Felix Hausch

FK506 binding protein 5 shapes stress responsiveness: modulation of neuroendocrine reactivity and coping behavior.

BACKGROUND The Hsp90 cochaperone FK506 binding protein 5 (FKBP5) is an established regulator of the glucocorticoid receptor (GR), and numerous genetic studies have linked it to stress-related diseases such as major depression or posttraumatic stress disorder. However, translational studies including genetic animal models are lacking. METHODS Mice deficient of FKBP5 were generated and analyzed in comparison with wildtype littermates. They were subjected to several test paradigms characterizing their emotionality, stress reactivity, and coping behavior as well as hypothalamus-pituitary-adrenal axis function and regulation. Moreover, protein expression of GR and FKBP5 was determined in different brain structures 8 days after stress exposure. The combined dexamethasone/corticotropin-releasing hormone test was performed both in mice and healthy human subjects of different FKBP5 genotypes. The GR function was evaluated by reporter gene assays. RESULTS Under basal conditions, deletion of FKBP5 did not change exploratory drive, locomotor activity, anxiety-related behavior, stress-coping, or depression-like behavior. After exposure to different acute stressors of sufficient intensity, however, it led to a more active coping behavior. Moreover, loss of FKBP5 decreased hypothalamus-pituitary-adrenal axis reactivity and GR expression changes in response to stressors. In mice and humans, the FKBP5 genotype also determined the outcome of the dexamethasone/corticotropin-releasing hormone test. CONCLUSIONS This study in mice and humans presents FKBP5 as a decisive factor for the physiological stress response, shaping neuroendocrine reactivity as well as coping behavior. This lends strong support to the concept emerging from human studies of FKBP5 as important factor governing gene-environment interactions relevant for the etiology of affective disorders.

Design, Synthesis, and Evaluation of Gluten Peptide Analogs as Selective Inhibitors of Human Tissue Transglutaminase

Recent studies have implicated a crucial role for tissue transglutaminase (TG2) in the pathogenesis of Celiac Sprue, a disorder of the small intestine triggered in genetically susceptible individuals by dietary exposure to gluten. Proteolytically stable peptide inhibitors of human TG2 were designed containing acivicin or alternatively 6-diazo-5-oxo-norleucine (DON) as warheads. In biochemical and cell-based assays, the best of these inhibitors, Ac-PQP-(DON)-LPF-NH(2), was considerably more potent and selective than other TG2 inhibitors reported to date. Selective pharmacological inhibition of extracellular TG2 should be useful in exploring the mechanistic implications of TG2-catalyzed modification of dietary gluten, a phenomenon of considerable relevance in Celiac Sprue.

Selective inhibitors of the FK506-binding protein 51 by induced fit

The FK506-binding protein 51 (FKBP51, encoded by the FKBP5 gene) is an established risk factor for stress-related psychiatric disorders such as major depression. Drug discovery for FKBP51 has been hampered by the inability to pharmacologically differentiate against the structurally similar but functional opposing homolog FKBP52, and all known FKBP ligands are unselective. Here, we report the discovery of the potent and highly selective inhibitors of FKBP51, SAFit1 and SAFit2. This new class of ligands achieves selectivity for FKBP51 by an induced-fit mechanism that is much less favorable for FKBP52. By using these ligands, we demonstrate that selective inhibition of FKBP51 enhances neurite elongation in neuronal cultures and improves neuroendocrine feedback and stress-coping behavior in mice. Our findings provide the structural and functional basis for the development of mechanistically new antidepressants.

Large FK506-binding proteins shape the pharmacology of rapamycin.

ABSTRACT The immunosuppressant and anticancer drug rapamycin works by inducing inhibitory protein complexes with the kinase mTOR, an important regulator of growth and proliferation. The obligatory accessory partner of rapamycin is believed to be FK506-binding protein 12 (FKBP12). Here we show that rapamycin complexes of larger FKBP family members can tightly bind to mTOR and potently inhibit its kinase activity. Cocrystal structures with FKBP51 and FKBP52 reveal the modified molecular binding mode of these alternative ternary complexes in detail. In cellular model systems, FKBP12 can be functionally replaced by larger FKBPs. When the rapamycin dosage is limiting, mTOR inhibition of S6K phosphorylation can be enhanced by FKBP51 overexpression in mammalian cells, whereas FKBP12 is dispensable. FKBP51 could also enable the rapamycin-induced hyperphosphorylation of Akt, which depended on higher FKBP levels than rapamycin-induced inhibition of S6K phosphorylation. These insights provide a mechanistic rationale for preferential mTOR inhibition in specific cell or tissue types by engaging specific FKBP homologs.

