Alternative translation initiation generates a novel isoform of insulin-degrading enzyme targeted to mitochondria

Somatostatin: a novel substrate and a modulator of insulin-degrading enzyme activity

Insulin-degrading enzyme (IDE) is an interesting pharmacological target for Alzheimer's disease (AD), since it hydrolyzes beta-amyloid, producing non-neurotoxic fragments. It has also been shown that the somatostatin level reduction is a pathological feature of AD and that it regulates the neprilysin activity toward beta-amyloid. In this work, we report for the first time that IDE is able to hydrolyze somatostatin [k(cat) (s(-1))=0.38 (+/-0.05); K(m) (M)=7.5 (+/-0.9) x 10(-6)] at the Phe6-Phe7 amino acid bond. On the other hand, somatostatin modulates IDE activity, enhancing the enzymatic cleavage of a novel fluorogenic beta-amyloid through a decrease of the K(m) toward this substrate, which corresponds to the 10-25 amino acid sequence of the Abeta(1-40). Circular dichroism spectroscopy and surface plasmon resonance imaging experiments show that somatostatin binding to IDE brings about a concentration-dependent structural change of the secondary and tertiary structure(s) of the enzyme, revealing two possible binding sites. The higher affinity binding site disappears upon inactivation of IDE by ethylenediaminetetraacetic acid, which chelates the catalytic Zn(2+) ion. As a whole, these features suggest that the modulatory effect is due to an allosteric mechanism: somatostatin binding to the active site of one IDE subunit (where somatostatin is cleaved) induces an enhancement of IDE proteolytic activity toward fluorogenic beta-amyloid by another subunit. Therefore, this investigation on IDE-somatostatin interaction contributes to a more exhaustive knowledge about the functional and structural aspects of IDE and its pathophysiological implications in the amyloid deposition and somatostatin homeostasis in the brain.

Expression of metalloprotease insulin-degrading enzyme insulysin in normal and malignant human tissues

Insulin-degrading enzyme (IDE, insulysin, insulinase; EC 3.4.22.11), a thiol metalloendopeptidase, is involved in intracellular degradation of insulin, thereby inhibiting its translocation and accumulation to the nucleus. Recently, protein expression of IDE has been demonstrated in the epithelial ducts of normal breast and breast cancer tissue. Utilizing four different antibodies generated against different epitopes of the IDE molecule, we performed Western blot analysis and immunohistochemical staining on several normal human tissues, on a plethora of tumor cell lines of different tissue origin, and on malignant breast and ovarian tissue. Applying the four IDE-directed antibodies, we demonstrated IDE expression at the protein level, by means of immunoblotting and immunocytochemistry, in each of the tumor cell lines analyzed. Insulin-degrading enzyme protein expression was found in normal tissues of the kidney, liver, lung, brain, breast and skeletal muscle, as well as in breast and ovarian cancer tissues. Immunohistochemical visualization of IDE indicated cytoplasmic localization of IDE in each of the cell lines and tissues assessed. In conclusion, we performed for the first time a wide-ranging survey on IDE protein expression in normal and malignant tissues and cells thus extending our knowledge on the cellular and tissue distribution of IDE, an enzyme which to date has mainly been studied in connection with Alzheimer's disease and diabetes but not in cancer.

The AbetaCs of Abeta-cleaving proteases

The amyloid beta-protein (Abeta), which accumulates abnormally in Alzheimer disease (AD), is degraded by a diverse set of proteolytic enzymes. Abeta-cleaving proteases, largely ignored until only recently, are now known to play a pivotal role in the regulation of cerebral Abeta levels and amyloid plaque formation in animal models, and accumulating evidence suggests that defective Abeta proteolysis may be operative in many AD cases. This review summarizes the growing body of evidence supporting the involvement of specific Abeta-cleaving proteases in the etiology and potential treatment of AD. Recognition of the importance of Abeta degradation to the overall economy of Abeta has revised our thinking about the mechanistic basis of AD pathogenesis and identified a novel class of enzymes that may serve as both therapeutic targets and therapeutic agents.

