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© The Rockefeller University Press,
0021-9525/1999//1 $5.00
The Journal of Cell Biology, Volume 147, Number 1,
, 1999 1-5
Mini-Review |
Regulated Secretion from Hemopoietic Cells
gillian.griffiths{at}path.ox.ac.uk
© 1999 The Rockefeller University Press
THE process of regulated secretion (reviewed in Bugress and Kelly, 1987) is critical for the correct biological functioning of many different cells of the immune system, most of which are derived from the hemopoietic lineage (Table ). For example, T lymphocytes use regulated secretion to selectively destroy appropriate targets recognized by the T cell receptor, while mast cells degranulate in response to IgE cross-linking to counter parasitic infection. Unlike conventional secretory cells (e.g., exocrine and endocrine cells) which use a separate organelle for the storage and release of their secretory products (Fig. 1 a), cells of the hemopoietic lineage use lysosomes to store and release their secretory products (Fig. 1 b; Griffiths 1996). These organelles have been termed secretory lysosomes. Although the lysosomal nature of the secretory granules found in several hemopoietic cells has been known for many years, recent evidence supports the idea that secretory lysosomes may use specialized mechanisms for sorting and secretion, which differ from those found in conventional secretory cells. Interestingly, a small number of nonhemopoietic cells also use these mechanisms, suggesting that secretory lysosomes may represent an early form of regulated secretion.
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| Secretory Lysosomes: A Mixture of Two Organelles |
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The mechanisms that regulate the sorting of proteins to the secretory lysosomes are also a mixture of those used to target lysosomal proteins and secretory granule proteins in conventional cells, although in hemopoietic cells the proteins are targeted to the same organelle. For example, the soluble secretory granzymes of T lymphocytes follow the mannose-6-phosphate pathway used by lysosomal hydrolases to reach the granules (Griffiths and Isaaz 1993). Other soluble proteins of secretory lysosomes, such as perforin in T lymphocytes, are able to complex with proteoglycans (Masson et al. 1990) and may be sorted by selective condensation into the dense core, as has been suggested for chromogranins in endocrine secretory cells (Chanat and Huttner 1991). Membrane-bound proteins that are expressed only in cells with secretory lysosomes, such as GMP-17 (Medley et al. 1996) or CTLA-4 (McNeil and Steinhardt 1997), possess the tyrosine-based sorting motifs found in lysosomal membrane proteins that enable selective sorting to the secretory lysosome. Recent data suggest that specialized mechanisms for sorting to secretory lysosomes may also exist. This arises from the observation that the membrane-bound protein of T lymphocytes, Fas ligand, is differentially sorted in hemopoietic and nonhemopoietic cells. The cytosolic tail of this protein preferentially sorts Fas ligand to secretory lysosomes in hemopoietic cells, but is unable to do so in nonhemopoietic cells in which it is expressed directly on the cell surface (Dossi and Griffiths, 1999). Mutagenesis of the tail demonstrates that a proline-rich domain is required for sorting to secretory lysosomes (Bossi, G., and G.M. Griffiths, unpublished observation). One possible mechanism for the differential sorting might, therefore, involve the interaction of this proline-rich domain with an SH3-domain containing protein that is preferentially expressed in cells with secretory lysosomes.
| The Advantages of Secretory Lysosomes for Hemopoietic Cells |
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In some cells it appears that not only the lysosome can be released but other prelysosomal compartments can also fuse with and deliver proteins to the plasma membrane. This mechanism is particularly important in hemopoietic cells expressing MHC class II. MHC class II is present within the cell in a prelysosomal compartment, termed the MIIC, and can be relocalized to the cell surface (Rodriguez et al. 1995; Wubbolts et al. 1996; Raposo et al. 1997). Since MHC class II presents peptides from extracellular pathogens that need to be taken up and degraded by the cell, the exocytosis of a compartment from the degradative pathway on the way to the lysosome is therefore ideal for efficient antigen presentation. Curiously, secretion of this multivesicular compartment not only translocates proteins of the outer membrane to the cell surface but also results in the release of small internal vesicles, termed exosomes, which may themselves have important biological effects (Zitvogel et al. 1998).
| The Secretory Machinery Required for Lysosome Release |
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400 kD. The gene is expressed in the majority of tissues examined, consistent with the abnormally sized lysosomes found in all CHS cell types. The most direct clue as to the function of the protein comes from experiments in which the wild-type protein is overexpressed in mutant fibroblasts. This results in the production of abnormally small lysosomes, suggesting that the protein is involved in lysosomal fission (Perou et al. 1997). | Albinism and Lysosome Secretion: What's the Link? |
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Recent studies on two human autosomal recessive diseases that give rise to partial albinism have produced some intriguing findings concerning the link between albinism and secretory lysosome biogenesis. The first is Griscelli's syndrome that has a mouse homologue known as dilute. Griscelli's syndrome is clinically related to CHS in that the patients show selective immunodeficiency and partial albinism (Klein et al. 1994). The defective gene in the dilute mouse was shown to be the myosin Va heavy chain (Mercer et al. 1991) and the human lesion has been shown to encode the same protein (Pastural et al. 1997). In wild-type melanocytes, melanosomes are concentrated in the peripheral dendrites from which they are released. However, in dilute mice the melanosomes are more concentrated in the center of the cell. In wild-type cells, myosin Va colocalizes with melanosomes in the dendrites (Wu et al. 1997). Recent studies suggest that myosin Va is important in capturing melanosomes that reach the periphery, since overexpression of a dominant negative myosin Va in wild-type cells dramatically depletes melanosome accumulation at the periphery (Wu et al. 1998). The selective nature of these diseases, which affect both hemopoietic cells and melanocytes, raises the intriguing possibility that myosin Va may also play a critical role in the secretory lysosome polarization which is required for secretion in many hemopoietic cell types.
