Structural basis of HIV-1 tethering to membranes by the BST-2/tetherin ectodomain

Separable determinants of subcellular localization and interaction account for the inability of group O HIV-1 Vpu to counteract tetherin

Tetherin (BST-2/CD317) is thought to restrict retroviral particle release by cross-linking nascent viral and cellular membranes. Unlike the Vpu proteins encoded by human immunodeficiency virus type 1 (HIV-1) group M strains (M-Vpu), those from the nonpandemic HIV-1 group O (O-Vpu) are not able to counteract tetherin activity. Here, we characterized the basis of this defect in O-Vpu. O-Vpu differs from M-Vpu in that it fails to interact with tetherin and downregulate it from the cell surface. Unlike M-Vpu, O-Vpu localizes to the endoplasmic reticulum (ER) rather than the trans-Golgi network (TGN). Interestingly M-Vpu bearing an ER retention signal at the C terminus localizes similarly to O-Vpu. While it still interacts with tetherin, it fails to promote virus release, suggesting that O-Vpu deficiency correlates with its cellular distribution in the endoplasmic reticulum as well as its failure to bind tetherin. O-Vpu-M-Vpu chimeras were designed to identify the minimal changes required to restore tetherin antagonism. While several chimeric proteins bearing residues of the M-Vpu transmembrane domain into the O-Vpu transmembrane domain recovered tetherin binding in coimmunoprecipitation studies, efficient antagonism required an additional glutamic acid-to-lysine change in the membrane-proximal hinge region of the O-Vpu cytoplasmic tail that was sufficient to abolish ER retention and permit TGN localization.

Host factors involved in retroviral budding and release

The plasma membrane is the final barrier that enveloped viruses must cross during their egress from the infected cell. Here, we review recent insights into the cell biology of retroviral assembly and release; these insights have driven a new understanding of the host proteins, such as the ESCRT machinery, that are used by retroviruses to promote their final separation from the host cell. We also review antiviral host factors such as tetherin, which can directly inhibit the release of retroviral particles. These studies have illuminated the role of the lipid bilayer as the unexpected target for virus restriction by the innate immune response.

The role of BST2/tetherin in feline retrovirus infection

Pathogenic retroviral infections of mammals have induced the evolution of cellular anti-viral restriction factors and have shaped their biological activities. This intrinsic immunity plays an important role in controlling viral replication and imposes a barrier to viral cross-species transmission. Well-studied examples of such host restriction factors are TRIM5α, an E3 ubiquitin ligase that binds incoming retroviral capsids in the cytoplasm via its C-terminal PRY/SPRY (B30.2) domain and targets them for proteasomal degradation, and APOBEC3 proteins, cytidine deaminases that induce hypermutation and impair viral reverse transcription. Tetherin (BST-2, CD317) is an interferon-inducible transmembrane protein that potently inhibits the release of nascent retrovirus particles in single-cycle replication assays. However, whether the primary biological activity of tetherin in vivo is that of a restriction factor remains uncertain as recent studies on human tetherin suggest that it is unable to prevent spreading infection of human immunodeficiency virus type 1 (HIV-1). The feline tetherin homologue resembles human tetherin in amino acid sequence, protein topology and anti-viral activity. Transiently expressed feline tetherin displays potent inhibition of feline immunodeficiency virus (FIV) and HIV-1 particle release. However, stable ectopic expression of feline tetherin in a range of feline cell lines has no inhibitory effect on the growth of either primary or cell culture-adapted strains of FIV. By comparing and contrasting the activities of the felid and primate tetherins against their respective immunodeficiency-causing lentiviruses we may gain insight into the contribution of tetherins to the control of lentiviral replication and the evolution of lentiviral virulence.

Intracellular defenses against HIV, viral evasion and novel therapeutic approaches

Human immunodeficiency virus (HIV), the causative agent of AIDS, is a retrovirus. It is estimated that, while in the cell, it interacts with almost 10% of cellular proteins. Several of these have evolved to protect the cell from infection with retroviruses and are known as "restriction factors". Restriction factors tell us much about how the virus functions and open up new paradigms for exploring novel antiviral therapeutics. This article gives an update on the three best studied restriction factors, their putative mechanisms of action and how the virus has overcome their effects, together with an indication of novel therapeutic approaches based on this knowledge.

