Publications Cell Physics Division

Devices implanted into the body become encapsulated due to a foreign body reaction. In the central nervous system (CNS), this can lead to loss of functionality in electrodes used to treat disorders. Around CNS implants, glial cells are activated, undergo gliosis and ultimately encapsulate the electrodes. The primary cause of this reaction is unknown. Here we show that the mechanical mismatch between nervous tissue and electrodes activates glial cells. Both primary rat microglial cells and astrocytes responded to increasing the contact stiffness from physiological values (G' similar to 100 Pa) to shear moduli G' >= 10 kPa by changes in morphology and upregulation of inflammatory genes and proteins. Upon implantation of composite foreign bodies into rat brains, foreign body reactions were significantly enhanced around their stiff portions in vivo. Our results indicate that CNS glial cells respond to mechanical cues, and suggest that adapting the surface stiffness of neural implants to that of nervous tissue could minimize adverse reactions and improve biocompatibility. (C) 2014 The Authors. Published by Elsevier Ltd. All rights reserved.

The classical purpose of optical fibres is delivery of either optical power, as for welding, or temporal information, as for telecommunication. Maximum performance in both cases is provided by the use of single-mode optical fibres. However, transmitting spatial information, which necessitates higher-order modes, is difficult because their dispersion relation leads to dephasing and a deterioration of the intensity distribution with propagation distance. Here we consciously exploit the fundamental cause of the beam deterioration-the dispersion relation of the underlying vectorial electromagnetic modes-by their selective excitation using adaptive optics. This allows us to produce output beams of high modal purity, which are well defined in three dimensions. The output beam distribution is even robust against significant bending of the fibre. The utility of this approach is exemplified by the controlled rotational manipulation of live cells in a dual-beam fibre-optical trap integrated into a modular lab-on-chip system.

This article presents a summary and critical review of what is known about the 'grouped retina', a peculiar type of retinal organization in fish in which groups of photoreceptor cell inner and outer segments are arranged in spatially separated bundles. In most but not all cases, these bundles are embedded in light-reflective cups that are formed by the retinal pigment epithelial cells. These cups constitute a specialized type of retinal tapetum (i.e., they are biological 'mirrors' that cause eye shine) and appear to be optimized for different purposes in different fishes. Generally, the large retinal pigment epithelial cells are filled with light-reflecting photonic crystals that consist of guanine, uric acid, or pteridine depending on species, and which ensure that the incoming light becomes directed onto the photoreceptor outer segments. This structural specialization has so far been found in representatives of 17 fish families; of note, not all members of a given family must possess a grouped retina, and the 17 families are not all closely related to each other. In many cases (e.g., in Osteoglossomorpha and Aulopiformes) the inner surface of the cup is formed by three to four layers of strikingly regularly shaped and spaced guanine platelets acting as an optical multilayer. It has been estimated that this provides an up to 10fold increase of the incident light intensity. In certain deep-sea fish (many Aulopiformes and the Polymixidae), small groups of rods are embedded in such 'parabolic mirrors'; most likely, this is an adaptation to the extremely low light intensities available in their habitat. Some of these fishes additionally possess similar tapetal cups that surround individual cones and, very likely, also serve as amplifiers of the weak incident light. In the Osteoglossomorpha, however, that inhabit the turbid water of rivers or streams, the structure of the cups is more complex and undergoes adaptation-dependent changes. At dim daylight, probably representing the usual environmental conditions of the fish, the outer segments of up to 30 cone cells are placed at the bottom of the cup where light intensity is maximized. Strikingly, however, a large number of rod receptor cells are positioned behind each mirroring cup. This peculiar arrangement (i) allows vision at deep red wavelenghts, (ii) matches the sensitivity of rod and cone photoreceptors, and (iii) facilitates the detection of low-contrast and color-mixed stimuli, within the dim, turbid habitat. Thus, for these fish the grouped retina appears to aid in reliable and quick detection of large, fast moving, biologically relevant stimuli such as predators. Overall, the grouped retina appears as a peculiar type of general retinal specialization in a variety of fish species that is adaptive in particular habitats such as turbid freshwater but also the deep-sea. The authors were prompted to write this review by working on the retina of Gnathonemus petersii; the data resulting from this work (Landsberger et al., 2008; Kreying et al., 2012) are included in the present review. (C) 2013 Elsevier Ltd. All rights reserved.

