Hepcidin is a key regulator of systemic iron homeostasis. Hepcidin deficiency induces iron overload, whereas hepcidin excess induces anemia. Mutations in the gene encoding hemojuvelin (HFE2, also known as HJV) cause severe iron overload and correlate with low hepcidin levels, suggesting that hemojuvelin positively regulates hepcidin expression. Hemojuvelin is a member of the repulsive guidance molecule (RGM) family, which also includes the bone morphogenetic protein (BMP) coreceptors RGMA and DRAGON (RGMB). Here, we report that hemojuvelin is a BMP coreceptor and that hemojuvelin mutants associated with hemochromatosis have impaired BMP signaling ability. Furthermore, BMP upregulates hepatocyte hepcidin expression, a process enhanced by hemojuvelin and blunted in Hfe2-/- hepatocytes. Our data suggest a mechanism by which HFE2 mutations cause hemochromatosis: hemojuvelin dysfunction decreases BMP signaling, thereby lowering hepcidin expression.

Peripheral inflammation induces p38 MAPK activation in the soma of C fiber nociceptors in the dorsal root ganglion (DRG) after 24 hr. Inflammation also increases protein, but not mRNA levels, of the heat-gated ion channel TRPV1 (VR1) in these cells, which is then transported to peripheral but not central C fiber terminals. Inhibiting p38 activation in the DRG reduces the increase in TRPV1 in the DRG and inflamed skin and diminishes inflammation-induced heat hypersensitivity without affecting inflammatory swelling or basal pain sensitivity. p38 activation in the DRG is secondary to peripheral production of NGF during inflammation and is required for NGF-induced increases in TRPV1. The activation of p38 in the DRG following retrograde NGF transport, by increasing TRPV1 levels in nociceptor peripheral terminals in a transcription-independent fashion, contributes to the maintenance of inflammatory heat hypersensitivity.

Inflammation causes the induction of cyclooxygenase-2 (Cox-2), leading to the release of prostanoids, which sensitize peripheral nociceptor terminals and produce localized pain hypersensitivity. Peripheral inflammation also generates pain hypersensitivity in neighbouring uninjured tissue (secondary hyperalgesia), because of increased neuronal excitability in the spinal cord (central sensitization), and a syndrome comprising diffuse muscle and joint pain, fever, lethargy and anorexia. Here we show that Cox-2 may be involved in these central nervous system (CNS) responses, by finding a widespread induction of Cox-2 expression in spinal cord neurons and in other regions of the CNS, elevating prostaglandin E2 (PGE2) levels in the cerebrospinal fluid. The major inducer of central Cox-2 upregulation is interleukin-1beta in the CNS, and as basal phospholipase A2 activity in the CNS does not change with peripheral inflammation, Cox-2 levels must regulate central prostanoid production. Intraspinal administration of an interleukin-converting enzyme or Cox-2 inhibitor decreases inflammation-induced central PGE2 levels and mechanical hyperalgesia. Thus, preventing central prostanoid production by inhibiting the interleukin-1beta-mediated induction of Cox-2 in neurons or by inhibiting central Cox-2 activity reduces centrally generated inflammatory pain hypersensitivity.

Endocannabinoids (eCBs) function as retrograde signaling molecules at synapses throughout the brain, regulate axonal growth and guidance during development, and drive adult neurogenesis. There remains a lack of genetic evidence as to the identity of the enzyme(s) responsible for the synthesis of eCBs in the brain. Diacylglycerol lipase-␣ (DAGL␣) and -␤ (DAGL␤) synthesize 2-arachidonoyl-glycerol (2-AG), the most abundant eCB in the brain. However, their respective contribution to this and to eCB signaling has not been tested. In the present study, we show ϳ80% reductions in 2-AG levels in the brain and spinal cord in DAGL␣ Ϫ/Ϫ mice and a 50% reduction in the brain in DAGL␤ Ϫ/Ϫ mice. In contrast, DAGL␤ plays a more important role than DAGL␣ in regulating 2-AG levels in the liver, with a 90% reduction seen in DAGL␤ Ϫ/Ϫ mice. Levels of arachidonic acid decrease in parallel with 2-AG, suggesting that DAGL activity controls the steady-state levels of both lipids. In the hippocampus, the postsynaptic release of an eCB results in the transient suppression of GABAmediated transmission at inhibitory synapses; we now show that this form of synaptic plasticity is completely lost in DAGL␣ Ϫ/Ϫ animals and relatively unaffected in DAGL␤ Ϫ/Ϫ animals. Finally, we show that the control of adult neurogenesis in the hippocampus and subventricular zone is compromised in the DAGL␣ Ϫ/Ϫ and/or DAGL␤ Ϫ/Ϫ mice. These findings provide the first evidence that DAGL␣ is the major biosynthetic enzyme for 2-AG in the nervous system and reveal an essential role for this enzyme in regulating retrograde synaptic plasticity and adult neurogenesis.

