Prominent Postsynaptic and Dendritic Exocytosis of Endogenous BDNF Vesicles in BDNF-GFP Knock-in Mice

Clustering of synaptic engram: Functional and structural basis of memory

Studies on memory engram have demonstrated how experience and learning can be allocated at a neuronal level for centuries. Recently emerging evidence narrowed down further to the synaptic connections and their patterned allocation on dendrites. Notably, groups of synapses within a specific range within dendrites known as 'synaptic clusters' have been revealed in association with learning and memory. Previous investigations have shown that a variety of factors mediated by both presynaptic inputs and postsynaptic dendrites contribute to clustering. Here, we review the neural mechanism of synaptic clustering and its correlation with memory. We highlight the recent findings about the clustering of synaptic engrams and memory formation and discuss future directions.

Retrograde adenosine/A2A receptor signaling facilitates excitatory synaptic transmission and seizures

Retrograde signaling at the synapse is a fundamental way by which neurons communicate and neuronal circuit function is fine-tuned upon activity. While long-term changes in neurotransmitter release commonly rely on retrograde signaling, the mechanisms remain poorly understood. Here, we identified adenosine/A2A receptor (A2AR) as a retrograde signaling pathway underlying presynaptic long-term potentiation (LTP) at a hippocampal excitatory circuit critically involved in memory and epilepsy. Transient burst activity of a single dentate granule cell induced LTP of mossy cell synaptic inputs, a BDNF/TrkB-dependent form of plasticity that facilitates seizures. Postsynaptic TrkB activation released adenosine from granule cells, uncovering a non-conventional BDNF/TrkB signaling mechanism. Moreover, presynaptic A2ARs were necessary and sufficient for LTP. Lastly, seizure induction released adenosine in a TrkB-dependent manner, while removing A2ARs or TrkB from the dentate gyrus had anti-convulsant effects. By mediating presynaptic LTP, adenosine/A2AR retrograde signaling may modulate dentate gyrus-dependent learning and promote epileptic activity.

Neurotrophin signalling in the human nervous system

This review focuses on neurotrophins and their tyrosine kinase receptors, with an emphasis on their relevance to the function and dysfunction in the human nervous system. It also deals with measurements of BDNF levels and highlights recent findings from our laboratory on TrkB and TrkC signalling in human neurons. These include ligand selectivity and Trk activation by neurotrophins and non-neurotrophin ligands. The ligand-induced down-regulation and re-activation of Trk receptors is also discussed.

CLARITY increases sensitivity and specificity of fluorescence immunostaining in long-term archived human brain tissue

BACKGROUND

Post mortem human brain tissue is an essential resource to study cell types, connectivity as well as subcellular structures down to the molecular setup of the central nervous system especially with respect to the plethora of brain diseases. A key method is immunostaining with fluorescent dyes, which allows high-resolution imaging in three dimensions of multiple structures simultaneously. Although there are large collections of formalin-fixed brains, research is often limited because several conditions arise that complicate the use of human brain tissue for high-resolution fluorescence microscopy.

RESULTS

In this study, we developed a clearing approach for immunofluorescence-based analysis of perfusion- and immersion-fixed post mortem human brain tissue, termed human Clear Lipid-exchanged Acrylamide-hybridized Rigid Imaging / Immunostaining / In situ hybridization-compatible Tissue-hYdrogel (hCLARITY). hCLARITY is optimized for specificity by reducing off-target labeling and yields very sensitive stainings in human brain sections allowing for super-resolution microscopy with unprecedented imaging of pre- and postsynaptic compartments. Moreover, hallmarks of Alzheimer's disease were preserved with hCLARITY, and importantly classical 3,3'-diaminobenzidine (DAB) or Nissl stainings are compatible with this protocol. hCLARITY is very versatile as demonstrated by the use of more than 30 well performing antibodies and allows for de- and subsequent re-staining of the same tissue section, which is important for multi-labeling approaches, e.g., in super-resolution microscopy.

CONCLUSIONS

Taken together, hCLARITY enables research of the human brain with high sensitivity and down to sub-diffraction resolution. It therefore has enormous potential for the investigation of local morphological changes, e.g., in neurodegenerative diseases.

