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| Morphological and physiological properties of the hippocampal mossy fiber pathway. (A) Three-dimensional visualization of the anatomical location of the dorsal hippocampus in mouse brain. (B) Schematic representation of hippocampal sub-regions, with emphasis on the input and output to and from the DG. (C) Schematic representation of the unique anatomical and morphological structure of the MF-CA3 synapse. Insert: mossy fiber bouton (MFB, orange) and postsynaptic thorny excrescence (TE, light green). (D) A representative confocal image of the hippocampus following injection of AAV-DIO-EF1a-tdTomato into the DG of a Prox1-cre transgenic mouse (top). The images below show the MF tract at higher magnification (tdTomato, orange), following immunolabeling for VGluT1 (VGluT1, cyan), and demonstrate the size differences between the large MF terminals (white arrows) and the small S.R. terminals (magenta arrows). Scale bars represent 200, 10 and 2 µm for the top, right column and left column images, respectively. (E,F) MF-CA3 short-term plasticity demonstrated by measurements of paired-pulse (E) and a high-frequency burst (F) stimulation, delivered electrically to the DG while recording fEPSPs from the S.L. (G,H) MF-CA3 long-term plasticity demonstrated by measurements of FSK-(G) and tetanus-(H) induced potentiation, with subsequent application of DCG-IV, blocking synaptic transmission. (I). MF-CA3 long-term plasticity (LTD) following a prolonged low-frequency stimulation, with subsequent application of DCG-IV, blocking synaptic transmission. Images in (A) were adapted from the Allen institute's Brain Explorer 2 (http://mouse.brain-map.org/static/brainexplorer).
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Cyclic adenosine monophosphate (cAMP) is a crucial second messenger involved in both pre- and postsynaptic plasticity in many neuronal types across species. In the hippocampal mossy fiber (MF) synapse, cAMP mediates presynaptic long-term potentiation and depression. The main cAMP-dependent signaling pathway linked to MF synaptic plasticity acts via...
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... to this notion of hippocampal information flow, neuronal activity originating in the adjacent entorhinal cortex (EC) is relayed via the perforant pathway (PP), primarily to the hippocampal dentate gyrus (DG), where it is processed and relayed to CA3 and CA2 pyramidal neurons via the mossy fibers (MF) pathway. From these regions, which are internally connected in an auto-associative network, the information is delivered almost exclusively to CA1 pyramidal neurons via Schaffer's collaterals (SC), which finally redistribute the processed signals across cortical and sub-cortical regions (Figures 1A,B; Lieberman, 1965;Witter, 2007). The MF synapse, corresponding to the second synapse in this circuit, is generally considered to be an important locus for the formation, storage and retrieval of contextual and episodic memories in mammals (Lieberman, 1965;Neves et al., 2008;Lisman et al., 2017), through a computational process termed "pattern separation" ( Leutgeb et al., 2007;Schmidt et al., 2012;Rolls, 2013). ...
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... MF pathway is characterized by non-myelinated axons that originate in dentate gyrus granule cells (DGCs) and travel immediately above and below the CA3 Stratum Pyramidale (S.P.), forming the Stratum Lucidum (S.L.). There, each axon forms about a dozen enormous synapses (up to several micrometers in diameter (Rollenhagen et al., 2007), termed mossy fiber boutons (MFB), onto the large proximal dendritic spines of CA3 neurons termed thorny excrescences (TEs) (Amaral and Dent, 1981; Figures 1C,D). A single large MFB contains 25 active zones on average and harbors some 16,000 synaptic vesicles, of which only about 600 are located within 60 nm from the AZ and are considered part of the readily releasable pool of vesicles, while an additional 4,000 vesicles are found at a short distance of 200 nm from the AZ and are considered part of the recycling pool ( Hallermann et al., 2003;Rollenhagen et al., 2007). ...
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... contrast to its prominent size, the MF synapse is characterized by a very low basal P r ( Jonas et al., 1993) and as a result, following a single action potential (AP) elicits weak excitatory post-synaptic potentials (EPSPs) in CA3 neurons ( Lysetskiy et al., 2005). However, following a short train of high-frequency APs, the accumulation of Ca 2+ in the MF synapse produces a dramatic increase in P r , manifested as robust short-term synaptic facilitation (Figures 1E,F). This facilitation, together with the strategic location of the synapse in proximity to the CA3 somata, and its multiple release sites, allow a single MF synapse to elicit APs in its postsynaptic target following a short train of APs ( Henze et al., 2000;Evstratova and Tóth, 2014;Chamberland et al., 2018). ...
