Insulin/IGF-1 signaling (IIS) is a critical regulator of an organism’s most important biological decisions, from growth, development, and metabolism to reproduction and longevity. It primarily does so through the activity of the DAF-16/FOXO transcription factor, whose global targets were identified in C. elegans using whole-worm transcriptional analyses more than a decade ago1. IIS and FOXO also regulate important neuronal and adult behavioral phenotypes, such as the maintenance of memory2 and axon regeneration3 with age, in both mammals4 and C. elegans, but the neuron-specific IIS/FOXO targets that regulate these functions are still unknown. By isolating adult C. elegans neurons for transcriptional profiling, we identified both the wild-type and IIS/FOXO adult neuronal transcriptomes for the first time. IIS/FOXO neuron-specific targets are distinct from canonical IIS/FOXO-regulated longevity and metabolism targets, and are required for IIS/daf-2 mutants’ extended memory. We also discovered that the activity of the forkhead transcription factor FKH-9 in neurons is required for daf-2’s ability to regenerate axons with age, and its activity in non-neuronal tissues is required for daf-2’s long lifespan. Together, neuron-specific and canonical IIS/FOXO-regulated targets enable the coordinated extension of neuronal activities, metabolism, and longevity under low insulin-signaling conditions.

SUMMARY Induced CREB activity is a hallmark of long-term memory, but the full repertoire of CREB transcriptional targets required specifically for memory is not known in any system. To obtain a more complete picture of the mechanisms involved in memory, we combined memory training with genome-wide transcriptional analysis of C. elegans CREB mutants. This approach identified 757 significant CREB/memory-induced targets and confirmed the involvement of known memory genes from other organisms, but also suggested new mechanisms and novel components that may be conserved through mammals. CREB mediates distinct basal and memory transcriptional programs at least partially through spatial restriction of CREB activity: basal targets are regulated primarily in nonneuronal tissues, while memory targets are enriched for neuronal expression, emanating from CREB activity in AIM neurons. This suite of novel memory-associated genes will provide a platform for the discovery of orthologous mammalian long-term memory components.

Fibroblast growth factors (FGF) receptors FgfR1, FgfR2 and FgfR3 are differentially regulated during oligodendrocyte (OL) maturation in vitro: FgfR3 is expressed by OL progenitors whereas FgfR2 is expressed by differentiated OLs[Mol Cell Neurosci 1996;7:263–275], and we have recently shown that FgfR3 is required for the timely differentiation of OLs in vivo [J Neurosci 2003;23:883–894].Here we have used in situhybridization to investigate the expression patterns of FgfR1–3 and compare them to the putative OL progenitor markers Olig2, Pdgfrαand Plp/dm20 as a function of development in vivo, in particular at sites of OL specification, migration or differentiation in the mouse forebrain and cerebellum. We show that at early stages FgfR1–3 expression overlaps with that of Olig2 in the embryonic ventricular zone of the lateral and medial ganglionic eminences. Further, a scattered population of cells expressing FgfR3 (but not FgfR1 or FgfR2) in the ventral telencephalon appear to arise from the ventricular zone, and at later stages are found more dorsally in the cortex, in an overall pattern similar to Olig2 and/or Pdgfrα. Postnatal expression of FgfR2 increases with age, more prominently in specific regions, including the cortical and cerebellar white matter and optic nerve. Thus, the differential expression pattern of FgfR2 and FgfR3 observed in vivo suggests that their expression is developmentally regulated in a manner consistent with the pattern of their expression in culture. These data provide further insights into role of FgfRs in OL development, and they emphasize that these receptors are positioned both spatially and temporally to impact OL generation in vivo.

Olfactory sensory neurons expressing particular olfactory receptors project to specific reproducible locations within the bulb. The axonal guidance cues that organize this precise projection pattern are only beginning to be identified. To aid in their identification and characterization, we generated a transgenic zebrafish line, OR111-7:IRES:Gal4, in which a small subset of olfactory sensory neurons is labeled. Most sensory neurons expressing the OR111-7 transgene project to a specific location within the bulb, the central zone protoglomerulus, while a smaller number project to the LG1 protoglomerulus. Inhibiting netrin/DCC signaling perturbs the ability of OR111-7 expressing axons to enter the olfactory bulb and alters their patterns of termination within the bulb. The netrin receptor DCC is expressed in olfactory sensory neurons around the time that they elaborate their axons, netrin1a is expressed near the medial-most margin of the olfactory bulb, and netrin1b is expressed within the ventral region of the bulb. Loss of netrin/DCC signaling components causes some OR111-7 expressing sensory axons to wander posteriorly after exiting the olfactory pit, away from netrin expressing areas in the bulb. OR111-7 expressing axons that enter the bulb target the central zone less precisely than normal, spreading away from netrin expressing regions. These pathfinding errors can be corrected by the re-expression of DCC within OR111-7 transgene expressing neurons in DCC morphant embryos. These findings implicate netrins as the only known attractants for olfactory sensory neurons, first drawing OR111-7 expressing axons into the bulb and then into the ventromedially positioned central zone protoglomerulus.

The thalamocortical tract is the primary source of sensory information to the cerebral cortex, but the mechanisms regulating its pathfinding are not completely understood. LIM-homeodomain (LIM-HD) gene Lhx2 has been proposed to participate in a combinatorial "code" to regulate dorsal thalamic patterning and also the topography of thalamocortical projections. Here, we report that Lhx2-/- embryos exhibit a gross disruption in the early development of the thalamocortical tract, such that thalamic axons are unable to enter the ventral telencephalon. A possible cause for this deficit is a severe reduction of "pioneer" cells in the mutant ventral telencephalon that constitutes a putative mechanism for guiding the entry of the thalamocortical tract into this structure in vivo. However, in vitro, the thalamocortical tract is able to enter the ventral telencephalon, and this permitted an examination of whether thalamocortical topography is normal in the Lhx2 mutant. Contrary to hypotheses that proposed a cell-autonomous role for Lhx2 in the thalamus, Lhx2-/- thalamic explants generate a normal topography of projections in control ventral telencephalic preparations. This is consistent with our findings of normal patterning of the Lhx2 mutant dorsal thalamus using a wide array of markers. In the reverse experiment, however, control thalamic explants display aberrant topography in Lhx2-/- telencephalic preparations. This perturbation is restricted to projections from caudal thalamic explants, while rostral and middle explants project normally. Thus Lhx2 is required for multiple steps in thalamocortical tract pathfinding, but these functions appear localized in the ventral telencephalon rather than in the dorsal thalamic neurons. Furthermore, the absence of Lhx2 in the ventral telencephalon selectively disrupts a subset of thalamic axon topography, indicating a specific rather than a general perturbation of cues in this structure.