Alternative translation start sites and hidden coding potential of eukaryotic mRNAs

It is widely suggested that a eukaryotic mRNA typically contains one translation start site and encodes a single functional protein product. However, according to current points of view on translation initiation mechanisms, eukaryotic ribosomes can recognize several alternative translation start sites and the number of experimentally verified examples of alternative translation is growing rapidly. Also, the frequent occurrence of alternative translation events and their functional significance are supported by the results of computational evaluations. The functional role of alternative translation and its contribution to eukaryotic proteome complexity are discussed.

Amyloid beta-degrading cryptidases: insulin degrading enzyme, presequence peptidase, and neprilysin

The accumulation of aggregates of amyloidogenic peptides is associated with numerous human diseases. One well studied example is the association between deposition of amyloid beta (Abeta) and Alzheimer's disease. Insulin degrading enzyme and neprilysin are involved in the clearance of Abeta, and presequence peptidase is suggested to play a role in the degradation of mitochondrial Abeta. Recent structural analyses reveal that these three peptidases contain a catalytic chamber (crypt) that selectively encapsulates and cleaves amyloidogenic peptides, hence the name cryptidase. The substrate selectivity of these cryptidases is determined by the size and charge distribution of their crypt as well as the conformational flexibility of substrates. The interaction of Abeta with the catalytic core of these cryptidases is controlled by conformational changes that make the catalytic chambers accessible for Abeta binding. These new structural and biochemical insights into cryptidases provide potential therapeutic strategies for the control of Abeta clearance.

Molecular basis for the thiol sensitivity of insulin-degrading enzyme

Insulin-degrading enzyme (IDE) is a ubiquitous zinc-metalloprotease that hydrolyzes several pathophysiologically relevant peptides, including insulin and the amyloid beta-protein (Abeta). IDE is inhibited irreversibly by compounds that covalently modify cysteine residues, a mechanism that could be operative in the etiology of type 2 diabetes mellitus (DM2) or Alzheimer's disease (AD). However, despite prior investigation, the molecular basis underlying the sensitivity of IDE to thiol-alkylating agents has not been elucidated. To address this topic, we conducted a comprehensive mutational analysis of the 13 cysteine residues within IDE. Our analysis implicates C178, C812, and C819 as the principal residues conferring thiol sensitivity. The involvement of C812 and C819, residues quite distant from the catalytic zinc atom, provides functional evidence that the active site of IDE comprises two separate domains that are operational only in close apposition. Structural analysis and other evidence predict that alkylation of C812 and C819 disrupts substrate binding, whereas alkylation of C178 interferes with the apposition of active-site domains and subtly repositions zinc-binding residues. Unexpectedly, alkylation of C590 was found to activate hydrolysis of Abeta significantly, while having no effect on insulin, demonstrating that chemical modulation of IDE can be both bidirectional and highly substrate selective. Our findings resolve a long-standing riddle about the basic enzymology of IDE with important implications for the etiology of DM2 and AD. Moreover, this work uncovers key details about the mechanistic basis of the unusual substrate selectivity of IDE that may aid the development of pharmacological agents or IDE mutants with therapeutic value.

Protein multifunctionality: principles and mechanisms

In the review, the nature of protein multifunctionality is analyzed. In the first part of the review the principles of structural/functional organization of protein are discussed. In the second part, the main mechanisms involved in development of multiple functions on a single gene product(s) are analyzed. The last part represents a number of examples showing that multifunctionality is a basic feature of biologically active proteins.

Insights into amyloid-beta-induced mitochondrial dysfunction in Alzheimer disease

Amyloid-beta has long been implicated in the pathogenesis of Alzheimer disease. The focus was initially on the extracellular fibrillar deposits of amyloid-beta but more recently has shifted to intracellular oligomeric forms of amyloid-beta. Unfortunately, the mechanism(s) by which either extracellular or intracellular amyloid-beta induces neuronal toxicity remains unclear. That said, a number of recent studies indicate that mitochondria might be an important target of amyloid-beta. Neurons rely heavily on mitochondria for energy and it is well established that mitochondrial dysfunction might be an important target of amyloid-beta. Mechanistically, amyloid-beta aggregates in mitochondria to impair function, leading to energy hypometabolism and elevated reactive oxygen species production. Additionally, amyloid-beta affects the balance of mitochondrial fission/fusion and mitochondrial transport, negatively impacting a host of cellular functions of neurons. Here, we review the role that amyloid-beta plays in mitochondrial structure and function of neurons and the importance of this in the pathogenesis of Alzheimer disease.

