Issue published July 1, 2004

A cute renal failure (ARF), characterized by sudden loss of the ability of the kidneys to excrete wastes, concentrate urine, conserve electrolytes, and maintain fluid balance, is a frequent clinical problem, particularly in the intensive care unit, where it is associated with a mortality of between 50% and 80%. In this review, the epidemiology and pathophysiology of ARF are discussed, including the vascular, tubular, and inflammatory perturbations. The clinical evaluation of ARF and implications for potential future therapies to decrease the high mortality are described.

A lthough the mechanisms of immune-mediated pregnancy loss are unknown, investigations are currently focused on mediators of immune activation and tissue injury at the maternal-fetal interface. A new study, however, demonstrates that systemic inflammatory mediators can induce pregnancy failure in a different way, by inhibiting ovarian hormone production, and identifies links between the immune and reproductive endocrine systems.

C irculating platelets are continually replenished by fragmentation of terminally differentiated megakaryocytes. Processes disrupted in inherited thrombocytopenias frequently shed light on normal thrombopoietic mechanisms. An especially rare condition called Paris-Trousseau syndrome (PTS) seems to occur by virtue of hemizygous loss of the FLI1 transcription factor gene. Provocative new data suggest that FLI1 shows monoallelic expression during a brief window in megakaryocyte differentiation, which thus explains the dominant inheritance pattern of PTS despite the presence of one normal FLI1 allele .

I n several clinical disorders, there are interactions between inflammation-dependent tissue injury and thrombin formation, fibrin deposition, and impaired fibrinolysis. New evidence generated from a mouse model of allergic airway hyperreactivity suggests that disordered coagulation and fibrinolysis may contribute to the pathogenesis of asthma . The inflammatory mechanisms that lead to airway smooth muscle contraction and airway hyperresponsiveness may be associated with accumulation of extravascular fibrin, plasma exudates, and inflammatory cells that can lead to airway closure.

I ncreasing evidence suggests that selective neuronal loss in neurodegenerative diseases involves activation of cysteine aspartyl proteases (caspases), which initiate and execute apoptosis. In Alzheimer disease both extracellular amyloid deposits and intracellular amyloid β protein may activate caspases, leading to cleavage of nuclear and cytoskeletal proteins, including tau protein. Proteolysis of tau may be critical to neurofibrillary degeneration, which correlates with dementia.

U nder normal circumstances, the respiratory tract maintains immune tolerance in the face of constant antigen provocation. Using a murine model of tolerance induced by repeated exposure to a low dose of aerosolized antigen, we show an important contribution by CD4⁺ T cells in the establishment and maintenance of tolerance. The CD4⁺ T cells expressed both cell surface and soluble TGF-β and inhibited the development of an allergic phenotype when adoptively transferred to naive recipient mice. While cells expressing cell surface TGF-β were detectable in mice with inflammation, albeit at a lower frequency compared with that in tolerized mice, only those from tolerized mice expressed FOXP3. Blockade of TGF-β in vitro and in vivo interfered with immunosuppression. Although cells that expressed TGF-β on the cell surface (TGF-β⁺), as well as the ones that did not (TGF-β^–), secreted equivalent levels of soluble TGF-β, only the former were able to blunt the development of an allergic phenotype in mice. Strikingly, separation of the TGF-β⁺ cells from the rest of the cells allowed the TGF-β^– cells to proliferate in response to antigen. We propose a model of antigen-induced tolerance that involves cell-cell contact with regulatory CD4⁺ T cells that coexpress membrane-bound TGF-β and FOXP3.

W e describe a murine model of early pregnancy failure induced by systemic activation of the CD40 immune costimulatory pathway. Although fetal loss involved an NK cell intermediate, it was not due to lymphocyte-mediated destruction of the fetus and placenta. Rather, pregnancy failure resulted from impaired progesterone synthesis by the corpus luteum of the ovary, an endocrine defect in turn associated with ovarian resistance to the gonadotropic effects of prolactin. Pregnancy failure also required the proinflammatory cytokine TNF-α and correlated with the luteal induction of the prolactin receptor signaling inhibitors suppressor of cytokine signaling 1 (Socs1) and Socs3. Such links between immune activation and reproductive endocrine dysfunction may be relevant to pregnancy loss and other clinical disorders of reproduction.

