Endothelial cells are an important source of BDNF in rat skeletal muscle

BDNF (brain-derived neurotrophic factor) is present in skeletal muscle, controlling muscular metabolism, strength and regeneration processes. However, there is no consensus on BDNF cellular source. Furthermore, while endothelial tissue expresses BDNF in large amount, whether endothelial cells inside muscle expressed BDNF has never been explored. The aim of the present study was to provide a comprehensive analysis of BDNF localization in rat skeletal muscle. Cellular localization of BDNF and activated Tropomyosin-related kinase B (TrkB) receptors was studied by immunohistochemical analysis on soleus (SOL) and gastrocnemius (GAS). BDNF and activated TrkB levels were also measured in muscle homogenates using Western blot analysis and/or Elisa tests. The results revealed BDNF immunostaining in all cell types examined with a prominent staining in endothelial cells and a stronger staining in type II than type I muscular fibers. Endothelial cells but not other cells displayed easily detectable activated TrkB receptor expression. Levels of BDNF and activated TrkB receptors were higher in SOL than GAS. In conclusion, endothelial cells are an important and still unexplored source of BDNF present in skeletal muscle. Endothelial BDNF expression likely explains why oxidative muscle exhibits higher BDNF levels than glycolytic muscle despite higher the BDNF expression by type II fibers.

Endocytic BDNF secretion regulated by Vamp3 in astrocytes

Brain-derived neurotrophic factor (BDNF) regulates diverse brain functions via TrkB receptor signaling. Due to the expression of TrkB receptors, astrocytes can internalize extracellular BDNF proteins via receptor-mediated endocytosis. Endocytosed BDNF can be re-secreted upon stimulation, but the molecular mechanism underlying this phenomenon remains unrecognized. Our study reveals that vesicle-associated membrane protein 3 (Vamp3) selectively regulates the release of endocytic BDNF from astrocytes. By using quantum dot (QD)-conjugated mature BDNF (QD-BDNF) as a proxy for the extracellular BDNF protein, we monitored the uptake, transport, and secretion of BDNF from cultured cortical astrocytes. Our data showed that endocytic QD-BDNF particles were enriched in Vamp3-containing vesicles in astrocytes and that ATP treatment sufficiently triggered either the antero- or retrograde transport and exocytosis of QD-BDNF-containing vesicles. Downregulation of Vamp3 expression disrupted endocytic BDNF secretion from astrocytes but did not affect uptake or transport. Collectively, these results provide evidence of the selective ability of astrocytic Vamp3 to control endocytic BDNF secretion during BDNF recycling.

Physiological basis underlying antidepressant-induced activation of TrkB receptors

We show that both pharmacological and non-pharmacological treatments of depression activate TrkB receptors - a well-established target of antidepressants - by inducing a physiological response coupled to sedation. Several rapid-acting antidepressants trigger TrkB signaling by evoking a state associated with electroencephalographic slow-wave activity, behavioral immobility, reduced cerebral glucose utilization, and lowered body temperature. Remarkably, antidepressant-induced TrkB signaling was not compromised in animals exhibiting reduced activity-dependent release of BDNF but was diminished by maintaining animals in warm ambient temperature. Most importantly, prevention of the hypothermic response attenuated the behavioral effects produced by rapid-acting antidepressant nitrous oxide. Our results suggest that the phenomenon underlying TrkB transactivation - changes in energy expenditure and thermoregulation - is essential, but not sufficient, for antidepressant responses. Indeed, regardless of differential clinical and pharmacodynamic properties, all drugs that disrupt energy metabolism and induce hypothermia activated TrkB. This study challenges pharmacology-centric hypotheses regarding antidepressant effects and highlight the role of complex changes in bioenergetics and thermoregulation.

Vamp3-dependent secretion of endocytic BDNF from astrocytes

Brain-derived neurotrophic factor (BDNF) regulates diverse brain functions via TrkB receptor signaling. Due to the expression of TrkB receptors, astrocytes can internalize extracellular BDNF proteins via receptor-mediated endocytosis. Endocytosed BDNF can be re-secreted upon stimulation, but the molecular mechanism underlying this phenomenon remains unrecognized. Our study reveals that vesicle-associated membrane protein 3 (Vamp3) selectively regulates the endocytic release of BDNF from astrocytes. By using quantum dot (QD)-conjugated mature BDNF (QD-BDNF) as a proxy for the extracellular BDNF protein, we monitored the uptake, transport, and secretion of BDNF from cultured cortical astrocytes. Our data showed that endocytic QD-BDNF particles were enriched in Vamp3-containing vesicles in astrocytes and that ATP treatment sufficiently triggered either the antero- or retrograde transport and exocytosis of QD-BDNF-containing vesicles. Downregulation of Vamp3 expression disrupted endocytic BDNF secretion from astrocytes but did not affect uptake or transport. Collectively, these results provide evidence of the selective ability of astrocytic Vamp3 to control endocytic BDNF secretion during BDNF recycling.

