Summary Disease tolerance is the ability of the host to reduce the impact of infection on host fitness. Analysis of disease tolerance pathways could provide new approaches for treating infections and other inflammatory diseases. Typically, an initial exposure to bacterial lipopolysaccharide (LPS) induces a state of refractoriness to further LPS challenge (“endotoxin tolerance”). We found that a first exposure to LPS activated the ligand-operated transcription factor aryl hydrocarbon receptor (AhR) and the hepatic enzyme tryptophan 2,3-dioxygenase 2, which provided an activating ligand to the former, to downregulate early inflammatory gene expression. However, on LPS rechallenge, AhR engaged in long-term regulation of systemic inflammation only in the presence of indoleamine 2,3-dioxygenase 1 (IDO1). AhR complex-associated Src kinase activity promoted IDO1 phosphorylation and signaling ability. The resulting endotoxin-tolerant state was found to protect mice against immunopathology in gram-negative and gram-positive infections, pointing to a role for AhR in contributing to host fitness.

Abstract. The neurotrophin family includes NGF, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4). Previous studies have demonstrated that expression of NGF and its low-affinity receptor is induced in nonneuronal cells of the distal segment of the transected sciatic nerve suggesting a role for NGF during axonal regeneration (Johnson, E. M., M. Taniuchi, and P. S. DeStefano. 1988. Trends Neurosci. 11:299-304). To assess the role of the other neurotrophins and the members of the family of Trk signaling neurotrophin receptors, we have here quantified the levels of mRNAs for BDNF, NT-3, and NT-4 as well as mRNAs for trkA, trkB, and trkC at different times after transection of the sciatic nerve in adult rats. A marked increase of BDNF and NT-4 mRNAs in the distal segment of the sciatic nerve was seen 2 wk after the lesion. The increase in BDNF mRNA was mediated by a selective activation of the BDNF exon IV promoter and adrenalectomy attenuated this increase by 50%. NT-3 mRNA, on the other hand, decreased shortly after the transection but returned to control levels 2 wk later. In Schwann Cells ensheathing the sciatic nerve, only trkB mRNA encoding truncated TrkB receptors was detected with reduced levels in the distal part of the lesioned nerve. Similar results were seen using a probe that detects all forms of trkC mRNA. In the denervated gastrocnemius muscle, the level of BDNF mRNA increased, NT-3 mRNA did not change, while NT-4 mRNA decreased. In the spinal cord, only small changes were seen in the levels of neurotrophin and trk mRNAs. These results show that expression of mRNAs for neurotrophins and their Trk receptors is differentially regulated after a peripheral nerve injury. Based on these results a model is presented for how the different neurotrophins could cooperate to promote regeneration of injured peripheral nerves.

Glial cell line-derived neurotrophic factor (GDNF), the most potent trophic factor yet described for both dopaminergic neurons of the substantia nigra and spinal motorneurons, has recently been shown to signal through a multireceptor complex composed of a novel glycosylphosphatidylinositol-anchored GDNF receptor-␣ (GDNFR-␣) and the receptor tyrosine kinase product of the c-ret proto-oncogene (RET). Despite its importance, the individual expression patterns and the relationships between domains of expression of the different components of this trophic system are not understood. We show here by in situ hybridization that GDNF mRNA is expressed in the normal adult rat brain in several targets of substantia nigra neurons, including striatum, nucleus accumbens, thalamic nuclei, olfactory tubercle, hippocampus, cerebellum, and cingulate cortex as well as in the internal granular cell layer of the olfactory bulb. Within the basal ganglia we observe a pronounced segregation of regions expressing GDNF from those expressing GDNF receptors, suggesting that within these structures GDNF is functioning in its anticipated role as a target-derived trophic factor. In addition, the expression of GDNF and both GDNF receptors within the cerebellum, hippocampus, and olfactory bulb may indicate a paracrine mode of action. Importantly, we also see expression of RET mRNA in cellular populations within the cerebellum and the glomerular layer of the olfactory bulb, as well as in the subthalamic nucleus, which lack GDNFR-␣ expression, indicating that RET functions either independently of GDNFR-␣ or with GDNFR-␣ presented in trans. Conversely, GDNFR-␣ is widely expressed in many regions in which RET expression is absent, suggesting that GDNFR-␣ may associate with additional signaling receptors. Finally, RET and GDNFR-␣ show distinct patterns of regulated expression in the brain after kainic acid stimulation and in the sciatic nerve after nerve transection. Taken together these findings indicate that GDNF, RET, and GDNFR-␣ utilize multiple mechanisms to comprise physiologically relevant trophic circuits for different neuronal populations.

