A Regulatory Cysteine Residue Mediates Reversible Inactivation of NAD+-Dependent Aldehyde Dehydrogenases to Promote Oxidative Stress Response

Thinking outside the CaaX-box: an unusual reversible prenylation on ALDH9A1

Protein lipidation is a post-translational modification that confers hydrophobicity on protein substrates to control their cellular localization, mediate protein trafficking, and regulate protein function. In particular, protein prenylation is a C-terminal modification on proteins bearing canonical motifs catalyzed by prenyltransferases. Prenylated proteins have been of interest due to their numerous associations with various diseases. Chemical proteomic approaches have been pursued over the last decade to define prenylated proteomes (prenylome) and probe their responses to perturbations in various cellular systems. Here, we describe the discovery of prenylation of a non-canonical prenylated protein, ALDH9A1, which lacks any apparent prenylation motif. This enzyme was initially identified through chemical proteomic profiling of prenylomes in various cell lines. Metabolic labeling with an isoprenoid probe using overexpressed ALDH9A1 revealed that this enzyme can be prenylated inside cells but does not respond to inhibition by prenyltransferase inhibitors. Site-directed mutagenesis of the key residues involved in ALDH9A1 activity indicates that the catalytic C288 bears the isoprenoid modification likely through an NAD+-dependent mechanism. Furthermore, the isoprenoid modification is also susceptible to hydrolysis, indicating a reversible modification. We hypothesize that this modification originates from endogenous farnesal or geranygeranial, the established degradation products of prenylated proteins and results in a thioester form that accumulates. This novel reversible prenoyl modification on ALDH9A1 expands the current paradigm of protein prenylation by illustrating a potentially new type of protein-lipid modification that may also serve as a novel mechanism for controlling enzyme function.

Can nicotinamide riboside protect against cognitive impairment?

PURPOSE OF REVIEW

The present review aims to address the clinical benefits of using nicotinamide riboside, a precursor to the essential pyridine nucleotide, nicotinamide adenine dinucleotide (NAD+) as a therapeutic agent to attenuate age-related cognitive decline.

RECENT FINDINGS

Oral supplementation with nicotinamide riboside can inhibit the accumulation of pathological hallmarks of Alzheimer's disease and improve learning and memory in various murine models for dementia. Nicotinamide riboside can also reduce DNA damage, neuroinflammation, apoptosis, and improved hippocampal synaptic plasticity in diabetic mice, and another Alzheimer's disease mouse model. The cognitive benefits of nicotinamide riboside in Alzheimer's disease models may be modulated in part by upregulation of proliferator-activated-γ coactivator 1α-mediated β-secretase 1(BACE-1) ubiquitination and degradation, preventing Aβ production in the brain. Nicotinamide riboside also maintained blood-brain barrier integrity and maintained the gut microbiota in a mouse model for cerebral small vessel disease and alcohol-induced depression, respectively. Oral nicotinamide riboside has been shown to be bioavailable and well tolerated in humans with limited adverse effects compared to other NAD+ precursors.

SUMMARY

Oral nicotinamide riboside may represent a promising stratagem to improve cognitive decline during 'normal' ageing, Alzheimer's disease and other diseases. Results from recent clinical trials are needed to enumerate the preclinical benefits in humans.

The Two-Species Model of transketolase explains donor substrate-binding, inhibition and heat-activation

We recently characterised a low-activity form of E. coli transketolase, TK_low, which also binds the cofactor thiamine pyrophosphate (TPP) with an affinity up to two-orders of magnitude lower than the previously known high TPP-affinity and high-activity form, TK_high, in the presence of Mg²⁺. We observed previously that partial oxidation was responsible for increased TK_high activity, while low-activity TK_low was unmodified. In the present study, the fluorescence-based cofactor-binding assay was adapted to detect binding of the β-hydroxypyruvate (HPA) donor substrate to wild-type transketolase and a variant, S385Y/D469T/R520Q, that is active towards aromatic aldehydes. Transketolase HPA affinity again revealed the two distinct forms of transketolase at a TK_high:TK_low ratio that matched those observed previously via TPP binding to each variant. The HPA dissociation constant of TK_low was comparable to the substrate-inhibition dissociation constant, K_i^HPA, determined previously. We provide evidence that K_i^HPA is a convolution of binding to the low-activity TK_low-TK_low dimer, and the TK_low subunit of the partially-active TK_high-TK_low mixed dimer, where HPA binding to the TK_low subunit of the mixed dimer results in inhibition of the active TK_high subunit. Heat-activation of transketolase was similarly investigated and found to convert the TK_low subunit of the mixed dimer to have TK_high-like properties, but without oxidation.