Morphology of Lyotropic Myelin Figures Stained with a Fluorescent Dye

Three-Dimensional Imaging of Bioinspired Lipidic Mesophases Using Multicolored Light-Emitting Carbon Nanodots

Recent progress in the design of carbon nanostructures exhibiting strong multiphoton-excited emission opens new pathways to explore the self-organization of lipids found in living organisms. Phospholipid-based lyotropic myelin figures (MFs) are promising materials as simplified models of biomembranes due to their structural resemblance to a multilamellar sheath insulating the axon. This study demonstrates the possibility of selective labeling of MFs by strongly emitting multicolor phloroglucinol-derived carbon nanodots (PG CNDs). Such dopants are efficiently excited by visible and near-infrared light; therefore, one- and two-photon fluorescence microscopies are incorporated to gain 3D insights into the MFs. Combining nondestructive fluorescence microscopy and spectroscopy techniques along with polarized light microscopy gives details on the stability and morphology of lipidic mesophases. Our findings suggest that PG CNDs can be a viable and simple alternative to conventional fluorescent lipid stains to image biologically relevant phospholipid-based structures.

Reconfigurable Droplet-Droplet Communication Mediated by Photochemical Marangoni Flows

Droplets are attractive building blocks for dynamic matter that organizes into adaptive structures. Communication among collectively operating droplets opens untapped potential in settings that vary from sensing, optics, protocells, computing, or adaptive matter. Inspired by the transmission of signals among decentralized units in slime mold Physarum polycephalum, we introduce a combination of surfactants, self-assembly, and photochemistry to establish chemical signal transfer among droplets. To connect droplets that float at an air-water interface, surfactant triethylene glycol monododecylether (C12E3) is used for its ability to self-assemble into wires called myelins. We show how the trajectory of these myelins can be directed toward selected photoactive droplets upon UV exposure. To this end, we developed a strategy for photocontrolled Marangoni flow, which comprises (1) the liquid crystalline coating formed at the surface of an oleic acid/sodium oleate (OA/NaO) droplet when in contact with water, (2) a photoacid generator that protonates sodium oleate upon UV exposure and therefore disintegrates the coating, and (3) the surface tension gradient that is generated upon depletion of the surfactant from the air-water interface by the uncoated droplet. Therefore, localized UV exposure of selected OA/NaO droplets results in attraction of the myelins such that they establish reconfigurable connections that self-organize among the C12E3 and OA/NaO droplets. As an example of communication, we demonstrate how the myelins transfer fluorescent dyes, which are selectively delivered in the droplet interior upon photochemical regulation of the liquid crystalline coating.

Immunological Regulation of Gut-Tropic Immune Cells by Extracellular Vesicles

The remarkable diversity of lymphocytes, essential components of the immune system, serves as an ingenious mechanism for maximizing the efficient utilization of limited host defense resources. While cell adhesion molecules, notably in gut-tropic T cells, play a central role in this mechanism, the counterbalancing molecular details have remained elusive. Conversely, we've uncovered the molecular pathways enabling extracellular vesicles secreted by lymphocytes to reach the gut's mucosal tissues, facilitating immunological regulation. This discovery sheds light on immune fine-tuning, offering insights into immune regulation mechanisms.

Strongly Emitting Folic Acid-Derived Carbon Nanodots for One- and Two-Photon Imaging of Lyotropic Myelin Figures

Non-invasive imaging of morphological changes in biologically relevant lipidic mesophases is essential for the understanding of membrane-mediated processes. However, its methodological aspects need to be further explored, with particular attention paid to the design of new excellent fluorescent probes. Here, we have demonstrated that bright and biocompatible folic acid-derived carbon nanodots (FA CNDs) may be successfully applied as fluorescent markers in one- and two-photon imaging of bioinspired myelin figures (MFs). Structural and optical properties of these new FA CNDs were first extensively characterized; they revealed remarkable fluorescence performance in linear and non-linear excitation regimes, justifying further applications. Then, confocal fluorescence microscopy and two-photon excited fluorescence microscopy were used to investigate a three-dimensional distribution of FA CNDs within the phospholipid-based MFs. Our results showed that FA CNDs are effective markers for imaging various forms and parts of multilamellar microstructures.

Surfactant-Mediated Solubilization of Myelin Figures: A Multistep Morphological Cascade

Lamellar mesophases of insoluble lipids are readily solubilized by the micellar mesophases of soluble surfactants. This simple process underscores a broad array of biochemical methodologies, including purification, reconstitution, and crystallization of membrane proteins, as well as the isolation of detergent-resistant membrane fractions. Although much is now known about the thermodynamic driving forces of membrane solubilization, the kinetic pathways by which the surfactant alters vesicular mesophases are only beginning to be appreciated. Little is known about how these interactions affect the solubilization of more complex, multilamellar mesophases. Here, we investigate how a common zwitterionic detergent affects the solubilization of a smectic, multilamellar, cylindrical mesophase of lipids, called the myelin figure. Our results reveal that myelin solubilization occurs in a multistep manner, producing a well-defined sequence of morphologically distinct intermediates en route to complete solubilization. The kinetic processes producing these intermediates include (1) coiling, which encompasses the formation, propagation, and tightening of extended helices; (2) thinning, which reflects the unbinding of lamellae in the smectic stacks; and (3) detachment or retraction, which either dissociates the myelinic protrusion from the source lipid mass or returns the myelinic protrusion to the source lipid mass─all in transit toward complete solubilization. These occasionally overlapping steps are most pronounced in single-lipid component myelins, while compositionally graded multicomponent myelins inhibit the coiling step and detach more frequently. Taken together, the appearance of these intermediates during the solubilization of myelins suggests a complex free-energy landscape characterizing myelin solubilization populated by metastable, morphological intermediates correlated with locally minimized changes in energy dependent upon the mesophase's composition. This then predicts the accessibility of structurally distinct, kinetic intermediates─such as loose and tight coiled helices, peeled myelins, retracted tubes, and detached protrusions─before reaching the stable ground state corresponding to a dissolved suspension of mixed surfactant-lipid micelles.

Growth of Myelin Figures from Parent Multilamellar Vesicles

We examine the formation and growth of isolated myelin figures and microscale multilamellar tubules from isotropic micellar solutions of an anionic surfactant. Upon cooling, surfactant micelles transform into multilamellar vesicles (MLVs) whose contact is found to trigger the unidirectional growth of myelins. While the MLV diameter grows as d_MLV ∝ t^1/2, myelins grow linearly in time as L_M ∝ t¹, with a fixed diameter. Combining time-resolved small-angle neutron scattering (SANS) and optical microscopy, we demonstrate that the microscopic growth of spherical MLVs and cylindrical myelins stems from the same nanoscale molecular mechanism, namely, the surfactant exchange from micelles into curved lamellar structures at a constant volumetric rate. This mechanism successfully describes the growth rate of (nonequilibrium) myelin figures based on a population balance at thermodynamic equilibrium.