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V Venkataraman

Publications and source records attributed to V Venkataraman.

At least 19 recordsLinked to original sources
PubMed2001

A novel calcium-regulated membrane guanylate cyclase transduction system in the olfactory neuroepithelium.

This report defines the identity of a calcium-regulated membrane guanylate cyclase transduction system in the cilia of olfactory sensory neurons, which is the site of odorant transduction. The membrane fraction of the neuroepithelial layer of the rat exhibited Ca(2+)-dependent guanylate cyclase activity, which was eliminated by the addition of EGTA. This indicated that the cyclase did not represent a rod outer segment guanylate cyclase (ROS-GC), which is inhibited by free Ca(2+). This interpretation was supported by studies with the Ca(2+) binding proteins, GCAPs (guanylate cyclase activating proteins), which stimulate photoreceptor ROS-GC in the absence of Ca(2+). They did not stimulate the olfactory neuroepithelial membrane guanylate cyclase. The olfactory neuroepithelium contained a Ca(2+) binding protein, neurocalcin, which stimulated the cyclase in a Ca(2+)-dependent fashion. The cyclase was cloned from the neuroepithelium and was found to be identical in structure to that of the previously cloned cyclase termed GC-D. The cyclase was expressed in a heterologous cell system, and was reconstituted with its Ca(2+)-dependent activity in the presence of recombinant neurocalcin. The reconstituted cyclase mimicked the native enzyme. Immunocytochemical studies showed that the guanylate cyclase coexists with neurocalcin in the apical region of the cilia. Deletion analysis showed that the neurocalcin-regulated domain resides at the C-terminal region of the cyclase. The findings establish the biochemical, molecular, and functional identity of a novel Ca(2+)-dependent membrane guanylate cyclase transduction system in the cilia of the olfactory epithelium, suggesting a mechanism of the olfactory neuroepithelial guanylate cyclase regulation fundamentally distinct from the phototransduction-linked ROS-GC.

Animals↗
PubMed2001

Negatively calcium-modulated membrane guanylate cyclase signaling system in the rat olfactory bulb.

The mechanism by which the individual odor signals are translated into the perception of smell in the brain is unknown. The signal processing occurs in the olfactory system which has three major components: olfactory neuroepithelium, olfactory bulb, and olfactory cortex. The neuroepithelial layer is composed of ciliated sensory neurons interspersed among supportive cells. The sensory neurons are the sites of odor transduction, a process that converts the odor signal into an electrical signal. The electrical signal is subsequently received by the neurons of the olfactory bulb, which process the signal and then relay it to the olfactory cortex in the brain. Apart from information about certain biochemical steps of odor transduction, there is almost no knowledge about the means by which the olfactory bulb and cortical neurons process this information. Through biochemical, functional, and immunohistochemical approaches, this study shows the presence of a Ca(2+)-modulated membrane guanylate cyclase (mGC) transduction system in the bulb portion of the olfactory system. The mGC is ROS-GC1. This is coexpressed with its specific modulator, guanylate cyclase activating protein type 1 (GCAP1), in the mitral cells. Thus, a new facet of the Ca(2+)-modulated GCAP1--ROS-GC1 signaling system, which, until now, was believed to be unique to phototransduction, has been revealed. The findings suggest a novel role for this system in the polarization and depolarization phenomena of mitral cells and also contradict the existing belief that no mGC besides GC-D exists in the olfactory neurons.

Animals↗
PubMed2001

Three dimensional atomic model and experimental validation for the ATP-Regulated Module (ARM) of the atrial natriuretic factor receptor guanylate cyclase.