Cost minimization of ribosomal frameshifts

Properties of mRNA leading regions that modulate protein synthesis are little known (besides effects of their secondary structure). Here I explore how coding properties of leading regions may account for their disparate efficiencies. Trinucleotides that form off frame stop codons decrease costs of ribosomal slippages during protein synthesis: protein activity (as a proxy of gene expression, and as measured in experiments using artificial variants of 5' leading sequences of beta galactosidase in Escherichia coli) increases proportionally to the number of stop motifs in any frame in the 5' leading region. This suggests that stop codons in the 5' leading region, upstream of the recognized coding sequence, terminate eventual translations that sometimes start before ribosomes reach the mRNA's recognized start codon, increasing efficiency. This hypothesis is confirmed by further analyses: mRNAs with 5' leading regions containing in the same frame a start preceding a stop codon (in any frame) produce less enzymatic activity than those with the stop preceding the start. Hence coding properties, in addition to other properties, such as the secondary structure of the 5' leading region, regulate translation. This experimentally (a) confirms that within coding regions, off frame stops increase protein synthesis efficiency by early stopping frameshifted translation; (b) suggests that this occurs for all frames also in 5' leading regions and that (c) several alternative start codons that function at different probabilities should routinely be considered for all genes in the region of the recognized initiation codon. An unknown number of short peptides might be translated from coding and non-coding regions of RNAs.

Regulation of Insulin Secretion by SIRT4, a Mitochondrial ADP-ribosyltransferase

Sirtuins are homologues of the yeast transcriptional repressor Sir2p and are conserved from bacteria to humans. We report that human SIRT4 is localized to the mitochondria. SIRT4 is a matrix protein and becomes cleaved at amino acid 28 after import into mitochondria. Mass spectrometry analysis of proteins that coimmunoprecipitate with SIRT4 identified insulindegrading enzyme and the ADP/ATP carrier proteins, ANT2 and ANT3. SIRT4 exhibits no histone deacetylase activity but functions as an efficient ADP-ribosyltransferase on histones and bovine serum albumin. SIRT4 is expressed in islets of Langerhans and colocalizes with insulin-expressing beta cells. Depletion of SIRT4 from insulin-producing INS-1E cells results in increased insulin secretion in response to glucose. These observations define a new role for mitochondrial SIRT4 in the regulation of insulin secretion.

Amyloid precursor protein and mitochondrial dysfunction in Alzheimer's disease

Growing evidence suggests that mitochondrial dysfunction is one of the key intracellular lesions associated with the pathogenesis of Alzheimer's disease (AD). Mitochondria, the powerhouses of the cell, participate in a number of physiological functions that include calcium homeostasis, signal transduction, and apoptosis. However, the pathophysiological mechanisms underlying the decline of mitochondrial vital functions leading to the dysfunction of mitochondria during AD are not well understood. Recent literature has observed the accumulation of Alzheimer's amyloid precursor protein (APP) and its C-terminal-cleaved product beta-amyloid (Abeta) in the mitochondrial compartment. Furthermore, evidence also implicates that the accumulation of full-length APP and Abeta in the mitochondrial compartment has a causative role in impairing mitochondrial physiological functions. Here, we review the mode of mitochondrial transport of full-length APP and Abeta and its pathological implications in bringing about mitochondrial dysfunction as seen in AD.

Mitochondrial cascade hypothesis of Alzheimer's disease: myth or reality?

Mitochondria are recognized to play a pivotal role in neuronal cell survival or death because they are regulators of both energy metabolism and apoptotic pathways. Morphologic, biochemical, and molecular genetic studies suggest that mitochondria might be a convergence point for neurodegeneration, including Alzheimer's disease (AD). The functions and properties of mitochondria might render subsets of selectively vulnerable neurons intrinsically susceptible to cellular aging and stress. However, the question, "Is mitochondrial dysfunction a necessary step in neurodegeneration?" is still unanswered. This review presents the ways in which malfunctioning mitochondria and oxidative stress might contribute to neuronal death in AD.

