PCSK9 is not involved in the degradation of LDL receptors and BACE1 in the adult mouse brain.

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a secreted protein that regulates hepatic low-density lipoprotein receptor (LDLR) levels in humans. PCSK9 has also been shown to regulate the levels of additional membrane-bound proteins in vitro, including the very low-density lipoprotein receptor (VLDLR), apolipoprotein E receptor 2 (ApoER2) and the beta-site amyloid precursor protein (APP)-cleaving enzyme 1 (BACE1), which are all highly expressed in the CNS and have been implicated in Alzheimer's disease. To better understand the role of PCSK9 in regulating these additional target proteins in vivo, their steady-state levels were measured in the brain of wild-type, PCSK9-deficient, and human PCSK9 overexpressing transgenic mice. We found that while PCSK9 directly bound to recombinant LDLR, VLDLR, and apoER2 protein in vitro, changes in PCSK9 expression did not alter the level of these receptors in the mouse brain. In addition, we found no evidence that PCSK9 regulates BACE1 levels or APP processing in the mouse brain. In conclusion, our results suggest that while PCSK9 plays an important role in regulating circulating LDL cholesterol levels by reducing the number of hepatic LDLRs, it does not appear to modulate the levels of LDLR and other membrane-bound proteins in the adult mouse brain.

Structural and biochemical characterization of the wild type PCSK9-EGF(AB) complex and natural familial hypercholesterolemia mutants.

PCSK9 regulates low density lipoprotein receptor (LDLR) levels and consequently is a target for the prevention of atherosclerosis and coronary heart disease. Here we studied the interaction, of LDLR EGF(A/AB) repeats with PCSK9. We show that PCSK9 binds the EGF(AB) repeats in a pH-dependent manner. Although the PCSK9 C-terminal domain is not involved in LDLR binding, PCSK9 autocleavage is required. Moreover, we report the x-ray structure of the PCSK9DeltaC-EGF(AB) complex at neutral pH. Compared with the low pH PCSK9-EGF(A) structure, the new structure revealed rearrangement of the EGF(A) His-306 side chain and disruption of the salt bridge with PCSK9 Asp-374, thus suggesting the basis for enhanced interaction at low pH. In addition, the structure of PCSK9DeltaC bound to EGF(AB)(H306Y), a mutant associated with familial hypercholesterolemia (FH), reveals that the Tyr-306 side chain forms a hydrogen bond with PCSK9 Asp-374, thus mimicking His-306 in the low pH conformation. Consistently, Tyr-306 confers increased affinity for PCSK9. Importantly, we found that although the EGF(AB)(H306Y)-PCSK9 interaction is pH-independent, LDLR(H306Y) binds PCSK9 50-fold better at low pH, suggesting that factors other than His-306 contribute to the pH dependence of PCSK9-LDLR binding. Further, we determined the structures of EGF(AB) bound to PCSK9DeltaC containing the FH-associated D374Y and D374H mutations, revealing additional interactions with EGF(A) mediated by Tyr-374/His-374 and providing a rationale for their disease phenotypes. Finally, we report the inhibitory properties of EGF repeats in a cellular assay measuring LDL uptake.

Rifamycin antibiotic resistance by ADP-ribosylation: Structure and diversity of Arr.

The rifamycin antibiotic rifampin is important for the treatment of tuberculosis and infections caused by multidrug-resistant Staphylococcus aureus. Recent iterations of the rifampin core structure have resulted in new drugs and drug candidates for the treatment of a much broader range of infectious diseases. This expanded use of rifamycin antibiotics has the potential to select for increased resistance. One poorly characterized mechanism of resistance is through Arr enzymes that catalyze ADP-ribosylation of rifamycins. We find that genes encoding predicted Arr enzymes are widely distributed in the genomes of pathogenic and nonpathogenic bacteria. Biochemical analysis of three representative Arr enzymes from environmental and pathogenic bacterial sources shows that these have equally efficient drug resistance capacity in vitro and in vivo. The 3D structure of one of these orthologues from Mycobacterium smegmatis was determined and reveals structural homology with ADP-ribosyltransferases important in eukaryotic biology, including poly(ADP-ribose) polymerases (PARPs) and bacterial toxins, despite no significant amino acid sequence homology with these proteins. This work highlights the extent of the rifamycin resistome in microbial genera with the potential to negatively impact the expanded use of this class of antibiotic.