Biological barriers to drug transport prevent successful accumulation of nanotherapeutics specifically at diseased sites, limiting efficacious responses in disease processes ranging from cancer to inflammation. Although substantial research efforts have aimed to incorporate multiple functionalities and moieties within the overall nanoparticle design, many of these strategies fail to adequately address these barriers. Obstacles, such as nonspecific distribution and inadequate accumulation of therapeutics, remain formidable challenges to drug developers. A reimagining of conventional nanoparticles is needed to successfully negotiate these impediments to drug delivery. Site-specific delivery of therapeutics will remain a distant reality unless nanocarrier design takes into account the majority, if not all, of the biological barriers that a particle encounters upon intravenous administration. By successively addressing each of these barriers, innovative design features can be rationally incorporated that will create a new generation of nanotherapeutics, realizing a paradigmatic shift in nanoparticle-based drug delivery.

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Principles of nanoparticle design for overcoming biological barriers to drug delivery

In the 1980s, exosomes were described as vesicles of endosomal origin secreted from reticulocytes. Interest increased around these extracellular vesicles, as they appeared to participate in several cellular processes. Exosomes bear proteins, lipids, and RNAs, mediating intercellular communication between different cell types in the body, and thus affecting normal and pathological conditions. Only recently, scientists acknowledged the difficulty of separating exosomes from other types of extracellular vesicles, which precludes a clear attribution of a particular function to the different types of secreted vesicles. To shed light into this complex but expanding field of science, this review focuses on the definition of exosomes and other secreted extracellular vesicles. Their biogenesis, their secretion, and their subsequent fate are discussed, as their functions rely on these important processes.

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Biogenesis, Secretion, and Intercellular Interactions of Exosomes and Other Extracellular Vesicles

Biogenesis, Secretion, and Intercellular Interactions of Exosomes and Other

Thapsigargin, a tumor-promoting sesquiterpene lactone, discharges intracellular Ca2+ in rat hepatocytes, as it does in many vertebrate cell types. It appears to act intracellularly, as incubation of isolated rat liver microsomes with thapsigargin induces a rapid, dose-dependent release of stored Ca2+. The thapsigargin-releasable pool of microsomal Ca2+ includes the pools sensitive to inositol 1,4,5-trisphosphate and GTP. Thapsigargin pretreatment of microsomes blocks subsequent loading with 45Ca2+, suggesting that its target is the ATP-dependent Ca2+ pump of endoplasmic reticulum. This hypothesis is strongly supported by the demonstration that thapsigargin causes a rapid inhibition of the Ca2(+)-activated ATPase activity of rat liver microsomes, with an identical dose dependence to that seen in whole cell or isolated microsome Ca2+ discharge. The inhibition of the endoplasmic reticulum isoform of the Ca2(+)-ATPase is highly selective, as thapsigargin has little or no effect on the Ca2(+)-ATPases of hepatocyte or erythrocyte plasma membrane or of cardiac or skeletal muscle sarcoplasmic reticulum. These results suggest that thapsigargin increases the concentration of cytosolic free Ca2+ in sensitive cells by an acute and highly specific arrest of the endoplasmic reticulum Ca2+ pump, followed by a rapid Ca2+ leak from at least two pharmacologically distinct Ca2+ stores. The implications of this mechanism of action for the application of thapsigargin in the analysis of Ca2+ homeostasis and possible forms of Ca2+ control are discussed.

https://www.pnas.org/content/pnas/87/7/2466.full.pdf

Thapsigargin, a tumor promoter, discharges intracellular Ca2+ stores by specific inhibition of the endoplasmic reticulum Ca2(+)-ATPase.

Rab GTPases control intracellular vesicle traffic by acting as regulatable switches that recruit effector molecules when in their GTP-bound form. The functional coupling between multiple Rab GTPases ensures the spatiotemporally coordinated regulation of vesicle traffic. Membrane trafficking between organelles by vesiculotubular carriers is fundamental to the existence of eukaryotic cells. Central in ensuring that cargoes are delivered to their correct destinations are the Rab GTPases, a large family of small GTPases that control membrane identity and vesicle budding, uncoating, motility and fusion through the recruitment of effector proteins, such as sorting adaptors, tethering factors, kinases, phosphatases and motors. Crosstalk between multiple Rab GTPases through shared effectors, or through effectors that recruit selective Rab activators, ensures the spatiotemporal regulation of vesicle traffic. Functional impairments of Rab pathways are associated with diseases, such as immunodeficiencies, cancer and neurological disorders.

