The histochemical and cytochemical localization of proteases

Enzyme histochemistry of cutaneous sensory nerve formations

The cutaneous sensory nerve formations belong to the structures which are studied intensely by the enzyme activity histochemistry since the early history of this technique. The histochemical localization of the activities of nonspecific cholinesterase, alkaline phosphatases, acid phosphatase, adenosine tri- and diphosphatases, adenylate cyclase, and dipeptidylpeptidase-IV in the cutaneous sensory nerve formations, mainly sensory corpuscles, is reviewed. The histochemical approach has brought new knowledge of both morphological building of these unique structures and their biochemical constituents. Taken together, the present results of enzyme histochemistry provide insight into the function of enzymes, and disclose new relationships between the sensory terminals and auxiliary structures in the cutaneous sensory nerve formations.

The role of the intralysosomal pH in the control of autophagic proteolytic flux in rat hepatocytes

Current methods to estimate changes in intralysosomal pH in hepatocytes do not discriminate between lysosomes and other intracellular acidic compartments. To obtain selective information on the change in lysosomal function in response to a change in lysosomal pH we have used the liberation of fluorescent 4-methoxy-2-naphthylamide from low concentrations of lysyl-alanyl-4-methoxy-2-naphthylamide, a substrate of lysosomal dipeptidylpeptidase II. Using permeabilized and intact hepatocytes, the activity of this enzyme in response to manipulations meant to increase the intralysosomal pH was compared with intralysosomal protein degradation and with the accumulation of [14C]chloroquine as a pH indicator of intracellular acidic compartments. The data show that changes in intralysosomal pH are indicated by changes in dipeptidylpeptidase II activity and that these are mainly due to a pH-dependent change in substrate accumulation in the lysosomes. Subsequently, the method was applied to establishing the extent to which an increase in intralysosomal pH can contribute to the inhibition of autophagic proteolysis in intact hepatocytes caused by a decrease in intracellular ATP, by an increase in amino acid concentration and by hypo-osmotic cell swelling. The following observations were made. (a) Moderate changes in intracellular ATP do not affect the lysosomal pH. (b) Hypo-osmotic cell swelling, which promotes inhibition of proteolysis by amino acids in freshly isolated hepatocytes, does not affect the lysosomal pH. (c) In addition to their known inhibitory effect on autophagic sequestration, amino acids (leucine in particular) can increase the lysosomal pH and thus inhibit intralysosomal protein degradation directly. (d) Low concentrations of the acidotropic agent methylamine increase the lysosomal pH without having an effect on autophagic proteolytic flux. It is concluded that autophagic proteolysis is not controlled by changes in the lysosomal pH.

Aminopeptidase M and dipeptidyl peptidase IV activity in epithelial skin tumors: a histochemical study

The activities of microsomal alanylaminopeptidase (APM EC 3.4.11.2) and of dipeptidyl dipeptidase IV (DPP IV EC 3.4.14.5) were histochemically studied in frozen sections of normal skin, seborrheic keratosis, basal cell carcinoma, solar keratosis, Bowen's disease and squamous cell carcinoma using amino acid- or peptide-4-methoxy-2-naphthylamides as specific chromogenic substrates. Compared to biochemical and immunohistochemical methods, the histochemical technique used in this study allows distinct localization of protease activity within the tumor tissue and the tumor-associated stroma. Strong APM activity was detectable only in the stroma of basal cell carcinoma, a result which reflects the particular tumor-stroma interaction of this semimalignant tumor. APM activity was not detectable in either healthy epidermis or the tumor parenchyma. Altered activity of DPP IV was found in the tumor cells as well as in the surrounding connective tissue: precancerous dermatoses and basal cell carcinomas had higher levels of DPP IV-activity than normal skin or benign seborrheic keratosis. Poorly differentiated malignant squamous cell carcinomas, however, showed no histochemically detectable DPP IV-activity at all. This result is in line with reports of decreased activity of this enzyme in cases of malignancy.

