Advances in Mechanical Thrombectomy for Acute Ischemic Stroke Due to Large Vessel Occlusion

Acute ischemic stroke (AIS) is a significant cause of global morbidity and mortality, with functional implications for quality of life and long-term disability. The limitations of intravenous thrombolytic therapy for the treatment of AIS, especially for emergent large vessel occlusion (ELVO), have paved the way for alternative therapies and the rapidly evolving landscape of endovascular therapy (EVT). Here, we summarize the major landmark trials that have advanced the field largely due to ongoing biomedical engineering device development that have translated into significantly improved clinical outcomes. Our review describes the clinical success of EVT, and current and future trends.

Plasma membrane guanylate cyclase is a multimodule transduction system

This minireview highlights the studies which suggest that guanylate cyclase is a single-component transducing system, containing distinct signaling modules in a single membrane-spanning protein. A guanylate cyclase signaling model is proposed which envisions the following sequential events: (1) a signal is initiated by the binding of the hormone to the ligand binding module; (2) the signal is potentiated by ATP at ARM; and (3) the amplified signal is finally transduced at the catalytic site. All of these signaling steps together constitute a switch, which when turned on, generates the second messenger cyclic GMP.

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.

Allosteric regulatory step and configuration of the ATP-binding pocket in atrial natriuretic factor receptor guanylate cyclase transduction mechanism

The atrial natriuretic factor (ANF) signal transduction mechanism consists of the transformation of the signal information into the production of cyclic GMP. The binding of ANF to its receptor, which is also a guanylate cyclase, generates the signal. This cyclase has been termed atrial natriuretic factor receptor guanylate cyclase, ANF-RGC. ANF-RGC is a single transmembrane-spanning protein. The ANF receptor domain resides in the extracellular region of the protein, and the catalytic domain is located in the intracellular region at the C-terminus of the protein. Thus, the signal is relayed progressively from the receptor domain to the catalytic domain, where it is converted into the formation of cyclic GMP. The first transduction step is the direct binding of ATP with ANF-RGC. This causes allosteric regulation of the enzyme and primes it for the activation of its catalytic moiety. The partial structural motif of the ATP binding domain in ANF-RGC has been elucidated, and it has been named ATP regulatory module (ARM). In this presentation, we provide a brief review of the ATP-regulated transduction mechanism and the ARM model. The model depicts a configuration of the ATP-binding pocket that has been experimentally validated, and the model shows that the ATP-dependent transduction process is a two- (or more) step event. The first step involves the binding of ATP with its ARM. This partially activates the cyclase and prepares it for the subsequent steps, which are consistent with its being phosphorylated and attaining the fully activated state.

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.

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.

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.

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.

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.

Regions in vertebrate photoreceptor guanylyl cyclase ROS-GC1 involved in Ca(2+)-dependent regulation by guanylyl cyclase-activating protein GCAP-1

The membrane bound guanylyl cyclase (GC) photoreceptor membrane GC1 (ROS-GCI) of photoreceptor cells synthesizes cGMP, the intracellular transmitter of vertebrate phototransduction. The activity of ROS-GCI is controlled by small Ca(2+)-binding proteins, named GC-activating proteins (GCAPs). We identified and characterized two short regulatory regions (M445-L456 and L503-1522) in the juxtamembrane domain (JMD) of ROS-GC1 by peptide competition and mutagenesis studies. Both regions are critical for the activation of ROS-GCI by GCAP-1.

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.

A second calcium regulator of rod outer segment membrane guanylate cyclase, ROS-GC1: neurocalcin

ROS-GC represents a membrane guanylate cyclase subfamily whose distinctive feature is that it transduces diverse intracellularly generated Ca(2+) signals into the production of the second messenger cyclic GMP. An intriguing feature of the first subfamily member, ROS-GC1, is that it is both stimulated and inhibited by these signals. The inhibitory signals are processed by the cyclase activating proteins, GCAPs. The only known stimulatory signal is by the Ca(2+)-dependent guanylate cyclase activating protein, CD-GCAP. There are two GCAPs, 1 and 2, which link the cyclase with phototransduction, and one CD-GCAP, which is predicted to link ROS-GC1 with its retinal synaptic activity. Individual switches for these GCAPs and CD-GCAP have been respectively defined as CRM1, CRM3, and CRM2. This report defines the identity of a new ROS-GC1 regulator: neurocalcin. A surprising feature of the regulator is that it structurally is a GCAP but functionally behaves as a CD-GCAP. Recombinant neurocalcin stimulates ROS-GC1 in a dose-dependent fashion; the stimulation is Ca(2+)-dependent with an EC(50) of 20 microM; and the modulated domain resides at the C-terminal segment, between amino acids 731 and 1054. Previously, the residence of CRM2 has also been defined in this segment of the cyclase. However, the present study shows that the neurocalcin-regulated domain is distinct from CRM2. This is now designated as CRM4. Thus, the signal transduction mechanisms of neurocalcin and CD-GCAP are different, occurring through different modules of ROS-GC1. Neurocalcin signaling of ROS-GC1 is highly specific. It does not influence the activity of its second subfamily member, ROS-GC2, and of the other retinal guanylate cyclase, atrial natriuretic factor-receptor guanylate cyclase. In conclusion, the findings extend the concept of ROS-GC1's sensing diverse Ca(2+) signals, reveal the identity of its unexpected new Ca(2+) regulator, and show that the regulator acts through its specific cyclase domain. This represents an additional transduction mechanism of Ca(2+) signaling via ROS-GC1.

