Immune activation state modulates infant engram expression across development

Infantile amnesia is possibly the most ubiquitous form of memory loss in mammals. We investigated how memories are stored in the brain throughout development by integrating engram labeling technology with mouse models of infantile amnesia. Here, we found a phenomenon in which male offspring in maternal immune activation models of autism spectrum disorder do not experience infantile amnesia. Maternal immune activation altered engram ensemble size and dendritic spine plasticity. We rescued the same apparently forgotten infantile memories in neurotypical mice by optogenetically reactivating dentate gyrus engram cells labeled during complex experiences in infancy. Furthermore, we permanently reinstated lost infantile memories by artificially updating the memory engram, demonstrating that infantile amnesia is a reversible process. Our findings suggest not only that infantile amnesia is due to a reversible retrieval deficit in engram expression but also that immune activation during development modulates innate, and reversible, forgetting switches that determine whether infantile amnesia will occur.

Engram reactivation in the RSC as a percentage of (J) ChR2-EYFP + and (K) DAPI + cells.

Fig. S2 .
Fig. S2.MIA female offspring do not demonstrate social interaction deficits or repetitive behavior but do show infantile amnesia for a fear memory.(A) IL-17a cytokine levels in blood serum of female C57 mice injected with PBS (Ctrl n=5) or Poly I:C (PIC n=5).(B, E) Total distance (m) traveled during social interaction test in adult (P63) C57BL/6J (B) male (n=10) and (E) female (n=10) MIA offspring.(C, F) Heat map analysis of time spent in each chamber during social interaction task.(D) Social interaction index (%) of adult (P63) C57BL/6J female MIA offspring (n=10).(G) Number of marbles buried by adult female MIA offspring during marble burying task (n= 8-10).(H) Memory recall for CFC in context A for male MIA offspring testing 1 d after training (n=10).(I, J) Recall for CFC in context A for female MIA offspring tested (I) 1 d (n=9-10) or (J) 8 d (n= 9) after training.(K) Memory recall for CFC in context A for male MIA offspring tested 8 d (P25) (n=10) or 15 d (P32) (n=13) after training.(L) Memory recall comparison in male and female MIA offspring 8 d after training.P < 0.05, P < 0.01, P < 0.001 calculated by (A) Student t-test, (K) nested Student t-test, (B, D, E, G-J) nested ANOVA, or (L) two-way ANOVA with Bonferroni post hoc tests.Data presented as ±SEM.

Fig. S5 .
Fig. S5.Engram reactivation is increased in the DG and AMG after natural recall at P20. (A) Behavioral paradigm.(B-O) Histological analysis of cell counts after recall at P20. Percentage of (B) ChR2-EYFP + and (C) c-Fos + cells in the DG (N=5-6, n=4).There was a significant increase in c-Fos + cells in the DG after recall at P20 (p < 0.05).(D) Percentage of DAPI cells positive for both ChR2-EYFP and c-Fos.(D) Engram reactivation as a percentage of DAPI + was significantly higher in the DG after recall at P20 (p < 0.05).Percentage of (E) ChR2-EYFP + and (F) c-Fos + cells in the AMG (N=4, n=4).There was a significant increase in c-Fos + cells in the AMG after recall at P20 (p < 0.05).(G) Percentage of DAPI cells positive for both ChR2-EYFP and c-Fos.

Fig. S7 .
Fig. S7.MIA female offspring do not show an increased number of EYFP + cells in the DG.(A) Behavioral paradigm for labeling engram cells in MIA female Ai32-FosTRAP offspring.(B) ChR2-EYFP + cell counts in the DG (N= 4, n=4) of female MIA offspring after recall at P25. (C) Engram reactivation in the DG of male MIA offspring as a percentage of DAPI + cells.P < 0.05, P < 0.01, P < 0.001 calculated by (B, C) nested Student t-test.Data presented as ±SEM.

Fig. S8
Fig. S8 Optogenetic reactivation of an infant engram in the CA1 elicits freezing behavior in adult mice.(A) Behavioral schedule for engram reactivation of a neutral context B in the DG of Ai32-FosTRAP male mice.The black lightning symbol represents foot shocks.The syringe symbol represents 4-OHT injection 2 h after context exposure.(B) Memory recall in context C (engram reactivation) with light-off and light-on epochs (n=9).(C) Freezing for the two light-off and lighton epochs averaged.(D) Behavioural schedule for infant engram reactivation in the CA1 of adult Ai32-FosTRAP male mice (n=7) (NS), (n=9) (S).(E, G) Freezing levels during natural memory recall.(F, H) Memory recall in context C during engram reactivation at (F) 20 Hz or (H) 4 Hz with light-off and light-on epochs.(I) Freezing for the two light-off and light-on epochs during 4 Hz stimulation averaged.P < 0.05, P < 0.01, P < 0.001 calculated by (C, G) Student t-test or (E, I) two-way ANOVA with Bonferroni post hoc tests.Data presented as ±SEM.

Fig. S10
Fig. S10 Optogenetic stimulation of an CFC engram encoded after the infantile period causes an increase in activity in the amygdala in adult mice.(A) Behavioral schedule for optogenetic reactivation of DG engram cells, labeled at P29, in adult (P63) Ai32-FosTRAP mice.The black lightning symbol represents foot shocks.The syringe symbol represents 4-OHT injection 2 h after training.Percentage of ChR2-EYFP + cells labeled

Fig. S11 .
Fig. S11.Infant mice can form context-specific memories that can be behaviorally updated.(A) Behavioral schedule for artificial updating of an adult engram in Ai32-FosTRAP mice.(B) Freezing levels during recall in context B and context A (n=10).Experimental light group froze significantly more in context A (P < 0.001).(C) Behavioral schedule for updating paradigm in C57BL/6J infant (P17) mice.(D) Freezing levels (n=10) during recall tests at P19 (Recall 1), P20 (CxtB), and P63 (Recall 2).Infant mice that were pre-exposed (PE) to context A froze significantly (P < 0.01) more during recall 1. (E) Behavioral schedule for updating paradigm when update occurs after the infantile amnesia period.(F) Freezing levels (n=9) during recall tests.P < 0.05, P < 0.01, P < 0.001 calculated by (B, D, F) two-way ANOVA with Bonferroni post hoc tests.Data presented as ±SEM.