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... LTP ( Zalutsky and Nicoll, 1990;Johnston et al., 1992;Huang et al., 1994;Salin et al., 1996b;Castillo, 2012). Long-term potentiation in the MF synapse (MF-LTP, Figure 1G) manifests as a long-term increase in the presynaptic P r and is mediated by cAMP, evident by the robust potentiation observed also following application of the adenylyl cyclase (AC) agonist forskolin (FSK, Figure 1H; Weisskopf et al., 1994;Salin et al., 1996b;Villacres et al., 1998;Castillo, 2012). Using pharmacological tools that control cAMP levels, it was demonstrated that both cAMP and PKA are important for the induction and maintenance of MF-LTP ( Huang et al., 1994;Weisskopf et al., 1994). ...
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... LTP ( Zalutsky and Nicoll, 1990;Johnston et al., 1992;Huang et al., 1994;Salin et al., 1996b;Castillo, 2012). Long-term potentiation in the MF synapse (MF-LTP, Figure 1G) manifests as a long-term increase in the presynaptic P r and is mediated by cAMP, evident by the robust potentiation observed also following application of the adenylyl cyclase (AC) agonist forskolin (FSK, Figure 1H; Weisskopf et al., 1994;Salin et al., 1996b;Villacres et al., 1998;Castillo, 2012). Using pharmacological tools that control cAMP levels, it was demonstrated that both cAMP and PKA are important for the induction and maintenance of MF-LTP ( Huang et al., 1994;Weisskopf et al., 1994). ...
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... addition to MF-LTP, which is induced by a short train of high-frequency activity, long-term depression in the MF synapse (MF-LTD) can also be elicited, by applying a prolonged (15 min) low-frequency stimulation ( Figure 1I). Like MF-LTP, MF-LTD is also NMDAR-independent, however, it is mediated by a reduction in cAMP levels and is manifested as a decrease in P r ( Tzounopoulos et al., 1998;Kobayashi, 2010). ...
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... induction can be blocked by the metabotropic glutamate receptor (mGluR) antagonist MCPG ( Fitzjohn et al., 1998) or mGluR2/3 KO (Lyon et al., 2011). At the same time, application of the mGluR2/3 agonist DCG-IV completely blocks MF synaptic transmission synapses ( Figures 1G-I), while having little to no effect on other hippocampal synapses ( Yoshino et al., 1996;Kamiya and Ozawa, 1999). Since mGluR2/3 inhibits cAMP synthesis, it can be inferred that bidirectional changes in cAMP concentration control P r at the MF synapse and can lead to LTP or LTD, depending on the direction of the change. ...
Citations
... Conversely, low-frequency stimulation of mossy fibres induces presynaptic LTD by inactivating AC via metabotropic glutamate receptors (Tzounopoulos et al., 1998). PKA is considered to be by far the most major molecule that mediates LTP/LTD at the mossy fibre terminals, and the target molecules include many active zone and vesicular proteins such as Rab3a, synapsin, RIM1a, synaptotagmin and tomosyn (Shahoha et al., 2022). Some of these proteins are also implicated in memory defects in Drosophila (Chen et al., 2011;Knapek et al., 2010;Niewalda et al., 2015;Sachidanandan et al., 2023). ...
Activation of the cAMP pathway is one of the common mechanisms underlying long‐term potentiation (LTP). In the Drosophila mushroom body, simultaneous activation of odour‐coding Kenyon cells (KCs) and reinforcement‐coding dopaminergic neurons activates adenylyl cyclase in KC presynaptic terminals, which is believed to trigger synaptic plasticity underlying olfactory associative learning. However, learning induces long‐term depression (LTD) at these synapses, contradicting the universal role of cAMP as a facilitator of transmission. Here, we developed a system to electrophysiologically monitor both short‐term and long‐term synaptic plasticity at KC output synapses and demonstrated that they are indeed an exception in which activation of the cAMP–protein kinase A pathway induces LTD. Contrary to the prevailing model, our cAMP imaging found no evidence for synergistic action of dopamine and KC activity on cAMP synthesis. Furthermore, we found that forskolin‐induced cAMP increase alone was insufficient for plasticity induction; it additionally required simultaneous KC activation to replicate the presynaptic LTD induced by pairing with dopamine. On the other hand, activation of the cGMP pathway paired with KC activation induced slowly developing LTP, proving antagonistic actions of the two second‐messenger pathways predicted by behavioural study. Finally, KC subtype‐specific interrogation of synapses revealed that different KC subtypes exhibit distinct plasticity duration even among synapses on the same postsynaptic neuron. Thus, our work not only revises the role of cAMP in synaptic plasticity by uncovering the unexpected convergence point of the cAMP pathway and neuronal activity, but also establishes the methods to address physiological mechanisms of synaptic plasticity in this important model. image
Key points
Although presynaptic cAMP increase generally facilitates synapses, olfactory associative learning in Drosophila, which depends on dopamine and cAMP signalling genes, induces long‐term depression (LTD) at the mushroom body output synapses.
