Substructure and responses of cholinergic synaptic vesicles in the atomic force microscope

Extracellular vesicle-matrix interactions

The extracellular matrix in microenvironments harbors a variety of signals to control cellular functions and the materiality of tissues. Most efforts to synthetically reconstitute the matrix by biomaterial design have focused on decoupling cell-secreted and polymer-based cues. Cells package molecules into nanoscale lipid membrane-bound extracellular vesicles and secrete them. Thus, extracellular vesicles inherently interact with the meshwork of the extracellular matrix. In this Review, we discuss various aspects of extracellular vesicle-matrix interactions. Cells receive feedback from the extracellular matrix and leverage intracellular processes to control the biogenesis of extracellular vesicles. Once secreted, various biomolecular and biophysical factors determine whether extracellular vesicles are locally incorporated into the matrix or transported out of the matrix to be taken up by other cells or deposited into tissues at a distal location. These insights can be utilized to develop engineered biomaterials where EV release and retention can be precisely controlled in host tissue to elicit various biological and therapeutic outcomes.

Physical characterization methods for iron oxide contrast agents encapsulated within a targeted liposome-based delivery system

Intact liposome-based targeted nanoparticle delivery systems (NDS) are immobilized by non-selective binding and characterized by scanning probe microscopy (SPM) in a fluid imaging environment. The size, size distribution, functionality, and stability of an NDS with a payload consisting of a super-paramagnetic iron oxide contrast agent for magnetic resonance imaging are determined. SPM results are combined with information obtained by more familiar techniques such as superconducting quantum interference device (SQUID) magnetometry, dynamic light scattering, and electron microscopy. By integrating the methods presented in this work into the NDS formulation and manufacturing process, size-dependent statistical properties of the complex can be obtained and the structure-function relationship of individual, multi-component nanoscale entities can be assessed in a reliable and reproducible manner.

Influence of synapsin I on synaptic vesicles: an analysis by force-volume mode of the atomic force microscope and dynamic light scattering

Synaptic vesicles (SVs) are small neuronal organelles that store neurotransmitters and release them by exocytosis into the synaptic cleft for signal transmission between nerve cells. They consist of a highly curved membrane composed of different lipids containing several proteins with specific functions. A family of abundant extrinsic SV proteins, the synapsins, interact with SV proteins and phospholipids and play an important role in the regulation of SV trafficking and stability. We investigated the interactions of one these proteins with the SV membrane using atomic force microscope and dynamic light scattering. We examined SVs isolated from rat forebrain both under native conditions and after depletion of endogenous synapsin I. We used the atomic force microscope in two modes: imaging mode for characterizing the shape and size of SVs, and force-volume mode for characterizing their stiffness. Synapsin-depleted SVs were larger in size and showed a higher tendency to aggregate than native vesicles, although their stiffness was not significantly different. Because synapsins are believed to cross-link SV to each other and to the actin cytoskeleton, we also measured the SV aggregation kinetics induced by synapsin I by dynamic light scattering and atomic force microscopy and found that the addition of synapsin I promotes a rapid aggregation of SVs. The data indicate that synapsin directly affects SV stability and aggregation state and support the physiological role of synapsins in the assembly and regulation of SV pools within nerve terminals.

Nanofabricated carbon-based detector

The procedure for the fabrication of a detector termed NACAD, nanofabricated carbon-based detector, is described. I characterized the mechanical properties of this detector using atomic force and scanning electron microscopy techniques. This detector exhibits a smooth surface (rms 2-4 nm) and easy access for atomic force microscopy tips, necessary features for biological studies of secretory granules and vesicles, which store transmitters and release them via exocytosis. The NACAD does not impair the elastic properties of granular matrixes deposited onto a detector, as they show their typical Young's moduli and ion exchanger properties; divalent histamine shrinks them, while monovalent sodium causes their swelling. Additionally, the NACADs' electrochemical properties allowed amperometric measurements of serotonin released from intact secretory granules isolated from mast cells, after removal of their granular membranes using a mild detergent treatment. Thus, this detector will aid future studies of single secretory granules and vesicles and their insoluble matrixes.