Antiviral inhibition of enveloped virus release by tetherin/BST-2: action and counteraction

Tetherin (BST2/CD317) has been recently recognized as a potent interferon-induced antiviral molecule that inhibits the release of diverse mammalian enveloped virus particles from infected cells. By targeting an immutable structure common to all these viruses, the virion membrane, evasion of this antiviral mechanism has necessitated the development of specific countermeasures that directly inhibit tetherin activity. Here we review our current understanding of the molecular basis of tetherin's mode of action, the viral countermeasures that antagonize it, and how virus/tetherin interactions may affect viral transmission and pathogenicity.

Ex vivo gene therapy for HIV-1 treatment

Until recently, progress in ex vivo gene therapy (GT) for human immunodeficiency virus-1 (HIV-1) treatment has been incremental. Long-term HIV-1 remission in a patient who received a heterologous stem cell transplant for acquired immunodeficiency syndrome-related lymphoma from a CCR5(-/-) donor, even after discontinuation of conventional therapy, has energized the field. We review the status of current approaches as well as future directions in the areas of therapeutic targets, combinatorial strategies, vector design, introduction of therapeutics into stem cells and enrichment/expansion of gene-modified cells. Finally, we discuss recent advances towards clinical application of HIV-1 GT.

Identification of Residues in the BST-2 TM Domain Important for Antagonism by HIV-1 Vpu Using a Gain-of-Function Approach

The HIV-1 Vpu protein enhances the release of viral particles from the cell-surface in a cell-type specific manner. In the absence of Vpu, nascent virions remain tethered to the cell-surface in restricted cell-types. Recently, the human host factor BST-2/CD317/tetherin was found to be responsible for the inhibition of virus release. It was also reported that HIV-1 Vpu can target human BST-2 but is unable to interfere with the function of murine or simian BST-2. We performed a gain-of-function study to determine which of the differences between human and rhesus BST-2 account for the differential sensitivity to Vpu. We transferred human BST-2 sequences into rhesus BST-2 and assessed the resulting chimeras for inhibition of HIV-1 virus release and sensitivity to Vpu. We found that rhesus BST-2 carrying the transmembrane (TM) domain of human BST-2 is susceptible to HIV-1 Vpu. Finally, a single-amino-acid change in the rhesus BST-2 TM domain was sufficient to confer Vpu sensitivity.

Structural and biophysical analysis of BST-2/tetherin ectodomains reveals an evolutionary conserved design to inhibit virus release

BST-2/tetherin is a host antiviral molecule that functions to potently inhibit the release of enveloped viruses from infected cells. In return, viruses have evolved antagonists to this activity. BST-2 traps budding virions by using two separate membrane-anchoring regions that simultaneously incorporate into the host and viral membranes. Here, we detailed the structural and biophysical properties of the full-length BST-2 ectodomain, which spans the two membrane anchors. The 1.6-Å crystal structure of the complete mouse BST-2 ectodomain reveals an ∼145-Å parallel dimer in an extended α-helix conformation that predominantly forms a coiled coil bridged by three intermolecular disulfides that are required for stability. Sequence analysis in the context of the structure revealed an evolutionarily conserved design that destabilizes the coiled coil, resulting in a labile superstructure, as evidenced by solution x-ray scattering displaying bent conformations spanning 150 and 180 Å for the mouse and human BST-2 ectodomains, respectively. Additionally, crystal packing analysis revealed possible curvature-sensing tetrameric structures that may aid in proper placement of BST-2 during the genesis of viral progeny. Overall, this extended coiled-coil structure with inherent plasticity is undoubtedly necessary to accommodate the dynamics of viral budding while ensuring separation of the anchors.

BST-2 is rapidly down-regulated from the cell surface by the HIV-1 protein Vpu: evidence for a post-ER mechanism of Vpu-action

Recent evidence suggests that transmembrane domain (TMD) interactions are essential for HIV-1 Vpu-mediated antagonism of the restriction factor BST-2/tetherin. We made Vpu TMD mutants to study the mechanism of BST-2 antagonism. Vpu-I17A, -A18F, -W22L, and -S23L co-localized with BST-2 within endosomal membranes while effectively enhancing virion release and down-regulating surface BST-2. However, Vpu-A18H was confined to an endoplasmic reticulum (ER)-like distribution, resulting in impaired down-regulation of BST-2 and reduced virion release. Brefeldin A confined wild type Vpu to the ER, resulting in a similarly impaired phenotype, as did the addition of a C-terminal ER-retention signal to Vpu. We determined the half-life of cell-surface BST-2 to be ~8 hours, whereas Vpu mediated an ~80% reduction of surface BST-2 within 6 hours, suggesting that TMD interactions between Vpu and BST-2 occur within post-ER membranes to directly and rapidly remove BST-2 from the cell surface and relieve restricted virion release.

