From glycosylation disorders to dolichol biosynthesis defects: a new class of metabolic diseases

Reduction hits the sweet spot: A revised biosynthesis of dolichol

Dolichol is a lipid that is involved in protein glycosylation, a process that is essential for all eukaryotic life. In this issue of Cell, Wilson and coworkers1 report how a rare human genetic disorder led to the discovery of dolichol biosynthesis.

A pseudoautosomal glycosylation disorder prompts the revision of dolichol biosynthesis

Dolichol is a lipid critical for N-glycosylation as a carrier for activated sugars and nascent oligosaccharides. It is commonly thought to be directly produced from polyprenol by the enzyme SRD5A3. Instead, we found that dolichol synthesis requires a three-step detour involving additional metabolites, where SRD5A3 catalyzes only the second reaction. The first and third steps are performed by DHRSX, whose gene resides on the pseudoautosomal regions of the X and Y chromosomes. Accordingly, we report a pseudoautosomal-recessive disease presenting as a congenital disorder of glycosylation in patients with missense variants in DHRSX (DHRSX-CDG). Of note, DHRSX has a unique dual substrate and cofactor specificity, allowing it to act as a NAD+-dependent dehydrogenase and as a NADPH-dependent reductase in two non-consecutive steps. Thus, our work reveals unexpected complexity in the terminal steps of dolichol biosynthesis. Furthermore, we provide insights into the mechanism by which dolichol metabolism defects contribute to disease.

The tunicamycin derivative TunR2 exhibits potent antibiotic properties with low toxicity in an in vivo Mycobacterium marinum-zebrafish TB infection model

Tunicamycins (TUN) are well-defined, Streptomyces-derived natural products that inhibit protein N-glycosylation in eukaryotes, and by a conserved mechanism also block bacterial cell wall biosynthesis. TUN inhibits the polyprenylphosphate-N-acetyl-hexosamine-1-phospho-transferases (PNPT), an essential family of enzymes found in both bacteria and eukaryotes. We have previously published the development of chemically modified TUN, called TunR1 and TunR2, that have considerably reduced activity on eukaryotes but that retain the potent antibacterial properties. A mechanism for this reduced toxicity has also been reported. TunR1 and TunR2 have been tested against mammalian cell lines in culture and against live insect cells but, until now, no in vivo evaluation has been undertaken for vertebrates. In the current work, TUN, TunR1, and TunR2 are investigated for their relative toxicity and antimycobacterial activity in zebrafish using a well-established Mycobacterium marinum (M. marinum) infection system, a model for studying human Mycobacterium tuberculosis infections. We also report the relative ability to activate the unfolded protein response (UPR), the known mechanism for the eukaryotic toxicity observed with TUN treatment. Importantly, TunR1 and TunR2 retained their antimicrobial properties, as evidenced by a reduction in M. marinum bacterial burden, compared to DMSO-treated zebrafish. In summary, findings from this study highlight the characteristics of recently developed TUN derivatives, mainly TunR2, and its potential for use as a novel anti-bacterial agent for veterinary and potential medical purposes.

Intramembrane protease SPP defines a cholesterol-regulated abundance control of the mevalonate pathway enzyme squalene synthase

Intramembrane proteolysis regulates important processes such as signaling and transcriptional and posttranslational abundance control of proteins with key functions in metabolic pathways. This includes transcriptional control of mevalonate pathway genes, thereby ensuring balanced biosynthesis of cholesterol and other isoprenoids. Our work shows that, at high cholesterol levels, signal peptide peptidase (SPP) cleaves squalene synthase (SQS), an enzyme that defines the branching point for allocation of isoprenoids to the sterol and nonsterol arms of the mevalonate pathway. This intramembrane cleavage releases SQS from the membrane and targets it for proteasomal degradation. Regulation of this mechanism is achieved by the E3 ubiquitin ligase TRC8 that, in addition to ubiquitinating SQS in response to cholesterol levels, acts as an allosteric activator of SPP-catalyzed intramembrane cleavage of SQS. Cellular cholesterol levels increase in the absence of SPP activity. We infer from these results that, SPP-TRC8 mediated abundance control of SQS acts as a regulation step within the mevalonate pathway.

Aptamer-Based Imaging of Polyisoprenoids in the Malaria Parasite

Dolichols are isoprenoid end-products of the mevalonate and 2C-methyl-D-erythritol-4-phosphate pathways. The synthesis of dolichols is initiated with the addition of several molecules of isopentenyl diphosphate to farnesyl diphosphate. This reaction is catalyzed by a cis-prenyltransferase and leads to the formation of polyprenyl diphosphate. Subsequent steps involve the dephosphorylation and reduction of the α-isoprene unit by a polyprenol reductase, resulting in the generation of dolichol. The size of the dolichol varies, depending on the number of isoprene units incorporated. In eukaryotes, dolichols are synthesized as a mixture of four or more different lengths. Their biosynthesis is predicted to occur in the endoplasmic reticulum, where dolichols play an essential role in protein glycosylation. In this study, we have developed a selection of aptamers targeting dolichols and enhanced their specificity by incorporating fatty acids for negative selection. One aptamer showed high enrichment and specificity for linear polyisoprenoids containing at least one oxygen atom, such as an alcohol or aldehyde, in the α-isoprene unit. The selected aptamer proved to be a valuable tool for the subcellular localization of polyisoprenoids in the malaria parasite. To the best of our knowledge, this is the first time that polyisoprenoids have been localized within a cell using aptamer-based imaging techniques.

Identification of non-conventional small molecule degraders and stabilizers of squalene synthase

Squalene synthase (SQS) is an essential enzyme in the mevalonate pathway, which controls cholesterol biosynthesis and homeostasis. Although catalytic inhibitors of SQS have been developed, none have been approved for therapeutic use so far. Herein we sought to develop SQS degraders using targeted protein degradation (TPD) to lower overall cellular cholesterol content. We found that KY02111, a small molecule ligand of SQS, selectively causes SQS to degrade in a proteasome-dependent manner. Unexpectedly, compounds based on the same scaffold linked to E3 ligase recruiting ligands led to SQS stabilization. Proteomic analysis found KY02111 to reduce only the levels of SQS, while lipidomic analysis determined that KY02111-induced degradation lowered cellular cholesteryl ester content. Stabilizers shielded SQS from its natural turnover without recruiting their matching E3 ligase or affecting enzymatic target activity. Our work shows that degradation of SQS is possible despite a challenging biological setting and provides the first chemical tools to degrade and stabilize SQS.

Precursor-Directed Biosynthesis and Biological Testing of omega-Alicyclic- and neo-Branched Tunicamycin N-Acyl Variants

Tunicamycins (TUNs) are Streptomyces-derived natural products, widely used to block protein N-glycosylation in eukaryotes or cell wall biosynthesis in bacteria. Modified or synthetic TUN analogues that uncouple these activities have considerable potential as novel mode-of-action antibacterial agents. Chemically modified TUNs reported previously with attenuated activity on yeast have pinpointed eukaryotic-specific chemophores in the uridyl group and the N-acyl chain length and terminal branching pattern. A small molecule screen of fatty acid biosynthetic primers identified several novel alicyclic- and neo-branched TUN N-acyl variants, with primer incorporation at the terminal omega-acyl position. TUNs with unique 5- and 6-carbon ω-cycloalkane and ω-cycloalkene acyl chains are produced under fermentation and in yields comparable with the native TUN. The purification, structural assignments, and the comparable antimicrobial properties of 15 of these compounds are reported, greatly extending the structural diversity of this class of compounds for potential medicinal and agricultural applications.

