Perivascular microglia activation is a hallmark of inflammatory demyelination in multiple sclerosis (MS), but the mechanisms underlying microglia activation and specific strategies to attenuate their activation remain elusive. Here, we identify fibrinogen as a novel regulator of microglia activation and show that targeting of the interaction of fibrinogen with the microglia integrin receptor Mac-1 (αMβ2, CD11b/CD18) is sufficient to suppress experimental autoimmune encephalomyelitis in mice that retain full coagulation function. We show that fibrinogen, which is deposited perivascularly in MS plaques, signals through Mac-1 and induces the differentiation of microglia to phagocytes via activation of Akt and Rho. Genetic disruption of fibrinogen–Mac-1 interaction in fibrinogen-γ390-396A knock-in mice or pharmacologically impeding fibrinogen–Mac-1 interaction through intranasal delivery of a fibrinogen-derived inhibitory peptide (γ377-395) attenuates microglia activation and suppresses relapsing paralysis. Because blocking fibrinogen–Mac-1 interactions affects the proinflammatory but not the procoagulant properties of fibrinogen, targeting the γ377-395 fibrinogen epitope could represent a potential therapeutic strategy for MS and other neuroinflammatory diseases associated with blood-brain barrier disruption and microglia activation.

Activation of innate immunity and deposition of blood-derived fibrin in the central nervous system (CNS) occur in autoimmune and neurodegenerative diseases, including multiple sclerosis (MS) and Alzheimer’s disease (AD). However, mechanisms linking blood-brain barrier (BBB) disruption with neurodegeneration are poorly understood, and exploration of fibrin as a therapeutic target has been limited by its beneficial clotting functions. Here we report the generation of monoclonal antibody 5B8 targeted against the cryptic fibrin epitope γ 377–395 to selectively inhibit fibrin-induced inflammation and oxidative stress without interfering with clotting. 5B8 suppressed fibrin-induced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and proinflammatory gene expression. In animal models of MS and AD, 5B8 entered the CNS and bound to parenchymal fibrin, and its therapeutic administration reduced innate immune activation and neurodegeneration. Thus, fibrin-targeting immunotherapy inhibits autoimmune- and amyloid-driven neurotoxicity and may have clinical benefit without globally suppressing innate immunity or interfering with coagulation in diverse neurological diseases.

Differentiation of hepatic stellate cells (HSCs) to extracellular matrix- and growth factor-producing cells supports liver regeneration through promotion of hepatocyte proliferation. We show that the neurotrophin receptor p75NTR, a tumor necrosis factor receptor superfamily member expressed in HSCs after fibrotic and cirrhotic liver injury in humans, is a regulator of liver repair. In mice, depletion of p75NTR exacerbated liver pathology and inhibited hepatocyte proliferation in vivo. p75NTR-/- HSCs failed to differentiate to myofibroblasts and did not support hepatocyte proliferation. Moreover, inhibition of p75NTR signaling to the small guanosine triphosphatase Rho resulted in impaired HSC differentiation. Our results identify signaling from p75NTR to Rho as a mechanism for the regulation of HSC differentiation to regeneration-promoting cells that support hepatocyte proliferation in the diseased liver.

Clearance of fibrin through proteolytic degradation is a critical step of matrix remodeling that contributes to tissue repair in a variety of pathological conditions, such as stroke, atherosclerosis, and pulmonary disease. However, the molecular mechanisms that regulate fibrin deposition are not known. Here, we report that the p75 neurotrophin receptor (p75NTR), a TNF receptor superfamily member up-regulated after tissue injury, blocks fibrinolysis by down-regulating the serine protease, tissue plasminogen activator (tPA), and up-regulating plasminogen activator inhibitor-1 (PAI-1). We have discovered a new mechanism in which phosphodiesterase PDE4A4/5 interacts with p75NTR to enhance cAMP degradation. The p75NTR-dependent down-regulation of cAMP results in a decrease in extracellular proteolytic activity. This mechanism is supported in vivo in p75NTR-deficient mice, which show increased proteolysis after sciatic nerve injury and lung fibrosis. Our results reveal a novel pathogenic mechanism by which p75NTR regulates degradation of cAMP and perpetuates scar formation after injury.

Summary Obesity and metabolic syndrome reflect the dysregulation of molecular pathways that control energy homeostasis. Here we show that upon high-fat diet (HFD), the p75 neurotrophin receptor (p75NTR) controls energy expenditure in obese mice. Despite no changes in food intake, p75NTR-null mice were protected from HFD-induced obesity and remained lean due to increased energy expenditure, without developing insulin resistance or liver steatosis. p75NTR directly interacts with the catalytic subunit of protein kinase A (PKA) and regulates cAMP signaling in adipocytes, leading to decreased lipolysis and thermogenesis. Adipocyte-specific depletion of p75NTR or transplantation of p75NTR-null white adipose tissue (WAT) into wild-type mice fed a HFD protected against weight gain and insulin resistance. Our results reveal that signaling from p75NTR to cAMP/PKA regulates energy balance and suggest that non-neuronal functions of neurotrophin receptor signaling could be a new target for treating obesity and the metabolic syndrome.