By combining electrophysiology, pharmacology and optogenetics, we directly demonstrate that these synapses are an exception where activation of the cAMP–protein kinase A pathway leads to presynaptic LTD.
Dopamine‐ or forskolin‐induced cAMP increase alone is not sufficient for LTD induction; neuronal activity, which has been believed to trigger cAMP synthesis in synergy with dopamine input, is required in the downstream pathway of cAMP.
In contrast to cAMP, activation of the cGMP pathway paired with neuronal activity induces presynaptic long‐term potentiation, which explains behaviourally observed opposing actions of transmitters co‐released by dopaminergic neurons.
Our work not only revises the role of cAMP in synaptic plasticity, but also provides essential methods to address physiological mechanisms of synaptic plasticity in this important model system.
... 69,70 Therefore, NaChBac expression may contribute to accelerated neuron maturation 41 or modulate neuronal plasticity. 71,72 In adult neuronal repair, the conditioning lesion paradigm (a phenomenon where a previous insult to the peripheral axon of DRG neurons leads to a switch in the regenerative capacity of the central axon) is mediated by the upregulated expression of several regeneration-associated genes after increased levels of intracellular Ca 2+ and cAMP within DRG neurons. [73][74][75][76] Since NaChBac expression increased both Ca 2+ and cAMP in DRGs, we hypothesized that NaChBac could induce a pro-regenerative state of DRG neurons transplanted within the SCI. ...
... In addition to tetanic stimulation, pharmacological activation of cAMP/PKA pathway with cAMP analogues or an adenylate cyclase activator, forskolin, has also been used for potentiation ("chemical potentiation") Midorikawa and Sakaba, 2017;Fukaya et al., 2021;Orlando et al., 2021). Although the chemical potentiation is robust and prevails throughout the preparation, it unlikely shares the induction pathway completely with mfPTP or mfLTP: Some presynaptic molecules, such as Rab3a and RIM1alpha, are responsible for mfLTP, not for chemical potentiation (Castillo et al., 1997(Castillo et al., , 2002, and vice versa (Shahoha et al., 2022). Vandael et al. (2020) induced mfPTP via a cell-attached presynaptic patch electrode and analyzed postsynaptic currents recorded from the paired CA3 pyramidal cell. ...
Presynaptic plasticity is an activity-dependent change in the neurotransmitter release and plays a key role in dynamic modulation of synaptic strength. Particularly, presynaptic potentiation mediated by cyclic adenosine monophosphate (cAMP) is widely seen across the animals and thought to contribute to learning and memory. Hippocampal mossy fiber-CA3 pyramidal cell synapses have been used as a model because of robust presynaptic potentiation in short- and long-term forms. Moreover, direct presynaptic recordings from large mossy fiber terminals allow one to dissect the potentiation mechanisms. Recently, super-resolution microscopy and flash-and-freeze electron microscopy have revealed the localizations of release site molecules and synaptic vesicles during the potentiation at a nanoscale, identifying the molecular mechanisms of the potentiation. Incorporating these growing knowledges, we try to present plausible mechanisms underlying the cAMP-mediated presynaptic potentiation.
... Despite about 2.2% of the world's population (more than 140 million people) living permanently at a HA [24], the impact of reduced SpO 2 on cognition remains poorly explored. SpO 2 values decrease with increasing altitude to a median of 96% (95)(96)(97) at 2500 m, 92% (90)(91)(92)(93) at 3600 m, 87% (85)(86)(87)(88)(89) at 4100, and 81% (78)(79)(80)(81)(82)(83)(84) at 5100 m, with increasing variability at higher altitudes [25]. The reduced SpO 2 at HAs induces a rapid increase in cerebral edema (HACE). ...