Atrial natriuretic factor (ANF) receptor guanylate cyclase (ANF-RGC) is a single chain transmembrane-spanning protein, containing both ANF binding and catalytic activities. ANF binding to the extracellular receptor domain activates the cytosolic catalytic domain, generating the second messenger cyclic GMP. Obligatory in this activation process is an intervening transduction step, which is regulated by the binding of ATP to the cyclase. The partial structural motif of the ATP binding domain of the cyclase has been elucidated and has been termed ATP Regulatory Module (ARM). The crystal structures of the tyrosine kinase domains of the human insulin receptor and haematopoietic cell kinase were used to derive a homology-based model of the ARM domain of ANF-RGC. The model identifies the precise configuration of the ATP-binding pocket in the ARM domain, accurately represents its ATP-dependent features, and shows that the ATP-dependent transduction phenomenon is a two-step mechanism. In the first step, ATP binds to its pocket and changes its configuration; in the second step, via an unknown protein kinase, it phosphorylates the cyclase for its full activation.

Adenosine Triphosphate↗
PubMed2001

Idiopathic cardiac asystole presenting as an intractable adult onset partial seizure disorder.

Asystole can occur during partial seizures. Conversely, asystole may produce clinical features associated semiologically with partial or secondarily generalized tonic--clonic seizures. Management is so profoundly different that accurate diagnosis is critical. We performed simultaneous scalp video electroencephalographic (EEG) and electrocardiographic (ECG) recordings in three patients who presented with an intractable seizure disorder. Habitual events were captured and reviewed. These revealed similar clinical features and temporal association with the ECG and EEG findings. Idiopathic asystole was detected as the cause in all three. All underwent emergency cardiac pacemaker implantation with resultant cessation of their paroxysmal episodes. If this diagnostic study had been delayed or not performed, all of our patients could have presented as sudden unexpected death in epilepsy (SUDEP). Early video-EEG and ECG monitoring is essential in adults with intractable seizure disorders.

Adult↗
PubMed2001

Mapping of expressive language cortex using magnetic source imaging.

The accurate localization of primary language cortex is one of the goals in the evaluation of brain surgery candidates. In this paper we describe the localization of expressive language cortex using magnetic source imaging (MSI) in a patient with refractory epilepsy caused by a tumor affecting the left inferior frontal gyrus. The magnetoencephalographic recordings during an expressive language task were co-registered with a magnetic resonance imaging (MRI) scan obtained after subdural grid placement and provided a cluster of sources of brain activation in the vicinity of the lesion. This map of expressive language provided by MSI was verified with electrocortical stimulation before the operation. No speech problems were reported in our patient after the resection of the lesion, suggesting that MSI is an accurate non-invasive method for the pre-surgical mapping of expressive language in cases where there is clear functional risk during tumor resection.

Adolescent↗
PubMed2001

Language dominance in children as determined by magnetic source imaging and the intracarotid amobarbital procedure: a comparison.

This study evaluated the validity of data derived from magnetic source imaging regarding hemispheric dominance for language in children and adolescents with intractable seizure disorder by comparison with results of the intracarotid amobarbital procedure. Functional imaging of the receptive language cortex using a whole-head neuromagnetometer was performed in 19 consecutive epilepsy patients, ages 8 to 18 years, who also underwent the intracarotid amobarbital procedure. During magnetic source imaging recordings, patients engaged in a continuous recognition memory task for words in visual and auditory modalities. This task has previously been shown to be valid for the purpose of lateralization and localization of language cortex in adult epilepsy patients who undergo the intracarotid amobarbital procedure and intraoperative language mapping allowing confirmation of magnetic source imaging findings. Results indicated that language laterality indices formed for the intracarotid amobarbital procedure and magnetic source imaging procedures were highly correlated (R = .87). In addition, clinical judgments regarding cerebral dominance for language made by independent raters using the two methods were in excellent agreement. We conclude that magnetic source imaging is a promising method for determination of cerebral dominance for language in children and adults.

Adolescent↗
PubMed2000

Impairment of the rod outer segment membrane guanylate cyclase dimerization in a cone-rod dystrophy results in defective calcium signaling.

