Memory enhancement: consolidation, reconsolidation and insulin-like growth factor 2

Abstract

Life and societies would change significantly if memory capacity or persistence in health and disease could be enhanced. It has been known for many years that memory can be improved and strengthened. Substances known to enhance memory include hormones, neurotransmitters, neuropeptides and metabolic substrates. Recently, attention has been given to identifying the molecular mechanisms and targets whereby memory enhancement can be achieved. One approach would be to target the physiological changes that are induced by learning and naturally required for memory strengthening via consolidation and reconsolidation. Here, we review approaches that boost memories by targeting the cAMP response element binding protein-CCAAT enhancer binding protein (CREB-C/EBP) pathway and/or its recently identified target gene insulin-like growth factor 2 (IGF2).

Figure 1

Schematic representation of the cAMP response element binding protein (CREB)-CCAAT enhancer binding protein (C/EBP) pathway activated during memory formation. Neurotransmitters, growth factors, and membrane depolarization are examples of the stimuli that activate intracellular signal transduction pathways that can lead to the activation of the CREB-dependent pathway. Growth factors, signal via receptor tyrosine kinase (RTK), which upon ligand binding and dimerization, induces activation of two pathways: the Ras/Raf/ mitogen-activated protein kinase (MAPK)/ MAP kinase kinase (MEK) and the phosphoinositide 3-kinase (PI3K)-dependent pathways. Activations of these pathways recruit additional protein kinases including p90 ribosomal S6 kinase (RSK2) and mitogen- and stress-activated protein kinase (MSK) for the MAPK-dependent pathway and Akt and p70S6 kinase(p70S6K) for the PI3K-dependent pathway to catalyze phosphoryation of CREB (pCREB) in its Ser-133 residue, which is an important step for its activation . CREB phosphoryation can also be achieved by many neurotransmitters binding to their receptors. Via these receptors, neurotransmitters can couple of cyclic adenosine monophosphate (cAMP) by regulating adenylyl cyclase activity. cAMP recruits protein kinase A (PKA) as the main kinase for CREB phosphorylation. Phosphodiesterase (PDE) can catalyze the hydrolysis of cAMP and inhibit its signaling. Additionally, increases in intracellular Ca²⁺ influx through voltage- or ligand-gated cation channels such as voltage-sensitive calcium channels (VSCCs) or NMDA receptors (NMDARs) can also lead to CREB phosphorylation via different calcium-dependent protein kinases . Once phosphorylated, CREB recruits its transcription coactivator CREB-binding protein (CBP) to promote transcription . The functional activation of CREB leads to the expression of target genes, among which are immediate early genes (IEGs), such as the transcription factor C/EBP ^, , which, in turn, regulates the expression of late response genes including insulin-like growth factor 2 (IGF-2) . Transcriptional regulation is further regulated by the chromatin state. In general, histone acetyl transferases (HATs) acetylate histone tails and promote a relaxed chromatin state and transcription. Histone deacetylases (HDACs), on the other hand, de-acetylate histones and promote a condensed chromatin state and genes silencing . Abbreviations: AC,, adenylyl cyclase; ATP, adenosine triphosphate.

Figure 2

The insulin-like growth factor (IGF)-related system components. The IGF-related system is composed of three ligands: insulin, IGF-1 and IGF-2. IGF-1 and IGF-2 levels in circulation are regulated by IGF binding proteins (BPs). There are multiple receptor conformations. Insulin and IGF-1 receptor (IR and IGF-1R) are tyrosine kinase receptors. Two isoforms of IR, IR-A and IR-B, are found in the brain . For simplicity, only signaling initiated by the activated IGF-1R is shown. Activation of IR or IGF-1R leads to phosphorylation of adaptor proteins belonging to the insulin receptor substrate (IRS) family or Src homology 2 domain containing transforming protein (SHC) . Activation of IRS and SHC leads to activation of Raf/Ras/ mitogen-activated protein kinase (MAPK)/ MAP kinase kinase (MEK) and phosphoinositide 3-kinase (PI3K)/Akt pathways. Phosphorylation of Akt leads to subsequent activation of mammalian target of rapamycin (mTOR), eukaryotic translation initiation factor 4E (eIF4E), and p70S6 kinase (S6K). Activation of these pathways leads to enhanced proliferation, survival and metastasis in cancer cells . IGF-2 binds IR-A , IGF-1R, and IGF-2R , with the highest affinity to IGF-2R. IGF-2R is structurally distinct from IR and IGF-1R and is not a receptor tyrosine kinase. Once IGF-2 binds, IGF-2R targets IGF-2 to endocytosis-mediated lysosomal degradation as well as effecting signal transduction (see Figure 4). Abbreviations: Grb2, growth factor receptor-bound protein 2; PDK1, phosphoinositide-dependent kinase-1; PIP3, Phosphatidylinositol (3,4,5)-trisphosphate.

