Investigating synaptopathy following traumatic brain injury in a preclinical model

Abstract

There is growing evidence that neural network disruption has a major contribution to Traumatic Brain Injury (TBI) morbidity. Understanding more about the impact of TBI on synaptic structure and function may help elucidate post-injury neural network disturbance. In this thesis we aimed to achieve this by studying two postsynaptic density proteins: post-synaptic density protein 95 (PSD95) and Synapse Associated Protein 102 (SAP102). These proteins are major scaffold proteins that assemble neurotransmitter receptors, channels and enzymes into multi-protein signalling complexes. We aimed to pursue the hypothesis that TBI disrupts the levels and location of PSD95 and SAP102 and thereby impairs synapse function contributing to post-injury neural network disruption.

In this thesis, we aimed to investigate mild TBI as it constitutes over three quarters of cases, patients can suffer from a range of symptoms and a substantial number do not get back to their pre-injury function. To do this, we validated a model of mild TBI using the Lateral Fluid Percussion Injury (LFPI) device. In wildtype mice, we found that a single mild LFPI led to an increased righting time (a simple behavioural assay) in the injury cohort. Histopathological analysis showed evidence of dysmorphic cortical cells and traumatic axonal pathology in the corpus callosum. Coupled to this, there was a significant inflammatory response within the injury cohort with elevated numbers of astrocytes and microglia. Together, this data showed evidence of behavioural, axonal and inflammatory changes after a mild LFPI.

The project utilized mice that had enhanced green fluorescent protein (eGFP) fused with the C-terminus of the endogenous PSD-95 protein and kusabira orange (mKO2) fused to SAP102. Male PSD95-eGFP and mKO2-SAP102 mice aged 8-16 weeks were randomised to a mild LFPI or sham and followed up to 7 or 28 days. Using high resolution confocal microscopy and machine learning approaches, PSD95 and SAP102 synaptome maps for puncta density, size and intensity were created. We found a significant reduction in synaptic puncta density at 28 days post-injury. This was evident in brain regions distal to the injury site including the contralateral cortex and hippocampus. We also observed evidence of synapse density recovery in the ipsilateral cortex between 7 and 28 days indicating synaptic recovery following a traumatic insult. There were differential patterns of change between PSD95 and SAP102 with evidence of more pronounced PSD95 puncta loss and recovery suggesting SAP102 is less vulnerable to TBI. We found evidence of a chronic inflammatory response with elevated numbers of microglia at 28 days. There was a negative association between puncta density and microglia numbers which may indicate a role for microglia in synapse removal post-TBI.

In conclusion, using a brain-wide unbiased synaptic mapping approach, we interrogated the impact of a mild traumatic injury on the postsynaptic density proteins PSD95 and SAP102. We observed a reorganization of the synaptome following injury which was progressive and involved brain regions distal from the injury site. Our study also highlighted the capacity for synaptic recovery post-injury and pointed towards a potential role of chronic inflammation on post-TBI synaptopathy.