Activation of retinal guanylyl cyclase-1 by Ca2+-binding proteins involves its dimerization

Retinal guanylyl cyclase-1 (retGC-1), a key enzyme in phototransduction, is activated by guanylyl cyclase-activating proteins (GCAPs) if [Ca2+] is less than 300 nM. The activation is believed to be essential for the recovery of photoreceptors to the dark state; however, the molecular mechanism of the activation is unknown. Here, we report that dimerization of retGC-1 is involved in its activation by GCAPs. The GC activity and the formation of a 210-kDa cross-linked product of retGC-1 were monitored in bovine rod outer segment homogenates, GCAPs-free bovine rod outer segment membranes and recombinant bovine retGC-1 expressed in COS-7 cells. In addition to recombinant bovine GCAPs, constitutively active mutants of GCAPs that activate retGC-1 in a [Ca2+]-independent manner and bovine brain S100b that activates retGC-1 in the presence of approximately 10 microM [Ca2+] were used to investigate whether these activations take place through a similar mechanism, and whether [Ca2+] is directly involved in the dimerization. We found that a monomeric form of retGC-1 ( approximately 110 kDa) was mainly observed whenever GC activity was at basal or low levels. However, the 210-kDa product was increased whenever the GC activity was stimulated by any Ca2+-binding proteins used. We also found that [Ca2+] did not directly regulate the formation of the 210-kDa product. The 210-kDa product was detected in a purified GC preparation and did not contain GCAPs even when the formation of the 210-kDa product was stimulated by GCAPs. These data strongly suggest that the 210-kDa cross-linked product is a homodimer of retGC-1. We conclude that inactive retGC-1 is predominantly a monomeric form, and that dimerization of retGC-1 may be an essential step for its activation by active forms of GCAPs.

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.

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.

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.

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.

Calcium modulated signaling site in type 2 rod outer segment membrane guanylate cyclase (ROS-GC2)

The ROS-GC subfamily of membrane guanylate cyclases is at present represented by two members: ROS-GC1 and ROS-GC2. A unique functional feature of this subfamily is that it is intracellularly modulated in low Ca2+ concentration by calmodulin-like Ca(2+)-binding proteins termed GCAPs, 1 and 2, and the modulation is consistent with its linkage to phototransduction. The present study shows that: (1) GCAP2 is a specific modulator of ROS-GC2; (2) through systematic remodeling of ROS-GC modules, the study also shows that the modulated domain resides within the amino acid segment 736-1020. This domain is distinct form the corresponding GCAP1-modulated ROS-GC1 domain. Thus, GCAP1 and GCAP2 act through different ROS-GCs and through two different cyclase domains.

Differential activation of rod outer segment membrane guanylate cyclases, ROS-GC1 and ROS-GC2, by CD-GCAP and identification of the signaling domain

The ROS-GC is one of the two subfamilies of membrane guanylate cyclases. It distinguishes itself from the other surface receptor subfamily in that its members are not regulated by extracellular peptides; instead, they are modulated by intracellular Ca2+ signals. There are two members of the subfamily, ROS-GC1 and ROS-GC2. An intriguing feature of ROS-GC1 is that it has two Ca2+ switches. One switch inhibits the enzyme at micromolar concentrations of Ca2+, and the other stimulates. The inhibitory switch is linked to phototransduction, and it is likely that the stimulatory switch is linked to retinal synaptic activity. Ca2+ acts indirectly via Ca(2+)-binding proteins, GCAPs and CD-GCAP. GCAPs modulate the inhibitory switching component of the cyclase and CD-GCAP turns on the activation signaling switch. The activating switch of ROS-GC2 has not so far been scrutinized. The present study shows that CD-GCAP is linked to the activation signaling switch of ROS-GC2, but the linkage is about 10-fold weaker than that of the ROS-GC1. Thus, CD-GCAP is a specific ROS-GC1 activator. Furthermore, through a series of expression studies on the mutants involving deletion, building of hybrids, and reconstruction of a heterologous cyclase, the study confirms that the CD-GCAP regulated switch resides within the amino acid segment 736-1053 of the cyclase.

Third calcium-modulated rod outer segment membrane guanylate cyclase transduction mechanism

Ca2+-modulated rod outer segment membrane guanylate cyclase (ROS-GC1) has been cloned and reconstituted to show that it is regulated by two processes: one inhibitory, the other stimulatory. The inhibitory process is consistent with its linkage to phototransduction; the physiology of the stimulatory process is probably linked to neuronal transmission. In both regulatory processes, calcium modulation of the cyclase takes place through the calcium binding proteins; guanylate cyclase activating proteins (GCAP1 and GCAP2) in the case of the phototransduction process and calcium-dependent GCAP (CD-GCAP) in the case of the stimulatory process. The cyclase domains involved in the two processes are located at two different sites on the ROS-GC1 intracellular region. The GCAP1-modulated domain resides within the aa 447-730 segment of ROS-GC1 and the CD-GCAP-modulated domain resides within the aa 731-1054 segment. In the present study the GCAP2-dependent Ca2+ modulation of the cyclase activity has been reconstituted using recombinant forms of GCAP2 and ROS-GC1, and its mutants. The results indicate that consistent to phototransduction, GCAP2 at low Ca2+ concentration (10 nM) maximally stimulates the cyclase activity of the wild-type and its mutants: ext (deleted aa 8-408), kin (deleted aa 447-730) and hybrid consisting of the ext, transmembrane and kin domains of ANF-RGC and the C-terminal domain, aa 731-1054, of ROS-GC1. In all cases, it inhibits the cyclase activity with an IC50 of about 140 nM. A previous study has shown that under identical conditions the kin and the hybrid mutant are at best only minimally stimulated. Thus, the GCAP1 and GCAP2 signal transduction mechanisms are different, occurring through different modules of ROS-GC1. These findings also demonstrate that the intracellular region of ROS-GC1 is composed of multiple modules, each designed to mediate a particular calcium-specific signalling pathway.

A role for amino acid residues in the third cytoplasmic loop in defining the ligand binding characteristics of the alpha2D-adrenergic receptor

In this report, 5 amino acid residues (aa) in the third cytoplasmic loop of the alpha 2D-adrenergic receptor are identified which (individually or together) alter its ligand-binding characteristics. An important structural discrepancy exists in the third cytoplasmic loop of the alpha 2D-ARs encoded by the rat cDNA and the rat gene--five aa are different. The newly identified bovine receptor as well as the mouse receptor contained the 5 aa identical to that encoded by the rat cDNA. Site-directed mutation of these residues to those of the rat gene encoded receptor resulted in alteration of binding characteristics: significant changes in the ability of the mutant receptor to bind to a number of agonists and antagonists were observed--ranging from a decrease by half in the case of oxymetazoline, to near total loss of binding in the case of prazosin. Thus, the mutant receptor was no longer pure alpha 2D-AR. This indicated a hitherto unrealized role of the third cytoplasmic loop in defining the ligand-binding characteristics of the receptor, and also suggested that the rat gene sequence was most probably in error.

