The niche matters: origin, function and fate of CNS-associated macrophages during health and disease.

There are several cellular and acellular structural barriers associated with the brain interfaces, which include the dura, the leptomeninges, the perivascular space and the choroid plexus epithelium. Each structure is enriched by distinct myeloid populations, which mainly originate from erythromyeloid precursors (EMP) in the embryonic yolk sac and seed the CNS during embryogenesis. However, depending on the precise microanatomical environment, resident myeloid cells differ in their marker profile, turnover and the extent to which they can be replenished by blood-derived cells. While some EMP-derived cells seed the parenchyma to become microglia, others engraft the meninges and become CNS-associated macrophages (CAMs), also referred to as border-associated macrophages (BAMs), e.g., leptomeningeal macrophages (MnMΦ). Recent data revealed that MnMΦ migrate into perivascular spaces postnatally where they differentiate into perivascular macrophages (PvMΦ). Under homeostatic conditions in pathogen-free mice, there is virtually no contribution of bone marrow-derived cells to MnMΦ and PvMΦ, but rather to macrophages of the choroid plexus and dura. In neuropathological conditions in which the blood-brain barrier is compromised, however, an influx of bone marrow-derived cells into the CNS can occur, potentially contributing to the pool of CNS myeloid cells. Simultaneously, resident CAMs may also proliferate and undergo transcriptional and proteomic changes, thereby, contributing to the disease outcome. Thus, both resident and infiltrating myeloid cells together act within their microenvironmental niche, but both populations play crucial roles in the overall disease course. Here, we summarize the current understanding of the sources and fates of resident CAMs in health and disease, and the role of the microenvironment in influencing their maintenance and function.

Reduction of body weight by increased loading is associated with activation of norepinephrine neurones in the medial nucleus of the solitary tract.

We previously provided evidence supporting the existence of a novel leptin-independent body weight homeostat ("the gravitostat") that senses body weight and then initiates a homeostatic feed-back regulation of body weight. We, herein, hypothesize that this feed-back regulation involves a CNS mechanism. To identify populations of neurones of importance for the putative feed-back signal induced by increased loading, high-fat diet-fed rats or mice were implanted intraperitoneally or subcutaneously with capsules weighing ∼15% (Load) or ∼2.5% (Control) of body weight. At 3-5 days after implantation, neuronal activation was assessed in different parts of the brain/brainstem by immunohistochemical detection of FosB. Implantation of weighted capsules, both subcutaneous and intraperitoneal, induced FosB in specific neurones in the medial nucleus of the solitary tract (mNTS), known to integrate information about the metabolic status of the body. These neurones also expressed tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DbH), a pattern typical of norepinephrine neurones. In functional studies, we specifically ablated norepinephrine neurones in mNTS, which attenuated the feed-back regulation of increased load on body weight and food intake. In conclusion, increased load appears to reduce body weight and food intake via activation of norepinephrine neurones in the mNTS.

A Body Weight Sensor Regulates Prepubertal Growth via the Somatotropic Axis in Male Rats.

In healthy conditions, prepubertal growth follows an individual specific growth channel. Growth hormone (GH) is undoubtedly the major regulator of growth. However, the homeostatic regulation to maintain the individual specific growth channel during growth is unclear. We recently hypothesized a body weight sensing homeostatic regulation of body weight during adulthood, the gravitostat. We now investigated if sensing of body weight also contributes to the strict homeostatic regulation to maintain the individual specific growth channel during prepubertal growth. To evaluate the effect of increased artificial loading on prepubertal growth, we implanted heavy (20% of body weight) or light (2% of the body weight) capsules into the abdomen of 26-day-old male rats. The body growth, as determined by change in biological body weight and growth of the long bones and the axial skeleton, was reduced in rats bearing a heavy load compared with light load. Removal of the increased load resulted in a catch-up growth and a normalization of body weight. Loading decreased hypothalamic growth hormone releasing hormone mRNA, liver insulin-like growth factor (IGF)-1 mRNA, and serum IGF-1, suggesting that the reduced body growth was caused by a negative feedback regulation on the somatotropic axis and this notion was supported by the fact that increased loading did not reduce body growth in GH-treated rats. Based on these data, we propose the gravitostat hypothesis for the regulation of prepubertal growth. This states that there is a homeostatic regulation to maintain the individual specific growth channel via body weight sensing, regulating the somatotropic axis and explaining catch-up growth.

