Basic fibroblast growth factor regulates ionic currents and excitability of fetal rat carotid body chemoreceptors

Growth Factors in the Carotid Body-An Update

The carotid body may undergo plasticity changes during development/ageing and in response to environmental (hypoxia and hyperoxia), metabolic, and inflammatory stimuli. The different cell types of the carotid body express a wide series of growth factors and corresponding receptors, which play a role in the modulation of carotid body function and plasticity. In particular, type I cells express nerve growth factor, brain-derived neurotrophic factor, neurotrophin 3, glial cell line-derived neurotrophic factor, ciliary neurotrophic factor, insulin-like-growth factor-I and -II, basic fibroblast growth factor, epidermal growth factor, transforming growth factor-α and -β, interleukin-1β and -6, tumor necrosis factor-α, vascular endothelial growth factor, and endothelin-1. Many specific growth factor receptors have been identified in type I cells, indicating autocrine/paracrine effects. Type II cells may also produce growth factors and express corresponding receptors. Future research will have to consider growth factors in further experimental models of cardiovascular, metabolic, and inflammatory diseases and in human (normal and pathologic) samples. From a methodological point of view, microarray and/or proteomic approaches would permit contemporary analyses of large groups of growth factors. The eventual identification of physical interactions between receptors of different growth factors and/or neuromodulators could also add insights regarding functional interactions between different trophic mechanisms.

Receptor-Receptor Interactions of G Protein-Coupled Receptors in the Carotid Body: A Working Hypothesis

In the carotid body (CB), a wide series of neurotransmitters and neuromodulators have been identified. They are mainly produced and released by type I cells and act on many different ionotropic and metabotropic receptors located in afferent nerve fibers, type I and II cells. Most metabotropic receptors are G protein-coupled receptors (GPCRs). In other transfected or native cells, GPCRs have been demonstrated to establish physical receptor–receptor interactions (RRIs) with formation of homo/hetero-complexes (dimers or receptor mosaics) in a dynamic monomer/oligomer equilibrium. RRIs modulate ligand binding, signaling, and internalization of GPCR protomers and they are considered of relevance for physiology, pharmacology, and pathology of the nervous system. We hypothesize that RRI may also occur in the different structural elements of the CB (type I cells, type II cells, and afferent fibers), with potential implications in chemoreception, neuromodulation, and tissue plasticity. This ‘working hypothesis’ is supported by literature data reporting the contemporary expression, in type I cells, type II cells, or afferent terminals, of GPCRs which are able to physically interact with each other to form homo/hetero-complexes. Functional data about cross-talks in the CB between different neurotransmitters/neuromodulators also support the hypothesis. On the basis of the above findings, the most significant homo/hetero-complexes which could be postulated in the CB include receptors for dopamine, adenosine, ATP, opioids, histamine, serotonin, endothelin, galanin, GABA, cannabinoids, angiotensin, neurotensin, and melatonin. From a methodological point of view, future studies should demonstrate the colocalization in close proximity (less than 10 nm) of the above receptors, through biophysical (i.e., bioluminescence/fluorescence resonance energy transfer, protein-fragment complementation assay, total internal reflection fluorescence microscopy, fluorescence correlation spectroscopy and photoactivated localization microscopy, X-ray crystallography) or biochemical (co-immunoprecipitation, in situ proximity ligation assay) methods. Moreover, functional approaches will be able to show if ligand binding to one receptor produces changes in the biochemical characteristics (ligand recognition, decoding, and trafficking processes) of the other(s). Plasticity aspects would be also of interest, as development and environmental stimuli (chronic continuous or intermittent hypoxia) produce changes in the expression of certain receptors which could potentially invest the dynamic monomer/oligomer equilibrium of homo/hetero-complexes and the correlated functional implications.

Trophic factors in the carotid body

The aim of the present study is to provide a review of the expression and action of trophic factors in the carotid body. In glomic type I cells, the following factors have been identified: brain-derived neurotrophic factor, glial cell line-derived neurotrophic factor, artemin, ciliary neurotrophic factor, insulin-like growth factors-I and -II, basic fibroblast growth factor, epidermal growth factor, transforming growth factor-alpha and -beta1, interleukin-1beta and -6, tumour necrosis factor-alpha, vascular endothelial growth factor, and endothelin-1 (ET-1). Growth factor receptors in the above cells include p75LNGFR, TrkA, TrkB, RET, GDNF family receptors alpha1-3, gp130, IL-6Ralpha, EGFR, FGFR1, IL1-RI, TNF-RI, VEGFR-1 and -2, ETA and ETB receptors, and PDGFR-alpha. Differential local expression of growth factors and corresponding receptors plays a role in pre- and postnatal development of the carotid body. Their local actions contribute toward producing the morphologic and molecular changes associated with chronic hypoxia and/or hypertension, such as cellular hyperplasia, extracellular matrix expansion, changes in channel densities, and neurotransmitter patterns. Neurotrophic factor production is also considered to play a key role in the therapeutic effects of intracerebral carotid body grafts in Parkinson's disease. Future research should also focus on trophic actions on carotid body type I cells by peptide neuromodulators, which are known to be present in the carotid body and to show trophic effects on other cell populations, that is, angiotensin II, adrenomedullin, bombesin, calcitonin, calcitonin gene-related peptide, cholecystokinin, erythropoietin, galanin, opioids, pituitary adenylate cyclase-activating polypeptide, atrial natriuretic peptide, somatostatin, tachykinins, neuropeptide Y, neurotensin, and vasoactive intestinal peptide.