T hrombolysis is widely used to intervene in acute ischemic stroke, but reestablishment of circulation may paradoxically initiate a reperfusion injury. Here we describe studies with mice lacking protein kinase Cδ (PKCδ) showing that absence of this enzyme markedly reduces reperfusion injury following transient ischemia. This was associated with reduced infiltration of peripheral blood neutrophils into infarcted tissue and with impaired neutrophil adhesion, migration, respiratory burst, and degranulation in vitro. Total body irradiation followed by transplantation with bone marrow from PKCδ-null mice donors reduced infarct size and improved neurological outcome in WT mice, whereas marrow transplantation from WT donors increased infarction and worsened neurological scores in PKCδ-null mice. These results indicate an important role for neutrophil PKCδ in reperfusion injury and strongly suggest that PKCδ inhibitors could prove useful in the treatment of stroke.

G hrelin, a recently described endogenous ligand for the growth hormone secretagogue receptor (GHS-R), is produced by stomach cells and is a potent circulating orexigen, controlling energy expenditure, adiposity, and growth hormone secretion. However, the functional role of ghrelin in regulation of immune responses remains undefined. Here we report that GHS-R and ghrelin are expressed in human T lymphocytes and monocytes, where ghrelin acts via GHS-R to specifically inhibit the expression of proinflammatory anorectic cytokines such as IL-1β, IL-6, and TNF-α. Ghrelin led to a dose-dependent inhibition of leptin-induced cytokine expression, while leptin upregulated GHS-R expression on human T lymphocytes. These data suggest the existence of a reciprocal regulatory network by which ghrelin and leptin control immune cell activation and inflammation. Moreover, ghrelin also exerts potent anti-inflammatory effects and attenuates endotoxin-induced anorexia in a murine endotoxemia model. We believe this to be the first report demonstrating that ghrelin functions as a key signal, coupling the metabolic axis to the immune system, and supporting the potential use of ghrelin and GHS-R agonists in the management of disease-associated cachexia.

B one marrow of breast cancer patients was found to contain CD8⁺ T cells specific for peptides derived from breast cancer–associated proteins MUC1 and Her-2/neu. Most of these cells had a central or effector memory phenotype (CD45RA^–CD62L⁺ or CD45RA^–CD62L^–, respectively). To test their in vivo function, we separated bone marrow–derived CD45RA⁺ naive or CD45RA^–CD45RO⁺ memory T cells, stimulated them with autologous dendritic cells pulsed with tumor lysate, and transferred them into NOD/SCID mice bearing autologous breast tumors and normal skin transplants. CD45RA^– memory but not CD45RA⁺ naive T cells infiltrated autologous tumor but not skin tissues after the transfer. These tumor-infiltrating cells had a central or effector memory phenotype and produced perforin. Many of them expressed the P-selectin glycoprotein ligand 1 and were found around P-selectin⁺ tumor endothelium. Tumor infiltration included cluster formation in tumor tissue by memory T cells with cotransferred dendritic cells. It was associated with the induction of tumor cell apoptosis and significant tumor reduction. We thus demonstrate selective homing of memory T cells to human tumors and suggest that tumor rejection is based on the recognition of tumor-associated antigens on tumor cells and dendritic cells by autologous specifically activated central and effector memory T cells.

P aris-Trousseau syndrome (PTS; also known as Jacobsen syndrome) is characterized by several congenital anomalies including a dysmegakaryopoiesis with two morphologically distinct populations of megakaryocytes (MKs). PTS patients harbor deletions on the long arm of chromosome 11, including the FLI1 gene, which encodes a transcription factor essential for megakaryopoiesis. We show here that lentivirus-mediated overexpression of FLI1 in patient CD34⁺ cells restores the megakaryopoiesis in vitro, indicating that FLI1 hemizygous deletion contributes to the PTS hematopoietic defects. FISH analysis on pre-mRNA and single-cell RT-PCR revealed that FLI1 expression is mainly monoallelic in CD41⁺CD42^– progenitors, while it is predominantly biallelic in the other stages of megakaryopoiesis. In PTS cells, the hemizygous deletion of FLI1 generates a subpopulation of CD41⁺CD42^– cells completely lacking FLI1 transcription. We propose that the absence of FLI1 expression in these CD41⁺CD42^– cells might prevent their differentiation, which could explain the segregation of the PTS MKs into two subpopulations: one normal and one composed of small immature MKs undergoing a massive lysis, presumably originating from either FLI1⁺ or FLI1^– CD41⁺CD42^– cells, respectively. Thus, we point to the role of transient monoallelic expression of a gene essential for differentiation in the genesis of human haploinsufficiency-associated disease and suggest that such a mechanism may be involved in the pathogenesis of other congenital or acquired genetic diseases.

