Zinc ions (Zn2+) are ingested through diet and can be found in a variety of foods – including the water supply. Zinc is the second most common transition metal ion found in the body (below iron) and is used as a cofactor in hundreds of proteins – including DNA-binding zinc-finger domains and carbonic anhydrase (converts CO2 into bicarbonate in metabolizing tissue) to name but a few. There are no mammalian zinc storage proteins (such as myoglobin-mediated molecular oxygen storage in metabolizing tissue). Therefore, direct transport of zinc to proteins for cofactor-use requires the careful and tissue-specific zinc transmembrane uniporters – ZIP (Zrt and Irt-like protein) zinc importers and ZnT (zinc transporter) zinc exporters [1]. The latter has been better characterized than the former at atomic resolution. The divide-and-conquer approach is still being applied to understand ZIP-mediated zinc import at atomic resolution. Recent crystal structures for the ZIP4 extracellular domain (ECD) [2] and transmembrane domains (TMD) [3] further our mechanistic understanding of zinc transport in the epidermal context. ZIP4 mutations (functional and AE-related) in ECD subdomains in the PAL-motif containing domain (PCD) and helix-rich domain (HRD, bridging region) significantly detrimented (HEK293T) cellular uptake of radiolabeled zinc. Moreover, these mutations lowered the Vmax but not binding constant Km – suggesting the ZIP4-ECD is critical for optimum cellular zinc uptake but not zinc specificity. This kinetic conclusion supports the structural model of the protein – the ECD must bind and pass zinc ions down to TMD which has the selectivity filter controlling specificity and rate of protein-bound transmembrane ion transport.
ZIP4-ECD and -TMD mutations have causative links with the rare autosomal recessive skin disease Acrodermatitis Enteropathica (AE) [4,5]. The extracellular domain is hypothesized to sense and pass down extracellular zinc ions to the TMD in order to transport zinc ions across the membrane – in this case, keratinocyte endosomal ZIP4 gathers zinc for organellular transport throughout the undifferentiated epidermis. There are more than a dozen zinc transporting proteins in the epidermis – proper zinc homeostasis is critical for a healthy skin barrier against pathogens and allergens.
ZIP4 mutations in AE disease severely disrupts zinc homeostasis. Impaired zinc endosomal uptake diminishes TGF-β1 (transforming growth factor beta 1) expression resulting in a loss of Langerhans Cell (innate immune response) – the sole supplier of epidermal CD39 transmembrane enzyme. ATP can no longer be hydrolyzed and so accumulates above threshold to induce an inflammatory response. In AE, this occurs predominantly at the mouth and genitals (periorificial dermatitis) – causing rashes, lesions, and diminished protection against pathogenic bacteria – dermatitis. Such severe AE symptoms present early in life (0-5 years) and can often be controlled with oral zinc therapy (to account for diminished epidermal zinc uptake), topical antibiotics (to kill the dermatitis-causing bacteria) and steroids (to control the rash).
Epidermal zinc transport is complex and tightly regulated – zinc deficiency by certain zinc transporting proteins (such as ZIP4) are not fully understood. For example, when endosomal ZIP4 is impaired, how can oral zinc therapy successfully control impaired epidermal zinc uptake? How do ZIP4 mutations of the ECD and TMD compare structurally and phenotypically and what are the compound effects (between ZIP4-ECD and TMD as well as between ZIP4 and the other epidermal zinc transporters)? Do other epidermal zinc transporters perform compensatory effects to control zinc homeostasis? Functional zinc-uptake cellular assays paired with structural characterization of zinc transporters at atomic resolution will lead to a greater understanding of mutation-causing zinc deficiency and disease treatment.
[1] A.K. Baltaci, K. Yuce, Zinc Transporter Proteins, Neurochem. Res. 43 (2018) 517–530. doi:10.1007/s11064-017-2454-y.
[2] T. Zhang, D. Sui, J. Hu, Structural insights of ZIP4 extracellular domain critical for optimal zinc transport, Nat. Commun. 7 (2016). doi:10.1038/ncomms11979.
[3] T. Zhang, J. Liu, M. Fellner, C. Zhang, D. Sui, J. Hu, Crystal structures of a ZIP zinc transporter reveal a binuclear metal center in the transport pathway, Sci. Adv. 3 (2017) e1700344. doi:10.1126/sciadv.1700344.
[4] N. Nistor, L. Ciontu, O.E. Frasinariu, V.V. Lupu, A. Ignat, V. Streanga, Acrodermatitis Enteropathica, Med. (United States). 95 (2016) 1–4. doi:10.1097/MD.0000000000003553.
[5] Y. Ogawa, M. Kinoshita, S. Shimada, T. Kawamura, Zinc and skin disorders, Nutrients. 10 (2018). doi:10.3390/nu10020199.