A human brain L-3-hydroxyacyl-coenzyme A dehydrogenase is identical to an amyloid beta-peptide-binding protein involved in Alzheimer's disease

Involvement of Type 10 17β-Hydroxysteroid Dehydrogenase in the Pathogenesis of Infantile Neurodegeneration and Alzheimer's Disease

Type 10 17β-hydroxysteroid dehydrogenase (17β-HSD10) is the HSD17B10 gene product playing an appreciable role in cognitive functions. It is the main hub of exercise-upregulated mitochondrial proteins and is involved in a variety of metabolic pathways including neurosteroid metabolism to regulate allopregnanolone homeostasis. Deacetylation of 17β-HSD10 by sirtuins helps regulate its catalytic activities. 17β-HSD10 may also play a critical role in the control of mitochondrial structure, morphology and dynamics by acting as a member of the Parkin/PINK1 pathway, and by binding to cyclophilin D to open mitochondrial permeability pore. 17β-HSD10 also serves as a component of RNase P necessary for mitochondrial tRNA maturation. This dehydrogenase can bind with the Aβ peptide thereby enhancing neurotoxicity to brain cells. Even in the absence of Aβ, its quantitative and qualitative variations can result in neurodegeneration. Since elevated levels of 17β-HSD10 were found in brain cells of Alzheimer's disease (AD) patients and mouse AD models, it is considered to be a key factor in AD pathogenesis. Since data underlying Aβ-binding-alcohol dehydrogenase (ABAD) were not secured from reported experiments, ABAD appears to be a fabricated alternative term for the HSD17B10 gene product. Results of this study would encourage researchers to solve the question why elevated levels of 17β-HSD10 are present in brains of AD patients and mouse AD models. Searching specific inhibitors of 17β-HSD10 may find candidates to reduce senile neurodegeneration and open new approaches for the treatment of AD.

Infantile Neurodegeneration Results from Mutants of 17β-Hydroxysteroid Dehydrogenase Type 10 Rather Than Aβ-Binding Alcohol Dehydrogenase

Type 10 17β-hydroxysteroid dehydrogenase (17β-HSD10), a homo-tetrameric multifunctional protein with 1044 residues encoded by the HSD17B10 gene, is necessary for brain cognitive function. Missense mutations result in infantile neurodegeneration, an inborn error in isoleucine metabolism. A 5-methylcytosine hotspot underlying a 388-T transition leads to the HSD10 (p.R130C) mutant to be responsible for approximately half of all cases suffering with this mitochondrial disease. Fewer females suffer with this disease due to X-inactivation. The binding capability of this dehydrogenase to Aβ-peptide may play a role in Alzheimer's disease, but it appears unrelated to infantile neurodegeneration. Research on this enzyme was complicated by reports of a purported Aβ-peptide-binding alcohol dehydrogenase (ABAD), formerly referred to as endoplasmic-reticulum-associated Aβ-binding protein (ERAB). Reports concerning both ABAD and ERAB in the literature reflect features inconsistent with the known functions of 17β-HSD10. It is clarified here that ERAB is reportedly a longer subunit of 17β-HSD10 (262 residues). 17β-HSD10 exhibits L-3-hydroxyacyl-CoA dehydrogenase activity and is thus also referred to in the literature as short-chain 3-hydorxyacyl-CoA dehydrogenase or type II 3-hydorxyacyl-CoA dehydrogenase. However, 17β-HSD10 is not involved in ketone body metabolism, as reported in the literature for ABAD. Reports in the literature referring to ABAD (i.e., 17β-HSD10) as a generalized alcohol dehydrogenase, relying on data underlying ABAD's activities, were found to be unreproducible. Furthermore, the rediscovery of ABAD/ERAB's mitochondrial localization did not cite any published research on 17β-HSD10. Clarification of the purported ABAD/ERAB function derived from these reports on ABAD/ERAB may invigorate this research field and encourage new approaches to the understanding and treatment of HSD17B10-gene-related disorders. We establish here that infantile neurodegeneration is caused by mutants of 17β-HSD10 but not ABAD, and so we conclude that ABAD represents a misnomer employed in high-impact journals.

3-Hydroxyacyl-CoA and Alcohol Dehydrogenase Activities of Mitochondrial Type 10 17β-Hydroxysteroid Dehydrogenase in Neurodegeneration Study

Background:

Mitochondrial 17β-hydroxysteroid dehydrogenase type 10 (17β-HSD10) is necessary for brain cognitive function, but its studies were confounded by reports of Aβ-peptide binding alcohol dehydrogenase (ABAD), formerly endoplasmic reticulum-associated Aβ-peptide binding protein (ERAB), for two decades so long as ABAD serves as the alternative term of 17β-HSD10.

Objective:

To determine whether those ABAD reports are true or false, even if they were published in prestigious journals.

Methods:

6xHis-tagged 17β-HSD10 was prepared and characterized by well-established experimental procedures.

Results:

The N-terminal 6xHis tag did not significantly interfere with the dehydrogenase activities of 17β-HSD10, but the kinetic constants of its 3-hydroxyacyl-CoA dehydrogenase activity are drastically distinct from those of ABAD, and it was not involved in ketone body metabolism as previously reported for ABAD. Furthermore, it was impossible to measure its generalized alcohol dehydrogenase activities underlying the concept of ABAD because the experimental procedures described in ABAD reports violated basic chemical and/or biochemical principles. More incredibly, both authors and journals had not yet agreed to make any corrigenda of ABAD reports.

Conclusion:

Brain 17β-HSD10 plays a key role in neurosteroid metabolism and further studies in this area may lead to potential treatments of neurodegeneration including AD.

ost in promiscuity? An evolutionary and biochemical evaluation of HSD10 function in cardiolipin metabolism

Multifunctional proteins are challenging as it can be difficult to confirm pathomechanisms associated with disease-causing genetic variants. The human 17β-hydroxysteroid dehydrogenase 10 (HSD10) is a moonlighting enzyme with at least two structurally and catalytically unrelated functions. HSD10 disease was originally described as a disorder of isoleucine metabolism, but the clinical manifestations were subsequently shown to be linked to impaired mtDNA transcript processing due to deficient function of HSD10 in the mtRNase P complex. A surprisingly large number of other, mostly enzymatic and potentially clinically relevant functions have been attributed to HSD10. Recently, HSD10 was reported to exhibit phospholipase C-like activity towards cardiolipins (CL), important mitochondrial phospholipids. To assess the physiological role of the proposed CL-cleaving function, we studied CL architectures in living cells and patient fibroblasts in different genetic backgrounds and lipid environments using our well-established LC-MS/MS cardiolipidomic pipeline. These experiments revealed no measurable effect on CLs, indicating that HSD10 does not have a physiologically relevant function towards CL metabolism. Evolutionary constraints could explain the broad range of reported substrates for HSD10 in vitro. The combination of an essential structural with a non-essential enzymatic function in the same protein could direct the evolutionary trajectory towards improvement of the former, thereby increasing the flexibility of the binding pocket, which is consistent with the results presented here.

The enzyme activity of mitochondrial trifunctional protein is not altered by lysine acetylation or lysine succinylation

Mitochondrial trifunctional protein (TFP) is a membrane-associated heterotetramer that catalyzes three of the four reactions needed to chain-shorten long-chain fatty acids inside the mitochondria. TFP is known to be heavily modified by acetyllysine and succinyllysine post-translational modifications (PTMs), many of which are targeted for reversal by the mitochondrial sirtuin deacylases SIRT3 and SIRT5. However, the functional significance of these PTMs is not clear, with some reports showing TFP gain-of-function and some showing loss-of-function upon increased acylation. Here, we mapped the known SIRT3/SIRT5-targeted lysine residues onto the recently solved TFP crystal structure which revealed that many of the target sites are involved in substrate channeling within the TFPα subunit. To test the effects of acylation on substate channeling through TFPα, we enzymatically synthesized the physiological long-chain substrate (2E)-hexadecenoyl-CoA. Assaying TFP in SIRT3 and SIRT5 knockout mouse liver and heart mitochondria with (2E)-hexadecenoyl-CoA revealed no change in enzyme activity. Finally, we investigated the effects of lysine acylation on TFP membrane binding in vitro. Acylation did not alter recombinant TFP binding to cardiolipin-containing liposomes. However, the presence of liposomes strongly abrogated the acylation reaction between succinyl-CoA and TFP lysine residues. Thus, TFP in the membrane-bound state may be protected against lysine acylation.

