Computational Fluid Dynamics Modeling of Hollow Membrane Filtration for Concentration Polarization

Based on CFD and film theory, filtration’s two-dimensional CFD model of the hollow membrane was established by integrating the mass transformation and the hydrodynamic transportation. Parameters of concentration polarization in the membrane channel (i.e., solute mass concentration, concentration polarization factors, and concentration polarization layer thickness) were estimated under different hydraulic conditions. In addition, the algorithm for the thickness of the concentration polarization layer has been improved. The results showed that decreasing the feed Reynolds number or increasing the transmembrane pressure can enhance the concentration polarization phenomena. Concentration polarization parameters increased sharply at the initial place (X/H < 25, where H is the entrance width, X is the distance from entrance) and then flatten out (X/H > 25) along the membrane channel; solute concentration and concentration polarization factors were arranged in a U-shape in the membrane channel’s cross-section. The improved algorithm could match well with cross section data, δ2H at X/H = 1, 25, and 200 are 0.038, 0.11, and 0.25, respectively, which can reasonably reflect the distribution of the concentration polarization phenomenon in the membrane channel.

Arsenic leads to autophagy of keratinocytes by increasing aquaporin 3 expression

Exposure to arsenic, a ubiquitous metalloid on Earth, results in human cancers. Skin cancer is the most common arsenical cancers. Both autophagy and aquaporin pathway are known to promote carcinogenesis. However, the mechanisms by which arsenic regulates aquaporin and autophagy in arsenical skin cancers remain elusive. This study aims to address how arsenic regulates aquaporin-3, the predominant aquaporin in epidermal keratinocytes, and how this process would induce autophagy. Quantitative real-time PCR and immunofluorescence were used to measure the expression of aquaporin 3 in arsenical skin cancers and arsenic-treated keratinocytes. Beclin-1 expression and autophagy were measured. We examined if blocking aquaporin 3 could interfere arsenic-induced autophagy in keratinocytes. Expression of aquaporin 3 is increased in arsenical cancers and in arsenic-treated keratinocytes. Arsenic induced autophagy in primary human keratinocytes. Notably, the arsenic-induced autophagy was inhibited by pretreatment of keratinocytes with aquaporin inhibitors Auphen or AgNO3, or RNA interference against aquaporin 3. The data indicates that the aquaporin 3 is an important cell membrane channel to mediate arsenic uptake and contributes to the arsenic-induced autophagy.

Astrocyte Pannexin 1 Suppresses LPS-Induced Inflammatory Responses to Protect Neuronal SH-SY5Y Cells

Reactive astrogliosis is a key hallmark of inflammatory responses in the pathogenesis of brain injury, including Parkinson’s disease (PD), but its role and regulatory mechanisms are not fully understood. Pannexin 1 (Panx 1) is a membrane channel that mediates substance release in many neurodegenerative diseases. However, the role of astrocyte Panx 1 in the regulation of PD-like neuroinflammation remains elusive. Here, we characterized the expression of Panx 1 in isolated primary astrocytes and a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD model. The functions of Panx 1 in inflammatory cytokines expression and the viability of neuronal SH-SY5Y cells were examined in cultured cells treated with lipopolysaccharide (LPS) and 1-methyl-4-phenylpyridinium (MPP+). We found that Panx 1 expression was significantly increased under both LPS- and MPP+-treated conditions. Panx 1 downregulation suppressed LPS-induced pro-inflammatory cytokine expression but did not significantly affect MPP+-induced astrocyte apoptosis or inflammatory cytokine expression through treatment with the Panx 1 inhibitor carbenoxolone (CBX) and Panx 1 siRNA. Moreover, silencing Panx 1 in reactive astrocytes had a potentially protective effect on the viability of neuronal SH-SY5Y cells. Therefore, we propose that Panx 1 may serve as a key regulator in reactive astrocytes to intervene in the inflammatory response and maintain neuronal viability in the context of PD-like conditions.

Osj10gBTF3-Mediated Import of Chloroplast Protein Is Essential for Pollen Development in Rice

Chloroplasts are crucial organelles for the generation of fatty acids and starch required for plant development. Nascent polypeptide-associated complex (NAC) proteins have been implicated in development as transcription factors. However, their chaperone roles in chloroplasts and their relationship with pollen development in plants remain to be elucidated. Here, we demonstrated that Osj10gBTF3, a NAC protein, regulates pollen and chloroplast development in rice by coordinating with a Hsp90 family chaperone OsHSP82 to mediate chloroplast import. Knockout of Osj10gBTF3 affects pollen and chloroplast development and significantly reduces the accumulation of fertility-related chloroplast protein OsPPR676. Both Osj10gBTF3 and OsHSP82 interact with OsPPR676. Interestingly, the interaction between OsHSP82 and OsPPR676 is only found in the cytoplasm, while the interaction between Osj10gBTF3 and OsPPR676 also occurs inside the chloroplast. The chloroplast stroma chaperone OsCpn60 can also be co-precipitated with Osj10gBTF3, but not with OsHSP82. Further investigation indicates that Osj10gBTF3 enters the chloroplast stroma possibly through the inner chloroplast membrane channel protein Tic110 and then recruits OsCpn60 for the folding or assembly of OsPPR676. Our results reveal a chaperone role of Osj10gBTF3 in chloroplast import different from Hsp90 and provide a link between chloroplast transport and pollen development in rice.

