Immunochemical characterization of bovine brain cathepsin D

Proteolysis of microtubule-associated protein 2 and tubulin by cathepsin D

The in vitro degradation of microtubule-associated protein 2 (MAP-2) and tubulin by the lysosomal aspartyl endopeptidase cathepsin D was studied. MAP-2 was very sensitive to cathepsin D-induced hydrolysis in a relatively broad, acidic pH range (3.0-5.0). However, at a pH value of 5.5, cathepsin D-mediated hydrolysis of MAP-2 was significantly reduced and at pH 6.0 only a small amount of MAP-2 was degraded at 60 min. Interestingly, the two electrophoretic forms of MAP-2 showed different sensitivities to cathepsin D-induced degradation, with MAP-2b being significantly more resistant to hydrolysis than MAP-2a. To our knowledge, this is the first clear demonstration that MAP-2 is a substrate in vitro for cathepsin D. In contrast to MAP-2, tubulin was relatively resistant to cathepsin D-induced hydrolysis. At pH 3.5 and an enzyme-to-substrate ratio of 1: 20, only 35% of the tubulin was degraded by cathepsin D at 60 min. The cathepsin D-mediated hydrolysis of tubulin was optimal only at pH 4.5. These results demonstrate that MAP-2 and tubulin are unequally susceptible to degradation by cathepsin D. These data also imply a potential for rapid degradation of MAP-2 in vivo by cathepsin D either in lysosomes or perhaps autophagic vacuoles of the neuron.

Changes induced in astrocyte cathepsin D by cytokines and leupeptin

Cathepsin D is widely, but unevenly, distributed among cells and is capable of degrading a number of neural peptides and proteins. The present study was undertaken to examine the level of cathepsin D in astrocytes that might be relevant to its induction in inflammatory demyelination. Primary astrocytes were cultured from neonatal rat cerebrums according to the method of McCarthy and de Vellis. Based on staining for cell markers, cultures were greater than 95% astrocytes and less than 3% microglia. Under serum-free conditions, leupeptin induced a 1.4- to 2.0-fold increase, maximal by 48 hours, in cathepsin D protein quantified by a radioimmunoassay. Cathepsin D enzymatic activity, inhibitable by pepstatin, also increased. Northern blot analysis demonstrated that leupeptin also increased cathepsin D mRNA expression. Kinetic analysis indicated that maximal cathepsin D mRNA levels are detected 24 h after stimulation with leupeptin. Exposure of astrocytes under the same conditions to rat recombinant interferon-gamma, human recombinant tumor necrosis factor-alpha, human recombinant interleukin-1 beta, lipopolysaccharide, calcium ionophore, or a combination of these reagents did not increase the level of cathepsin D above controls. These results indicate that astrocytic cathepsin D mRNA and protein can be induced by selected materials. Furthermore, the effects attributed to leupeptin as a proteinase inhibitor may be modified by its ability to increase cathepsin D activity.

Cathepsin D from human leukocytes. Purification by affinity chromatography and properties of the enzyme

Cathepsin D of human leukocytes was isolated and characterized. Purified leukocytes were lysed under nitrogen pressure and the proteinase activity precipitated by centrifugation at 48,000 x g. The precipitate was extracted by various buffers. The yield of cathepsin D was almost pH-independent but could be increased by Triton X-100. Employing gel chromatography the activity was found at a molecular mass close to 42,000 Da. Purification of the enzyme was performed by a two-step procedure using pepstatin-Sepharose chromatography and ion exchange chromatography. Three multiple forms of the enzyme were separated by ion exchange chromatography. The isoelectric points of the three forms of the enzyme were close to pH 5.0. The enzyme showed the typical characteristics of the acid proteinase cathepsin D. Enzyme activity was influenced by heavy metals such as Hg2 and Fe3 as well as by typical inhibitors for carboxyl-proteinases such as diazoacetyl-DL-norleucine methyl ester, 1,2-epoxy-3-(4-nitrophenoxy)propane and 4-bromo-phenacylbromide. An immunological comparison with cathepsin D from human liver by immunodiffusion and immunoelectrophoresis indicates identity of the two enzymes.

Alterations of the posttranslational processing of a lysosomal enzyme in C6 glioma cells

Cathepsin D was assessed in C6 glioma cells grown in medium with an intermediate- or low-percent composition of serum. The amount, form, and subcellular location of cathepsin D differed after treatment with cyanate or monensin in cells grown in a low-serum, growth-factor-supplemented medium. Immunoblotting showed that cathepsin D in the lysosomal fraction of the C6 cell line had a molecular weight (Mr) of 42 kD, whereas that in the microsomal fraction had Mr's of 42, 47, and 78 kD. After treatment for 1 to 16 hr with 4 mmol/L cyanate and subcellular fractionation, the molecular weight of lysosomal cathepsin D was the same in treated and untreated cells, but more enzyme was found in lysosomes of treated cells at 8 and 16 hr. In the microsomal fraction, the amounts of both the 42 and 47 kD forms were increased after 1 to 16 hr of treatment. When exposed to 20 mmol/L cyanate, C6 cells remained viable, but compared with untreated cells, they showed 25% less lysosomal cathepsin D, with increased amounts found in the microsomal fraction. The 78 kD protein detected by immunoblotting was present in both the lysosomal and microsomal fractions but was predominant in the latter. The apparent molecular weight of this protein was the same after cyanate but differed with monensin, where Mr's of 39, 42, and 73 kD were found. Monensin-treated cells had less lysosomal cathepsin D and relatively more microsomal enzyme. The differing molecular weights of cathepsin D from cyanate- and monensin-treated cells suggest that their inhibitions occur at different processing loci in distal elements of the Golgi stacks. The differences in the pI of cathepsin D and the number of its forms from cyanate- and monensin-treated cells are also consistent with interference in the late stages of glycoprotein maturation. In this paper we show that the amount, molecular form, and consequent intracellular location of cathepsin D in cells of the C6 line can be affected by agents that selectively disrupt stages in Golgi-related protein modification and transport.

