A novel protein modification generating an aldehyde group in sulfatases: its role in catalysis and disease

In multiple sulfatase deficiency, a rare human lysosomal storage disorder, all known sulfatases are synthesized as catalytically poorly active polypeptides. Analysis of the latter has shown that they lack a protein modification that was detected in all members of the sulfatase family. This novel protein modification generates a 2-amino-3-oxopropanoic acid (C alpha-formylglycine) residue by oxidation of the thiol group of a cysteine that is conserved among all eukaryotic sulfatases. The oxidation occurs in the endoplasmic reticulum at a stage when the nascent polypeptide is not yet folded. The aldehyde is part of the catalytic site and is likely to act as an aldehyde hydrate. One of the geminal hydroxyl groups accepts the sulfate during sulfate ester cleavage leading to the formation of a covalently sulfated enzyme intermediate. The other hydroxyl is required for the subsequent elimination of the sulfate and regeneration of the aldehyde group. In some prokaryotic members of the sulfatase gene family, the DNA sequence predicts a serine residue, and not a cysteine. Analysis of one of these prokaryotic sulfatases, however, revealed the presence of the C alpha-formylglycine indicating that the aldehyde group is essential for all members of the sulfatase family and that it can be generated from either cysteine or serine.

Sulfatases, trapping of the sulfated enzyme intermediate by substituting the active site formylglycine

Sulfatases contain an active site formylglycine residue that is generated by post-translational modification. Crystal structures of two lysosomal sulfatases revealed significant similarity to the catalytic site of alkaline phosphatase containing a serine at the position of formylglycine. To elucidate the catalytic mechanism of sulfate ester hydrolysis, the formylglycine of arylsulfatases A and B was substituted by serine. These mutants upon incubation with substrate were covalently sulfated at the introduced serine. This sulfated enzyme intermediate was stable at pH 5. At alkaline pH it was slowly hydrolyzed. These characteristics are analogous to that of alkaline phosphatase which forms a phosphoserine intermediate that is stable at pH 5, but is hydrolyzed at alkaline pH. In wild-type sulfatases the hydroxyl needed for formation of the sulfated enzyme intermediate is provided by the aldehyde hydrate of the formylglycine. The second, non-esterified hydroxyl of the aldehyde hydrate is essential for rapid desulfation of the enzyme at acidic pH, which most likely occurs by elimination. The lack of this second hydroxyl in the serine mutants explains the trapping of the sulfated enzyme intermediate. Thus, in acting as a geminal diol the formylglycine residue allows for efficient ester hydrolysis in an acidic milieu.

Arylsulfatase from Klebsiella pneumoniae carries a formylglycine generated from a serine

Eukaryotic sulfatases share an unusual posttranslational protein modification, which converts a cysteine into alpha-formylglycine. The alpha-formylglycine is essential for the catalytic activity. Klebsiella pneumoniae expresses an inducible arylsulfatase for which the DNA predicts a serine at the position occupied by the alpha-formylglycine residue in eukaryotic sulfatases. Structural analysis showed that the majority of the arylsulfatase polypeptides from K. pneumoniae carries the alpha-formylglycine, whereas the remaining arylsulfatase polypeptides contain the predicted serine residue. This demonstrates the evolutionary conservation between prokaryotes and eukaryotes of this novel protein modification that so far has been found only in sulfatases. alpha-Formylglycine in Klebsiella is generated from a serine and not from a cysteine as in eukaryotes.

Conversion of cysteine to formylglycine in eukaryotic sulfatases occurs by a common mechanism in the endoplasmic reticulum

Sulfatases undergo an unusual protein modification leading to conversion of a specific cysteine residue into alpha-formylglycine. This conversion is essential for catalytic activity. In arylsulfatase A the alpha-formylglycine is generated inside the endoplasmic reticulum at a late stage of protein translocation. Using in vitro translation in the presence of transport-competent microsomes we found that arylsulfatase B is also modified in a similar way by the formylglycine-generating machinery. Modification depended on protein transport and on the correct position of the relevant cysteine. Arylsulfatase A and B did not compete for modification, as became apparent in co-expression experiments. This could argue for an association of the modification machinery with the protein translocation apparatus.

