Adenoviral Vector-Mediated Expression of Neurotrophin-3 Increases Neuronal Survival in Suprachiasmatic Nucleus Grafts

Constitutive activation of the ERK-MAPK pathway in the suprachiasmatic nuclei inhibits circadian resetting

Circadian entrainment involves photic stimulation of the suprachiasmatic molecular oscillator, including activation of the ERK/MAP kinase, which is phosphorylated endogenously during the day and in response to light during the night. We aimed to disrupt the diurnal cycle of ERK phosphorylation by in vivo transfection of a constitutively active form of MEK, a MAPK kinase. This procedure did not affect normal circadian parameters, but completely inhibited light-induced phase advances. Therefore, circadian regulation of the ERK pathway is not essential for the normal mechanism of the biological clock, but it is fundamental as an interface with environmental entrainment by light.

Gene delivery to the spinal cord: comparison between lentiviral, adenoviral, and retroviral vector delivery systems

Viral gene delivery for spinal cord injury (SCI) is a promising approach for enhancing axonal regeneration and neuroprotection. An understanding of spatio-temporal transgene expression in the spinal cord is essential for future studies of SCI therapies. Commonly, intracellular marker proteins (e.g., EGFP) were used as indicators of transgene levels after viral delivery, which may not accurately reflect levels of secreted transgene. This study examined transgene expression using ELISA after viral delivery of D15A, a neurotrophin with BDNF and NT-3 activities, at 1, 2, and 4weeks after in vivo and ex vivo delivery using lentiviral, adenoviral, and retroviral vectors. Further, the inflammatory responses and viral infection patterns after in vivo delivery were examined. Lentiviral vectors had the most stable pattern of gene expression, with D15A levels of 536 +/- 38 and 363 +/- 47 pg/mg protein seen at 4 weeks after the in vivo and ex vivo delivery, respectively. Our results show that protein levels downregulate disproportionately to levels of EGFP after adenoviral vectors both in vivo and ex vivo. D15A dropped from initial levels of 422 +/- 87 to 153 +/- 18 pg/mg protein at 4 weeks after in vivo administration. Similarly, ex vivo retrovirus-mediated transgene expression exhibited rapid downregulation by 2 weeks post-grafting. Compared to adenoviral infection, macrophage activation was attenuated after lentiviral infection. These results suggest that lentiviral vectors are most suitable in situations where stable long-term transgene expression is needed. Retroviral ex vivo delivery is optional when transient expression within targeted spinal tissue is desired, with adenoviral vectors in between.

Actions of neurotrophic factors and their signaling pathways in neuronal survival and axonal regeneration

Adult axons in the mammalian central nervous system do not elicit spontaneous regeneration after injury, although many affected neurons have survived the neurotrauma. However, axonal regeneration does occur under certain conditions. These conditions include: (a) modification of regrowth environment, such as supply of peripheral nerve bridges and transplantation of Schwann cells or olfactory ensheathing glia to the injury site; (b) application of neurotrophic factors at the cell soma and axon tips; (c) blockade of growth-inhibitory molecules such as Nogo-A, myelin-associated glycoprotein, and oligodendrocyte-myelin glycoprotein; (d) prevention of chondroitin-sulfate-proteoglycans-related scar tissue formation at the injury site using chondroitinase ABC; and (e) elevation of intrinsic growth potential of injured neurons via increasing intracellular cyclic adenosine monophosphate level. A large body of evidence suggests that these conditions achieve enhanced neuronal survival and axonal regeneration through sometimes overlapping and sometimes distinct signal transduction mechanisms, depending on the targeted neuronal populations and intervention circumstances. This article reviews the available information on signal transduction pathways underlying neurotrophic-factor-mediated neuronal survival and neurite outgrowth/axonal regeneration. Better understanding of signaling transduction is important in helping us develop practical therapeutic approaches for encouraging neuronal survival and axonal regeneration after traumatic injury in clinical context.

Apoptotic cell death following traumatic injury to the central nervous system

Apoptotic cell death is a fundamental and highly regulated biological process in which a cell is instructed to actively participate in its own demise. This process of cellular suicide is activated by developmental and environmental cues and normally plays an essential role in eliminating superfluous, damaged, and senescent cells of many tissue types. In recent years, a number of experimental studies have provided evidence of widespread neuronal and glial apoptosis following injury to the central nervous system (CNS). These studies indicate that injury-induced apoptosis can be detected from hours to days following injury and may contribute to neurological dysfunction. Given these findings, understanding the biochemical signaling events controlling apoptosis is a first step towards developing therapeutic agents that target this cell death process. This review will focus on molecular cell death pathways that are responsible for generating the apoptotic phenotype. It will also summarize what is currently known about the apoptotic signals that are activated in the injured CNS, and what potential strategies might be pursued to reduce this cell death process as a means to promote functional recovery.

