PARL Leu262Val is not associated with fasting insulin levels in UK populations

Mitochondrial Processing Peptidases-Structure, Function and the Role in Human Diseases

Mitochondrial proteins are encoded by both nuclear and mitochondrial DNA. While some of the essential subunits of the oxidative phosphorylation (OXPHOS) complexes responsible for cellular ATP production are synthesized directly in the mitochondria, most mitochondrial proteins are first translated in the cytosol and then imported into the organelle using a sophisticated transport system. These proteins are directed mainly by targeting presequences at their N-termini. These presequences need to be cleaved to allow the proper folding and assembly of the pre-proteins into functional protein complexes. In the mitochondria, the presequences are removed by several processing peptidases, including the mitochondrial processing peptidase (MPP), the inner membrane processing peptidase (IMP), the inter-membrane processing peptidase (MIP), and the mitochondrial rhomboid protease (Pcp1/PARL). Their proper functioning is essential for mitochondrial homeostasis as the disruption of any of them is lethal in yeast and severely impacts the lifespan and survival in humans. In this review, we focus on characterizing the structure, function, and substrate specificities of mitochondrial processing peptidases, as well as the connection of their malfunctions to severe human diseases.

Mitochondrial quality control pathways: PINK1 acts as a gatekeeper

Mitochondria have a pivotal role in the maintenance of cell homeostasis and survival. Mitochondria are involved in processes such as ATP production, reactive oxygen species production, apoptosis induction, calcium homeostasis and protein degradation. Thus, mechanisms that regulate the intrinsic quality of mitochondria have a crucial role in dictating overall cell fate. The importance of these well-regulated mechanisms is highlighted in disease scenarios such as neurodegeneration, cancer and neuromuscular atrophy. How mitochondria senses and regulates their intrinsic quality control, and consequently cell survival, is still not fully understood. In this review, we discuss the pathways that are at present considered as state-of-the-art for mitochondria quality control regulation, and highlight a mitochondrial protein-PINK1-that has revealed to act as a mitochondrial gatekeeper able to sense the presence of healthy or damaged mitochondria.

Rhomboid proteases in human disease: Mechanisms and future prospects

Rhomboids are intramembrane serine proteases that cleave the transmembrane helices of substrate proteins, typically releasing luminal/extracellular domains from the membrane. They are conserved in all branches of life and there is a growing recognition of their association with a wide range of human diseases. Human rhomboids, for example, have been implicated in cancer, metabolic disease and neurodegeneration, while rhomboids in apicomplexan parasites appear to contribute to their invasion of host cells. Recent advances in our knowledge of the structure and the enzyme function of rhomboids, and increasing efforts to identify specific inhibitors, are beginning to provide important insight into the prospect of rhomboids becoming future therapeutic targets. This article is part of a Special Issue entitled: Proteolysis as a Regulatory Event in Pathophysiology edited by Stefan Rose-John.

PARL: The mitochondrial rhomboid protease

The rhomboid family comprises evolutionary conserved intramembrane proteases involved in a wide spectrum of biologically relevant activities. A mitochondrion-localized rhomboid, called PARL in mammals, and conserved in yeast and Drosophila as RBD1/PCP1 and rho-7, respectively, plays an indispensable role in cell homeostasis as illustrated by the severe phenotypes caused by its genetic ablation in the various investigated species. Although several substrates of PARL have been proposed to explain these phenotypes, there remains a lot of controversy in this important area of research. We review here the putative functions and substrates of PARL and its orthologues in different species, highlighting areas of uncertainty, and discuss its potential involvement in some prevalent diseases such as type II diabetes and Parkinson's disease.

ABCC5 transporter is a novel type 2 diabetes susceptibility gene in European and African American populations

Numerous functional studies have implicated PARL in relation to type 2 diabetes (T2D). We hypothesised that conflicting human association studies may be due to neighbouring causal variants being in linkage disequilibrium (LD) with PARL. We conducted a comprehensive candidate gene study of the extended LD genomic region that includes PARL and transporter ABCC5 using three data sets (two European and one African American), in relation to healthy glycaemic variation, visceral fat accumulation and T2D disease. We observed no evidence for previously reported T2D association with Val262Leu or PARL using array and fine-map genomic and expression data. By contrast, we observed strong evidence of T2D association with ABCC5 (intron 26) for European and African American samples (P = 3E-07) and with ABCC5 adipose expression in Europeans [odds ratio (OR) = 3.8, P = 2E-04]. The genomic location estimate for the ABCC5 functional variant, associated with all phenotypes and expression data (P = 1E-11), was identical for all samples (at Chr3q 185,136 kb B36), indicating that the risk variant is an expression quantitative trait locus (eQTL) with increased expression conferring risk of disease. That the association with T2D is observed in populations of disparate ancestry suggests the variant is a ubiquitous risk factor for T2D.

