Visualization of self-delivering hydrophobically modified siRNA cellular internalization.

siRNAs are a new class of therapeutic modalities with promising clinical efficacy that requires modification or formulation for delivery to the tissue and cell of interest. Conjugation of siRNAs to lipophilic groups supports efficient cellular uptake by a mechanism that is not well characterized. Here we study the mechanism of internalization of asymmetric, chemically stabilized, cholesterol-modified siRNAs (sd-rxRNAs®) that efficiently enter cells and tissues without the need for formulation. We demonstrate that uptake is rapid with significant membrane association within minutes of exposure followed by the formation of vesicular structures and internalization. Furthermore, sd-rxRNAs are internalized by a specific class of early endosomes and show preferential association with epidermal growth factor (EGF) but not transferrin (Tf) trafficking pathways as shown by live cell TIRF and structured illumination microscopy (SIM). In fixed cells, we observe ∼25% of sd-rxRNA co-localizing with EGF and <5% with Tf, which is indicative of selective endosomal sorting. Likewise, preferential sd-rxRNA co-localization was demonstrated with EEA1 but not RBSN-containing endosomes, consistent with preferential EGF-like trafficking through EEA1-containing endosomes. sd-rxRNA cellular uptake is a two-step process, with rapid membrane association followed by internalization through a selective, saturable subset of the endocytic process. However, the mechanistic role of EEA1 is not yet known. This method of visualization can be used to better understand the kinetics and mechanisms of hydrophobic siRNA cellular uptake and will assist in further optimization of these types of compounds for therapeutic intervention.

Ack1 is a dopamine transporter endocytic brake that rescues a trafficking-dysregulated ADHD coding variant.

The dopamine (DA) transporter (DAT) facilitates high-affinity presynaptic DA reuptake that temporally and spatially constrains DA neurotransmission. Aberrant DAT function is implicated in attention-deficit/hyperactivity disorder and autism spectrum disorder. DAT is a major psychostimulant target, and psychostimulant reward strictly requires binding to DAT. DAT function is acutely modulated by dynamic membrane trafficking at the presynaptic terminal and a PKC-sensitive negative endocytic mechanism, or "endocytic brake," controls DAT plasma membrane stability. However, the molecular basis for the DAT endocytic brake is unknown, and it is unknown whether this braking mechanism is unique to DAT or common to monoamine transporters. Here, we report that the cdc42-activated, nonreceptor tyrosine kinase, Ack1, is a DAT endocytic brake that stabilizes DAT at the plasma membrane and is released in response to PKC activation. Pharmacologic and shRNA-mediated Ack1 silencing enhanced basal DAT internalization and blocked PKC-stimulated DAT internalization, but had no effects on SERT endocytosis. Both cdc42 activation and PKC stimulation converge on Ack1 to control Ack1 activity and DAT endocytic capacity, and Ack1 inactivation is required for stimulated DAT internalization downstream of PKC activation. Moreover, constitutive Ack1 activation is sufficient to rescue the gain-of-function endocytic phenotype exhibited by the ADHD DAT coding variant, R615C. These findings reveal a unique endocytic control switch that is highly specific for DAT. Moreover, the ability to rescue the DAT(R615C) coding variant suggests that manipulating DAT trafficking mechanisms may be a potential therapeutic approach to correct DAT coding variants that exhibit trafficking dysregulation.

NPHP4 controls ciliary trafficking of membrane proteins and large soluble proteins at the transition zone.

The protein nephrocystin-4 (NPHP4) is widespread in ciliated organisms, and defects in NPHP4 cause nephronophthisis and blindness in humans. To learn more about the function of NPHP4, we have studied it in Chlamydomonas reinhardtii. NPHP4 is stably incorporated into the distal part of the flagellar transition zone, close to the membrane and distal to CEP290, another transition zone protein. Therefore, these two proteins, which are incorporated into the transition zone independently of each other, define different domains of the transition zone. An nphp4-null mutant forms flagella with nearly normal length, ultrastructure and intraflagellar transport. When fractions from isolated wild-type and nphp4 flagella were compared, few differences were observed between the axonemes, but the amounts of certain membrane proteins were greatly reduced in the mutant flagella, and cellular housekeeping proteins >50 kDa were no longer excluded from mutant flagella. Therefore, NPHP4 functions at the transition zone as an essential part of a barrier that regulates both membrane and soluble protein composition of flagella. The phenotypic consequences of NPHP4 mutations in humans likely follow from protein mislocalization due to defects in the transition zone barrier.

