NEWS & VIEWS
NE UR O SCIENCE

Memory andthe single molecule
Sustained activity of the brain-specific enzyme PKM-ζ is thought to underlie
the maintenance of long-term memories. Studies in PKM-ζ-deficient mice,
however, cast the importance of this protein into question.
PA U L W. F R A N K L A N D & S H E E N A A . J O S S E LY NM

emory traces are thought to beformed by the strengthening ofsynaptic connections between
particular collections of neurons. A major
goal of neuroscience research is to identify themol­e cular machinery that sustains thesestrengthened connections once they areformed. Many molecules that were once
thought to have starring roles in memory
have since been relegated to supporting, albeit
still important, roles. Two papers publishedon Nature’s website today (Volk et al.1 and Lee
et al.2) challenge the star status of yet anothermemory molecule — protein kinase M-ζ.
The human genome encodes more than
500 protein kinase enzymes, and many of these
are known to mediate memory formation.
Protein kinase M-ζ (PKM-ζ) is special inthat it is proposed to maintain the enhanced
synaptic strength associated with memory
formation — unlike most other kinases, which
are involved in synaptic strengthening per se.
Moreover, in contrast to other kinases, PKM-ζ
is always active, a perfect feature for a moleculethat retains memory. It has therefore been considered truly deserving of ‘memory molecule’billing.
One of the first hints of the involvementof PKM-ζ in sustaining synaptic strength
came from a study3 in which the function of
the enzyme was disrupted after induction of
long-term potentiation (LTP, a form of synaptic strengthening that is often used as acellular proxy for memory formation) in thebrain’s hippocampus region. This investigation
found that both an inhibitory form of PKM-ζ
and a pharmacological inhibitor of this protein
called ZIP reversed previously established LTP.
(ZIP stands for zeta inhibitory peptide and is a
13-amino-acid sequence thought to mimic thenatural substrate that turns PKM-ζ off.)
This finding received considerable attention,
but the true red-carpet moment for PKM-ζ
came when another paper4 showed that intrahippocampal microinjections of ZIP — but
not of a scrambled ZIP peptide that did notaffect PKM-ζ and so was used as a control —reversed not only established LTP but also
an established memory in rats. That disrupting the function of a molecule could erase amemory created a great buzz and sparked
numerous sequels. Indeed, microinjectionof ZIP into many different brain regions was
subsequently shown to disrupt establishedspatial, fear, appetitive, habit and sensori­motor memories in rodents and sensitization
memory in the sea slug Aplysia5.
Using different strategies to geneticallydelete PKM-ζ, Volk et al. and Lee et al. now
challenge the conclusions of these previous studies. They
“Asking whether question both thespecificity of ZIP andPKM-ζ could
the importance ofreally be
PKM-ζ in maintain‘the’ memory
ing LTP and memory.
molecule isVolk and colleagues
probably not the created a line of miceright question.” in which Pkm-ζ and aclosely related gene,Pkc-ζ, were deleted throughout the animalin embryonic development. Despite having
no PKM-ζ, these mice seemed normal, andtheir hippocampal LTP was indistinguishable
both in magnitude and duration from LTP innormal mice.
Because absence of a gene since early
development may result in compensation for
its function by other genes, the authors also
created a line of mice in which PKM-ζ could
be deleted specifically in excitatory forebrainneurons in adult animals. These mice similarly showed normal LTP. The authors went
on to perform a clever experiment in which
they examined the effects of ZIP on maintenance of LTP in mice that entirely lacked
PKM-ζ. Remarkably, ZIP (and scrambledZIP), at doses used in previous experiments,
disrupted established LTP not only in normal
mice, but also in PKM-ζ-deficient mice. This
result indicates that ZIP disrupts established
LTP by PKM-ζ-independent mechanisms andbrings into question the role of PKM-ζ in themaintenance of LTP.

