Jean-François Hernandez

Structural biology of C1: dissection of a complex molecular machinery

The classical pathway of complement is initiated by the C1 complex, a multimolecular protease comprising a recognition subunit (C1q) and two modular serine proteases (C1r and C1s) associated as a Ca2+‐dependent tetramer (C1s‐C1r‐C1r‐C1s). Early studies have allowed identification of specialized functional domains in these proteins and have led to low‐resolution models of the C1 complex. The objective of current studies is to gain deeper insights into the structure of C1, and the strategy used for this purpose mainly consists of dissecting the C1 components into modular fragments, in order to solve their three‐dimensional structure and establish the structural correlates of their function. The aim of this article is to provide an overview of the structural and functional information generated by this approach, with particular emphasis on the domains involved in the assembly, the recognition function, and the highly specific proteolytic properties of C1.

The Crystal Structure of the Epstein–Barr Virus Protease Shows Rearrangement of the Processed C Terminus

Epstein-Barr virus (EBV) belongs to the gamma-herpesvirinae subfamily of the Herpesviridae. The protease domain of the assemblin protein of herpesviruses forms a monomer-dimer equilibrium in solution. The protease domain of EBV was expressed in Escherichia coli and its structure was solved by X-ray crystallography to 2.3A resolution after inhibition with diisopropyl-fluorophosphate (DFP). The overall structure confirms the conservation of the homodimer and its structure throughout the alpha, beta, and gamma-herpesvirinae. The substrate recognition could be modelled using information from the DFP binding, from a crystal contact, suggesting that the substrate forms an antiparallel beta-strand extending strand beta5, and from the comparison with the structure of a peptidomimetic inhibitor bound to cytomegalovirus protease. The long insert between beta-strands 1 and 2, which was disordered in the KSHV protease structure, was found to be ordered in the EBV protease and shows the same conformation as observed for proteases in the alpha and beta-herpesvirus families. In contrast to previous structures, the long loop located between beta-strands 5 and 6 is partially ordered, probably due to DFP inhibition and a crystal contact. It also contributes to substrate recognition. The protease shows a specific recognition of its own C terminus in a binding pocket involving residue Phe210 of the other monomer interacting across the dimer interface. This suggests conformational changes of the protease domain after its release from the assemblin precursor followed by burial of the new C terminus and a possible effect onto the monomer-dimer equilibrium. The importance of the processed C terminus was confirmed using a mutant protease carrying a C-terminal extension and a mutated release site, which shows different solution properties and a strongly reduced enzymatic activity.

JLK isocoumarin inhibitors: Selective γ-secretase inhibitors that do not interfere with Notch pathway in vitro or in vivo

γ‐Secretase activity is involved in the generation of Aβ and therefore likely contributes to the pathology of Alzheimers disease. Blocking this activity was seen as a major therapeutic target to slow down or arrest Aβ‐related AD progression. This strategy seemed more doubtful when it was established that γ‐secretase also targets other substrates including Notch, a particularly important transmembrane protein involved in vital functions, at both embryonic and adulthood stages. We have described previously new non‐peptidic inhibitors able to selectively inhibit Aβ cellular production in vitro without altering Notch pathway. We show here that in vivo, these inhibitors do not alter the Notch pathway responsible for somitogenesis in the zebrafish embryo. In addition, we document further the selectivity of JLK inhibitors by showing that, unlike other described γ‐secretase inhibitors, these agents do not affect E‐cadherin processing. Finally, we establish that JLKs do not inhibit β‐site APP cleaving enzymes (BACE) 1 and BACE2, α‐secretase, the proteasome, and GSK3β kinase. Altogether, JLK inhibitors are the sole agents to date that are able to prevent Aβ production without triggering unwanted cleavages of other proteins.

Design and characterization of a new cell‐permeant inhibitor of the β‐secretase BACE1