Case analysis of epilepsy, neurodevelopmental disorder, and motor disorders associated with mutations in the dehydrodolichyl diphosphate synthase gene

The objective of this study is to analyze the role of dehydrodolichyl diphosphate synthase (DHDDS), a crucial enzyme in the mevalonate pathway, and its encoded mutations in the onset of developmental delay and seizures, with or without movement abnormalities. Its genotype-phenotype characteristics are still inconclusive. We analyzed the clinical characteristics of epilepsy, and neurodevelopmental and motor disorders related to DHDDS gene mutations and report the genotype-phenotype characteristics of a child with epilepsy caused by DHDDS gene mutation, providing a summary and a statistical analysis of epilepsy cases associated with DHDDS gene mutation up until February 2022.

METHODS

Using "DHDDS; epilepsy; neurodevelopmental disorder" as the keywords, the literature relevant to DHDDS gene mutations up until February 2022 was reviewed. A total of 25 cases were retrieved, among which 21 cases with complete data were included in the chi-squared test. The clinical characteristics of DHDDS gene-related cases were summarized and analyzed.

RESULTS

The onset of epilepsy caused by mutations of the DHDDS gene typically occurs during infancy. Predominantly, the mutation occurs in the locus of c.632G>A p.R211Q. Myoclonus is frequently the initial manifestation of epilepsy; it frequently coexists with neurodevelopmental disorder and intellectual disability, and patients have no specific type of motor disorder. Cranial magnetic resonance imaging (MRI) reveals no abnormalities, whereas electroencephalogram (EEG) frequently exhibits abnormalities. Valproic acid (VPA) yields good curative effects.

CONCLUSION

Mutations in the DHDDS gene are associated with congenital glycosylation disorder, autosomal recessive retinitis pigmentosa, and epilepsy. According to statistical analysis using the chi-squared test, for pediatric patients with mutations in this gene locus, most of the epilepsy types are myoclonic epilepsies with intellectual disability and neurodevelopmental disorders. They have normal brain MRIs and abnormal EEGs. VPA produces beneficial therapeutic results and the differences are all statistically significant. The current diagnosis still relies on next-generation sequencing or whole-exome sequencing.

Unveiling the Differences in Signaling and Regulatory Mechanisms between Dopamine D2 and D3 Receptors and Their Impact on Behavioral Sensitization

Dopamine receptors are classified into five subtypes, with D2R and D3R playing a crucial role in regulating mood, motivation, reward, and movement. Whereas D2R are distributed widely across the brain, including regions responsible for motor functions, D3R are primarily found in specific areas related to cognitive and emotional functions, such as the nucleus accumbens, limbic system, and prefrontal cortex. Despite their high sequence homology and similar signaling pathways, D2R and D3R have distinct regulatory properties involving desensitization, endocytosis, posttranslational modification, and interactions with other cellular components. In vivo, D3R is closely associated with behavioral sensitization, which leads to increased dopaminergic responses. Behavioral sensitization is believed to result from D3R desensitization, which removes the inhibitory effect of D3R on related behaviors. Whereas D2R maintains continuous signal transduction through agonist-induced receptor phosphorylation, arrestin recruitment, and endocytosis, which recycle and resensitize desensitized receptors, D3R rarely undergoes agonist-induced endocytosis and instead is desensitized after repeated agonist exposure. In addition, D3R undergoes more extensive posttranslational modifications, such as glycosylation and palmitoylation, which are needed for its desensitization. Overall, a series of biochemical settings more closely related to D3R could be linked to D3R-mediated behavioral sensitization.

Ichthyosis

The ichthyoses are a large, heterogeneous group of skin cornification disorders. They can be inherited or acquired, and result in defective keratinocyte differentiation and abnormal epidermal barrier formation. The resultant skin barrier dysfunction leads to increased transepidermal water loss and inflammation. Disordered cornification is clinically characterized by skin scaling with various degrees of thickening, desquamation (peeling) and erythema (redness). Regardless of the type of ichthyosis, many patients suffer from itching, recurrent infections, sweating impairment (hypohidrosis) with heat intolerance, and diverse ocular, hearing and nutritional complications that should be monitored periodically. The characteristic clinical features are considered to be a homeostatic attempt to repair the skin barrier, but heterogeneous clinical presentation and imperfect phenotype-genotype correlation hinder diagnosis. An accurate molecular diagnosis is, however, crucial for predicting prognosis and providing appropriate genetic counselling. Most ichthyoses severely affect patient quality of life and, in severe forms, may cause considerable disability and even death. So far, treatment provides only symptomatic relief. It is lifelong, expensive, time-consuming, and often provides disappointing results. A better understanding of the molecular mechanisms that underlie these conditions is essential for designing pathogenesis-driven and patient-tailored innovative therapeutic solutions.

Vertebrate Animal Models of RP59: Current Status and Future Prospects

Retinitis pigmentosa-59 (RP59) is a rare, recessive form of RP, caused by mutations in the gene encoding DHDDS (dehydrodolichyl diphosphate synthase). DHDDS forms a heterotetrameric complex with Nogo-B receptor (NgBR; gene NUS1) to form a cis-prenyltransferase (CPT) enzyme complex, which is required for the synthesis of dolichol, which in turn is required for protein N-glycosylation as well as other glycosylation reactions in eukaryotic cells. Herein, we review the published phenotypic characteristics of RP59 models extant, with an emphasis on their ocular phenotypes, based primarily upon knock-in of known RP59-associated DHDDS mutations as well as cell type- and tissue-specific knockout of DHDDS alleles in mice. We also briefly review findings in RP59 patients with retinal disease and other patients with DHDDS mutations causing epilepsy and other neurologic disease. We discuss these findings in the context of addressing "knowledge gaps" in our current understanding of the underlying pathobiology mechanism of RP59, as well as their potential utility for developing therapeutic interventions to block the onset or to dampen the severity or progression of RP59.

Acetazolamide treatment in late onset CDG type 1 due to biallelic pathogenic DHDDS variants

Pathogenic variants in DHDDS have been associated with either autosomal recessive retinitis pigmentosa or DHDDS-CDG. Heterozygous variants in DHDDS have been described in patients with a progressive neurodegenerative disease. Here we report on an individual presenting with a multisystem CDG phenotype who was diagnosed with known homozygous pathogenic DHDDS variants, previously associated with isolated retinitis pigmentosa.

An adult Ashkenazi Jewish female developed multiple symptoms of late onset type 1 CDG including seizures, ataxia, protein losing enteropathy, tremor, and titubation in association with elevated mono-oligo/di-oligo transferrin ratio in blood, and classic retinitis pigmentosa. She was diagnosed by whole exome sequencing with the common Ashkenazi Jewish, homozygous p.K42E variants in DHDDS. She was started on Acetazolamide and responded well to the treatment which improved her titubation, tremor, and generalized edema.

Reviewing the literature, families with DHDDS variants and multisystem presentation were different from our patient's presentation in terms of clinical manifestations, severity, genetic defect, and mode of inheritance. In previously reported patients with neurologic symptoms including seizures, movement abnormalities, and global development delay, the phenotype was caused by heterozygous pathogenic variants in DHDDS. The infant who was reported with a multisystem phenotype and fatal type 1 CDG had compound heterozygosity for a nonsense and a splice site variant in DHDDS, resulting in DHDDS-CDG.

The discovery of the novel phenotype associated with the common p.K42E pathogenic variant in DHDDS expands the spectrum of CDG and further enhances our understanding on the role of DHDDS in glycosylation beyond the retina.

