Andjelka Ćelić

Domain Mapping of the Polycystin-2 C-terminal Tail Using de Novo Molecular Modeling and Biophysical Analysis

In polycystic kidney disease (PKD), polycystin-2 (PC2) is frequently mutated or truncated in the C-terminal cytoplasmic tail (PC2-C). The currently accepted model of PC2-C consists of an EF-hand motif overlapping with a short coiled coil; however, this model fails to explain the mechanisms by which PC2 truncations C-terminal to this region lead to PKD. Moreover, direct PC2 binding to inositol 1,4,5-trisphosphate receptor, KIF3A, and TRPC1 requires residues in PC2-C outside this region. To address these discrepancies and investigate the role of PC2-C in PC2 function, we performed de novo molecular modeling and biophysical analysis. De novo molecular modeling of PC2-C using the ROBETTA server predicts two domains as follows: an EF-hand motif (PC2-EF) connected by a linker to a previously unidentified C-terminal coiled coil (PC2-CC). This model differs substantially from the current model and correlates with limited proteolysis, matrix-assisted laser desorption/ionization mass spectroscopy, N-terminal sequencing, and improved coiled coil prediction algorithms. PC2-C is elongated and oligomerizes through PC2-CC, as measured by analytical ultracentrifugation and size exclusion chromatography, whereas PC2-EF is globular and monomeric. We show that PC2-C and PC2-EF have micromolar affinity for calcium (Ca2+) by isothermal titration calorimetry and undergo Ca2+-induced conformational changes by circular dichroism. Mutation of predicted EF-hand loop residues in PC2 to alanine abolishes Ca2+ binding. Our results suggest that PC2-CC is involved in PC2 oligomerization, and PC2-EF is a Ca2+-sensitive switch. PKD-associated PC2 mutations are located in regions that may disrupt these functions, providing structural insight into how PC2 mutations lead to disease.

Structure of the EF-hand domain of polycystin-2 suggests a mechanism for Ca2+-dependent regulation of polycystin-2 channel activity

The C-terminal cytoplasmic tail of polycystin-2 (PC2/TRPP2), a Ca2+-permeable channel, is frequently mutated or truncated in autosomal dominant polycystic kidney disease. We have previously shown that this tail consists of three functional regions: an EF-hand domain (PC2-EF, 720–797), a flexible linker (798–827), and an oligomeric coiled coil domain (828–895). We found that PC2-EF binds Ca2+ at a single site and undergoes Ca2+-dependent conformational changes, suggesting it is an essential element of Ca2+-sensitive regulation of PC2 activity. Here we describe the NMR structure and dynamics of Ca2+-bound PC2-EF. Human PC2-EF contains a divergent non-Ca2+-binding helix-loop-helix (HLH) motif packed against a canonical Ca2+-binding EF-hand motif. This HLH motif may have evolved from a canonical EF-hand found in invertebrate PC2 homologs. Temperature-dependent steady-state NOE experiments and NMR R1 and R2 relaxation rates correlate with increased molecular motion in the EF-hand, possibly due to exchange between apo and Ca2+-bound states, consistent with a role for PC2-EF as a Ca2+-sensitive regulator. Structure-based sequence conservation analysis reveals a conserved hydrophobic surface in the same region, which may mediate Ca2+-dependent protein interactions. We propose that Ca2+-sensing by PC2-EF is responsible for the cooperative nature of PC2 channel activation and inhibition. Based on our results, we present a mechanism of regulation of the Ca2+ dependence of PC2 channel activity by PC2-EF.

