A new age in understanding adult hippocampal neurogenesis in Alzheimer's disease

Several lines of evidence have established that proliferation and differentiation of neural stem cells into neurons within the sub-granular zone of the dentate gyrus, a process named adult hippocampal neurogenesis, contribute to maintaining healthy cognitive functions throughout life. The rate of adult hippocampal neurogenesis decreases with aging and a premature impairment of adult hippocampal neurogenesis has been observed both in animal models of Alzheimer’s disease and human post-mortem tissues. The causal relationship between adult hippocampal neurogenesis and the development of Alzheimer’s disease pathology has, however, not been established. This is partly due to the limitation of recapitulating the development of Alzheimer’s disease pathology in rodent models and the lack of translatable biomarkers to identify tractable targets in humans. While it is tempting to postulate that adult hippocampal neurogenesis could be leveraged to improve cognitive deficits in Alzheimer’s disease, consensual results have yet to be reached to fully explore this hypothesis. In this review, we discuss how the recent progress in identifying molecular pathways in adult hippocampal neurogenesis provides a good framework to initiate strategies for drug-based intervention in neurodegenerative diseases, especially in Alzheimer’s disease. We outline how discrepancies in pre-clinical disease models and experimental methodology have resulted in contradictory findings and propose a shift towards using more translatable approaches to model neurogenesis in Alzheimer’s disease. In particular, we review how exploring novel experimental paradigms including the use of human induced pluripotent stem cells and more complex cell culture systems, as well as standardizing protocols used to investigate evidence of neurogenesis in human tissues, could deliver deeper mechanistic insights that would kick-start innovative drug discovery efforts to promote healthy aging and cellular rejuvenation.

Targeting impaired adult hippocampal neurogenesis in ageing by leveraging intrinsic mechanisms regulating Neural Stem Cell activity

Deficits in adult neurogenesis may contribute to the aetiology of many neurodevelopmental, psychiatric and neurodegenerative diseases. Genetic ablation of neurogenesis provides proof of concept that adult neurogenesis is required to sustain complex and dynamic cognitive functions, such as learning and memory, mostly by providing a high degree of plasticity to neuronal circuits. In addition, adult neurogenesis is reactive to external stimuli and the environment making it particularly susceptible to impairment and consequently contributing to comorbidity. In the human brain, the dentate gyrus of the hippocampus is the main active source of neural stem cells that generate granule neurons throughout life. The regulation and preservation of the pool of neural stem cells is central to ensure continuous and healthy adult hippocampal neurogenesis (AHN). Recent advances in genetic and metabolic profiling alongside development of more predictive animal models have contributed to the development of new concepts and the emergence of molecular mechanisms that could pave the way to the implementation of new therapeutic strategies to treat neurological diseases. In this review, we discuss emerging molecular mechanisms underlying AHN that could be embraced in drug discovery to generate novel concepts and targets to treat diseases of ageing including neurodegeneration. To support this, we review cellular and molecular mechanisms that have recently been identified to assess how AHN is sustained throughout life and how AHN is associated with diseases. We also provide an outlook on strategies for developing correlated biomarkers that may accelerate the translation of pre-clinical and clinical data and review clinical trials for which modulation of AHN is part of the therapeutic strategy.

Using Tetracysteine-Tagged TDP-43 with a Biarsenical Dye To Monitor Real-Time Trafficking in a Cell Model of Amyotrophic Lateral Sclerosis

TAR DNA-binding protein 43 (TDP-43) has been identified as the major constituent of the proteinaceous inclusions that are characteristic of most forms of amyotrophic lateral sclerosis (ALS) and ubiquitin positive frontotemporal lobar degeneration (FTLD). Wild type TDP-43 inclusions are a pathological hallmark of >95% of patients with sporadic ALS and of the majority of familial ALS cases, and they are also found in a significant proportion of FTLD cases. ALS is the most common form of motor neuron disease, characterized by progressive weakness and muscular wasting, and typically leads to death within a few years of diagnosis. To determine how the translocation and misfolding of TDP-43 contribute to ALS pathogenicity, it is crucial to define the dynamic behavior of this protein within the cellular environment. It is therefore necessary to develop cell models that allow the location of the protein to be defined. We report the use of TDP-43 with a tetracysteine tag for visualization using fluorogenic biarsenical compounds and show that this model displays features of ALS observed in other cell models. We also demonstrate that this labeling procedure enables live-cell imaging of the translocation of the protein from the nucleus into the cytosol.

The relevance of contact-independent cell-to-cell transfer of TDP-43 and SOD1 in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease involving the formation of cytoplasmic aggregates by proteins including TDP-43 and SOD1, in affected cells in the central nervous system (CNS). Pathology spreads from an initial site of onset to contiguous anatomical regions. There is evidence that for disease-associated proteins, including TDP-43 and SOD1, non-native protein conformers can promote misfolding of the natively folded counterparts, and cell-to-cell transfer of pathological aggregates may underlie the spread of the disease throughout the CNS. A variety of studies have demonstrated that SOD1 is released by neuron-like cells into the surrounding culture medium, either in their free state or encapsulated in extracellular vesicles such as exosomes. Extracellular SOD1 can then be internalised by naïve cells incubated in this conditioned medium, leading to the misfolding and aggregation of endogenous intracellular SOD1; an effect that propagates over serial passages. A similar phenomenon has also been observed with other proteins associated with protein misfolding and progressive neurological disorders, including tau, α-synuclein and both mammalian and yeast prions. Conditioned media experiments using TDP-43 have been less conclusive, with evidence for this protein undergoing intercellular transfer being less straightforward. In this review, we describe the properties of TDP-43 and SOD1 and look at the evidence for their respective abilities to participate in cell-to-cell transfer via conditioned medium, and discuss how variations in the nature of cell-to-cell transfer suggests that a number of different mechanisms are involved in the spreading of pathology in ALS.

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Protein aggregates transfer between cells in motor neuron disease.

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Cell contact-independent mechanisms may be a route of transfer.

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SOD1 undergoes cell-to-cell transfer via conditioned medium in cell culture.

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It is still unclear whether TDP-43 consistently undergoes cell-to-cell transfer

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Differences between the two proteins may explain this observation.

Flow cytometric measurement of the cellular propagation of TDP-43 aggregation

Amyotrophic lateral sclerosis is a devastating neuromuscular degenerative disease characterized by a focal onset of motor neuron loss, followed by contiguous outward spreading of pathology including TAR DNA-binding protein of 43 kDa (TDP-43) aggregates. Previous work suggests that TDP-43 can move between cells. Here we used a novel flow cytometry technique (FloIT) to analyze TDP-43 inclusions and propagation. When cells were transfected to express either mutant G294A TDP-43 fused to GFP or wild type TDP-43fused to tomato red and then co-cultured, flow cytometry detected intact cells containing both fusion proteins and using FloIT detected an increase in the numbers of inclusions in lysates from cells expressing wild type TDP-43-tomato. Furthermore, in this same model, FloIT analyses detected inclusions containing both fusion proteins. These results imply the transfer of TDP-43 fusion proteins between cells and that this process can increase aggregation of wild-type TDP-43 by a mechanism involving co-aggregation with G294A TDP-43.