The effects of sucrose loading on lysosomal hydrolases

Chloroquine and bafilomycin A mimic lysosomal storage disorders and impair mTORC1 signalling

Autophagy is dependent upon lysosomes, which fuse with the autophagosome to complete the autophagic process and whose acidic interior permits the activity of their intraluminal degradative enzymes. Chloroquine (CQ) and bafilomycin A1 (BafA) each cause alkalinisation of the lumen and thus impair lysosomal function, although by distinct mechanisms. CQ diffuses into lysosomes and undergoes protonation, while BafA inhibits the ability of the vacuolar type H⁺-ATPase (v-ATPase) to transfer protons into the lysosome. In the present study, we examine the impact of CQ and BafA on the activity of mammalian target of rapamycin complex 1 (mTORC1), inhibition of which is an early step in promoting autophagy. We find each compound inhibits mTORC1 signalling, without affecting levels of protein components of the mTORC1 signalling pathway. Furthermore, these effects are not related to these agents’ capacity to inhibit autophagy or the reduction in amino acid supply from lysosomal proteolysis. Instead, our data indicate that the reduction in mTORC1 signalling appears to be due to the accumulation of lysosomal storage material. However, there are differences in responses to these agents, for instance, in their abilities to up-regulate direct targets of transcription factor EB (TFEB), a substrate of mTORC1 that drives transcription of many lysosomal and autophagy-related genes. Nonetheless, our data imply that widely used agents that alkalinise intralysosomal pH are mimetics of acute lysosomal storage disorders (LSDs) and emphasise the importance of considering the result of CQ and BafA on mTORC1 signalling when interpreting the effects of these agents on cellular physiology.

The gene for the lysosomal protein LAMP3 is a direct target of the transcription factor ATF4

Autophagy and lysosomal activities play a key role in the cell by initiating and carrying out the degradation of misfolded proteins. Transcription factor EB (TFEB) functions as a master controller of lysosomal biogenesis and function during lysosomal stress, controlling most but, importantly, not all lysosomal genes. Here, we sought to better understand the regulation of lysosomal genes whose expression does not appear to be controlled by TFEB. Sixteen of these genes were screened for transactivation in response to diverse cellular insults. mRNA levels for lysosomal-associated membrane protein 3 (LAMP3), a gene that is highly up-regulated in many forms of cancer, including breast and cervical cancers, were significantly increased during the integrated stress response, which occurs in eukaryotic cells in response to accumulation of unfolded and misfolded proteins. Of note, results from siRNA-mediated knockdown of activating transcription factor 4 (ATF4) and overexpression of exogenous ATF4 cDNA indicated that ATF4 up-regulates LAMP3 mRNA levels. Finally, ChIP assays verified an ATF4-binding site in the LAMP3 gene promoter, and a dual-luciferase assay confirmed that this ATF4-binding site is indeed required for transcriptional up-regulation of LAMP3 These results reveal that ATF4 directly regulates LAMP3, representing the first identification of a gene for a lysosomal component whose expression is directly controlled by ATF4. This finding may provide a key link between stresses such as accumulation of unfolded proteins and modulation of autophagy, which removes them.

A lysosome-plasma membrane-sphingolipid axis linking lysosomal storage to cell growth arrest

Lysosomal accumulation of undegraded materials is a common feature of lysosomal storage diseases, neurodegenerative disorders, and the aging process. To better understand the role of lysosomal storage in the onset of cell damage, we used human fibroblasts loaded with sucrose as a model of lysosomal accumulation. Sucrose-loaded fibroblasts displayed increased lysosomal biogenesis followed by arrested cell proliferation. Notably, we found that reduced lysosomal catabolism and autophagy impairment led to an increase in sphingolipids ( i.e., sphingomyelin, glucosylceramide, ceramide, and the gangliosides GM3 and GD3), at both intracellular and plasma membrane (PM) levels. In addition, we observed an increase in the lysosomal membrane protein Lamp-1 on the PM of sucrose-loaded fibroblasts and a greater release of the soluble lysosomal protein cathepsin D in their extracellular medium compared with controls. These results indicate increased fusion between lysosomes and the PM, as also suggested by the increased activity of lysosomal glycosphingolipid hydrolases on the PM of sucrose-loaded fibroblasts. The inhibition of β-glucocerebrosidase and nonlysosomal glucosylceramidase, both involved in ceramide production resulting from glycosphingolipid catabolism on the PM, partially restored cell proliferation. Our findings indicate the existence of a new molecular mechanism underlying cell damage triggered by lysosomal impairment.-Samarani, M., Loberto, N., Soldà, G., Straniero, L., Asselta, R., Duga, S., Lunghi, G., Zucca, F. A., Mauri, L., Ciampa, M. G., Schiumarini, D., Bassi, R., Giussani, P., Chiricozzi, E., Prinetti, A., Aureli, M., Sonnino, S. A lysosome-plasma membrane-sphingolipid axis linking lysosomal storage to cell growth arrest.

