Molecular aspects of the endocytic pathway

A retrotransposon-derived DNA zip code internalizes myeloma cells through Clathrin-Rab5a-mediated endocytosis

Introduction

We have demonstrated that transposons derived from ctDNA can be transferred between cancer cells. The present research aimed to investigate the cellular uptake and intracellular trafficking of Multiple Myeloma-zip code (MM-ZC), a cell-specific zip code, in myeloma cell lines. We demonstrated that MM-ZC uptake by myeloma cells was concentration-, time- and cell-type-dependent.

Methods

Flow cytometry and confocal microscopy methods were used to identify the level of internalization of the zip codes in MM cells. To screen for the mechanism of internalization, we used multiple inhibitors of endocytosis. These experiments were followed by biotin pulldown and confocal microscopy for validation. Single interference RNA (siRNA) targeting some of the proteins involved in endocytosis was used to validate the role of this pathway in ZC cell internalization.

Results

Endocytosis inhibitors identified that Monensin and Chlorpromazine hydrochloride significantly reduced MM-ZC internalization. These findings suggested that Clathrin-mediated endocytosis and endosomal maturation play a crucial role in the cellular uptake of MM-ZC. Biotin pulldown and confocal microscopic studies revealed the involvement of proteins such as Clathrin, Rab5a, Syntaxin-6, and RCAS1 in facilitating the internalization of MM-ZC. Knockdown of Rab5a and Clathrin proteins reduced cellular uptake of MM-ZC and conclusively demonstrated the involvement of Clathrin-Rab5a pathways in MM-ZC endocytosis. Furthermore, both Rab5a and Clathrin reciprocally affected their association with MM-ZC when we depleted their proteins by siRNAs. Additionally, the loss of Rab5a decreased the Syntaxin-6 association with MMZC but not vice versa. Conversely, MM-ZC treatment enhanced the association between Clathrin and Rab5a.

Conclusion

Overall, the current study provides valuable insights into the cellular uptake and intracellular trafficking of MM-ZC in myeloma cells. Identifying these mechanisms and molecular players involved in MM-ZC uptake contributes to a better understanding of the delivery and potential applications of cell-specific Zip-Codes in gene delivery and drug targeting in cancer research.

A genome-wide CRISPR functional survey of the human phagocytosis molecular machinery

Phagocytosis, the process by which cells engulf large particles, plays a vital role in driving tissue clearance and host defense. Its dysregulation is connected to autoimmunity, toxic accumulation of proteins, and increased risks for infections. Despite its importance, we lack full understanding of all molecular components involved in the process. To create a functional map in human cells, we performed a genome-wide CRISPRko FACS screen that identified 716 genes. Mapping those hits to a comprehensive protein-protein interaction network annotated for functional cellular processes allowed retrieval of protein complexes identified multiple times and detection of missing phagocytosis regulators. In addition to known components, such as the Arp2/3 complex, the vacuolar-ATPase-Rag machinery, and the Wave-2 complex, we identified and validated new phagocytosis-relevant functions, including the oligosaccharyltransferase complex (MAGT1/SLC58A1, DDOST, STT3B, and RPN2) and the hypusine pathway (eIF5A, DHPS, and DOHH). Overall, our phagocytosis network comprises elements of cargo uptake, shuffling, and biotransformation through the cell, providing a resource for the identification of potential novel drivers for diseases of the endo-lysosomal system. Our approach of integrating protein-protein interaction offers a broadly applicable way to functionally interpret genome-wide screens.

Seneca Valley virus enters cells through multiple pathways and traffics intracellularly via the endolysosomal pathway

Seneca Valley virus (SVV, also known as Senecavirus A), an oncolytic virus, is a nonenveloped, positive-strand RNA virus and the sole member of the genus Senecavirus within the family Picornaviridae. The mechanisms of SVV entry into cells are currently almost unknown. In the present study, we found that SVV entry into HEK293T cells is acidic pH-dependent by using ammonium chloride (NH₄Cl) and chloroquine, both of which could inhibit SVV infection. We confirmed that dynamin II is required for SVV entry by using dynasore, silencing the dynamin II protein, or expressing the dominant-negative (DN) K44A mutant of dynamin II. Then, we discovered that chlorpromazine (CPZ) treatment or knockdown of the clathrin heavy chain (CLTC) protein significantly inhibited SVV infection. In addition, overexpression of CLTC promoted SVV infection. Caveolin-1 and membrane cholesterol were also required for SVV endocytosis. Notably, utilizing genistein, EIPA or nocodazole, we observed that macropinocytosis and microtubules are not involved in SVV entry. Furthermore, overexpression of the Rab7 and Rab9 proteins but not the Rab5 or Rab11 proteins promoted SVV infection. The findings were further validated by the knockdown of four Rabs and Lamp1 proteins, indicating that after internalization, SVV is transported from late endosomes to the trans-Golgi network (TGN) or lysosomes, respectively, eventually releasing its RNA into the cytosol from the lysosomes. Our findings concretely revealed SVV endocytosis mechanisms in HEK293T cells and provided an insightful theoretical foundation for further research into SVV oncolytic mechanisms.

Copper dependent ERK1/2 phosphorylation is essential for the viability of neurons and not glia

Intracellular copper [Cu(I)] has been hypothesized to play role in the differentiation of the neurons. This necessitates understanding the role of Cu(I) not only in the neurons but also in the glia considering their anatomical proximity, contribution towards ion homeostasis, and neurodegeneration. In this study, we did a systematic investigation of the changes in the cellular copper homeostasis during neuronal and glial differentiation and the pathways triggered by them. Our study demonstrates increased mRNA for the plasma membrane copper transporter CTR1 leading to increased Cu(I) during the neuronal (PC-12) differentiation. ATP7A is retained in the trans-Golgi network (TGN) despite high Cu(I) demonstrating its utilization towards the neuronal differentiation. Intracellular copper triggers pathways essential for neurite generation and ERK1/2 activation during the neuronal differentiation. ERK1/2 activation also accompanies the differentiation of the foetal brain derived neuronal progenitor cells. The study demonstrates that ERK1/2 phosphorylation is essential for the viability of the neurons. In contrast, differentiated C-6 (glia) cells contain low intracellular copper and significant downregulation of the ERK1/2 phosphorylation demonstrating that ERK1/2 activation does not regulate the viability of the glia. But ATP7A shows vesicular localization despite low copper in the glia. In addition to the TGN, ATP7A localizes into RAB11 positive recycling endosomes in the glial neurites. Our study demonstrates the role of copper dependent ERK1/2 phosphorylation in the neuronal viability. Whereas glial differentiation largely involves sequestration of Cu(I) into the endosomes potentially (i) for ready release and (ii) rendering cytosolic copper unavailable for pathways like the ERK1/2 activation.

