Gayle H. Doherty

Leptin prevents hippocampal synaptic disruption and neuronal cell death induced by amyloid β

Accumulation of amyloid-β (Aβ) is a key event mediating the cognitive deficits in Alzheimers disease (AD) as Aβ promotes synaptic dysfunction and triggers neuronal death. Recent evidence has linked the hormone leptin to AD as leptin levels are markedly attenuated in AD patients. Leptin is also a potential cognitive enhancer as it facilitates the cellular events underlying hippocampal learning and memory. Here we show that leptin prevents the detrimental effects of Aβ(1-42) on hippocampal long-term potentiation. Moreover leptin inhibits Aβ(1-42)-driven facilitation of long-term depression and internalization of the 2-amino-3-(5-methyl-3-oxo-1,2- oxazol-4-yl)propanoic acid (AMPA) receptor subunit, GluR1, via activation of PI3-kinase. Leptin also protects cortical neurons from Aβ(1-42)-induced cell death by a signal transducer and activator of transcription-3 (STAT-3)-dependent mechanism. Furthermore, leptin inhibits Aβ(1-42)-mediated upregulation of endophilin I and phosphorylated tau in vitro, whereas cortical levels of endophilin I and phosphorylated tau are enhanced in leptin-insensitive Zucker fa/fa rats. Thus leptin benefits the functional characteristics and viability of neurons that degenerate in AD. These novel findings establish that the leptin system is an important therapeutic target in neurodegenerative conditions.

Nitric oxide in neurodegeneration: potential benefits of non-steroidal anti-inflammatories

The cellular messenger nitric oxide (NO) has been linked to neurodegenerative disorders due to the increased expression of the enzymes that catalyze its synthesis in postmortem tissues derived from sufferers of these diseases. Nitrated proteins have also been detected in these samples, revealing that NO is biologically active in regions damaged during neurodegeneration. Modulation of NO levels has been reported not only in the neurons of the central nervous system, but also in the glial cells (microglia and astroglia) activated during the neuroinflammatory response. Neuroinflammation has been found in some neurodegenerative conditions, and inhibition of these neuroinflammatory signals has been shown to delay the progress of such disorders. Thus NO and the pathways triggering its release are emerging as an important research focus in the search for strategies to prevent, halt or cure neurodegenerative diseases.摘要一氧化氮(nitric oxide, NO)是一类胞内信使。研究表明, 神经退行性病人脑组织中催化合成NO的酶的表达水平显著提高, 提示NO与神经退行性疾病密切相关。此外, 在这些组织中还检测到硝化的蛋白, 提示NO在这些组织中具有生物活性。在神经免疫应答中, 神经元和胶质细胞(包括小胶质细胞和星形胶质细胞)内都发生了NO 水平的改变。很多神经退行性疾病都伴随有神经炎症, 抑制神经炎症的信号通路能延迟这些疾病的发展。因此, NO及其释放通路已逐渐成为神经退行性疾病研究领域的热点, 对它们的理解能帮助我们找到合适的方案来预防、减缓或者治愈这些疾病。

Neuroprotective actions of leptin on central and peripheral neurons in vitro

Neuronal cell death and its regulation have been extensively studied as an essential process of both neurodevelopment and neurodegenerative conditions. However it is not clear how circulating hormones influence such processes. Therefore we aimed to determine whether the anti-obesity hormone leptin could promote the survival of murine central and peripheral neurons in vitro. Thus we established primary neuronal cultures of dopaminergic midbrain neurons and trigeminal sensory neurons and induced cell death via either toxic insult or growth factor withdrawal. We demonstrate that leptin promotes the survival of developing peripheral and central neurons via activation of mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3-kinase)/Akt/nuclear factor kappa B (NF-kappaB) -dependent signaling cascades. Specifically, leptin protects dopaminergic midbrain neurons from the apoptotic stimuli, tumor necrosis factor alpha (TNF-alpha) and 6-hydroxydopamine (6-OHDA). In addition, it promotes the survival of postnatal, but not embryonic, trigeminal sensory neurons following neurotrophin withdrawal. Our data reveal a novel neuroprotective role for leptin in the peripheral nervous system while expanding on the known anti-apoptotic role of leptin in the CNS. These findings have important implications for our understanding of neuronal viability.

