Issue published February 1, 2003

D eletions in the DAP12 gene in humans result in Nasu-Hakola disease, characterized by a combination of bone fractures and psychotic symptoms similar to schizophrenia, rapidly progressing to presenile dementia. However, it is not known why these disorders develop upon deficiency in DAP12, an immunoreceptor signal activator protein initially identified in the immune system. Here we show that DAP12-deficient (DAP12^–/–) mice develop an increased bone mass (osteopetrosis) and a reduction of myelin (hypomyelinosis) accentuated in the thalamus. In vitro osteoclast induction from DAP12^–/– bone marrow cells yielded immature cells with attenuated bone resorption activity. Moreover, immature oligodendrocytes were arrested in the vicinity of the thalamus, suggesting that the primary defects in DAP12^–/– mice are the developmental arrest of osteoclasts and oligodendrocytes. In addition, the mutant mice also showed synaptic degeneration, impaired prepulse inhibition, which is commonly observed in several neuropsychiatric diseases in humans including schizophrenia, and aberrant electrophysiological profiles in the thalami. These results provide a molecular basis for a unique combination of skeletal and psychotic characteristics of Nasu-Hakola disease as well as for schizophrenia and presenile dementia.

T he hallmark of early atherosclerosis is the accumulation of lipid-laden macrophages in the subendothelial space. Circulating monocytes are the precursors of these “foam cells,” and recent evidence suggests that chemokines play important roles in directing monocyte migration from the blood to the vessel wall. Fractalkine (FK) is a structurally unusual chemokine that can act either as a soluble chemotactic factor or as a transmembrane-anchored adhesion receptor for circulating leukocytes. A polymorphism in the FK receptor, CX₃CR1, has been linked to a decrease in the incidence of coronary artery disease. To determine whether FK is critically involved in atherogenesis, we deleted the gene for CX₃CR1 and crossed these mice into the apoE^–/– background. Here we report that FK is robustly expressed in lesional smooth muscle cells, but not macrophages, in apoE^–/– mice on a high-fat diet. CX₃CR1^–/– mice have a significant reduction in macrophage recruitment to the vessel wall and decreased atherosclerotic lesion formation. Taken together, these data provide strong evidence that FK plays a key role in atherogenesis.

O ur understanding of the genetic basis of disease has expanded with the identification of rare DNA sequence variations (“mutations”) that evoke inherited syndromes such as cystic fibrosis, congenital epilepsy, and cardiac arrhythmias. Common sequence variants (“polymorphisms”) have also been implicated as risk factors in multiple diseases. Mutations in SCN5A, the cardiac Na⁺ channel gene, that cause a reduction in Na⁺ current may evoke severe, life-threatening disturbances in cardiac rhythm (i.e., Brugada syndrome), isolated cardiac conduction disease, or combinations of these disorders. Conduction disease is manifest clinically as heart rate slowing (bradycardia), syncope, or “lightheadedness”. Recent electrophysiologic studies reveal that mutations in particular families exhibiting cardiac conduction disease cause marked effects on several competing voltage-dependent gating processes, but nonetheless cause a mild “net” reduction in Na⁺ current. Here we show that a common SCN5A polymorphism (H558R) in the Na⁺ channel I-II interdomain cytoplasmic linker, present in 20% of the population, can mitigate the in vitro effects of a nearby mutation (T512I) on Na⁺ channel function. The mutation and the polymorphism were both found in the same allele of a child with isolated conduction disease, suggesting a direct functional association between a polymorphism and a mutation in the same gene. J. Clin. Invest. 111:341–346 (2003). doi:10.1172/JCI200316879.

A new member of the lipase gene family, initially termed endothelial lipase (gene nomenclature, LIPG; protein, EL), is expressed in a variety of different tissues, suggesting a general role in lipid metabolism. To assess the hypothesis that EL plays a physiological role in lipoprotein metabolism in vivo, we have used gene targeting of the native murine locus and transgenic introduction of the human LIPG locus in mice to modulate the level of EL expression. Evaluation of these alleles in a C57Bl/6 background revealed an inverse relationship between HDL cholesterol level and EL expression. Fasting plasma HDL cholesterol was increased by 57% in LIPG^–/– mice and 25% in LIPG^+/– mice and was decreased by 19% in LIPG transgenic mice as compared with syngeneic controls. Detailed analysis of lipoprotein particle composition indicated that this increase was due primarily to an increased number of HDL particles. Phospholipase assays indicated that EL is a primary contributor to phospholipase activity in mouse. These data indicate that expression levels of this novel lipase have a significant effect on lipoprotein metabolism.