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We report on the first individual carrying homozygous p.K42E variants in DHDDS associated with protein losing enteropathy, seizures, and ataxia.

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We observed familial variability in association with p.K42E and progressive ataxia in siblings with in DHDDS-CDG.

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The novel DHDDS-CDG patient phenotype broadens the current spectrum of CDG.

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Acetazolamide was successful in treating titubation and recurrent edema in our DHDDS-CDG patient of Ashkenazi Jewish descent.

De novo DHDDS variants cause a neurodevelopmental and neurodegenerative disorder with myoclonus

Subcellular membrane systems are highly enriched in dolichol, whose role in organelle homeostasis and endosomal-lysosomal pathway remains largely unclear besides being involved in protein glycosylation. DHDDS encodes for the catalytic subunit (DHDDS) of the enzyme cis-prenyltransferase (cis-PTase), involved in dolichol biosynthesis and dolichol-dependent protein glycosylation in the endoplasmic reticulum. An autosomal recessive form of retinitis pigmentosa (retinitis pigmentosa 59) has been associated with a recurrent DHDDS variant. Moreover, two recurring de novo substitutions were detected in a few cases presenting with neurodevelopmental disorder, epilepsy and movement disorder. We evaluated a large cohort of patients (n = 25) with de novo pathogenic variants in DHDDS and provided the first systematic description of the clinical features and long-term outcome of this new neurodevelopmental and neurodegenerative disorder. The functional impact of the identified variants was explored by yeast complementation system and enzymatic assay. Patients presented during infancy or childhood with a variable association of neurodevelopmental disorder, generalized epilepsy, action myoclonus/cortical tremor and ataxia. Later in the disease course, they experienced a slow neurological decline with the emergence of hyperkinetic and/or hypokinetic movement disorder, cognitive deterioration and psychiatric disturbances. Storage of lipidic material and altered lysosomes were detected in myelinated fibres and fibroblasts, suggesting a dysfunction of the lysosomal enzymatic scavenger machinery. Serum glycoprotein hypoglycosylation was not detected and, in contrast to retinitis pigmentosa and other congenital disorders of glycosylation involving dolichol metabolism, the urinary dolichol D18/D19 ratio was normal. Mapping the disease-causing variants into the protein structure revealed that most of them clustered around the active site of the DHDDS subunit. Functional studies using yeast complementation assay and in vitro activity measurements confirmed that these changes affected the catalytic activity of the cis-PTase and showed growth defect in yeast complementation system as compared with the wild-type enzyme and retinitis pigmentosa-associated protein. In conclusion, we characterized a distinctive neurodegenerative disorder due to de novo DHDDS variants, which clinically belongs to the spectrum of genetic progressive encephalopathies with myoclonus. Clinical and biochemical data from this cohort depicted a condition at the intersection of congenital disorders of glycosylation and inherited storage diseases with several features akin to of progressive myoclonus epilepsy such as neuronal ceroid lipofuscinosis and other lysosomal disorders.

Advances in understanding N-glycosylation structure, function, and regulation in health and disease

N-linked glycosylation is a post-translational modification crucial for membrane protein folding, stability and other cellular functions. Alteration of membrane protein N-glycans is implicated in wide range of pathological conditions including cancer metastasis, chronic inflammatory diseases, and viral pathogenesis. Even though the roles of N-glycans have been studied extensively, our knowledge of their mechanisms remains unclear due to the lack of detailed structural analysis of the N-glycome. Mapping the N-glycome landscape will open new avenues to explore disease mechanisms and identify novel therapeutic targets. This review discusses the diverse structure of N-linked glycans, the function and regulation of N-glycosylation in health and disease, and ends with a focus on recent approaches to target N-glycans in rheumatoid arthritis and cancer metastasis.

A central role for polyprenol reductase in plant dolichol biosynthesis

Dolichol is an essential polyisoprenoid within the endoplasmic reticulum of all eukaryotes. It serves as a membrane bound anchor onto which N-glycans are assembled prior to being transferred to nascent polypeptides, many of which enter the secretory pathway. Historically, it has been posited that the accumulation of dolichol represents the 'rate-limiting' step in the evolutionary conserved process of N-glycosylation, which ultimately affects the efficacy of approximately one fifth of the entire eukaryotic proteome. Therefore, this study aimed to enhance dolichol accumulation by manipulating the enzymes involved in its biosynthesis using an established Nicotiana benthamiana platform. Co-expression of a Solanum lycopersicum (tomato) cis-prenyltransferase (CPT) and its cognate partner protein, CPT binding protein (CPTBP), that catalyze the antepenultimate step in dolichol biosynthesis led to a 400-fold increase in the levels of long-chain polyprenols but resulted in only modest increases in dolichol accumulation. However, when combined with a newly characterized tomato polyprenol reductase, dolichol biosynthesis was enhanced by approximately 20-fold. We provide further evidence that in the aquatic macrophyte, Lemna gibba, dolichol is derived exclusively from the mevalonic acid (MVA) pathway with little participation from the evolutionary co-adopted non-MVA pathway. Taken together these results indicate that to effectively enhance the in planta accumulation of dolichol, coordinated synthesis and reduction of polyprenol to dolichol, is strictly required.

Chemo-enzymatic synthesis of the ALG1-CDG biomarker and evaluation of its immunogenicity

Congenital disorders of glycosylation (CDG) are a growing group diseases that result from defects in genes involved in glycan biosynthesis pathways. One tetrasaccharide, i.e., Neu5Ac-α2, 6-Gal-β1, 4-GlcNAc-β1, 4-GlcNAc, was recently reported as the biomarker of ALG1-CDG, the disease caused by ALG1 deficiency. To develop a novel diagnostic method for ALG1-CDG, chemo-enzymatic synthesis of the tetrasaccharide biomarker linked to phytanyl phosphate and the biomarker's immune stimulation were investigated in this study. The immunization study using liposomes bearing phytanyl-linked tetrasaccharide revealed that they stimulated a moderate immune response. The induced antibody showed strong binding specificity for the ALG1-CDG biomarker, indicating its potential in medical applications.

CbrA Mediates Colicin M Resistance in Escherichia coli through Modification of Undecaprenyl-Phosphate-Linked Peptidoglycan Precursors

Colicin M is an enzymatic bacteriocin produced by some Escherichia coli strains which provokes cell lysis of competitor strains by hydrolysis of the cell wall peptidoglycan undecaprenyl-PP-MurNAc(-pentapeptide)-GlcNAc (lipid II) precursor. The overexpression of a gene, cbrA (formerly yidS), was shown to protect E. coli cells from the deleterious effects of this colicin, but the underlying resistance mechanism was not established. We report here that a major structural modification of the undecaprenyl-phosphate carrier lipid and of its derivatives occurred in membranes of CbrA-overexpressing cells, which explains the acquisition of resistance toward this bacteriocin. Indeed, a main fraction of these lipids, including the lipid II peptidoglycan precursor, now displayed a saturated isoprene unit at the α-position, i.e., the unit closest to the colicin M cleavage site. Only unsaturated forms of these lipids were normally detectable in wild-type cells. In vitro and in vivo assays showed that colicin M did not hydrolyze α-saturated lipid II, clearly identifying this substrate modification as the resistance mechanism. These saturated forms of undecaprenyl-phosphate and lipid II remained substrates of the different enzymes participating in peptidoglycan biosynthesis and carrier lipid recycling, allowing this colicin M-resistance mechanism to occur without affecting this essential pathway.IMPORTANCE Overexpression of the chromosomal cbrA gene allows E. coli to resist colicin M (ColM), a bacteriocin specifically hydrolyzing the undecaprenyl-PP-MurNAc(-pentapeptide)-GlcNAc (lipid II) peptidoglycan precursor of targeted cells. This resistance results from a CbrA-dependent modification of the precursor structure, i.e., reduction of the α-isoprenyl bond of C₅₅-carrier lipid moiety that is proximal to ColM cleavage site. This modification, observed here for the first time in eubacteria, annihilates the ColM activity without affecting peptidoglycan biogenesis. These data, which further increase our knowledge of the substrate specificity of this colicin, highlight the capability of E. coli to generate reduced forms of C₅₅-carrier lipid and its derivatives. Whether the function of this modification is only relevant with respect to ColM resistance is now questioned.