Interleukin-6 in the central amygdala is bioactive and co-localised with glucagon-like peptide-1 receptor.

Neuronal circuits involving the central amygdala (CeA) are gaining prominence as important centres for regulation of metabolic functions. As a part of the subcortical food motivation circuitry, CeA is associated with food motivation and hunger. We have previously shown that interleukin (IL)-6 can act as a downstream mediator of the metabolic effects of glucagon-like peptide-1 (GLP-1) receptor (R) stimulation in the brain, although the sites of these effects are largely unknown. In the present study, we used the newly generated and validated RedIL6 reporter mouse strain to investigate the presence of IL-6 in the CeA, as well as possible interactions between IL-6 and GLP-1 in this nucleus. IL-6 was present in the CeA, mostly in cells in the medial and lateral parts of this structure, and a majority of IL-6-containing cells also co-expressed GLP-1R. Triple staining showed GLP-1 containing fibres co-staining with synaptophysin close to or overlapping with IL-6 containing cells. GLP-1R stimulation enhanced IL-6 mRNA levels. IL-6 receptor-alpha (IL-6Rα) was found to a large part in neuronal CeA cells. Using electrophysiology, we determined that cells with neuronal properties in the CeA could be rapidly stimulated by IL-6 administration in vitro. Moreover, microinjections of IL-6 into the CeA could slightly reduce food intake in vivo in overnight fasted rats. In conclusion, IL-6 containing cells in the CeA express GLP-1R, are close to GLP-1-containing synapses, and demonstrate increased IL-6 mRNA in response to GLP-1R agonist treatment. IL-6, in turn, exerts biological effects in the CeA, possibly via IL-6Rα present in this nucleus.

Glucagon-Like Peptide-1-, but not Growth and Differentiation Factor 15-, Receptor Activation Increases the Number of Interleukin-6-Expressing Cells in the External Lateral Parabrachial Nucleus.

Interleukin (IL)-6 in the hypothalamus and hindbrain is an important downstream mediator of suppression of body weight and food intake by glucagon-like peptide-1 (GLP-1) receptor stimulation. CNS GLP-1 is produced almost exclusively in prepro-glucagon neurons in the nucleus of the solitary tract. These neurons innervate energy balance-regulating areas, such as the external lateral parabrachial nucleus (PBNel); essential for induction of anorexia. Using a validated novel IL-6-reporter mouse strain, we investigated the interactions in PBNel between GLP-1, IL-6, and calcitonin gene-related peptide (CGRP, a well-known mediator of anorexia). We show that PBNel GLP-1R-containing cells highly (to about 80%) overlap with IL-6-containing cells on both protein and mRNA level. Intraperitoneal administration of a GLP-1 analogue exendin-4 to mice increased the proportion of IL-6-containing cells in PBNel 3-fold, while there was no effect in the rest of the lateral parabrachial nucleus. In contrast, injections of an anorexigenic peptide growth and differentiation factor 15 (GDF15) markedly increased the proportion of CGRP-containing cells, while IL-6-containing cells were not affected. In summary, GLP-1R are found on IL-6-producing cells in PBNel, and GLP-1R stimulation leads to an increase in the proportion of cells with IL-6-reporter fluorescence, supporting IL-6 mediation of GLP-1 effects on energy balance.

The Gravitostat Regulates Fat Mass in Obese Male Mice While Leptin Regulates Fat Mass in Lean Male Mice.

Leptin has been the only known homeostatic regulator of fat mass, but we recently found evidence for a second one, named the gravitostat. In the current study, we compared the effects of leptin and increased loading (gravitostat stimulation) on fat mass in mice with different levels of body weight (lean, overweight, and obese). Leptin infusion suppressed body weight and fat mass in lean mice given normal chow but not in overweight or obese mice given a high-fat diet for 4 and 8 weeks, respectively. The maximum effect of leptin on body weight and fat mass was obtained already at <44 ng/mL of serum leptin. Increased loading using intraperitoneal capsules with different weights decreased body weight in overweight and obese mice. Although the implantation of an empty capsule reduced the body weight in lean mice, only a nonsignificant tendency of a specific effect of increased loading was observed in the lean mice. These findings demonstrate that the gravitostat regulates fat mass in obese mice, whereas leptin regulates fat mass only in lean mice with low endogenous serum leptin levels. We propose that activation of the gravitostat primarily protects against obesity, whereas low levels of leptin protect against undernutrition.