This commentary highlights the research entitled: Transplantation of platelet-derived mitochondria alleviates cognitive impairment and mitochondrial dysfunction in db/db mice, presented by Ma et al. appearing in Clinical Science (2020) 134(16), https://doi.org/10.1042/CS20200530. The authors evaluated the effect of xenograft transplantation of mitochondria isolated from peripheral blood platelets in an animal model of type II diabetes and evaluated the effects of transplantation on diabetes-associated cognitive impairment (DACI). They showed cognitive and molecular improvement in response to mitochondrial transplantation to db/db mice brains. Besides, they showed better internalization of the transplanted mitochondria into the diseased animals’ hippocampal cells compared with the healthy normal control.
The mitochondria, the cellular organelles concerned with energy and regulating Ca2+ buffering, can be involved in many disorders. Mitochondria are not static structures, and they are relocated between cells (this is valid also for neurons and neuroglia) by different molecular connections like, junctions and cell fusion .
Sources of the transplanted mitochondria from the periphery include skeletal muscles; however, it is invasive and may result in cosmetic complications  and mesenchymal stem cells (MSCs) . Some protocols and strategies increase the efficiency of the isolated mitochondria transplantation like peptide-mediated delivery (PMD)  and the use of Dextran as a polymer conjugate . Mitochondrial fluorescent probes as MitoTracker Orange CMTMRos can assay the isolated mitochondria viability before transplantation . Mitochondrial dysfunction has been reported in Alzheimer’s disease (AD), Parkinson’s disease (PD), Schizophrenia, Huntington’s disease, and diabetes mellitus (DM) .
DM is a common endocrine disorder with multisystem complications affecting the central and peripheral nervous system. Diabetes-associated cognitive impairment (DACI) is a part of the neurological complications of DM that impairs the quality of life of diabetic patients. DACI results from microvascular, macrovascular, and molecular abnormalities that affect the neurons and the supporting glial cells. Furthermore, DACI is associated with impaired metabolic energy coupling between astrocytes and neurons, impaired neurovascular coupling that synchronizes neuronal activity to the blood flow, dysfunction of astrocytes, and mitochondrial dysfunctions . In addition to the previously mentioned effects of diabetes and insulin resistance, the brain has specific characteristics that make it more vulnerable to oxidative damage. The brain has a high content of mitochondria that produce higher levels of reactive oxygen species (ROS), easily peroxidizable unsaturated fatty acids, high oxygen consumption rate, and relatively less content of antioxidant enzymes .
In volume 134, issue 16 of Clinical Science, Ma et al.  investigated the effect of transplantation of platelets-derived mitochondria in an animal model of type II diabetes (leptin-resistant db/db mice). The selected mitochondria source is practical, convenient, and easy to obtain fully functional mitochondria compared with other described sources.
Summary of the main findings of the study by Ma et al. (2020) and the major points that need further investigation
Ma et al.  transplanted mitochondria into the brain by intracerebroventricular (icv) injection (in vivo), and this provided a better evaluation of the functional outcome in vivo rather than treating cell cultures with the transplanted mitochondria. They evaluated the integrity and viability of the isolated mitochondria by JC-1 dye, membrane potential by flow cytometry, morphology by transmission electron microscopy (TEM). Furthermore, they used immunofluorescence for cytochrome c oxidase subunits, complex IV expression as a mitochondrial marker, and followed the direction of the transplanted mitochondria by MitoTracker.
Although the injection of the mitochondria into the brain ventricle is invasive and requires specific skills and stereotaxis preparation, it provided effective homing into the hippocampus as demonstrated by TEM, immunofluorescence, evaluation of the oxidative phosphorylation capacity, oxidative stress, and apoptosis markers .
Diabetes is associated with activation of microglia and astrocytes  and excessive amyloid-β (Aβ) deposition. The icv injection of mitochondria by Ma et al.  restored microglia activation as concluded from ionized calcium-binding adapter molecule 1 (IBA-1) expression, and decreased Aβ content of the hippocampus, restored total and phosphorylated τ. These results agree and support the improvement of spatial memory assessed by Morris Water Maze. However, the icv injection of mitochondria did not restore Glial Fibrillary Acidic Protein (GFAP) expression, the astrogliosis marker . Reactive astrocytosis can be seen in transplantation, especially in xenograft transplantation, which is the case in the present study by Ma et al. ; the astrogliosis may result from the trauma of injection or reaction of the brain tissue to the transplantation . In accordance with that, Lin et al.  reported an immune response to isolated mitochondria in a mouse model of a xenograft heart transplant.
Ma et al. paved the way to search for molecular changes affecting micropinocytosis and endocytosis during the internalization process. By labeling the transplanted platelets-derived mitochondria with MitoTracker red, they revealed a higher percentage of internalization of mitochondria in the diseased model than the healthy control in the acute phase (24 h) and after 1 month of injection. This finding is worth further investigation.
Maybe this is related to the development of tunneling nanotubes (TNTs) by the hippocampal neurons, which are thin membranes developed from the insulted cells that facilitate communication and transport between the stressed, injured cell and surrounding normal cells. It was reported that the formation of TNTs is stimulated by high glucose . Moreover, maybe that higher internalization into the injured neurons is caused by the release of growth factors by these neurons facilitating the injected mitochondria’s macropinocytosis.
It is interesting to know that when neurons exposed to ischemia, the astrocytes donate their mitochondria to the neighboring neurons, and mitochondria released by the astrocytes were detected in cerebrospinal fluid (CSF) in subarachnoid hemorrhage; this may provide a mean to neurorestoration by mitochondrial transplantation through the CSF and the transplanted mitochondria might be delivered into neurons and neuroglia .
Ma et al.’s findings  struck the light for an accessible platelets source and represented a promising start for further studies to meet the questions and unresolved mechanisms aroused by their results.
Further studies are required to assess the molecular mechanism of increased uptake in the diseased model, compare allogenic graft versus xenogeneic graft, and impact on astrocytes and other markers representing a reaction to the transplanted mitochondria. Besides, assessment of other aspects of cognition is recommended as DACI negatively affects different cognitive domains, not only spatial memory.
For more practical and clinical application, further manipulation of the isolated mitochondria to increase permeability and enhance delivery across the blood–brain barrier (BBB), maybe by conjugation to certain particles or nanomedicine techniques (Figure 1).
The author declares that there are no competing interests associated with the manuscript.
diabetes-associated cognitive impairment
glial fibrillary acidic protein
ionized calcium-binding adapter molecule 1
mesenchymal stem cells
reactive oxygen species
transmission electron microscopy