Intraperitoneal injection of the Gram-negative bacterial endotoxin lipopolysaccharide (LPS) elicits a rapid innate immune response. While this systemic inflammatory response can be destructive, tolerable low doses of LPS render the brain transiently resistant to subsequent injuries. However, the mechanism by which microglia respond to LPS stimulation and participate in subsequent neuroprotection has not been documented. In this study, we first established a novel LPS treatment paradigm where mice were injected intraperitoneally with 1.0 mg/kg LPS for four consecutive days to globally activate CNS microglia. By using a reciprocal bone marrow transplantation procedure between wild-type and Toll-like receptor 4 (TLR4) mutant mice, we demonstrated that the presence of LPS receptor (TLR4) is not required on hematogenous immune cells but is required on cells that are not replaced by bone marrow transplantation, such as vascular endothelia and microglia, to transduce microglial activation and neuroprotection. Furthermore, we showed that activated microglia physically ensheathe cortical projection neurons, which have reduced axosomatic inhibitory synapses from the neuronal perikarya. In line with previous reports that inhibitory synapse reduction protects neurons from degeneration and injury, we show here that neuronal cell death and lesion volumes are significantly reduced in LPS-treated animals following experimental brain injury. Together, our results suggest that activated microglia participate in neuroprotection and that this neuroprotection is likely achieved through reduction of inhibitory axosomatic synapses. The therapeutic significance of these findings rests not only in identifying neuroprotective functions of microglia, but also in establishing the CNS location of TLR4 activation.

Microglia actively survey the brain microenvironment and play essential roles in sculpting synaptic connections during brain development. While microglial functions in the adult brain are less clear, activated microglia can closely appose neuronal cell bodies and displace axosomatic presynaptic terminals. Microglia-mediated stripping of presynaptic terminals is considered neuroprotective, but the cellular and molecular mechanisms are poorly defined. Using 3D electron microscopy, we demonstrate that activated microglia displace inhibitory presynaptic terminals from cortical neurons in adult mice. Electrophysiological recordings further establish that the reduction in inhibitory GABAergic synapses increased synchronized firing of cortical neurons in γ-frequency band. Increased neuronal activity results in the calcium-mediated activation of CaM kinase IV, phosphorylation of CREB, increased expression of antiapoptotic and neurotrophic molecules and reduced apoptosis of cortical neurons following injury. These results indicate that activated microglia can protect the adult brain by migrating to inhibitory synapses and displacing them from cortical neurons.

Recent studies have described significant demyelination and microglial activation in the cerebral cortex of brains from multiple sclerosis patients. To date, however, experimental models of cortical demyelination or cortical inflammation have not been extensively studied. In this report we describe focal cortical inflammation induced by stereotaxic injection of killed bacteria (BCG), followed 1 month later by subcutaneous injection of the same antigen, a protocol that overcomes the immune privilege of the cortex. Intracerebral BCG injection produced focal microglial activation at the injection site (termed acute lesion). Ten days after peripheral challenge (termed immune-mediated lesion), larger areas and higher densities of activated microglia were found near the injection site. In both paradigms, activated microglia and/or their processes closely apposed neuronal perikarya and apical dendrites. In the immune-mediated lesions, 45% of the axosomatic synapses was displaced by activated microglia. Upon activation, therefore, cortical microglial migrate to and strip synapses from neuronal perikarya. Since neuronal pathology was not a feature of either the acute or immune-mediated lesion, synaptic stripping by activated microglia may have neuroprotective consequences. V V C 2006 Wiley-Liss, Inc.

Preconditioning is a phenomenon in which the brain protects itself against future injury by adapting to low doses of noxious insults. Preconditioning stimuli include ischemia, low doses of endotoxin, hypoxia, hypothermia and hyperthermia, cortical spreading depression, anesthetics, and 3-nitropropionic acid, among others. Understanding of the mechanisms underlying preconditioning has been elusive, but NMDA receptor activation, nitric oxide, inflammatory cytokines, and suppression of the innate immune system appear to have a role. Elucidation of the endogenous cell survival pathways involved in preconditioning has significant clinical implications for preventing neuronal damage in susceptible patients. All authors reported that they have no financial relationships that pose a potential conflict of interest with this article.

The medial nucleus of the trapezoid body (MNTB) in the superior olivary complex (SOC) is an inhibitory hub considered critical for binaural sound localization. We show that genetic ablation of MNTB neurons in mice only subtly affects this ability by prolonging the minimum time required to detect shifts in sound location. Furthermore, glycinergic innervation of the SOC is maintained without an MNTB, consistent with the existence of parallel inhibitory inputs. These findings redefine the role of MNTB in sound localization and suggest that the inhibitory network is more complex than previously thought.

Little is known about the genetic pathways and transcription factors that control development and maturation of central auditory neurons. En1, a gene expressed by a subset of developing and mature superior olivary complex (SOC) cells, encodes a homeodomain transcription factor important for neuronal development in the midbrain, cerebellum, hindbrain and spinal cord. Using genetic fate-mapping techniques, we show that all En1-lineal cells in the SOC are neurons and that these neurons are glycinergic, cholinergic and GABAergic in neurotransmitter phenotype. En1 deletion does not interfere with specification or neural fate of these cells, but does cause aberrant positioning and subsequent death of all En1-lineal SOC neurons by early postnatal ages. En1-null cells also fail to express the transcription factor FoxP1, suggesting that FoxP1 lies downstream of En1. Our data define important roles for En1 in the development and maturation of a diverse group of brainstem auditory neurons.

The ventral nuclei of the lateral lemniscus (VNLL) are part of the central auditory system thought to participate in temporal sound processing. While the timing and location of VNLL neurogenesis have been determined, the genetic factors that regulate VNLL neuron development are unknown. Here, we use genetic fate-mapping techniques to demonstrate that all glycinergic and glycinergic/GABAergic VNLL neurons derive from a cellular lineage that expresses the homeobox transcription factor Engrailed 1 (En1). We also show that En1 deletion does not affect migration or adoption of a neuronal cell fate but does lead to VNLL neuron death during development. Furthermore, En1 deletion blocks expression of the transcription factor FoxP1 in a subset of VNLL neurons. Together, these data identify En1 as a gene important for VNLL neuron development and survival.

A transgenic mouse line (Oligo-TTK) was established to monitor oligodendrocyte cell death and myelin formation in the CNS. The expression of a conditionally toxic gene, the herpes simplex virus-1 thymidine kinase (HSV1-TK), was made under control of the MBP (myelin basic protein) gene promoter. A truncated form of the HSV1-TK (TTK) gene was used to avoid both bystander effect resulting from leaking in thymidine kinase activity and sterility in transgenic males observed in previous transgenic mice. The transgene was expressed in the CNS with a restricted localization in oligodendrocytes. Oligodendrocyte proliferation and myelin formation are therefore tightly controlled experimentally by administration of ganciclovir (GCV) via the induction of oligodendrocyte cell death. The most severe and irreversible hypomyelination was obtained when GCV was given daily from postnatal day 1 (P1) to P30. Oligodendrocyte plasticity and myelin recovery were analyzed in another phenotype generated by GCV treatment from P1 to P15. In this model, after dysmyelination, an apparent normal behavior was restored with no visible pathological symptoms by P30. Proliferating cells, which may be implicated in myelin repair in this model, are detected primarily in myelin tracts expressing the oligodendrocyte phenotype. Therefore, the endogenous potential of oligodendrocytes to remyelinate was clearly demonstrated in the mice of this study.