Structural Insights into Azole-based Inhibitors of Heme Oxygenase-1: Development of Selective Compounds for Therapeutic Applications.

The development of isozyme-selective heme oxygenase (HO) inhibitors promises powerful pharmacological tools to elucidate the regulatory characteristics of the HO system. It is already known that HO has cytoprotective properties with a role in several disease states; thus, it is an enticing therapeutic target. Historically, the metalloporphyrins have been used as competitive HO inhibitors based on their structural similarity to the substrate, heme. However, heme's important role in several other proteins (e.g. cytochromes P450, nitric oxide synthase), results in non-selectivity being an unfortunate side effect. Reports that azalanstat and other non-porphyrin molecules inhibited HO led to a multi-faceted effort over a decade ago to develop novel compounds as potent, selective inhibitors of HO. The result was the creation of the first generation of non-porphyrin based, non-competitive inhibitors with selectivity for HO, including a subset with isozyme selectivity for HO-1. Using X-ray crystallography, the structures of several complexes of HO-1 with novel inhibitors have been elucidated and provided insightful information regarding the salient features required for inhibitor binding. This included the structural basis for non-competitive inhibition, flexibility and adaptability of the inhibitor binding pocket, and multiple, potential interaction subsites, all of which can be exploited in future drug-design strategies. Notably, HO-1 inhibitors are of particular interest for the treatment of hyperbilirubinemia and certain types of cancer. Key features based on this initial study have already been used by others to discover additional potential HO-1 inhibitors. Moreover, studies have begun to use selected compounds and determine their effects in some disease models.

Cysteine-independent activation/inhibition of heme oxygenase-2.

Reactive thiols of cysteine (cys) residues in proteins play a key role in transforming chemical reactivity into a biological response. The heme oxygenase-2 (HO-2) isozyme contains two cys residues that have been implicated in binding of heme and also the regulation of its activity. In this paper, we address the question of a role for cys residues for the HO-2 inhibitors or activators designed in our laboratory. We tested the activity of full length recombinant human heme oxygenase-2 (FL-hHO-2) and its analog in which cys265 and cys282 were both replaced by alanine to determine the effect on activation by menadione (MD) and inhibition by QC-2350. Similar inhibition by QC-2350 and almost identical activation by MD was observed for both recombinant FL-hHO-2s. Our findings are interpreted to mean that thiols of FL-hHO-2s are not involved in HO-2 activation or inhibition by the compounds that have been designed and identified by us. Activation or inhibition of HO-2 by our compounds should be attributed to a mechanism other than altering binding affinity of HO-2 for heme through cys265 and cys282.

Function coupling of otoferlin with GAD65 acts to modulate GABAergic activity.

Otoferlin, an integral membrane protein implicated in a late stage of exocytosis, has been reported to play a critical role in hearing although the underlying mechanisms remain elusive. However, its widespread tissue distribution infers a more ubiquitous role in synaptic vesicle trafficking. Glutamate, an excitatory neurotransmitter, is converted to its inhibitory counterpart, γ-aminobutyric acid (GABA), by L-glutamic acid decarboxylase (GAD), which exists in soluble (GAD67) and membrane-bound (GAD65) forms. For the first time, we have revealed a close association between otoferlin and GAD65 in both HEK293 and neuronal cells, including SH-SY5Y neuroblastoma and primary rat hippocampus cells, showing a direct interaction between GAD65 and otoferlin's C2 domains. In primary rat hippocampus cells, otoferlin and GAD65 co-localized in a punctate pattern within the cell body, as well as in the axon along the path of vesicular traffic. Significantly, GABA is virtually abolished in otoferlin-knockdown neuronal cells whereas otoferlin overexpression markedly increases endogenous GABA. GABA attenuation in otoferlin-knockdown primary cells is correlated with diminished L-type calcium current. This previously unknown and close correlation demonstrates that otoferlin, through GAD65, modulates GABAergic activity. The discovery of otoferlin-GAD65 functional coupling provides a new avenue for understanding the molecular mechanism by which otoferlin functions in neurological pathways.