Identification of Novel BDNF-Specific Corticostriatal Circuitries

Brain-derived neurotrophic factor (BDNF) is released from axon terminals originating in the cerebral cortex onto striatal neurons. Here, we characterized BDNF neurons in the corticostriatal circuitry. First, we used BDNF-Cre and Ribotag transgenic mouse lines to label BDNF-positive neurons in the cortex and detected BDNF expression in all the subregions of the prefrontal cortex (PFC). Next, we used a retrograde viral tracing strategy, in combination with BDNF-Cre knock-in mice, to map the cortical outputs of BDNF neurons in the dorsomedial and dorsolateral striatum (DMS and DLS, respectively). We found that BDNF-expressing neurons located in the medial prefrontal cortex (mPFC) project mainly to the DMS, and those located in the primary and secondary motor cortices (M1 and M2, respectively) and agranular insular cortex (AI) project mainly to the DLS. In contrast, BDNF-expressing orbitofrontal cortical (OFC) neurons differentially target the dorsal striatum (DS) depending on their mediolateral and rostrocaudal location. Specifically, the DMS is mainly innervated by the medial and ventral part of the orbitofrontal cortex (MO and VO, respectively), whereas the DLS receives projections specifically from the lateral part of the OFC (LO). Together, our study uncovers previously unknown BDNF corticostriatal circuitries. These findings could have important implications for the role of BDNF signaling in corticostriatal pathways.

The Role of BDNF as a Biomarker in Cognitive and Sensory Neurodegeneration

Brain-derived neurotrophic factor (BDNF) has a crucial function in the central nervous system and in sensory structures including olfactory and auditory systems. Many studies have highlighted the protective effects of BDNF in the brain, showing how it can promote neuronal growth and survival and modulate synaptic plasticity. On the other hand, conflicting data about BDNF expression and functions in the cochlear and in olfactory structures have been reported. Several clinical and experimental research studies showed alterations in BDNF levels in neurodegenerative diseases affecting the central and peripheral nervous system, suggesting that BDNF can be a promising biomarker in most neurodegenerative conditions, including Alzheimer's disease, shearing loss, or olfactory impairment. Here, we summarize current research concerning BDNF functions in brain and in sensory domains (olfaction and hearing), focusing on the effects of the BDNF/TrkB signalling pathway activation in both physiological and pathological conditions. Finally, we review significant studies highlighting the possibility to target BDNF as a biomarker in early diagnosis of sensory and cognitive neurodegeneration, opening new opportunities to develop effective therapeutic strategies aimed to counteract neurodegeneration.

Brain-derived neurotrophic factor expression in serotonergic neurons improves stress resilience and promotes adult hippocampal neurogenesis

The neurotrophin brain-derived neurotrophic factor (BDNF) stimulates adult neurogenesis, but also influences structural plasticity and function of serotonergic neurons. Both, BDNF/TrkB signaling and the serotonergic system modulate behavioral responses to stress and can lead to pathological states when dysregulated. The two systems have been shown to mediate the therapeutic effect of antidepressant drugs and to regulate hippocampal neurogenesis. To elucidate the interplay of both systems at cellular and behavioral levels, we generated a transgenic mouse line that overexpresses BDNF in serotonergic neurons in an inducible manner. Besides displaying enhanced hippocampus-dependent contextual learning, transgenic mice were less affected by chronic social defeat stress (CSDS) compared to wild-type animals. In parallel, we observed enhanced serotonergic axonal sprouting in the dentate gyrus and increased neural stem/progenitor cell proliferation, which was uniformly distributed along the dorsoventral axis of the hippocampus. In the forced swim test, BDNF-overexpressing mice behaved similarly as wild-type mice treated with the antidepressant fluoxetine. Our data suggest that BDNF released from serotonergic projections exerts this effect partly by enhancing adult neurogenesis. Furthermore, independently of the genotype, enhanced neurogenesis positively correlated with the social interaction time after the CSDS, a measure for stress resilience.

