Characterization of mitochondrial calpain-5

Treatment and Visual Outcomes in Pediatric Patients with Autosomal Dominant Neovascular Inflammatory Vitreoretinopathy: A Cohort Study

BACKGROUND

Autosomal dominant neovascular inflammatory vitreoretinopathy (NIV), formerly called "ADNIV," is a rare autoinflammatory condition mainly of adulthood caused by mutations in calcium-activated calpain-5 protease (CAPN5). Our aim is to report the treatment and visual outcomes of children newly diagnosed with NIV after systemic treatment.

METHODS

We reviewed charts of patients ≤18 years old with CAPN5 gene mutation, ocular findings consistent with NIV, and treated with systemic immunosuppression for a minimum of 6 months. Treatment response was based on ophthalmic examination, ultra-widefield fluorescein-angiography (UWFFA), and optical coherence tomography (OCT).

RESULTS

Eight children (16 eyes) were diagnosed with NIV at a median age of 14 (Range [R] 9-16) years, with a median follow-up of 18 months (R6-20). At diagnosis, one patient had impaired visual acuity (VA > 0.4), eight had vascular leakage, two had neovascularization, and three had macular edema. All responded to oral or local glucocorticoids but was not sustained. Systemic immunosuppression was started in seven patients with methotrexate and infliximab after a median time from diagnosis of 1.5 months (R0.5-2) and 3.2 months (R2.5-3.1), respectively. Infliximab was discontinued in all after a median time of 7 months (R3.5-10) for ineffectiveness, and 5/7 switched to tocilizumab and 1 to adalimumab. Five failed to respond (4 tocilizumab, 1 adalimumab) and one had a minimal response to tocilizumab.

CONCLUSIONS

We report on the systemic treatment response of seven children with ADNIV treated with methotrexate, infliximab, and tocilizumab. None were able to control disease. Further studies are needed to understand long-term outcomes and the utility of systemic immunosuppression.

Proteolysis of mitochondrial calpain-13 in cerebral ischemia-reperfusion injury

Calpains are calcium-dependent cysteine proteases activated by intracellular Ca2+. Although calpains mainly exist in the cytosol, calpain-13 is present in the mitochondria in mouse brains; however, the enzymatic properties and physiological functions of calpain-13 remain unknown. Hence, in this study, we predicted and evaluated the enzymatic properties of calpain-13. Based on our bioinformatic approaches, calpain-13 possessed a catalytic triad and EF-hand domain, similar to calpain-1, a well-studied calpain. Therefore, we hypothesized that calpain-13 had calpain-1-like enzymatic properties; however, calpain-13 was not proteolyzed in C57BL/6J mouse brains. Subsequently, cerebral ischemia/reperfusion (I/R) injury caused proteolysis of mitochondrial calpain-13. Thus, our study showed that mitochondrial calpain-13 was proteolyzed in the mitochondria of the I/R injured mouse brain. This finding could be valuable in further research elucidating the involvement of calpain-13 in cell survival or death in brain diseases, such as cerebral infarction.

Role of calpain-5 in cerebral ischemia and reperfusion injury

BACKGROUND

Ischemia and reperfusion (I/R) injury exacerbate the prognosis of ischemic diseases. The cause of this exacerbation is partly a mitochondrial cell death pathway. Mitochondrial calpain-5 is proteolyzed/autolyzed under endoplasmic reticulum stress, resulting in inflammatory caspase-4 activation. However, the role of calpain-5 in I/R injury remains unclear. We hypothesized that calpain-5 is involved in ischemic brain disease.

METHODS

Mitochondria from C57BL/6J mice were extracted via centrifugation with/without proteinase K treatment. The expression and proteolysis/autolysis of calpain-5 were determined using western blotting. The mouse and human brains with I/R injury were analyzed using hematoxylin and eosin staining and immunohistochemistry. HT22 cells were treated with tunicamycin and CAPN5 siRNA.

RESULTS

Calpain-5 was expressed in the mitochondria of mouse tissues. Mitochondrial calpain-5 in mouse brains was responsive to calcium earlier than cytosolic calpain-5 in vitro calcium assays and in vivo bilateral common carotid artery occlusion model mice. Immunohistochemistry revealed that neurons were positive for calpain-5 in the normal brains of mice and humans. The expression of calpain-5 was increased in reactive astrocytes at human infarction sites. The knockdown of calpain-5 suppressed of cleaved caspase-11.