Rod outer segment membrane guanylate cyclase1 (ROS-GC1) is the original member of the membrane guanylate cyclase subfamily whose distinctive feature is that it transduces diverse intracellularly generated Ca(2+) signals in the sensory neurons. In the vertebrate retinal neurons, ROS-GC1 is pivotal for the operations of phototransduction and, most likely, of the synaptic activity. The phototransduction- and the synapse-linked domains are separate, and they are located in the intracellular region of ROS-GC1. These domains sense Ca(2+) signals via Ca(2+)-binding proteins. These proteins are ROS-GC activating proteins, GCAPs. GCAPs control ROS-GC1 activity through two opposing regulatory modes. In one mode, at nanomolar concentrations of Ca(2+), the GCAPs activate the cyclase and as the Ca(2+) concentrations rise, the cyclase is progressively inhibited. This mode operates in phototransduction via two GCAPs: 1 and 2. The second mode occurs at micromolar concentrations of Ca(2+) via S100beta. Here, the rise of Ca(2+) concentrations progressively stimulates the enzyme. This mode is linked with the retinal synaptic activity. In both modes, the final step in Ca(2+) signal transduction involves ROS-GC dimerization, which causes the cyclase activation. The identity of the dimerization domain is not known. A heterozygous, triple mutation -E786D, R787C, T788M- in ROS-GC1 has been connected with autosomal cone-rod dystrophy in a British family. The present study shows the biochemical consequences of this mutation on the phototransduction- and the synapse-linked components of the cyclase. (1) It severely damages the intrinsic cyclase activity. (2) It significantly raises the GCAP1- and GCAP2-dependent maximal velocity of the cyclase, but this compensation, however, is not sufficient to override the basal cyclase activity. (3) It converts the cyclase into a form that only marginally responds to S100beta. The mutant produces insufficient amounts of the cyclic GMP needed to drive the machinery of phototransduction and of the retinal synapse at an optimum level. The underlying cause of the breakdown of both types of machinery is that, in contrast to the native ROS-GC1, the mutant cyclase is unable to change from its monomeric to the dimeric form, the form required for the functional integrity of the enzyme. The study defines the CORD in molecular terms, at a most basic level identifies a region that is critical in its dimer formation, and, thus, discloses a single unifying mechanistic theme underlying the complex pathology of the disease.

Amino Acid Substitution↗
PubMed2000

Rod outer segment membrane guanylate cyclase type 1-linked stimulatory and inhibitory calcium signaling systems in the pineal gland: biochemical, molecular, and immunohistochemical evidence.

Recent evidence indicates the presence of a novel alpha(2D/A)-adrenergic receptor (alpha(2D/A)-AR) linked membrane guanylate cyclase signal transduction system in the pineal gland. This system operates via a Ca(2+)-driven rod outer segment membrane guanylate cyclase (ROS-GC). In the present study, this transduction system has been characterized via molecular, immunohistochemical, and biochemical approaches. The two main components of the system are ROS-GC1 and its Ca(2+) regulator, S100B. Both components coexist in pinealocytes where the signaling component alpha(2D/A)-AR also resides. The presence of ROS-GC2 was not detected in the pineal gland. Thus, transduction components involved in processing alpha(2D/A)-AR-mediated signals are Ca(2+), S100B, and ROS-GC1. During this investigation, an intriguing observation was made. In certain pinealocytes, ROS-GC1 coexisted with its other Ca(2+) modulator, guanylate cyclase activating protein type 1 (GCAP1). In these pinealocytes, S100B was not present. The other GCAP protein, GCAP2, which is also a known modulator of ROS-GC in photoreceptors, was not present in the pineal gland. The results establish the identity of an alpha(2D/A)-AR-linked ROS-GC1 transduction system in pinealocytes. Furthermore, the findings show that ROS-GC1, in a separate subpopulation of pinealocytes, is associated with an opposite Ca(2+) signaling pathway, which is similar to phototransduction in retina. Thus, like photoreceptors, pinealocytes sense both positive and negative Ca(2+) signals, where ROS-GC1 plays a pivotal role; however, unlike photoreceptors, the pinealocyte is devoid of the ROS-GC2/GCAP2 signal transduction system.

