Brain-Resident Microglia and Blood-Borne Macrophages Orchestrate Central Nervous System Inflammation in Neurodegenerative Disorders and Brain Cancer

The Multifarious Role of Microglia in Brain Metastasis

The immune landscape in brain metastasis is a very heterogeneous framework. Amongst a broad plethora of cells within the tumor microenvironment, the presence of activated microglia has been perfectly described. The innate role of microglial cells is to detect and eliminate any insults that may disturb the regular behavior of the brain. As part of its defensive role, it releases pro- and anti-inflammatory cytokines that aim to modulate the inflammatory scenario at the metastatic foci. However, the long term effects that these cells may exert on the metastatic progression is not clear. One of the biggest challenges in the field is to distinguish between brain resident microglial cells and infiltrated bone-marrow derived macrophages. Part of this issue is the fact that both cell types share similar phenotypes. Current studies are based on the modulation of the immune response against cancer cells (immunotherapy). However, most of current clinical trials and newly developed drugs focus on the adaptive immune response (e.g., immune blockade check-points). Additionally, the unique structure of the central nervous system with the presence of the blood-brain barrier have hindered a significant advance in novel therapies against brain metastasis. In this manuscript, we describe current advances in characterization of tumor-associated microglia and macrophages, the importance of microglia during the anti-cancerous response, and the future direction for the development of new strategies against this complex disease.

Role of the CD200-CD200R Axis During Homeostasis and Neuroinflammation

Microglia are considered to be the resident macrophages of the CNS and main effector of immune brain function. Due to their essential role in the regulation of neuroinflammatory response, microglia constitute an important target for neurological diseases, such as multiple sclerosis, Alzheimer's or Parkinson's disease. The communication between neurons and microglia contributes to a proper maintenance of homeostasis in the CNS. Research developed in the last decade has demonstrated that this interaction is mediated by "Off-signals" - molecules exerting immune inhibition - and "On signals" - molecules triggering immune activation. Among "Off signals", molecular pair CD200 and its CD200R receptor, expressed mainly in the membrane of neurons and microglia, respectively, have centered our attention due to its unexplored and powerful immunoregulatory functions. In this review, we will offer an updated global view of the CD200-CD200R role in the microglia-neuron crosstalk during homeostasis and neuroinflammation. Specifically, the effects of CD200-CD200R in the inhibition of pro-inflammatory microglial activation will be explained, and their involvement in other functions such as homeostasis preservation, tissue repair, and brain aging, among others, will be pointed out. In addition, we will depict the effects of CD200-CD200R uncoupling in the etiopathogenesis of autoimmune and neurodegenerative diseases. Finally, we will explore how to translate the scientific evidence of CD200-CD200R interaction into possible clinical therapeutic strategies to tackle neuroinflammatory CNS diseases.

Increased Microglial Activity, Impaired Adult Hippocampal Neurogenesis, and Depressive-like Behavior in Microglial VPS35-Depleted Mice

Vacuolar sorting protein 35 (VPS35) is a critical component of retromer, which is essential for selective endosome-to-Golgi retrieval of membrane proteins. VPS35 deficiency is implicated in neurodegenerative disease pathology, including Alzheimer's disease (AD). However, exactly how VPS35 loss promotes AD pathogenesis remains largely unclear. VPS35 is expressed in various types of cells in the brain, including neurons and microglia. Whereas neuronal VPS35 plays a critical role in preventing neurodegeneration, the role of microglial VPS35 is largely unknown. Here we provide evidence for microglial VPS35's function in preventing microglial activation and promoting adult hippocampal neurogenesis. VPS35 is expressed in microglia in various regions of the mouse brain, with a unique distribution pattern in a brain region-dependent manner. Conditional knocking out of VPS35 in microglia of male mice results in regionally increased microglial density and activity in the subgranular zone of the hippocampal dentate gyrus (DG), accompanied by elevated neural progenitor proliferation, but decreased neuronal differentiation. Additionally, newborn neurons in the mutant DG show impaired dendritic morphology and reduced dendritic spine density. When examining the behavioral phenotypes of these animals, microglial VPS3S-depleted mice display depression-like behavior and impairment in long-term recognition memory. At the cellular level, VPS35-depleted microglia have grossly enlarged vacuolar structures with increased phagocytic activity toward postsynaptic marker PSD95, which may underlie the loss of dendritic spines observed in the mutant DG. Together, these findings identify an important role of microglial VPS35 in suppressing microglial activation and promoting hippocampal neurogenesis, which are both processes involved in AD pathogenesis.SIGNIFICANCE STATEMENT The findings presented here provide the first in vivo evidence that Vacuolar sorting protein 35 (VPS35)/retromer is essential for regulating microglial function and that when microglial retromer mechanics are disrupted, the surrounding brain tissue can be affected in a neurodegenerative manner. These findings present a novel, microglial-specific role of VPS35 and raise multiple questions regarding the mechanisms underlying our observations. These findings also have myriad implications for the field of retromer research and the role of retromer dysfunction in neurodegenerative pathophysiology. Furthermore, they implicate a pivotal role of microglia in the regulation of adult hippocampal neurogenesis and the survival/integration of newborn neurons in the adult hippocampus.