It is clear that in both human development and in vitro, LMNs and UMNs require significantly more time for neurogenesis and maturation to occur than murine counterparts. Aside from apparent challenges such as increased time in culture, study length and cost, the ability of in vitro hPSC-derived cells to exhibit mature, native functional properties are crucial for accurate in vitro modeling of adult-onset diseases. An interesting approach to artificially accelerate the aging process genetically manipulates iPSC-derived neurons to express progerin, a mutated form of lamin A, that causes the premature aging disease Progeria129 (link). Upon progerin overexpression, iPSC-derived dopaminergic neurons exhibit age-associated changes, however the non-age related effects of artificial Lamin A expression in neurons is unknown. By interfering with Notch signaling, small molecule γ-secretase inhibitors, namely DAPT and Compound E, have been shown to accelerate neuronal differentiation by delaying G1/S phase long enough to commit neurons to neurogenesis130 (link), 131 (link). These inhibitors have been shown to be effective in hPSC-LMN protocols as well25 . While these compounds have proven useful in promoting cell cycle exit, the consequences of this treatment in MN diversity is unexplored. As different MN subtype fates emerge at distinct developmental timepoints in vivo, premature cell cycle exit could obstruct later MN subtype programs.
An alternate method to generate human MNs in vitro is described as induced-MNs (iMN)28 (link). This process circumvents reprogramming to pluripotency by directly converting patient somatic cells to MNs through transgenic expression of transcription factors that drive MN differentiation. By avoiding the epigenetic “reset” that occurs during reprogramming to pluripotency132 (link), this approach has been shown to maintain age-related epigenetic signatures accrued over the lifetime of the patient133 . iMNs display unique age-related cellular defects not observed in hPSC-MN approaches. However, these approaches are challenged by genomic instability resulting from increased somatic cell expansion requirements, as well as deficient MN maturation in vitro.
It is widely accepted that the maturation status of iPSC-derived cells remains a significant hurdle to the field of regenerative medicine at large, and yet it remains largely under-explored in humans4 (link). The ability to harness cell signaling to promote maturation in vitro, rests on increased understanding of anatomical, molecular and electrophysiological data from fetal, adult and aged human spinal post mortem tissue. Projects such as the Allen Brain Atlas provide valuable templates to guide anatomical- and genomic-level evaluation of native human MN maturation and aging. However, detailed functional data on human MNs are lacking and therefore, comparative evaluation of functional maturity relies largely upon the characterization of other mammalian species, often at cost to fidelity.
An alternate method to generate human MNs in vitro is described as induced-MNs (iMN)28 (link). This process circumvents reprogramming to pluripotency by directly converting patient somatic cells to MNs through transgenic expression of transcription factors that drive MN differentiation. By avoiding the epigenetic “reset” that occurs during reprogramming to pluripotency132 (link), this approach has been shown to maintain age-related epigenetic signatures accrued over the lifetime of the patient133 . iMNs display unique age-related cellular defects not observed in hPSC-MN approaches. However, these approaches are challenged by genomic instability resulting from increased somatic cell expansion requirements, as well as deficient MN maturation in vitro.
It is widely accepted that the maturation status of iPSC-derived cells remains a significant hurdle to the field of regenerative medicine at large, and yet it remains largely under-explored in humans4 (link). The ability to harness cell signaling to promote maturation in vitro, rests on increased understanding of anatomical, molecular and electrophysiological data from fetal, adult and aged human spinal post mortem tissue. Projects such as the Allen Brain Atlas provide valuable templates to guide anatomical- and genomic-level evaluation of native human MN maturation and aging. However, detailed functional data on human MNs are lacking and therefore, comparative evaluation of functional maturity relies largely upon the characterization of other mammalian species, often at cost to fidelity.