Metabolomics profiling reveals new aspects of dolichol biosynthesis in Plasmodium falciparum

The cis-polyisoprenoid lipids namely polyprenols, dolichols and their derivatives are linear polymers of several isoprene units. In eukaryotes, polyprenols and dolichols are synthesized as a mixture of four or more homologues of different length with one or two predominant species with sizes varying among organisms. Interestingly, co-occurrence of polyprenols and dolichols, i.e. detection of a dolichol along with significant levels of its precursor polyprenol, are unusual in eukaryotic cells. Our metabolomics studies revealed that cis-polyisoprenoids are more diverse in the malaria parasite Plasmodium falciparum than previously postulated as we uncovered active de novo biosynthesis and substantial levels of accumulation of polyprenols and dolichols of 15 to 19 isoprene units. A distinctive polyprenol and dolichol profile both within the intraerythrocytic asexual cycle and between asexual and gametocyte stages was observed suggesting that cis-polyisoprenoid biosynthesis changes throughout parasite’s development. Moreover, we confirmed the presence of an active cis-prenyltransferase (PfCPT) and that dolichol biosynthesis occurs via reduction of the polyprenol to dolichol by an active polyprenol reductase (PfPPRD) in the malaria parasite.

Glycan Mimetics from Natural Products: New Therapeutic Opportunities for Neurodegenerative Disease

Neurodegenerative diseases (NDs) affect millions of people worldwide. Characterized by the functional loss and death of neurons, NDs lead to symptoms (dementia and seizures) that affect the daily lives of patients. In spite of extensive research into NDs, the number of approved drugs for their treatment remains limited. There is therefore an urgent need to develop new approaches for the prevention and treatment of NDs. Glycans (carbohydrate chains) are ubiquitous, abundant, and structural complex natural biopolymers. Glycans often covalently attach to proteins and lipids to regulate cellular recognition, adhesion, and signaling. The importance of glycans in both the developing and mature nervous system is well characterized. Moreover, glycan dysregulation has been observed in NDs such as Alzheimer's disease (AD), Huntington's disease (HD), Parkinson's disease (PD), multiple sclerosis (MS), and amyotrophic lateral sclerosis (ALS). Therefore, glycans are promising but underexploited therapeutic targets. In this review, we summarize the current understanding of glycans in NDs. We also discuss a number of natural products that functionally mimic glycans to protect neurons, which therefore represent promising new therapeutic approaches for patients with NDs.

Crosstalk between protein N-glycosylation and lipid metabolism in Saccharomyces cerevisiae

The endoplasmic reticulum (ER) is a multi functional organelle and plays a crucial role in protein folding and lipid biosynthesis. The SEC59 gene encodes dolichol kinase, required for protein glycosylation in the ER. The mutation of sec59-1 caused a protein N-glycosylation defect mediated ER stress resulting in increased levels of phospholipid, neutral lipid and sterol, whereas growth was reduced. In the sec59-1∆ cell, the N-glycosylation of vacuolar carboxy peptidase-Y (CPY) was significantly reduced; whereas the ER stress marker Kar2p and unfolded protein response (UPR) were significantly increased. Increased levels of Triacylglycerol (TAG), sterol ester (SE), and lipid droplets (LD) could be attributed to up-regulation of DPP1, LRO1, and ARE2 in the sec 59-1∆ cell. Also, the diacylglycerol (DAG), sterol (STE), and free fatty acids (FFA) levels were significantly increased, whereas the genes involved in peroxisome biogenesis and Pex3-EGFP levels were reduced when compared to the wild-type. The microarray data also revealed increased expression of genes involved in phospholipid, TAG, fatty acid, sterol synthesis, and phospholipid transport resulting in dysregulation of lipid homeostasis in the sec59-1∆ cell. We conclude that SEC59 dependent N-glycosylation is required for lipid homeostasis, peroxisome biogenesis, and ER protein quality control.

High N-glycan multiplicity is critical for neuronal adhesion and sensitizes the developing cerebellum to N-glycosylation defect

Proper brain development relies highly on protein N-glycosylation to sustain neuronal migration, axon guidance and synaptic physiology. Impairing the N-glycosylation pathway at early steps produces broad neurological symptoms identified in congenital disorders of glycosylation. However, little is known about the molecular mechanisms underlying these defects. We generated a cerebellum specific knockout mouse for Srd5a3, a gene involved in the initiation of N-glycosylation. In addition to motor coordination defects and abnormal granule cell development, Srd5a3 deletion causes mild N-glycosylation impairment without significantly altering ER homeostasis. Using proteomic approaches, we identified that Srd5a3 loss affects a subset of glycoproteins with high N-glycans multiplicity per protein and decreased protein abundance or N-glycosylation level. As IgSF-CAM adhesion proteins are critical for neuron adhesion and highly N-glycosylated, we observed impaired IgSF-CAM-mediated neurite outgrowth and axon guidance in Srd5a3 mutant cerebellum. Our results link high N-glycan multiplicity to fine-tuned neural cell adhesion during mammalian brain development.

Neuromelanin organelles are specialized autolysosomes that accumulate undegraded proteins and lipids in aging human brain and are likely involved in Parkinson's disease

During aging, neuronal organelles filled with neuromelanin (a dark-brown pigment) and lipid bodies accumulate in the brain, particularly in the substantia nigra, a region targeted in Parkinson's disease. We have investigated protein and lipid systems involved in the formation of these organelles and in the synthesis of the neuromelanin of human substantia nigra. Membrane and matrix proteins characteristic of lysosomes were found in neuromelanin-containing organelles at a lower number than in typical lysosomes, indicating a reduced enzymatic activity and likely impaired capacity for lysosomal and autophagosomal fusion. The presence of proteins involved in lipid transport may explain the accumulation of lipid bodies in the organelle and the lipid component in neuromelanin structure. The major lipids observed in lipid bodies of the organelle are dolichols with lower amounts of other lipids. Proteins of aggregation and degradation pathways were present, suggesting a role for accumulation by this organelle when the ubiquitin-proteasome system is inadequate. The presence of proteins associated with aging and storage diseases may reflect impaired autophagic degradation or impaired function of lysosomal enzymes. The identification of typical autophagy proteins and double membranes demonstrates the organelle's autophagic nature and indicates that it has engulfed neuromelanin precursors from the cytosol. Based on these data, it appears that the neuromelanin-containing organelle has a very slow turnover during the life of a neuron and represents an intracellular compartment of final destination for numerous molecules not degraded by other systems.

