Project Summary/Abstract
The urea cycle is the major pathway for detoxification of ammonia in mammals. In humans, arginase 1 deficiency
is characterized clinically by progressive mental impairment, spasticity, and growth retardation, with only periodic
episodes of hyperammonemia unlike the other urea cycle disorders where this is much more common. In recent
experiments, we have found substantial anatomical, ultrastructural and electrophysiological differences between
knockout animals and wild type controls. This includes decreased intrinsic excitability, altered functional synaptic
transmission, decreased dendritic arborization and decreased synapse density. These measurable differences at
the neuron, synapse, and circuit level have begun to elucidate the functional abnormalities in arginase
deficiency. However, a human model is essential as there are critical differences between mice and humans in
disease presentation for this disorder. Such a model will allow establishment of high-throughput screens to
determine the compounds responsible for the neuronal abnormalities detected and to discover compounds that
may improve cognitive and motor functions. In this proposal the Lipshutz Lab will examine the role arginase
plays in the normal development of neurons and circuits as they have hypothesized that anatomic and functional
abnormalities develop in neurons of arginase deficient patients and that such changes are responsible for the
phenotype detected in humans with homozygous deficiency of arginase 1. In addition, the lab will study
metabolites hypothesized to be involved in the pathogenesis of the disorder, thus opening doors to potential
pharmacological interventions. Preliminary data: This research group has (amongst other findings): 1)
constructed and characterized the arginase 1 knockout mouse; 2) demonstrated long-term survival and rescue
with liver-specific arginase 1 expression by recombinant adeno-associated viral vectors; 3) demonstrated that
only low-level ureagenesis is necessary for long-term survival; 4) shown that single copy (heterozygotes) or
double copy (homozygotes) of loss of arginase gene expression results in abnormalities of intrinsic excitability
and the dendritic arbor of murine neurons; 5) shown, using an array of tests, that AAV-treated liver-specific
arginase knockout animals have persistent abnormalities of neurons at the synapse and dendritic arbor level and
in their electrophysiologic response; and 6) shown that peripheral metabolism can result in control of circulating
plasma arginine. In Aim 1, the hypothesis that human pluripotent stem cell (hPSC)-derived neurons with deficient
arginase expression will show deficits in synaptogenesis, excitability, and neurite development will be tested. In
Aim 2, we will determine the effect of arginine and other presumptive causative agents on the intrinsic excitability
and synaptic physiology of arginase-deficient hPSC-derived neurons. This proposal from a team of investigators
with complementary expertise in arginase deficiency, stem cell biology with in vitro neuron development, and
electrophysiology will be critical for building and validating this human neuronal culture model.
