Structural insights into the calcium-dependent interaction between calbindin-D28K and caspase-3

Regulation and role of calcium in cellular senescence

Cellular senescence is a state of stable cell proliferation arrest accompanied by a distinct secretory program impacting the senescent cell microenvironment. This phenotype can be induced by many stresses, including telomere shortening, oncogene activation, oxidative or genotoxic stress. Cellular senescence plays a key role in the organism throughout life, with beneficial effects at a young age for instance in embryonic development and wound healing, and deleterious effects during aging and in aging-related diseases. In the last decade calcium and calcium signaling have been established as critical factors in the implementation and regulation of cellular senescence. In this review we will present and discuss the main discoveries in this field, from the observation of an increased intracellular calcium concentration in senescent cells to the identification of calcium-binding proteins, calcium channels (TRP, ITPR, …) and MERCs (mitochondria-endoplasmic reticulum contact sites) as key players in this context.

Expression of the Calcium-Binding Protein CALB1 Is Induced and Controls Intracellular Ca2+ Levels in Senescent Cells

In response to many stresses, such as oncogene activation or DNA damage, cells can enter cellular senescence, a state of proliferation arrest accompanied by a senescence-associated secretory phenotype (SASP). Cellular senescence plays a key role in many physiopathological contexts, including cancer, aging and aging-associated diseases, therefore, it is critical to understand how senescence is regulated. Calcium ions (Ca²⁺) recently emerged as pivotal regulators of cellular senescence. However, how Ca²⁺ levels are controlled during this process is barely known. Here, we report that intracellular Ca²⁺ contents increase in response to many senescence inducers in immortalized human mammary epithelial cells (HMECs) and that expression of calbindin 1 (CALB1), a Ca²⁺-binding protein, is upregulated in this context, through the Ca²⁺-dependent calcineurin/NFAT pathway. We further show that overexpression of CALB1 buffers the rise in intracellular Ca²⁺ levels observed in senescent cells. Finally, we suggest that increased expression of Ca²⁺-binding proteins calbindins is a frequent mark of senescent cells. This work thus supports that, together with Ca²⁺channels, Ca²⁺-binding proteins modulate Ca²⁺ levels and flux during cellular senescence. This opens potential avenues of research to better understand the role of Ca²⁺ and of Ca²⁺-binding proteins in regulating cellular senescence.

Cytotoxicity of Moringa oleifera fruits on human liver cancer and molecular docking analysis of bioactive constituents against caspase-3 enzyme

Moringa oleifera is an excellent source of phenolics and flavonoids comprise various pharmacological activities. The fourth widespread leading cause of the patients' death is liver cancer. This study was formulated to perform the antiproliferative activity of Moringa oleifera fruit (MOF) extract on human liver cancer HepG2 cells and computational validation of cell death. HepG2 cell line was treated with 25, 50, 75, 100, and 200 µg/ml of MOF extract for 48 hr, and antiproliferative activity was analyzed using MTT assay, nuclear condensation, annexin V-FITC/PI double stain, ROS generation, and apoptosis executioner enzyme caspase-3. MOF extract reduced the cell viability significantly (p ˂ .05) by increasing cellular apoptosis which was confirmed by annexin V-FITC/PI staining assay. In addition, MOF stimulated intracellular ROS production and subsequently induced caspase-3 activity depending upon dose. In silico analysis revealed the good binding interaction between amino acid residues of caspase-3 (PDB ID: 1GFW) protein and selected active constituents of MOF. PASS analyses of the phytoconstituents showed no violation of Lipinski's rule of five. Analysis of drug-likeness and toxicity measurement exhibited drug-like candidates with no predicted toxicity. In conclusion, this study showed the potential anticancer activity of MOF extract which may be valuable source for anticancer drug development. PRACTICAL APPLICATIONS: Moringa oleifera fruit extract induced the anti-proliferative activity against human hepatocellular carcinoma HepG2 cells through ROS-mediated apoptosis and activation of caspase-3 enzyme. Structure-based virtual screening study between bioactive components of Moringa oleifera fruits and apoptosis executioner caspase-3 enzyme has validated the anti-proliferative activity of Moringa oleifera fruit extract. Interestingly, active phytoconstituents of Moringa oleifera fruit exhibited drug-like candidates with no predicted toxicity. Thus, Moringa oleifera fruit could be used as valuable source for anticancer drug development against human liver cancer with relatively non-toxic to healthy cells.

The Impact of the Ketogenic Diet on Glial Cells Morphology. A Quantitative Morphological Analysis

Ketogenic diet is reported to protect against cognitive decline, drug-resistant epilepsy, Alzheimer's Disease, damaging effect of ischemic stroke and many neurological diseases. Despite mounting evidence that this dietary treatment works, the exact mechanism of its protective activity is largely unknown. Ketogenic diet acts systemically, not only changing GABA signaling in neurons, but also influencing the reliance on mitochondrial respiration, known to be disrupted in many neurological diseases. Normally, human body is driven by glucose while ketogenic diet mimics starvation and energy required for proper functioning comes from fatty acids oxidation. In the brain astrocytes are believed to be the sole neural cells capable of fatty oxidation. Here we try to explain that not exclusively neurons, but also morphological changes of astroglia and/or microglia due to different metabolic state are important for the mechanism underlying the protective role of ketogenic diet. By quantifying different parameters describing cellular morphology like ramification index or fractal dimension and using Principal Component Analysis to discover the regularities between them, we demonstrate that in normal adult rat brain, ketogenic diet itself is able to change glial morphology, indicating an important role of these underappreciated cells in the brain metabolism.