The bovine alpha 2D-adrenergic receptor gene: structure, expression in retina, and pharmacological characterization of the encoded receptor

This report describes cloning of the bovine alpha 2D-adrenergic receptor (alpha 2D-AR) gene and determination of the transcription start site, unequivocal presence of the alpha 2D-AR transcript in the retina, and pharmacological characteristics of the encoded product. Furthermore, expression of the gene in selected bovine tissues has also been scrutinized. A genomic clone was isolated from lambda EMBL3 library and a 3 kb fragment was subcloned and sequenced. This fragment contained the putative TATA box and the coding region. The encoded receptor was transiently expressed in COS cells. The recombinant receptor expressed pharmacological characteristics almost identical to the wild-type bovine retinal receptor, which were typical of the alpha 2D-AR subtype. RNase protection analysis confirmed the expression of the gene in the retina. The bovine receptor was structurally close to its rat analogue which also encodes the alpha 2D-AR, but, the highest homology was observed with the porcine receptor expressing alpha 2A-AR pharmacological characteristics. Certain structural features of the bovine gene were unique to itself and not shared by any other alpha2-AR subtype. Among the tissues tested using reverse transcriptase-polymerase chain reaction (RT-PCR), the alpha 2D-AR message was the most abundant in retina, followed by the brain and olfactory lobe. Thus, the availability of the bovine receptor gene probe will become an important additional tool in the elucidation of molecular mechanisms behind the alpha 2D-AR physiology in neurosensory processes such as those occurring in the eye and the brain.

Structural and functional characterization of retinal calcium-dependent guanylate cyclase activator protein (CD-GCAP): identity with S100beta protein

Calcium-dependent guanylate cyclase activator protein (CD-GCAP) is a low-molecular-weight retinal calcium-binding protein which activates rod outer segment guanylate cyclase (ROS-GC) in a calcium-dependent manner. This investigation was undertaken to determine the protein's structure and identity. Partial amino acid sequencing (72% of the protein), mass spectral analysis, cloning, and immunological studies revealed that CD-GCAP is identical to S100beta, another low-molecular-weight calcium-binding protein whose structure was known. We had shown earlier that the latter protein, which is usually called S100b (S100betabeta or dimer of S100beta), also activates ROS-GC but that the Vmax of activated cyclase was about 50% lower than when stimulated by CD-GCAP. S100b also required about 15 times more calcium (3.2 x 10(-)5 vs 1.5 x 10(-)6 M) for half-maximal stimulation of cyclase. To investigate the possibility that CD-GCAP is a post-translationally modified form of S100b, both proteins were treated with 1 M hydroxylamine which is known to deacylate proteins. After the treatment, CD-GCAP did not activate cyclase while S100b activation remained unaffected suggesting that CD-GCAP could not be a modified form of S100b. Hydroxylamine also broke down CD-GCAP into smaller fragments while leaving S100b intact. It therefore appeared that in spite of identical primary structures, the conformations of the two proteins were different. We then investigated the possibility that the purification procedures of the two proteins, which were quite different, could have contributed to such conformational differences: CD-GCAP purification included a step of heating at 75 degrees C in 5 mM Ca, while S100b purification included zinc affinity chromatography. To test the influence of these treatments on the properties of the proteins, CD-GCAP was subjected to zinc affinity chromatography and purified as S100b (CD-GCAP-->S100b) and S100b was heated in Ca and purified as CD-GCAP (S100b-->CD-GCAP). Cyclase activation, calcium-sensitivity, and hydroxylamine-lability measurements revealed that CD-GCAP-->S100b is identical to S100b and that S100b-->CD-GCAP is identical to CD-GCAP. Taken together the results demonstrate that CD-GCAP and S100b are one and the same protein and that their functional differences are due to different interconvertible conformational states.

Photoreceptor guanylate cyclases: a review

Almost three decades of research in the field of photoreceptor guanylate cyclases are discussed in this review. Primarily, it focuses on the members of membrane-bound guanylate cyclases found in the outer segments of vertebrate rods. These cyclases represent a new guanylate cyclase subfamily, termed ROS-GC, which distinguishes itself from the peptide receptor guanylate cyclase family that it is not extracellularly regulated. It is regulated, instead, by the intracellularly-generated Ca2+ signals. A remarkable feature of this regulation is that ROS-GC is a transduction switch for both the low and high Ca2+ signals. The low Ca2+ signal transduction pathway is linked to phototransduction, but the physiological relevance of the high Ca2+ signal transduction pathway is not yet clear; it may be linked to neuronal synaptic activity. The review is divided into eight sections. In Section I, the field of guanylate cyclase is introduced and the scope of the review is briefly explained; Section II covers a brief history of the investigations and ideas surrounding the discovery of rod guanylate cyclase. The first five subsections of Section III review the experimental efforts to quantify the guanylate cyclase activity of rods, including in vitro and in situ biochemistry, and also the work done since 1988 in which guanylate cyclase activity has been determined. In the remaining three subsections an analytical evaluation of the Ca2+ modulation of the rod guanylate cyclase activity related to phototransduction is presented. Section IV deals with the issues of a biochemical nature: isolation and purification, subcellular localization and functional properties of rod guanylate cyclase. Section V summarizes work on the cloning of the guanylate cyclases, analysis of their primary structures, and determination of their location with in situ hybridization. Section VI summarizes studies on the regulation of guanylate cyclases, with a focus on guanylate cyclases activating proteins. In Section VII, the evidence about the localization and functional role of guanylate cyclases in other retinal cells, especially in "on-bipolar" cells, in which guanylate cyclase most likely plays a critical role in electrical signaling, is discussed. The review concludes with Section VIII, with remarks about the future directions of research on retinal guanylate cyclases.

