Before experiments with the PKM-ζ inhibitor ZIP, there was no evidence that long-term memories were maintained by a specific molecular mechanism. The effect of ZIP on memory is quite specific and unique—memories from one day to three months old are erased, whereas there is no effect on short-term memory, initial learning, or baseline synaptic transmission measured in the living animal (1). Until ZIP, there was no agent known to erase memory long after its consolidation without hampering the ability to acquire new memories later on. So the action of ZIP is not anything like an anesthetic. PKM-ζ increases during memory storage and enhances synaptic transmission (2). PKM-ζ inhibitors also reverse the maintenance of long-term potentiation, again the only agents known to do so (2). Moreover, molecularly jamming the mechanism by which PKM-ζ-mediated synaptic enhancement can be reversed prevents ZIP from erasing memories, strongly supporting that ZIP is working on PKM-ζ (3). All these studies, along with others using additional inhibitors and dominant negative versions of PKM-ζ (4),...  Read more

Before experiments with the PKM-ζ inhibitor ZIP, there was no evidence that long-term memories were maintained by a specific molecular mechanism. The effect of ZIP on memory is quite specific and unique—memories from one day to three months old are erased, whereas there is no effect on short-term memory, initial learning, or baseline synaptic transmission measured in the living animal (1). Until ZIP, there was no agent known to erase memory long after its consolidation without hampering the ability to acquire new memories later on. So the action of ZIP is not anything like an anesthetic. PKM-ζ increases during memory storage and enhances synaptic transmission (2). PKM-ζ inhibitors also reverse the maintenance of long-term potentiation, again the only agents known to do so (2). Moreover, molecularly jamming the mechanism by which PKM-ζ-mediated synaptic enhancement can be reversed prevents ZIP from erasing memories, strongly supporting that ZIP is working on PKM-ζ (3). All these studies, along with others using additional inhibitors and dominant negative versions of PKM-ζ (4), indicate that PKM-ζ maintains long-term memory in normal animals, from mice to invertebrates such as flies (5) and the mollusk Aplysia (6).

What is new in these two studies is that they use genetically modified mice in which the gene that makes PKM-ζ had been completely deleted at the animals’ conception in order to study learning and memory. They find that the mice’s memory is intact. This is not too surprising because compensation for one gene by another, which might have taken place here but may have escaped detection, is a routine observation in knockout mice. If so, the compensatory gene product would subserve a similar function and exhibit learning-induced upregulation.

So what is the backup mechanism for maintaining memory? The authors do not say, but a clue can be obtained by their observations that ZIP still works to erase memory and reverse long-term potentiation. ZIP is a pseudo-substrate inhibitor of the catalytic domain of atypical PKCs, and effectively inhibits PKM-ζ and the other atypical PKC found in forebrain, PKC-ι/λ. PKC-ι and PKC-λ are different names for the same gene—the human gene is termed PKC-ι and the mouse gene PKC-λ. Before vertebrates, there was only one atypical PKC gene, and a PKM form is made from it (7,8). But with the earliest vertebrates, this function was split into two very closely related atypical PKC genes. In the forebrain, the ζ gene became dedicated to making only PKM-ζ (7), but PKC-ι/λ can also make a PKM form similar to the way it is made in invertebrates—by shortening the PKC-ι/λ protein by proteolysis (8). Genetic studies have shown that these two isoforms can compensate for each other’s functions (9), and both ζ and ι/λ can do the same thing in neurons to enhance excitability (10). We have previously shown that PKC-ι/λ is also activated in LTP (11), and that there is normally a small amount of PKM-ι/λ present in the brain as well (12). The methods in the new papers may not have been optimized to detect synaptic PKM-ι/λ in the complete knockout after learning or LTP. So it is indeed possible that PKC/PKM-ι/λ is the backup mechanism for memory that is normally maintained by PKM-ζ. In such a case, the ability of ZIP to block LTP and memory, reported in the papers, is actually expected and would be strong support for the crucial role of atypical PKCs in maintaining long-term memories.

