Giant lysosomes, a cytoplasmic marker in osteoclasts of beige mice

The lysosomal trafficking regulator "LYST": an 80-year traffic jam

Lysosomes and lysosome related organelles (LROs) are dynamic organelles at the intersection of various pathways involved in maintaining cellular hemostasis and regulating cellular functions. Vesicle trafficking of lysosomes and LROs are critical to maintain their functions. The lysosomal trafficking regulator (LYST) is an elusive protein important for the regulation of membrane dynamics and intracellular trafficking of lysosomes and LROs. Mutations to the LYST gene result in Chédiak-Higashi syndrome, an autosomal recessive immunodeficiency characterized by defective granule exocytosis, cytotoxicity, etc. Despite eight decades passing since its initial discovery, a comprehensive understanding of LYST's function in cellular biology remains unresolved. Accumulating evidence suggests that dysregulation of LYST function also manifests in other disease states. Here, we review the available literature to consolidate available scientific endeavors in relation to LYST and discuss its relevance for immunomodulatory therapies, regenerative medicine and cancer applications.

Formation and function of the ruffled border in osteoclasts

Osteoclasts are multinucleated hematopoietic cells specialised for bone resorption. Dissolution of the inorganic fraction of the bone matrix is mediated by acidification of the bone surface in contact with the osteoclast whereas secreted lysosomal enzymes digest organic components. Through massive exocytosis, the plasma membrane in contact with the bone surface enlarges into the ruffled border, which has unusual features more similar to endosomal/lysosomal membranes. Maintenance of the ruffled border during resorption is achieved through a balance between exocytosis and endocytosis. Inactivation of proteins necessary for the extracellular acidification or of the proteases involved in matrix degradation leads to osteopetrosis; a disease characterised by dense bones.

Use of a phage display technique to identify potential osteoblast binding sites within osteoclast lacunae

There is a temporal coupling between the processes of bone resorption and bone formation in normal skeletal remodeling. That is, osteoblastic activity usually follows episodes of osteoclastic activity. However, what has not been universally appreciated is that there also is a spatial coupling between these processes. Bone formation only occurs in the immediate vicinity of the resorptive event. In this study, we describe a phage display technique that has been used to identify the mechanisms by which osteoblasts recognize components of the prior resorbed lacunar surface. Using a type V tartrate-resistant acid phosphatase (TRAP) as the bait and a random peptide M13 phage display library as the probe, we have identified specific sequences that show a very high affinity for TRAP. One of these peptides, designated clone 5, has a subnanomolar Kd for TRAP, interacts with TRAP in a Far-Western assay, binds exclusively to TRAP within osteoclast lacunae, is present in osteoblasts, and can effectively block osteoblast binding to resorption surfaces. The clone 5 peptide shows a high homology to glypican 4 (GPC4), a proteoglycan attachment receptor found in a number of cell types.

Pluripotent hemopoietic stem cells give rise to osteoclasts in vitro: effects of rGM-CSF

Studies involving bone marrow transplantation of osteopetrotic rodents have provided evidence for the lineage of the osteoclast. Recent investigations have demonstrated that pluripotent hemopoietic stem cells (PHSC) isolated from the bone marrow of normal animals cure the skeletal sclerosis and result in the formation of normal osteoclasts when transplanted into ia osteopetrotic rats. A criticism of these findings is that the microenvironment of the osteopetrotic bone and the bone marrow compartment may be unique in its ability to induce the differentiation of these stem cells into osteoclasts. To test this hypothesis, PHSC were co-cultured with fetal metatarsal bones from normal animals. PHSC were isolated from normal bone marrow using FITC-labelled monoclonal antibodies directed against rat Thy 1.1 and fluorescence-activated cell sorting. The PHSC or whole mononuclear bone marrow were co-cultured with 20-day fetal rat metatarsal rudiments. In some cultures, recombinant mouse granulocyte-macrophage colony-stimulating factor (rGM-CSF) (250 U per culture) was added in addition to the PHSC. After 7 days the fetal bones were prepared for light and electron microscopy and the number of osteoclasts generated in vitro was determined. The PHSC isolate generated as many osteoclasts as the whole mononuclear bone marrow. The addition of rGM-CSF did not enhance the generation of osteoclasts in either control bones or in bones cultured with PHSC. These results are equivalent to those reported in the osteopetrotic transplant system.