The Prospect of FKBP51 as a Drug Target

The FK506 binding protein 51 (FKBP51) is best known as an Hsp90‐associated co‐chaperone that regulates the responsiveness of steroid hormone receptors. In human genetic association studies, FKBP51 has repeatedly been associated with emotion processing and numerous stress‐related affective disorders. It has also been implicated in contributing to the glucocorticoid hyposensitivity observed in New World primates. More recently, several research groups have consistently shown a protective effect of FKBP51 knockout or knockdown on stress endocrinology and stress‐coping behavior in animal models of depression and anxiety. The principal druggability of FKBP51 is exemplified by the prototypic FKBP ligands FK506 and rapamycin. Moreover, FKBP51 is highly suited for X‐ray co‐crystallography, which should facilitate the rational drug design of improved FKBP51 ligands. In summary, FKBP51 has emerged as a promising new drug target for stress‐related disorders that should be amenable to drug discovery.

Corticotropin releasing factor receptor antagonists for major depressive disorder

Introduction: Major depressive disorder is a serious and common psychiatric illness, and many of the depressive patients benefit from pharmacological treatment. Available antidepressants produce remission in only about 30 – 40% of the patients. Therefore, new concepts are being explored for the development of innovative antidepressants with higher efficacy. Areas covered: The use of corticotropin releasing factor type 1 (CRF1) receptor antagonists for depression is supported by abundant evidence of target validation, the availability of in vitro and in vivo assays and specific small ligands. Some of these compounds have advanced to clinical studies, with discouraging results so far in depression. This review covers the development of CRF1 receptor antagonists at different stages of the development pipeline of the pharmaceutical industry and its bottlenecks. Most of the available CRF1 receptor antagonists known so far share a common chemical scaffold. We present possible strategies to overcome obstacles in the discovery and development process at the levels of library screenings and clinical studies to find more diverse compounds. Expert opinion: CRF1 receptor antagonists are expected to be beneficial only for those patients with CRF overexpression and the need for tests to identify these individuals is discussed. New technical developments and diagnostic tools might eventually lead to a more successful treatment of major depression with CRF1 receptor antagonists.

Evaluation of Synthetic FK506 Analogues as Ligands for the FK506-Binding Proteins 51 and 52.

The FK506-binding proteins (FKBP) 51 and 52 are cochaperones that modulate the signal transduction of steroid hormone receptors. Both proteins have been implicated in prostate cancer. Furthermore, single nucleotide polymorphisms in the gene encoding FKBP51 have been associated with a variety of psychiatric disorders. Rapamycin and FK506 are two macrocyclic natural products that bind to these proteins indiscriminately but with nanomolar affinity. We here report the cocrystal structure of FKBP51 with a simplified α-ketoamide analogue derived from FK506 and the first structure-activity relationship analysis for FKBP51 and FKBP52 based on this compound. In particular, the tert-pentyl group of this ligand was systematically replaced by a cyclohexyl ring system, which more closely resembles the pyranose ring in the high-affinity ligands rapamycin and FK506. The interaction with FKBPs was found to be surprisingly tolerant to the stereochemistry of the attached cyclohexyl substituents. The molecular basis for this tolerance was elucidated by X-ray cocrystallography.

Fluorescent probes to characterise FK506-binding proteins.

Talented all‐rounders: Fluorescence polarisation assays were developed for members of the FK506‐binding protein family by using fluorescent rapamycin analogues (demonstrated in the figure). These tracers retain medium to high affinity to all tested proteins (FKBP12, ‐12.6, ‐13, ‐25, ‐51, ‐52). They can be used for active‐site titrations, competition assays with unlabelled ligands and enable a robust, miniaturized assay adequate for high‐throughput screening.

The Chemical Biology of Immunophilin Ligands

The immunophilin ligands cyclosporin A, FK506 and rapamycin are best known for their immunosuppressive properties and their clinical use in transplantation medicine. These compounds or their analogs are also clinically used or investigated in various types of cancer, coronary angioplasty, dermatology, hepatitis C infections, and neuroprotection. Furthermore, the role of immunophilins in various pathologies is increasingly being recognized, supporting the preclinical drug development for novel immunophilin targets. Finally, immunophilin ligands are widely used as sophisticated tools in chemical biology. This review shows the progress on three major areas made in the last five years. An update of the immunosuppressive ligands and their clinical applications is discussed in the first part of the review, followed by a discussion about the emerging immunophilin targets and their respective ligands. The final section gives a detailed assessment of immunophilin ligand-based tools.

XAP2 inhibits glucocorticoid receptor activity in mammalian cells.

XAP2 is member of a protein family sharing the TPR protein interaction motif. It displays close homology to the immunophilins FKBP51 and FKBP52 that act via the Hsp90 folding machinery to regulate the glucocorticoid receptor (GR). We show that XAP2 inhibits GR by reducing its responsiveness to hormone in transcriptional activation. The effect of XAP2 on GR requires its interaction with Hsp90 through the TPR motif. The PPIase‐like region turned out to be enzymatically inactive. Thus, PPIase activity is not essential for the action of XAP2 on GR, similarly to FKBP51 and FKBP52.