Complex Phenotypes in Inborn Errors of Metabolism: Overlapping Presentations in Congenital Disorders of Glycosylation and Mitochondrial Disorders

Congenital disorders of glycosylation (CDG) and mitochondrial disorders have overlapping clinical features, including central nervous system, cardiac, gastrointestinal, hepatic, muscular, endocrine, and psychiatric disease. Specific abnormalities orienting the clinician toward the right diagnostic approach include abnormal fat distribution, coagulation abnormalities, together with anticoagulation abnormalities, hyperinsulinism, and congenital malformations in CDG. Diabetes, sensorineural deafness, and depression are very rare in CDG but common in mitochondrial disease. Chronic lactic acidosis is highly suggestive of mitochondrial dysfunction. Serum transferrin isoform analysis is specific for glycosylation abnormalities but not abnormal in all types of CDG.

Cardiac complications of congenital disorders of glycosylation (CDG): a systematic review of the literature

Congenital disorders of glycosylation (CDG) are inborn errors of metabolism due to protein and lipid hypoglycosylation. This rapidly growing family of genetic diseases comprises 103 CDG types, with a broad phenotypic diversity ranging from mild to severe poly-organ -system dysfunction. This literature review summarizes cardiac involvement, reported in 20% of CDG. CDG with cardiac involvement were divided according to the associated type of glycosylation: N-glycosylation, O-glycosylation, dolichol synthesis, glycosylphosphatidylinositol (GPI)-anchor biosynthesis, COG complex, V-ATPase complex, and other glycosylation pathways. The aim of this review was to document and interpret the incidence of heart disease in CDG patients. Heart disorders were grouped into cardiomyopathies, structural defects, and arrhythmogenic disorders. This work may contribute to improved early management of cardiac complications in CDG.

Crystal structure and biochemical characterization of the transmembrane PAP2 type phosphatidylglycerol phosphate phosphatase from Bacillus subtilis

Type 2 phosphatidic acid phosphatases (PAP2s) can be either soluble or integral membrane enzymes. In bacteria, integral membrane PAP2s play major roles in the metabolisms of glycerophospholipids, undecaprenyl-phosphate (C55-P) lipid carrier and lipopolysaccharides. By in vivo functional experiments and biochemical characterization we show that the membrane PAP2 coded by the Bacillus subtilis yodM gene is the principal phosphatidylglycerol phosphate (PGP) phosphatase of B. subtilis. We also confirm that this enzyme, renamed bsPgpB, has a weaker activity on C55-PP. Moreover, we solved the crystal structure of bsPgpB at 2.25 Å resolution, with tungstate (a phosphate analog) in the active site. The structure reveals two lipid chains in the active site vicinity, allowing for PGP substrate modeling and molecular dynamic simulation. Site-directed mutagenesis confirmed the residues important for substrate specificity, providing a basis for predicting the lipids preferentially dephosphorylated by membrane PAP2s.

Lipid sugar carriers at the extremes: The phosphodolichols Archaea use in N-glycosylation

N-glycosylation, a post-translational modification whereby glycans are covalently linked to select Asn residues of target proteins, occurs in all three domains of life. Across evolution, the N-linked glycans are initially assembled on phosphorylated cytoplasmically-oriented polyisoprenoids, with polyprenol (mainly C₅₅ undecaprenol) fulfilling this role in Bacteria and dolichol assuming this function in Eukarya and Archaea. The eukaryal and archaeal versions of dolichol can, however, be distinguished on the basis of their length, degree of saturation and by other traits. As is true for many facets of their biology, Archaea, best known in their capacity as extremophiles, present unique approaches for synthesizing phosphodolichols. At the same time, general insight into the assembly and processing of glycan-bearing phosphodolichols has come from studies of the archaeal enzymes responsible. In this review, these and other aspects of archaeal phosphodolichol biology are addressed.

Early evolution of polyisoprenol biosynthesis and the origin of cell walls

After being a matter of hot debate for years, the presence of lipid membranes in the last common ancestor of extant organisms (i.e., the cenancestor) now begins to be generally accepted. By contrast, cenancestral cell walls have attracted less attention, probably owing to the large diversity of cell walls that exist in the three domains of life. Many prokaryotic cell walls, however, are synthesized using glycosylation pathways with similar polyisoprenol lipid carriers and topology (i.e., orientation across the cell membranes). Here, we provide the first systematic phylogenomic report on the polyisoprenol biosynthesis pathways in the three domains of life. This study shows that, whereas the last steps of the polyisoprenol biosynthesis are unique to the respective domain of life of which they are characteristic, the enzymes required for basic unsaturated polyisoprenol synthesis can be traced back to the respective last common ancestor of each of the three domains of life. As a result, regardless of the topology of the tree of life that may be considered, the most parsimonious hypothesis is that these enzymes were inherited in modern lineages from the cenancestor. This observation supports the presence of an enzymatic mechanism to synthesize unsaturated polyisoprenols in the cenancestor and, since these molecules are notorious lipid carriers in glycosylation pathways involved in the synthesis of a wide diversity of prokaryotic cell walls, it provides the first indirect evidence of the existence of a hypothetical unknown cell wall synthesis mechanism in the cenancestor.

A theoretical estimate for nucleotide sugar demand towards Chinese Hamster Ovary cellular glycosylation

Glycosylation greatly influences the safety and efficacy of many of the highest-selling recombinant therapeutic proteins (rTPs). In order to define optimal cell culture feeding strategies that control rTP glycosylation, it is necessary to know how nucleotide sugars (NSs) are consumed towards host cell and rTP glycosylation. Here, we present a theoretical framework that integrates the reported glycoproteome of CHO cells, the number of N-linked and O-GalNAc glycosylation sites on individual host cell proteins (HCPs), and the carbohydrate content of CHO glycosphingolipids to estimate the demand of NSs towards CHO cell glycosylation. We have identified the most abundant N-linked and O-GalNAc CHO glycoproteins, obtained the weighted frequency of N-linked and O-GalNAc glycosites across the CHO cell proteome, and have derived stoichiometric coefficients for NS consumption towards CHO cell glycosylation. By combining the obtained stoichiometric coefficients with previously reported data for specific growth and productivity of CHO cells, we observe that the demand of NSs towards glycosylation is significant and, thus, is required to better understand the burden of glycosylation on cellular metabolism. The estimated demand of NSs towards CHO cell glycosylation can be used to rationally design feeding strategies that ensure optimal and consistent rTP glycosylation.

Pharmacological research on natural substances in Latvia: Focus on lunasin, betulin, polyprenol and phlorizin

In this concise review the current research in plant bioactive compound studies in Latvia is described. The paper summarizes recent studies on substances from edible plants (e.g., cereals and apples) or their synthetic analogues, such as peptide lunasin, as well as substances isolated from inedible plants (e.g., birch and conifer), such as pentacyclic triterpenes (e.g., betulin, betulinic acid, and lupeol) and polyprenols. Latvian researchers have been first to demonstrate the presence of lunasin in triticale and oats. Additionally, the impact of genotype on the levels of lunasin in cereals was shown. Pharmacological studies have revealed effects of lunasin and synthetic triterpenes on the central nervous system in rodents. We were first to show that synthetic lunasin causes a marked neuroleptic/cataleptic effect and that betulin antagonizes bicuculline-induced seizures (a GABA A receptor antagonist). Studies on the mechanisms of action showed that lunasin binds to dopamine D1 receptors and betulin binds to melanocortin and gamma-aminobutyric acid A receptors therefore we suggest that these receptors play an essential role in lunasin's and betulin's central effects. Recent studies on conifer polyprenols demonstrated the ability of polyprenols to prevent statin-induced muscle weakness in a rat model. Another study on plant compounds has demonstrated the anti-hyperglycemic activity of phlorizin-containing unripe apple pomace in healthy volunteers. In summary, research into plant-derived compounds in Latvia has been focused on fractionating, isolating and characterizing of lunasin, triterpenes, polyprenols and phlorizin using in vitro, and in vivo assays, and human observational studies.

