Chemical chaperones protect from effects of apoptosis-inducing mutation in carbonic anhydrase IV identified in retinitis pigmentosa 17

4-Phenylbutyrate restores the functionality of a misfolded mutant low-density lipoprotein receptor

Familial hypercholesterolemia is an autosomal dominant disease caused by mutations in the gene encoding the low-density lipoprotein receptor. To date, more than 900 different mutations have been described. Transport-defective mutations (class 2) causing partial or complete retention of the receptor in the endoplasmic reticulum are the predominant class of mutations. In a cell culture system (Chinese hamster ovary cells), we show that chemical chaperones are able to mediate rescue of a transport-defective mutant (G544V), and that the ability to obtain rescue is mutation dependent. In particular, the low molecular mass fatty acid derivative 4-phenylbutyrate mediated a marked increase in the transport of G544V-mutant low-density lipoprotein receptor to the plasma membrane. Thirty per cent of the mutant receptor was able to escape from the endoplasmic reticulum and reach the cell surface. The rescued receptor had reduced stability, but was found to be as efficient as the wild-type low-density lipoprotein receptor in binding and internalizing low-density lipoprotein. In addition to 4-phenylbutyrate, we also studied 3-phenylpropionate and 5-phenylvalerate, and compared their effect on rescue of the G544V-mutant low-density lipoprotein receptor with their ability to increase overall gene expression caused by their histone deacetylase inhibitor activity. No correlation was found. Our results indicate that the effect of these agents was not solely mediated by their ability to induce gene expression of proteins involved in intracellular transport, but rather could be due to a direct chemical chaperone activity. These data suggest that rescue of mutant low-density lipoprotein receptor is possible and that it might be feasible to develop pharmacologic chaperones to treat familial hypercholesterolemia patients with class 2 mutations.

Chemically chaperoning the actions of insulin

The role of inflammation as a mediator of insulin resistance in type 2 diabetes and obesity has been a major focus of studies over the past ten years. In mouse models of obesity and type 2 diabetes, the development of insulin resistance correlates with elevated levels of endoplasmic reticulum stress and induction of the unfolded protein response. Activation of N-terminal C-Jun kinase is known to be associated with unfolded protein response activation, and has been shown to participate in the inhibition of insulin action by stimulating serine phosphorylation of the insulin receptor substrate 1, an event that attenuates insulin signaling. 'Chemical chaperones' are small molecules that have been shown to attenuate unfolded protein response activation. The exciting new findings of Ozcan et al. indicate that chemical chaperones improve glucose tolerance and insulin action in a mouse model of type 2 diabetes. These findings offer a potential new target for therapeutic strategies designed to improve insulin action and glucose tolerance in diabetic individuals.

Effects of chemical chaperones on oxidative stress and detergent-insoluble species formation following conditional expression of amyloid precursor protein carboxy-terminal fragment

Oxidative stress, protein misfolding, protein complex formation, and detergent insolubility are biochemical features of Alzheimer's disease (AD). We tested the cause-and-effect relationships among these using MC65 human neuroblastoma cells that exhibit toxicity upon conditional expression of carboxy-terminal fragments (CTFs) of the human amyloid precursor protein (APP). Treatments with three different antioxidants (alpha-tocopherol, N-acetyl cysteine, and alpha-lipoic acid) or three different compounds (glycerol, trimethylamine-N-oxide, and 4-phenylbutyric acid) that have been described to have a "chemical chaperone" function in promoting protein folding all had a protective effect on MC65 cells and decreased markers of oxidative damage and accumulation of high molecular weight amyloid (A) beta-immunoreactive (IR) species. However, chaperones partially reduced detergent insolubility of the remaining Abeta-IR species, while antioxidants did not. These results suggest that protein misfolding associated with overexpression of APP CTFs promotes oxidative stress and cytotoxicity and contributes to formation of detergent-insoluble species that appear unrelated to cytotoxicity.

Hydrophilic iminosugar active-site-specific chaperones increase residual glucocerebrosidase activity in fibroblasts from Gaucher patients

Gaucher disease is an autosomal recessive lysosomal storage disorder caused by the deficient activity of glucocerebrosidase. Accumulation of glucosylceramide, primarily in the lysosomes of cells of the reticuloendothelial system, leads to hepatosplenomegaly, anemia and skeletal lesions in type I disease, and neurologic manifestations in types II and III disease. We report herein the identification of hydrophilic active-site-specific chaperones that are capable of increasing glucocerebrosidase activity in the cultured fibroblasts of Gaucher patients. Screening of a variety of natural and synthetic alkaloid compounds showed isofagomine, N-dodecyl deoxynojirimycin, calystegines A3, B1, B2 and C1, and 1,5-dideoxy-1,5-iminoxylitol to be potent inhibitors of glucocerebrosidase. Among them, isofagomine was the most potent inhibitor of glucocerebrosidase in vitro, and the most effective active-site-specific chaperone capable of increasing residual glucocerebrosidase activity in fibroblasts established from Gaucher patients with the most prevalent Gaucher disease-causing mutation (N370S). Intracellular enzyme activity increased approximately two-fold after cells had been incubated with isofagomine, and the increase in glucocerebrosidase activity was both dose-dependent and time-dependent. Western blotting demonstrated that there was a substantial increase in glucocerebrosidase protein in cells after isofagomine treatment. Immunocytochemistry revealed an improvement in the glucocerebrosidase trafficking pattern, which overlaps that of lysosome-associated membrane protein 2 in Gaucher fibroblasts cultivated with isofagomine, suggesting that the transport of mutant glucocerebrosidase is at least partially improved in the presence of isofagomine. The hydrophilic active-site-specific chaperones are less toxic to cultured cells. These results indicate that these hydrophilic small molecules are suitable candidates for further drug development for the treatment of Gaucher disease.