Brain-derived neurotrophic factor-induced regulation of RNA metabolism in neuronal development and synaptic plasticity

The neurotrophin brain-derived neurotrophic factor (BDNF) plays multiple roles in the nervous system, including in neuronal development, in long-term synaptic potentiation in different brain regions, and in neuronal survival. Alterations in these regulatory mechanisms account for several diseases of the nervous system. The synaptic effects of BDNF mediated by activation of tropomyosin receptor kinase B (TrkB) receptors are partly mediated by stimulation of local protein synthesis which is now considered a ubiquitous feature in both presynaptic and postsynaptic compartments of the neuron. The capacity to locally synthesize proteins is of great relevance at several neuronal developmental stages, including during neurite development, synapse formation, and stabilization. The available evidence shows that the effects of BDNF-TrkB signaling on local protein synthesis regulate the structure and function of the developing and mature synapses. While a large number of studies have illustrated a wide range of effects of BDNF on the postsynaptic proteome, a growing number of studies also point to presynaptic effects of the neurotrophin in the local regulation of the protein composition at the presynaptic level. Here, we will review the latest evidence on the role of BDNF in local protein synthesis, comparing the effects on the presynaptic and postsynaptic compartments. Additionally, we overview the relevance of BDNF-associated local protein synthesis in neuronal development and synaptic plasticity, at the presynaptic and postsynaptic compartments, and their relevance in terms of disease. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications RNA Export and Localization > RNA Localization.

Computation, wiring, and plasticity in synaptic clusters

Synaptic clusters on dendrites are extraordinarily compact computational building blocks. They contribute to key local computations through biophysical and biochemical signaling that utilizes convergence in space and time as an organizing principle. However, these computations can only arise in very special contexts. Dendritic cluster computations, their highly organized input connectivity, and the mechanisms for their formation are closely linked, yet these have not been analyzed as parts of a single process. Here, we examine these linkages. The sheer density of axonal and dendritic arborizations means that there are far more potential connections (close enough for a spine to reach an axon) than actual ones. We see how dendritic clusters draw upon electrical, chemical, and mechano-chemical signaling to implement the rules for formation of connections and subsequent computations. Crucially, the same mechanisms that underlie their functions also underlie their formation.

Stress-Related Dysfunction of Adult Hippocampal Neurogenesis-An Attempt for Understanding Resilience?

Newborn neurons in the adult hippocampus are regulated by many intrinsic and extrinsic cues. It is well accepted that elevated glucocorticoid levels lead to downregulation of adult neurogenesis, which this review discusses as one reason why psychiatric diseases, such as major depression, develop after long-term stress exposure. In reverse, adult neurogenesis has been suggested to protect against stress-induced major depression, and hence, could serve as a resilience mechanism. In this review, we will summarize current knowledge about the functional relation of adult neurogenesis and stress in health and disease. A special focus will lie on the mechanisms underlying the cascades of events from prolonged high glucocorticoid concentrations to reduced numbers of newborn neurons. In addition to neurotransmitter and neurotrophic factor dysregulation, these mechanisms include immunomodulatory pathways, as well as microbiota changes influencing the gut-brain axis. Finally, we discuss recent findings delineating the role of adult neurogenesis in stress resilience.

Long-term depression at hippocampal mossy fiber-CA3 synapses involves BDNF but is not mediated by p75NTR signaling

