BBA - Molecular Cell Research (v.1793, #4)

Lysosomes by Volkmar Gieselmann; Thomas Braulke (603-604).

Sorting of lysosomal proteins by Thomas Braulke; Juan S. Bonifacino (605-614).
Lysosomes are composed of soluble and transmembrane proteins that are targeted to lysosomes in a signal-dependent manner. The majority of soluble acid hydrolases are modified with mannose 6-phosphate (M6P) residues, allowing their recognition by M6P receptors in the Golgi complex and ensuing transport to the endosomal/lysosomal system. Other soluble enzymes and non-enzymatic proteins are transported to lysosomes in an M6P-independent manner mediated by alternative receptors such as the lysosomal integral membrane protein LIMP-2 or sortilin. Sorting of cargo receptors and lysosomal transmembrane proteins requires sorting signals present in their cytosolic domains. These signals include dileucine-based motifs, DXXLL or [DE]XXXL[LI], and tyrosine-based motifs, YXXØ, which interact with components of clathrin coats such as GGAs or adaptor protein complexes. In addition, phosphorylation and lipid modifications regulate signal recognition and trafficking of lysosomal membrane proteins. The complex interaction of both luminal and cytosolic signals with recognition proteins guarantees the specific and directed transport of proteins to lysosomes.
Keywords: Lysosomal protein; Posttranslational modification; GIcNac-1-phosphotransferase; Mannose 6-phosphate; Adaptor protein complex;

Delivery of endocytosed membrane proteins to the lysosome by Paul R. Pryor; J. Paul Luzio (615-624).
The delivery of proteins from the plasma membrane to the lysosome for degradation is essential for normal cellular function. There is now a good understanding of the protein complexes involved in sorting proteins at the plasma membrane and into the intralumenal vesicles of the multi-vesicular body. A combination of cell free content mixing assays and live-cell imaging has dissected out the final step in delivery of macromolecules to the lysosome from the multi-vesicular body and provided insights into the molecular mechanisms by which late endosomes and lysosomes exchange lumenal contents. The endocytic pathway has provided a platform with which to understand the autophagic and phagocytic pathways, but the fine details of how traffic through these pathways is regulated remain to be determined.
Keywords: Endocytosis; Late endosomes; Lysosomes; Multivesicular body; Membrane fusion;

Proteomics of the lysosome by Torben Lübke; Peter Lobel; David E. Sleat (625-635).
Defects in lysosomal function have been associated with numerous monogenic human diseases typically classified as lysosomal storage diseases. However, there is increasing evidence that lysosomal proteins are also involved in more widespread human diseases including cancer and Alzheimer disease. Thus, there is a continuing interest in understanding the cellular functions of the lysosome and an emerging approach to this is the identification of its constituent proteins by proteomic analyses. To date, the mammalian lysosome has been shown to contain ∼ 60 soluble luminal proteins and ∼ 25 transmembrane proteins. However, recent proteomic studies based upon affinity purification of soluble components or subcellular fractionation to obtain both soluble and membrane components suggest that there may be many more of both classes of protein resident within this organelle than previously appreciated. Discovery of such proteins has important implications for understanding the function and the dynamics of the lysosome but can also lead the way towards the discovery of the genetic basis for human diseases of hitherto unknown etiology. Here, we describe current approaches to lysosomal proteomics and data interpretation and review the new lysosomal proteins that have recently emerged from such studies.
Keywords: Lysosomal protein; Proteomic; Mass spectrometry; Lysosomal storage disease; Mannose-6 phosphate receptor; subcellular fractionation;

