Transdifferentiating Astrocytes Into Neurons Using ASCL1 Functionalized With a Novel Intracellular Protein Delivery Technology

Advances in neuronal reprogramming for neurodegenerative diseases: Strategies, controversies, and opportunities

Neuronal death is often observed in central nervous system injuries and neurodegenerative diseases. The mammalian central nervous system manifests limited neuronal regeneration capabilities, and traditional cell therapies are limited in their potential applications due to finite cell sources and immune rejection. Neuronal reprogramming has emerged as a novel technology, in which non-neuronal cells (e.g. glial cells) are transdifferentiated into mature neurons. This process results in relatively minimal immune rejection. The present review discuss the latest progress in this cutting-edge field, including starter cell selection, innovative technical strategies and methods of neuronal reprogramming for neurodegenerative diseases, as well as the potential problems and controversies. The further development of neuronal reprogramming technology may pave the way for novel therapeutic strategies in the treatment of neurodegenerative diseases.

A promise for neuronal repair: reprogramming astrocytes into neurons in vivo

Massive loss of neurons following brain injury or disease is the primary cause of central nervous system dysfunction. Recently, much research has been conducted on how to compensate for neuronal loss in damaged parts of the nervous system and thus restore functional connectivity among neurons. Direct somatic cell differentiation into neurons using pro-neural transcription factors, small molecules, or microRNAs, individually or in association, is the most promising form of neural cell replacement therapy available. This method provides a potential remedy for cell loss in a variety of neurodegenerative illnesses, and the development of reprogramming technology has made this method feasible. This article provides a comprehensive review of reprogramming, including the selection and methods of reprogramming starting cell populations as well as the signaling methods involved in this process. Additionally, we thoroughly examine how reprogramming astrocytes into neurons can be applied to treat stroke and other neurodegenerative diseases. Finally, we discuss the challenges of neuronal reprogramming and offer insights about the field.

Transcription Factors in Brain Regeneration: A Potential Novel Therapeutic Target

Transcription factors play a crucial role in providing identity to each cell population. To maintain cell identity, it is essential to balance the expression of activator and inhibitor transcription factors. Cell plasticity and reprogramming offer great potential for future therapeutic applications, as they can regenerate damaged tissue. Specific niche factors can modify gene expression and differentiate or transdifferentiate the target cell to the required fate. Ongoing research is being carried out on the possibilities of transcription factors in regenerating neurons, with neural stem cells (NSCs) being considered the preferred cells for generating new neurons due to their epigenomic and transcriptome memory. NEUROD1/ASCL1, BRN2, MYTL1, and other transcription factors can induce direct reprogramming of somatic cells, such as fibroblasts, into neurons. However, the molecular biology of transcription factors in reprogramming and differentiation still needs to be fully understood.

Direct Cell Conversion of Somatic Cells into Dopamine Neurons: Achievements and Perspectives

In the last decade, direct reprogramming has emerged as a novel strategy to obtain mature and functional dopamine neurons from somatic cells. This approach could overcome issues linked to the use of human pluripotent stem cells such as ethical concerns and safety problems that can arise from the overgrowth of undifferentiated cells after transplantation. Several conversion methodologies have been developed to obtain induced DA neurons (iDANs) or induced DA neuron progenitors (iDPs). iDANs have also proved to successfully integrate in mice striatum, alleviating Parkinson's disease (PD) motor symptoms. In the next decade, human iDANs and/or iDPs could be translated to clinic to achieve a patient-tailored therapy, but current critical issues hinder this goal, such as the low conversion rate of adult human fibroblasts and the risks associated with lentiviral delivery of conversion factors. In this study, we summarize the strategies and recent improvements developed for the generation of mouse and human iDANs/iDPs. Furthermore, we discuss the more recent application of in vivo direct conversion, which may enable clinical therapies for PD by means of brain in situ delivery of dopaminergic reprogramming transcription factors.

MicroRNA Roles in Cell Reprogramming Mechanisms

Cell reprogramming is a groundbreaking technology that, in few decades, generated a new paradigm in biomedical science. To date we can use cell reprogramming to potentially generate every cell type by converting somatic cells and suitably modulating the expression of key transcription factors. This approach can be used to convert skin fibroblasts into pluripotent stem cells as well as into a variety of differentiated and medically relevant cell types, including cardiomyocytes and neural cells. The molecular mechanisms underlying such striking cell phenotypes are still largely unknown, but in the last decade it has been proven that cell reprogramming approaches are significantly influenced by non-coding RNAs. Specifically, this review will focus on the role of microRNAs in the reprogramming processes that lead to the generation of pluripotent stem cells, neurons, and cardiomyocytes. As highlighted here, non-coding RNA-forced expression can be sufficient to support some cell reprogramming processes, and, therefore, we will also discuss how these molecular determinants could be used in the future for biomedical purposes.

Induced Neurons for Disease Modeling and Repair: A Focus on Non-fibroblastic Cell Sources in Direct Reprogramming

Direct cellular reprogramming exhibits distinct advantages over reprogramming from an induced pluripotent stem cell intermediate. These include a reduced risk of tumorigenesis and the likely preservation of epigenetic data. In vitro direct reprogramming approaches primarily aim to model the pathophysiological development of neurological disease and identify therapeutic targets, while in vivo direct reprogramming aims to develop treatments for various neurological disorders, including cerebral injury and cancer. In both approaches, there is progress toward developing increased control of subtype-specific production of induced neurons. A majority of research primarily utilizes fibroblasts as the donor cells. However, there are a variety of other somatic cell types that have demonstrated the potential for reprogramming into induced neurons. This review highlights studies that utilize non-fibroblastic cell sources for reprogramming, such as astrocytes, olfactory ensheathing cells, peripheral blood cells, Müller glia, and more. We will examine benefits and obstructions for translation into therapeutics or disease modeling, as well as efficiency of the conversion. A summary of donor cells, induced neuron types, and methods of induction is also provided.