BDNF plays a crucial role in the regulation of synaptic plasticity. It is synthesized as a precursor (proBDNF) that can be proteolytically cleaved to mature BDNF (mBDNF). Previous studies revealed a bidirectional mode of BDNF actions, where long-term potentiation (LTP) was mediated by mBDNF through tropomyosin related kinase (Trk) B receptors whereas long-term depression (LTD) depended on proBDNF/p75 neurotrophin receptor (p75NTR) signaling. While most experimental evidence for this BDNF dependence of synaptic plasticity in the hippocampus was derived from Schaffer collateral (SC)-CA1 synapses, much less is known about the mechanisms of synaptic plasticity, in particular LTD, at hippocampal mossy fiber (MF) synapses onto CA3 neurons. Since proBDNF and mBDNF are expressed most abundantly at MF-CA3 synapses in the rodent brain and we had shown previously that MF-LTP depends on mBDNF/TrkB signaling, we now explored the role of proBDNF/p75NTR signaling in MF-LTD. Our results show that neither acute nor chronic inhibition of p75NTR signaling impairs MF-LTD, while short-term plasticity, in particular paired-pulse facilitation, at MF-CA3 synapses is affected by a lack of functional p75NTR signaling. Furthermore, MF-CA3 synapses showed normal LTD upon acute inhibition of TrkB receptor signaling. Nonetheless, acute inhibition of plasminogen activator inhibitor-1 (PAI-1), an inhibitor of both intracellular and extracellular proBDNF cleavage, impaired MF-LTD. This seems to indicate that LTD at MF-CA3 synapses involves BDNF, however, MF-LTD does not depend on p75NTRs. Altogether, our experiments demonstrate that p75NTR signaling is not warranted for all glutamatergic synapses but rather needs to be checked separately for every synaptic connection.

Function of Drosophila Synaptotagmins in membrane trafficking at synapses

The Synaptotagmin (SYT) family of proteins play key roles in regulating membrane trafficking at neuronal synapses. Using both Ca²⁺-dependent and Ca²⁺-independent interactions, several SYT isoforms participate in synchronous and asynchronous fusion of synaptic vesicles (SVs) while preventing spontaneous release that occurs in the absence of stimulation. Changes in the function or abundance of the SYT1 and SYT7 isoforms alter the number and route by which SVs fuse at nerve terminals. Several SYT family members also regulate trafficking of other subcellular organelles at synapses, including dense core vesicles (DCV), exosomes, and postsynaptic vesicles. Although SYTs are linked to trafficking of multiple classes of synaptic membrane compartments, how and when they interact with lipids, the SNARE machinery and other release effectors are still being elucidated. Given mutations in the SYT family cause disorders in both the central and peripheral nervous system in humans, ongoing efforts are defining how these proteins regulate vesicle trafficking within distinct neuronal compartments. Here, we review the Drosophila SYT family and examine their role in synaptic communication. Studies in this invertebrate model have revealed key similarities and several differences with the predicted activity of their mammalian counterparts. In addition, we highlight the remaining areas of uncertainty in the field and describe outstanding questions on how the SYT family regulates membrane trafficking at nerve terminals.

Hypothalamic gene transfer of BDNF promotes healthy aging

The aging process and age-related diseases all involve metabolic decline and impaired ability to cope with adversity. Environmental enrichment (EE)-a housing environment which recapitulates aspects of active lifestyle-exerts a wide range of health benefits in laboratory rodents. Brain-derived neurotrophic factor (BDNF) in the hypothalamus orchestrates autonomic and neuroendocrine processes, serving as one key brain mediator of EE-induced resistance to obesity, cancer, and autoimmunity. Recombinant adeno-associated virus (AAV)-mediated hypothalamic BDNF gene transfer alleviates obesity, diabetes, and metabolic syndromes in both diet-induced and genetic models. One recent study by our lab demonstrates the efficacy and safety of a built-in autoregulatory system to control transgene BDNF expression, mimicking the body's natural feedback systems in middle-age mice. Twelve-month old mice were treated with autoregulatory BDNF vector and monitored for 7months. BDNF gene transfer prevented age-associated metabolic decline by: reducing adiposity, preventing the decline of brown fat activity, increasing adiponectin while reducing leptin and insulin in circulation, improving glucose tolerance, increasing energy expenditure, alleviating hepatic steatosis, and suppressing inflammatory genes in the hypothalamus and adipose tissues. Furthermore, BDNF treatment reduced anxiety-like and depression-like behaviors. This chapter summarizes this work and discusses potential roles that hypothalamic BDNF might play in promoting healthy aging.

