Cathepsin S cannibalism of cathepsin K as a mechanism to reduce type I collagen degradation.

Cathepsins S and K are potent mammalian proteases secreted into the extracellular space and have been implicated in elastin and collagen degradation in diseases such as atherosclerosis and osteoporosis. Studies of individual cathepsins hydrolyzing elastin or collagen have provided insight into their binding and kinetics, but cooperative or synergistic activity between cathepsins K and S is less described. Using fluorogenic substrate assays, Western blotting, cathepsin zymography, and computational analyses, we uncovered cathepsin cannibalism, a novel mechanism by which cathepsins degrade each other as well as the substrate, with cathepsin S predominantly degrading cathepsin K. As a consequence of these proteolytic interactions, a reduction in total hydrolysis of elastin and type I collagen was measured compared with computationally predicted values derived from individual cathepsin assays. Furthermore, type I collagen was preserved from hydrolysis when a 10-fold ratio of cathepsin S cannibalized the highly collagenolytic cathepsin K, preventing its activity. Elastin was not preserved due to strong elastinolytic ability of both enzymes. Together, these results provide new insight into the combined proteolytic activities of cathepsins toward substrates and each other and present kinetic models to consider for more accurate predictions and descriptions of these systems.

Detection of femtomole quantities of mature cathepsin K with zymography.

Cathepsin K, the most potent mammalian collagenase, has been implicated in osteoporosis, cancer metastasis, atherosclerosis, and arthritis. Although procathepsin K is stable and readily detected, the active mature cathepsin K eludes detection by in vitro methods due to its shorter half-life and inactivation at neutral pH. We describe, for the first time, reliable detection, visualization, and quantification of mature cathepsin K to femtomole resolution using gelatin zymography. The specificity of the method was validated with cathepsin K knockdown using small interfering RNA (siRNA) transfection of human monocyte-derived macrophages, and enzymatic activity confirmed with benzyloxycarbonyl-glycine-proline-arginine-7-amino-4-methylcoumarin (Z-GPR-AMC) substrate hydrolysis was fit to a computational model of enzyme kinetics. Furthermore, cathepsin K zymography was used to show that murine osteoclasts secrete more cathepsin K than is stored intracellularly, and this was opposite to the behavior of the macrophages from which they were differentiated. In summary, this inexpensive, species-independent, antibody-free protocol describes a sensitive method with broad potential to elucidate previously undetectable cathepsin K activity.

PACMANS: A bioinformatically informed algorithm to predict, design, and disrupt protease-on-protease hydrolysis.

Multiple proteases in a system hydrolyze target substrates, but recent evidence indicates that some proteases will degrade other proteases as well. Cathepsin S hydrolysis of cathepsin K is one such example. These interactions may be uni- or bi-directional and change the expected kinetics. To explore potential protease-on-protease interactions in silico, a program was developed for users to input two proteases: (1) the protease-ase that hydrolyzes (2) the substrate, protease. This program identifies putative sites on the substrate protease highly susceptible to cleavage by the protease-ase, using a sliding-window approach that scores amino acid sequences by their preference in the protease-ase active site, culled from MEROPS database. We call this PACMANS, Protease-Ase Cleavage from MEROPS ANalyzed Specificities, and test and validate this algorithm with cathepsins S and K. PACMANS cumulative likelihood scoring identified L253 and V171 as sites on cathepsin K subject to cathepsin S hydrolysis. Mutations made at these locations were tested to block hydrolysis and validate PACMANS predictions. L253A and L253V cathepsin K mutants significantly reduced cathepsin S hydrolysis, validating PACMANS unbiased identification of these sites. Interfamilial protease interactions between cathepsin S and MMP-2 or MMP-9 were tested after predictions by PACMANS, confirming its utility for these systems as well. PACMANS is unique compared to other putative site cleavage programs by allowing users to define the proteases of interest and target, and can also be employed for non-protease substrate proteins, as well as short peptide sequences.

VEGF internalization is not required for VEGFR-2 phosphorylation in bioengineered surfaces with covalently linked VEGF.

Vascular endothelial growth factor (VEGF) is known to activate proliferation, migration, and survival pathways in endothelial cells through phosphorylation of VEGF receptor-2 (VEGFR-2). VEGF has been incorporated into biomaterials through encapsulation, electrostatic sequestration, and covalent attachment, but the effect of these immobilization strategies on VEGF signaling has not been thoroughly investigated. Further, although growth factor internalization along with the receptor generally occurs in a physiological setting, whether this internalization is needed for receptor phosphorylation is not entirely clear. Here we show that VEGF covalently bound through a modified heparin molecule elicits an extended response of pVEGFR-2 in human umbilical vein endothelial cells (HUVECs) and that the covalent linkage reduces internalization of the growth factor during receptor endocytosis. Optical tweezer measurements show that the rupture force required to disrupt the heparin-VEGF-VEGFR-2 interaction increases from 3-8 pN to 6-12 pN when a covalent bond is introduced between VEGF and heparin. Importantly, by covalently binding VEGF to a heparin substrate, the stability (half-life) of VEGF is extended over three-fold. Here, mathematical models support the biological conclusions, further suggesting that VEGF internalization is significantly reduced when covalently bound, and indicating that VEGF is available for repeated phosphorylation events.