The use of proteolysis to study the structure of nardilysin

Ultrastructural localization and distribution of Nardilysin in mammalian male germ cells

Background

NRD convertase, also termed Nardilysin, is a Zn⁺⁺ metalloendopeptidase that specifically cleaves the N-terminus of arginine and lysine residues into dibasic moieties. Although this enzyme was found located within the testis, its function in male reproduction is largely unknown. In addition, the precise distribution of this enzyme within germ cells remains to be determined.

Methods

To answer these questions, we developed an immuno-gold electron microscopy analysis to detect Nardilysin at ultrastructural level in mice. In addition, we performed a quantitative analysis of these gold particles to statistically estimate the distribution of Nardilysin in the different subcellular compartments of differentiating late spermatids/spermatozoa.

Results

Expression of Nardilysin in wild-type mice was restricted to germ cells and markedly increased during the last steps of spermiogenesis. In elongated spermatids, we found the enzyme mainly localized in the cytoplasm, more precisely associated with two microtubular structures, the manchette and the axoneme. No labelling was detected over the membranous organelles of the spermatids. To test whether this localization is dependent of the functional microtubules organization of the flagella, we analysed the localization into a specific mouse mutant ebo/ebo (ébouriffé) known to be sterile due to an impairment of the final organization of the flagellum. In the ebo/ebo, the enzyme was still localized over the microtubules of the axoneme and over the isolated cytoplasmic microtubules doublets. Quantification of gold particles in wild-type and mutant flagella revealed the specific association of the enzyme within the microtubular area of the axoneme.

Conclusions

The strong and specific accumulation of Nardilysin in the manchette and axoneme suggests that the enzyme probably contributes either to the establishment of these specific microtubular structures and/or to their functional properties.

Histochemical evidence for wide expression of the metalloendopeptidase nardilysin in human brain neurons

Nardilysin is a metalloendopeptidase that in vitro cleaves peptides such as dynorphin-A, somatostatin-28, alpha-neoendorphin and glucagon at the N-terminus of arginine and lysine residues in dibasic moieties. The enzyme is highly expressed in many endocrine tissues. Nardilysin has also been found in the brain. Previously, we have detected that nardilysin interacts with brain-specific proteins, i.e. p42(IP4)/centaurin-alpha1 [Stricker R, Chow KM, Walther D, Hanck T, Hersh LB, Reiser G (2006) Interaction of the brain specific protein p42(IP4)/centaurin-alpha1 with the peptidase nardilysin is regulated by the cognate ligands of p42(IP4), PtdIns(3,4,5)P(3) and Ins(1,3,4,5)P(4), with stereospecificity. J Neurochem 98:343-354]. However, very little is known about the distribution of nardilysin in the brain. The aim of the present study was to reveal its regional distribution and cellular localization in developing and adult human brain. Using immunohistochemistry and Western blot analysis we demonstrate that the enzyme is widely, but unevenly, expressed in the human brain. We found high staining intensity in the hypothalamus, neocortex and brain stem nuclei. The cellular localization is almost exclusively confined to neurons. In pre- and perinatal human brain cortex, most neurons express the enzyme. In cortical neurons nardilysin protein was found to be partially co-localized with parvalbumin but not calretinin. No co-expression was seen with somatostatin-28 immunoreactivity. A considerable overlap was revealed between p42(IP4) and nardilysin. Our data support the hypothesis that nardilysin might possibly play a role in brain development, whereas its putative function in brain peptide metabolism remains to be clarified further.

Interaction of the brain-specific protein p42IP4/centaurin-alpha1 with the peptidase nardilysin is regulated by the cognate ligands of p42IP4, PtdIns(3,4,5)P3 and Ins(1,3,4,5)P4, with stereospecificity

The brain-specific protein p42IP4, also called centaurin-alpha1, specifically binds phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3] and inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4]. Here, we investigate the interaction of p42IP4/centaurin-alpha1 with nardilysin (NRDc), a member of the M16 family of zinc metalloendopeptidases. Members of this peptidase family exhibit enzymatic activity and also act as receptors for other proteins. We found that p42IP4/centaurin-alpha1 binds specifically to NRDc from rat brain. We further detected that centaurin-alpha2, a protein that is highly homologous to p42IP4/centaurin-alpha1 and expressed ubiquitously, also binds to NRDc. In vivo interaction was demonstrated by co-immunoprecipitation of p42IP4/centaurin-alpha1 with NRDc from rat brain. The acidic domain of NRDc (NRDc-AD), which does not participate in catalysis, is sufficient for the protein interaction with p42IP4. Interestingly, preincubation of p42IP4 with its cognate ligands D-Ins(1,3,4,5)P4 and the lipid diC8PtdIns(3,4,5)P3 negatively modulates the interaction between the two proteins. D-Ins(1,3,4,5)P4 and diC8PtdIns(3,4,5)P3 suppress the interaction with virtually identical concentration dependencies. This inhibition is highly ligand specific. The enantiomer L-Ins(1,3,4,5)P4 is not effective. Similarly, the phosphoinositides diC8PtdIns(3,4)P2, diC8PtdIns(3,5)P2 and diC8PtdIns(4,5)P2 all have no influence on the interaction. Further experiments revealed that endogenous p42IP4 from rat brain binds to glutathione-S-transferase (GST)-NRDc-AD. The proteins dissociate from each other when incubated with D-Ins(1,3,4,5)P4, but not with inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. In summary, we demonstrate that p42IP4 binds to NRDc via the NRDc-AD, and that this interaction is controlled by the cognate cellular ligands of p42IP4/centaurin-alpha1. Thus, specific ligands of p42IP4 can modulate the recruitment of proteins, which are docked to p42IP4, to specific cellular compartments.