Effects of Alzheimer's amyloid-beta and tau protein on mitochondrial function -- role of glucose metabolism and insulin signalling

Alzheimer's disease (AD) is the most frequent form of dementia among the elderly and is characterized by neuropathological hallmarks of extracellular amyloid-beta (Abeta) plaques and intracellular neurofibrillary tangles composed of abnormally hyperphosphorylated microtubular protein tau in the brains of AD patients. Of note, current data illustrate a complex interplay between the amyloid and tau pathology during the course of the disease. We hypothesize a direct impact of abnormally phosphorylated tau and Abeta on proteins/enzymes involved in metabolism, respiratory chain function and cellular detoxification. Probably at the level of mitochondria, both Alzheimer proteins exhibit synergistic effects finally leading to/accelerating neurodegenerative mechanisms. Moreover, accumulating evidence that mitochondria failure, reduced glucose utilization and deficient energy metabolism occur already very early in the course of the disease suggests a role of impaired insulin signalling in the pathogenesis of AD. Thus, this review addresses also the question if mitochondrial dysfunction may represent a link between diabetes and AD.

Histochemical evidence for wide expression of the metalloendopeptidase nardilysin in human brain neurons

Nardilysin is a metalloendopeptidase that in vitro cleaves peptides such as dynorphin-A, somatostatin-28, alpha-neoendorphin and glucagon at the N-terminus of arginine and lysine residues in dibasic moieties. The enzyme is highly expressed in many endocrine tissues. Nardilysin has also been found in the brain. Previously, we have detected that nardilysin interacts with brain-specific proteins, i.e. p42(IP4)/centaurin-alpha1 [Stricker R, Chow KM, Walther D, Hanck T, Hersh LB, Reiser G (2006) Interaction of the brain specific protein p42(IP4)/centaurin-alpha1 with the peptidase nardilysin is regulated by the cognate ligands of p42(IP4), PtdIns(3,4,5)P(3) and Ins(1,3,4,5)P(4), with stereospecificity. J Neurochem 98:343-354]. However, very little is known about the distribution of nardilysin in the brain. The aim of the present study was to reveal its regional distribution and cellular localization in developing and adult human brain. Using immunohistochemistry and Western blot analysis we demonstrate that the enzyme is widely, but unevenly, expressed in the human brain. We found high staining intensity in the hypothalamus, neocortex and brain stem nuclei. The cellular localization is almost exclusively confined to neurons. In pre- and perinatal human brain cortex, most neurons express the enzyme. In cortical neurons nardilysin protein was found to be partially co-localized with parvalbumin but not calretinin. No co-expression was seen with somatostatin-28 immunoreactivity. A considerable overlap was revealed between p42(IP4) and nardilysin. Our data support the hypothesis that nardilysin might possibly play a role in brain development, whereas its putative function in brain peptide metabolism remains to be clarified further.

Decreased Catalytic Activity of the Insulin-degrading Enzyme in Chromosome 10-Linked Alzheimer Disease Families

Insulin-degrading enzyme (IDE) is a zinc metalloprotease that degrades the amyloid beta-peptide, the key component of Alzheimer disease (AD)-associated senile plaques. We have previously reported evidence for genetic linkage and association of AD on chromosome 10q23-24 in the region harboring the IDE gene. Here we have presented the first functional assessment of IDE in AD families showing the strongest evidence of the genetic linkage. We have examined the catalytic activity and expression of IDE in lymphoblast samples from 12 affected and unaffected members of three chromosome 10-linked AD pedigrees in the National Institute of Mental Health AD Genetics Initiative family sample. We have shown that the catalytic activity of cytosolic IDE to degrade insulin is reduced in affected versus unaffected subjects of these families. Further, we have shown the decrease in activity is not due to reduced IDE expression, suggesting the possible defects in IDE function in these AD families. In attempts to find potential mutations in the IDE gene in these families, we have found no coding region substitutions or alterations in splicing of the canonical exons and exon 15b of IDE. We have also found that total IDE mRNA levels are not significantly different in sporadic AD versus age-matched control brains. Collectively, our data suggest that the genetic linkage of AD in this set of chromosome 10-linked AD families may be the result of systemic defects in IDE activity in the absence of altered IDE expression, further supporting a role for IDE in AD pathogenesis.