Cytochemical localization of dipeptidyl peptidase II activity in rat incisor tooth ameloblasts

Dipeptidyl peptidase II (DPP II), E.C. 3.4.14.2, a serine class endopeptidase, is widely used as a lysosomal marker in cytochemical studies. To date most ultrastructural studies of ameloblasts use the presence of acid phosphatase activity to identify cellular organelles to be lysosomal. Using decalcified rat mandibles, with kidney tissue as a positive control, DPP II activity, was assessed with specific substrate Lysyl-alanine-4-methoxy-2-naphthylamide in ameloblasts at an ultrastructural level. Reaction product (RP) indicative of DPP II activity was observed only within lysosome-like organelles. These RP-labelled organelles were only localized in the supra- or para-nuclear regions of the ameloblasts, which corresponds with previous studies using acid phosphatase cytochemistry. However, in contrast with these studies, RP was not detected in the distal region of the ameloblasts, viz., in the Tomes' processes of the secretory ameloblasts or near the ruffled border in the maturation ameloblasts. The transitional ameloblasts were notable for the intensity of staining of their RP-labelled organelles. We propose that DPP II may have a role in programmed cell death which is thought to occur in this transition zone. Biochemical analysis of rat incisor enamel organ homogenates, indicated tissue fixation resulted in an 82% reduction in DPP II activity, although the specific activity of DPP II was not affected.

A histochemical study by light and electron microscopy of the distribution of dipeptidyl peptidase-IV activity in the human dental pulp

This activity was demonstrated in blood vessels, fibroblast-like cells, odontoblasts and Schwann cells surrounding non-myelinated axons. It was present on both the luminal and abluminal plasma membrane of endothelial cells. The plasma membrane of fibroblast-like cells and their processes had patches of fine electron-dense end-product corresponding to DPP-IV activity, while the plasma membrane of odontoblasts was loaded with a consistent reaction product. DPP-IV activity in the non-myelin-forming Schwann cells was in the plasma membrane of the free surface in contact with the extracellular matrix as well as in the plasma membrane in close contact with the axolemma. The plasma membrane of Schwann cells producing myelin sheath had no positive reaction for DPP-IV. The DPP-IV activity in the plasma membrane of fibroblast-like cells and odontoblasts may be associated with fibronectin-mediated adhesion to collagen, whereas the membrane-bound DPP-IV activity in endothelium and non-myelin-forming Schwann cells may be involved in cleavage of neuropeptides of other biologically active peptides.

Catalytic histochemistry of acid and neutral hydrolases in plant seedlings

In contrast to human and animal tissues, little information is available on the activity, distribution and functional role of acid and neutral hydrolases in plant cells and tissues. Because it is known that these enzymes are relatively active during germination, they were analysed histochemically during this process using light microscope azo, azoindoxyl, indigogenic and tetrazolium methods. Proteases, glucosidases and glucuronidases could not be detected. Non-specific acid phosphatases were species-independent and showed considerable activities in aleuron and nutritional cells, in other cell types of cotyledon or endosperm tissue and in different types of embryonic cells. Acid glycosidases and non-specific esterases, in contrast, displayed a species-dependent activity and differences in localization. Of the glycosidases, alpha-D-galactosidase was the most active. Non-specific esterases, acid phosphatase and glucosaminidase were also present in the extracellular matrix. During germination, acid hydrolase activity either decreased or increased, depending on the seedling species and enzyme.