Ontogeny of calcium-binding proteins in the cingulate cortex of the guinea pig: The same onset but different developmental patterns

This paper compared the density of calbindin D28k (CB), calretinin (CR) and parvalbumin (PV) containing neurons in prenatal, newborn and postnatal periods in the cingulate cortex (CC) of the guinea pig as an animal model. The distribution and co-distribution among calcium-binding proteins (CaBPs) was also investigated during the entire ontogeny. The study found that CB-positive neurons exhibited the highest density in the developing CC. The CC development in the prenatal period took place with a high level of CB and CR immunoreactivity and both of these proteins reached peak density during fetal life. The density of PV-positive neurons, in contrast to CB and CR-positive neurons, reached high levels postnatally. The observed changes of the CaBPs-positive neuron density in the developing CC coincide with developmental events in the guinea pig. E.g. the eyes opening moment may be preceded by elevated levels of CB and CR at E50, whereas high immunoreactivity of PV from P10 to P40 with a peak at P20 may indicate the participation of PV in enhancement of the inhibitory cortical pathway maturation.

The X-ray structure of human calbindin-D28K: an improved model

Calbindin-D28K is a widely expressed calcium-buffering cytoplasmic protein that is involved in many physiological processes. It has been shown to interact with other proteins, suggesting a role as a calcium sensor. Many of the targets of calbindin-D28K are of therapeutic interest: for example, inositol monophosphatase, the putative target of lithium therapy in bipolar disorder. Presented here is the first crystal structure of human calbindin-D28K. There are significant deviations in the tertiary structure when compared with the NMR structure of rat calbindin-D28K (PDB entry 2g9b), despite 98% sequence identity. Small-angle X-ray scattering (SAXS) indicates that the crystal structure better predicts the properties of calbindin-D28K in solution compared with the NMR structure. Here, the first direct visualization of the calcium-binding properties of calbindin-D28K is presented. Four of the six EF-hands that make up the secondary structure of the protein contain a calcium-binding site. Two distinct conformations of the N-terminal EF-hand calcium-binding site were identified using long-wavelength calcium single-wavelength anomalous dispersion (SAD). This flexible region has previously been recognized as a protein-protein interaction interface. SAXS data collected in both the presence and absence of calcium indicate that there are no large structural differences in the globular structure of calbindin-D28K between the calcium-loaded and unloaded proteins.

Rare human Caspase-6-R65W and Caspase-6-G66R variants identify a novel regulatory region of Caspase-6 activity

The cysteine protease Caspase-6 (Casp6) is a potential therapeutic target of Alzheimer Disease (AD) and age-dependent cognitive impairment. To assess if Casp6 is essential to human health, we investigated the effect of CASP6 variants sequenced from healthy humans on Casp6 activity. Here, we report the effects of two rare Casp6 amino acid polymorphisms, R65W and G66R, on the catalytic function and structure of Casp6. The G66R substitution eliminated and R65W substitution significantly reduced Casp6 catalytic activity through impaired substrate binding. In contrast to wild-type Casp6, both Casp6 variants were unstable and inactive in transfected mammalian cells. In addition, Casp6-G66R acted as a dominant negative inhibitor of wild-type Casp6. The R65W and G66R substitutions caused perturbations in substrate recognition and active site organization as revealed by molecular dynamics simulations. Our results suggest that full Casp6 activity may not be essential for healthy humans and support the use of Casp6 inhibitors against Casp6-dependent neurodegeneration in age-dependent cognitive impairment and AD. Furthermore, this work illustrates that studying natural single amino acid polymorphisms of enzyme drug targets is a promising approach to uncover previously uncharacterized regulatory sites important for enzyme activity.

Modifying caspase-3 activity by altering allosteric networks

Caspases have several allosteric sites that bind small molecules or peptides. Allosteric regulators are known to affect caspase enzyme activity, in general, by facilitating large conformational changes that convert the active enzyme to a zymogen-like form in which the substrate-binding pocket is disordered. Mutations in presumed allosteric networks also decrease activity, although large structural changes are not observed. Mutation of the central V266 to histidine in the dimer interface of caspase-3 inactivates the enzyme by introducing steric clashes that may ultimately affect positioning of a helix on the protein surface. The helix is thought to connect several residues in the active site to the allosteric dimer interface. In contrast to the effects of small molecule allosteric regulators, the substrate-binding pocket is intact in the mutant, yet the enzyme is inactive. We have examined the putative allosteric network, in particular the role of helix 3, by mutating several residues in the network. We relieved steric clashes in the context of caspase-3(V266H), and we show that activity is restored, particularly when the restorative mutation is close to H266. We also mimicked the V266H mutant by introducing steric clashes elsewhere in the allosteric network, generating several mutants with reduced activity. Overall, the data show that the caspase-3 native ensemble includes the canonical active state as well as an inactive conformation characterized by an intact substrate-binding pocket, but with an altered helix 3. The enzyme activity reflects the relative population of each species in the native ensemble.