AraC binds the p75NTR transmembrane domain to induce neurodegeneration in mature neurons

BackgroundCytosine arabinoside (AraC) is one of the main therapeutic treatments for several types of cancer including acute myeloid leukaemia. However, after high dose AraC chemotherapy regime, patients develop severe neurotoxicity and neurodegeneration in the central nervous system leading to cerebellar ataxia, dysarthria, nystagmus, somnolence and drowsiness. AraC induces apoptosis in dividing cells, however, the mechanism by which it leads to neurite degeneration and cell death in mature neurons remains unclear. We hypothesized that the upregulation of the death receptor p75NTR is responsible for AraC-mediated neurodegeneration and cell death in leukemia patients undergoing AraC treatment.MethodsTo determine the role of AraC-p75NTR signalling in degeneration of mature cerebellar granule neurons, we used primary cultures from p75NTR knockout and p75NTRCys259 mice. Evaluation of neurodegeneration, cell death and p75NTR signalling was done by immunohistochemistry and immunoblotting. To assess the direct interaction between AraC and p75NTR, we performed isothermal dose response-cellular thermal shift and AraTM assays as well as Homo-FRET anisotropy imaging.ResultsWe show that AraC induces neurite degeneration and programmed cell death of mature cerebellar granule neurons in a p75NTR-dependent manner. Mechanistically, AraC binds to Proline 252 and Cysteine 256 of the p75NTR transmembrane domain and selectively uncouples p75NTR from the NFκB survival pathway. This in turn, exacerbates the activation of the cell death/JNK pathway by recruitment of TRAF6 to p75NTR.ConclusionOur findings identify p75NTR as a novel molecular target to develop treatments to counteract AraC-mediated neurodegeneration.

Plexin-B2 controls the timing of differentiation and the motility of cerebellar granule neurons

Plexin-B2 deletion leads to aberrant lamination of cerebellar granule neurons (CGNs) and Purkinje cells. Although in the cerebellum Plexin-B2 is only expressed by proliferating CGN precursors in the outer external granule layer (oEGL), its function in CGN development is still elusive. Here, we used 3D imaging, in vivo electroporation and live-imaging techniques to study CGN development in novel cerebellum-specific Plxnb2 conditional knockout mice. We show that proliferating CGNs in Plxnb2 mutants not only escape the oEGL and mix with newborn postmitotic CGNs. Furthermore, motility of mitotic precursors and early postmitotic CGNs is altered. Together, this leads to the formation of ectopic patches of CGNs at the cerebellar surface and an intermingling of normally time-stamped parallel fibers in the molecular layer (ML), and aberrant arborization of Purkinje cell dendrites. There results suggest that Plexin-B2 restricts CGN motility and might have a function in cytokinesis.

Differential ROS-Mediated Phosphorylation of Drp1 in Mitochondrial Fragmentation Induced by Distinct Cell Death Conditions in Cerebellar Granule Neurons

Reactive oxygen species (ROS) production has been associated with neuronal death. ROS are also involved in mitochondrial fission, which is mediated by Dynamin-related protein 1 (Drp1). The regulation of mitochondrial fragmentation mediated by Drp1 and its relationship to mitochondrial ROS (mtROS) in neuronal death have not been completely clarified. The aim of this study is to evaluate the role of mtROS in cell death and their involvement in the activation of Drp1 and mitochondrial fission in a model of cell death of cultured cerebellar granule neurons (CGN). Neuronal death of CGN induced by potassium deprivation (K5) and staurosporine (ST) triggers mitochondrial ROS production and mitochondrial fragmentation. K5 condition evoked an increase of Drp1 phosphorylation at Ser616, but ST treatment led to a decrease of Drp1 phosphorylation. Moreover, the death of CGN induced by both K5 and ST was markedly reduced in the presence of MitoTEMPO; however, mitochondrial morphology was not recovered. Here, we show that the mitochondria are the initial source of ROS involved in the neuronal death of CGN and that mitochondrial fragmentation is a common event in cell death; however, this process is not mediated by Drp1 phosphorylation at Ser616.

Binding of Amyloid β(1–42)-Calmodulin Complexes to Plasma Membrane Lipid Rafts in Cerebellar Granule Neurons Alters Resting Cytosolic Calcium Homeostasis

Lipid rafts are a primary target in studies of amyloid β (Aβ) cytotoxicity in neurons. Exogenous Aβ peptides bind to lipid rafts, which in turn play a key role in Aβ uptake, leading to the formation of neurotoxic intracellular Aβ aggregates. On the other hand, dysregulation of intracellular calcium homeostasis in neurons has been observed in Alzheimer’s disease (AD). In a previous work, we showed that Aβ(1–42), the prevalent Aβ peptide found in the amyloid plaques of AD patients, binds with high affinity to purified calmodulin (CaM), with a dissociation constant ≈1 nM. In this work, to experimentally assess the Aβ(1–42) binding capacity to intracellular CaM, we used primary cultures of mature cerebellar granule neurons (CGN) as a neuronal model. Our results showed a large complexation of submicromolar concentrations of Aβ(1–42) dimers by CaM in CGN, up to 120 ± 13 picomoles of Aβ(1–42) /2.5 × 106 cells. Using fluorescence microscopy imaging, we showed an extensive co-localization of CaM and Aβ(1–42) in lipid rafts in CGN stained with up to 100 picomoles of Aβ(1–42)-HiLyteTM-Fluor555 monomers. Intracellular Aβ(1–42) concentration in this range was achieved by 2 h incubation of CGN with 2 μM Aβ(1–42), and this treatment lowered the resting cytosolic calcium of mature CGN in partially depolarizing 25 mM potassium medium. We conclude that the primary cause of the resting cytosolic calcium decrease is the inhibition of L-type calcium channels of CGN by Aβ(1–42) dimers, whose activity is inhibited by CaM:Aβ(1–42) complexes bound to lipid rafts.

BMP signalling facilitates transit amplification in the developing cerebellum

ABSTRACTThe external granule layer (EGL) is a transient proliferative layer that gives rise to cerebellar granule neurons and drives the foliation of amniote cerebella. The formation of and differentiation from the EGL is incompletely understood, though BMP signalling has been implicated. Here, we characterise active BMP signalling during cerebellar development in chick and human and show that while in chick BMP signalling correlates with EGL formation, humans maintain BMP signalling throughout the EGL after the onset of foliation. Using in ovo electroporation in chick, we show that BMP signalling is necessary for EGL formation, but not for granule neuron fate. Our data are also consistent with a second role for BMP signalling in driving differentiation of granule progenitors in the EGL. These results elucidate two key, temporally distinct roles for BMP signalling in organising first the assembly of the EGL and then the tempo of granule neuron production within it.