Calpain activation is essential for membrane fusion of erythrocytes in the presence of exogenous Ca2+

Redirecting E3 ubiquitin ligases for targeted protein degradation with heterologous recognition domains

Targeted protein degradation (TPD) mediated by proteolysis targeting chimeras or molecular glues is an emerging therapeutic strategy. Despite greater than 600 E3 ligases and their associated components, a limited number have been deployed in TPD. Those commonly used include cereblon and von Hippel-Lindau tumor suppressor (VHL), which is expressed widely and for which high affinity ligands are available. Limiting TPD to specific cells or tissues would be desirable in many settings. To this goal we have determined the potential of two erythroid cell-enriched E3 ligases, TRIM10 and TRIM58, to degrade a protein of interest, BCL11A, a validated therapeutic target for the β-hemoglobinopathies. We established a general strategy in which heterologous recognition domains replace the PRY-SPRY domain of TRIM10 and TRIM58. Recruitment of TRIM10 or TRIM58 to BCL11A by coiled-coil peptides, nanobodies, or the substrate recognition domain of cereblon led to its degradation. Our findings illustrate a strategy that may be widely useful in evaluating the TPD potential of other E3 ubiquitin ligases and provide a rationale for discovery of ligands for TRIM10 and TRIM58 for erythroid-selective depletion of proteins of interest.

Integrative Transcriptomics Data Mining to Explore the Functions of TDP1α and TDP1β Genes in the Arabidopsis thaliana Model Plant

The tyrosyl-DNA phosphodiesterase 1 (TDP1) enzyme hydrolyzes the phosphodiester bond between a tyrosine residue and the 3'-phosphate of DNA in the DNA-topoisomerase I (TopI) complex, being involved in different DNA repair pathways. A small TDP1 gene subfamily is present in plants, where TDP1α has been linked to genome stability maintenance, while TDP1β has unknown functions. This work aimed to comparatively investigate the function of the TDP1 genes by taking advantage of the rich transcriptomics databases available for the Arabidopsis thaliana model plant. A data mining approach was carried out to collect information regarding gene expression in different tissues, genetic backgrounds, and stress conditions, using platforms where RNA-seq and microarray data are deposited. The gathered data allowed us to distinguish between common and divergent functions of the two genes. Namely, TDP1β seems to be involved in root development and associated with gibberellin and brassinosteroid phytohormones, whereas TDP1α is more responsive to light and abscisic acid. During stress conditions, both genes are highly responsive to biotic and abiotic treatments in a time- and stress-dependent manner. Data validation using gamma-ray treatments applied to Arabidopsis seedlings indicated the accumulation of DNA damage and extensive cell death associated with the observed changes in the TDP1 genes expression profiles.

KSHV-encoded vCyclin can modulate HIF1α levels to promote DNA replication in hypoxia

The cellular adaptive response to hypoxia, mediated by high HIF1α levels includes metabolic reprogramming, restricted DNA replication and cell division. In contrast to healthy cells, the genome of cancer cells, and Kaposi’s sarcoma associated herpesvirus (KSHV) infected cells maintains replication in hypoxia. We show that KSHV infection, despite promoting expression of HIF1α in normoxia, can also restrict transcriptional activity, and promoted its degradation in hypoxia. KSHV-encoded vCyclin, expressed in hypoxia, mediated HIF1α cytosolic translocation, and its degradation through a non-canonical lysosomal pathway. Attenuation of HIF1α levels by vCyclin allowed cells to bypass the block to DNA replication and cell proliferation in hypoxia. These results demonstrated that KSHV utilizes a unique strategy to balance HIF1α levels to overcome replication arrest and induction of the oncogenic phenotype, which are dependent on the levels of oxygen in the microenvironment.

Loss of UCHL1 rescues the defects related to Parkinson's disease by suppressing glycolysis

The role of ubiquitin carboxyl-terminal hydrolase L1 (UCHL1; also called PARK5) in the pathogenesis of Parkinson's disease (PD) has been controversial. Here, we find that the loss of UCHL1 destabilizes pyruvate kinase (PKM) and mitigates the PD-related phenotypes induced by PTEN-induced kinase 1 (PINK1) or Parkin loss-of-function mutations in Drosophila and mammalian cells. In UCHL1 knockout cells, cellular pyruvate production and ATP levels are diminished, and the activity of AMP-activated protein kinase (AMPK) is highly induced. Consequently, the activated AMPK promotes the mitophagy mediated by Unc-51-like kinase 1 (ULK1) and FUN14 domain-containing 1 (FUNDC1), which underlies the effects of UCHL1 deficiency in rescuing PD-related defects. Furthermore, we identify tripartite motif-containing 63 (TRIM63) as a previously unknown E3 ligase of PKM and demonstrate its antagonistic interaction with UCHL1 to regulate PD-related pathologies. These results suggest that UCHL1 is an integrative factor for connecting glycolysis and PD pathology.

