Memory processing by hippocampal adult-born neurons

This review provides an integrative overview of the functional roles of adult neurogenesis in the hippocampal dentate gyrus (DG), focusing specifically on its impact on memory processes across the lifespan. A distinguishing feature of this review is its systematic approach, organizing the contributions of adult-born neurons (ABNs) chronologically through the stages of memory-from initial encoding, through sleep-dependent consolidation, retrieval, and finally forgetting. Although the existence and extent of adult neurogenesis in the human DG remain debated, accumulating evidence suggests that ABNs support cognitive functions throughout adulthood. This perspective gains particular importance when considering cognitive decline associated with aging and neurological disorders such as Alzheimer's disease, which are linked to substantial reductions in adult neurogenesis. We compare traditional models of DG function with emerging evidence highlighting both shared and unique contributions of ABNs. For example, the DG is well-established for its role in pattern separation, and as key mediators of this function, ABNs-due to their transiently heightened plasticity and excitability-appear critical for discriminating novel or similar experiences. On the other hand, recent findings underscore the distinct and essential role of ABNs in memory consolidation during REM sleep, suggesting specialized functions of ABNs that are absent in developmentally born granule cells in the DG. Clinically, the potential therapeutic importance of enhancing neurogenesis in memory-related disorders, including post-traumatic stress disorder (PTSD), is emphasized, highlighting promising treatments such as memantine. Lastly, we outline key unresolved questions, advocating for future research aimed at understanding ABN-specific mechanisms. Far from being a mere evolutionary vestige, hippocampal ABNs represent dynamic and essential elements of neural plasticity that are critical for memory formation, adaptation, and resilience across the lifespan.