Functional and transcriptional profiling of microglial activation during the chronic phase of TBI identifies an age-related driver of poor outcome in old mice

Abstract

Elderly patients with traumatic brain injury (TBI) have greater mortality and poorer outcomes than younger individuals. The extent to which old age alters long-term recovery and chronic microglial activation after TBI is unknown, and evidence for therapeutic efficacy in aged mice is sorely lacking. The present study sought to identify potential inflammatory mechanisms underlying age-related outcomes late after TBI. Controlled cortical impact was used to induce moderate TBI in young and old male C57BL/6 mice. At 12 weeks post-injury, aged mice exhibited higher mortality, poorer functional outcomes, larger lesion volumes, and increased microglial activation. Transcriptomic analysis identified age- and TBI-specific gene changes consistent with a disease-associated microglial signature in the chronically injured brain, including those involved with complement, phagocytosis, and autophagy pathways. Dysregulation of phagocytic and autophagic function in microglia was accompanied by increased neuroinflammation in old mice. As proof-of-principle that these pathways have functional importance, we administered an autophagic enhancer, trehalose, in drinking water continuously for 8 weeks after TBI. Old mice treated with trehalose showed enhanced functional recovery and reduced microglial activation late after TBI compared to the sucrose control group. Our data indicate that microglia undergo chronic changes in autophagic regulation with both normal aging and TBI that are associated with poorer functional outcome. Enhancing autophagy may therefore be a promising clinical therapeutic strategy for TBI, especially in older patients.

Brain injury accelerates the onset of a reversible age-related microglial phenotype associated with hyperphagocytosis and inflammatory neurodegeneration

“…For each animal, the mean value of four regions on the same side was taken into statistical analysis. The signal colocalization of autofluorescence with Iba1 or CD68 was confirmed by the “Orthogonal View” function in ImageJ ( 36, 64 ). All IHC experiments were performed with appropriate positive control tissue, as well as primary/secondary only negative controls.…”

“…Moreover, microglial phagocytosis of apoptotic neurons and synapses was evident for up to eight months after TBI, coincident with decreased synaptic protein expression in neurons ( 35 ). An age-related increase in chronic engulfment of neuronal and myelin antigens late after TBI has also been demonstrated and such changes were accompanied by an age-related increase in the tissue expression of DAM genes, microglial ROS production, white matter loss, and hippocampal neuron death ( 36 ). Thus, the AF phenotype may identify a subset of dysfunctional or neurotoxic microglia responsible for these molecular signatures within heterogenous cell and tissue samples.…”

“…Sham animals underwent the same procedure as CCI mice except for craniotomy and cortical impact. In the 14d and Hv1 TBI cohorts, an injury of moderate severity was induced by a TBI-0310 Head Impactor (Precision Systems and Instrumentation) with a 3.5-mm diameter tip followed by impact velocity of 4.3 m/s and a displacement depth of 1.2 mm ( 36, 47 ).…”

“…Grip strength was measured using a digital grip strength meter (Bioseb BP, In Vivo Research Instruments) as previously described ( 36, 60 ). Forelimb grip strength was measured from the mouse using both the ipsilateral and contralateral forepaws together.…”

“…For testing non-hippocampal mediated memory, mice in Cohort 3 underwent NOR as previously described ( 36, 62 ). Mice were tested in an open field apparatus after a 5 minute habituation period on the first day.…”