Purification and characterization of recombinant human cathepsin E

Purification and characterization of recombinant human cathepsin E expressed in human kidney cell line 293

A cDNA encoding human prepro-cathepsin E was introduced into the adenovirus-transformed HEK-293 (human embryonic kidney) cell line. The construct contained both a V5 peptide epitope and histidine tags at the carboxy terminus. Transfected cells efficiently secreted recombinant pro-cathepsin E into the culture medium. The secreted pro-cathepsin E was purified in a single step using Ni affinity chromatography yielding a protein of about 92 kDa under non-reducing conditions. The amino-terminal sequence of the purified protein began at Ser20, suggesting human cathepsin E accumulated in the culture supernatant as the pro-enzyme. The purified protein was rapidly and completely converted to the active form by treatment at pH 4.0 or below. Steady state kinetic parameters for hydrolysis of the fluorogenic peptide substrate MOCAc-Gly-Lys-Pro-Ile-Leu-Phe-Phe-Arg-Leu-Lys(Dnp)-d-Arg-NH2 (cleavage at the Phe-Phe bond) were consistent with previously reported values for purified human enzyme (kc/Ki= 53 x 10(6) M(-1) s(-1), Km= 6.3 microM, and kcat= 3 x 10(2) s(-1)). The activated protein was potently inhibited by pepstatin with Ki= 0.2 nM, as well as a reported beta secretase inhibitor. This work demonstrates the potential for producing large quantities of highly purified human cathepsin E from HEK-293 cells in quantities to support both biochemical and structural characterization of the enzyme.

Cathepsin D, but not cathepsin E, degrades desmosomes during epidermal desquamation

BACKGROUND

We previously reported that an ambient aspartic proteinase is crucial to desquamation of the stratum corneum at pH 5. Identification of this aspartic proteinase by using enzyme inhibitors suggested it to be cathepsin D, although we could not exclude cathepsin E.

OBJECTIVES

To determine the identity of this aspartic proteinase and its distribution within the stratum corneum.

METHODS

We measured enzyme activities of cathepsin D and cathepsin E in the salt and detergent extracts from callus stratum corneum, using a fluorogenic peptide as a substrate and comparing the effect of addition of Ascaris pepsin inhibitor (specific for cathepsin E) with that of pepstatin A (which inhibits both cathepsin D and cathepsin E). Both enzymes were then extracted and purified from plantar stratum corneum samples and identified by Western blotting. Immunofluorescence microscopy was used to investigate the localization of proteinases within human plantar stratum corneum sample sections.

RESULTS

We found that 20% of total aspartic proteinase activity could be attributed to cathepsin E, the remainder to cathepsin D. Two subunits of cathepsin D were identified, a mature active form at 33 kDa and an intermediate active form at 48 kDa; cathepsin E was also identified at 48 kDa, although in a stained band 10-fold weaker in the immunoblot. Immunofluorescence microscopy showed the antibody to cathepsin D to be localized in the lipid envelopes of the stratum corneum, whereas that to cathepsin E stained the tissue diffusely. The labelling for cathepsin D was similar to that observed for desmosomes, and immunoelectron microscopy confirmed that cathepsin D was present on desmosomes. On the other hand, cathepsin E occurred intracellularly within the squames.

CONCLUSIONS

We conclude that cathepsin D, and not cathepsin E, causes desquamation by degrading desmosomes.

Pro-domain removal in ASP-2 and the cleavage of the amyloid precursor are influenced by pH

BACKGROUND

One of the signatures of Alzheimer's disease is the accumulation of aggregated amyloid protein, Abeta, in the brain. Abeta arises from cleavage of the Amyloid Precursor protein by beta and gamma secretases, which present attractive candidates for therapeutic targeting. Two beta-secretase candidates, ASP-1 and ASP-2, were identified as aspartic proteases, both of which cleave the amyloid precursor at the beta-site. These are produced as immature transmembrane proteins containing a pro-segment.

RESULTS

ASP-2 expressed in HEK293-cells cleaved the Swedish mutant amyloid precursor at different beta-sites at different pHs in vitro. Recent reports show that furin cleaves the pro-peptide of ASP-2, whereas ASP-1 undergoes auto-catalysis. We show that purified recombinant ASP-2 cleaves its own pro-peptide at ph 5 but not pH 8.5 as seen by mass spectrometry, electrophoresis and N-terminal sequencing.

CONCLUSION

We suggest that ASP-2 processing as well as activity are influenced by pH, and hence the cellular localisation of the protein may have profound effects on the production of Abeta. These factors should be taken into consideration in the design of potential inhibitors for these enzymes.