The effects of transplantation of granulocyte-macrophage progenitors on bone resorption in osteopetrotic rats

Osteopetrosis in the ia (incisors absent) rat is the result of reduced bone resorption due to abnormal osteoclasts. This mutant and others have been used to determine the precursor(s) to osteoclasts. Hemopoietic stem cells, isolated from bone marrow of normal littermates, cure the skeletal sclerosis and result in the formation of normal osteoclasts when transplanted into ia rats. These studies were conducted to define further the precursor to the osteoclast by evaluating the effects of the transplantation of granulocyte-macrophage progenitors on bone resorption in the ia rat. Granulocyte-colony forming cells (G-CFC), granulocyte-macrophage-colony forming cells (GM-CFC), and macrophage-colony forming cells (M-CFC) were isolated from normal bone marrow using an FITC-labeled monoclonal antibody directed against rat Thy-1.1 and fluorescence-activated cell sorting. The isolates were evaluated in soft agar culture; granulocyte isolates generated 71% G-CFC of all colonies formed and were enhanced 27 times over unfractionated cells. Mixed isolates generated 57% GM-CFC of all colonies formed and were 15 times enhanced, while macrophage isolates were 75% M-CFC with an enhancement factor of 18. The isolated populations were injected into 3-week-old ia recipients and evaluated for the ability of these cellular isolates to correct the bone resorption defect by measuring the size of the tibial marrow cavity and by identifying morphologically normal osteoclasts. In addition, isolated populations of cells were labeled with FITC and injected into ia donors to determine if labeled osteoclasts developed.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of cholera toxin on cyclic AMP accumulation and bone resorption in cultured mouse calvaria

We have utilized the adenylate cyclase stimulator, cholera toxin, as a tool to test the role of cyclic AMP as a mediator of the effects on bone resorption by the calcium-regulating hormones, parathyroid hormone (PTH) and calcitonin. The effects on bone resorption were studied in an organ culture system using calvarial bones from newborn mice. Cyclic AMP response was assayed in calvarial bone explants and isolated osteoblasts from neonatal mouse calvaria. Cholera toxin caused a dose-dependent cAMP response in calvarial bones, seen at and above approx. 1-3 ng/ml and calculated half-maximal stimulation (EC50) at 18 ng/ml. The stimulatory effect of cholera toxin could be potentiated by the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX, 0.2 mmol/l). Cyclic AMP accumulation in the bones was maximal after 4-6 h, and thereafter declined. However, activation of the adenylate cyclase was irreversible and the total amount (bone + medium) of cAMP produced, in the presence of IBMX (0.2 mmol/l), increased with time, for at least 48 h. In osteoblast-like cells cholera toxin (1 microgram/ml) stimulated the cellular levels of cAMP with a peak after 60-120 min, which could be potentiated with IBMX. The total cAMP accumulation indicated an irreversible response. In short-term bone organ cultures (at most, 24 h) cholera toxin, at and above 3 ng/ml, inhibited the stimulatory effect of PTH (10 nmol/l) on 45Ca release from prelabelled calvarial bones. The inhibitory effect of cholera toxin (0.1 microgram/ml) on 45Ca release was significant after 6 h and the calculated IC50 value at 24 h was 11.2 ng/ml. Cholera toxin (0.1 microgram/ml) also inhibited PTH-stimulated (10 nmol/l) release of Ca2+, inorganic phosphate (Pi), beta-glucuronidase, beta-N-acetylglucosaminidase and degradation of organic matrix (release of 3H from [3H]proline-labelled bones) in 24 h cultures. 45Ca release from bones stimulated by prostaglandin E2 (1 mumol/l) and 1 alpha-hydroxyvitamin D3 (0.1 mumol/l) was also inhibited by cholera toxin (0.3 microgram/ml) in 24-h cultures. The inhibitory effect of cholera toxin on bone resorption was transient, and in long-term cultures (120 h) cholera toxin caused a dose-dependent, delayed stimulation of mineral mobilization (Ca2+, 45Ca, Pi), degradation of matrix and release of the lysosomal enzymes beta-glucuronidase and beta-N-acetylglucosaminidase.(ABSTRACT TRUNCATED AT 400 WORDS)

