Henrik Jensen

Requirement of subunit co-assembly and ankyrin-G for M-channel localization at the axon initial segment

The potassium channel subunits KCNQ2 and KCNQ3 are believed to underlie the M current of hippocampal neurons. The M-type potassium current plays a key role in the regulation of neuronal excitability; however, the subcellular location of the ion channels underlying this regulation has been controversial. We report here that KCNQ2 and KCNQ3 subunits are localized to the axon initial segment of pyramidal neurons of adult rat hippocampus and in cultured hippocampal neurons. We demonstrate that the localization of the KCNQ2/3 channel complex to the axon initial segment is favored by co-expression of the two channel subunits. Deletion of the ankyrin-G-binding motif in both the KCNQ2 and KCNQ3 C-terminals leads to the disappearance of the complex from the axon initial segment, albeit the channel complex remains functional and still reaches the plasma membrane. We further show that although heteromeric assembly of the channel complex favours localization to the axon initial segment, deletion of the ankyrin-G-binding motif in KCNQ2 alone does not alter the subcellular localization of KCNQ2/3 heteromers. By contrast, deletion of the ankyrin-G-binding motif in KCNQ3 significantly reduces AIS enrichment of the complex, implicating KCNQ3 as a major determinant of M channel localization to the AIS.

Basolateral localisation of KCNQ1 potassium channels in MDCK cells: molecular identification of an N-terminal targeting motif

KCNQ1 potassium channels are expressed in many epithelial tissues as well as in the heart. In epithelia KCNQ1 channels play an important role in salt and water transport and the channel has been reported to be located apically in some cell types and basolaterally in others. Here we show that KCNQ1 channels are located basolaterally when expressed in polarised MDCK cells. The basolateral localisation of KCNQ1 is not affected by co-expression of any of the five KCNE β-subunits. We characterise two independent basolateral sorting signals present in the N-terminal tail of KCNQ1. Mutation of the tyrosine residue at position 51 resulted in a non-polarized steady-state distribution of the channel. The importance of tyrosine 51 in basolateral localisation was emphasized by the fact that a short peptide comprising this tyrosine was able to redirect the p75 neurotrophin receptor, an otherwise apically located protein, to the basolateral plasma membrane. Furthermore, a di-leucine-like motif at residues 38-40 (LEL) was found to affect the basolateral localisation of KCNQ1. Mutation of these two leucines resulted in a primarily intracellular localisation of the channel.

A small molecule activator of Nav1.1 channels increases fast-spiking interneuron excitability and GABAergic transmission in vitro and has anti-convulsive effects in vivo

Nav1.1 (SCN1A) channels primarily located in gamma‐aminobutyric acid (GABA)ergic fast‐spiking interneurons are pivotal for action potential generation and propagation in these neurons. Inappropriate function of fast‐spiking interneurons, leading to disinhibition of pyramidal cells and network desynchronization, correlates with decreased cognitive capability. Further, reduced functionality of Nav1.1 channels is linked to various diseases in the central nervous system. There is, at present, however no subtype selective pharmacological activators of Nav1.1 channels available for studying pharmacological modulation of interneuron function. In the current study, we identified a small molecule Nav1.1 activator, 3‐amino‐5‐(4‐methoxyphenyl)thiophene‐2‐carboxamide, named AA43279, and provided an in vitro to in vivo characterization of the compound. In HEK‐293 cells expressing human Nav1.1 channels, AA43279 increased the Nav1.1‐mediated current in a concentration‐dependent manner mainly by impairing the fast inactivation kinetics of the channels. In rat hippocampal brain slices, AA43279 increased the firing activity of parvalbumin‐expressing, fast‐spiking GABAergic interneurons and increased the spontaneous inhibitory post‐synaptic currents (sIPSCs) recorded from pyramidal neurons. When tested in vivo, AA43279 had anti‐convulsive properties in the maximal electroshock seizure threshold test. AA43279 was tested for off‐target effects on 72 different proteins, including Nav1.2, Nav1.4, Nav1.5, Nav1.6 and Nav1.7 and exhibited reasonable selectivity. Taken together, AA43279 might constitute a valuable tool compound for revealing biological functions of Nav1.1 channels.

