Converting Astrocytes Into Neurons Reverses Motor Deficits in a Model of Parkinson’s Disease

Post by Amanda McFarlan

What's the science?

Parkinson’s disease is a neurodegenerative disorder associated with motor deficits caused by the substantial loss of dopaminergic neurons in the substantia nigra (a region involved in reward and movement). Recent studies have shown that it is possible to reprogram fibroblasts (cells involved in synthesizing the extracellular matrix and collagen) to become neurons by downregulating the RNA-binding proteins polypyrimidine tract-binding protein (PTB) and nPTB (another member of the PTB family) since both PTB and nPTB suppress factors that prevent cells from developing into a neuron. Like fibroblasts, glial cells in the brain known as astrocytes are also flexible with respect to cell fate, however, whether they can be converted to neurons is still unknown. This week in Nature, Qian and colleagues investigated whether astrocytes in the substantia nigra could be converted into functional dopaminergic neurons and whether this conversion could restore neuronal populations and motor deficits in a chemically-induced model of Parkinson’s disease.

How did they do it?

To determine whether astrocytes could be converted into neurons, the authors used reverse transcription qPCR to measure the expression levels of neuron-specific transcription factors and microRNAs in mouse astrocytes from the cortex and midbrain. They hypothesized that, unlike fibroblasts, the knockdown of PTB alone would be sufficient to convert astrocytes into neurons. To test this hypothesis, the authors disrupted PTB signalling in-vitro in cell cultures by transducing astrocytes from both mouse and human tissue with a virus containing a small hairpin RNA against PTB. Eight weeks later, the authors determined whether the astrocytes had been converted to neurons by using patch clamp recordings to assess their electrophysiological properties. Then, the authors investigated whether astrocytes could be converted into neurons in vivo by using viral injections to induce PTB knockdown in astrocytes in the mouse midbrain. Ten weeks after the viral injections, the authors confirmed whether the astrocytes in the substantia nigra of the midbrain had been successfully converted into dopaminergic neurons by assessing their morphological properties and expression of neuronal markers. They then investigated whether the conversion of astrocytes into dopaminergic neurons in the substantia nigra could be used to reconstitute the neuronal population in this area following an injury. They used the 6-hydroxydopamine (a neurotoxin that destroys dopaminergic neurons) model of Parkinson’s disease to deplete the substantia nigra of dopamine neurons and then performed viral injections to induce PTB knockdown in astrocytes as previously described. Finally, the authors used chemogenetics and behavioural tests to determine whether injury-induced impairments to mouse motor function can be restored by converting astrocytes in the injured area into neurons.

What did they find?

The authors determined that up to 80% of astrocytes displayed neuronal morphologies only four weeks after in-vitro viral transduction. These cells also exhibited neuron-specific electrophysiological properties including voltage-gated sodium and potassium channels and action potential firing, suggesting that PTB knockdown is sufficient to convert astrocytes into neurons in both mouse and human tissue. Then, the authors revealed that the in vivo conversion of astrocytes to neurons in the substantia nigra was a gradual process, with 20% conversion at three weeks, 60% conversion at five weeks, and finally, 80% conversion at ten weeks post-injection. Moreover, they observed that these dopaminergic neurons projected to areas of the nigrostriatal pathway including the caudate, putamen, and the nucleus accumbens. Together, these findings suggest that astrocytes in the substantia nigra can be converted into dopamine neurons that are re-incorporated into the nigrostriatal pathway in a time-dependent manner.

Next, the authors showed that the conversion of astrocytes to neurons restored the population of dopaminergic neurons in the substantia nigra to 33% of the initial number prior to injury. Injury-induced motor deficits resulting from the depletion of dopaminergic neurons were gradually restored to almost normal levels following the viral transduction of astrocytes. Furthermore, chemogenetic manipulations confirmed that these improvements in motor deficits were due to the reconstitution of the dopaminergic neuronal population in the substantia nigra. 

What’s the impact?

This is the first study to show that it is possible to reprogram astrocytes in the substantia nigra of the mouse brain to become functional dopaminergic neurons. Moreover, the authors demonstrated that this conversion of astrocyte to neuron could be used to partially restore the population of dopaminergic neurons in the substantia nigra as well as improve motor deficits following a chemically induced lesion to this area. Together, these findings provide exciting new insights into possible treatments that could be developed to treat individuals with neurodegenerative disorders, like Parkinson’s disease, that are associated with a substantial loss of neuronal populations. 

Qian et al. Reversing a model of Parkinson’s disease with in situ converted nigral neurons. Nature (2020). Access the original scientific publication here.