Cognitive dysfunction and reactive microglia are hallmarks of traumatic brain injury (TBI), yet whether these cells contribute to cognitive deficits and secondary inflammatory pathology remains poorly understood. Here, we show that removal of microglia from the mouse brain has little effect on the outcome of TBI, but inducing the turnover of these cells through either pharmacologic or genetic approaches can yield a neuroprotective microglial phenotype that profoundly aids recovery. The beneficial effects of these repopulating microglia are critically dependent on interleukin-6 (IL-6) trans-signaling via the soluble IL-6 receptor (IL-6R) and robustly support adult neurogenesis, specifically by augmenting the survival of newborn neurons that directly support cognitive function. We conclude that microglia in the mammalian brain can be manipulated to adopt a neuroprotective and pro-regenerative phenotype that can aid repair and alleviate the cognitive deficits arising from brain injury.

BackgroundComponents of the innate immune complement system have been implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS) specifically using hSOD1 transgenic animals; however, a comprehensive examination of complement expression in other transgenic ALS models has not been performed. This study therefore aimed to determine the expression of several key complement components and regulators in the lumbar spinal cord and tibialis anterior muscle of TDP-43Q331K mice during different disease ages.MethodsNon-transgenic, TDP-43WT and TDP-43Q331K mice were examined at three different ages of disease progression. Expression of complement components and their regulators were examined using real-time quantitative PCR and enzyme-linked immunosorbent assay. Localisation of terminal complement component receptor C5aR1 within the lumbar spinal cord was also investigated using immunohistochemistry.ResultsAltered levels of several major complement factors, including C5a, in the spinal cord and tibialis anterior muscle of TDP-43Q331K mice were observed as disease progressed, suggesting overall increased complement activation in TDP-43Q331K mice. C5aR1 increased during disease progression, with immuno-localisation demonstrating expression on motor neurons and expression on microglia surrounding the regions of motor neuron death. There was a strong negative linear relationship between spinal cord C1qB, C3 and C5aR1 mRNA levels with hind-limb grip strength.ConclusionsThese results indicate that similar to SOD1 transgenic animals, local complement activation and increased expression of C5aR1 may contribute to motor neuron death and neuromuscular junction denervation in the TDP-43Q331K mouse ALS model. This further validates C5aR1 as a potential therapeutic target for ALS.

Transactive response DNA-binding protein-43 (TDP-43) is involved in gene regulation via the control of RNA transcription, splicing, and transport. TDP-43 is a major protein component of ubiquinated inclusions that are found in amyotrophic lateral sclerosis (ALS); however, the function of TDP-43 at the neuromuscular junction (NMJ) and its role in ALS pathogenesis is largely unknown. Here, we show that TDP-43 mutation in mice resulted in impaired neurotransmission by age 3 mo, preceding deficits in motor function and motor neuron loss, which were observed from age 10 mo. These defects were in the effective fusion and release of synaptic vesicles within the motor nerve terminal and manifested in decreased quantal content and reduced probability of quantal release. We observed morphologic alterations that were associated with the TDP-43 mutation, such as aberrant innervation patterns and the distribution of synaptic vesicle-related proteins, which is indicative of a failing NMJ undergoing synaptic remodeling. These findings support a growing acceptance that dysregulation of the NMJ function is a key early event in the pathology of ALS.-Chand, K. K., Lee, K. M., Lee, J. D., Qiu, H., Willis, E. F., Lavidis, N. A., Hilliard, M. A., Noakes, P. G. Defects in synaptic transmission at the neuromuscular junction precede motor deficits in a TDP-43 transgenic mouse model of amyotrophic lateral sclerosis.

Studies on the role of the hippocampus in higher cognitive functions such as spatial learning and memory in rodents are reliant upon robust and objective behavioral tests. This protocol describes one such test—the active place avoidance (APA) task. This behavioral task involves the mouse continuously integrating visual cues to orientate itself within a rotating arena in order to actively avoid a shock zone, the location of which remains constant relative to the room. This protocol details the step-by-step procedures for a novel paradigm of the hippocampal-dependent APA task, measuring acquisition of spatial learning during a single 20-min trial (i.e., short-term memory), with spatial memory encoding and retrieval (i.e., long-term memory) assessed by trials conducted over consecutive days. Using the APA task, cognitive flexibility can be assessed using the reversal learning paradigm, as this increases the cognitive load required for efficient performance in the task. In addition to a detailed experimental protocol, this paper also describes the range of its possible applications, the expected key results, as well as the analytical methods to assess the data, and the pitfalls/troubleshooting measures. The protocol described herein is highly robust and produces replicable results, thus presenting an important paradigm that enables the assessment of subtle short-term changes in spatial learning and memory, such as those observed for many experimental interventions.

Microglia, the resident immune cells of the CNS, have emerged as key regulators of neural precursor cell activity in the adult brain. However, the microglia-derived factors that mediate these effects remain largely unknown. In the present study, we investigated a role for microglial brain-derived neurotrophic factor (BDNF), a neurotrophic factor with well known effects on neuronal survival and plasticity. Surprisingly, we found that selective genetic ablation of BDNF from microglia increased the production of newborn neurons under both physiological and inflammatory conditions (e.g., LPS-induced infection and traumatic brain injury). Genetic ablation of BDNF from microglia otherwise also interfered with self-renewal/proliferation, reducing their overall density. In conclusion, we identify microglial BDNF as an important factor regulating microglia population dynamics and states, which in turn influences neurogenesis under both homeostatic and pathologic conditions.