Figure 3

Insulin-like growth factor 2 (IGF-2) enhances fear as well as fear extinction memories. a) Rats underwent inhibitory avoidance training. Immediately after training, they received bilateral intra-hippocampal injections of IGF-1, IGF-2, or vehicle solution (indicated by ↓). Memory retention was tested at 24 hours (Test 1) and 7 days (Test 2) after training. Rats that received IGF-2, compared to IGF-1 and vehicle solution, had significantly higher memory retention at both tests . Similar intra-hippocampal injections of IGF-2 immediately after inhibitory avoidance training, compared to vehicle solution, significantly prevented memory forgetting. Rats injected with IGF-2 immediately after inhibitory avoidance training showed significantly higher memory retention at 3 weeks after training (Test) while the rats that received vehicle solution showed a significant memory retention decay . b). Rats underwent contextual/auditory fear conditioning, in which the rats associate a context and a tone to a footshock and freeze in subsequent exposure to the training context or to the tone in a different context. Immediately after conditioning, the rats received bilateral intra-hippocampal injections of IGF-2 or vehicle (indicated by ↓). Rats that were injected with IGF-2, compared to vehicle solution, had significantly higher freezing score when they were tested in the training context. In contrast, no effect was seen on the auditory fear-conditioning test, as rats injected with IGF-2 had freezing scores similar to rats injected with vehicle. c) Rats underwent inhibitory avoidance training and were divided into two groups. Twenty-four hours later, one group received a bilateral intra-hippocampal injection of either IGF-2 or IGF-1 whereas the other group underwent inhibitory avoidance memory retrieval (Test 1) and immediately thereafter received similar bilateral intra-hippocampal injections (indicated by ↓). All groups were tested for memory retention 48 hours after training (Final Test). IGF-2 had no effect when given 24 hours after training (NoR Final test), but, if given in concert with memory retrieval, compared to IGF-1, significantly enhanced memory retention . d) Mice underwent contextual fear conditioning. Context-dependent freezing was assessed 24 hour later. Extinction of contextual fear was performed on consecutive days at 24-hour intervals consisting of re-exposure to the training context without footshock. Fear extinction was significantly enhanced in mice that received injections of IGF-2 into the dorsal hippocampus immediately after each extinction trial (indicated by ↓) compared to the vehicle-injected group . Data are shown as mean latency or % freezing ± s.e.m.; *P < 0.05, **P< 0.01, ***P< 0.001. Adapted, with permission, from [26] (A–C) and [82] (D).

Figure 4

Insulin-like growth factor 2 receptor (IGF-2R)-dependent lysosomal enzyme trafficking, signaling transduction and endocytosis. Intracellular or extracellular lysosomal enzymes such as acid hydrolase are transported to late endosomes by IGF-2R (also known as mannose 6-phosphate receptor) . Lysosomal enzymes dissociate from IGF-2R within the low-pH environment of late endosomes and are subsequently delivered to lysosomes. IGF-2R is recycled to the Golgi or to the cell surface . At the cell surface, IGF-2 binding to IGF-2R is followed by endocytosis to form early endosomes. From the early endosomes, IGF-2R dissociates from IGF-2: IGF-2R is recycled to cell surface while IGF-2 is targeted to late endosome for lysosomal degradation . A recent study suggests that, in the hippocampus, learning leads to IGF-2 binding to cell surface IGF-2R which results in glycogen synthase kinase 3β (GSK3β) activation and activity-regulated cytoskeletal-associated protein (Arc) synthesis, both of which have been implicated in regulating AMPA receptor (AMPAR) endocytosis ^, . Internalized AMPARs can recycle to cell surface or be targeted for degradation . IGF-2/IGF-2Rs could also directly regulate AMPAR endocytosis independently of GSK3β and Arc.