Effect of chromogranin A on central autonomic control of blood pressure

Vasostatin I (CgA17-76) vasoconstricts rat splanchnic vascular bed but does not affect central cardiovascular function

Vasostatin I (CgA(1-76)) is a naturally occurring biologically active peptide derived from chromogranin A (CgA), and is so named for its inhibitory effects on vascular tension. CgA mRNA is expressed abundantly in sympathoexcitatory catecholaminergic neurons of the rostral ventrolateral medulla (RVLM). CgA microinjection into the RVLM decreases blood pressure (BP), heart rate (HR) and sympathetic nerve activity (SNA). Proteolytic fragments of CgA are thought to be responsible for the cardiovascular effects observed. We hypothesised that vasostatin I is one of the fragments responsible for the central effects of CgA. We examined the role of a vasostatin I fragment, CgA(17-76) (VS-I((CgA17-76))), containing the portion important for biological effects. The effects of VS-I((CgA17-76)) delivered by intrathecal injection, or microinjection into the RVLM, on cardio-respiratory function in urethane anaesthetised, vagotomised, mechanically ventilated Sprague-Dawley rats (n=21) were evaluated. The effects of intrathecal VS-I((CgA17-76)) on the somato-sympathetic, baroreceptor and peripheral chemoreceptor reflexes were also examined. At the concentrations used (10, 100 or 200 μM, intrathecal; or 5 μM, RVLM microinjection) VS-I((CgA17-76)) produced no change in mean arterial pressure, HR, splanchnic SNA, phrenic nerve amplitude or phrenic nerve frequency. All reflexes examined were unchanged following intrathecal VS-I((CgA17-76)). In the periphery, VS-I((CgA17-76)) potentiated the contractile effects of noradrenaline on rat mesenteric arteries (n=6), with a significant left-shift in the dose response curve to noradrenaline (3.7×10(-7) vs 7.7×10(-7)). Our results indicate that VS-I((CgA17-76)) is active in the periphery but not centrally, and is not a central modulator of cardiorespiratory function and physiological reflexes.

Catestatin in rat RVLM is sympathoexcitatory, increases barosensitivity, and attenuates chemosensitivity and the somatosympathetic reflex

The fundamental role and corollary effects of neuropeptides that govern cardiorespiratory control in the brain stem are poorly understood. One such regulatory peptide, catestatin [Cts, human chromogranin A-(352-372)], noncompetitively inhibits nicotinic-cholinergic-stimulated catecholamine release. Previously, we demonstrated the presence of chromogranin A mRNA in brain stem neurons that are important for the maintenance of arterial pressure. In the present study, using immunofluorescence histochemistry, we show that Cts immunoreactivity is colocalized with tyrosine hydroxylase in C1 neurons of the rostral ventrolateral medulla (RVLM, n = 3). Furthermore, we investigated the effects of Cts on resting blood pressure, splanchnic sympathetic nerve activity, phrenic nerve activity, heart rate, and adaptive reflexes. Cts (1 mM in 50 nl or 100 μM in 50-100 nl) was microinjected into the RVLM in urethane-anesthetized, vagotomized, ventilated Sprague-Dawley rats (n = 19). Cardiovascular responses to stimulation of carotid baroreceptors, peripheral chemoreceptors, and the sciatic nerve (somatosympathetic reflex) were analyzed. Cts (1 mM in 50 nl) increased resting arterial pressure (28 ± 3 mmHg at 2 min postinjection), sympathetic nerve activity (15 ± 3% at 2 min postinjection), and phrenic discharge amplitude (31 ± 4% at 10 min postinjection). Cts increased sympathetic barosensitivity 40% (slope increased from -0.05 ± 0.01 before Cts to -0.07 ± 0.01 after Cts) and attenuated the somatosympathetic reflex [1st peak: 36% (from 132 ± 32.1 to 84.0 ± 17.0 μV); 2nd peak: 44% (from 65.1 ± 21.4 to 36.6 ± 14.1 μV)] and chemoreflex (blood pressure response to anoxia decreased 55%, sympathetic response decreased 46%). The results suggest that Cts activates sympathoexcitatory bulbospinal neurons in the RVLM and plays an important regulatory role in adaptive reflexes.