Structural and functional characterization of a second subfamily member of the calcium-modulated bovine rod outer segment membrane guanylate cyclase, ROS-GC2

A native bovine calcium-modulated rod outer segment membrane guanylate cyclase (ROS-GC) has been cloned and reconstituted to show its linkage consistent to the process of phototransduction. In the present study, a second form of the membrane guanylate cyclase has been cloned from the bovine retina. This cyclase shares a high sequence identity with ROS-GC, is specifically expressed in the bovine retina, and, like ROS-GC, is modulated in low Ca2+ by a calmodulin-like Ca2+-binding protein, termed GCAP2. For this reason, this cyclase has now been named ROS-GC2 and the previously described ROS-GC as ROS-GC1. The tail end of ROS-GC2 contains a stretch of five amino acids, a structural feature unique to itself. These findings support the existence of a calcium-modulated subfamily of ROS-GC and indicate that ROS-GC2 embodies a five amino acid signature element at its tail end.

Membrane guanylate cyclase signal transduction system

The suspect role of the receptor-mediated cyclic GMP signaling pathway was dispelled by the discovery of a membrane guanylate cyclase that was also an atrial natriuretic factor receptor. It is now established that the membrane guanylate cyclase transduction system is linked to the signaling of natriuretic factors, guanylin/enterotoxin, and emerging evidence suggests that a new neural tissue-specific subfamily of membrane guanylate cyclases exists whose mechanism of signal transduction is different from those of the other membrane cyclases. This review will briefly discuss the fascinating, albeit turbulent, history of this signal transduction field, which will be followed by its current status and finally the direction it is heading.

Distinct inhibitory ATP-regulated modulatory domain (ARMi) in membrane guanylate cyclases

Depending upon the cofactors Mg2+ or Mn2+, ATP stimulates or inhibits the signal transduction activities of the natriuretic factor receptor guanylate cyclases, ANF-RGC and CNP-RGC: there is stimulation in the presence of Mg2+ and inhibition in the presence of Mn2+. A defined core ATP-regulated modulatory (ARM) sequence motif within the intracellular 'kinase-like' domain of the cyclases is critical for stimulation, but the mechanism of the inhibitory transduction process is not known. In addition, ATP inhibits the basal cyclase activity of a rod outer segment membrane guanylate cyclase (ROS-GC). The mechanism of this inhibitory transduction process is also not known. These issues have been addressed in the present investigation through a program of deletion mutagenesis/expression studies of the cyclases. The study shows that the ATP-mediated inhibitory transduction processes of the natriuretic factor receptor cyclases and of ROS-GC are identical. The ATP-regulated inhibitory domain of all these cyclases resides within the C-terminal segment of the cyclase. This domain is in a different location from the one representing the ATP-stimulatory ARM. The identification of the inhibitory domain in the C-terminal segment of the cyclase indicates that this segment is composed of two separate domains: one representing a catalytic cyclase domain and the other an ATP-regulated inhibitory (ARMi) domain. These findings establish a novel ATP-mediated inhibitory transduction mechanism of the membrane guanylate cyclases which is distinct from that of its counterpart, the stimulatory ATP-mediated hormonal signal transduction mechanism. Thus, they define a new paradigm of guanylate cyclase-linked signal transduction pathways.

Calcium modulation of bovine photoreceptor guanylate cyclase

Bovine photoreceptor guanylate cyclase (ROS-GC) consists of a single transmembrane polypeptide chain with extracellular and intracellular domains. In contrast to non-photoreceptor guanylate cyclases (GCs) which are activated by hormone peptides, ROS-GC is modulated in low Ca2+ by calmodulin-like Ca(2+)-binding proteins termed GCAPs (guanylate cyclase-activating proteins). In this communication we show that, like the native system, ROS-GC expressed in COS cells is activated 4-6-fold by recombinant GCAP1 at 10 nM Ca2+ and that the reconstituted system is inhibited at physiological levels of Ca2+ (1 microM). A mutant ROS-GC in which the extracellular domain was deleted was stimulated by GCAP1 indistinguishable from native ROS-GC indicating that this domain is not involved in Ca2+ modulation. Deletion of the intracellular kinase-like domain diminished the stimulation by GCAP1, indicating that this domain is at least in part involved in Ca2+ modulation. Replacement of the catalytic domain in a non-photoreceptor GC by the catalytic domain of ROS-GC yielded a chimeric GC that was sensitive to ANF/ATP and to a lesser extent to GCAP1. The results establish that GCAP1 acts at an intracellular domain, suggesting a mechanism of photoreceptor GC stimulation fundamentally distinct from hormone peptide stimulation of other cyclase receptors.

Molecular and pharmacological identity of the alpha 2D-adrenergic receptor subtype in bovine retina and its photoreceptors

The rat cA2-47 gene encodes the pharmacologically defined alpha 2D-adrenergic receptor (alpha 2D-AR) subtype. Previously, the expression of its mRNA was shown in bovine retina by amplification through the reverse transcription-polymerase chain reaction (RT-PCR) of a region corresponding to the rat alpha 2D-AR, amino acid (aa) residues 382-439, indicating the presence of this subtype in this neural tissue. In the present study, the structure of this gene has been probed and the encoded receptor subtype has been characterized in bovine retina and its photoreceptor cells. The deduced aa sequence of the two bovine gene fragments, aa residues 290-375 and aa residues 392-434, demonstrates 77% overall identity with the rat alpha 2D-AR subtype and 80% overall identity with the mouse alpha 2D-AR. The receptor encoded by the bovine gene was expressed in the retina and its photoreceptors with the typical pharmacological characteristics established for the rat alpha 2D-AR subtype: The receptor bound rauwolscine with a KD of 14 nM in the retina and with that of 19 nM in the photoreceptor cells; the binding association rate constant, k+1, for the ligand was 0.012 min-1, the dissociation rate constant, k-1, was 0.14 min-1 and the half-time for dissociation was 5 min. Oxymetazoline displaced the bound [3H]-rauwolscine with an EC50 value of 85 nM, while SK & F 104078, and prazosin displaced the bound [3H]-rauwolscine with the respective IC50 values of 900 nM and 3000 nM. The other alpha 2-AR subtypes -alpha 2A-AR, alpha 2B-AR, alpha 2C-AR-were not detected in the retina and its photoreceptors. Thus, this study shows that the bovine alpha 2D-AR gene is a structural variant of the rat and mouse genes, that the bovine gene encodes the typical pharmacologically defined alpha 2D-AR subtype, that this subtype is present in its exclusive form in the bovine retina and its photoreceptors, where it may be presynaptic in nature.

