Axotomy-induced c-JUN expression in young medial septal neurons is regulated by nerve growth factor

Transcription factor LIM homeobox 7 (Lhx7) maintains subtype identity of cholinergic interneurons in the mammalian striatum

The generation and maintenance of a plethora of neuronal subtypes is essential for normal brain function. Nevertheless, little is known about the molecular mechanisms that maintain the defining characteristics of neurons following their initial postmitotic specification. Using conditional gene ablation in mice, we demonstrate here that the homeodomain protein LIM homeobox (Lhx)7 is essential for maintaining the morphological and molecular characteristics of cholinergic interneurons of the striatum. Lhx7-depleted cholinergic interneurons extinguish expression of several subtype-specific markers, including choline acetyl transferase and Isl1, and are respecified into Lhx6-expressing mature GABAergic interneurons. Additional expression studies support a model where Lhx7 controls the choice between cholinergic or GABAergic identity by gating a cross inhibitory regulation between Isl1 and Lhx6. By demonstrating that the switch between alternative striatal interneuron fates depends on persistent activity of a single transcription factor, we provide evidence that the intrinsic plasticity of mammalian forebrain neuronal subtypes is maintained after the initial specification and lineage commitment and possibly throughout life.

Regulation of intrinsic neuronal properties for axon growth and regeneration

Regulation of neuritic growth is crucial for neural development, adaptation and repair. The intrinsic growth potential of nerve cells is determined by the activity of specific molecular sets, which sense environmental signals and sustain structural extension of neurites. The expression and function of these molecules are dynamically regulated by multiple mechanisms, which adjust the actual growth properties of each neuron population at different ontogenetic stages or in specific conditions. The neuronal potential for axon elongation and regeneration are restricted at the end of development by the concurrent action of several factors associated with the final maturation of neurons and of the surrounding tissue. In the adult, neuronal growth properties can be significantly modulated by injury, but they are also continuously tuned in everyday life to sustain physiological plasticity. Strict regulation of structural remodelling and neuritic elongation is thought to be required to maintain specific patterns of connectivity in the highly complex mammalian CNS. Accordingly, procedures that neutralize such mechanisms effectively boost axon growth in both intact and injured nervous system. Even in these conditions, however, aberrant connections are only formed in the presence of unusual external stimuli or experience. Therefore, growth regulatory mechanisms play an essentially permissive role by setting the responsiveness of neural circuits to environmental stimuli. The latter exert an instructive action and determine the actual shape of newly formed connections. In the light of this notion, efficient therapeutic interventions in the injured CNS should combine targeted manipulations of growth control mechanisms with task-specific training and rehabilitation paradigms.

Reparative mechanisms in the cerebellar cortex

In the adult brain, different neuronal populations display different degrees of plasticity. Here, we describe the highly different plastic properties of inferior olivary neurones and Purkinje cells. Olivary neurones show a basal expression of growth-associated proteins, such as GAP-43 and Krox24/EGR-1, and remarkable remodelling capabilities of their terminal arbour. They also regenerate their transected neurites into growth-permissive territories and may reinnervate the lost target. Sprouting and regrowing olivary axons are able to follow specific positional information cues to establish new connections according to the original projection map. In addition, they set a strong cell body reaction to injury, which in specific olivary subsets is regulated by inhibitory target-derived cues. In contrast, Purkinje cells do not have a constitutive level of growth-associated genes, and show little cell body reaction, no axonal regeneration after axotomy, and weak sprouting capabilities. Block of myelin-derived signals allows terminal arbour remodelling, but not regeneration, while selective over-expression of GAP-43 induces axonal sprouting along the axonal surface and at the level of the lesion. We suggest that the high constitutive intrinsic plasticity of the inferior olive neurones allows their terminal arbour to sustain the activity-dependent ongoing competition with the parallel fibres in order to maintain the post-synaptic territory, and possibly underlies mechanisms of learning and memory. Such a plasticity is used also as a reparative mechanism following axotomy. In contrast, in Purkinje cells, poor intrinsic regenerative capabilities and myelin-derived signals stabilise the mature connectivity and prevent axonal regeneration after lesion.

MIXED-LINEAGEKINASES: A Target for the Prevention of Neurodegeneration

The activation of the c-Jun N-terminal kinase (JNK) pathway is critical for naturally occurring neuronal cell death during development and may be important for the pathological neuronal cell death of neurodegenerative diseases. The small molecule inhibitor of the mixed-lineage kinase (MLK) family of kinases, CEP-1347, inhibits the activation of the JNK pathway and, consequently, the cell death in many cell culture and animal models of neuronal death. CEP-1347 has the ability not only to inhibit cell death but also to maintain the trophic status of neurons in culture. The possible importance of the JNK pathway in neurodegenerative diseases such as Alzheimer's and Parkinson's diseases provides a rationale for the use of CEP-1347 for the treatment of these diseases. CEP-1347 has the potential of not only retarding disease progression but also reversing the severity of symptoms by improving the function of surviving neurons.

