Vesicles stimuli-responsive

In addition, it can be foreseen that supramolecular chemistry will play a key role in the development of stimuli-responsive vesicles. Stimuli-responsive vesicles will be important elements of sensors, nanoreactors, and drug... 

In a more general way, the two major driving forces for the design of novel micellar systems are the control over morphology (spheres, vesicles, rods, tubules etc. with controlled size) and function (stimulus-responsive materials, biological functions). Both of these aspects are intimately related since a given morphology can induce a specific function. 

A sensor is a system that displays a readily detectable response in the presence of a specific analyte. Indeed, stimulus-responsive vesicles have been tailor-made to function as highly specific sensors. The overwhelming majority of vesicle-based sensors are based on a very simple type of amphiphile polydiacetylenes. These polymeric amphiphiles are easily formed from simple diacelylene amphiphiles by in situ photopolymerization of vesicles. If the vesicles are additionally equipped with ligand or receptor groups, the absorbance and fluorescence of the conjugated polymer backbone is highly sensitive to the presence of metal ions, anions, and small as well as large... 

Stimulus-responsiveness of the membrane is a specific property that is important in drug delivery systems in the presence of stimuli [e.g. the acidic environment of a tumor) the membrane is disrupted or changes its architecture e.g. from vesicle to micelle). Responsiveness of hydrophobic blocks can be chemically achieved for various stimuli, such as pH, redox conditions, enzyme-degradation, light, temperature, and changes in a magnetic field. ... 

In another experiment, photo-controlled inclusion and exclusion reaction of azobenzene-containing polydiacetylene vesicles with alpha-cyclodextrin were used to act as driving force to induce chromatic transition of PDA vesicles, which provided a novel model system that combines photochemistry and host-guest chemistry for a photo-stimulus-responsive vesicle [137]. 

At least two classes of regulated secretion can be defined [54]. The standard regulated secretion pathway is common to all secretory cells (i.e. adrenal chromaffin cells, pancreatic beta cells, etc.) and works on a time scale of minutes or even longer in terms of both secretory response to a stimulus and reuptake of membranes after secretion. The second, much faster, neuron-specific form of regulated secretion is release of neurotransmitters at the synapse. Release of neurotransmitters may occur within fractions of a second after a stimulus and reuptake is on the order of seconds. Indeed, synaptic vesicles may be recycled and ready for another round of neurotransmitter release within 1-2 minutes [64]. These two classes of regulated secretion will be discussed separately after a consideration of secretory vesicle biogenesis. 

Measuring muscle-evoked responses to repetitive motor nerve electrical stimulation permits detection of presyn-aptic neuromuscular junction dysfunction. In botulism and the Lambert-Eaton syndrome, repetitive stimulation elicits a smaller than normal skeletal muscle response at the beginning of the stimulus train, due to impaired initial release of acetylcholine-containing vesicles from presyn-aptic terminals of motor neurons followed by a normal or accentuated incremental muscle response during repeated stimulation. This incremental response to repetitive stimulation in presynaptic neuromuscular disorders can be distinguished from the decremental response that characterizes autoimmune myasthenia gravis, which affects the postsynaptic component of neuromuscular junctions. 

P cells of the pancreatic islets in combination with atoms of zinc, but when required to regulate blood glucose concentration, the prohormone is cleaved and functional insulin is released into the circulation along with the C-peptide. This example of post-translational processing is mediated by peptidases which are contained in the vesicles along with the proinsulin. The fusion of the secretory vesicles with the cell membrane and activation of the peptidase prior to exocytosis of the insulin are prompted by an influx of calcium ions into the P-cell in response to the appropriate stimulus. Similarly, catecholamines are synthesized and held within the cell by attachment to proteins called chromogranins. 

The era of biomimetic peptide- and sugar-based polymer vesicles has just begun and seems very promising. Bioinspired vesicles are mainly applied for drug deliv-ery/release and the fabrication of composite materials, but could readily be used for biomimetic materials science, biomineralization, and so on. Especially interesting are smart vesicles changing properties in response to an external stimulus (temperature, pH, ions). 

As noted In the chapter Introduction, all eukaryotic cells continuously secrete certain proteins, a process commonly called constitutive secretion. Specialized secretory cells also store other proteins In vesicles and secrete them only when triggered by a specific stimulus. One example of such regulated secretion occurs In pancreatic p cells, which store newly made Insulin In special secretory vesicles and secrete Insulin In response to an elevation In blood glucose (see Figure 15-7). These and other secretory cells simultaneously utilize two different types of vesicles to move proteins from the trans-Golgi network to the cell surface regulated transport vesicles, often simply called secretory vesicles, and unregulated transport vesicles, also called constitutive secretory vesicles. 

The mechanism by which the association of elevated plasma dopa hydroxylase and defective response to the sympathetic stimulus is achieved is unknown. The hydroxylase could be readily released either from synaptic vesicles and adrenals prematurely so that its action on the substrate is prevented, catecholamines themselves could be overproduced and unable to bind to the proper receptor, or the receptor could be defective [219]. 

Furthermore, these block copolymers may be designed to contain stimuli-responsive components such that the assembly process may be triggered through the application of a stimulus. With unique properties and a host of potential applications, stimuli-responsive assemblies such as micelles, vesicles, bioconjugates, films, networks, and patterned surfaces have been prepared from block copolymers synthesized via controlled polymerization methods (Fig. 3.5) [1, 42]. 

Self-organization of amphiphilic (co)polymers has resulted in assemblies such as micelles, vesicles, fibers, helical superstructures, and macroscopic tubes [174, 175]. These nanoscale to macroscale morphologies are of interest in areas ranging from material science to biology [176]. Stimuli-responsive versions of these assemblies are likely to further enhance their scope as smart materials. Thermo- or pH-sensitive polymer micelles [177] and vesicles [178] have been reported in which the nature of the functionality at the corona changes in response to the stimulus. Some attention has been also paid to realize an environment-dependent switch from a micelle-type assembly with a hydrophilic corona to an inverted micelle-type assembly with a lipophilic corona [179]. 

Figure 2. Diagrammatic representation of a pancreatic exocrine cell. Secretory precursor proteins are synthesized by the RER and sequestered in the cisternae of that organelle (1). The material is then transferred by way of small vesicles to mature Golgi saccules and vacuoles (2). Within the vacuoles the secretory precursors are concentrated with the removal of water (3) and the Golgi vacuole is transformed into a mature zymogen granule (4). In response to the appropriate stimulus, the membrane of the zymogen granule fuses with the apical plasma membrane (5), and the final secretory product is released to the lumen of the gland.

---