(S)-3-Hydroxybutyryl-CoA Dehydrogenase From the Autotrophic 3-Hydroxypropionate/4-Hydroxybutyrate Cycle in Nitrosopumilus maritimus

Ammonia-oxidizing archaea of the phylum Thaumarchaeota are among the most abundant organisms that exert primary control of oceanic and soil nitrification and are responsible for a large part of dark ocean primary production. They assimilate inorganic carbon via an energetically efficient version of the 3-hydroxypropionate/4-hydroxybutyrate cycle. In this cycle, acetyl-CoA is carboxylated to succinyl-CoA, which is then converted to two acetyl-CoA molecules with 4-hydroxybutyrate as the key intermediate. This conversion includes the (S)-3-hydroxybutyryl-CoA dehydrogenase reaction. Here, we heterologously produced the protein Nmar_1028 catalyzing this reaction in thaumarchaeon Nitrosopumilus maritimus, characterized it biochemically and performed its phylogenetic analysis. This NAD-dependent dehydrogenase is highly active with its substrate, (S)-3-hydroxybutyryl-CoA, and its low K_m value suggests that the protein is adapted to the functioning in the 3-hydroxypropionate/4-hydroxybutyrate cycle. Nmar_1028 is homologous to the dehydrogenase domain of crotonyl-CoA hydratase/(S)-3-hydroxybutyryl-CoA dehydrogenase that is present in many Archaea. Apparently, the loss of the dehydratase domain of the fusion protein in the course of evolution was accompanied by lateral gene transfer of 3-hydroxypropionyl-CoA dehydratase/crotonyl-CoA hydratase from Bacteria. Although (S)-3-hydroxybutyryl-CoA dehydrogenase studied here is neither unique nor characteristic for the HP/HB cycle, Nmar_1028 appears to be the only (S)-3-hydroxybutyryl-CoA dehydrogenase in N. maritimus and is thus essential for the functioning of the 3-hydroxypropionate/4-hydroxybutyrate cycle and for the biology of this important marine archaeon.

Surface plasmon resonance biosensors for detection of Alzheimer's biomarkers; an effective step in early and accurate diagnosis

The rapid and direct detection of biomarkers in biofluids at clinically relevant concentrations faces serious limitations to develop diagnostic criteria for neurodegenerative diseases such as Alzheimer's disease (AD). In this regard, the early detection of biomarkers correlated with AD using novel modalities and instruments is at the center of attention. Recently, some newly invented optical-based biosensors namely Surface Plasmon Resonance (SPR) has been extensively investigated for the detection of biomarkers using a label-free method or by checking interaction between ligand and analyte. These approaches can sense a very small amount of target molecules in the blood and cerebrospinal fluids samples. In this review, the different hypothesis related to AD, and the structural properties of AD biomarkers was introduced. Also, we aim to highlight the specific role of available SPR-based sensing methods for early detection of AD biomarkers such as aggregated β-amyloid and tau proteins. Efforts to better understand the accuracy and efficiency of optical-based biosensors in the field of neurodegenerative disease enable us to accelerate the advent of novel modalities in the clinical setting for therapeutic and diagnostic purposes.

Friend or enemy? Review of 17β-HSD10 and its role in human health or disease

17β-hydroxysteroid dehydrogenase (17β-HSD10) is a multifunctional human enzyme with important roles both as a structural component and also as a catalyst of many metabolic pathways. This mitochondrial enzyme has important functions in the metabolism, development and aging of the neural system, where it is involved in the homeostasis of neurosteroids, especially in regard to estradiol, changes in which make it an essential part of neurodegenerative pathology. These roles therefore, indicate that 17β-HSD10 may be a possible druggable target for neurodegenerative diseases including Alzheimer's disease (AD), and in hormone-dependent cancer. The objective of this review was to provide a summary about physiological functions and pathological roles of 17β-HSD10 and the modulators of its activity.

Mitochondria as intracellular signaling platforms in health and disease

Mitochondria, long viewed solely in the context of bioenergetics, are increasingly emerging as critical hubs for intracellular signaling. Due to their bacterial origin, mitochondria possess their own genome and carry unique lipid components that endow these organelles with specialized properties to help orchestrate multiple signaling cascades. Mitochondrial signaling modulates diverse pathways ranging from metabolism to redox homeostasis to cell fate determination. Here, we review recent progress in our understanding of how mitochondria serve as intracellular signaling platforms with a particular emphasis on lipid-mediated signaling, innate immune activation, and retrograde signaling. We further discuss how these signaling properties might potentially be exploited to develop new therapeutic strategies for a range of age-related conditions.

Benzothiazolyl Ureas are Low Micromolar and Uncompetitive Inhibitors of 17β-HSD10 with Implications to Alzheimer's Disease Treatment

Human 17β-hydroxysteroid dehydrogenase type 10 is a multifunctional protein involved in many enzymatic and structural processes within mitochondria. This enzyme was suggested to be involved in several neurological diseases, e.g., mental retardation, Parkinson's disease, or Alzheimer's disease, in which it was shown to interact with the amyloid-beta peptide. We prepared approximately 60 new compounds based on a benzothiazolyl scaffold and evaluated their inhibitory ability and mechanism of action. The most potent inhibitors contained 3-chloro and 4-hydroxy substitution on the phenyl ring moiety, a small substituent at position 6 on the benzothiazole moiety, and the two moieties were connected via a urea linker (4at, 4bb, and 4bg). These compounds exhibited IC₅₀ values of 1-2 μM and showed an uncompetitive mechanism of action with respect to the substrate, acetoacetyl-CoA. These uncompetitive benzothiazolyl inhibitors of 17β-hydroxysteroid dehydrogenase type 10 are promising compounds for potential drugs for neurodegenerative diseases that warrant further research and development.

17β-Hydroxysteroid dehydrogenases and neurosteroid metabolism in the central nervous system

17β-Hydroxysteroid dehydrogenases are indispensable for downstream enzyme steps of the neurosteroidogenesis. Neurosteroids are synthesized de novo in neurons and glia from cholesterol transported into mitochondria, or by conversion from proneurosteroids, e. g. dehydroepiandrosterone (DHEA) and pregnenolone, through the same metabolic pathway as revealed in the de novo neurosteroidogenesis. Hormonal steroids generated from endocrine glands are transported into brain from the circulation to exert neuronal activity via genomic pathway, whereas neurosteroids produced in brain cells without genomic targets identified could bind to cell surface targets, e.g., GABA_A or NMDA receptors and elicit antidepressant, anxiolytic, anticonvulsant and anesthetic effects by regulating neuroexcitability. In a broad sense, neurosteroids include hormonal steroids in brain, and they are irrespective of origin playing important roles in brain development including neuroprotection, neurogenesis and neural plasticity. They are also a critical element in cognitive and memory functions. Mitochondrial 17β-HSD10, encoded by the HSD17B10 gene mapping to Xp11.2, is found in various brain regions, essential for the maintenance of neurosteroid homeostasis. Mutations identified in this gene resulted in two distinct brain diseases, namely HSD10 deficiency and MRXS10, of which clinical presentations and pathogenetic mechanisms are quite different. Since elevated levels of 17β-HSD10 was found in brains of Alzheimer's disease patients and AD mouse model, it may also act as an adverse factor in the AD pathogenesis due to an imbalance of neurosteroid metabolism.

