James C. Cronk

Wild-type microglia arrest pathology in a mouse model of Rett syndrome

Rett syndrome is an X-linked autism spectrum disorder. The disease is characterized in most cases by mutation of the MECP2 gene, which encodes a methyl-CpG-binding protein. Although MECP2 is expressed in many tissues, the disease is generally attributed to a primary neuronal dysfunction. However, as shown recently, glia, specifically astrocytes, also contribute to Rett pathophysiology. Here we examine 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, markedly attenuated disease symptoms. Thus, through 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, apnoeas were reduced, body weight was increased to near that of wild type, and locomotor activity was improved. Mecp2+/− females also showed 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 by 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 the pathophysiology of this devastating disorder, and suggest that bone marrow transplantation might offer a feasible therapeutic approach for it.

IL-4 in the Brain: A Cytokine To Remember

IL-4 has been extensively studied in the context of its role in immunity. Accumulating evidence indicates, however, that it also plays a critical role in higher functions of the normal brain, such as memory and learning. In this review, we summarize current knowledge of the basic immunology of IL-4, describe how and where this cytokine appears to operate in normal brain function, and propose a hypothesis concerning its potential role in neurological pathologies.

The role of microglia in brain maintenance: implications for Rett syndrome

The role of microglia in central nervous system (CNS) pathology has been studied extensively, and more recently, examination of microglia in the healthy brain has yielded important insights into their many functions. It was long assumed that microglia were essentially quiescent cells, unless provoked into activation, which was considered a hallmark of disease. More recently, however, it has become increasingly clear that they are extraordinarily dynamic cells, constantly sampling their environment and adjusting to exquisitely delicate stimuli. Along these lines, our laboratory has identified a new and unexpected role for microglial phagocytosis - or lack thereof - in the pathophysiology of Rett syndrome, a neurodevelopmental disease caused by mutation of the gene encoding methyl-CpG binding protein (MECP)2. We have shown that specific expression of wild type Mecp2 in myeloid cells of Mecp2-null mice is sufficient to arrest major symptoms associated with this devastating disease. This beneficial effect, however, is abolished if phagocytic activity of microglia is inhibited. Here, we discuss microglial origins, the role of microglia in brain development and maintenance, and the phenomenon of microglial augmentation by myeloid progenitor cells in the adult brain. Finally, we address in some detail the beneficial roles of microglia as clinical targets in Rett syndrome and other neurological disorders.

Unexpected cellular players in Rett syndrome pathology

Rett syndrome is a devastating neurodevelopmental disorder, primarily caused by mutations of methyl CpG-binding protein 2 (MeCP2). Although the genetic cause of disease was identified over a decade ago, a significant gap still remains in both our clinical and scientific understanding of its pathogenesis. Neurons are known to be primary players in pathology, with their dysfunction being the key in Rett syndrome. While studies in mice have demonstrated a clear causative - and potential therapeutic - role for neurons in Rett syndrome, recent work has suggested that other tissues also contribute significantly to progression of the disease. Indeed, Rett syndrome is known to present with several common peripheral pathologies, such as osteopenia, scoliosis, gastrointestinal problems including nutritional defects, and general growth deficit. Mouse models assessing the potential role of non-neuronal cell types have confirmed both roles in disease and potential therapeutic targets. A new picture is emerging in which neurons both initiate and drive pathology, while dysfunction of other cell types and peripheral tissues exacerbate disease, possibly amplifying further neurologic problems, and ultimately result in a positive feedback loop of progressively worsening symptoms. Here, we review what is known about neuronal and non-neuronal cell types, and discuss how this new, integrative understanding of the disease may allow for additional clinical and scientific pathways for treating and understanding Rett syndrome.

Peripherally derived macrophages can engraft the brain independent of irradiation and maintain an identity distinct from microglia

Peripherally derived macrophages infiltrate the brain after bone marrow transplantation and during central nervous system (CNS) inflammation. It was initially suggested that these engrafting cells were newly derived microglia and that irradiation was essential for engraftment to occur. However, it remains unclear whether brain-engrafting macrophages (beM&phgr;s) acquire a unique phenotype in the brain, whether long-term engraftment may occur without irradiation, and whether brain function is affected by the engrafted cells. In this study, we demonstrate that chronic, partial microglia depletion is sufficient for beM&phgr;s to populate the niche and that the presence of beM&phgr;s does not alter behavior. Furthermore, beM&phgr;s maintain a unique functional and transcriptional identity as compared with microglia. Overall, this study establishes beM&phgr;s as a unique CNS cell type and demonstrates that therapeutic engraftment of beM&phgr;s may be possible with irradiation-free conditioning regimens.

Microglia – the brain’s busy bees

Recent years have seen significant changes in the way scientists view microglia and their role in health and disease. For decades, it was presumed that microglia were stationary, inactive immune cells in the brain, waiting for an immunologic insult to call them into action. In contrast, modern imaging techniques have revealed that microglia are constantly in motion, surveying their environment. Lineage tracing studies have led to the understanding that microglia are part of a larger family of cells, called tissue-resident macrophages, which arise from early yolk sac progenitors during embryogenesis and engraft nearly every organ in the body. Microglia, and all tissue-resident macrophages, rely on signaling through CD115 (the colony stimulating factor 1 receptor) for survival, primarily through the ligand, macrophage colony-stimulating factor. However, it is now understood that some microglia have a specific need for another CD115 ligand, Interleukin-34, which is only shared with Langerhans cells in the skin. In contrast to classical views, recent evidence suggests that the primary functions of microglia may occur during postnatal neurodevelopment and adult homeostasis, as absence or impairment of microglia results in a pathology separate from inflammatory immune function. In summary, these advances suggest that microglia might eventually be utilized or targeted to improve disease outcomes via encouraging or enhancing their health-promoting homeostatic functions.

Influenza A induces dysfunctional immunity and death in MeCP2-overexpressing mice

Loss of function or overexpression of methyl-CpG-binding protein 2 (MeCP2) results in the severe neurodevelopmental disorders Rett syndrome and MeCP2 duplication syndrome, respectively. MeCP2 plays a critical role in neuronal function and the function of cells throughout the body. It has been previously demonstrated that MeCP2 regulates T cell function and macrophage response to multiple stimuli, and that immune-mediated rescue imparts significant benefit in Mecp2-null mice. Unlike Rett syndrome, MeCP2 duplication syndrome results in chronic, severe respiratory infections, which represent a significant cause of patient morbidity and mortality. Here, we demonstrate that MeCP2Tg3 mice, which overexpress MeCP2 at levels 3- to 5-fold higher than normal, are hypersensitive to influenza A/PR/8/34 infection. Prior to death, MeCP2Tg3 mice experienced a host of complications during infection, including neutrophilia, increased cytokine production, excessive corticosterone levels, defective adaptive immunity, and vascular pathology characterized by impaired perfusion and pulmonary hemorrhage. Importantly, we found that radioresistant cells are essential to infection-related death after bone marrow transplantation. In all, these results demonstrate that influenza A infection in MeCP2Tg3 mice results in pathology affecting both immune and nonhematopoietic cells, suggesting that failure to effectively respond and clear viral respiratory infection has a complex, multicompartment etiology in the context of MeCP2 overexpression.