Abstract. Glial cell line-derived neurotrophic factor (GDNF) is a neurotrophic polypeptide, distantly related to transforming growth factor- , originally isolated by virtue of its ability to induce dopamine uptake and cell survival in cultures of embryonic ventral midbrain dopaminergic neurons, and more recently shown to be a potent neurotrophic factor for motorneurons. The biological activities and distribution of this molecule outside the central nervous system are presently unknown. We report here on the mRNA expression, biological activities and initial receptor binding characterization of GDNF and a shorter spliced variant termed GDNF[3 in different organs and peripheral neurons of the developing rat. Both GDNF mRNA forms were found to be most highly expressed in developing skin, whisker pad, kidney, stomach and testis. Lower expression was also detected in developing skeletal muscle, ovary, lung, and adrenal gland. Developing spinal cord, superior cervical ganglion (SCG) and dorsal root ganglion (DRG) also expressed low levels of GDNF mRNA. Two days after nerve transection, GDNF mRNA levels increased dramatically in the sciatic nerve. Overall, GDNF mRNA expression was significantly higher in peripheral organs than in neuronal tissues. Expression of either GDNF mRNA isoform in insect cells resulted in the production of indistinguishable mature GDNF polypeptides. Purified recombinant GDNF promoted neurite outgrowth and survival of embryonic chick sympathetic neurons. GDNF produced robust bundle-like, fasciculated outgrowth from chick sympathetic ganglion explants. Although GDNF displayed only low activity on survival of newborn rat SCG neurons, this protein was found to increase the expression of vasoactive intestinal peptide and preprotachykinin-A mRNAs in cultured SCG neurons. GDNF also promoted survival of about half of the neurons in embryonic chick nodose ganglion and a small subpopulation of embryonic sensory neurons in chick dorsal root and rat trigeminal ganglia. Embryonic chick sympathetic neurons expressed receptors for GDNF with Kd 1-5 × 10 -9 M, as measured by saturation and displacement binding assays. Our findings indicate GDNF is a new neurotrophic factor for developing peripheral neurons and suggest possible non-neuronal roles for GDNF in the developing reproductive system. T HE development and maintenance of the vertebrate nervous system requires the activity of a range of polypeptides known as neurotrophic factors. These molecules have been shown to control the generation, survival, differentiation and regeneration of neurons in the peripheral and central nervous systems (Barde, 1989;Thoenen, 1991). They include members of the neurotrophin family, including NGF and related molecules (Thoenen, 1991: Persson andIb~ifiez, 1993), members of the neurokine family, such as ciliary neurotrophic factor (CNTF), 1

The production of neurotrophin-4 (NT-4) in rat skeletal muscle was found to depend on muscle activity. The amounts of NT-4 messenger RNA present decreased after blockade of neuromuscular transmission with alpha-bungarotoxin and increased during postnatal development and after electrical stimulation in a dose-dependent manner. NT-4 immunoreactivity was detected in slow, type I muscle fibers. Intramuscular administration of NT-4 induced sprouting of intact adult motor nerves. Thus, muscle-derived NT-4 acted as an activity-dependent neurotrophic signal for growth and remodeling of adult motor neuron innervation. NT-4 may thus be partly responsible for the effects of exercise and electrical stimulation on neuromuscular performance.