Cysteine protease-mediated cytoskeleton interactions with LFA-1 promote T-cell morphological changes

Cysteine Cathepsins as Therapeutic Targets in Immune Regulation and Immune Disorders

Cysteine cathepsins, as the most abundant proteases found in the lysosomes, play a vital role in several processes-such as protein degradation, changes in cell signaling, cell morphology, migration and proliferation, and energy metabolism. In addition to their lysosomal function, they are also secreted and may remain functional in the extracellular space. Upregulation of cathepsin expression is associated with several pathological conditions including cancer, neurodegeneration, and immune-system dysregulation. In this review, we present an overview of cysteine-cathepsin involvement and possible targeting options for mitigation of aberrant function in immune disorders such as inflammation, autoimmune diseases, and immune response in cancer.

Low level lysosomal membrane permeabilization for limited release and sub-lethal functions of cathepsin proteases in the cytosol and nucleus

For a long time, lysosomes were purely seen as organelles in charge of garbage disposal within the cell. They destroy any cargo delivered into their lumen with a plethora of highly potent hydrolytic enzymes, including various proteases. In case of damage to their limiting membranes, the lysosomes release their soluble content with detrimental outcomes for the cell. In recent years however, this view of the lysosome changed towards acknowledging it as a platform for integration of manifold intra- and extracellular signals. Even impaired lysosomal membrane integrity is no longer considered to be a one-way street to cell death. Increasing evidence suggests that lysosomal enzymes, mainly cathepsin proteases, can be released in a spatially and temporarily restricted manner that is compatible with cellular survival. This way, cathepsins can act in the cytosol and the nucleus, where they affect important cellular processes such as cell division. Here, we review this evidence and discuss the routes and molecular mechanisms by which the cathepsins may reach their unusual destination.

Control of mitosis, inflammation, and cell motility by limited leakage of lysosomes

Lysosomal membrane permeabilization and subsequent leakage of lysosomal hydrolases into the cytosol are considered as the major hallmarks of evolutionarily conserved lysosome-dependent cell death. Contradicting this postulate, new sensitive methods that can detect a minimal lysosomal membrane damage have demonstrated that lysosomal leakage does not necessarily equal cell death. Notably, cells are not only able to survive minor lysosomal membrane permeabilization, but some of their normal functions actually depend on leaked lysosomal hydrolases. Here we discuss emerging data suggesting that spatially and temporally controlled lysosomal leakage delivers lysosomal hydrolases to specific subcellular sites of action and controls at least three essential cellular processes, namely mitotic chromosome segregation, inflammatory signaling, and cellular motility.

Cathepsin H mediates the processing of talin and regulates migration of prostate cancer cells

The cytoskeletal protein talin, an actin- and β-integrin tail-binding protein, plays an important role in cell migration by promoting integrin activation and focal adhesion formation. Here, we show that talin is a substrate for cathepsin H (CtsH), a lysosomal cysteine protease with a strong aminopeptidase activity. Purified active CtsH sequentially cleaved a synthetic peptide representing the N terminus of the talin F0 head domain. The processing of talin by CtsH was determined also in the metastatic PC-3 prostate cancer cell line, which exhibits increased expression of CtsH. The attenuation of CtsH aminopeptidase activity by a specific inhibitor or siRNA-mediated silencing significantly reduced the migration of PC-3 cells on fibronectin and invasion through Matrigel. We found that in migrating PC-3 cells, CtsH was co-localized with talin in the focal adhesions. Furthermore, specific inhibition of CtsH increased the activation of α(v)β(3)-integrin on PC-3 cells. We propose that CtsH-mediated processing of talin might promote cancer cell progression by affecting integrin activation and adhesion strength.

Ostrinia furnacalis integrin β1 may be involved in polymerization of actin to modulate spreading and encapsulation of plasmatocytes

Insect hemocytes must change their state from non-adhesive to adhesive when they spread on or encapsulate foreign invaders. Although integrin β has been reported to play an important role in hemocyte spreading and encapsulation in several insects, how it is involved in the encapsulation process is still unclear. Here we report that integrin β1 of Ostrinia furnacalis (Ofint β1) may modulate plasmatocyte spreading by regulating polymerization of F-actin and further affecting formation of capsules. In the Sephadex A-25 bead-injected larvae, hemocytes forming capsules expressed approximately ten times more Ofint β1 than hemocytes that are free in circulation in hemolymph. When the expression of Ofint β1 in hemocytes was inhibited by dsRNA of Ofint β1 (dsINT), polymerization of F-actin in hemocytes, especially in plasmatocytes, was significantly decreased, spreading of plasmatocytes was inhibited, and encapsulation rate of Sephadex beads was also significantly decreased. Furthermore, hemocytes formed individual aggregates on beads in the dsINT injected larvae, while hemocytes formed complete capsules surrounding the beads in the control larvae; and most of the hemocytes on the beads in the dsINT-injected larvae assumed round forms rather than spread forms. Based on these results, we speculate that integrins on cellular membranes may modulate hemocyte spreading by regulating polymerization of F-actin and further affecting encapsulation of foreign objects.

LFA-1 fine-tuning by cathepsin X

The adhesion molecule lymphocyte function-associated antigen (LFA)-1 plays a key role in immune surveillance and response. Its conformation is spatially and temporally regulated, enabling adhesion and deadhesion during T-cell migration. LFA-1 adhesion to its major ligand intercellular adhesion molecule 1 is controlled by adaptor proteins which bind the cytoplasmic tail of the β (2) subunit. Cathepsin X, a cysteine carboxypeptidase, promotes T-cell migration and morphological changes by cleaving the β (2) cytoplasmic tail of LFA-1. In this way, it modulates the affinity of LFA-1 for structural adaptors talin-1 and α-actinin-1 and enables the stepwise transition between intermediate and high-affinity conformations of LFA-1, an event that is necessary for effective T-cell function. Cathepsin X regulation that would allow precise modulation of LFA-1 affinity has a great potential for anti-LFA-1 therapy.

A proteomic and cellular analysis of uropods in the pathogen Entamoeba histolytica

Exposure of Entamoeba histolytica to specific ligands induces cell polarization via the activation of signalling pathways and cytoskeletal elements. The process leads to formation of a protruding pseudopod at the front of the cell and a retracting uropod at the rear. In the present study, we show that the uropod forms during the exposure of trophozoites to serum isolated from humans suffering of amoebiasis. To investigate uropod assembly, we used LC-MS/MS technology to identify protein components in isolated uropod fractions. The galactose/N-acetylgalactosamine lectin, the immunodominant antigen M17 (which is specifically recognized by serum from amoeba-infected persons) and a few other cells adhesion-related molecules were primarily involved. Actin-rich cytoskeleton components, GTPases from the Rac and Rab families, filamin, α-actinin and a newly identified ezrin-moesin-radixin protein were the main factors found to potentially interact with capped receptors. A set of specific cysteine proteases and a serine protease were enriched in isolated uropod fractions. However, biological assays indicated that cysteine proteases are not involved in uropod formation in E. histolytica, a fact in contrast to the situation in human motile immune cells. The surface proteins identified here are testable biomarkers which may be either recognized by the immune system and/or released into the circulation during amoebiasis.