Early endosome autoantigen 1 regulates IL-1β release upon caspase-1 activation independently of gasdermin D membrane permeabilization

Unconventional protein secretion represents an important process of the inflammatory response. The release of the pro-inflammatory cytokine interleukin (IL)-1β which burst during pyroptosis as a consequence of gasdermin D plasma membrane pore formation, can also occur through other unconventional secretion pathways dependent on caspase-1 activation. However, how caspase-1 mediates cytokine release independently of gasdermin D remains poorly understood. Here we show that following caspase-1 activation by different inflammasomes, caspase-1 cleaves early endosome autoantigen 1 (EEA1) protein at Asp^127/132. Caspase-1 activation also results in the release of the endosomal EEA1 protein in a gasdermin D-independent manner. EEA1 knock-down results in adecreased release of caspase-1 and IL-1β, but the pyroptotic release of other inflammasome components and lactate dehydrogenase was not affected. This study shows how caspase-1 control the release of EEA1 and IL-1β in a pyroptotic-independent manner.

Fluvastatin inhibits Rab5-mediated IKs internalization caused by chronic Ca2+-dependent PKC activation

Statins, in addition to their cholesterol lowering effects, can prevent isoprenylation of Rab GTPase proteins, a key protein family for the regulation of protein trafficking. Rab-GTPases have been shown to be involved in the control of membrane expression level of ion channels, including one of the major cardiac repolarizing channels, IKs. Decreased IKs function has been observed in a number of disease states and associated with increased propensity for arrhythmias, but the mechanism underlying IKs decrease remains elusive. Ca²⁺-dependent PKC isoforms (cPKC) are chronically activated in variety of human diseases and have been suggested to acutely regulate IKs function. We hypothesize that chronic cPKC stimulation leads to Rab-mediated decrease in IKs membrane expression, and that can be prevented by statins. In this study we show that chronic cPKC stimulation caused a dramatic Rab5 GTPase-dependent decrease in plasma membrane localization of the IKs pore forming subunit KCNQ1, reducing IKs function. Our data indicates fluvastatin inhibition of Rab5 restores channel localization and function after cPKC-mediated channel internalization. Our results indicate a novel statin anti-arrhythmic effect that would be expected to inhibit pathological electrical remodeling in a number of disease states associated with high cPKC activation. Because Rab-GTPases are important regulators of membrane trafficking they may underlie other statin pleiotropic effects.

Impaired endo-lysosomal membrane integrity accelerates the seeding progression of α-synuclein aggregates

In neurodegenerative diseases, seeding is a process initiated by the internalization of exogenous protein aggregates. Multiple pathways for internalization of aggregates have been proposed, including direct membrane penetration and endocytosis. To decipher the seeding mechanisms of alpha-synuclein (αS) aggregates in human cells, we visualized αS aggregation, endo-lysosome distribution, and endo-lysosome rupture in real-time. Our data suggest that exogenous αS can seed endogenous cytoplasmic αS by either directly penetrating the plasma membrane or via endocytosis-mediated endo-lysosome rupture, leading to formation of endo-lysosome-free or endo-lysosome-associated αS aggregates, respectively. Further, we demonstrate that αS aggregates isolated from postmortem human brains with diffuse Lewy body disease (DLBD) preferentially show endocytosis-mediated seeding associated with endo-lysosome rupture and have significantly reduced seeding activity compared to recombinant αS aggregates. Colocalization of αS pathology with galectin-3 (a marker of endo-lysosomal membrane rupture) in the basal forebrain of DLBD, but not in age-matched controls, suggests endo-lysosome rupture is involved in the formation of αS pathology in humans. Interestingly, cells with endo-lysosomal membrane permeabilization (LMP) are more vulnerable to the seeding effects of αS aggregates. This study suggests that endo-lysosomal impairment in neurons might play an important role in PD progression.

Regulating the Efficacy of Inhibition Through Trafficking of γ-Aminobutyric Acid Type A Receptors

Trafficking of anesthetic-sensitive receptors within the plasma membrane, or from one cellular component to another, occurs continuously. Changes in receptor trafficking have implications in altering anesthetic sensitivity. γ-Aminobutyric acid type A receptors (GABAARs) are anion-permeable ion channels and are the major class of receptor in the adult mammalian central nervous system that mediates inhibition. GABAergic signaling allows for precise synchronized firing of action potentials within brain circuits that is critical for cognition, behavior, and consciousness. This precision depends upon tightly controlled trafficking of GABAARs into the membrane. General anesthetics bind to and allosterically enhance GABAARs by prolonging the open state of the receptor and thereby altering neuronal and brain circuit activity. Subunit composition and GABAAR localization strongly influence anesthetic end points; therefore, changes in GABAAR trafficking could have significant consequences to anesthetic sensitivity. GABAARs are not static membrane structures but are in a constant state of flux between extrasynaptic and synaptic locations and are continually endocytosed and recycled from and to the membrane. Neuronal activity, posttranslational modifications, and some naturally occurring and synthetic compounds can influence the expression and trafficking of GABAARs. In this article, we review GABAARs, their trafficking, and how phosphorylation of GABAAR subunits can influence the surface expression and function of the receptor. Ultimately, alterations of GABAAR trafficking could modify anesthetic end points, both unintentionally through pathologic processes but potentially as a therapeutic target to adjust anesthetic-sensitive GABAARs.

An endocytosis pathway initiated through neuropilin-1 and regulated by nutrient availability

Neuropilins (NRPs) are trans-membrane receptors involved in axon guidance and vascular development. Many growth factors and other signalling molecules bind to NRPs through a carboxy (C)-terminal, basic sequence motif (C-end Rule or CendR motif). Peptides with this motif (CendR peptides) are taken up into cells by endocytosis. Tumour-homing CendR peptides penetrate through tumour tissue and have shown utility in enhancing drug delivery into tumours. Here we show, using RNAi screening and subsequent validation studies, that NRP1-mediated endocytosis of CendR peptides is distinct from known endocytic pathways. Ultrastructurally, CendR endocytosis resembles macropinocytosis, but is mechanistically different. We also show that nutrient-sensing networks such as mTOR signalling regulate CendR endocytosis and subsequent intercellular transport of CendR cargo, both of which are stimulated by nutrient depletion. As CendR is a bulk transport pathway, our results suggest a role for it in nutrient transport; CendR-enhanced drug delivery then makes use of this natural pathway.

