Importance of Tyr409 and Tyr414 in constructing the substrate pocket of human aminopeptidase B.

Aminopeptidase B (APB, EC 3.4.11.6) preferentially hydrolyzes basic amino acids of synthetic substrates and requires a physiological concentration of chloride anions for optimal activity. Several amino acid residues of APB responsible for its enzymatic activity have been elucidated. In this study, we further searched for residues critical to its enzymatic activity, especially toward peptide substrates. APB residues Tyr409 (Y409) and Tyr414 (Y414), both of which were critical to its hydrolytic activity toward synthetic substrates, were predicted by molecular modeling to be involved in cleaving peptide substrates via its interaction with amino acids in the P1' cleavage site. Using site-directed mutagenesis, several mutant APBs were prepared. In contrast to synthetic substrates, wild-type and Y409F/Y414F double mutant enzymes showed P1'-dependent cleavage of peptide substrates, indicating that both tyrosine residues were not indispensable for hydrolytic activity toward peptide substrates. Moreover, the Y409F/Y414F double mutant enzyme cleaved peptides with a Pro residue at the P1' site, which is uncommon among the M1 family of aminopeptidases. These results suggested that Tyr409 and Tyr414 of APB play important roles in enzymatic function and characteristic properties of APB via proper formation of the S1' site.

Involvement of Phenylalanine 297 in the Construction of the Substrate Pocket of Human Aminopeptidase B.

Aminopeptidase B (APB, EC 3.4.11.6) preferentially hydrolyzes the N-terminal basic amino acids of synthetic and peptide substrates and requires a physiological concentration of NaCl for optimal activity. In this study, we used site-directed mutagenesis and molecular modeling to search for an amino acid residue that is critical for the enzymatic properties of human APB. Substitution of Phe297 with Tyr caused a significant decrease in hydrolytic activity toward synthetic and peptide substrates as well as chloride anion sensitivity. Molecular modeling suggests that Phe297 contributes to the construction of the substrate pocket of APB, which is wide enough to hold a chloride anion and allow the interaction of Gln169 with the N-terminal Arg residue of the substrate through bridging with the chloride anion. These results indicate that Phe297 is crucial for the optimal enzymatic activity and chloride anion sensitivity of APB via formation of the optimal structure of the catalytic pocket.

Role of glutamine-169 in the substrate recognition of human aminopeptidase B.

BACKGROUND: Aminopeptidase B (EC 3.4.11.6, APB) preferentially hydrolyzes N-terminal basic amino acids of synthetic and peptide substrates. APB is involved in the production and maturation of peptide hormones and neurotransmitters such as miniglucagon, cholecystokinin and enkephalin by cleaving N-terminal basic amino acids in extended precursor proteins. Therefore, the specificity for basic amino acids is crucial for the biological function of APB. METHODS: Site-directed mutagenesis and molecular modeling of the S1 site were used to identify amino acid residues of the human APB responsible for the basic amino acid preference and enzymatic efficiency. RESULTS: Substitution of Gln169 with Asn caused a significant decrease in hydrolytic activity toward the fluorescent substrate Lys-4-methylcoumaryl-7-amide (MCA). Substantial retardation of enzyme activity was observed toward Arg-MCA and substitution with Glu caused complete loss of enzymatic activity of APB. Substitution with Asn led to an increase in IC50 values of inhibitors that interact with the catalytic pocket of APB. The EC50 value of chloride ion binding was also found to increase with the Asn mutant. Gln169 was required for maximal cleavage of the peptide substrates. Molecular modeling suggested that interaction of Gln169 with the N-terminal Arg residue of the substrate could be bridged by a chloride anion. CONCLUSION: Gln169 is crucial for obtaining optimal enzymatic activity and the unique basic amino acid preference of APB via maintaining the appropriate catalytic pocket structure and thus for its function as a processing enzyme of peptide hormones and neurotransmitters.

Right frontopolar cortex activity correlates with reliability of retrospective rating of confidence in short-term recognition memory performance.