Membrane channel hypothesis of lysosomal permeabilization by beta-amyloid

Alzheimer's disease (AD) is the most common cause of dementia affecting millions of people. Neuronal death in AD is initiated by oligomeric amyloid-β (Aβ) peptides. Recently, we proposed the amyloid degradation toxicity hypothesis, which explains multiple major observations associated with AD - such as autophagy failure and a decreased metabolism. According to the hypothesis, the key event in the cellular toxicity of amyloid is the formation of non-selective membrane channels in lysosomal membranes by amyloid fragments that are produced by the digestion of Aβ previously absorbed by endocytosis. Electrophysiological data suggest that amyloid-formed channels have different sizes, which can be explained by the fact that barrel-shaped amyloid aggregates which create channels can consist of different number of monomers. To estimate the ability of channels to leak molecules of various molecular weights, we modeled the channels as saline-filled cylinders in non-conductive membranes that pass spheres with a density of average globular proteins. As a basis, we used the conductance distribution taken from the previously published experimental dataset, in which single channels with a conductance reaching one nanosiemens were registered. Our calculations show that channels with a giant conductance can allow for passing macromolecules such as lysosomal cathepsins implicated in the activation of apoptosis. The formation of giant channels is disproportionally promoted in an acidic environment. Also, amyloid fragments leaking from permeabilized lysosomes can reach the internal leaflet of the plasma membrane and permeabilize it. We conclude that while dissipation of the proton gradient by any - even the smallest amyloid channel - readily explains lysosomal failure, the relatively rare events of lysosomal permeabilization to large macromolecules can be an alternative mechanism of cellular death induced by exposure to Aβ.

UBA6 and NDFIP1 regulate the degradation of ferroportin

Hepcidin regulates iron homeostasis by controlling the level of ferroportin, the only membrane channel that facilitates export of iron from within cells. Binding of hepcidin to ferroportin induces the ubiquitination of ferroportin at multiple lysine residues and subsequently causes the internalization and degradation of the ligand-channel complex within lysosomes. The objective of this study was to identify components of the ubiquitin system that are involved in ferroportin degradation. A HepG2 cell line, which inducibly expresses ferroportin-GFP (FPN-GFP), was established to test the ability of siRNAs directed against components of the ubiquitin system to prevent BMP6- and exogenous hepcidin-induced ferroportin degradation. Of the 88 siRNAs directed against components of the ubiquitin pathway that were tested, siRNAmediated depletion of the alternative E1 enzyme UBA6 as well as the adaptor protein NDFIP1 prevented BMP6- and hepcidin- induced degradation of ferroportin in vitro. A third component of the ubiquitin pathway, ARIH1, indirectly inhibited ferroportin degradation by impairing BMP6 mediated induction of hepcidin. In mice, the AAVmediated silencing of Ndfip1 in the murine liver increased the level of hepatic ferroportin and increased circulating iron. The results suggest that the E1 enzyme UBA6 and the adaptor protein NDFIP1 are involved in iron homeostasis by regulating the degradation of ferroportin. These specific components of the ubiquitin system may be promising targets for the treatment of iron related diseases, including iron overload and anemia of inflammation.

Introduction to the Theme on Membrane Channels

This volume of the Annual Review of Biochemistry contains three reviews on membrane channel proteins: the first by Szczot et al., titled The Form and Function of PIEZO2; the second by Ruprecht & Kunji, titled Structural Mechanism of Transport of Mitochondrial Carriers; and the third by McIlwain et al., titled Membrane Exporters of Fluoride Ion. These reviews provide nice illustrations of just how far evolution has been able to play with the basic helix-bundle architecture of integral membrane proteins to produce membrane channels and transporters of widely different functions.

Conserved Aquaporins in Gossypium and Silencing of Ghpip2;7 and Ghtip2;1 Decreased Salt and Osmotic Stress Tolerances in Upland Cotton

Aquaporins (aquaporin), membrane channel proteins, facilitate the transport of water and small molecules across intrinsic membranes and play a critical role in abiotic stresses. Here, 111, 54, and 56 candidate AQP genes were identified in Gossypium hirsutum (AD1), G. arboreum (A2), and G. raimondii (D5), and classified into five subfamilies including PIP, TIP, NIP, SIP, and XIP, respectively. Some GhPIPs and GhTIPs exhibited high expression levels in drought and salt stress verified by transcriptome analysis and Quantitative Real-time PCR (qRT-PCR). The chlorophyll content, SOD activity, and POD activity were decreased in GhPIP2;7 gene-silenced plants versus the control (using blank vector) under 400 mM NaCl treatment, which indicated a negative role of GhPIP2;7 in cotton salt tolerance. Comparing with mock plants, GhTIP2;1 silenced cotton plant was more sensitive to osmotic stress. Overexpressed GhTIP2;1 plants exhibited less accumulation of H2O2 and MDA with higher proline contents detected under osmotic stress. Taken together these results, provide in-depth knowledge into plant response to abiotic stress and gene resource.