Aberrant cerebellar Purkinje cell function repaired in vivo by fusion with infiltrating bone marrow-derived cells.

Bone marrow-derived cells are known to infiltrate the adult brain and fuse with cerebellar Purkinje cells. Histological observations that such heterotypic cell fusion events are substantially more frequent following cerebellar injury suggest they could have a role in the protection of mature brain neurons. To date, the possibility that cell fusion can preserve or restore the structure and function of adult brain neurons has not been directly addressed; indeed, though frequently suggested, the possibility of benefit has always been rather speculative. Here we report, for the first time, that fusion of a bone marrow-derived cell with a neuron in vivo, in the mature brain, results in the formation of a spontaneously firing neuron. Notably, we also provide evidence supporting the concept that heterotypic cell fusion acts as a biological mechanism to repair pathological changes in Purkinje cell structure and electrophysiology. We induced chronic central nervous system inflammation in chimeric mice expressing bone marrow cells tagged with enhanced green fluorescent protein. Subsequent in-depth histological analysis revealed significant Purkinje cell injury. In addition, there was an increased incidence of cell fusion between bone marrow-derived cells and Purkinje cells, revealed as enhanced green fluorescent protein-expressing binucleate heterokaryons. These fused cells resembled healthy Purkinje cells in their morphology, soma size, ability to synthesize the neurotransmitter gamma-aminobutyric acid, and synaptic innervation from neighbouring cells. Extracellular recording of spontaneous firing ex vivo revealed a shift in the predominant mode of firing of non-fused Purkinje cells in the context of cerebellar inflammation. By contrast, the firing patterns of fused Purkinje cells were the same as in healthy control cerebellum, indicating that fusion of bone marrow-derived cells with Purkinje cells mitigated the effects of cell injury on electrical activity. Together, our histological and electrophysiological results provide novel fundamental insights into physiological processes by which nerve cells are protected in adult life.

Mesenchymal Stem Cell-Derived Factors Restore Function to Human Frataxin-Deficient Cells.

Friedreich's ataxia is an inherited neurological disorder characterised by mitochondrial dysfunction and increased susceptibility to oxidative stress. At present, no therapy has been shown to reduce disease progression. Strategies being trialled to treat Friedreich's ataxia include drugs that improve mitochondrial function and reduce oxidative injury. In addition, stem cells have been investigated as a potential therapeutic approach. We have used siRNA-induced knockdown of frataxin in SH-SY5Y cells as an in vitro cellular model for Friedreich's ataxia. Knockdown of frataxin protein expression to levels detected in patients with the disorder was achieved, leading to decreased cellular viability, increased susceptibility to hydrogen peroxide-induced oxidative stress, dysregulation of key anti-oxidant molecules and deficiencies in both cell proliferation and differentiation. Bone marrow stem cells are being investigated extensively as potential treatments for a wide range of neurological disorders, including Friedreich's ataxia. The potential neuroprotective effects of bone marrow-derived mesenchymal stem cells were therefore studied using our frataxin-deficient cell model. Soluble factors secreted by mesenchymal stem cells protected against cellular changes induced by frataxin deficiency, leading to restoration in frataxin levels and anti-oxidant defences, improved survival against oxidative stress and stimulated both cell proliferation and differentiation down the Schwann cell lineage. The demonstration that mesenchymal stem cell-derived factors can restore cellular homeostasis and function to frataxin-deficient cells further suggests that they may have potential therapeutic benefits for patients with Friedreich's ataxia.

Human mesenchymal stem cells increase anti-oxidant defences in cells derived from patients with Friedreich's ataxia.

Friedreich's ataxia (FRDA) is a progressive neurodegenerative disorder which is, at present, incurable. Oxidative damage and inhibition of mitochondrial function are key determinants of cellular damage in FRDA, since there is greater sensitivity to oxidative stress in cells with frataxin deficiency. In addition, frataxin-deficient cells have an impaired ability to recruit antioxidant defences against endogenous oxidative stress. We have recently shown that factors derived from bone marrow-derived mesenchymal stem cells (MSCs) increase hydrogen peroxide scavenging enzymes and offer protection against hydrogen peroxide-mediated injury in cells derived from patients with FRDA. Here we extend these studies and have performed a series of experiments showing that expression of superoxide dismutase (1 and 2) enzymes is reduced in FRDA cells but can be restored by treatment with conditioned medium from human MSCs. Furthermore, we have demonstrated that exposure to factors secreted by MSCs increases resistance to nitric oxide-induced oxidative stress in FRDA fibroblasts through, at least in part, restoring the expression of the superoxide dismuting enzymes and via modulation of PI(3) kinase/Akt pathways. These findings suggest that MSCs secrete factors that improve the cellular homeostasis of cells derived from FRDA patients and provide suitable support for their enhanced survival. This study further suggests the potential therapeutic use of MSCs in patients with FRDA.