Rescue of the Functional Alterations of Motor Cortical Circuits in Arginase Deficiency by Neonatal Gene Therapy

Abstract

Arginase 1 deficiency is a urea cycle disorder associated with hyperargininemia, spastic diplegia, loss of ambulation, intellectual disability, and seizures. To gain insight on how loss of arginase expression affects the excitability and synaptic connectivity of the cortical neurons in the developing brain, we used anatomical, ultrastructural, and electrophysiological techniques to determine how single-copy and double-copy arginase deletion affects cortical circuits in mice. We find that the loss of arginase 1 expression results in decreased dendritic complexity, decreased excitatory and inhibitory synapse numbers, decreased intrinsic excitability, and altered synaptic transmission in layer 5 motor cortical neurons. Hepatic arginase 1 gene therapy using adeno-associated virus rescued nearly all these abnormalities when administered to neonatal homozygous knock-out animals. Therefore, gene therapeutic strategies can reverse physiological and anatomical markers of arginase 1 deficiency and therefore may be of therapeutic benefit for the neurological disabilities in this syndrome.

Hepatic arginase deficiency fosters dysmyelination during postnatal CNS development

“…P2 AAV hepatic gene therapy rescued the defects associated with myelinated axons in the regions examined and strongly implicates the functional recovery of oligodendrocytes after gene therapy. Secondly, by extending our previous studies (18), we further demonstrated that P2 AAV hepatic gene therapy can rescue the defects of excitatory synaptic density and perforated synapses in layer V of the Arg1-KO motor cortex up to 4 months of age, strongly indicating that early postnatal treatment can drastically change the synaptic organization in layer V and ultimately lead to recovery of function in the corticospinal pathway. Dendritic arborization and soma size are altered in Arg1 deficiency.…”

“…Hypothesis I: Deficiency of arginase 1 results in CNS dysmyelination. In our previous study (18), we demonstrated that, in motor cortex layer V of Arg1-KO mice at P15, a significant decrease in both the density of asymmetrical synapses and AMPA receptor-enriched (AMPAR-enriched) perforated synapses (19)(20)(21) corresponded with a malfunction in glutamatergic excitatory synaptic transmission. We also found in that study that intrinsic excitability is altered in proportion to the copy number loss of arginase and that the circuit connectivity is altered in KO neurons, as shown in the electrophysiological studies by decreased frequencies of miniature excitatory postsynaptic currents (mEPSCs) and the decrease in amplitude of miniature inhibitory postsynaptic currents (mIPSCs).…”

“…Until recently, little was known of the neurological compromise that occurs in arginase deficiency. Our group had previously found decreased dendritic complexity, synapse number, and intrinsic excitability, as well as the presence of functional synaptic deficits in homozygous Arg1-KO mice; remarkably, hepatic-based gene therapy on P2 led to substantial improvement in these abnormalities when examined at P14 (18). However, those studies did not explore the etiology of the unique neuromotor findings, nor did they examine the longterm effectiveness of a therapy initiated during the neonatal period, when neurodevelopment is incomplete.…”

“…The greatest differences in the Sholl analysis were detected as the distance increased in relationship to the cell body; most distally, there were relatively less differences between the 3 groups. Thus, the basal dendritic complexity, which was shown to be dramatically decreased by loss of Arg1 in a graded manner (18), can be sub- stantially rescued and maintained long-term with neonatal hepatic-based Arg1 gene therapy.…”

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“…The reduction of the neurological symptoms and the decrease of the levels of arginine and ammonia are essential [49,50].…”