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Myelin is required for the function of neuronal axons in the central nervous system, but the mechanisms that support myelin health are unclear. Although macrophages in the central nervous system have been implicated in myelin health1, it is unknown which macrophage populations are involved and which aspects they influence. Here we show that resident microglia are crucial for the maintenance of myelin health in adulthood in both mice and humans. We demonstrate that microglia are dispensable for developmental myelin ensheathment. However, they are required for subsequent regulation of myelin growth and associated cognitive function, and for preservation of myelin integrity by preventing its degeneration. We show that loss of myelin health due to the absence of microglia is associated with the appearance of a myelinating oligodendrocyte state with altered lipid metabolism. Moreover, this mechanism is regulated through disruption of the TGFβ1–TGFβR1 axis. Our findings highlight microglia as promising therapeutic targets for conditions in which myelin growth and integrity are dysregulated, such as in ageing and neurodegenerative disease2,3.

Cerebral small vessel disease (SVD) is the leading cause of vascular dementia, causes a quarter of strokes, and worsens stroke outcomes. The disease is characterised by patchy cerebral small vessel and white matter pathology, but the underlying mechanisms are poorly understood. This microvascular and tissue damage has been classically considered secondary to extrinsic factors, such as hypertension, but this fails to explain the patchy nature of the disease, the link to endothelial cell (EC) dysfunction even when hypertension is absent, and the increasing evidence of high heritability to SVD-related brain damage. We have previously shown the link between deletion of the phospholipase flippase Atp11b and EC dysfunction in an inbred hypertensive rat model with SVD-like pathology and a single nucleotide polymorphism (SNP) in ATP11B associated with human sporadic SVD. Here, we generated a novel normotensive transgenic rat model, where Atp11b is deleted, and show pathological, imaging and behavioural changes typical of those in human SVD, but that occur without hypertension. Atp11bKO rat brain and retinal small vessels show ECs with molecular and morphological changes of dysfunction, with myelin disruption in a patchy pattern around some but not all brain small vessels, similar to the human brain. We show that ATP11B/ATP11B is heterogeneously expressed in ECs in normal rat and human brain even in the same transverse section of the same blood vessel, suggesting variable effects of the loss of ATP11B on each vessel and an explanation for the patchy nature of the disease. This work highlights a link between inherent EC dysfunction and vulnerability to SVD white matter damage with a marked heterogeneity of ECs in vivo which modulates this response, occurring even in the absence of hypertension. These findings refocus our strategies for therapeutics away from antihypertensive (and vascular risk factor) control alone and towards ECs in the effort to provide alternative targets to prevent a major cause of stroke and dementia.

Visual Abstract, Myocardial infarction (MI) is one of the leading causes of death worldwide, and inflammation is central to tissue response and patient outcomes. The 18-kDa translocator protein (TSPO) has been used in PET as an inflammatory biomarker. The aims of this study were to screen novel, fluorinated, TSPO radiotracers for susceptibility to the rs6971 genetic polymorphism using in vitro competition binding assays in human brain and heart; assess whether the in vivo characteristics of our lead radiotracer, 18F-LW223, are suitable for clinical translation; and validate whether 18F-LW223 can detect macrophage-driven inflammation in a rat MI model. Methods: Fifty-one human brain and 29 human heart tissue samples were screened for the rs6971 polymorphism. Competition binding assays were conducted with 3H-PK11195 and the following ligands: PK11195, PBR28, and our novel compounds (AB5186 and LW223). Naïve rats and mice were used for in vivo PET kinetic studies, radiometabolite studies, and dosimetry experiments. Rats underwent permanent coronary artery ligation and were scanned using PET/CT with an invasive input function at 7 d after MI. For quantification of PET signal in the hypoperfused myocardium, K1 (rate constant for transfer from arterial plasma to tissues) was used as a surrogate marker of perfusion to correct the binding potential for impaired radiotracer transfer from plasma to tissue (BPTC). Results: LW223 binding to TSPO was not susceptible to the rs6971 genetic polymorphism in human brain and heart samples. In rodents, 18F-LW223 displayed a specific uptake consistent with TSPO expression, a slow metabolism in blood (69% of parent at 120 min), a high plasma free fraction of 38.5%, and a suitable dosimetry profile (effective dose of 20.5–24.5 μSv/MBq). 18F-LW223 BPTC was significantly higher in the MI cohort within the infarct territory of the anterior wall relative to the anterior wall of naïve animals (32.7 ± 5.0 vs. 10.0 ± 2.4 cm3/mL/min, P ≤ 0.001). Ex vivo immunofluorescent staining for TSPO and CD68 (macrophage marker) resulted in the same pattern seen with in vivo BPTC analysis. Conclusion: 18F-LW223 is not susceptible to the rs6971 genetic polymorphism in in vitro assays, has favorable in vivo characteristics, and is able to accurately map macrophage-driven inflammation after MI.

