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Traumatic Brain Injury (TBI) is an enormous global socio-economic burden since, apart from its high death rate, it is the primary cause of coma worldwide and a prevalent cause of long-term disability. Until today there is no established treatment for dealing with the long-term outcomes of TBI despite many years of research. Although a lot is known about the pathophysiology of TBI in the damaged tissue and the surrounding area in case of focal lesion, only few studies have investigated the structural and functional integrity of the contralateral intact cortex. In order to explore this territory, this study employs a well-established and widely used animal model of focal open skull TBI known as the Controlled Cortical Impact (CCI) model. The first aim of this study was to systematically characterize a specific neuronal population, the transcallosal projection neurons, as they are the ones connecting the intact cortex with the lesioned cortex. The description of the organization of transcallosal neurons and their axonal projections at the contralateral hemisphere was carried out in healthy, non-injured C57Bl6 mice. Retrograde and anterograde tracing methods were implemented to label transcallosal cell bodies and their axonal projections, respectively. In addition, different injection coordinates were used in order to label transcallosal connections at distinct brain regions, including the motor cortex (M1), somatosensory cortex (S1), and barrel cortex, rostral and caudal to Bregma. In agreement with previous research, I observed that transcallosal projections are organized homotopically across the various brain regions, with the axonal terminals spanning the entire cortical column. Interestingly my study describes for the first time a non-negligible fraction of heterotopic transcallosal neurons that, in addition, display a slightly less strict layer distribution pattern compared to the homotopic ones. After the initial characterization of transcallosal neuron organization, I proceeded by investigating how these neurons with projections at the injury site are affected at various timepoints following focal TBI. I used GFPM mice to visualize dendrites and spines of transcallosal and non-transcallosal neurons, in order to examine their structural integrity at different timepoints post-injury. I detected significant differences in dendritic spine density and morphology between controls and injured mice, which were time-dependent. More specifically, the dendritic spine density in transcallosal neurons was strongly decreased as soon as 7days following injury. Interestingly, spine density in non-transcallosal neurons was not changed following TBI. In terms of spine shape, I found a morphological shift only for the apical tuft segments. These results point towards a general sensitivity of transcallosal spines to TBI-induced damage, where loss of spines (preferentially mature) seems to take place at 1-2 weeks post-injury and resolve at 3-6 weeks post-injury, indicative of late plasticity processes. As the anatomically connected neuronal population seems to recover overtime I then decided to further explore whether transcallosal circuit remodeling takes place after TBI. To do so I used the retrograde mono-trans-synaptic tracer SADΔG-GFP (EnvA) Rabies virus. In that way, I was able to distinctively label transcallosal neurons and their presynaptic partners and obtain an overview of the presynaptic population throughout the cortex across brain regions at different post-injury timepoints. This study demonstrates that spine plasticity did not result in adaptive circuit plasticity with the recruitment of other brain regions but rather that initial circuits were re-established. In brief, during this thesis I have demonstrated the adaptive plastic capacities of anatomically connected neurons to the brain injury. I believe that this knowledge may help in unraveling further compensatory plastic mechanisms that could then be therapeutically targeted to improve the outcome following brain injury. |
