Live imaging of Toll-like receptor 2 response in cerebral ischaemia reveals a role of olfactory bulb microglia as modulators of inflammation

In vivo bioluminescent imaging (BLI): noninvasive visualization and interrogation of biological processes in living animals

In vivo bioluminescent imaging (BLI) is increasingly being utilized as a method for modern biological research. This process, which involves the noninvasive interrogation of living animals using light emitted from luciferase-expressing bioreporter cells, has been applied to study a wide range of biomolecular functions such as gene function, drug discovery and development, cellular trafficking, protein-protein interactions, and especially tumorigenesis, cancer treatment, and disease progression. This article will review the various bioreporter/biosensor integrations of BLI and discuss how BLI is being applied towards a new visual understanding of biological processes within the living organism.

Lipopolysaccharide-QD micelles induce marked induction of TLR2 and lipid droplet accumulation in olfactory bulb microglia

The intranasal entry of biological and artificial nanoparticles can induce inflammatory responses both locally and more widely in surrounding tissues. The aim of this study was to assess the microglia activation induced by nanoparticles with different surfaces in (i) a transgenic mouse (Toll-like receptor (TLR)-2-luciferase (Luc) reporter) which allowed the biophotonic imaging of microglial activation/innate immune response after intranasal delivery of nanoparticles and (ii) in microglial dispersed cells in vitro. Cadmium selenide nanoparticles (quantum dots, QD), surface-exchanged with lipopolysaccharide (LPS) to form micelles, were tested to assess microglia activation and lipid droplet formation in both model systems. In vivo imaging revealed a robust increase in the extent of microglial activation/TLR2 response, initially in the olfactory bulb, but also in other more caudal brain regions. The increased TLR2 expression was complemented with enhanced CD68 expression in activated microglia in the same regions. Intense in vitro microglial activation by LPS-QD micelles was accompanied by a significant enhancement of nitric oxide production and formation of large lipid droplets, suggesting the possibility of this organelle acting as an inflammatory biomarker in response to nanoparticles, and not simply as a storage site in fat tissues.

Microglia dynamics and function in the CNS

Microglial cells constitute the resident immune cell population of the mammalian central nervous system. One striking feature of these cells is their highly dynamic nature under both normal and pathological brain conditions. The highly branched processes of resting microglia display a constitutive mobility and undergo rapid directional movement towards sites of acute tissue disruption. Microglia can be converted by a large number of different stimuli to a chronically activated state by signaling through both purinergic and Toll-like receptor systems, among others. Recent work has uncovered some of the mechanisms underlying microglia dynamics and shed new light into the functional significance of this enigmatic member of the glial cell family.

Microglial response to gold nanoparticles

Given the emergence of nanotherapeutics and nanodiagnostics as key tools in today's medicine, it has become of critical importance to define precisely the interactions of nanomaterials with biological systems and to characterize the resulting cellular response. We report here the interactions of microglia and neurons with gold nanoparticles (GNPs) of three morphologies, spheres, rods, and urchins, coated with poly(ethylene glycol) (PEG) or cetyl trimethylammonium bromide (CTAB). Microglia are the resident immune cells of the brain, primarily involved in surveillance, macrophagy, and production of cytokines and trophic factors. Analysis by dark-field microscopy and by two-photon-induced luminescence (TPL) indicates that the exposure of neural cells to GNPs resulted in (i) GNP internalization by both microglial cells and primary hippocampal neurons, as revealed by dark-field microscopy and by two-photon-induced luminescence (TPL), (ii) transient toll-like receptor 2 (TLR-2) up-regulation in the olfactory bulb, after intranasal administration in transgenic mice, in vivo, in real time, and (iii) differential up-regulation in vitro of TLR-2 together with interleukin 1 alpha (IL-1alpha), granulocyte macrophage colony-stimulating factor (GM-CSF) and nitric oxide (NO) in microglia. The study demonstrates that GNP morphology and surface chemistry strongly influence the microglial activation status and suggests that interactions between GNPs and microglia can be differentially regulated by tuning GNP nanogeometry.

Microglia: biology and pathology

The past 20 years have seen a gain in knowledge on microglia biology and microglia functions in disease that exceeds the expectations formulated when the microglia "immune network" was introduced. More than 10,000 articles have been published during this time. Important new research avenues of clinical importance have opened up such as the role of microglia in pain and in brain tumors. New controversies have also emerged such as the question of whether microglia are active or reactive players in neurodegenerative disease conditions, or whether they may be victims themselves. Premature commercial interests may be responsible for some of the confusion that currently surrounds microglia in both the Alzheimer and Parkinson's disease research fields. A critical review of the literature shows that the concept of "(micro)glial inflammation" is still open to interpretation, despite a prevailing slant towards a negative meaning. Perhaps the most exciting foreseeable development concerns research on the role of microglia in synaptic plasticity, which is expected to yield an answer to the question whether microglia are the brain's electricians. This review provides an analysis of the latest developments in the microglia field.

Regulation of innate immune responses in the brain

Microglial cells are the main innate immune cells of the complex cellular structure of the brain. These cells respond quickly to pathogens and injury, accumulate in regions of degeneration and produce a wide variety of pro-inflammatory molecules. These observations have resulted in active debate regarding the exact role of microglial cells in the brain and whether they have beneficial or detrimental functions. Careful targeting of these cells could have therapeutic benefits for several types of trauma and disease specific to the central nervous system. This Review discusses the molecular details underlying the innate immune response in the brain during infection, injury and disease.

Inflammation, plasticity and real-time imaging after cerebral ischemia

With an incidence of approximately 350 in 100,000, stroke is the third leading cause of death and a major cause of disability in industrialized countries. At present, although progress has been made in understanding the molecular pathways that lead to ischemic cell death, the current clinical treatments remain poorly effective. There is mounting evidence that inflammation plays an important role in cerebral ischemia. Experimentally and clinically, brain response to ischemic injury is associated with an acute and prolonged inflammatory process characterized by the activation of resident glial cells, production of inflammatory cytokines as well as leukocyte and monocyte infiltration in the brain, events that may contribute to ischemic brain injury and affect brain recovery and plasticity. However, whether the post-ischemic inflammatory response is deleterious or beneficial to brain recovery is presently a matter of debate and controversies. Here, we summarize the current knowledge on the molecular mechanisms underlying post-ischemic neuronal plasticity and the potential role of inflammation in regenerative processes and functional recovery after stroke. Furthermore, because of the dynamic nature of the brain inflammatory response, we highlight the importance of the development of novel experimental approaches such as real-time imaging. Finally, we discuss the novel transgenic reporter mice models that have allowed us to visualize and to analyze the processes such as neuroinflammation and neuronal repair from the ischemic brains of live animals.