Molecular and cellular basis of lysosomal transmembrane protein dysfunction by Raquel Ruivo; Christine Anne; Corinne Sagné; Bruno Gasnier (636-649).
Lysosomal membrane proteins act at several crucial steps of the lysosome life cycle, including lumen acidification, metabolite export, molecular motor recruitment and fusion with other organelles. This review summarizes the molecular mechanisms of lysosomal storage diseases caused by defective transport of small molecules or ions across the lysosomal membrane, as well as Danon disease. In cystinosis and free sialic acid storage diseases, transporters for cystine and acidic monosaccharides, respectively, are blocked or retarded. A putative cobalamin transporter and a hybrid transporter/transferase of acetyl groups are defective in cobalamin F type disease and mucopolysaccharidosis type IIIC, respectively. In neurodegenerative forms of osteopetrosis, mutations of a proton/chloride exchanger impair the charge balance required for sustained proton pumping by the V-type ATPase, thus resulting in bone-resorption lacuna neutralization. However, the mechanism leading to lysosomal storage and neurodegeneration remains unclear. Mucolipidosis type IV is caused by mutations of a lysosomal cation channel named TRPML1; its gating properties are still poorly understood and the ion species linking this channel to lipid storage and membrane traffic defects is debated. Finally, the autophagy defect of Danon disease apparently arises from a role of LAMP2 in lysosome/autophagosome fusion, possibly secondary to a role in dynein-based centripetal motility.
Keywords: Lysosomal storage disorder; Lysosomal membrane; Transporter; Channel; Cystinosis; Salla disease; Infantile sialic acid disorder; Cobalamin F type disease; Mucopolysaccharidosis; Sanfilippo C syndrome; Osteopetrosis; Mucolipidosis type IV; Danon disease; Cystinosin; Sialin; HGSNAT; ClC-7; Ostm1; TRPML1; Mucolipin; LAMP2;

The yeast lysosome-like vacuole: Endpoint and crossroads by Sheena Claire Li; Patricia M. Kane (650-663).
Fungal vacuoles are acidic organelles with degradative and storage capabilities that have many similarities to mammalian lysosomes and plant vacuoles. In the past several years, well-developed genetic, genomic, biochemical and cell biological tools in S. cerevisiae have provided fresh insights into vacuolar protein sorting, organelle acidification, ion homeostasis, autophagy, and stress-related functions of the vacuole, and these insights have often found parallels in mammalian lysosomes. This review provides a broad overview of the defining features and functions of S. cerevisiae vacuoles and compares these features to mammalian lysosomes. Recent research challenges the traditional view of vacuoles and lysosomes as simply the terminal compartment of biosynthetic and endocytic pathways (i.e. the “garbage dump” of the cell), and suggests instead that these compartments are unexpectedly dynamic and highly regulated.
Keywords: Yeast; Vacuole; V-ATPase; Acidification; Autophagy; Trafficking; Morphology; Stress;

Autophagy delivers cytoplasmic material and organelles to lysosomes for degradation. The formation of autophagosomes is controlled by a specific set of autophagy genes called atg genes. The magnitude of autophagosome formation is tightly regulated by intracellular and extracellular amino acid concentrations and ATP levels via signaling pathways that include the nutrient sensing kinase TOR. Autophagy functions as a stress response that is upregulated by starvation, oxidative stress, or other harmful conditions. Remarkably, autophagy has been shown to possess important housekeeping and quality control functions that contribute to health and longevity. Autophagy plays a role in innate and adaptive immunity, programmed cell death, as well as prevention of cancer, neurodegeneration and aging. In addition, impaired autophagic degradation contributes to the pathogenesis of several human diseases including lysosomal storage disorders and muscle diseases.
Keywords: Autophagy; Lysosome; Phagocytosis; Cancer; Cell death; Neurodegeneration; Aging;

Principles of lysosomal membrane degradation by Heike Schulze; Thomas Kolter; Konrad Sandhoff (674-683).
Cellular membranes enter the lysosomal compartment by endocytosis, phagocytosis, or autophagy. Within the lysosomal compartment, membrane components of complex structure are degraded into their building blocks. These are able to leave the lysosome and can then be utilized for the resynthesis of complex molecules or can be further degraded. Constitutive degradation of membranes occurs on the surface of intra-endosomal and intra-lysosomal membrane structures. Many integral membrane proteins are sorted to the inner membranes of endosomes and lysosome after ubiquitinylation. In the lysosome, proteins are degraded by proteolytic enzymes, the cathepsins. Phospholipids originating from lipoproteins or cellular membranes are degraded by phospholipases. Water-soluble glycosidases sequentially cleave off the terminal carbohydrate residues of glycoproteins, glycosaminoglycans, and glycosphingolipids. For glycosphingolipids with short oligosaccharide chains, the additional presence of membrane-active lysosomal lipid-binding proteins is required. The presence of lipid-binding proteins overcomes the phase problem of water soluble enzymes and lipid substrates by transferring the substrate to the degrading enzyme or by solubilizing the internal membranes. The lipid composition of intra-lysosomal vesicles differs from that of the plasma membrane. To allow at least glycosphingolipid degradation by hydrolases and activator proteins, the cholesterol content of these intraorganellar membranes decreases during endocytosis and the concentration of bis(monoacylglycero)phosphate, a stimulator of sphingolipid degradation, increases. A considerable part of our current knowledge about mechanism and biochemistry of lysosomal lipid degradation is derived from a class of human diseases, the sphingolipidoses, which are caused by inherited defects within sphingolipid and glycosphingolipid catabolism.
Keywords: Lysosome; Glycosphingolipid; Sphingolipid activator protein;