CONCLUSIONS

The neurons of human and mouse brains express calpain-5, which is proteolyzed/autolyzed in the mitochondria in the early stage of I/R injury and upregulated in reactive astrocytes in the end-stage.

GENERAL SIGNIFICANCE

Our results provide a comprehensive understanding of the mechanisms underlying I/R injury. Targeting the expression or activity of mitochondrial calpain-5 may suppress the inflammation during I/R injuries such as cerebrovascular diseases.

Membrane-anchored calpains - hidden regulators of growth and development beyond plants?

Calpains are modulatory proteases that modify diverse cellular substrates and play essential roles in eukaryots. The best studied are animal cytosolic calpains. Here, we focus on enigmatic membrane-anchored calpains, their structural and functional features as well as phylogenetic distribution. Based on domain composition, we identified four types of membrane-anchored calpains. Type 1 and 2 show broad phylogenetic distribution among unicellular protists and streptophytes suggesting their ancient evolutionary origin. Type 3 and 4 diversified early and are present in brown algae and oomycetes. The plant DEK1 protein is the only representative of membrane-anchored calpains that has been functionally studied. Here, we present up to date knowledge about its structural features, putative regulation, posttranslational modifications, and biological role. Finally, we discuss potential model organisms and available tools for functional studies of membrane-anchored calpains with yet unknown biological role. Mechanistic understanding of membrane-anchored calpains may provide important insights into fundamental principles of cell polarization, cell fate control, and morphogenesis beyond plants.

Impaired activity and membrane association of most calpain-5 mutants causal for neovascular inflammatory vitreoretinopathy

Neovascular inflammatory vitreoretinopathy (NIV) is a rare eye disease that ultimately leads to complete blindness and is caused by mutations in the gene encoding calpain-5 (CAPN5), with six pathogenic mutations identified. In transfected SH-SY5Y cells, five of the mutations resulted in decreased membrane association, diminished S-acylation, and reduced calcium-induced autoproteolysis of CAPN5. CAPN5 proteolysis of the autoimmune regulator AIRE was impacted by several NIV mutations. R243, L244, K250 and the adjacent V249 are on β-strands in the protease core 2 domain. Conformational changes induced by Ca2+binding result in these β-strands forming a β-sheet and a hydrophobic pocket which docks W286 side chain away from the catalytic cleft, enabling calpain activation based on comparison with the Ca2+-bound CAPN1 protease core. The pathologic variants R243L, L244P, K250N, and R289W are predicted to disrupt the β-strands, β-sheet, and hydrophobic pocket, impairing calpain activation. The mechanism by which these variants impair membrane association is unclear. G376S impacts a conserved residue in the CBSW domain and is predicted to disrupt a loop containing acidic residues which may contribute to membrane binding. G267S did not impair membrane association and resulted in a slight but significant increase in autoproteolytic and proteolytic activity. However, G267S is also identified in individuals without NIV. Combined with the autosomal dominant pattern of NIV inheritance and evidence that CAPN5 may dimerize, the results are consistent with a dominant negative mechanism for the five pathogenic variants which resulted in impaired CAPN5 activity and membrane association and a gain-of-function for the G267S variant.

Novel dicarbonyl metabolic pathway via mitochondrial ES1 possessing glyoxalase III activity

