Current controversies in glia-to-neuron conversion therapy in neurodegenerative diseases

Peng Cao,Jianan Li,Zhuxi Liu,Guobiao Liang

Loss of neurons and disruption of neural circuits are associated with many neurological diseases,including neurodegenerative diseases and mental disorders.The most prevalent pathological feature of neurodegenerative diseases is the aggregate loss of certain neuronal populations.For example,the loss of dopamine (DA) neurons in the substantia nigra pars compacta has been defined as a pathological hallmark of Parkinson’s disease (PD;Kamath et al.,2022).Therefore,methods to reverse neuronal reduction in an effort to rebuild neural circuits and improve function have become an important research focus.In the past decade,regenerative medicine has taken a momentous leap forward with a groundbreaking discovery in the field of neuroregeneration.In vivoglia-to-neuron conversion therapy has presented itself as a potential solution (Qian et al.,2020).This is a paradigm-shifting regenerative strategy to reprogram resident glial cells in situ into neuronsin vivo(Figure 1).This method is obviously superior to the strategy of inducing stem cells to differentiate into neuronsin vitroforin vivotransplantation,avoiding various problems,such as difficult integration,cancer degeneration,and immunogenicity.However,the evidence supporting the replacement of lost neurons with trans-differentiated cells and the reconstitution of neuronal circuits remains limited.

Figure 1｜Glia-to-neuron conversion by down-regulation of PTBP1 or NeuroD1 may be a potential solution in Parkinson’s disease.

In a groundbreaking study,Qian et al.(2020) shed light on a novel strategy for directly converting astrocytes into DA neurons in the substantia nigra.By depleting the RNA binding protein PTB,the researchers induced the conversion of astrocytes into functional DA neurons,leading to the restoration of striatal dopamine levels and the reversal of PD-relevant motor phenotypes in a mouse model (Wang and Zhang,2022).

Astrocytes,which are non-neuronal cells that are abundant and highly plastic in the brain,offer several advantages forin vivoreprogramming (Yu et al.,2020).The researchers discovered that astrocytes exhibit characteristics similar to both fibroblasts and neurons in key regulatory loops controlled by PTB and its neuronal analog nPTB (Xue et al.,2016).This unique property of astrocytes allowed for the successful conversion into functional neurons byPTBP1(the gene that encodes PTB) knockdown alone,both in mouse and human cells.The study also highlighted the potential of antisense oligonucleotides as a therapeutic strategy for treating PD and other neurodegenerative diseases.The researchers provided evidence for the feasibility of using an anti-PTB antisense oligonucleotide as a single-step approach to deplete PTB and induce astrocyteto-neuron conversion.Antisense oligonucleotides have shown the ability to modulate brain disorders,and this study further supports their potential as a gene expression manipulation target in neurodegenerative diseases.

Similarly,another study conducted by Zhou et al.(2020) reported that downregulation of the genePtbp1,usingin vivoviral delivery of a recently developed RNA-targeting CRISPR system CasRx,resulted in the conversion of Müller glia into retinal ganglion cells with a high efficiency and induced neurons with dopaminergic features in a PD mice model.They further confirmed that glia-to-neuron conversion mediated by Ptbp1 knockdown represents a promisingin vivogenetic approach for treating a variety of diseases caused by neuron loss.

This innovative in situ transformation therapy technique of glia-to-neuron conversion holds immense potential for addressing disorders characterized by cell loss,particularly neurodegenerative diseases,such as PD.However,Wang and Zhang (2022) proposed a key question that has been completely overlooked.Specifically,how the manipulation of a generic factor,such as PTBP1,could theoretically generate diverse neuronal subtypes with precise regional identity and connectivity.Wang et al.(2021) also suggested that purportedly astrocyte-converted neurons were actually endogenous neurons by using stringent lineage tracing in the mouse brain,and that,despite efficient knockdown of PTBP1in vivo,genetically traced resident astrocytes were not converted into neurons.Their results do not support the phenomenal claims that resident astrocytes can be directly and efficiently converted into mature neurons.Coincidentally,Hoang et al.(2023) published an article claiming thatPtbp1deletion did not induce astrocyteto-neuron conversion.Moreover,Yang et al.(2023) overturned their early research conclusion and pointed out that Cas13X cannot induce astrocyte-to-neuron conversionin vivoafterPtbp1knockdown.