1 The β‐secretase BACE1 is one of the enzymes that contribute to the production of the Aβ peptide, in vitro and in vivo. JMV1195 was previously shown to inhibit BACE activity in vitro but was unable to block cellular BACE activity. We have designed a new permeable inhibitor, JMV2764 that corresponds to a derivative of JMV1195 to which a penetratin sequence had been added at its N‐terminus. We have assessed the ability of JMV2764 to affect BACE1 activity in vitro, and to modify Aβ production in various cell systems. 2 Endogenous β‐secretase or BACE1 activities were monitored in vitro by means of two distinct fluorimetric substrates in HEK293 extracts of cells expressing either wild‐type βAPP, Swedish mutated βAPP or SPA4CT constructs. Aβ40 recovery was monitored by immunoprecipitation and Western blot analysis. 3 JMV2764 and JMV1195 inhibited endogenous β‐secretase activity of HEK293 cellular homogenates with IC50s of 0.8 and 6.6 μM, respectively. Interestingly, JMV2764 also inhibited β‐secretase activity after preincubation with intact cells while JMV1195 was inactive, indicating that unlike JMV1195, JMV2764 could penetrate into the cells. 4 JMV2764 but not JMV1195 also prevented Aβ production by HEK293 cells overexpressing wild‐type and Swedish‐mutated βAPP. However, JMV2764 was unable to affect Aβ production from cells expressing SPA4CT, a βAPP‐derived sequence that does not need β‐secretase to produce Aβ. 5 Altogether, we have designed a new cell‐permeable BACE1 inhibitor that allows to envision to prevent Aβ production in vivo. Work is in progress to assess the potential of these compounds to prevent Aβ production in transgenic mice overproducing Aβ.

Melatonin stimulates the nonamyloidogenic processing of βAPP through the positive transcriptional regulation of ADAM10 and ADAM17.

Melatonin controls many physiological functions including regulation of the circadian rhythm and clearance of free radicals and neuroprotection. Importantly, melatonin levels strongly decrease as we age and patients with Alzheimers disease (AD) display lower melatonin than age‐matched controls. Several studies have reported that melatonin can reduce aggregation and toxicity of amyloid‐β peptides that are produced from the β‐amyloid precursor protein (βAPP). However, whether melatonin can directly regulate the βAPP‐cleaving proteases (‘secretases’) has not been investigated so far. In this study, we establish that melatonin stimulates the α‐secretase cleavage of βAPP in cultured neuronal and non‐neuronal cells. This effect is fully reversed by ADAM10‐ and ADAM17‐specific inhibitors and requires both plasma membrane‐located melatonin receptor activation, and ERK1/2 phosphorylation. Moreover, we demonstrate that melatonin upregulates both ADAM10 and ADAM17 catalytic activities and endogenous protein levels. Importantly, genetic depletion of one or the other protease in mouse embryonic fibroblasts prevents melatonin stimulating constitutive and PKC‐regulated sAPPα secretion and ADAM10/ADAM17 catalytic activities. Furthermore, we show that melatonin induces ADAM10 and ADAM17 promoter transactivation, and we identify the targeted promoter regions. Finally, we correlate melatonin‐dependent sAPPα production with a protection against staurosporine‐induced apoptosis. Altogether, our results provide the first demonstration that melatonin upregulates the nonamyloidogenic ADAM10 and ADAM17 proteases through melatonin receptor activation, ERK phosphorylation and the transactivation of some specific regions of their promoters and further underline the preventive rather than curative nature of melatonin regarding AD treatment.

A peptide from the adenovirus fiber shaft forms amyloid-type fibrils

The fiber protein of adenovirus consists of a C‐terminal globular head, a shaft and a short N‐terminal tail. The crystal structure of a stable domain comprising the head plus a part of the shaft of human adenovirus type 2 fiber has recently been solved at 2.4 Å resolution [van Raaij et al. (1999) Nature 401, 935–938]. A peptide corresponding to the portion of the shaft immediately adjacent to the head (residues 355–396) has been synthesized chemically. The peptide failed to assemble correctly and instead formed amyloid‐type fibrils as assessed by electron microscopy, Congo red binding and X‐ray diffraction. Peptides corresponding to the fiber shaft could provide a model system to study mechanisms of amyloid fibril formation.

Structure and functions of the interaction domains of C1r and C1s: keystones of the architecture of the C1 complex

C1r and C1s, the proteases responsible for activation and proteolytic activity of the C1 complex of complement, share similar overall structural organizations featuring five nonenzymic protein modules (two CUB modules surrounding a single EGF module, and a pair of CCP modules) followed by a serine protease domain. Besides highly specific proteolytic activities, both proteases exhibit interaction properties associated with their N-terminal regions. These properties include the ability to bind Ca2+ ions with high affinity, to associate with each other within a Ca2+-dependent C1s-C1r-C1r-C1s tetramer, and to interact with C1q upon C1 assembly. Precise functional mapping of these regions has been achieved recently, allowing identification of the domains responsible for these interactions, and providing a comprehensive picture of their structure and function. The objective of this article is to provide a detailed and up-to-date overview of the information available on these domains, which are keystones of the assembly of C1, and appear to play an essential role at the interface between the recognition function of C1 and its proteolytic activity.

