BFU2016-77363-R. Regulation of autophagy through dopaminergic D2R/D3R agonists: Induction or inhibition, selectivity for D2R or D3R and neuroprotection. (D2RD3Rautophagy)

[vc_row][vc_column][vc_tta_accordion shape=»square» c_icon=»chevron» c_position=»right» active_section=»» no_fill=»true» collapsible_all=»true»][vc_tta_section title=»Resumen» tab_id=»resumen»][vc_column_text]

Autophagy dysfunction plays a fundamental role in the pathogenesis of neurodegenerative diseases (NDDs) by leading to the formation of misfolded protein aggregates that, in their oligomeric phase, are cytotoxic, causing neuronal death. Therefore, inducing autophagy by preventing the formation of protein aggregates is a promising therapeutic alternative in NDDs. However, depending on the cellular environment, autophagy can also cause cell death. This is particularly relevant in NDDs because they require prolonged treatment, and currently available autophagy inducers act non-selectively on all cells. It would be beneficial to find autophagy inducers with a high therapeutic index (therapeutic dose/toxic dose ratio) that act selectively on specific neuronal populations. Mesencephalic dopaminergic neurons and striatal median spiny neurons, the main targets of degeneration in Parkinson's disease (PD) and Huntington's disease (HD), respectively, express D2 (D2R) and D3 (D3R) dopaminergic receptors. Recent studies indicate that D2R/D3R ligands modulate autophagy; however, findings regarding their inductive or inhibitory capacity are contradictory. Both induction and inhibition/disruption of autophagy have been described following administration of D2R/D3R agonists or antagonists. These discrepancies may be due to differences in both experimental conditions and the interpretation of autophagy markers. Data from our laboratory using pramipexole (PPX), a D2R/D3R-preferential dopaminergic agonist, in cell and animal models show that: 1. at low doses, PPX induces autophagy via the classical (mTOR-dependent) pathway mediated by D3R, and 2. at higher doses, PPX induces changes in autophagy markers not mediated by D3R. These changes may: a) correspond to either activation of autophagy via the mTOR-independent pathway or dysregulation (inhibition) of autophagy, b) be mediated by D2R or by non-DAergic mechanisms, and c) have a protective or toxic effect. Our hypothesis is that the neuroprotective effect of D2R/D3R agonists as autophagy inducers depends on the D3R selectivity in their signaling. Preferential D3R agonists, administered at low doses (selective D3R signaling), prevent the formation of protein aggregates by autophagy activation via the classical (mTOR-dependent) pathway. At higher doses (non-D3R selective), acting either through D2R or non-DAergic mechanisms, they produce changes in autophagy markers that may be related to reactive autophagy to cell damage, autophagy dysregulation, and/or apoptosis activation. The objective of this project is:

1. To demonstrate that D2R/D3R DAergic agonists induce autophagy in mesencephalic and striatal medium spiny DAergic neurons through D3R and mTOR inhibition,
2. To demonstrate that this effect prevents the formation of aggregates of α-synuclein and mutated huntingtin in animal models of PD and HD, respectively and
3. Investigate whether non-D3R (D2R or non-DAergic) signaling from D2R/D3R agonists causes mTOR-independent autophagy, disruption/inhibition of autophagy and/or cell damage.

[/vc_column_text][/vc_tta_section][vc_tta_section title=»Abstract» tab_id=»abstract»][vc_column_text]

Autophagy dysfunction plays a central role in the pathogenesis of neurodegenerative diseases (NDD) by provoking the aggregation of misfolded proteins which are cytotoxic in their oligomeric form and lead to cell death. So, autophagy induction can prevent the formation of protein aggregates and is a promising therapy in NDD. However, depending on the cellular environment, autophagy can also promote cell death. This fact is particularly relevant in NDD because they require long-time treatment and autophagy inducers available nowadays are not selective for specific cell populations. It would be interesting to find autophagy inducers with a high therapeutic index (therapeutic dose/toxic dose ratio) and which act selectively on specific neuronal populations. Midbrain dopaminergic (DAergic) neurons and striatal medium spiny neurons, the main target of degeneration in Parkinsons disease (PD) and Huntingtons disease (HD), respectively, express the DAergic D2 receptor (D2R) and D3 receptor (D3R). Recent reports show that autophagy can be regulated by D2R/D3R ligands. However, the findings about their capacity as inducers or inhibitors are contradictory. Both induction and inhibition/disruption have been described after agonist or antagonist administration. Discrepancies may be due to differences in experimental conditions as well as in the interpretation of autophagy markers. Our experiments using pramipexole (PPX), a D3R preferential D2R/D3R agonist, in cell and animal models show that: 1. At low doses, PPX induces D3R-mediated classical (mTOR-dependent) autophagy, and 2: At high doses, PPX induces non D3R-mediated changes in autophagy markers. These changes may: a) reflect either activation of non-classical autophagy (mTOR-independent) or autophagy dysregulation/inhibition, b) be mediated by D2R or by non-DAergic mechanisms, and c) have a protective or toxic effect. Our hypothesis is that the neuroprotective effect of D2R/D3R agonists as autophagy inducers depends on their selectivity for D3R signaling. At low doses (D3R selective signaling), D3R preferential agonists prevent the formation of protein aggregates through the activation of classical (mTOR-dependent) autophagy. At high doses (D3R non-selective signaling), acting through either D2R or nonDAergic mechanisms, these agonists promote changes in autophagy markers that can be due to autophagy activation in response to cell damage, autophagy dysregulation or apoptosis activation. Our aim is:

1. To demonstrate that DAergic D2R/D3R agonists induce autophagy in midbrain dopaminergic (DAergic) neurons and striatal medium spiny neurons through D3R signaling and mTOR inhibition,
2. To demonstrate that this effect prevents the formation of α-synuclein and mutant huntingtin aggregates in animal models of PD and HD, respectively, and
3. To investigate whether non-D3R (D2R or non-DAergic) signaling of D2R/D3R agonsits promotes mTOR-independent autophagy, autophagy inhibition/disruption and/or cell damage.

[/vc_column_text][/vc_tta_section][/vc_tta_accordion][/vc_column][/vc_row]

Researcher at the University of La Laguna

• Tomás H. González Hernández