Gd loading by hypotonic swelling: an efficient and safe route for cellular labeling

Cells incubated in hypo-osmotic media swell and their membranes become leaky. The flow of water that enters the cells results in the net transport of molecules present in the incubation medium directly into the cell cytoplasm. This phenomenon has been exploited to label cells with MRI Gd-containing contrast agents. It has been found that, in the presence of 100 mM Gd-HPDO3A in an incubation medium characterized by an overall osmolarity of 160 mOsm l⁻¹, each cell is loaded with amounts of paramagnetic complex ranging from 2 × 10⁹ to 2 × 10¹⁰ depending on the cell type. To obtain more insight into the determinants of cellular labeling by the 'hypo-osmotic shock' methodology, a study on cell viability, proliferation rate and cell morphology was carried out on J774A.1 and K562 cells as representative of cells grown in adhesion and suspended ones, respectively. Moreover a comparison of the efficiency of the proposed method with established cell labeling procedures such as pinocytosis and electroporation was carried out. Finally, the effects of the residual electric charge, the size and some structural features of the metal complex were investigated. In summary, the 'hypotonic shock' methodology appears to be an efficient and promising tool to pursue cellular labeling with paramagnetic complexes. Its implementation is straightforward and one may foresee that it will be largely applied in in vitro cellular labeling of many cell types.

Kinetic analysis of mouse brain proteome alterations following Chikungunya virus infection before and after appearance of clinical symptoms

Recent outbreaks of Chikungunya virus (CHIKV) infection have been characterized by an increasing number of severe cases with atypical manifestations including neurological complications. In parallel, the risk map of CHIKV outbreaks has expanded because of improved vector competence. These features make CHIKV infection a major public health concern that requires a better understanding of the underlying physiopathological processes for the development of antiviral strategies to protect individuals from severe disease. To decipher the mechanisms of CHIKV infection in the nervous system, a kinetic analysis on the host proteome modifications in the brain of CHIKV-infected mice sampled before and after the onset of clinical symptoms was performed. The combination of 2D-DIGE and iTRAQ proteomic approaches, followed by mass spectrometry protein identification revealed 177 significantly differentially expressed proteins. This kinetic analysis revealed a dramatic down-regulation of proteins before the appearance of the clinical symptoms followed by the increased expression of most of these proteins in the acute symptomatic phase. Bioinformatic analyses of the protein datasets enabled the identification of the major biological processes that were altered during the time course of CHIKV infection, such as integrin signaling and cytoskeleton dynamics, endosome machinery and receptor recycling related to virus transport and synapse function, regulation of gene expression, and the ubiquitin-proteasome pathway. These results reveal the putative mechanisms associated with severe CHIKV infection-mediated neurological disease and highlight the potential markers or targets that can be used to develop diagnostic and/or antiviral tools.

Exocytosis of nanoparticles from cells: role in cellular retention and toxicity

Over the past decade, nanoparticles (NPs) have been increasingly developed in various biomedical applications such as cell tracking, biosensing, contrast imaging, targeted drug delivery, and tissue engineering. Their versatility in design and function has made them an attractive, alternative choice in many biological and biomedical applications. Cellular responses to NPs, their uptake, and adverse biological effects caused by NPs are rapidly-growing research niches. However, NP excretion and its underlying mechanisms and cell signaling pathways are yet elusive. In this review, we present an overview of how NPs are handled intracellularly and how they are excreted from cells following the uptake. We also discuss how exocytosis of nanomaterials impacts both the therapeutic delivery of nanoscale objects and their nanotoxicology.

A dileucine motif is involved in plasma membrane expression and endocytosis of rat sodium taurocholate cotransporting polypeptide (Ntcp)

The sodium taurocholate cotransporting polypeptide (Ntcp) is the major uptake transporter for bile salts into liver parenchymal cells, and PKC-mediated endocytosis was shown to regulate the number of Ntcp molecules at the plasma membrane. In this study, mechanisms of Ntcp internalization were analyzed by flow cytometry, immunofluorescence, and Western blot analyses in HepG2 cells. PKC activation induced endocytosis of Ntcp from the plasma membrane by ~30%. Endocytosis of Ntcp was clathrin dependent and was followed by lysosomal degradation. A dileucine motif located in the third intracellular loop of Ntcp was essential for endocytosis but also for processing and plasma membrane targeting, suggesting a dual function of this motif for intracellular trafficking of Ntcp. Mutation of two of five potential phosphorylation sites surrounding the dileucine motif (Thr225 and Ser226) inhibited PKC-mediated endocytosis. In conclusion, we could identify a motif, which is critical for Ntcp plasma membrane localization. Endocytic retrieval protects hepatocytes from elevated bile salt concentrations and is of special interest, because NTCP has been identified as a receptor for the hepatitis B and D virus.

Internalization of Met requires the co-receptor CD44v6 and its link to ERM proteins

Receptor Tyrosine Kinases (RTKs) are involved in many cellular processes and play a major role in the control of cell fate. For these reasons, RTK activation is maintained under tight control. Met is an essential RTK that induces proliferation, differentiation, migration, survival and branching morphogenesis. Deregulation of Met by overexpression, amplification or lack of effective degradation leads to cancer and metastasis. We have shown that Met relies on CD44v6 for its activation and for signaling in several cancer cell lines and also in primary cells. In this paper, we show that internalization of Met is dependent on CD44v6 and the binding of Ezrin to the CD44v6 cytoplasmic domain. Both CD44v6 and Met are co-internalized upon Hepatocyte Growth Factor induction suggesting that Met-induced signaling from the endosomes relies on its collaboration with CD44v6 and the link to the cytoskeleton provided by ERM proteins.

Productive entry pathways of human rhinoviruses

Currently, complete or partial genome sequences of more than 150 human rhinovirus (HRV) isolates are known. Twelve species A use members of the low-density lipoprotein receptor family for cell entry, whereas the remaining HRV-A and all HRV-B bind ICAM-1. HRV-Cs exploit an unknown receptor. At least all A and B type viruses depend on receptor-mediated endocytosis for infection. In HeLa cells, they are internalized mainly by a clathrin- and dynamin-dependent mechanism. Upon uptake into acidic compartments, the icosahedral HRV capsid expands by ~4% and holes open at the 2-fold axes, close to the pseudo-3-fold axes and at the base of the star-shaped dome protruding at the vertices. RNA-protein interactions are broken and new ones are established, the small internal myristoylated capsid protein VP4 is expelled, and amphipathic N-terminal sequences of VP1 become exposed. The now hydrophobic subviral particle attaches to the inner surface of endosomes and transfers its genomic (+) ssRNA into the cytosol. The RNA leaves the virus starting with the poly(A) tail at its 3′-end and passes through a membrane pore contiguous with one of the holes in the capsid wall. Alternatively, the endosome is disrupted and the RNA freely diffuses into the cytoplasm.

Imaging lysosomal enzyme activity in live cells using self-quenched substrates

Endocytosis, the internalization and transport of extracellular cargo, is an essential cellular process. The ultimate step in endocytosis is the intracellular degradation of extracellular cargo for use by the cell. While live cell imaging and single particle tracking have been well-utilized to study the internalization and transport of cargo, the final degradation step has required separate biochemical assays. We describe the use of self-quenched endocytic cargo to image the intracellular transport and degradation of endocytic cargo directly in live cells. We first outline the fluorescent labeling and quantification of two common endocytic cargos: a protein, bovine serum albumin, and a lipid nanoparticle, low-density lipoprotein. In vitro measurements confirm that self-quenching is a function of the number of fluorophores bound to the protein or particle and that recovery of the fluorescent signal occurs in response to enzymatic degradation. We then use confocal fluorescence microscopy and flow cytometry to demonstrate the use of self-quenched bovine serum albumin with standard fluorescence techniques. Using live cell imaging and single particle tracking, we find that the degradation of bovine serum albumin occurs in an endo-lysosomal vesicle that is positive for LAMP1.