Site-Specific Quantification of Surface N-Glycoproteins in Statin-Treated Liver Cells

The frequent modification of cell-surface proteins by N-linked glycans is known to be correlated with many biological processes. Aberrant glycosylation on surface proteins is associated with different cellular statuses and disease progression. However, it is extraordinarily challenging to comprehensively and site-specifically analyze glycoproteins located only on the cell surface. Currently mass spectrometry (MS)-based proteomics provides the possibility to analyze the N-glycoproteome, but effective separation and enrichment methods are required for the analysis of surface glycoproteins prior to MS measurement. The introduction of bio-orthogonal groups into proteins accelerates research in the robust visualization, identification, and quantification of proteins. Here we have comprehensively evaluated different sugar analogs in the analysis of cell-surface N-glycoproteins by combining copper-free click chemistry and MS-based proteomics. Comparison of three sugar analogs, N-azidoacetylgalactosamine (GalNAz), N-azidoacetylglucosamine (GlcNAz), and N-azidoacetylmannosamine (ManNAz), showed that metabolic labeling with GalNAz resulted in the greatest number of glycoproteins and glycosylation sites in biological duplicate experiments. GalNAz was then employed for the quantification experiment in statin-treated HepG2 liver cells, and 280 unique N-glycosylated sites were quantified from 168 surface proteins. The quantification results demonstrated that many glycosylation sites on surface proteins were down-regulated in statin-treated cells compared to untreated cells because statin prevents the synthesis of dolichol, which is essential for the formation of dolichol-linked precursor oligosaccharides. Several glycosylation sites in proteins that participate in the Alzheimer's disease pathway were down-regulated. This method can be extensively applied for the global analysis of the cell-surface N-glycoproteome.

Identification and characterization of transcriptional control region of the human beta 1,4-mannosyltransferase gene

All asparagine-linked glycans (N-glycans) on the eukaryotic glycoproteins are primarily derived from dolichol-linked oligosaccharides (DLO), synthesized on the rough endoplasmic reticulum membrane. We have previously reported cloning and identification of the human gene, HMT-1, which encodes chitobiosyldiphosphodolichol beta-mannosyltransferase (β1,4-MT) involved in the early assembly of DLO. Considering that N-glycosylation is one of the most ubiquitous post-translational modifications for many eukaryotic proteins, the HMT-1 could be postulated as one of the housekeeping genes, but its transcriptional regulation remains to be investigated. Here we screened a 1 kb region upstream from HMT-1 open reading frame (ORF) for transcriptionally regulatory sequences by using chloramphenicol acetyl transferase (CAT) assay, and found that the region from -33 to -1 positions might act in HMT-1 transcription at basal level and that the region from -200 to -42 should regulate its transcription either positively or negatively. In addition, results with CAT assays suggested the possibility that two GATA-1 motifs and an Sp1 motif within a 200 bp region upstream from HMT-1 ORF might significantly upregulate HMT-1 transcription. On the contrary, the observations obtained from site-directed mutational analyses revealed that an NF-1/AP-2 overlapping motif located at -148 to -134 positions should serve as a strong silencer. The control of the HMT-1 transcription by these motifs resided within the 200 bp region could partially explain the variation of expression level among various human tissues, suggesting availability and importance of this region for regulatory role in HMT-1 expression.

In vitro and in vivo downregulation of the ATP binding cassette transporter B1 by the HMG-CoA reductase inhibitor simvastatin

Extrusion of chemotherapeutics by ATP-binding cassette (ABC) transporters like ABCB1 (P-glycoprotein) represents a crucial mechanism of multidrug resistance in cancer therapy. We have previously shown that the 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor simvastatin directly inhibits ABCB1, alters the glycosylation of the transporter, and enhances the intracellular accumulation of doxorubicin with subsequent anti-cancer action. Here, we show that simvastatin reduces endogenous dolichol levels and ABCB1 in human neuroblastoma SH-SY5Y cells. Coapplication with dolichol prevents the downregulation of the ABCB1 transporter. Importantly, dolichol also attenuated simvastatin-induced apoptosis, unmasking involvement of unfolded protein response. Direct monitoring of the fluorescent fusion protein YFP-ABCB1 further confirms concentration-dependent reduction of ABCB1 in HEK293 cells by simvastatin. In simvastatin-treated murine xenografts, ABCB1 was also reduced in the liver and rhabdomyosarcoma but did not reach significance in neuroblastoma. Nevertheless, the in vivo anti-cancer effects of simvastatin are corroborated by increased apoptosis in tumor tissues. These findings provide experimental evidence for usage of simvastatin in novel chemotherapeutic regimens and link dolichol depletion to simvastatin-induced anti-cancer activity.

Hamster SRD5A3 lacks steroid 5α-reductase activity in vitro

According to current knowledge, two steroid 5α-reductases, designated type 1 (SRD5A1) and type 2 (SRD5A2), are present in all species examined to date. These isozymes play a central role in steroid hormone physiology by catalyzing the reduction of 3-keto-4-ene-steroids into more active 5α-reduced derivatives, including the conversion of testosterone (T) to dihydrotestosterone (DHT). A third 5α-reductase (SRD5A3, -type 3), which is overexpressed in hormone-refractory prostate cancer cells, has been identified; however, its enzymatic characteristics are practically unknown. Here, we isolated a cDNA encoding hamster Srd5a3 (hSrd5a3) and performed functional metabolic assays to investigate its biochemical properties. The cloned cDNA encodes a 330 amino acid protein that is 87% identical to the homologous protein in mice and 78% to that in humans. However, hSrd5a3 exhibits low sequence homology with its counterparts hSrd5a1 (19%) and hSrd5a2 (17%). A fusion protein consisting of hSrd5a3 and green fluorescent protein provided evidence for cytoplasmic localization in transfected mammalian cells. Real-time PCR analysis revealed that, Srd5a3 mRNA was present in nearly all hamster tissues, with high expression in the cerebellum, Harderian gland and testis. Functional assays expressing hSrd5a3 cDNA in HEK-293 cells revealed that this isozyme is unable to reduce T into DHT. Further expression assays confirmed that similar to testosterone, progesterone, androstenedione and corticosterone are not reduced by hSrd5a3 or human SRD5A3. Together, these results indicate that hSrd5a3 lacks the catalytic activity to transform 3-keto-4-ene-compounds; therefore 5α-reductase type 3 may not be involved in 5α-reduction of steroids.

Genetic defects in dolichol metabolism

Congenital disorders of glycosylation (CDG) comprise a group of inborn errors of metabolism with abnormal glycosylation of proteins and lipids. Patients with defective protein N-glycosylation are identified in routine metabolic screening via analysis of serum transferrin glycosylation. Defects in the assembly of the dolichol linked Glc(3)Man(9)GlcNAc(2) glycan and its transfer to proteins lead to the (partial) absence of complete glycans on proteins. These defects are called CDG-I and are located in the endoplasmic reticulum (ER) or cytoplasm. Defects in the subsequent processing of protein bound glycans result in the presence of truncated glycans on proteins. These defects are called CDG-II and the enzymes involved are located mainly in the Golgi apparatus. In recent years, human defects have been identified in dolichol biosynthesis genes within the group of CDG-I patients. This has increased interest in dolichol metabolism, has resulted in specific recognizable clinical symptoms in CDG-I and has offered new mechanistic insights in dolichol biosynthesis. We here review its biosynthetic pathways, the clinical and biochemical phenotypes in dolichol-related CDG defects, up to the formation of dolichyl-P-mannose (Dol-P-Man), and discuss existing evidence of regulatory networks in dolichol metabolism to provide an outlook on therapeutic strategies.