MicroRNA-Independent Modulation of DICER1 Expression by hAgo2

Many proteins, including DICER1 and hAgo2, are involved in the biogenesis of microRNAs (miRNAs). Whether hAgo2 regulates DICER1 expression is unknown. Exogenously overexpressed hAgo2 suppressed DICER1 expression at the levels of both protein and mRNA, and the reduction in hAgo2 expression enhanced DICER1 expression. Precursor miRNA processing mediated by DICER1 was also modulated by hAgo2. However, hAgo2 protein did not suppress DICER1 promoter activity. Therefore, hAgo2 protein probably regulates DICER1 expression at the posttranscriptional level. Indeed, hAgo2 protein inhibited the reporter assay of the DICER1 mRNA 3' untranslated region (3'-UTR). Previous reports have demonstrated that miRNAs (e.g., let-7 and miR-103/107) inhibited DICER1 expression posttranscriptionally. However, hAgo2 still suppressed DICER1 expression in the cells depleted of these miRNAs. Moreover, the reporter activities of the DICER1 mRNA 3'-UTR without these miRNA binding sites were still suppressed by hAgo2. Therefore, in addition to an miRNA-dependent pathway, hAgo2 can also modulate DICER1 expression through an miRNA-independent mechanism. Downregulation of DICER1 expression was further proven to be dependent on both hAgo2 and AUF1 proteins. Interactions of hAgo2 and AUF1 proteins were demonstrated by the coimmunoprecipitation assay. As expected, hAgo2 could not suppress the DICER1 mRNA 3'-UTR reporter with a mutation in the potential AUF1-binding site. Thus, downregulation of DICER1 expression through the 3'-UTR requires both hAgo2 and AUF1.

Vitamin K3 chloro derivative (VKT-2) inhibits HDAC6, activates autophagy and apoptosis, and inhibits aggresome formation in hepatocellular carcinoma cells

Epigenetics plays a vital role in regulating gene expression and determining the specific phenotypes of eukaryotic cells. Histone deacetylases (HDACs) are important epigenetic regulatory proteins effecting multiple biological functions. Particularly, HDAC6 has become a promising anti-cancer drug target because of its regulation of cell mobility, protein trafficking, degradation of misfolded proteins, cell growth, apoptosis, and metastasis. In this study, we identified one out of six vitamin K₃ derivatives, VKT-2, as HDAC6 inhibitor using molecular docking and cell viability assays in HDAC6-overexpressing HuH-7 cancer cells. Microscale thermophoresis and HDAC6 enzymatic assays revealed that VKT-2 bound to HDAC6 and inhibited its function. We further identified its cytotoxic activity. VKT-2 hyperacetylated HDAC6 substrates and disturbed tubulin integrity leading to significant inhibition of tumor migration in both HuH-7 spheroids and U2OS-GFP-α-tubulin cells. Moreover, VKT-2 induced autophagic and apoptotic cell death in HuH-7, while aggresome formation was restrained after VKT-2 treatment. A HuH-7 cell-xenograft model in zebrafish larvae provided evidence that VKT-2 inhibited the tumor growth in vivo. To best of our knowledge, it is the first time to demonstrate that vitamin k₃ derivatives (VKT-2) inhibits HDAC6 in solid tumor cells. These unique findings suggested that VKT-2 is a promising anti-cancer agent targeting HDAC6.

VCP/p97 targets the nuclear export and degradation of p27Kip1 during G1 to S phase transition

One of the critical regulatory mechanisms for cell cycle progression is the timely degradation of CDK inhibitors, including p21Cip1 and p27Kip1 . VCP/p97, an AAA-ATPase, is reported to be overexpressed in many types of cancers. Here, we found that treatment of MCF-7 human breast cancer cells with DBeQ, a VCP inhibitor, or VCP knockdown in MCF-7 cells arrested cells at G1 phase, accompanied with the blockage of both p21 and p27 degradation. Whereas, double knockdown of p21 and p27 in MCF-7 cells rendered cells refractory to DBeQ-induced G1 arrest. Moreover, inhibition or knockdown of VCP or UFD1, one of VCP's co-factors, in MCF-7, NIH3T3, or HEK293T cells blocked the nuclear export of p27 during earlier G1 phase after mitogen stimulation. We also identified the nuclear localization sequence (NLS) of VCP, and found that adding back wild-type VCP, not the NLS-deleted VCP mutant, restored the nuclear export and degradation of p27 in VCP knockout MCF-7 cells. Importantly, we found that VCP inhibition sensitized breast cancer cells to the treatment of several anticancer therapeutics both in vitro and in vivo. Taken together, our study not only uncovers the mechanisms underlying VCP-mediated cell proliferation control but also provides potential therapeutic option for cancer treatment.

