The sequential limited degradation of bovine myelin basic protein by bovine brain cathepsin D

[Functional degradation of myelin basic protein. Proteomic approach]

Proteolytic degradation of autoantigens is of prime importance in current biochemistry and immunology. The most fundamental issue in this field is the functional role of peptides produced when the specificity of hydrolysis changes during the shift from health to disease and from normal state to pathology. The identification of specific peptide fragments in many cases proposes the diagnostic and prognostic criterion in the pathology progression. The aim of this work is comparative study of the degradation peculiarities of one of the main neuroantigen, myelin basic protein by proteases, activated during progress of pathological demyelinating process, and by proteasome of different origin. The comparison of specificity of different studied biocatalysts gives reason to discuss the critical change in the set of myelin basic protein fragments capable to be presented by major histocompatibility complex class I during neurodegeneration, which can promote the progress of autoimmune pathological process.

Cathepsin D: the lysosomal aspartic proteinase

Cathepsin D was originally known simply as 'cathepsin' and was first purified in the late 1930s. Nowadays the enzyme is purified by conventional column chromatography, and by isoelectric focusing (which resolves isoforms), but affinity chromatography with pepstatin--Sepharose is also important. Cathepsin D is a glycoprotein of about 42,000 molecular weight; sometimes it comprises a single polypeptide chain but often this is found to have been 'nicked' about two-thirds of the way from one end. Cathepsin D is an 'aspartic proteinase' and may be one of the more primitive members of the family. The activity of cathepsin D is expressed exclusively at acidic pH values and the specificity shows a strong preference for cleavage near hydrophobic amino acids. Specific inhibition of cathepsin D with antibodies and pepstatin has provided strong evidence that the enzyme plays a part in intralysosomal proteolysis but there is as yet little evidence for extracellular activity.

Increased citrullinated glial fibrillary acidic protein in secondary progressive multiple sclerosis

In this study, we demonstrate that grossly unaffected white matter from secondary progressive multiple sclerosis (SP-MS) patients is heavily citrullinated, as compared to normal white matter from control patients. Citrullination was most pronounced at plaque interfaces and was shown to colocalize with glial fibrillary acidic protein (GFAP)-immunoreactivity using dual color immunofluorescence. In contrast, the plaques themselves weakly stained for citrullinated proteins compared to control white matter and usually contained a blood vessel with surrounding astrocytes that were positive both for citrullinated proteins and GFAP. In SP-MS brain samples, but not in normal brains, long fibers of colocalized GFAP- and citrullinated proteins extended into the gray matter. Increased numbers of astrocytes containing citrullinated proteins and GFAP were also present at the junction between the gray and white matter in SP-MS brains. Western blot analysis of acidic brain proteins from nonplaque-containing white matter showed upregulation of multiple citrullinated GFAP proteins in SP-MS brains as compared to controls. Our results demonstrate that increased amounts of citrullinated GFAP are present in SP-MS brains, but also shows that these proteins are present in areas of MS brains that were grossly normal appearing. These data raise the possibility that citrullination of GFAP contributes to the pathophysiology of MS.

Immunohistochemical localization of citrullinated proteins in adult rat brain

By using hybridoma technology, an IgM monoclonal antibody (F95) against multiple citrullinated synthetic and natural peptides was recently developed and used to stain immunohistochemically subsets of astrocytes and myelin basic protein (MBP) from selected regions of human brain (Nicholas and Whitaker [2002] Glia 37:328-336). With this antibody, the present study provides a more detailed localization of citrullinated epitopes in the central nervous system (CNS) by examining immunohistochemical staining patterns for F95 in the normal adult rat brain. Thus, immunohistochemical labeling for citrullinated epitopes was seen in white matter areas consistent with myelin staining; however, in general, it was more prominent and uniform in the caudal CNS (spinal cord, medulla oblongata, pons, and cerebellum) than in more rostral areas. F95 staining was also seen in cells and fibers often intimately associated with blood vessels and/or ventricular surfaces. By using dual-color immunofluorescence, the vast majority of this latter staining was colocalized within a subset of astrocytes also immunoreactive for glial fibrillary acidic protein (GFAP). By using Western blot analysis of rat brain proteins, multiple GFAP- and MBP-immunoreactive proteins and peptide fragments were seen, and many of them were also reactive with the F95 antibody. Thus, the present study not only demonstrates that citrullinated epitopes in normal rat brain are most concentrated in subsets of myelin and astrocytes but also provides evidence that GFAP, like MBP, may be present as multiple citrullinated isoforms.

Expression of neuropsin in oligodendrocytes after injury to the CNS

Proteases are involved in a variety of processes including demyelination after injury to the central nervous system. Neuropsin is a serine protease, which is constitutively expressed in the neurons of the limbic system. In the present study, intrahippocampal kainate injection and enucleation were performed on adult mice. Neuropsin mRNA and protein expression was detected by in situ hybridization and immunohistochemistry. Double in situ hybridization confirmed that the mRNA expression was induced in oligodendrocytes. One day after kainate injection to the hippocampus, neuropsin mRNA was expressed, peaking 4-8 days postoperatively and disappearing at 14 days. Immunohistochemistry and immunoelectron microscopy revealed that neuropsin was expressed in the cell body of oligodendrocytes and myelin. To see if neuropsin degrades myelin protein, purified myelin was incubated with recombinant neuropsin. A decrease in the intensity of the bands of myelin basic protein was observed. These results indicate that neuropsin is involved in demyelination.