Animals↗
PubMed2000

Three dimensional atomic model and experimental validation for the ATP-Regulated Module (ARM) of the atrial natriuretic factor receptor guanylate cyclase.

Atrial natriuretic factor (ANF) receptor guanylate cyclase (ANF-RGC) is a single chain transmembrane-spanning protein, containing both ANF binding and catalytic activities. ANF binding to the extracellular receptor domain activates the cytosolic catalytic domain, generating the second messenger cyclic GMP. Obligatory in this activation process is an intervening transduction step, which is regulated by the binding of ATP to the cyclase. The partial structural motif of the ATP binding domain of the cyclase has been elucidated and has been termed ATP Regulatory Module (ARM). The crystal structures of the tyrosine kinase domains of the human insulin receptor and haematopoietic cell kinase were used to derive a homology-based model of the ARM domain of ANF-RGC. The model identifies the precise configuration of the ATP-binding pocket in the ARM domain, accurately represents its ATP-dependent features, and shows that the ATP-dependent transduction phenomenon is a two-step mechanism. In the first step, ATP binds to its pocket and changes its configuration; in the second step, via an unknown protein kinase, it phosphorylates the cyclase for its full activation.

Adenosine Triphosphate↗
PubMed1999

Mutations in the rod outer segment membrane guanylate cyclase in a cone-rod dystrophy cause defects in calcium signaling.

Rod outer segment guanylate cyclase 1 (ROS-GC1) is a member of the subfamily of Ca(2+)-regulated membrane guanylate cyclases; and it is pivotal for vertebrate phototransduction. Two opposing regulatory modes control the activity of ROS-GC1. At nanomolar concentrations of Ca(2+), ROS-GC1 is activated by Ca(2+)-binding proteins named guanylate cyclase activating proteins (GCAPs). However, at micromolar concentrations of Ca(2+), ROS-GC1 is stimulated by S100beta [also named calcium-dependent (CD) GCAP]. This mode is not linked with phototransduction; instead, it is predicted to be involved in retinal synaptic activity. Two point mutations, E786D and R787C, in ROS-GC1 have been connected with cone-rod dystrophy (CORD6), with only one type of point mutation occurring in each family. The present study shows that the E786D mutation has no effect on the basal catalytic activity of ROS-GC1 and on its activation by GCAP1 and S100beta; however, the mutated cyclase becomes more activated by GCAP2. The R787C mutation has three consequences: (1) it causes major damage to the basal cyclase activity, (2) it makes the cyclase 5-fold more sensitive to activation by GCAP1; and 3) converts the cyclase into a form that is less sensitive to activation by GCAP2 and S100beta. Thus, the two CORD6-linked mutations in ROS-GC1, which occur at adjacent positions, result in vastly different biochemical phenotypes, and they are connected with very specific molecular defects in the Ca(2+) switching components of the cyclase. These defects, in turn, are proposed to have a profound effect on both the machinery of phototransduction and the retinal synapse. The study for the first time defines the biochemistry of CORD6 pathology in precise molecular terms.

Animals↗
PubMed1999

Functional consequences of a rod outer segment membrane guanylate cyclase (ROS-GC1) gene mutation linked with Leber's congenital amaurosis.

ROS-GC1 is the original member of the subfamily of membrane guanylate cyclases with two Ca2+ switches, which have been defined as CRM1 and CRM2. These are separately located within the intracellular domain of the cyclase. CRM1 switches on the enzyme at nanomolar concentrations of Ca2+ and is linked with phototransduction; the other stimulates at micromolar Ca2+ concentrations and is predicted to be linked with retinal synaptic activity. Ca2+ acts indirectly via Ca2+-binding proteins, GCAP1 and CD-GCAP. GCAP1 is a modulator of the CRM1 switch, and CD-GCAP turns on the CRM2 switch. A Leber's congenital amaurosis, termed LCA1, involves F514S point mutation in ROS-GC1. The present study shows that the mutation severely damages its intrinsic cyclase activity and inactivates its CRM1 switch but does not affect the CRM2 switch. In addition, on the basis of the established modulatory features of ROS-GC1, it is predicted that, in two other forms of LCA1 involving deletion of nt 460C or 693C, there is a frameshift in ROS-GC1 gene, which results in the nonexpression of the cyclase. For the first time, the findings define the linkage of distinct molecular forms of LCA to ROS-GC1 in precise biochemical terms; they also explain the reasons for the insufficient production of cyclic GMP in photoreceptors to sustain phototransduction, which ultimately leads to the degeneration of the photoreceptors.