Human memory systems contain self-monitoring mechanisms for evaluating their progress. People can change their learning strategy on the basis of confidence in their performance at that time. However, it has not been fully understood how the brain is engaged in reliable rating of confidence in past recognition memory performance. We measured the brain activity by fMRI while healthy subjects performed a visual short-term recognition memory test and then rated their confidence in their answers as high, middle, or low. As shown previously, their behavioral performance in the confidence rating widely varied; some showed a positive confidence-recognition correlation (i.e., "rate reliably") while others did not. Among brain regions showing greater activity during rating their confidence relative to during a control, non-metamemory task (discriminating brightness of words), only a posterior-dorsal part of the right frontopolar cortex exhibited higher activity as the confidence level better correlated with actual recognition memory performance. These results suggest that activation in the right frontopolar cortex is key to a reliable, retrospective rating of confidence in short-term recognition memory performance.

Paralimbic system and striatum are involved in motivational behavior.

Goal-directed rewarded behavior and goal-directed non-rewarded behavior are concerned with motivation. However, the neural substrates involved in goal-directed non-rewarded behaviors are unknown. Using functional magnetic resonance imaging, we investigated the brain activities of healthy individuals during a novel tool use (turning a screwdriver) to elucidate the relationship between the brain mechanism relevant to goal-directed non-rewarded behavior and motivation. We found that our designed behavioral task evoked activities in the orbitofrontal cortex, striatum, anterior insula, lateral prefrontal cortex, and anterior cingulate cortex compared with a meaningless task. These results suggest that activation in these cerebral regions play important roles in motivational behavior without tangible rewards.

Anatomical segregation of representations of personally familiar and famous people in the temporal and parietal cortices.

Person recognition has been assumed to entail many types of person-specific cognitive responses, including retrieval of knowledge, episodic recollection, and emotional responses. To demonstrate the cortical correlates of this modular structure of multimodal person representation, we investigated neural responses preferential to personally familiar people and responses dependent on familiarity with famous people in the temporal and parietal cortices. During functional magnetic resonance imaging (fMRI) measurements, normal subjects recognized personally familiar names (personal) or famous names with high or low degrees of familiarity (high or low, respectively). Effects of familiarity with famous people (i.e., high-low) were identified in the bilateral angular gyri, the left supramarginal gyrus, the middle part of the bilateral posterior cingulate cortices, and the left precuneus. Activation preferentially relevant to personally familiar people (i.e., personal-high) was identified in the bilateral temporo-parietal junctions, the right anterolateral temporal cortices, posterior middle temporal gyrus, posterior cingulate cortex (with a peak in the posterodorsal part), and the left precuneus; these activation foci exhibited varying degrees of activation for high and low names. An equivalent extent of activation was observed for all familiar names in the bilateral temporal poles, the left orbito-insular junction, the middle temporal gyrus, and the anterior part of the posterior cingulate cortex. The results demonstrated that distinct cortical areas supported different types of cognitive responses, induced to different degrees during recognition of famous and personally familiar people, providing neuroscientific evidence for the modularity of multimodal person representation.

Is Broca's area involved in the processing of passive sentences? An event-related fMRI study.

We used functional magnetic resonance imaging (fMRI) to investigate whether activation in Broca's area is greater during the processing of passive versus active sentences in the brains of healthy subjects. Twenty Japanese native speakers performed a visual sentence comprehension task in which they were asked to read a visually presented sentence and to identify the agent or the patient in the sentence by pressing a button. We found that the processing of passive sentences elicited no greater activation than that of active sentences in Broca's area. However, passive sentences elicited greater activation than active sentences in the left frontal operculum and the inferior parietal lobule. Thus, our neuroimaging results suggest that deficits in the comprehension of passive sentences in Japanese aphasics are induced not by lesions to Broca's area, but to the left frontal operculum and/or the inferior parietal lobule.

Cortical mechanisms of person representation: recognition of famous and personally familiar names.

Personally familiar people are likely to be represented more richly in episodic, emotional, and behavioral contexts than famous people, who are usually represented predominantly in semantic context. To reveal cortical mechanisms supporting this differential person representation, we compared cortical activation during name recognition tasks between personally familiar and famous names, using an event-related functional magnetic resonance imaging (fMRI). Normal subjects performed familiar- or unfamiliar-name detection tasks during visual presentation of personally familiar (Personal), famous (Famous), and unfamiliar (Unfamiliar) names. The bilateral temporal poles and anterolateral temporal cortices, as well as the left temporoparietal junction, were activated in the contrasts Personal-Unfamiliar and Famous-Unfamiliar to a similar extent. The bilateral occipitotemporoparietal junctions, precuneus, and posterior cingulate cortex showed activation in the contrasts Personal-Unfamiliar and Personal-Famous. Together with previous findings, differential activation in the occipitotemporoparietal junction, precuneus, and posterior cingulate cortex between personally familiar and famous names is considered to reflect differential person representation. The similar extent of activation for personally familiar and famous names in the temporal pole and anterolateral temporal cortex is consistent with the associative role of the anterior temporal cortex in person identification, which has been conceptualized as a person identity node in many models of person identification. The left temporoparietal junction was considered to process familiar written names. The results illustrated the neural correlates of the person representation as a network of discrete regions in the bilateral posterior cortices, with the anterior temporal cortices having a unique associative role.

