Cardiolipin Stabilizes and Increases Catalytic Efficiency of Carnitine Palmitoyltransferase II and Its Variants S113L, P50H, and Y479F.

Muscle carnitine palmitoyltransferase II (CPT II) deficiency is associated with various mutations in CPT2 gene. In the present study, the impact of the two CPT II variants P50H and Y479F were characterized in terms of stability and activity in vitro in comparison to wildtype (WT) and the well investigated variant S113L. While the initial enzyme activity of all variants showed wild-type-like behavior, the activity half-lives of the variants at different temperatures were severely reduced. This finding was validated by the investigation of thermostability of the enzymes using nano differential scanning fluorimetry (nanoDSF). Further, it was studied whether the protein stabilizing diphosphatidylglycerol cardiolipin (CL) has an effect on the variants. CL indeed had a positive effect on the stability. This effect was strongest for WT and least pronounced for variant P50H. Additionally, CL improved the catalytic efficiency for CPT II WT and the investigated variants by twofold when carnitine was the varied substrate due to a decrease in KM. However, there was no influence detected for the variation of substrate palmitoyl-CoA. The functional consequences of the stabilization by CL in vivo remain open.

Formation of HERV-K and HERV-Fc1 Envelope Family Members is Suppressed on Transcriptional and Translational Level.

The human genome comprises 8% sequences of retroviral origin, so-called human endogenous retroviruses (HERVs). Most of these proviral sequences are defective, but some possess open reading frames. They can lead to the formation of viral transcripts, when activated by intrinsic and extrinsic factors. HERVs are thought to play a pathological role in inflammatory diseases and cancer. Since the consequences of activated proviral sequences in the human body are largely unexplored, selected envelope proteins of human endogenous retroviruses associated with inflammatory diseases, namely HERV-K18, HERV-K113, and HERV-Fc1, were investigated in the present study. A formation of glycosylated envelope proteins was demonstrated in different mammalian cell lines. Nevertheless, protein maturation seemed to be incomplete as no transport to the plasma membrane was observed. Instead, the proteins remained in the ER where they induced the expression of genes involved in unfolded protein response, such as HSPA5 and sXBP1. Furthermore, low expression levels of native envelope proteins were increased by codon optimization. Cell-free expression systems showed that both the transcriptional and translational level is affected. By generating different codon-optimized variants of HERV-K113 envelope, the influence of single rare t-RNA pools in certain cell lines was demonstrated. The mRNA secondary structure also appears to play an important role in the translation of the tested viral envelope proteins. In summary, the formation of certain HERV proteins is basically possible. However, their complete maturation and thus full biologic activity seems to depend on additional factors that might be disease-specific and await elucidation in the future.

Lack of activation of the S113L variant of carnitine palmitoyltransfersase II by cardiolipin.

The phospholipid environment of the mitochondrial inner membrane, which contains large amounts of cardiolipin, could play a key role in transport of the long chain fatty acids. In the present study, the pre-incubation of cardiolipin with the wild type carnitine palmitoyltransferase (CPT) II led to a more than 1.5-fold increase of enzyme activity at physiological temperatures. At higher temperatures, however, there was a pronounced loss of activity. The most frequent variant S113L showed even at 37 °C a great activity loss. Pre-incubation of the wild type with both malonyl-CoA and cardiolipin counteracted the positive effect of cardiolipin. Malonyl-CoA, however, showed no inhibition effect on the variant in presence of cardiolipin. The activity loss in presence of cardiolipin at fever simulating situations was more pronounced for the variant comparing to the wild type. The reason might be a disturbed membrane association or a blockage of the active center of the mutated enzyme.

Normal Thermostability of p.Ser113Leu and p.Arg631Cys Variants of Mitochondrial Carnitine Palmitoyltransferase II (CPT II) in Human Muscle Homogenate.

Previous fibroblast and recombinant enzyme studies showed a markedly thermolabile p.Ser113Leu variant compared to the wild-type (WT) in muscle carnitine palmitoyltransferase II (CPT II) deficiency. Additionally, it has been shown that cardiolipin (CLP) stimulated or inhibited the p.Ser113Leu recombinant variant depending on the pre-incubation temperatures. In this study, the thermolabilities of mitochondrial enzyme CPT II in muscle homogenates of patients with the p.Ser113Leu (n = 3) and p.Arg631Cys (n = 2) variants were identified to be similar to that of WT. Pre-incubation with CLP on ice stimulated the WT enzyme more than both variants. However, CLP stimulated the variants and WT at 46 °C to about 6-18-fold. The present data indicate that the thermostability of CPT II variant in muscle homogenate is similar to that of WT. This is in contrast to the increased thermolability of enzymes derived from fibroblast and that of recombinant enzymes. Hence, it can be speculated that the disruption of the compartmentation in muscle homogenate mediates a protective effect on the thermolability of the native variant. However, the exact mechanism remains unclear. However, the activating effect of CLP on CPT II in muscle homogenate seems to align with those on recombinant enzymes.