Perfluoroalkyl Substance Exposure and the BDNF Pathway in the Placental Trophoblast

BackgroundPer- and polyfluoroalkyl substances (PFAS) are persistent organic pollutants that have become globally ubiquitous in humans and the environment. In utero PFAS exposure is associated with neurodevelopmental effects; however, the mechanism is poorly understood. Brain-derived neurotrophic factor (BDNF) signaling is critical to fetal neurodevelopment during pregnancy and maintains important regulatory roles later in life. This study aims to characterize placental BDNF signaling and investigate whether PFAS exposure disrupts the signaling pathway in placental trophoblast cells.MethodsThe expression and localization of BDNF receptors–p75NTR and TrkB–in first trimester and term human placentas and trophoblast cells were investigated by immunofluorescence staining. To assess the effects of PFAS exposure on the BDNF pathway, BeWo cells were treated with PFAS mixtures that mimicked blood levels in a highly exposed population and major PFAS compounds in the mixture at 0.01, 0.1, 1, and 10 µM concentrations. Changes in pro-BDNF levels and phosphorylation of TrkB receptors were examined by Western blot.ResultsIn first trimester human placentas, TrkB and p75NTR receptors were primarily localized to syncytiotrophoblast and cytotrophoblast cells. At term, TrkB and p75NTR receptors were primarily observed in the placental villous stroma. TrkB receptor staining in trophoblasts was reduced at term, while p75NTR receptor staining was negative. TrkB receptors were confined to the nuclear and perinuclear spaces, and phosphorylation occurred at the Tyr816 residue in BeWo cells. Exposure to PFOS, PFOA, PFBS, and the six-PFAS mixture did not significantly affect BDNF levels or activation (phosphorylation) of TrkB. Treating cells with 1 μM and 10 μM of PFNA resulted in increased TrkB phosphorylation compared to unexposed controls, but BDNF levels were unchanged.ConclusionsBDNF receptors are present in different regions of human placental villi, indicating diverse functions of BDNF signaling in placental development. Our findings suggest that the BDNF pathway in placental trophoblast cells is not disrupted by exposures to PFOS, PFOA, PFBS, and a PFAS mixture, but may be affected by PFNA exposures. Further investigation is needed on how PFAS affects other critical signaling pathways during fetal neurodevelopment.

Depletion of TrkB Receptors From Adult Serotonergic Neurons Increases Brain Serotonin Levels, Enhances Energy Metabolism and Impairs Learning and Memory

Neurotrophin brain-derived neurotrophic factor (BDNF) and neurotransmitter serotonin (5-HT) regulate each other and have been implicated in several neuronal mechanisms, including neuroplasticity. We have investigated the effects of BDNF on serotonergic neurons by deleting BDNF receptor TrkB from serotonergic neurons in the adult brain. The transgenic mice show increased 5-HT and Tph2 levels with abnormal behavioral phenotype. In spite of increased food intake, the transgenic mice are significantly leaner than their wildtype littermates, which may be due to increased metabolic activity. Consistent with increased 5-HT, the proliferation of hippocampal progenitors is significantly increased, however, long-term survival of newborn cells is unchanged. Our data indicates that BDNF-TrkB signaling regulates the functional phenotype of 5-HT neurons with long-term behavioral consequences.

Adolescent fluoxetine treatment mediates a persistent anxiety-like outcome in female C57BL/6 mice that is ameliorated by fluoxetine re-exposure in adulthood

The objective of this study was to evaluate whether juvenile fluoxetine (FLX) exposure induces long-term changes in baseline responses to anxiety-inducing environments, and if so, whether its re-exposure in adulthood would ameliorate this anxiety-like phenotype. An additional goal was to assess the impact of adolescent FLX pretreatment, and its re-exposure in adulthood, on serotonin transporters (5-HTT) and brain-derived-neurotrophic-factor (BDNF)-related signaling markers (TrkB-ERK1/2-CREB-proBDNF-mBDNF) within the hippocampus and prefrontal cortex. To do this, female C57BL/6 mice were exposed to FLX in drinking water during postnatal-days (PD) 35–49. After a 21-day washout-period (PD70), mice were either euthanized (tissue collection) or evaluated on anxiety-related tests (open field, light/dark box, elevated plus-maze). Juvenile FLX history resulted in a persistent avoidance-like profile, along with decreases in BDNF-signaling markers, but not 5-HTTs or TrkB receptors, within both brain regions. Interestingly, FLX re-exposure in adulthood reversed the enduring FLX-induced anxiety-related responses across all behavioral tasks, while restoring ERK2-CREB-proBDNF markers to control levels and increasing mBDNF within the prefrontal cortex, but not the hippocampus. Collectively, these results indicate that adolescent FLX history mediates neurobehavioral adaptations that endure into adulthood, which are indicative of a generalized anxiety-like phenotype, and that this persistent effect is ameliorated by later-life FLX re-exposure, in a prefrontal cortex-specific manner.

Rab10 regulates the sorting of internalised TrkB to retrograde axonal transport

The extreme, complex morphology of neurons provides an unrivalled model to study the coordination between local signalling and long-range cell responses. A cogent example is provided by the binding of brain-derived neurotrophic factor (BDNF) to its receptor TrkB, which triggers signalling cascades at axon terminals that result in responses at the level of the cell body, including modulation of gene expression. Retrograde propagation of these critical signals relies on the sorting of activated TrkB receptors to retrograde axonal transport organelles termed signalling endosomes. In this work, we show that the small GTPase Rab10 is critical for the sorting of activated TrkB receptors to axonal retrograde carriers and the propagation of neurotrophin signalling from the axon terminal to the soma. Moreover, our data indicate that Rab10 defines a novel class of axonal organelles that are mobilised towards the axon terminal upon BDNF stimulation, thus enabling the axon to dynamically adjust the retrograde signalling flow to changes in BDNF availability at the synapse.