In vitro processing of the human alkyl-dihydroxyacetonephosphate synthase precursor

Alkyl-dihydroxyacetonephosphate synthase, a peroxisomal enzyme involved in the biosynthesis of ether phospholipids, is synthesized with a cleavable N-terminal presequence containing the peroxisomal targeting signal type 2. The human alkyl-dihydroxyacetonephosphate synthase precursor produced in vitro or expressed in Escherichia coli could be processed to a lower molecular weight protein by incubation at 37 degrees C with a guinea pig liver fraction, enriched in mitochondria, lysosomes, and peroxisomes. This lower molecular weight protein was identified as the mature human alkyl-dihydroxyacetonephosphate synthase by radiosequencing, indicating that the processing protease is present in this organellar fraction. Characterization of the processing protease indicated that it is a cysteine protease with a pH optimum of 6.5. Furthermore, it was demonstrated that exogenously added pre-alkyl-dihydroxyacetonephosphate synthase was imported and processed in purified peroxisomes in vitro. Processing of alkyl-dihydroxyacetonephosphate synthase did not increase the activity of the enzyme. This indicates that the presence of the presequence does not affect the activity of the enzyme.

Rapid release and unusual stability of immunodominant peptide 45-89 from citrullinated myelin basic protein

Myelin basic protein (MBP) exists in a population of isoforms and isomers. The 18.5 kDa MBP-C1, the main human adult isoform, has 170 residues and is relatively unmodified, whereas the same isoform can be citrullinated on six arginine residues to create the MBP-C8 (MBP Cit6) isomer. MBP Cit6 dominates in MS brain, accounting for 45% rather than 25% of the population of MBP isomers. In the fulminant form of MS, known as Marburg's Disease, 18 of the 19 arginines in MBP are citrullinated (MBP Cit18). Citrullination of MBP could lead to instability of myelin or limited remyelination. In this investigation, the susceptibilities to degradation by cathepsin D of MBP Cit6 and MBP-C1, both from normal and MS brain tissue, and Marburg MBP Cit18 were compared. The pattern of digestion was similar, and no differences of corresponding isomers in normal and MS brain were noted. However, normal MBP Cit6 was degraded 10-fold more rapidly than MBP-C1, and MBP Cit18 was degraded even more rapidly. MBP peptide 45-89 was preserved regardless of isomer type or source. Its generation was directly related to the citrulline content of the MBP substrate being 4 times faster in normal MBP Cit6 and 35 times faster in Marburg MBP Cit18 than in normal MBP-C1. Peptide 45-89 from a citrullinated MBP exhibited more deamidation, and, regardless of source, showed an alpha-helix structure in a lipid mimetic environment. We postulate that the generation of MBP peptides, including those that are dominant and encephalitogenic, is directly related to deimination of arginine to citrulline in MBP.

Analysis of the membrane-interacting domains of myelin basic protein by hydrophobic photolabeling

Myelin basic protein is a water soluble membrane protein which interacts with acidic lipids through some type of hydrophobic interaction in addition to electrostatic interactions. Here we show that it can be labeled from within the lipid bilayer when bound to acidic lipids with the hydrophobic photolabel 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine (TID) and by two lipid photolabels. The latter included one with the reactive group near the apolar/polar interface and one with the reactive group linked to an acyl chain to position it deeper in the bilayer. The regions of the protein which interact hydrophobically with lipid to the greatest extent were determined by cleaving the TID-labeled myelin basic protein (MBP) with cathepsin D into peptides 1-43, 44-89, and 90-170. All three peptides from lipid-bound protein were labeled much more than peptides from the protein labeled in solution. However, the peptide labeling pattern was similar for both environments. The two peptides in the N-terminal half were labeled similarly and about twice as much as the C-terminal peptide indicating that the N-terminal half interacts hydrophobically with lipid more than the C-terminal half. MBP can be modified post-translationally in vivo, including by deamidation, which may alter its interactions with lipid. However, deamidation had no effect on the TID labeling of MBP or on the labeling pattern of the cathepsin D peptides. The site of deamidation has been reported to be in the C-terminal half, and its lack of effect on hydrophobic interactions of MBP with lipid are consistent with the conclusion that the N-terminal half interacts hydrophobically more than the C-terminal half. Since other studies of the interaction of isolated N-terminal and C-terminal peptides with lipid also indicate that the N-terminal half interacts hydrophobically with lipid more than the C-terminal half, these results from photolabeling of the intact protein suggest that the N-terminal half of the intact protein interacts with lipid in a similar way as the isolated peptide. The similar behavior of the intact protein to that of its isolated peptides suggests that when the purified protein binds to acidic lipids, it is in a conformation which allows both halves of the protein to interact independently with the lipid bilayer. That is, it does not form a hydrophobic domain made up from different parts of the protein.

Citrullinated myelin basic protein induces experimental autoimmune encephalomyelitis in Lewis rats through a diverse T cell repertoire

An increased proportion of citrullinated MBP (MBP-C8) occurs in the brains of multiple sclerosis (MS) patients. In this study, MBP-C8 from guinea pig (GP) brains was isolated and found encephalitogenic in Lewis rats upon immunization. An encephalitogenic T cell line selected with MBP-C8 preferentially reacted with MBP-C8 over unmodified MBP. This T cell line responded weakly to the dominant encephalitogenic epitope, GP-MBP peptide 70-88, and did not display restricted TCR beta-chain usage (such as Vbeta88.2). The distinctive features of MBP-C8 were also demonstrated by its ability to reinduce active EAE in 70% of rats which had recovered from unmodified MBP induced EAE. These findings raise the possibility that citrullinated MBP may elicit a different pathogenic T cell repertoire for the recurrent phases of inflammatory demyelination.