Human adipose tissue expansion in pregnancy is impaired in gestational diabetes mellitus.

AIMS/HYPOTHESIS: During pregnancy, adipose tissue (AT) must expand to support the growing fetus and the future nutritional needs of the offspring. Limited expandability of AT is associated with insulin resistance, attributed to ectopic lipid deposition. This study aimed to investigate human AT expandability during pregnancy and its role in the pathogenesis of gestational diabetes mellitus (GDM). METHODS: This cross-sectional study of omental (OM) and subcutaneous (SQ) AT collected at Caesarean delivery included 11 pregnant and three non-pregnant women with normal glucose tolerance (NGT), five with GDM, three with type 2 diabetes mellitus. Adipocyte size, capillary density, collagen content and capillary growth were measured. Affymetrix arrays and real-time PCR studies of gene expression were performed. RESULTS: Mean OM adipocyte size was greater in women with GDM than in those with NGT (p = 0.004). Mean OM and SQ capillary density was lower in GDM compared with NGT (p = 0.015). Capillary growth did not differ significantly between groups. The most differentially expressed AT transcript when comparing non-pregnant and pregnant women corresponded to the IGF binding protein (IGFBP)-5, the expression levels of which was found by subsequent quantitative real-time PCR to be lower in women with GDM vs women with NGT (p < 0.0001). CONCLUSIONS/INTERPRETATION: The relative OM adipocyte hypertrophy and decreased OM and SQ capillary density are consistent with impaired AT expandability in GDM. The induction of adipose tissue IGFBP5 in pregnancy and its decrease in GDM point to the importance of the IGF-1 signalling pathway in AT expansion in pregnancy and GDM susceptibility.

Rabenosyn-5 defines the fate of the transferrin receptor following clathrin-mediated endocytosis.

Cell surface receptors and other proteins internalize through diverse mechanisms at the plasma membrane and are sorted to different destinations. Different subpopulations of early endosomes have been described, raising the question of whether different internalization mechanisms deliver cargo into different subsets of early endosomes. To address this fundamental question, we developed a microscopy platform to detect the precise position of endosomes relative to the plasma membrane during the uptake of ligands. Axial resolution is maximized by concurrently applied total internal reflection fluorescence and epifluorescence-structured light. We found that transferrin receptors are delivered selectively from clathrin-coated pits on the plasma membrane into a specific subpopulation of endosomes enriched in the multivalent Rab GTPase and phosphoinositide-binding protein Rabenosyn-5. Depletion of Rabenosyn-5, but not of other early endosomal proteins such as early endosome antigen 1, resulted in impaired transferrin uptake and lysosomal degradation of transferrin receptors. These studies reveal a critical role for Rabenosyn-5 in determining the fate of transferrin receptors internalized by clathrin-mediated endocytosis and, more broadly, a mechanism whereby the delivery of cargo from the plasma membrane into specific early endosome subpopulations is required for its appropriate intracellular traffic.

Type 1 ryanodine receptor knock-in mutation causing central core disease of skeletal muscle also displays a neuronal phenotype.