Calreticulin protects insulin against reductive stress in vitro and in MIN6 cells

Oxidative folding of proinsulin in the endoplasmic reticulum (ER) is critical for the proper sorting and secretion of insulin from pancreatic β-cells. Here, by using non-cell-based insulin aggregation assays and mouse insulinoma-derived MIN6 cells, we searched for a candidate molecular chaperone for (pro)insulin when its oxidative folding is compromised. We found that interaction between insulin and calreticulin (CRT), a lectin that acts as an ER-resident chaperone, was enhanced by reductive stress in MIN6 cells. Co-incubation of insulin with recombinant CRT prevented reductant-induced aggregation of insulin. Furthermore, lysosomal degradation of proinsulin, which was facilitated by dithiothreitol-induced reductive stress, depended on CRT in MIN6 cells. Together, our results suggest that CRT may be a protective molecule against (pro)insulin aggregation when oxidative folding is defective, e.g. under reductive stress conditions, in vitro and in cultured cells. Because CRT acts as a molecular chaperone for not only glycosylated proteins but also non-glycosylated polypeptides, we also propose that (pro)insulin is a novel candidate client of the chaperone function of CRT.

Celastrol-induced degradation of FANCD2 sensitizes pediatric high-grade gliomas to the DNA-crosslinking agent carboplatin

Background

Pediatric high-grade gliomas (pHGG) are the leading cause of cancer-related death during childhood. Due to their diffuse growth characteristics, chemoresistance and location behind the blood-brain barrier (BBB), the prognosis of pHGG has barely improved in the past decades. As such, there is a dire need for new therapies that circumvent those difficulties. Since aberrant expression of DNA damage-response associated Fanconi anemia proteins play a central role in the onset and therapy resistance of many cancers, we here investigated if FANCD2 depletion could sensitize pHGG to additional DNA damage.

Methods

We determined the capacity of celastrol, a BBB-penetrable compound that degrades FANCD2, to sensitize glioma cells to the archetypical DNA-crosslinking agent carboplatin in vitro in seven patient-derived pHGG models. In addition, we tested this drug combination in vivo in a patient-derived orthotopic pHGG xenograft model. Underlying mechanisms to drug response were investigated using mRNA expression profiling, western blotting, immunofluorescence, FANCD2 knockdown and DNA fiber assays.

Findings

FANCD2 is overexpressed in HGGs and depletion of FANCD2 by celastrol synergises with carboplatin to induce cytotoxicity. Combination therapy prolongs survival of pHGG-bearing mice over monotherapy and control groups in vivo (P<0.05). In addition, our results suggest that celastrol treatment stalls ongoing replication forks, causing sensitivity to DNA-crosslinking in FANCD2-dependent glioma cells.

Interpretation

Our results show that depletion of FANCD2 acts as a chemo-sensitizing strategy in pHGG. Combination therapy using celastrol and carboplatin might serve as a clinically relevant strategy for the treatment of pHGG.

Funding

This study was funded by a grant from the Children Cancer-Free Foundation (KIKA, project 210). The disclosed funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Disproportionately high levels of HGF induce the degradation of the c-met receptor through the proteasomal degradation pathway

Hepatocyte growth factor (HGF) receptor is a member of the receptor tyrosine kinases (RTKs) and has been reported to perform diverse functions in various cell types during both the developmental and adult stages. Among different roles, HGF is best known for its angiogenic effects of inducing the migration of endothelial cells. Because angiogenesis is one of the prerequisite steps for tumor metastasis, HGF-dependent cell migration has to be tightly controlled. However, the underlying mechanisms regulating the optimum level of HGF/c-met signaling have been poorly understood. In this study, we tested whether the migration of endothelial cells is regulated by a negative feedback mechanism under disproportionately large amounts of HGF. Data from endothelial cell migration assays showed that HGF activity increased as its concentration increased, but declined beyond a certain point. Under limiting conditions, amounts of phosphorylated Erk and Akt surged, reaching a plateau in which the enhanced level was more or less maintained. The c-met receptor was degraded when unnecessarily large amounts of HGF were present. Under these conditions, HGF could no longer activate downstream signaling pathways even if cells were re-treated with optimal amounts of HGF. Excessive doses of HGF increased the phosphorylation of tyrosine residue 1003 involved in the ubiquitination of c-met, and phosphorylated c-met was diverted toward the proteasomal degradation pathway. Taken together, HGF/c-met signaling is tightly regulated by a negative feedback loop through an ubiquitin-proteasomal degradation pathway.

