Kristin E. Larsen

α-Synuclein Overexpression in PC12 and Chromaffin Cells Impairs Catecholamine Release by Interfering with a Late Step in Exocytosis

α-Synuclein (α-syn), a protein implicated in Parkinsons disease pathogenesis, is a presynaptic protein suggested to regulate transmitter release. We explored how α-syn overexpression in PC12 and chromaffin cells, which exhibit low endogenous α-syn levels relative to neurons, affects catecholamine release. Overexpression of wild-type or A30P mutant α-syn in PC12 cell lines inhibited evoked catecholamine release without altering calcium threshold or cooperativity of release. Electron micrographs revealed that vesicular pools were not reduced but that, on the contrary, a marked accumulation of morphologically “docked” vesicles was apparent in the α-syn-overexpressing lines. We used amperometric recordings from chromaffin cells derived from mice that overexpress A30P or wild-type (WT) α-syn, as well as chromaffin cells from control and α-syn null mice, to determine whether the filling of vesicles with the transmitter was altered. The quantal size and shape characteristics of amperometric events were identical for all mouse lines, suggesting that overexpression of WT or mutant α-syn did not affect vesicular transmitter accumulation or the kinetics of vesicle fusion. The frequency and number of exocytotic events per stimulus, however, was lower for both WT and A30P α-syn-overexpressing cells. The α-syn-overexpressing cells exhibited reduced depression of evoked release in response to repeated stimuli, consistent with a smaller population of readily releasable vesicles. We conclude that α-syn overexpression inhibits a vesicle “priming” step, after secretory vesicle trafficking to “docking” sites but before calcium-dependent vesicle membrane fusion.

Methamphetamine-Induced Degeneration of Dopaminergic Neurons Involves Autophagy and Upregulation of Dopamine Synthesis

Methamphetamine (METH) selectively injures the neurites of dopamine (DA) neurons, generally without inducing cell death. It has been proposed that METH-induced redistribution of DA from the vesicular storage pool to the cytoplasm, where DA can oxidize to produce quinones and additional reactive oxygen species, may account for this selective neurotoxicity. To test this hypothesis, we used mice heterozygous (+/−) or homozygous (−/−) for the brain vesicular monoamine uptake transporter VMAT2, which mediates the accumulation of cytosolic DA into synaptic vesicles. In postnatal ventral midbrain neuronal cultures derived from these mice, METH-induced degeneration of DA neurites and accumulation of oxyradicals, including metabolites of oxidized DA, varied inversely with VMAT2 expression. METH administration also promoted the synthesis of DA via upregulation of tyrosine hydroxylase activity, resulting in an elevation of cytosolic DA even in the absence of vesicular sequestration. Electron microscopy and fluorescent labeling confirmed that METH promoted the formation of autophagic granules, particularly in neuronal varicosities and, ultimately, within cell bodies of dopaminergic neurons. Therefore, we propose that METH neurotoxicity results from the induction of a specific cellular pathway that is activated when DA cannot be effectively sequestered in synaptic vesicles, thereby producing oxyradical stress, autophagy, and neurite degeneration.

Proteasomal inhibition leads to formation of ubiquitin/α-synuclein-immunoreactive inclusions in PC12 cells

Proteasomal dysfunction has been recently implicated in the pathogenesis of several neurodegenerative diseases, including Parkinsons disease and diffuse Lewy body disease. We have developed an in vitro model of proteasomal dysfunction by applying pharmacological inhibitors of the proteasome, lactacystin or ZIE[O‐tBu]‐A‐leucinal (PSI), to dopaminergic PC12 cells. Proteasomal inhibition caused a dose‐dependent increase in death of both naive and neuronally differentiated PC12 cells, which could be prevented by caspase inhibition or CPT‐cAMP. A percentage of the surviving cells contained discrete cytoplasmic ubiquitinated inclusions, some of which also contained synuclein‐1, the rat homologue of human α‐synuclein. However the total level of synuclein‐1 was not altered by proteasomal inhibition. The ubiquitinated inclusions were present only within surviving cells, and their number was increased if cell death was prevented. We have thus replicated, in this model system, the two cardinal pathological features of Lewy body diseases, neuronal death and the formation of cytoplasmic ubiquitinated inclusions. Our findings suggest that inclusion body formation and cell death may be dissociated from one another.