Basic FGF localization in rat carotid body: paracrine role in O2 -chemoreceptor survival

Exposure of perinatal rat carotid body (CB) O2-chemoreceptors to basic fibroblast growth factor (bFGF) or hypoxia in vitro increases mitotic activity. Using double-label immunofluorescence, we localized bFGF and its receptor (FGFR) to tyrosine hydroxylase-positive (TH+) chemoreceptors in vitro; bFGF immunoreactivity also localized to chemoreceptors in CB tissue sections. Mitotic activity, measured as percentage TH+ cells that took up bromodeoxyuridine, was relatively constant ( approximately 29%) in normoxic (20% O2) cultures grown with or without bFGF neutralizing antibody (nAb). However, the number of surviving chemoreceptors was significantly reduced in nAb-treated cultures. Under chronic hypoxia (6% O2), the presence of nAb significantly reduced chemoreceptor survival to approximately 70% of control, without affecting mitotic activity. Thus, autocrine/ paracrine actions of endogenous bFGF may help promote CB chemoreceptor survival.

Bladder afferent pathway and spinal cord injury: possible mechanisms inducing hyperreflexia of the urinary bladder

Lower urinary tract dysfunction is a common problem in patients with spinal cord injury (SCI). Since the coordination of the urinary bladder and urethra is controlled by the complex mechanisms in spinal and supraspinal neural pathways, SCI rostral to the lumbosacral level disrupts voluntary and supraspinal control of voiding and induces a considerable reorganization of the micturition reflex pathway. Following SCI, the urinary bladder is initially areflexic. but then becomes hyperreflexic because of the emergence of a spinal micturition reflex pathway. Recent electrophysiologic and histologic studies in rats have revealed that chronic SCI induces various phenotypic changes in bladder afferent neurons such as: (1) somal hypertrophy along with increased expression of neurofilament protein; and (2) increased excitability due to the plasticity of Na+ and K+ ion channels. These results have now provided detailed information to support the previous notion that capsaicin-sensitive, unmyelinated C-fiber afferents innervating the urinary bladder change their properties after SCI and are responsible for inducing bladder hyperreflexia in both humans and animals. It is also suggested that the changes in bladder reflex pathways following SCI are influenced by neural-target organ interactions probably mediated by neurotrophic signals originating in the hypertrophied bladder. Thus, increased knowledge of the plasticity in bladder afferent pathways may help to explain the pathogenesis of lower urinary tract dysfunctions after SCI and may provide valuable insights into new therapeutic strategies for urinary symptoms in spinal cord-injured patients.

Differential regulation of potassium currents by FGF-1 and FGF-2 in embryonic Xenopus laevis myocytes

1. Fibroblast growth factors (FGFs) are involved in the regulation of many aspects of muscle development. This study investigated their role in regulating voltage-dependent K+ currents in differentiating Xenopus laevis myocytes. Both FGF-1 and FGF-2 are expressed by developing muscle cells, so their actions were compared. Experiments were performed on cultured myocytes isolated from stage 15 embryos. 2. Long-term exposure of the embryonic myocytes to FGF-1 downregulated inward rectifier K+ current (IK(IR)) density as well as both sustained and inactivating voltage-dependent outward K+ currents (IK,S and IK,I, respectively) and their densities. In contrast, FGF-2 upregulated these currents, although, because of an increase in capacitance caused by FGF-2, current density did not change with this factor. 3. The regulation of IK(IR) by FGF-1 was prevented by the cytoplasmic tyrosine kinase inhibitor herbimycin A, but that of IK,S and IK,I was unaffected, indicating that FGF-1 achieves its regulatory effects on electrical development via separate signalling pathways. The receptor tyrosine kinase inhibitor genistein in isolation suppressed K+ currents, but this may have occurred through a channel-blocking mechanism. 4. In many cells, IK, S was found to be composed of two components with differing voltage dependencies of activation. The FGFs brought about an alteration in the amount of total IK,S by equal effects on each component. Conversely, herbimycin A increased the proportion of low voltage-activated current without affecting total current amplitude. Therefore, we suggest that a single species of channel whose voltage dependence is shifted by tyrosine phosphorylation generates IK,S. 5. In summary, FGF-1 and FGF-2 exert opposite effects on voltage-dependent K+ currents in embryonic myocytes and, furthermore, FGF-1 achieves its effects on different K+ currents via separate second messenger pathways.

