Contribution of a dysfunctional variant of ALDH4A1 to Alzheimer’s Disease pathology through altered metabolism and mitochondrial impairment

Abstract

AbstractBackgroundAldehyde dehydrogenases (ALDH) are enzymes which oxidize toxic aldehydic species into carboxylates. ALDH4A1 is a mitochondrial, NAD+‐dependent, dimeric enzyme and oxidizes the toxic aldehyde glutamic‐g‐semialdehyde (GSA) to glutamate. ALDH4A1 lies at the nexus of proline and arginine metabolism, the TCA cycle, and mitochondrial health (Fig. 1). The substrate of ALDH4A1, GSA, inactivates vitamin B6 (Farrant et al., 2001), an essential cofactor for amino acid and neurotransmitter biosynthesis. Toxic aldehydes like 4‐hydroxynonenal (4‐HNE), are elevated in Alzheimer’s Disease (AD) patients (Markesbery & Lovell, 1998) and promote amyloid‐b aggregation (Siegel et al., 2007). A recent genetic analysis identified a missense variant of ALDH4A1 as a risk factor for Late Onset Alzheimer’s Disease (LOAD) (Wang et al., 2015). Our previous analysis found that ALDH4A1 is an essential enzyme with no common loss of function variants (Chen et al., 2021). Our work elucidates the contribution of ALDH4A1 to AD pathology.MethodRecombinant protein was characterized to determine changes in protein activity and stability. The AD risk factor mutation was knocked‐in to the endogenous locus of induced pluripotent cells to monitor mitochondrial health, metabolism, and AD biomarkers. Mouse embryonic fibroblasts that do not express ALDH4A1 were used to monitor protein stability in cellulo.ResultThe AD associated variant of ALDH4A1 has decreased activity compared to wildtype (WT) (Fig. 2). ALDH4A1 can oxidize the toxic AD‐associated aldehyde 4‐HNE, and the variant has 50% of the activity of WT with 4‐HNE (Fig. 2). The variant enzyme is less stable in cellulo; variant ALDH4A1 is degraded within a few hours compared to WT that remains stable up to 24 hours. Knock‐in cells accumulate 25% more extracellular proline, and show a trend towards increased intracellular proline, indicating faulty proline metabolism in cells expressing the AD‐associated mutant ALDH4A1.ConclusionALDH4A1 oxidizes AD associated aldehydes (i.e., 4‐HNE) in addition to its known substrate. The AD‐associated variant of ALDH4A1 is less catalytically active and less stable than the WT enzyme. Cells expressing this variant have dysregulated proline metabolism. These data indicate that ALDH4A1 variants may contribute to AD through altered metabolism and increased aldehydic load from dysregulated protein activity and stability.