Brain activation during the course of sentence comprehension.

The purpose of this study is to determine, by functional magnetic resonance imaging, how the activated regions of the brain change as a Japanese sentence is presented in a grammatically correct order. In this study, we presented constituents of a sentence to Japanese participants one by one at regular intervals. The results showed that the left lingual gyrus was significantly activated at the beginning of the sentence, then the left inferior frontal gyrus and left supplementary motor area, in the middle of the sentence, and the left inferior temporal gyrus, at the end of the sentence. We suggest that these brain areas are involved in sentence comprehension in this temporal order.

Cortical activation during reading of ancient versus modern Japanese texts: fMRI study.

The purpose of this study was to investigate human brain activity during the reading of ancient Japanese texts using functional magnetic resonance imaging. Thirty right-handed normal Japanese subjects performed two reading tasks: covert reading of (1) ancient and (2) modern Japanese text. Common areas are activated during both tasks. Activity in the left inferior frontal cortices increased during the reading of ancient Japanese text compared with the reading of modern Japanese text, whereas occipital activity increased during the reading of modern Japanese text. Our results indicate that ancient Japanese language may be processed as a foreign language.

Cortical mechanisms of visual self-recognition.

Several lines of evidence have suggested that visual self-recognition is supported by a special brain mechanism; however, its functional anatomy is of great controversy. We performed an event-related functional magnetic resonance imaging (fMRI) study to identify brain regions selectively involved in recognition of one's own face. We presented pictures of each subject's own face (SELF) and a prelearned face of an unfamiliar person (CONT), as well as two personally familiar faces with high and low familiarity (HIGH and LOW, respectively) to test selectivity of activation to the SELF face. Compared with the CONT face, activation selective to the SELF face was observed in the right occipito-temporo-parietal junction and frontal operculum, as well as in the left fusiform gyrus. On the contrary, the temporoparietal junction in both the hemispheres and the left anterior temporal cortex, which were activated during recognition of HIGH and/or LOW faces, were not activated during recognition of the SELF face. The results confirmed the partial distinction of the brain mechanism involved in recognition of personally familiar faces and that in recognition of one's own face. The right occipito-temporo-parietal junction and frontal operculum appear to compose a network processing motion-action contingency, a role of which in visual self-recognition has been suggested in previous behavioral studies. Activation of the left fusiform gyrus selective to one's own face was consistent with the results of two previous functional imaging studies and a neuropsychological report, possibly suggesting its relationship with lexical processing.

Different roles of the frontal and parietal regions in memory-guided saccade: a PCA approach on time course of BOLD signal changes.

Although involvement of the frontoparietal regions in visually guided saccade and visuospatial attention has been established, functional difference of the frontal and parietal regions suggested in neuropsychological observations and lesion studies in animals has not been explicitly supported by functional imaging studies. Considering a possible disadvantage of cognitive subtraction in an interregional comparison, we directly compared the time course of BOLD signal changes across regions. Normal subjects performed a modified version of a memory-guided saccade task in which saccade was performed both during encoding and execution phases. In addition, the delay period was fixed and the peripheral target was presented also during the execution phase together with distracters. Therefore, visuospatial representation was likely maintained in the sensory domain during the delay phase. A principal component analysis on the time-course data separated the 20 activated areas into three groups, which largely coincided with the cerebral lobes. The frontal group included the putative human FEF and SEF, and the parietal group PEF. The frontal and occipital groups exhibited the time course of activation with two peaks corresponding to neural responses during the encoding and execution phases, and the parietal group exhibited a single-humped activation pattern corresponding to neural activity during the delay phase. The results suggest that the frontal regions are more associated with the execution of saccade, and the parietal regions with visuospatial representation, presumably in the sensory domain.

