Murine dopaminergic Müller cells restore motor function in a model of Parkinson's disease

Müller cells constitute the main glial cell type in the retina where it interacts with virtually all cells displaying relevant functions to retinal physiology. Under appropriate stimuli, Müller cells may undergo dedifferentiation, being able to generate other neural cell types. Here, we show that purified mouse Müller cells in culture express a group of proteins related to the dopaminergic phenotype, including the nuclear receptor-related 1 protein, required for dopaminergic differentiation, as well the enzyme tyrosine hydroxylase. These dopaminergic components are active, since Müller cells are able to synthesize and release dopamine to the extracellular medium. Moreover, Müller-derived tyrosine hydroxylase can be regulated, increasing its activity because of phosphorylation of serine residues in response to agents that increase intracellular cAMP levels. These observations were extended to glial cells obtained from adult monkey retinas with essentially the same results. To address the potential use of dopaminergic Müller cells as a source of dopamine in cell therapy procedures, we used a mouse model of Parkinson's disease, in which mouse Müller cells with the dopaminergic phenotype were transplanted into the striatum of hemi-parkinsonian mice generated by unilateral injection of 6-hydroxydopamine. These cells fully decreased the apomorphine-induced rotational behavior and restored motor functions in these animals, as measured by the rotarod and the forelimb-use asymmetry (cylinder) tests. The data indicate local restoration of dopaminergic signaling in hemi-parkinsonian mice confirmed by measurement of striatal dopamine after Müller cell grafting. Müller cells are the main glial cells in the retina. When these cells are cultured in the absence of neurons, they spontaneously express proteins of the dopaminergic phenotype, including the enzymes tyrosine hydroxylase (TH), L-DOPA-decarboxylase (DDC) and the dopamine transport system (DAT). In this study, we show this phenomenon is observed with Müller cells obtained from different species, including primates, and address the therapeutic potential of these cells, using a mouse model of Parkinson's disease (PD). ‘Dopaminergic Müller cells’ synthesize dopamine and release most of this neurotransmitter to the extracellular space, constituting a natural dopaminergic ‘pump’. When transplanted to the striatum of PD mice, Müller cells decreased their apomorphine-induced rotational behavior and restored their overall motor functions, measured by rotarod and forelimb use asymmetry tests. Local restoration of dopaminergic signaling was also observed in grafted PD mice, by measuring striatum dopamine and its metabolite (DOPAC) levels (SB: 20µm)