The Nuclear Zinc Finger Protein Zfat Maintains FoxO1 Protein Levels in Peripheral T Cells by Regulating the Activities of Autophagy and the Akt Signaling Pathway

Forkhead box O1 (FoxO1) is a key molecule for the development and functions of peripheral T cells. However, the precise mechanisms regulating FoxO1 expression in peripheral T cells remain elusive. We previously reported that Zfat(f/f)-CD4Cre mice showed a marked decline in FoxO1 protein levels in peripheral T cells, partially through proteasomal degradation. Here we have identified the precise mechanisms, apart from proteasome-mediated degradation, of the decreased FoxO1 levels in Zfat-deficient T cells. First, we confirmed that tamoxifen-inducible deletion of Zfat in Zfat(f/f)-CreERT2 mice coincidently decreases FoxO1 protein levels in peripheral T cells, indicating that Zfat is essential for maintaining FoxO1 levels in these cells. Although the proteasome-specific inhibitors lactacystin and epoxomicin only moderately increase FoxO1 protein levels, the inhibitors of lysosomal proteolysis bafilomycin A1 and chloroquine restore the decreased FoxO1 levels in Zfat-deficient T cells to levels comparable with those in control cells. Furthermore, Zfat-deficient T cells show increased numbers of autophagosomes and decreased levels of p62 protein, together indicating that Zfat deficiency promotes lysosomal FoxO1 degradation through autophagy. In addition, Zfat deficiency increases the phosphorylation levels of Thr-308 and Ser-473 of Akt and the relative amounts of cytoplasmic to nuclear FoxO1 protein levels, indicating that Zfat deficiency causes Akt activation, leading to nuclear exclusion of FoxO1. Our findings have demonstrated a novel role of Zfat in maintaining FoxO1 protein levels in peripheral T cells by regulating the activities of autophagy and the Akt signaling pathway.

Selective inhibition of caspases in skeletal muscle reverses the apoptotic synaptic degeneration in slow-channel myasthenic syndrome

Slow-channel syndrome (SCS) is a congenital myasthenic disorder caused by point mutations in subunits of skeletal muscle acetylcholine receptor leading to Ca(2+) overload and degeneration of the postsynaptic membrane, nuclei and mitochondria of the neuromuscular junction (NMJ). In both SCS muscle biopsies and transgenic mouse models for SCS (mSCS), the endplate regions are shrunken, and there is evidence of DNA damage in the subsynaptic region. Activated caspase-9, -3 and -7 are intensely co-localized at the NMJ, and the Ca(2+)-activated protease, calpain, and the atypical cyclin-dependent kinase (Cdk5) are overactivated in mSCS. Thus, the true mediator(s) of the disease process is not clear. Here, we demonstrate that selective inhibition of effector caspases, caspase-3 and -7, or initiator caspase, caspase-9, in limb muscle in vivo by localized expression of recombinant inhibitor proteins dramatically decreases subsynaptic DNA damage, increases endplate area and improves ultrastructural abnormalities in SCS transgenic mice. Calpain and Cdk5 are not affected by this treatment. On the other hand, inhibition of Cdk5 by expression of a dominant-negative form of Cdk5 has no effect on the degeneration. Together with previous studies, these results indicate that focal activation of caspase activity at the NMJ is the principal pathological process responsible for the synaptic apoptosis in SCS. Thus, treatments that reduce muscle caspase activity are likely to be of benefit for SCS patients.

Calpain-1 knockout reveals broad effects on erythrocyte deformability and physiology

Pharmacological inhibitors of cysteine proteases have provided useful insights into the regulation of calpain activity in erythrocytes. However, the precise biological function of calpain activity in erythrocytes remains poorly understood. Erythrocytes express calpain-1, an isoform regulated by calpastatin, the endogenous inhibitor of calpains. In the present study, we investigated the function of calpain-1 in mature erythrocytes using our calpain-1-null [KO (knockout)] mouse model. The calpain-1 gene deletion results in improved erythrocyte deformability without any measurable effect on erythrocyte lifespan in vivo. The calcium-induced sphero-echinocyte shape transition is compromised in the KO erythrocytes. Erythrocyte membrane proteins ankyrin, band 3, protein 4.1R, adducin and dematin are degraded in the calcium-loaded normal erythrocytes but not in the KO erythrocytes. In contrast, the integrity of spectrin and its state of phosphorylation are not affected in the calcium-loaded erythrocytes of either genotype. To assess the functional consequences of attenuated cytoskeletal remodelling in the KO erythrocytes, the activity of major membrane transporters was measured. The activity of the K+-Cl- co-transporter and the Gardos channel was significantly reduced in the KO erythrocytes. Similarly, the basal activity of the calcium pump was reduced in the absence of calmodulin in the KO erythrocyte membrane. Interestingly, the calmodulin-stimulated calcium pump activity was significantly elevated in the KO erythrocytes, implying a wider range of pump regulation by calcium and calmodulin. Taken together, and with the atomic force microscopy of the skeletal network, the results of the present study provide the first evidence for the physiological function of calpain-1 in erythrocytes with therapeutic implications for calcium imbalance pathologies such as sickle cell disease.