Pluripotent hemopoietic stem cells give rise to osteoclasts

Osteopetrosis in the ia (incisors absent) rat is the result of reduced bone resorption due to abnormal osteoclasts. The mutant osteoclasts lack a ruffled border--the membrane specialization involved in osteolysis. Studies in the ia mutant have shown that when pluripotent hemopoietic stem cells from normal littermates are transplanted into ia recipients, normal osteoclasts are formed and the skeletal sclerosis is eventually cured. The present study was conducted to provide evidence for the mechanism of the cure. Do the transplanted stem cells provide a helper function, i.e. secrete soluble factor(s) which transform pre-existing osteoclasts, or do they fuse with each other or pre-existing osteoclasts, or do they fuse with each other or pre-existing osteoclasts to form functional osteoclasts? Using the procedures described by Gold-schneider and co-workers, and fluorescence-activated cell sorting (FACS), pluripotent hemopoietic stem cells were isolated from normal rat bone marrow, labeled with saturated FITC, and injected intravenously into irradiated ia rats. After 48 hr, the recipients' long bones were removed and split longitudinally, and the endosteal surface was scraped. The resulting cellular suspension containing osteoclasts was examined by phase contrast and fluorescence microscopy. Fluorescing mononuclear cells of donor origin that had homed to the bone marrow demonstrated moderate cytoplasmic fluorescence. Approximately 30% of the osteoclasts observed demonstrated light cytoplasmic fluorescence. When cellular pools incapable of curing osteopetrosis (thymocytes) were labeled and injected into ia recipients, no labeled osteoclasts were observed. These studies indicated that pluripotent hemopoietic stem cells, when transplanted into ia hosts, fuse with each other and differentiate into osteoclasts or fuse with pre-existing osteoclasts.

Differences in basal and stimulated cyclic AMP content in calvaria bones from normal mice and mice with an impaired lysosomal function (beige mice)

Earlier we have shown that bone resorption is impaired in cultured calvaria from beige mice (an animal equivalent of the Chediak-Higashi syndrome in man). In the present study we have compared the concentrations of cyclic AMP and cyclic GMP in calvarial bones from beige mice with the nucleotide concentrations in bones from corresponding normal mice. In six independent experiments the basal concentrations of cyclic AMP was significantly (P less than 0.01) higher in bones from beige mice (on an average 50% augmented). The ratio of cyclic AMP/cyclic GMP was 2.43 times higher (P less than 0.01) in bones from beige mice. After stimulation with the phosphodiesterase inhibitor isobutylmethylxanthine and prostaglandin E2 no significant differences of cyclic AMP concentrations between beige and control mice could be registered. The response to adenosine was significantly higher (P less than 0.005) in bones from beige mice (4.3 +/- 0.4-fold of basal cyclic AMP concentrations, mean +/- SE) compared to control mice (1.9 +/- 0.4-fold of basal, mean +/- SE). The increased basal concentration of cyclic AMP in calvaria from beige mice may be due to increased sensitivity to some agonists, such as adenosine, rather than simply being a function of cell mass.

The lifespan of osteoclasts: experimental studies using the giant granule cytoplasmic marker characteristic of beige mice

Osteoclasts are large multinucleated skeletal cells that form by fusion of bloodborne mononuclear precursors. Fusion with mononuclear precursors occurs throughout life, and survival of osteoclasts is believed to be dependent upon continued replenishment by fusion. This study examined osteoclast lifespan, defined as maximal survival without fusion, in normal mice irradiated to eliminate host stem cells and rescued with stem cells from beige (bg) mice whose osteoclasts have a distinctive phenotype. Osteoclasts of donor phenotype appeared during the second week and progressively increased so that by the sixth week no osteoclasts of host phenotype were present. Radiation alone did not produce any change in osteoclast phenotype. These data are interpreted to indicate that the maximal survival of osteoclasts without fusion of precursors is less than 6 weeks.

Immunological competence in osteopetrotic (ia) rats

Osteopetrosis in ia rats is characterized by excessive skeletal mass and reduced bone resorption. The skeletal defects can be corrected by the transfer of mononuclear spleen cells from normal littermates. These studies suggest that osteopetrotic mutants may also have defective immune functions. The op, osteopetrotic, rat demonstrates early thymic atrophy and immune function which decreases with age. Several studies have shown significantly reduced responses to T and B cell mitogens by spleen cells from osteopetrotic mutant mice. The problem with these latter studies is that different populations of cells have been compared in mutants and normal littermates because the spleen is a focus of extramedullary hemopoiesis in osteopetrotic animals. To circumvent this problem, the Ficoll-Hypaque, mononuclear isolate of spleen and mesenteric lymph node from 5-week-old ia and normal littermates were compared. Under appropriate culture conditions the cells were exposed to Con A, PHA, and LPS for 3 days and 3H-thymidine for the last 24 hours. In all cases, the response to optimal concentrations of the 3 mitogens was similar for ia and normal spleen and lymph node cells (ia/control ratios ranged from 0.6 to 1.2). The cellular composition of the samples tested in the mitogen assays were also evaluated by fluorescent microscopy using FITC-conjugated monoclonal antibodies directed against specific cell surface markers. The percentage of B cells, macrophages, total T cells, and helper T cells were found to be similar in the Ficoll-Hypaque isolate of ia and normal spleen and lymph nodes. Likewise, the ia mutant does not show any signs of abnormal thymic involution. These results indicate normal immune function in the ia mutant when similar populations of cells are compared.

