Severe dermatitis with loss of epidermal Langerhans cells in human and mouse zinc deficiency

Genetic causes and gene–nutrient interactions in mammalian zinc deficiencies: acrodermatitis enteropathica and transient neonatal zinc deficiency as examples

Discovering genetic causes of zinc deficiency has been a remarkable scientific journey. It started with the description of a rare skin disease, its treatment with various agents, the successful therapy with zinc, and the identification of mutations in a zinc transporter causing the disease. The journey continues with defining the molecular and cellular pathways that lead to the symptoms caused by zinc deficiency. Remarkably, at least two zinc transporters from separate protein families are now known to be involved in the genetics of zinc deficiency. One is ZIP4, which is involved in intestinal zinc uptake. Its mutations can cause acrodermatitis enteropathica (AE) with autosomal recessive inheritance. The other one is ZnT2, the transporter responsible for supplying human milk with zinc. Mutations in this transporter cause transient neonatal zinc deficiency (TNZD) with symptoms similar to AE but with autosomal dominant inheritance. The two diseases can be distinguished in affected infants. AE is fatal if zinc is not supplied to the infant after weaning, whereas TNZD is a genetic defect of the mother limiting the supply of zinc in the milk, and therefore the infant usually will obtain enough zinc once weaned. Although these diseases are relatively rare, the full functional consequences of the numerous mutations in ZIP4 and ZnT2 and their interactions with dietary zinc are not known. In particular, it remains unexplored whether some mutations cause milder disease phenotypes or increase the risk for other diseases if dietary zinc requirements are not met or exceeded. Thus, it is not known whether widespread zinc deficiency in human populations is based primarily on a nutritional deficiency or determined by genetic factors as well. This consideration becomes even more significant with regard to mutations in the other 22 human zinc transporters, where associations with a range of diseases, including diabetes, heart disease, and mental illnesses have been observed. Therefore, clinical tests for genetic disorders of zinc metabolism need to be developed.

Current understanding of ZIP and ZnT zinc transporters in human health and diseases

Zinc transporters, the Zrt-, Irt-like protein (ZIP) family and the Zn transporter (ZnT) family transporters, are found in all aspects of life. Increasing evidence has clarified the molecular mechanism, in which both transporters play critical roles in cellular and physiological functions via mobilizing zinc across the cellular membrane. In the last decade, mutations in ZIP and ZnT transporter genes have been shown to be implicated in a number of inherited human diseases. Moreover, dysregulation of expression and activity of both transporters has been suggested to be involved in the pathogenesis and progression of chronic diseases including cancer, immunological impairment, and neurodegenerative diseases, although comprehensive understanding is far from complete. The diverse phenotypes of diseases related to ZIP and ZnT transporters reflect the multifarious biological functions of both transporters. The present review summarizes the current understanding of ZIP and ZnT transporter functions from the standpoint of human health and diseases. The study of zinc transporters is currently of great clinical interest.

Zinc transporter SLC39A10/ZIP10 controls humoral immunity by modulating B-cell receptor signal strength

The humoral immune response, also called the antibody-mediated immune response, is one of the main adaptive immune systems. The essential micronutrient zinc (Zn) is known to modulate adaptive immune responses, and dysregulated Zn homeostasis leads to immunodeficiency. However, the molecular mechanisms underlying this Zn-mediated modulation are largely unknown. Here, we show that the Zn transporter SLC39A10/ZIP10 plays an important role in B-cell antigen receptor (BCR) signal transduction. Zip10-deficiency in mature B cells attenuated both T-cell-dependent and -independent immune responses in vivo. The Zip10-deficient mature B cells proliferated poorly in response to BCR cross-linking, as a result of dysregulated BCR signaling. The perturbed signaling was found to be triggered by a reduction in CD45R phosphatase activity and consequent hyperactivation of LYN, an essential protein kinase in BCR signaling. Our data suggest that ZIP10 functions as a positive regulator of CD45R to modulate the BCR signal strength, thereby setting a threshold for BCR signaling in humoral immune responses.

The enzyme Cyp26b1 mediates inhibition of mast cell activation by fibroblasts to maintain skin-barrier homeostasis

Mast cells (MCs) mature locally, thus possessing tissue-dependent phenotypes for their critical roles in both protective immunity against pathogens and the development of allergy or inflammation. We previously reported that MCs highly express P2X7, a receptor for extracellular ATP, in the colon but not in the skin. The ATP-P2X7 pathway induces MC activation and consequently exacerbates the inflammation. Here, we identified the mechanisms by which P2X7 expression on MCs is reduced by fibroblasts in the skin, but not in the other tissues. The retinoic-acid-degrading enzyme Cyp26b1 is highly expressed in skin fibroblasts, and its inhibition resulted in the upregulation of P2X7 on MCs. We also noted the increased expression of P2X7 on skin MCs and consequent P2X7- and MC-dependent dermatitis (so-called retinoid dermatitis) in the presence of excessive amounts of retinoic acid. These results demonstrate a unique skin-barrier homeostatic network operating through Cyp26b1-mediated inhibition of ATP-dependent MC activation by fibroblasts.

Mast cells play a key role in host defense against herpes simplex virus infection through TNF-α and IL-6 production

The essential contribution of mast cells (MCs) to bacterial host defense has been well established; however, little is known about their role in viral infections in vivo. Here, we found that intradermal injection with herpes simplex virus 2 (HSV-2) into MC-deficient Kit(W/Wv) mice led to increased clinical severity and mortality with elevated virus titers in HSV-infected skins. Ex vivo HSV-specific tetramer staining assay demonstrated that MC deficiency did not affect the frequency of HSV-specific cytotoxic T lymphocytes (CTLs) in draining lymph nodes. Moreover, the high mortality in Kit(W/W-v) mice was completely reversed by intradermal reconstitution with bone marrow-derived MCs (BMMCs) from wild-type, but not TNF(-/-) or IL-6(-/-) mice, indicating that MCs or, more specifically, MC-derived tumor necrosis factor (TNF) and IL-6 can protect mice from HSV-induced mortality. However, HSV did not directly induce TNF-α or IL-6 production by BMMCs; supernatants from HSV-infected keratinocytes induced the production of these cytokines by BMMCs without degranulation. Furthermore, IL-33 expression was induced in HSV-infected keratinocytes, and blocking the IL-33 receptor T1/ST2 on BMMCs significantly reduced TNF-α and IL-6 production by BMMCs. These results indicate the involvement of MCs in host defense at HSV-infected sites through TNF-α and IL-6 production, which is induced by keratinocyte-derived IL-33.