Calcium-induced Conformational Changes in C-terminal Tail of Polycystin-2 Are Necessary for Channel Gating

Background: Polycystin-2, a calcium-permeable TRP channel, is mutated in autosomal dominant polycystic kidney disease. Results: Calcium binding by the polycystin-2 EF-hand domain induces discrete conformational and oligomerization state transitions that impact channel gating. Conclusion: Polycystin-2 channel activity is regulated by cytoplasmic calcium-induced conformational changes. Significance: These studies provide a structural and mechanistic understanding for the impact of calcium binding on channel regulation. Polycystin-2 (PC2) is a Ca2+-permeable transient receptor potential channel activated and regulated by changes in cytoplasmic Ca2+. PC2 mutations are responsible for ∼15% of autosomal dominant polycystic kidney disease. Although the C-terminal cytoplasmic tail of PC2 has been shown to contain a Ca2+-binding EF-hand domain, the molecular basis of PC2 channel gating by Ca2+ remains unknown. We propose that the PC2 EF-hand is a Ca2+ sensor required for channel gating. Consistent with this, Ca2+ binding causes a dramatic decrease in the radius of gyration (Rg) of the PC2 EF-hand by small angle x-ray scattering and significant conformational changes by NMR. Furthermore, increasing Ca2+ concentrations cause the C-terminal cytoplasmic tail to transition from a mixture of extended oligomers to a single compact dimer by analytical ultracentrifugation, coupled with a >30 Å decrease in maximum interatomic distance (Dmax) by small angle x-ray scattering. Mutant PC2 channels unable to bind Ca2+ via the EF-hand are inactive in single-channel planar lipid bilayers and inhibit Ca2+ release from ER stores upon overexpression in cells, suggesting dominant negative properties. Our results support a model where PC2 channels are gated by discrete conformational changes in the C-terminal cytoplasmic tail in response to changes in cytoplasmic Ca2+ levels. These properties of PC2 are lost in autosomal dominant polycystic kidney disease, emphasizing the importance of PC2 to kidney cell function. We speculate that PC2 and the Ca2+-dependent transient receptor potential channels in general are regulated by similar conformational changes in their cytoplasmic domains that are propagated to the channel pore.

Analysis of the cytoplasmic interaction between polycystin-1 and polycystin-2

Autosomal dominant polycystic kidney disease (ADPKD) arises following mutations of either Pkd1 or Pkd2. The proteins these genes encode, polycystin-1 (PC1) and polycystin-2 (PC2), form a signaling complex using direct intermolecular interactions. Two distinct domains in the C-terminal tail of PC2 have recently been identified, an EF-hand and a coiled-coil domain. Here, we show that the PC2 coiled-coil domain interacts with the C-terminal tail of PC1, but that the PC2 EF-hand domain does not. We measured the K0.5 of the interaction between the C-terminal tails of PC1 and PC2 and showed that the direct interaction of these proteins is abrogated by a PC1 point mutation that was identified in ADPKD patients. Finally, we showed that overexpression of the PC1 C-terminal tail in MDCK cells alters the Ca2+ response, but that overexpression of the PC1 C-terminal tail containing the disease mutation does not. These results allow a more detailed understanding of the mechanism of pathogenic mutations in the cytoplasmic regions of PC1 and PC2.

17(E)-picolinylidene androstane derivatives as potential inhibitors of prostate cancer cell growth: antiproliferative activity and molecular docking studies.

We report a rapid and efficient synthesis of A-ring modified 17α-picolyl and 17(E)-picolinylidene androstane derivatives from dehydroepiandrosterone. Compounds were validated spectroscopically and structurally characterized by X-ray crystallography. Virtual screening by molecular docking against clinical targets of steroidal anticancer drugs (ERα, AR, Aromatase and CYP17A1) suggests that 17(E)-picolinylidene, but not 17α-picolyl androstanes could specifically interact with CYP17A1 (17α-hydroxylase) with similar geometry and affinity as Abiraterone, a 17-pyridinyl androstane drug clinically used in the treatment of prostate cancer. In addition, several 17(E)-picolinylidene androstanes demonstrated selective antiproliferative activity against PC3 prostate cancer cells, which correlates with Abiraterone antiproliferative activity and predicted CYP17A1 binding affinities. Based on these preliminary results, 17(E)-picolinylidene androstane derivatives could be a promising starting point for the development of new compounds for the treatment of prostate cancer.

