When Is a Reaction Network a Metabolism? Criteria for Simple Metabolisms That Support Growth and Division of Protocells

With the aim of better understanding the nature of metabolism in the first cells and the relationship between the origin of life and the origin of metabolism, we propose three criteria that a chemical reaction system must satisfy in order to constitute a metabolism that would be capable of sustaining growth and division of a protocell. (1) Biomolecules produced by the reaction system must be maintained at high concentration inside the cell while they remain at low or zero concentration outside. (2) The total solute concentration inside the cell must be higher than outside, so there is a positive osmotic pressure that drives cell growth. (3) The metabolic rate (i.e., the rate of mass throughput) must be higher inside the cell than outside. We give examples of small-molecule reaction systems that satisfy these criteria, and others which do not, firstly considering fixed-volume compartments, and secondly, lipid vesicles that can grow and divide. If the criteria are satisfied, and if a supply of lipid is available outside the cell, then continued growth of membrane surface area occurs alongside the increase in volume of the cell. If the metabolism synthesizes more lipid inside the cell, then the membrane surface area can increase proportionately faster than the cell volume, in which case cell division is possible. The three criteria can be satisfied if the reaction system is bistable, because different concentrations can exist inside and out while the rate constants of all the reactions are the same. If the reaction system is monostable, the criteria can only be satisfied if there is a reason why the rate constants are different inside and out (for example, the decay rates of biomolecules are faster outside, or the formation rates of biomolecules are slower outside). If this difference between inside and outside does not exist, a monostable reaction system cannot sustain cell growth and division. We show that a reaction system for template-directed RNA polymerization can satisfy the requirements for a metabolism, even if the small-molecule reactions that make the single nucleotides do not.

Mitotic Spindle Positioning (MISP) is an actin bundler that selectively stabilizes the rootlets of epithelial microvilli

Microvilli are conserved actin-based surface protrusions that have been repurposed throughout evolution to fulfill diverse cell functions. In the case of transporting epithelia, microvilli are supported by a core of actin filaments bundled in parallel by villin, fimbrin, and espin. Remarkably, microvilli biogenesis persists in mice lacking all three of these factors, suggesting the existence of unknown bundlers. We identified Mitotic Spindle Positioning (MISP) as an actin binding factor that localizes specifically to the rootlet end of the microvillus. MISP promotes rootlet elongation in cells, and purified MISP exhibits potent filament bundling activity in vitro. MISP-bundled filaments also recruit fimbrin, which further elongates and stabilizes bundles. MISP confinement to the rootlet is enforced by ezrin, which prevents decoration of the membrane-wrapped distal end of the core bundle. These discoveries reveal how epithelial cells optimize apical membrane surface area and offer insight on the remarkable robustness of microvilli biogenesis.

Effect of Acid Flow Rate, Membrane Surface Area, and Capture Solution on the Effectiveness of Suspended GPM Systems to Recover Ammonia

Ammonia losses from manure pose serious problems for ecosystems and human and animal health. Gas-permeable membranes (GPMs) constitute a promising approach to address the challenge of reducing farm ammonia emissions and to attain the EU’s Clean Air Package goals. In this study, the effect of NH3-N concentration, membrane surface area, acid flux, and type of capture solution on ammonia recovery was investigated for a suspended GPM system through three experiments, in which ammonia was released from a synthetic solution (NH4Cl + NaHCO3 + allylthiourea). The effect of two surface areas (81.7 and 163.4 cm2) was first evaluated using three different synthetic N emitting concentrations (3000, 6000, and 12,000 mg NH3-N∙L−1) and keeping the flow of acidic solution (1N H2SO4) constant (0.8 L·h−1). A direct relationship was found between the amount of NH3 captured and the NH3-N concentration in the N-emitting solution, and between the amount of NH3 captured and the membrane surface area at the two lowest concentrations. Nonetheless, the use of a larger membrane surface barely improved ammonia capture at the highest concentration, pointing to the existence of other limiting factors. Hence, ammonia capture was then studied using different acid flow rates (0.8, 1.3, 1.6, and 2.1 L∙h−1) at a fixed N emitting concentration of 6000 mg NH3-N∙L−1 and a surface area of 122.5 cm2. A higher acid flow rate (0.8–2.1 L∙h−1) resulted in a substantial increase in ammonia absorption, from 165 to 262 mg of NH3∙d−1 over a 14-day period. Taking the parameters that led to the best results in experiments 1 and 2, different types of ammonia capture solutions (H2SO4, water and carbonated water) were finally compared under refrigeration conditions (at 2 °C). A high NH3 recovery (81% in 7 days), comparable to that obtained with the H2SO4 solution (88%), was attained when chilled water was used as the capture solution. The presented results point to the need to carefully optimize the emitter concentration, flow rate, and type of capture solution to maximize the effectiveness of suspended GPM systems, and suggest that chilled water may be used as an alternative to conventional acidic solutions, with associated savings.

