Building out the FBXL4-NIX-mitophagy axis

Mitochondria are the power stations within nearly all human cells. From time to time these are decommisoned and new ones are made. The old or damaged mitochondria are delivered to the cell breakers yard or lysosome in a process called mitophagy. Mitophagy is important in developmental pathways but also in suppressing neurodegeneration and other pathophysological conditions. Sometimes mitophagy can be over-active and patients are left with too few mitochondria for the body's energy needs and suffer from mitochondrial depletion syndrome. This is the case for patients with a mutation in a gene called FBXL4. We have recently figured out how such mutations lead to this excess of mitophagy, by showing that FBXL4 controls the levels of another protein called NIX, which can dive into the mitochondria and effectively act as a notice of condemnation. Now we want to build on these findings and provide a more integrated view of the mitophagy process that looks at how the function of FBXL4 is choreographed with other proteins and modifications. Our work combines hypothesis-driven and more open-ended discovery based research which may lead to new ways of influencing mitochondrial health for the benefit of patients.

Mitophagy is essential for the maintenance of mitochondrial health and for metabolic remodelling. Failure of mitophagy has been linked to several neurodegenerative conditions, Parkinson's, ALS and Alzheimer's Disease. Other conditions where mitophagy is critically important include pulmonary fibrosis, acute kidney injury and recovery from cardiac ischemia. FBXL4 is a ubiquitin E3 ligase that is depleted in Mitochondrial depletion syndrome 13, an autosomal recessive disorder characterised by early infantile onset of encephalopathy, hypotonia, lactic acidosis, and severe global developmental delay. We have recently shown that FBXL4 controls the stability of the mitochondrial adaptor proteins NIX and BNIP3. Here we propose to provide a more detailed and enlarged view of this circuitry through the systematic analysis of several variables and incorporation of other proteins implicated in the process (PPTC7 and TMEM11). Our discovery of FBXL4 control of mitophagy stemmed from an E3-ligase targeted CRISPR screen. We will integrate other hits from this screen into our scheme of mitophagy regulation and conduct orthologous screens using a deubiquitylase (DUB) library that represents a family of druggable targets. In particular we aim to identify DUBs which oppose the ligase activity of FBXL4. At all opportunities we will test and validate our findings in patient derived cell lines bearing FBXL4 mutations.