Synthesis and anticancer cell potential of steroidal 16,17-seco-16,17a-dinitriles: Identification of a selective inhibitor of hormone-independent breast cancer cells

We report the synthesis of steroidal 16,17-seco-16,17a-dinitriles and investigate their antitumor cell properties. Compounds were evaluated for anticancer potential by in vitro antiproliferation studies, molecular docking and virtual screening. Several compounds inhibit the growth of breast and prostate cancer cell lines (MCF-7, MDA-MB-231 and PC3), and/or cervical cancer cells (HeLa). Supporting this, molecular docking predicts that steroidal 16,17-seco-16,17a-dinitriles could bind with high affinity to multiple molecular targets of breast and prostate cancer treatment (aromatase, estrogen receptor α, androgen receptor and 17α-hydroxylase) facilitated by D-seco flexibility and nitrile-mediated contacts. Thus, 16,17-seco-16,17a-dinitriles may be useful for the design of inhibitors of multiple steroidogenesis pathways. Strikingly, 10, a 1,4-dien-3-on derivative, displayed selective submicromolar antiproliferative activity against hormone-dependent (MCF-7) and -independent (MDA-MB-231) breast cancer cells (IC50 0.52, 0.11μM, respectively). Ligand-based 3D similarity searches suggest AKR1C, 17β-HSD and/or 3β-HSD subfamilies as responsible for this antiproliferative activity, while fast molecular docking identified AKR1C and ERβ as potential binders-both targets in the treatment of hormone-independent breast cancers.

DISCRETE PROTEOLYSIS OF NEURONAL CALCIUM SENSOR 1 (NCS-1) BY μ-CALPAIN DISRUPTS CALCIUM BINDING

Neuronal calcium sensor-1 (NCS-1) is a high-affinity, low-capacity Ca(2+)-binding protein expressed in many cell types. We previously showed that NCS-1 interacts with inositol 1,4,5-trisphosphate receptor (InsP(3)R) and modulates Ca(2+)-signaling by enhancing InsP3-dependent InsP(3)R channel activity and intracellular Ca(2+) transients. Recently we reported that the chemotherapeutic agent, paclitaxel (taxol) triggers mu-calpain dependent proteolysis of NCS-1, leading to reduced Ca(2+)-signaling within the cell. Degradation of NCS-1 may be critical in the induction of peripheral neuropathy associated with taxol treatment for breast and ovarian cancer. To begin to design strategies to protect NCS-1, we treated NCS-1 with mu-calpain in vitro and identified the cleavage site by N-terminal sequencing and MALDI mass spectroscopy. mu-Calpain cleavage of NCS-1 occurs within an N-terminal pseudoEF-hand domain, which by sequence analysis appears to be unable to bind Ca(2+). Our results suggest a role for this pseudoEF-hand in stabilizing the three functional EF-hands within NCS-1. Using isothermal titration calorimetry (ITC) we found that loss of the pseudoEF-hand markedly decreased NCS-1s affinity for Ca(2+). Physiologically, this significant decrease in Ca(2+) affinity may render NCS-1 incapable of responding to changes in Ca(2+) levels in vivo. The reduced ability of mu-calpain treated NCS-1 to bind Ca(2+) may explain the altered Ca(2+) signaling in the presence of taxol and suggests a strategy for therapeutic intervention of peripheral neuropathy in cancer patients undergoing taxol treatment.

