Synaptic morphology, function, and regulation in a paediatric-onset neurodegenerative disorder

Abstract

The mucopolysaccharidoses are a group of inherited metabolic diseases wherein undegraded substrate accumulates within lysosomes as a result of dysfunctional lysosomal protein. The most common sub-group is mucopolysaccharidosis (MPS) III (collectively 1 in 70,000). In Australia, MPS IIIA is the most common sub-type and the focus of this study. MPS IIIA is caused by an autosomal recessive mutation in the sulphamidase gene, leading to accumulation of heparan sulphate. Progressive neurological symptoms are the primary manifestation, including impaired control of motor function. Despite the abnormal intracellular pathology, negligible neuronal cell loss occurs. This suggests that neurological dysfunction does not arise from cellular loss, but rather discrete structural or functional changes in the neurons themselves. Disease pathogenesis is presently not well understood, and the molecular basis of cognitive impairment is unknown. In this study, an MPS IIIA mouse model was utilised to: 1. Establish the time-course of changes in the morphology and number of dendritic spines on layer 5 pyramidal neurons in the cerebral motor cortex. 2. Elucidate the biological impact of synaptic dysfunction on neuronal activity by electrophysiology. 3. Determine the underlying mechanisms contributing to dendritic spine dysgenesis/loss. 4. Determine if synaptic degeneration can be prevented by administering AAV9-based gene therapy prior to symptom-onset. The hypothesis was that changes in synaptic morphology and function underlie cognitive decline in MPS IIIA. The aim of the study was to increase understanding of neurological dysfunction in MPS IIIA, and potentially provide therapeutic targets for this presently untreatable disease. Dendritic spine deficiencies were evident in the pre-symptomatic MPS IIIA mouse brain, with loss of immature and mature spine sub-types, and reductions in overall density. The deficits persisted with disease progression to at least 20-weeks of age. Whilst this did not alter electrophysiological action potential frequency/kinetics, reductions in excitatory post-synaptic current events and altered event kinetics were observed in these same neurons in the pre-symptomatic disease stage. Therefore, these structural and functional changes may be contributing to symptom onset and could help explain the neurological decline seen in MPS IIIA. Immunofluorescence-based techniques demonstrated that pro-inflammatory glial cells were closely apposed to layer 5 pyramidal neurons in the MPS IIIA mouse motor cortex from a pre-symptomatic age when synaptic loss also occurs. Additionally, the recently identified A1-type reactive astrocyte was highly prevalent in the MPS IIIA mouse motor cortex. These cells have been linked to/associated with higher expression of inflammatory complement components which bind to and identify synapses for removal by microglia. This suggests over-active synaptic pruning could be contributing to the observed synaptic loss via pro-inflammatory marker expression. Gene therapy delivered intraventricularly to MPS IIIA mice at birth resulted in prevention/delay of heparan sulphate accumulation and activation of astrocytes and microglia and appeared to normalise dendritic spine density and maturation to 20-weeks of age. However, longer-term studies are needed to establish the longevity of this effect. The findings, coupled with existing literature, indicate that synaptic dysfunction occurs in various neurons in the MPS III brain, and suggests that the evaluation of novel therapies should examine discrete changes to the brain architecture.