The answer is in the mitochondria
Many brain diseases, injuries and neurodegenerative processes are characterized by the loss of neurons, which is not replenished. Regenerative medicine techniques are aimed at restoring functioning neurons through transplantation, stem cell differentiation, or direct conversion of endogenous non-neuronal cells into functioning neurons.
Researchers from the Munich Research Center named after Helmholtz Zentrum München and Ludwig Maximilian University of Munich are working in depth in the field of direct conversion of glial cells into neurons. Glial cells are the most common type of brain cells and can regenerate when injured. Currently, researchers have learned how to convert glial cells into neurons, but in the process of reprogramming, many cells die. This means that only a small part of the glial cells turn into functioning neurons, which makes the process ineffective.
Learning new methods
Magdalena Goetz and her group investigated potential obstacles in the transformation process and took a new path: most of the work focused on the genetic aspects of direct reprogramming of neurons, and they decided to study the role of mitochondria and cellular metabolism in this process. In her previous study, Goetz, together with the group of Markus Conrad from Helmholtz Zentrum München, found that cells die due to excessive amounts of reactive oxygen species during the transformation process.
Scientists hypothesized that if the metabolism of glial cells were reprogrammed towards the metabolism of a neuron, this could increase the efficiency of conversion. Given their previous data, the researchers focused on mitochondria. They extracted mitochondria from neurons and astrocytes (a type of glial cell) of mice and compared the proteins expressed. It was found that the mitochondrial proteome of neurons and astrocytes differs by 20%.
Reprogrammed neurons express neuronal mitochondrial proteins at a late stage
Knowing how much the mitochondrial proteome of neurons differs from astrocytes, Goetz had to make sure whether neurons that transform from astrocytes actually acquire the mitochondrial proteome of a neuron. In the standard reprogramming process, astrocytes turn into neurons within a few days and develop into functioning neurons within two weeks. Strikingly, the mitochondrial proteins typical of neurons appeared in these cells relatively late in the reprogramming process – only after one week. Given that most cells die before this time, this may be the cause of failures. In addition, the cells that failed to be reprogrammed still expressed astrocytic mitochondrial proteins. Thanks to this new understanding, the researchers hypothesized that the absence of mitochondrial proteins of neurons can block the conversion process.
Improvement and acceleration of conversion due to metabolism
To overcome this obstacle, the group used CRISPR/Cas9 technology in collaboration with the groups of Stefan Stricker and Wolfgang Wurst at Helmholtz Zentrum München. With the help of new gene activation tools developed by this group, the synthesis of neuronal mitochondrial proteins was activated at an early stage of the process of reprogramming astrocytes into neurons. By controlling one or two mitochondrial proteins, the researchers obtained four times as many reprogrammed neurons. In addition, neurons appeared and matured faster, as evidenced by continuous visualization in real time.
Thus, a change in the expression of several mitochondrial proteins greatly affected the reprogramming rate. This proves how important the intercellular differences of mitochondrial proteins are. The authors found other differences in the proteins of organelles of different cell types, which reach 70%. Perhaps this will become the basis for further improvement of the process of reprogramming neurons, so that after a head injury it was possible to obtain in vivo cells as similar as possible to endogenous neurons.
Article by G.L.Russo et al. CRISPR-Mediated Induction of Neuron-Enriched Mitochondrial Proteins Boosts Direct Glia-to-Neuron Conversion is published in the journal Cell Stem Cell.
Aminat Adzhieva, portal "Eternal Youth" http://vechnayamolodost.ru based on Helmholtz Zentrum München: Researchers Improve Neuronal Reprogramming by Manipulating Mitochondria.