Differential roles of the mevalonate pathway in the development and survival of mouse Purkinje cells in culture

The cerebellum is an important locus for motor learning and higher cognitive functions, and Purkinje cells constitute a key component of its circuit. Biochemically, significant turnover of cholesterol occurs in Purkinje cells, causing the activation of the mevalonate pathway. The mevalonate pathway has important roles in cell survival and development. In this study, we investigated the outcomes of mevalonate inhibition in immature and mature mouse cerebellar Purkinje cells in culture. Specifically, we found that the inhibition of the mevalonate pathway by mevastatin resulted in cell death, and geranylgeranylpyrophosphate (GGPP) supplementation significantly enhanced neuronal survival. The surviving immature Purkinje cells, however, exhibited dendritic developmental deficits. The morphology of mature cells was not affected. The inhibition of squalene synthase by zaragozic acid caused impaired dendritic development, similar to that seen in the GGPP-rescued Purkinje cells. Our results indicate GGPP is required for cell survival and squalene synthase for the cell development of Purkinje cells. Abnormalities in Purkinje cells are linked to motor-behavioral learning disorders such as cerebellar ataxia. Thus, serious caution should be taken when using drugs that inhibit geranylgeranylation or the squalene-cholesterol branch of the pathway in the developing stage.

cis-Prenyltransferase atCPT6 produces a family of very short-chain polyisoprenoids in planta

cis-Prenyltransferases (CPTs) comprise numerous enzymes synthesizing isoprenoid hydrocarbon skeleton with isoprenoid units in the cis (Z) configuration. The chain-length specificity of a particular plant CPT is in most cases unknown despite thecomposition of the accumulated isoprenoids in the tissue of interest being well established. In this report AtCPT6, one of the nine Arabidopsis thaliana CPTs, is shown to catalyze the synthesis of a family of very short-chain polyisoprenoid alcohols of six, seven, and eight isoprenoid units, those of seven units dominating The product specificity of AtCPT6 was established in vivo following its expression in the heterologous system of the yeast Saccharomyces cerevisiae and was confirmed by the absence of specific products in AtCPT6 T-DNA insertion mutants and their overaccumulation in AtCPT6-overexpressing plants. These observations are additionally validated in silico using an AtCPT6 model obtained by homology modeling. AtCPT6 only partially complements the function of the yeast homologue of CPT-Rer2 since it restores the growth but not protein glycosylation in rer2delta yeast.This is the first in planta characterization of specific products of a plant CPT producing polyisoprenoids. Their distribution suggests that a joint activity of several CPTs is required to produce the complex mixture of polyisoprenoid alcohols found in Arabidopsis roots.

Deciphering the metabolism of undecaprenyl-phosphate: the bacterial cell-wall unit carrier at the membrane frontier

During the biogenesis of bacterial cell-wall polysaccharides, such as peptidoglycan, cytoplasmic synthesized precursors should be trafficked across the plasma membrane. This essential process requires a dedicated lipid, undecaprenyl-phosphate that is used as a glycan lipid carrier. The sugar is linked to the lipid carrier at the inner face of the membrane and is translocated toward the periplasm, where the glycan moiety is transferred to the growing polymer. Undecaprenyl-phosphate originates from the dephosphorylation of its precursor undecaprenyl-diphosphate, with itself generated by de novo synthesis or by recycling after the final glycan transfer. Undecaprenyl-diphosphate is de novo synthesized by the cytosolic cis-prenyltransferase undecaprenyl-diphosphate synthase, which has been structurally and mechanistically characterized in great detail highlighting the condensation process. In contrast, the next step toward the formation of the lipid carrier, the dephosphorylation step, which has been overlooked for many years, has only started revealing surprising features. In contrast to the previous step, two unrelated families of integral membrane proteins exhibit undecaprenyl-diphosphate phosphatase activity: BacA and members of the phosphatidic acid phosphatase type 2 super-family, raising the question of the significance of this multiplicity. Moreover, these enzymes establish an unexpected link between the synthesis of bacterial cell-wall polymers and other biological processes. In the present review, the current knowledge in the field of the bacterial lipid carrier, its mechanism of action, biogenesis, recycling, regulation, and future perspective works are presented.

Congenital disorders of glycosylation and intellectual disability

The congenital disorders of glycosylation (CDG) are a rapidly growing group of inborn errors of metabolism that result from defects in the synthesis of glycans. Glycosylation is a major post-translational protein modification and an estimated 2% of the human genome encodes proteins for glycosylation. The molecular bases for the current 60 disorders, affecting approximately 800 individuals, have been identified, many in the last 5 years. CDG should be considered in any multi-system syndrome or single tissue disorder not explained by the identification of another disorder. The initial clinical presentation varies significantly among individuals, even between affected siblings. However, two thirds of the known CDGs are associated with intellectual disabilities and most affected individuals need support services throughout their lives. Additional disorders of glycosylation are likely to be characterized over time.

Severe, fatal multisystem manifestations in a patient with dolichol kinase-congenital disorder of glycosylation

Congenital disorders of glycosylation are a group of metabolic disorders with an expansive and highly variable clinical presentation caused by abnormal glycosylation of proteins and lipids. Dolichol kinase (DOLK) catalyzes the final step in biosynthesis of dolichol phosphate (Dol-P), which is the oligosaccharide carrier required for protein N-glycosylation. Human DOLK deficiency, also known as DOLK-CDG or CDG-Im, results in a syndrome that has been reported to manifest with dilated cardiomyopathy of variable severity. A male neonate born to non-consanguineous parents of Palestinian origin presented with dysmorphic features, genital abnormalities, talipes equinovarus, and severe, refractory generalized seizures. Additional multi-systemic manifestations developed including dilated cardiomyopathy, hepatomegaly, severe insulin-resistant hyperglycemia, and renal failure, which were ultimately fatal at age 9months. Electrospray ionization mass spectrometric (ESI-MS) analysis of transferrin identified a type I congenital disorder of glycosylation; next-generation sequencing demonstrated homozygous p.Q483K DOLK mutations that were confirmed in patient fibroblasts to result in severely reduced substrate binding and catalytic activity. This patient expands the phenotype of DOLK-CDG to include anatomic malformations and multi-systemic dysfunction.

Conservation of the PTEN catalytic motif in the bacterial undecaprenyl pyrophosphate phosphatase, BacA/UppP

Isoprenoid lipid carriers are essential in protein glycosylation and bacterial cell envelope biosynthesis. The enzymes involved in their metabolism (synthases, kinases and phosphatases) are therefore critical to cell viability. In this review, we focus on two broad groups of isoprenoid pyrophosphate phosphatases. One group, containing phosphatidic acid phosphatase motifs, includes the eukaryotic dolichyl pyrophosphate phosphatases and proposed recycling bacterial undecaprenol pyrophosphate phosphatases, PgpB, YbjB and YeiU/LpxT. The second group comprises the bacterial undecaprenol pyrophosphate phosphatase, BacA/UppP, responsible for initial formation of undecaprenyl phosphate, which we predict contains a tyrosine phosphate phosphatase motif resembling that of the tumour suppressor, phosphatase and tensin homologue (PTEN). Based on protein sequence alignments across species and 2D structure predictions, we propose catalytic and lipid recognition motifs unique to BacA/UppP enzymes. The verification of our proposed active-site residues would provide new strategies for the development of substrate-specific inhibitors which mimic both the lipid and pyrophosphate moieties, leading to the development of novel antimicrobial agents.

