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

β-arrestin1-E2F1-ac axis regulates physiological apoptosis and cell cycle exit in cellular models of early postnatal cerebellum

Development of the cerebellum is characterized by rapid proliferation of cerebellar granule cell precursors (GCPs) induced by paracrine stimulation of Sonic hedgehog (Shh) signaling from Purkinje cells, in the external granular layer (EGL). Then, granule cell precursors differentiate and migrate into the inner granular layer (IGL) of the cerebellum to form a terminally differentiated cell compartment. Aberrant activation of Sonic hedgehog signaling leads to granule cell precursors hyperproliferation and the onset of Sonic hedgehog medulloblastoma (MB), the most common embryonal brain tumor. β-arrestin1 (ARRB1) protein plays an important role downstream of Smoothened, a component of the Sonic hedgehog pathway. In the medulloblastoma context, β-arrestin1 is involved in a regulatory axis in association with the acetyltransferase P300, leading to the acetylated form of the transcription factor E2F1 (E2F1-ac) and redirecting its activity toward pro-apoptotic gene targets. This axis in the granule cell precursors physiological context has not been investigated yet. In this study, we demonstrate that β-arrestin1 has antiproliferative and pro-apoptotic functions in cerebellar development. β-arrestin1 silencing increases proliferation of Sonic hedgehog treated-cerebellar precursor cells while decreases the transcription of E2F1-ac pro-apoptotic targets genes, thus impairing apoptosis. Indeed, chromatin immunoprecipitation experiments show a direct interaction between β-arrestin1 and the promoter regions of the pro-apoptotic E2F1 target gene and P27, indicating the double role of β-arrestin1 in controlling apoptosis and cell cycle exit in a physiological context. Our data elucidate the role of β-arrestin1 in the early postnatal stages of cerebellar development, in those cell compartments that give rise to medulloblastoma. This series of experiments suggests that the physiological function of β-arrestin1 in neuronal progenitors is to directly control, cooperating with E2F1 acetylated form, transcription of pro-apoptotic genes.

Genes involved in paediatric apnoea and death based on knockout animal models: Implications for sudden infant death syndrome (SIDS)

The mechanism of death in Sudden infant death syndrome (SIDS) remains unknown but it is hypothesised that cardiorespiratory failure of brainstem origin results in early post-natal death. For a subset of SIDS infants, an underlying genetic cause may be present, and genetic abnormalities affecting brainstem respiratory control may result in abnormalities that are detectable before death. Genetic knockout mice models were developed in the 1990s and have since helped to elucidate the physiological roles of a number of genes. This systematic review aimed to identify which genes, when knocked out, result in the phenotypes of abnormal cardiorespiratory control and/or early post-natal death. Three major genes were identified: Pet1- a serotonin transcription factor, the neurotrophin pituitary adenylate cyclase activating polypeptide (PACAP) and its receptor (PAC1). Knockouts targeting these genes had blunted hypercapnic and/or hypoxic responses and early post-natal death. The hypothesis that these genes have a role in SIDS is supported by their being identified as abnormal in SIDS cohorts. Future research in SIDS cohorts will be important to determine whether these genetic abnormalities coexist and their potential applicability as biomarkers.

Caspase-3 Mediated Cell Death in the Normal Development of the Mammalian Cerebellum

Caspase-3, onto which there is a convergence of the intrinsic and extrinsic apoptotic pathways, is the main executioner of apoptosis. We here review the current literature on the intervention of the protease in the execution of naturally occurring neuronal death (NOND) during cerebellar development. We will consider data on the most common altricial species (rat, mouse and rabbit), as well as humans. Among the different types of neurons and glia in cerebellum, there is ample evidence for an intervention of caspase-3 in the regulation of NOND of the post-mitotic cerebellar granule cells (CGCs) and Purkinje neurons, as a consequence of failure to establish proper synaptic contacts with target (secondary cell death). It seems possible that the GABAergic interneurons also undergo a similar type of secondary cell death, but the intervention of caspase-3 in this case still remains to be clarified in full. Remarkably, CGCs also undergo primary cell death at the precursor/pre-migratory stage of differentiation, in this instance without the intervention of caspase-3. Glial cells, as well, undergo a process of regulated cell death, but it seems possible that expression of caspase-3, at least in the Bergmann glia, is related to differentiation rather than death.

