Selective expression of hyaluronan and receptor for hyaluronan mediated motility (Rhamm) in the adult mouse subventricular zone and rostral migratory stream and in ischemic cortex.

Hyaluronan is a large glycosaminoglycan, which is abundant in the extracellular matrix of the developing rodent brain. In the adult brain however, levels of hyaluronan are significantly reduced. In this study, we used neurocan-GFP as a histochemical probe to analyze the distribution of hyaluronan in the adult mouse subventricular zone (SVZ), as well as in the rostral migratory stream (RMS). Interestingly, we observed that hyaluronan is generally downregulated in the adult brain, but notably remains at high levels in the SVZ and RMS; areas in which neural stem/progenitor cells (NSPCs) persist, proliferate and migrate throughout life. In addition, we found that the receptor for hyaluronan-mediated motility (Rhamm) was expressed in migrating neuroblasts in these areas, indicating that Rhamm could be involved in regulating hyaluronan-mediated cell migration. Hyaluronan levels are balanced by synthesis through hyaluronan synthases (Has) and degradation by hyaluronidases (Hyal). We found that Has1 and Has2, as well as Hyal1 and Hyal2 were expressed in GFAP positive cells in the adult rodent SVZ and RMS, indicating that astrocytes could be regulating hyaluronan-mediated functions in these areas. We also demonstrate that hyaluronan levels are substantially increased at six weeks following a photothrombotic stroke lesion to the adult mouse cortex. Furthermore, GFAP positive cells in the peri-infarct area express Rhamm. Thus, hyaluronan may be involved in regulating cell migration in the normal SVZ and RMS and could also be responsible for priming the peri-infarct area following an ischemic lesion for cell migration.

NFIA controls telencephalic progenitor cell differentiation through repression of the Notch effector Hes1.

The balance between self-renewal and differentiation of neural progenitor cells is an absolute requirement for the correct formation of the nervous system. Much is known about both the pathways involved in progenitor cell self-renewal, such as Notch signaling, and the expression of genes that initiate progenitor differentiation. However, whether these fundamental processes are mechanistically linked, and specifically how repression of progenitor self-renewal pathways occurs, is poorly understood. Nuclear factor I A (Nfia), a gene known to regulate spinal cord and neocortical development, has recently been implicated as acting downstream of Notch to initiate the expression of astrocyte-specific genes within the cortex. Here we demonstrate that, in addition to activating the expression of astrocyte-specific genes, Nfia also downregulates the activity of the Notch signaling pathway via repression of the key Notch effector Hes1. These data provide a significant conceptual advance in our understanding of neural progenitor differentiation, revealing that a single transcription factor can control both the activation of differentiation genes and the repression of the self-renewal genes, thereby acting as a pivotal regulator of the balance between progenitor and differentiated cell states.

The role of p-c-Jun in survival and outgrowth of developing sensory neurons.

c-Jun activation has been implicated not only in neuronal apoptosis, but also in survival and regeneration. This Janus facet of c-Jun activation could be related to neuronal cell type or to the developmental stage of the neuron. We investigated c-Jun activation in E18 sensory neurons. Cultures of rat dorsal root ganglia neurons were maintained with or without the addition of nerve growth factor or the c-Jun N-terminal kinase inhibitor, (D)-JNKI1. Few dorsal root ganglia neurons survived nerve growth factor deprivation, whereas neurons supplied with nerve growth factor survived and exhibited extensive axonal outgrowth. Activated c-Jun was present in the nuclei of neurons with regenerating axons, but not in apoptotic neurons. c-Jun N-terminal kinase inhibition reduced the number of p-c-Jun immunoreactive and regenerating neurons, and increased cell death. Thus, activation of c-Jun seems to be required for survival and regeneration of developing sensory neurons.

The Janus role of c-Jun: cell death versus survival and regeneration of neonatal sympathetic and sensory neurons.

We investigated the functional outcome of c-Jun activation in sympathetic and sensory neurons of neonatal rat superior cervical ganglion (SCG) and dorsal root ganglion (DRG), respectively. Distinctly different roles of c-Jun activation have been suggested for these two types of neurons. In dissociated sympathetic neurons, c-Jun has been demonstrated to promote apoptosis, whereas in sensory neurons it stimulates axonal outgrowth. In organ-cultured ganglia, we found that c-Jun was activated within 24 h of explantation in both types of neurons, and that the JNK inhibitor SP600125 could mitigate this response. In both types of neurons, c-Jun activation was also reduced by NGF treatment. Inhibition of c-Jun activation did not affect the viability of sympathetic neurons, whereas the number of apoptotic sensory neurons increased. Furthermore, inhibition of c-Jun reduced axonal outgrowth from both SCG and DRG. Thus, in organ culture, c-Jun activation may be required for axonal outgrowth and, at least in sensory neurons, it promotes survival. The role of ATF3, a neuronal marker of injury and a c-Jun dimerization partner, was also examined. We found an ATF3 induction in both SCG and DRG neurons, a response, which was reduced by JNK inhibition. The reduction of ATF3 upon JNK inhibition was much larger in DRG than in SCG, a result which might account for the higher number of apoptotic neurons in JNK inhibitor exposed DRG. Taken together, and contrary to our expectations, neonatal sympathetic and sensory neurons seem to respond to axonal injury similarly with respect to c-Jun activation, and in no case was this activation pro-apoptotic.

Retrograde axonal transport of JNK signaling molecules influence injury induced nuclear changes in p-c-Jun and ATF3 in adult rat sensory neurons.

In the present study, we investigated if the previously observed JNK-mediated activation of c-Jun and induction of ATF3 could be ascribed to axonal transport of JNK signaling components, or if axonal transport of the transcription factors themselves contributes to the nuclear changes in injured sensory neurons. We observed retrograde axonal transport of a number of JNK upstream kinases in ligated rat sciatic nerve. In these preparations, axonal transport of JNK/p-JNK, the JNK scaffolding protein JIP, and the transcription factors ATF3 and ATF2/p-ATF2 was also found. No or little retrograde transport of c-Jun and p-c-Jun was found, whereas an anterograde transport of Hsp27, a protein previously reported in the context of p-c-Jun and ATF3, was observed. In separate experiments, we found that in vitro inhibition of axonal transport or axonal inhibition of JNK reduced the number of p-c-Jun- and ATF3-positive neuronal nuclei. The results suggest that retrograde axonal transport of JNK signaling components contributes to the injury induced c-Jun phosphorylation and ATF3 induction.