Automated coating procedures to produce poly(ethylene glycol) brushes in fused‐silica capillaries

Many bioanalytical methods rely on electrophoretic separation of structurally labile and surface active biomolecules such as proteins and peptides. Often poor separation efficiency is due to surface adsorption processes leading to protein denaturation and surface fouling in the separation channel. Flexible and reliable approaches for preventing unwanted protein adsorption in separation science are thus in high demand. We therefore present new coating approaches based on an automated in-capillary surface-initiated atom transfer radical polymerization process (covalent coating) as well as by electrostatically adsorbing a presynthesized polymer leading to functionalized molecular brushes. The electroosmotic flow was measured following each step of the covalent coating procedure providing a detailed characterization and quality control. Both approaches resulted in good fouling resistance against the four model proteins cytochrome c, myoglobin, ovalbumin, and human serum albumin in the pH range 3.4-8.4. Further, even samples containing 10% v/v plasma derived from human blood did not show signs of adsorbing to the coated capillaries. The covalent as well as the electrostatically adsorbed coating were both found to be stable and provided almost complete suppression of the electroosmotic flow in the pH range 3.4-8.4. The coating procedures may easily be integrated in fully automated capillary electrophoresis methodologies.

Phase separation of in situ forming poly (lactide-co-glycolide acid) implants investigated using a hydrogel-based subcutaneous tissue surrogate and UV–vis imaging

Graphical abstract Figure. No Caption available. HighlightsInvestigation of in situ PLGA implant formation using UV–vis imaging.UV–vis imaging phase separation of PLGA‐NMP and PLGA‐TA systems in hydrogel.Changes in hydrogel (agarose) concentrations affect implant morphology.UV–vis imaging is useful for characterizing phase inverting injectable solutions. Abstract Phase separation of in situ forming poly (lactide‐co‐glycolide acid) (PLGA) implants with agarose hydrogels as the provider of nonsolvent (water) mimicking subcutaneous tissue was investigated using a novel UV–vis imaging‐based analytical platform. In situ forming implants of PLGA‐1‐methyl‐2‐pyrrolidinone and PLGA‐triacetin representing fast and slow phase separating systems, respectively, were evaluated using this platform. Upon contact with the agarose hydrogel, the phase separation of the systems was followed by the study of changes in light transmission and absorbance as a function of time and position. For the PLGA‐1‐methyl‐2‐pyrrolidinone system, the rate of spatial phase separation was determined and found to decrease with increasing the PLGA concentration from 20% to 40% (w/w). Hydrogels with different agarose concentrations (1% and 10% (w/v)) were prepared for providing the nonsolvent, water, to the in situ forming PLGA implants simulating the injection site environment. The resulting implant morphology depended on the stiffness of hydrogel matrix, indicating that the matrix in which implants are formed is of importance. Overall, the work showed that the UV–vis imaging‐based platform with an agarose hydrogel mimicking the subcutaneous tissue holds potential in providing bio‐relevant and mechanistic information on the phase separation processes of in situ forming implants.

Concomitant monitoring of implant formation and drug release of in situ forming poly (lactide-co-glycolide acid) implants in a hydrogel matrix mimicking the subcutis using UV–vis imaging

