DNA repair process key to memory formation, study finds

In a recent study published in the journal NatureThe researchers found that the recruitment of neurons into memory circuits is preceded by a cascade of molecular events triggered during learning, which includes double-stranded deoxyribonucleic acid (DNA) damage in hippocampal neuronal clusters and toll-like receptor 9 (TLR9)-mediated repair ).

Study: Formation of memory complexes through the DNA-sensing TLR9 pathway. Image credit: Billion Photos / Shutterstock

Record

Memories are formed when neurons in the hippocampus undergo long-term molecular adaptations to form cortical microcircuits in response to stimuli. This process is energy intensive and involves substantial morphological and biochemical changes. These molecular changes are thought to cause transient breaks in double-stranded DNA.

Studies have also investigated the role of intrinsic neuronal and preexisting developmental programs in memory formation and have found that transcription factors such as cyclic adenosine monophosphate (cAMP) response element binding protein (CREB) are involved in the process. Recent research has also focused on understanding how intraneuronal perineural networks control inhibitory inputs to neuronal assemblies to stabilize memory circuits.

About the study

In the present study, the researchers sought to understand and identify any general processes that incorporated preexisting developmental mechanisms and pathways initiated by stimuli that influenced neurons to commit to memory-specific assemblies or microcircuits.

Mouse models were used to analyze transcriptional profiles of neurons in the dorso-hippocampal regions over 48 hours to understand immediate, early and late gene expressions and protein signaling. For this analysis, mice underwent contextual fear conditioning, and hippocampal samples obtained either four or 21 days after conditioning were used for ribonucleic acid (RNA) mass sequencing.

Since transient DNA double-strand breaks are known to be induced during neuronal activity to induce immediate early gene expression, they hypothesized that DNA damage induced by learning activity may be more extensive and persist in distinct populations neurons. Immunofluorescence labeling was performed using antibodies specific for the binding of phospho-histone γH2AX to double-stranded DNA fragments to understand the origin of fear-generated extranuclear double-stranded DNA fragments.

Brain sections were also collected one hour after fear conditioning to analyze γH2AX signals associated with immediate early gene expression. In addition, the basal expression of CREB, which is already recognized to play a role in memory, was also analyzed using immunostaining. The researchers also examined the upregulation of Fos protein during memory reactivation and the respective roles of immediate early gene expression and DNA damage repair.

Based on their identification of inflammatory signaling in these neuronal populations, the researchers further investigated whether these inflammatory responses were a result of DNA double-strand breaks induced during learning or whether inflammation had a specific role to play in memory formation. Given the role of TLR9 in these inflammatory responses, they conducted TLR9 knockout experiments in specific neurons to determine how it affected memory formation.

Additionally, single nuclear RNA sequencing was performed to characterize gene expression changes in hippocampal neuronal and non-neuronal cell populations due to the effect of contextual fear conditioning and neuron-specific knockout of TLR9. The researchers also examined the contribution of infiltrating immune cells and cell-free DNA from the blood to memory formation and upregulation of TLR9 signaling.

Results

The study found that learning and memory formation involved ruptures in the nuclear envelope, histone release in the perinuclear region, and persistent DNA double-strand breaks in groups of neurons in the Cornu Ammonis area 1 (CA1) of the hippocampus. Moreover, these damages to double-stranded DNA and nuclear envelope were followed by activation of TLR9 signaling, resulting inflammatory response and accumulation of centrosomal complexes to repair the damaged double-stranded DNA.

The role of TLR9-related inflammatory responses in learning-induced memory establishment was confirmed when TLR9 knockout in specific neurons resulted in memory impairments and blunting of gene expression changes associated with fear conditioning. TLR9 was also found to play an important role in DNA damage formation, centrosomal complex repair, ciliagenesis, and perineuronal network construction.

The results indicate that learning-related stimuli triggered a cascade of molecular events involving double-stranded DNA damage and TLR9-mediated DNA repair in specific neuronal clusters in the hippocampus that recruited these neurons to memory formation. The researchers also hypothesized that when TLR9 function is compromised, errors in this fundamental mechanism could lead to cognitive impairment, psychiatric disorders, accelerated aging, and neurodegenerative disorders.

conclusions

In summary, the study found that learning-related stimuli trigger a TLR9-mediated cascade of DNA damage and DNA repair that engage hippocampal neuronal clusters in memory formation. TLR9-mediated inflammatory responses have a vital role in memory formation, and impairments in TLR9 function could be implicated in cognitive, neurodegenerative, and psychiatric disorders.