PRG-1 transcriptional regulation independent from Nex1/Math2-mediated activation

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

Acute intermittent porphyria (AIP) is an autosomal dominant inborn error of heme biosynthesis due to a pathogenic mutation in the Hmbs gene, resulting in half-normal activity of hydroxymethylbilane synthase. Factors that induce hepatic heme biosynthesis induce episodic attacks in heterozygous patients. The clinical presentation of acute attacks involves the signature neurovisceral pain and may include psychiatric symptoms. Here we used a knock-in mouse line that is biallelic for the Hmbs c.500G > A (p.R167Q) mutation with ~ 5% of normal hydroxymethylbilane synthase activity to unravel the consequences of severe HMBS deficiency on affective behavior and brain physiology. Hmbs knock-in mice (KI mice) model the rare homozygous dominant form of AIP and were used as tool to elucidate the hitherto unknown pathophysiology of the behavioral manifestations of the disease and its neural underpinnings. Extensive behavioral analyses revealed a selective depression-like phenotype in Hmbs KI mice; transcriptomic and immunohistochemical analyses demonstrated aberrant myelination. The uncovered compromised mitochondrial function in the hippocampus of knock-in mice and its ensuing neurogenic and neuroplastic deficits lead us to propose a mechanistic role for disrupted mitochondrial energy production in the pathogenesis of the behavioral consequences of severe HMBS deficiency and its neuropathological sequelae in the brain.

Current views on regulation and function of plasticity-related genes (PRGs/LPPRs) in the brain

Plasticity-related genes (PRGs, Lipid phosphate phosphatase-related proteins LPPRs) are a defined as a subclass of the lipid phosphate phosphatase (LPP) superfamily, comprising so far five brain- and vertebrate-specific membrane-spanning proteins. LPPs interfere with lipid phosphate signaling and are thereby involved in mediating the extracellular concentration and signal transduction of lipid phosphate esters such as lysophosphatidate (LPA) and spingosine-1 phosphate (S1P). LPPs dephosphorylate their substrates through extracellular catalytic domains, thus making them ecto-phosphatases. PRGs/LPPRs are structurally similar to the other LPP family members in general. They are predominantly expressed in the CNS in a subtype specific pattern rather than having a wide tissue distribution. In contrast to LPPs, PRGs/LPPRs may act by modifying bioactive lipids and their signaling pathways, rather than possessing an ecto-phosphatase activity. However, the exact functional roles of PRGs/LPPRs have just begun to be explored. Here, we discuss new findings on the neuron-specific transcriptional regulation of PRG1/LPPR4 and new insights into protein-protein interaction and signaling pathway regulation. Further, we start to shed light on the subcellular localization and the resulting functional modulatory influence of PRG1/LPPR4 expression in excitatory synaptic transmission to the established neural effects such as promotion of filopodia formation, neurite extension, axonal sprouting and reorganization after lesion. This range of effects suggests an involvement in the pathogenesis and/or reparation attempts in disease. Therefore, we summarize available data on the association of PRGs/LPPRs with several neurological and other diseases in humans and experimental animals. Finally we highlight important open questions and emerging future directions of research. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.