... In the brain, cAMP leads to increased expression of the N-methyl-D-aspartate (NMDA) receptor subunit GluN1 in the hippocampus, an area involved in learning and memory [84]. cAMP can also regulate the activity of various downstream effectors involved in synaptic plasticity and memory formation, such as the protein kinase A (PKA) [85] and the cAMP response elementbinding protein (CREB) [86]. Moreover, the cAMP signaling pathway can modulate other molecular targets in the brain, such as mitochondrial function and oxidative stress, by activating CREB and the peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1α) [87]. ...
The brain requires over one-fifth of the total body oxygen demand for normal functioning. At high altitude (HA), the lower atmospheric oxygen pressure inevitably challenges the brain, affecting voluntary spatial attention, cognitive processing, and attention speed after short-term, long-term, or lifespan exposure. Molecular responses to HA are controlled mainly by hypoxia-inducible factors. This review aims to summarize the cellular, metabolic, and functional alterations in the brain at HA with a focus on the role of hypoxia-inducible factors in controlling the hypoxic ventilatory response, neuronal survival, metabolism, neurogenesis, synaptogenesis, and plasticity.
... Despite about 2.2% of the world's population (more than 140 million people) living permanently at a HA [24], the impact of reduced SpO 2 on cognition remains poorly explored. SpO 2 values decrease with increasing altitude to a median of 96% (95)(96)(97) at 2500 m, 92% (90)(91)(92)(93) at 3600 m, 87% (85)(86)(87)(88)(89) at 4100, and 81% (78)(79)(80)(81)(82)(83)(84) at 5100 m, with increasing variability at higher altitudes [25]. The reduced SpO 2 at HAs induces a rapid increase in cerebral edema (HACE). ...
... In the brain, cAMP leads to increased expression of the N-methyl-D-aspartate (NMDA) receptor subunit GluN1 in the hippocampus, an area involved in learning and memory [84]. cAMP can also regulate the activity of various downstream effectors involved in synaptic plasticity and memory formation, such as the protein kinase A (PKA) [85] and the cAMP response elementbinding protein (CREB) [86]. Moreover, the cAMP signaling pathway can modulate other molecular targets in the brain, such as mitochondrial function and oxidative stress, by activating CREB and the peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1α) [87]. ...
The brain requires over one-fifth of the total body oxygen demand for normal functioning. At high altitude (HA), the lower atmospheric oxygen pressure inevitably challenges the brain, affecting voluntary spatial attention, cognitive processing, and attention speed after short-term, long-term, or lifespan exposure. Molecular responses to HA are controlled mainly by hypoxia-inducible factors. This review aims to summarize the cellular, metabolic, and functional alterations in the brain at HA with a focus on the role of hypoxia-inducible factors in controlling the hypoxic ventilatory response, neuronal survival, metabolism, neurogenesis, synaptogenesis, and plasticity.
Synapsins are highly abundant presynaptic proteins that play a crucial role in neurotransmission and plasticity via the clustering of synaptic vesicles. The synapsin III isoform is usually downregulated after development, but in hippocampal mossy fiber boutons it persists in adulthood. Mossy fiber boutons express presynaptic forms of short- and long-term plasticity, which are thought to underlie different forms of learning. Previous research on synapsins at this synapse focused on synapsin isoforms I and II. Thus, a complete picture regarding the role of synapsins in mossy fiber plasticity is still missing. Here, we investigated presynaptic plasticity at hippocampal mossy fiber boutons by combining electrophysiological field recordings and transmission electron microscopy in a mouse model lacking all synapsin isoforms. We found decreased short-term plasticity - i.e. decreased facilitation and post-tetanic potentiation - but increased long-term potentiation in male synapsin triple knockout mice. At the ultrastructural level, we observed more dispersed vesicles and a higher density of active zones in mossy fiber boutons from knockout animals. Our results indicate that all synapsin isoforms are required for fine regulation of short- and long-term presynaptic plasticity at the mossy fiber synapse.
Significance statement Synapsins cluster vesicles at presynaptic terminals and shape presynaptic plasticity at giant hippocampal mossy fiber boutons. Deletion of all synapsin isoforms results in decreased short- but increased long-term plasticity.
