Background: In eukaryotic cells, apoptosis signaling pathways are controlled mainly by aspartic acid cysteine proteases (caspases). However, certain unicellular microorganisms, such as Giardia duodenalis, lack these proteins. Thus, other cysteine proteases may play an important role in the parasite apoptosis signaling pathway.

Objective: To understand the effect of cathepsin B-like inhibition on the cell viability of Giardia duodenalis and its cell death process.

Methods: Bioinformatics analysis was performed to identify apoptotic proteases. Analysis showed that cathepsin B-like protease genes from G. duodenalis were the best candidate. A homology modeling technique was used to explore in silico the inhibitory effect of E-64 against cathepsin B-like proteases from G. duodenalis genome and to examine the effect of curcumin on cathepsin B-like activity regulation. In addition, the effect of E-64 on parasite survival and DNA fragmentation was tested.

Results: Eight cathepsin B-like protease coding genes were identified in silico. Interestingly, while these sequences lacked the cathepsin B characteristic occluding loop, they maintained the catalytic active- site responsible for cathepsin B activity, which was evidenced by the increase in the degradation of the Z-RR-AMC substrate, suggesting the upregulation of the activity of these proteins. Additionally, inhibition of E-64 against G. duodenalis trophozoites caused a decrease in DNA fragmentation compared to control cells and had a positive effect on parasite survival after exposure to curcumin.

Conclusion: Overall, these results suggested that Giardia duodenalis might have a cell death mechanism in which cathepsin B-like proteases play an important role.

背景：在真核细胞中，凋亡信号通路主要由天冬氨酸半胱氨酸蛋白酶（胱天蛋白酶）控制。但是，某些单细胞微生物（例如十二指肠贾第虫）缺乏这些蛋白质。因此，其他半胱氨酸蛋白酶可能在寄生虫凋亡信号通路中起重要作用。

目的：了解组织蛋白酶B样抑制对十二指肠贾第鞭毛虫细胞活力及其细胞死亡过程的影响。

方法：进行生物信息学分析以鉴定凋亡蛋白酶。分析表明，十二指肠球菌的组织蛋白酶B样蛋白酶基因是最好的候选基因。同源性建模技术被用于计算机研究E-64对十二指肠球菌基因组中组织蛋白酶B样蛋白酶的抑制作用，并研究姜黄素对组织蛋白酶B样活性调节的作用。另外，测试了E-64对寄生虫存活和DNA片段化的影响。

结果：在计算机上鉴定了八个组织蛋白酶B样蛋白酶编码基因。有趣的是，尽管这些序列缺少组织蛋白酶B的特征性闭环，但它们仍维持着负责组织蛋白酶B活性的催化活性位点，这可通过Z-RR-AMC底物降解的增加来证明，表明该活性的上调这些蛋白质。此外，与对照细胞相比，抑制E-64对抗十二指肠营养滋养体的DNA片段减少，并且对暴露于姜黄素的寄生虫存活有积极影响。

结论：总的来说，这些结果表明十二指肠菊可能具有细胞死亡机制，其中组织蛋白酶B样蛋白酶起着重要的作用。