Protein Kinase Mζ-Dependent Maintenance of GluA2 at the Synapse: A Possible Target for Preventing or Treating Age-Related Memory Decline?

Introduction

C onverging data from studies in humans and monkeys indicate that age-related functional alterations in the perforant path projection from the entorhinal cortex to the dentate gyrus (DG) of the hippocampus play a major role in age-related memory impairments, ^1,2 but little is known about the molecular mechanisms responsible for these changes. A better understanding may help to identify new molecular targets that can be addressed to ameliorate memory performances during aging.

AMPA-type glutamate receptors (α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors, AMPARs) mediate the majority of fast excitatory synaptic transmission in the brain and play a critical role in synaptic plasticity, i.e., in the dynamic changes in synaptic efficacy underlying information coding and storage in learning and memory. Increases in AMPAR function through changes in their number and location at synaptic sites, subunit composition, and/or properties result in long-term potentiation (LTP) of synaptic efficacy. Conversely, decreases in AMPAR function result in long-term depression (LTD). Regulated AMPAR trafficking is also involved in slower, non-Hebbian mechanisms of plasticity. ³ AMPARs are tetrameric complexes of combinations of four separate subunits (GluA1–4), assembled as two identical heterodimers. The expression of these subunits is developmentally regulated and is region specific. For example, GluA1/2 is the most predominant AMPAR subtype in hippocampal pyramidal neurons, followed by GluA2/3 heteromers. GluA2 is the most abundant subunit in the adult brain; its presence has a profound impact on the biophysical properties of receptors. GluA2-containing AMPARs are Ca²⁺-impermeable with a linear current–voltage relationship, whereas GluA2-lacking receptors are Ca²⁺-permeable and have an inwardly rectifying current–voltage relationship. ⁴ Recent evidence indicates that the GluA2 subunit plays a major role in long-term memory maintenance, by regulating AMPAR trafficking at synaptic sites. ^5,6

Protein kinase Mζ (PKMζ) is a protein kinase C (PKC) isoform expressed exclusively in the brain, particularly in the hippocampus and neocortex. It plays a critical role in long-term memory because it blocks a N-ethylmaleimide–sensitive factor/GluA2-dependent pathway that removes postsynaptic GluA2-containing AMPARs, resulting in a persistent increase of these receptors at postsynaptic sites. ^{5 –7} Notably, PKMζ's capability for maintaining long-term memories persists for a long time after its formation. ^{7 –9}

Synaptic Distributions of GluA2 and PKMζ in the Monkey Dentate Gyrus and Their Relationships with Aging and Memory

In a recent interesting study, Hara and colleagues ¹⁰ tested young adult and aged monkeys on the visual recognition memory task “delayed non-matching-to-sample” (DNMS). Electron microscopy immunocytochemistry was carried out in the hippocampal DG to investigate the subcellular distribution of the GluA2 subunit of the AMPARs and PKMζ. Young adult monkeys showed better learning performances compared to the aged ones and had better accuracy scores at all the tested delays. Electron microscopy immunocytochemistry showed that: (1) GluA2 and PKMζ double-labeled dendritic spines in DG had larger post-synaptic densities (PSDs) than GluA2 single- and PKMζ single-labeled spines, regardless of age; (2) compared with young adults, aged monkeys had lower densities of synaptic GluA2 in double-labeled spines and a lower density of total GluA2 in double- and single-labeled spines. Fast recognition memory acquisition (i.e., fast DNMS task acquisition) was associated with (1) high densities of cytoplasmic, plasmalemmal, and total GluA2, and (2) high densities of synaptic, cytoplasmic, and total PKMζ in double-labeled spines. The integrity of recognition memory, measured as DNMS accuracy scores averaged across increasing retention intervals (15–600 sec), resulted and was positively associated with cytoplasmic GluA2 and total PKMζ in double-labeled spines and with densities of total GluA2 and total PKMζ in single-labeled spines. Altogether, the results obtained by Hara and colleagues ¹⁰ suggest that age-related deficits in visual recognition memory are coupled with impairment in PKMζ-dependent maintenance of GluA2 at the synapse, consistent with previous studies demonstrating a significant role of PKMζ in memory storage ^6,7 and with the recent evidence that long-term memory can be enhanced by PKMζ overexpression. ⁹

Medical Implications

The study of Hara and colleagues, ¹⁰ together with previous evidence of the critical role of PKMζ in memory consolidation, ^{6 –9} render this enzyme an attractive potential therapeutic target for preventing or treating age-related memory decline. Several other molecules have been shown to be involved in AMPAR trafficking, including synaptic-associated protein 97KDa (SAP97), protein 4.1N, glutamate receptor interacting protein (GRIP), protein interacting with C-kinase 1 (PICK1), AP-2, BRAG2, and the N-ethylmaleimide-sensitive factor (NSF). The latter is an essential component of SNARE-mediated fusion machinery, which directly binds to the carboxy-terminal juxta-membrane region of GluA2. This interaction is required for direct insertion of GluA2 into the plasma membrane, as well as for its rapid incorporation and stabilization at synapses. ⁴ The noradrenergic input to the hippocampus plays a critical role in memory formation ¹² and is involved in age-related working memory decline ^13,14 ; notably, emotions enhance learning via norepinephrine regulation of AMPAR trafficking. ¹⁵

A major role in AMPAR trafficking is also played by brain-derived neurotrophic factor (BDNF), which up-regulates the total cellular levels of AMPAR subunits GluA1–A4 and induces the surface expression of newly synthesized GluA1. ¹⁶ Induction and maintenance of LTP require an increase in secretion of BDNF, ^17,18 which leads to morphologic changes of synaptic contacts needed for memory consolidation. ¹⁹ Indeed, the preservation of structural plasticity at synaptic sites during aging is critical for maintaining the ability to consolidate memory traces. ²⁰ Aged rats with good memory retention have higher expression in genes involved in the regulation of cytoskeleton and synaptic vesicle pools. ^21,22 BDNF binds to TrkB receptors and induces activation of phosphoinositide 3-kinase (PI3K), which in turn activates AKT and the subsequent phosphorylation and activation of mammalian target of rapamycin complex 1 (mTORC1). ^23,24 Both BDNF ²⁵ and mTOR ^{26 –28} are potential therapeutic targets for treating age-related memory impairments. Notably, increases in BDNF levels and beneficial effects on cognition can be induced by intense physical exercise. ^29,30

Disruption of AMPAR trafficking by soluble amyloid β (Aβ) oligomers is a major factor in synaptic dysfunction in Alzheimer disease. ^31,32 The effect is partly due to competition with proteolytic maturation of BDNF, which is required for its effect on synaptic potentiation. ³³ Thus, therapeutic intervention on AMPA trafficking during aging may also reduce soluble Aβ oligomers-induced synaptic dysfunctions.

Conclusions

The discovery by Hara and colleagues ¹⁰ that age-related recognition memory impairment is coupled with changes in the subcellular distribution of GluA2 and PKMζ in DG synapses extends our previous knowledge regarding the critical role of PKMζ in memory consolidation, and renders this enzyme an attractive potential therapeutic target for preventing or treating age-related memory decline. Moreover, these new data support the view that the pharmacological manipulation of AMPAR trafficking in the synapses may provide new insights in the search of memory enhancers for aged individuals, including those affected by Alzheimer disease.