doi:10.1038/nature11850

Lee et al. also created mice that lacked
PKM-ζ from early in development and, using
an exhaustive battery of behavioural tests,probed their memory. The mice could form
persistent memories in the various tests — anobservation that is independently reported byVolk and co-workers. Lee et al. also examinedthe effects of ZIP on existing memories. For
this, they microinjected ZIP into the brain ofPKM-ζ knockout mice after formation of aplace memory. As a striking counterpart to theLTP results of Volk et al., ZIP erased memory
in mice lacking PKM-ζ. Although the specificity of ZIP (and of scrambled ZIP) has been
debated6,7 and the jury is still out on the precise molecular targets of these peptides, thesedata convincingly show that ZIP disrupts bothestablished LTP and memory independentlyof PKM-ζ.
These results, however, do not entirely
exclude the possibility that PKM-ζ is a key
player in memory maintenance. They do notaccount for two previous studies in which
genetic tools were used to manipulate PKM-ζ
function following learning. One of thesestudies8 found that acute expression of amouse PKM-ζ transgene in the fruitfly Drosophila 30 minutes after training enhanced
odour memory measured one or four days
after training. The second study9 showedthat micro­injecting a lentivirus expressing
PKM-ζ into the brain of rats six days after
training enhanced a taste-aversion memory.
It also found that disrupting PKM-ζ function
by inducing expression of an inhibitory formof the enzyme days after training was sufficient to erase an established taste-aversion
memory.
How might these findings1,2,8,9 be reconciled?
Hundreds of molecules are probably involvedin the formation and maintenance of memory, and within this ‘mnemome’ there may be
a fair degree of redundancy and degeneracy— whereby related molecules compensate foreach others’ deficiency10. Such mechanisms
could make up for long-term deficiency of asingle molecule, allowing formation of stable
memories. Acute, post-training manipulations
may reduce the likelihood of this type of compensation but might not eliminate it.
So asking whether PKM-ζ could really be‘the’ memory molecule is probably not theright question, as it fails to capture the complexity of the interactions between the molecules within the mnemome. Given evolution’s
penchant for redundancy, it seems unlikelythat any single molecule will play this part solo.
Rather, the question should be one of degree— is a molecule a lead player (and thereforeirreplaceable) or does it have more of a supporting role? The casting debate on PKM-ζwill, no doubt, continue. ■| NAT U R E | 1© 2013 Macmillan Publishers Limited. All rights reserved

RESEARCH NEWS & VIEWSPaul W. Frankland and Sheena A. Josselyn
are in the Program in Neurosciences andMental Health, Hospital for Sick Children,Toronto, Ontario M5G 1X8, Canada. They
are also in the Departments of Psychology andof Physiology and at the Institute of MedicalScience, University of Toronto.
e-mail: paul.frankland@sickkids.ca

1. Volk, L. J., Bachman, J. L., Johnson, R., Yu, Y. &Huganir, R. L. Nature http://dx.doi.org/10.1038/
nature11802 (2013).
2. Lee, A. M. et al. Nature http://dx.doi.org/10.1038/
nature11803 (2013).
3. Ling, D. S. F. et al. Nature Neurosci. 5, 295–296(2002).
4. Pastalkova, E. et al. Science 313, 1141–1144(2006).
5. Sacktor, T. C. Nature Rev. Neurosci. 12, 9–15 (2011).

2 | NAT U R E |© 2013 Macmillan Publishers Limited. All rights reserved

6. Wu-Zhang, A. X., Schramm, C. L., Nabavi, S.,
Malinow, R. & Newton, A. C. J. Biol. Chem. 287,12879–12885 (2012).
7. Yao, Y. et al. Neuropharmacology 64, 37–44(2013).
8. Drier, E. A. et al. Nature Neurosci. 5, 316–324(2002).
9. Shema, R. et al. Science 331, 1207–1210 (2011).
10. Sanes, J. R. & Lichtman, J. W. Nature Neurosci. 2,597–604 (1999).