17B-hydroxysteroid dehydrogenases as acyl thioester metabolizing enzymes

17β-Hydroxysteroid dehydrogenases (HSD17B) catalyze the oxidation/reduction of 17β-hydroxy/keto group in position C17 in C18- and C19 steroids. Most HSD17Bs are also catalytically active with substrates other than steroids. A subset of these enzymes is able to process thioesters of carboxylic acids. This group of enzymes includes HSD17B4, HSD17B8, HSD17B10 and HSD17B12, which execute reactions in intermediary metabolism, participating in peroxisomal β-oxidation of fatty acids, mitochondrial oxidation of 3R-hydroxyacyl-groups, breakdown of isoleucine and fatty acid chain elongation in endoplasmic reticulum. Divergent substrate acceptance capabilities exemplify acquirement of catalytic site adaptiveness during evolution. As an additional common feature these HSD17Bs are multifunctional enzymes that arose either via gene fusions (HSD17B4) or are incorporated as subunits into multifunctional protein complexes (HSD17B8 and HSD17B10). Crystal structures of HSD17B4, HSD17B8 and HSD17B10 give insight into their structure-function relationships. Thus far, deficiencies of HSD17B4 and HSD17B10 have been assigned to inborn errors in humans, underlining their significance as enzymes of metabolism.

Clinical and molecular analysis of 6 Chinese patients with isoleucine metabolism defects: identification of 3 novel mutations in the HSD17B10 and ACAT1 gene

Hydroxysteroid (17β) dehydrogenase 10 (HSD10) and mitochondrial acetoacetyl-CoA thiolase (β-KT) are two adjacent enzymes for the degradation of isoleucine, thus HSD10 and β-KT deficiencies are confusing at an early stage because of nearly the same elevation of typical metabolites in urine, such as 2-methyl-3-hydroxybutyric acid (2M3HBA) and tiglylglycine (TG). In order to better understand the differences between these two disorders, we described the clinical and molecular characteristics of two HSD10 deficiency patients and four β-KT deficiency patients. β-KT deficiency patients had a much more favorable outcome than that of HSD10 deficiency patients, indicating that the multifunction of HSD10, especially neurosteroid metabolic activity, other than only enzymatic degradation of isoleucine, is involved in the pathogenesis of HSD10 deficiency. Two different mutations, a novel mutation p.Ile175Met and a reported mutation p.Arg226Gln, were detected in the HSD17B10 gene of HSD10 deficiency patients. Six different mutations, including four known mutations: p.Ala333Pro, p.Thr297Lys, c.83_84delAT, c.1006-1G > C, and two novel mutations: p.Thr277Pro and c.121-3C > G were identified in the ACAT1 gene of β-KT deficiency patients. In general, DNA diagnosis played an important role in distinguishing between these two disorders.

Novel patient missense mutations in the HSD17B10 gene affect dehydrogenase and mitochondrial tRNA modification functions of the encoded protein

MRPP2 (also known as HSD10/SDR5C1) is a multifunctional protein that harbours both catalytic and non-catalytic functions. The protein belongs to the short-chain dehydrogenase/reductases (SDR) family and is involved in the catabolism of isoleucine in vivo and steroid metabolism in vitro. MRPP2 also moonlights in a complex with the MRPP1 (also known as TRMT10C) protein for N1-methylation of purines at position 9 of mitochondrial tRNA, and in a complex with MRPP1 and MRPP3 (also known as PRORP) proteins for 5'-end processing of mitochondrial precursor tRNA. Inherited mutations in the HSD17B10 gene encoding MRPP2 protein lead to a childhood disorder characterised by progressive neurodegeneration, cardiomyopathy or both. Here we report two patients with novel missense mutations in the HSD17B10 gene (c.34G>C and c.526G>A), resulting in the p.V12L and p.V176M substitutions. Val12 and Val176 are highly conserved residues located at different regions of the MRPP2 structure. Recombinant mutant proteins were expressed and characterised biochemically to investigate their effects towards the functions of MRPP2 and associated complexes in vitro. Both mutant proteins showed significant reduction in the dehydrogenase, methyltransferase and tRNA processing activities compared to wildtype, associated with reduced stability for protein with p.V12L, whereas the protein carrying p.V176M showed impaired kinetics and complex formation. This study therefore identified two distinctive molecular mechanisms to explain the biochemical defects for the novel missense patient mutations.

A novel HSD17B10 mutation impairing the activities of the mitochondrial RNase P complex causes X-linked intractable epilepsy and neurodevelopmental regression

We report a Caucasian boy with intractable epilepsy and global developmental delay. Whole-exome sequencing identified the likely genetic etiology as a novel p.K212E mutation in the X-linked gene HSD17B10 for mitochondrial short-chain dehydrogenase/reductase SDR5C1. Mutations in HSD17B10 cause the HSD10 disease, traditionally classified as a metabolic disorder due to the role of SDR5C1 in fatty and amino acid metabolism. However, SDR5C1 is also an essential subunit of human mitochondrial RNase P, the enzyme responsible for 5'-processing and methylation of purine-9 of mitochondrial tRNAs. Here we show that the p.K212E mutation impairs the SDR5C1-dependent mitochondrial RNase P activities, and suggest that the pathogenicity of p.K212E is due to a general mitochondrial dysfunction caused by reduction in SDR5C1-dependent maturation of mitochondrial tRNAs.

Determination of small molecule ABAD inhibitors crossing blood-brain barrier and pharmacokinetics

A major obstacle to the development of effective treatment of Alzheimer's disease (AD) is successfully delivery of drugs to the brain. We have previously identified a series of benzothiazole phosphonate compounds that block the interaction of amyloid-β peptide with amyloid-β binding alcohol dehydrogenase (ABAD). A selective and sensitive method for the presence of three new benzothiazole ABAD inhibitors in mouse plasma, brain, and artificial cerebrospinal fluid has been developed and validated based on high performance liquid chromatography tandem mass spectrometry. Mass spectra were generated using Micromass Quattro Ultima "triple" quadrupole mass spectrometer equipped with an Electrospray Ionization interface. Good linearity was obtained over a concentration range of 0.05-2.5 μg/ml. The lowest limit of quantification and detection was found to be 0.05 μg/ml. All inter-day accuracies and precisions were within ± 15% of the nominal value and ± 20%, respectively, at the lower limit of quantitation. The tested compounds were stable at various conditions with recoveries >90.0% (RSD <10%). The method used for pharmacokinetic studies of compounds in mouse cerebrospinal fluid, plasma, and brain is accurate, precise, and specific with no matrix effect. Pharmacokinetic data showed that these compounds penetrate the blood-brain barrier (BBB) yielding 4-50 ng/ml peak brain concentrations and 2 μg/ml peak plasma concentrations from a 10 mg/kg dose. These results indicate that our newly synthesized small molecule ABAD inhibitors have a good drug properties with the ability to cross the blood-brain barrier, which holds a great potential for AD therapy.

