Letters in 2012

Shin'ichi Takeda and colleagues show that loading of muscle results in nitric oxide creation and its conversion to peroxynitrite. The latter molecule then activates TRPV1 in the muscle, leading to increased cytoplasmic concentrations of calcium and activation of mTOR and, thus, muscle hypertrophy. They could replicate these effects without overload by treating mice with a TRPV1 agonist, suggesting a possible therapy for muscle wasting.

Resting CD4⁺ T cells are resistant to HIV-1 infection, but the underlying reasons for this lack of permissiveness have not been clear. Oliver Fackler and colleagues now report that SAMHD1, the deoxynucleoside triphosphate triphosphohydrolase responsible for restriction of HIV-1 infection in myeloid cells, also restricts infection of resting CD4⁺ T cells. The findings shed new light on the mechanisms of cellular and molecular regulation of HIV-1 infection.

A minority of HIV-1–infected individuals develop broadly neutralizing antibodies, which are considered an important goal of many HIV vaccine strategies. Moore et al. now report their study of the evolution of a broadly neutralizing antibody response targeting a glycan on the viral envelope in two HIV-1–infected individuals. Their findings show that the targeted glycan is absent early in acute infection but develops over time as the virus escapes initial antibody-mediated pressure.

The authors uncover a new mechanism for the regulation of the activity of leukemia-initiating cells in T-ALL. A subpopulation of stem cells with low amounts of reactive oxygen species (ROS) is enriched in their ability to reconstitute disease in mouse models, and this effect is regulated by repression of PKC-θ, which increases ROS production. Moreover, oncogenic NOTCH, a common T-ALL–driving alteration, regulates stem cell activity by increasing RUNX3 expression, which represses RUNX1, a PCK-θ activator, a pathway that is conserved in human patients.

Anaplastic large cell lymphomas (ALCL) often express the oncoprotein NPM-ALK. This study shows that the activator protein 1 family members JUN and JUNB promote lymphoma development through platelet-derived growth factor receptor-β (PDGFRB). Inhibition of PDGFRB prolonged survival of NPM-ALK transgenic mice and increased the efficacy of an ALK-specific inhibitor against transplanted NPM-ALK tumors. Inhibition of PDGFR in a patient with ALCL also resulted in rapid, complete and sustained remission.

House dust promotes allergic responses to inhaled allergens, but it remains unclear what microbial components in dust are required for this activity. Donald N. Cook and his colleagues show that the bacterial protein flagellin stimulates allergic airway responses, and its receptor, Toll-like receptor 5, is required to elicit airway eosinophilia and hyperreactivity in response to dust. Individuals with asthma have higher serum antibody titers to flagellin, suggesting exposure to flagellin is associated with allergic airway disease.

Individuals with hemophilia A lack the coagulation factor FVIII and are treated with frequent intravenous injections of FVIII agents. However, many individuals develop antibodies to FVIII and can no longer be treated by FVIII injection. Takehisa Kitazawa and his colleagues report the development of a bispecific antibody to FIXa and FX that mimics the function of FVIII. This antibody reduces bleeding in a nonhuman primate model of hemophilia A, is resistant to the inhibitory effects of FVIII-specific antibodies and has a long half-life after subcutaneous injection.

Animals use their muscles to shiver to generate heat when exposed to the cold. But this is a short-term adaptation. Long term, it is believed the body relies on the brown adipose tissue (BAT) to generate heat in a nonshivering fashion. New work from Muthu Periasamy and colleagues challenge this BAT-centric view by showing that the muscle is also a key site of nonshivering thermogenesis.

Ciliopathies are caused by alterations in the development and function of cilia. Now Jeffrey Martens and his colleagues demonstrate anatomic and functional rescue of cilia development in mature, differentiated neurons by adenovirus-mediated restoration of expression of the wild-type protein intraflagellar transport protein 88 (Ift88) and show restoration of olfactory function in a mouse model of ciliopathy. A loss-of-function mutation in IFT88 is also identified in individuals with ciliopathies.