The physiology of regulated BDNF release

The neurotrophic factor BDNF is an important regulator for the development of brain circuits, for synaptic and neuronal network plasticity, as well as for neuroregeneration and neuroprotection. Up- and downregulations of BDNF levels in human blood and tissue are associated with, e.g., neurodegenerative, neurological, or even cardiovascular diseases. The changes in BDNF concentration are caused by altered dynamics in BDNF expression and release. To understand the relevance of major variations of BDNF levels, detailed knowledge regarding physiological and pathophysiological stimuli affecting intra- and extracellular BDNF concentration is important. Most work addressing the molecular and cellular regulation of BDNF expression and release have been performed in neuronal preparations. Therefore, this review will summarize the stimuli inducing release of BDNF, as well as molecular mechanisms regulating the efficacy of BDNF release, with a focus on cells originating from the brain. Further, we will discuss the current knowledge about the distinct stimuli eliciting regulated release of BDNF under physiological conditions.

Journey of brain-derived neurotrophic factor: from intracellular trafficking to secretion

Brain-derived neurotrophic factor (BDNF) is known to control a wide variety of brain functions, ranging from memory formation to food intake. However, since the BDNF levels are extremely low in the nervous system, the dynamics in neurons from intracellular trafficking to secretion is absolutely complicated; the understanding is not fully promoted. We here review the findings of those critical mechanisms from intracellular trafficking to the secretion of BDNF. Furthermore, to solve this issue, technological advances for the detection, measurement, and imaging of this growth factor are essential. We believe that this review helps the study of these complex but critical mechanisms of BDNF.

Lactate and BDNF: Key Mediators of Exercise Induced Neuroplasticity?

Accumulating evidence from animal and human studies supports the notion that physical exercise can enhance neuroplasticity and thus reduce the risk of several neurodegenerative diseases (e.g., dementia). However, the underlying neurobiological mechanisms of exercise induced neuroplasticity are still largely unknown. One potential mediator of exercise effects is the neurotrophin BDNF, which enhances neuroplasticity via different pathways (e.g., synaptogenesis, neurogenesis, long-term potentiation). Current research has shown that (i) increased peripheral lactate levels (following high intensity exercise) are associated with increased peripheral BDNF levels, (ii) lactate infusion at rest can increase peripheral and central BDNF levels and (iii) lactate plays a very complex role in the brain’s metabolism. In this review, we summarize the role and relationship of lactate and BDNF in exercise induced neuroplasticity.

BDNF impact on synaptic dynamics: extra or intracellular long-term release differently regulates cultured hippocampal synapses

Brain Derived Neurotrophic Factor (BDNF) signalling contributes to the formation, maturation and plasticity of Central Nervous System (CNS) synapses. Acute exposure of cultured brain circuits to BDNF leads to up-regulation of glutamatergic neuro-transmission, by the accurate tuning of pre and post synaptic features, leading to structural and functional synaptic changes. Chronic BDNF treatment has been comparatively less investigated, besides it may represent a therapeutic option to obtain rescue of post-injury alterations of synaptic networks. In this study, we used a paradigm of BDNF long-term (4 days) incubation to assess in hippocampal neurons in culture, the ability of such a treatment to alter synapses. By patch clamp recordings we describe the augmented function of excitatory neurotransmission and we further explore by live imaging the presynaptic changes brought about by long-term BDNF. In our study, exogenous long-term BDNF exposure of post-natal neurons did not affect inhibitory neurotransmission. We further compare, by genetic manipulations of cultured neurons and BDNF release, intracellular overexpression of this neurotrophin at the same developmental age. We describe for the first-time differences in synaptic modulation by BDNF with respect to exogenous or intracellular release paradigms. Such a finding holds the potential of influencing the design of future therapeutic strategies.

Direct current stimulation boosts hebbian plasticity in vitro

BACKGROUND

There is evidence that transcranial direct current stimulation (tDCS) can improve learning performance. Arguably, this effect is related to long term potentiation (LTP), but the precise biophysical mechanisms remain unknown.

HYPOTHESIS

We propose that direct current stimulation (DCS) causes small changes in postsynaptic membrane potential during ongoing endogenous synaptic activity. The altered voltage dynamics in the postsynaptic neuron then modify synaptic strength via the machinery of endogenous voltage-dependent Hebbian plasticity. This hypothesis predicts that DCS should exhibit Hebbian properties, namely pathway specificity and associativity.