Molecular insights into HSD10 disease: impact of SDR5C1 mutations on the human mitochondrial RNase P complex

SDR5C1 is an amino and fatty acid dehydrogenase/reductase, moonlighting as a component of human mitochondrial RNase P, which is the enzyme removing 5′-extensions of tRNAs, an early and crucial step in tRNA maturation. Moreover, a subcomplex of mitochondrial RNase P catalyzes the N¹-methylation of purines at position 9, a modification found in most mitochondrial tRNAs and thought to stabilize their structure. Missense mutations in SDR5C1 cause a disease characterized by progressive neurodegeneration and cardiomyopathy, called HSD10 disease. We have investigated the effect of selected mutations on SDR5C1's functions. We show that pathogenic mutations impair SDR5C1-dependent dehydrogenation, tRNA processing and methylation. Some mutations disrupt the homotetramerization of SDR5C1 and/or impair its interaction with TRMT10C, the methyltransferase subunit of the mitochondrial RNase P complex. We propose that the structural and functional alterations of SDR5C1 impair mitochondrial RNA processing and modification, leading to the mitochondrial dysfunction observed in HSD10 patients.

Roles of 17β-hydroxysteroid dehydrogenase type 10 in neurodegenerative disorders

17β-Hydroxysteroid dehydrogenase type 10 (17β-HSD10) is encoded by the HSD17B10 gene mapping at Xp11.2. This homotetrameric mitochondrial multifunctional enzyme catalyzes the oxidation of neuroactive steroids and the degradation of isoleucine. This enzyme is capable of binding to other peptides, such as estrogen receptor α, amyloid-β, and tRNA methyltransferase 10C. Missense mutations of the HSD17B10 gene result in 17β-HSD10 deficiency, an infantile neurodegeneration characterized by progressive psychomotor regression and alteration of mitochondria morphology. 17β-HSD10 exhibits only a negligible alcohol dehydrogenase activity, and is not localized in the endoplasmic reticulum or plasma membrane. Its alternate name - Aβ binding alcohol dehydrogenase (ABAD) - is a misnomer predicated on the mistaken belief that this enzyme is an alcohol dehydrogenase. Misconceptions about the localization and function of 17β-HSD10 abound. 17β-HSD10's proven location and function must be accurately identified to properly assess this enzyme's important role in brain metabolism, especially the metabolism of allopregnanolone. The brains of individuals with Alzheimer's disease (AD) and of animals in an AD mouse model exhibit abnormally elevated levels of 17β-HSD10. Abnormal expression, as well as mutations of the HSD17B10 gene leads to impairment of the structure, function, and dynamics of mitochondria. This may underlie the pathogenesis of the synaptic and neuronal deficiency exhibited in 17β-HSD10 related diseases, including 17β-HSD10 deficiency and AD. Restoration of steroid homeostasis could be achieved by the supplementation of neuroactive steroids with a proper dosing and treatment regimen or by the adjustment of 17β-HSD10 activity to protect neurons. The discovery of this enzyme's true function has opened a new therapeutic avenue for treating AD.

Dimerization interface of 3-hydroxyacyl-CoA dehydrogenase tunes the formation of its catalytic intermediate

3-hydroxyacyl-CoA dehydrogenase (HAD, EC 1.1.1.35) is a homodimeric enzyme localized in the mitochondrial matrix, which catalyzes the third step in fatty acid β-oxidation. The crystal structures of human HAD and subsequent complexes with cofactor/substrate enabled better understanding of HAD catalytic mechanism. However, numerous human diseases were found related to mutations at HAD dimerization interface that is away from the catalytic pocket. The role of HAD dimerization in its catalytic activity needs to be elucidated. Here, we solved the crystal structure of Caenorhabditis elegans HAD (cHAD) that is highly conserved to human HAD. Even though the cHAD mutants (R204A, Y209A and R204A/Y209A) with attenuated interactions on the dimerization interface still maintain a dimerization form, their enzymatic activities significantly decrease compared to that of the wild type. Such reduced activities are in consistency with the reduced ratios of the catalytic intermediate formation. Further molecular dynamics simulations results reveal that the alteration of the dimerization interface will increase the fluctuation of a distal region (a.a. 60–80) that plays an important role in the substrate binding. The increased fluctuation decreases the stability of the catalytic intermediate formation, and therefore the enzymatic activity is attenuated. Our study reveals the molecular mechanism about the essential role of the HAD dimerization interface in its catalytic activity via allosteric effects.

Steroid signaling: ligand-binding promiscuity, molecular symmetry, and the need for gating

Steroid/sterol-binding receptors and enzymes are remarkably promiscuous in the range of ligands they can bind to and, in the case of enzymes, modify - raising the question of how specific receptor activation is achieved in vivo. Estrogen receptors (ER) are modulated by 27-hydroxycholesterol and 5α-androstane-3β,17β-diol (Adiol), in addition to estradiol (E2), and respond to diverse small molecules such as bisphenol A. Steroid-modifying enzymes are also highly promiscuous in ligand binding and metabolism. The specificity problem is compounded by the fact that the steroid core (hydrogenated cyclopentophenanthrene ring system) has several planes of symmetry. Ligand binding can be in symmetrical East-West (rotation) and North-South (inversion) orientations. Hydroxysteroid dehydrogenases (HSDs) can modify symmetrical 7 and 11, also 3 and 17/20, positions, exemplified here by yeast 3α,20β-HSD and mammalian 11β-HSD and 17β-HSD enzymes. Faced with promiscuity and symmetry, other strategies are clearly necessary to promote signaling selectivity in vivo. Gating regulates hormone access via enzymes that preferentially inactivate (or activate) a subclass of ligands, thereby governing which ligands gain receptor access - exemplified by 11β-HSD gating cortisol access to the mineralocorticoid receptor, and P450 CYP7B1 gating Adiol access to ER. Counter-intuitively, the specificity of steroid/sterol action is achieved not by intrinsic binding selectivity but by the combination of local metabolism and binding affinity.

Transcription start sites and epigenetic analysis of the HSD17B10 proximal promoter

BACKGROUND

Hydroxysteroid (17beta) dehydrogenase X (HSD10) is a multifunctional protein encoded by the HSD17B10 gene at Xp11.2. In response to stress or hypoxia-ischemia its levels increase rapidly. Expression of this gene is also elevated significantly in colonic mucosa of the inactive ulcerative colitis patients. However, accurate information about its several transcripts is still lacking, and additional evidence for its escape from X-chromosome inactivation remains to be obtained in order to help settle a debate (He XY, Dobkin C, Yang SY: Does the HSD17B10 gene escape from X-inactivation? Eur J Hum Genet 2011, 19: 123-124).

RESULTS

Two major HSD17B10 transcription start sites were identified by primer extension at -37 and -6 as well as a minor start site at -12 nucleotides from the initiation codon ATG. Epigenetic analysis of the 5'-flanking region of the HSD17B10 gene showed that there was little 5-methylcytosine (< 3%) in a normal male, and that none of CpG dinucleotides in the CpG island approached 50% methylation in females.

CONCLUSION

The actual length of first exon of the HSD17B10 gene was found to be about a quarter larger than that originally reported. Its transcripts result from a slippery transcription complex. The hypomethylation of the CpG island provides additional evidence for the variable escape of the HSD17B10 gene from X-chromosome inactivation which could influence the range of severity of HSD10 deficiency, an inherited error in isoleucine metabolism, in heterozygous females.

The amyloid-β-SDR5C1(ABAD) interaction does not mediate a specific inhibition of mitochondrial RNase P

The amyloid-β peptide (Aβ) is suggested to cause mitochondrial dysfunction in Alzheimer's disease. The mitochondrial dehydrogenase SDR5C1 (also known as ABAD) was shown to bind Aβ and was proposed to thereby mediate mitochondrial toxicity, but the molecular mechanism has not been clarified. We recently identified SDR5C1 as an essential component of human mitochondrial RNase P and its associated tRNA:m¹R9 methyltransferase, the enzymes responsible for tRNA 5'-end processing and methylation of purines at tRNA position 9, respectively. With this work we investigated whether SDR5C1's role as a subunit of these two tRNA-maturation activities represents the mechanistic link between Aβ and mitochondrial dysfunction. Using recombinant enzyme components, we tested RNase P and methyltransferase activity upon titration of Aβ. Micromolar concentrations of monomeric or oligomerized Aβ were required to inhibit tRNA 5'-end processing and position 9 methylation catalyzed by the SDR5C1-containing enzymes, yet similar concentrations of Aβ also inhibited related RNase P and methyltransferase activities, which do not contain an SDR5C1 homolog. In conclusion, the proposed deleterious effect of Aβ on mitochondrial function cannot be explained by a specific inhibition of mitochondrial RNase P or its tRNA:m¹R9 methyltransferase subcomplex, and the molecular mechanism of SDR5C1-mediated Aβ toxicity remains unclear.

