Microglia contribute to circuit defects in Mecp2 null mice independent of microglia-specific loss of Mecp2 expression

Microglia, the resident CNS macrophages, have been implicated in the pathogenesis of Rett Syndrome (RTT), an X-linked neurodevelopmental disorder. However, the mechanism by which microglia contribute to the disorder is unclear and recent data suggest that microglia do not play a causative role. Here, we use the retinogeniculate system to determine if and how microglia contribute to pathogenesis in a RTT mouse model, the Mecp2 null mouse (Mecp2tm1.1Bird/y). We demonstrate that microglia contribute to pathogenesis by excessively engulfing, thereby eliminating, presynaptic inputs at end stages of disease (≥P56 Mecp2 null mice) concomitant with synapse loss. Furthermore, loss or gain of Mecp2 expression specifically in microglia (Cx3cr1CreER;Mecp2fl/yor Cx3cr1CreER; Mecp2LSL/y) had little effect on excessive engulfment, synapse loss, or phenotypic abnormalities. Taken together, our data suggest that microglia contribute to end stages of disease by dismantling neural circuits rendered vulnerable by loss of Mecp2 in other CNS cell types. DOI: http://dx.doi.org/10.7554/eLife.15224.00

A Diet With Docosahexaenoic and Arachidonic Acids as the Sole Source of Polyunsaturated Fatty Acids Is Sufficient to Support Visual, Cognitive, Motor, and Social Development in Mice

Polyunsaturated fatty acids serve multiple functions in neurodevelopment and neurocognitive function. Intravenous lipid emulsions are administered to children that are dependent on parenteral nutrition to provide the essential fatty acids needed to sustain growth and development. One of these emulsions, derived from fish-oil, is particularly poor in the traditional essential fatty acids, linoleic and alpha-linolenic acids. However, it does contain adequate amounts of its main derivatives, arachidonic acid (ARA) and docosahexaenoic acid (DHA), respectively. This skewed composition has raised concern about the sole use of fish-oil based lipid emulsions in children and how its administration can be detrimental to their neurodevelopment. Using a custom-made diet that contains ARA and DHA as a sole source of polyunsaturated fatty acids, we bred and fed mice for multiple generations. Compared to adult, chow-fed mice, animals maintained on this special diet showed similar outcomes in a battery of neurocognitive tests performed under controlled conditions. Chow-fed mice did perform better in the rotarod test for ataxia and balance, although both experimental groups showed a conserved motor learning capacity. Conversely, mice fed the custom diet rich in DHA and ARA showed less neophobia than the chow-fed animals. Results from these experiments suggest that providing a diet where ARA and DHA are the sole source of polyunsaturated fatty acids is sufficient to support gross visual, cognitive, motor, and social development in mice

Sensory Deprivation Induces Microglial Synapse Engulfment

Synaptic connectivity is highly plastic in early development and undergoes extensive remodeling in response to changes in neuronal activity and sensory experience. Microglia, the resident central nervous system macrophages, participate in shaping mature neuronal circuits by dynamically surveying the brain parenchyma and pruning away less active synaptic connections. However, it is unknown how changes in neuronal activity regulates microglial pruning within circuits and whether this activity-dependent pruning is necessary to achieve plasticity. Using the rodent somatosensory circuit, I identified that microglia engulf and eliminate synapses in the cortex following early postnatal (P4) unilateral removal of mouse whiskers. I found this early life microglial synaptic remodeling requires specific chemokine signaling between neurons and microglia. Mice that lack expression of either the neuronal chemokine CX3CL1 (fractalkine), or its microglial receptor CX3CR1, have significantly reduced microglial synapse engulfment and fail to eliminate synapses following whisker removal. To gain more insight into how this signaling is regulated, I performed both single-cell RNA sequencing of the primary somatosensory cortex as well as microglia-specific Translating Ribosome Affinity Purification (TRAP) sequencing. I identified that the majority of central nervous system (CNS) cell populations in the somatosensory cortex, including microglia, undergo transcriptional changes following whisker removal. Further, the transcriptional changes in microglia after whisker cauterization require expression of the receptor CX3CR1. Importantly, I also found that Adam10, a gene encoding the metalloprotease known to post-translationally cleave CX3CL1 into a soluble chemokine, is upregulated in the deprived cortex after whisker ablation. Pharmacological inhibition of ADAM10 inhibits microglia-mediated removal of synapses in the deprived cortex. These data support a mechanism by which cleavage of membrane-bound CX3CL1 by ADAM10 is necessary for neuronal signaling to microglia via CX3CR1 to induce transcriptional changes within microglia upstream of synaptic engulfment and elimination following sensory deprivation

Mechanisms governing activity-dependent synaptic pruning in the developing mammalian CNS