Glycation, caused by reactive dicarbonyls, plays a role in various diseases by forming advanced glycation end products. In live cells, reactive dicarbonyls such as glyoxal (GO) and methylglyoxal (MGO) are produced during cell metabolism, and these should be removed consistently. However, the dicarbonyl metabolic system in the mitochondria remains unclear. It has been speculated that the mammalian mitochondrial protein ES1 is a homolog of bacterial elbB possessing glyoxalase III (GLO3) activity. Therefore, in this study, to investigate ES1 functions and GLO3 activity, we generated ES1-knockout (KO) mice and recombinant mouse ES1 protein and investigated the biochemical and histological analyses. In the mitochondrial fraction obtained from ES1-KO mouse brains, the GO metabolism and cytochrome c oxidase activity were significantly lower than those in the mitochondrial fraction obtained from wildtype (WT) mouse brains. However, the morphological features of the mitochondria did not change noticeably in the ES1-KO mouse brains compared with those in the WT mouse brains. The mitochondrial proteome analysis showed that the MGO degradation III pathway and oxidative phosphorylation-related proteins were increased. These should be the response to the reduced GO metabolism caused by ES1 deletion to compensate for the dicarbonyl metabolism and damaged cytochrome c oxidase by elevated GO. Recombinant mouse ES1 protein exhibited catalytic activity of converting GO to glycolic acid. These results indicate that ES1 possesses GLO3 activity and modulates the metabolism of GO in the mitochondria. To our knowledge, this is the first study to show a novel metabolic pathway for reactive dicarbonyls in mitochondria.

Olfactory marker protein is unlikely to be cleaved by calpain 5

Olfactory maturation marker protein (OMP) is expressed in olfactory receptor neurons and hypothalamic neurons. OMP is a nested gene located in the intron of calpain 5 (CAPN5), a Ca²⁺-dependent cysteine protease. Despite being located at the same genomic locus, genetic regulation of the reciprocal expression of OMP and CAPN5 has been suggested. By performing a motif search, we detected possible calpain cleavage sites in OMP. However, the direct proteolytic regulation of OMP by CAPN5 is unclear. Here, we generated OMP fused with Myc-tag and His-tag at its N- and C-termini and examined whether CAPN5 cleaves OMP into fragments by detecting immunoreactivity against Myc, OMP and His. Western blotting demonstrated that OMP was unlikely to be cleaved even in the presence of Ca²⁺ in vitro. We expressed OMP and CAPN5 in HEK293T cells and applied a calcium ionophore under physiological conditions in cellulo, which resulted in no apparent fragmentation of OMP. We also applied liquid chromatography/mass spectrometry to the electrophoresed fractions smaller than the uncut Myc-OMP-His signals, which demonstrated no significant fragmentation of OMP. These results collectively indicate that OMP is unlikely to be cleaved by CAPN5.

S-acylation regulates the membrane association and activity of Calpain-5

Calpain-5 (CAPN5) is a member of the calpain family of calcium-activated neutral thiol proteases. CAPN5 is partly membrane associated, despite its lack of a transmembrane domain. Unlike classical calpains, CAPN5 contains a C-terminal C2 domain. C2 domains often have affinity to lipids, mediating membrane association. We recently reported that the C2 domain of CAPN5 was essential for its membrane association and the activation of its autolytic activity. However, despite the removal of the C2 domain by autolysis, the N-terminal fragment of CAPN5 remained membrane associated. S-acylation, also referred to as S-palmitoylation, is a reversible post-translational lipid modification of cysteine residues that promotes membrane association of soluble proteins. In the present study several S-acylated cysteine residues were identified in CAPN5 with the acyl-PEG exchange method. Data reported here demonstrate that CAPN5 is S-acylated on up to three cysteine residues including Cys-4 and Cys-512, and likely Cys-507. The D589N mutation in a potential calcium binding loop within the C2 domain interfered with the S-acylation of CAPN5, likely preventing initial membrane association. Mutating specific cysteine residues of CAPN5 interfered with both its membrane association and the activation of CAPN5 autolysis. Taken together, our results suggest that the S-acylation of CAPN5 is critical for its membrane localization which appears to favor its enzymatic activity.

Lentiviral expression of calpain-1 C2-like domain peptide prevents glutamate-induced cell death in mouse hippocampal neuronal HT22 cells

Glutamate neurotoxicity is involved in neurodegenerative diseases, including Alzheimer's and Parkinson's diseases. Excess glutamate causes caspase-independent programmed cell death via oxidative stress and calcium influx. Our previous study showed that calpain-1 localizes to both the cytoplasm and mitochondria, where apoptosis-inducing factor (AIF) is cleaved by calpain-1 and translocates to the nucleus to induce DNA fragmentation. The autoinhibitory region of calpain-1 conjugated with the cell-penetrating peptide HIV1-Tat (namely Tat-μCL) specifically prevents the activity of mitochondrial calpain-1 and attenuates neuronal cell death in animal models of retinitis pigmentosa, as well as glutamate-induced cell death in mouse hippocampal HT22 cells. In the present study, we constructed a lentiviral vector expressing the Tat-μCL peptide and evaluated its protective effect against glutamate-induced cell death in HT22 cells. Lentiviral transduction with Tat-μCL significantly suppressed glutamate-induced nuclear translocation of AIF and DNA fragmentation. The findings of the present study suggest that the stable expression of Tat-μCL may be a potential gene therapy modality for neurodegenerative diseases.