Animals↗
PubMed1999

Safety of rapid intravenous infusion of valproate loading doses in epilepsy patients.

IV valproate may be given to patients requiring rapid elevation of serum valproate or for patients unable to take oral medication. Previously we established the loading dose of IV valproate needed to achieve serum concentrations of > 100 ug/ml. This study evaluated the safety of rapid infusion of valproate to achieve high therapeutic levels. Twenty-four infusions of IV valproate were carried out electively in twenty-one patients with epilepsy (ages 2-54 years). The dose ranged from 21-28 mg/kg (mean 24.2 mg/kg). Target infusion rates were 3 or 6 mg/kg per min, yielding an infusion duration of 4.17 or 8.34 min. ECG was monitored; blood pressure was measured before and after infusion. Post infusion serum valproate concentrations were 64-204.1 ug/ml (mean 132.6). There were no significant changes in blood pressure and no ECG abnormalities observed. Transient pain occurred at the site of injection in five patients, associated with redness in two. This appeared to related to the concentration of valproic acid in the infusion fluid. We conclude that a loading dose of IV valproate can be administered safely and rapidly. This finding enables further studies evaluating IV valproate as a non-sedative anticonvulsant in the management of status epilepticus.

Adolescent↗
PubMed1999

alpha2D/A-adrenergic receptor gene induction in the retina by phorbol ester: involvement of an AP-2 element.

BACKGROUND: The alpha2-adrenergic receptor (alpha2-AR) expressed in the bovine retina has been demonstrated to be of the alpha2D subtype. The bovine alpha2D-adrenergic receptor (alpha2D/A-AR) gene has been cloned and characterized. This report describes the induction of this gene by phorbol- 12, 13-myristate acetate (PMA), an activator of protein kinase C (PKC). RESULTS: Treatment of the bovine retina for 60 min with PMA (1 micrometer) resulted in significant and similar increases in alpha2D/A-AR mRNA level and gene transcription. This indicates that PMA causes alpha2D/A-AR gene induction and that this induction takes place directly at the transcriptional level. In C6 cells, treatment with PMA at a concentration which was as low as 0.1 micrometer induced endogenous alpha2D/A-AR mRNA after 60 min. Luciferase reporter assays in C6 cells mapped the PMA-responsive element to a region between -247 bp and -163 bp on the alpha2D/A-AR promoter. Electrophoretic mobility shift assays showed an increased binding of nuclear factor(s) from PMA-treated bovine retina to this promoter region. Competition assays indicate that an AP-2 element may be involved in the PMA-dependent induction. CONCLUSION: These findings demonstrate for the first time, the direct induction of the alpha2D/A-AR gene by PMA and support a role for an AP-2 element in the induction mechanism.

Animals↗
PubMed1999

New formulations of drugs in epilepsy.

The advent of numerous new treatment options in epilepsy therapy over the last decade is enabling a more flexible and individualized approach to patients with seizures. For some patients, these products offer added efficacy, reduction of troublesome side effects associated with standard anticonvulsants, and control over acute seizure exacerbations. This review profiles new formulations of anti-epileptic drugs. Tegretol-XR (TXR) and Carbatrol (CBTL), two extended-release preparations of carbamazepine (CBZ), which allow twice daily administration, minimising drug toxicity and improving efficacy. Topiramate sprinkles and lamotrigine chewable dispersible tablets allow easier administration in children. The rectal gel preparation of diazepam (Diastat) is useful for parents of patients with acute seizure exacerbations. Intravenous valproate (Depacon) and fosphenytoin (Cerebyx) provide parenteral treatment of acute seizures, without sedation or significant peripheral venous side effects. All of these new formulations expand treatment options for patients with epilepsy, who will benefit from them.