One of the experiments in one of the papers used a drug that binds to the estrogen receptor in a technique to reduce gene expression of PKM-ζ after a mouse’s initial development, and this is a particularly interesting experiment because they still saw synaptic long-term potentiation. They didn’t look at the mouse’s behavior. However, there was still PKM-ζ around in the brain of the mouse and, more importantly, we don’t know if any new PKM-ζ was made during the LTP from the PKM-ζ mRNA that was still around. Moreover, the estrogen receptor binds to and activates atypical PKC (13), so it is not clear how chronic exposure to the drug they used would affect PKM-ζ or PKC-ι/λ activity or turnover.

So I think future research from these studies will be to try to find the backup mechanisms for memory and to determine what role they play, along with PKM-ζ, in memory in normal animals. I am gratified that our research has stimulated the experiments of others to identify the molecular mechanisms that store long-term memories—one of the fundamental questions in neuroscience.

References:
1. Sacktor TC. How does PKM-ζ maintain long-term memory? Nat Rev Neurosci. 2011 Jan;12(1):9-15. Abstract

2. Ling DS, Benardo LS, Serrano PA, Blace N, Kelly MT, Crary JF, Sacktor TC. Protein kinase Mzeta is necessary and sufficient for LTP maintenance. Nat Neurosci. 2002 Apr;5(4):295-6. Abstract

3. Migues PV, Hardt O, Wu DC, Gamache K, Sacktor TC, Wang YT, Nader K. PKMzeta maintains memories by regulating GluR2-dependent AMPA receptor trafficking. Nat Neurosci. 2010 May;13(5):630-4. Abstract

4. Shema R, Haramati S, Ron S, Hazvi S, Chen A, Sacktor TC, Dudai Y. Enhancement of consolidated long-term memory by overexpression of protein kinase Mzeta in the neocortex. Science. 2011 Mar 4;331(6021):1207-10. Abstract

5. Drier EA, Tello MK, Cowan M, Wu P, Blace N, Sacktor TC, Yin JC. Memory enhancement and formation by atypical PKM activity in Drosophila melanogaster. Nat Neurosci. 2002 Apr;5(4):316-24. Abstract

6. Cai D, Pearce K, Chen S, Glanzman DL. Protein kinase M maintains long-term sensitization and long-term facilitation in aplysia. J Neurosci. 2011 Apr 27;31(17):6421-31. Abstract

7. Sacktor TC. Memory maintenance by PKM-ζ - an evolutionary perspective. Mol Brain. 2012;5:31. Abstract

8. Bougie JK, Cai D, Hastings M, Farah CA, Chen S, Fan X, McCamphill PK, Glanzman DL, Sossin WS. Serotonin-induced cleavage of the atypical protein kinase C Apl III in Aplysia. J Neurosci. 2012 Oct 17;32(42):14630-40. Abstract

9. Kishikawa M, Suzuki A, Ohno S. aPKC enables development of zonula adherens by antagonizing centripetal contraction of the circumferential actomyosin cables. J Cell Sci. 2008 Aug 1;121(Pt 15):2481-92. Abstract

10. Zhang YH, Kays J, Hodgdon KE, Sacktor TC, Nicol GD. Nerve growth factor enhances the excitability of rat sensory neurons through activation of the atypical protein kinase C isoform, PKM-ζ. J Neurophysiol. 2012 Jan;107(1):315-35. Abstract

11. Kelly MT, Crary JF, Sacktor TC. Regulation of protein kinase Mzeta synthesis by multiple kinases in long-term potentiation. J Neurosci. 2007 Mar 28;27(13):3439-44. Abstract

12. Naik MU, Benedikz E, Hernandez I, Libien J, Hrabe J, Valsamis M, Dow-Edwards D, Osman M, Sacktor TC. Distribution of protein kinase Mzeta and the complete protein kinase C isoform family in rat brain. J Comp Neurol. 2000 Oct 16;426(2):243-58. Abstract

13. Castoria G, Migliaccio A, Di Domenico M, Lombardi M, de Falco A, Varricchio L, Bilancio A, Barone MV, Auricchio F. Role of atypical protein kinase C in estradiol-triggered G1/S progression of MCF-7 cells. Mol Cell Biol. 2004 Sep;24(17):7643-53. Abstract

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