Neurotrophic effects of leptin on cerebellar Purkinje but not granule neurons in vitro

As recent evidence has revealed a pro-survival role for the anti-obesity hormone leptin in the nervous system, we investigated the generality of this finding on cerebellar Purkinje and granule neurons in vitro. We found that whilst leptin promoted cerebellar Purkinje neuron survival, it had no affect on cerebellar granule cells. In addition, we discovered that leptin promoted both the outgrowth of neurites from cerebellar Purkinje neurons and increased the complexity of the neurite arbor. Thus, leptin has different effects on two neighbouring populations of neurons within the cerebellum implying specificity of its actions in the central nervous system.

Neurotoxic effects of homocysteine on cerebellar purkinje neurons in vitro

Whilst a plethora of studies that describe the toxicity of homocysteine to CNS neurons have been published, the effects of homocysteine on the Purkinje neurons of the cerebellum that play a vital role in motor function remain wholly unexplored. We have therefore established cultures of embryonic cerebellar Purkinje neurons and exposed them to a range of concentrations of homocysteine and determined its effects on their survival. The experiments revealed that all concentrations of homocysteine studied, from 50 to 500microM, caused a significant decrease in cerebellar Purkinje neuron number. This loss could be counteracted by the pan-caspase inhibitor z-VAD-fmk in the first 24h following homocysteine exposure, revealing that the initial loss was apoptotic. However, z-VAD-fmk could not prevent homocysteine-mediated loss of cerebellar Purkinje neurons in the longer term, after 6 days in vitro. In addition to its effects on Purkinje neuron survival, homocysteine markedly reduced both the overall magnitude and the complexity of the neurite arbor extended by the cerebellar Purkinje neurons, following 6 days incubation with this agent in vitro. Taken together our data reveal that homocysteine is toxic to cerebellar Purkinje neurons in vitro, inhibiting both their survival and the outgrowth of neurites.

Developmental switch in the effects of TNFα on ventral midbrain dopaminergic neurons

The cytokine tumour necrosis factor-alpha (TNFalpha) has been implicated in the pathogenesis of neurodegenerative conditions as well as in the establishment of neural networks during development. This study investigated the in vitro effects of TNFalpha on embryonic dopaminergic neurons of the ventral mesencephalon. TNFalpha treatment enhanced the number of dopaminergic neurons in cultures derived from E12.5 mice embryos in a dose-dependent manner. In order to achieve this effect TNFalpha signalled via NF-kappaB. This enhancement in cell number was found to be due to TNFalpha promoting the differentiation of dopaminergic neurons rather than to an increase in cell survival. In contrast, TNFalpha-treated cultures derived from E14 or E16 mice demonstrated a decrease in dopaminergic neurons, and this loss was negated by pharmacological inhibition of caspases. The data presented demonstrate that during embryonic development, dopaminergic ventral mesencephalic neurons switch their in vitro response to TNFalpha from neurotrophic to neurotoxic. This is the first report of a population of neurons exhibiting this switch in TNFalpha responsiveness during neurodevelopment.

Obesity and the ageing brain: could leptin play a role in neurodegeneration?

Obesity and ageing are both characteristics of the human population that are on the increase across the globe. It has long been established that ageing is the major risk factor for neurodegenerative conditions such as Alzheimers disease, and it is becoming increasingly evident that obesity is another such factor. Leptin resistance or insensitivity has been uncovered as a cause of obesity, and in addition the leptin signalling system is less potent in the elderly. Taken together, these findings reveal that this molecule may be a link between neurodegeneration and obesity or ageing. It is now known that leptin has beneficial effects on both the survival and neurophysiology of the neurons that are lost in Alzheimers disease suggesting that it may be an important research target in the quest for strategies to prevent, halt, or cure this condition.

