The adverse neurocognitive sequelae following clinical radiotherapy (RT) for central nervous system (CNS) malignancies are often longlasting without any clinical recourse. Despite recent progress, the cellular mechanisms mediating RT-induced cognitive deficits (RICD) are poorly understood. The complement system is an immediate sensor of a disturbed inflammatory environment and a potent mediator of gliosis with a range of nonimmune functions in the CNS, including synaptic pruning, which is detrimental if dysregulated. We hypothesize that complement-mediated changes in glial cell function significantly contribute to RICD. The underlying alterations in CNS complement cascade proteins (C1q, C3), TLR4, and colabeling with glia (IBA1, GFAP) were examined using gene expression, immunofluorescence, and in silico modeling approaches in the adult mouse brain following 9 Gy cranial RT. Three-dimensional volumetric quantification showed elevated molecular signatures of gliosis at shortand long-term post-RT times. We found significant elevations in complement C1q, C3, and TLR4 post-RT accompanied by increased colabeling of astrocytes and microglia. To address the mechanism of RT-induced complement cascade activation, neuroinflammation, and cognitive dysfunction, we used a genetic approach-conditional, microglia-selective C1q (Flox) knockdown mice-to determine whether a glia-specific, upstream complement cascade contributes to RICD. C1q-Flox mice exposed to cranial RT showed no cognitive deficits compared with irradiated WT mice. Further, irradiated C1q-Flox mice were protected from RT-induced microglial activation and synaptic loss, elevation of anaphylatoxin C5a receptor, astrocytic-C3, and microglial-TLR4 expression in the brain. Our findings demonstrate for the first time a microglia-specific mechanism of RICD involving an upstream complement cascade component, C1q.Significance: Clinically-relevant radiotherapy induces aberrant complement activation, leading to brain injury. Microglia-selective genetic deletion of CNS complement C1q ameliorates radiationinduced cognitive impairments, synaptic loss, and neuroinflammation, highlighting the potential for C1q as a novel therapeutic target.See related commentary by Korimerla and Wahl, p. 1635

The radiation fields in space define tangible risks to the health of astronauts, and significant work in rodent models has clearly shown a variety of exposure paradigms to compromise central nervous system (CNS) functionality. Despite our current knowledge, sex differences regarding the risks of space radiation exposure on cognitive function remain poorly understood, which is potentially problematic given that 30% of astronauts are women. While work from us and others have demonstrated pronounced cognitive decrements in male mice exposed to charged particle irradiation, here we show that female mice exhibit significant resistance to adverse neurocognitive effects of space radiation. The present findings indicate that male mice exposed to low doses (≤30 cGy) of energetic (400 MeV/n) helium ions (4 He) show significantly higher levels of neuroinflammation and more extensive cognitive deficits than females. Twelve weeks following 4 He ion exposure, irradiated male mice demonstrated significant deficits in object and place recognition memory accompanied by activation of microglia, marked upregulation of hippocampal Toll-like receptor 4 (TLR4), and increased expression of the pro-inflammatory marker high mobility group box 1 protein (HMGB1). Additionally, we determined that exposure to 4 He ions caused a significant decline in the number of dendritic branch points and total dendritic length along with the hippocampus neurons in female mice. Interestingly, only male mice showed a significant decline of dendritic spine density following irradiation. These data indicate that fundamental differences in inflammatory cascades between male and female mice may drive divergent CNS radiation responses that differentially impact the structural plasticity of neurons and neurocognitive outcomes following cosmic radiation exposure.

Cancer-related cognitive impairment (CRCI) considerably affects the quality of life of millions of cancer survivors. Brain-derived neurotrophic factor (BDNF) has been shown to promote survival, differentiation, and maintenance of in vivo dentate neurogenesis, and chemotherapy induces a plethora of physiological and cellular alterations, including a decline in neurogenesis and increased neuroinflammation linked with cognitive impairments. In our clinical studies, breast cancer patients treated with doxorubicin (Adriamycin®, ADR) experienced a significant reduction in the blood levels of BDNF that was associated with a higher risk of CRCI. Our past rodent studies in CRCI have also shown a significant reduction in dentate neurogenesis accompanied by cognitive impairment. In this study, using a female mouse model of ADR-induced cognitive decline, we tested the impact of riluzole (RZ), an orally active BDNF-enhancing medication that is FDA-approved for amyotrophic lateral sclerosis. ADR-treated mice receiving RZ in the drinking water for 1 month showed significant improvements in hippocampal-dependent learning and memory function (spatial recognition), fear extinction memory consolidation, and reduced anxiety-like behavior. RZ prevented chemotherapy-induced reductions of BDNF levels in the hippocampus. Importantly, RZ mitigated chemotherapy-induced loss of newly born, immature neurons, dentate neurogenesis, and neuroinflammation. In conclusion, this data provides pre-clinical evidence for a translationally feasible approach to enhance the neuroprotective effects of RZ treatment to prevent CRCI.

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<div>Abstract<p>The adverse neurocognitive sequelae following clinical radiotherapy (RT) for central nervous system (CNS) malignancies are often long-lasting without any clinical recourse. Despite recent progress, the cellular mechanisms mediating RT-induced cognitive deficits (RICD) are poorly understood. The complement system is an immediate sensor of a disturbed inflammatory environment and a potent mediator of gliosis with a range of nonimmune functions in the CNS, including synaptic pruning, which is detrimental if dysregulated. We hypothesize that complement-mediated changes in glial cell function significantly contribute to RICD. The underlying alterations in CNS complement cascade proteins (C1q, C3), TLR4, and colabeling with glia (IBA1, GFAP) were examined using gene expression, immunofluorescence, and <i>in silico</i> modeling approaches in the adult mouse brain following 9 Gy cranial RT. Three-dimensional volumetric quantification showed elevated molecular signatures of gliosis at short- and long-term post-RT times. We found significant elevations in complement C1q, C3, and TLR4 post-RT accompanied by increased colabeling of astrocytes and microglia. To address the mechanism of RT-induced complement cascade activation, neuroinflammation, and cognitive dysfunction, we used a genetic approach—conditional, microglia-selective C1q (Flox) knockdown mice—to determine whether a glia-specific, upstream complement cascade contributes to RICD. C1q-Flox mice exposed to cranial RT showed no cognitive deficits compared with irradiated WT mice. Further, irradiated C1q-Flox mice were protected from RT-induced microglial activation and synaptic loss, elevation of anaphylatoxin C5a receptor, astrocytic-C3, and microglial-TLR4 expression in the brain. Our findings demonstrate for the first time a microglia-specific mechanism of RICD involving an upstream complement cascade component, C1q.</p>Significance:<p>Clinically-relevant radiotherapy induces aberrant complement activation, leading to brain injury. Microglia-selective genetic deletion of CNS complement C1q ameliorates radiation-induced cognitive impairments, synaptic loss, and neuroinflammation, highlighting the potential for C1q as a novel therapeutic target.</p><p><i>See related commentary by Korimerla and Wahl, p. 1635</i></p></div>