Messenger RNAs in synaptosomal fractions from rat brain

Functional Differences between Synaptic Mitochondria from the Striatum and the Cerebral Cortex

Mitochondrial dysfunction has been shown to play a major role in neurodegenerative disorders such as Huntington's disease, Alzheimer's disease and Parkinson's disease. In these and other neurodegenerative disorders, disruption of synaptic connectivity and impaired neuronal signaling are among the early signs. When looking for potential causes of neurodegeneration, specific attention is drawn to the function of synaptic mitochondria, as the energy supply from mitochondria is crucial for normal synaptic function. Mitochondrial heterogeneity between synaptic and non-synaptic mitochondria has been described, but very little is known about possible differences between synaptic mitochondria from different brain regions. The striatum and the cerebral cortex are often affected in neurodegenerative disorders. In this study we therefore used isolated nerve terminals (synaptosomes) from female mice, striatum and cerebral cortex, to investigate differences in synaptic mitochondrial function between these two brain regions. We analyzed mitochondrial mass, citrate synthase activity, general metabolic activity and mitochondrial respiration in resting as well as veratridine-activated synaptosomes using glucose and/or pyruvate as substrate. We found higher mitochondrial oxygen consumption rate in both resting and activated cortical synaptosomes compared to striatal synaptosomes, especially when using pyruvate as a substrate. The higher oxygen consumption rate was not caused by differences in mitochondrial content, but instead corresponded with a higher proton leak in the cortical synaptic mitochondria compared to the striatal synaptic mitochondria. Our results show that the synaptic mitochondria of the striatum and cortex differently regulate respiration both in response to activation and variations in substrate conditions.

Intra-axonal protein synthesis in development and beyond

Proteins can be locally produced in the periphery of a cell, allowing a rapid and spatially precise response to the changes in its environment. This process is especially relevant in highly polarized and morphologically complex cells such as neurons. The study of local translation in axons has evolved from being primarily focused on developing axons, to the notion that also mature axons can produce proteins. Axonal translation has been implied in several physiological and pathological conditions, and in all cases it shares common molecular actors and pathways as well as regulatory mechanisms. Here, we review the main findings in these fields, and attempt to highlight shared principles.

NMDA-mediated regulation of DSCAM dendritic local translation is lost in a mouse model of Down's syndrome

Down's syndrome cell adhesion molecule (DSCAM) belongs to the Down's syndrome critical region of human chromosome 21, and it encodes a cell adhesion molecule involved in dendrite morphology and neuronal wiring. Although the function of DSCAM in the adult brain is unknown, its expression pattern suggests a role in synaptic plasticity. Local mRNA translation is a key process in axonal growth, dendritogenesis, and synaptogenesis during development, and in synaptic plasticity in adulthood. Here, we report the dendritic localization of DSCAM mRNA in the adult mouse hippocampus, where it associates with CPEB1 [cytoplasmic polyadenylation element (CPE) binding protein 1], an important regulator of mRNA transport and local translation. We identified five DSCAM isoforms produced by alternative polyadenylation bearing different combinations of regulatory CPE motifs. Overexpression of DSCAM in hippocampal neurons inhibited dendritic branching. Interestingly, dendritic levels of DSCAM mRNA and protein were increased in hippocampal neurons from Ts1Cje mice, a model of Down's syndrome. Most importantly, DSCAM dendritic translation was rapidly induced by NMDA in wild-type, but not in Ts1Cje neurons. We propose that impairment of the NMDA-mediated regulation of DSCAM translation may contribute to the alterations in dendritic morphology and/or synaptic plasticity in Down's syndrome.

Dendritic localization and activity-dependent translation of Engrailed1 transcription factor

Engrailed1 (En1) is a homeoprotein transcription factor expressed throughout adulthood in several midbrain cells, including the dopaminergic neurons of the substantia nigra. Here we report the presence of Engrailed protein and En1 mRNA in proximal dendrites of these neurons and of En1 mRNA in ventral midbrain synaptoneurosomes. We show that the 3' untranslated region of En1 mRNA contains a functional cytoplasmic polyadenylation element (CPE), suggesting that its dendritic localization is regulated by CPE binding protein (CPEB). In order to evaluate activity-regulated translation, conditions were developed using primary midbrain neurons. With this in vitro model, En1 mRNA translation is increased by depolarization in a polyadenylation dependent manner. Furthermore, En1 translation is prevented by rapamycin, implicating the mTOR pathway, which is known to regulate dendritic translation. Together, these results suggest an activity-dependent role for Engrailed in midbrain dopaminergic neuron physiology.

Synaptosomal protein synthesis is selectively modulated by learning

Synaptosomes from rat brain have long been used to investigate the properties of synaptic protein synthesis. Comparable analyses have now been made in adult male rats trained for a two-way active avoidance task to examine the hypothesis of its direct participation in brain plastic events. Using Ficoll-purified synaptosomes from neocortex, hippocampus and cerebellum, our data indicate that the capacity of synaptosomal protein synthesis and the specific activity of newly synthesized proteins were not different in trained rats in comparison with home-caged control rats. On the other hand, the synthesis of two proteins of 66.5 kDa and 87.6 kDa separated by SDS-PAGE and analyzed by quantitative densitometry was selectively enhanced in trained rats. In addition, the synthesis of the 66.5 kDa protein, but not of the 87.6 kDa protein, correlated with avoidances and escapes and inversely correlated with freezings in the neocortex, while in the cerebellum it correlated with avoidances and escapes. The data demonstrate the participation of synaptic protein synthesis in plastic events of behaving rats, and the selective, region-specific modulation of the synthesis of a synaptic 66.5 kDa protein by the newly acquired avoidance response and by the reprogramming of innate neural circuits subserving escape and freezing responses.