Molecular characterization of S100A1-S100B protein in retina and its activation mechanism of bovine photoreceptor guanylate cyclase

In contrast to the membrane guanylate cyclases which are stimulated by extracellular ligands, rod outer segment membrane guanylate cyclase (ROS-GC) activity is modulated intracellularly by calcium in two ways: one, where it is inhibited, and the other, where it is stimulated. The former way is linked to the phototransduction, and physiology of the second is unknown. In both ways calcium modulation of the cyclase occurs through the calcium binding proteins: through guanylate cyclase activating proteins (GCAPs) in the case of phototransduction, and through the recently discovered calcium-dependent GCAP (CD-GCAP) in the case of the other way. The kinase-like domain of ROS-GC is critical for the phototransduction-linked process. The present study shows the expression of alpha and beta chains of S100A1-S100B protein in the bovine retina and demonstrates that this protein stimulates ROS-GC activity in a dose-dependent fashion, that the stimulation is calcium dependent with an EC50 of 17 microM, and that the kinase-like domain is not involved in the calcium-modulated cyclase activation. Instead the involved domain resides at the C-terminal segment, between amino acids 731 and 1054. Thus, this S100A1-S100B protein-mediated calcium-modulated signal transduction mechanism is novel. Furthermore, this study provides the molecular understanding of the two transduction processes mediated by the same ROS-GC, one linked to the low and the other to the high calcium levels.

ATP modulation of the ligand binding and signal transduction activities of the type C natriuretic peptide receptor guanylate cyclase

The type C natriuretic peptide (CNP)-activated guanylate cyclase (CNP-RGC) is a single-chain transmembrane-spanning protein, containing both CNP binding and catalytic cyclase activities. Upon binding CNP to the extracellular receptor domain, the cytosolic catalytic domain of CNP-RGC is activated, generating the second messenger cyclic GMP. Obligatory in this activation process is an intervening signal transduction step which is regulated by ATP binding to the cyclase. This bridges the events of ligand binding and cyclase activation. A defined sequence motif (Gly499-Xa-Xa-Xa-Gly503), termed ATP regulatory module (ARM), is critical for this step. The present study shows that ATP not only amplifies the signal transduction step, it also concomitantly reduces the ligand binding activity of CNP-RGC. Reduction in the ligand binding activity is a consequence of the transformation of the high affinity receptor-form to the low affinity receptor-form. A single ARM residue Gly499 is critical in the mediation of both ATP effects, signal transduction and ligand binding activity of the receptor. Thus, this residue represents an ATP bimodal switch to turn the CNP signal on and off.

A novel calcium-dependent activator of retinal rod outer segment membrane guanylate cyclase

The membrane guanylate cyclase in retinal rod outer segments (ROS-GC) is known to be negatively regulated by calcium; when the calcium concentration is reduced below the dark-adapted level of about 500 nM, the enzyme is activated by a soluble protein. We now report that the enzyme is also positively regulated by calcium; a novel soluble protein is identified and purified from bovine retina which activates ROS-GC, with half-maximal activation occurring at 2-5 microM calcium. The activation is dose-dependent, and at its maximum, cyclase is stimulated up to 25-fold. The activator has a molecular mass of about 40 kDa and is a multimer of a 6-7 kDa peptide.

Single amino acid residue-linked signaling shifts in the transduction activities of atrial and type C natriuretic factor receptor guanylate cyclases

The type A (ANF) and the type C (CNP) natriuretic factor-activated guanylate cyclases, respectively termed as ANF-RGC and CNP-RGC, are single-chain transmembrane-spanning proteins, containing ligand binding and catalytic cyclase domains at two opposite ends of the protein. The binding activity resides at the N-terminal extracellular region and the catalytic cyclase activity at the carboxyl end. The ANF-RGC residue Leu-364, residing in the extracellular region, is critical for the ANF-binding activity; the CNP-RGC residue Glu-332 is critical for the CNP-binding activity. The counter part of CNP-RGC-Glu-332 residue is the ANF-RGC residue Gln-338 and of ANF-RGC-Leu-364 residue in CNP-RGC is the Valine-358. The present study shows a remarkable signal switching phenomenon associated with these residues. By changing the ANF-RGC residue Gln-338 to Glu, ANF-RGC switches from no to significant CNP signal transduction activity; similarly, a change from Valine-358 to Leu generates ANF signal transduction activity in CNP-RGC. These acquired signal transduction activities in the cyclases are in addition to their natural signal transduction activities. Thus, these new cyclases show both ANF and CNP signaling activities.

Regulation of bovine rod outer segment membrane guanylate cyclase by ATP, phosphodiesterase and metal ions

In vertebrate retina, rod outer segment is the site of visual transduction. The inward cationic current in the dark-adapted outer segment is regulated by cyclic GMP. A light flash on the outer segment activates a cyclic GMP phosphodiesterase resulting in rapid hydrolysis of the cyclic nucleotide which in turn causes a decrease in the dark current. Restoration of the dark current requires inactivation of the phosphodiesterase and synthesis of cyclic GMP. The latter is accomplished by the enzyme guanylate cyclase which catalyzes the formation of cyclic GMP from GTP. Therefore, factors regulating the cyclase activity play a critical role in visual transduction. But regulation of the cyclase by some of these factors--phosphodiesterase, ATP, the soluble proteins and metal cofactors (Mg and Mn)--is controversial. The availability of different types of cyclase preparations, dark-adapted rod outer segments with fully inhibited phosphodiesterase activity, partially purified cyclase without PDE contamination, cloned rod outer segment cyclase free of other rod outer segment proteins, permitted us to address these controversial issues. The results show that ATP inhibits the basal cyclase activity but enhances the stimulation of the enzyme by soluble activator, that cyclase can be activated in the dark at low calcium concentrations under conditions where phosphodiesterase activity is fully suppressed, and that greater activity is observed with manganese as cofactor than magnesium. These results provide a better understanding of the controls on cyclase activity in rod outer segments and suggest how regulation of this cyclase by ATP differs from that of other known membrane guanylate cyclases.