METHODS

We studied the effects of DCS applied during the induction of LTP in the CA1 region of rat hippocampal slices and using a biophysical computational model.

RESULTS

DCS enhanced LTP, but only at synapses that were undergoing plasticity, confirming that DCS respects Hebbian pathway specificity. When different synaptic pathways cooperated to produce LTP, DCS enhanced this cooperation, boosting Hebbian associativity. Further slice experiments and computer simulations support a model where polarization of postsynaptic pyramidal neurons drives these plasticity effects through endogenous Hebbian mechanisms. The model is able to reconcile several experimental results by capturing the complex interaction between the induced electric field, neuron morphology, and endogenous neural activity.

CONCLUSIONS

These results suggest that tDCS can enhance associative learning. We propose that clinical tDCS should be applied during tasks that induce Hebbian plasticity to harness this phenomenon, and that the effects should be task specific through their interaction with endogenous plasticity mechanisms. Models that incorporate brain state and plasticity mechanisms may help to improve prediction of tDCS outcomes.

A New Mouse Line Reporting the Translation of Brain-Derived Neurotrophic Factor Using Green Fluorescent Protein

While BDNF is receiving considerable attention for its role in synaptic plasticity and in nervous system dysfunction, identifying brain circuits involving BDNF-expressing neurons has been challenging. BDNF levels are very low in most brain areas, except for the large mossy fiber terminals in the hippocampus where BDNF accumulates at readily detectable levels. This report describes the generation of a mouse line allowing the detection of single brain cells synthesizing BDNF. A bicistronic construct encoding BDNF tagged with a P2A sequence preceding GFP allows the translation of BDNF and GFP as separate proteins. Following its validation with transfected cells, this construct was used to replace the endogenous Bdnf gene. Viable and fertile homozygote animals were generated, with the GFP signal marking neuronal cell bodies translating the Bdnf mRNA. Importantly, the distribution of immunoreactive BDNF remained unchanged, as exemplified by its accumulation in mossy fiber terminals in the transgenic animals. GFP-labeled neurons could be readily visualized in distinct layers in the cerebral cortex where BDNF has been difficult to detect with currently available reagents. In the hippocampal formation, quantification of the GFP signal revealed that <10% of the neurons do not translate the Bdnf mRNA at detectable levels, with the highest proportion of strongly labeled neurons found in CA3.

Genetic Increases in Olfactory Bulb BDNF Do Not Enhance Survival of Adult-Born Granule Cells

Adult-born neurons produced in the dentate gyrus subgranular zone (SGZ) develop as excitatory hippocampal granule cells (GCs), while those from the subventricular zone (SVZ) migrate to the olfactory bulb (OB), where most develop as GABAergic olfactory GCs. Both types of neurons express TrkB as they mature. Normally ~50% of new olfactory GCs survive, but survival declines if sensory drive is reduced. Increases in endogenous brain-derived neurotrophic factor (BDNF) in hippocampus, particularly with wheel running, enhance dentate GC survival. Whether survival of new olfactory GCs is impacted by augmenting BDNF in the OB, where they mature and integrate, is not known. Here, we determined if increasing OB BDNF expression enhances survival of new GCs, and if it counters their loss under conditions of reduced sensory activity. Neurogenesis was assessed under normal conditions, and following unilateral naris occlusion, in mice overexpressing BDNF in the granule cell layer (GCL). OB BDNF levels were significantly higher in transgenic mice compared to controls, and this was maintained following sensory deprivation. Bromodeoxyuridine (BrdU) cell birth dating showed that at 12-14 days post-BrdU, numbers of new GCs did not differ between genotypes, indicating normal recruitment to the OB. At later intervals, transgenic and control mice showed levels of GC loss in deprived and nondeprived animals that were indistinguishable, as was the incidence of apoptotic cells in the GCL. These results demonstrate that, in contrast to new dentate GCs, elevations in endogenous BDNF do not enhance survival of adult-born olfactory GCs.