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. Because I810N/+ mice die prematurely from tonic-clonic seizures from about 4 weeks of age, with a lifespan typically under 1 year, the I810N mutation is classified as harmful with a severe phenotype. The current estimate of the number of mice required for the AHC research in the Clapcote lab is about 800 mice per year or 4000 mice over a 5-year period. Similar studies in 8 other laboratories in Europe and the USA with highly related research interests would require a similar number of mice, giving a total usage of 36,000 mice over a 5-year period.
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 locomotory 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. Despite this AHC-relevant phenotype of ATPalpha mutant flies, and the structural and functional similarities between the NKA alpha subunits of mammals and insects, Drosophila has not been widely adopted as a model system for ATP1A3-related encephalopathy. This is likely because ATPalpha is orthologous to all mammalian NKA alpha subunits, such that ATPalpha is not a specific one-to-one orthologue of ATP1A3. 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 replacement model system in the study of AHC. The student will use a variety of experimental approaches to assess whether NKA alpha subunit-modified flies could be a valid invertebrate model of AHC. Critically, the project will determine whether WT and G755S human ATP1A3 cDNA transgenes are able to, respectively, rescue or phenocopy ATPalpha mutant flies, thus interrogating the functional equivalence of the proteins encoded by the ATPalpha (Drosophila) and ATP1A3 (human) genes. The heads of other laboratories have reported that they would consider replacing mice in their AHC studies if a valid Drosophila model of AHC were available.