ATP bimodal switch that regulates the ligand binding and signal transduction activities of the atrial natriuretic factor receptor guanylate cyclase

Atrial natriuretic factor (ANF)-dependent guanylate cyclase (ANF-RGC) is a single-chain transmembrane-spanning protein, containing both ANF binding and catalytic cyclase activity. 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 step regulated by the ATP binding to the cyclase. This is a signal transduction step that bridges the events of ligand binding and cyclase activation. A defined structural motif (Gly503-Xa-Gly505-Xa-Xa-Xa-Gly509), termed ATP regulatory module (ARM), is critical for this step. The present study shows that the ARM-Gly505 residue acts as an ATP bimodal switch in regulating both the ligand binding and signal transduction activities of ANF-RGC, thus representing a critical site to turn the hormone signal on and off.

Structural, genetic and pharmacological identity of the rat alpha 2-adrenergic receptor subtype cA2-47 and its molecular characterization in rat adrenal, adrenocortical carcinoma and bovine retina

Subsequent to the first alpha 2-adrenergic receptor (alpha 2-AR) gene cloning of alpha 2-C10 from human platelet, cloning of the first rodent alpha 2-AR cDNA, cA2-47, was reported. Based on the structural and limited pharmacological comparison, it was concluded that the rodent receptor is a molecular and pharmacological analog of the human receptor, which is pharmacologically classified as the alpha 2A-AR. A later study slightly revised the structure of the human receptor. Thus, the precise structural comparison of the rat receptor to the human platelet receptor is no longer valid. Another rat alpha 2-AR gene, RG20, was then cloned and was also found to be a structural analog of the human alpha 2-C10. It, however, varied slightly from the alpha 2A subtype pharmacology, but matched the newly defined alpha 2D subtype pharmacology. It was, therefore, concluded that RG20 encodes the alpha 2D subtype. The structural and pharmacological relationship of RG20 with cA2-47 is not known, although it has been tacitly assumed that both are the identical alpha 2D receptor subtypes. The present study addresses this and other issues relating to the precise structural, genetic and pharmacological relationship of cA2-47 with the human platelet alpha 2-C10 receptor, and also the localization of cA2-47 transcript in certain rat tissues. The results show that the cA2-47 receptor shows a high degree of sequence identity to the alpha 2-C10 receptor, yet important differences exist between them. The sequence identity of cA2-47 receptor to the RG20 receptor is almost, but not quite complete. The cA2-47 gene is not present in the human and the human gene is not present in the rat; that cA2-47 receptor subtype is pharmacologically similar to the RG20 receptor subtype, both being of the alpha 2D subtype. The cA2-47 receptor transcript in addition to being found in the rat brain is present in the rat adrenal gland, testes, adrenocortical carcinoma and the bovine retina.

Structural and functional characterization of the rod outer segment membrane guanylate cyclase

In the vertebrate photoreceptor cell, rod outer segment (ROS) is the site of visual signal-transduction process, and a pivotal molecule that regulates this process is cyclic GMP. Cyclic GMP controls the cationic conductance into the ROS, and light causes a decrease in the conductance by activating hydrolysis of the cyclic nucleotide. The identity of the granylate cyclase (ROS-GC) that synthesizes this pool of cyclic GMP is unknown. We now report the cloning, expression and functional characterization of a DNA from bovine retina that encodes ROS-GC.

Glutamic acid-332 residue of the type C natriuretic peptide receptor guanylate cyclase is important for signaling

The type C natriuretic peptide (CNP)-activated guanylate cyclase (CNP-RGC) is a single-chain transmembrane-spanning protein, predicted to contain both ligand binding and catalytic activities. Upon binding CNP, CNP-RGC catalyzes the formation of cyclic GMP. We now show that the Glu-332 residue residing in the extracellular region of CNP-RGC plays an important role in signal transduction. Deletion of the CNP-RGC intracellular region resulted in the CNP receptor which lacked cyclase activity; deletion or substitution of Glu-332 with His or Lys resulted in almost total loss of both CNP binding and the CNP-dependent cyclase activity without affecting the basal cyclase activity of the mutant proteins. These observations support the general signal transduction model of the subfamily of natriuretic factor receptor cyclases where it is predicted that ligand binding to the extracellular receptor domain of the protein activates the cytosolic catalytic domain, generating the second-messenger cyclic GMP, and identify an amino acid residue of CNP-RGC that plays an important role in CNP signaling.

The glycine residue of ATP regulatory module in receptor guanylate cyclases that is essential in natriuretic factor signaling

Atrial natriuretic factor (ANF) and C-type natriuretic peptide (CNP)-activated guanylate cyclases are single-chain transmembrane-spanning proteins, containing both ligand binding and catalytic activities. In both proteins, ligand binding to the extracellular receptor domain activates the cytosolic catalytic domain, generating the second messenger cyclic GMP. Obligatory in this activation process is an ATP-dependent step. ATP directly binds to a defined ATP-regulatory module (ARM) sequence motif in the cyclases and through ARM bridges the events of ligand binding and signal transduction. These ARM sequence motifs are respectively represented by Gly503-Xa-Gly505-Xa-Xa-Xa-Gly509 and Gly499-Xa-Xa-Xa-Gly503 in the case of ANF receptor guanylate cyclase (ANF-RGC) and CNP receptor guanylate cyclase (CNP-RGC). Through genetic remodeling techniques, we now show that ARM-Gly505 in ANF-RGC and the corresponding ARM-Gly499 in CNP-RGC are critical for ANF and CNP signaling, and other ARM-Gly residues have minimal effect in the respective signaling processes.

