Memorable proteins: Spatial learning instigates short-term changes in protein abundances in the brain

Memory effects

The way we learn and remember things relies on the part of the brain known as the hippocampus, and specifically on the dentate gyrus. In this section, new neurons are formed to play a prominent role in the creation of new memories.

Neurons are linked to one another by synapses which change their shape and/or function in response to signals from the neurons. These changes can occur over a few seconds, a few hours, or longer, and they are linked to the formation of memories. This synaptic plasticity is governed by changes in gene and protein synthesis and is also associated with protein degradation.

There have been several reports on the changes in mammalian gene expression during learning tasks such as spatial learning, fear conditioning and eye blink conditioning but there are no comparable studies on the changes in protein expression.

Now, a new study has examined the changes in protein levels in the hippocampus dentate gyrus in rats over time following a single spatial learning experience. Marco Monopoli and colleagues from University College Dublin, Innovative Medicines AstraZeneca, Mereside, Alderley Park, UK, and Pfizer Worldwide Research & Development at Groton, CT and Cambridge, MA, published their findings in Proteomics.

Water maze test

The learning experiment was the water maze paradigm, a popular tool for testing spatial learning and memory in which rats must learn the position of a submerged platform in a circular water pool.

Each rat was placed into the pool and the time taken to find the platform was recorded up to a maximum of 60 s. If the animal did not manage to find the platform in that time, it was placed on the platform for 10 seconds. The trial was then repeated for up to five times with a break of 300 seconds between each trial.

All of the animals learned the task quickly with platform location times falling sharply by the second trial and continuing to fall thereafter.

Control rats were allowed to swim in the water maze for matching times but in the absence of a platform, to counter the effects of stress and other factors which might come into play.

The rats were sacrificed at different times following the trial to try and detect changes in the protein signatures. The dentate gyrus was extracted and the proteins present were labelled with the CyDye florescent dyes for 2D differential gel electrophoresis and image analysis. Any protein spots of interest were removed from the gel for identification by mass spectrometry.

Remembering protein changes in the brain

Each gel contained about 1500 resolved protein spots, 43 of them being either up- or down-regulated as a result of learning. The patterns fell into two major periods of activity after 3 and 12 hours, in which many proteins were differentially regulated. At 6 and 24 hours, the number of protein changes was markedly lower. During the first peak, at 3 hours, down-regulated proteins were in the minority but this was reversed at 12 hours.

Following their identification, it became clear that most of the proteins connected with learning were associated with cellular structure or cellular metabolism. Other recognised functional categories were protein metabolism, cellular signalling, secreted carriers and cell proliferation.

The three major structural components of the neuronal cytoskeleton are microfilaments, microtubules and neurofilaments. Their core proteins are polymers of actins, tubulins and class IV intermediate filaments, respectively and all three were regulated after spatial learning. This observation implicates the adaptation of the monomer/polymer balance in the memory process.

Phosphorylation was also implicated by the prolonged up-regulation of dihydropyrimidinase-related protein 2, which is found in the growing axons and binds to tubulin filaments to facilitate microtubule elongation. The researchers proposed that phosphorylation might regulate the neuron-synaptic cytoskeleton after learning.

Of more interest was the discovery of several differentially expressed proteins that had not been previously associated with a temporal role in the memory effect. They included Pea-15, transthyretin and serum albumin.

Levels of non-phosphorylated Pea-15 were reduced 3 hours after training but returned to normal levels at the 6 hour stage, whereas the phosphorylated form of the protein remained stable throughout the test period. So, phosphorylation-dependent degradation of Pea-15 in neurons of the dentate gyrus accompanied spatial memory formation.

The secreted proteins transthyretin and serum albumin were both up-regulated during memory consolidation. Their increases were attributed to an increased supply of thyroid hormone and retinoic acid to the hippocampus and, in the case of transthyretin, to a protection role by clearing amyloid beta fragments.

The researchers recommended that more detailed work on particular cell subtypes and the roles of phosphoproteins should be carried out to gain further insight. In the meantime, their study sheds a little light on the types of protein changes that occur in the brain during spatial memory formation, implicating cellular structure and metabolism.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.