Anticonvulsants↗
PubMed1999

Socioeconomic aspects of peritoneal dialysis in North America: role of non medical factors in the choice of dialysis.

Patients initiating dialysis therapy must make a choice between hemodialysis (HD) and peritoneal dialysis (PD). Controversy persists over the relative merits of each modality in the treatment of end-stage renal disease (ESRD). Issues relating to survival, morbidity, economics, and patient characteristics will all determine the final choice of therapy. Non medical factors are the most important determinant of dialysis modality selection. In the United States, HD has been the more commonly used modality, while PD is underrepresented. This disparity arises from multiple factors including reactions (sometimes incorrect) to the healthcare financing structure, physician biases, and changing demographic patterns in the ESRD population. We discuss these issues and present collected evidence showing that increased use of PD may have substantial overall benefit.

Age Factors↗
PubMed1998

The alpha(2D/A)-adrenergic receptor-linked membrane guanylate cyclase: a new signal transduction system in the pineal gland.

In the pineal gland, the membrane guanylate cyclase activity was specifically stimulated by alpha(2D/A)-adrenergic receptor (alpha(2D/A)-AR) agonists. The agonists, however, did not stimulate the cyclase activity in the cell-free membranes. It was possible to stimulate the cyclase in cell-free membranes by the addition of the pineal soluble fraction, but this stimulation was Ca2+-dependent and alpha(2D/A)-agonist-independent. It was also possible to achieve Ca2+-dependent stimulation of the cyclase by the direct addition of CD-GCAP to the isolated pineal membranes. CD-GCAP is a Ca2+-binding protein and is a specific activator of one of the two members of the ROS-GC subfamily of membrane guanylate cyclases, ROS-GC1. The soluble fraction of the pineal gland stimulated recombinant ROS-GC1 in a Ca2+-dependent fashion. The direct presence of both ROS-GC1 and CD-GCAP in the pineal was established by molecular cloning/PCR studies. The findings demonstrate the existence of a novel signal transduction mechanism--the linkage of the alpha(2D/A)-AR signaling system with ROS-GC1 transduction system, occurring through intracellular Ca2+ via CD-GCAP.

Animals↗
PubMed1998

Rod outer segment membrane guanylate cyclase type 1 (ROS-GC1) gene: structure, organization and regulation by phorbol ester, a protein kinase C activator.

At present there are two recognized members of the ROS-GC subfamily of membrane guanylate cyclases. They are ROS-GC1 and ROS-GC2. A distinctive feature of this family is that its members are not switched on by the extracellular peptide hormones; instead, they are modulated by intracellular Ca2+ signals, consistent to their linkage with phototransduction. An intriguing feature of ROS-GC1, which distinguishes it from ROS-GC2, is that it has two Ca2+ switches. One switch inhibits the enzyme at micromolar concentrations of Ca2+, as in phototransduction; the other, stimulates. The stimulatory switch, most likely, is linked to retinal synaptic activity. Thus, ROS-GC1 is linked to both phototransduction and the synaptic activity. The present study describes (1) the almost complete structural identity of 18.5 kb ROS-GC1 gene; (2) its structural organization: the gene is composed of 20 exons and 19 introns with classical GT/AG boundaries; (3) the activity of the ROS-GC1 promoter assayed through luciferase reporter in COS cells; and (4) induction of the gene by phorbol ester, a protein kinase C (PKC) activator. The co-presence of PKC and ROS-GC1 in photoreceptors suggests that regulation of the ROS-GC1 gene by PKC might be a physiologically relevant phenomenon.

Animals↗