Dept. of Pharmacology, Medical School University of Crete, Institute of Molecular Biology & Biotechnology, Foundation of Research & Technology-Hellas, Heraklion 71110, Greece
Neurotrophins hold a pivotal role in brain development and maintenance, been also involved in the pathophysiology of neurodegenerative diseases. The effectiveness of neurotrophins in controlling neuronal apoptosis in various experimental animal models of neurodegenerative conditions has not yet been translated to clinical use, because of their inability to pass the blood-brain-barrier (BBB) and their unstable serum pharmacokinetics and bioavailability (Gravanis et al. Oncotarget 2017). We provide evidence that BNN27, a novel, BBB-permeable C17-spiroepoxy steroid derivative of steroid dehydroepiandrosterone (Charalampopoulos et al. PNAS 2004, Calogeropoulou et al. J Med Chem 2009, Lazaridis et al. PLoS Biol 2011), specifically interacts with and activates the TrkA receptor of neurotrophin NGF, inducing phosphorylation of TrkA tyrosine residues and down-stream neuronal survival-related kinase signaling (Pediaditakis et al., Neuropharmacology 2016). Additionally, microneurotrophin BNN27 potentiates the efficacy of low levels of NGF, by facilitating its binding to the TrkA receptors and differentially inducing fast return of internalized TrkA receptors into neuronal cell membranes. Furthermore, BNN27 synergizes with NGF in promoting axonal outgrowth, effectively rescues from apoptosis NGF-dependent and TrkA positive sympathetic and sensory neurons, in vitro, ex vivo and in vivo in NGF null mice. Interestingly, BNN27 does not possess the hyperalgesic properties of NGF. Microneurotrophin BNN27 binds also to pan-neurotrophin p75NTR receptor, known to control neuronal cell fate after its selective activation by immature and mature isoforms of all neurotrophins. We tested the p75NTR-mediated effects of BNN27 in mouse Cerebellar Granule Neurons (CGNs), expressing p75NTR, but not TrkA receptors (Pediaditakis et al., Front Pharmacol 2017). Our findings show that BNN27 physically interacts with p75NTR receptors in specific amino-residues of its extracellular domain, inducing the recruitment of p75NTR receptor to its effector protein RIP2 and the simultaneous release of RhoGDI in primary neuronal cells. Activation of the p75NTR receptor by BNN27 reverses serum deprivation-induced apoptosis of CGNs resulting in the decrease of the phosphorylation of pro-apoptotic JNK kinase and of the cleavage of Caspase-3, effects completely abolished in CGNs, isolated from p75NTR null mice.
We are now testing the efficacy of BNN27 in various animal models of neurodegenerative conditions:
1) The effect of BNN27 on oligodendrocytes in vivo in the cuprizone mouse model of demyelination is investigated. In this model, that does not directly involve the immune system, BNN27 is able to protect from demyelination without affecting the remyelinating process (Bonetto et al. submitted). BNN27 preserves mature oligodendrocyte during demyelination in vivo, while reducing microgliosis and astrogliosis. We present also evidence that BNN27 provides trophic action (rescue from apoptosis), in a TrkA-dependent manner, to mature oligodendrocytes when they are challenged with the cuprizone toxin in vitro. BNN27 treatment also increases oligodendrocyte process branching. Deletion of p75NTR decreases oligodendrocyte arborization, not affecting the protective role of BNN27.
2) The effects of BNN27 on neural/glial cell function and apoptosis in the experimental rat streptozotocin (STZ)-model of diabetic retinopathy (DR) were also investigated (Liza et al. submitted). Administration of the compound for four weeks post STZ-injection reversed the diabetes-induced glial activation and loss of function of amacrine cells (bNOS and TH expression) and ganglion cell axons via a TrkA- dependent mechanism. BNN27 activated/phosphorylated the TrkA residue, ERK1/2 kinases and reversed the SAPK/JNK phosphorylation, in the absence but not the presence of TrkA inhibitor.
3) The effects of BNN27 on recognition memory in rats is also investigated (Pitsikas & Gravanis Neurobiol Learn Mem 2017). Intraperitoneal administration of BNN27 antagonized the TrkA-positive, cholinergic-dependent, cognition deficits in the normal rat, suggesting that this derivative affected acquisition, storage and retrieval of information. In addition, BNN27 counteracted the scopolamine-induced non-spatial and spatial recognition memory deficits.
In conclusion, BNN27 represents a lead molecule for the development of novel, non-toxic, BBB-permeable neurotrophin receptor ligands, controlling neuronal cell fate and function, with potential applications in therapeutics of neurodegenerative diseases.
 Calogeropoulou et al. J Med Chem 52:6569-6587, 2009
 Charalampopoulos et al., PNAS 101:8209-8218, 2004,
 Gravanis et al., Oncotarget 8:9005-9006, 2017
 Lazaridis et al., PLoS Biol (4):e10010512011
 Pediaditakis et al., Neuropharmacology 111:266-282, 2016
 Pediaditakis et al., Front Pharmacol 7:512-526, 2017
 Pitsikas & Gravanis, Neurobiol Learn Mem, 140:145-153, 2017