45. Rescue of the Functional Alterations of Motor Cortical Circuits in Arginase 1 Deficiency with AAV-Based Gene Therapy

Abstract

The urea cycle is the main mechanism for terrestrial mammals to detoxify excess nitrogen. Disorders of the proximal urea cycle characteristically have periodic episodes of hyperammonemia leading to often severe and permanent neurological deterioration & disability. Ammonia has been implicated by compromising potassium buffering of astrocytic membranes and causing clinical neurological abnormalities by impairing cortical inhibition. Complete arginase 1 (Arg1) deficiency, a distal urea cycle disorder, is the least severe of these abnormalities, demonstrating neurological impairment including spasticity, loss of ambulation and seizures; while characterized by the presence of hyperargininemia, hyperammonemia is not a frequent clinical finding. While mortality is unfortunately common due to acute episodes of hyperammonemia in proximal urea cycle disorders, patients with hyperargininemia often are long-lived, however, suffering from progressive intellectual disability and spastic diplegia, and the mechanisms underlying the neurological dysfunction are not understood. To gain better insight on how the loss of arginase expression causes dysfunction in the developing brain, and if gene therapy could prevent these abnormalities, we studied how the excitability and functional and anatomical connectivity of motor cortical neurons are altered in the disorder using the murine knockout model. In addition, we examined if AAV expressing Arg1, administered IV on postnatal day 2, could rescue these findings. Results: Single- and double-copy loss of Arg1 caused dose-dependent decreases in intrinsic excitability, dendritic arborization complexity, and synapses in motor cortex layer V neurons. These findings show that 1) the intrinsic excitability of neurons of homozygous Arg1 knockout mice is abnormal and that, unexpectedly, heterozygous neurons (single copy loss) exhibit an intermediate phenotype compared to wild type and homozygous knockouts (double copy loss) (Fig. AFig. A); corresponding loss of Arg1 decreased the frequency of miniature excitatory postsynaptic currents and the amplitude of miniature inhibitory postsynaptic currents; 2) neuronal branching and spine phenotypes differ between genotypes with, unexpectedly, an intermediate phenotype for heterozygotes (Fig. BFig. B); and 3) with electron microscopic analysis and comparison of layer V synapses from arginase 1 knockout, heterozygous, and wild type mice, there is a very low density of excitatory (i.e. asymmetrical) synapses (Fig. CFig. C) in the knockout and decreased number of inhibitory (perisomatic) synapses (i.e. symmetrical) on somata of pyramidal cells, both dramatic findings. Finally, changes in synaptic morphology and abnormal ultrastructural features were found in knockout mice, also suggesting neuronal degeneration and inflammation. Neonatal intravenous administration on the second postnatal day with AAV expressing arginase 1 by a hepatocyte-specific promoter led to a near-resolution of these abnormalities when administered to homozygous Arg1 knockout animals. Summary: Our studies suggest that arginase 1 deficiency leads to severe and specific changes to intrinsic excitability and synaptic connectivity of motor cortical circuits. Importantly, we find that neonatal AAV-based Arg1 gene expression is effective in reversing both the physiological and anatomical hallmarks of the disorder.View Large Image | Download PowerPoint Slide