High yield synthesis and characterization of phosphorylated recombinant human procathepsin D expressed in mammalian cells

Knock-down of cathepsin D affects the retinal pigment epithelium, impairs swim-bladder ontogenesis and causes premature death in zebrafish

The lysosomal aspartic protease Cathepsin D (CD) is ubiquitously expressed in eukaryotic organisms. CD activity is essential to accomplish the acid-dependent extensive or partial proteolysis of protein substrates within endosomal and lysosomal compartments therein delivered via endocytosis, phagocytosis or autophagocytosis. CD may also act at physiological pH on small-size substrates in the cytosol and in the extracellular milieu. Mouse and fruit fly CD knock-out models have highlighted the multi-pathophysiological roles of CD in tissue homeostasis and organ development. Here we report the first phenotypic description of the lack of CD expression during zebrafish (Danio rerio) development obtained by morpholino-mediated knock-down of CD mRNA. Since the un-fertilized eggs were shown to be supplied with maternal CD mRNA, only a morpholino targeting a sequence containing the starting ATG codon was effective. The main phenotypic alterations produced by CD knock-down in zebrafish were: 1. abnormal development of the eye and of retinal pigment epithelium; 2. absence of the swim-bladder; 3. skin hyper-pigmentation; 4. reduced growth and premature death. Rescue experiments confirmed the involvement of CD in the developmental processes leading to these phenotypic alterations. Our findings add to the list of CD functions in organ development and patho-physiology in vertebrates.

Akt induces apoptosis in neuroblastoma cells expressing a C98X vasopressin mutant following autophagy suppression

Mutations in the arginine vasopressin (AVP)-neurophysin II (NP-II) gene that affect the folding and transport of the prohormone result in loss of secretion of the anti-diuretic hormone AVP from pituitary nerve terminals and cause autosomal dominant familial neurohypophyseal diabetes insipidus (adFNDI). One such mutation consists of the replacement of a Cys residue at position 98 with a stop codon (C98X) in the AVP precursor (corresponding to C67X in NP domain). In neuroblastoma cells over-expressing this truncated AVP precursor autophagy, a macromolecular degradation process, was shown to be essential for assuring cell survival. In the present study, we investigated the role of the Akt pro-survival signalling in the regulation of autophagy and of apoptosis linked with the handling of C98X AVP. Impairing autophagy-lysosomal sequestration or cathepsin D (CD)-mediated proteolysis triggered the activation of the intrinsic death pathway of apoptosis in C98X-expressing cells, but not in the wild-type -AVP-expressing cells. This was shown by the expression of a Vps34 dominant negative, which down-regulates the PI3k class III-dependent signalling needed for autophagosome (APH) formation, by genetic silencing as a result of RNA interference (RNAi) of Lamp2, a protein indispensable for the fusion of APHs with lysosomes, and by RNAi silencing of the lysosomal protease CD. Ectopic expression of either the wild-type or the mutated C98X AVP altered neither the expression nor the phosphorylation of the pro-survival signalling molecule Akt. Strikingly, the ectopic adenoviral-directed expression of a constitutively active Akt, instead of preserving cell survival, resulted in the suppression of autophagy, and precipitated Bax-mediated cell death. The present data demonstrate the need for autophagy-mediated degradation of mutated C98X peptides, which otherwise become toxic to the cell, and suggest that, in the presence of mis-folded proteins, the stimulation of the Akt signalling counteracts the beneficial effects of autophagy and precipitates cell death. It follows that growth factors impinging on the Akt pathway may have deleterious effect in neurones expressing mutant neuropeptides. This can provide an explanation for the late onset and progressive neuronal cell loss observed in hypothalamic magnocellular neurones of adFNDI patients.

Chemotherapy drug response in ovarian cancer cells strictly depends on a cathepsin D-Bax activation loop