Immunological effects of an arginine side chain contaminating synthetically prepared peptides

The side chain, 4-methoxy-2,3,6-trimethylbenzenesulphonyl (Mtr), is a protective group coupled to arginine to mask the omega-nitrogen, in order to protect the guanidino function during peptide synthesis by the 9-fluorenylmethoxycarbonyl (Fmoc) procedure (Walker, 1994). This group is removed at the completion of peptide synthesis; however, the cleavage process can be incomplete. We have found that animals injected with a mixed population of pure, i.e. unmodified, and Mtr-containing MBP peptides have an immunodominant humoral response to the Mtr-bearing peptide. This response is dependent on the characteristics of the MBP peptide involved. For two MBP peptides, the Mtr-containing peptide had increased binding to antibody over pure peptide. For two other peptides, only the Mtr-containing peptide bound antibody while the unmodified peptide did not. In a separate system involving a polyclonal response to an unrelated peptide from beta2-microglobulin (beta2 m), the dominance of the Mtr group was also evident. These results provide further evidence that a small side chain on a single amino acid in a peptide can markedly alter the immunogenicity and antigenicity of that peptide for antibody reactivity. This evidence emphasizes the need for a critical awareness of each component of peptide synthesis and its potential to alter the immunoreactivity of the final product.

Evidence supporting membrane fusion as the mechanism of myelin basic protein-induced insulin release from rat pancreatic islets

In order to clarify insulinotropic effects of the myelin basic protein (MBP) we studied mode of association and distribution of MBP in the pancreatic islets and tested the insulin-releasing activity of various MBP peptides. Rat pancreatic islets were first stimulated in a static incubation with 10 microM bovine MBP (bMBP) at a substimulatory (3.5 mM) glucose concentration. The islets exposed to MBP released significantly more insulin and glucagon in a second incubation in the absence of added stimulant and in the presence of 11.5 mM arginine than the incubated, non-stimulated islets and islets initially stimulated with 15 mM glucose. Response to stimulation with 15 mM glucose in the second incubation by islets exposed first to MBP was impaired compared to incubated, non-stimulated islets. Immunoelectron microscopy showed that MBP had entered into the islet cells and associated with membranes of intracellular vacuoles, most of which represented enlarged, often fused insulin granules. MBP was also present at the islet edge and in the intercellular spaces. Of the purified MBP peptides of sizes of 4.8-13.6 kDa, produced from the digestion with brain acid proteinase and with pepsin and covering the entire bMBP sequence, only the large peptides (1-88, 9.8 kDa and 43-169, 13.6 kDa) stimulated insulin secretion significantly. Heterogeneous peptide mixtures, obtained from a time-course digestion of bMBP by myelin calcium-activated neutral protease, consisting of peptides of approximate molecular weights of 8-11 kDa and larger, also stimulated insulin release. The glucagon-releasing activity of MBP peptides was low and followed the same pattern as the insulin-releasing activity. The present results suggest that MBP-induced fusion of the membranes of hormone granules is involved in MBP-induced insulin release. The hormone-releasing activity of the large peptides in addition to that of the intact molecule is explained as being due to the ability of these peptides to associate with membranes. MBP-induced hormone release and related effects could be associated with neuropathological conditions such as stroke and multiple sclerosis.

Human procathepsin D: three-dimensional model and isolation

Human procathepsin D was isolated from medium of human breast cancer cell line ZR-75-1 potentiated with estrogen. The isolation involved both immunoaffinity chromatography and ion-exchange chromatography. The affinity chromatography employed polyclonal antibodies raised against a synthetic activation peptide of human cathepsin D. We have started preliminary crystallization trials using the isolated material. A model of human procathepsin D was also built using coordinates of human cathepsin D and pig pepsinogen. The model aids understanding of multiple roles played by activation peptides of aspartic proteinases and will be used as a starting model for molecular replacement.

Limited proteolysis of myelin basic protein in a system mimetic of the myelin interlamellar aqueous space

We have investigated the early steps of myelin basic protein (MBP) degradation in a membrane mimetic system (reverse micelles), resembling the interlamellar aqueous spaces where the protein is located in the myelin sheath. MBP, unfolded in buffer, refolds on incorporation into the micelles, resulting in reduced accessibility to three proteolytic enzymes, trypsin, cathepsin D, and Staphylococcus aureus V8 protease, in comparison with aqueous solution. Eleven cleavage sites seen in buffer are removed from proteolytic attack in micellar solution. These sites delineate a protected protein domain displaying a potential beta-sheet structure capable of interacting with the myelin membrane. An additional site not seen in buffer is attacked in the micelles. Experiments with a structure inducer, 15% 1-propanol in buffer, reveal that the refolding pattern of MBP in reverse micelles is specific to the membrane biomimetic system and is not produced by organic solvent per se. Micellar digestions of MBP generate long peptides, two of which, isolated after tryptic digestion, have been found to be immunodominant in multiple sclerosis patients. The findings suggest the structure induced in MBP by the micelles resembles that leading to production of the self-peptides recognized by T cells during proteolytic breakdown of MBP in autoimmune demyelinating diseases.

Exploration of subsite binding specificity of human cathepsin D through kinetics and rule-based molecular modeling

The family of aspartic proteinases includes several human enzymes that may play roles in both physiological and pathophysiological processes. The human lysosomal aspartic proteinase cathepsin D is thought to function in the normal degradation of intracellular and endocytosed proteins but has also emerged as a prognostic indicator of breast tumor invasiveness. Presented here are results from a continuing effort to elucidate the factors that contribute to specificity of ligand binding at individual subsites within the cathepsin D active site. The synthetic peptide Lys-Pro-Ile-Glu-Phe*Nph-Arg-Leu has proven to be an excellent chromogenic substrate for cathepsin D yielding a value of kcat/Km = 0.92 x 10(-6) s-1 M-1 for enzyme isolated from human placenta. In contrast, the peptide Lys-Pro-Ala-Lys-Phe*Nph-Arg-Leu and all derivatives with Ala-Lys in the P3-P2 positions are either not cleaved at all or cleaved with extremely poor efficiency. To explore the binding requirements of the S3 and S2 subsites of cathepsin D, a series of synthetic peptides was prepared with systematic replacements at the P2 position fixing either Ile or Ala in P3. Kinetic parameters were determined using both human placenta cathepsin D and recombinant human fibroblast cathepsin D expressed in Escherichia coli. A rule-based structural model of human cathepsin D, constructed on the basis of known three-dimensional structures of other aspartic proteinases, was utilized in an effort to rationalize the observed substrate selectivity.