Structural and biochemical identity of retinal rod outer segment membrane guanylate cyclase

Recent molecular cloning reports show that there are at least three membrane guanylate cyclases in vertebrate retina: (1) atrial natriuretic factor receptor guanylate cyclase (ANF-RGC), (2) C-type natriuretic peptide receptor guanylate cyclase (CNP-RGC), and (3) "retinal guanylate cyclase" (RetGC). The specific cellular localization of the first two cyclases is unknown, but RetGC is apparently localized in photoreceptor cells, suggesting that it participates in visual transduction. With the overall objective of identifying the guanylate cyclase that is linked to phototransduction, we compared the structural and regulatory properties of the biochemically characterized 112 kDa bovine rod outer segment membrane guanylate cyclase (ROS-GC) with those of RetGC, ANF-RGC and CNP-RGC. The N-terminal and two internal peptide sequences of purified ROS-GC had about 90% similarity with the corresponding sequences of the RetGC; the sequence identity with natriuretic peptide receptor cyclases was about 30%. A 19 amino acid long sequence from a tryptic peptide of ROS-GC had no corresponding sequence in the other three cyclases. ROS-GC was inhibited by ATP but ANF-RGC and CNP-RGC were activated by ATP in the presence of the respective peptide hormones. These results suggest that ROS-GC represents a new subtype of the membrane guanylate cyclase family that is structurally and biochemically distinct from the other retinal cyclases.

Interaction of atrial natriuretic factor and endothelin-1 signals through receptor guanylate cyclase in pulmonary artery endothelial cells

The endothelial cell has a unique intrinsic feature: it produces a most potent vasopressor peptide hormone, endothelin (ET-1), yet it also contains a signaling system of an equally potent hypotensive hormone, atrial natriuretic factor (ANF). This raises two related curious questions: does the endothelial cell also contain an ET-1 signaling system? If yes, how do the two systems interact with each other? The present investigation was undertaken to determine such a possibility. Bovine pulmonary artery endothelial (BPAE) cells were chosen as a model system. Identity of the ANF receptor guanylate cyclase was probed with a specific polyclonal antibody to the 180 kDa membrane guanylate cyclase (mGC) ANF receptor. A Western-blot analysis of GTP-affinity-purified endothelial cell membrane proteins recognized a 180 kDa band; the same antibody inhibited the ANF-stimulated guanylate cyclase activity; the ANF-dependent rise of cyclic GMP in the intact cells was dose-dependent. By affinity cross-linking technique, a predominant 55 kDa membrane protein band was specifically labeled with [125I]ET-1. ET-1 treatment of the cells showed a migration of the protein kinase C (PKC) activity from cytosol to the plasma membrane; ET-1 inhibited the ANF-dependent production of cyclic GMP in a dose-dependent fashion with an EC50 of 100 nM. This inhibitory effect was duplicated by phorbol 12-myristate 13-acetate (PMA), a known PKC-activator. The EC50 of PMA was 5 nM. A PKC inhibitor, 1-(5-isoquinolinyl-sulfonyl)-2-methyl piperazine (H-7), blocked the PMA-dependent attenuation of ANF-dependent cyclic GMP formation. These results demonstrate that the 180 kDa mGC-coupled ANF and ET-1 signaling systems coexist in endothelial cells and that the ET-1 signal negates the ANF-dependent guanylate cyclase activity and cyclic GMP formation. Furthermore, these results support the paracrine and/or autocrine role of ET-1.

Cloning and expression of an ATP-regulated human retina C-type natriuretic factor receptor guanylate cyclase

The natriuretic factors are structurally related polypeptide hormones that regulate the hemodynamics of the physiological processes of diuresis, water balance, and blood pressure. Presumably, these hormones act through the activation of guanylate cyclases which are also the specific receptors of these hormones. Two such structurally similar cell surface receptors are known; the ligand for one is atrial natriuretic factor (ANF) and for the other is C-type natriuretic peptide (CNP). Studies with ANF receptor guanylate cyclase (ANF-RGC) have indicated that its ligand binding site is extracellular and the catalytic site is intracellular, but the mere ligand binding to the receptor domain does not activate the cytosolic catalytic domain. An intervening ATP-mediated event is obligatory: ATP binds to a defined ATP-regulated module (ARM) sequence and bridges the events of ligand binding and signal transduction. The mechanism of CNP signaling is not known, although CNP in intact cells transfected with CNP receptor guanylate cyclase (CNP-RGC) stimulates the formation of cyclic GMP. Furthermore, there is no prior evidence of the presence of CNP signal transduction system in retina, although the presence of ANF-RGC has been documented. We now report the molecular cloning and expression of CNP-RGC from human retina and show that ATP is obligatory in CNP signaling also.(ABSTRACT TRUNCATED AT 250 WORDS)

Core sequence of ATP regulatory module in receptor guanylate cyclases

Atrial natriuretic factor (ANF) and C-type natriuretic peptide (CNP)-activated guanylate cyclases are single-chain transmembrane-spanning proteins, containing both ligand binding and catalytic activities. In both proteins, ligand binding to the extracellular receptor domain activates the cytosolic catalytic domain, generating the second messenger cyclic GMP. Studies with ANF receptor guanylate cyclase (ANF-RGC) have indicated that obligatory in this activation process is an ATP-dependent step. ATP directly binds to the cyclase and bridges the events of ligand binding and signal transduction. A defined ATP-regulated module (ARM) sequence (Gly503-Arg-Gly-Ser-Asn-Tyr-Gly509) in the cyclase is critical in the ATP-mediated event. Through genetic remodeling techniques, we have now identified the core ARM sequence that is essential in both ANF and CNP signaling. This sequence is Gly-Xa-Xa-Xa-Gly, represented by Gly505-Ser-Asn-Tyr-Gly509 in the case of ANF-RGC ARM and by Gly499-Ser-Ser-Tyr-Gly503 in the CNP receptor guanylate cyclase ARM.

A structural motif that defines the ATP-regulatory module of guanylate cyclase in atrial natriuretic factor signalling

Atrial natriuretic factor (ANF)-dependent guanylate cyclase is a single-chain transmembrane-spanning protein, containing an ANF receptor and having catalytic activity. ANF binding to the receptor domain activates the catalytic domain, generating the second messenger cyclic GMP. Obligatory in this activation process is an intervening step regulated by ATP, but its mechanism is not known. Through a programme of site-directed and deletion mutagenesis/expression studies, we report herein the identity of a structural motif (Gly503-Arg-Gly-Ser-Asn-Tyr-Gly509) that binds ATP and amplifies the ANF-dependent cyclase activity; this, therefore, represents an ATP-regulatory module (ARM) of the enzyme, which plays a pivotal role in ANF signalling.