Brain mitochondrial dysfunction as a link between Alzheimer's disease and diabetes

It has been argued that in late-onset Alzheimer's disease a disturbance in the control of neuronal glucose metabolism consequent to impaired insulin signalling strongly resembles the pathophysiology of type 2 diabetes in non-neural tissue. The fact that mitochondria are the major generators and direct targets of reactive oxygen species led several investigators to foster the idea that oxidative stress and damage in mitochondria are contributory factors to several disorders including Alzheimer's disease and diabetes. Since brain possesses high energetic requirements, any decline in brain mitochondria electron chain could have a severe impact on brain function and particularly on the etiology of neurodegenerative diseases. This review is primarily focused in the discussion of brain mitochondrial dysfunction as a link between diabetes and Alzheimer's disease.

Eclipsed distribution: A phenomenon of dual targeting of protein and its significance

One of the surprises from genome sequencing projects is the apparently small number of predicted genes in different eukaryotic cells, particularly human. One possible reason for this 'shortage' of genes is multiple distribution of proteins; a single protein is targeted to more than one subcellular compartment and consequently participates in different biochemical pathways and might have completely different functions. Indeed, in recent years, there have been reports on proteins that were found to be localized in cellular compartments other than those initially attributed to them. Furthermore, the phenomenon of highly uneven isoprotein distribution was recently observed and termed 'eclipsed distribution'. In these cases, the amount of one of the isoproteins, in one of the locations, is significantly minute and its detection by standard biochemical and visualization methods is masked by the presence of the dominant isoprotein. In fact, the minute amounts of eclipsed proteins can be essential. Since detecting eclipsed distribution is difficult, we assume that this phenomenon is probably more common than currently recorded. Hence, developing methods for localization and functional detection of eclipsed proteins is a challenge in cell biology research. Finally, eclipsed distribution may lead to cellular pathologies as has been suggested to occur in human disorders such as Prion diseases and Alzheimer. This review provides a short description of the eclipsed distribution phenomenon followed by an overview of protein distribution mechanisms, examples of eclipsed distribution and experimental approaches for revealing these elusive proteins.

Insulin degrading enzyme is localized predominantly at the cell surface of polarized and unpolarized human cerebrovascular endothelial cell cultures

Insulin degrading enzyme (IDE) is expressed in the brain and may play an important role there in the degradation of the amyloid beta peptide (Abeta). Our results show that cultured human cerebrovascular endothelial cells (HCECs), a primary component of the blood-brain barrier, express IDE and may respond to exposure to low levels of Abeta by upregulating its expression. When radiolabeled Abeta is introduced to the medium of cultured HCECs, it is rapidly degraded to smaller fragments. We believe that this degradation is largely the result of the action of IDE, as it can be substantially blocked by the presence of insulin in the medium, a competitive substrate of IDE. No inhibition is seen when an inhibitor of neprilysin, another protease that may degrade Abeta, is present in the medium. Our evidence suggests that the action of IDE occurs outside the cell, as inhibitors of internalization fail to affect the rate of the observed degradation. Further, our evidence suggests that degradation by IDE occurs on the plasma membrane, as much of the IDE present in HCECs was biotin-labeled by a plasma membrane impermeable reagent. This activity seems to be polarity dependent, as measurement of Abeta degradation by each surface of differentiated HCECs shows greater degradation on the basolateral (brain-facing) surface. Thus, IDE could be an important therapeutic target to decrease the amount of Abeta in the cerebrovasculature.

Degradation of the amyloid beta-protein by the novel mitochondrial peptidasome, PreP

Recently we have identified the novel mitochondrial peptidase responsible for degrading presequences and other short unstructured peptides in mitochondria, the presequence peptidase, which we named PreP peptidasome. In the present study we have identified and characterized the human PreP homologue, hPreP, in brain mitochondria, and we show its capacity to degrade the amyloid beta-protein (Abeta). PreP belongs to the pitrilysin oligopeptidase family M16C containing an inverted zinc-binding motif. We show that hPreP is localized to the mitochondrial matrix. In situ immuno-inactivation studies in human brain mitochondria using anti-hPreP antibodies showed complete inhibition of proteolytic activity against Abeta. We have cloned, overexpressed, and purified recombinant hPreP and its mutant with catalytic base Glu(78) in the inverted zinc-binding motif replaced by Gln. In vitro studies using recombinant hPreP and liquid chromatography nanospray tandem mass spectrometry revealed novel cleavage specificities against Abeta-(1-42), Abeta-(1-40), and Abeta Arctic, a protein that causes increased protofibril formation an early onset familial variant of Alzheimer disease. In contrast to insulin degrading enzyme, which is a functional analogue of hPreP, hPreP does not degrade insulin but does degrade insulin B-chain. Molecular modeling of hPreP based on the crystal structure at 2.1 A resolution of AtPreP allowed us to identify Cys(90) and Cys(527) that form disulfide bridges under oxidized conditions and might be involved in redox regulation of the enzyme. Degradation of the mitochondrial Abeta by hPreP may potentially be of importance in the pathology of Alzheimer disease.