Applying Membrane Distillation for the Recovery of Nitrate from Saline Water Using PVDF Membranes Modified as Superhydrophobic Membranes

In this study, a flat sheet direct contact membrane distillation (DCMD) module was designed to eliminate nitrate from water. A polyvinylidene fluoride (PVDF) membrane was used in a DCMD process at an ambient pressure and at a temperature lower than the boiling point of water. The electrical conductivity of the feed containing nitrate increased, while the electrical conductivity of the permeate remained constant during the entire process. The results indicated that the nitrate ions failed to pass through the membrane and their concentration in the feed increased as pure water passed through the membrane. Consequently, the membrane was modified using TiO2 nanoparticles to make a hierarchical surface with multi-layer roughness on the micro/nanoscales. Furthermore, 1H,1H,2H,2H-Perfluorododecyltrichlorosilane (FTCS) was added to the modified surface to change its hydrophobic properties into superhydrophobic properties and to improve its performance. The results for both membranes were compared and reported on a pilot scale using MATLAB. In the experimental scale (a membrane surface area of 0.0014 m2, temperature of 77 °C, nitrate concentration of 0.9 g/Kg, and flow rate of 0.0032 Kg/s), the flux was 2.3 Kgm−2h−1. The simulation results of MATLAB using these data showed that for the removal of nitrate (with a concentration of 35 g/Kg) from the intake feed with a flow rate of 1 Kg/s and flux of 0.96 Kgm−2h−1, a membrane surface area of 0.5 m2 was needed.

Three-dimensional ATUM-SEM reconstruction and analysis of hepatic endoplasmic reticulum-organelle interactions

The endoplasmic reticulum (ER) is a contiguous and complicated membrane network in eukaryotic cells, and membrane contact sites (MCSs) between the ER and other organelles perform vital cellular functions, including lipid homeostasis, metabolite exchange, calcium level regulation, and organelle division. Here, we establish a whole pipeline to reconstruct all ER, mitochondria, lipid droplets, lysosomes, peroxisomes, and nuclei by automated tape-collecting ultramicrotome scanning electron microscopy (ATUM-SEM) and deep-learning techniques, which generates an unprecedented 3D model for mapping liver samples. Furthermore, the morphology of various organelles is systematically analyzed. We found that the ER presents with predominantly flat cisternae and is knitted tightly all throughout the intracellular space and around other organelles. In addition, the ER has a smaller volume-to-membrane surface area ratio than other organelles, which suggests that the ER could be more suited for functions that require a large membrane surface area. Moreover, the MCSs between the ER and other organelles are explored. Our data indicate that ER-mitochondrial contacts are particularly abundant, especially for branched mitochondria. In addition, ER contacts with lipid droplets, lysosomes, and peroxisomes are also plentiful. In summary, we design an efficient method for obtaining a 3D reconstruction of biological structures at a nanometer resolution. Our study also provides the first 3D reconstruction of various organelles in liver samples together with important information fundamental for biochemical and functional studies in the liver.

Uneven Distribution of Astrocyte Membranes over the Layers of the Rat Primary Somatosensory Cortex