Tetherin and its viral antagonists

Restriction factors comprise an important layer of host defense to fight against viral infection. Some restriction factors are constitutively expressed whereas the majority is induced by interferon to elicit innate immunity. In addition to a number of well-characterized interferon-inducible antiviral factors such as RNaseL/OAS, ISG15, Mx, PKR, and ADAR, tetherin (BST-2/CD317/HM1.24) was recently discovered to block the release of enveloped viruses from the cell surface, which is regarded as a novel antiviral mechanism induced by interferon. Here, we briefly review the history of tetherin discovery, discuss how tetherin blocks virus production, and highlight the viral countermeasures to evade tetherin restriction.

HIV-1 Vpu blocks recycling and biosynthetic transport of the intrinsic immunity factor CD317/tetherin to overcome the virion release restriction

The intrinsic immunity factor CD317 (BST-2/HM1.24/tetherin) imposes a barrier to HIV-1 release at the cell surface that can be overcome by the viral protein Vpu. Expression of Vpu results in a reduction of CD317 surface levels; however, the mechanism of this Vpu activity and its contribution to the virological antagonism are incompletely understood. Here, we characterized the influence of Vpu on major CD317 trafficking pathways using quantitative antibody-based endocytosis and recycling assays as well as a microinjection/microscopy-based kinetic de novo expression approach. We report that HIV-1 Vpu inhibited both the anterograde transport of newly synthesized CD317 and the recycling of CD317 to the cell surface, while the kinetics of CD317 endocytosis remained unaffected. Vpu trapped trafficking CD317 molecules at the trans-Golgi network, where the two molecules colocalized. The subversion of both CD317 transport pathways was dependent on the highly conserved diserine S52/S56 motif of Vpu; however, it did not require recruitment of the diserine motif interactor and substrate adaptor of the SCF-E3 ubiquitin ligase complex, β-TrCP. Treatment of cells with the malaria drug primaquine resulted in a CD317 trafficking defect that mirrored that induced by Vpu. Importantly, primaquine could functionally replace Vpu as a CD317 antagonist and rescue HIV-1 particle release.

IMPORTANCE

HIV efficiently replicates in the human host and induces the life-threatening immunodeficiency AIDS. Mammalian genomes encode proteins such as CD317 that can inhibit viral replication at the cellular level. As a countermeasure, HIV has evolved genes like vpu that can antagonize these intrinsic immunity factors. Investigating the mechanism by which Vpu overcomes the virion release restriction imposed by CD317, we find that Vpu subverts recycling and anterograde trafficking pathways of CD317, resulting in surface levels of the restriction factor insufficient to block HIV-1 spread. This describes a novel mechanism of immune evasion by HIV.

Differential effects of human immunodeficiency virus type 1 Vpu on the stability of BST-2/tetherin

BST-2/CD317/tetherin is a host factor that inhibits the release of HIV-1 and other unrelated viruses. A current model proposes that BST-2 physically tethers virions to the surface of virus-producing cells. The HIV-1-encoded Vpu protein effectively antagonizes the activity of BST-2. How Vpu accomplishes this task remains unclear; however, it is known that Vpu has the ability to down-modulate BST-2 from the cell surface. Here we analyzed the effects of Vpu on BST-2 by performing a series of kinetic studies with HeLa, 293T, and CEMx174 cells. Our results indicate that the surface downregulation of BST-2 is not due to an accelerated internalization or reduced recycling of internalized BST-2 but instead is caused by interference with the resupply of newly synthesized BST-2 from within the cell. While our data confirm previous reports that the high-level expression of Vpu can cause the endoplasmic reticulum (ER)-associated degradation of BST-2, we found no evidence that Vpu targets endogenous BST-2 in the ER in the course of a viral infection. Instead, we found that Vpu acts in a post-ER compartment and increases the turnover of newly synthesized mature BST-2. Our observation that Vpu does not affect the recycling of BST-2 suggests that Vpu does not act directly at the cell surface but may interfere with the trafficking of newly synthesized BST-2 to the cell surface, resulting in the accelerated targeting of BST-2 to the lysosomal compartment for degradation.