Mental visual synthesis is originated in the fronto-temporal network of the left hemisphere.

Mental visual synthesis is the capacity for experiencing, constructing, or manipulating 'mental imagery'. To investigate brain networks involved in mental visual synthesis, brain activity was measured in right-handed healthy volunteers during mental imagery tasks, in which the subjects were instructed to imagine a novel object, that does not exist in the real world, by composing it from two visually presented words associated with a real object or two achromatic line drawings of a real object, using functional magnetic resonance imaging (fMRI). Both tasks activated the same areas in the inferior frontal and inferior temporal cortices of the left hemisphere. Our results indicate that the source of mental visual synthesis may be formed by activity of a brain network consisting of these areas, which are also involved in semantic operations and visual imagery.

The human parietal cortex is involved in spatial processing of tongue movement-an fMRI study.

The human tongue is so sensitive and dexterous that spatial representations of the inside of the oral cavity for the tongue movement are naturally expected to exist. In the present study, we examined the brain activity associated with spatial processing during tongue movements using a functional magnetic resonance imaging technique. Twenty-four normal subjects participated in the study, which consisted of a periodic series of three blocks; resting of the tongue, tongue movement (pressing the inside of a tooth with the tip of the tongue), and tongue retraction. The cerebral fields of activation during the tongue movement to the left and right side relative to those during rest were found in the primary sensorimotor area and supplementary motor area bilaterally, and in the left inferior parietal lobule (IPL). The activation areas during the tongue retraction relative to those during rest were almost the same, except that activation in the left IPL was not observed. The fields of activation during tongue movement to the left and right side relative to those during tongue retraction were found bilaterally in the dorsal premotor area, superior parietal lobule (SPL), and the IPL. The results indicate that the bilateral SPL and IPL were specifically involved in the processing for human tongue movement. Although no significant laterality was observed, the left parietal area tended to show greater activation in statistical values and area than the right parietal area, thus indicating the possibility that this processing for human tongue movement is related to that for language.

Voxel-based morphometry of human brain with age and cerebrovascular risk factors.

The objectives of this study were to evaluate the correlations of the volumes of the gray matter and white matter with age, and the correlations of the tissue probabilities of the gray matter and white matter with age and several cerebrovascular risk factors. We obtained magnetic resonance (MR) images of the brain and clinical information from 769 normal Japanese subjects. We processed the MR images automatically by correcting for inter-individual differences in brain size and shape, and by segmenting the MR images into the gray matter and white matter. Volumetry of the brain revealed a significant negative correlation between the gray matter volume and age, which was not observed between white matter volume and age. Voxel-based morphometry showed that age, systolic blood pressure, and alcohol drinking correlated with the regional tissue probabilities of the gray matter and white matter.

A state-space model of the hemodynamic approach: nonlinear filtering of BOLD signals.

In this paper, a new procedure is presented which allows the estimation of the states and parameters of the hemodynamic approach from blood oxygenation level dependent (BOLD) responses. The proposed method constitutes an alternative to the recently proposed Friston [Neuroimage 16 (2002) 513] method and has some advantages over it. The procedure is based on recent groundbreaking time series analysis techniques that have been, in this case, adopted to characterize hemodynamic responses in functional magnetic resonance imaging (fMRI). This work represents a fundamental improvement over existing approaches to system identification using nonlinear hemodynamic models and is important for three reasons. First, our model includes physiological noise. Previous models have been based upon ordinary differential equations that only allow for noise or error to enter at the level of observation. Secondly, by using the innovation method and the local linearization filter, not only the parameters, but also the underlying states of the system generating responses can be estimated. These states can include things like a flow-inducing signal triggered by neuronal activation, de-oxyhemoglobine, cerebral blood flow and volume. Finally, radial basis functions have been introduced as a parametric model to represent arbitrary temporal input sequences in the hemodynamic approach, which could be essential to understanding those brain areas indirectly related to the stimulus. Hence, thirdly, by inferring about the radial basis parameters, we are able to perform a blind deconvolution, which permits both the reconstruction of the dynamics of the most likely hemodynamic states and also, to implicitly reconstruct the underlying synaptic dynamics, induced experimentally, which caused these states variations. From this study, we conclude that in spite of the utility of the standard discrete convolution approach used in statistical parametric maps (SPM), nonlinear BOLD phenomena and unspecific input temporal sequences must be included in the fMRI analysis.

