The Impact of FKBP5 Deficiency in Glucocorticoid Receptor Mediated Regulation of Synaptic Transmission in the Medial Prefrontal Cortex

Genetically engineered mouse models of FK506-binding protein 5

FK506 binding protein 51 (FKBP51) is a molecular chaperone that influences stress response. In addition to having an integral role in the regulation of steroid hormone receptors, including glucocorticoid receptor, FKBP51 has been linked with several biological processes including metabolism and neuronal health. Genetic and epigenetic alterations in the gene that encodes FKBP51, FKBP5, are associated with increased susceptibility to multiple neuropsychiatric disorders, which has fueled much of the research on this protein. Because of the complexity of these processes, animal models have been important in understanding the role of FKBP51. This review examines each of the current mouse models of FKBP5, which include whole animal knockout, conditional knockout, overexpression, and humanized mouse models. The generation of each model and observational details are discussed, including behavioral phenotypes, molecular changes, and electrophysiological alterations basally and following various challenges. While much has been learned through these models, there are still many aspects of FKBP51 biology that remain opaque and future studies are needed to help illuminate these current gaps in knowledge. Overall, FKBP5 continues to be an exciting potential target for stress-related disorders.

Multimodal Associations of FKBP5 Methylation With Emotion-Regulatory Brain Circuits

BACKGROUND

Understanding the biological processes that underlie individual differences in emotion regulation and stress responsivity is a key challenge for translational neuroscience. The gene FKBP5 is a core regulator in molecular stress signaling that is implicated in the development of psychiatric disorders. However, it remains unclear how FKBP5 DNA methylation in peripheral blood is related to individual differences in measures of neural structure and function and their relevance to daily-life stress responsivity.

METHODS

Here, we characterized multimodal correlates of FKBP5 DNA methylation by combining epigenetic data with neuroimaging and ambulatory assessment in a sample of 395 healthy individuals.

RESULTS

First, we showed that FKBP5 demethylation as a psychiatric risk factor was related to an anxiety-associated reduction of gray matter volume in the ventromedial prefrontal cortex, a brain area that is involved in emotion regulation and mental health risk and resilience. This effect of epigenetic upregulation of FKBP5 on neuronal structure is more pronounced where FKBP5 is epigenetically downregulated at baseline. Leveraging 208 functional magnetic resonance imaging scans during a well-established emotion-processing task, we found that FKBP5 DNA methylation in peripheral blood was associated with functional differences in prefrontal-limbic circuits that modulate affective responsivity to daily stressors, which we measured using ecological momentary assessment in daily life.

CONCLUSIONS

Overall, we demonstrated how FKBP5 contributes to interindividual differences in neural and real-life affect regulation via structural and functional changes in prefrontal-limbic brain circuits.

Reducing FKBP51 Expression in the Ventral Hippocampus Decreases Auditory Fear Conditioning in Male Rats

Fear conditioning evokes a physiologic release of glucocorticoids that assists learning. As a cochaperone in the glucocorticoid receptor complex, FKBP51 modulates stress-induced glucocorticoid signaling and may influence conditioned fear responses. This study combines molecular and behavioral approaches to examine whether locally reducing FKBP51 expression in the ventral hippocampus is sufficient to affect fear-related behaviors. We hypothesized that reducing FKBP51 expression in the VH would increase glucocorticoid signaling to alter auditory fear conditioning. Adult male rats were injected with an adeno-associated virus (AAV) vector expressing short hairpin - RNAs (shRNA) targeting FKBP5 into the ventral hippocampus to reduce FKBP5 levels or a control AAV. Infusion of FKBP5-shRNA into the ventral hippocampus decreased auditory fear acquisition and recall. Although animals injected with FKBP5-shRNA showed less freezing during extinction recall, the difference was due to a reduced fear recall rather than improved extinction. Reducing ventral hippocampus FKBP51 did not affect exploratory behavior in either the open field test or the elevated zero maze test but did increase passive behavior in the forced swim test, suggesting that the reduction in auditory fear recall was not due to more active responses to acute stress. Furthermore, lower ventral hippocampus FKBP51 levels did not alter corticosterone release in response to restraint stress, suggesting that the reduced fear recall was not due to lower corticosterone release. Our findings suggest FKBP51 in the ventral hippocampus plays a selective role in modulating fear-learning processes and passive behavioral responses to acute stress rather than hypothalamic-pituitary-adrenal axis reactivity or exploratory responses.

