Christian Namendorf

Polymorphisms in the Drug Transporter Gene ABCB1 Predict Antidepressant Treatment Response in Depression

The clinical efficacy of a systemically administered drug acting on the central nervous system depends on its ability to pass the blood-brain barrier, which is regulated by transporter molecules such as ABCB1 (MDR1). Here we report that polymorphisms in the ABCB1 gene predict the response to antidepressant treatment in those depressed patients receiving drugs that have been identified as substrates of ABCB1 using abcb1ab double-knockout mice. Our results indicate that the combined consideration of both the medications capacity to act as an ABCB1-transporter substrate and the patients ABCB1 genotype are strong predictors for achieving a remission. This finding can be viewed as a further step into personalized antidepressant treatment.

Individual Stress Vulnerability Is Predicted by Short-Term Memory and AMPA Receptor Subunit Ratio in the Hippocampus

Increased vulnerability to aversive experiences is one of the main risk factors for stress-related psychiatric disorders as major depression. However, the molecular bases of vulnerability, on the one hand, and stress resilience, on the other hand, are still not understood. Increasing clinical and preclinical evidence suggests a central involvement of the glutamatergic system in the pathogenesis of major depression. Using a mouse paradigm, modeling increased stress vulnerability and depression-like symptoms in a genetically diverse outbred strain, and we tested the hypothesis that differences in AMPA receptor function may be linked to individual variations in stress vulnerability. Vulnerable and resilient animals differed significantly in their dorsal hippocampal AMPA receptor expression and AMPA receptor binding. Treatment with an AMPA receptor potentiator during the stress exposure prevented the lasting effects of chronic social stress exposure on physiological, neuroendocrine, and behavioral parameters. In addition, spatial short-term memory, an AMPA receptor-dependent behavior, was found to be predictive of individual stress vulnerability and response to AMPA potentiator treatment. Finally, we provide evidence that genetic variations in the AMPA receptor subunit GluR1 are linked to the vulnerable phenotype. Therefore, we propose genetic variations in the AMPA receptor system to shape individual stress vulnerability. Those individual differences can be predicted by the assessment of short-term memory, thereby opening up the possibility for a specific treatment by enhancing AMPA receptor function.

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.

Pharmacokinetics of acute and sub-chronic aripiprazole in P-glycoprotein deficient mice

BACKGROUND P-glycoprotein (P-gp), an efflux transporter localized in the blood-brain barrier, limits the access of multiple xenobiotics to the central nervous system (CNS). For the new antipsychotic aripiprazole and its active metabolite dehydroaripiprazole differences in disposition in blood and brain were investigated after acute and sub-chronic administration in a P-gp knockout mouse model. METHODS Serum and brain concentrations of both drugs were measured at several time points 1-24h after i.p. injection of 10mg/kg aripiprazole and after 11 days of sub-chronic administration in several tissues. Moreover, the expression of P-gp was determined by Western blot analysis after sub-chronic administration of the drug. RESULTS In both wild type and abcb1ab (-/-) mice concentration of aripiprazole in brain were up to 9 fold higher than in serum. For dehydroaripiprazole the mean brain to serum ratios were below two. Brain to serum concentrations of both substances were significantly higher after acute and sub-chronic administration in connection to the expression of P-gp indicated by higher levels in abcb1ab (-/-) mice especially for dehydroaripiprazole. Sub-chronic aripiprazole treatment in WT animals had no effect on P-gp expression in the blood-brain barrier. CONCLUSIONS Aripiprazole and, even more pronounced its active metabolite dehydroaripiprazole could be identified as substrates of P-gp. The efflux transporter P-gp must therefore be considered as a relevant factor that contributes to wanted or unwanted clinical effects in patients treated with aripiprazole.

Vasopressin in CSF and plasma in depressed suicide attempters: preliminary results

Increased plasma arginine vasopressin (AVP) concentrations have been reported in depressed suicide attempters. Plasma AVP is primarily produced by the magnocellular system in response to increased plasma osmolality, and central AVP may be independently regulated. In the present study we investigated cerebrospinal fluid (CSF) and plasma AVP concentrations in depressed patients and controls. Nineteen drug-free depressed psychiatric inpatients (nine suicide attempters) and nine neurological control subjects underwent lumbar puncture and psychiatric evaluation. CSF and plasma concentrations of AVP, serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), homovanillic acid (HVA), and cortisol were assayed. In 15 depressed patients (eight suicide attempters), the combined dexamethasone/corticotropin-releasing hormone (Dex/CRH) test was performed to examine the hypothalamic-pituitary-adrenocortical (HPA) system. There were no differences between depressed subjects and controls in all parameters measured. Suicide attempters did not differ from nonattempters. In depressed patients, plasma AVP correlated positively with cortisol. There was no relationship between CSF AVP and monoamine metabolites in CSF.

Pharmacological Inhibition of the Psychiatric Risk Factor FKBP51 Has Anxiolytic Properties

Anxiety-related psychiatric disorders represent one of the largest health burdens worldwide. Single nucleotide polymorphisms of the FK506 binding protein 51 (FKBP51) gene have been repeatedly associated with anxiety-related disorders and stress sensitivity. Given the intimate relationship of stress and anxiety, we hypothesized that amygdala FKBP51 may mediate anxiety-related behaviors. Mimicking the stress effect by specifically overexpressing FKBP51 in the basolateral amygdala (BLA) or central amygdala resulted in increased anxiety-related behavior, respectively. In contrast, application of a highly selective FKBP51 point mutant antagonist, following FKBP51mut BLA-overexpression, reduced the anxiogenic phenotype. We subsequently tested a novel FKBP51 antagonist, SAFit2, in wild-type mice via BLA microinjections, which reduced anxiety-related behavior. Remarkably, the same effect was observed following peripheral administration of SAFit2. To our knowledge, this is the first in vivo study using a specific FKBP51 antagonist, thereby unraveling the role of FKBP51 and its potential as a novel drug target for the improved treatment of anxiety-related disorders.