Membrane structures in normal and essential fatty acid-deficient stratum corneum: characterization by ruthenium tetroxide staining and x-ray diffraction

Despite the importance of intercellular lamellar bilayers for stratum corneum (SC) barrier function, knowledge about the structure of these bilayers is limited due to their poor visualization and/or retention. Whereas substitution of ruthenium tetroxide (RuO4) for osmium tetroxide fixation provides clear images of these bilayers, the usefulness of RuO4 has been limited by its slow penetration and cytotoxicity. Utilizing a new fixation protocol for RuO4, we obtained clear images of lamellar domains at all levels of murine SC. Computer-aided image reconstructions demonstrated a lamellar spacing of 129 +/- 2 A, which agreed with x-ray diffraction data from parallel, unfixed samples (131 +/- 2 A), a spacing not affected by hydration. Furthermore, novel structures were seen in the intercellular spaces of normal SC. Finally, in murine essential fatty acid deficiency (EFAD), the overall lamellar spacing is comparable to normal [127 +/- 7 A by computer transform vs. 131.9 +/- 2 A (hydrated) and 129.6 +/- 2.2 A (dry) by x-ray diffraction]. Yet, these domains are structurally abnormal, displaying regions with either an excess or absence of lamellae. The new RuO4 protocol provides quantitative information about SC lamellar dimensions and morphologic abnormalities in bilayer distribution and substructure in EFAD stratum corneum that are not detected by either x-ray diffraction or computer-aided image reconstruction. Thus, the barrier abnormality in EFAD stratum corneum can be ascribed either to focal depletion of lamellae or abnormalities in lamellar substructure.

Histochemical and biochemical studies of dipeptidyl peptidase I (DPP I) in laboratory rodents

Different from other proteases the halide- and thiol-dependent lysosomal dipeptidyl peptidase I (DPP I, cathepsin C, EC 3.4.14.1.) was not thoroughly analysed up to now. Therefore, using complementary methods of catalytic histochemistry and biochemistry, DPP I was studied in many rat and mouse organs and compared with DPP II, another lysosomal protease, whose biochemical and histochemical behaviour is known. Continuous (kinetic) fluorometric measurements revealed Gly-Arg naphthylamine (NA) as the substrate with the highest rate of hydrolysis in which, however, also microsomal alanyl aminopeptidase may participate. A more specific substrate appeared to be Pro-Arg-NA; Gly-Phe-NA, and Ser-Tyr-NA were inferior substrates. The Km values were 0.1 mmol/l and 0.2 mmol/l for Gly-Arg-NA and Pro-Arg-NA. The optimal substrate concentration was between 1 and 2.5 mmol/l and the optimal pH value between 5 less than or equal to pH less than or equal to 5.5. The highest reaction velocity was measured with cacodylate or phosphate buffer. Chloride ions and thiol reagents increased the rate of hydrolysis, but an absolute chloride or thiol dependence was not found. Formaldehyde or glutaraldehyde inhibited DPP I depending on the type and concentration of aldehyde. The DPP I activities in rat and mouse organs varied species-dependently and were highest in the extraorbital gland, liver, jejunum, and kidney. In many of the organs, DPP I activity differed considerably from that of DPP II. For catalytic DPP I histochemistry and localization of the enzyme in lysosomes and secretion granules freeze-dried celloidin-mounted cryosections incubated with Gly-Arg-4-methoxy-2-naphthylamine in the presence of Fast Garnet GBC or Fast Blue B were the method of choice in comparison with other tissue pretreatments, Pro-Arg-MNA as substrate and other coupling agents. Use of thiol reagents and chloride ions did not improve the histochemical results. Using this method DPP I was visualized in many cells of rat and mouse organs where its existence was not yet known before. At many sites, DPP I and DPP II distribution patterns differed considerably. In conclusion, a histochemical chromogenic method is now available, which allows the reliable detection of DPP I as was already possible for DPP II using an MNA substrate.

