TGME49_249670

Toxoplasma gondii cathepsin B (TgCPB/toxopain-1) is a single-copy cysteine endopeptidase that supports parasite invasion and intracellular growth (PMID:12000756, PMID:15102804). It is synthesized as a zymogen that cannot self-mature and instead depends on TgCPL for proteolytic activation, and it localizes to the lysosome-like vacuolar compartment (VAC) together with TgCPL rather than to the rhoptries proposed in the initial characterization (PMID:23250753). Consistent with this revised localization, TgCPB knock-out parasites retain normal rhoptry morphology and rhoptry protein maturation, showing it is dispensable for those processes (PMID:23250753). Its activity is restrained by endogenous cysteine proteinase inhibitors (toxostatins) acting in the nanomolar range (PMID:19111576). In infected human placental cells, TgCPB is released into the host cytosol downstream of ROS to drive NLRP1/NLRP3/NLRC4/AIM2 inflammasome activation and pyroptosis (PMID:37741453).

1. 2002 High

   Establishing whether T. gondii encodes a functional cathepsin B and what it does required cloning and characterizing the enzyme; this defined TgCPB as an active cysteine protease and first linked it to invasion.

   Evidence Gene cloning, recombinant refolding with substrate profiling and homology modeling, immunolocalization, pulse-chase, and inhibitor treatment of infected fibroblasts

   PMID:12000756

   Open questions at the time

   • Initial rhoptry localization was later contradicted
   • Inhibitor effects on invasion are pharmacological, not genetic in this study
2. 2004 Medium

   To move from correlation to functional requirement, antisense knockdown tested whether depleting TgCPB impairs the parasite, confirming its contribution to invasion and multiplication.

   Evidence Antisense knockdown of mRNA/protein with in vitro invasion/multiplication assays and a chicken embryo congenital toxoplasmosis model

   PMID:15102804

   Open questions at the time

   • Incomplete (>60%) knockdown
   • Mechanism by which loss impairs invasion not resolved
3. 2006 Medium

   Classifying TgCPB's catalytic mode distinguished it as an endopeptidase and revealed stage-specific expression, refining its enzymatic identity relative to other parasite cathepsins.

   Evidence Substrate specificity classification of recombinant enzymes and real-time PCR expression analysis

   PMID:17164247

   Open questions at the time

   • Physiological substrates not identified
   • Basis of bradyzoite silencing unknown
4. 2008 Medium

   Identifying endogenous regulators addressed how TgCPB activity is controlled; toxostatins inhibit it at nanomolar potency, though overexpression did not alter invasion or growth.

   Evidence Cloning and recombinant expression of ICPs/toxostatins, enzyme inhibition assays, and transgenic toxostatin-1 overexpression with phenotypic readouts

   PMID:19111576

   Open questions at the time

   • Lack of phenotype leaves physiological role of the inhibitor unclear
   • Specificity of toxostatins for TgCPB versus other proteases not fully resolved
5. 2012 High

   Determining how TgCPB is activated and where it acts revised the field: it requires TgCPL for maturation and resides in the VAC, not the rhoptries, and is dispensable for rhoptry biogenesis.

   Evidence In vitro recombinant maturation assay, validated antibody immunolocalization, and TgCPB knock-out analysis of rhoptry morphology and protein processing

   PMID:23250753

   Open questions at the time

   • VAC-resident substrates of TgCPB not defined
   • How TgCPB contributes to invasion/growth given VAC localization is unresolved
6. 2023 Medium

   Extending TgCPB's relevance to host response, its release into the host cytosol was placed downstream of ROS and upstream of inflammasome-driven pyroptosis in infected placental cells.

   Evidence Cathepsin B inhibitor and ROS scavenger treatment, inflammasome-specific siRNA, and assays of caspase-1 cleavage, IL-1β maturation, and gasdermin D cleavage in infected trophoblast/amniotic cells

   PMID:37741453

   Open questions at the time

   • Mechanism of TgCPB release into host cytosol not defined
   • Direct molecular link between TgCPB and each inflammasome sensor not established
   • Pharmacological/siRNA readouts only, single cell context

• The physiological substrates of TgCPB within the VAC and the molecular basis by which it promotes invasion and intracellular growth remain undefined.
• No endogenous VAC substrate identified
• Causal mechanism linking VAC localization to invasion phenotype unresolved