Review on PACAP-Induced Transcriptomic and Proteomic Changes in Neuronal Development and Repair

Pituitary adenylate cyclase activating polypeptide (PACAP) is a neuropeptide with widespread occurrence and diverse biological effects. Among its several different effects, of special importance is the action of PACAP on neuronal proliferation, differentiation and migration, and neuroprotection. The neuroprotective mechanism of PACAP is both direct and indirect, via neuronal and non-neuronal cells. Several research groups have performed transcriptomic and proteomic analysis on PACAP-mediated genes and proteins. Hundreds of proteins have been described as being involved in the PACAP-mediated neuroprotection. In the present review we summarize the few currently available transcriptomic data potentially leading to the proteomic changes in neuronal development and protection. Proteomic studies focusing on the neuroprotective role of PACAP are also reviewed and discussed in light of the most intriguing and promising effect of this neuropeptide, which may possibly have future therapeutic potential.

Early Neurobehavioral Development of Mice Lacking Endogenous PACAP

Pituitary adenylate cyclase activating polypeptide (PACAP) is a multifunctional neuropeptide. In addition to its diverse physiological roles, PACAP has important functions in the embryonic development of various tissues, and it is also considered as a trophic factor during development and in the case of neuronal injuries. Data suggest that the development of the nervous system is severely affected by the lack of endogenous PACAP. Short-term neurofunctional outcome correlates with long-term functional deficits; however, the early neurobehavioral development of PACAP-deficient mice has not yet been evaluated. Therefore, the aim of the present study was to describe the postnatal development of physical signs and neurological reflexes in mice partially or completely lacking PACAP. We examined developmental hallmarks during the first 3 weeks of the postnatal period, during which period most neurological reflexes and motor coordination show most intensive development, and we describe the neurobehavioral development using a complex battery of tests. In the present study, we found that PACAP-deficient mice had slower weight gain throughout the observation period. Interestingly, mice partially lacking PACAP weighed significantly less than homozygous mice. There was no difference between male and female mice during the first 3 weeks. Some other signs were also more severely affected in the heterozygous mice than in the homozygous mice, such as air righting, grasp, and gait initiation reflexes. Interestingly, incisor teeth erupted earlier in mice lacking PACAP. Motor coordination, shown by the number of foot-faults on an elevated grid, was also less developed in PACAP-deficient mice. In summary, our results show that mice lacking endogenous PACAP have slower weight gain during the first weeks of development and slower neurobehavioral development regarding a few developmental hallmarks.

Early Disruption of Extracellular Pleiotrophin Distribution Alters Cerebellar Neuronal Circuit Development and Function

The cerebellum is a structure of the central nervous system involved in balance, motor coordination, and voluntary movements. The elementary circuit implicated in the control of locomotion involves Purkinje cells, which receive excitatory inputs from parallel and climbing fibers, and are regulated by cerebellar interneurons. In mice as in human, the cerebellar cortex completes its development mainly after birth with the migration, differentiation, and synaptogenesis of granule cells. These cellular events are under the control of numerous extracellular matrix molecules including pleiotrophin (PTN). This cytokine has been shown to regulate the morphogenesis of Purkinje cells ex vivo and in vivo via its receptor PTPζ. Since Purkinje cells are the unique output of the cerebellar cortex, we explored the consequences of their PTN-induced atrophy on the function of the cerebellar neuronal circuit in mice. Behavioral experiments revealed that, despite a normal overall development, PTN-treated mice present a delay in the maturation of their flexion reflex. Moreover, patch clamp recording of Purkinje cells revealed a significant increase in the frequency of spontaneous excitatory postsynaptic currents in PTN-treated mice, associated with a decrease of climbing fiber innervations and an abnormal perisomatic localization of the parallel fiber contacts. At adulthood, PTN-treated mice exhibit coordination impairment on the rotarod test associated with an alteration of the synchronization gait. Altogether these histological, electrophysiological, and behavior data reveal that an early ECM disruption of PTN composition induces short- and long-term defaults in the establishment of proper functional cerebellar circuit.

Exogenous administration of PACAP alleviates traumatic brain injury in rats through a mechanism involving the TLR4/MyD88/NF-κB pathway

