Limitations and Challenges in Modeling Diseases Involving Spinal Motor Neuron Degeneration in Vitro

Abstract

Pathogenic conditions involving degeneration of spinal motor neurons (MNs), such as amyotrophic lateral sclerosis, sarcopenia, and spinal cord injury, mostly occur in individuals whose spinal MNs are fully mature. There is currently no effective treatment to prevent death or promote axonal regeneration of the spinal MNs affected in these patients. To increase our understanding and find a cure for such conditions, easily controllable and monitorable cell culture models allow for a better dissection of certain molecular and cellular events that cannot be teased apart in whole organism models. To date, various types of spinal MN cultures have been described. Yet these models are all based on the use of immature neurons or neurons uncharacterized for their degree of maturity after being isolated and cultured. Additionally, studying only MNs cannot give a comprehensive and complete view of the neurodegenerative processes usually involving other cell types. To date, there is no confirmed in vitro model faithfully emulating disease or injury of the mature spinal MNs. In this review, we summarize the different limitations of currently available culture models, and discuss the challenges that have to be overcome for developing more reliable and translational platforms for the in vitro study of spinal MN degeneration.

In vitro Neuromuscular Junction Induced from Human Induced Pluripotent Stem Cells

“…Various strategies have been published for generating in vitro NMJ of different maturation stages, but the in vitro NMJ generated in our system showed the significant maturity, as we can observe the transition of gamma to epsilon subunits of AChR during the culture 18 . Maturity is an important consideration for disease modeling with in vitro NMJ 21 .…”

Challenges and Future Prospects on 3D in-vitro Modeling of the Neuromuscular Circuit

“…The co-culture of main cell-types participating in the spinal-locomotion circuit is mandatory to provide a native microenvironment, including the inherent release of growth-factors, as well as to support the viability and maturation of both muscle and neurons, and axon elongation of MNs (Gingras et al, 2008). For instance, spinal MNs cannot achieve proper maturation even after long-term maintenance, unless cultured with muscle-cells, and Schwann cells, as previously reviewed (Bucchia et al, 2018).…”

“…Besides, both SN and MN could be altered in particular NMDs (Jablonka et al, 2006; Rumsey et al, 2010; Guo et al, 2017), but not being many available studies focused on the muscle spindle (Taylor et al, 2005; Dagberg and Alstermark, 2006; Rumsey et al, 2010; Bewick and Banks, 2015; Matthews, 2015; Guo et al, 2017) challenges the task of mimicking and characterizing the mechanosensory spinal-locomotion circuit. Additionally, glial cells are also affected and involved in several neuromuscular pathologies (Lobsiger and Cleveland, 2007; Vilmont et al, 2016; Bucchia et al, 2018). Yet, most publications do not consider them.…”

“…Indeed, studying only functional changes in MNs cannot give a comprehensive and complete picture of the process as it is also regulated by non-neuronal cells (Lobsiger and Cleveland, 2007; Bucchia et al, 2018; Maimon et al, 2018). For instance, a recent study performed by Maimon et al (2018) demonstrated that axon degeneration only occurred with both MN and muscle cells carrying the genetic mutation indicative of the disorder.…”

“…For instance, a recent study performed by Maimon et al (2018) demonstrated that axon degeneration only occurred with both MN and muscle cells carrying the genetic mutation indicative of the disorder. Furthermore, the morphology of spinal MN axons in-vitro differs from the one presented in-vivo : contrary to in-vivo , axonal terminals in-vitro manifest growth-cones, and are prone to regenerate and lengthen in response to neurotrophic factors required for the in-vitro maintenance of the culture (Bucchia et al, 2018). On top of that, another caveat is the fact that extrapolating effects of short-term studies to longer-term disease processes is often not correlated.…”