Direct Reprogramming of Somatic Cells to Neurons: Pros and Cons of Chemical Approach

Translating successful preclinical research in neurodegenerative diseases into clinical practice has been difficult. The preclinical disease models used for testing new drugs not always appear predictive of the effects of the agents in the human disease state. Human induced pluripotent stem cells, obtained by reprogramming of adult somatic cells, represent a powerful system to study the molecular mechanisms of the disease onset and pathogenesis. However, these cells require a long time to differentiate into functional neural cells and the resetting of epigenetic information during reprogramming, might miss the information imparted by age. On the contrary, the direct conversion of somatic cells to neuronal cells is much faster and more efficient, it is safer for cell therapy and allows to preserve the signatures of donors’ age. Direct reprogramming can be induced by lineage-specific transcription factors or chemical cocktails and represents a powerful tool for modeling neurological diseases and for regenerative medicine. In this Commentary we present and discuss strength and weakness of several strategies for the direct cellular reprogramming from somatic cells to generate human brain cells which maintain age‐related features. In particular, we describe and discuss chemical strategy for cellular reprogramming as it represents a valuable tool for many applications such as aged brain modeling, drug screening and personalized medicine.

Neuronal Reprogramming for Tissue Repair and Neuroregeneration

Stem cell and cell reprogramming technology represent a rapidly growing field in regenerative medicine. A number of novel neural reprogramming methods have been established, using pluripotent stem cells (PSCs) or direct reprogramming, to efficiently derive specific neuronal cell types for therapeutic applications. Both in vitro and in vivo cellular reprogramming provide diverse therapeutic pathways for modeling neurological diseases and injury repair. In particular, the retina has emerged as a promising target for clinical application of regenerative medicine. Herein, we review the potential of neuronal reprogramming to develop regenerative strategy, with a particular focus on treating retinal degenerative diseases and discuss future directions and challenges in the field.

Next-generation disease modeling with direct conversion: a new path to old neurons

Within just over a decade, human reprogramming-based disease modeling has developed from a rather outlandish idea into an essential part of disease research. While iPSCs are a valuable tool for modeling developmental and monogenetic disorders, their rejuvenated identity poses limitations for modeling age-associated diseases. Direct cell-type conversion of fibroblasts into induced neurons (iNs) circumvents rejuvenation and preserves hallmarks of cellular aging. iNs are thus advantageous for modeling diseases that possess strong age-related and epigenetic contributions and can complement iPSC-based strategies for disease modeling. In this review, we provide an overview of the state of the art of direct iN conversion and describe the key epigenetic, transcriptomic, and metabolic changes that occur in converting fibroblasts. Furthermore, we summarize new insights into this fascinating process, particularly focusing on the rapidly changing criteria used to define and characterize in vitro-born human neurons. Finally, we discuss the unique features that distinguish iNs from other reprogramming-based neuronal cell models and how iNs are relevant to disease modeling.

Combination of Chemical and Neurotrophin Stimulation Modulates Neurotransmitter Receptor Expression and Activity in Transdifferentiating Human Adipose Stromal Cells

Adipose stromal cells are promising tools for clinical applications in regeneration therapies, due to their ease of isolation from tissue and its high yield; however, their ability to transdifferentiate into neural phenotypes is still a matter of controversy. Here, we show that combined chemical and neurotrophin stimulation resulted in neuron-like morphology and regulated expression and activity of several genes involved in neurogenesis and neurotransmission as well as ion currents mediated by NMDA and GABA receptors. Among them, expression patterns of genes coding for kinin-B1 and B2, α7 nicotinic, M1, M3 and M4 muscarinic acetylcholine, glutamatergic (AMPA2 and mGlu2), purinergic P2Y1 and P2Y4 and GABAergic (GABA-A, β3-subunit) receptors and neuronal nitric oxide synthase were up-regulated compared to levels of undifferentiated cells. Simultaneously, expression levels of P2X1, P2X4, P2X7 and P2Y6 purinergic and M5 muscarinic acetylcholine receptors were down-regulated. Agonist-induced activity levels of the studied receptor classes also augmented during neuronal transdifferentiation. Transdifferentiated cells expressed high levels of neuronal β3-tubulin, NF-H, NeuN and MAP-2 proteins as well as increased ASCL1, MYT1 and POU3F2 gene expression known to drive neuronal fate determination. The presented work contributes to a better understanding of transdifferentiation induced by neurotrophins for a prospective broad spectrum of medical applications.

Lipo-PEG-PEI complex as an intracellular transporter for protein therapeutics

Background

Protein or peptide drugs are emerging therapeutics for treating human diseases. However, current protein drugs are typically limited to acting on extracellular/cell membrane components associated with the diseases, while intracellular delivery of recombinant proteins replaces or replenishes faulty/missing proteins and remains inadequate. In this study, we developed a convenient and efficient intracellular protein delivery vehicle.

Materials and methods

A cationic liposomal polyethylenimine and polyethylene glycol complex (LPPC) was developed to noncovalently capture proteins for protein transfer into cells via endocytosis. β-glucuronidase (βG) was used in vitro and in vivo as a model enzyme to demonstrate the enzymatic activity of the intracellular transport of a protein.

Results

The endocytosed protein/LPPC complexes escaped from lysosomes, and the bound protein dissociated from LPPC in the cytosol. The enzymatic activity of βG was well preserved after intracellular delivery in vitro and in vivo.

Conclusion

Using LPPC as an intracellular protein transporter for protein therapeutics, we illustrated that LPPC may be an effective and convenient tool for studying diseases and developing therapeutics.