Modulation of HIV-1-host interaction: role of the Vpu accessory protein

Viral protein U (Vpu) is a type 1 membrane-associated accessory protein that is unique to human immunodeficiency virus type 1 (HIV-1) and a subset of related simian immunodeficiency virus (SIV). The Vpu protein encoded by HIV-1 is associated with two primary functions during the viral life cycle. First, it contributes to HIV-1-induced CD4 receptor downregulation by mediating the proteasomal degradation of newly synthesized CD4 molecules in the endoplasmic reticulum (ER). Second, it enhances the release of progeny virions from infected cells by antagonizing Tetherin, an interferon (IFN)-regulated host restriction factor that directly cross-links virions on host cell-surface. This review will mostly focus on recent advances on the role of Vpu in CD4 downregulation and Tetherin antagonism and will discuss how these two functions may have impacted primate immunodeficiency virus cross-species transmission and the emergence of pandemic strain of HIV-1.

The interferon-inducible host factor bone marrow stromal antigen 2/tetherin restricts virion release, but is it actually a viral restriction factor?

Viruses face a variety of obstacles when infecting a new host. The past few years have brought exciting new insights into the function of restriction factors, which form part of the host's innate immune system. One of the most recently identified restriction factors is bone marrow stromal antigen 2 (BST-2)/tetherin. BST-2 is an interferon-inducible gene whose expression dramatically reduces the release of viruses from infected cells. This effect of BST-2 is not specific to human immunodeficiency virus but affects a broad range of enveloped viruses. Since the identification of BST-2 as a restriction factor in 2008, much progress has been made in understanding the molecular properties and functional characteristics of this host factor. The goal of this review was to provide an update on our current understanding of the role of BST-2 in regulating virus release and to discuss its role in controlling virus spread during productive infection with special emphasis on human immunodeficiency virus-1.

Tethered virions are intermediates in the assembly and release of HIV-1 particles

Vpu enhances HIV-1 release by suppressing CD317-mediated tethering of virions to the cell surface. In HeLa H1 cells, Vpu(+) infection produces significant amounts of tethered virions, although efficient virus release requires Vpu. We have analyzed membrane targeting and assembly of newly synthesized Gag in infected HeLa H1 cells by quantitative pulse-chase assays in both Vpu(+) and ΔVpu virus backgrounds and in the presence and absence of CD317. Our results show that formation of tethered virions precedes release of viruses to the culture medium in the Vpu(+) infection, and CD317 knockdown reduces tethering in both Vpu(+) and ΔVpu virus backgrounds. Significantly, our results indicate that tethered Vpu(+) viruses represent precursors for extracellular viruses, and Vpu thus appears to reverse tethering in HeLa H1 cells after budding.

Endogenous CD317/Tetherin limits replication of HIV-1 and murine leukemia virus in rodent cells and is resistant to antagonists from primate viruses

Human CD317 (BST-2/tetherin) is an intrinsic immunity factor that blocks the release of retroviruses, filoviruses, herpesviruses, and arenaviruses. It is unclear whether CD317 expressed endogenously in rodent cells has the capacity to interfere with the replication of the retroviral rodent pathogen murine leukemia virus (MLV) or, in the context of small-animal model development, contributes to the well-established late-phase restriction of human immunodeficiency virus type 1 (HIV-1). Here, we show that small interfering RNA (siRNA)-mediated knockdown of CD317 relieved a virion release restriction and markedly enhanced the egress of HIV-1, HIV-2, and simian immunodeficiency virus (SIV) in rat cells, including primary macrophages. Moreover, rodent CD317 potently inhibited MLV release, and siRNA-mediated depletion of CD317 in a mouse T-cell line resulted in the accelerated spread of MLV. Several virus-encoded antagonists have recently been reported to overcome the restriction imposed by human or monkey CD317, including HIV-1 Vpu, envelope glycoproteins of HIV-2 and Ebola virus, Kaposi's sarcoma-associated herpesvirus K5, and SIV Nef. In contrast, both rat and mouse CD317 showed a high degree of resistance to these viral antagonists. These data suggest that CD317 is a broadly acting and conserved mediator of innate control of retroviral infection and pathogenesis that restricts the release of retroviruses and lentiviruses in rodents. The high degree of resistance of the rodent CD317 restriction factors to antagonists from primate viruses has implications for HIV-1 small-animal model development and may guide the design of novel antiviral interventions.