Release of intracellular calcium stores facilitates coxsackievirus entry into polarized endothelial cells

Group B coxsackieviruses (CVB) are associated with viral-induced heart disease and are among the leading causes of aseptic meningitis worldwide. Here we show that CVB entry into polarized brain microvasculature and aortic endothelial cells triggers a depletion of intracellular calcium stores initiated through viral attachment to the apical attachment factor decay-accelerating factor. Calcium release was dependent upon a signaling cascade that required the activity of the Src family of tyrosine kinases, phospholipase C, and the inositol 1,4,5-trisphosphate receptor isoform 3. CVB-mediated calcium release was required for the activation of calpain-2, a calcium-dependent cysteine protease, which controlled the vesicular trafficking of internalized CVB particles. These data point to a specific role for calcium signaling in CVB entry into polarized endothelial monolayers and highlight the unique signaling mechanisms used by these viruses to cross endothelial barriers.

Enteroviruses are associated with a number of diverse syndromes such as myocarditis, febrile illness, and are the main causative agents of aseptic meningitis. No effective therapeutics exist to combat non-poliovirus enterovirus infections. A better understanding of the mechanisms by which these viruses infect host cells could lead to the design of effective therapeutic interventions. In this study, we found that intracellular calcium stores in polarized endothelial monolayers are depleted upon exposure to coxsackievirus B (CVB) and that this release is mediated by viral attachment to its receptor decay-accelerating factor. We also discovered that the calcium release requires the activation of signaling molecules involved in calcium signaling such as Src tyrosine kinases, phospholipase C, and the inositol 1,4,5-trisphosphate receptor isoform 3 on the ER membrane. Furthermore, we found that a calcium-activated cystein protease, calpain-2, was activated and necessary for proper viral trafficking inside the cell. Interestingly, we found that this signaling cascade was critical for CVB internalization into the endothelium, but was not involved in CVB entry into the epithelium. This is an important advance in our understanding of how enteroviruses hijack host endothelial cell signaling mechanisms in order to facilitate their entry and eventual spread.

Inhibition of rat liver cathepsins B and L by the peptide aldehyde benzyloxycarbonyl-leucyl-leucyl-leucinal and its analogues

Cathepsins B and L belong to the papain superfamily of cysteine proteases and play important roles in various physiological and pathological processes. In the course of studies on their inhibitors, we examined the inhibitory effects of the peptide aldehyde benzyloxycarbonyl-leucyl-leucyl-leucinal (ZLLLal) and its analogues. As a result, rat liver cathepsins B and L were shown to be strongly inhibited by them. The concentration required for 50% inhibition (IC(50)) by ZLLLal was 88 nM for cathepsin B and 163 nM for cathepsin L. Moreover, various analogues of ZLLLal, including 2-furancarbonyl-, nicotinyl-, isonicotinyl- and 4-morpholinylsuccinyl-LLLals, and some acetyl-Pro (AcP)-containing analogues, AcPLLLal and AcPPLLLal, were shown to inhibit both enzymes more strongly than ZLLLal. Among them, isonicotinyl-LLLal was most inhibitory against both cathepsins B (IC(50), 12 nM) and L (IC(50), 20 nM). Several of these inhibitors were indicated to be somewhat more soluble in aqueous media than ZLLLal.

Oxidants and Ca+2 induce PGC-1alpha degradation through calpain

Peroxisome proliferator activator receptor gamma coactivator 1alpha (PGC-1alpha) is a transcriptional coactivator known to mediate mitochondrial biogenesis. Whereas PGC-1alpha transcription is regulated by a variety of signaling cascades, the mechanisms of PGC-1alpha degradation have received less investigation. Thus, we investigated the mechanisms responsible for PGC-1alpha degradation in renal proximal tubular cells (RPTC). Amino acid sequence analysis of the PGC-1alpha protein revealed three PEST sequence-rich regions, predictive of proteolysis by calpains and/or the proteasome. Under basal conditions, treatment with the protein synthesis inhibitor cycloheximide resulted in rapid degradation of PGC-1alpha (t(1/2)=38 min), which was blocked by the proteasome inhibitor epoxomicin, but not the calpain inhibitor calpeptin. Oxidant exposure resulted in the degradation of both endogenous and adenovirally over-expressed PGC-1alpha, which was inhibited by calpeptin but not epoxomicin. Thapsigargin-induced release of ER Ca(2+) also stimulated calpain-dependent, epoxomicin-independent degradation of PGC-1alpha. Finally, Ca(2+) addition to lysates of RPTC over-expressing PGC-1alpha resulted in calpeptin-sensitive, epoxomicin-insensitive degradation of PGC-1alpha. In summary, we suggest two distinct mechanisms regulate PGC-1alpha: basal PGC-1alpha turnover by proteasome degradation and oxidant- and Ca(2+)-mediated PGC-1alpha degradation through calpain.