Pharmacological evidences for DFK167-sensitive presenilin-independent γ-secretase-like activity

Amyloid‐β (Aβ) peptides production is thought to be a key event in the neurodegenerative process ultimately leading to Alzheimer’s disease (AD) pathology. A bulk of studies concur to propose that the C‐terminal moiety of Aβ is released from its precursor β‐amyloid precursor protein by a high molecular weight enzymatic complex referred to as γ‐secretase, that is composed of at least, nicastrin (NCT), Aph‐1, Pen‐2, and presenilins (PS) 1 or 2. They are thought to harbor the γ‐secretase catalytic activity. However, several lines of evidence suggest that additional γ‐secretase‐like activities could potentially contribute to Aβ production. By means of a quenched fluorimetric substrate (JMV2660) mimicking the β‐amyloid precursor protein sequence targeted by γ‐secretase, we first show that as expected, this probe allows monitoring of an activity detectable in several cell systems including the neuronal cell line telencephalon specific murine neurons (TSM1). This activity is reduced by DFK167, N‐[N‐(3,5‐difluorophenacetyl)‐L‐alanyl]‐S‐phenylglycine t‐butyl ester (DAPT), and LY68458, three inhibitors known to functionally interact with PS. Interestingly, JMV2660 but not the unrelated peptide JMV2692, inhibits Aβ production in an in vitroγ‐secretase assay as expected from a putative substrate competitor. This activity is enhanced by PS1 and PS2 mutations known to be responsible for familial forms of AD and reduced by aspartyl mutations inactivating PS or in cells devoid of PS or NCT. However, we clearly establish that residual JMV2660‐hydrolysing activity could be recovered in PS‐ and NCT‐deficient fibroblasts and that this activity remained inhibited by DFK167. Overall, our study describes the presence of a proteolytic activity displaying γ‐secretase‐like properties but independent of PS and still blocked by DFK167, suggesting that the PS‐dependent complex could not be the unique γ‐secretase activity responsible for Aβ production and delineates PS‐independent γ‐secretase activity as a potential additional therapeutic target to fight AD pathology.

Activation of α-secretase by curcumin-aminoacid conjugates.

The extracellular senile plaques observed in Alzheimers disease (AD) patients are mainly composed of amyloid peptides produced from the β-amyloid precursor protein (βAPP) by β- and γ-secretases. A third non-amyloidogenic α-secretase activity performed by the disintegrins ADAM10 and ADAM17 occurs in the middle of the amyloid-β peptide Aβ and liberates the large sAPPα neuroprotective fragment. Since the activation of α-secretase recently emerged as a promising therapeutic approach to treat AD, the identification of natural compounds able to trigger this cleavage is highly required. Here we describe new curcumin-based modified compounds as α-secretase activators. We established that the aminoacid conjugates curcumin-isoleucine, curcumin-phenylalanine and curcumin-valine promote the constitutive α-secretase activity and increase ADAM10 immunoreactivity. Strickingly, experiments carried out under conditions mimicking the PKC/muscarinic receptor-regulated pathway display different patterns of activation by these compounds. Altogether, our data identified new lead natural compounds for the future development of powerful and stable α-secretase activators and established that some of these molecules are able to discriminate between the constitutive and regulated α-secretase pathways.

Dipeptide mimic oligomer transporter mediates intracellular delivery of Cathepsin D inhibitors: a potential target for cancer therapy.

Implication of the intracellular proteolytic activity of Cathepsin D (CathD), a lysosomal aspartyl-protease overexpressed in numerous solid tumors, has been evidenced on tumor growth. Its intracellular inhibition by potent inhibitors such as pepstatin constitutes a relevant but challenging molecular target. Indeed the potential of pepstatin as a therapeutic molecule is hampered by its too low intracellular penetration. We addressed this limitation by designing and developing a bioconjugate combining a pepstatin derivative with a new vector of cell penetration (CPNP) specifically targeting the endolysosomal compartment. We showed that this pepstatin conjugate (JMV4463) exhibited high anti-proliferative effect on tumor cell cultures via intracellular CathD inhibition and altered cell cycle associated with apoptotic events in vitro. When tested in mice xenografted with breast cancer cells, JMV4463 delayed tumor emergence and growth.