A mitochondrial paradigm of metabolic and degenerative diseases, aging, and cancer: a dawn for evolutionary medicine

Life is the interplay between structure and energy, yet the role of energy deficiency in human disease has been poorly explored by modern medicine. Since the mitochondria use oxidative phosphorylation (OXPHOS) to convert dietary calories into usable energy, generating reactive oxygen species (ROS) as a toxic by-product, I hypothesize that mitochondrial dysfunction plays a central role in a wide range of age-related disorders and various forms of cancer. Because mitochondrial DNA (mtDNA) is present in thousands of copies per cell and encodes essential genes for energy production, I propose that the delayed-onset and progressive course of the age-related diseases results from the accumulation of somatic mutations in the mtDNAs of post-mitotic tissues. The tissue-specific manifestations of these diseases may result from the varying energetic roles and needs of the different tissues. The variation in the individual and regional predisposition to degenerative diseases and cancer may result from the interaction of modern dietary caloric intake and ancient mitochondrial genetic polymorphisms. Therefore the mitochondria provide a direct link between our environment and our genes and the mtDNA variants that permitted our forbears to energetically adapt to their ancestral homes are influencing our health today.

Gradual alteration of mitochondrial structure and function by beta-amyloids: importance of membrane viscosity changes, energy deprivation, reactive oxygen species production, and cytochrome c release

Intracellular amyloid beta-peptide (A beta) accumulation is considered to be a key pathogenic factor in sporadic Alzheimer's disease (AD), but the mechanisms by which it triggers neuronal dysfunction remain unclear. We hypothesized that gradual mitochondrial dysfunction could play a central role in both initiation and progression of sporadic AD. Thus, we analyzed changes in mitochondrial structure and function following direct exposure to increasing concentrations of A beta(1--42) and A beta(25--35) in order to look more closely at the relationships between mitochondrial membrane viscosity, ATP synthesis, ROS production, and cytochrome c release. Our results show the accumulation of monomeric A beta within rat brain and muscle mitochondria. Subsequently, we observed four different and additive modes of action of A beta, which were concentration dependent: (i) an increase in mitochondrial membrane viscosity with a concomitant decrease in ATP/O, (ii) respiratory chain complexes inhibition, (iii) a potentialization of ROS production, and (iv) cytochrome c release.

Amyloid beta-protein is degraded by cellular angiotensin-converting enzyme (ACE) and elevated by an ACE inhibitor

Human genetic data have associated angiotensin-converting enzyme (ACE) with Alzheimer disease (AD), and purified ACE has been reported to cleave synthetic amyloid beta-protein (Abeta) in vitro. Whether deficiency in ACE activity, arising from genetic alteration or pharmacological inhibition, can decrease Abeta degradation and allow Abeta accumulation in intact cells is unknown. We cloned ACE from human neuroblastoma cells and showed that it had posttranslational processing and enzymatic activity typical of the endogenous protease. Cellular expression of ACE promoted degradation of naturally secreted Abeta40 and Abeta42, leading to significant clearance of both species. Using site-directed mutagenesis, we determined that both active sites within ACE contribute to Abeta clearance, and an ACE construct bearing mutations in each catalytic domain had no effect on Abeta levels. Pharmacological inhibition of ACE with a widely prescribed drug, captopril, promoted the accumulation of cell-derived Abeta in the media of beta-amyloid precursor-protein expressing cells. Together, these results show that ACE can lower the levels of secreted Abeta in living cells and that this effect is blocked by inhibiting the protease's activity with an ACE inhibitor. This work, combined with the genetic studies, supports the hypothesis that ACE may modulate the susceptibility to and progression of AD via degradation of Abeta. Our data encourage further analyses of the ACE gene for disease association and raise the question of whether currently prescribed ACE inhibitors could elevate cerebral Abeta levels in humans.