Interstrain cross-reactive idiotypes on monoclonal antibodies to an encephalitogenic myelin basic protein peptide

In order to assess the role of idiotype (Id) and the anti-Id network in murine experimental autoimmune encephalomyelitis (EAE), Id-bearing monoclonal antibodies (mAb) to human myelin basic protein (MBP) peptide acetyl 1-9, as well as mAb anti-Id, were developed in EAE-susceptible PL/J mice (H-2u). These mice recognize MBP residues acetyl 1-9 as an encephalitogenic determinant. Reactivities of PL/J Id-bearing mAbs to MBP and to MBP peptides were identical to those of mAbs generated against the same MBP peptide in EAE-resistant BALB/c mice (H-2d), even though isotypes of the mAbs differed. By using an inhibitory ELISA and immunoblotting, it was demonstrated that one PL/J mAb anti-Id recognized a public or framework Id, whereas another PL/J mAb-anti Id was directed to a private Id more restricted to the paratopic site. Two Id-bearing PL/J mAbs shared a cross-reactive Id (IdX) on the light chain, and an interstrain IdX was present on both the heavy and light chains of mAbs raised in PL/J and BALB/c mice to the same MBP peptide. The PL/J mAb anti-Id was capable of cross-regulating the production of Id-bearing mAbs by hybridomas across murine strains. These findings suggest that a restrictive family of germ-line genes encode for these Id-bearing antibodies to MBP peptide, irrespective of whether the MBP peptide is encephalitogenic in the murine strain immunized. Manipulation of the Id network may provide a means for modifying autoimmune demyelinating diseases of the central nervous system.

An immunochemical comparison of human myelin basic protein and its modified, citrullinated form, C8

An immunochemical analysis was conducted to compare the C1 isomer of human myelin basic protein (MBP) with the newly described and less cationic, citrullinated isomer of MBP referred to as C8. Ten polyclonal antisera directed at multiple epitopes or restricted regions of MBP were used in radioimmunoassays to examine MBP-C1 and MBP-C8. Antisera reactive with MBP peptide 1-14 clearly distinguished MBP-C1 from MBP-C8. Antisera to human MBP peptides 10-19 and 90-170, but not to MBP peptide 69-89, showed modest differences between MBP-C1 and MBP-C8. The MBP-C8s from multiple sclerosis (MS) and non-MS brain reacted essentially the same. With murine monoclonal antibodies and enzyme-linked immunosorbent assay (ELISA), differences between MBP-C8 and other isomers were shown for anti-MBP 10-19 but not for anti-MBP 1-9 or anti-MBP 80-89. These findings imply differences in sequence or conformation in the structure of MBP-C7 compared to MBP-C1, most notably near the amino terminus.

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.

Conformational correlates of the epitopes of human myelin basic protein peptide 80-89

Different epitopes residing within the decapeptide of residues 80-89 of human myelin basic protein (MBP) exist in the MBP-like material detected in human CSF and urine. In the present study, the structure of human MBP peptide 80-89 was examined by a combination of physical measurements and correlated with its varying immunochemical reaction with three polyclonal antisera. At least two epitopes are present in the decapeptide. Progressive shortening and reduction in net negative charge of MBP peptide 80-89 to form peptides 81-89, 82-89, 83-89, and 84-89 revealed an epitope not present in intact MBP. Circular dichroism and Fourier-transform infrared of these MBP peptides in water demonstrated random structure that was partially changed to beta-structure in the shorter peptides. In methanol, used as a model for a lipid environment, the random structure was diminished and was replaced by alpha-helix and beta-structure, especially in the shorter peptides. The findings indicate that the range of epitopes present in this decapeptide is influenced by conformation, which, unexpectedly, becomes progressively less random as the peptide becomes smaller, especially in a hydrophobic environment. This behavior has implications for the immunochemical detection of small antigens or antibodies to them in tissue extracts or body fluids.

Postnatal changes of cathepsin D activity in rat liver and brain

Total and specific activity of cathepsin D (EC. 3.4.23.5) were measured in rat liver and brain from 1 to 98 days of age. The activity of cathepsin D in the liver of adult and newborn rats was the same while in the rat brain it was higher in adult than in newborn rats. In the liver maximum specific activity of cathepsin D occurred on the 10th postnatal day and minimum on the fourth day of age. In the brain maximum specific activity of the enzyme occurred on the 14th postnatal day. Total activity of cathepsin D increased after birth in rat liver and brain. These results are discussed in relation to the functional role of cathepsin D in the rat liver and the brain.

Antibody to peptides of human myelin basic protein in post-rabies vaccine encephalomyelitis sera

Development of neurologic complications after Semple rabies vaccine is closely linked to development of antibody to myelin basic protein (MBP). The portions of MBP against which the antibodies are directed were analyzed by enzyme immunoassay in sera and cerebrospinal fluid from 27 patients with vaccine complications. Most of the antibody was directed to regions of MBP peptides 45-89 and 90-170. There was no apparent correlation between antibody specificity for MBP peptides 1-44, 45-89 and 90-170 and the type of post-vaccinal neurologic complication. We conclude that the immunoglobulin repertoire in human B lymphocytes for responding to human MBP favors the portion of the MBP molecule containing residues 45-170.

Immunolocalization of cathepsin D in dental tissues

Cathepsin D antigenicity was localized at the light and electron microscopic levels within dental cells, but not in extracellular matrix. Different intracellular sites for cathepsin D were found depending on the cell type: the enzyme was detected in secretory vesicles of the odontoblasts and in the lysosome-like structures of the ameloblasts. Otherwise, these results suggest that the secretory vesicles of the odontoblasts may contain both cathepsin D and type I collagen. These data might implicate cathepsin D in the enamel and the dentin formations.