Genetically tailored atrial natriuretic factor-dependent guanylate cyclase. Immunological and functional identity with 180 kDa membrane guanylate cyclase and ATP signaling site

Biochemical and immunological studies have established that one of the signal transducers of atrial natriuretic factor (ANF) is a 180 kDa membrane guanylate cyclase (180 kDa mGC), which is also an ANF receptor; obligatory in the transduction process is an intervening ATP-regulated step, but its mechanism is not known. GC alpha is a newly discovered member of the guanylate cyclase family whose activity is independent of the known natriuretic peptides, and the enzyme is not an ANF receptor. The genetically tailored GC alpha, GC alpha-DmutGln338Leu364, however, is not only a guanylate cyclase but also an ANF receptor and is structurally and functionally identical to the cloned wild-type ANF receptor guanylate cyclase, GC-A. We now report that the ANF-dependent guanylate cyclase activity in the particulate fractions of cells transfected with GC alpha-DmutGln338Leu364 was inhibited by the 180 kDa mGC polyclonal antibody, and with this antibody probe it was possible to purify the 130 kDa expressed receptor; the hormone-dependent cyclase activity of this receptor was exclusively dependent upon ATP; and through site-directed mutational studies with GC alpha mutants, the signaling sequence that defines ATP binding site was identified. We thus conclude that 180 kDa mGC and the mutant protein are immunologically similar, both proteins are linked to the ANF signal in the generation of cyclic GMP synthesis; and in both the ligand binding and catalytic activities are bridged through a defined ATP binding module.

Site-directed mutational analysis of a membrane guanylate cyclase cDNA reveals the atrial natriuretic factor signaling site

Natriuretic peptides are structurally related hormones that regulate hemodynamics of the physiological processes of diuresis, water balance, and blood pressure. One of the second messengers of these hormones is cGMP, and the type of receptor that is involved in the generation of cGMP is also a guanylate cyclase. Recent genetic evidence has revealed such a receptor family; two family members, GC-A and GC-B, have been cloned. We now describe the molecular cloning, sequencing, and expression of a cDNA clone from rat adrenal gland that encodes a membrane guanylate cyclase, GC alpha, that, with the exception of two amino acids, is structurally identical to GC-A and conforms to the purported topographical model of GC-A. The two amino acid changes are the substitutions Gln338----His338 and Leu364----Pro364, involving single nucleotide changes, CAG----CAC and CTG----CCG, respectively. Expression studies indicate that GC alpha cyclase activity is independent of the known natriuretic peptides, and direct binding studies demonstrate that GC alpha is not an ANF receptor. To determine the importance of Gln338 and Leu364 in ANF signaling, the GC alpha cDNA regions encoding amino acid residues 338 and 364 were remodeled by oligonucleotide-directed mutagenesis. A double mutant encoding Gln338 and Leu364, and a single-substitution mutant encoding Leu364 expressed both ANF binding and ANF-dependent cyclase activities, but the mutant encoding Gln338 and a deletion mutant lacking residue 364 did not express either of the above activities. These results define the critical role of Leu364 in ANF signal transduction.

Molecular cloning, sequencing and expression of an alpha 2-adrenergic receptor complementary DNA from rat brain

We have isolated a cDNA clone from rat brain using a human platelet alpha 2-adrenergic receptor genomic clone as a probe. Comparison of the deduced amino acid sequence (450 residues) corresponding to the rat brain cDNA with that of the human platelet and human kidney alpha 2-adrenergic receptors showed 84% and 44% sequence similarity, respectively. The major sequence difference between the rat brain and human platelet proteins, was a stretch of 48 amino acids within the third cytosolic loop in which the similarity was only 42%. Analysis of the 48 amino acid-region indicated that the two receptors significantly differ in terms of their primary amino acid sequence and the predicted secondary and tertiary structural features. There was no sequence similarity between the human platelet and rat brain clone over the 177 bases of 3'-noncoding sequence and a less than 50% similarity over a stretch of 210 nucleotides in the 5'-untranslated region. Southern-blot analysis with a human platelet alpha 2-adrenergic receptor probe revealed the existence of a single 5.2 kb restriction fragment (KpnI/SacI) in both human and rat genomic DNA; the rat brain alpha 2-receptor probe, however, hybridized to a single 1.9 kb band in rat DNA. Northern-blot analysis of rat brain poly(A+) RNA with the rat brain cDNA probe under stringent hybridization conditions revealed a single 4.5 kb mRNA; none was detected by the human platelet receptor probe. The rat brain 4.5 kb mRNA was not detected in any (other than brain) tested rat tissues utilizing either rat brain or human platelet DNA probes. The rat brain cDNA was expressed in a mammalian cell line (COS-2A) and found to bind the alpha 2-adrenergic antagonist [3H]yohimbine; based on the binding-affinity for prazosin, the presently cloned receptor was pharmacologically closer to the alpha 2A subclass. We conclude that the rat brain cDNA encodes a new alpha 2-adrenergic receptor subtype that may be brain-specific.

Regulation of guanylate cyclase activity by atrial natriuretic factor and protein kinase C

The putative 'second messenger' of certain atrial natriuretic factor (ANF) signal transductions is cyclic GMP. Recently, we purified a 180-kDa protein, apparently containing both ANF receptor and guanylate cyclase activities, and hypothesized that this is one of the cyclic GMP transmembrane signal transducers. The enzyme is ubiquitous and appears to be conserved. Utilizing the 180-kDa membrane guanylate cyclase, we now show that the 180-kDa guanylate cyclase is regulated in opposing fashions by two receptor signals--ANF stimulating it and protein kinase C inhibiting it. Furthermore, protein kinase C phosphorylates the 180-kDa enzyme. This suggests a novel 'switch on' and 'switch off' mechanism of the cyclic GMP signal transduction. 'Switch off' represents the phosphorylation while 'switch on' the dephosphorylation of the enzyme.