Aminopeptidase in bone cells

OSTEOCLASTS: ORGANIZATION IN CHICK EMBRYO BONE

A multinucleated endosteum was observed in the long bones of chick embryos aged 14 to 17 days. The endosteum appears to be responsible for extensive bone resorption; the bulk of the diaphyseal bone is almost completely replaced during this particular 3-day interval. Eventually, the endosteum appears to be transformed into numerous individual osteoclasts. Both the endosteum and the osteoclasts are found almost entirely on the medial surfaces of the innermost bone spicules and they seem to destroy collagen by extracellular proteolysis.

Function of osteocytes in bone

Although the structural design of cellular bone (i.e., bone containing osteocytes that are regularly spaced throughout the bone matrix) dates back to the first occurrence of bone as a tissue in evolution, and although osteocytes represent the most abundant cell type of bone, we know as yet little about the role of the osteocyte in bone metabolism. Osteocytes descend from osteoblasts. They are formed by the incorporation of osteoblasts into the bone matrix. Osteocytes remain in contact with each other and with cells on the bone surface via gap junction-coupled cell processes passing through the matrix via small channels, the canaliculi, that connect the cell body-containing lacunae with each other and with the outside world. During differentiation from osteoblasts to mature osteocyte the cells lose a large part of their cell organelles. Their cell processes are packed with microfilaments. In this review we discuss the various theories on osteocyte function that have taken in consideration these special features of osteocytes. These are 1) osteocytes are actively involved in bone turnover; 2) the osteocyte network is through its large cell-matrix contact surface involved in ion exchange; and 3) osteocytes are the mechanosensory cells of bone and play a pivotal role in functional adaptation of bone. In our opinion, especially the last theory offers an exciting concept for which some biomechanical, biochemical, and cell biological evidence is already available and which fully warrants further investigations.

Osteoclast differentiation

The osteoclast is the primary bone resorbing cell. It is a highly specialized multinucleated cell whose primary function is to help in the control of calcium homeostasis. The osteoclast has been very difficult to study because of its relative inaccessability, low numbers, and fragility when isolated from bone. Recently, techniques have been developed to study the cell biology of the osteoclast that have expanded our ability to understand the biological and functional properties of osteoclasts. In this article, studies on the origin of the osteoclast are reviewed and the differentiation markers that are used to detect cells in the osteoclast lineage are discussed. Factors that affect osteoclast differentiation are presented and model systems currently in use for studying osteoclast differentiation are evaluated for their relative strengths and weaknesses. In addition, osteoclast differentiation during tooth eruption and root resorption and the effects of bone matrix elements on osteoclast differentiation are reviewed.

Hormonal regulation of acid phosphatase release by osteoclasts disaggregated from neonatal rat bone

Osteoclasts disaggregated from neonatal rat long bones and incubated on plastic or glass substrates were found to release a considerable proportion of tartrate-resistant acid phosphatase into culture supernatants. Enzyme release was detectable in the supernatant medium of cultures containing as few as ten cells after 1 hr of incubation and proceeded in a linear manner for the ensuing 6 hr. Calcitonin (1 pg/ml) and cytochalasin B (5 micrograms/ml) inhibited release into the supernatant, suggesting that release represents enzyme secretion. Prostaglandin E1 induced transient inhibition followed by recovery; parathyroid hormone and 1,25(OH)2 vitamin D3 were without influence. Acid phosphatase release in these cultures shows a pattern of hormone responsiveness that coincides with the effects of these hormones on bone resorption by isolated osteoclasts. The extent of acid phosphatase release and its regulation by calciotropic hormones imply a central role for acid hydrolase secretion in osteoclastic bone resorption. The experimental system described in this study may facilitate analysis of the pharmacological hormonal and cellular regulation of osteoclastic function.