The calpain system

The calpain system originally comprised three molecules: two Ca2+-dependent proteases, mu-calpain and m-calpain, and a third polypeptide, calpastatin, whose only known function is to inhibit the two calpains. Both mu- and m-calpain are heterodimers containing an identical 28-kDa subunit and an 80-kDa subunit that shares 55-65% sequence homology between the two proteases. The crystallographic structure of m-calpain reveals six "domains" in the 80-kDa subunit: 1). a 19-amino acid NH2-terminal sequence; 2). and 3). two domains that constitute the active site, IIa and IIb; 4). domain III; 5). an 18-amino acid extended sequence linking domain III to domain IV; and 6). domain IV, which resembles the penta EF-hand family of polypeptides. The single calpastatin gene can produce eight or more calpastatin polypeptides ranging from 17 to 85 kDa by use of different promoters and alternative splicing events. The physiological significance of these different calpastatins is unclear, although all bind to three different places on the calpain molecule; binding to at least two of the sites is Ca2+ dependent. Since 1989, cDNA cloning has identified 12 additional mRNAs in mammals that encode polypeptides homologous to domains IIa and IIb of the 80-kDa subunit of mu- and m-calpain, and calpain-like mRNAs have been identified in other organisms. The molecules encoded by these mRNAs have not been isolated, so little is known about their properties. How calpain activity is regulated in cells is still unclear, but the calpains ostensibly participate in a variety of cellular processes including remodeling of cytoskeletal/membrane attachments, different signal transduction pathways, and apoptosis. Deregulated calpain activity following loss of Ca2+ homeostasis results in tissue damage in response to events such as myocardial infarcts, stroke, and brain trauma.

Involvement of the calcium-specific protease, calpain, in the fertilizing capacity of human spermatozoa

We recently reported the novel finding that human spermatozoa contain the calcium (Ca2+)-dependent protease, calpain. In somatic cells this protease mediates several cellular activities regulated by Ca2+ including membrane fusibility during cell-to-cell interactions. In this paper we examined the participation of sperm calpain in sperm-oocyte penetration, a process that is dependent on Ca2+ and involves membrane fusion between the two cells. Oocyte penetration was assessed using ejaculated spermatozoa from fertile men and zona-free hamster oocytes. Penetration rate was impaired by the presence of the active-site calpain inhibitor, calpain inhibitor-I, in a dose-dependent manner. At 1 mM, penetration scores were reduced by 65% (p < 0.01; n=5). The effects did not involve the oocyte, nor did the inhibitor alter sperm motility. Similar inhibitory effects on sperm penetration capacity were observed with specific antibodies directed either against calpain-I or calpain-II, the two forms of calpains described in somatic cells. At 1:1000 antibody dilution, penetration was inhibited 50 and 60% with anti-calpain-I and anti-calpain-II antibodies, respectively (p < 0.01; n=6). Furthermore, a combination of these two antibodies reduced the penetration rates by 75% (p < 0.01; n=6). We conclude that calpain inhibitor and anti-calpain antibodies impair human sperm capacity to fuse and penetrate the oocyte. These findings suggest that sperm calpain is a novel component of the biochemical processes that regulate the fertilizing capacity of human spermatozoa.

The calpain-calpastatin system and protein degradation in fusing myoblasts

Calpain (Ca(2+)-activated cysteine protease) induced proteolysis has been suggested to play a role in myoblast fusion. We previously found that calpastatin (the endogenous inhibitor of calpain) diminishes markedly in myoblasts during myoblast differentiation just prior to the start of fusion, allowing Ca(2+)-induced calpain activation at that stage. Here, we show that a limited degradation of some proteins occurs within the myoblasts undergoing fusion, but not in proliferating myoblasts. The protein degradation is observed at the stage when calpastatin is low. Protein degradation within the myoblasts and myoblast fusion are inhibited by EGTA, by the cysteine protease inhibitors calpeptin and E-64d and by calpastatin. The degradation appears to be selective for certain myoblast proteins. Integrin beta 1 subunit, talin and beta-tropomyosin are degraded in the fusing myoblasts, whereas alpha-actinin, beta-tubulin and alpha-tropomyosin are not. A similar pattern of degradation is observed in lysates of proliferating myoblasts when Ca2+ and excess calpain are added, a degradation that is inhibited by calpastatin. The results support the notion that degradation of certain proteins is required for myoblast fusion and that calpain participates in the fusion-associated protein degradation. Participation of calpain is made possible by a change in calpain/calpastatin ratio, i.e., by a diminution in calpastatin level from a high level in the proliferating myoblasts to a low level in the differentiating myoblasts. Degradation of certain proteins, known to be responsible for the stability of the membrane-skeleton organization and for the interaction of the cell with the extracellular matrix, would allow destabilization of the membrane and the creation of membrane fusion-potent regions.