Imaging the Selective Binding of Synapsin to Anionic Membrane Domains

Synapsins are membrane-associated proteins that cover the surface of synaptic vesicles and are responsible for maintaining a pool of neurotransmitter-loaded vesicles for use during neuronal activity. We have used atomic force microscopy (AFM) to study the interaction of synapsin I with negatively charged lipid domains in phase-separated supported lipid bilayers prepared from mixtures of phosphatidylcholines (PCs) and phosphatidylserines (PSs). The results indicate a mixture of electrostatic binding to anionic PS-rich domains as well as some nonspecific binding to the PC phase. Interestingly, both protein binding and scanning with synapsin-coated AFM tips can be used to visualize charged lipid domains that cannot be detected by topography alone.

Loading and recycling of synaptic vesicles in the Torpedo electric organ and the vertebrate neuromuscular junction

In vertebrate motor nerve terminals and in the electromotor nerve terminals of Torpedo there are two major pools of synaptic vesicles: readily releasable and reserve. The electromotor terminals differ in that the reserve vesicles are twice the diameter of the readily releasable vesicles. The vesicles contain high concentrations of ACh and ATP. Part of the ACh is brought into the vesicle by the vesicular ACh transporter, VAChT, which exchanges two protons for each ACh, but a fraction of the ACh seems to be accumulated by different, unexplored mechanisms. Most of the vesicles in the terminals do not exchange ACh or ATP with the axoplasm, although ACh and ATP are free in the vesicle interior. The VAChT is controlled by a multifaceted regulatory complex, which includes the proteoglycans that characterize the cholinergic vesicles. The drug (-)-vesamicol binds to a site on the complex and blocks ACh exchange. Only 10-20% of the vesicles are in the readily releasable pool, which therefore is turned over fairly rapidly by spontaneous quantal release. The turnover can be followed by the incorporation of false transmitters into the recycling vesicles, and by the rate of uptake of FM dyes, which have some selectivity for the two recycling pathways. The amount of ACh loaded into recycling vesicles in the readily releasable pool decreases during stimulation. The ACh content of the vesicles can be varied over eight-fold range without changing vesicle size.

Control of neurotransmitter release by an internal gel matrix in synaptic vesicles

Neurotransmitters are stored in synaptic vesicles, where they have been assumed to be in free solution. Here we report that in Torpedo synaptic vesicles, only 5% of the total acetylcholine (ACh) or ATP content is free, and that the rest is adsorbed to an intravesicular proteoglycan matrix. This matrix, which controls ACh and ATP release by an ion-exchange mechanism, behaves like a smart gel. That is, it releases neurotransmitter and changes its volume when challenged with small ionic concentration change. Immunodetection analysis revealed that the synaptic vesicle proteoglycan SV2 is the core of the intravesicular matrix and is responsible for immobilization and release of ACh and ATP. We suggest that in the early steps of vesicle fusion, this internal matrix regulates the availability of free diffusible ACh and ATP, and thus serves to modulate the quantity of transmitter released.

Physical morphology and surface properties of unsaturated Pseudomonas putida biofilms

Unsaturated biofilms of Pseudomonas putida, i.e., biofilms grown in humid air, were analyzed by atomic force microscopy to determine surface morphology, roughness, and adhesion forces in the outer and basal cell layers of fresh and desiccated biofilms. Desiccated biofilms were equilibrated with a 75.5% relative humidity atmosphere, which is far below the relative humidity of 98 to 99% at which these biofilms were cultured. In sharp contrast to the effects of drying on biofilms grown in fluid, we observed that drying caused little change in morphology, roughness, or adhesion forces in these unsaturated biofilms. Surface roughness for moist and dry biofilms increased approximately linearly with increasing scan sizes. This indicated that the divides between bacteria contributed more to overall roughness than did extracellular polymeric substances (EPS) on individual bacteria. The EPS formed higher-order structures we termed mesostructures. These mesostructures are much larger than the discrete polymers of glycolipids and proteins that have been previously characterized on the outer surface of these gram-negative bacteria.