GPER activation is effective in protecting against inflammation-induced nigral dopaminergic loss and motor function impairment.

Increasing evidence suggest that excessive inflammatory responses from overactivated microglia play a critical role in Parkinson's disease (PD), contributing to, or exacerbating, nigral dopaminergic (DA) degeneration. Recent results from our group and others demonstrated that selective activation of G protein-coupled estrogen receptor (GPER) with the agonist G1 can protect DA neurons from 1-methyl-4-phenylpyridinium (MPP+) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxins. However, it is not known whether modulation of microglial responses is one of the mechanisms by which G1 exerts its DA neuroprotective effects. We analyzed, in the N9 microglial cell line, the effect of G1 on microglial activation induced by lipopolysaccharide (LPS) exposure. The results revealed that G1 significantly decrease phagocytic activity, expression of inducible nitric oxide synthase (iNOS) and release of nitric oxide (NO) induced by LPS. To determine the relevance of this anti-inflammatory effect to the protection of nigral DA cells, the effect of G1 was analyzed in male mice injected unilaterally in the substantia nigra (SN) with LPS. Although G1 treatment did not decrease LPS-induced increase of ionized calcium binding adaptor molecule 1 (iba-1) positive cells it significantly reduced interleukin-1beta (IL-1ß), cluster of differentiation 68 (CD68) and iNOS mRNA levels, and totally inhibited nigral DA cell loss and, as a consequence, protected the motor function. In summary, our findings demonstrated that the G1 agonist is able to modulate microglial responses and to protect DA neurons and motor functions against a lesion induced by an inflammatory insult. Since G1 lacks the feminizing effects associated with agonists of the classical estrogen receptors (ERs), the use of G1 to selectively activate the GPER may be a promising strategy for the development of new therapeutics for the treatment of PD and other neuroinflammatory diseases.

GPER: A new tool to protect dopaminergic neurons?

Parkinson's disease (PD) is characterized by a selective degeneration of nigrostriatal dopaminergic pathway. Epidemiological studies revealed a male predominance of the disease that has been attributed to the female steroid hormones, mainly the estrogen. Estrogen neuroprotective effects have been shown in several studies, however the mechanisms responsible by these effects are still unclear. Previous data from our group revealed that glial cell line-derived neurotrophic factor (GDNF) is crucial to the dopaminergic protection provided by 17ß-estradiol, and also suggest that the intracellular estrogen receptors (ERs) are not required for that neuroprotective effects. The present study aimed to investigate the contribution of the G protein-coupled ER (GPER) activation in estrogen-mediated dopaminergic neuroprotection against an insult induced by 1-methyl-4-phenylpyridinium (MPP(+)), and whether GPER neuroprotective effects involve the regulation of GDNF expression. Using primary mesencephalic cultures, we found that GPER activation protects dopaminergic neurons from MPP(+) toxicity in an extent similar to the promoted by a 17ß-estradiol. Moreover, GPER activation promotes an increase in GDNF levels. Both, GDNF antibody neutralization or RNA interference-mediated GDNF knockdown prevented the GPER-mediated dopaminergic protection verified in mesencephalic cultures challenged with MPP(+). Overall, these results revealed that G1, a selective agonist of GPER, is able to protect dopaminergic neurons and that GDNF overexpression is a key feature to GPER induced the neuroprotective effects.

STEP61 is a substrate of the E3 ligase parkin and is upregulated in Parkinson's disease.

Parkinson's disease (PD) is characterized by the degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNc). The loss of SNc dopaminergic neurons affects the plasticity of striatal neurons and leads to significant motor and cognitive disabilities during the progression of the disease. PARK2 encodes for the E3 ubiquitin ligase parkin and is implicated in genetic and sporadic PD. Mutations in PARK2 are a major contributing factor in the early onset of autosomal-recessive juvenile parkinsonism (AR-JP), although the mechanisms by which a disruption in parkin function contributes to the pathophysiology of PD remain unclear. Here we demonstrate that parkin is an E3 ligase for STEP61 (striatal-enriched protein tyrosine phosphatase), a protein tyrosine phosphatase implicated in several neuropsychiatric disorders. In cellular models, parkin ubiquitinates STEP61 and thereby regulates its level through the proteasome system, whereas clinically relevant parkin mutants fail to do so. STEP61 protein levels are elevated on acute down-regulation of parkin or in PARK2 KO rat striatum. Relevant to PD, STEP61 accumulates in the striatum of human sporadic PD and in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned mice. The increase in STEP61 is associated with a decrease in the phosphorylation of its substrate ERK1/2 and the downstream target of ERK1/2, pCREB [phospho-CREB (cAMP response element-binding protein)]. These results indicate that STEP61 is a novel substrate of parkin, although further studies are necessary to determine whether elevated STEP61 levels directly contribute to the pathophysiology of PD.