced swimming in 20 degrees C water for 2 minutes. Notably, this stress-induced locomotor impairment is well recapitulated in Drosophila melanogaster fruit flies heterozygous for mutation G744S in ATPalpha, orthologous to the NKA alpha subunit, at an amino acid residue equivalent to that affected by recurrent mutations in NKA alpha3 in AHC patients. The Drosophila NKA alpha subunit has an amino acid sequence identity of 76% with the NKA alpha3 of humans and mice, but also has amino acid sequence identities of 75-76% with alpha1 and alpha2. Previous work in the Clapcote lab showed that NKA alpha3 is the functionally dominant alpha-isoform in the postnatal mammalian brain, whereas alpha1 and alpha2 have more significant contributions during development in utero.

The overall goal of the PhD project is to interrogate the power and advantages of Drosophila as a model system in the study of AHC. The key objectives are to:
1) Quantify the extent that ATPalpha mutant flies phenocopy NKA alpha3 mutant mice and AHC patients;
2) Generate WT and G755S human cDNA ATP1A3 transgenic lines in Drosophila;
3) Determine whether WT and G755S ATP1A3 transgenes are able to rescue or phenocopy ATPalpha mutant flies.