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Role of the mitochondrial Translocator Protein (mTSPO) in Brain Cellular Physiology: a Neglected Pathway in Signalling and Self-conservation Mechanisms

Final Report Summary - TSPO & BRAIN (Role of the mitochondrial Translocator Protein (mTSPO) in Brain Cellular Physiology: a Neglected Pathway in Signalling and Self-conservation Mechanisms)

The outcome of the years of work possible via this grant is that the 18kDa protein TSPO acts as efficient limiting factor of the mitochondrial quality control process in specialised cell models such as neurons and glia. It operates by inhibiting the PARK2-mediated ubiquitination of mitochondria meant to be disposed via targeted autophagy (or mitophagy).
Along with this core mechanistic finding we also found that TSPO is overexpressed in stressed neuronal cells (e.g. following glutamate treatment) leading to deregulation of mitochondrial Ca2+ handling and increased redox-stress which induces subtle, detrimental citotoxicity. The TSPO mediated impairment of mitochondrial Ca2+ signaling and consequently homeostasis of the Reactive Oxygen Species (ROS) is the means –according to our findings- via which the efficiency of mitochondrial poly-ubiquitination is limited thus blocking mitochondrial quality control and therefore underlying cellular pathology.

These data have therefore the merit to describe TSPO as a novel element in the regulation of mitochondrial response to neurotoxicity and suggest this to be mediated by a selective limitation of the targeted autophagy and consequent impaired cell signalling.
This outcome aligns though the clinical evidences on increased TSPO expression in aged brain indicating that toxicity caused by impaired homeostasis of neurotransmitters can be both primed and detected via TSPO.
The work develops in this way a fundamental understanding of cellular processes linked to TSPO, which will underpin the development of future studies and targeted interventions in human conditions.

It warrented successfully testing of the hypotheses paved in the program of work by achieving the following scientific goals:
I. In neuronal cells, the onset and progression of mitochondrial autophagy is regulated by TSPO level of expression.
II. Both mitochondrial coupling efficiency and structural remodeling are affected by the molecular and pharmacological modulation of TSPO.
III. TSPO didactics mitochondrial and hence cellular commitment to demise by indirectly modifying VDAC function
IV. TSPO stands as molecular regulator of the mitochondrial mechanisms to neurotoxocity.
Namely we succeeded to:

a. Manipulate the endogenous expression of TSPO by recombinant overexpression (+TSPO) and transient deletion (-TSPO) in U-87MG and SY-SH5Y.
b. Evaluate the level of basal and triggered autophagy

c. Assess mitochondrialCa2+ signaling

d. Measure the rate of cytosolic ROS generation

e. Test the TSPO role in regulating the neuroglial cells’ susceptibility to glutamate-induced toxicity.
f,g. Furthermore, we could also generate a U-87MG line stably downregulated for TSPO and testing the direct contribution in Apoptotic cell death.
h, i. Finally proved the relation between TSPO and neurotoxicity.

Our findings will have a tangible benefit for european science since the advanced understanding of framed molecular cell biology of the mitochondrial quality control pivotal for an improved clinical understanding of TSPO in health and disease states of the brain. An impact beyond the most immediate academic beneficiaries could therefore be foreseen.