GRP78 Overexpression Triggers PINK1-IP3R-Mediated Neuroprotective Mitophagy.

Tatiana Leiva-Rodríguez,Joaquim Forés,Valerie Petegnief,David Romeo-Guitart,Miriam Polo,Nadezda Apostolova,Assumpció Bosch,Mireia Herrando-Grabulosa,Caty Casas,Celia Bañuls,Jose Miguel Lizcano,Pau Muñoz-Guardiola

doi:10.3390/biomedicines9081039

Abstract

An experimental model of spinal root avulsion (RA) is useful to study causal molecular programs that drive retrograde neurodegeneration after neuron-target disconnection. This neurodegenerative process shares common characteristics with neuronal disease-related processes such as the presence of endoplasmic reticulum (ER) stress and autophagy flux blockage. We previously found that the overexpression of GRP78 promoted motoneuronal neuroprotection after RA. After that, we aimed to unravel the underlying mechanism by carrying out a comparative unbiased proteomic analysis and pharmacological and genetic interventions. Unexpectedly, mitochondrial factors turned out to be most altered when GRP78 was overexpressed, and the abundance of engulfed mitochondria, a hallmark of mitophagy, was also observed by electronic microscopy in RA-injured motoneurons after GRP78 overexpression. In addition, GRP78 overexpression increased LC3-mitochondria tagging, promoted PINK1 translocation, mitophagy induction, and recovered mitochondrial function in ER-stressed cells. Lastly, we found that GRP78-promoted pro-survival mitophagy was mediated by PINK1 and IP3R in our in vitro model of motoneuronal death. This data indicates a novel relationship between the GRP78 chaperone and mitophagy, opening novel therapeutical options for drug design to achieve neuroprotection.

Highlights

Disruption of the functional neuronal connectivity is a common early characteristic of neurodegenerative processes [1]
In order to identify the molecular mechanisms leading to neuroprotection mediated by GRP78 overexpression, we performed a comparative label-free proteomic analysis to identify both quantitative and qualitative differences between root avulsion (RA)-injured animals infected with adenoviruses for overexpression of GRP78 (Ad-GRP78), previously found to be neuroprotective of MNs, or beta-galactosidase (Ad-ß-Gal) as control [20]
RA model, we observed that signaling events were similar until 7 dpi, when divergent pro-degenerative events emerged after RA [20,25]

Summary

Introduction

Disruption of the functional neuronal connectivity is a common early characteristic of neurodegenerative processes [1]. In the face of damage, neurons react, activating endogenous mechanisms of neuroprotection such as the unfolded protein response (UPR), the heat-shock response, the autophagy pathway, the ubiquitinproteasome system, chaperone expression, the endoplasmic reticulum (ER) associated degradation machinery (ERAD), and the antioxidant defense. Their precise activation is effective in recovering the cell, excessive damage as well as aging can result in defective functioning of one or more of those programs. GRP78, known as BiP or heat shock protein 5a (HSP5a), is a multifunctional protein with critical functions in endogenous mechanisms of neuroprotection [4]. GRP78 orchestrates the UPR, which is activated after ER stress, has ATPase activity, and is a Ca2+

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