Abstract
Matrix metalloproteinase-9 is a secreted endopeptidase targeting extracellular matrix proteins, creating permissive environments for neuronal development and plasticity. Developmental dysregulation of MMP-9 may also lead to neurodevelopmental disorders (ND). Here we test the hypothesis that chronically elevated MMP-9 activity during early neurodevelopment is responsible for neural circuit hyperconnectivity observed in Xenopus tadpoles after early exposure to valproic acid (VPA), a known teratogen associated with ND in humans. In Xenopus tadpoles, VPA exposure results in excess local synaptic connectivity, disrupted social behavior and increased seizure susceptibility. We found that overexpressing MMP-9 in the brain copies effects of VPA on synaptic connectivity, and blocking MMP-9 activity either pharmacologically or genetically reverses effects of VPA on physiology and behavior. We further show that during normal neurodevelopment MMP-9 levels are tightly regulated by neuronal activity and required for structural plasticity. These studies show a critical role for MMP-9 in both normal and abnormal development.
Summary Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by deficits in social and cognitive functions. Prenatal exposure to valproic acid (VPA) results in ASD in humans and in neurodevelopmental abnormalities in other animal models. Similarly, exposure to VPA during a critical developmental period in Xenopus tadpoles causes behavioral and electrophysiological abnormalities consistent with hyperconnected neural networks. VPA exposure results in upregulation of matrix metalloproteinase-9 (MMP-9) levels in tadpole brains, suggesting a role of MMP-9 in VPA-induced neurodevelopmental disorders. MMP-9 is a matrix metalloproteinase that cleaves various components of the extracellular matrix, enabling synaptic and circuit level reorganization and plasticity. We found that experimental upregulation of MMP-9 mimics VPA-induced effects while downregulation rescues these effects. Overexpression of MMP-9 resulted in significant increase in the frequency of sEPSCs and sIPSCs, similar to the VPA-effects while inhibition of MMP-9 rescued these effects, as well as behavioral effects, without altering the basal transmission. To test the effects of MMP-9 during normal activity-induced structural plasticity, we exposed tadpoles to enhanced visual stimulation which resulted in both elevated MMP-9 levels and increased tectal cell dendritic growth. This growth could be arrested by using MMP-9 inhibitors, indicating that transient expression of MMP-9 promotes growth. Taken together, our findings suggest that early VPA exposure results in chronically elevated MMP-9 levels. This results in hyperplasticity leading to hyperconnected neural networks which elicit ASD-related behaviors. This study demonstrates that dysregulation of MMPs during early brain development can be an important contributor to the etiology of neurodevelopmental disorders.
Competing Interest Statement
The authors have declared no competing interest.