Nitric oxide and peroxynitrite signalling triggers homocysteine-mediated apoptosis in trigeminal sensory neurons in vitro

The neurotoxic actions of homocysteine on central nervous system neurons have been well established, yet its effects on the neurons of the peripheral nervous system remain largely unknown. We analysed the consequences of homocysteine exposure for the in vitro survival of embryonic and postnatal murine trigeminal sensory neurons from E14 to P1, and also quantified the effects of homocysteine exposure on neurite outgrowth. We discovered that homocysteine was toxic to these neurons when they were grown with NGF, or, in the case of P1 trigeminal neurons, with CNTF. Cell death induced by homocysteine was blocked using caspase inhibitors indicating that this cell loss was apoptotic. In addition, we demonstrated that homocysteine toxicity was mediated through the actions of the NMDA receptor, nitric oxide and peroxynitrite. We found that homocysteine had no effect on neurite outgrowth. Taken together our data show that homocysteine induces apoptosis in trigeminal sensory neurons via a nitric oxide-dependent mechanism. These data represent the first demonstration that homocysteine is toxic to a population of cranial sensory neurons and elucidate key components of the signalling pathway engaged to bring this about. These findings are of importance to our understanding of homocysteines influence on neurodevelopment and on peripheral neuropathies.

Effects of Tumour Necrosis Factor-alpha on Developing Cerebellar Granule and Purkinje Neurons In Vitro

Tumour necrosis factor-alpha (TNF-α) has been widely implicated in both neurodevelopment and neurodegeneration, yet its effects on individual populations of cerebellar neurons as they develop have not been fully elucidated. Therefore, we established primary neuronal cultures of developing murine cerebellar Purkinje neurons and postnatal cerebellar granule cells to determine the consequences of TNF-α exposure for their survival. We discovered that TNF-α did not affect the viability of cerebellar granule neurons at any of the ages studied, even though TNF-α and its receptors, TNFR1 and TNFR2, are widely expressed in the postnatal cerebellum. In addition, TNF-α was neither able to ameliorate, nor enhance, cell death in cerebellar granule cells elicited by a variety of stimuli including homocysteine and alcohol exposure. In contrast, in cultures established at embryonic day 16, TNF-α enhanced the number of cerebellar Purkinje neurons in vitro but this effect was not observed in embryonic day 19 cultures. Thus, TNF-α has differential and highly specific effects on different populations of cerebellar neurons as they develop.

A Leptin Fragment Mirrors the Cognitive Enhancing and Neuroprotective Actions of Leptin

A key pathology of Alzheimers disease (AD) is amyloid β (Aβ) accumulation that triggers synaptic impairments and neuronal death. Metabolic disruption is common in AD and recent evidence implicates impaired leptin function in AD. Thus the leptin system may be a novel therapeutic target in AD. Indeed, leptin has cognitive enhancing properties and it prevents the aberrant effects of Aβ on hippocampal synaptic function and neuronal viability. However, as leptin is a large peptide, development of smaller leptin-mimetics may be the best therapeutic approach. Thus, we have examined the cognitive enhancing and neuroprotective properties of known bioactive leptin fragments. Here we show that the leptin (116-130) fragment, but not leptin (22-56), mirrored the ability of leptin to promote AMPA receptor trafficking to synapses and facilitate activity-dependent hippocampal synaptic plasticity. Administration of leptin (116-130) also mirrored the cognitive enhancing effects of leptin as it enhanced performance in episodic-like memory tests. Moreover, leptin (116-130) prevented hippocampal synaptic disruption and neuronal cell death in models of amyloid toxicity. These findings establish further the importance of the leptin system as a therapeutic target in AD.