Issue published April 15, 2001

W e have developed a transgenic mouse line, NJ.1638, which expresses high levels of IL-5 from T cells, with profound hematological consequences. Eosinophils comprise more than 60% of circulating white blood cells in these animals, with the total peripheral white blood cell counts increasing more than 40-fold relative to wild-type littermates. This extraordinary proliferative capacity is sustained by expanded sites of extramedullary hematopoiesis and is accompanied by multifocal, ectopic bone formation in the spleen. Histology of the splenic nodules revealed the presence of osteoid matrices and osteocytes trapped within mineralized trabecular plates. In addition, polarized light microscopy of calcified tissue sections revealed both woven bone and areas of organized lamellar bone. Morphometric assessments demonstrated that both the growth and mineralization of splenic bone occurred at rates nearly an order of magnitude higher than in skeletal bone. Skeletal bone metabolic parameters were also perturbed. We also observed heterotopic ossification of the spleen and perturbation of skeletal bone homeostasis following adoptive engraftment of transgenic marrow to wild-type recipients. These data suggest that IL-5 overexpression mediates bone formation through the mobilization of marrow-derived osteogenic progenitors and/or the inhibition of recruited osteoclasts.

T argeted ablation of the vitamin D receptor (VDR) results in hypocalcemia, hypophosphatemia, hyperparathyroidism, rickets, osteomalacia, and alopecia — the last a consequence of defective anagen initiation. To investigate whether the markedly elevated levels of 1,25-dihydroxyvitamin D led to the alopecia, we raised VDR-null mice in a ultraviolet light–free environment and fed them chow lacking vitamin D for five generations. Despite undetectable circulating levels of 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D, alopecia persisted in the VDR-null mice, demonstrating that the alopecia was not secondary to toxic levels of 1,25-dihydroxyvitamin D interacting with an alternative receptor. Furthermore, alopecia was not seen in control littermates, suggesting that absence of ligand and absence of receptor cause different phenotypes. To identify the cell population responsible for the alopecia, we performed hair-reconstitution assays in nude mice and observed normal hair follicle morphogenesis, regardless of the VDR status of the keratinocytes and dermal papilla cells. However, follicles reconstituted with VDR-null keratinocytes demonstrated a defective response to anagen initiation. Hence, alopecia in the VDR-null mice is due to a defect in epithelial-mesenchymal communication that is required for normal hair cycling. Our results also identify the keratinocyte as the cell of origin of the defect and suggest that this form of alopecia is due to absence of ligand-independent receptor function.

T he medical treatment of chronic heart failure has undergone a dramatic transition in the past decade. Short-term approaches for altering hemodynamics have given way to long-term, reparative strategies, including β-adrenergic receptor (βAR) blockade. This was once viewed as counterintuitive, because acute administration causes myocardial depression. Cardiac myocytes from failing hearts show changes in βAR signaling and excitation-contraction coupling that can impair cardiac contractility, but the role of these abnormalities in the progression of heart failure is controversial. We therefore tested the impact of different manipulations that increase contractility on the progression of cardiac dysfunction in a mouse model of hypertrophic cardiomyopathy. High-level overexpression of the β₂AR caused rapidly progressive cardiac failure in this model. In contrast, phospholamban ablation prevented systolic dysfunction and exercise intolerance, but not hypertrophy, in hypertrophic cardiomyopathy mice. Cardiac expression of a peptide inhibitor of the βAR kinase 1 not only prevented systolic dysfunction and exercise intolerance but also decreased cardiac remodeling and hypertrophic gene expression. These three manipulations of cardiac contractility had distinct effects on disease progression, suggesting that selective modulation of particular aspects of βAR signaling or excitation-contraction coupling can provide therapeutic benefit.