The closed structure of presequence protease PreP forms a unique 10,000 Angstroms3 chamber for proteolysis

Presequence protease PreP is a novel protease that degrades targeting peptides as well as other unstructured peptides in both mitochondria and chloroplasts. The first structure of PreP from Arabidopsis thaliana refined at 2.1 Angstroms resolution shows how the 995-residue polypeptide forms a unique proteolytic chamber of more than 10,000 Angstroms(3) in which the active site resides. Although there is no visible opening to the chamber, a peptide is bound to the active site. The closed conformation places previously unidentified residues from the C-terminal domain at the active site, separated by almost 800 residues in sequence to active site residues located in the N-terminal domain. Based on the structure, a novel mechanism for proteolysis is proposed involving hinge-bending motions that cause the protease to open and close in response to substrate binding. In support of this model, cysteine double mutants designed to keep the chamber covalently locked show no activity under oxidizing conditions. The manner in which substrates are processed inside the chamber is reminiscent of the proteasome; therefore, we refer to this protein as a peptidasome.

Subcellular localization of nardilysin during mouse oocyte maturation

We have previously shown that the peptidase, nardilysin, contains a bipartite nuclear localization signal that permits the enzyme to cycle between the nucleus and cytoplasm. In the present study, we report that nardilysin accumulates in the nucleus of an oocyte as a function of its maturation. Nardilysin is predominantly localized in the cytoplasm of an oocyte when initially placed into culture. The enzyme starts to accumulate in the nucleus within 30 min of in vitro culture. After 3 h, nardilysin is found as a spherical structure surrounded by condensed chromosomal DNA. After 18 h of in vitro culture, it co-localizes with beta-tubulin at the spindle apparatus. Cilostamide, a phosphodiesterase 3A inhibitor that inhibits meiosis, blocks accumulation of nuclear nardilysin. This finding demonstrates that the nuclear entry of nardilysin is tightly controlled in the oocyte. Taken together, these experiments strongly suggest a role for nardilysin in meiosis through its dynamic translocation from cytosol to nucleus, and then to the spindle apparatus.

Nuclear shuttling of the peptidase nardilysin

The metalloendopeptidase nardilysin contains a putative N-terminal nuclear localization signal. The functionality of this sequence was tested with nardilysin-GFP fusion constructs. Expression in NIH3T3 cells showed approximately 90-95% of nardilysin-GFP as cytoplasmic. However, 3-6% of transfected cells showed both cytosolic and nuclear staining, while 2-4% showed predominantly nuclear staining. A nuclear localization signal mutant and an N-terminally truncated nardilysin-GFP with the nuclear localization signal deleted were completely cytoplasmic. Although endogenous nardilysin was barely detectable in the nucleus, after treatment with leptomycin B, nuclear nardilysin rose to approximately 15% and to over 25% after addition of spermine. The ability of a methionine 49 to act as the sole initiator methionine, as previously proposed, was tested by inserting a c-myc epitope between leucine28 and glycine29. Expression in HEK293 cells showed the presence of the c-myc tag, demonstrating that the enzyme can be translated from the first methionine and contains the nuclear localization signal.

Substrate activation of insulin-degrading enzyme (insulysin). A potential target for drug development

The rate of the insulin-degrading enzyme (IDE)-catalyzed hydrolysis of the fluorogenic substrate 2-aminobenzoyl-GGFLRKHGQ-ethylenediamine-2,4-dinitrophenyl is increased 2-7-fold by other peptide substrates but not by peptide non-substrates. This increased rate is attributed to a decrease in Km with little effect on Vmax. An approximately 2.5-fold increase in the rate of amyloid beta peptide hydrolysis is produced by dynorphin B-9. However, with insulin as substrate, dynorphin B-9 is inhibitory. Immunoprecipitation of differentially tagged IDE and gel filtration analysis were used to show that IDE exists as a mixture of dimers and tetramers. The equilibrium between dimer and tetramer is concentration-dependent, with the dimer the more active form. Bradykinin shifted the equilibrium toward dimer. Activation of substrate hydrolysis is not seen with a mixed dimer of IDE containing one active subunit and one subunit that is catalytically inactive and deficient in substrate binding. On the other hand, a mixed dimer containing one active subunit and one subunit that is catalytically inactive but binds substrate with normal affinity is activated by peptides. These findings suggest that peptides bind to one subunit of IDE and induce a conformational change that shifts the equilibrium to the more active dimer as well as activates the adjacent subunit. The selective activation of IDE toward amyloid beta peptide relative to insulin suggests the potential for development of compounds that increase IDE activity toward amyloid beta peptide as a therapeutic intervention for the treatment of Alzheimer's disease.