Purification, crystallization and preliminary crystallographic analysis of 3-hydroxyacyl-CoA dehydrogenase from Caenorhabditis elegans

3-Hydroxyacyl-CoA dehydrogenase (HAD; EC 1.1.1.35) is the enzyme that catalyzes the third step in fatty-acid β-oxidation, oxidizing the hydroxyl group of 3-hydroxyacyl-CoA to a keto group. The 3-hydroxyacyl-CoA dehydrogenase from Caenorhabditis elegans (cHAD) was cloned, overexpressed in Escherichia coli and purified to homogeneity for crystallography. Initial crystals were obtained by the hanging-drop vapour-diffusion method. Optimization of the precipitant concentration and the pH yielded two types of well diffracting crystals with parallelepiped and cuboid shapes, respectively. Complete diffraction data sets were collected and processed from both crystal types. Preliminary crystallographic analysis indicated that the parallelepiped-shaped crystal belonged to space group P1, while the cuboid-shaped crystal belonged to space group P212121. Analyses of computed Matthews coefficient and self-rotation functions suggested that there are two cHAD molecules in one asymmetric unit in both crystals, forming identical dimers but packing in distinct manners.

A 5-methylcytosine hotspot responsible for the prevalent HSD17B10 mutation

Approximately half of the cases of hydroxysteroid (17β) dehydrogenase X (HSD10) deficiency are due to a missense C>T mutation in exon 4 of the HSD17B10 gene. The resulting HSD10 (p.R130C) loses most or all catalytic functions, and the males with this mutation have a much more severe clinical phenotype than those carrying p.V65A, p.L122V, or p.E249Q mutations. We found that the mutated cytosine which is +2259 nucleotide from the ATG of the gene, is >90% methylated in both the active and inactive X chromosomes in two normal females as well as in the X chromosome of a normal male. Since 5-methylcytosine is prone to conversion to thymine by deamination, the methylation of this cytosine in normal X chromosomes provides an explanation for the prevalence of the p.R130C mutation among patients with HSD10 deficiency. The substitution of arginine for cysteine eliminates several hydrogen bonds and reduces the van der Waals interaction between HSD10 subunits. The resulting disruption of protein structure impairs some if not all of the catalytic and non-enzymatic functions of HSD10. A meta-analysis of residual HSD10 activity in eight patients with the p.R130C mutation showed an average 2-methyl-3-hydroxybutyryl-CoA dehydrogenase (MHBD) activity of only 6 (±5) % of the normal control level. This is significantly lower than in cells of patients with other, clinically milder mutations and suggests that the loss of HSD10/MHBD activity is a marker for the disorder.

Gene network effects on brain microstructure and intellectual performance identified in 472 twins

A major challenge in neuroscience is finding which genes affect brain integrity, connectivity, and intellectual function. Discovering influential genes holds vast promise for neuroscience, but typical genome-wide searches assess approximately one million genetic variants one-by-one, leading to intractable false positive rates, even with vast samples of subjects. Even more intractable is the question of which genes interact and how they work together to affect brain connectivity. Here, we report a novel approach that discovers which genes contribute to brain wiring and fiber integrity at all pairs of points in a brain scan. We studied genetic correlations between thousands of points in human brain images from 472 twins and their nontwin siblings (mean age: 23.7 ± 2.1 SD years; 193 male/279 female). We combined clustering with genome-wide scanning to find brain systems with common genetic determination. We then filtered the image in a new way to boost power to find causal genes. Using network analysis, we found a network of genes that affect brain wiring in healthy young adults. Our new strategy makes it computationally more tractable to discover genes that affect brain integrity. The gene network showed small-world and scale-free topologies, suggesting efficiency in genetic interactions and resilience to network disruption. Genetic variants at hubs of the network influence intellectual performance by modulating associations between performance intelligence quotient and the integrity of major white matter tracts, such as the callosal genu and splenium, cingulum, optic radiations, and the superior longitudinal fasciculus.

HSD10 disease: clinical consequences of mutations in the HSD17B10 gene

The HSD17B10 gene is located on chromosome Xp11.2 and codes for a multifunctional protein called 17β-hydroxysteroid dehydrogenase type 10 (HSD10). This protein catalyzes the 2-methyl-3-hydroxybutyryl-CoA dehydrogenation (MHBD) reaction in isoleucine metabolism and is an essential component of mitochondrial RNase P required for the processing of mtDNA transcripts. HSD10 is required for normal mitochondrial maintenance, and complete loss of HSD10 is incompatible with life. Mutations in the HSD17B10 gene have been reported in 19 families. The classical infantile form of what is best named HSD10 disease is characterized by a period of more or less normal development in the first 6-18 months of life. Some patients showed transient metabolic derangement in the neonatal period, with good clinical recovery but often persistent lactate elevation. Usually from age 6-18 months affected boys show a progressive neurodegenerative disease course in conjunction with retinopathy and cardiomyopathy leading to death at age 2-4 years or later. A more severe presentation in the neonatal period with little neurological development, severe progressive cardiomyopathy, and early death, is denoted neonatal form. Juvenile and atypical/asymptomatic forms of HSD10 disease have been recognized. Heterozygous females often show non-progressive developmental delay and intellectual disability but may also be clinically normal. The pathogenesis is poorly understood but is unrelated to MHBD function. Diagnosis is based on typical abnormalities in urinary organic acid analysis and molecular studies. The same de novo mutation p.R130C was found in over half of patient families; it is associated with the infantile disease form. There is no effective treatment.

A novel mutation in the HSD17B10 gene of a 10-year-old boy with refractory epilepsy, choreoathetosis and learning disability

Hydroxysteroid (17beta) dehydrogenase 10 (HSD10) is a mitochondrial multifunctional enzyme encoded by the HSD17B10 gene. Missense mutations in this gene result in HSD10 deficiency, whereas a silent mutation results in mental retardation, X-linked, syndromic 10 (MRXS10). Here we report a novel missense mutation found in the HSD17B10 gene, namely c.194T>C transition (rs104886492), brought about by the loss of two forked methyl groups of valine 65 in the HSD10 active site. The affected boy, who possesses mutant HSD10 (p.V65A), has a neurological syndrome with metabolic derangements, choreoathetosis, refractory epilepsy and learning disability. He has no history of acute decompensation or metabolic acidosis whereas his urine organic acid profile, showing elevated levels of 2-methyl-3-hydroxybutyrate and tiglylglycine, is characteristic of HSD10 deficiency. His HSD10 activity was much lower than the normal control level, with normal β-ketothiolase activity. The c.194T>C mutation in HSD17B10 can be identified by the restriction fragment polymorphism analysis, thereby facilitating the screening of this novel mutation in individuals with intellectual disability of unknown etiology and their family members much easier. The patient's mother is an asymptomatic carrier, and has a mixed ancestry (Hawaiian, Japanese and Chinese). This demonstrates that HSD10 deficiency patients are not confined to a particular ethnicity although previously reported cases were either Spanish or German descendants.