Although DA positive neurons have been observed after astrocyte PTBP1 inhibition through delivery of an adeno-associated virus expressing a reporter with shRNA or CRISPR-CasRx,the possibility of this adeno-associated virus leaking into endogenous DA neurons cannot be excluded without using a reliable lineage tracing method.One of these previous studies concluded that reports of astrocyte-to-neuron conversion likely represented leaky neuronal expression of the GFAP-minipromoter-based constructs used to label astrocytes (Hoang et al.,2023).This may have led to false positive results.Unfortunately,consistent with the report of Wang et al.(2021),Chen et al.(2022) also confirmed that repressing PTBP1 failed to convert reactive astrocytes to DA neurons,regardless of physiological or PD-related pathological conditions.

In response to these doubts,Hao et al.(2023) explained thatPtbp1knockoutversus Ptbp1knockdown may produce distinct phenotypes.Although it remains to be ruled out that the observed astrocyte-to-neuron conversion may have resulted from leakage into existing neurons,they firmly believe in the existence of a subclass of astrocytes or astrocyte-like cells with a higher propensity than mature astrocytes to be redirected to the neuronal lineage.

In addition to PTBP1,Guo et al.(2014) and Chen et al.(2020) have recently demonstrated a direct conversion of reactive astrocytes into functional neurons by a single transcription factor,NeuroD1,in the mouse brain.Together,their studies suggest that internal neuroregeneration usingin situcell conversion technology may be an effective approach for functional brain repair after injury.

The conversion of astrocytes into functional neurons represents a remarkable advance in the field of neuroregeneration.This breakthrough study not only unveils the potential of cell conversion therapy for treating neurodegenerative diseases,but also sheds light on the intricate cellular processes governing neuronal induction and maturation.This is undoubtedly a major breakthrough;however,the emergence of any innovative technology is always accompanied by voices of skepticism.Here we summarize the current important research,including both positive and negative results (Table 1).Throughout the research process,it is especially important to be wary of false positive results and erroneous conclusions caused by unreliable detection systems.Although this pioneering study propels the field of regenerative medicine forward,several challenges and questions remain.Long-term stability,integration of the converted neurons into existing neuronal circuits,and potential side effects need to be thoroughly investigated.Additionally,optimization of conversion efficiency and exploring the approach’s applicability to other neurodegenerative disorders are crucial avenues for future research.

Table 1｜Current controversies in in situ glia-to-neuron conversion therapy

In summary,as researchers continue to unravel the mysteries of neuroregeneration,the hope of developing transformative therapies for millions of individuals impacted by neurodegenerative conditions grows stronger.

This work was supported by the National Natural Science Foundation of China,No.81 971133;Liaoning Key Research and Development Project,No.2021JH2/10 300059;Liaoning Revitalization Talents Prograrn,No.XLYC2002109 (all to GL).

Peng Cao*,Jianan Li,Zhuxi Liu,Guobiao Liang

Department of Neurosurgery,General Hospital of the Northern Theater Command of Chinese People’s Liberation Army,Shenyang,Liaoning Province,China (Cao P,Li J,Liang G)

Key Laboratory of Precision Medical Research on Major Chronic Disease,Shengjing Hospital of China Medical University,Shenyang,Liaoning Province,China (Liu Z)

*Correspondence to:Peng Cao,MD,pengcao518@163.com.

https://orcid.org/0000-0002-2809-506X (Peng Cao)

Date of submission:June 20,2023

Date of decision:July 5,2023

Date of acceptance:July 10,2023

Date of web publication:August 14,2023

https://doi.org/10.4103/1673-5374.382251

How to cite this article:Cao P,Li J,Liu Z,Liang G (2024) Current controversies in glia-to-neuron conversion therapy in neurodegenerative diseases.Neural Regen Res 19(4):723-724.

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