Cathepsin D activity in human peripheral blood mononuclear leukocytes

To investigate the cellular origins of cathepsin D (CD) in inflammatory lesions, the CD content of lymphocyte subsets, monocytes, and macrophages were compared. Human monocytes, B lymphocytes, CD4+ T lymphocytes, and CD8+ T lymphocytes were separated from peripheral blood of normal donors. CD content was 0.13 +/- .01 micrograms equivalents of CD per million cells and significant differences between different cell types were not found. To determine the CD content of macrophages, differentiation of peripheral blood monocytes was induced by either in vitro culture or treatment with 4 beta-phorbol-12-myristate-13-acetate (PMA). Macrophages induced by five-day culture contained four times more CD than unstimulated monocytes, and macrophages induced by 18-h treatment with 20 mg/ml 4 beta-PMA contained nine times more CD than monocytes treated with 4 alpha-PMA, an inactive stereoisomer of 4 beta-PMA. These results suggest that macrophages are one of the enriched sources of CD in inflammatory lesions.

Peptides of myelin basic protein stimulate T lymphocytes from patients with multiple sclerosis

Peripheral blood T lymphocytes from patients with multiple sclerosis (MS) and other neurological diseases (OND) were tested for primary in vitro proliferation in response to four synthetic peptides derived from the sequence of human myelin basic protein (HuMBP) and to HuMBP 45-89 peptide fragment, using a [3H]thymidine incorporation assay. The synthetic peptides used corresponded to residues HuMBP 15-31, 75-96, 83-96 and 131-141 of human myelin basic protein. Significant proliferation of T lymphocytes to peptides was noted only in the MS group (with the exception of peptide 131-141): the majority of control subjects and OND patients did not respond to the above-mentioned peptides. The sensitized T lymphocytes in MS patients displayed the inducer/helper phenotype and required autologous monocytes for optimal proliferation. An anti-HLA-DR monoclonal antibody, directed against a monomorphic determinant of DR molecules, was able to block the responses in a dose-dependent fashion. These results suggest that autoimmune inducer/helper T lymphocytes in the peripheral blood of MS patients may initiate and/or regulate the demyelination process in patients with MS. Furthermore, our data demonstrate that monocytes and HLA-DR molecules are essential for activation of these cells. Finally primary in vitro T cell proliferation to HuMBP synthetic peptide may be used as an additional diagnostic test in MS.

Activation of astrocytic lysosomal proteinases by factors released by mononuclear leukocytes

Lysosomal proteinases are increased in the tissue lesions of experimental allergic encephalomyelitis and have been implicated in the degradation of myelin proteins. The cellular origins of the increased proteinases are not known but reactive astrocytes found in areas of increased activity are candidate cells. To evaluate the potential of astrocytes as the source of these proteinases, cathepsin B (CB) and cathepsin D (CD) levels were measured in lysates of cultured astrocytes from neonatal rats. Because astrocytes are activated by inflammatory mediators in demyelinating lesions the effect of activation on proteinase levels was examined. Culture supernatants from mononuclear leukocytes stimulated with either concanavalin A or phytohemagglutinin (PHA) induced significant increases in the astrocytic proteinases. Neither PHA alone, interleukin-1, interleukin-2, nor gamma-interferon induced significant increases. Fractions of the supernatant from PHA stimulated mononuclear leukocytes were tested and activity was found in fractions corresponding to a molecular weight of 45-50,000. These studies demonstrate that astrocytes contain significant amounts of CB and CD activity which can be increased by a factor or factors released by activated mononuclear leukocytes.

Human myelin basic protein peptide 69-89: immunochemical features and use in immunoassays of cerebrospinal fluid

The characteristics and heterogeneity of the myelin basic protein (MBP)-like material appearing in cerebrospinal fluid (CSF) after acute central nervous system (CNS) myelin injury are unresolved. Antigenic material containing an epitope in the carboxyl-terminal portion of human MBP peptide 45-89 (from the intact molecule of 170 residues) is a prominent species of the MBP-like material present. In an effort to define further the MBP-like material in CSF and to enhance its detection, a modified double-antibody radioimmunoassay has been developed using a radioligand of human MBP synthetic peptide 69-89. This assay is more sensitive with results paralleling those of previously used MBP assays for CNS myelin damage. Results with this assay provide additional confirmation of the presence of an epitope of MBP in the decapeptide of MBP 80-89 but in a conformation simulating that of intact MBP in CSF after CNS myelin injury. Unexpected buffer effects were noted to influence the immunochemical behavior of some of the small peptides of MBP.

Cation-mediated release and proteolytic cleavage of basic protein of isolated human myelin at acid pH

Myelin from human brain was incubated at pH 4.4 with metal salts, including KCl, NaCl, CaCl2, and MgSO4, to elicit cation-dependent autoproteolysis of myelin proteins. Incubation of myelin resulted in soluble proteolytic breakdown products of Mr smaller than those of the three original myelin basic proteins (MBPs). Comparable polypeptides were essentially absent from residual myelin. Proteolysis was strongly stimulated by increasing millimolar concentrations of K+, Na+, and Mg2+ and only moderately by Ca2+. Breakdown products were traced to MBP by immunostaining. Their origin from MBP was also indicated by identical electrophoretic cleavage patterns from endogenous myelin protein and exogenous MBP. All four metal salts, in addition to activating endogenous proteolysis, also caused a biphasic extraction of MBP. Electrophoresis of myelin revealed a quick initial and a slow further loss of protein, eventually leading to the removal of up to 78% of original MBP. The results are consistent with a concurrent extraction of MBP and activation of latent-bound acid protease activity by metal cations. It is therefore suggested that, in particular disease states, unfavorable changes in electrolytes and pH of white matter may cause a selective loss and proteolytic cleavage of MBP.