An antisense oligodeoxyribonucleotide to m-calpain mRNA inhibits secretion from alveolar epithelial type II cells

We investigated the effect of translational suppression of m-calpain on [3H]-phosphatidylcholine (PC) secretion utilising an antisense oligodexoyribonucleotide (oligo) directed against mRNA encoding m-calpain catalytic subunit. Two types of oligo, sense (S) and antisense (AS), to a portion of exon 12 of rat m-calpain catalytic subunit mRNA were tested. Constitutive secretion was decreased by 23% by AS-oligo (1 microM) treatment, while S-oligo (1 microM) had no effect. TPA-stimulated secretion was inhibited about 50-60% by AS-oligo (1-3 microM) and the inhibition was concentration-dependent, while S-oligo (1 microM) only inhibited about 10% of TPA-stimulated secretion. Northern and Western blot analyses revealed that the AS-oligo treatment reduced m-calpain mRNA and protein levels by 32% and 78%, respectively. The data indicate that antisense strategy is effective in suppressing calpain expression and type II cell secretion.

Calpain and calpastatin in myoblast differentiation and fusion: effects of inhibitors

Myoblast differentiation and fusion to multinucleated muscle cells can be studied in myoblasts grown in culture. Calpain (Ca(2+)-activated thiol protease) induced proteolysis has been suggested to play a role in myoblast fusion. We previously showed that calpastatin (the endogenous inhibitor of calpain) plays a role in cell membrane fusion. Using the red cell as a model, we found that red cell fusion required calpain activation and that fusibility depended on the ratio of cell calpain to calpastatin. We found recently that calpastatin diminishes markedly in myoblasts during myoblast differentiation just prior to the start of fusion, allowing calpain activation at that stage; calpastatin reappears at a later stage (myotube formation). In the present study, the myoblast fusion inhibitors TGF-beta, EGTA and calpeptin (an inhibitor of cysteine proteases) were used to probe the relation of calpastatin to myoblast fusion. Rat L8 myoblasts were induced to differentiate and fuse in serum-poor medium containing insulin. TGF-beta and EGTA prevented the diminution of calpastatin. Calpeptin inhibited fusion without preventing diminution of calpastatin, by inhibiting calpain activity directly. Protein levels of mu-calpain and m-calpain did not change significantly in fusing myoblasts, nor in the inhibited, non-fusing myoblasts. The results indicate that calpastatin level is modulated by certain growth and differentiation factors and that its continuous presence results in the inhibition of myoblast fusion.

Calpain activity promotes the sealing of severed giant axons

A barrier (seal) must form at the cut ends of a severed axon if a neuron is to survive and eventually regenerate. Following severance of crayfish medial giant axons in physiological saline, vesicles accumulate at the cut end and form a barrier (seal) to ion and dye diffusion. In contrast, squid giant axons do not seal, even though injury-induced vesicles form after axonal transection and accumulate at cut axonal ends. Neither axon seals in Ca2+-free salines. The addition of calpain to the bath saline induces the sealing of squid giant axons, whereas the addition of inhibitors of calpain activity inhibits the sealing of crayfish medial giant axons. These complementary effects involving calpain in two different axons suggest that endogenous calpain activity promotes plasmalemmal repair by vesicles or other membranes which form a plug or a continuous membrane barrier to seal cut axonal ends.

Apoptosis induction resulting from proteasome inhibition

Proteases are known to be involved in the apoptotic pathway. We report here that benzyloxycarbonyl (Z)-Leu-Leu-leucinal(ZLLLal), a leupeptin analogue, can induce apoptosis in MOLT-4 and L5178Y cells. ZLLLal is a cell-permeant inhibitor of proteasome. Among the protease inhibitors tested, only calpain inhibitor I (acetyl-Leu-Leu-norleucinal) and ZLLLal caused a marked induction of apoptosis in MOLT-4 cells. In contrast Z-Leu-leucinal, a specific inhibitor of calpain, did not induce apoptosis. When MOLT-4 cells were incubated in the presence of ZLLLal, p53 accumulated in the cells. These results strongly suggest that inhibition of proteasome induces p53-dependent apoptosis and that proteasome can protect cell from apoptosis.

Inhibition of secretion from isolated rat alveolar epithelial type II cells by the cell permeant calpain inhibitor II (N-acetyl-leucyl-leucyl-methioninal)

Although several signal transduction pathways, including activation of specific protein kinases have been proposed and studied for the secretory processes of lung surfactant from alveolar epithelial type II cells, the role of proteolytic processing by calpains (calcium-activated neutral proteases) in secretion has not been investigated. Therefore, we examined the effect of cell permeable calpain inhibitor I (N-acetyl-leucyl-leucyl-norleucinal) and II (N-acetyl-leucyl-leucyl-methioninal) on secretion to test the hypothesis that calpains participate in the secretory processes of alveolar epithelial type II cells. Calpain inhibitor I preferentially inhibits micro (mu)-calpain while inhibitor II inhibits milli (m)-calpain. Isolated type II cells were prelabelled with [3H]-choline for 18-24 h. To measure secretion, [3H]-labelled disaturated phosphatidylcholine (DSPC) released in the medium was monitored. Basal secretion of DSPC was maximally (87%) depressed by the presence of 10 microM inhibitor II. Secretagogue-stimulated secretion was also modulated by inhibitor II treatment. Stimulation with calcium ionophore A23187 enhanced secretion 3-fold. However, cells pre-exposed to inhibitor II displayed a 90% reduction of calcium-stimulated secretion. Terbutaline (10 microM) and ATP (1 mM) each increased secretion 2- and 4-fold, respectively. However, the inhibitor-treated cells, exposed to the same stimuli, attained only 53 or 62% of these increases. Calpain inhibitor I, on the other hand, inhibited neither basal nor stimulated secretion. The results suggest that m-calpain, the major isozyme of lung calpain requiring mM calcium for activity in vitro, is involved in the secretory pathways of alveolar epithelial type II cells.