Introduction Hepatorenal syndrome (HRS) is a feared complication of cirrhosis characterised by intense renal vasoconstriction. The pathophysiology remains unclear, pharmacotherapy is limited and mortality is high. We investigated vascular responsiveness and the pathogenesis of renal vasoconstriction in models of advanced rat cirrhosis. Additionally,we determined the mechanism of action of the vasoactive peptide relaxin (RLN), previously shown to increase renal blood flow (RBF) in experimental cirrhosis (Snowdon V et al ., BSG 2013). Methods We induced cirrhosis,reduced RBF and renal dysfunction in male SD rats by carbon tetrachloride (16 wk) or bile duct ligation (4 wk). Arteries from renal (renal, segmental, interlobar) and splanchnic circulation were isolated for functional assessment using wire myography. qPCR array for vasoactive signalling genes,western blot for eNOS signalling proteins and NOS activity assay were undertaken in cirrhotic and control kidneys. Markers of oxidative stress and inflammatory cytokines were measured in serum by ELISA. We studied the effects of s.c. infusion of recombinant human RLN(seralaxin;72 h, 4 µg/h) on these parameters. Doppler USS measured changes in cardiac output (CO) and renal arterial resistive index (RRI) in response to i.v. RLN (4 µg). Kidney endothelial morphology was assessed by electron microscopy, H+E and PAS stained kidney by light microscopy. Results In renal arteries from control and cirrhotic rats endothelial vasodilatation was eNOS-dependent. In cirrhotic rats endothelium-dependent relaxation (acetylcholine; 10–9–3 × 10–5 M) was dramatically reduced (p Conclusion Renal vascular endothelial dysfunction characterises experimental cirrhosis, through a reduction in renal eNOS activity. This impairment may contribute to the renal vasoconstriction seen in cirrhosis and is a promising target for therapeutic modulation. RLN treatment restored renal endothelial vasodilatation. The potential for recombinant forms of RLN as a haemodynamic modulator in human cirrhosis and HRS merits investigation in translational studies. Disclosure of Interest None Declared.