Regulation of translation via mRNA structure in prokaryotes and eukaryotes

The mechanism of initiation of translation differs between prokaryotes and eukaryotes, and the strategies used for regulation differ accordingly. Translation in prokaryotes is usually regulated by blocking access to the initiation site. This is accomplished via base-paired structures (within the mRNA itself, or between the mRNA and a small trans-acting RNA) or via mRNA-binding proteins. Classic examples of each mechanism are described. The polycistronic structure of mRNAs is an important aspect of translational control in prokaryotes, but polycistronic mRNAs are not usable (and usually not produced) in eukaryotes. Four structural elements in eukaryotic mRNAs are important for regulating translation: (i) the m7G cap; (ii) sequences flanking the AUG start codon; (iii) the position of the AUG codon relative to the 5' end of the mRNA; and (iv) secondary structure within the mRNA leader sequence. The scanning model provides a framework for understanding these effects. The scanning mechanism also explains how small open reading frames near the 5' end of the mRNA can down-regulate translation. This constraint is sometimes abrogated by changing the structure of the mRNA, sometimes with clinical consequences. Examples are described. Some mistaken ideas about regulation of translation that have found their way into textbooks are pointed out and corrected.

Alternative splicing of human insulin-degrading enzyme yields a novel isoform with a decreased ability to degrade insulin and amyloid beta-protein

Deletion of insulin-degrading enzyme (IDE) in mice causes accumulation of cerebral amyloid beta-protein (Abeta), hyperinsulinemia, and glucose intolerance. Together with genetic linkage and allelic association of IDE to Alzheimer's disease (AD) and type 2 diabetes mellitus (DM2), these findings suggest that IDE hypofunction could mediate human disease. To date, no coding mutations have been found in the canonical isoform of IDE, suggesting that pathological mutations could exist in undiscovered exons or regulatory regions, including untranslated regions (UTRs). However, neither isoforms arising from alternative splicing nor the UTRs have been described. Here, we systematically characterize human IDE mRNAs, identify a novel splice form, and compare its subcellular distribution, kinetic properties, and ability to degrade Abeta to the known isoform. Six distinct human IDE transcripts were identified, with most of the variance attributable to alternative polyadenylation sites. In the novel spliceoform, an exon we designate "15b" replaces the canonical exon "15a", and the resultant variant is widely expressed. Subcellular fractionation, immunofluorescent confocal microscopy, and immunogold-electron microscopy reveal that the 15b-IDE protein occurs in both cytosol and mitochondria. Organelle targeting of both isoforms is determined by which of two translation start sites is used, and only those isoforms utilizing the second site regulate levels of secreted Abeta. 15b-IDE can exist as a heterodimer with the 15a isoform or as a homodimer. The apparent K(m) values of recombinant 15b-IDE for both insulin and Abeta are significantly higher and the k(cat) and catalytic efficiency markedly lower than those of 15a-IDE. In accord, cells coexpressing beta-amyloid precursor protein (APP) and 15b-IDE accumulated significantly more Abeta in their media than those expressing APP and 15a-IDE. Our results identify a novel, catalytically inefficient form of IDE expressed in brain and non-neural tissues and recommend novel regions of the IDE gene in which to search for mutations predisposing patients to AD and DM2.

Mitochondria take center stage in aging and neurodegeneration

A critical role of mitochondrial dysfunction and oxidative damage has been hypothesized in both aging and neurodegenerative diseases. Much of the evidence has been correlative, but recent evidence has shown that the accumulation of mitochondrial DNA mutations accelerates normal aging, leads to oxidative damage to nuclear DNA, and impairs gene transcription. Furthermore, overexpression of the antioxidant enzyme catalase in mitochondria increases murine life span. There is strong evidence from genetics and transgenic mouse models that mitochondrial dysfunction results in neurodegeneration and may contribute to the pathogenesis of Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, hereditary spastic paraplegia, and cerebellar degenerations. Therapeutic approaches targeting mitochondrial dysfunction and oxidative damage in these diseases therefore have great promise.