Rett syndrome is an X-linked autism spectrum disorder. The disease is characterized in the majority of cases by mutation of the MECP2 gene, which encodes a methyl-CpG-binding protein 1–5. Although MeCP2 is expressed in many tissues, the disease is generally attributed to a primary neuronal dysfunction 6. However, as shown recently, glia, specifically astrocytes, also contribute to Rett pathophysiology. Here we examined the role of another form of glia, microglia, in a murine model of Rett syndrome. Transplantation of wild type bone marrow into irradiation-conditioned Mecp2-null hosts resulted in engraftment of brain parenchyma by bone marrow-derived myeloid cells of microglial phenotype, and arrest of disease development. However, when cranial irradiation was blocked by lead shield, and microglial engraftment was prevented, disease was not arrested. Similarly, targeted expression of Mecp2 in myeloid cells, driven by Lysmcre on an Mecp2-null background, dramatically attenuated disease symptoms. Thus, via multiple approaches, wild type Mecp2-expressing microglia within the context of an Mecp2-null male mouse arrested numerous facets of disease pathology; lifespan was increased; breathing patterns were normalized; apneas were reduced; body weight was increased to near wild type, and locomotor activity was improved. Mecp2+/− females also exhibited significant improvements as a result of wild type microglial engraftment. These benefits mediated by wild type microglia, however, were diminished when phagocytic activity was inhibited pharmacologically using annexin V to block phosphatydilserine residues on apoptotic targets, thus preventing recognition and engulfment by tissue-resident phagocytes. These results suggest the importance of microglial phagocytic activity in Rett syndrome. Our data implicate microglia as major players in Rett pathophysiology, and suggest that bone marrow transplantation might offer a feasible therapeutic approach for this devastating disorder.

The retrotrapezoid "nucleus" (RTN), located in the rostral ventrolateral medullary reticular formation, contains a bilateral cluster of ϳ1000 glutamatergic noncatecholaminergic Phox2b-expressing propriobulbar neurons that are activated by CO 2 in vivo and by acidification in vitro. These cells are thought to function as central respiratory chemoreceptors, but this theory still lacks a crucial piece of evidence, namely that stimulating these particular neurons selectively in vivo increases breathing. The present study performed in anesthetized rats seeks to test whether this expectation is correct. We injected into the left RTN a lentivirus that expresses the lightactivated cationic channel ChR2 (channelrhodopsin-2) (H134R mutation; fused to the fluorescent protein mCherry) under the control of the Phox2-responsive promoter PRSx8. Transgene expression was restricted to 423 Ϯ 38 Phox2b-expressing neurons per rat consisting of noncatecholaminergic and C1 adrenergic neurons (3:2 ratio). Photostimulation delivered to the RTN region in vivo via a fiberoptic activated the CO 2 -sensitive neurons vigorously, produced a long-lasting (t 1/2 ϭ 11 s) increase in phrenic nerve activity, and caused a small and short-lasting cardiovascular stimulation. Selective lesions of the C1 cells eliminated the cardiovascular response but left the respiratory stimulation intact. In rats with C1 cell lesions, the mCherry-labeled axon terminals originating from the transfected noncatecholaminergic neurons were present exclusively in the lower brainstem regions that contain the respiratory pattern generator. These results provide strong evidence that the Phox2b-expressing noncatecholaminergic neurons of the RTN region function as central respiratory chemoreceptors.

The C1 neurons reside in the rostral and intermediate portions of the ventrolateral medulla (RVLM, IVLM). They use glutamate as a fast transmitter and synthesize catecholamines plus various neuropeptides. These neurons regulate the hypothalamic pituitary axis via direct projections to the paraventricular nucleus and regulate the autonomic nervous system via projections to sympathetic and parasympathetic preganglionic neurons. The presympathetic C1 cells, located in the RVLM, are probably organized in a roughly viscerotopic manner and most of them regulate the circulation. C1 cells are variously activated by hypoglycemia, infection or inflammation, hypoxia, nociception, and hypotension and contribute to most glucoprivic responses. C1 cells also stimulate breathing and activate brain stem noradrenergic neurons including the locus coeruleus. Based on the various effects attributed to the C1 cells, their axonal projections and what is currently known of their synaptic inputs, subsets of C1 cells appear to be differentially recruited by pain, hypoxia, infection/inflammation, hemorrhage, and hypoglycemia to produce a repertoire of stereotyped autonomic, metabolic, and neuroendocrine responses that help the organism survive physical injury and its associated cohort of acute infection, hypoxia, hypotension, and blood loss. C1 cells may also contribute to glucose and cardiovascular homeostasis in the absence of such physical stresses, and C1 cell hyperactivity may contribute to the increase in sympathetic nerve activity associated with diseases such as hypertension. C1 neurons; blood pressure; brain stem BEST KNOWN for their contribution to the control of arterial pressure (AP), the C1 neurons have also been implicated in many other physiological processes ranging from neuroendocrine responses to infection and inflammation, glucose homeostasis, reproduction, breathing, thermoregulation, hypothalamo-pituitary axis (HPA)-mediated stress responses, and food consumption. The purpose of this review is to summarize the most salient information concerning the C1 cells, to point out some of the remaining gaps in our current knowledge, and to suggest a few unifying physiological principles that could account for these seemingly disparate observations. Based on the various effects attributed to the C1 cells and what is currently known of their synaptic inputs, we propose that these neurons are, figuratively speaking, the body's "emergency medical technicians." By this we imply that these neurons produce stereotyped autonomic, metabolic, and neuroendocrine responses designed to help the organism survive major acute physical stresses such as accidental, pathological, or dive-related hypoxia or physical injury and its associated cohort of acute infection, blood loss, and hypotension. These emergency responses include, in the short term and depending on the stress, vasoconstriction, cardioinhibition, or acceleration, breathing stimulation, antidiuresis, changes in metabolism, and gastrointestinal (GI) functions designed to conserve pe...

Blood gas and tissue pH regulation depend on the ability of the brain to sense CO2 and/or H+ and alter breathing appropriately, a homeostatic process called central respiratory chemosensitivity. We show that selective expression of the proton-activated receptor GPR4 in chemosensory neurons of the mouse retrotrapezoid nucleus (RTN) is required for CO2-stimulated breathing. Genetic deletion of GPR4 disrupted acidosis-dependent activation of RTN neurons, increased apnea frequency and blunted ventilatory responses to CO2. Reintroduction of GPR4 into RTN neurons restored CO2-dependent RTN neuronal activation and rescued the ventilatory phenotype. Additional elimination of TASK-2, a pH-sensitive K+ channel expressed in RTN neurons, essentially abolished the ventilatory response to CO2. The data identify GPR4 and TASK-2 as distinct, parallel and essential central mediators of respiratory chemosensitivity.