Pituitary adenylate cyclase-activating polypeptide (PACAP) is effective in reducing axonal damage associated with traumatic brain injury (TBI), and has immunomodulatory properties. Toll-like receptor 4 (TLR4) is an important mediator of the innate immune response. It significantly contributes to neuroinflammation induced by brain injury. However, it remains unknown whether exogenous PACAP can modulate TBI through the TLR4/adapter protein myeloid differentiation factor 88 (MyD88)/nuclear factor-κB (NF-κB) signaling pathway. In this study, we investigated the potential neuroprotective mechanisms of PACAP pretreatment in a weight-drop model of TBI. PACAP38 was microinjected intracerebroventricularly before TBI. Brain samples were extracted from the pericontusional area in the cortex and hippocampus. We found that TBI induced significant upregulation of TLR4, with peak expression occurring 24 h post-trauma, and that pretreatment with PACAP significantly improved motor and cognitive dysfunction, attenuated neuronal apoptosis, and decreased brain edema. Pretreatment with PACAP inhibited upregulation of TLR4 and its downstream signaling molecules MyD88, p-IκB, and NF-κB, and suppressed increases in the levels of the downstream inflammatory agents interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α), in the brain tissue around the injured cortex and in the hippocampus. Administration of PACAP both in vitro and in vivo attenuated the ability of the TLR4 agonist lipopolysaccharide (LPS) to increase TLR4 protein levels. Therefore, PACAP exerts a neuroprotective effect in this rat model of TBI, by inhibiting a secondary inflammatory response mediated by the TLR4/MyD88/NF-κB signaling pathway in microglia and neurons, thereby reducing neuronal death and improving the outcome following TBI.

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.

Effects of PACAP on intracellular signaling pathways in human retinal pigment epithelial cells exposed to oxidative stress

The integrity of retinal pigment epithelial cells is critical for photoreceptor survival and vision. Pituitary adenylate cyclase activating polypeptide (PACAP) exerts retinoprotective effects against several types of injuries in vivo, including optic nerve transection, retinal ischemia, excitotoxic injuries, UVA-induced lesion, and diabetic retinopathy. In a recent study, we have proven that PACAP is also protective in oxidative stress-induced injury in human pigment epithelial cells (ARPE-19 cells). The aim of the present study was to investigate the possible mechanisms of this protection. ARPE cells were exposed to a 24-h hydrogen peroxide treatment. Expressions of kinases and apoptotic markers were studied by complex array kits and Western blot. Oxidative stress induced the activation of several apoptotic markers, including Bad, Bax, HIF-1α, several heat shock proteins, TNF-related apoptosis-inducing ligand, and Fas-associated protein with death domain, while PACAP treatment decreased them. The changes in the expression of MAP kinases showed that PACAP activated the protective ERK1/2 and downstream CREB, and decreased the activation of the pro-apoptotic p38MAPK and c-Jun N-terminal kinase, an effect opposite to that observed with only oxidative stress. Furthermore, PACAP increased the activation of the protective Akt pathway. In addition, the effects of oxidative stress on several other signaling molecules were counteracted by PACAP treatment (Chk2, Yes, Lyn, paxillin, p53, PLC, STAT4, RSK). These play a role in cell death, cell cycle, inflammation, adhesion, differentiation and proliferation. In summary, PACAP, acting at several levels, influences the balance between pro- and anti-apoptotic factors in favor of anti-apoptosis, thereby providing protection in oxidative stress-induced injury of human retinal pigment epithelial cells.

Pituitary adenylate cyclase-activating polypeptide protects astroglial cells against oxidative stress-induced apoptosis

Oxidative stress, associated with a variety of disorders including neurodegenerative diseases, results from accumulation of reactive oxygen species (ROS). Oxidative stress is not only responsible for neuron apoptosis, but can also provoke astroglial cell death. Numerous studies indicate that pituitary adenylate cyclase-activating polypeptide (PACAP) promotes neuron survival, but nothing is known regarding the action of PACAP on astroglial cell survival. Thus, the purpose of the present study was to investigate the potential glioprotective effect of PACAP on H(2)O(2)-induced astrocyte death. Pre-treatment of cultured rat astrocytes with nanomolar concentrations of PACAP prevented cell death provoked by H(2)O(2) (300 μM), whereas vasoactive intestinal polypeptide was devoid of protective activity. The effect of PACAP on astroglial cell survival was abolished by the type 1 PACAP receptor antagonist, PACAP6-38. The protective action of PACAP was blocked by the protein kinase A inhibitor H89, the protein kinase C inhibitor chelerythrine and the mitogen-activated protein (MAP)-kinase kinase (MEK) inhibitor U0126. PACAP stimulated glutathione formation, and blocked H(2)O(2)-evoked ROS accumulation and glutathione content reduction. In addition, PACAP prevented the decrease of mitochondrial activity and caspase 3 activation induced by H(2)O(2). Taken together, these data indicate for the first time that PACAP, acting through type 1 PACAP receptor, exerts a potent protective effect against oxidative stress-induced astrocyte death. The anti-apoptotic activity of PACAP on astrocytes is mediated through the protein kinase A, protein kinase C and MAPK transduction pathways, and can be accounted for by inhibition of ROS-induced mitochondrial dysfunctions and caspase 3 activation.