Basic protein dissociating from myelin membranes at physiological ionic strength and pH is cleaved into three major fragments

Experiments were performed with isolated human myelin membrane preparations to analyse factors that may modulate association of myelin basic protein (MBP) with the membranes and could contribute to demyelinating processes. Transfer of membranes (5 mg protein ml-1) at 37 degrees C and pH 7.4 from a hypotonic medium, in which they were relatively stable, to one of physiological ionic strength produced three major effects: (1) initial dissociation of MBP from the membranes by a nonenzymatic process that was doubled in the presence of millimolar Ca2+/Mg2+; (2) within 10 min, the appearance in the medium of three major MBP fragments (14.4, 10.3, and 8.4 kilodaltons); and (3) progressive acidification of dissociated MBP and its fragments, probably due to deamidation. Between 1 and 6 h a steady state was apparent in which protein equivalent to 15% of the MBP originally bound to the membranes was found in the medium. The three major MBP fragments formed two-thirds of this solubilised material and appeared metabolically stable for 24 h. The kinetics of peptide formation suggested that dissociated, rather than membrane-bound, MBP was cleaved by myelin-associated neutral proteases. Two-dimensional electrophoresis and immunoblotting using two monoclonal antibodies indicated that proteolysis occurred in the vicinity of residues 35 and 75. Evidence was also obtained for removal of C-terminal arginines and relatively rapid deamidation in the C-terminal half of MBP. These modifications of MBP might also occur if extracellular fluid gained access to the compacted cytoplasmic space of the myelin sheath.

Demyelinating canine distemper encephalomyelitis: measurement of myelin basic protein in cerebrospinal fluid

Beagle dogs were experimentally infected with the Cornell A75-17 strain of canine distemper virus. At three time points post-infection (PI), immunoreactive myelin basic protein (MBP) was measured in cerebrospinal fluid (CSF). Levels were correlated with neuropathological findings, interferon in CSF and virus isolation from the brain. CSF from animals inoculated with Cornell A75-17 strain often showed detectable immunoreactive MBP late in the disease course. As anticipated from earlier morphological studies, CSF drawn around day 20 PI lacked MBP while subsequent samples were positive. Dogs with severe demyelination had elevated values of immunoreactive MBP while dogs with only mild inflammation had little or none. Release of MBP or MBP peptides into CSF of dogs with canine distemper may be a valuable laboratory test in studies of the natural history of this disease and in assessing the response to treatment. Whether an immune response to MBP plays an immunopathogenic role in the chronic, demyelinating phase of canine distemper encephalitis remains to be determined.

Two-dimensional electrophoretic characterization of microheterogeneous myelin basic protein fragments

A new two-dimensional polyacrylamide gel electrophoresis (PAGE) system with minislab gel apparatus was devised for the rapid (4 h) analysis of peptide fragments derived from the enzymic digestion of myelin basic protein (MBP). The first dimension consisted of 5% polyacrylamide running gels in 1.9 M potassium glycinate, pH 7.3, with 4.3% stacking gels in 0.08 M potassium glycinate, pH 10.3. Anodic and cathodic buffer chambers contained 38 mM glycine/5 mM Tris, pH 8.3, and 10 mM Tris-HCl, pH 8.1, respectively. This system fractionated MBP peptides on the basis of charge. By contrast, acid-urea 15% PAGE separated MBP peptides by both charge and size. A two-dimensional system of 5% PAGE followed by sodium dodecylsulfate 15% PAGE (Laemmli) was used to resolve MBP fragments from pepsin and cathepsin D digests; this analysis indicated that cathodic mobilities could be predicted by the ratio of basic to acidic amino acids in each peptide. This method should be particularly powerful in combination with immunoblotting to identify microheterogenous fragments arising from normal and pathological metabolism of MBP.

Epitopes of immunoreactive myelin basic protein in human cerebrospinal fluid

To define in more detail the features of the immunoreactive myelin basic protein (MBP) present in cerebrospinal fluid (CSF) of humans following acute injury to central nervous system myelin, the epitopes of MBP recognized by three different antisera, each capable of detecting immunoreactive MBP in CSF, were examined. All three antisera reacted well with human MBP and human MBP peptide 45-89. Only in radioimmunoassays in which the MBP peptide 45-89 served as the radioligand could clearly elevated values of immunoreactive MBP be measured in CSF specimens from 5 patients with multiple sclerosis during or immediately after an exacerbation. The two antisera that reacted well with MBP peptide 80-89 resulted in higher levels of immunoreactive MBP measured in CSF. An epitope present in human MBP peptide 80-89 but sharing a conformation with both MBP and MBP peptide 45-89 is present in CSF following acute central nervous system myelin damage in multiple sclerosis.

Analysis of the primary sequence of human myelin basic protein peptides 1-44 and 90-170 by fast atom bombardment mass spectrometry

The originally described sequence of human myelin basic protein peptide 45-89 has recently been shown to contain two errors which have now been resolved. In the present study fast atom bombardment mass spectrometry was utilized to analyze the primary sequence of the other portions, peptides 1-44 and 90-170 of human myelin basic protein. The results obtained confirm the accuracy of the primary sequence published for both of these terminal peptides.