E ndothelial lipase (EL) is a recently discovered member of the lipoprotein lipase gene family that hydrolyzes HDL phospholipids ex vivo and reduces HDL cholesterol (HDL-C) levels when overexpressed in vivo in mice. To gain further insight into the physiological role of EL in the metabolism of HDL in vivo, studies were performed in which EL was inhibited in wild-type, hepatic lipase knockout (HL^–/–), and human apoA-I transgenic mice by intravenous infusion of a polyclonal antibody inhibitory to murine EL. As compared with infusion of a control antibody, infusion of the inhibitory antibody resulted in a 25–60% increase in HDL-C levels in the three mouse models, with the peak HDL-C levels occurring at 48 hours after injection. Inhibition of EL also generated larger HDL particles in the HL^–/– mice. The clearance of HDL phospholipid was significantly slower in human apoA-I transgenic mice injected with an antibody against murine EL (mEL) than in mice injected with a control antibody. We conclude that inhibition of EL results in increased HDL-C levels and that EL is an important enzyme in the physiological regulation of HDL metabolism.

A pathogenic hallmark of rheumatoid arthritis (RA) is persistent activation of self-reactive CD4⁺ T cells. The cause of this aberrant activity remains elusive. We report here detection of autoantibodies against B7-H1, a recently described member of the B7 family, in 29% of patients with RA versus 4% of healthy donors. High-level expression of cell surface B7-H1 are found on activated human CD4⁺, CD8⁺, and CD45RO⁺ T cells. Immobilized autoantibodies to B7-H1 are capable of costimulating the proliferation of CD4⁺ T cells in vitro, and the presence of these autoantibodies correlates with active disease status. Using immobilized B7-H1 mAb’s and programmed death 1Ig, we demonstrate that engagement of B7-H1 on CD4⁺ T cells costimulates proliferation and secretion of IL-10, and subsequently leads to programmed cell death, accompanied with upregulated expression of TNF-related apoptosis–inducing ligand and activation of caspase-3. Taken together with our previous findings, these data indicate a bidirectional signaling role of B7-H1 in T cell costimulation and apoptosis and implicate B7-H1 autoantibodies as contributing to the progression of RA by inducing aberrant T cell responses.

E xtracellular nucleotides are important regulators of epithelial ion transport. Here we investigated nucleotide-mediated effects on colonic NaCl secretion and the signal transduction mechanisms involved. Basolateral UDP induced a sustained activation of Cl^– secretion, which was completely inhibited by 293B, a specific inhibitor of cAMP-stimulated basolateral KCNQ1/KCNE3 K⁺ channels. We therefore speculated that a basolateral P2Y₆ receptor could increase cAMP. Indeed UDP elevated cAMP in isolated crypts. We identified an epithelial P2Y₆ receptor using crypt [Ca²⁺]_i measurements, RT-PCR, and immunohistochemistry. To investigate whether the rat P2Y₆elevates cAMP, we coexpressed the P2Y₁ or P2Y₆ receptor together with the cAMP-regulated cystic fibrosis transmembrane conductance regulator (CFTR) Cl^– channel in Xenopus oocytes. A two-electrode voltage clamp was used to monitor nucleotide-induced Cl^– currents. In oocytes expressing the P2Y₁ receptor, ATP transiently activated the endogenous Ca²⁺-activated Cl^– current, but not CFTR. In contrast, in oocytes expressing the P2Y₆receptor, UDP transiently activated the Ca²⁺-activated Cl^– current and subsequently CFTR. CFTR Cl^– currents were identified by their halide conductance sequence. In summary we find a basolateral P2Y₆ receptor in colonic epithelial cells stimulating sustained NaCl secretion by way of a synergistic increase of [Ca²⁺]_i and cAMP. In support of these data P2Y₆ receptor stimulation differentially activates CFTR in Xenopus oocytes.

W e have previously described the identification of Artemis, a factor involved in the nonhomologous end joining (NHEJ) phase of V(D)J recombination of T and B cell receptor genes. Null mutations of the Artemis gene result in a complete absence of T and B lymphocytes that is associated with increased cell radiosensitivity, causing the radiosensitive T^–B^– SCID (RS-SCID) condition. We presently report the occurrence of hypomorphic mutations of the Artemis gene in four patients from two kindreds. Partially preserved in vivo activity of Artemis is associated with the presence of polyclonal T and B lymphocyte populations, albeit in reduced numbers, along with chromosomal instability and development of EBV-associated lymphoma in two of four patients. This syndrome emphasizes the role of Artemis in the NHEJ pathway of DNA repair and suggests that other, yet ill-defined, conditions associating immunodeficiency and lymphoma could be caused by mutations in genes encoding NHEJ factors.