EEG slowing and cerebrospinal fluid tau levels in patients with cognitive decline

We explored the relationship between cerebrospinal fluid (CSF) tau levels as indirect markers of tau-related pathology in Alzheimer's disease (AD) and EEG slowing, a typical neurophysiological finding in the disease. A positive correlation between CSF tau levels and ratio of alpha/delta global field power was found in 14 AD patients (r = 0.65, p = 0.01). This relationship was better approximated by polynomial fit of 2nd degree (p = 0.002). A subgroup of AD patients (n = 7) with higher tau levels and shorter duration of illness showed a strong relationship between CSF tau levels and alpha/theta (r = 0.83, p = 0.02), and alpha/delta (r = 0.87, p = 0.01) ratios of the global field power. There were no significant correlations between EEG slowing and CSF tau levels in 12 patients with mild cognitive dysfunction or in 14 healthy control subjects. That a strong inverse linear correlation exists in AD patients with higher levels of tau and shorter duration of illness may imply that with longer illness duration CSF tau levels decrease due to neuronal death.

Clinical efficacy of Ginkgo biloba special extract EGb 761 in dementia of the Alzheimer type

Among the psychiatric illnesses associated with old age primary degenerative dementia of the Alzheimer type (DAT) has gained increasing importance in recent years. Even though a curative treatment of the disease is currently impossible, various drugs can be used to slow down its progression. In the present study the influence of oral treatment with 240 mg/day of Ginkgo bilabo special extract EGb 761 (Tebonin forte, manufactured by Dr Willmar Schwabe, Karlsruhe) on the clinical course of DAT was investigated in a double-blind, randomized, placebo-controlled parallel-group design in 20 outpatients. The duration of treatment was 3 months. The primary outcome variable was the sum score in the SKT-test for the determination of attention and memory. Other psychometric tests (trailmaking test, ADAS, CGI) and electrophysiological investigations (EEG topography) were evaluated descriptively. Although the active-treatment group, with a mean sum score of 19.67 points in the, S.K.T., had a poorer baseline level than the placebo group (18.11 points), it experienced an improvement to 16.78 points under treatment with EGb 761 whereas the placebo group deteriorated to 18.89 points. The differences between the baseline and final values formed the basis for a statistical group comparison, which gave a result favourable to EGb 761, at a significance level of p < .013. In addition to this psychometric confirmation of efficacy, certain descriptive trends were found at the psychopathological (Clinical Global Impression) and dynamic functional (EEG findings) levels, which can be interpreted as evidence of effectiveness of Ginkgo biloba special extract EGb 761 in mild to moderate dementia and of local effects in the central nervous system. Inter-group differences in the ADAS cognitive and non-cognitive subscales did not reach statistical significance, probably because of the small sample size.

Conversion of cysteine to formylglycine: a protein modification in the endoplasmic reticulum

In sulfatases a Calpha-formylglycine residue is found at a position where their cDNA sequences predict a cysteine residue. In multiple sulfatase deficiency, an inherited lysosomal storage disorder, catalytically inactive sulfatases are synthesized which retain the cysteine residue, indicating that the Calpha-formylglycine residue is required for sulfatase activity. Using in vitro translation in the absence or presence of transport competent microsomes we found that newly synthesized sulfatase polypeptides carry a cysteine residue and that the oxidation of its thiol group to an aldehyde is catalyzed in the endoplasmic reticulum. A linear sequence of 16 residues surrounding the Cys-69 in arylsulfatase A is sufficient to direct the oxidation. This novel protein modification occurs after or at a late stage of cotranslational protein translocation into the endoplasmic reticulum when the polypeptide is not yet folded to its native structure.

EEG-microstates in mild memory impairment and Alzheimer's disease: possible association with disturbed information processing

The only available functional neuroimaging methods reaching the time resolution of human information processing are EEG and MEG. Since spectral analysis implies analysis of longer time epochs, the high temporal resolution of EEG is partly lost. By dividing the EEG in the time-domain into segments of similar spatial distribution on the scalp (microstates) it has been possible to assess patterns of neuronal activity representing the information process currently performed by the brain. In the present study alterations of EEG microstates in subjective (n = 31) and objective (n = 38) memory impairment as well as in probable Alzheimer disease (DAT: n = 64) compared to healthy controls (n = 21) were investigated. The main findings were reduced segment durations and a more anterior center of gravity of the microstate topography in DAT. With more pronounced cognitive dysfunction larger window sizes were found. Shorter microstates and larger windows reflect more rapidly changing spatial activation patterns, and are interpreted as an impaired capability to establish stable brain states necessary for normal brain function. The anteriorization of the microstates is consistent with results in the frequency domain and may reflect neuropathological changes in DAT.

