Balanced effect of PACAP and FasL on granule cell death during cerebellar development: a morphological, functional and behavioural characterization.

It is now established that the development of the CNS requires equilibrium between cell survival and apoptosis. Pituitary adenylate cyclase-activating polypeptide (PACAP) exerts a powerful protective effect on cerebellar granule cells by inhibiting the caspase 3. In contrast, Fas ligand (FasL) plays an essential role during ontogenesis in eliminating supernumerary neurons by apoptosis. To determine if PACAP and FasL interact during cerebellar development, we characterized the effects of these factors on cerebellar morphogenesis and caspase 3 activity in PACAP+/+ and PACAP-/- mice. First, we demonstrated in vivo that PACAP is able to reverse the diminution of internal granule cell layer thickness induced by FasL in PACAP+/+ and PACAP-/- mice. Second, ex vivo and immunohistochemical studies revealed that interaction between FasL and PACAP occurs through the caspase 3 activity. Third, behavioural study showed a significant difference for the PACAP + FasL group in the righting reflex test at P8 which does not persist at P60. Finally, a time course study revealed that the pro-apoptotic effect of FasL characterized at P8 was followed by a progressive compensatory mechanism in caspase 3 activity and bromodeoxyuridine incorporation. These data suggest that PACAP and FasL interact during cerebellar development to control apoptosis of granule cells and may affect some motor cerebellar functions.

Spatio-temporal characterization of the pleiotrophinergic system in mouse cerebellum: evidence for its key role during ontogenesis.

The development of the central nervous system requires an appropriate micro-environment that is conditioned by a combination of various extracellular components. Most of the known signaling factors, such as neurotransmitters or neuropeptides, are soluble and diffuse into the extracellular matrix. However, other secreted molecules like proteoglycans or glycosaminoglycans anchor in the extracellular matrix to influence cerebral ontogenesis. As such, pleiotrophin (PTN), which binds the proteoglycans syndecan-3 (SDC3) and protein tyrosine phosphatase zeta (PTPζ), has been described as a pro-migratory and a pro-differentiating secreted cytokine on cortical neurons. In rat cerebellum, PTN is highly expressed during the first postnatal week, suggesting that this cytokine could participate to the development of the cerebellar cortex. According to this hypothesis, our spatio-temporal cartography of PTN, PTPζ and SDC3 indicated that, in mouse, the PTNergic system was present in the cerebellum at least from the first postnatal day (P0). Until P12, PTN was mainly expressed by granule cell precursors and located in the extracellular matrix, while SDC3 was expressed by Purkinje cells, Golgi cells and granule cell precursors, and PTPζ was present on Purkinje cells and Bergmann fibers. In vitro studies confirmed the presence of SDC3 on immature granule cells and demonstrated that PTN could stimulate directly their velocity in culture. In contrast, subarachnoidal injection of PTN in the cerebellum significantly reduced the rate of migration of granule cells, exacerbated their apoptosis and induced an atrophy of the Purkinje cell dendritic tree. Since differentiated granule cells did not express SDC3 or PTPζ, the PTN effect observed on migration and apoptosis may be indirectly mediated by Purkinje and/or Bergmann cells. From P21 to adulthood, the distribution of PTN, SDC3 and PTPζ changed and their expression dramatically decreased even if they were still detectable. PTN and SDC3 immunolabeling was restricted around Purkinje cell bodies and Golgi cells, whereas PTPζ was located around interneurons. These data suggested that, in the cerebellum of adult mice, PTN participates to the perineuronal nets that control neuronal plasticity. To conclude, the present work represents the first spatio-temporal characterization of the PTNergic system in the mouse cerebellum and indicates that PTN may contribute to cerebellum ontogenesis during the postnatal development as well as to neuronal plasticity at adulthood.

Selective impairment of some forms of synaptic plasticity by oligomeric amyloid-ß peptide in the mouse hippocampus: implication of extrasynaptic NMDA receptors.

Alzheimer's disease is characterized by the loss of memory and synaptic damage. Evidence is accumulating for a causal role of soluble oligomeric species of amyloid-ß peptide (Aßo) in the impairment of synaptic plasticity and cognition but the precise mechanisms underlying these effects are still not clear. Synaptic plasticity such as long-term potentiation is thought to underlie learning and memory. While the effect of Aß on long-term potentiation is well documented, a more general understanding of Aß action on various aspects of plasticity involving synaptic and extrasynaptic receptors and the nature of the mechanisms involved in its effects are lacking. Using a combination of electrophysiological and biochemical techniques in mouse hippocampal slices, we show here that Aßo drastically affects synaptic plasticities induced by high stimulation frequencies through the involvement of extrasynaptic glutamate receptors. Experiments on hippocampal slices as well as on cultured cortical neurons show that Aßo potentiates extrasynaptic NMDA receptors-mediated responses. Pharmacological characterization indicates that GluN2B-containing NMDARs are involved in these responses. When synaptic and extrasynaptic glutamate receptor-mediated effects are dissociated using cortical neurons in culture, it appears that Aßo has differential effects on these two receptors types. We conclude that the pool of extrasynaptic GluN2B-containing NMDARs is a major target of Aßo in the hippocampus. During high frequency stimulation, Aßo dramatically impairs long-term neuronal responses.