Plant polyisoprenoids and control of cholesterol level

The ability of plant polyisoprenoids (polyprenols and polyprenyl phosphates) to diminish the levels of serum cholesterol affecting its biosynthetic pathway are highlighted here. Possible mechanism of such process is discussed. It is also noted that polyisoprenoids can prevent toxic injuries of the liver and restore disturbed hepatic functions. The possibility of polyprenyl phosphates to reveal at the same time anti-inflammatory action suppressing lipoxygenase activity and lowering the levels of proinflammatory cytokines will be illustrated. Attention will be focused on the potential usefulness of plant polyisoprenoids in the course of prevention and treatment of hypercholesterolemia. High efficiency for combined use of polyprenyl phosphate and β-sitosterol, which leads to substantial enhancement of the ability to overcome hypercholesterolemia versus the individual constituents will be demonstrated.

Disorders of phospholipids, sphingolipids and fatty acids biosynthesis: toward a new category of inherited metabolic diseases

We wish to delineate a novel, and rapidly expanding, group of inborn errors of metabolism with neurological/muscular presentations: the defects in phospholipids, sphingolipids and long chain fatty acids biosynthesis. At least 14 disorders have been described so far. Clinical presentations are diverse but can be divided into (1) diseases of the central nervous system; (2) peripheral neuropathies; and (3) muscular/cardiac presentations. (1) Leukodystrophy and/or iron deposits in basal ganglia is a common feature of phospholipase A2 deficiency, fatty acid hydroxylase deficiency, and pantothenate kinase-associated neurodegeneration. Infantile epilepsy has been reported in GM3 synthetase deficiency. Spastic quadriplegia with ichthyosis and intellectual disability are the presenting signs of the elongase 4 deficiency and the Sjogren-Larsson syndrome caused by fatty aldehyde dehydrogenase deficiency. Spastic paraplegia and muscle wasting are also seen in patients with mutations in the neuropathy target esterase gene. (2) Peripheral neuropathy is a prominent feature in PHARC syndrome due to α/β-hydrolase 12 deficiency, and in hereditary sensory autonomic neuropathy type I due to serine palmitoyl-CoA transferase deficiency. (3) Muscular/cardiac presentations include recurrent myoglobinuria in phosphatidate phosphatase 1 (Lipin1) deficiency; cardiomyopathy and multivisceral involvement in Barth syndrome secondary to tafazzin mutations; congenital muscular dystrophy due to choline kinase deficiency, Sengers syndrome due to acylglycerol kinase deficiency and Chanarin Dorfman syndrome due to α/β- hydrolase 5 deficiency. These synthesis defects of complex lipid molecules stand at the frontier between classical inborn errors of metabolism and other genetic diseases involving the metabolism of structural proteins.

N-linked protein glycosylation in the ER

N-linked protein glycosylation in the endoplasmic reticulum (ER) is a conserved two phase process in eukaryotic cells. It involves the assembly of an oligosaccharide on a lipid carrier, dolichylpyrophosphate and the transfer of the oligosaccharide to selected asparagine residues of polypeptides that have entered the lumen of the ER. The assembly of the oligosaccharide (LLO) takes place at the ER membrane and requires the activity of several specific glycosyltransferases. The biosynthesis of the LLO initiates at the cytoplasmic side of the ER membrane and terminates in the lumen where oligosaccharyltransferase (OST) selects N-X-S/T sequons of polypeptide and generates the N-glycosidic linkage between the side chain amide of asparagine and the oligosaccharide. The N-glycosylation pathway in the ER modifies a multitude of proteins at one or more asparagine residues with a unique carbohydrate structure that is used as a signalling molecule in their folding pathway. In a later stage of glycoprotein processing, the same systemic modification is used in the Golgi compartment, but in this process, remodelling of the N-linked glycans in a protein-, cell-type and species specific manner generates the high structural diversity of N-linked glycans observed in eukaryotic organisms. This article summarizes the current knowledge of the N-glycosylation pathway in the ER that results in the covalent attachment of an oligosaccharide to asparagine residues of polypeptide chains and focuses on the model organism Saccharomyces cerevisiae. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

From discrete dilated cardiomyopathy to successful cardiac transplantation in congenital disorders of glycosylation due to dolichol kinase deficiency (DK1-CDG)

Congenital disorders of glycosylation are a growing group of inborn errors of protein glycosylation. Cardiac involvement is frequently observed in the most common form, PMM2-CDG, especially hypertrophic cardiomyopathy. Dilated cardiomyopathy, however, has been only observed in a few CDG subtypes, usually with a lethal outcome. We report on cardiac pathology in nine patients from three unrelated Israeli families, diagnosed with dolichol kinase deficiency, due to novel, homozygous DK1 gene mutations. The cardiac symptoms varied from discrete, mild dilation to overt heart failure with death. Two children died unexpectedly with acute symptoms of heart failure before the diagnosis of DK1-CDG and heart transplantation could take place. Three other affected children with mild dilated cardiomyopathy at the time of the diagnosis deteriorated rapidly, two of them within days after an acute infection. They all went through successful heart transplantation; one died unexpectedly and 2 others are currently (after 1-5 years) clinically stable. The other 4 children diagnosed with mild dilated cardiomyopathy are doing well on supportive heart failure therapy. In most cases, the cardiac findings dominated the clinical picture, without central nervous system or multisystem involvement, which is unique in CDG syndrome. We suggest to test for DK1-CDG in patients with dilated cardiomyopathy. Patients with discrete cardiomyopathy may remain stable on supportive treatment while others deteriorate rapidly. Our paper is the first comprehensive study on the phenotype of DK1-CDG and the first successful organ transplantation in CDG syndrome.

Understanding human glycosylation disorders: biochemistry leads the charge

Nearly 70 inherited human glycosylation disorders span a breathtaking clinical spectrum, impacting nearly every organ system and launching a family-driven diagnostic odyssey. Advances in genetics, especially next generation sequencing, propelled discovery of many glycosylation disorders in single and multiple pathways. Interpretation of whole exome sequencing results, insights into pathological mechanisms, and possible therapies will hinge on biochemical analysis of patient-derived materials and animal models. Biochemical diagnostic markers and readouts offer a physiological context to confirm candidate genes. Recent discoveries suggest novel perspectives for textbook biochemistry and novel research opportunities. Basic science and patients are the immediate beneficiaries of this bidirectional collaboration.

Heritable disorders in the metabolism of the dolichols: A bridge from sterol biosynthesis to molecular glycosylation

Dolichols, polyisoprene alcohols derived from the mevalonate pathway of cholesterol synthesis, serve as carriers of glycan precursors for the formation of oligosaccharides important in protein glycosylation. Seven autosomal-recessively inherited disorders in the metabolism (synthesis, utilization, recycling) of the dolichols have recently been described, and all are associated with decreased lipid-linked oligosaccharides leading to underglycosylated proteins or lipids which facilitate their detection in the diagnostic laboratory. Multisystem pathology encompasses developmental delays and eye, heart, skin and muscle abnormalities; outcomes range from death in infancy to mild, late-onset disease.