M ice lacking natriuretic peptide receptor A (NPRA) have marked cardiac hypertrophy and chamber dilatation disproportionate to their increased blood pressure (BP), suggesting, in support of previous in vitro data, that the NPRA system moderates the cardiac response to hypertrophic stimuli. Here, we have followed the changes in cardiac function in response to altered mechanical load on the heart of NPRA-null mice (Npr1^–/–). Chronic treatment with either enalapril, furosemide, hydralazine, or losartan were all effective in reducing and maintaining BP at normal levels without affecting heart weight/body weight. In the reverse direction, we used transverse aortic constriction (TAC) to induce pressure overload. In the Npr1^–/– mice, TAC resulted in a 15-fold increase in atrial natriuretic peptide (ANP) expression, a 55% increase in left ventricular weight/body weight (LV/BW), dilatation of the LV, and significant decline in cardiac function. In contrast, banded Npr1^+/+ mice showed only a threefold increase in ANP expression, an 11% increase in LV/BW, a 0.2 mm decrease in LV end diastolic dimension, and no change in fractional shortening. The activation of mitogen-activated protein kinases that occurs in response to TAC did not differ in the Npr1^+/+ and Npr1^–/– mice. Taken together, these results suggest that the NPRA system has direct antihypertrophic actions in the heart, independent of its role in BP control.

I n Chagas’ disease caused by Trypanosoma cruzi, a paradigm of autoimmune disease, both autoantibodies and autoreactive T cells have been described. We have identified a novel dominant autoantigen, named Cha, recognized by the majority of sera from T. cruzi–infected humans and mice. We noted significant homologies between amino acids 120-129 of Cha, where the B-cell epitope maps, and an expressed sequence tag from T. cruzi, and also between amino acids 254-273 of Cha and a repeated amino acid sequence from the shed acute-phase antigen (SAPA) of T. cruzi. Moreover, T. cruzi–infected mice contain autoreactive T cells that can cross-react with Cha and the SAPA homologous peptides. Transfer of T cells from infected mice triggered anti-Cha (120-129) Ab production in naive recipients. Interestingly, heart tissue sections from those adoptive transferred mice showed cardiac pathology similar to T. cruzi–infected mice. Our results demonstrate the presence of both T- and B-cell cross-reactive epitopes in the Cha antigen. This dual mimicry may lead to T/B cell cooperation and give rise to a pathological immunodominant response against Cha in T. cruzi infected animals.

I nitial migration of encephalitogenic T cells to the central nervous system (CNS) in relapsing experimental autoimmune encephalomyelitis (R-EAE), an animal model of multiple sclerosis (MS), depends on the interaction of the α4 integrin (VLA-4) expressed on activated T cells with VCAM-1 expressed on activated cerebrovascular endothelial cells. Alternate homing mechanisms may be employed by infiltrating inflammatory cells after disease onset. We thus compared the ability of anti–VLA-4 to regulate proteolipid protein (PLP) 139-151–induced R-EAE when administered either before or after disease onset. Preclinical administration of anti–VLA-4 either to naive recipients of primed encephalitogenic T cells or to mice 1 week after peptide priming, i.e., before clinical disease onset, inhibited the onset and severity of clinical disease. In contrast, Ab treatment either at the peak of acute disease or during remission exacerbated disease relapses and increased the accumulation of CD4⁺ T cells in the CNS. Most significantly, anti–VLA-4 treatment either before or during ongoing R-EAE enhanced Th1 responses to both the priming peptide and endogenous myelin epitopes released secondary to acute tissue damage. Collectively, these results suggest that treatment with anti–VLA-4 Ab has multiple effects on the immune system and may be problematic in treating established autoimmune diseases such as MS.

W e used Hoxa3 knockout mice and other mouse models to study the role of the fetal parathyroids in fetal calcium homeostasis. Hoxa3-null fetuses lack parathyroid glands, and absence of parathyroid hormone (PTH) was confirmed with a rodent PTH immunoradiometric assay. The ionized calcium level of Hoxa3-null fetuses was significantly lower than that of wild-type or heterozygous littermates or of the mother. Both the rate of placental calcium transfer and the plasma PTHrP level were normal in Hoxa3 mutants and their heterozygous siblings. Because we had previously observed an increase in placental calcium transfer in PTH/PTHrP receptor 1–null (Pthr1-null) fetuses, we assayed plasma PTHrP in those mice. Pthr1-null fetuses had plasma PTHrP levels 11-fold higher than those of their littermates. Northern analysis, immunohistochemical, and in situ hybridization studies of Pthr1-null fetuses indicated that liver and placenta had increased expression of PTHrP. In summary, loss of fetal parathyroids in Hoxa3-null fetuses caused marked hypocalcemia but did not alter placental calcium transfer or the circulating PTHrP level. The findings in the Pthr1-null fetuses indicate that several tissues may contribute to the circulating PTHrP level in fetal mice.