The fusion of early endosomes induces molecular-motor-driven tubule formation and fission

Organelles in the endocytic pathway interact and communicate through the crucial mechanisms of fusion and fission. However, any specific link between fusion and fission has not yet been determined. To study the endosomal interactions with high spatial and temporal resolution, we enlarged the endosomes by two mechanistically different methods: by expression of the MHC-class-II-associated chaperone invariant chain (Ii; or CD74) or Rab5, both of which increased the fusion rate of early endosomes and resulted in enlarged endosomes. Fast homotypic fusions were studied, and immediately after the fusion a highly active and specific tubule formation and fission was observed. These explosive tubule formations following fusion seemed to be a direct effect of fusion. The tubule formations were dependent on microtubule interactions, and specifically controlled by Kif16b and dynein. Our results show that fusion of endosomes is a rapid process that destabilizes the membrane and instantly induces molecular-motor-driven tubule formation and fission.

Endo-lysosomal vesicles positive for Rab7 and LAMP1 are terminal vesicles for the transport of dextran

The endo-lysosomal pathway is essential for intracellular transport and the degradation of extracellular cargo. The relationship between three populations of endo-lysosomal vesicles--Rab7-positive, LAMP1-positive, and both Rab7- and LAMP1-postive--was probed with fluorescence microscopy and single particle tracking. Of specific interest was determining if these vesicles were intermediate or terminal vesicles in the transport of extracellular cargo. We find that the major organelle in the endo-lysosomal pathway, both in terms of population and cargo transport, is positive for Rab7 and LAMP1. Dextran, a fluid phase cargo, shifts from localization within all three populations of vesicles at 30 minutes and 1 hour to primarily LAMP1- and Rab7/LAMP1-vesicles at longer times. This demonstrates that LAMP1- and Rab7/LAMP1-vesicles are terminal vesicles in the endo-lysosomal pathway. We tested two possible mechanisms for this distribution of cargo, delivery to mannose 6-phosphate receptor (M6PR)-negative vesicles and the fusion dynamics of individual vesicles. We find no correlation with M6PR but do find that Rab7-vesicles undergo significantly fewer fusion events than LAMP1- or Rab7/LAMP1-vesicles suggesting that the distribution of fluid phase cargo is driven by vesicle dynamics.

The dynamic modulation of GABA(A) receptor trafficking and its role in regulating the plasticity of inhibitory synapses

Inhibition in the adult mammalian central nervous system (CNS) is mediated by γ-aminobutyric acid (GABA). The fast inhibitory actions of GABA are mediated by GABA type A receptors (GABA(A)Rs); they mediate both phasic and tonic inhibition in the brain and are the principle sites of action for anticonvulsant, anxiolytic, and sedative-hypnotic agents that include benzodiazepines, barbiturates, neurosteroids, and some general anesthetics. GABA(A)Rs are heteropentameric ligand-gated ion channels that are found concentrated at inhibitory postsynaptic sites where they mediate phasic inhibition and at extrasynaptic sites where they mediate tonic inhibition. The efficacy of inhibition and thus neuronal excitability is critically dependent on the accumulation of specific GABA(A)R subtypes at inhibitory synapses. Here we evaluate how neurons control the number of GABA(A)Rs on the neuronal plasma membrane together with their selective stabilization at synaptic sites. We then go on to examine the impact that these processes have on the strength of synaptic inhibition and behavior.

The blood-testis barrier: the junctional permeability, the proteins and the lipids

The elucidation of how individual components of the Sertoli cell junctional complexes form and are dismantled to allow not only individual cells but whole syncytia of germinal cells to migrate from the basal to the lumenal compartment of the seminiferous epithelium without causing a permeability leak in the blood-testis barrier is amongst the most enigmatic yet, challenging and timely questions in testicular physiology. The intriguing key event in this process is how the barrier modulates its permeability during the periods of formation and dismantling of individual Sertoli cell junctions. The purpose of this review is therefore to first provide a reliable account on the normal formation, maintenance and dismantling process of the Sertoli cells junctions, then to assess the influence of the expression of their individual proteins, of the cytoskeleton associated with the junctions, and of the lipid content in the seminiferous tubules on the regulation of the their permeability barrier function. To help focus on the formation and dismantling of the Sertoli cell junctions, several considerations are based on data gleaned not only from rodents but from seasonal breeders as well because these animal models are characterized by exhaustive periods of junction assembly during development and the onset of the seasonal re-initiation of spermatogenesis as well as by an extensive junction dismantling period at the beginning of testicular regression, something unavailable in normal physiological conditions in continual breeders. Thus, the modulation of the permeability barrier function of the Sertoli cell junctions is analyzed in the physiological context of the blood-epidydimis barrier and in particular of the blood-testis barrier rather than in the context of a detailed account of the molecular composition and signalisation pathways of cell junctions. Moreover, the considerations discussed in this review are based on measurements performed on seminiferous tubule-enriched fractions gleaned at regular time intervals during development and the annual reproductive cycle.

Cell surface binding and internalization of aβ modulated by degree of aggregation

The amyloid peptides, Aβ40 and Aβ42, are generated through endoproteolytic cleavage of the amyloid precursor protein. Here we have developed a model to investigate the interaction of living cells with various forms of aggregated Aβ40/42. After incubation at endosomal pH 6, we observed a variety of Aβ conformations after 3 (Aβ³), 24 (Aβ²⁴), and 90 hours (Aβ⁹⁰). Both Aβ42²⁴ and Aβ40²⁴ were observed to rapidly bind and internalize into differentiated PC12 cells, leading to accumulation in the lysosome. In contrast, Aβ40/42⁹⁰ were both found to only weakly associate with cells, but were observed as the most aggregated using dynamic light scattering and thioflavin-T. Internalization of Aβ40/42²⁴ was inhibited with treatment of monodansylcadaverine, an endocytosis inhibitor. These studies indicate that the ability of Aβ40/42 to bind and internalize into living cells increases with degree of aggregation until it reaches a maximum beyond which its ability to interact with cells diminishes drastically.

Single particle tracking as a method to resolve differences in highly colocalized proteins

Single particle tracking fluorescence microscopy was used to study two late endosomal proteins, Rab7 and LAMP1, that appear to be highly colocalized in static fluorescence microscopy images. Imaging these proteins simultaneously reveals that Rab7 and LAMP1 undergo periods of separation within the cell. Single particle tracking carried out during these periods of separation shows that Rab7-vesicles have greater velocities, but undergo less efficient transport than LAMP1-vesicles. This research demonstrates the use of single particle tracking as a tool to resolve functional differences in highly colocalized proteins in intact live cells.