The ovarian cancer cell lines A2780 (wild-type p53) and NIHOVCAR3 (mutated p53) showed, respectively, sensitivity and resistance towards several chemotherapy drugs. We hypothesized that the two cell lines differ in their ability to activate the intrinsic death pathway and have, therefore, dissected the lysosome-mitochondrion signalling pathway by pharmacological inhibition or genetic manipulation of key regulators and executioners. Biochemical and morphological confocal fluorescence studies showed that: (1) In A2780 cells bcl-2 is expressed at an undetectable level, whereas Bax is expressed at a rather high level; by contrast, bcl-2 is highly expressed and Bax is expressed at extremely low levels in NIHOVCAR3 cells; (2) Chemotherapy treatment reduced the expression of bcl-2 in NIHOVCAR3 cells, yet these cells resisted to drug toxicity; (3) Cathepsin D (CD), not cathepsin B or L, mediates the activation of the mitochondrial intrinsic death pathway in A2780 cells; (4) Lysosome leakage and cytosolic relocation of CD occurs in the chemosensitive A2780 cells, not in the chemoresistant NIHOVCAR3 cells; (5) Bax is essential for the permeabilization of both lysosomes and mitochondria in A2780 cells exposed to chemotherapy drugs; (6) CD activity is mandatory for the oligomerization of Bax on both mitochondrial and lysosomal membranes; (7) Bax activation did not occur in the resistant NIHOVCAR3 cells despite their high content in CD. The present data are consistent with a model in which on treatment with a cytotoxic drug the activation of a CD-Bax loop leads to the generalized permeabilization of lysosomes and eventually of mitochondria, thus reaching the point of no return, and culminates with the activation of the caspase cascade. Our data also imply that dysfunctional permeabilization of lysosomes contributes to the development of chemoresistance.

Profiling of proteolytic enzymes in the gut of the tick Ixodes ricinus reveals an evolutionarily conserved network of aspartic and cysteine peptidases

Background

Ticks are vectors for a variety of viral, bacterial and parasitic diseases in human and domestic animals. To survive and reproduce ticks feed on host blood, yet our understanding of the intestinal proteolytic machinery used to derive absorbable nutrients from the blood meal is poor. Intestinal digestive processes are limiting factors for pathogen transmission since the tick gut presents the primary site of infection. Moreover, digestive enzymes may find practical application as anti-tick vaccine targets.

Results

Using the hard tick, Ixodes ricinus, we performed a functional activity scan of the peptidase complement in gut tissue extracts that demonstrated the presence of five types of peptidases of the cysteine and aspartic classes. We followed up with genetic screens of gut-derived cDNA to identify and clone genes encoding the cysteine peptidases cathepsins B, L and C, an asparaginyl endopeptidase (legumain), and the aspartic peptidase, cathepsin D. By RT-PCR, expression of asparaginyl endopeptidase and cathepsins B and D was restricted to gut tissue and to those developmental stages feeding on blood.

Conclusion

Overall, our results demonstrate the presence of a network of cysteine and aspartic peptidases that conceivably operates to digest host blood proteins in a concerted manner. Significantly, the peptidase components of this digestive network are orthologous to those described in other parasites, including nematodes and flatworms. Accordingly, the present data and those available for other tick species support the notion of an evolutionary conservation of a cysteine/aspartic peptidase system for digestion that includes ticks, but differs from that of insects relying on serine peptidases.

Folding, activity and targeting of mutated human cathepsin D that cannot be processed into the double-chain form

The precursor of human cathepsin D (CD) is converted into the single-chain and the double-chain active polypeptides by subsequent proteolysis reactions taking place in the endosomal-lysosomal compartment and involving specific aminoacid sequences. We have mutagenized the region of aminoacids (comprising the beta-hairpin loop) involved in the latter proteolytic maturation step and generated a mutant CD that cannot be converted into the mature double-chain form. This mutant CD expressed in rodent cells reaches the lysosome and is stable as single-chain polypeptide, bears high-mannose type sugars, binds to pepstatin A and is enzymatically active, indicating that it is correctly folded. The present work provides new insights on the aminoacid region involved in the terminal processing of human CD and on the function of the processing beta-hairpin loop.

Cathepsin D expression is decreased in Alzheimer's disease fibroblasts

Cathepsin D (CTSD), a protease detectable in different cell types whose primary function is to degrade proteins by bulk proteolysis in lysosomes, has been suggested to be involved in Alzheimer's disease (AD). In fact, there is increasing evidence that disturbance of the normal balance and localization of cathepsins may contribute to neurodegeneration in AD [Nakanishi H. Neuronal and microglial cathepsins in aging and age-related diseases. Aging Res Rev 2003; 2(4):367-81]. Here, we provide evidence of an altered balance of CTSD in skin fibroblasts from patients affected either by sporadic or familial forms of AD. In particular, we demonstrate that CTSD is down regulated at both transcriptional and translational level and its processing is altered in AD fibroblasts. The oncogene Ras is involved in the regulation of CTSD, as high expression level of the constitutively active form of Ras in normal or AD fibroblasts induces CTSD down-regulation. p38 MAPK signalling pathway also appears to down-modulate CTSD level. Overall results reinforce the hypothesis that a lysosomal impairment may be involved in AD pathogenesis and can be detected not only in the CNS but also at a peripheral level.