This is a summary of Sonsalla G, Malpartida AB, Riedemann T, Gusic M, Rusha E, Bulli G, Najas S, Janjic A, Hersbach BA, Smialowski P, Drukker M, Enard W, Prehn JHM, Prokisch H, Götz M, Masserdotti G. Direct neuronal reprogramming of NDUFS4 patient cells identifies the unfolded protein response as a novel general reprogramming hurdle. Neuron. doi: 10.1016/j.neuron.2023.12.020.
The challenge
Neurodegenerative diseases and traumatic brain injury are characterized by the irreversible loss of neurons, leading to a decline in cognitive abilities and functional impairments. Replacing lost or damaged neurons is, therefore, an important therapeutic goal. Neuronal reprogramming, a technique that transforms non-neuronal cells into neurons, addresses this need. It can, for example, turn astrocytes, a type of glial cells, into functional neurons. However, little is known about the conversion of human astrocytes and the role of mitochondria in neuronal reprogramming. Mitochondria are the cell's powerhouse, and their dysfunction is common in neurological disorders, both in disorders of developmental origin (e.g., Leigh syndrome) and related to aging and neurodegenerative disorders. We, therefore, studied the role of the mitochondria in direct neuronal reprogramming.
Our approach
To model the role of dysfunctional mitochondria, we used astrocytes with mutations in the NDUFS4 gene, which is associated with Leigh Syndrome and mitochondrial C-I deficiency – a dysfunction in the Complex I of the mitochondrial respiratory chain. We obtained these astrocytes from human induced Pluripotent Stem Cells (iPSCs) from NDUFS4 patients and a control group. Then, we reprogrammed these iPSC-derived astrocytes into neurons by the forced expression of various transcription factors and found that astrocytes carrying mutations in NDUFS4 generated fewer and non-functional neurons.
Our findings
We identified the unfolded protein response (UPR) and integrated stress response (ISR) as novel significant hurdles in direct neuronal conversion. Both UPR and ISR are emergency systems activated in response to the accumulation of unfolded or misfolded proteins, helping repair and remove these proteins. We observed more mature and functional neurons when we transiently blocked these emergency processes not only in patient but also in control astrocytes, leading to a highly efficient conversion rate. Similar results were obtained when fibroblasts were reprogrammed, thus indicating that UPR and ISR are general roadblocks towards the direct generation of human neurons. Moreover, there was no increase of misfolded proteins in the new neurons despite blocking the UPR and ISR.
The implications
Our results emphasize the importance of protein balance (proteostasis) mechanisms in direct neuronal reprogramming. They also emphasize the reliability of using direct neuronal reprogramming as a method for modeling mitochondrial disease. Specifically, our findings show that mitochondrial deficits negatively affect the direct conversion into neurons, but transient treatment can counter this. It thus opens a new path for treating neurodevelopmental disorders with mitochondrial deficits and for neuronal replacement therapy.
Creating SyNergies
This research was led by Magdalena Götz from SyNergy together with Giacomo Masserdotti from Helmholtz/LMU. Within SyNergy, our next steps will be to translate these findings into an in vivo model system and screen for small molecules that could further improve the conversion rate of human cells into functional neurons.