A possible role of myristoylated alanine-rich C kinase substrate in endocytic pathway of Alzheimer's disease

It is believed that amyloid-beta peptide (Abeta) plays a central role in the pathogenesis of Alzheimer's disease (AD). Thus, the process of amyloid precursor protein (APP) cleavage is a key event and has raised much attention in the field of AD research. It is proposed that APP, beta- and gamma-secretases are all located on the lipid raft, and the meeting of them is an indispensable step for Abeta generation. Endocytosis can lead to clustering of APP, beta- and gamma-secretases from separate smaller lipid rafts into a larger one. On the other hand, for myristoylated alanine-rich C kinase substrate (MARCKS), phosphorylation by protein kinase C (PKC) or interaction with Ca(2+) can lead to its release from membrane into cytoplasm. This process induces the release of actins and phosphatidylinositol 4, 5-bisphosphate (PIP2), which are important factors for endocytosis. Thus, the present review proposes that MARCKS may be implicated in Abeta generation, by modulating free PIP2 level and actin movement, causing endocytosis.

Intracellular degradation of low-density lipoprotein probed with two-color fluorescence microscopy

The intracellular vesicle-mediated degradation of extracellular cargo is an essential cellular function. Using two-color single particle tracking fluorescence microscopy, we have probed the intracellular degradation of low-density lipoprotein (LDL) in living cells. To detect degradation, individual LDL particles were heavily labeled with multiple fluorophores resulting in a quenched fluorescent signal. The degradation of the LDL particle then resulted in an increase in fluorescence. Endocytic vesicles were fluorescently labeled with variants of GFP. We imaged the transient colocalization of LDL with endocytic vesicles while simultaneously measuring the intensity of the LDL particle as an indicator of degradation. These studies demonstrate that late endosomes are active sites of degradation for LDL. Measurement of the time from colocalization with lysosome-associated membrane protein 1 (LAMP1) vesicles to degradation suggests that LAMP1-vesicles initiate the degradative event. Observing degradation as it occurs in living cells makes it possible to describe the complete endocytic pathway of LDL from internalization to degradation. More generally, this research provides a model for the intracellular degradation of extracellular cargo and a method for its study in living cells.

Intracellular trafficking of silicon particles and logic-embedded vectors

Mesoporous silicon particles show great promise for use in drug delivery and imaging applications as carriers for second-stage nanoparticles and higher order particles or therapeutics. Modulation of particle geometry, surface chemistry, and porosity allows silicon particles to be optimized for specific applications such as vascular targeting and avoidance of biological barriers commonly found between the site of drug injection and the final destination. In this study, the intracellular trafficking of unloaded carrier silicon particles and carrier particles loaded with secondary iron oxide nanoparticles was investigated. Following cellular uptake, membrane-encapsulated silicon particles migrated to the perinuclear region of the cell by a microtubule-driven mechanism. Surface charge, shape (spherical and hemispherical) and size (1.6 and 3.2 microm) of the particle did not alter the rate of migration. Maturation of the phagosome was associated with an increase in acidity and acquisition of markers of late endosomes and lysosomes. Cellular uptake of iron oxide nanoparticle-loaded silicon particles resulted in sorting of the particles and trafficking to unique destinations. The silicon carriers remained localized in phagosomes, while the second stage iron oxide nanoparticles were sorted into multi-vesicular bodies that dissociated from the phagosome into novel membrane-bound compartments. Release of iron from the cells may represent exocytosis of iron oxide nanoparticle-loaded vesicles. These results reinforce the concept of multi-functional nanocarriers, in which different particles are able to perform specific tasks, in order to deliver single- or multi-component payloads to specific sub-cellular compartments.

Stimuli-responsive smart nanogels for cancer diagnostics and therapy

This article discusses stimuli-responsive poly(ethylene glycol) (PEG)-coated (PEGylated) nanogels and their biomedical applications. Preparation and characterization of stimuli-responsive PEGylated nanogels composed of a crosslinked poly(2-[N,N-diethylamino]ethyl methacrylate) (PEAMA) core and PEG tethered chains are initially described. Stimuli-responsive PEGylated nanogels show unique properties and functions in synchronizing with the reversible volume phase transition of the PEAMA core in response to the extracellular pH (7-6.5) of a tumor environment as well as endosomal/lysosomal pH (6.5-5.0) and temperature. We list several biomedical applications of stimuli-responsive PEGylated nanogels, including (19)F magnetic resonance spectroscopic imaging (MRS/I) probe to visualize acidosis (tumor tissue), intracellular drug and siRNA delivery, antennas for cancer photothermal therapy and apoptosis probe for monitoring response to cancer therapy. Thus, stimuli-responsive PEGylated nanogels can be utilized as smart nanomedicines for cancer diagnostics and therapy.

Metal ions and electrolytes regulate the dissociation of heme from human hemopexin at physiological pH

The stability of the hemopexin-heme (Hx-heme) complex to dissociation of the heme prosthetic group has been examined in bicarbonate buffers in the presence and absence of various divalent metal ions. In NH(4)HCO(3) buffer (pH 7.4, 20 mm, 25 degrees C) containing Zn(2+) (100 microm), 14% of the heme dissociates from this complex (4.5 microm) within 10 min, and 50% dissociates within 2 h. In the absence of metal ions, the rate of dissociation of this complex is far lower, is decreased further in KHCO(3) solution, and is minimal in NaHCO(3). In NH(4)HCO(3) buffer, dissociation of the Hx-heme complex is accelerated by addition of divalent metals with decreasing efficiency in the order Zn(2+) > Cu(2+) >> Ni(2+) > Co(2+)>>Mn(2+). Addition of Ca(2+) prior to addition of Zn(2+) stabilizes the Hx-heme complex to dissociation of the heme group, and addition of Ca(2+) after Zn(2+)-induced dissociation of the Hx-heme complex results in re-formation of the Hx-heme complex. These effects are greatly accelerated at 37 degrees C and diminished in other buffers. Overall, the solution conditions that promote formation of the Hx-heme complex are similar to those found in blood plasma, and conditions that promote release of heme are similar to those that the Hx-heme complex should encounter in endosomes following endocytosis of the complex formed with its hepatic receptor.