Astrocytes are the main glial cells maintaining water-electrolyte and energy balance in the brain. Today, astroglia is also believed to take a direct part in the regulation of synaptic transmission and in enabling synchronous operation of neurons at large distances. Astrocytes fulfil their functions through numerous processes that penetrate the entire neuropil. The authors believe that changes in the astrocyte membrane surface area per unit volume of neuropil directly reflect changes in the intensity of the astrocyte–neuron interaction. Strengthening or weakening of astrocyte regulation, undoubtedly, affect the functioning of neural circuits. Nevertheless, in spite of the growing popularity of research into the glia–neuron relations, this aspect remains insufficiently studied when it comes to the cerebral cortex. The purpose of this study was to layer-by-layer determine the astrocyte membrane surface per unit volume in the neuropil of the rat primary somatosensory cortex. The research was conducted on samples of the primary somatosensory cortex obtained from 5 white male rats (P60–80). After immune labeling against astrocytic marker S100B using the pre-embedding method, the samples were prepared for transmission electron microscopy according to the standard technique. In total, 250 electron micrographs were obtained for each layer of the primary somatosensory cortex, which were then used to determine the astrocyte membrane surface area per unit volume in the neuropil by means of the random secant method. The research found that this indicator is the minimum in the first and maximum in the fifth layers of the cortical column. In addition, the article discusses the possible functional consequences of uneven distribution of astrocytic membranes in the neocortex.

Experimental Investigation of Spiral Wound Module Under Pressure Retarded Osmosis Process

The performance of a spiral wound module operating under pressure retarded osmosis (PRO) is measured experimentally with a feed water back pressure. The module has a diameter of 0.1 m, a length of 1.02 m, and total membrane surface area of 7.45 m2 capsulated in a stainless-steel pressure vessel. The module is commercially designed for a reverse osmosis (RO) system however, a feed water outlet is added, and a back pressure is applied to adapt it for the PRO process. The experimental results of the spiral wound PRO module with a back pressure in the feed water side showed low power density values (about 0.35 W/m2) compared with those commonly found in the literature, demonstrating the geometric inefficiency of RO modules for the PRO process.

High-efficiency, high-flux in-line hemofiltration using a high blood flow extracorporeal circuit

The ability of current renal replacement therapy modalities to achieve rapid solute removal is limited by membrane surface area and blood flow rate. Extracorporeal membrane oxygenation offers high blood flow and hemodynamic support that may be harnessed to overcome limitations in traditional renal replacement therapy. Using an extracorporeal membrane oxygenation circuit, we describe a high blood flow, high-efficiency hemofiltration technique using in-line hemofilters (hemoconcentrators) and standard replacement fluid to enhance solute clearance. Using this approach and a total of 5 L of replacement volume per treatment, creatinine (Cr) clearances of 8.3 L/hour and 11.2 L/hour using one and two hemoconcentrators, respectively, were achieved. With use of a high blood flow rate of up to 5 L/min, this hemofiltration technique can potentially offer clearance of 30 times that of continuous renal replacement therapy and of 6 times that of hemodialysis which may expand the ability to remove substances traditionally not considered removable via existing extracorporeal therapies.

Rab28 is localised to photoreceptor outer segments, regulates outer segment shedding but is not linked to retinal degeneration in zebrafish

The photoreceptor outer segment is the canonical example of a modified and highly specialised cilium, with an expanded membrane surface area in the form of discs or lamellae for efficient light detection. Many ciliary proteins are essential for normal photoreceptor function and cilium dysfunction often results in retinal degeneration leading to impaired vision. Herein, we investigate the function and localisation of the ciliary G-protein RAB28 in zebrafish cone photoreceptors. CRISPR-Cas9 generated rab28 mutant zebrafish display a reduction in shed outer segment material in the RPE at 1 month post fertilisation (mpf), but otherwise normal retinal structure and visual function up to 12 mpf. Cone photoreceptor-specific transgenic reporter lines show Rab28 localises almost exclusively to outer segments, independently of nucleotide binding. Co-immunoprecipitation analysis demonstrates tagged Rab28 interacts with components of the phototransduction cascade, including opsins, Phosphodiesterase 6C and Guanylate Cyclase 2D. Our data shed light on RAB28 function in cones and provide a model for RAB28-associated cone-rod dystrophy.

Ceramic Membranes Photocatalytically Functionalized on the Permeate Side and Their Application to Water Treatment

This work deals with direct coupling of membrane separation and photocatalytic degradation by using photocatalytic ceramic membranes. An unusual configuration is considered here, with the irradiation applied on the permeate side of the membrane in order to mineralize small organic molecules not retained by the membrane. Different types of such membranes are presented. Their functional performance is quantified thanks to a simple experimental method enabling the estimation of the specific degradation rate δ, i.e., the quantity of destroyed organic molecules per unit of time and of membrane surface area. The relevance of δ for the design and scale-up of purification units is then illustrated. Finally, current technological challenges and potential solutions concerning the industrial implementation of such photocatalytic membranes are discussed.