Increased concentrations of immunoreactive cathepsin D in supraoptic nucleus of the Brattleboro rat

The distribution of cathepsin D, an aspartyl endopeptidase, was measured in selected, discrete nuclei of the forebrain of the Brattleboro rat by means of microdissection and radioimmunoassay. The results indicate that cathepsin D is widely distributed, but in varying amounts among nuclear groups in this region of the brain. High concentrations were detected in the supraoptic and paraventricular nuclei. In studies of the vasopressin-deficient Brattleboro rat, an increased content of cathepsin D in the supraoptic nucleus was observed compared to the heterozygous control. No differences were detected between homozygous and heterozygous Brattleboro rats in the caudate, medial preoptic, suprachiasmatic or paraventricular nuclei or globus pallidus. These results raise the possibility that brain cathepsin D may be involved in the physiological events related to fluid homeostasis.

The distribution of cathepsin D in rat tissues determined by immunocytochemistry

The distribution of cathepsin D (CD) was surveyed in rat tissues by light microscopic immunocytochemistry. Although immunoreactive CD was detected in all tissues examined, there was a marked difference in the amount in the cytoplasm of different cell types of the same organ. In the retina large amounts of CD were present in the pigment epithelium, ganglion cells, and Müller cells. Moderate to large amounts of CD were also found in neuronal perikarya of the gastrointestinal tract and adrenal medulla; in macrophages in the lung, liver, and spleen; in some secretory cells of the submandibular and lacrimal glands; in parts of renal distal convoluted and collecting tubules; and in the surface transitional epithelium of the calyx, ureter, and urinary bladder. Other cells adjacent to cells containing large amounts of the enzyme had little or no detectable CD themselves. These included hepatocytes, the proximal tubular cells of the kidney, selected cells of the submandibular gland, cells of the zona glomerulosa of the adrenal cortex, and lymphocytes in lymphoid organs. The localization of CD indicates that its degradative effect is exerted preferentially in certain cell types and suggests that physiological influences on CD may have a variety of effects in different organs.

Immunocytochemical studies of cathepsin D in human skeletal muscle

The distribution of cathepsin D, an acidic endopeptidase, was localized by immunocytochemistry in human skeletal muscle obtained from 34 persons with a variety of neuromuscular disorders. Normal human skeletal muscle contained small amounts of cathepsin D, all of which was found close to the sarcolemmal membrane. Immunoreactive cathepsin D was present in the cytoplasm of many infiltrating phagocytic cells and was increased in skeletal muscle fibers from patients with muscular dystrophies, inflammatory myopathies, rhabdomyolysis, acid maltase deficiency, and neurogenic atrophy. In cases of Duchenne type muscular dystrophy, the increase in cathepsin D was especially prominent in small regenerating fibers, in which it was visualized at the ultrastructural level in lysosome-like organelles and extralysosomal locations. The function of cathepsin D in skeletal muscle is unclear, but the present findings suggest a possible role in muscle regeneration and repair. Such a role would necessitate careful selection of drugs which interfere with proteolytic activity if they are to be used as therapeutic agents in treating neuromuscular diseases.

Immunocytochemical localization of cathepsin D in rat neural tissue

The localization of cathepsin D (CD) in normal adult rat neural tissue was determined with an indirect immunoperoxidase technique utilizing rabbit anti rat brain CD followed by a horseradish peroxidase conjugate of the Fab portion of goat antirabbit IgG. The immunoreactive enzyme protein was distributed predominantly in a granular pattern, presumably lysosomal, in neurons and choroid plexus epithelium. Smaller amounts were detected in oligodendrocytes and ependymal cells. The neuronal localization included the perikaryon and its processes, was widely distributed, and displayed a range of staining intensities in different anatomical areas. Immunoreactive CD was heavily concentrated in brain stem and spinal cord motoneurons, large neurons of the caudate nucleus, neurons of several nuclear groups, especially the paraventricular and supraoptic, in the hypothalamus, and neurons of superior cervical and dorsal root ganglia. CD was also readily detected in brain stem sensory neurons, pyramidal cells of the hippocampus, inferior olive and Purkinje cells, but was absent or present in very small quantities in the granule cells of the cerebellar cortex, the more superficial layers of the neocortex, and smaller neurons of the caudate nucleus. This distribution suggests that CD may have a major role in specific chemical events in neural functions and peptidergic pathways and could be involved in the alterations of certain neural structures in disease states.