A cardinal feature of inflammation is heightened pain sensitivity at the site of the inflamed tissue. This results from the local release by immune and injured cells of nociceptor sensitizers, including prostaglandin E 2 , bradykinin, and nerve growth factor, that reduce the threshold and increase the excitability of the peripheral terminals of nociceptors so that they now respond to innocuous stimuli: the phenomenon of peripheral sensitization. We show here that the proinflammatory cytokine interleukin-1␤ (IL-1␤), in addition to producing inflammation and inducing synthesis of several nociceptor sensitizers, also rapidly and directly activates nociceptors to generate action potentials and induce pain hypersensitivity. IL-1␤ acts in a p38 mitogen-activated protein kinase (p38 MAP kinase)-dependent manner, to increase the excitability of nociceptors by relieving resting slow inactivation of tetrodotoxin-resistant voltage-gated sodium channels and also enhances persistent TTX-resistant current near threshold. By acting as an IL-1␤ sensor, nociceptors can directly signal the presence of ongoing tissue inflammation.

In the adult mouse, single and compound null mutations in the genes for retinoic acid receptor beta and retinoid X receptors beta and gamma resulted in locomotor defects related to dysfunction of the mesolimbic dopamine signaling pathway. Expression of the D1 and D2 receptors for dopamine was reduced in the ventral striatum of mutant mice, and the response of double null mutant mice to cocaine, which affects dopamine signaling in the mesolimbic system, was blunted. Thus, retinoid receptors are involved in the regulation of brain functions, and retinoic acid signaling defects may contribute to pathologies such as Parkinson's disease and schizophrenia.

Dopamine receptors have been implicated in the behavioural response to drugs of abuse. These responses are mediated particularly by the mesolimbic dopaminergic pathway arising in the ventral tegmental area and projecting to the limbic system. The rewarding properties of opiates and the somatic expression of morphine abstinence have been related to changes in mesolimbic dopaminergic activity that could constitute the neural substrate for opioid addiction. These adaptive responses to repeated morphine administration have been investigated in mice with a genetic disruption of the dopaminergic D2 receptors. Although the behavioural expression of morphine withdrawal was unchanged in these mice, a total suppression of morphine rewarding properties was observed in a place-preference test. This effect is specific to the drug, as mice lacking D2 receptors behaved the same as wild-type mice when food is used as reward. We conclude that the D2 receptor plays a crucial role in the motivational component of drug addiction.

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor (TGF)␤ superfamily of ligands that regulate many crucial aspects of embryonic development and organogenesis. Unlike other TGF␤ ligands, co-receptors for BMP ligands have not been described. Here we show that DRAGON, a glycosylphosphatidylinositol-anchored member of the repulsive guidance molecule family, which is expressed early in the developing nervous system, enhances BMP but not TGF␤ signaling. DRAGON binds directly to BMP2 and BMP4 but not to BMP7 or other TGF␤ ligands. The enhancing action of DRAGON on BMP signaling is also reduced by administration of Noggin, a soluble BMP antagonist, indicating that the action of DRAGON is ligand-dependent. DRAGON associates directly with BMP type I (ALK2, ALK3, and ALK6) and type II (ActRII and ActRIIB) receptors, and its signaling is reduced by dominant negative Smad1 and ALK3 or -6 receptors. In the Xenopus embryo, DRAGON both reduces the threshold of the ability of Smad1 to induce mesodermal and endodermal markers and alters neuronal and neural crest patterning. The direct interaction of DRAGON with BMP ligands and receptors indicates that it is a BMP co-receptor that potentiates BMP signaling. Transforming growth factor beta (TGF␤)1 superfamily ligands that include the TGF␤, bone morphogenetic protein (BMP), growth and differentiation factor, and nodal-related families play a pleiotropic role in vertebrate development by influencing cell specification, differentiation, proliferation, patterning, and migration (1, 2). These functions require the tight control of ligand production, ensuring a highly ordered spatiotemporal distribution and specific activation, via receptor complexes, of particular intracellular signaling pathways. The TGF␤/activin/nodal ligand subfamily contributes to the specification of endoderm and mesoderm in pregastrula embryos and at gastrula stages, to dorsal mesoderm formation and anterior-posterior patterning (3, 4). Later, TGF␤ ligands influence the body axis and patterning of the nervous system (5). BMPs, a second major ligand subfamily, contribute to the ventralization of germ layers in the early embryo and suppress the default neural cell fate of the ectoderm (6). BMPs also participate later in development in the formation and patterning of the neural crest, heart, blood, kidney, limb, muscle, and skeletal system (7).Signal transduction in the BMP subfamily is initiated by ligand binding to a receptor complex composed of two type I and two type II receptors. Three different BMP type I receptors (activin receptor-like kinase ALK2, ALK3, and ALK6) and three BMP type II receptors (BMP type II receptor (BMPRII), activin type IIA receptor (ActRIIA), activin type IIB receptor (ActRIIB)), each with intracellular serine/threonine kinase domains, have been identified (8). Ligand binding induces phosphorylation of the type I receptor by the type II receptor, which leads to phosphorylation of cytoplasmic receptor-activated Smads. The BMP subfamily signals through one set...