For poly (lactide-co-glycolide acid) (PLGA)-based in situ forming implants, the rate of implant formation plays an important role in determining the overall drug release kinetics. Currently, in vitro techniques capable of characterizing the processes of drug release and implant formation at the same time are not available. A hydrogel-based in vitro experimental setup was recently developed requiring only microliter of formulation and forming a closed system potentially suitable for interfacing with various spectroscopic techniques. The aim of the present proof-of-concept study was to investigate the feasibility of concomitant UV imaging, Vis imaging and light microscopy for detailed characterization of the behavior of in situ forming PLGA implants in the hydrogel matrix mimicking the subcutis. The model compounds, piroxicam and α-lactalbumin were added to PLGA-1-methyl-2-pyrrolidinone and PLGA-triacetin solutions. Upon bringing the PLGA-solvent-compound pre-formulation in contact with the hydrogel, Vis imaging and light microscopy were applied to visualize the depot formation and UV imaging was used to quantify drug transport in the hydrogel. As compared to piroxicam, the α-lactalbumin invoked an acceleration of phase separation and an increase of implant size. α-Lactalbumin was released faster from the PLGA-1-methyl-2-pyrrolidinone system than the PLGA-triacetin system opposite to the piroxicam release pattern. A linear relationship between the rate of implant formation and initial compound release within the first 4h was established for the PLGA-NMP systems. This implies that phase separation may be one of the controlling factors in drug release. The rate of implant formation may be an important parameter for predicting and tailoring drug release. The approach combining UV imaging, Vis imaging and light microscopy may facilitate understanding of release processes and holds potential for becoming a useful tool in formulation development of in situ forming implants.

Continuous electromembrane extraction coupled with mass spectrometry – Perspectives and challenges

This tutorial discusses continuous electromembrane extraction (c-EME) coupled directly to mass spectrometry (MS), and the applicability of such systems for on-line and real-time monitoring of in-vitro drug metabolism. Parent drug substances and corresponding drug metabolites are extracted from the metabolic reaction mixture, through a supported liquid membrane (SLM), and into an acceptor solution on the other side. Extraction is accomplished using an external electrical field sustained over the SLM. The acceptor solution is continuously pumped into the mass spectrometer, and the decline of parent drug as well as the development of metabolites is followed directly with the mass spectrometer. The purpose of the extraction is to avoid proteins and salts from the reaction mixture from entering the mass spectrometer. This tutorial first discusses the principles and theory of operation. Second, technical development is highlighted with special focus on major challenges associated with c-EME-MS systems. Third, operational parameters and performance are discussed, and finally future perspectives and challenges are considered.

Separation of Peptides with Forward Osmosis Biomimetic Membranes

Forward osmosis (FO) membranes have gained interest in several disciplines for the rejection and concentration of various molecules. One application area for FO membranes that is becoming increasingly popular is the use of the membranes to concentrate or dilute high value compound solutions such as pharmaceuticals. It is crucial in such settings to control the transport over the membrane to avoid losses of valuable compounds, but little is known about the rejection and transport mechanisms of larger biomolecules with often flexible conformations. In this study, transport of two chemically similar peptides with molecular weight (Mw) of 375 and 692 Da across a thin film composite Aquaporin Inside™ Membrane (AIM) FO membrane was investigated. Despite the relative large size, both peptides were able to permeate the dense active layer of the AIM membrane and the transport mechanism was determined to be diffusion-based. Interestingly, the membrane permeability increased 3.65 times for the 692 Da peptide (1.39 × 10−12 m2·s−1) compared to the 375 Da peptide (0.38 × 10−12 m2·s−1). This increase thus occurs for an 85% increase in Mw but only for a 34% increase in peptide radius of gyration (Rg) as determined from molecular dynamics (MD) simulations. This suggests that Rg is a strong influencing factor for membrane permeability. Thus, an increased Rg reflects the larger peptide chains ability to sample a larger conformational space when interacting with the nanostructured active layer increasing the likelihood for permeation.

KCNQ Channels are Sensors of Cell Volume

Many important physiological processes involve changes in cell volume, e.g., the transport of salt and water in epithelial cells and the contraction of muscle cells. These cells respond to swelling with a so-called regulatory volume decrease which involves the activation of K+ channels. However, the molecular identity of the involved K+ channels has not been clear, and in particular, the mechanism for activation has been obscure.