Structural basis of calcineurin activation by calmodulin.

Calcineurin is the only known calmodulin (CaM) activated protein phosphatase, which is involved in the regulation of numerous cellular and developmental processes and in calcium-dependent signal transduction. Although commonly assumed that CaM displaces the autoinhibitory domain (AID) blocking substrate access to its active site, the structural basis underlying activation remains elusive. We have created a fused ternary complex (CBA) by covalently linking three polypeptides: CaM, calcineurin regulatory B subunit (CnB) and calcineurin catalytic A subunit (CnA). CBA catalytic activity is comparable to that of fully activated native calcineurin in the presence of CaM. The crystal structure showed virtually no structural change in the active site and no evidence of CaM despite being covalently linked. The asymmetric unit contains four molecules; two parallel CBA pairs are packed in an antiparallel mode and the large cavities in crystal packing near the calcineurin active site would easily accommodate multiple positions of AID-bound CaM. Intriguingly, the conformation of the ordered segment of AID is not altered by CaM; thus, it is the disordered part of AID, which resumes a regular α-helical conformation upon binding to CaM, which is displaced by CaM for activation. We propose that the structural basis of calcineurin activation by CaM is through displacement of the disordered fragment of AID which otherwise impedes active site access.

Free cysteine modulates the conformation of human C/EBP homologous protein.

The C/EBP Homologous Protein (CHOP) is a nuclear protein that is integral to the unfolded protein response culminating from endoplasmic reticulum stress. Previously, CHOP was shown to comprise extensive disordered regions and to self-associate in solution. In the current study, the intrinsically disordered nature of this protein was characterized further by comprehensive in silico analyses. Using circular dichroism, differential scanning calorimetry and nuclear magnetic resonance, we investigated the global conformation and secondary structure of CHOP and demonstrated, for the first time, that conformational changes in this protein can be induced by the free amino acid L-cysteine. Addition of L-cysteine caused a significant dose-dependent decrease in the protein helicity--dropping from 69.1% to 23.8% in the presence of 1 mM of L-cysteine--and a sequential transition to a more disordered state, unlike that caused by thermal denaturation. Furthermore, the presence of small amounts of free amino acid (80 µM, an 8:1 cysteine∶CHOP ratio) during CHOP thermal denaturation altered the molecular mechanism of its melting process, leading to a complex, multi-step transition. On the other hand, high levels (4 mM) of free L-cysteine seemed to cause a complete loss of rigid cooperatively melting structure. These results suggested a potential regulatory function of L-cysteine which may lead to changes in global conformation of CHOP in response to the cellular redox state and/or endoplasmic reticulum stress.

Fibroblast growth factor 9 delivery during angiogenesis produces durable, vasoresponsive microvessels wrapped by smooth muscle cells.

The therapeutic potential of angiogenic growth factors has not been realized. This may be because formation of endothelial sprouts is not followed by their muscularization into vasoreactive arteries. Using microarray expression analysis, we discovered that fibroblast growth factor 9 (FGF9) was highly upregulated as human vascular smooth muscle cells (SMCs) assemble into layered cords. FGF9 was not angiogenic when mixed with tissue implants or delivered to the ischemic mouse hind limb, but instead orchestrated wrapping of SMCs around neovessels. SMC wrapping in implants was driven by sonic hedgehog-mediated upregulation of PDGFRß. Computed tomography microangiography and intravital microscopy revealed that microvessels formed in the presence of FGF9 had enhanced capacity to receive flow and were vasoreactive. Moreover, the vessels persisted beyond 1 year, remodeling into multilayered arteries paired with peripheral nerves. This mature physiological competency was attained by targeting mesenchymal cells rather than endothelial cells, a finding that could inform strategies for therapeutic angiogenesis and tissue engineering.