The origin of osteoclasts: evidence, clinical implications and investigative challenges of an extra-skeletal source

Recent evidence for an extraskeletal origin of osteoclasts and the historical record of the genesis of osteoclasts are examined critically. Reviews of the structure, function and development of osteoclasts from mononuclear precursors, the local regulation of bone resorption and the coupling of bone formation to preceding resorption are presented as a background for discussing the clinical implications for management of osteolytic bone diseases. The roles of osteoclasts and macrophages as phagocytes are compared and contrasted, and recent evidence for macrophage heterogeneity resulting from site-specific monoblastic precursors is reviewed. The implications of these recent developments in macrophage biology are extrapolated to osteoclasts and the existence of site-specific, extraskeletal osteoclast precursors is proposed. Finally, the investigative challenges inherent in these perspectives are discussed.

Current views on the ontogeny of macrophages and the humoral regulation of monocytopoiesis

Macrophages do not constitute a stable population but are constantly renewed by an influx of monocytes and by local division of immature mononuclear phagocytes that have also recently arrived in the tissues. The calculated mean turn-over time of tissue macrophages lies between four and six days. During an inflammatory response the production of monocytes in the bone marrow is increased. This process is regulated by a humoral factor, FIM, which is formed and secreted by tissue macrophages. The response of monocyte precursors to FIM is under genetic control.

Morphological evidence of reduced bone resorption in osteopetrotic (op) mice

Osteopetrosis, a metabolic bone disease in which a generalized accumulation of bone mass reduces or obliterates marrow cavities, is inherited as an autosomal recessive in several mammalian species. A recently discovered mutation in mice, the osteopetrosic (op) mutation, exhibits an elevation in bone matrix synthesis and a resistance to the hypercalcemic effects of exogenous parathyroid extract when young mutants are compared with normal littermates. This investigation examined the number, cytology, and ultrastructure of osteoclasts and the structure of bone surfaces in op mice in a morphologic assessment of bone resorption. Compared with normal littermates, op mice have a severe deficiency of osteoclasts, which also contain unusual toluidine blue-positive and electron-dense cytoplasmic inclusions and hypertrophy of clear zones and ruffled borders. Marrow spaces in op mice contained large numbers of megakaryocytes and large lipoid masses. Bone surfaces exhibiting evidence of resorption by scanning electron microscopy in normal littermates showed no evidence of resorption in op mice. Instead, these areas were characterized by morphologic features of bone formation. These data offer morphologic evidence of a reduction of bone resorption in this mutation. They are interpreted to mean 1) that op mice have a severe reduction in numbers or proliferative capacity of osteoclast precursors, which may be related to the cellular inclusions in the osteoclast population, and 2) that hypertrophy of the ruffled borders and clear zones of op osteoclasts is a compensatory attempt to increase bone resorption.

The hematogenous origin of osteoclasts: experimental evidence from osteopetrotic (microphthalmic) mice treated with spleen cells from beige mouse donors

The excessive skeletal mass and reduced bone resorption characteristic of osteopetrosis in microphthalmic (mi) mice can be corrected by irradiation and transfer of spleen cells from a normal littermate. Osteoclasts in beige (bg) mice, a mutation without osteopetrosis, have giant lysosomal granules. These two facts were exploited to trace osteoclast lineage. Microphthalmic mice treated with whole-body irradiation and spleen cells from a beige donor resorbed the excessive skeletal mass and recovered from osteopetrosis. Furthermore, osteoclasts in treated mi mice had giant lysosomal granules and resembled those found in bg donors when examined by light and transmission electron microscopy. These data provide direct evidence for a hematogenous origin of osteoclasts in mammals.

The origin of phagocytic cells in the joint and bone

Studies dealing with the concept of the mononuclear phagocyte system are described, and the origin of synovial type A cells and osteoclasts is discussed in some detail. All of the available evidence indicates that both derive from precursor cells in the bone marrow. It seems justifiable to conclude that monocytes transform into type A synovial cells and fuse to form osteoclasts.