The type 1 ryanodine receptor (RyR1) is expressed widely in the brain, with high levels in the cerebellum, hippocampus, and hypothalamus. We have shown that L-type Ca(2+) channels in terminals of hypothalamic magnocellular neurons are coupled to RyRs, as they are in skeletal muscle, allowing voltage-induced Ca(2+) release (VICaR) from internal Ca(2+) stores without Ca(2+) influx. Here we demonstrate that RyR1 plays a role in VICaR in nerve terminals. Furthermore, in heterozygotes from the Ryr1(I4895T/WT) (IT/+) mouse line, carrying a knock-in mutation corresponding to one that causes a severe form of human central core disease, VICaR is absent, demonstrating that type 1 RyR mediates VICaR and that these mice have a neuronal phenotype. The absence of VICaR was shown in two ways: first, depolarization in the absence of Ca(2+) influx elicited Ca(2+)syntillas (scintilla, spark, in a nerve terminal, a SYNaptic structure) in WT, but not in mutant terminals; second, in the presence of extracellular Ca(2+), IT/+ terminals showed a twofold decrease in global Ca(2+) transients, with no change in plasmalemmal Ca(2+) current. From these studies we draw two conclusions: (i) RyR1 plays a role in VICaR in hypothalamic nerve terminals; and (ii) a neuronal alteration accompanies the myopathy in IT/+ mice, and, possibly in humans carrying the corresponding RyR1 mutation.

Dopamine transporter endocytic trafficking in striatal dopaminergic neurons: differential dependence on dynamin and the actin cytoskeleton.

Dopaminergic signaling profoundly impacts rewarding behaviors, movement, and executive function. The presynaptic dopamine (DA) transporter (DAT) recaptures released DA, thereby limiting synaptic DA availability and maintaining dopaminergic tone. DAT constitutively internalizes and PKC activation rapidly accelerates DAT endocytosis, resulting in DAT surface loss. Longstanding evidence supports PKC-stimulated DAT trafficking in heterologous expression studies. However, PKC-stimulated DAT internalization is not readily observed in cultured dopaminergic neurons. Moreover, conflicting reports implicate both classic and nonclassic endocytic mechanisms mediating DAT trafficking. Prior DAT trafficking studies relied primarily upon chronic gene disruption and dominant-negative protein expression, or were performed in cell lines and cultured neurons, yielding results difficult to translate to adult dopaminergic neurons. Here, we use newly described dynamin inhibitors to test whether constitutive and PKC-stimulated DAT internalization are dynamin-dependent in adult dopaminergic neurons. Ex vivo biotinylation studies in mouse striatal slices demonstrate that acute PKC activation drives native DAT surface loss, and that surface DAT surprisingly partitions between endocytic-willing and endocytic-resistant populations. Acute dynamin inhibition reveals that constitutive DAT internalization is dynamin-independent, whereas PKC-stimulated DAT internalization is dynamin-dependent. Moreover, total internal reflection fluorescence microscopy experiments demonstrate that constitutive DAT internalization occurs equivalently from lipid raft and nonraft microdomains, whereas PKC-stimulated DAT internalization arises exclusively from lipid rafts. Finally, DAT endocytic recycling relies on a dynamin-dependent mechanism that acts in concert with the actin cytoskeleton. These studies are the first comprehensive investigation of native DAT trafficking in ex vivo adult neurons, and reveal that DAT surface dynamics are governed by complex multimodal mechanisms.

Catecholamine exocytosis during low frequency stimulation in mouse adrenal chromaffin cells is primarily asynchronous and controlled by the novel mechanism of Ca2+ syntilla suppression.

Adrenal chromaffin cells (ACCs), stimulated by the splanchnic nerve, generate action potentials (APs) at a frequency near 0.5 Hz in the resting physiological state, at times described as 'rest and digest'. How such low frequency stimulation in turn elicits sufficient catecholamine exocytosis to set basal sympathetic tone is not readily explained by the classical mechanism of stimulus-secretion coupling, where exocytosis is synchronized to AP-induced Ca(2+) influx. By using simulated action potentials (sAPs) at 0.5 Hz in isolated patch-clamped mouse ACCs, we show here that less than 10% of all catecholaminergic exocytosis, measured by carbon fibre amperometry, is synchronized to an AP. The asynchronous phase, the dominant phase, of exocytosis does not require Ca(2+) influx. Furthermore, increased asynchronous exocytosis is accompanied by an AP-dependent decrease in frequency of Ca(2+) syntillas (i.e. transient, focal Ca(2+) release from internal stores) and is ryanodine sensitive. We propose a mechanism of disinhibition, wherein APs suppress Ca(2+) syntillas, which themselves inhibit exocytosis as they do in the case of spontaneous catecholaminergic exocytosis.