W hile platelet-activating factor (PAF) is produced in various diseases associated with bone resorption, its functions in bone metabolism remain unknown. Using PAF receptor–deficient mice, we evaluated the role of PAF in the development of bone resorption following ovariectomy, a model of postmenopausal osteoporosis. Through observations of bone mineral density and histomorphometric parameters, it was found that bone resorption was markedly attenuated in PAF receptor–deficient mice, indicating that PAF links estrogen depletion and osteoporosis in vivo. Osteoclasts expressed higher amounts of the enzymes required for PAF biosynthesis than osteoblasts. TNF-α and IL-1β increased the acetyl-coenzyme A:lyso-PAF acetyltransferase activity in osteoclasts. Osteoclasts, but not osteoblasts, expressed the functional PAF receptor. PAF receptor stimulation prolonged the survival of osteoclasts in vitro. Furthermore, osteoclasts treated with a PAF receptor antagonist, and also those from PAF receptor–deficient mice, showed reductions in survival rate and Ca resorption activity. Consistently, in organ cultures, bone resorption was significantly suppressed by a PAF receptor antagonist treatment or genetic PAF receptor deficiency. Thus, these results suggest that, through the inflammatory cytokines, estrogen depletion enhances PAF production as a unique autocrine factor for osteoclast functions. Inhibition of PAF function might pave the way for a new strategy to prevent postmenopausal bone loss without disturbing osteoblast functions.

G lycerol, a product of adipose tissue lipolysis, is an important substrate for hepatic glucose synthesis. However, little is known about the regulation of hepatic glycerol metabolism. Here we show that several genes involved in the hepatic metabolism of glycerol, i.e., cytosolic and mitochondrial glycerol 3-phosphate dehydrogenase (GPDH), glycerol kinase, and glycerol transporters aquaporin 3 and 9, are upregulated by fasting in wild-type mice but not in mice lacking PPARα. Furthermore, expression of these genes was induced by the PPARα agonist Wy14643 in wild-type but not PPARα−null mice. In adipocytes, which express high levels of PPARγ, expression of cytosolic GPDH was enhanced by PPARγ and β/δ agonists, while expression was decreased in PPARγ^+/– and PPARβ/δ^–/– mice. Transactivation, gel shift, and chromatin immunoprecipitation experiments demonstrated that cytosolic GPDH is a direct PPAR target gene. In line with a stimulating role of PPARα in hepatic glycerol utilization, administration of synthetic PPARα agonists in mice and humans decreased plasma glycerol. Finally, hepatic glucose production was decreased in PPARα-null mice simultaneously fasted and exposed to Wy14643, suggesting that the stimulatory effect of PPARα on gluconeogenic gene expression was translated at the functional level. Overall, these data indicate that PPARα directly governs glycerol metabolism in liver, whereas PPARγ regulates glycerol metabolism in adipose tissue.

M echanisms underlying airway hyperresponsiveness are not yet fully elucidated. One of the manifestations of airway inflammation is leakage of diverse plasma proteins into the airway lumen. They include fibrinogen and thrombin. Thrombin cleaves fibrinogen to form fibrin, a major component of thrombi. Fibrin inactivates surfactant. Surfactant on the airway surface maintains airway patency by lowering surface tension. In this study, immunohistochemically detected fibrin was seen along the luminal surface of distal airways in a patient who died of status asthmaticus and in mice with induced allergic airway inflammation. In addition, we observed altered airway fibrinolytic system protein balance consistent with promotion of fibrin deposition in mice with allergic airway inflammation. The airways of mice were exposed to aerosolized fibrinogen, thrombin, or to fibrinogen followed by thrombin. Only fibrinogen followed by thrombin resulted in airway hyperresponsiveness compared with controls. An aerosolized fibrinolytic agent, tissue-type plasminogen activator, significantly diminished airway hyperresponsiveness in mice with allergic airway inflammation. These results are consistent with the hypothesis that leakage of fibrinogen and thrombin and their accumulation on the airway surface can contribute to the pathogenesis of airway hyperresponsiveness.

