Sarah J. Harper

Signalling for survival and death in neurones: the role of stress-activated kinases, JNK and p38.

The pathways involved in neuronal survival or death have been extensively studied mainly in cell lines. Recent evidence has suggested that activation of the stress activated pathways, jun N-terminal kinase (JNK) and p38 may play important roles in neuronal cell death or regeneration. In this review we will discuss these pahtways in detail. We will examine the evidence that these pathways are important in neuronal cell death. Finally we will review the evidence that inhibitors of these pathways have a neuroprotective effect both in vitro and in vivo.

MAPKs: new targets for neurodegeneration

Neurodegenerative diseases remain a huge unmet pharmaceutical need. For some diseases such as Parkinson’s disease, there are currently only palliative therapies, and for others such as Alzheimer’s disease there are no proven therapies on the market that have any significant impact on disease progression. Recent work has suggested that cell death may play a key role in a number of neurodegenerative diseases, and halting this aberrant cell death may halt disease progression. Kinases identified in cell death pathways may be attractive targets for neurodegenerative diseases. In this review, the authors will focus on three families of related mitogen-activated protein kinases (MAPKs), namely, the extracellular signal-regulated protein kinases (ERKs), the c-Jun N-terminal kinases (JNKs) and the p38 MAPKs. The evidence for activation of each of these pathways in disease states and in models of neurodegenerative disorders will be examined. Effects of inhibitors, where available, will be discussed, and potential problems and side effects of kinase inhibitors as therapeutics will be addressed.

A rapid method for semi-quantitative analysis of neurite outgrowth from chick DRG explants using image analysis

Neurite outgrowth from dorsal root ganglion (DRG) explants is a method of evaluating neurotrophic activity of growth factors and neurotrophin mimetics. The drawbacks to this approach are the difficulties in quantifying the response. Neurite counts are time consuming and labour intensive, and the accuracy is often questionable due to branching and fasciculation of the neurites. We report here a method of semi-quantitative analysis of neurite outgrowth from chick DRG explants, using image analysis to quantify the area occupied by neurites emanating from the ganglion. This method is rapid, takes into account both the length and number of neurites, and is unaffected by neurite fasciculation or branching. Primary explants of chick DRGs were treated with the neurotrophins nerve growth factor (NGF) or neurotrophin-3 (NT-3) and with the compound K252a. K252b was tested for potentiation of the response to NT-3. The results show a dose dependent outgrowth of neurites from explants treated with NGF, NT-3 and K252a, and potentiation of the NT-3 response by K252b. These responses were quantified by neurite area quantification using image analysis. We conclude that neurite area measurement using image analysis provides a robust means of evaluating neurotrophic activity of growth factors and neurotrophin mimetics in vitro.

Caspase Inhibitors Attenuate 1-Methyl-4-Phenylpyridinium Toxicity in Primary Cultures of Mesencephalic Dopaminergic Neurons

Parkinsons disease is characterized by a loss of dopaminergic nigrostriatal neurons. This neuronal loss is mimicked by the neurotoxin 1-methyl-4-phenylpyridinium (MPP+). MPP+ toxicity is mediated through inhibition of mitochondrial complex I, decreasing ATP production, and upregulation of oxygen radicals. There is evidence that the cell death induced by MPP+ is apoptotic and that inhibition of caspases may be neuroprotective. In primary cultures of rat mesencephalic dopaminergic neurons, MPP+ treatment decreased the number of surviving dopaminergic neurons in the cultures and the ability of the neurons to take up [3H]dopamine ([3H]DA). Caspase inhibition using the broad-spectrum inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (zVAD-fmk) spared MPP+-treated dopaminergic neurons and increased somatic size. There was a partial restoration of neurite length in zVAD-fmk-treated cultures, but little restoration of [3H]DA uptake. Peptide inhibitors of caspases 2, 3, and 9, but not of caspase 1, caused significant neuroprotection. Two novel caspase inhibitors were tested for neuroprotection, a broad spectrum inhibitor and a selective caspase 3 inhibitor; both inhibitors increased survival to >90% of control. No neuroprotection was observed with an inactive control compound. MPP+treatment caused chromatin condensation in dopaminergic neurons and increased expression of activated caspase 3. Inhibition of caspases with either zVAD-fmk or a selective caspase 3 inhibitor decreased the number of apoptotic profiles, but not expression of the active caspase. We conclude that MPP+ toxicity in primary dopaminergic neurons involves activation of a pathway terminating in caspase 3 activation, but that other mechanisms may underlie the neurite loss.