Extrinsic regulation of injury/growth-related gene expression in the inferior olive of the adult rat

Successful axon regeneration relies on the capability of the lesioned neurons to up-regulate a specific set of injury/growth-associated genes. In the adult central nervous system, the strength of the cell body response is generally related to the distance of the injury site from the perikaryon, being stronger for proximal lesions. Nevertheless, inferior olive (IO) cells react to injury and regenerate their axons even after distal transections. To investigate the mechanisms that regulate the IO growth properties, we examined the expression of injury/growth markers (nitric oxide synthase, growth-associated protein 43 and c-Jun) after target deletion or axotomy performed at different sites along the olivocerebellar pathway. Both axon injury and target loss disclose two subsets of IO neurons distributed within precise subnuclei: one subset up-regulates all markers in all conditions, whereas the other shows a mild c-Jun expression but remains unresponsive even after a very proximal axotomy. These observations indicate that distinct subpopulations of IO cells respond to different regulatory strategies. Unresponsive neurons appear insensitive to environmental positive or negative cues, suggesting that they are intrinsically unable to set up a cellular reaction to injury. In contrast, cell body changes in reactive neurons are elicited after the removal of retrogradely transported target-derived inhibitory signals. Target loss also induces degeneration of IO cells, whose survival remains partially dependent on Purkinje targets in adulthood. Thus, the intrinsic regenerative potential of a functionally homogeneous population is regulated by multiple mechanisms, specific for distinct neuronal subsets.

Long-term injured purkinje cells are competent for terminal arbor growth, but remain unable to sustain stem axon regeneration

Long-distance axon regeneration requires the activation of a specific set of neuronal growth-associated genes. Adult Purkinje cells fail to upregulate these molecules in response to axotomy and show extremely weak regenerative properties. Nevertheless, starting from several months after injury, transected Purkinje axons undergo spontaneous sprouting. Here, we asked whether long-term injured Purkinje cells acquire novel intrinsic growth properties that enable them to upregulate growth-associated genes and sustain axon regeneration. To test this hypothesis, we examined axon growth and cell body changes in adult rat Purkinje neurons following axotomy and implantation of embryonic neocortical tissue or Schwann cells into the injury track. Purkinje cells that survived over 6 months after injury/transplantation displayed profuse sprouting in the injured cerebellum and developed extensive networks of terminal branches into embryonic neocortical grafts. In addition, severed Purkinje axons exposed to these transplants 6 months after injury grew faster than their counterparts confronted with the same environment immediately after axotomy. Nevertheless, long-term injured Purkinje cells failed to regenerate stem neurites into Schwann cell grafts, and, under all experimental conditions, they did not upregulate growth-associated molecules, including c-Jun, GAP-43, SNAP-25, and NADPH-diaphorase. These results indicate that the long-term injured Purkinje cells remain unable to activate the gene program required to sustain axon regeneration and their plasticity is restricted to terminal arbor remodeling. We propose that the delayed growth of injured Purkinje cells reflects an adaptive phenomenon by which the severed axon stump develops a new terminal arbor searching for alternative connections with local partners.

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.

Up-regulation of growth-associated protein 43 mRNA in rat medial septum neurons axotomized by fimbria-fornix transection

Transection of septohippocampal fibres is widely used to study the response of CNS neurons to axotomy. Septohippocampal projection neurons survive axotomy and selectively up-regulate the transcription factor c-Jun. In the present study we investigated whether these cells concomitantly up-regulate the growth-associated protein-43 (GAP-43), a potential target gene of c-Jun implicated in axonal growth and regeneration. Using in situ hybridization histochemistry (ISHH) it was demonstrated that postlesional c-jun mRNA expression is accompanied by an increased expression of GAP-43 mRNA in the medial septum 3 days following fimbria-fornix transection (FFT). The increase reached a maximum at 7 days and gradually declined thereafter (17 days, 3 weeks). Retrograde prelabeling with Fluoro-Gold followed by axotomy and ISHH revealed that GAP-43 mRNA was up-regulated in septohippocampal projection neurons. Colocalization of GAP-43 mRNA and choline acetyltransferase protein showed that GAP-43 mRNA was expressed by cholinergic medial septal neurons after axotomy. Selective immunolesioning of the cholinergic component of the septohippocampal projection with 192 IgG-saporin followed by FFT demonstrated that GAP-43 mRNA was also synthesized by axotomized GABAergic neurons. These results demonstrate an up-regulation of GAP-43 mRNA in axotomized septohippocampal projection neurons independent of their transmitter phenotype which is closely correlated with c-Jun expression. Because the GAP-43 gene contains an AP-1 site, we hypothesize a c-Jun-driven up-regulation of GAP-43 in lesioned medial septal neurons that may contribute to their survival and regenerative potential following axotomy.

c-Jun promotes neurite outgrowth and survival in PC12 cells

We investigated the function of c-Jun in PC12 cells by transfecting them with a plasmid containing a c-Jun cDNA transcription cassette. Transfected cells expressed high levels of c-Jun mRNA and protein and demonstrated an increase in both AP-1 DNA binding and gene activation. The c-Jun over-expressing cells showed marked neurite outgrowth but no evidence of spontaneous cell death. In fact, c-Jun over-expressing cells were more resistant to okadaic acid-induced apoptosis. The process outgrowth was not indicative of a full neuronal differentiation response as the transfected PC12 cells did not display action potentials when examined with whole-cell patch-clamping. The phosphorylation of c-Jun on serine 73 appears to be important for this neurite sprouting effect as mutagenesis at this site reduced sprouting whereas a serine 63 mutant tended to increase sprouting. Thus, in PC12 cells c-Jun expression does not induce apoptosis, but rather functions as a neurite outgrowth and neuronal survival signal.

Role of NGF in axotomy-induced c-Jun expression in medial septal cholinergic neurons

The extent of neuronal regeneration after axotomy largely depends on the survival capacity of the injured cell. It has been shown for a long time that nerve fiber transection results in retrograde changes in the parent neuronal cell body, and that these changes may eventually lead to neuronal degeneration. At present, little is known about the sequence of events initiated in a nerve cell body by the transection of its axonal process. In this report, we will focus on an interaction of nerve growth factor (NGF) with the transcription factor c-Jun in intact and axotomized septohippocampal projection neurons.