cAMP-Dependent Synaptic Plasticity at the Hippocampal Mossy Fiber Terminal

Abstract

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 the activation of the protein kinase A (PKA) molecular cascade. Accordingly, various downstream putative synaptic PKA target proteins have been linked to cAMP-dependent MF synaptic plasticity, such as synapsin, rabphilin, synaptotagmin-12, RIM1a, tomosyn, and P/Q-type calcium channels. Regulating the expression of some of these proteins alters synaptic release probability and calcium channel clustering, resulting in short- and long-term changes to synaptic efficacy. However, despite decades of research, the exact molecular mechanisms by which cAMP and PKA exert their influences in MF terminals remain largely unknown. Here, we review current knowledge of different cAMP catalysts and potential downstream PKA-dependent molecular cascades, in addition to non-canonical cAMP-dependent but PKA-independent cascades, which might serve as alternative, compensatory or competing pathways to the canonical PKA cascade. Since several other central synapses share a similar form of presynaptic plasticity with the MF, a better description of the molecular mechanisms governing MF plasticity could be key to understanding the relationship between the transcriptional and computational levels across brain regions.

Keywords: LTP; PKA; cAMP; forskolin-induced potentiation; mossy fiber synapse; synaptic plasticity.

FIGURE 1

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).

FIGURE 2

Expression of AC isoforms across hippocampal sub-regions. (A) Distribution pattern of the ten AC isoforms in the hippocampus of adult mice following in situ hybridization (Lein et al., 2007). (B) A heat map showing the relative mRNA expression levels of Adcy isoforms 1–9 across different hippocampal sub-regions. fpkm—fragments per kilobase of transcript per million mapped reads (Cembrowski et al., 2016). (C) as in (B), a heat map showing relative Adcy isoform 1–9 mRNA expression levels for the main cell clusters across several central brain regions. The right-most column specifies for which cell clusters evidence supports the existence of presynaptic LTP (Saunders et al., 2018). Data in (A) were adapted from the Allen institutes ISH brain atlas (http://mouse.brain-map.org/), data in (B) were adapted from hipposeq.janelia.org/and data in (C) were adapted from Dropviz.org.

FIGURE 3

cAMP cascades and cAMP-dependent synaptic plasticity in the MF synapse. (A) Cascades of activation of cAMP, PKA, and Epac2 in the MF terminal. A train of action potentials arriving at the MF synapse triggers a calcium influx through voltage-dependent calcium channels and subsequent activation of CaM that activates cAMP synthesis by AC1. AC1 acts on PKA or Epac2 to up regulate synaptic transmission (dashed green arrows between vesicles). Application of FSK activates both AC1 and AC2 and elevates cAMP. AC2 is also activated by PKC following the activation by a G-coupled protein. Application of DCG-IV, an agonist to mGluR2/3, leads to inhibition of AC1. Green arrows: activation, red arrow: inhibition, blue arrows: application of pharmacological reagents. (B) Downstream PKA cascades and their relation to synaptic plasticity. PKA activity is dependent on the anchoring protein AKAP7, and is inhibited by the negative regulator PKAα. PKA, in turn, phosphorylates multiple proteins including synapsin, rabphilin, tomosyn-1, synaptotagmin12 (Syt12) and RIM1a. Of these, deletion of synapsin and rabphilin does not impair MF synaptic plasticity. Ablation of RAB3A, although not a direct PKA target, and RIM1a abolishes MF-LTP but does not affect STP and FSK-induced potentiation (FIP). Synaptotagmin-12 KO abolishes MF-LTP and impairs FSK-induced potentiation but do not affect STP, while deletion of tomosyn-1 abolishes MF-LTP, impairs FSK-induced potentiation and also affects STP. Epac2 manipulation also abolishes LTP and impairs FSK-induced potentiation. STP- short-term potentiation, LTP- long-term potentiation, FIP- FSK-induced potentiation. Orange arrow represent PKA-independent pathway, Green arrows shades represent different pathways.

FIGURE 4

Model of molecular mechanisms of cAMP-dependent LTP. Three cAMP-dependent effectors, Epac, PKA, and Rapgef2, give rise to three parallel molecular pathways. Each of the three cAMP-dependent molecular pathways include a transcription factor that upon activation leads to long-term effects on gene regulation that support synaptic plasticity.