BDNF exerts contrasting effects on peripheral myelination of NGF-dependent and BDNF-dependent DRG neurons.

Although brain-derived neurotrophic factor (BDNF) has been shown to promote peripheral myelination during development and remyelination after injury, the precise mechanisms mediating this effect remain unknown. Here, we determine that BDNF promotes myelination of nerve growth factor-dependent neurons, an effect dependent on neuronal expression of the p75 neurotrophin receptor, whereas BDNF inhibits myelination of BDNF-dependent neurons via the full-length TrkB receptor. Thus, BDNF exerts contrasting effects on Schwann cell myelination, depending on the complement of BDNF receptors that are expressed by different subpopulations of dorsal root ganglion neurons. These results demonstrate that BDNF exerts contrasting modulatory roles in peripheral nervous system myelination, and that its mechanism of action is acutely regulated and specifically targeted to neurons.

Autophagy generates retrogradely transported organelles: a hypothesis.

Nerve cells require trophic signals transmitted from the nerve terminal via the axon in order to survive and develop normally. As the axon may be more than a meter long, specialised mechanisms are needed to transmit these signals. This involves the retrograde axonal transport of signalling endosomes containing nerve growth factor (NGF) and other synaptically derived molecules. These are large, double membrane multivesicular bodies containing a mixture of all vesicle types seen in the nerve terminal. How this signalling endosome is formed and targeted for retrograde axonal transport, however, remains an open question. Here we show that members of the Rab family of proteins that are retrogradely transported indicate that the signalling endosome contains both early and recycling endosomes. In addition, we show that retrogradely transported labelled antibody to dopamine beta-hydroxylase, a marker for synaptic vesicles, co-localizes within the same signalling endosome as NGF. We further show that LC3, a marker for autophagosomes, is retrogradely transported and associates with retrogradely transported NGF. We propose that neurons have exploited the mechanism of autophagy to engulf a sample of the cytoplasmic contents of the nerve terminal to transport back to the cell body. This sample of cytoplasmic contents relays a reliable snapshot of the totality of signalling events occurring in the nerve terminal at that instant in time.

Diazepam binding inhibitor overexpression in mice causes hydrocephalus, decreases plasticity in excitatory synapses and impairs hippocampus-dependent learning.

Diazepam binding inhibitor (DBI) and its processing products are endogenous modulators of GABAA and linked to various brain disorders ranging from anxiety and drug dependence to epilepsy. To investigate the physiological role of endogenously expressed DBI in the brain we created a transgenic mouse line overexpressing DBI gene. Transgenic mice had a 37x increased protein expression and immunohistochemistry showed excessive glial expression in the infragranular region of the dentate gyrus. Transgenic animals had significantly larger lateral ventricles and decreased plasticity of excitatory synapses without affecting either inhibitory or excitatory synaptic transmission. In behavioral tests transgenic animals had no differences in motor and exploratory activity, yet impaired hippocampus-dependent learning and memory. Overexpression did not cause anxiety or proconflict behavior, nor influenced kainic acid or pentylenetetrazole induced seizure activity. Our transgenic mouse line demonstrates that endogenously overexpressed DBI impairs hippocampus-dependent learning without anxiety or proconflict behavior.

Retrograde axonal transport of dopamine beta hydroxylase antibodies by neurons in the trigeminal ganglion.

In this study we describe a population of neurons in the adult rat trigeminal ganglion (TG) that express dopamine beta-hydroxylase (DBH) and tyrosine hydroxylase (TH), and transport anti-DBH from their terminals. We have used NGF and NT3 labeled with biotin and anti-p75NTR labeled with FITC to examine the transport of neurotrophins and their receptors by these cells. In both the superior cervical ganglion (SCG) and the TG all neurons that transported anti-DBH transported NGF. While 100% of the DBH positive neurons in the TG also transported NT3, approximately 25% of these neurons in the SCG failed to transport NT3. In the SCG virtually all the neurons transported anti-p75NTR with the neurotrophins while in the TG more than 25% of these neurons failed to transport anti-p75NTR with the neurotrophins. These findings suggest that DBH positive neurons in the TG depend upon target-derived NGF and NT3 for their noradrenergic phenotype.

Spermidine/spermine N1-acetyltransferase overexpression in mice induces hypoactivity and spatial learning impairment.