Insulin stimulates membrane fusion and GLUT4 accumulation in clathrin coats on adipocyte plasma membranes.

Total internal reflection fluorescence (TIRF) microscopy reveals highly mobile structures containing enhanced green fluorescent protein-tagged glucose transporter 4 (GLUT4) within a zone about 100 nm beneath the plasma membrane of 3T3-L1 adipocytes. We developed a computer program (Fusion Assistant) that enables direct analysis of the docking/fusion kinetics of hundreds of exocytic fusion events. Insulin stimulation increases the fusion frequency of exocytic GLUT4 vesicles by approximately 4-fold, increasing GLUT4 content in the plasma membrane. Remarkably, insulin signaling modulates the kinetics of the fusion process, decreasing the vesicle tethering/docking duration prior to membrane fusion. In contrast, the kinetics of GLUT4 molecules spreading out in the plasma membrane from exocytic fusion sites is unchanged by insulin. As GLUT4 accumulates in the plasma membrane, it is also immobilized in punctate structures on the cell surface. A previous report suggested these structures are exocytic fusion sites (Lizunov et al., J. Cell Biol. 169:481-489, 2005). However, two-color TIRF microscopy using fluorescent proteins fused to clathrin light chain or GLUT4 reveals these structures are clathrin-coated patches. Taken together, these data show that insulin signaling accelerates the transition from docking of GLUT4-containing vesicles to their fusion with the plasma membrane and promotes GLUT4 accumulation in clathrin-based endocytic structures on the plasma membrane.

Dihydropyridine receptors and type 1 ryanodine receptors constitute the molecular machinery for voltage-induced Ca2+ release in nerve terminals.

Ca2+ stores were studied in a preparation of freshly dissociated terminals from hypothalamic magnocellular neurons. Depolarization from a holding level of -80 mV in the absence of extracellular Ca2+ elicited Ca2+ release from intraterminal stores, a ryanodine-sensitive process designated as voltage-induced Ca2+ release (VICaR). The release took one of two forms: an increase in the frequency but not the quantal size of Ca2+ syntillas, which are brief, focal Ca2+ transients, or an increase in global [Ca2+]. The present study provides evidence that the sensors of membrane potential for VICaR are dihydropyridine receptors (DHPRs). First, over the range of -80 to -60 mV, in which there was no detectable voltage-gated inward Ca2+ current, syntilla frequency was increased e-fold per 8.4 mV of depolarization, a value consistent with the voltage sensitivity of DHPR-mediated VICaR in skeletal muscle. Second, VICaR was blocked by the dihydropyridine antagonist nifedipine, which immobilizes the gating charge of DHPRs but not by Cd2+ or FPL 64176 (methyl 2,5 dimethyl-4[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylate), a non-dihydropyridine agonist specific for L-type Ca2+ channels, having no effect on gating charge movement. At 0 mV, the IC50 for nifedipine blockade of VICaR in the form of syntillas was 214 nM in the absence of extracellular Ca2+. Third, type 1 ryanodine receptors, the type to which DHPRs are coupled in skeletal muscle, were detected immunohistochemically at the plasma membrane of the terminals. VICaR may constitute a new link between neuronal activity, as signaled by depolarization, and a rise in intraterminal Ca2+.

Adipose tissue angiogenesis assay.

Changes in adipose tissue mass must be accompanied by parallel changes in microcirculation. Investigating the mechanisms that regulate adipose tissue angiogenesis could lead to better understanding of adipose tissue function and reveal new potential therapeutic strategies. Angiogenesis is defined as the formation of new capillaries from existing microvessels. This process can be recapitulated in vitro, by incubation of tissue in extracellular matrix components in the presence of pro-angiogenic factors. Here, we describe a method to study angiogenesis from adipose tissue fragments obtained from mouse and human tissue. This assay can be used to define effects of diverse factors added in vitro, as well as the role of endogenously produced factors on angiogenesis. We also describe approaches to quantify angiogenic potential for the purpose of enabling comparisons between subjects, thus providing information on the role of physiological conditions of the donor on adipose tissue angiogenic potential.

