Science - USA (2020-01-03)

couldnotberetrievedbynaturalmeans.Silent
DG engram cells showed weaker physiological
(increased synaptic strength) and structural
(increased dendritic spine density) alterations
than normal engram cells (in control mice),
suggesting that a silent engram may be the
result of disrupting the synaptic strengthening
normally induced by training. That optoge-
netic activation of DG engram cells was able
to induce memory retrieval suggests that di-
rect optogenetic activation was able to cir-
cumvent this requirement for synaptic and

structural plasticity within engram cells. Con-

sistent with this, genetic restoration of spine

density [targeted overexpression of p-21 acti-

vated kinase (PAK 1)] also allowed a silent en-

gram to be reactivated and memory expressed

by natural retrieval cues ( 160 ).

Theideathatengramsmaybesilencedby

disrupting synaptic efficacy and spine den-

sity and reawakened by enhancing synaptic

plasticity is consistent with findings from a

nonengram study examining auditory fear

conditioning ( 161 ). Rats were trained in a

variant of an auditory fear conditioning task

in which the tone conditioned stimulus was

replaced by optogenetic activation of LA axon

terminals from neurons originating in the

medial geniculate nucleus and auditory cor-

tex. Immediately after conditioning, long-term

depression (LTD)–like optogenetic stimula-

tion was administered. LTD is thought to

weaken synaptic efficacy and decrease spine

density ( 162 – 165 ). Consistent with the inter-

pretation that LTD-likestimulation silenced

the engram, this opto-LTD stimulation im-

paired subsequent memory recall. However,

LTP-like optogenetic stimulation allowed the

memory to be retrieved (consistent with the

interpretation that the engram was“unsi-

lenced”). Again, subsequent LTD-like opto-

genetic stimulation silenced this memory,

whereas LTP-like optogenetic stimulation al-

lowed recovery of this memory.

These findings raise thequestionofwhether

engrams (and the memories they support) in

other amnesic conditions are truly“lost”or are

simply inaccessible such that they cannot be

retrieved under naturalconditions. Silent en-

grams were reactivated by artificially stim-

ulating engram cells in amnestic mice used to

study the early stages of Alzheimer’sdisease

(AD) ( 166 , 167 ). These transgenic mice [APP/

PS1 mice containing human transgenes with

the familial AD mutation in both amyloid pre-

cursor protein (APP) and presenilin 1 (PSEN1)]

showed contextual fear memory deficits ( 166 ).

However, optogenetic reactivation of ChR2-

labeled DG engram cells induced robust freez-

ing comparable to control mice ( 166 ). Consistent

with other examples of silent engram cells,

DG engram cells in these mice used to study

AD showed decreased spine density. However,

LTP-like optogenetic stimulation at entorhinal

cortex engram cell inputs onto DG engram

cells restored not only spine density in DG en-

gram cells but also the ability of natural re-

trieval cues to elicit memory retrieval (thereby

unsilencing the engram) ( 166 ). These findings

inmice are consistent with reports that mem-

ory retrieval in people with early-stage AD

may be enhanced by particular retrieval cues

( 168 , 169 ). Therefore, under certain conditions,

a previously inaccessible memory may be re-

trieved in human AD, consistent with the in-

terpretation that some engrams in early-AD

brains may be silent rather than lost.

Apart from clinical implications, the finding

of silent engrams is relevant to discussions

on the role of protein synthesis–dependent

cellular consolidation in terms of memory stor-

age versus retrieval. There has been persistent

debate on this issue ( 170 – 173 ). The majority

of neuroscientists examining cellular memory

consolidation may favor the view that disrupt-

ing protein synthesis disrupts memory storage.

However, in many amnesia experiments, mem-

ory storage is conflated with memory retrieval.

Josselynet al.,Science 367 , eaaw4325 (2020) 3 January 2020 7of14

Non engram cell Silent engram cell Reactivated silent engram cell Active engram cell Reactivated active engram cell

Freezing No Freezing Freezing

Spine density

HIGH

Spine density

LOW

Spine density

LOW

Natural recall Artificial recall

(ChR2 + blue laser)

Active Engram Silent Engram

Natural recall

Context A Context A Context B

In amnesia and early Alzheimer’s disease

A

Spine density: HIGH

Basal level Recent natural recall Remote natural recall

CFC

Dox(-)

Spine density: LOW

Dematuration

B

During Systems Consolidation

No Freezing Freezing Freezing

Spine density: LOW

CFC

Dox(-)

Spine density: HIGH

Maturation

Hippocampus

mPFC

Fig. 4. Active and silent engram cells in amnesia and during memory systems consolidation.
(A) Active engram cells have higher spine density and are activated in the conditioned context A to
produce the conditioned response, freezing. Silent engram cells generated in amnesia and in a mouse designed
to model early Alzheimer’s disease show lower spine density and cannot be activated in the conditioned
context A to produce a conditioned response but can be activated by blue light in an unconditioned context B if
they were tagged with ChR2 during encoding. (B) During memory systems consolidation, active engram
cells with high spine density are formed in the hippocampus during contextual fear conditioning and for several
days, the conditioned context can evoke a conditioned response. However, by two weeks (remote recall),
these hippocampal engram cells demature to become silent, with reduced spine density. In the mPFC, engram
cells are formed during CFC but are silent with low spine density. During the following 2 weeks, these mPFC
silent engram cells acquire higher spine density and become active engram cells.

RESEARCH | REVIEW