A ngiotensin I–converting enzyme (ACE) inhibitors are thought to lower blood pressure in hypertensive patients, mainly by decreasing angiotensin II (Ang II) formation. Chymase, a human mast cell protease, has recently been proposed to play a role in blood pressure regulation because of its Ang II–forming activity. Here we show that the predominant chymase mRNA species in the mouse aorta are those for types 4 and 5 isoforms, and that both are efficient Ang II–forming enzymes. Evaluation of ACE-dependent and ACE-independent Ang II–forming pathways in mast cell–deficient (Kit^w/Kit^w-v) mice and their mast cell–sufficient littermate (MC^+/+) controls revealed that, in contrast to the latter, Kit^w/Kit^w-v mice fail to express chymase mRNAs in the vasculature and have almost no ACE-independent Ang II–forming activity in either isolated blood vessels or homogenates. Moreover, in MC^+/+ but not in Kit^w/Kit^w-v mice, a contribution of ACE-independent Ang II generation to blood pressure regulation was evident by a 1.6-fold greater maximal reduction in mean arterial pressure with acute ACE inhibition plus AT₁ receptor blockade than with ACE inhibition alone. Thus, mast cells are the source of the vascular ACE-independent pathway, and the antihypertensive benefit of combining ACE inhibitor therapy with AT₁ receptor antagonist therapy is most likely due to negation of chymase-catalyzed Ang II generation.

N eurofibrillary tangles (NFTs) are composed of abnormal aggregates of the cytoskeletal protein tau. Together with amyloid β (Aβ) plaques and neuronal and synaptic loss, NFTs constitute the primary pathological hallmarks of Alzheimer disease (AD). Recent evidence also suggests that caspases are activated early in the progression of AD and may play a role in neuronal loss and NFT pathology. Here we demonstrate that tau is cleaved at D421 (ΔTau) by executioner caspases. Following caspase-cleavage, ΔTau facilitates nucleation-dependent filament formation and readily adopts a conformational change recognized by the early pathological tau marker MC1. ΔTau can be phosphorylated by glycogen synthase kinase-3β and subsequently recognized by the NFT antibody PHF-1. In transgenic mice and AD brains, ΔTau associates with both early and late markers of NFTs and is correlated with cognitive decline. Additionally, ΔTau colocalizes with Aβ_1–42 and is induced by Aβ_1–42 in vitro. Collectively, our data imply that Aβ accumulation triggers caspase activation, leading to caspase-cleavage of tau, and that this is an early event that may precede hyperphosphorylation in the evolution of AD tangle pathology. These results suggest that therapeutics aimed at inhibiting tau caspase-cleavage may prove beneficial not only in preventing NFT formation, but also in slowing cognitive decline.

R etinitis pigmentosa (RP) is a clinically and genetically heterogeneous degenerative eye disease. Mutations at Arg135 of rhodopsin are associated with a severe form of autosomal dominant RP. This report presents evidence that Arg135 mutant rhodopsins (e.g., R135L, R135G, and R135W) are hyperphosphorylated and bind with high affinity to visual arrestin. Mutant rhodopsin recruits the cytosolic arrestin to the plasma membrane, and the rhodopsin-arrestin complex is internalized into the endocytic pathway. Furthermore, the rhodopsin-arrestin complexes alter the morphology of endosomal compartments and severely damage receptor-mediated endocytic functions. The biochemical and cellular defects of Arg135 mutant rhodopsins are distinct from those previously described for class I and class II RP mutations, and, hence, we propose that they be named class III. Impaired endocytic activity may underlie the pathogenesis of RP caused by class III rhodopsin mutations.