The vertebrate retina is inverted with respect to its optical function, which requires light to pass through the entire tissue prior to detection. The last significant barrier for photons to overcome is the outer nuclear layer formed by photoreceptor cell (PRC) nuclei. Here we experimentally characterise the optical properties of PRC nuclei using bright-field defocusing microscopy to capture near-field intensity distributions behind individual nuclei. We find that some nuclei efficiently focus incident light confirming earlier predictions based on comparative studies of chromatin organisation in nocturnal and diurnal mammals. The emergence of light focusing during the development of mouse nuclei highlights the acquired nature of the observed lens-like behaviour. Optical characterisation of these nuclei is an important first step towards an improved understanding of how light transmission through the retina is influenced by its constituents. (C) 2014 Optical Society of America

Salmonellosis is one of the leading causes of food poisoning worldwide. Controlling bacterial burden is essential to surviving infection. Nucleotide-binding oligomerization domain-like receptors (NLRs), such as NLRC4, induce inflammasome effector functions and play a crucial role in controlling Salmonella infection. Inflammasome-dependent production of IL-1 beta recruits additional immune cells to the site of infection, whereas inflammasome-mediated pyroptosis of macrophages releases bacteria for uptake by neutrophils. Neither of these functions is known to directly kill intracellular salmonellae within macrophages. The mechanism, therefore, governing how inflammasomes mediate intracellular bacterial-killing and clearance in host macrophages remains unknown. Here, we show that actin polymerization is required for NLRC4-dependent regulation of intracellular bacterial burden, inflammasome assembly, pyroptosis, and IL-1 beta production. NLRC4-induced changes in actin polymerization are physically manifested as increased cellular stiffness, and leads to reduced bacterial uptake, production of antimicrobial molecules, and arrested cellular migration. These processes act in concert to limit bacterial replication in the cell and dissemination in tissues. We show, therefore, a functional link between innate immunity and actin turnover in macrophages that underpins a key host defense mechanism for the control of salmonellosis.

Determining cell mechanical properties is increasingly recognized as a marker-free way to characterize and separate biological cells. This emerging realization has led to the development of a plethora of appropriate measurement techniques. Here, we use a fairly novel approach, deterministic lateral displacement (DLD), to separate blood cells based on their mechanical phenotype with high throughput. Human red blood cells were treated chemically to alter their membrane deformability and the effect of this alteration on the hydrodynamic behaviour of the cells in a DLD device was investigated. Cells of defined stiffness (glutaraldehyde cross-linked erythrocytes) were used to test the performance of the DLD device across a range of cell stiffness and applied shear rates. Optical stretching was used as an independent method for quantifying the variation in stiffness of the cells. Lateral displacement of cells flowing within the device, and their subsequent exit position from the device were shown to correlate with cell stiffness. Data showing how the isolation of leucocytes from whole blood varies with applied shear rate are also presented. The ability to sort leucocyte sub-populations (T-lymphocytes and neutrophils), based on a combination of cell size and deformability, demonstrates the potential for using DLD devices to perform continuous fractionation and/or enrichment of leucocyte sub-populations from whole blood.

A cell is a complex material whose mechanical properties are essential for its normal functions. Heating can have a dramatic effect on these mechanical properties, similar to its impact on the dynamics of artificial polymer networks. We investigated such mechanical changes by the use of a microfluidic optical stretcher, which allowed us to probe cell mechanics when the cells were subjected to different heating conditions at different time scales. We find that HL60/ S4 myeloid precursor cells become mechanically more compliant and fluid-likewhen subjected to either a sudden laser-induced temperature increase or prolonged exposure to higher ambient temperature. Above a critical temperature of 52 +/- 18 degrees C, we observed active cell contraction, which was strongly correlatedwith calciuminflux through temperature-sensitive transient receptor potential vanilloid 2 (TRPV2) ion channels, followed by a subsequent expansion in cell volume. The change from passive to active cellular response can be effectively described by a mechanical model incorporating both active stress and viscoelastic components. Our work highlights the role of TRPV2 in regulating the thermomechanical response of cells. It also offers insights into how cortical tension and osmotic pressure govern cell mechanics and regulate cell-shape changes in response to heat and mechanical stress.