Developmental patterns in the rat cerebellum after cis-dichlorodiammineplatinum treatment

A cytochemical study was made of some metabolic enzymes in the cerebellar neurons during postnatal ontogenesis after injection of cis-dichlorodiammineplatinum into 10-day-old rats. The profiles during development of neuron-specific enolase immunoreactivity (involved in the glycolytic pathway), dihydrofolate reductase activity (involved in the metabolism of nucleic acids and folate) and dipeptidylaminopeptidase II activity were determined in lobules V-VII of cerebellar vermis. At different developmental stages, treated rats had folia in which the morphology and cytochemical responses of Purkinje neurons were greatly affected. On postinjection day 1 (PD 11), only neuron-specific enolase immunoreactivity was changed, reactions being more intense at the basal pole, which was abnormally enlarged in several neurons. Seven days after treatment (PD 17), the dihydrofolate reductase reaction showed weakly positive cells with small grains of formazan in the perinuclear regions and dipeptidylaminopeptidase II activity, which had appeared at this time in some cells of the controls, was not observed. On PD 25 and PD 35, Purkinje cells, probably undergoing clear degeneration, were negative or very weakly positive in all the reactions. Some tracts of folia had no Purkinje cells. These results suggest that cis-dichlorodiammineplatinum affects the differentiation of Purkinje neurons and interferes first with the glycolytic enzyme and then with some enzymes of the synthetic and catabolic machinery, leading to cellular dysfunction and degeneration.

A histochemical study of the effects of high doses of sodium fluoride on dipeptidyl peptidase II activity in the rat incisor ameloblast

Female Wistar rats, 3 weeks old, were given sodium fluoride in saline solution (isotonic) by intraperitoneal injection at a dose of either 0, 10 or 20 mg per kg body weight. This treatment was given 9 times over 4.5 days. After fixation by perfusion and demineralization in neutral EDTA, hemi-mandibles were sectioned in a cryostat. Sections were stained for dipeptidyl peptidase II activity, using the specific substrate Lys-Ala-MNA and the coupler Fast Blue B for histochemical localization. Staining indicative of dipeptidyl peptidase II was found in the enamel organ of the incisor, particularly in cells of the stratum intermedium and in both secretory and maturation ameloblasts. This staining was markedly reduced in ameloblasts of rats given either 10 or 20 mg sodium fluoride per kg body weight.

Proteinases and their inhibitors in cells and tissues

A large body of evidence has been assembled to indicate the substantial importance of proteolytic processes in various physiological functions. It has recently become clear too that endo-acting peptide bond hydrolases provisionally characterized and classified at present as serine, cysteine, aspartic and metallo together with unknown catalytic mechanism proteinases sometimes act in cascades. They are controlled by natural proteinase inhibitors present in cells and body fluids. In the first part of the present monograph the author was concerned to present an overview on the morphological and physiological approach to localization, surveying reaction principles and methods suitable for visualization of proteolytic enzymes and their natural and synthetic inhibitors. In the second part the roles played by proteinases have been summarized from the point of view of cell biology. The selection of earlier and recent data reviewed on the involvement of proteolysis in the behavior of individual cells reveals that enzymes, whether they be exogeneous or intrinsic, can be effective and sensitive modulators of cellular growth and morphology. There exists a close correlation between malignant growth and degradation of cells. It appears likely that as yet unknown or at least so far inadequately characterized factors that influence the survival or the death of cells may turn out to be proteinases. The causal role of extracellular proteolysis in cancer cell metastases, in stopping cancer cell growth and in cytolysis remains for further investigated. Ovulation, fertilization and implantation are basic biological functions in which proteolytic enzymes play a key role. The emergence of new approaches in reproductive biology and a growing factual basis will inevitably necessitate a reevaluation of present knowledge of proteolytic processes involved. The molecular aspects of intracellular protein catabolism have been discussed in terms of the inhibition of lysosomal and/or non-lysosomal protein breakdown. Peptide and protein hormone biosynthesis and inactivation are still at the centre of interest in cell biology, and a number of proteinases have been implicated in both processes. A number of conjectures partly based on the author's own work have been discussed which suggest the possibility of the involvement of proteolysis in exocytosis and endocytosis. The author's optimistic conclusion is that through the common action of biochemists, cell biologists, cytochemists, and pharmacologists the mystery of cellular proteolysis is beginning to be solved.