Almost 60 years have passed since the initial discovery by Hubel and Wiesel that changes in neuronal activity can elicit developmental rewiring of the central nervous system (CNS). Over this period, we have gained a more comprehensive picture of how both spontaneous neural activity and sensory experience-induced changes in neuronal activity guide CNS circuit development. Here we review activity-dependent synaptic pruning in the mammalian CNS, which we define as the removal of a subset of synapses, while others are maintained, in response to changes in neural activity in the developing nervous system. We discuss the mounting evidence that immune and cell-death molecules are important mechanistic links by which changes in neural activity guide the pruning of specific synapses, emphasizing the role of glial cells in this process. Finally, we discuss how these developmental pruning programmes may go awry in neurodevelopmental disorders of the human CNS, focusing on autism spectrum disorder and schizophrenia. Together, our aim is to give an overview of how the field of activity-dependent pruning research has evolved, led to exciting new questions and guided the identification of new, therapeutically relevant mechanisms that result in aberrant circuit development in neurodevelopmental disorders

Luciferase shRNA Presents off-Target Effects on Voltage-Gated Ion Channels in Mouse Hippocampal Pyramidal Neurons

RNA interference (RNAi) is a straightforward approach to study gene function from the in vitro cellular level to in vivo animal behavior. Although RNAi-mediated gene knockdown has become essentially routine in neuroscience over the past ten years, off-target effects of short hairpin RNAs (shRNAs) should be considered as the proper choice of control shRNA is critical in order to perform meaningful experiments. Luciferase shRNA (shLuc), targeting firefly luciferase, and scrambled shRNAs (shScrs) have been widely used as controls for vertebrate cell research. However, thorough validation of control shRNAs has not been made to date. Here, we performed thorough physiological and morphological studies against control shRNAs in mouse hippocampal CA1 pyramidal neurons. As expected, all control shRNAs exhibited normal basal synaptic transmission and dendritic morphology. However, to our surprise, shLuc exerted severe off-target effects on voltage-gated ion channel function, while the shScr had no detectable changes. These results indicate that thorough validation of shRNA is imperative and, in the absence of such validation, that shScr is the best available negative control for gene knockdown studies

Sensory lesioning induces microglial synapse elimination via ADAM10 and fractalkine signaling

Microglia rapidly respond to changes in neural activity and inflammation to regulate synaptic connectivity. The extracellular signals, particularly neuron-derived molecules, that drive these microglial functions at synapses remain a key open question. Here we show that whisker lesioning, known to dampen cortical activity, induces microglia-mediated synapse elimination. This synapse elimination is dependent on signaling by CX3CR1, the receptor for microglial fractalkine (also known as CXCL1), but not complement receptor 3. Furthermore, mice deficient in CX3CL1 have profound defects in synapse elimination. Single-cell RNA sequencing revealed that Cx3cl1 is derived from cortical neurons, and ADAM10, a metalloprotease that cleaves CX3CL1 into a secreted form, is upregulated specifically in layer IV neurons and in microglia following whisker lesioning. Finally, inhibition of ADAM10 phenocopies Cx3cr1(-/-) and Cx3cl1(-/-) synapse elimination defects. Together, these results identify neuron-to-microglia signaling necessary for cortical synaptic remodeling and reveal that context-dependent immune mechanisms are utilized to remodel synapses in the mammalian brain

Loss of hypothalamic corticotropin-releasing hormone markedly reduces anxiety behaviors in mice

A long-standing paradigm posits that hypothalamic corticotropin-releasing hormone (CRH) regulates neuroendocrine functions such as adrenal glucocorticoid release, while extra-hypothalamic CRH plays a key role in stressor-triggered behaviors. Here we report that hypothalamus-specific Crh knockout mice (Sim1CrhKO mice, created by crossing Crhflox with Sim1Cre mice) have absent Crh mRNA and peptide mainly in the paraventricular nucleus of the hypothalamus (PVH) but preserved Crh expression in other brain regions including amygdala and cerebral cortex. As expected, Sim1CrhKO mice exhibit adrenal atrophy as well as decreased basal, diurnal and stressor-stimulated plasma corticosterone secretion and basal plasma ACTH, but surprisingly, have a profound anxiolytic phenotype when evaluated using multiple stressors including open field, elevated plus maze, holeboard, light-dark box, and novel object recognition task. Restoring plasma corticosterone did not reverse the anxiolytic phenotype of Sim1CrhKO mice. Crh-Cre driver mice revealed that PVHCrh fibers project abundantly to cingulate cortex and the nucleus accumbens shell, and moderately to medial amygdala, locus coeruleus, and solitary tract, consistent with the existence of PVHCrh-dependent behavioral pathways. Although previous, nonselective attenuation of CRH production or action, genetically in mice and pharmacologically in humans, respectively, has not produced the anticipated anxiolytic effects, our data show that targeted interference specifically with hypothalamic Crh expression results in anxiolysis. Our data identify neurons that express both Sim1 and Crh as a cellular entry point into the study of CRH-mediated, anxiety-like behaviors and their therapeutic attenuation