Severe hydroxymethylbilane synthase deficiency causes depression-like behavior and mitochondrial dysfunction in a mouse model of homozygous dominant acute intermittent porphyria

The metabolic regulator USF-1 is involved in the control of affective behaviour in mice

Epidemiological studies indicate a bidirectional association between metabolic disturbances, including obesity and related pathological states, and mood disorders, most prominently major depression. However, the biological mechanisms mediating the comorbid relationship between the deranged metabolic and mood states remain incompletely understood. Here, we tested the hypothesis that the enhanced activation of brown fat tissue (BAT), known to beneficially regulate obesity and accompanying dysfunctional metabolic states, is also paralleled by an alteration of affective behaviour. We used upstream stimulatory factor 1 (USF-1) knock-out (KO) mice as a genetic model of constitutively activated BAT and positive cardiometabolic traits and found a reduction of depression-like and anxiety-like behaviours associated with USF-1 deficiency. Surgical removal of interscapular BAT did not impact the behavioural phenotype of USF-1 KO mice. Further, the absence of USF-1 did not lead to alterations of adult hippocampal neural progenitor cell proliferation, differentiation, or survival. RNA-seq analysis characterised the molecular signature of USF-1 deficiency in the hippocampus and revealed a significant increase in the expression of several members of the X-linked lymphocyte-regulated (xlr) genes, including xlr3b and xlr4b. Xlr genes are the mouse orthologues of the human FAM9 gene family and are implicated in the regulation of dendritic branching, dendritic spine number and morphology. The transcriptional changes were associated with morphological alterations in hippocampal neurons, manifested in reduced dendritic length and complexity in USF-1 KO mice. Collectively these data suggest that the metabolic regulator USF-1 is involved in the control of affective behaviour in mice and that this modulation of mood states is unrelated to USF-1-dependent BAT activation, but reflected in structural changes in the brain.

Cutting-Edge Therapies and Novel Strategies for Acute Intermittent Porphyria: Step-by-Step towards the Solution

Acute intermittent porphyria (AIP) is an autosomal dominant disease caused by the hepatic deficiency of porphobilinogen deaminase (PBGD) and the slowdown of heme biosynthesis. AIP symptomatology includes life-threatening, acute neurovisceral or neuropsychiatric attacks manifesting in response to precipitating factors. The latter promote the upregulation of 5-aminolevulinic acid synthase-1 (ALAS1), the first enzyme of heme biosynthesis, which promotes the overload of neurotoxic porphyrin precursors. Hemin or glucose infusions are the first-line therapies for the reduction of ALAS1 levels in patients with mild to severe AIP, while liver transplantation is the only curative treatment for refractory patients. Recently, the RNA-interference against ALAS1 was approved as a treatment for adult and adolescent patients with AIP. These emerging therapies aim to substitute dysfunctional PBGD with adeno-associated vectors for genome editing, human PBGD mRNA encapsulated in lipid nanoparticles, or PBGD protein linked to apolipoprotein A1. Finally, the impairment of glucose metabolism linked to insulin resistance, and mitochondrial aberrations during AIP pathophysiology provided new therapeutic targets. Therefore, the use of liver-targeted insulin and insulin-mimetics such as α-lipoic acid may be useful for overcoming metabolic dysfunction in these subjects. Herein, the present review aims to provide an overview of AIP pathophysiology and management, focusing on conventional and recent therapeutical approaches.

α-Lipoic Acid Improves Hepatic Metabolic Dysfunctions in Acute Intermittent Porphyria: A Proof-of-Concept Study

BACKGROUND

Acute intermittent porphyria (AIP) is caused by the haploinsufficiency of porphobilinogen deaminase (PBGD) enzymatic activity. Acute attacks occur in response to fasting, and alterations in glucose metabolism, insulin resistance, and mitochondrial turnover may be involved in AIP pathophysiology. Therefore, we investigated the metabolic pathways in PBGD-silenced hepatocytes and assessed the efficacy of an insulin mimic, α-lipoic acid (α-LA), as a potential therapeutic strategy.

METHODS

HepG2 cells were transfected with siRNA-targeting PBGD (siPBGD). Cells were cultured with low glucose concentration to mimic fasting and exposed to α-LA alone or with glucose.

RESULTS

At baseline, siPBGD cells showed a lower expression of genes involved in glycolysis and mitochondrial dynamics along with reduced total ATP levels. Fasting further unbalanced glycolysis by inducing ATP shortage in siPBGD cells and activated DRP1, which mediates mitochondrial separation. Consistently, siPBGD cells in the fasted state showed the lowest protein levels of Complex IV, which belongs to the oxidative phosphorylation (OXPHOS) machinery. α-LA upregulated glycolysis and prompted ATP synthesis and triglyceride secretion, thus possibly providing energy fuels to siPBGD cells by improving glucose utilization. Finally, siPBGD exposed to α-LA plus glucose raised mitochondrial dynamics, OXPHOS activity, and energy production.

CONCLUSIONS

α-LA-based therapy may ameliorate glucose metabolism and mitochondrial dysfunctions in siPBGD hepatocytes.

Acute intermittent porphyria: focus on possible mechanisms of acute and chronic manifestations

Porphyrias are a group of inherited metabolic diseases that include eight types, each of which is caused by a mutation that affects an enzyme of the heme biosynthetic pathway. When an enzyme defect has physiological significance, it leads to overproduction of pathway precursors prior to the defective step. The partial absence of the third enzyme in the heme biosynthetic pathway, porphobilinogen deaminase (PBGD) also known as hydroxymethylbilane synthase (HMBS), results in acute intermittent porphyria (AIP), which affects mainly women. Subjects who had AIP symptoms were deemed to have manifest AIP (MAIP). Clinical manifestations are usually diverse and non-specific. Acute AIP episodes may present with abdominal pain, nausea, and vomiting, and repeated episodes may result in a series of chronic injuries. Therefore, studying the mechanisms of acute and chronic manifestations of AIP is of great significance. This review aims to summarize the possible mechanisms of acute and chronic manifestations in patients with AIP.