Cell-penetrating peptides (CPPs): From delivery of nucleic acids and antigens to transduction of engineered nucleases for application in transgenesis

Cell-penetrating peptides (CPPs) have been studied for their capacity to translocate across the lipid membrane of several cell types. In membrane translocation, these peptides can remarkably transport biologically active hydrophilic molecules, such as pharmaceuticals, nucleic acids (DNA and RNA) and even high-molecular-weight proteins, Fig. 3 into the cell cytoplasm and organelles. The development of CPPs as transduction agents includes the modification of gene and protein expression, the reprogramming and differentiation of induced pluripotent stem cells and the preparation of cellular vaccines. A relatively recent field of CPP application is the transduction of plasmid DNA vectors and CPP-fusion proteins to modify genomes and introduce new traits in cells and organisms. CPP-mediated transduction of components for genome editing is an advantageous alternative to viral DNA vectors. Engineered site-specific nucleases, such as Cre recombinase, ZFN, TALENs and CRISPR associated protein (Cas), have been coupled to CPPs, and the fused proteins have been used to permeate targeted cells and tissues. The functionally active fusion CPP-nucleases subsequently home to the nucleus, incise genomic DNA at specific sites and induce repair and recombination. This review has the objective of discussing CPPs and elucidating the prospective use of CPP-mediated transduction technology, particularly in genome modification and transgenesis.

Sucrose induces vesicle accumulation and autophagy

It has been shown that the treatment of mammalian cells with sucrose leads to vacuole accumulation associated with lysosomes and upregulation of lysosomal enzyme expression and activity. Autophagy is an evolutionarily conserved homeostatic process by which cells deliver cytoplasmic material for degradation into lysosomes, thus it is probable that sucrose affects the autophagic activity. The role of sucrose in autophagy is unknown; however, another disaccharide, trehalose has been shown to induce autophagy. In the current study, we used mouse embryonic fibroblasts to investigate whether sucrose induces autophagy and whether vesicle formation is associated with autophagy. The results showed that sucrose induces autophagy while being accumulated within the endosomes/lysosomes. These vesicles were swollen and packed within the cytoplasm. Furthermore, trehalose and the trisaccharide raffinose, which are not hydrolyzed in mammalian cells, increased the rate of vesicles accumulation and LC3-II level (a protein marker of autophagy). However, fructose and maltose did not show the same effects. The correlation between the two processes, vesicle accumulation and autophagy induction, was confirmed by treatment of cells with sucrose plus invertase, or maltose plus acarbose-the α-glucosidase inhibitor-and by sucrose deprivation. Results also showed that vesicle accumulation was not affected by autophagy inhibition. Therefore, the data suggest that sucrose-induced autophagy through accumulation of sucrose-containing vesicles is caused by the absence of hydrolysis enzymes.

Recent advances in live cell imaging of hepatoma cells

Live cell imaging enables the study of dynamic processes of living cells in real time by use of suitable reporter proteins and the staining of specific cellular structures and/or organelles. With the availability of advanced optical devices and improved cell culture protocols it has become a rapidly growing research methodology. The success of this technique relies mainly on the selection of suitable reporter proteins, construction of recombinant plasmids possessing cell type specific promoters as well as reliable methods of gene transfer. This review aims to provide an overview of the recent developments in the field of marker proteins (bioluminescence and fluorescent) and methodologies (fluorescent resonance energy transfer, fluorescent recovery after photobleaching and proximity ligation assay) employed as to achieve an improved imaging of biological processes in hepatoma cells. Moreover, different expression systems of marker proteins and the modes of gene transfer are discussed with emphasis on the study of lipid droplet formation in hepatocytes as an example.

Internalization of p53(14-29) peptide amphiphiles and subsequent endosomal disruption results in SJSA-1 cell death

In vivo peptide inhibition of tumor suppressor p53 binding to the protein MDM2 is hampered by inefficient delivery of the peptide. Our approach to couple a hydrophobic lipid-like tail on the inhibitory peptide p53(14-29) allowed its intracellular delivery in vitro, in a panel of different cell lines. The constructed chimeric molecules, termed peptide amphiphiles, further self-assembled into supramolecular structures, identified as elongated wormlike micelles. Internalization of peptides occurred following micelle disassembly, partly via clathrin-mediated endocytosis of monomers. Incubation of SJSA-1 cells in hypertonic culture media, aimed to disrupt endocytic vesicles, resulted in peptide amphiphile-mediated cell death. Our results provide the basis for the construction of novel therapeutic supramolecular nanoparticles and suggest hydrophobic modification of peptides as a promising strategy for enhancing delivery of impermeable peptides.