Lysosome-dependent Ca(2+) release response to Fas activation in coronary arterial myocytes through NAADP: evidence from CD38 gene knockouts

Activation of the death receptor Fas has been implicated in the development of vascular injury or disease, but most studies have focused on its role in the regulation of cell apoptosis and growth. The present study was designed to examine the early response of coronary artery to Fas activation by its ligand, FasL. The hypothesis being tested is that CD38 signaling pathway mediates FasL-induced intracellular Ca(2+) release through nicotinic acid adenine dinucleotide phosphate (NAADP) in mouse coronary arterial myocytes (CAMs) and thereby produces vasoconstriction in coronary arteries. HPLC analysis demonstrated that FasL markedly increased NAADP production in CAMs from wild-type mice (CD38(+/+)) but not in cells from CD38 knockout (CD38(-/-)) mice. Using fluorescent Ca(2+) imaging analysis, we found that FasL (10 ng/ml) significantly increased Ca(2+) release from 142.5 +/- 22.5 nM at the basal level to 509.4 +/- 64.3 nM in CD38(+/+) CAMs but not in CD38(-/-) CAMs. However, direct delivery of NAADP, the CD38 metabolite, into CD38(-/-) CAMs still markedly increased Ca(2+) release, which could be significantly attenuated by a lysosomal function inhibitor, bafilomycin A1 (Baf), or a NAADP antagonist, pyridoxalphosphate-6-azophenyl-2-disulfonic acid. Confocal microscopy further demonstrated that FasL produced a typical two-phase Ca(2+) release with a local Ca(2+) burst from lysosomes, followed by a global Ca(2+) response in CD38(+/+) CAMs. In isolated perfused septal coronary arteries from CD38(+/+) mice, FasL was found to significantly increase U-46619-induced vasoconstriction from 29.2 +/- 7.3 to 63.2 +/- 10.3%, which was abolished by Baf (100 nM). These results strongly indicate that the early response of CAMs to FasL is to increase intracellular Ca(2+) levels and enhance the vascular reactivity through stimulation of NAADP production and lysosome-associated two-phase Ca(2+) release in coronary arteries.

Cellular endocytosis and gene delivery

Endocytosis is the process by which cells take up macromolecules from the surrounding medium. The best-characterized process is the so-called clathrin-dependent endocytosis, although much is also currently known about clathrin-independent endocytic processes such as those involving caveolae and lipid rafts. An understanding of endocytosis and the cellular trafficking that occurs thereafter has a great deal of relevance to current molecular medicine. Gene therapy, which is presently being investigated for its therapeutic potential in treating immunodeficiency and metabolic diseases, cancer and heart disease, employs a variety of viral and nonviral vectors, which can be delivered to the target cells of the body and are subsequently endocytosed and dissembled. A variety of vectors can be used to deliver genes to organs in vivo or cells ex vivo. Various routes of vector delivery have been investigated. The mechanisms by which vectors such as adenoviruses, adeno-associated viruses, retroviruses and liposomes enter the cell are increasingly being investigated as the effort to increase the efficiency of gene therapy continues. This review focuses on mechanisms of endocytosis and how they relate to the internal trafficking of viral and nonviral vectors in gene therapy.

Visualization of phospholipid particle fusion induced by duramycin

We visualized nanometer-scale phospholipid particle fusion by scanning tunneling microscopy (STM) on an alkanethiol-modified gold substrate, induced by duramycin, a tetracyclic antibiotic peptide with 19 amino residues. Ultrasonic homogenization generated a suspension mainly consisting of minimal lipid particles (MLP) from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) in a phosphate buffer solution, confirmed by dynamic light scattering (DLS). In situ STM discerned individual MLP as particles (diameter approximately 8 nm) spread on Au(111), modified with alkanethiol, within the suspension. The MLP became fragile by the presence of duramycin, and the MLP were easily scratched by the scanning tip into multilayers along the surface. This process of particle fusion on the gold surface coincides with the aggregation of MLP in the suspension, observed by DLS. It was demonstrated that STM is capable of discerning and monitoring the nanometer-scale features of phospholipid particles altered by antibiotics with biochemical impact. STM might allow in situ, real-space, nanometer-scale observations of minute particles composed of phospholipids within the real cells with the highest magnification ratio.

Sdmg1 is a component of secretory granules in mouse secretory exocrine tissues

Sdmg1 is a conserved eukaryotic transmembrane protein that is mainly expressed in the gonads where it may have a role in mediating signaling between somatic cells and germ cells. In this study we demonstrate that secretory exocrine cells in the pancreas, salivary gland, and mammary gland also express Sdmg1. Furthermore, we show that Sdmg1 expression is up-regulated during pancreas development when regulated secretory granules start to appear, and that Sdmg1 colocalizes with secretory granule markers in adult pancreatic acinar cells. In addition, we show that Sdmg1 co-purifies with secretory granules during subcellular fractionation of the pancreas and that Sdmg1 and the secretory granule marker Vamp2 are localized to distinct subdomains in the secretory granule membrane. These data suggest that Sdmg1 is a component of regulated secretory granules in exocrine secretory cells and that the developmental regulation of Sdmg1 expression is related to a role for Sdmg1 in post-Golgi membrane trafficking.

GABA(A) Receptor Dynamics and Constructing GABAergic Synapses

GABA_A receptors are located on the majority of neurons in the central and peripheral nervous system, where they mediate important actions of the neurotransmitter gamma-aminobutyric acid. Early in development the trophic properties of GABA allow a healthy development of the nervous system. Most neurons have a high intracellular Cl-concentration early in life due to the late functional expression of the Cl-pump KCC2, therefore GABA has excitatory effects at this stage. Upon higher expression and activation of KCC2 GABA takes on its inhibitory effects while glutamate functions as the major excitatory neurotransmitter. Like all multisubunit membrane proteins the GABA_A receptor is assembled in the ER and travels through the Golgi and remaining secretory pathway to the cell surface, where it mediates GABA actions either directly at the synapses or at extrasynaptic sites responding to ambient GABA to provide a basal tonic inhibitory state. In order to adapt to changing needs and information states, the GABAergic system is highly dynamic. That includes subtype specific trafficking to different locations in the cell, regulation of mobility by interaction with scaffold molecules, posttranslational modifications, that either directly affect channel function or the interaction with other proteins and finally the dynamic exchange between surface and intracellular receptor pools, that either prepare receptors for recycling to the surface or degradation. Here we give an overview of the current understanding of GABA_A receptor functional and molecular dynamics that play a major part in maintaining the balance between excitation and inhibition and in changes in network activity.

From endocytosis to tumors through asymmetric cell division of stem cells

Recent studies in vertebrate and invertebrate model organisms uncover the importance of endocytosis for biased signaling during asymmetric cell division. In stem cells, perturbing polarity and asymmetric division affect their selfrenewal causing exponential proliferation, thereby giving rise to cancer. An emerging pattern is that endocytosis controls asymmetric cell division, which underlies stem cell selfrenewal and defective selfrenewal is on the basis of tumorigenesis caused by cancer stem cells.