Enzyme patterns in human endocytotic multinucleate giant cells--a histochemical study

A series of human multinucleate giant cells (MGCs) of the endocytotic type were studied using enzyme histochemical methods for dehydrogenases, glycosidases, phosphatases, and peptidases. Several enzyme patterns were found. The subgroup of MGCs associated with inflammatory granulomatous processes (sarcoidosis, granulomatous myositis, familial granulomatosis, lymphogranuloma, granulomatous cholangitis) was characterized by high activities of nonspecific esterase (NE) and tartrate-sensitive acid phosphatase (AcPase-Ts). There was no detectable activity of peptidases or tartrate-resistant isoenzyme of acid phosphatase (AcPase-Tr). This enzyme equipment was indistinguishable from that in mononuclear precursors in the granulomas. The other MGCs of the series displayed enzyme patterns substantially different from their monocytic precursors (blood monocytes and Langerhans cells). The subgroup of foreign body associated MGCs (resorption of fat, keratin, and suture material) was characterized by high activities of NE, AcPase-Tr, and greatly variable activities of both peptidases studied. The latter lacked predilection for certain subcellular regions. The subgroup of osteoclasts and so-called giant cell tumours (osteoclastoma, giant cell tumour of soft parts, giant cell epulis of peripheral, and central types) displayed very low activity of NE, high activity of AcPase-Tr, and strong activities of peptidases. The latter were localized near the surface membrane of the polykarya. MGCs in histiocytosis X (HX) differed from the previous group by higher values of NE in average. All MGC types had common denominator in the absence of alkaline phosphatase activity, on average intense dehydrogenase activities, mostly low beta-glucuronidase and highly variable alpha-mannosidase activities. The enzyme pattern heterogeneity is discussed with regard to the phenomenon of enzyme induction and depression occurring in course of polykaryon production. The variability of phenomenon may reflect reactive adaptation to varying functional demands imposed on MGCs under different conditions.

Neutral peptidase activities in matrix vesicles from bovine fetal alveolar bone and dog osteosarcoma

Extracellular matrix vesicles from bovine fetal alveolar bone and from a dog osteosarcoma were isolated by differential centrifugation and then fractionated on a discontinuous sucrose density gradient. The fractions were examined by electron microscopy and were analyzed for protein, alkaline phosphatase, aminotripeptidase, and four different beta-naphthylamidase activities. The low-density peak of enzyme activities was shown by electron microscopy to be much more homogeneous than the crude matrix vesicle fraction. Two major peaks of protein and enzyme activities were present, one in the high and one in the low density layers. There was good correlation between the activities of alkaline phosphatase and the various peptidases in the fractions from the sucrose density gradient. These results indicate a coexistence of peptidase and alkaline phosphatase in matrix vesicles. On the other hand, there was generally no correlation between the peptidase and alkaline phosphatase activities in vesicular specimens from bovine liver obtained in the same way. Most of the peptidase activity and about half of the alkaline phosphatase activity were solubilized from bone matrix vesicles by detergents. The extracted alkaline phosphatase and alanyl beta-naphthylamidase activities were separated from each other on a DEAE-cellulose column.

Enzyme histochemical characteristics of human and kitten odontoclasts and kitten osteoclasts: a comparative study using whole cells

Methods for the histochemical demonstration of enzymes in whole cell preparations of odontoclasts and osteoclasts are described. Enzyme histochemical characteristics of human and kitten odontoclasts from resorbing primary teeth and of osteoclasts from kitten femur metaphyses were determined and compared. The enzyme profiles, times for the appearance of detectable reaction product, intensity of the reactions and localization of the reaction products were similar in all three types of giant cell. These findings suggest that odontoclasts have enzyme properties and metabolic functions similar to those of osteoclasts. Species differences appear to be minor, although the NADP-dependent enzymes are less active in human than in kitten odontoclasts. Both odontoclasts and osteoclasts are rich in enzymes concerned with energy production and possess considerable activity of enzymes usually associated with catabolic functions. Metabolic pathways are well developed in respect of the utilization of succinic, malic, glutamic, lactic and isocitric acids, beta-hydroxybutyric acid and glucose-6-phosphate, and they also possess phosphatases, non-specific esterases and leucine naphthylamidase. The distribution of enzyme reaction products for the individual enzymes demonstrated is consistent with the presence in these cells of large numbers of mitochondria and lysosome-like organelles. Considerable phosphatase activity is demonstrable in both odontoclasts and osteoclasts at both neutral and acid pH.

Leucineaminopeptidase in rheumatoid arthritis. Localization in subchondral bone and in synovial fluid cells

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