Peptides for specific intracellular delivery and targeting of nanoparticles: implications for developing nanoparticle-mediated drug delivery

The use of peptides to mediate the delivery and uptake of nanoparticle (NP) materials by mammalian cells has grown significantly over the past 10 years. This area of research has important implications for the development of new therapeutic materials and for the emerging field of NP-mediated drug delivery. In this review, we highlight recent advances in the delivery of various NPs by some of the more commonly employed cellular delivery peptides and discuss important related factors such as NP–peptide bioconjugation, uptake efficiency, intracellular fate and toxicity. We also highlight various demonstrations of therapeutic applications of NP–peptide conjugates where appropriate. The paper concludes with a brief forward-looking perspective discussing what can be expected as this field develops in the coming years.

Delivering quantum dot-peptide bioconjugates to the cellular cytosol: escaping from the endolysosomal system

For luminescent quantum dots (QDs) to realize their full potential as intracellular labeling, imaging and sensing reagents, robust noninvasive methods for their delivery to the cellular cytosol must be developed. Our aim in this study was to explore a range of methods aimed at delivering QDs to the cytosol. We have previously shown that QDs functionalized with a polyarginine 'Tat' cell-penetrating peptide (CPP) could be specifically delivered to cells via endocytic uptake with no adverse effects on cellular proliferation. We began by assessing the long-term intracellular fate and stability of these QD-peptide conjugates. We found that the QDs remained sequestered within acidic endolysosomal vesicles for at least three days after initial uptake while the CPP appeared to remain stably associated with the QD throughout this time. We next explored techniques designed to either actively deliver QDs directly to the cytosol or to combine endocytosis with subsequent endosomal escape to the cytosol in several eukaryotic cell lines. Active delivery methods such as electroporation and nucleofection delivered only modest amounts of QDs to the cytosol as aggregates. Delivery of QDs using a variety of transfection polymers also resulted in primarily endosomal sequestration of QDs. However, in one case the commercial PULSin reagent did facilitate a modest cytosolic dispersal of QDs, but only after several days in culture and with significant polymer-induced cytotoxicity. Finally, we demonstrated that an amphiphilic peptide designed to mediate cell penetration and vesicle membrane interactions could mediate rapid QD uptake by endocytosis followed by a slower efficient endosomal release which peaked at 48 h after initial delivery. Importantly, this QD-peptide bioconjugate elicited minimal cytotoxicity in the cell lines tested.

Endosome trapping limits the efficiency of splicing correction by PNA-oligolysine conjugates

Splicing correction by steric-blocking oligonucleotides (ON) might lead to important clinical applications but requires efficient delivery to cell nuclei. The conjugation of short oligolysine tails has been used to deliver a correcting peptide nucleic acid (PNA) sequence in a positive readout assay in which ON hybridization to the cryptic splice site is strictly required for the expression of a luciferase reporter gene. We have investigated the mechanism of cellular uptake and the efficiency of a (Lys)(8)-PNA-Lys construction in this model system. Cell uptake is temperature-dependent and leads to sequestration of the conjugate in cytoplasmic vesicles in keeping with an endocytic mechanism of internalization. Accordingly a significant and sequence-specific splicing correction is achieved only in the presence of endosome-disrupting agents as chloroquine or 0.5 M sucrose. These endosome-disrupting agents do not affect the activity of free PNA, and do not increase (Lys)(8)-PNA-Lys uptake.

Endosome disruption enhances the functional nuclear delivery of Tat-fusion proteins

Tat protein from human immunodeficiency virus can deliver biologically active proteins in vivo and is of considerable interest for protein therapeutics. The mechanism responsible for Tat-fusion protein internalization is still poorly understood and controversial. The punctuate distribution, timing, and temperature sensitivity observed in our experiments with Tat-fusion proteins are consistent with endocytosis. After a few hours, Tat-fusion proteins accumulated around the nucleus without any significant visible nuclear targeting. Using a Cre/Lox based functional assay, lysosomotropic agents known to disrupt endosome integrity, increased by up to 23-fold the nuclear delivery of functional Tat-Cre recombinase without increasing cell uptake in a similar fashion. This shows that endosome disruption can significantly increase Tat-fusion protein access to the cytosol and nucleus. In addition, we found that internalized Tat-fusion proteins persisted several hours and that inhibitors of lysosome acidification did not increase functional nuclear delivery of Tat-Cre. This suggests that Tat-fusion proteins enter via the endosomal pathway, circumvent lysosomal degradation, and are then sequestered in the periphery of the nucleus. Most importantly, our work indicates that an inadequate intracellular trafficking is the main factor limiting the efficiency of protein cargo delivery using Tat.