Suppression of Ca2+ syntillas increases spontaneous exocytosis in mouse adrenal chromaffin cells.

A central concept in the physiology of neurosecretion is that a rise in cytosolic [Ca(2+)] in the vicinity of plasmalemmal Ca(2+) channels due to Ca(2+) influx elicits exocytosis. Here, we examine the effect on spontaneous exocytosis of a rise in focal cytosolic [Ca(2+)] in the vicinity of ryanodine receptors (RYRs) due to release from internal stores in the form of Ca(2+) syntillas. Ca(2+) syntillas are focal cytosolic transients mediated by RYRs, which we first found in hypothalamic magnocellular neuronal terminals. (scintilla, Latin for spark; found in nerve terminals, normally synaptic structures.) We have also observed Ca(2+) syntillas in mouse adrenal chromaffin cells. Here, we examine the effect of Ca(2+) syntillas on exocytosis in chromaffin cells. In such a study on elicited exocytosis, there are two sources of Ca(2+): one due to influx from the cell exterior through voltage-gated Ca(2+) channels, and that due to release from intracellular stores. To eliminate complications arising from Ca(2+) influx, we have examined spontaneous exocytosis where influx is not activated. We report here that decreasing syntillas leads to an increase in spontaneous exocytosis measured amperometrically. Two independent lines of experimentation each lead to this conclusion. In one case, release from stores was blocked by ryanodine; in another, stores were partially emptied using thapsigargin plus caffeine, after which syntillas were decreased. We conclude that Ca(2+) syntillas act to inhibit spontaneous exocytosis, and we propose a simple model to account quantitatively for this action of syntillas.

Sorting of EGF and transferrin at the plasma membrane and by cargo-specific signaling to EEA1-enriched endosomes.

The biological function of receptors is determined by their appropriate trafficking through the endosomal pathway. Following internalization, the transferrin (Tf) receptor quantitatively recycles to the plasma membrane, whereas the epidermal growth factor (EGF) receptor undergoes degradation. To determine how Tf and EGF engage these two different pathways we imaged their binding and early endocytic pathway in live cells using total internal reflection fluorescence microscopy (TIRF-M). We find that EGF and Tf bind to distinct plasma membrane regions and are incorporated into different endocytic vesicles. After internalization, both EGF-enriched and Tf-enriched vesicles interact with endosomes containing early endosome antigen 1 (EEA1). EGF is incorporated and retained in these endosomes, while Tf-containing vesicles rapidly dissociate and move to a juxtanuclear compartment. Endocytic vesicles carrying EGF recruit more Rab5 GTPase than those carrying Tf, which, by strengthening their association with EEA1-enriched endosomes, may provide a mechanism for the observed cargo-specific sorting. These results reveal pre-endocytic sorting of Tf and EGF, a specialized role for EEA1-enriched endosomes in EGF trafficking, and a potential mechanism for cargo-specified sorting of endocytic vesicles by these endosomes.

Plasma membrane domains specialized for clathrin-mediated endocytosis in primary cells.

Clathrin assembly at the plasma membrane is a fundamental process required for endocytosis. In cultured cells, most of the clathrin is localized to large patches that display little lateral mobility. The functional role of these regions is not clear, and it has been thought that they may represent artifacts of cell adhesion of cultured cells. Here we have analyzed clathrin organization in primary adipose cells isolated from mice, which are nonadherent and fully differentiated. The majority of clathrin on the plasma membrane of these cells (>60%) was found in large clathrin patches that displayed virtually no lateral mobility and persisted for many minutes, and a smaller amount was found in small spots that appeared and disappeared rapidly. Direct visualization of transferrin revealed that it bound onto large arrays of clathrin, internalizing through vesicles that emerge from these domains. High resolution imaging (50 images/s) revealed fluorescence intensity fluctuations consistent with the formation and detachment of coated vesicles from within large patches. These results reveal that large clathrin assemblies are active regions of endocytosis in mammalian cells and highlight the importance of understanding the mechanistic basis for this organization.