The mineralocorticoid receptor, targeted by drugs commonly used to treat hypertension, is generally thought to contribute to hypertension by altering kidney function. Using mice lacking the mineralocorticoid receptor specifically in smooth muscle cells, Iris Jaffe and her colleagues show that it also controls many aspects of vascular aging, including blood vessel tone, and that these vascular effects contribute to the mineralocorticoid receptor's prohypertensive actions.

Complement components activate and recruit immune cells, promoting host defense and inflammatory disease. Jörg Köhl and his colleagues demonstrate that IgG1 immune complexes inhibit C5a-mediated inflammatory responses and disease. The inhibitory effect of IgG1 immune complexes requires galactosylation of the antibody, binding to the inhibitory IgG receptor FcγRIIB and the association of FcγRIIB with the C-type lectin–like receptor dectin-1.

Epha4 is a receptor involved in axonal repulsion. Wim Robberecht and his colleagues report that genetic or pharmacological inhibition of Epha4 is protective in rodent and zebrafish models of amyotrophic lateral sclerosis. In humans, expression of Epha4 inversely correlates with disease onset and survival, and in two patients, mutations in Epha4 are associated with longer survival, suggesting Epha4 may be targeted therapeutically to prevent axonal degeneration.

Ronald Duman and colleagues report that synapse number is reduced in subjects with major depressive disorder. This is associated with decreased expression of synapse-related genes and increased expression of the transcriptional repressor, GATA1. Expression of GATA1 in prefrontal cortex neurons decreases the expression of synapse-related genes, reduces dendrite branching and produces depressive behavior in a rat model of depression.

Infiltration of various immune cell types into the fat tissue and liver has been implicated in obesity-induced insulin resistance. Jerry Olefsky and his colleagues now show that neutrophils are one of the earliest immune cells to arrive in these tissues, that they release the protease neutrophil elastase and that this enzyme degrades IRS-1, a key member of the insulin signaling pathway. These results show that neutrophils contribute to insulin resistance and how they may do so.

Excess free fatty acids (FFAs) are known to induce insulin resistance, and a role for TLR4 has been implicated in this process. But FFAs are believed to be incapable of binding TLR4 directly. In a new study, Samir Bhattacharya and colleagues show that fetuin-A acts as an intermediary in this process by bindings FFAs and presenting them to TLR4. These results suggest fetuin-A as a new target to treat insulin resistance and diabetes.

Ultraviolet radiation induces an inflammatory response in the skin, but it remains unclear how cells in the skin detect this damage and trigger an inflammatory response. Richard L. Gallo and his colleagues report that ultraviolet radiation damages self noncoding RNA. These modified RNA are released from irradiated keratinocytes and act as a danger signal that is detected by Toll-like receptor 3 (TLR3), which is required for the induction of proinflammatory cytokine release and for radiation-induced immune suppression.

Activation of the epithelial Na⁺ channel ENaC in mouse endometrium triggers Ca²⁺ influx and upregulation of cyclooxygenase 2, an enzyme key for prostaglandin production and for embryo implantation.

Using an unbiased genetic screen in mice, Hartmut Geiger et al. found that the thrombomodulin–activated protein C pathway, which controls blood coagulation among other functions, also protects from radiation damage. Administration of a recombinant variant of thrombomodulin or of recombinant activated protein C protected mice from total body irradiation, pointing to potential therapeutic applications.

Pelizaeus-Merzbacher disease (PMD) is a neurological disease caused by loss of myelin. Now, Gesine Saher et al. show that a cholesterol-enriched diet can ameliorate disease in a mouse model of PMD.

This report identifies upregulation of HGF as an autocrine growth pathway in several subsets of AML. Ligand-dependent activation of MET represents a new oncogenic stimulus, and the dynamic regulation of HGF can overcome the effects of MET inhibition. These results suggest that combination treatments may be needed to disrupt this autocrine signaling loop and quell the growth of AML.