α-Synuclein Overexpression Increases Cytosolic Catecholamine Concentration

Dysregulation of dopamine homeostasis and elevation of the cytosolic level of the transmitter have been suggested to underlie the vulnerability of catecholaminergic neurons in Parkinson’s disease. Because several known mutations in α-synuclein or overexpression of the wild-type (WT) protein causes familial forms of Parkinson’s disease, we investigated possible links between α-synuclein pathogenesis and dopamine homeostasis. Chromaffin cells isolated from transgenic mice that overexpress A30P α-synuclein displayed significantly increased cytosolic catecholamine levels as measured by intracellular patch electrochemistry, whereas cells overexpressing the WT protein and those from knock-out animals were not different from controls. Likewise, catechol concentrations were higher in l-DOPA-treated PC12 cells overexpressing A30P or A53T compared with those expressing WT α-synuclein, although the ability of cells to maintain a low cytosolic dopamine level after l-DOPA challenge was markedly inhibited by either protein. We also found that incubation with low-micromolar concentrations of WT, A30P, or A53T α-synuclein inhibited ATP-dependent maintenance of pH gradients in isolated chromaffin vesicles and that the WT protein was significantly less potent in inducing the proton leakage. In summary, we demonstrate that overexpression of different types of α-synuclein disrupts vesicular pH and leads to a marked increase in the levels of cytosolic catechol species, an effect that may in turn trigger cellular oxyradical damage. Although multiple molecular mechanisms may be responsible for the perturbation of cytosolic catecholamine homeostasis, this study provides critical evidence about how α-synuclein might exert its cytotoxicity and selectively damage catecholaminergic cells.

Increased Expression of Rat Synuclein in the Substantia Nigra Pars Compacta Identified by mRNA Differential Display in a Model of Developmental Target Injury

Abstract : Human α‐synuclein was identified on the basis of proteolytic fragments derived from senile plaques of Alzheimers disease, and it is the locus of mutations in some familial forms of Parkinsons disease. Its normal function and whether it may play a direct role in neural degeneration remain unknown. To explore cellular responses to neural degeneration in the dopamine neurons of the substantia nigra, we have developed a rodent model of apoptotic death induced by developmental injury to their target, the striatum. We find by mRNA differential display that synuclein is up‐regulated in this model, and thus it provides an opportunity to examine directly whether synuclein plays a role in the death of these neurons or, alternatively, in compensatory responses. Up‐regulation of mRNA is associated with an increase in the number of neuronal profiles immunostained for synuclein protein. At a cellular level, synuclein is almost exclusively expressed in normal neurons, rather than apoptotic profiles. Synuclein is up‐regulated throughout normal postnatal development of substantia nigra neurons, but it is not further up‐regulated during periods of natural cell death. We conclude that up‐regulation of synuclein in the target injury model is unlikely to mediate apoptotic death and propose that it may be due to a compensatory response in neurons destined to survive.

Brain-derived neurotrophic factor inhibits apoptosis and dopamine-induced free radical production in striatal neurons but does not prevent cell death

In hereditary Huntingtons disease, a triplet repeat disease, there is extensive loss of striatal neurons. It has been shown that brain-derived neurotrophic factor (BDNF) protects striatal neurons against a variety of insults. We confirmed that BDNF enhances survival and DARPP-32 expression in primary striatal cultures derived from postnatal mice. Furthermore, BDNF inhibited intracellular oxyradical stress triggered by dopamine, and partially blocked basal and dopamine-induced apoptosis. Nevertheless, BDNF failed to rescue striatal neurons from dopamine-induced cell death. Therefore, BDNF inhibits free radical and apoptotic pathways in medium spiny neurons, but does so downstream from the point of commitment to cell death.

A glial cell line-derived neurotrophic factor (GDNF):tetanus toxin fragment C protein conjugate improves delivery of GDNF to spinal cord motor neurons in mice

Glial cell line-derived neurotrophic factor (GDNF) has shown robust neuroprotective and neuroreparative activities in various animal models of Parkinsons Disease or amyotrophic lateral sclerosis (ALS). The successful use of GDNF as a therapeutic in humans, however, appears to have been hindered by its poor bioavailability to target neurons in the central nervous system (CNS). To improve delivery of exogenous GDNF protein to CNS motor neurons, we employed chemical conjugation techniques to link recombinant human GDNF to the neuronal binding fragment of tetanus toxin (tetanus toxin fragment C, or TTC). The predominant species present in the purified conjugate sample, GDNF:TTC, had a molecular weight of approximately 80 kDa as determined by non-reducing SDS-PAGE. Like GDNF, addition of GDNF:TTC to culture media of neuroblastoma cells expressing GFRalpha-1/c-RET produced a dose-dependent increase in cellular phospho-c-RET levels. Treatment of cultured midbrain dopaminergic neurons with either GDNF or the conjugate similarly promoted both DA neuron survival and neurite outgrowth. However, in contrast to mice treated with GDNF by intramuscular injection, mice receiving GDNF:TTC revealed intense GDNF immunostaining associated with spinal cord motor neurons in fixed tissue sections. That GDNF:TTC provided neuroprotection of axotomized motor neurons in neonatal rats further revealed that the conjugate retained its GDNF activity in vivo. These results indicate that TTC can serve as a non-viral vehicle to substantially improve the delivery of functionally active growth factors to motor neurons in the mammalian CNS.