Oestrogen synthesis in the hippocampus: role in axon outgrowth

Ovarian oestrogens have been postulated to be neuroprotective. It has also been shown that considerable amounts of oestrogens are synthesised in hippocampal neurones. In the present study, we focused on a potential role of hippocampus-derived oestradiol compared to gonad-derived oestradiol on axon outgrowth of hippocampal neurones. To address the role of hippocampus-derived oestradiol, we inhibited oestrogen synthesis by treatment of neonatal hippocampal cell cultures with letrozole, a specific aromatase inhibitor. As an alternative, we used siRNA against steroidogenic acute regulatory protein (StAR). Axon outgrowth and GAP-43 expression were significantly down-regulated in response to letrozole and in siRNA-StAR transfected cells. The effects after inhibition of oestrogen synthesis in response to letrozole and in siRNA-StAR transfected cells were reversed by oestrogen supplementation. No difference was found between ovariectomised animals, cycling animals at pro-oestrus and ovariectomised and subsequently oestradiol-treated animals. However, high pharmacological doses of oestradiol promoted axon outgrowth, which was possible to abolish by the oestrogen receptor antagonist ICI 182,780. Our results show that oestradiol-induced neurite outgrowth is very likely mediated by genomic oestrogen receptors and requires higher doses of oestradiol than physiological serum concentrations derived from the gonads.

Expression of high affinity choline transporter during mouse development in vivo and its upregulation by NGF and BMP-4 in vitro

An important feature of cholinergic neurons is high-affinity choline transport, which allows them to reuse choline for the synthesis of ACh needed to support cholinergic neurotransmission. The choline transporter, designated CHT, was recently cloned. We applied RT/PCR to monitor the expression of CHT in the developing mouse CNS from embryonic day 14 (E14) to postnatal day 30 (P30). We found that CHT was expressed early in development, predominantly in the regions containing cholinergic neurons. In the spinal cord, CHT mRNA was present at close to adult levels at the earliest time point examined (E14) and showed almost no changes after birth. In the striatum and the septum, CHT mRNA increased steadily during embryonic stages and leveled off after birth. Surprisingly, CHT mRNA expression was also detected in other brain regions, notably in the cerebellum, where it peaked on E19, and then rapidly declined during postnatal development. CHT protein was detected by Western blotting as a band of apparent molecular weight of 70 kDa. The accumulation of this protein during development lagged behind mRNA accumulation in all tissues. We also examined the effects of NGF and BMP-4, the potent inducers of choline acetyltransferase and vesicular acetylcholine transporter genes, on CHT expression. Both factors increased CHT mRNA accumulation in primary septal cultures. The effect of NGF was dependent on the PI3K signaling, as it was abolished by the PI3K inhibitor LY294002. This result indicates that some of the signals regulating other cholinergic-specific genes also control CHT expression.

Selective regulation of presynaptic calcium/calmodulin-dependent protein kinase II by psychotropic drugs

BACKGROUND

Changes in neuroplasticity have been involved in the pathogenesis of psychiatric disorders as well as in psychotropic drug action. Calcium/calmodulin-dependent protein kinase II (CaM kinase II), an enzyme with a pivotal role in synaptic plasticity and cognitive functions, has been implicated in the action of anticonvulsants, benzodiazepines, and antidepressants, but little is known as to its role in the action of different drugs employed for treatment of psychiatric disorders.

METHODS

We studied the function and expression of CaM kinase II following chronic treatment of rats with two antidepressants, fluvoxamine and desipramine, a typical antipsychotic drug, haloperidol, and the typical medication for manic-depressive disorder, lithium.

RESULTS

Antidepressants significantly increased the kinase activity in presynaptic vesicles of frontal/prefrontal cortex. Haloperidol induced no change, whereas lithium significantly decreased the activity. Kinase activation by antidepressants was further demonstrated by increased phosphorylation of exogenously added recombinant synaptotagmin. Immunoreactivity of vesicular kinase (alpha-isoform) was significantly increased by reuptake blockers but not by the two other drugs. Kinetic analysis showed that limiting value of enzymatic velocity (Vmax) of the kinase for substrate was also increased by reuptake blockers and decreased by lithium; however, neither messenger ribonucleic acid nor protein expression level of the kinase was increased in frontal/prefrontal cortex homogenates of antidepressant-treated rats, suggesting the involvement of local synaptic mechanisms.

CONCLUSIONS

These findings show that functional regulation of presynaptic CaM kinase II is selectively affected by different psychotropic drugs, and suggest local synaptic mechanisms for pharmacological regulation of the kinase.

Axonal and presynaptic protein synthesis: new insights into the biology of the neuron

The presence of a local mRNA translation system in axons and terminals was proposed almost 40 years ago. Over the ensuing period, an impressive body of evidence has grown to support this proposal -- yet the nerve cell body is still considered to be the only source of axonal and presynaptic proteins. To dispel this lingering neglect, we now present the wealth of recent observations bearing on this central idea, and consider their impact on our understanding of the biology of the neuron. We demonstrate that extrasomatic translation sites, which are now well recognized in dendrites, are also present in axonal and presynaptic compartments.