Mipsagargin: The Beginning-Not the End-of Thapsigargin Prodrug-Based Cancer Therapeutics

Søren Brøgger Christensen isolated and characterized the cell-penetrant sesquiterpene lactone Thapsigargin (TG) from the fruit Thapsia garganica. In the late 1980s/early 1990s, TG was supplied to multiple independent and collaborative groups. Using this TG, studies documented with a large variety of mammalian cell types that TG rapidly (i.e., within seconds to a minute) penetrates cells, resulting in an essentially irreversible binding and inhibiting (IC50~10 nM) of SERCA 2b calcium uptake pumps. If exposure to 50-100 nM TG is sustained for >24-48 h, prostate cancer cells undergo apoptotic death. TG-induced death requires changes in the cytoplasmic Ca2+, initiating a calmodulin/calcineurin/calpain-dependent signaling cascade that involves BAD-dependent opening of the mitochondrial permeability transition pore (MPTP); this releases cytochrome C into the cytoplasm, activating caspases and nucleases. Chemically unmodified TG has no therapeutic index and is poorly water soluble. A TG analog, in which the 8-acyl groups is replaced with the 12-aminododecanoyl group, afforded 12-ADT, retaining an EC50 for killing of <100 nM. Conjugation of 12-ADT to a series of 5-8 amino acid peptides was engineered so that they are efficiently hydrolyzed by only one of a series of proteases [e.g., KLK3 (also known as Prostate Specific Antigen); KLK2 (also known as hK2); Fibroblast Activation Protein Protease (FAP); or Folh1 (also known as Prostate Specific Membrane Antigen)]. The obtained conjugates have increased water solubility for systemic delivery in the blood and prevent cell penetrance and, thus, killing until the TG-prodrug is hydrolyzed by the targeting protease in the vicinity of the cancer cells. We summarize the preclinical validation of each of these TG-prodrugs with special attention to the PSMA TG-prodrug, Mipsagargin, which is in phase II clinical testing.

Calpain-1 C2L domain peptide protects mouse hippocampus-derived neuronal HT22 cells against glutamate-induced oxytosis

Calpains are Ca²⁺-dependent cysteine proteases; their aberrant activation is associated with several neurodegenerative diseases. The μ-calpain catalytic subunit, calpain-1, is located in the cytoplasm as well as in the mitochondria. Mitochondrial calpain-1 cleaves apoptosis-inducing factor (AIF), leading to apoptotic cell death. We have previously reported that short peptides of calpain-1 C2-like domain conjugated with cell penetrating peptide HIV-Tat (Tat-μCL) selectively inhibit mitochondrial calpain-1 and effectively prevent neurodegenerative diseases of the eye. In this study, we determined whether mitochondrial calpain-1 mediates oxytosis (oxidative glutamate toxicity) in hippocampal HT22 cells using Tat-μCL and newly generated polyhistidine-conjugated μCL peptide and compared their efficacies in preventing oxytosis. TUNEL assay and single strand DNA staining revealed that both μCL peptides inhibited glutamate-induced oxytosis. Additionally, both the peptides suppressed the mitochondrial AIF translocation into the nucleus. All polyhistidine-μCL peptides (containing 4–16 histidine residues) showed higher cell permeability than Tat-μCL. Notably, tetrahistidine (H4)-μCL exerted the highest cytoprotective activity. Thus, H4-μCL may be a potential peptide drug for calpain-1-mediated neurodegenerative diseases such as Alzheimer's disease.

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Mitochondrial calpain-1 mediates glutamate-induced oxytosis in HT22 cells.

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CPP-μCL inhibits AIF translocation and DNA fragmentation in oxytosis.

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Polyhistidine conjugation enhances intracellular μCL peptide uptake efficiency.