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Commentary
Commentary Brilliant Blue G: What a Little More Colour Can Be Iwamaru, Y.; Takenouchi, T.; Murayama, Y.; Okada, H.; Imamura, M.; Shimizu, Y.; Hashimoto, M.; Mohri, S.; Yokoyama, T.; Kitani, H. Anti-prion activity of Brilliant Blue G. PLoS One, 2012, 7(5), e37896.
Neurodegenerative diseases are characterized by pathological features including intracellular inclusion bodies, extracellular protein deposits, axonal swelling, synaptic loss and last, but not least, neuronal cell death. Progressive accumulation of misfolded protease-resistant proteins leading to the formation of neurotoxic aggregates likely plays a central role in the pathogenesis of such diseases, since it causes also secondary neuroinflammation sustained by astrocytes and microglia through dysregulation of cell-specific signal transduction pathways. In this complex scenario the dye Brilliant Blue G (BBG), commonly used for protein staining is already employed in clinical practice for vitreoctomy [1]. In addition to being a well-known antagonist for the purinergic ionotropic P2X7 receptor, BBG is a structural analogue of a Food and Drug Administration-approved food dye, with no toxicity reported so far and high bloodbrain barrier permeability. Thanks to these properties and colourful behaviour, BBG has proven to elicit interesting effects in animal models of neurodegenerative diseases [2]. In particular, BBG can remodel amyloid fibril formation in vitro, in a physiologically relevant manner, and furthermore reduce amyloid -peptide-associated cytotoxicity in vivo, by promoting the formation of non-toxic aggregates [3,4]. This supports the hypothesis that by interfering with aggregate formation, small aromatic compounds such as BBG might constitute an effective strategy for reducing amyloid -peptide cytotoxicity in Alzheimer’s disease. Moreover, BBG acting as inhibitor of P2X7 receptors has shown efficacy against traumatic brain injury [5], a neurodegenerative disease with a strong inflammatory component, while its role in spinal cord injury is still under scrutiny [6,7]. Neuroinflammation plays an important role also in cerebral ischemia/reperfusion injury, where BBG ameliorates brain damage by modulating inflammatory responses [8,9]. Similar results were found after intrathecal administration of BBG in the sciatic nerve injury rat model, reversing chronic constriction-induced neuropathic pain linked to microglia activation [10]. Finally, BBG improved Huntington’s disease symptoms in mice [11]. However, the in vivo functions of BBG don’t finish here, as described by Iwamaru and co-authors in their PlosOne (2012) publication. Prion diseases are fatal neurodegenerative disorders characterized by accumulation of protease-resistant prion protein isoforms (PrPres) accompanied by strong astrogliosis and neuronal cell loss. Based on previous evidence for involvement of the P2X7 receptor in prion disease [12], and sustained by the chemical structure of BBG closely resembling some known anti-prion molecules, the authors hypothesised an anti-prion activity of BBG and examined its inhibitory activity on prion replication and accumulation of pathogenic PrPres. They infected, respectively, microglia and neuronal cell lines with the ME7 (ScMG20) and Chandler (ScN2a) murine strains of scrapie, demonstrating that BBG prevents PrPres accumulation and prion replication in both cases – in a P2X7 receptor-independent fashion. Iwamaru and colleagues next investigated whether administration of BBG in vivo in mice pre-infected with Fukuoka-1 strain of Gerstmann-Sträussler-Scheinker prion disease could affect disease progression, and found that BBG efficiently reduced cerebral accumulation of PrPres. That synaptic loss and gliosis are hallmarks of prion pathology and are proportional to its severity led the authors to test the effectiveness of BBG on these same parameters. Unexpectedly BBG, rather than affecting disease progression, enhanced synaptic loss and astrocytosis, while P2X7 receptor expression slightly decreased. The authors explained this failure by the fact that neuroinflammation in prion diseases is manly sustained by the astrocytic component, while BBG exerts anti-inflammatory effects by inhibiting the P2X7 receptor known to be mainly expressed by microglia. This hypothesis is consistent with the lack of beneficial effects exerted by pharmacological or genetic ablation of P2X7 receptor in an in vivo model of Parkinson’s disease characterized by an atypical microglia inflammatory response [13]. On the whole, these results would suggest that P2X7 receptor involvement is strictly correlated to the specific space-time role that each different cell phenotype can acquire in several different neurodegenerative diseases. When neuroinflammation is mainly sustained by microglia, we would expect a strong participation of P2X7 receptors, whereas in conditions of prominent neuronal cell damage accompanied by more robust astrocytosis, as in prion diseases, P2X7 receptors appear to play a more subtle and complex role. The results presented by Iwamaru and co-authors thus open a new field of investigation on the anti-aggregation properties of a colourful and versatile molecule such as BBG, and its potential efficacy and therapeutic role in prion diseases. Moreover, they suggest that a more comprehensive way of thinking must be adopted with regard to the involvement of the P2X7 receptor in neurodegenerative/neuroinflammatory diseases, besides its simplistic toxic or trophic function. 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Pelayes, D.E.; Kuhn, F.; Folgar, A.M.; Takahashi, W.; Bastien, A.; Vinicius, P.N.; Zarate, J.O. Staining of the internal limiting membrane with the use of heavy brilliant blue G. Ophthalmic Res., 2012, 48(1), 21-25. Takenouchi, T.; Sekiyama, K.; Sekigawa, A.; Fujita, M.; Waragai, M.; Sugama, S.; Iwamaru, Y.;Kitani, H.; Hashimoto, M. P2X7 receptor signaling pathway as a therapeutic target for neurodegenerative diseases. Arch. Immunol. Ther. Exp. (Warsz.), 2010, 58(2), 91-96.
Commentary [3]. [4]. [5]. [6]. [7]. [8]. [9]. [10]. [11].
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Savina Apolloni and Cinzia Volonté* (*Editorial Advisory Board Member) Cellular Biology and Neurobiology Institute, CNR Fondazione Santa Lucia Rome Italy E-mail:
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PMID: 23746137