Introduction Hepatorenal syndrome (HRS) is a feared complication of cirrhosis with a high mortality rate and limited treatment options. The hallmark features of HRS are profound renal vasoconstriction, resulting in a functional renal failure but with normal kidney histology. The peptide hormone relaxin (RLN) mediates maternal haemodynamic adaptations to pregnancy, including increased renal blood flow (RBF) and glomerular filtration rate (GFR). We hypothesised that RLN could beneficially modulate RBF in cirrhosis and HRS. Methods Cirrhosis, with reduced RBF, was induced in rats by 16 weeks of intraperitoneal (i.p.) carbon tetrachloride (CCl4) and decompensated biliary cirrhosis by 3 weeks bile duct ligation (BDL). We measured the effect of acute intravenous (i.v.) and extended (72 hr) subcutaneous (s.c.) RLN on systemic haemodynamics, RBF, GFR and organ histology. Subgroups of rats were co-treated with the nitric oxide (NO) synthase inhibitor L-NAME. Blood oxygen dependent-magnetic resonance imaging (BOLD-MRI) was used to quantify changes in renal oxygenation. Tissue expression and distribution of RLN receptor (RXFP1) was determined by qPCR and immunofluorescence. Expression of vasoconstrictor genes was quantified by qPCR array. Results RXFP1 was detected on glomerular podocytes, renal pericytes, and endothelial cells of the renal, segmental and interlobar arteries of cirrhotic rats. In CCl4 cirrhosis, acute i.v. RLN (4µg) induced a 50% increase in RBF after 60 minutes ( p n = 6). BOLD-MRI showed increased tissue oxygenation at the same timepoint in renal cortex and medulla. Extended s.c. RLN increased RBF by 54% in CCl4 ( p n = 8) and 57% in BDL ( p n = 5) and increased GFR by 138% in CCl4 ( p n = 8) and 103% in BDL ( p n = 5). Mean arterial pressure was unaffected by RLN. L-NAME (250mg/L) orally (p.o.) abrogated the effect of RLN on RBF and GFR. The relative expression of vasoconstrictor genes in the kidney was markedly reduced by RLN treatment. Conclusion RLN increases RBF in experimental cirrhosis. Crucially, RLN also improves renal function and oxygenation but does not induce systemic hypotension even in decompensated disease. The effects of RLN are mediated via augmentation of NO and downregulation of vasoconstrictor genes known to be important in the pathogenesis of HRS. RLN has potential as a treatment for HRS and further translational studies are warranted. Disclosure of Interest None Declared.

Background Hepatorenal syndrome is a feared complication of cirrhosis, with a high mortality rate and limited treatment options. The hallmarks of the disorder are functional renal failure, normal kidney histological findings, and profound renal vasoconstriction. The hormone relaxin mediates haemodynamic adaptations to pregnancy including increased renal blood flow and glomerular filtration rate (GFR). We hypothesised that relaxin could be used to modulate renal blood flow in cirrhosis.

Background Hepatorenal syndrome is a feared complication of cirrhosis, with a high mortality rate and limited treatment options. The hallmarks of the disorder are functional renal failure, normal kidney histological findings, and profound renal vasoconstriction. The hormone relaxin mediates haemodynamic adaptations to pregnancy including increased renal blood flow and glomerular filtration rate (GFR). We hypothesised that relaxin could be used to modulate renal blood flow in cirrhosis. Methods Cirrhosis was induced in male rats by 16 weeks of carbon tetrachloride (intraperitoneally) and decompensated biliary cirrhosis by bile duct ligation. The effect of acute intravenous or extended (72 h) subcutaneous relaxin versus placebo was determined by analysis of systemic haemodynamics, renal blood flow, GFR, and organ histology. The effect of co-treatment with L-NG-nitroarginine methyl ester (L-NAME) was measured in subgroups of relaxin or placebo treated rats. Blood oxygen dependent-MRI (BOLD-MRI) was used to non-invasively detect changes to renal oxygenation. Hepatic and renal expression of RXFP1 (relaxin receptor) was determined by IHC and quantitative PCR (qPCR). A qPCR array was used to quantify changes in renal vasodilator/vasoconstrictor gene expression in response to relaxin. Findings RXFP1 was detected in glomerular podocytes, renal pericytes, and renal, segmental, and interlobar arteries of cirrhotic rats. In carbon tetrachloride (CCl 4 ) induced cirrhosis, acute intravenous relaxin (4 μg) induced a 50% increase in renal blood flow after 60 min (p 4 (p 4 (p vs placebo, n=8) and 70% in bile duct ligated animals (p vs placebo, n=3). Mean arterial pressure was unaffected by relaxin. L-NAME (250 mg/L) orally abrogated the effect of relaxin on renal blood flow and GFR. Relative expression of vasoconstrictor genes in kidney was markedly reduced by relaxin treatment. Interpretation Relaxin increases renal blood flow in experimental models of cirrhosis. Crucially, relaxin also improves renal function and oxygenation and does not induce systemic hypotension even in advanced disease. These effects are modulated via augmentation of nitric oxide and downregulation of vasoconstrictors, pivotal in the pathogenesis of hepatorenal syndrome. Relaxin has potential as a novel therapy for hepatorenal syndrome, and further translational studies are warranted. Funding Wellcome Trust through the Scottish Translational Medicine and Therapeutics Initiative.