Effects of Corticosterone on the Excitability of Glutamatergic and GABAergic Neurons of the Adolescent Mouse Superficial Dorsal Horn

Stress evokes age-dependent effects on pain sensitivity and commonly occurs during adolescence. However, the mechanisms linking adolescent stress and pain remain poorly understood, in part due to a lack of information regarding how stress hormones modulate the function of nociceptive circuits in the adolescent CNS. Here we investigate the short- and long-term effects of corticosterone (CORT) on the excitability of GABAergic and presumed glutamatergic neurons of the spinal superficial dorsal horn (SDH) in Gad1-GFP mice at postnatal days (P)21-P34. In situ hybridization revealed that glutamatergic SDH neurons expressed significantly higher mRNA levels of both glucocorticoid receptors (GR) and mineralocorticoid receptors (MR) compared to adjacent GABAergic neurons. The incubation of spinal cord slices with CORT (90 min) evoked select long-term changes in spontaneous synaptic transmission across both cell types in a sex-dependent manner, without altering the intrinsic firing of either Gad1-GFP+ or GFP- neurons. Meanwhile, the acute bath application of CORT significantly decreased the frequency and amplitude of miniature excitatory postsynaptic currents (mEPSCs), as well as the frequency of miniature inhibitory postsynaptic currents (mIPSCs), in both cell types leading to a net reduction in the balance of spontaneous excitation vs. inhibition (E:I ratio). This CORT-induced reduction in the E:I ratio was not prevented by selective antagonists of either GR (mifepristone) or MR (eplerenone), although eplerenone blocked the effect on mEPSC amplitude. Collectively, these data suggest that corticosterone modulates synaptic function within the adolescent SDH which could influence the overall excitability and output of the spinal nociceptive network.

Molecular pathways of major depressive disorder converge on the synapse

Major depressive disorder (MDD) is a psychiatric disease of still poorly understood molecular etiology. Extensive studies at different molecular levels point to a high complexity of numerous interrelated pathways as the underpinnings of depression. Major systems under consideration include monoamines, stress, neurotrophins and neurogenesis, excitatory and inhibitory neurotransmission, mitochondrial dysfunction, (epi)genetics, inflammation, the opioid system, myelination, and the gut-brain axis, among others. This review aims at illustrating how these multiple signaling pathways and systems may interact to provide a more comprehensive view of MDD's neurobiology. In particular, considering the pattern of synaptic activity as the closest physical representation of mood, emotion, and conscience we can conceptualize, each pathway or molecular system will be scrutinized for links to synaptic neurotransmission. Models of the neurobiology of MDD will be discussed as well as future actions to improve the understanding of the disease and treatment options.

Inescapable footshocks induce molecular changes in the prefrontal cortex of rats in an amyloid-beta-42 model of Alzheimer's disease

Alzheimer's disease (AD) affects several brain areas, including the prefrontal cortex (PFC) involved in execution, working memory, and fear extinction. Despite these critical roles, the PFC is understudied in AD pathology. People with post-traumatic stress disorder (PTSD) have twice the risk of developing AD, and the underlying mechanisms linking these two diseases are less understood. Here, we investigated the effect of footshock stress on behavioural vis-a-vis molecular changes in the PFC of an amyloid-beta (Aβ)-42 lesion rat model of AD. Trauma-like conditions were induced by exposing the animals to several footshocks. AD-like condition was induced via intra-hippocampal injection of Aβ-42 peptide. Following Aβ-42 injections, animals were tested for behavioural changes using the Open Field Test (OFT) and Y-maze test. The PFC was later harvested for neurochemical analyses. Our results showed an interactive effect of footshocks and Aβ-42 lesion on: reduced percentage alternation in the Y-maze test, suggesting memory impairment; reduced number of line crosses and time spent in the centre square of the OFT, indicating anxiogenic responses. Similarly, there was an interactive effect of footshocks and Aβ-42 lesion on: increased FK506 binding protein 51 (FKBP5) expression, which can be associated with stress-induced anxiogenic behaviours; and increased neuronal apoptosis in the PFC of the animals. In addition, footshocks, as well as Aβ-42 lesion, reduced superoxide dismutase levels and Bridging Integrator-1 (BIN1) expression in the PFC of the animals, which can be linked to the observed memory impairment. In conclusion, our findings indicate that footshocks exaggerate PFC-associated behavioural and molecular changes induced by an AD-like pathology.

Interaction Between Glucocorticoid Receptors and FKBP5 in Regulating Neurotransmission of the Hippocampus

FK501 binding protein 51 (FKBP5) is a stress response prolyl isomerase that inhibits the translocation of the glucocorticoid receptor (GR) heterocomplex to the nucleus. Previous studies have shown that the expression levels of FKBP5 are positively correlated with psychiatric disorders, including depression and post-traumatic stress disorder. In rodents, FKBP5 deletion in the brain leads to be resilient to stress-induced depression. The hippocampus is known to be one of the primary locations mediating stress responses in the brain by providing negative feedback signals to the hypothalamus-pituitaryadrenal gland axis. Therefore, we aimed to investigate the role of FKBP5 and its interaction with GRs in the hippocampus. We observed that FKBP5 deletion in the hippocampus resulted in a minimal change in synaptic transmission. In the hippocampus, GR activation alters the release probability in inhibitory synapses as well as the postsynaptic contribution of glutamate receptors in excitatory synapses; however, no such alterations were induced in the absence of FKBP5. FKBP5 deficiency causes insensitivity to activated GRs in the hippocampus suggesting that FKBP5 mediates synaptic changes caused by GR activation. Our study provides electrophysiological evidence of stress resilience observed in FKBP5-deficient mice.