Cystatin B deficiency results in sustained histone H3 tail cleavage in postnatal mouse brain mediated by increased chromatin-associated cathepsin L activity

Abstract

Cystatin B (CSTB) is a cysteine cathepsin inhibitor whose biallelic loss-of-function mutations in human result in defects in brain development and in neurodegeneration. The physiological function of CSTB is largely unknown, and the mechanisms underlying the human brain diseases remain poorly understood. We previously showed that CSTB modulates the proteolysis of the N-terminal tail of histone H3 (H3cs1) during in vitro neurogenesis. Here we investigated the significance of this mechanism in postnatal mouse brain. Spatiotemporal analysis of H3cs1 intensity showed that while H3cs1 in wild-type (wt) mice was found at varying levels during the first postnatal month, it was virtually absent in adult brain. We further showed that the high level of H3cs1 coincides with chromatin association of de novo synthesized cathepsin L suggesting a role for nuclear cathepsin L in brain development and maturation. On the contrary, the brains of Cstb–/– mice showed sustained H3cs1 proteolysis to adulthood with increased chromatin-associated cathepsin L activity, implying that CSTB regulates chromatin-associated cathepsin L activity in the postnatal mouse brain. As H3 tail proteolysis has been linked to cellular senescence in vitro, we explored the presence of several cellular senescence markers in the maturing Cstb–/– cerebellum, where we see increased levels of H3cs1. While several markers showed alterations in Cstb–/– mice, the results remained inconclusive regarding the association of deficient CSTB function with H3cs1-induced senescence. Together, we identify a molecular role for CSTB in brain with implications for brain development and disease.

The MMP-2 histone H3 N-terminal tail protease is selectively targeted to the transcription start sites of active genes

“…To this end, the native ChIP-Seq technique was used to map MMP-2, in part, because nuclear MMP-2 was found to bind chromatin with high affinity. This has similarly been reported for other H3NT proteases, including CSTL, MMP-9 and the neutrophil serine proteases, supporting high affinity chromatin binding as a conserved property of H3NT proteases [ 19 , 20 , 22 , 31 ]. This is an unexpected trait since the canonical functions and substrates of all these proteases are extra-nuclear and, furthermore, that the proteases lack any canonical DNA binding domains or nuclear protein–protein interaction domains.…”

“…The lysosomal cathepsin L protease (CTSL) was the first H3NT protease discovered in differentiating mouse embryonic stem cells [ 14 ]. CSTL was reported to “clip” the first 21 amino acids from H3, generating a H3∆21 product within chromatin, in several different cell types including fibroblasts, melanocytes, developing brains and villi of small intestines [ 14 , 18 , 20 , 23 ]. A recent study demonstrated that several neutrophil serine proteases coordinate H3NT proteolysis in monocytes resulting in multiple H3cl products [ 19 ].…”