Identification and characterization of membrane-bound calpains in clathrin-coated vesicles from bovine brain

Calpains are intracellular cysteine proteases activated in a Ca(2+)-dependent manner. Previously, we found that the differentiation of K562 cells induced by 4 beta-phorbol 12-myristate 13-acetate treatment is accompanied by an increase in m-calpain levels and, at the same time, m-calpain becomes localized on the inside of plasma membranes, coated pits, and coated vesicles [Nakamura, M., Mori, M., Morishita, Y., Mori, S. & Kawashima, S. (1992) Exp. Cell Res. 200, 513-522]. We also reported that mu-calpain plays an essential role in morphological changes and membrane fusion of erythrocytes through the degradation of spectrin, a lining protein [Hayashi, M., Saito, Y. & Kawashima, S. (1992) Biochim. Biophys. Res. Commun. 182, 939-946]. Thus, calpains are implicated in endocytosis and/or exocytosis, processes stimulated by Ca2+ and involving intracellular membrane fusion. In this study, we report the biochemical characterization of calpains as components of purified coated vesicles from bovine brain. It was found by Western-blot analysis and chemical cross-linking of proteins that calpains are bound to the membranes of coated vesicles, and not to the coats. The binding of m-calpain to vesicles is Ca(2+)-dependent, while that of mu-calpain is less dependent on the presence of Ca2+. We also identified substrate proteins for calpains in coated vesicles. Upon activation of endogenous calpains, component proteins of coated vesicles such as the clathrin light chain, tubulins, and adaptins, but not the clathrin heavy chain, are highly sensitive to calpain digestion. In the case of exogenously added calpains, low concentrations degraded the same protein components. The degradation pattern differs slightly between added mu-calpain and m-calpain. These results strongly suggest that calpains are involved in the formation of coated vesicles and/or vesicle fusion to endosomes.

Calpain (Ca(2+)-dependent thiol protease) in erythrocytes of young and old individuals

Limited proteolysis by calpain (Ca(2+)-activated protease; EC 3.4.22.17) is believed to regulate the function of membrane enzymes and modify the behavior of membrane structural proteins. Calpain is activated by autolysis. The degradation of band 3 protein by mu-calpain is known to be enhanced in erythrocyte membranes from human individuals > 70 years old (old) as compared with that from individuals 20-30 years old (young). In the present study, monoclonal antibody to mu-calpain was used to study the behavior of calpain in erythrocytes of young and old individuals. Less calpain was found in erythrocyte cytosol and membranes from old than in those from young. Increasing the erythrocyte Ca2+ induced translocation of calpain to the cell membrane and autolysis of the enzyme. Alkylation of erythrocyte thiols also promoted translocation of calpain to the membrane, especially in the presence of Ca2+. When calpain was added to erythrocyte membranes, initial binding was greater and subsequent autolysis faster in old than in young individuals, possibly arising from alterations in cell membranes of old individuals. The enhanced calpain autolysis was accompanied by enhanced degradation of band 3 protein in the old. The results suggest that calpain in old individuals is translocated to the cell membrane and is activated by autolysis, resulting in degradation of certain membrane proteins and loss of calpain. Enhanced calpain-induced membrane proteolysis may play a role in abnormal cell destruction (e.g., shortening the life span of erythrocytes in the aged, neuronal degeneration, etc). The erythrocyte membrane provides a convenient model for the study of age-associated alterations in cell membranes and in calpain behavior.

Replacement of m-calpain by mu-calpain during maturation of megakaryocytes and possible involvement in platelet formation

Localization of calpains in human bone marrow cells was studied immunohistochemically employing monoclonal antibodies against the high-Ca(2+)-requiring form (m-calpain) and the low-Ca(2+)-requiring form (mu-calpain). Most cells were stained with anti-m-calpain more strongly than with anti-mu-calpain, and staining with anti-mu-calpain was prominent only in megakaryocytes. To confirm the result, megakaryoblastic cell line (T-33) cells were subjected to immunoblot analysis. However no immunoreactivity to mu-calpain was seen in T-33 cells. Bone marrow from a patient with idiopathic thrombocytopenic purpura showed immature megakaryocytes (stage II) strongly stained by anti-m-calpain antibody while mature cells (stage III) were strongly stained by anti-mu-calpain antibody. These results suggest that mu-calpain plays a crucial role in mature megakaryocytes, possibly in platelet production.