Suppressive effects of 4-phenylbutyrate on the aggregation of Pael receptors and endoplasmic reticulum stress

Endoplasmic reticulum (ER) stress is defined as an accumulation of unfolded proteins in the endoplasmic reticulum. 4-phenylbutyrate (4-PBA) has been demonstrated to promote the normal trafficking of the DeltaF508 cystic fibrosis transmembrane conductance regulator (CFTR) mutant from the ER to the plasma membrane and to restore activity. We have reported that 4-PBA protected against cerebral ischemic injury and ER stress-induced neuronal cell death. In this study, we revealed that 4-PBA possesses chemical chaperone activity in vitro, which prevents the aggregation of denatured alpha-lactalbumin and bovine serum albumin (BSA). Furthermore, we investigated the effects of 4-PBA on the accumulation of Parkin-associated endothelin receptor-like receptor (Pael-R) pathologically relevant to the loss of dopaminergic neurons in autosomal recessive juvenile parkinsonism (AR-JP). Interestingly, 4-PBA restored the normal expression of Pael-R protein and suppressed ER stress induced by the overexpression of Pael-R. In addition, we showed that 4-PBA attenuated the activation of ER stress-induced signal transduction pathways and subsequent neuronal cell death. Moreover, 4-PBA restored the viability of yeasts that fail to induce an ER stress response under ER stress conditions. These results suggest that 4-PBA suppresses ER stress by directly reducing the amount of misfolded protein, including Pael-R accumulated in the ER.

Carbonic anhydrase XIV identified as the membrane CA in mouse retina: strong expression in Müller cells and the RPE

The presence of carbonic anhydrase (CA) activity in the neural retina has been known for several decades. CA-II, a soluble cytoplasmic isoform expressed by Müller cells and a subset of amacrine cells, was thought to be the sole source of CA activity in the neural retina. However, CA-II deficient mice retain CA activity in the neural retina, which implies that another isoform must be present in that tissue. Recently CA-XIV, an integral membrane protein, was cloned and characterized. We, therefore, sought to determine whether CA-XIV is expressed in the neural retina, and hence is responsible for the CA activity observed in CA-II null animals. Immunohistochemical analyses of histological sections from CA-II null, CA-XIV null, and control mice were performed to localize the CA-XIV isoform, as well as other known retinal markers. Immunoblotting and real-time RT-PCR analyses were also performed to test for CA-XIV expression in retina and other mouse tissues. We determined herein that CA-XIV, a approximately 45kDa membrane protein, is expressed in retina, as it is in kidney. In the retina, CA-XIV is expressed on the plasma membrane of Müller cells. CA-XIV is also found on both the apical and basal membranes of the retinal pigmented epithelium. The data presented here indicate that like CA-II, CA-XIV is highly expressed in the neural retina and, like CA-II, more specifically by the Müller cells. The cellular compartmentalization of the two isoforms in the Müller cell-one cytoplasmic and the other on the plasma membrane-suggest that the two enzymes have specific and unique functions. Future studies will be necessary to assign functions to CA-II and CA-XIV in the mouse neural retina.

Characterization of CA XV, a new GPI-anchored form of carbonic anhydrase

The main function of CAs (carbonic anhydrases) is to participate in the regulation of acid-base balance. Although 12 active isoenzymes of this family had already been described, analyses of genomic databases suggested that there still exists another isoenzyme, CA XV. Sequence analyses were performed to identify those species that are likely to have an active form of this enzyme. Eight species had genomic sequences encoding CA XV, in which all the amino acid residues critical for CA activity are present. However, based on the sequence data, it was apparent that CA XV has become a non-processed pseudogene in humans and chimpanzees. RT-PCR (reverse transcriptase PCR) confirmed that humans do not express CA XV. In contrast, RT-PCR and in situ hybridization performed in mice showed positive expression in the kidney, brain and testis. A prediction of the mouse CA XV structure was performed. Phylogenetic analysis showed that mouse CA XV is related to CA IV. Therefore both of these enzymes were expressed in COS-7 cells and studied in parallel experiments. The results showed that CA XV shares several properties with CA IV, i.e. it is a glycosylated glycosylphosphatidylinositol-anchored membrane protein, and it binds CA inhibitor. The catalytic activity of CA XV is low, and the correct formation of disulphide bridges is important for the activity. Both specific and non-specific chaperones increase the production of active enzyme. The results suggest that CA XV is the first member of the alpha-CA gene family that is expressed in several species, but not in humans and chimpanzees.

Light in retinitis pigmentosa

Retinitis pigmentosa (RP) is one of the most genetically heterogeneous inherited disorders. Twelve genes have now been identified in the autosomal dominant form of the disease, including some recently characterized genes that show unprecedented and fascinating traits in both their function and in their expression profiles. These include many widely expressed genes encoding components of the spliceosome and a guanine nucleotide synthesis gene. Intriguingly, the most recently identified dominant gene does not appear to be expressed in the neuronal retina but is expressed in the capillaries of the choroid. In attempting to understand the effects of mutations in these genes, investigators are forced to re-evaluate their thinking on the molecular mechanisms of genetic blindness and to undertake an increasingly inter-disciplinary approach in their analysis of this disease. Recently, this has resulted in significant developments in the elucidation of the molecular pathogenesis of RP.