P-glycoprotein is a factor in the uptake of dextromethorphan, but not of melperone, into the mouse brain: evidence for an overlap in substrate specificity between P-gp and CYP2D6

In this study, the role of P-glycoprotein (P-gp) for the pharmacokinetics of dextromethorphan, a CYP2D6 substrate, and of melperone, a CYP2D6 inhibitor, was investigated. The substances were administered subcutaneously near the nape of the neck of wild-type mice and of abcb1ab (-/-) mice. One hour after injection, concentrations of the two drugs in cerebrum, plasma and in different organs were measured by high-performance liquid chromatography. No significant differences between wild-type mice and abcb1ab (-/-) mice were observed for melperone, suggesting that P-gp is not involved in the uptake of melperone into the brain or other organs of mice. The concentration of dextromethorphan in the brain was more than twice as high in abcb1ab (-/-) mice compared to wild-type mice. Therefore, P-gp appears to be a factor in the uptake of dextromethorphan into the mouse brain, and abcb1-polymorphisms need to be considered for CYP2D6 phenotyping experiments with this drug. There is an overlap in substrate specificity between P-gp and CYP2D6. P-gp is a factor in the uptake of dextromethorphan, but not of melperone.

The amino acid transporter SLC6A15 is a regulator of hippocampal neurochemistry and behavior

Although mental disorders as major depression are highly prevalent worldwide their underlying causes remain elusive. Despite the high heritability of depression and a clear genetic contribution to the disease, the identification of genetic risk factors for depression has been very difficult. The first published candidate to reach genome-wide significance in depression was SLC6A15, a neuronal amino acid transporter. With a reported 1,42 fold increased risk of suffering from depression associated with a single nucleotide polymorphism (SNP) in a regulatory region of SLC6A15, the polymorphism was also found to affect hippocampal morphology, integrity, and hippocampus-dependent memory. However, the function of SLC6A15 in the brain is so far largely unknown. To address this question, we investigated if alterations in SLC6A15 expression, either using a full knockout or a targeted hippocampal overexpression, affect hippocampal neurochemistry and consequently behavior. We could show that a lack of SLC6A15 reduced hippocampal tissue levels of proline and other neutral amino acids. In parallel, we observed a decreased overall availability of tissue glutamate and glutamine, while at the same time the basal tone of extracellular glutamate in the hippocampus was increased. By contrast, SLC6A15 overexpression increased glutamate/glutamine tissue concentrations. These neurochemical alterations could be linked to behavioral abnormalities in sensorimotor gating, a key translational endophenotype relevant for many psychiatric disorders. Overall, our data supports SLC6A15 as a crucial factor controlling amino acid content in the hippocampus, thereby likely interfering with glutamatergic transmission and behavior. These findings emphasize SLC6A15 as pivotal risk factor for vulnerability to psychiatric diseases.

Stress-responsive FKBP51 regulates AKT2-AS160 signaling and metabolic function

The co-chaperone FKBP5 is a stress-responsive protein-regulating stress reactivity, and its genetic variants are associated with T2D related traits and other stress-related disorders. Here we show that FKBP51 plays a role in energy and glucose homeostasis. Fkbp5 knockout (51KO) mice are protected from high-fat diet-induced weight gain, show improved glucose tolerance and increased insulin signaling in skeletal muscle. Chronic treatment with a novel FKBP51 antagonist, SAFit2, recapitulates the effects of FKBP51 deletion on both body weight regulation and glucose tolerance. Using shorter SAFit2 treatment, we show that glucose tolerance improvement precedes the reduction in body weight. Mechanistically, we identify a novel association between FKBP51 and AS160, a substrate of AKT2 that is involved in glucose uptake. FKBP51 antagonism increases the phosphorylation of AS160, increases glucose transporter 4 expression at the plasma membrane, and ultimately enhances glucose uptake in skeletal myotubes. We propose FKBP51 as a mediator between stress and T2D development, and potential target for therapeutic approaches.Stress is recognized as risk factor for the development of type 2 diabetes. Here Balsevich et al. show that the stress responsive co-chaperone FKBP5 regulates glucose metabolism in mice by modulating AS160 phosphorylation, glucose transporter expression and muscle glucose uptake.

Remote and reversible inhibition of neurons and circuits by small molecule induced potassium channel stabilization

Manipulating the function of neurons and circuits that translate electrical and chemical signals into behavior represents a major challenges in neuroscience. In addition to optogenetic methods using light-activatable channels, pharmacogenetic methods with ligand induced modulation of cell signaling and excitability have been developed. However, they are largely based on ectopic expression of exogenous or chimera proteins. Now, we describe the remote and reversible expression of a Kir2.1 type potassium channel using the chemogenetic technique of small molecule induced protein stabilization. Based on shield1-mediated shedding of a destabilizing domain fused to a protein of interest and inhibition of protein degradation, this principle has been adopted for biomedicine, but not in neuroscience so far. Here, we apply this chemogenetic approach in brain research for the first time in order to control a potassium channel in a remote and reversible manner. We could show that shield1-mediated ectopic Kir2.1 stabilization induces neuronal silencing in vitro and in vivo in the mouse brain. We also validated this novel pharmacogenetic method in different neurobehavioral paradigms.The DD-Kir2.1 may complement the existing portfolio of pharmaco- and optogenetic techniques for specific neuron manipulation, but it may also provide an example for future applications of this principle in neuroscience research.