P rotease inhibitors decrease the viral load in HIV patients, however the patients develop hypertriglyceridemia, hypercholesterolemia, and atherosclerosis. It has been assumed that protease inhibitor–dependent increases in atherosclerosis are secondary to the dyslipidemia. Incubation of THP-1 cells or human PBMCs with protease inhibitors caused upregulation of CD36 and the accumulation of cholesteryl esters. The use of CD36-blocking antibodies, a CD36 morpholino, and monocytes isolated from CD36 null mice demonstrated that protease inhibitor–induced increases in cholesteryl esters were dependent on CD36 upregulation. These data led to the hypothesis that protease inhibitors induce foam cell formation and consequently atherosclerosis by upregulating CD36 and cholesteryl ester accumulation independent of dyslipidemia. Studies with LDL receptor null mice demonstrated that low doses of protease inhibitors induce an increase in the level of CD36 and cholesteryl ester in peritoneal macrophages and the development of atherosclerosis without altering plasma lipids. Furthermore, the lack of CD36 protected the animals from protease inhibitor–induced atherosclerosis. Finally, ritonavir increased PPAR-γ and CD36 mRNA levels in a PKC- and PPAR-γ–dependent manner. We conclude that protease inhibitors contribute to the formation of atherosclerosis by promoting the upregulation of CD36 and the subsequent accumulation of sterol in macrophages.

T he availability of mice lacking the mitochondrial uncoupling protein UCP1, has provided an opportunity to analyze the relationship between the capacity for energy expenditure and the development of obesity in response to a high-fat, high-sucrose diet. Congenic UCP1-deficient mice on a C57BL/6J genetic background show a temperature-dependent resistance to diet-induced obesity when compared with wild-type mice. This resistance, which occurs at 20°C, is quickly reversed when the ambient temperature is increased to 27°C. At 20°C, total oxygen consumption and physical activity of mutant and wild-type mice are indistinguishable; however, body temperature is higher in UCP1-deficient mice by 0.1–0.3°C, and respiratory quotient is slightly reduced. A reduced respiratory quotient, together with elevated β-hydroxybutyrate and reduced plasma fatty acid levels, suggests that the mutants oxidize a greater proportion of fat than wild-type mice, and that this possibly accounts for the resistance to diet-induced obesity. Although shivering is one alternative mechanism of thermogenesis that is probably used in UCP1-deficient mice, whether there are others remains to be determined. Nevertheless, our study underscores the paradox that elimination of the major thermogenic mechanism in the animal reduces rather than increases metabolic efficiency. We propose that in the absence of nonshivering thermogenesis, alternative, calorically more costly pathways of metabolism must be used to maintain body temperature.

D imeric Fc receptor (FcR) nonbinding anti-CD3 antibodies have been developed to minimize toxicities associated with classical anti-CD3 monoclonal antibodies (e.g., OKT3). Studies with murine analogs of non-FcR–binding antibodies have shown reduced mitogenicity compared to OKT3. In a trial of an FcR nonbinding humanized anti-CD3 mAb hOKT3γ1(Ala-Ala) for treatment of patients with type 1 diabetes, we found significant increases in IL-10 and IL-5 in the serum of 63% and 72% of patients, respectively, and TNF-α and IL-6 levels that were lower than those previously reported following OKT3 therapy. The activation signal delivered by hOKT3γ1(Ala-Ala) was associated with calcium signaling and cytokine production by previously activated human cells in vitro. However, the production of IL-10, compared to IFN-γ on a molar basis, was greater after culture with hOKT3γ1(Ala-Ala) than with OKT3. Flow cytometric studies confirmed that OKT3 induced IFN-γ and IL-10 production, but hOKT3γ1(Ala-Ala) induced only detectable IL-10 production in CD45RO⁺ cells. Moreover, in vivo, we found IL-10⁺CD4⁺ T cells after drug treatment. These cells were heterogeneous but generally CD45RO⁺, CTLA-4^–, and expressed CCR4. A subgroup of these cells expressed TGF-β. Thus, the non-FcR binding anti-CD3 mAb, hOKT3γ1(Ala-Ala) delivers an activation signal to T cells that is quantitatively and qualitatively different from OKT3. It leads to the generation of T cells that might inhibit the autoimmune response and may be involved in the beneficial effect on β cell destruction in Type 1 diabetes.

L ipoprotein lipase is the principal enzyme that hydrolyzes circulating triglycerides and liberates free fatty acids that can be used as energy by cardiac muscle. Although lipoprotein lipase is expressed by and is found on the surface of cardiomyocytes, its transfer to the luminal surface of endothelial cells is thought to be required for lipoprotein lipase actions. To study whether nontransferable lipoprotein lipase has physiological actions, we placed an α-myosin heavy-chain promoter upstream of a human lipoprotein lipase minigene construct with a glycosylphosphatidylinositol anchoring sequence on the carboxyl terminal region. Hearts of transgenic mice expressed the altered lipoprotein lipase, and the protein localized to the surface of cardiomyocytes. Hearts, but not postheparin plasma, of these mice contained human lipoprotein lipase activity. More lipid accumulated in hearts expressing the transgene; the myocytes were enlarged and exhibited abnormal architecture. Hearts of transgenic mice were dilated, and left ventricular systolic function was impaired. Thus, lipoprotein lipase expressed on the surface of cardiomyocytes can increase lipid uptake and produce cardiomyopathy.