Long-term upregulation of inflammation and suppression of cell proliferation in the brain of adult rats exposed to traumatic brain injury using the controlled cortical impact model

Abstract

The long-term consequences of traumatic brain injury (TBI), specifically the detrimental effects of inflammation on the neurogenic niches, are not very well understood. In the present in vivo study, we examined the prolonged pathological outcomes of experimental TBI in different parts of the rat brain with special emphasis on inflammation and neurogenesis. Sixty days after moderate controlled cortical impact injury, adult Sprague-Dawley male rats were euthanized and brain tissues harvested. Antibodies against the activated microglial marker, OX6, the cell cycle-regulating protein marker, Ki67, and the immature neuronal marker, doublecortin, DCX, were used to estimate microglial activation, cell proliferation, and neuronal differentiation, respectively, in the subventricular zone (SVZ), subgranular zone (SGZ), striatum, thalamus, and cerebral peduncle. Stereology-based analyses revealed significant exacerbation of OX6-positive activated microglial cells in the striatum, thalamus, and cerebral peduncle. In parallel, significant decrements in Ki67-positive proliferating cells in SVZ and SGZ, but only trends of reduced DCX-positive immature neuronal cells in SVZ and SGZ were detected relative to sham control group. These results indicate a progressive deterioration of the TBI brain over time characterized by elevated inflammation and suppressed neurogenesis. Therapeutic intervention at the chronic stage of TBI may confer abrogation of these deleterious cell death processes.

Figure 1. Upregulation of MHCll+ activated microglia cells in gray matter in chronic TBI.

Results indicate that there is a clear exacerbation of activated microglia cells in ipsilateral side of subcortical gray matter regions in chronic TBI relative to contralateral side and sham control. After 8 weeks from initial TBI injury, asterisks denote significant upregulation on the volume of MHC II expressing cells in A) cortex, B) striatum, C) thalamus. While contralateral side present an estimated volume of activated microglia cells similar to sham control animals. ANOVA revealed significant treatment effects as follows: cortex, F_2,45 = 18.49; ***p<0.005; striatum, F_2,45 = 15.71, ***p<0.005, and; thalamus, F_2,45 = 12.23, ***p<0.005. Photomicrographs correspond to representative gray matter in coronal sections stained with OX6 (MHC ll) from ipsilateral sham control and TBI rats, cortex (Figure 1D, E), striatum (Figure 1F, G), thalamus (Figure 1H, I). Scale bars for D, E, F, G, H, I = 1 µm.

Figure 2. Upregulation of MHCll+ activated microglia cells in white matter in chronic TBI.

Results indicate that there is an upregulation of activated microglia cells after 8 weeks post TBI in proximal white matter areas. There is an upregulation of MHCll+ cells in the ipsilateral and contralateral side of corpus callosum relative to sham control (Figure 2A). In contrast, upregulation of MHCll+ activated microglia cells in the cerebral peduncle (Figure 2B) and fornix (Figure 2C) is only present in the ipsilateral side as compared with the contralateral and sham control. There were no significant differences between contralateral side and sham control animals in (Figure 2B) and (Figure 2C). ANOVA revealed significant treatment effects as follows: corpus callosum, F_2,45 = 5.656; *p<0.05; cerebral peduncle, F_2,45 = 27.39, ***p<0.0005, and; fornix, F_2,45 = 5.541, *p<0.05. Representative photomicrographs, ipsilateral corpus callosum, sham-control Figure 2E and TBI Figure 2F, ipsilateral cerebral peduncle, sham-control Figure 2G and TBI Figure 2H, and ipsilateral Fornix, sham-control Figure 2I and TBI Figure 2J. Scale bars for Figure 2E, F, G, H, I, J = 1 µm. A summary of MHCll+ estimated volume is presented capturing different subcortical regions; including those proximal and distal from TBI insult (Figure 2D). Chronic TBI greatly upregulates the neuroinflammation in the thalamus expressing the highest upregulation of MHCll+ activated microglia cells, despite its distal subcortical location. Strong expression of MHCll+ activated microglia cells is also detected in the corpus callosum and striatum (Figure 2D).

Figure 3. Hippocampal CA3 cell loss and downregulation of cell proliferation.

H&E staining revealed a significant cell loss in the hippocampal CA3 region after chronic TBI (Figure 3A). Ki67, a cell proliferation marker, revealed a significant chronic TBI-related decrease in the SVZ of cell proliferation only in the ipsilateral side relative to contralateral side and sham control animals (Figure 3B). Contralateral measurements revealed that cell proliferation also decrease, but it does not show significant differences when compared with sham control animals (Figure 3B). Also, Ki67 revealed a significant decrease in cell proliferation in the SGZ of the hippocampus in the ipsilateral side compared to sham control (Figure 3C). In summary, ANOVA revealed significant treatment effects as follows: Hippocampal CA3 neurons, F_2,9 = 10.78, ***p<0.005; SVZ, F_2,45 = 10.45, ***p<0.005, and; SGZ, F_2,45 = 3.755, ***p<0.005. Representative photomicrographs from coronal sections ipsilateral CA3 region stained with hematoxylin/eosin in sham control and TBI rats (Figure 3D, E). Ipsilateral SVZ from sham-control and TBI rats (Figure 3F, G) and ipsilateral SGZ from sham-control and TBI rats (Figure 3H, I) are shown. Scale bars for Figures 3D, E, F, G, H, I = 50 µm.

Figure 4. Neural differentiation is not affected by chronic TBI.

DCX staining, neural differentiation marker revealed that there is not significant impairment in neural differentiation in either SVZ of the lateral ventricle, or the SGZ of the hippocampus relative to contralateral side and sham control animals. The “ns” denotes non-significant differences (p>0.05). Representative coronal sections from ipsilateral SVZ stained with DCX in sham control and TBI rats (Figure 4C, D) and SGZ from sham-control and TBI rats (Figure 4E, F) are shown. Scale bars for Figure 4C, D, E, F = 50 µm.