Immune evasion and counteraction of restriction factors by HIV-1 and other primate lentiviruses

Retroviruses have evolved effective strategies to evade the host immune response, such as high variability and latent infection. In addition, primate lentiviruses, such as HIV-1, have acquired several "accessory" genes that antagonize antiviral host restriction factors and facilitate viral immune evasion, thereby allowing continuous and efficient viral replication despite apparently strong innate and acquired immune responses. Here, I summarize some of our current knowledge on the acquisition and function of the viral vif, vpr, vpu, and nef genes, with a particular focus on the evolution and specific properties of pandemic HIV-1 strains that may contribute to their efficient spread and high virulence.

Structural insight into the mechanisms of enveloped virus tethering by tetherin

Tetherin/BST2 is a type-II membrane protein that inhibits the release of a range of enveloped viruses, including HIV-1. Here we report three crystal structures of human tetherin, including the full-length ectodomain, a triple cysteine mutant and an ectodomain truncation. These structures show that tetherin forms a continuous alpha helix encompassing almost the entire ectodomain. Tetherin helices dimerize into parallel coiled coils via interactions throughout the C-terminal portion of the ectodomain. A comparison of the multiple structures of the tetherin dimer reveals inherent constrained flexibility at two hinges positioned at residues A88 and G109. In the crystals, two tetherin ectodomain dimers associate into a tetramer by forming an antiparallel four-helix bundle at their N termini. However, mutagenesis studies suggest that the tetrametric form of tetherin, although potentially contributing to, is not essential for its antiviral activity. Nonetheless, the structural and chemical properties of the N terminus of the ectodomain are important for optimal tethering function. This study provides detailed insight into the mechanisms by which this broad-spectrum antiviral restriction factor can function.

Determinants of tetherin antagonism in the transmembrane domain of the human immunodeficiency virus type 1 Vpu protein

Tetherin (BST2/CD317) potently restricts the particle release of human immunodeficiency virus type 1 (HIV-1) mutants defective in the accessory gene vpu. Vpu antagonizes tetherin activity and induces its cell surface downregulation and degradation in a manner dependent on the transmembrane (TM) domains of both proteins. We have carried out extensive mutagenesis of the HIV-1 NL4.3 Vpu TM domain to identify three amino acid positions, A14, W22, and, to a lesser extent, A18, that are required for tetherin antagonism. Despite the mutants localizing indistinguishably from the wild-type (wt) protein and maintaining the ability to multimerize, mutation of these positions rendered Vpu incapable of coimmunoprecipitating tetherin or mediating its cell surface downregulation. Interestingly, these amino acid positions are predicted to form one face of the Vpu transmembrane alpha helix and therefore potentially contribute to an interacting surface with the transmembrane domain of tetherin either directly or by modulating the conformation of Vpu oligomers. While the equivalent of W22 is invariant in HIV-1/SIVcpz Vpu proteins, the positions of A14 and A18 are highly conserved among Vpu alleles from HIV-1 groups M and N, but not those from group O or SIVcpz that lack human tetherin (huTetherin)-antagonizing activity, suggesting that they may have contributed to the adaption of HIV-1 to human tetherin.

HIV-1 Vpu targets cell surface markers CD4 and BST-2 through distinct mechanisms

Vpu is a small integral membrane protein encoded by HIV-1 and some SIV isolates. The protein is known to induce degradation of the viral receptor molecule CD4 and to enhance the release of newly formed virions from the cell surface. Vpu accomplishes these two functions through two distinct mechanisms. In the case of CD4, Vpu acts as a molecular adaptor to connect CD4 to an E3 ubiquitin ligase complex resulting in CD4 degradation by cellular proteasomes. This requires signals located in Vpu's cytoplasmic domain. Enhancement of virus release on the other hand involves the neutralization of a cellular host factor, BST-2 (also known as CD317, HM1.24, or tetherin) and requires Vpu's TM domain. The current review discusses recent advances on the role of Vpu in controlling degradation of CD4 and in regulating virus release.

