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Analysis of protein function in a cellular context ideally requires physiologically representative levels of that protein. Thus conventional nucleic acid-based transfection methods are far from ideal owing to the over expression that generally results. Likewise, fusions with protein transduction domains can be problematic whilst delivery via liposomes/nanoparticles typically results in endosomal localisation. Recently, polymer microspheres have been reported to be highly effective at delivering proteins into cells and thus provide a viable new alternative for protein delivery (protein transduction). Herein we describe the successful delivery of active ribonuclease A into HeLa cells via novel polymer core–silica shell microspheres. Specifically, poly(styrene-co-vinylbenzylisothiouronium chloride) core particles, generated by dispersion polymerisation, were coated with a poly(styrene-co-trimethoxysilylpropyl methacrylate) shell. The resultant core–shell morphology was characterised by transmission electron, scanning electron and confocal fluorescence microscopies, whilst size and surface charge was assessed by dynamic light scattering and zeta-potential measurements, respectively. Subsequently, ribonuclease A was coupled to the microspheres using simple carbodiimide chemistry. Gel electrophoresis confirmed and quantified the activity of the immobilised enzyme against purified HeLa RNA. Finally, the polymer–protein particles were evaluated as protein-transduction vectors in vitro to deliver active ribonuclease A to HeLa cells. Cellular uptake of the microspheres was successful and resulted in reduced levels of both intracellular RNA and cell viability.

The control of cellular water flow is mediated by the aquaporin (AQP) family of membrane proteins. The structural features of the family and the mechanism of selective water passage through the AQP pore are established, but there remains a gap in our knowledge of how water transport is regulated. Two broad possibilities exist. One is controlling the passage of water through the AQP pore, but this only has been observed as a phenomenon in some plant and microbial AQPs. An alternative is controlling the number of AQPs in the cell membrane. Here, we describe a novel pathway in mammalian cells whereby a hypotonic stimulus directly induces intracellular calcium elevations through transient receptor potential channels, which trigger AQP1 translocation. This translocation, which has a direct role in cell volume regulation, occurs within 30 s and is dependent on calmodulin activation and phosphorylation of AQP1 at two threonine residues by protein kinase C. This direct mechanism provides a rationale for the changes in water transport that are required in response to constantly changing local cellular water availability. Moreover, because calcium is a pluripotent and ubiquitous second messenger in biological systems, the discovery of its role in the regulation of AQP translocation has ramifications for diverse physiological and pathophysiological processes, as well as providing an explanation for the rapid regulation of water flow that is necessary for cell homeostasis.

Membrane proteins are drug targets for a wide range of diseases. Having access to appropriate samples for further research underpins the pharmaceutical industry's strategy for developing new drugs. This is typically achieved by synthesizing a protein of interest in host cells that can be cultured on a large scale, allowing the isolation of the pure protein in quantities much higher than those found in the protein's native source. Yeast is a popular host as it is a eukaryote with similar synthetic machinery to that of the native human source cells of many proteins of interest, while also being quick, easy and cheap to grow and process. Even in these cells, the production of human membrane proteins can be plagued by low functional yields; we wish to understand why. We have identified molecular mechanisms and culture parameters underpinning high yields and have consolidated our findings to engineer improved yeast host strains. By relieving the bottlenecks to recombinant membrane protein production in yeast, we aim to contribute to the drug discovery pipeline, while providing insight into translational processes.

Background: Emerging evidence supports the view that (AQP) aquaporin water channels are regulators of transcellular\ud water flow. Consistentwith their expression in most tissues, AQPs are associatedwith diverse physiological\ud and pathophysiological processes.\ud Scope of review: AQP knockout studies suggest that the regulatory role of AQPs, rather than their action as passive\ud channels, is their critical function. Transport through all AQPs occurs by a common passive mechanism, but their\ud regulation and cellular distribution varies significantly depending on cell and tissue type; the role of AQPs in cell\ud volumeregulation (CVR) is particularly notable. This reviewexamines the regulatory role of AQPs in transcellular\ud water flow, especially in CVR.We focus on key systems of the human body, encompassing processes as diverse as\ud urine concentration in the kidney to clearance of brain oedema.\ud Major conclusions: AQPs are crucial for the regulation of water homeostasis, providing selective pores for the\ud rapidmovement ofwater across diverse cellmembranes and playing regulatory roles in CVR. Gatingmechanisms\ud have been proposed for human AQPs, but have only been reported for plant andmicrobial AQPs. Consequently, it\ud is likely that the distribution and abundance of AQPs in a particular membrane is the determinant of membrane\ud water permeability and a regulator of transcellular water flow.\ud General significance: Elucidating the mechanisms that regulate transcellular water flow will improve our understanding\ud of the human body in health and disease. The central role of specific AQPs in regulating water homeostasis\ud will provide routes to a range of novel therapies. This article is part of a Special Issue entitled Aquaporins.