Spatial Coding Dysfunction and Network Instability in the Aging Medial Entorhinal Cortex.

Across species, spatial memory declines with age, possibly reflecting altered hippocampal and medial entorhinal cortex (MEC) function. However, the integrity of cellular and network-level spatial coding in aged MEC is unknown. Here, we leveraged in vivo electrophysiology to assess MEC function in young, middle-aged, and aged mice navigating virtual environments. In aged grid cells, we observed impaired stabilization of context-specific spatial firing, correlated with spatial memory deficits. Additionally, aged grid networks shifted firing patterns often but with poor alignment to context changes. Aged spatial firing was also unstable in an unchanging environment. In these same mice, we identified 458 genes differentially expressed with age in MEC, 61 of which had expression correlated with spatial firing stability. These genes were enriched among interneurons and related to synaptic transmission. Together, these findings identify coordinated transcriptomic, cellular, and network changes in MEC implicated in impaired spatial memory in aging.

Endogenous neuronal DNA double-strand breaks are not sufficient to drive brain aging and neurodegeneration.

Loss of genomic information due to the accumulation of somatic DNA damage has been implicated in aging and neurodegeneration 1-3 . Somatic mutations in human neurons increase with age 4 , but it is unclear whether this is a cause or a consequence of brain aging. Here, we clarify the role of endogenous, neuronal DNA double-strand breaks (DSBs) in brain aging and neurodegeneration by generating mice with post-developmental inactivation of the classical non-homologous end-joining (C-NHEJ) core factor Xrcc4 in forebrain neurons. Xrcc4 is critical for the ligation step of C-NHEJ and has no known function outside of DSB repair 5,6 . We find that, unlike their wild-type counterparts, C-NHEJ-deficient neurons accumulate high levels of DSB foci with age, indicating that neurons undergo frequent DSBs that are typically efficiently repaired by C-NHEJ across their lifespan. Genome-wide mapping reveals that endogenous neuronal DSBs preferentially occur in promoter regions and other genic features. Analysis of 3-D genome organization shows intra-chromosomal clustering and loop extrusion of neuronal DSB regions. Strikingly, however, DSB accumulation caused by loss of C-NHEJ induces only minor epigenetic alterations and does not significantly affect gene expression, 3-D genome organization, or mutational outcomes at neuronal DSBs. Despite extensive aging-associated accumulation of neuronal DSBs, mice with neuronal Xrcc4 inactivation do not show neurodegeneration, neuroinflammation, reduced lifespan, or impaired memory and learning behavior. We conclude that the formation of spontaneous neuronal DSBs caused by normal cellular processes is insufficient to cause brain aging and neurodegeneration, even in the absence of C-NHEJ, the principal neuronal DSB repair pathway.

Neuronal activation of Gαq EGL-30/GNAQ late in life rejuvenates cognition across species.

Loss of cognitive function with age is devastating. EGL-30/GNAQ and Gαq signaling pathways are highly conserved between C. elegans and mammals, and murine Gnaq is enriched in hippocampal neurons and declines with age. We found that activation of EGL-30 in aged worms triples memory span, and GNAQ gain of function significantly improved memory in aged mice: GNAQ(gf) in hippocampal neurons of 24-month-old mice (equivalent to 70- to 80-year-old humans) rescued age-related impairments in well-being and memory. Single-nucleus RNA sequencing revealed increased expression of genes regulating synaptic function, axon guidance, and memory in GNAQ-treated mice, and worm orthologs of these genes were required for long-term memory extension in worms. These experiments demonstrate that C. elegans is a powerful model to identify mammalian regulators of memory, leading to the identification of a pathway that improves memory in extremely old mice. To our knowledge, this is the oldest age at which an intervention has improved age-related cognitive decline.

Loss of neuronal Tet2 enhances hippocampal-dependent cognitive function.

DNA methylation has emerged as a critical modulator of neuronal plasticity and cognitive function. Notwithstanding, the role of enzymes that demethylate DNA remain to be fully explored. Here, we report that loss of ten-eleven translocation methylcytosine dioxygenase 2 (Tet2), which catalyzes oxidation of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC), in adult neurons enhances cognitive function. In the adult mouse hippocampus, we detected an enrichment of Tet2 in neurons. Viral-mediated neuronal overexpression and RNA interference of Tet2 altered dendritic complexity and synaptic-plasticity-related gene expression in vitro. Overexpression of neuronal Tet2 in adult hippocampus, and loss of Tet2 in adult glutamatergic neurons, resulted in differential hydroxymethylation associated with genes involved in synaptic transmission. Functionally, overexpression of neuronal Tet2 impaired hippocampal-dependent memory, while loss of neuronal Tet2 enhanced memory. Ultimately, these data identify neuronal Tet2 as a molecular target to boost cognitive function.

Microglial GPR56 is the molecular target of maternal immune activation-induced parvalbumin-positive interneuron deficits.

Parvalbumin-positive (PV+) interneurons play a critical role in maintaining circuit rhythm in the brain, and their reduction is implicated in autism spectrum disorders. Animal studies demonstrate that maternal immune activation (MIA) leads to reduced PV+ interneurons in the somatosensory cortex and autism-like behaviors. However, the underlying molecular mechanisms remain largely unknown. Here, we show that MIA down-regulates microglial Gpr56 expression in fetal brains in an interleukin-17a-dependent manner and that conditional deletion of microglial Gpr56 [Gpr56 conditional knockout (cKO)] mimics MIA-induced PV+ interneuron defects and autism-like behaviors in offspring. We further demonstrate that elevated microglial tumor necrosis factor-α expression is the underlying mechanism by which MIA and Gpr56 cKO impair interneuron generation. Genetically restoring Gpr56 expression in microglia ameliorates PV+ interneuron deficits and autism-like behaviors in MIA offspring. Together, our study demonstrates that microglial GPR56 plays an important role in PV+ interneuron development and serves as a salient target of MIA-induced neurodevelopmental disorders.

Context-Specific Transcription Factor Functions Regulate Epigenomic and Transcriptional Dynamics during Cardiac Reprogramming.

Ectopic expression of combinations of transcription factors (TFs) can drive direct lineage conversion, thereby reprogramming a somatic cell's identity. To determine the molecular mechanisms by which Gata4, Mef2c, and Tbx5 (GMT) induce conversion from a cardiac fibroblast toward an induced cardiomyocyte, we performed comprehensive transcriptomic, DNA-occupancy, and epigenomic interrogation throughout the reprogramming process. Integration of these datasets identified new TFs involved in cardiac reprogramming and revealed context-specific roles for GMT, including the ability of Mef2c and Tbx5 to independently promote chromatin remodeling at previously inaccessible sites. We also find evidence for cooperative facilitation and refinement of each TF's binding profile in a combinatorial setting. A reporter assay employing newly defined regulatory elements confirmed that binding of a single TF can be sufficient for gene activation, suggesting that co-binding events do not necessarily reflect synergy. These results shed light on fundamental mechanisms by which combinations of TFs direct lineage conversion.