Enhanced plasmid DNA transfection with lysosomotropic agents in cultured fibroblasts

Transfer of plasmid DNA into mammalian cells has posed major challenges for gene therapy. Most non-viral vectors are known to internalize in the cells by endocytosis. Therefore, low transfection efficiency of non-viral vectors may be due to intracellular degradation of input DNA in the endosomes and/or lysosomes. DNA degradation can be inhibited either by inactivating the lysosomal enzymes or obliterating endosome fusion to lysosomes using lysosomotropic agents. We report here the effects of individual lysosomotropic agents such as chloroquine, polyvinylpyrolidone (PVP) and sucrose on beta-gal expression in cultured fibroblasts COS, 293 and CHO. Cell viability was influenced by type, exposure time and concentration of lysosomotropic agents. Exposure to chloroquine at high concentration (1000 microM) or more than 4 h at any concentration (10-1000 microM) caused extensive cell death, however, cytotoxicity due to sucrose (5-500 mM) and PVP (0.01-1 mg/ml) was minimal in the cell lines tested. All the agents utilized in this study enhanced the gene expression and the transfection efficiency followed the order of sucrose>chloroquine>PVP at the concentrations used in all cell lines. Results suggest that lysosomotropic agents can enhance transfection efficiency but the degree of transgene expression may be cell- and agent-specific. Of the agents studied, sucrose appears to be an attractive agent in improving gene expression without toxic effect in the cultured fibroblasts. Thus, it can be used as an excipient in the formulation of new gene delivery systems.

Sustained delivery and expression of DNA encapsulated in polymeric nanoparticles

Sustained release polymeric gene delivery systems offer increased resistance to nuclease degradation, increased amounts of plasmid DNA (pDNA) uptake, and the possibility of control in dosing and sustained duration of pDNA administration. Furthermore, such a system lacks the inherent problems associated with viral vectors. Biodegradable and biocompatible poly(DL-lactide-co-glycolide) polymer was used to enacapsulate pDNA (alkaline phosphatase, AP, a reporter gene) in submicron size particles. Gene expression mediated by the nanoparticles (NP) was evaluated in vitro and in vivo in comparison to cationic-liposome delivery. Nano size range (600 nm) pDNA-loaded in poly(DL-lactide-co-glycolide) polymer particles with high encapsulation efficiency (70%) were formulated, exhibiting sustained release of pDNA of over a month. The entrapped plasmid maintained its structural and functional integrity. In vitro transfection by pDNA-NP resulted in significantly higher expression levels in comparison to naked pDNA. Furthermore, AP levels increased when the transfection time was extended, indicating sustained activity of pDNA. However, gene expression was significantly lower in comparison with standard liposomal transfection. Seven days after i.m. injections in rats, naked pDNA and pDNA-NP were found to be significantly more potent (1-2 orders of magnitude) than liposomal pDNA. Plasmid DNA-NP treatment exhibited increased AP expression after 7 and 28 days indicating sustained activity of the NP.

Change of isoforms' spectra of alpha-L-fucosidase from human skin fibroblasts in intracellular storage of nonhydrolyzable substances

The effect of exogenous and endogenous products storage in lysosomes on the activity and multiple forms of alpha-L-fucosidase from human skin fibroblasts was investigated. It was shown that sucrose load, modelling intralysosomal accumulation of nonhydrolyzable products, causes certain changes in secretion level of alpha-L-fucosidase and multiple forms' spectra of the intracellular and secreted enzymes. These changes were different for the enzyme from embryonal and postnatal normal fibroblasts. Some changes of secreted alpha-L-fucosidase isoforms' spectra were found in fibroblasts from a patient with Fabry's disease, characterized by the intralysosomal storage of di- and trihexosylceramides. The alterations of isoforms' profiles in Fabry fibroblasts at the early and late accumulation stages were similar to those in sucrose-loaded embryonal and postnatal fibroblasts, respectively. It is proposed that intralysosomal accumulation of nonhydrolyzable compounds influences the alpha-L-fucosidase posttranslational processing.