Cellular and functional recovery of Parkinsonian rats after intrastriatal transplantation of carotid body cell aggregates

We have tested the suitability of chromaffin-like carotid body glomus cells for dopamine cell replacement in Parkinsonian rats. Intrastriatal grafting of cell aggregates resulted in almost optimal abolishment of motor asymmetries and deficits of sensorimotor orientation. Recovery of transplanted animals was apparent 10 days after surgery and progressed throughout the 3 months of the study. The behavioral effects were correlated with the long survival of glomus cells in the host brain. In host tissue, glomus cells were organized into glomerulus-like structures and retained the ability to secrete dopamine. Several weeks after transplantation, dopaminergic fibers emerged from the graft, reinnervating the striatal gray matter. The special durability of grafted glomus cells in the conditions of brain parenchyma could be related to their sensitivity to hypoxia, which is known to induce cell growth, excitability, and dopamine synthesis. This work should stimulate research on the clinical applicability of carotid body autotransplants in Parkinson's disease.

Nicotinic acetylcholine sensitivity of rat petrosal sensory neurons in dissociated cell culture

Using whole-cell, patch-clamp techniques we investigated acetylcholine (ACh) sensitivity of dissociated sensory neurons from rat petrosal ganglia after 4 h-14 days in vitro. In approx. 68% of petrosal neurons (PN; n = 109) ACh, applied by fast perfusion or pressure ejection from a 'puffer' pipette, caused a rapid depolarization associated with a conductance increase. Under voltage clamp near the resting potential (approx. - 60 mV), ACh induced a hexamethonium-sensitive, inward current (IACh), mimicked by nicotine application, suggesting the presence of neuronal nicotinic acetylcholine receptors (nAChR). The reversal potential of IACh occurred near 0 mV (n = 4), a region where the I-V curve displayed a prominent rectification. The dose-response relation for IACh versus ACh concentration was fitted by the Hill equation with EC50 = approx. 33.9 microM and Hill coefficient = approx. 1.6. The activation phase of IACh was well fitted by a single exponential with mean (+/- S.E.M.) time constant of 102 +/- 82 ms (n = 6); the desensitization phase of IACh was best fitted by the sum of two exponentials, with time constant of 870 +/- 210 ms (n = 6) and 8576 +/- 1435 ms (at -70 mV). Fluctuation analysis yielded an apparent single-channel conductance of 21.6 +/- 10 pS (mean +/- S.E.M.; n = 4). These data indicate that a major subpopulation of sensory neurons in visceral petrosal ganglia of the rat express nAChR. Thus, if similar receptors are present on corresponding nerve terminals, they could mediate fast afferent excitation in response to ACh released at peripheral targets, e.g., the chemosensory carotid body.

Role of basic FGF and oxygen in control of proliferation, survival, and neuronal differentiation in carotid body chromaffin cells

Crest-derived glomus cells of the carotid body (CB) are O(2)-sensitive chemoreceptors, which resemble sympathoadrenal (SA) chromaffin cells. In this study, we tested whether perinatal rat glomus cells are sensitive to basic fibroblast growth factor (bFGF) in vitro and whether their sensitivity is regulated by oxygen. In chemically defined medium, bFGF (1-100 ng/ml) caused a significant, dose-dependent increase in the number of surviving tyrosine hydroxylase-positive (TH+) glomus cells in embryonic (E17-E19) CB cultures, following a 48-hr exposure. Though basic FGF (10 ng/ml) appeared mitogenic for these cells, based on stimulation of bromodeoxyuridine (BrdU) uptake, it supported survival of only approximately 60% of the initial TH+ population, suggesting that significant cell death was occurring. This apparent cell loss in E17 cultures could be largely prevented by combined treatment with bFGF and low oxygen (6% O(2)). In contrast, in early postnatal (P1) cultures, glomus cell number was relatively unchanged over 48 hr under control conditions or in presence of mitogenic activity from either bFGF or low oxygen. However, combined treatment with both bFGF and low oxygen stimulated proliferation of P1 glomus cells such that by 48 hr the TH+ population had increased to approximately 1.5x the initial density. Basic FGF (10 ng/ ml) also stimulated neurite outgrowth and neurofilament expression in E18-E19, but not P1-P3, glomus cells. In contrast to bFGF, treatment with nerve growth factor was ineffective. Taken together, these results suggest that bFGF and low oxygen are mitogens for perinatal CB chromaffin cells and interact cooperatively as survival factors. It is plausible that these mechanisms may operate to regulate chemoreceptor cell density, during the animal's transition from in utero (hypoxic) to ex utero (normoxic)life.

Loss of basic fibroblast growth factor in the subcommissural organ of old spontaneously hypertensive rats

The immunocytochemical localization of basic fibroblast growth factor (bFGF) was studied in subcommissural organ (SCO) of aged-matched normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SH) rats at 10, 14 and 18 months of age using a polyclonal antibody against bFGF. The bFGF-like immunoreactivity (bFGF-ir) was observed in SCO ependymal cells of young and old normotensive rats. However, a progressive loss of bFGF-immunopositive ependymal SCO cells was observed with age in SH rats (27.24, 57.5 and 96.9% in 10, 14 and 18 months old respectively) compared with aged-matched WKY normotensive rats. Considering the potential role of the SCO in sleep regulation and sodium homeostasis, which are altered in essential hypertension, these data show a new neuroendocrine anomaly to be added to the many others previously observed in this rat strain, when they develop hypertension as they get old.