Hydroxysteroid (17β) dehydrogenase X in human health and disease

Hydroxysteroid (17β) dehydrogenase 10 (HSD10), the HSD17B10 gene product, is a mitochondrial NAD(+)-dependent dehydrogenase. There are two outstanding features of this vital enzyme: (a) the versatility of its catalytic endowment is attributed to the flexibility of its active site to accommodate diverse substrates such as steroids, fatty acids, bile acid, and xenobiotics; (b) its capacity to bind other proteins and peptides. For example, it tightly binds with three identical subunits to compose a homotetramer. The homotetramer then binds with two other proteins, namely, RNA (guanine-9-)methyl-transferase domain containing-1 and KIAA0391, to form mitochondrial RNase P. Furthermore, various HSD10 functions are inhibited when the enzyme is bound by amyloid-β peptide or estrogen receptor alpha. Missense mutations of HSD10 may cause neurodegeneration related to HSD10 deficiency, whereas a silent mutation of HSD10 results in mental retardation, choreoathetosis and abnormal behavior (MRXS10). The clinical condition of some HSD10 patients mimics mitochondrial disorders. Since normal HSD10 function is essential for brain cognitive activity, elevated levels of HSD10 found in brains of Alzheimer disease (AD) patients and mouse AD model might counterbalance the inhibition of HSD10 by amyloid-β peptide. The investigation of HSD10 may lead to a better understanding of AD pathogenesis.

(-)-CHANA, a fluorogenic probe for detecting amyloid binding alcohol dehydrogenase HSD10 activity in living cells

The association of 17β-hydroxysteroid dehydrogenase 10 (HSD10) with β-amyloid in the brain is known to contribute to the progression of Alzheimer's disease. Further, it has been shown that the interaction between the purified HSD10 and β-amyloid inhibits its enzymatic activity. However, to date no system has been developed to enable the study of HSD10 activity in intact living cells. To address this significant shortcoming, we have developed a novel fluorogenic probe, (-)-cyclohexenyl amino naphthalene alcohol [(-)-CHANA], to observe and measure the activity of HSD10 in living cells. The oxidation of (-)-CHANA by HSD10 results in the production and accumulation of a fluorescent product, which can be measured using real-time fluorescence microscopy. This compound permits the measurement of mitochondrial HSD10 activity and its inhibition by both a small molecule HSD10 inhibitor and by β-amyloid, in living cells. Herein, we define the parameters under which this probe can be used. This compound is likely to prove useful in future investigations aimed at developing therapeutic compounds targeting the HSD10-β-amyloid association.

X-inactivation of HSD17B10 revealed by cDNA analysis in two female patients with 17β-hydroxysteroid dehydrogenase 10 deficiency

17β-Hydroxysteroid dehydrogenase 10 (HSD10) is a mitochondrial enzyme involved in the degradation pathway of isoleucine and branched-chain fatty acids. The gene encoding HSD10, HSD17B10, has been reported as one of the few genes that escapes X-inactivation. We previously studied two female patients with HSD10 deficiency, one of them was severely affected and the other presented a mild phenotype. To elucidate as to why these two carriers were so differently affected, cDNA analyses were performed. The HSD17B10 cDNA of eight control cell lines, two hemizygous patients and two carriers was obtained from cultured fibroblasts, amplified by PCR and sequenced by standard methods. All HSD17B10 cDNAs were quantified by real-time PCR. In the fibroblasts of the female patient who presented with the severe phenotype, only the mutant allele was identified in the cDNA sequence, which was further confirmed by relative quantification (RQ) of HSD17B10 cDNA. This is in agreement with an unfavourable X-inactivation. The other female patient, with slight clinical affectation, showed the presence of both mutant and wild-type alleles in the cDNA sequence, which was confirmed by RQ of HSD17B10 cDNA in fibroblasts. This is in line with normal X-inactivation and the expression of both alleles in different cells (functional mosaicism). RQ results of HSD17B10 cDNA did not differ significantly between male and female controls, which indicate that the genetic doses of mRNA of HSD17B10 was the same in both sexes. In conclusion, these results suggest that the HSD17B10 gene does not escape X-inactivation as has been reported previously.

The consequences of mitochondrial amyloid beta-peptide in Alzheimer's disease

The Abeta (amyloid-beta peptide) has long been associated with Alzheimer's disease, originally in the form of extracellular plaques. However, in the present paper we review the growing evidence for the role of soluble intracellular Abeta in the disease progression, with particular reference to Abeta found within the mitochondria. Once inside the cell, Abeta is able to interact with a number of targets, including the mitochondrial proteins ABAD (amyloid-binding alcohol dehydrogenase) and CypD (cyclophilin D), which is a component of the mitochondrial permeability transition pore. Interference with the normal functions of these proteins results in disruption of cell homoeostasis and ultimately cell death. The present review explores the possible mechanisms by which cell death occurs, considering the evidence presented on a molecular, cellular and in vivo level.

Mental retardation linked to mutations in the HSD17B10 gene interfering with neurosteroid and isoleucine metabolism

Mutations in the HSD17B10 gene were identified in two previously described mentally retarded males. A point mutation c.776G>C was found from a survivor (SV), whereas a potent mutation, c.419C>T, was identified in another deceased case (SF) with undetectable hydroxysteroid (17beta) dehydrogenase 10 (HSD10) activity. Protein levels of mutant HSD10(R130C) in patient SF and HSD10(E249Q) in patient SV were about half that of HSD10 in normal controls. The E249Q mutation appears to affect HSD10 subunit interactions, resulting in an allosteric regulatory enzyme. For catalyzing the oxidation of allopregnanolone by NAD+ the Hill coefficient of the mutant enzyme is approximately 1.3. HSD10(E249Q) was unable to catalyze the dehydrogenation of 2-methyl-3-hydroxybutyryl-CoA and the oxidation of allopregnanolone, a positive modulator of the gamma-aminobutyric acid type A receptor, at low substrate concentrations. Neurosteroid homeostasis is critical for normal cognitive development, and there is increasing evidence that a blockade of isoleucine catabolism alone does not commonly cause developmental disabilities. The results support the theory that an imbalance in neurosteroid metabolism could be a major cause of the neurological handicap associated with hydroxysteroid (17beta) dehydrogenase 10 deficiency.

Integrated view on 17beta-hydroxysteroid dehydrogenases

17beta-Hydroxysteroid dehydrogenases (17beta-HSDs) are important enzymes in steroid metabolism. Long known members of the protein family seemed to be well characterised concerning their role in the regulation of the biological potency of steroid hormones, but today more and more evidence points to pivotal contributions of these enzymes in a variety of other metabolic pathways. Therefore, studies on 17beta-HSDs develop towards metabolomic survey. Latest research results give new insights into the complex metabolic interconnectivity of the 17beta-HSDs. In this paper metabolic activities of 17beta-HSDs will be compared, their interplay with endogenous substrates summarised, and interlacing pathways depicted. Strategies on deciphering the physiological role of 17beta-HSDs and the genetic predisposition for associated diseases will be presented.

Recent advances in 17beta-hydroxysteroid dehydrogenases

The metabolism of steroids at position 17 is catalysed by a growing number of 17beta-hydroxysteroid dehydrogenases (17beta-HSDs). Several human diseases like breast or prostate cancer, endometriosis,metabolic syndrome and mental diseases were associated with dysfunctions of 17beta-HSDs, which consequently became drug targets. This review will focus on identities of 17beta-HSDs and recent advances in analyses of their physiological roles in steroid and lipid metabolism. It will also address the potential of metabolomics in drug development.