Isolation and characterization of cathepsin B from bovine brain

Cathepsin B (EC 3.4.22.1) was purified 746-fold with a 21% recovery from bovine brain by autolysis, fractional precipitation with acetone, carboxy-methyl-Sephadex chromatography, affinity chromatography on a cystamine containing column and gel filtration chromatography. The purified cathepsin B eluted on gel filtration with an apparent molecular weight of 27,000 but was resolved into three bands of 30,000, 25,000 and 5,000 molecular weight by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS-PAGE). Antibodies to cathepsin B, raised against the 30,000 dalton band, were shown by immunoblots to react with both the 30,000 and 25,000 dalton proteins with results suggesting that the former predominated as the immunoreactive form in bovine brain homogenates. Isoelectric focusing demonstrated multiple bands, ranging from pH 4.75-5.2 with the major band at pH 5.1-5.2, all of which were capable of degrading N alpha-carbobenzoxy-L-arginyl-L-arginine 4-methoxy-beta-naphthylamide. The cathepsin B activity against N alpha-benzoyl-DL-arginine beta-naphthylamide (BANA) and bovine myelin basic protein (MBP) had a pH optimum of pH 6.0. The Km for the degradation of BANA was 1.0 mM and 5.1 mM when assayed in the presence of 1% and 2.5% dimethylsulfoxide, respectively. Cathepsin B from bovine brain has many properties similar to cathepsin B isolated from other organs. The degradative effect of cathepsin B on MBP suggests a role for this proteinase in inflammatory demyelination.

Acidic lipids enhance cathepsin D cleavage of the myelin basic protein

Some acidic lipids including sulfatide and phosphatidylinositol were found to increase greatly the rate of cathepsin D cleavage of the myelin basic protein. Since a similar effect was seen when the substrate was changed to cytochrome C, but not when the enzyme was changed to pepsin, these acidic lipids seem to be acting on cathepsin D rather than on myelin basic protein itself. Even so, chemical modification studies suggest that this phenomenon is only seen when the myelin basic protein is in its native conformation. Succinylation of MBP increases its rate of cleavage by cathepsin D by at least tenfold and, in addition, with this modified and presumably denatured MBP as substrate, activation of cathepsin D is no longer seen with acidic lipids. These findings suggest that the native conformation of MBP is both an important determinant of its rate of cleavage by cathepsin D and is also essential for observing activation of this reaction by acidic lipids. The acidic lipids seem to alter the "extended active site" of cathepsin D in such a way as to enable this enzyme better to utilize the native myelin basic protein as a substrate. Cathepsin D has previously been implicated as the protease responsible for the release into cerebrospinal fluid in multiple sclerosis patients of an encephalitogenic fragment derived from myelin basic protein. It is possible that the elevated levels of cathepsin D and sulfatide that have previously been found associated with multiple sclerosis plaques in vivo act in concert to bring about the rapid cleavage and subsequent loss of the myelin basic protein from these localized regions in the myelin sheath.

Rat neural tissue cathepsin D: ultrastructural immunocytochemistry

The cellular and subcellular localization of cathepsin D, an aspartyl endopeptidase, was investigated in the central and peripheral nervous systems of the rat by light and electron microscopic immunocytochemistry. The reaction of rabbit anti-rat brain cathepsin D within ventral cervical spinal cord, cerebellum, corpus callosum, caudate nucleus, optic nerve, trigeminal ganglion, fifth cranial nerve and sciatic nerve was localized with an indirect immunoperoxidase technique. A number of tissue processing methods were utilized, but only in tissues fixed in paraformaldehyde-lysine-periodate and sectioned at thicknesses of 25-50 micron could antibody penetration, enzyme protein immunoreactivity and intact morphology be reliably attained. Immunoreactive cathepsin D was present in lysosomes and pleomorphic dense bodies of neurons in the anterior horn of spinal cord, cerebellar Purkinje and granule cell layers, caudate nucleus and trigeminal ganglion. Lysosomal localization of cathepsin D was also documented in oligodendrocytes, astrocytes, endothelial cells and Schwann cells. Reaction product was not observed in microglia although its presence there would be expected. With these methods, reaction product was not detected in the Golgi saccules of any cell type.

Sequential limited proteolysis of myelin basic protein by neutral protease activities of bovine brain

Acid extracts of delipidated white matter of bovine brain were prepared, and their proteolytic activities toward myelin basic protein (MBP) were evaluated at pH 3 and pH 7. This was done by measuring changes in total protein using a selective dye-binding assay, and by evaluating peptide patterns by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and densitometry. At pH 7 greater than 50% of total protein and about 75% of MBP were degraded after 48 h, whereas at pH 3 it was less than 20% altogether. Neutral proteolysis of MBP entailed up to 12 different proteolytic peptide fragments in the molecular weight range of 17.5 to 6 kd. Its enzymatic nature was verified using protease inhibitors, including N-ethylmaleimide, phenylmethylsulfonyl fluoride, o-phenanthroline, and EDTA, as well as pepstatin A and alpha 2-macroglobulin. Both transient changes in percentages of some intermediate peptides and differential effects of individual inhibitors on electrophoretic peptide patterns strongly suggest a sequential type of limited proteolysis. The results also indicate that acid extracts contained several endopeptidases of which a cysteine protease appears to initiate the breakdown of MBP.

Hormonal and drug effects on the degradation of human myelin basic protein peptide 43-88 by alkaline proteolytic activity in the rat kidney

The microsomal brush-border fraction of rat renal tissue contains enzymatic activity, optimally active at pH 9, that is capable of degrading human myelin basic protein (BP) peptide 43-88. In the present study, this degradation and the effect on it of selected drugs and hormones were examined further. Of the substances tested, 10(-2) M chloroquine and 10(-5) M ACTH 1-24 were found to be the most effective inhibitors followed by 10(-5) M ACTH 1-39; parathormone, glucagon and insulin were found to be inhibitors an order of magnitude weaker than ACTH 1-24. Hydrocortisone, dexamethasone, maleic acid and ACTH 4-10 were found to have minimal or no inhibitory effect on the peptide degrading activity. Gel filtration of the degradation products indicated that the rate of degradation of BP peptide 43-88 at pH 9 had been retarded by ACTH 1-24. These studies indicate that the clearance and catabolism of this peptide may be altered by available therapeutic agents.

