TY - JOUR

T1 - Reciprocal signals between microglia and neurons regulate alpha-synuclein secretion by exophagy through a neuronal cJU-N-Nterminal kinase-signaling axis

AU - Christensen, Dan Ploug

AU - Ejlerskov, Patrick

AU - Rasmussen, Izabela

AU - Vilhardt, Frederik

PY - 2016/3/8

Y1 - 2016/3/8

N2 - Background: Secretion of proteopathic alpha-synuclein (alpha-SNC) species from neurons is a suspected driving force in the propagation of Parkinson's disease (PD). We have previously implicated exophagy, the exocytosis of autophagosomes, as a dominant mechanism of alpha-SNC secretion in differentiated PC12 or SH-SY5Y nerve cells. Here we have examined the regulation of exophagy associated with different forms of nerve cell stress relevant to PD.Results: We identify cJUN-N-terminal kinase (JNK) activity as pivotal in the secretory fate of autophagosomes containing alpha-SNC. Pharmacological inhibition or genetic (shRNA) knockdown of JNK2 or JNK3 decreases alpha-SNC secretion in differentiated PC12 and SH-SY5Y cells, respectively. Conversely, expression of constitutively active mitogen-activated protein kinase kinase 7 (MKK7)-JNK2 and -JNK3 constructs augment secretion. The transcriptional activity of cJUN was not required for the observed effects. We establish a causal relationship between increased alpha-SNC release by exophagy and JNK activation subsequent to lysosomal fusion deficiency (overexpression of Lewy body-localized protein p25 alpha or bafilomycin A1). JNK activation following neuronal ER or oxidative stress was not correlated with exophagy, but of note, we demonstrate that reciprocal signaling between microglia and neurons modulates alpha-SNC secretion. NADPH oxidase activity of microglia cell lines was upregulated by direct co-culture with alpha-SNC-expressing PC12 neurons or by passive transfer of nerve cell-conditioned medium. Conversely, inflammatory factors secreted from activated microglia increased JNK activation and alpha-SNC secretion several-fold in PC12 cells. While we do not identify these factors, we extend our observations by showing that exposure of neurons in monoculture to TNF alpha, a classical pro-inflammatory mediator of activated microglia, is sufficient to increase alpha-SNC secretion in a mechanism dependent on JNK2 or JNK3. In continuation hereof, we show that also IFN beta and TGF beta increase the release of alpha-SNC from PC12 neurons.Conclusions: We implicate stress kinases of the JNK family in the regulation of exophagy and release of alpha-SNC following endogenous or exogenous stimulation. In a wider scope, our results imply that microglia not only inflict bystander damage to neurons in late phases of inflammatory brain disease but may also be active mediators of disease propagation

AB - Background: Secretion of proteopathic alpha-synuclein (alpha-SNC) species from neurons is a suspected driving force in the propagation of Parkinson's disease (PD). We have previously implicated exophagy, the exocytosis of autophagosomes, as a dominant mechanism of alpha-SNC secretion in differentiated PC12 or SH-SY5Y nerve cells. Here we have examined the regulation of exophagy associated with different forms of nerve cell stress relevant to PD.Results: We identify cJUN-N-terminal kinase (JNK) activity as pivotal in the secretory fate of autophagosomes containing alpha-SNC. Pharmacological inhibition or genetic (shRNA) knockdown of JNK2 or JNK3 decreases alpha-SNC secretion in differentiated PC12 and SH-SY5Y cells, respectively. Conversely, expression of constitutively active mitogen-activated protein kinase kinase 7 (MKK7)-JNK2 and -JNK3 constructs augment secretion. The transcriptional activity of cJUN was not required for the observed effects. We establish a causal relationship between increased alpha-SNC release by exophagy and JNK activation subsequent to lysosomal fusion deficiency (overexpression of Lewy body-localized protein p25 alpha or bafilomycin A1). JNK activation following neuronal ER or oxidative stress was not correlated with exophagy, but of note, we demonstrate that reciprocal signaling between microglia and neurons modulates alpha-SNC secretion. NADPH oxidase activity of microglia cell lines was upregulated by direct co-culture with alpha-SNC-expressing PC12 neurons or by passive transfer of nerve cell-conditioned medium. Conversely, inflammatory factors secreted from activated microglia increased JNK activation and alpha-SNC secretion several-fold in PC12 cells. While we do not identify these factors, we extend our observations by showing that exposure of neurons in monoculture to TNF alpha, a classical pro-inflammatory mediator of activated microglia, is sufficient to increase alpha-SNC secretion in a mechanism dependent on JNK2 or JNK3. In continuation hereof, we show that also IFN beta and TGF beta increase the release of alpha-SNC from PC12 neurons.Conclusions: We implicate stress kinases of the JNK family in the regulation of exophagy and release of alpha-SNC following endogenous or exogenous stimulation. In a wider scope, our results imply that microglia not only inflict bystander damage to neurons in late phases of inflammatory brain disease but may also be active mediators of disease propagation

KW - Inflammation

KW - Synucleinopathy

KW - Microglia

KW - Neuronal secretion

KW - JNK

KW - Parkinson's disease

U2 - 10.1186/s12974-016-0519-5

DO - 10.1186/s12974-016-0519-5

M3 - Journal article

C2 - 26957005

VL - 13

JO - Journal of Neuroinflammation

JF - Journal of Neuroinflammation

SN - 1742-2094

M1 - 59

ER -