Cellular and molecular characterization of an embryonic cell line (BME26) from the tick Rhipicephalus (Boophilus) microplus

The cellular and molecular characteristics of a cell line (BME26) derived from embryos of the cattle tick Rhipicephalus (Boophilus) microplus were studied. The cells contained glycogen inclusions, numerous mitochondria, and vesicles with heterogeneous electron densities dispersed throughout the cytoplasm. Vesicles contained lipids and sequestered palladium meso-porphyrin (Pd-mP) and rhodamine-hemoglobin, suggesting their involvement in the autophagic and endocytic pathways. The cells phagocytosed yeast and expressed genes encoding the antimicrobial peptides (microplusin and defensin). A cDNA library was made and 898 unique mRNA sequences were obtained. Among them, 556 sequences were not significantly similar to any sequence found in public databases. Annotation using Gene Ontology revealed transcripts related to several different functional classes. We identified transcripts involved in immune response such as ferritin, serine proteases, protease inhibitors, antimicrobial peptides, heat shock protein, glutathione S-transferase, peroxidase, and NADPH oxidase. BME26 cells transfected with a plasmid carrying a red fluorescent protein reporter gene (DsRed2) transiently expressed DsRed2 for up to 5 weeks. We conclude that BME26 can be used to experimentally analyze diverse biological processes that occur in R. (B.) microplus such as the innate immune response to tick-borne pathogens.

Autophagy generates retrogradely transported organelles: a hypothesis

Nerve cells require trophic signals transmitted from the nerve terminal via the axon in order to survive and develop normally. As the axon may be more than a meter long, specialised mechanisms are needed to transmit these signals. This involves the retrograde axonal transport of signalling endosomes containing nerve growth factor (NGF) and other synaptically derived molecules. These are large, double membrane multivesicular bodies containing a mixture of all vesicle types seen in the nerve terminal. How this signalling endosome is formed and targeted for retrograde axonal transport, however, remains an open question. Here we show that members of the Rab family of proteins that are retrogradely transported indicate that the signalling endosome contains both early and recycling endosomes. In addition, we show that retrogradely transported labelled antibody to dopamine beta-hydroxylase, a marker for synaptic vesicles, co-localizes within the same signalling endosome as NGF. We further show that LC3, a marker for autophagosomes, is retrogradely transported and associates with retrogradely transported NGF. We propose that neurons have exploited the mechanism of autophagy to engulf a sample of the cytoplasmic contents of the nerve terminal to transport back to the cell body. This sample of cytoplasmic contents relays a reliable snapshot of the totality of signalling events occurring in the nerve terminal at that instant in time.

Stable isotopic labeling by amino acids in cultured primary neurons: application to brain-derived neurotrophic factor-dependent phosphotyrosine-associated signaling

Cultured primary neurons are a well established model for the study of neuronal function in vitro. Here we demonstrated that stable isotope labeling by amino acids in cell culture (SILAC) can be applied to a differentiated, non-dividing cell type such as primary neurons, and we applied this technique to assess changes in the neuronal phosphotyrosine proteome in response to stimulation by brain-derived neurotrophic factor (BDNF), an important molecule for the development and regulation of neuronal connections. We found that 13 proteins had SILAC ratios above 1.50 or below 0.67 in phosphotyrosine immunoprecipitations comparing BDNF-treated and control samples, and an additional 18 proteins had ratios above 1.25 or below 0.80. These proteins include TrkB, the receptor tyrosine kinase for BDNF, and others such as hepatocyte growth factor-regulated tyrosine kinase substrate and signal-transducing adaptor molecule, which are proteins known to regulate intracellular trafficking of receptor tyrosine kinases. These results demonstrate that the combination of primary neuronal cell culture and SILAC can be a powerful tool for the study of the proteomes of neuronal molecular and cellular dynamics.

Processing of human cathepsin D is independent of its catalytic function and auto-activation: involvement of cathepsins L and B

The current mechanism proposed for the processing and activation of the 52 kDa lysosomal aspartic protease cathepsin D (cath-D) is a combination of partial auto-activation generating a 51 kDa pseudo-cath-D, followed by enzyme-assisted maturation involving cysteine and/or aspartic proteases and yielding successively a 48 kDa intermediate and then 34 + 14 kDa cath-D mature species. Here we have investigated the in vivo processing of human cath-D in a cath-D-deficient fibroblast cell line in order to determine whether its maturation occurs through already active cath-D and/or other proteases. We demonstrate that cellular cath-D is processed in a manner independent of its catalytic function and that auto-activation is not a required step. Moreover, the cysteine protease inhibitor E-64 partially blocks processing, leading to accumulation of 52-48 kDa cath-D intermediates. Furthermore, two inhibitors, CLICK148 and CA-074Met, specific for the lysosomal cath-L and cath-B cysteine proteases induce accumulation of 48 kDa intermediate cath-D. Finally, maturation of endocytosed pro-cath-D is also independent of its catalytic function and requires cysteine proteases. We therefore conclude that the mechanism of cath-D maturation involves a fully-assisted processing similar to that of pro-renin.

Cell-specific delivery of polymeric nanoparticles to carbohydrate-tagging cells

Carbohydrates on cell surfaces contribute a variety of communications between the cell and its environment, and they have been assumed to act as markers for cellular recognition. In this research, 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer nanoparticles, which can react with specific carbohydrates of target cells, were newly prepared to serve as novel drug carriers. A water-soluble MPC polymer bearing hydrazide groups (PMBH) was synthesized by conventional radical polymerization. The MPC polymer showed amphiphilic nature and worked as an emulsifier to form nanoparticles. The nanoparticles covered with PMBH were prepared by the solvent evaporation method and exhibited monodispersity. They were approximately 200 nm in diameter and -2.0 mV in surface potential. According to a surface analysis of the nanoparticles, phosphorylcholine and hydrazide groups were observed, and the surface was fully covered with PMBH. Unnatural carbohydrates having ketone groups on human cervical carcinoma cell (HeLa) surfaces were expressed by treatment with levulinoyl mannosamine (ManLev). When the PMBH nanoparticles were in contact with the ManLev-treated HeLa cells, they accumulated in the cells. In contrast, the nanoparticles were not observed in native HeLa cells (without unnatural carbohydrates). These results indicate that the hydrazide groups of the nanoparticles selectively reacted to the ketone groups of the carbohydrates on the cell surface. The PMBH nanoparticles immobilized with anticancer drugs such as doxorubicin or paclitaxel were in contact with either ManLev-treated or untreated HeLa cells. The viability of the ManLev-treated HeLa cells was effectively reduced, but that of the untreated cells was preserved. This indicated that the anticancer drugs were selectively delivered to the ManLev-treated cells. Nonspecific cellular uptake of the nanoparticles was effectively reduced by MPC polymer coating. Furthermore, the immobilization processes of the drugs differed because of the solubility of the drugs. In conclusion, cellular-specific drug delivery by means of the novel nanoparticles was demonstrated with the selective reaction between unnatural carbohydrates on the cell surface and the hydrazide groups bearing the phosphorylcholine polymer nanoparticles.