Transgenic approach reveals expression of the VPAC2 receptor in phenotypically defined neurons in the mouse suprachiasmatic nucleus and in its efferent target sites

Circadian rhythms in mammals depend on the properties of cells in the suprachiasmatic nucleus (SCN). The retino-recipient core of the mouse SCN is characterized by vasoactive intestinal peptide (VIP) neurons. Expression within the SCN of VPAC2, a VIP receptor, is required for circadian rhythmicity. Using transgenic mice with beta-galactosidase as a marker for VPAC2, we have phenotyped VPAC2-expressing cells within the SCN and investigated expression of the VPAC2 marker at sites previously shown to receive VIP-containing SCN efferents. In situ hybridization and immunohistochemistry demonstrated identical distributions for VPAC2 mRNA and beta-galactosidase and coexpression of the two signals in the SCN. Double-label confocal immunofluorescence identified beta-galactosidase in 32% of the VIP and 31% of the calretinin neurons in the SCN core. Of the arginine-vasopressin neurons that characterize the SCN shell, 45% expressed beta-galactosidase. In contrast, this marker was not apparent in astrocytes within the SCN core or shell. Cell bodies containing beta-galactosidase were detected at sites reportedly receiving VIP-containing SCN efferents, including the subparaventricular zone and lateral septum and the anteroventral periventricular, preoptic suprachiasmatic, medial preoptic and paraventricular hypothalamic nuclei. The detection of a marker for VPAC2 expression in the SCN in almost one-third of the VIP and calretinin core neurons and nearly half of the arginine-vasopressin shell neurons and also in cell bodies at sites receiving VIP-immunoreactive projections from the SCN indicates that VPAC2 may contribute to autoregulation and/or coupling within the SCN core and to the control of the SCN shell and sites distal to this nucleus.

Expression of VIP and/or PACAP receptor mRNA in peptide synthesizing cells within the suprachiasmatic nucleus of the rat and in its efferent target sites

The suprachiasmatic nucleus (SCN) contains the predominant circadian pacemaker in mammals. Considerable evidence indicates that VPAC(2) and PAC(1), receptors for vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating peptide (PACAP), play critical roles in maintaining and entraining circadian rhythms. Retinal projections to the rat SCN contain PACAP and terminate mostly in the ventral SCN, the site of VIP neurons. The incidence of VPAC(2) and PAC(1) mRNAs within distinct neuronal populations of the rat SCN has been determined using double-label in situ hybridization. VPAC(2) mRNA was detected in almost all arginine-vasopressin (AVP) neurons of the dorsomedial SCN and in 41% of the VIP neurons; somatostatin (SST) neurons, predominantly in dorsomedial and intermediate regions, showed a decreased incidence (23%). PAC(1) mRNA was present in nearly half of the VIP and SST neurons (45% and 40%, respectively) and in one-third of the AVP neurons (32%). Cells expressing VPAC(2) mRNA also were detected in diencephalic areas that receive VIP-immunoreactive SCN efferents, such as the peri-suprachiasmatic region, lateral subparaventricular zone, parvocellular hypothalamic paraventricular subdivisions, dorsomedial hypothalamic nucleus, and anterior thalamic paraventricular and paratenial nuclei. The extensive distribution of PAC(1) mRNA within the SCN suggests that actions of PACAP are not restricted to the predominantly retinorecipient region. The presence of VPAC(2) mRNA in nearly half the VIP neurons, in almost all the AVP neurons, and at sites receiving VIP-immunoreactive SCN efferents suggests that the SCN VIP neurons are coupled and/or autoregulated and also influence the AVP-containing dorsomedial SCN and distal sites via VPAC(2).

NGF and NT-3 exert differential effects on the expression of neuropeptides in the suprachiasmatic nucleus of rats withdrawn from ethanol treatment

Some neurotrophins have the capability of enhancing neuropeptide expression in several regions of the brain. It was also recently shown that NGF, infused over 1 month, offsets the decreased synthesis and expression of vasopressin (VP) and vasoactive intestinal polypeptide (VIP) in the suprachiasmatic nucleus (SCN) of rats submitted to chronic ethanol treatment and withdrawal. In the present study we examined the effectiveness of neutrotrophin-3 (NT-3) in promoting such effects, given that SCN neurons express both the high and the low affinity receptors for this neurotrophin. NT-3 was intraventricularly infused during 10 days to rats withdrawn from prolonged ethanol treatment. The total number, and the mean somatic volume, of VP- and VIP-immunoreactive neurons was compared with the estimates obtained from control rats and withdrawn rats treated with either NGF or cerebrospinal fluid during the same period. The infusion of cerebrospinal fluid and of NT-3 did not prevent the reduction in the number of peptide-producing neurons induced by withdrawal from ethanol treatment. Conversely, NGF infusion increased their number to control levels and led to neuronal hypertrophy. Our results show that, unlike NGF, NT-3 does not display the capacity of enhancing neuropeptide expression in the SCN. Because SCN neurons express the low affinity p75(NTR), which is equally activated by both neurotrophins, our results additionally indicate that the effects of NGF upon SCN neurons are not receptor-mediated. Taken together, our data suggest that indirect mechanisms, rather than direct neutrophin signaling, are likely to mediate the trophic effects exerted by NGF upon SCN neurons.