The AMDP modules I-IV: recommendations for a standardized acquisition of EEG data in psychiatry. Association for Methodology and Documentation in Psychiatry

During the last few years, the working group 'Psychophysiology' of the Association for Methodology and Documentation in Psychiatry (AMDP) discussed the possibility of the establishment of defined EEG modules in psychiatry. It was the aim to create a common data pool in order to be able to have access to larger data sets. The installation of such a common data pool was regarded as an important prerequisite for a future diagnostic application of EEG and EP data in clinical practice. The most relevant arguments are: From a statistical point of view, multivariate investigations can be improved when relatively large data sets are available. Subgrouping of patients is facilitated. Different centers have access to different populations of patients. Furthermore, compliance with the recommendations contributes to a reduction in misunderstanding and false interpretation of other investigators' results. The working group 'Psychophysiology' of the AMDP now recommends EEG modules to be registered in psychiatry. The recommendations are based on investigations using these modules in clinical research and practice as well as several years of discussion within the working group. Four AMDP modules (I-IV) are presented: MI: resting EEG (closed eyes), MII: resting EEG (eyes open), MIII: EEG during videotracking, and MIV: EEG during choice reaction time. Recommendations for additional modules are planned in the near future: MV: EEG during geometry test, MVI: EEG during labyrinth test, and MVII: amplitude stimulus intensity function. MIII-MVI is paralleled by an EEG recording so that psychomotor performance can be measured and EEG data under different activation conditions are available. Compliance with the recommendations guarantees the possibility of access to the common data pool. Computer software is available in Berlin.

A comparison of qEEG and HMPAO-SPECT in relation to the clinical severity of Alzheimer's disease

Electroencephalographical studies have disclosed correlations between topographical features of Fast Fourier Transformation maps and the severity of Alzheimer's disease (DAT). The object of the present study was to explore the relations of HMPAO-SPECT and quantitative EEG (qEEG) with the severity of dementia. Twenty-three patients were included in the study. Spectral and topographical EEG parameters were compared with global and regional cerebral blood flow, and with psychometric measures of clinical severity. None of the regions of interest of the SPECT scans were significantly correlated with clinical severity. Low values in delta- and theta bands, however, were related to high scores on the Mini-Mental-State examination (P < 0.01), whereas the Syndrome-Kurz test correlated inversely with the power values in the alpha and beta band. The global decrease in cerebral blood flow (CBF) was associated with a shift on the topographical alpha-centroids in the posterior direction (P < 0.01). In previous studies correlations between CBF and clinical severity were weak, indicating a high interindividual variance, or interactions with concomitant vascular lesions. Whereas SPECT is a well-established tool for the diagnosis of dementia, the present study indicates qEEG as a potential marker for the staging of the cognitive decline in DAT.

A microsomal ATP-binding protein involved in efficient protein transport into the mammalian endoplasmic reticulum

Protein transport into the mammalian endoplasmic reticulum depends on nucleoside triphosphates. Photoaffinity labelling of microsomes with azido-ATP prevents protein transport at the level of association of precursor proteins with the components of the transport machinery, Sec61alpha and TRAM proteins. The same phenotype of inactivation was observed after depleting a microsomal detergent extract of ATP-binding proteins by passage through ATP-agarose and subsequent reconstitution of the pass-through into proteoliposomes. Transport was restored by co-reconstitution of the ATP eluate. This eluate showed eight distinct bands in SDS gels. We identified five lumenal proteins (Grp170, Grp94, BiP/Grp78, calreticulin and protein disulfide isomerase), one membrane protein (ribophorin I) and two ribosomal proteins (L4 and L5). In addition to BiP (Grp78), Grp170 was most efficiently retained on ATP-agarose. Purified BiP did not stimulate transport activity. Sequence analysis revealed a striking similarity of Grp170 and the yeast microsomal protein Lhs1p which was recently shown to be involved in protein transport into yeast microsomes. We suggest that Grp170 mediates efficient insertion of polypeptides into the microsomal membrane at the expense of nucleoside triphosphates.

Luciferase assembly after transport into mammalian microsomes involves molecular chaperones and peptidyl-prolyl cis/trans-isomerases

The assembly of a heterodimeric luciferase was studied after de novo synthesis of corresponding precursor proteins in reticulocyte lysate and concomitant transport into dog pancreas microsomes. This cytosolic luciferase from a prokaryotic organism (Vibrio harveyi) was specifically used as a model protein to investigate (i) whether the eukaryotic cytosol and the microsomal lumen have similar folding capabilities and (ii) whether the requirements of a polypeptide for certain molecular chaperones and folding catalysts are determined by the polypeptide or the intracellular compartment. The two luciferase subunits were fused to the preprolactin signal peptide. Data indicate that efficient assembly of luciferase occurs in the mammalian microsomes. Furthermore, it was observed that luciferase assembly can be separated in time from synthesis and membrane transport, depends on ATP hydrolysis, is partially sensitive to cyclosporin A and FK506, and in the absence of lumenal proteins is less efficient as compared with the presence of lumenal proteins. Thus, heterodimeric luciferase depends on functionally related molecular chaperones and folding catalysts during its assembly in either the eukaryotic cytosol or the microsomal lumen.

