The authors successfully demonstrated the direct conversion of astrocytes to dopaminergic neurons of an in vitro mouse model of Parkinson. These newly-created neurons were shown to integrate into the dopamine pathway, functionally replacing lost neurons in the disease model. The authors were also able to recreate the same effects using an antisense oligonucleotide targeting PTB. 

4. Zhou, H. et al. Glia-to-neuron conversion by CRISPR-CasRx alleviates symptoms of neurological disease in mice. Cell 181, 590-603 (2020).

5. Beal, M. Parkinson's disease: a model dilemma. Nature 466, S8–S10 (2010). https://doi.org/10.1038/466S8a

6.  Gascón, S., Masserdotti, G., Russo, G. L. & Götz, M. Direct Neuronal Reprogramming: Achievements, Hurdles, and New Roads to Success. Cell Stem Cell 21, 18–34 (2017). 

Strengths of this paper include: detailed tracking of cell type and origin by monitoring the expression of numerous markers (as explained in methods), examining both the molecular mechanism of astrocyte conversion (the PTB pathway) as well as the rescuing effects on the disease model and disease related motor phenotypes (a clinically relevant application). As well, the authors verified results both in vivo and in vitro, removing any discrepancies that one method may have procured over the other. Another major strength of the paper is that the authors tested 2 month old mice and 1 year old mice. This is an age comparable in humans to the age of onset of Parkinson’s. This allows the authors to discern age-related effects/deficiencies on neuronal reprogramming. ASOs are able to diffuse t/membranes, do not have immunity response, have been used in treatment in other neurological diseases such as spinal muscular atrophy (Spinraza)². Furthermore, clinical tests confirm the safety of this method.

A major weakness of this paper is the incomplete recapitulation of disease phenotypes in the mouse model used by the authors. This includes the lack of Lewy body formation, bradykinesia and dementia. The lack of such phenotypes is important since they are major components of the disease and necessarily implies that there are molecular interactions seen in the disease that are unaccounted for in this study³. Essentially authors are held back by the lack of an optimal disease model. Dementia comorbid with Parkinson’s is known to have alpha-syn and tau pathologies, an aspect not taken into account in this study. Nevertheless, this model was the best option for the author’s means since it does recapitulate the degradation of the nigrostriatal dopamine pathway, while other models (i.e. genetic) are not consistent in that respect. Beal 2010

Antisense Oligonucleotides (ASOs) are short, synthetic, single-stranded oligodeoxynucleotides that can alter RNA and reduce, restore, or modify protein expression through several distinct mechanisms³. One of these mechanisms involves ASOs binding to a specific target mRNA which results in gene silencing. 

ASO is a suitable alternative to the AAV vector - good for administering in clinical setting 

Ultimately, incorporation of new DA neurons in nigrostriatal pathway was found to be time-dependent

* arrows= TH+ DA neurons
* arrowheads= RFP+ cells
Image from Qian et al., (2020) Figure 3g and Figure 9g,h