The hippocampal mossy fiber synapse, formed between axons of dentate gyrus granule cells and dendrites of CA3 pyramidal neurons, is a key synapse in the trisynaptic circuitry of the hippocampus. Because of its comparatively large size, this synapse is accessible to direct presynaptic recording, allowing a rigorous investigation of the biophysical mechanisms of synaptic transmission and plasticity. Furthermore, because of its placement in the very center of the hippocampal memory circuit, this synapse seems to be critically involved in several higher network functions, such as learning, memory, pattern separation, and pattern completion. Recent work based on new technologies in both nanoanatomy and nanophysiology, including presynaptic patch-clamp recording, paired recording, super-resolution light microscopy, and freeze-fracture and “flash-and-freeze” electron microscopy, has provided new insights into the structure, biophysics, and network function of this intriguing synapse. This brings us one step closer to answering a fundamental question in neuroscience: how basic synaptic properties shape higher network computations.
The transcription factor Bcl11b has been linked to neurodevelopmental and neuropsychiatric disorders associated with synaptic dysfunction. Bcl11b is highly expressed in dentate gyrus granule neurons and is required for the structural and functional integrity of mossy fiber-CA3 synapses. The underlying molecular mechanisms, however, remained unclear. We show that the synaptic organizer molecule C1ql2 is a direct functional target of Bcl11b that regulates synaptic vesicle recruitment and long-term potentiation at mossy fiber-CA3 synapses in vivo and in vitro. Furthermore, we demonstrate C1ql2 to exert its functions through direct interaction with a specific splice variant of neurexin-3, Nrxn3(25b+). Interruption of C1ql2-Nrxn3(25b+) interaction by expression of a non-binding C1ql2 mutant or by deletion of Nrxn3 in the dentate gyrus granule neurons recapitulates major parts of the Bcl11b as well as C1ql2 mutant phenotype. Together, this study identifies a novel C1ql2-Nrxn3(25b+)-dependent signaling pathway through which Bcl11b controls mossy fiber-CA3 synapse function. Thus, our findings contribute to the mechanistic understanding of neurodevelopmental disorders accompanied by synaptic dysfunction.
Compartmentalization by membranes is a common feature of eukaryotic cells and serves to spatiotemporally confine biochemical reactions to control physiology. Membrane-bound organelles such as the endoplasmic reticulum (ER), the Golgi complex, endosomes and lysosomes, and the plasma membrane, continuously exchange material via vesicular carriers. In addition to vesicular trafficking entailing budding, fission, and fusion processes, organelles can form membrane contact sites (MCSs) that enable the nonvesicular exchange of lipids, ions, and metabolites, or the secretion of neurotransmitters via subsequent membrane fusion. Recent data suggest that biomolecule and information transfer via vesicular carriers and via MCSs share common organizational principles and are often mediated by proteins with intrinsically disordered regions (IDRs). Intrinsically disordered proteins (IDPs) can assemble via low-affinity, multivalent interactions to facilitate membrane tethering, deformation, fission, or fusion. Here, we review our current understanding of how IDPs drive the formation of multivalent protein assemblies and protein condensates to orchestrate vesicular and nonvesicular transport with a special focus on presynaptic neurotransmission. We further discuss how dysfunction of IDPs causes disease and outline perspectives for future research.
The transcription factor Bcl11b has been linked to neurodevelopmental and neuropsychiatric disorders associated with synaptic dysfunction. Bcl11b is highly expressed in dentate gyrus granule neurons and is required for the structural and functional integrity of mossy fiber-CA3 synapses. The underlying molecular mechanisms, however, remained unclear. We show that the synaptic organizer molecule C1ql2 is a direct functional target of Bcl11b that regulates synaptic vesicle recruitment and long-term potentiation at mossy fiber-CA3 synapses in vivo and in vitro. Furthermore, we demonstrate C1ql2 to exert its functions through direct interaction with a specific splice variant of neurexin-3, Nrxn3(25b+). Interruption of C1ql2-Nrxn3(25b+) interaction by expression of a non-binding C1ql2 mutant or by deletion of Nrxn3 in the dentate gyrus granule neurons recapitulates major parts of the Bcl11b as well as C1ql2 mutant phenotype, and interferes with C1ql2 targeting to the synapse. Together, this study identifies a novel C1ql2-Nrxn3(25b+)-dependent signaling pathway through which Bcl11b controls mossy fiber-CA3 synapse function. Thus, our findings contribute to the mechanistic understanding of neurodevelopmental disorders accompanied by synaptic dysfunction.
