In the last few years, we have witnessed a tremendous advancement in cell culturing methods. The scene has been dominated by the optimization of 3D in vitro models either in the form of dissociated neuronal cultures or of the so-called organoids. The attempt of introducing new methods responds to the difficulty of capturing the multifactorial nature of diseases such as Alzheimer’s (AD) and Parkinson’s (PD) diseases in conventional models. There is certainly the need for developing more relevant pre-clinical set-ups and their use in association to stem cell technologies might provide previously unexplored modelling opportunities. The laboratories of professors Tanzi and Kim, first described in the context of AD, how the 3D environment allows recapitulating key features of AD pathology on-a-dish [1, 2]. Compared to standard 2D cultures, stem cell derived-neurons carrying familial AD mutations (APP and PS1), differentiated in 3D, show accumulation of critical amounts of amyloid-β peptide (Aβ) and hyperphosphorylated tau protein. In the context of PD, we show that 3D cultures of dopaminergic neurons carrying the LRRK2-G2019S mutation progressively degenerate in association with mitochondrial defects . In 2D cultures, similar degeneration was previously observed only with the use of stress factors. An explanation for this different behavior might lie in the fact that cells embedded in matrix surrogates experience a more physiological equilibration and transport of soluble factors compared to 2D-maintained cells [4, 5]. The more confined environment favors the synthesis of extracellular matrix proteins and facilitates cell-cell interaction. Interestingly, gene and protein expression profiles have shown to significantly differ when comparing 2D and 3D cultures [6, 7]. Cells cultured in 2D show approximately 30% of differentially expressed genes compared to cells in vivo . This is not surprising considering that the 2D environment provides cells with mechanical constraints and favors artificial geometries. Because of this fine mechanical and biochemical interplay between neurons and their surrounding matrix, disease-specific subtle phenotypes might be then missed when studied only in 2D.
A further step forward in disease-modelling optimization has been the use of cerebral organoids, first described in 2013 . These self-organizing brain tissues with mature cortical neuronal subtypes might represent a good starting point to study AD, but provided also the hippocampal area of the brain can be better developed in the future. In the context of PD, we and others have generated organoids resembling the midbrain area of the human brain [10, 11].
The next challenge in this exciting field will be the improvement of quality control measures and standards. This in particular with the brain organoids as major variability has been described during their generation. In addition, the use of stem cell 3D models for the study of neurodegenerative mechanisms has been often criticized because of the thought that neurons do not reach a sufficient degree of maturation. However, 3D conditions have been shown to promote neuronal maturity and to increase adult tau isoforms levels . That said, the field will need to introduce the aging component in the model to further refine the methods and capture faithfully more traits of neurodegenerative mechanisms. Although the technology currently presents several limitations, 3D is certainly a powerful approach which represents a step closer to the in vivo situation and can fuel new drug discovery options.
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