The present work addresses the role of polyamines in learning and general behavior by subjecting transgenic mice overexpressing polyamine catabolic enzyme, spermidine/spermine N(1)-acetyltransferase (SSAT) and their syngenic littermates to neurobehavioral profiling assessment (SHIRPA) and to radial eight-arm maze. The general health and physiological conditions as well as the entire behavioral battery comprising of 34 parameters were recorded. The eight-arm radial maze (8-RAM) task included an initial acquisition task for 9 days followed by a 2-day retention test after a 2-week break. In addition, blood samples were taken for hormone analysis. Transgenic mice, which showed reduced motor activity, aggression and muscle tone, spent more time in the radial maze during initial acquisition and retention tasks as compared with syngenic mice. Moreover, the learning performance of transgenic females was significantly inferior to syngenic females. Interestingly, the levels of several hormones were significantly altered in SSAT transgenic mice; circulating adrenocorticotropic hormone (ACTH) and corticosterone levels were markedly increased while testosterone and thyroidal hormone levels were decreased. These changes may be related to the dramatic increase in brain putrescine levels in SSAT-overexpressing (SSAT-OE) mice, but it is likewise possible that the behavioral changes and learning impairment are attributable to more peripheral mechanisms (such as alterations in steroid hormone metabolism), which in turn, could be a consequence of the disturbed polyamine homeostasis.

Overexpression of spermidine/spermine N1-acetyltransferase elevates the threshold to pentylenetetrazol-induced seizure activity in transgenic mice.

Activation of polyamine catabolism in transgenic mice through an overexpression of spermidine/spermine N(1)-acetyltransferase (SSAT) results in a massive overaccumulation of the diamine putrescine in most tissues including brain. Putrescine pool in transgenic animals was strikingly expanded in every six brain regions analyzed at present. Pons (23-fold), cerebellum (37-fold), cerebrum (34-fold), and hippocampus (16-fold) showed the greatest increases in putrescine levels. Moreover, the molar ratio of putrescine to spermidine was increased in the different brain regions of the transgenic animals on an average of nearly 40-fold. Upon an exposure of the animals to pentylenetetrazol (PTZ) infusions, a compound known to induce epilepsy-like seizure activity, the SSAT transgenic mice showed significantly elevated seizure threshold to both clonic and tonic convulsions in comparison with their syngenic littermates. This difference, however, disappeared when the animals were treated with ifenprodil prior to PTZ infusions. The latter compound acts as an antagonist of N-methyl-D-aspartate receptor by binding to the polyamine site of the receptor. Overexpression of SSAT likewise appeared to protect the transgenic animals from PTZ-induced neuron loss in the hippocampus. As putrescine is known to serve as a precursor to gamma-aminobutyric acid (GABA), we carried out (1)H NMR analyses the results of which revealed that the levels of the inhibitory amino acid GABA and its excitatory counterpart glutamate were indistinguishable in syngenic and transgenic animals in all brain regions analyzed. The present results suggest that the frequently observed enhanced accumulation of putrescine in response to brain insults belongs to neuroprotective measures rather than being a cause of the subsequent injury.

Induction and activation of protein kinase C delta in hippocampus and cortex after kainic acid treatment.

Various isoforms of protein kinase C (PKC), especially the novel PKC subtypes delta, epsilon, and the atypical subtype PKC zeta, are involved in delayed cell death. We studied the expression and late activation of the latter PKC isoforms in comparison with classic PKC alpha, beta, and gamma in the brains of rats exposed to systemic kainate injection. The expression of PKC delta mRNA was strikingly upregulated (13-fold) in the cortex and the CA1 and CA3 hippocampal regions on 1 day after kainate administration, whereas PKC zeta mRNA was only moderately increased (about 100%) in these three brain regions on day 2 following the drug. PKC epsilon mRNA was slightly increased only in the cortex on days 2 and 6, while the mRNA levels of the classic PKC subtypes (alpha, beta, and gamma) remained unchanged or decreased after the treatment. Immunoblotting analyses revealed that the level of PKC delta protein started to increase on day 1 after kainate and was significantly elevated on day 2 in both the membrane and cytosol fractions of cortex and hippocampus. PKC epsilon protein only showed a marginal increase and the level of PKC zeta protein remained unaltered in response to the treatment. Cortical and CA1-3 pyramidal neurons displayed strong immunoreactivity for PKC delta on days 1 and 2, and microglia on days 1, 2, and 4 after the drug. The results indicate that the expression of apoptosis-associated isoforms of PKC, most notably that of delta, but to lesser extent also that of epsilon and zeta, is increased during kainate-induced neuronal death. The predominant induction of PKC delta in neurons and microglia suggests that PKC delta could be the major mediator or modulator of apoptotic and inflammatory responses to excitotoxic insults.