Molecular dynamics simulations of the amyloid-beta binding alcohol dehydrogenase (ABAD) enzyme

In this work, we present 10 ns molecular dynamics simulations of the homotetramer of the ABAD enzyme, as well as of the structural units, dimer and monomer, that assemble to form the tetramer, in the presence and absence of a NAD-inhibitor adduct. The aim was to compare the stability of the different structures and to study the effects of the inhibitor binding on the flexibility of the enzyme structure. The results indicate that the tetramer, dimer and monomer show a comparable stability and that tetramerization stabilizes some regions of the protein that when exposed to the solvent in dimer and monomer become more flexible. Binding of the cofactor and inhibitor stabilizes the protein, the main effect being a stabilization of the substrate binding loop. In the absence of the ligand, this region was found to have a much higher flexibility and to adopt an open conformation. An interesting result emerging from this work is the conformational flexibility exhibited by the azepane and benzene rings of the inhibitor moiety of the adduct, which appears to be influenced by the mobility of the substrate binding loop. This highlights the importance of integrate the flexibility of the substrate binding loop into de novo design of inhibitors of ABAD.

Role of short-chain hydroxyacyl CoA dehydrogenases in SCHAD deficiency

Short-chain hydroxyacyl CoA dehydrogenase deficiency is an ill-defined, severe pediatric disorder of mitochondrial fatty acid beta-oxidation of short-chain hydroxyacyl CoAs. To understand the relative contributions of the two known short-chain hydroxyacyl CoA dehydrogenases (HADH) tissue biopsies of six distinct family individuals were analyzed and kinetic parameters were compared. Steady-state kinetic constants for HADH 1 and HADH 2 suggest that type 1 is the major enzyme involved in mitochondrial beta-oxidation of short-chain hydroxyacyl-CoAs. Two patients are heterozygous carriers of a HADH 1 polymorphism, whereas no mutation is detected in the HADH 2 gene of all patients. The data suggest that protein interactions rather than HADH mutations are responsible for the disease phenotype. Pull-down experiments of recombinant HADH 1 and 2 with human mitochondrial extracts reveal two proteins interacting with HADH 1, one of which was identified as glutamate dehydrogenase. This association provides a possible link between fatty acid metabolism and the hyperinsulinism/hyperammonia syndrome.

Amyloid-beta peptide binds to microtubule-associated protein 1B (MAP1B)

Extracellular and intraneuronal formation of amyloid-beta aggregates have been demonstrated to be involved in the pathogenesis of Alzheimer's disease. However, the precise mechanism of amyloid-beta neurotoxicity is not completely understood. Previous studies suggest that binding of amyloid-beta to a number of targets have deleterious effects on cellular functions. In the present study we have shown for the first time that amyloid-beta 1-42 bound to a peptide comprising the microtubule binding domain of the heavy chain of microtubule-associated protein 1B by the screening of a human brain cDNA library expressed on M13 phage. This interaction may explain, in part, the loss of neuronal cytoskeletal integrity, impairment of microtubule-dependent transport and synaptic dysfunction observed previously in Alzheimer's disease.

HSD17B10: a gene involved in cognitive function through metabolism of isoleucine and neuroactive steroids

The HSD17B10 gene maps on chromosome Xp11.2, a region highly associated with X-linked mental retardation. This gene encodes HSD10, a mitochondrial multifunctional enzyme that plays a significant part in the metabolism of neuroactive steroids and the degradation of isoleucine. The HSD17B10 gene is composed of six exons and five introns. Its exon 5 is an alternative exon such that there are several HSD17B10 mRNA isoforms in brain. A silent mutation (c.605C-->A) and three missense mutations (c.395C-->G; c.419C-->T; c.771A-->G), respectively, cause the X-linked mental retardation, choreoathetosis, and abnormal behavior (MRXS10) and the hydroxyacyl-CoA dehydrogenase II deficiency. The latter condition seems to be a multifactorial disease due to the disturbance of more than one metabolic pathway by the HSD10 deficiency. HSD10 inactivates the positive modulators of GABAA receptors, and plays a role in the maintenance of GABAergic neuronal function. This working model may account for the mental retardation of these patients. The dehydrogenase activity is slightly inhibited by the binding of amyloid-beta peptide to the loop D of HSD10. Elevated levels of HSD10 were observed in hippocampi of Alzheimer disease patients so this multifunctional enzyme may be related to Alzheimer disease pathogenesis; however, the molecular mechanism of its involvement remains to be ascertained.

Comparative evolutionary genomics of the HADH2 gene encoding Abeta-binding alcohol dehydrogenase/17beta-hydroxysteroid dehydrogenase type 10 (ABAD/HSD10)

Background

The Aβ-binding alcohol dehydrogenase/17β-hydroxysteroid dehydrogenase type 10 (ABAD/HSD10) is an enzyme involved in pivotal metabolic processes and in the mitochondrial dysfunction seen in the Alzheimer's disease. Here we use comparative genomic analyses to study the evolution of the HADH2 gene encoding ABAD/HSD10 across several eukaryotic species.

Results

Both vertebrate and nematode HADH2 genes showed a six-exon/five-intron organization while those of the insects had a reduced and varied number of exons (two to three). Eutherian mammal HADH2 genes revealed some highly conserved noncoding regions, which may indicate the presence of functional elements, namely in the upstream region about 1 kb of the transcription start site and in the first part of intron 1. These regions were also conserved between Tetraodon and Fugu fishes. We identified a conserved alternative splicing event between human and dog, which have a nine amino acid deletion, causing the removal of the strand β_F. This strand is one of the seven strands that compose the core β-sheet of the Rossman fold dinucleotide-binding motif characteristic of the short chain dehydrogenase/reductase (SDR) family members. However, the fact that the substrate binding cleft residues are retained and the existence of a shared variant between human and dog suggest that it might be functional. Molecular adaptation analyses across eutherian mammal orthologues revealed the existence of sites under positive selection, some of which being localized in the substrate-binding cleft and in the insertion 1 region on loop D (an important region for the Aβ-binding to the enzyme). Interestingly, a higher than expected number of nonsynonymous substitutions were observed between human/chimpanzee and orangutan, with six out of the seven amino acid replacements being under molecular adaptation (including three in loop D and one in the substrate binding loop).

Conclusion

Our study revealed that HADH2 genes maintained a reasonable conserved organization across a large evolutionary distance. The conserved noncoding regions identified among mammals and between pufferfishes, the evidence of an alternative splicing variant conserved between human and dog, and the detection of positive selection across eutherian mammals, may be of importance for further research on ABAD/HSD10 function and its implication in the Alzheimer's disease.

Systemic distribution and tissue localizations of human 17beta-hydroxysteroid dehydrogenase type 12

The 17beta-hydroxysteroid dehydrogenases (HSDs) are enzymes that catalyze the reduction of 17-ketosteroids or the oxidation of 17beta-hydroxysteroids. 17beta-HSD type 12, the most recently cloned member of this gene family, was classified into the 17beta-HSD family based on sequence homology, rather than steroid catalyzing activity. Meanwhile, it has been reported that 17beta-HSD type 12 may be involved in fatty acid synthesis. To better understand the role of 17beta-HSD type 12 in lipid metabolism, we determined the detailed systemic distribution and tissue localizations of 17beta-HSD type 12, which, due partly to the lack of antibodies, had not yet been studied. We carried out these investigations by quantitative reverse transcription (RT)-PCR, Northern blot analysis, and immunohistochemistry, using an antibody against 17beta-HSD type 12 that we have generated. 17beta-HSD type 12 is highly expressed in organs related to lipid metabolism such as liver, kidney, heart and skeletal muscle. 17beta-HSD type 12 is also detected in endocrine-related organs such as pancreas, pituitary gland, adrenal gland, testis and placenta, and in the gastrointestinal tract, which point to the possible involvement of 17beta-HSD type 12 in the regulation of lipid biosynthesis and steroid metabolism. These results support previous reports and solidify the possibility that 17beta-HSD type 12 may play critical roles in the physiological processes, such as fatty acid synthesis, in addition to the steroid metabolism.