Small molecule Trk receptor agonists and other neurotrophic factor mimetics.

Nerve growth factor belongs to a small family of proteins whose binding at the Trk and p75(NTR) transmembrane receptors triggers a cascade of signaling events that give rise to neurotrophic responses in neuronal cells and in vivo. Following their robust effects in animal models of neurodegeneration, neurotrophins have been evaluated for therapy for several human neurodegenerative diseases. However, due mainly to the poor pharmacokinetic behavior of these proteins, they have largely met without success in the clinic, making it desirable to develop small molecule neurotrophin mimetics. A range of compounds is described that achieves some of the neurotrophic and neuroprotective effects attributed to neurotrophins through a variety of mechanisms. These small molecules are divided into the following functional categories: (1). compounds that activate Trk receptors directly; (2). compounds that potentiate the actions of neurotrophins on Trk receptors; (3). compounds that activate Trk indirectly; (4). compounds that influence neurotrophin expression or secretion; and (5). a broad class of compounds that act downstream of, or independently of, Trk receptors. Unfortunately, most of the compounds that have been reported suffer from either lack of specificity for the desired mechanism/effect(s) or lack of efficacy of the compounds in appropriate in vivo models, or both. This second limitation has been particularly severe for compounds designed to mimic the neurotrophins in their interaction with Trk receptors, an ongoing and formidable challenge. Nevertheless, a small subset of the compounds, acting on intracellular signaling pathways downstream of Trk receptors, shows promise for the future treatment of neurodegenerative diseases.

CEP-1347 increases ChAT activity in culture and promotes cholinergic neurone survival following fimbria-fornix lesion.

Recent evidence suggests that the activation of the Jun N-terminal kinase (JNK) signal transduction pathway may be important in neuronal responses to stresses such as trophic factor deprivation. Preventing the activation of JNK and expression of c-Jun may, therefore, be neuroprotective. Here, we report that the small molecule CEP-1347, which has been shown to inhibit the JNK signalling pathway, promotes cholinergic activity in cultured embryonic septal neurones. In vivo, we have shown that CEP-1347, administered either by sub-cutaneous (s.c.) injection or by continuous infusion, is partially neuroprotective, for cholinergic neurones in the medial septum, following fimbria-fornix transection. These data suggest that small molecules such as CEP-1347 may have beneficial effects in treating neurodegenerative diseases.

CEP-1347 promotes survival of NGF responsive neurones in primary DRG explants

&NA; CEP‐1347 inhibits the signalling pathway of c‐jun‐N‐terminal kinase, and is neuroprotective in vivo and in vitro. Embryonic chick dorsal root ganglion neurones are dependent on NGF for survival and neurite outgrowth; NGF withdrawal results in apoptotic cell death. We examined the neuroprotective and neurite outgrowth promoting activity of CEP‐1347 in dissociated DRG neurones and in primary DRG explants. CEP‐1347 was as effective as NGF in promoting survival of dissociated DRG neurones, but caused only limited neurite outgrowth from DRG explants. When NGF was subsequently added to CEP‐1347 treated explants, the outgrowth increased to a similar level to explants which had received NGF throughout. CEP‐1347 may be a useful tool to maintain viable DRG explants to allow evaluation of neurite outgrowth promoting compounds and dissection of survival and neurite outgrowth signalling pathways.

A rapid method for determination of cell survival in primary neuronal DRG cultures

A simple and rapid enzyme-linked immunosorbent assay (ELISA) has been developed to provide an alternative to cell counting to detect increases in cell survival in primary neuronal cultures. This sensitive assay has the advantage of being less time consuming and labour intensive than cell counting, can be used to quantify cell survival and is more accurate than estimation methods of counting. The ELISA uses an antibody raised to GAP-43, a growth-associated protein which is strongly expressed by developing and regenerating neurones. The effects of nerve growth factor (NGF), neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF) on GAP-43 immunoreactivity in dissociated primary cultures of rat and chick dorsal root ganglia have been compared to results obtained by cell counting. Data show that human NGF produced the greatest increase in GAP-43-immunoreactive neurones in both species; this increase in immunoreactivity correlated well with the increased survival shown by cell count data. Results prove that the ELISA can also be used to accurately detect small changes in cell survival as seen with NT-3 and BDNF, or potentiation of the effects obtained with the trophic factor NT-3. In conclusion, this ELISA may be a useful tool to detect neurotrophic effects of unknown agents or novel neurotrophins.