Box 2. Outstanding Questions
- What molecular markers distinguish prenatal motor neurons from their postnatal counterparts?
Though studies of embryonic development have revealed molecular and functional markers of terminally differentiated motor neurons, our knowledge of in particular the molecular postnatal changes and distinguishing characteristics that arise over time is incomplete. While it is clear that important functional distinctions take place throughout postnatal development, further effort is needed to elucidate when and through what mechanism these changes are taking place. Stem cell differentiated Hb9 and Islet1 expressing embryonic motor neurons that are electro physiologically competent have yet to undergo maturation towards the various subclasses of neurons present in adult motor pools, and accomplishing this goal may greatly advance our ability to model degenerative phenotypes of motor neuron disease.
- Which methods of in vitro motor neuron aging will generate cellular phenotypes most true to those seen in natural aging?
Current methods of accelerating neural aging involve the use of stressors such as induction of oxidative stress, or exposure to aging associated proteins such as Progerin. While these methods are capable of inducing particular later stage phenotypes in some patient derived models of degenerative diseasesuch as DNA damage and cell death, these processes differ from typical progressions of aging, and therefore their legitimacy in producing cell types similar to the in vivo equivalents that they are intended to model requires further scrutiny. Alternatively, exposing early stage cultured neurons to aged serum and cerebrospinal fluid, which contain natural aging signals, or discovering and distilling the responsible factors present in these fluids for direct incubation with motor neurons may demonstrate to be effective methods of provoking an aged phenotype.
- What criteria best define aging in cultured motor neurons?
While aging is a key component in the onset of motor neuron degeneration disorders it remains scarcely understood. Changes in cell morphology, genomic integrity, and mitochondrial function are observed in late adult primary spinal neurons, and achieving these phenotypes is most often the focus of in vitro motor neuron aging, though this may not necessarily be the best representation and does not allow for a graded assessment of maturation. Obtaining a better profile of molecular alterations that take place throughout aging will allow for more physiologically accurate modeling by in vitro stem cell derived neurons.
- Which phenotypic, functional, and molecular changes that occur upon aging towards disease relevant stages correlate with, and are responsible for the degenerative effects seen in motor neuron disorder?
It remains puzzling how and why there is a timed penetrance in genetic disorders where the gene product itself is ever present throughout the lifetime of an organism. This occurrence alludes at the complex nature of these disorders, indicating multiple events are required to take place before these diseases manifest. Knowledge of the processes that are required for the onset of motor neuron disease may be of great value in preventing and reverting their degenerative pathology.