Lysosomal disorders: From storage to cellular damage by Andrea Ballabio; Volkmar Gieselmann (684-696).
Lysosomal storage diseases represent a group of about 50 genetic disorders caused by deficiencies of lysosomal and non-lysosomal proteins. Patients accumulate compounds which are normally degraded in the lysosome. In many diseases this accumulation affects various organs leading to severe symptoms and premature death. The revelation of the mechanism by which stored compounds affect cellular function is the basis for understanding pathophysiology underlying lysosomal storage diseases. In the past years it has become clear that storage compounds interfere with various processes on the cellular level. The spectrum covers e.g. receptor activation by non-physiologic ligands, modulation of receptor response and intracellular effectors of signal transduction cascades, impairment of autophagy, and others. Importantly, many of these processes are associated with accumulation of storage material in non-lysosomal compartments. Here we summarize current knowledge on the effects that storage material can elicit on the cellular level.
Keywords: Lysosomal storage diseases; Lipidoses; Mucopolysaccharidoses; Pathogenesis;

Neuronal ceroid lipofuscinoses by Anu Jalanko; Thomas Braulke (697-709).
The neuronal ceroid lipofuscinoses (NCL) are severe neurodegenerative lysosomal storage disorders of childhood, characterized by accumulation of autofluorescent ceroid lipopigments in most cells. NCLs are caused by mutations in at least ten recessively inherited human genes, eight of which have been characterized. The NCL genes encode soluble and transmembrane proteins, localized to the endoplasmic reticulum (ER) or the endosomal/lysosomal organelles. The precise function of most of the NCL proteins has remained elusive, although they are anticipated to carry pivotal roles in the central nervous system. Common clinical features in NCL, including retinopathy, motor abnormalities, epilepsia and dementia, also suggest that the proteins may be functionally linked. All subtypes of NCLs present with selective neurodegeneration in the cerebral and cerebellar cortices. Animal models have provided valuable data about the pathological characteristics of NCL and revealed that early glial activation precedes neuron loss in the thalamocortical system. The mouse models have also been efficiently utilized for the evaluation of therapeutic strategies. The tools generated by the accomplishments in genomics have further substantiated global analyses and these have initially provided new insights into the NCL field. This review summarizes the current knowledge of the NCL proteins, basic characteristics of each disease and studies of pathogenetic mechanisms in animal models of these diseases.
Keywords: NCL; Lysosome; Neurodegeneration; Lysosomal storage disease; Animal model;