The second disease to shed light on the link between albinism and cells with secretory lysosomes is Hermansky-Pudlak syndrome (HPS). HPS is an autosomal recessive disease of humans resulting in partial albinism and defects in lysosomal secretion which has several mouse models (reviewed in Swank et al. 1998). Since the defective genes in the different mouse models map to at least 10 different loci, it seems likely that defects in multiple genes result in a similar phenotype (Swank et al. 1998). Recent studies demonstrate that HPS and its mouse models reflect defects in a lysosomal sorting pathway. Two different proteins have been identified so far. One has been termed HPS1 and encodes a 79-kD novel transmembrane protein of unknown function (Kantheti et al. 1998; Feng et al. 1999). The HPS1 sequence provides few clues as to the function of this protein and there is no significant homology to other known proteins. The other protein that has been found to be defective is the adaptor protein AP3. The mocha mouse is defective in the
subunit of AP3, while pearl mice are defective in the β3A subunit (Barbosa et al. 1996; Gardner et al. 1997). Some HPS patients which show normal expression of HPS1 have also been shown to be defective in the β3A subunit of the AP3 adaptor protein (DellAngelica et al. 1999). Several lysosomal membrane proteins are mis-sorted in fibroblasts derived from these patients. Findings from earlier studies of HPS and its related mouse models which indicate that both melanocyte function and lysosome secretion are affected by these mutations, suggest that AP3 may be especially important in transporting proteins to both melanosomes and secretory lysosomes. Intriguingly, all of these mutants have also been reported to be defective in the secretion lysosomal hydrolases from kidneys, another cell type that uses secretory lysosomes (Swank et al. 1998). Together, these studies demonstrate that many of these albinism mutants may provide important clues in understanding the different steps of the secretory mechanisms used by cells with secretory lysosomes.
| Is Secretion a Property of All Lysosomes? |
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First, lysosomes from nonsecretory cells can be secreted. It has been shown that both Chinese hamster ovary and normal rat kidney cells can be induced to secrete their lysosomes in response to influx of high levels of calcium (Coorssen et al. 1996; Rodriguez et al. 1997). Although the percentage of the total lysosomal population responding to the signal is generally low (10%, compared with 60% in cells with secretory lysosomes) and high levels of calcium are required, these observations suggest that there may be secretion-competent lysosomes in these cells. Similarly, trypanosomes seem to be able to trigger calcium-mediated fusion of host cell lysosomes at the cell surface during their invasion into a variety of nonhemopoietic mammalian cells (Tardieux et al. 1992; Rodriguez et al. 1995, Rodriguez et al. 1996). Again only a sub-population of lysosomes respond to the trypanosomal signal in fibroblasts. These differences in the level of lysosomal secretion may simply reflect variations in the number of secretion-competent lysosomes in different cell types.
Second, there is evidence that repair of the plasma membrane may involve a process of lysosomal secretion. Wounding of the plasma membrane studied in many nonhemopoietic cell types results in a calcium flux which causes membranes of endocytic origin to fuse with the plasma membrane (Miyake and McNeil 1995). In this way, lysosomal exocytosis could be an important mechanism in wound healing required by all cell types (McNeil and Steinhardt 1997; Caler et al. 1998).
Studies from Dictyostelium suggest that secretion of lysosomes may represent a primitive secretory system. Dictyostelium are also able to secrete their lysosomal contents and a number of mutants in this process have been isolated, demonstrating that several groups of genes are required for secretion (Ebert et al. 1990). Many of these genes also appear to be important in the development of the multicellular stage of the Dictyostelium life cycle. Of particular interest is the observation that one of the genes required for cytokinesis bears strong homology to the gene that is defective in CHS (De Lozanne, A., personal communication). Many of the secretory mutants which block polarized membrane delivery in Saccharomyces are also required for cytokinesis (reviewed in Finger and Novick 1998) and may also be involved in polarized delivery of secretory lysosomes. The existence of genes required for lysosomal secretion in Dictyostelium related to those required for lysosomal secretion in mammals suggests a strong evolutionary conservation of these mechanisms.
Taken together, these findings suggest that the mechanism of regulated secretion of lysosomes used by hemopoietic cells is present in many cell types and may represent a primitive secretory system which has simply been enhanced in cells of the hemopoietic system. If this is the case, then it is likely that the specialized secretory granules used for regulated secretion from exocrine and endocrine cells are a later evolutionary development. Intriguingly, although conventional endocrine and exocrine secretory cells contain distinct lysosomes and secretory storage compartments they contain a post-Golgi intermediate, the immature granule, on the pathway to these distinct organelles in which the newly synthesized secretory and lysosomal proteins coexist (Klumperman et al. 1998; Kuliawat et al. 1997). This compartment has several features in common with secretory lysosomes: it is acidic, performs some proteolytic functions, and can be simulated to secrete in a calcium-dependent manner (reviewed in Arvan and Castle 1998). It may be that the immature granule is an evolutionary vestige of a secretory lysosome but that in these cells the secretory and lysosomal functions have subsequently become separated. In contrast, regulated secretory cells of the hemopoietic lineage have optimized a more primitive secretory mechanism and use it to regulate the release of soluble and membrane proteins.
| Acknowledgments |
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Submitted: 9 August 1999
Revised: 3 September 1999
Accepted: 3 September 1999
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