The PARL family of mitochondrial rhomboid proteases

Rhomboids are an ancient and conserved family of intramembrane-cleaving proteases, a small group of proteolytic enzymes capable of hydrolyzing a peptide bond within a transmembrane helix that anchors a substrate protein to the membrane. Mitochondrial rhomboids evolved in eukaryotes to coordinate a critical aspect of cell biology, the regulation of mitochondrial membranes dynamics. This function appears to have required the emergence of a structural feature that is unique among all other rhomboids: an additional transmembrane helix (TMH) positioned at the N-terminus of six TMHs that form the core proteolytic domain of all prokaryotic and eukaryotic rhomboids. This "1+6" structure, which is shared only among mitochondrial rhomboids, defines a subfamily of rhomboids with the prototypical family member being mammalian Parl. Here, we present the findings that in 11 years have elevated mitochondrial rhomboids as the gatekeepers of mitochondrial dynamics and apoptosis; further, we discuss the aspects of their biology that are bound to introduce new paradigm shifts in our understanding of how the organelle uses this unique type of protease to govern stress, signaling to the nucleus, and other key mitochondrial activities in health and disease.

The Leu262Val polymorphism of presenilin associated rhomboid like protein (PARL) is associated with earlier onset of type 2 diabetes and increased urinary microalbumin creatinine ratio in an Irish case-control population

AIMS

Environmental and genetic factors contribute to the evolution of type 2 diabetes (T2DM). Presenilin associated rhomboid like protein (PARL) is a mitochondrial protein that has been implicated in T2DM in both the rodent Psammomys obesus and in humans. The SNP variant (Leu262Val) in PARL has been shown to be associated with hyperinsulinaemia in an age-dependent manner in a US non-diabetic, cohort. However, this finding has not been replicated in UK cohorts. We studied Leu262Val associations in an Irish Caucasian T2DM case-control population.

METHODS

An RFLP-PCR assay using BstN I was used to assess Leu262Val genotype in a total of 613 subjects, 421 with T2DM and 192 controls.

RESULTS

In the control group genotype frequencies were as follows 27.37% (GG), 51.58% (CG) and 21.05% (CC), while in the group with T2DM 30.64% (GG), 47.74% (CG) and 21.62% (CC). We observed no association between Leu262Val variant and T2DM nor was there an association with plasma insulin concentrations or BMI. There was no interaction between age and fasting plasma insulin concentration. However, in the group with T2DM the C allele was associated with higher urinary albumin to creatinine ratio while the GG genotype was associated with an earlier age of onset of T2DM.

CONCLUSION

The Leu262Val polymorphism of PARL is not associated with markers of insulin resistance. However, in subjects with T2DM, genetic variation at this locus may indicate earlier onset of T2DM and increased susceptibility to nephropathy and cardiovascular complications.

Mitochondrial dynamics as a bridge between mitochondrial dysfunction and insulin resistance

Muscle from obese subjects or from type 2 diabetic patients show mitochondrial dysfunction, and this may participate in the insulin resistance in those conditions. The mechanisms involved in mitochondrial dysfunction are not completely understood. Dynamic mitochondrial filaments or networks form by mitochondrial fusion and fission events. There is substantial evidence that proteins participating in mitochondrial fusion or fission also have a role in metabolism. Thus, mitofusin-2 (Mfn2) a mitochondrial fusion protein, stimulates respiration, substrate oxidation and OXPHOS subunits expression. In this regard, muscle from obese subjects, or from type 2 diabetic patients, show a reduced expression of Mfn2 and, amelioration of insulin sensitivity by bariatric surgery is associated with an increased Mfn2 expression in muscle. Here, we propose the hypothesis that mitochondrial dynamics proteins play a role in mitochondrial dysfunction in obesity or in type 2 diabetes and that it may also participate in the development of insulin resistance.

Association of PARL rs3732581 genetic variant with insulin levels, metabolic syndrome and coronary artery disease

PARL (presenilin-associated rhomboid-like) is a mitochondrial protein involved in mitochondrial membrane remodelling, and maps to a quantitative trait locus (3q27) associated with metabolic traits. Recently the rs3732581 (Leu262Val) variant was found to be associated with increased levels of plasma insulin, a finding not replicated in a larger cohort. The aim of the current study was to investigate the associations between rs3732581 and levels of plasma insulin, metabolic syndrome (MetS) and its components, and cardiovascular disease. The CUPID population consisted of 556 subjects with angiographically proven CAD and the CUDAS cohort consisted of 1,109 randomly selected individuals from Perth, Western Australia. Samples were genotyped using mutation-specific PCR. No significant associations were observed between rs3732581 and levels of plasma insulin, glucose, BMI or MetS in either population. However, carriers of the minor allele had significantly lower mean intima-media thickness (IMT) [0.69 mm, 95% CI (0.69, 0.70 mm); P = 0.004], compared with major allele homozygotes [mean IMT = 0.71 mm, 95% CI (0.70, 0.72 mm)] in the CUDAS population. Further analysis using a recessive model showed homozygous carriers of the minor allele were predisposed to CAD [OR 1.55, 95% CI (1.11, 2.16); P = 0.01]. Despite the functional evidence for a role of PARL in regulating insulin levels, no association with rs3732581 was found in the current study. Additionally, there were no associations with glucose levels, BMI or MetS. There were significant effects of the variant on mean IMT and risk of CAD. A role for PARL in metabolic conditions cannot be excluded and more comprehensive genetic studies are warranted.