Cortical activation during reading aloud of long sentences: fMRI study.

The purpose of this study was to investigate human brain activity during the reading aloud of Japanese sentences using fMRI. Twenty-three right-handed normal Japanese subjects performed three reading tasks: covert reading of meaningful or meaningless sentences, and reading aloud of meaningful sentences. Areas in the bilateral frontal and temporal cortices were activated during the reading-aloud task compared with the covert reading task. In addition, activation of these brain areas showed significant positive correlation with the reading speed during the reading-aloud task. Our results indicate that bilateral frontal-temporal networks are involved in phonological processing during reading aloud.

Context-dependent cortical activation in response to financial reward and penalty: an event-related fMRI study.

An event-related fMRI technique was used to assess neural responses to financial reward and penalty during a simple gambling task. We attempted to determine whether brain activities are dependent on the unique context of an event sequence. Thirty-six healthy volunteers participated in the study. The task was to guess the color of the suit of a card on each trial and to respond by pressing a button. Every correct response ("win") and incorrect response ("loss") was associated with financial reward and penalty, respectively. The magnitude of reward or penalty in each trial did not change; however, the subjects' self-reported emotional arousal was significantly higher for the events of "the fourth win of four wins in a row" and "the fourth loss of four losses in a row." We also found that the bilateral anterior cingulate and medial prefrontal cortices were specifically activated when the subjects experienced "the fourth win of four wins in a row" and "the fourth loss of four losses in a row." When the subjects experienced "a win following four losses in a row" or "a loss following four wins in a row," the right dorsolateral prefrontal cortex was specifically activated. Our data indicate that there exist brain activities associated with the event-sequence context in which abstract reward or penalty is received. These context-dependent activities appear to be crucial for adapting oneself to new circumstances and may account for clinical symptoms of various mental illnesses in which dysfunction of these regions has been reported.

The human prefrontal and parietal association cortices are involved in NO-GO performances: an event-related fMRI study.

One of the important roles of the prefrontal cortex is inhibition of movement. We applied an event-related functional magnetic resonance imaging (fMRI) technique to observe changes in fMRI signals of the entire brain during a GO/NO-GO task to identify the functional fields activated in relation to the NO-GO decision. Eleven normal subjects participated in the study, which consisted of a random series of 30 GO and 30 NO-GO trials. The subjects were instructed to press a mouse button immediately after the GO signal was presented. However, they were instructed not to move when the NO-GO signal was presented. We detected significant changes in MR signals in relation to the preparation phases, GO responses, and NO-GO responses. The activation fields related to the NO-GO responses were located in the bilateral middle frontal cortices, left dorsal premotor area, left posterior intraparietal cortices, and right occipitotemporal area. The fields of activation in relation to the GO responses were found in the left primary sensorimotor, right cerebellar anterior lobule, bilateral thalamus, and the area from the anterior cingulate to the supplementary motor area (SMA). Brain activations related to the preparation phases were identified in the left dorsal premotor, left lateral occipital, right ventral premotor, right fusiform, and the area from the anterior cingulate to the SMA. The results indicate that brain networks consisting of the bilateral prefrontal, intraparietal, and occipitotemporal cortices may play an important role in executing a NO-GO response.

Direction of cross-modal information transfer affects human brain activation: a PET study.

The purpose of this study was to determine the functional organization of the human brain involved in cross-modal discrimination between tactile and visual information. Regional cerebral blood flow was measured by positron emission tomography in nine right-handed volunteers during four discrimination tasks; tactile-tactile (TT), tactile-visual (TV), visual-tactile (VT), and visual-visual (VV). The subjects were asked either to look at digital cylinders of different diameters or to grasp the digital cylinders with the thumb and index finger of the right hand using haptic interfaces. Compared with the motor control task in which the subjects looked at and grasped cylinders of the same diameter, the right lateral prefrontal cortex and the right inferior parietal lobule were activated in all the four discrimination tasks. In addition, the dorsal premotor cortex, the ventral premotor cortex, and the inferior temporal cortex of the right hemisphere were activated during VT but not during TV. Our results suggest that the human brain mechanisms underlying cross-modal discrimination have two different pathways depending on the temporal order in which stimuli are presented.