Molecular basis of multiple sulfatase deficiency, mucolipidosis II/III and Niemann–Pick C1 disease — Lysosomal storage disorders caused by defects of non-lysosomal proteins by Thomas Dierks; Lars Schlotawa; Marc-André Frese; Karthikeyan Radhakrishnan; Kurt von Figura; Bernhard Schmidt (710-725).
Multiple sulfatase deficiency (MSD), mucolipidosis (ML) II/III and Niemann–Pick type C1 (NPC1) disease are rare but fatal lysosomal storage disorders caused by the genetic defect of non-lysosomal proteins. The NPC1 protein mainly localizes to late endosomes and is essential for cholesterol redistribution from endocytosed LDL to cellular membranes. NPC1 deficiency leads to lysosomal accumulation of a broad range of lipids. The precise functional mechanism of this membrane protein, however, remains puzzling. ML II, also termed I cell disease, and the less severe ML III result from deficiencies of the Golgi enzyme N-acetylglucosamine 1-phosphotransferase leading to a global defect of lysosome biogenesis. In patient cells, newly synthesized lysosomal proteins are not equipped with the critical lysosomal trafficking marker mannose 6-phosphate, thus escaping from lysosomal sorting at the trans Golgi network. MSD affects the entire sulfatase family, at least seven members of which are lysosomal enzymes that are specifically involved in the degradation of sulfated glycosaminoglycans, sulfolipids or other sulfated molecules. The combined deficiencies of all sulfatases result from a defective post-translational modification by the ER-localized formylglycine-generating enzyme (FGE), which oxidizes a specific cysteine residue to formylglycine, the catalytic residue enabling a unique mechanism of sulfate ester hydrolysis. This review gives an update on the molecular bases of these enigmatic diseases, which have been challenging researchers since many decades and so far led to a number of surprising findings that give deeper insight into both the cell biology and the pathobiochemistry underlying these complex disorders. In case of MSD, considerable progress has been made in recent years towards an understanding of disease-causing FGE mutations. First approaches to link molecular parameters with clinical manifestation have been described and even therapeutical options have been addressed. Further, the discovery of FGE as an essential sulfatase activating enzyme has considerable impact on enzyme replacement or gene therapy of lysosomal storage disorders caused by single sulfatase deficiencies.
Keywords: Multiple sulfatase deficiency; Mucolipidosis II/III; Niemann–Pick disease type C; Formylglycine generating enzyme; Sulfatase; SUMF1;

Secondary lipid accumulation in lysosomal disease by Steven U. Walkley; Marie T. Vanier (726-736).
Lysosomal diseases are inherited metabolic disorders caused by defects in a wide spectrum of lysosomal and a few non-lysosomal proteins. In most cases a single type of primary storage material is identified, which has been used to name and classify the disorders: hence the terms sphingolipidoses, gangliosidoses, mucopolysaccharidoses, glycoproteinoses, and so forth. In addition to this primary storage, however, a host of secondary storage products can also be identified, more often than not having no direct link to the primary protein defect. Lipids – glycosphingolipids and phospholipids, as well as cholesterol – are the most ubiquitous and best studied of these secondary storage materials. While in the past typically considered nonspecific and nonconsequential features of these diseases, newer studies suggest direct links between secondary storage and disease pathogenesis and support the view that understanding all aspects of this sequestration process will provide important insights into the cell biology and treatment of lysosomal disease.
Keywords: Sphingolipid; Glycosphingolipid; Ganglioside; Lactosylceramide; Glucosylceramide; Sphingomyelin; Phospholipid; Bis(monoacylglycero)phosphate; Cholesterol;

Treating lysosomal storage disorders: Current practice and future prospects by Frances M. Platt; Robin H. Lachmann (737-745).
There are over 40 human disease states that are caused by defects in various aspects of lysosomal function. Over the past two decades there has been dramatic progress in the development and evaluation of therapies for lysosomal storage disorders, several of which are now in routine clinical use or in clinical trials. The greatest current challenge is in developing effective therapies for treating the CNS manifestations of these complex disorders. In this article, we will review the current therapies/approaches being considered for treating lysosomal storage diseases and give a perspective on the scientific, medical, social and ethical issues they raise.
Keywords: Lysosomal storage disease; Enzyme replacement; Substrate reduction; Gene therapy; Neurodegeneration;

Lysosomal involvement in cell death and cancer by Thomas Kirkegaard; Marja Jäättelä (746-754).
Lysosomes, with their arsenal of degradative enzymes are increasingly becoming an area of interest in the field of oncology. The changes induced in this compartment upon transformation are numerous and whereas most are viewed as pro-oncogenic the same processes also render cancer cells susceptible to lysosomal death pathways. This review will provide an overview of the pro- and anti-oncogenic potential of this compartment and how these might be exploited for cancer therapy, with special focus on lysosomal death pathways.
Keywords: Lysosomes; Cathepsins; Cancer; Apoptosis; Transformation; Cell death;