T hyroid hormone thyroxine (T₄) and tri-iodothyronine (T₃) production is regulated by feedback inhibition of thyrotropin (TSH) and thyrotropin-releasing hormone (TRH) synthesis in the pituitary and hypothalamus when T₃ binds to thyroid hormone receptors (TRs) interacting with the promoters of the genes for the TSH subunit and TRH. All of the TR isoforms likely participate in the negative regulation of TSH production in vivo, but the identity of the specific TR isoforms that negatively regulate TRH production are less clear. To clarify the role of the TR-β2 isoform in the regulation of TRH gene expression in the hypothalamic paraventricular nucleus, we examined preprothyrotropin-releasing hormone (prepro-TRH) expression in mice lacking the TR-β2 isoform under basal conditions, after the induction of hypothyroidism with propylthiouracil, and in response to T₃ administration. Prepro-TRH expression was increased in hypothyroid wild-type mice and markedly suppressed after T₃ administration. In contrast, basal TRH expression was increased in TR-β2–null mice to levels seen in hypothyroid wild-type mice and did not change significantly in response to induction of hypothyroidism or T₃ treatment. However, the suppression of TRH mRNA expression in response to leptin reduction during fasting was preserved in TR-β2–null mice. Thus TR-β2 is the key TR isoform responsible for T₃-mediated negative-feedback regulation by hypophysiotropic TRH neurons.

P PARα is a ligand-dependent transcription factor expressed at high levels in the liver. Its activation by the drug gemfibrozil reduces clinical events in humans with established atherosclerosis, but the underlying mechanisms are incompletely defined. To clarify the role of PPARα in vascular disease, we crossed PPARα-null mice with apoE-null mice to determine if the genetic absence of PPARα affects vascular disease in a robust atherosclerosis model. On a high-fat diet, concentrations of atherogenic lipoproteins were higher in PPARα^–/–apoE^–/– than in PPARα^+/+apoE^–/– mice, due to increased VLDL production. However, en face atherosclerotic lesion areas at the aortic arch, thoracic aorta, and abdominal aorta were less in PPARα-null animals of both sexes after 6 and 10 weeks of high-fat feeding. Despite gaining as much or more weight than their PPARα^+/+apoE^–/– littermates, PPARα^–/–apoE^–/– mice had lower fasting levels of glucose and insulin. PPARα-null animals had greater suppression of endogenous glucose production in hyperinsulinemic clamp experiments, reflecting less insulin resistance in the absence of PPARα. PPARα^–/–apoE^–/– mice also had lower blood pressures than their PPARα^+/+apoE^–/– littermates after high-fat feeding. These results suggest that PPARα may participate in the pathogenesis of diet-induced insulin resistance and atherosclerosis.

N itric oxide synthase (NOS) inhibitors have therapeutic potential in the management of numerous conditions in which NO overproduction plays a critical role. Identification of transport systems in the intestine that can mediate the uptake of NOS inhibitors is important to assess the oral bioavailability and therapeutic efficacy of these potential drugs. Here, we have cloned the Na⁺- and Cl^–-coupled amino acid transport system B^0,+ (ATB^0,+) from the mouse colon and investigated its ability to transport NOS inhibitors. When expressed in mammalian cells, ATB^0,+ can transport a variety of zwitterionic and cationic amino acids in a Na⁺- and Cl^–-coupled manner. Each of the NOS inhibitors tested compete with glycine for uptake through this transport system. Furthermore, using a tritiated analog of the NOS inhibitor N^G-nitro-L-arginine, we showed that Na⁺- and Cl^–-coupled transport occurs via ATB^0,+. We then studied transport of a wide variety of NOS inhibitors in Xenopus laevis oocytes expressing the cloned ATB^0,+ and found that ATB^0,+ can transport a broad range of zwitterionic or cationic NOS inhibitors. These data represent the first identification of an ion gradient–driven transport system for NOS inhibitors in the intestinal tract.