Rab GTPases as coordinators of vesicle traffic.

possess a transport system for glucose of the facilitative diffusion type; these transporters allow the movement of glucose across the plasma membrane down its chemical gradient either into or out of cells. These transporters are specific for the D-enantiomer of glucose and are not coupled to any energy-requiring components, such as ATP hydrolysis or a HI gradient [1]. The facilitative glucose transporters are distinct from the Na+dependent transporters, which actively accumulate glucose [2,3]. The importance of glucose as a cellular metabolite has led to

/pdf/the-glucose-transporter-family-structure-function-and-tissue-1kjd3397um.pdf

The glucose transporter family: structure, function and tissue-specific expression.

Low grade chronic inflammation as reflected by increased C-reactive protein (CRP) concentrations independently predicts those at risk for coronary heart disease (CHD) and type 2 diabetes. Women with polycystic ovarian syndrome (PCOS) are insulin resistant and have increased risk for CHD and type 2 diabetes, but currently there are no data on markers of inflammation in women with PCOS. Seventeen women with PCOS (defined on the basis of elevated testosterone and oligomenorrhea) and 15 healthy women matched as a group for body mass index were recruited. Measurement of CRP concentrations was made using a highly sensitive assay. Insulin resistance was assessed using the hyperinsulinemic euglycemic clamp technique. The women with PCOS had significantly elevated CRP concentrations relative to controls (geometric means, 2.12 and 0.67 mg/L, respectively; P = 0.016). Log CRP correlated with body mass index in both PCOS and controls (r = 0.58; P < 0.05 and r = 0.78; P < 0.01, respectively) and inversely with insulin sensitivity (r = -0.57; P < 0.05 and r = -0.69; P < 0.01). Total testosterone did not correlate with log CRP in either group. On adjustment for body mass index and age, there remained a significant difference in log CRP between PCOS and controls (t = 2.13; P < 0.05). On further adjustment for insulin sensitivity, log CRP was no longer significantly different between groups (t = 1.51; P = 0.14). We conclude that women with PCOS have significantly increased CRP concentrations relative to women with normal menstrual rhythm and normal androgen levels. We propose low grade chronic inflammation as a novel mechanism contributing to increased risk of CHD and type 2 diabetes in these women.

Low Grade Chronic Inflammation in Women with Polycystic Ovarian Syndrome

Structure and function of mammalian facilitative sugar transporters.

Lipid rafts are microdomains present within membranes of most cell types. These membrane microdomains, which are enriched in cholesterol and glycosphingolipids, have been implicated in the regulation of certain signal transduction and membrane traffic pathways. To investigate the possibility that lipid rafts organize exocytotic pathways in neuroendocrine cells, we examined the association of proteins of the exocytotic machinery with rafts purified from PC12 cells. The target soluble N-ethylmaleimide-sensitive factor attachment protein receptor (tSNARE) proteins syntaxin 1A and synaptosomal-associated protein of 25 kDa (SNAP-25) were both found to be highly enriched in lipid rafts ( approximately 25-fold). The vesicle SNARE vesicle-associated membrane protein (VAMP)2 was also present in raft fractions, but the extent of this recovery was variable. However, further analysis revealed that the majority of VAMP2 was associated with a distinct class of raft with different detergent solubility characteristics to the rafts containing syntaxin 1A and SNAP-25. Interestingly, no other studied secretory proteins were significantly associated with lipid rafts, including SNARE effector proteins such as nSec1. Chemical crosslinking experiments showed that syntaxin1A/SNAP-25 heterodimers were equally present in raft and nonraft fractions, whereas syntaxin1A/nSec1 complexes were detected only in nonraft fractions. SDS-resistance assays revealed that raft-associated syntaxin1A/SNAP-25 heterodimers were able to interact with VAMP2. Finally, reduction of cellular cholesterol levels decreased the extent of regulated exocytosis of dopamine from PC12 cells. The results described suggest that the interaction of SNARE proteins with lipid rafts is important for exocytosis and may allow structural and spatial organization of the secretory machinery.

https://www.pnas.org/content/pnas/98/10/5619.full.pdf

SNARE proteins are highly enriched in lipid rafts in PC12 cells: Implications for the spatial control of exocytosis

Human facilitative glucose transporters. Isolation, functional characterization, and gene localization of cDNAs encoding an isoform (GLUT5) expressed in small intestine, kidney, muscle, and adipose tissue and an unusual glucose transporter pseudogene-like sequence (GLUT6).

Gwyn W. Gould

Papers

The glucose transporter family: structure, function and tissue-specific expression.

Low Grade Chronic Inflammation in Women with Polycystic Ovarian Syndrome

Structure and function of mammalian facilitative sugar transporters.

SNARE proteins are highly enriched in lipid rafts in PC12 cells: Implications for the spatial control of exocytosis

Human facilitative glucose transporters. Isolation, functional characterization, and gene localization of cDNAs encoding an isoform (GLUT5) expressed in small intestine, kidney, muscle, and adipose tissue and an unusual glucose transporter pseudogene-like sequence (GLUT6).