Neurotrophin activation of catecholamine storage vesicle protein gene expression: signaling to chromogranin a biosynthesis

Nerve growth factor differentiates precursor cells into sympathetic neurons. Does acquisition of a "neuronal" phenotype after nerve growth factor involve biosynthesis of chromogranin A, the major soluble protein in chromaffin granule cores? Nerve growth factor activated chromogranin A gene expression 7.6-fold in PC12 pheochromocytoma cells, and similarly activated PC12-transfected mouse, rat or human chromogranin A promoter/reporter constructs. Chromogranin A promoter 5'-deletions narrowed the nerve growth factor response element to a region from - 77 to - 61 bp upstream of the cap site, a region containing the chromogranin A cyclic AMP response element (TGACGTAA). Three different site-directed mutations of the cyclic AMP response element each reduced the nerve growth factor effect by >90%. Transfer of the cyclic AMP response element to a heterologous (thymidine kinase) promoter activated that promoter approximately 5-fold after nerve growth factor, while transfer of a cyclic AMP response element point-gap mutant (TGA-GTAA) to a heterologous promoter abolished the nerve growth factor effect. These findings indicate that the cyclic AMP response element in cis is, at least in part, both necessary and sufficient to activate the chromogranin A gene. Chemical blockade of the nerve growth factor receptor TrkA or the mitogen-activated protein kinase pathway component MEK substantially diminished nerve growth factor-induced expression of chromogranin A. By contrast, the response of chromogranin A to nerve growth factor was not impaired after blockade of phospholipase C-gamma or phosphoinositide-3 kinase. Chemical blockade of TrkA, Ras, MEK or mitogen-activated protein kinase similarly inhibited nerve growth factor activation of chromogranin A. Expression of constitutively activated Ras, Raf or MEK mutants increased chromogranin A promoter activity. Expression of dominant negative (inhibitory) mutants of Sos, Ha-Ras, Rafl, mitogen-activated protein kinase, ribosomal protein S6 serine kinase II (CREB kinase) or CREB (KCREB) each inhibited the nerve growth factor-induced increase in chromogranin A promoter activity. Thus, each component of the mitogen-activated protein kinase pathway is crucially involved in relaying the nerve growth factor signal in trans to the chromogranin A gene, in the following proposed sequence: nerve growth factor --> TrkA --> Shc/Grb2/Sos --> Ras --> Raf --> MEK --> mitogen-activated protein kinase --> ribosomal protein S6 serine kinase II --> CREB cyclic AMP response element.

Characterization of a partial cDNA clone detected by imidazoline receptor-selective antisera

A cDNA clone has been isolated from a human hippocampal cDNA expression library by relying on the selectivity of two antisera that are specific for imidazoline binding proteins. A 1789 bp cDNA clone was sequenced and shown to contain a single open-reading frame that predicts a 66 kDa polypeptide, but it is truncated based on its lack of a stop codon and poly-A+ tail. Two regions of homology exist for the predicted amino acid sequence in common with chromogranin-A and B proteins, a zinc finger protein, and the ryanodine receptor. Northern blot analyses of poly-A+ mRNA from 36 human tissues indicated two differentially expressed transcripts of 6.0 and 9.5 kb. The 6.0 kb mRNA form was enriched in brain and endocrine tissues as compared to other tissues, but not in strict concordance with I1-imidazoline binding sites. The highest overall amounts of the combined transcripts were found in pituitary. In situ hybridization histochemistry revealed an enrichment of the message in neuronal cell bodies of the rat hippocampus and cerebellar cortex. This clone has some of the properties expected of an imidazoline receptor.

Chromogranin A induces a neurotoxic phenotype in brain microglial cells

Chromogranin A (CGA) belongs to a multifunctional protein family widely distributed in secretory vesicles in neurons and neuroendocrine cells. Within the brain, CGA is localized in neurodegenerative areas associated with reactive microglia. By using cultured rodent microglia, we recently described that CGA induces an activated phenotype and the generation of nitric oxide. These findings led us to examine whether CGA might affect neuronal survival, expression of neurofilaments, and high affinity gamma-aminobutyric acid uptake in neurons cultured in the presence or absence of microglial cells. We found that CGA was unable to exert a direct toxic effect on neurons but provoked neuronal injury and degeneration in the presence of microglial cells. These effects were observed with natural and recombinant CGA and with a recombinant N-terminal fragment corresponding to residues 1-78. CGA stimulated microglial cells to secrete heat-stable diffusible neurotoxic agents. CGA also induced a marked accumulation of nitric oxide and tumor necrosis factor-alpha by microglia, but we could not establish a direct correlation between the levels of nitric oxide and tumor necrosis factor-alpha and the neuronal damage. The possibility that CGA represents an endogenous factor that triggers the microglial responses responsible for the pathogenesis of neuronal degeneration is discussed.