The number and location of EF hand motifs dictates the calcium dependence of polycystin-2 function

Polycystin 2 (PC2) is a calcium‐dependent calcium channel, and mutations to human PC2 (hPC2) are associated with polycystic kidney disease. The C‐terminal tail of hPC2 contains 2 EF hand motifs, but only the second binds calcium. Here, we investigate whether these EF hand motifs serve as a calcium sensor responsible for the calcium dependence of PC2 function. Using NMR and bioinformatics, we show that the overall fold is highly conserved, but in evolutionarily earlier species, both EF hands bind calcium. To test whether the EF hand motif is truly a calcium sensor controlling PC2 channel function, we altered the number of calcium binding sites in hPC2. NMR studies confirmed that modified hPC2 binds an additional calcium ion. Single‐channel recordings demonstrated a leftward shift in the calcium dependence, and imaging studies in cells showed that calcium transients were enhanced compared with wild‐type hPC2. However, biophysics and functional studies showed that the first EF hand can only bind calcium and be functionally active if the second (native) calcium‐binding EF hand is intact. These results suggest that the number and location of calcium‐binding sites in the EF hand senses the concentration of calcium required for PC2 channel activity and cellular function.—Kuo, I. Y., Keeler, C., Corbin, R., Ćelić, A., Petri, E. T., Hodsdon, M. E., Ehrlich, B. E. The number and location of EF hand motifs dictates the calcium dependence of polycystin‐2 function. FASEB J. 28, 2332–2346 (2014). www.fasebj.org

Synthesis, structural analysis and antiproliferative activity of some novel D-homo lactone androstane derivatives

An efficient synthesis of several A,B-modified D-homo lactone androstane derivatives is reported. The synthetic scheme shows the transformation of 17-oxa-D-homoandrost-5-en-16-on-3β-yl acetate 1 into the 5α-hydroxy-17-oxa-D-homoandrostane-6,16-dion-3β-yl acetate (4). After the dehydration of 4, the newly synthesized 6-keto-androst-4-ene-3β-yl acetate derivative 5 was oximinated to give the 6-hydroximino derivative 6, which was converted to A,B-condensed isoxazole derivatives 7 and 8. Compound 4 was also converted (via 6(E)- and 6(Z)-hydroximino derivatives 9 and 10) to the B-seco-cyano derivative 11 under a Beckmann fragmentation, while compound 5 was transformed to the 4β,5β-epoxy derivative 12. Structures were confirmed by IR, 1H NMR, 13C NMR, and HRMS, and for 7 and 8 by X-ray crystallography. All compounds were tested in vitro on six malignant cell lines (MCF-7, MDA-MB-231, PC-3, HeLa, HT-29, K562) and one non-tumor MRC-5 cell line. Significant antiproliferative activity was observed for specific compounds against prostate (PC-3), cervical (HeLa) and colon (HT-29) cancer cells, while no compounds showed antiproliferative activity to non-cancerous control cells (MRC-5). Interestingly, 1–8 displayed selective antiproliferative activity against estrogen-independent (ER−, MDA-MB-231) breast cancer cells over estrogen-dependent (ER+, MCF-7) breast cancer cells.

Selective antitumour activity and ERα molecular docking studies of newly synthesized D-homo fused steroidal tetrazoles

Here we report a convenient “click” synthesis for D-homo fused steroidal tetrazoles 11–14 from androstane and estratriene 16,17-seco-16-nitrile-17-mesyloxy derivatives 5–8, via intramolecular 1,3-dipolar cycloaddition from in situ generated 16,17-seco-17-azido-16-nitriles 5a–8a. Products were validated by 1H/13C-NMR, IR, HRMS, and structurally characterized by X-ray crystallography and computational methods. Compounds were evaluated as potential anti-proliferative agents against a panel of human cancer cell lines. D-Homo fused steroidal tetrazoles 13 and 14 appear to have specific, selective anti-proliferative effects against estrogen receptor positive (ER+) breast adenocarcinoma cells, which correlate with binding energies calculated from molecular docking to the estrogen receptor α-ligand binding domain (ERα-LBD). Moreover, molecular docking suggests that D-ring fused steroidal tetrazoles 13 and 14 could bind to ERα-LBD in a manner similar to known anti-estrogenic compounds. Addition of a D-homo fused tetrazole group appears to structurally mimic the hydrogen bonding potential of β-estradiol, suggesting their general utility in designing novel steroidal tetrazole derivatives as anti-estrogens or inhibitors of steroidogenic enzymes.