Double deficiency of cathepsins B and L results in massive secretome alterations and suggests a degradative cathepsin-MMP axis.

Endolysosomal cysteine cathepsins functionally cooperate. Cathepsin B (Ctsb) and L (Ctsl) double-knockout mice die 4 weeks after birth accompanied by (autophago-) lysosomal accumulations within neurons. Such accumulations are also observed in mouse embryonic fibroblasts (MEFs) deficient for Ctsb and Ctsl. Previous studies showed a strong impact of Ctsl on the MEF secretome. Here we show that Ctsb alone has only a mild influence on extracellular proteome composition. Protease cleavage sites dependent on Ctsb were identified by terminal amine isotopic labeling of substrates (TAILS), revealing a prominent yet mostly indirect impact on the extracellular proteolytic cleavages. To investigate the cooperation of Ctsb and Ctsl, we performed a quantitative secretome comparison of wild-type MEFs and Ctsb (-/-) Ctsl (-/-) MEFs. Deletion of both cathepsins led to drastic alterations in secretome composition, highlighting cooperative functionality. While many protein levels were decreased, immunodetection corroborated increased levels of matrix metalloproteinase (MMP)-2. Re-expression of Ctsl rescues MMP-2 abundance. Ctsl and to a much lesser extent Ctsb are able to degrade MMP-2 at acidic and neutral pH. Addition of active MMP-2 to the MEF secretome degrades proteins whose levels were also decreased by Ctsb and Ctsl double deficiency. These results suggest a degradative Ctsl-MMP-2 axis, resulting in increased MMP-2 levels upon cathepsin deficiency with subsequent degradation of secreted proteins such as collagen α-1 (I).