Structural and functional studies on the extracellular domain of BST2/tetherin in reduced and oxidized conformations

HIV-1 and other enveloped viruses can be restricted by a host cellular protein called BST2/tetherin that prevents release of budded viruses from the cell surface. Mature BST2 contains a small cytosolic region, a predicted transmembrane helix, and an extracellular domain with a C-terminal GPI anchor. To advance understanding of BST2 function, we have determined a 2.6 Å crystal structure of the extracellular domain of the bacterially expressed recombinant human protein, residues 47-152, under reducing conditions. The structure forms a single long helix that associates as a parallel dimeric coiled coil over its C-terminal two-thirds, while the N-terminal third forms an antiparallel four-helix bundle with another dimer, creating a global tetramer. We also report the 3.45 Å resolution structure of BST2(51-151) prepared by expression as a secreted protein in HEK293T cells. This oxidized construct forms a dimer in the crystal that is superimposable with the reduced protein over the C-terminal two-thirds of the molecule, and its N terminus suggests pronounced flexibility. Hydrodynamic data demonstrated that BST2 formed a stable tetramer under reducing conditions and a dimer when oxidized to form disulfide bonds. A mutation that selectively disrupted the tetramer (L70D) increased protein expression modestly but only reduced antiviral activity by approximately threefold. Our data raise the possibility that BST2 may function as a tetramer at some stage, such as during trafficking, and strongly support a model in which the primary functional state of BST2 is a parallel disulfide-bound coiled coil that displays flexibility toward its N terminus.

BST-2 diminishes HIV-1 infectivity

Bone marrow stromal cell antigen 2 (BST-2, also known as tetherin/CD317/HM1.24) inhibits the release of human immunodeficiency virus type 1 (HIV-1) and other enveloped viruses by tethering virus particles to the cell surface. In this study, we provide evidence not only that the yield of cell-free HIV-1 particles is significantly reduced by BST-2 but also that the infectivity of these progeny virions is severely impaired. The lowered virion infectivity is due to the accumulation of pr55 Gag precursor and the p40Gag intermediates as well as to the loss of a mature core in the majority of HIV-1 particles. These data suggest that, in addition to impeding the release of HIV-1 particles from host cells, BST-2 may also interfere with the activation of viral protease and, as a result, impairs viral Gag processing as well as maturation of HIV-1 particles.

BST-2/tetherin: a new component of the innate immune response to enveloped viruses

The interferon-inducible, transmembrane protein BST-2 (CD317, tetherin) directly holds fully formed enveloped virus particles to the cells that produce them, inhibiting their spread. BST-2 inhibits members of the retrovirus, filovirus, arenavirus and herpesvirus families. These viruses encode a variety of proteins to degrade BST-2 and/or direct it away from its site of action at the cell surface. Viral antagonism has subjected BST-2 to positive selection, leading to species-specific differences that presented a barrier to the transmission of simian immunodeficiency viruses (SIVs) to humans. This barrier was crossed by HIV-1 when its Vpu protein acquired activity as a BST-2 antagonist. Here, we review this new host-pathogen relationship and discuss its impact on the evolution of primate lentiviruses and the origins of the HIV pandemic.

An imperfect rule for the particle roost

Tetherin inhibits HIV and other enveloped viruses by grasping particles at the budding site and preventing their release. An article by Hinz and coworkers (Hinz et al., 2010) in this issue of Cell Host & Microbe reveals remarkable irregularities within the coiled-coil domain of the tetherin dimer that enhance flexibility of the molecule and contribute to its function.

Cellular HIV-1 restriction factors: a new avenue for AIDS therapy?

The lack of an efficacious HIV-1 vaccine and the continued emergence of drug-resistant HIV-1 strains have pushed the research community to explore novel avenues for AIDS therapy. Over the last decade, one new avenue that has been realized involves cellular HIV-1 restriction factors, defined as host cellular proteins or factors that restrict or inhibit HIV-1 replication. Many of these factors are interferon-induced and inhibit specific stages of the HIV-1 lifecycle that are not targeted by current AIDS therapies. Our understanding of the molecular mechanisms underlying HIV-1 restriction is far from complete, but our current knowledge of these factors offers hope for the future development of novel therapeutic ideas.