Efficient folding of firefly luciferase after transport into mammalian microsomes in the absence of luminal chaperones and folding catalysts

Folding of polypeptides emerging from the protein translocase in the membrane of mammalian microsomes was analyzed after synthesis of corresponding precursor proteins in a mammalian translation system. Firefly luciferase was used as a model protein; the corresponding hybrid precursor contained the preprolactin signal peptide. The rates and efficiencies of folding of luciferase in microsomes were compared with those of folding of luciferase in the cytosol. Furthermore, folding of luciferase in microsomes was compared with that in proteoliposomes, i.e. in the absence of luminal molecular chaperones and folding catalysts. Folding in microsomes was less efficient compared with folding in the cytosol. Folding in the absence of luminal proteins was more efficient compared with folding in their presence and identical to folding in the cytosol. Thus, firefly luciferase emerging from translocase can efficiently fold to its native conformation without chaperoning by any luminal proteins. There may be molecular chaperones present in the microsomal membrane that can efficiently substitute for the cytosolic chaperone machinery comprising Hsp40, Hsp60, and Hsp70 with respect to folding of firefly luciferase.

Cyclosporin A inhibits the degradation of signal sequences after processing of presecretory proteins by signal peptidase

Targeting of presecretory proteins to, and insertion into, the microsomal membrane are mediated by signal sequences. These signal sequences are removed from presecretory proteins by signal peptidase. We demonstrate that the signal sequence of preprolactin, after translocation into microsomes and cleavage by signal peptidase, is converted to an intermediate form. This intermediate was found outside the microsomes, where it was degraded in the presence of cytosol. Degradation of the signal sequence of another presecretory protein, preprocecropinA, occurred even in the absence of cytosol. The immunosuppressant cyclosporin A inhibited trimming of the preprolactin signal sequence and degradation of the preprocecropinA signal sequence. We observed by cross-linking studies that cleaved signal sequences are bound to two microsomal proteins prior to degradation.

Early cerebral functional outcome after coronary artery bypass surgery using different acid-base management during hypothermic cardiopulmonary bypass

BACKGROUND

Cerebral injury can render meaningless an otherwise successful cardiac operation. As carbon dioxide management during hypothermic cardiopulmonary bypass has a major impact on cerebral blood flow during bypass, it may also influence postoperative cerebral function. The objective of this prospective, controlled, double-blind study was to compare the effect of pH-stat (temperature correction of blood gas analyses) or alpha-stat (no temperature correction) on brain function.

METHODS

Cerebral function was assessed non-invasively using neurological examination, a neuropsychological test battery, quantitative electroencephalogram and the cognitive evoked potential P300 preoperatively and on postoperative day 7 or 8. Forty-five patients undergoing elective aortocoronary bypass grafting in barbiturate/opiate anesthesia were randomised into a pH-stat-(n = 23) and an alpha-stat-group (n = 22). Membrane oxygenators with arterial line filtration and non-pulsatile flow at a perfussion pressure of 50-60 mmHg were used throughout.

RESULTS

One patient in each group developed a new paresis. Among 49 comparisons of cerebral function parameters, only 3 showed significant differences at the 5%-level, i.e. exactly what is expected purely by chance. These differences were too small to be clinically relevant and favored either pH-stat (theta-amplitude) or alpha-stat (subtest 3 of syndrome-short-test, number of mistakes in aiming).

CONCLUSION

Under the bypass conditions of this study there was no difference in early cerebral functional outcome between pH-or alpha-stat carbon dioxide management during hypothermic cardiopulmonary bypass.

Topography of the maximum of the amplitude of EEG frequency bands in dementia of the Alzheimer type

The localization of the maxima of activity of 36 patients with dementia of the Alzheimer type and 36 age-matched controls was calculated after Fourier transformation of EEG data in six frequency bands (4 Hz steps between 1 and 24 Hz). Patients were divided into three groups of severity (BCRS mean < 3,9, 4.0-4.9, and > 4.9). Significant differences were found in the beta frequency band. In the distribution of the maxima of the activity, the maxima of patients were located more frontally than the maxima of controls. The alterations were staged dependent. The beta activity in EEG is said to be correlated with cognitive processes; the correlation between cognitive decline and decreasing beta activity in dementia of the Alzheimer type might support this assumption. Differences in the location of the maxima between patients and controls might allow for differentiation between both groups.