Localization of cathepsin D in rat liver. Immunocytochemical study using post-embedding immunoenzyme and protein A-gold techniques

Light and electron microscopic localization of cathepsin D in rat liver was investigated by post-embedding immunoenzyme and protein A-gold techniques. By light microscopy, cytoplasmic granules of parenchymal cells and Kupffer cells were stained for cathepsin D. Weak staining was also noted in sinusoidal endothelial cells. In the parenchymal cells many of positive granules located around bile canaliculi. In the Kupffer cells and the endothelial cells, diffuse staining was noted in the cytoplasm in addition to granular staining. By electron microscopy, gold particles representing the antigenic sites for cathepsin D were seen in typical secondary lysosomes and some multivesicular bodies of the parenchymal cells and Kupffer cells. The lysosomes of the endothelial cells and fat-storing cells were weakly labeled. Quantitative analysis of the labeling density in the lysosomes of these three types of cells demonstrated that the lysosomes of parenchymal cells and Kupffer cells are main containers of cathepsin D in rat liver. The results suggest that cathepsin D functions in the intracellular digestive system of parenchymal cells and Kupffer cells but not so much in that of the endothelial cells.

Degradation of bovine P2 protein by bovine brain cathepsin D

Bovine brain cathepsin D cleaved bovine P2 protein to produce three major and several minor peptides. The major P2 peptides formed were shown by amino acid analysis and partial sequencing to be peptides 17-54, 20-58 and 65-131 with the latter predominating. In preliminary experiments, P2 peptide 65-131 did not induce experimental allergic neuritis in Lewis rats in equimolar amounts to the neuritogenic P2.

Cleavage of myelin basic protein by neutral protease activity of human white matter and myelin

Polypeptides arising from neutral in vitro proteolysis of myelin basic protein (MBP) of human brain were evaluated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. At pH 7 a marked breakdown of MBP resulted in the formation of 8-12 polypeptides ranging from 6 to 17 kd in molecular weight. As neutral proteolytic activity was not eliminated by either gel filtration or cation-exchange chromatography acid-soluble protease(s) involved probably have a size and electric charge similar to that of MBP. The enzymatic nature of neutral proteolysis was ascertained by heat inactivation and inhibition by alpha 2-macroglobulin. Incomplete inhibition of proteolysis and the failure of small peptides (less than 6 kd) to show up on electrophoresis seem to suggest that MBP was degraded by exopeptic proteases as well. Acid extracts of purified myelin yielded polypeptides similar to those of MBP of delipidated white matter. The results are consistent with a sequential limited proteolysis of MBP by neutral proteases probably associated with myelin and possibly related to the in situ catabolism of MBP in man.

A heme binding site on myelin basic protein: characterization, location, and significance

Myelin basic protein (MBP), an extrinsic membrane protein from the myelin sheath, binds dicyanohemin. The binding generates absorption bands in the Soret region and quenches the fluorescence emitted by the sole tryptophan residue. The absorption titration curves in the Soret demonstrate that the binding is stoichiometric, one heme per protein, with a large value of the extinction coefficient (8 X 10(4) M-1 cm-1 at 420 nm). Fluorescence quenching titration curves indicate an identical stoichiometry and a low quenching efficiency of 20%. From the heme titration curve the association constant between dicyanohemin and MBP is estimated to be greater than or equal to 10 nM-1 in 50 mM 4-morpholinepropanesulfonic acid buffer, pH 7.0, at 20 degrees C. Digestion of MBP by Staphylococcus aureus V8 protease yields a peptide (38-118) whose heme binding properties are identical to those of MBP. In contrast, peptides obtained by digestion of MBP with cathepsin D do not exhibit any specific binding of dicyanohemin. The cleavage of the Phe-Phe (42-43) bond appears to be critical in this respect. A comparison of the sequence immediately preceding, including these residues with a probable heme binding site of a mitochondrial cytochrome b, reveals a high degree of homology. The possible significance of heme binding is discussed.

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.

Separation of myelin basic protein peptide 43-88 and its fragments by analytic and preparative high-performance liquid chromatography

Reversed-phase (RP) high-performance liquid chromatography (HPLC) using a solvent mixture of triethylammonium formate buffer and methanol permitted the rapid separation of myelin basic protein peptide 43-88 and a mixture of synthetic fragments from this peptide. The elution times for some of the peptides were markedly affected by minimal changes in the solvent mixture. Attempts to separate the same peptides by gel permeation HPLC resulted in poor resolution and an aberrant elution pattern unrelated to molecular size. With the use of the volatile triethylammonium formate buffer in the RP-HPLC, material could be more readily separated and easily recovered by freeze-drying. Analysis of the components separated by this system of RP-HPLC demonstrated that the preparation of normal human BP peptide 43-88 results in an admixture of peptides 43-87 and 43-88. This procedure of RP-HPLC should make it possible to analyze the degradation of myelin basic protein peptide 43-88 and to isolate the degradation products for characterization.

Species comparison of renal proteolytic activity for human myelin basic protein peptide 43-88

1. A species comparison was conducted on the proteolytic activity in human, dog, rabbit, guinea-pig and rat kidney which can degrade human myelin basic protein peptide 43-88. 2. In rat kidney the degrading activity occurred over a pH range of 4-11.5 with the greatest activities at pH 5 and 9. The peptide degrading activity in human, dog, rabbit and guinea-pig kidney was considerably less than in the rat and occurred predominantly at pH 7 with lesser activity at pH 9. 3. The effects of inhibitors of proteolytic enzymes indicated that the peptide degrading activities at the same two pH's of dog, rabbit and guinea-pig were similar to one another but differed from that of human. 4. These results indicate that the activity for degrading a potential autoantigenic material is widespread in renal tissue among different species and that different enzymes are involved. More generally, these findings suggest that renal proteinases differ among commonly used laboratory animals and also differ from the human enzymes.