Translational Neuroscience Approaches: Stem Cell Exosomes for the Treatment of Brain Diseases

Exosomes are tiny, membrane-bound vesicles (50-200 nm in size) released by many types of cells including eukaryotic cells, into the extracellular environment.^1,2 They facilitate intercellular communication by transporting a diverse array of proteins, lipids, and nucleic acids from their parent cells. Exosomes are formed by the inward budding of the endosomal membrane, resulting in multivesicular bodies that when fused with the cell membrane, release their contents into the surrounding space. These small vesicles were found to have important functions in health and in disease. They play a crucial role in cell-to-cell communication by transferring proteins, lipids, and nucleic acids (like mRNAs and microRNAs) from one cell to another. This transfer can influence the behavior and function of recipient cells. Exosomes carry a variety of cargo,² which can include signaling molecules, enzymes, and genetic material. This cargo can impart specific biological signals to target cells, affecting processes like inflammation, immune responses, and cell survival.

Exosomes have been implicated in various diseases, including ASD and neurodegenerative disorders. For instance, they can facilitate the spread of toxic proteins in neurodegenerative diseases like Alzheimer’s and Parkinson’s disease.³ Due to their ability to transport biological molecules and influence cellular processes, exosomes are being explored as potential therapeutic agents. They are particularly attractive because they are generally well-tolerated by the immune system; they can be produced in large quantities, and can deliver therapeutic agents directly to target cells. Overall, exosomes are an important aspect of cellular communication and have significant implications for both health and disease states.

Numerous studies have highlighted the role of exosomes in mediating signaling during immune and inflammatory cellular responses. Furthermore, exosomes have been shown to carry misfolded or aggregated proteins linked to neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and amyotrophic lateral sclerosis (ALS), which can spread pathological characteristics to recipient cells.⁴ Previous investigations have also reported the advantageous properties of exosomes. For example, exosomes derived from oligodendrocytes can enhance neuronal survival under stress conditions.⁵ Exosomes have been shown to exert neuroprotective effects in models of Parkinson’s disease by promoting neuronal survival and reducing neuroinflammation.⁶ Additionally, research indicates that exosomes can facilitate intercellular communication and deliver therapeutic RNA molecules, offering innovative strategies for treating conditions such as Alzheimer’s disease and schizophrenia.^7,8 These findings suggest that exosome-based therapies could pave the way for novel treatment approaches in neurodegenerative and psychiatric disorders. Recent research has explored the application of exosomes in the treatment of multiple sclerosis (MS), showing that exosomes derived from MSCs can promote remyelination and modulate immune responses, thereby suggesting their potential as a therapeutic strategy in MS management.⁹ Additionally, studies have indicated that exosomes can carry protective factors that alleviate cognitive deficits in traumatic brain injury (TBI), highlighting their promise as a non-invasive treatment option for neurotrauma.¹⁰ Furthermore, exosomes have been utilized in the delivery of therapeutic agents for mood disorders, with findings demonstrating that exosomes can transport antidepressant compounds, offering a novel route for enhancing therapeutic efficacy in depression.¹¹

Induced pluripotent stem cells (iPSCs) are a groundbreaking type of stem cell created by reprogramming adult somatic cells to a pluripotent state,¹³ allowing them to develop into any cell type, with similar properties to embryonic stem cells. This reprogramming is achieved by introducing specific genes associated with maintaining the defining characteristics of pluripotency by transfection.¹⁴ iPSCs hold great promise for regenerative medicine and disease modeling because they can be derived from the patient’s own cells, reducing the risk of immune rejection and ethical concerns surrounding the use of embryonic stem cells. Importantly, they carry the patient’s DNA, thus allowing the generation of a perfectly matched DNA model. Their versatility allows researchers to investigate diseases, develop new therapies, and potentially create patient-specific cells for transplantation and treatment.^15-22 As research progresses, both MSCs and iPSCs continue to expand the horizons of regenerative medicine and therapeutic interventions.

MSC-derived exosomes are emerging as possible therapeutics for autism spectrum disorder (ASD) and other neurological disorders.²³ Intranasal administration of MSC-derived exosomes improved the social deficits in a SHANK3 KO mouse.²⁴ Exosomes obtained from mesenchymal stem cells reduce inflammation and demyelination in the central nervous system of experimental allergic encephalomyelitis (EAE) rats by influencing the polarization of microglia.²⁵ The number of studies working with exosomes derived from MSCs has been increasing dramatically over the past 3 years and consists of around 500 studies per year. MSCs are extracted from various tissues, usually bone marrow. They are collected under anaesthesia. They can be propagated a limited number of times (around 5). Human induced pluripotent stem cells (hiPSCs) are more easily collected; they require a small blood sample, and there are even protocols for preparing them from urine samples.²⁶ Once made, they can be split and propagated many times (around 50) and this makes them an almost indefinite source for exosome extractions. They are pluripotent and can be differentiated into any cell type in the body, while MSCs are limited to specific lineages. They are patient specific and can be generated from each patient’s own somatic cells, minimizing the risk of immune rejection. Tools such as CRISPR-Cas9, TALENs, and others are often optimized for iPSCs. These tools can be used to introduce, delete, or replace genetic material with high efficiency in iPSCs^27,28 This makes them a potentially better source of exosomes.

We subsequently conducted quantitative proteomics utilizing LC-MS/MS to analyze the protein cargo of these different types of exosomes. We found that the exosomes from SHANK3 mutant neurons contained a high number of proteins (216 different types) and those from MSCs had 217 types, in contrast to exosomes from control neurons (114 types of proteins) and iPSC-derived exosomes (32 types of proteins).³⁰ A cellular component analysis of the protein cargo from these exosomes showed a significant enrichment for “exosomes” and “extracellular” proteins, confirming the validity of the exosome isolation. Furthermore, neuron-derived exosomes exhibited enrichment for “cytoplasm”, which was not observed in the stem cell-derived exosomes.

We are now conducting experiments with SHANK3 KO mice and these are very promising. We use intranasal administration of exosomes derived from iPSCs. Additionally, electrophysiological recordings from cortical neurons derived from ASD patients with other mutations also show great promise for iPSC-derived exosomes as a treatment for ASD. It is clear that these exosomes can be genetically modified in a patient-specific manner to further improve ASD phenotypes. To summarize, iPSC-derived exosomes are a promising potential treatment avenue for ASD that offers a low risk compared to other current genetic treatments. Once these are FDA-approved, they can be relatively easily used to treat many patients in the clinic and even at home by parent-to-child administration intranasally.