Specialized roles for cysteine cathepsins in health and disease

Abstract

Cathepsins were originally identified as proteases that act in the lysosome. Recent work has uncovered nontraditional roles for cathepsins in the extracellular space as well as in the cytosol and nucleus. There is strong evidence that subspecialized and compartmentalized cathepsins participate in many physiologic and pathophysiologic cellular processes, in which they can act as both digestive and regulatory proteases. In this review, we discuss the transcriptional and translational control of cathepsin expression, the regulation of intracellular sorting of cathepsins, and the structural basis of cathepsin activation and inhibition. In particular, we highlight the emerging roles of various cathepsin forms in disease, particularly those of the cardiac and renal systems.

Figure 1. Phylogeny and nomenclature of cathepsins.

Phylogenic tree of mouse and human cathepsin proenzymes. Mouse cathepsin L, human cathepsin V, and human cathepsin L are compared with each other and with human and mouse cathepsin B in order to demonstrate the phylogenic distance to other prototypic members of the cysteine cathepsin family.

Figure 2. Cathepsin structures and traditional and nontraditional protease trafficking.

Cathepsins contain a signal peptide (blue) that directs insertion of the nascent polypeptide chain into the ER. Within the ER, the signal peptide is cleaved and the protein folds with the assistance of the proregion (red). Disulfide bond formation (indicated by S-S) and N-linked glycosylation with high-mannose glycans subsequently occurs in the ER. Within the Golgi, mannose residues are phosphorylated to form m6p, which is used to route the protein into the endosomal/lysosomal compartment via the m6p receptor. Upon initial acidification of the endosome, cathepsins are activated, which leads to cleavage of the proregion and further activation of the cathepsin, resulting in further proteolytic processing in the lysosome into heavy and light chains (yellow). A portion of the cathepsins is not converted to the m6p form and as a result is shunted into the exocytosis pathway. Conversion to m6p appears to be rate limiting, as overexpression of a given cathepsin greatly increases the proportion of the enzyme in this pathway. Ribbon diagrams depict the structure of mature cathepsin L in the extracellular matrix and in the lysosome. The ribbon colors correspond to the colors used in the diagram on the left.

Figure 3. Alternative translation from downstream AUG sites produces cytoplasmic and nuclear cathepsin L.

(A) Alternative translation from downstream AUG sites produces cathepsin L that is devoid of the leader sequence and that can be present outside the lysosome, including in the cytosol and the nucleus. (B) Alternative translation leads to peptides that lack the leader sequence and thus are targeted to the cytosol or nucleus. The initial folding of the protein requires an intact proregion, and the mature protein is stabilized by three disulfide bonds. Thus, this process may require the presence of yet to be identified chaperones to generate a functional enzyme.

Figure 4. Cytosolic cathepsin L and its function in proteinuric kidney disease.

Phosphorylation of synaptopodin by PKA or CaMKII promotes 14-3-3 binding, which protects synaptopodin and dynamin against cathepsin L–mediated cleavage, thereby contributing to a stationary podocyte phenotype and an intact glomerular filtration barrier. Dephosphorylation of synaptopodin by calcineurin abrogates the interaction with 14-3-3. This renders the cathepsin L cleavage sites of synaptopodin accessible and promotes the degradation of synaptopodin and dynamin, thereby promoting a motile phenotype and the development of proteinuria. The calcineurin inhibitor CsA and the cathepsin inhibitor E64 safeguard against proteinuria by stabilizing synaptopodin and dynamin protein levels in podocytes.