Infiltration of peripheral macrophages into the CNS has been well documented in various central neurological disorders, such as neurodegenerative diseases, psychiatry disorders and traumatic brain injuries [3–5]. However, little is known about the mechanisms responsible for the infiltration and its pathological roles in systemic inflammation-mediated neurological disorders. Here we demonstrated a dynamic interplay of peripheral tissue injury, systemic inflammation, neuroinflammation, apoptosis and macrophage infiltration using a zebrafish model. Most importantly, we demonstrated that il1b is a key regulator in mediating the infiltration of peripheral macrophages into the brain from neighboring tissues through circulation-independent migration, a process which is probably triggered by the accumulation of apoptotic cells in the brain [38]. The proteomic analysis also identified an elevation in an il1b-regulated protein, camk2g1 following peripheral tissue injury. This protein is elevated in the brain affected by neuropsychiatry disorders, and its increase accompanied with neuroinflammation, infiltrating macrophages and neuronal apoptosis might contribute to the hyperactive behavior of zebrafish observed in this study [3,40].
Many studies have reported that circulatory macrophages infiltrate the brain following peripheral inflammation and further contribute to the CNS diseases. However, they were limited to the several relatively rare disease states including arthritis and liver injury [24,25,41]. Our work provided experimental evidence of systemic inflammation induced infiltration of macrophages into the brain in response to peripheral tissue injury, and suggests a potential role of infiltrated macrophages in the development of peripheral trauma-induced neurological disorders such as PTSD and POCD [3,42]. Furthermore, we confirmed that il1b but not tnfa, the latter being generally recognized as a cytokine in systemic inflammation-induced infiltration of macrophages and neurological disorders, served as the key proinflammatory regulator under peripheral tissue injury [24,25]. While microglia-secreted CCL2, a chemokine attracting macrophages into the CNS [24,43], remained unchanged in this model, our results demonstrated that the accumulation of apoptotic cells in the brain triggered by il1b impaired phagocytosis, as characterized by reduced lysosomes in macrophage/microglia, and may contribute to the recruitment of macrophages/microglia following peripheral tissue injury [36,39,44]. These observations suggest that the mechanisms of the systemic inflammation mediated neurological disorders may vary between pathological conditions, which warrant further studies. It is widely believed that breakdown of BBB promotes circulatory macrophage infiltration, though infiltrating macrophages can be found in the CNS with intact BBB and migrating through the cerebrospinal fluid (CSF) [9,45]. In this current study, circulation-independent infiltration of macrophages was observed, where macrophages invaded the brain from the neighboring tissues through the lateral periphery of the hindbrain, specifically between metencephalon (or cerebellum) and myelencephalon, differs from the microglial invasion to the zebrafish midbrain during development [46]. This suggests that in addition to circulation, other less studied migration routes might be involved in the infiltration of macrophages into the brain under specific disease states.
Involved in the pathogenesis of multiple neurological disorders, infiltration of macrophages into the brain might result in seizure or sickness behaviour or anxiety in response to peripheral inflammation and neuroinflammation, respectively [3,24,47]. We demonstrated that peripheral tissue injury results in hyperactive behavior marked by elevated expression of camk2g1 in the context of both systemic inflammation and neuroinflammation. More importantly, il1b mutation mitigated the increased levels of camk2g1 and the hyperactive behavior. CaMK2G, the human ortholog of camk2g1, is associated with major neuropsychiatry disorders. Correlation between polymorphism in CaMK2G and human memory performance was reported previously [48]. In addition, abnormal levels of alternative CaMK2G splicing was found in the brain of patients with autism spectrum disorder (ASD) [49]. More importantly, the elevated CaMK2G-encoding protein (γCaMKII) level in the brain has been detected in rodent models of anxiety, schizophrenia and major depressive disorder (MDD) [50–52]. Thus, hyperactive behavior observed in our peripheral tissue injury model might represent the camk2g1-related neuropsychiatry disorder. While the roles of CaMK2G in macrophages infiltration and subsequent impact on neurological functions require further investigations [53], the il1b-mediated elevated expression of camk2g1 together with infiltration of macrophages, neuronal apoptosis and neuroinflammation might serve as the potential mechanism underlying peripheral tissue injury-induced neuropsychiatry disorders.