Fibroblast growth factor 2 requires complex formation with ATP for neuroprotective activity

Present and New Treatment Strategies in the Management of Glaucoma

Glaucoma is a neurodegenerative disease characterized by retinal ganglion cell (RGC) death and axonal loss. It remains a major cause of blindness worldwide. All current modalities of treatment are focused on lowering intraocular pressure (IOP), and it is evident that increased IOP is an important risk factor for progression of the disease. However, it is clear that a significant number of glaucoma patients show disease progression despite of pressure lowering treatments. Much attention has been given to the development of neuroprotective treatment strategies, but the identification of such has been hampered by lack of understanding of the etiology of glaucoma. Hence, in spite of many attempts no neuroprotective drug has yet been clinically approved. Even though neuroprotection is without doubt an important treatment strategy, many glaucoma subjects are diagnosed after substantial loss of RGCs. In this matter, recent approaches aim to rescue RGCs and regenerate axons in order to restore visual function in glaucoma. The present review seeks to provide an overview of the present and new treatment strategies in the management of glaucoma. The treatment strategies are divided into current available glaucoma medications, new pressure lowering targets, prospective neuroprotective interventions, and finally possible neuroregenrative strategies.

Nerve growth factor and brain-derived neurotrophic factor but not granulocyte colony-stimulating factor, nimodipine and dizocilpine, require ATP for neuroprotective activity after oxygen-glucose deprivation of primary neurons

In previous work, we have demonstrated by radiolabeling, mass spectrometry and site-directed mutagenesis that nerve growth factor (NGF) as well as brain-derived neurotrophic factor (BDNF) and fibroblast growth factor 2 (FGF2) are capable of ATP-binding and that this binding appears to be essential for their neuroprotective activity. In this study, we attempted to shed some light on the question whether ATP is a general prerequisite for neuroprotection. Therefore, we used the non-ATP-binding granulocyte colony-stimulating factor (GCSF), the calcium antagonist nimodipine and the NMDA antagonist dizocilpine to find out whether they need ATP for neuroprotection comparable to NGF and BDNF. However, ATP was not necessary for the neuroprotective effects of GCSF, nimodipine and dizocilpine on primary cultures of rat cortical neurons damaged by oxygen-glucose deprivation whereas neuroprotection was demonstrable for NGF and BDNF only when ATP was present in the culture medium at a concentration higher than ca. 0.4nmol/l. In circular dichroism studies ATP caused changes of the secondary structure of NGF but not of GCSF. Taken together, we suggest that ATP is not a general prerequisite for neuroprotectivity but some growth factors like NGF and BDNF can stimulate their receptors only if they have bound ATP.

ATP-NGF-complex, but not NGF, is the neuroprotective ligand

We have shown previously that nerve growth factor (NGF) requires only low nanomolar ATP concentrations in the cell culture medium to protect cortical rat neurons (CRN) from cellular damage induced by staurosporine (STS). We have also demonstrated before that NGF and other growth factors form stable non-covalent complexes with ATP. Here we demonstrated that 8N(1)ATP-NGF, but not NGF, protected CRN against damage. The photo-reactive ATP derivative 8N(3)ATP was incubated with NGF and was trapped in its position by UV irradiation forming a covalent bond. The cross-link with a molar ratio of 1:1 (8N(1)ATP:NGF) was confirmed by mass spectrometry. Circular dichroism experiments revealed that 8N(1)ATP altered the secondary structure of NGF in the same way as ATP did. Covalently bound 8N(1)ATP-NGF was shown to be stable in the presence of the ATP-hydrolyzing enzyme alkaline phosphatase while the non-covalent ATP-NGF-complex dissociated with the removal of free ATP from the solution. 8N(1)ATP-NGF protected CRN against damage by STS independently of free ATP in the culture medium. These results suggest that the ATP-NGF-complex, but not NGF, is the active ligand.

Binding of ATP to vascular endothelial growth factor isoform VEGF-A165 is essential for inducing proliferation of human umbilical vein endothelial cells

BACKGROUND

ATP binding is essential for the bioactivity of several growth factors including nerve growth factor, fibroblast growth factor-2 and brain-derived neurotrophic factor. Vascular endothelial growth factor isoform 165 (VEGF-A(165)) induces the proliferation of human umbilical vein endothelial cells, however a dependence on ATP-binding is currently unknown. The aim of the present study was to determine if ATP binding is essential for the bioactivity of VEGF-A(165).

RESULTS

We found evidence that ATP binding to VEGF-A(165) induced a conformational change in the secondary structure of the growth factor. This binding appears to be significant at the biological level, as we found evidence that nanomolar levels of ATP (4-8 nm) are required for the VEGF-A(165)-induced proliferation of human umbilical vein endothelial cells. At these levels, purinergic signaling by ATP via P2 receptors can be excluded. Addition of alkaline phosphate to cell culture lowered the ATP concentration in the cell culture medium to 1.8 nM and inhibited cell proliferation.

CONCLUSIONS

We propose that proliferation of endothelial cells is induced by a VEGF-A(165)-ATP complex, rather than VEGF-A(165) alone.

Interaction of ATP with fibroblast growth factor 2: biochemical characterization and consequence for growth factor stability

Background

Fibroblast growth factor 2, a well-characterized heparin-binding growth factor, is involved in many biological processes like embryogenesis, cell proliferation and angiogenesis. However, this growth factor is very unstable and shows rapid degradation in aqueous solution. Beside the well-known stabilization of FGF2 by heparin or heparan sulphate, the recently discovered binding to ATP also shows a stabilizing and protective effect on this growth factor.

Results

Here we determined the dissociation constant of ATP on FGF2 by equilibrium microdialysis (K_D: 59.8 μM) and analyzed the impact of this binding on secondary structure by CD-spectroscopy. ATP-binding to FGF2 significantly changed the secondary structure of this growth factor with a shift to random coil structure elements. We also analyzed the influence of this binding on the stability of FGF2 in aqueous solution over a period of 2 h. While the amount of untreated FGF2 is reduced drastically over this period of time, ATP-binding reduces the degradation considerably.

Conclusions

Taken together, our data suggest an important role of ATP in FGF2-stabilization beside the well known-role of heparin and heparan sulphate.