The predictive value of P300-amplitudes in the course of schizophrenic disorders

Auditory P300-amplitudes have been found to be correlated with the social functioning and with the impairment in daily life by negative symptoms in cross-sectional studies. In this prospective longitudinal study, the correlation of auditory P300-amplitudes registrated at the index examination was investigated with the clinical outcome after an average of 2.4 years. Based on previous studies, only schizophrenic patients who were in a stabilized residual state were included in the study. Reference-independent P300-parameters of the index examination were correlated with axis V of DSM-III-R (GAF), with the Brief Psychiatric Rating Scale (BPRS) and with the Scale for Assessment of Negative Symptoms (SANS) assessed at the follow-up examination. The correlation of index P300-amplitude with social functioning at follow-up was significant. No correlations of index P300 with the current symptomatology at follow-up, as expressed by BPRS and SANS was found, however. The results indicate a predictive value of the P300-amplitude on the clinical outcome in terms of social functioning of schizophrenic patients.

Plasma levels of the beta-carbolines harman and norharman in Parkinson's disease

Several lines of evidence suggest that endogenous and exogenous toxins may play a major role in the pathogenesis of Parkinson's disease (PD). In vivo aromatic beta-carbolines, like harman or norharman, may easily be formed by cyclization of indoleamines with e.g. aldehydes. Because of the structural similarity to MPTP, beta-carbolines have been proposed as endogenous toxins. For further elucidation of the role of beta-carbolines in neurodegenerative disorders, harman and norharman plasma levels were measured in 36 patients with PD and compared to an age- and sex-matched control group. Plasma levels of norharman in PD were significantly higher compared to the control group. Harman in the plasma of Parkinsonian patients was also elevated compared to controls, but this difference was not significant. Correlation of beta-carbolines with plasma levels of L-dopa, oral doses of bromocriptine and selegiline in treated Parkinsonian patients showed no significant results. On the one hand these results may suggest a possible role of beta-carbolines in the pathophysiological processes initiating PD, by, e.g., inducing mitochondrial respiratory inhibition like MPP+. One may speculate, however, that elevated levels of norharman and harman are due to an endogenous upregulation caused by unknown metabolic processes.

Correlation between cognitive brain function and electrical brain activity in dementia of Alzheimer type

Psychometric tests which assess cognitive brain function in dementia disorders are partly prone to artifacts, e.g., the experience of the investigator and the cooperation of the patient influences the results. An objective way to assess the degree of cognitive disturbance could be to measure neuronal activity represented by the electrical brain activity. The aim of the present study was to investigate how well cognitive function in dementia assessed by psychometric tests correlates with electrical brain activity (EEG). Multichannel EEG data was reduced into 3-D intracerebral equivalent dipole EEG generators allowing a more convenient statistical data management and valid physiological data interpretation. 35 patients suffering from dementia of Alzheimer type were investigated. An increase of dipole strength in the slow frequency bands, a more anterior equivalent dipole of alpha- and beta-activity, and a slowing of the EEG with increasing cognitive deterioration could be demonstrated. The results support the hypothesis that the amount of disturbance of cognitive function in dementia can be assessed by measuring the electrical activity of the brain.

Larger topographical variance and decreased duration of brain electric microstates in depression

The topographical configuration of the spontaneous brain electric fields is considered to contain relevant information about the pattern of the generating cortical electrochemical activation and the associated cognitive processes. Space oriented segmentation allows to break down the stream of the spontaneous EEG into brain electric microstates with stable configuration of the fields. It has been shown that the mean duration of the microstates was consistent with the duration of elementary steps of cognitive processes, and that different topographies of the microstates are associated with different cognitive modalities. Space-oriented segmentation was applied to the resting EEG of 22 depressive patients and of 22 controls. The topographical variance was larger, and the most prominent brain electrical microstates of the EEG epochs were significantly shorter in the depressive group than in controls. No differences were found for the shortest microstates. This result cannot be explained by group differences in the frequency domain of the EEG. No topographical differences were found between the microstates of depressives and those of controls. Based on previous results in healthy volunteers during spontaneous cognition and in schizophrenic patients, the findings indicate that formal aspects rather than the modalities of the stream of cognition are altered in depression. Automatic and schematic processing, and attentional deficits as described in depressive patients might account for the finding of less sustained brain electrical microstates.