Identification of amyloid-beta 1–42 binding protein fragments by screening of a human brain cDNA library

Extracellular and intraneuronal formation of amyloid-beta (Abeta) deposits have been demonstrated to be involved in the pathogenesis of Alzheimer's disease (AD). However, the precise mechanism of Abeta neurotoxicity is not completely understood. Previous studies suggest that binding of Abeta with a number of targets have deleterious effects on cellular functions. It has been shown that Abeta directly interacted with intracellular protein ERAB (endoplasmic reticulum amyloid beta-peptide-binding protein) also known as ABAD (Abeta-binding alcohol dehydrogenase) resulting in mitochondrial dysfunction and cell death. In the present study we have identified another mitochondrial enzyme, ND3 of the human complex I, that binds to Abeta1-42 by the screening of a human brain cDNA library expressed on M13 phage. Our results indicated a strong interaction between Abeta and a phage-displayed 25 amino acid long peptide TTNLPLMVMSSLLLIIILALSLAYE corresponding to C-terminal peptide domain of NADH dehydrogenase, subunit 3 (MTND3) encoded by mitochondrial DNA (mtDNA). This interaction may explain, in part, the inhibition of complex I activity in astrocytes and neurons in the presence of Abeta, described recently. To our knowledge, the present study is the first demonstration of interaction between Abeta and one of the subunits of the human complex I.

Structure and function of human 17beta-hydroxysteroid dehydrogenases

17Beta-hydroxysteroid dehydrogenases (17beta-HSDs) catalyze the NAD(P)(H) dependent oxidoreduction at C17 oxo/beta-hydroxyl groups of androgen and estrogen hormones. This reversible reaction constitutes an important pre-receptor control mechanism for nuclear receptor ligands, since the conversion "switches" between the 17beta-OH receptor ligands and their inactive 17-oxo metabolites. At present, 14 mammalian 17beta-HSDs are described, of which at least 11 exist within the human genome, encoded by different genes. The enzymes differ in their expression pattern, nucleotide cofactor preference, steroid substrate specificity and subcellular localization, and thus constitute a complex system ensuring cell-specific adaptation and regulation of sex steroid hormone levels. Broad and overlapping substrate specificities with enzymes involved in lipid metabolism suggest interactions of several 17beta-HSDs with other metabolic pathways. Several 17beta-HSDs enzymes constitute promising drug targets, of particular importance in cancer, metabolic diseases, neurodegeneration and possibly immunity.

Multifunctionality of human 17beta-hydroxysteroid dehydrogenases

17Beta-hydroxysteroid dehydrogenases (17beta-HSDs) belong to the family of short chain dehydrogenases/reductases (SDRs) and aldoketo-reductases (AKRs). Some of the enzymes were discovered and named due to their enzymatic activity on steroid substrates or according to their sequence homology to other 17beta-HSDs. During characterisation of these enzymes it turned out that their substrate specificity is broader than first expected and key functions of some 17beta-HSDs in vivo are probably not in steroid metabolism but in basic metabolic pathways. The issue of such multifunctionality is the topic of this review.

3-Hydroxyacyl-CoA dehydrogenase and short chain 3-hydroxyacyl-CoA dehydrogenase in human health and disease

3-Hydroxyacyl-CoA dehydrogenase (HAD) functions in mitochondrial fatty acid beta-oxidation by catalyzing the oxidation of straight chain 3-hydroxyacyl-CoAs. HAD has a preference for medium chain substrates, whereas short chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD) acts on a wide spectrum of substrates, including steroids, cholic acids, and fatty acids, with a preference for short chain methyl-branched acyl-CoAs. Therefore, HAD should not be referred to as SCHAD. SCHAD is not a member of the HAD family, but instead, belongs to the short chain dehydrogenase/reductase superfamily. Previously reported cases of SCHAD deficiency are due to an inherited HAD deficiency. SCHAD, also known as 17beta-hydroxysteroid dehydrogenase type 10, is important in brain development and aging. Abnormal levels of SCHAD in certain brain regions may contribute to the pathogenesis of some neural disorders. The human SCHAD gene and its protein product, SCHAD, are potential targets for intervention in conditions, such as Alzheimer's disease, Parkinson's disease, and an X-linked mental retardation, that may arise from the impaired degradation of branched chain fatty acid and isoleucine.

Molecular biomarkers and adaptation to environmental stress in moon jelly (Aurelia spp.)

We describe a strategy that identifies molecular biomarkers and links the study of abiotic stress to evolutionary history. By utilizing the moon jellyfish Aurelia spp. as a model, we identified genes differentially regulated in response to the chemical stressor tributyltin by means of complementary DNA subtraction analyses. Expression of 3 out of 25 identified candidate genes, one oxidative stress gene, one heat shock (hsp70) gene, and one GTP-binding gene, was quantified under laboratory conditions and in field tests using semiquantitative reverse transcriptase polymerase chain reaction. Differential expression patterns were found following exposure to tributyltin and temperature treatments. The findings suggest that the identified genes are involved in response to chemical as well as heat- induced stress and may serve as biomarkers for monitoring marine habitats. Gene regulatory patterns combined with phylogenetic inferences of the hsp70 gene support a possible role of ecologically driven divergence within the genus Aurelia. We show that added information on genetic variability can raise the predictive power of molecular biomarkers in studies of individual stress response.

Intracellular oxidation of allopregnanolone by human brain type 10 17beta-hydroxysteroid dehydrogenase

Allopregnanolone is a positive allosteric modulator of GABAA receptors, generated by the reduction of 5alpha-dihydroprogesterone (5alpha-DHP) in astrocytes. This neuroactive steroid can be inactivated by its 3alpha-oxidation to yield 5alpha-DHP. It was found that 5alpha-DHP levels in HEK293 cells expressing type 10 17beta-hydroxysteroid dehydrogenase (17beta-HSD10), but not its catalytic inactive mutant, increased significantly as allopregnanolone was added to culture media. The results demonstrate that mitochondrial 17beta-HSD10 effectively catalyzes the intracellular oxidation of allopregnanolone. Moreover, brain astrocytes contain a moderate level of 17beta-HSD10, which is elevated in activated astrocytes of brains with Alzheimer type pathology, including sporadic Alzheimer's disease (AD) and Down's syndrome with AD. The distribution of 17beta-HSD10 was found not to parallel that of 3alpha-HSD3. Cerebral cortex has the lowest level of 17beta-HSD10; whereas the hippocampus, hypothalamus, and amygdala possess relatively higher levels of this enzyme. The catalysis of 17beta-HSD10 appears to be essential for maintaining normal functions of GABAergic neurons. The elevated level of 17beta-HSD10 in activated astrocytes is a new feature found in brains of people with AD, and it may have important impact on AD pathogenesis.

Multiple functions of type 10 17beta-hydroxysteroid dehydrogenase

Human 17beta-hydroxysteroid dehydrogenase type 10 (17beta-HSD10) is a mitochondrial enzyme encoded by the SCHAD gene, which escapes chromosome X inactivation. 17Beta-HSD10/SCHAD mutations cause a spectrum of clinical conditions, from mild mental retardation to progressive infantile neurodegeneration. 17Beta-HSD10/SCHAD is essential for the metabolism of isoleucine and branched-chain fatty acids. It can inactivate 17beta-estradiol and steroid modulators of GABA(A) receptors, and convert 5alpha-androstanediol into 5alpha-dihydrotestosterone (DHT). Certain malignant prostatic epithelial cells contain high levels of 17beta-HSD10, generating 5alpha-DHT in the absence of testosterone. 17Beta-HSD10 has an affinity for amyloid-beta peptide, and might be linked to the mitochondrial dysfunction seen in Alzheimer's disease. This versatile enzyme might provide a new drug target for neuronal excitability control and for intervention in Alzheimer's disease and certain cancers.