Executive summary

Previous study has uncovered that post-mitotic neurons in the brain, when prompted to re-enter the cell cycle, undergo a swift transition into senescence in a laboratory mouse model (Chow et al., 2019). Recently, a new study has uncovered that this phenomenon is conserved in the human brain, through a new bioinformatics approach that also allows researchers, for the first time, to unbiasedly profile the nature of these cells (Wu et al., 2024). Notably, this re-entry phenomenon occurs more frequently in individuals affected by Alzheimer's disease. The findings hold promise for gaining deeper insights into the neurodegeneration process, and the robustness of this new bioinformatics-based study approach is validated in other disease models and in a cross-species settings.

Content
Research background
Alzheimer disease (AD) is the most common form of dementia yet there is currently no cure. Despite the recent approval of anti-amyloid-β (Aβ) monoclonal antibody Lecanemab has offered hope, its limited effectiveness to mainly early stages of the disease and concerns over brain swelling or bleeding side effects (van Dyck et al., 2023) suggest more alternative strategies are needed to help a wider spectrum of patients on the AD continuum (Aisen et al., 2017). Indeed, prevention trials grounding on risk reductions may hold greater promises in preserving neuronal health and brain functions (Crous-Bou et al., 2017).

Traditionally, it was widely believed that the majority of neurons in the brain, having reached a post-mitotic stage, no longer possessed the ability to divide. However, erroneous events of neuronal cell cycle re-entry are reported previously as a type of irreversible changes to a small subset of mature neurons (Wong and Chow, 2023), which emerge frequently at times long before the deposition of Aβ and tau neurofibrillary tangles – the two classic pathologies of AD (Wong and Chow, 2023). This suggested that any attempts of cell cycle re-entry may directly prime neuronal cell dyshomeostasis, which may promote aberrant processing of amyloid precursor protein (APP) and tau hyperphosphorylation that subsequently seed disease pathologies. Therefore, we believe that interventions designed against key regulators underlying such irreversible events in neurons are highly desirable as a novel strategy to prevent AD-related disease pathogenesis.

A need for a novel analytical methodology to objectively identify and profile these cells
For more than two decades ever since these cells were first detected and discovered (Smith and Lippa, 1995), both cytogenetic- and immune-histology technique remain as the mainstay experimental approaches for investigating neuronal cell cycle re-entry events. These techniques, however, are insufficient to dissect the global molecular changes occurred within these sub-populations of neurons. To overcome that, our team leveraged publicly available "snRNA-seq" datasets prepared from brain tissues harvested from human subjects where individual cell nuclei are isolated and their RNA is sequenced, providing a snapshot of the cell's activity at the time of sampling. The cell cycle itself consists of distinct phases, including growth, DNA synthesis, division-specific growth, and mitosis, and each phase is defined by a characteristic set of proteins required to execute that stage of the cycle. Therefore, by analysing the RNA profiles, we were able to assign each nucleus with a score based on the expression levels of approximately 350 cell cycle-related genes, using a bioinformatics-based approach. The analysis revealed that a distinct subpopulation of excitatory neurons had indeed re-entered the cell cycle. However, these cells did not typically progress successfully through the full cell cycle to generate daughter neurons. Instead, the cells that had re-initiated the cell cycle also exhibited elevated expression of genes associated with cellular senescence – a safeguarding mechanism a normal cell would normally use to permanently halt any aberrant cell cycle events. In the essence, the neurons had reawakened the cell cycle machinery, but this reactivation led them to enter a state of senescence rather than completing cell division.

Uncovering their clinical relevance in human neurodegenerative diseases
By far, due to the scarce prevalence and random spatial distribution of these cells within the brain, their molecular profiles and disease-specific heterogeneities remain unclear when the analyses were conducted with the traditional experimental approaches. With our bioinformatics-based tool, it allows our team for the first time to uncovered that that neurons in the brains of AD patients re-entered the cell cycle at an elevated rate. Furthermore, those neurons that had re-entered the cell cycle and aged exhibited increased expression of multiple genes associated with a higher risk of AD, including genes that directly contribute to the production of Aβ. To further test the board applicability of our newly devised pipeline, our team also conducted similar analyses in brains from individuals with Parkinson's disease and Lewy body dementia, which also displayed an increased proportion of re-entering neurons compared to healthy brains.

Conclusion

While the neurobiological implications of this elevated rate of neuronal cell cycle re-entry in the diseased brain remain unclear at this stage. However, the analytical methodology employed in this study offers the field a new tool to study these cells in the brain, it also provides deeper insights into distinct neuronal subpopulations within the brain, in addition to shedding light on the underlying disease mechanisms in neurodegenerative disorders.

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Author:
Professor Kim Chow Hei Man, Assistant Professor, School of Life Sciences, The Chinese University of Hong Kong

June 2024