RAB-10 regulates glutamate receptor recycling in a cholesterol-dependent endocytosis pathway

Regulated endocytosis of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptors (AMPARs) is critical for synaptic plasticity. However, the specific combination of clathrin-dependent and -independent mechanisms that mediate AMPAR trafficking in vivo have not been fully characterized. Here, we examine the trafficking of the AMPAR subunit GLR-1 in Caenorhabditis elegans. GLR-1 is localized on synaptic membranes, where it regulates reversals of locomotion in a simple behavioral circuit. Animals lacking RAB-10, a small GTPase required for endocytic recycling of intestinal cargo, are similar in phenotype to animals lacking LIN-10, a postsynaptic density 95/disc-large/zona occludens-domain containing protein: GLR-1 accumulates in large accretions and animals display a decreased frequency of reversals. Mutations in unc-11 (AP180) or itsn-1 (Intersectin 1), which reduce clathrin-dependent endocytosis, suppress the lin-10 but not rab-10 mutant phenotype, suggesting that LIN-10 functions after clathrin-mediated endocytosis. By contrast, cholesterol depletion, which impairs lipid raft formation and clathrin-independent endocytosis, suppresses the rab-10 but not the lin-10 phenotype, suggesting that RAB-10 functions after clathrin-independent endocytosis. Animals lacking both genes display additive GLR-1 trafficking defects. We propose that RAB-10 and LIN-10 recycle AMPARs from intracellular endosomal compartments to synapses along distinct pathways, each with distinct sensitivities to cholesterol and the clathrin-mediated endocytosis machinery.

An investigation of hemopexin redox properties by spectroelectrochemistry: biological relevance for heme uptake

Hemopexin (HPX) has two principal roles: it sequesters free heme in vivo for the purpose of preventing the toxic effects of this moiety, which is largely due to heme's ability to catalyze free radical formation, and it transports heme intracellularly thus limiting its availability as an iron source for pathogens. Spectroelectrochemistry was used to determine the redox potential for heme and meso-heme (mH) when bound by HPX. At pH 7.2, the heme-HPX assembly exhibits E (1/2) values in the range 45-90 mV and the mH-HPX assembly in the range 5-55 mV, depending on environmental electrolyte identity. The E (1/2) value exhibits a 100 mV positive shift with a change in pH from 7.2 to 5.5 for mH-HPX, suggesting a single proton dependent equilibrium. The E (1/2) values for heme-HPX are more positive in the presence of NaCl than KCl indicating that Na(+), as well as low pH (5.5) stabilizes ferro-heme-HPX. Furthermore, comparing KCl with K(2)HPO(4), the chloride salt containing system has a lower potential, indicating that heme-HPX is easier to oxidize. These physical properties related to ferri-/ferro-heme reduction are both structurally and biologically relevant for heme release from HPX for transport and regulation of heme oxygenase expression. Consistent with this, when the acidification of endosomes is prevented by bafilomycin then heme oxygenase-1 induction by heme-HPX no longer occurs.

Insulin-like Growth Factor Type-I Receptor Internalization and Recycling Mediate the Sustained Phosphorylation of Akt

Previously we demonstrated that insulin-like growth factor-I mediates the sustained phosphorylation of Akt, which is essential for long term survival and protection of glial progenitors from glutamate toxicity. These prosurvival effects correlated with prolonged activation and stability of the insulin-like growth factor type-I receptor. In the present study, we investigated the mechanisms whereby insulin-like growth factor-I signaling, through the insulin-like growth factor type-I receptor, mediates the sustained phosphorylation of Akt. We showed that insulin-like growth factor-I stimulation induced loss of receptors from the cell surface but that surface receptors recovered over time. Blocking receptor internalization inhibited Akt phosphorylation, whereas inhibition of receptor trafficking blocked receptor recovery at the cell surface and the sustained phosphorylation of Akt. Moreover the insulin-like growth factor type-I receptor localized with the transferrin receptor and Rab11-positive endosomes in a ligand-dependent manner, further supporting the conclusion that this receptor follows a recycling pathway. Our results provide evidence that ligand stimulation leads to internalization of the insulin-like growth factor type-I receptor, which mediates Akt phosphorylation, and that receptor recycling sustains Akt phosphorylation in glial progenitors. Mathematical modeling of receptor trafficking further supports these results and predicts an additional kinetic state of the receptor consistent with sustained Akt phosphorylation.

Regulation of excitation by GABA(A) receptor internalization

Neuronal inhibition is of paramount importance in maintaining the delicate and dynamic balance between excitatory and inhibitory influences in the central nervous system. GABA (gamma-aminobutyric acid), the primary inhibitory neurotransmitter in brain, exerts its fast inhibitory effects through ubiquitously expressed GABA(A) receptors. Activation of these heteropentameric receptors by GABA results in the gating of an integral chloride channel leading to membrane hyperpolarization and neuronal inhibition. To participate in neurotransmission, the receptor must reside on the cell surface. The trafficking of nascent receptors to the cell surface involves posttranslational modification and the interaction of the receptor with proteins that reside within the secretory pathway. The subsequent insertion of the receptor into specialized regions of the plasma membrane is dictated by receptor composition and other factors that guide insertion at synaptic or perisynaptic/extrasynaptic sites, where phasic and tonic inhibition are mediated, respectively. Once at the cell surface, the receptor is laterally mobile and subject to both constitutive and regulated endocytosis. Following endocytosis the receptor undergoes either recycling to the plasma membrane or degradation. These dynamic processes profoundly affect the strength of GABAergic signaling, neuronal inhibition, and presumably synaptic plasticity. Heritable channelopathies that affect receptor trafficking have been recently recognized and compelling evidence exists that mechanisms underlying acquired epilepsy involve GABA(A) receptor internalization. Additionally, GABA(A) receptor endocytosis has been identified as an early event in the ischemic response that leads to excitotoxicity and cell death. This chapter summarizes what is known regarding the regulation of receptor trafficking and cell surface expression and its impact on nervous system function from both cell biology and disease perspectives.

Fate of micelles and quantum dots in cells

Micelles and quantum dots have been used as experimental drug delivery systems and imaging tools both in vitro and in vivo. Investigations of their fate at the subcellular level require different surface-core modifications. Among the most common modifications are those with fluorescent probes, dense-core metals or radionucleids. Cellular fate of several fluorescent probes incorporated into poly(caprolactone)-b-copolymer micelles (PCL-b-PEO) was followed by confocal microscopy, and colloidal gold incorporated in poly 4-vinyl pyridine-PEO micelles were developed to explore micelle fate by electron microscopy. More recently, we have examined quantum dots (QDs) as the next-generation-labels for cells and nanoparticulate drug carriers amenable both to confocal and electron microscopic analyses. Effects of QDs at the cellular and subcellular levels and their integrity were studied. Results from different studies suggest that size, charge and surface manipulations of QDs may play a role in their subcellular distribution. Examples of pharmacological agents incorporated into block copolymer micelles, administered or attached to QD surfaces show how the final biological outcome (e.g. cell death, proliferation or differentiation) depends on physical properties of these nanoparticles.