Signalling Pathways Mediating the Effects of CD40-Activated CD40L Reverse Signalling on Inhibitory Medium Spiny Neuron Neurite Growth.

CD40-activated CD40L-mediated reverse signalling is a major physiological regulator of neurite growth from excitatory and inhibitory neurons in the developing central nervous system (CNS). Whereas in excitatory pyramidal neurons, CD40L reverse signalling promotes the growth and elaboration of dendrites and axons, in inhibitory GABAergic striatal medium spiny neurons (MSNs), it restricts neurite growth and branching. In pyramidal neurons, we previously reported that CD40L reverse signalling activates an interconnected and interdependent signalling network involving protein kinase C (PKC), extracellular regulated kinases 1 and 2 (ERK1/2), and c-Jun N-terminal kinase (JNK) signalling pathways that regulates dendrite and axon growth. Here, we have studied whether these signalling pathways also influence neurite growth from striatal inhibitory MSNs. To unequivocally activate CD40L reverse signalling, we treated MSN cultures from CD40-deficient mice with CD40-Fc. Here, we report that activation of CD40L reverse signalling in these cultures also increased the phosphorylation of PKC, ERK1/2, and JNK. Using pharmacological activators and inhibitors of these signalling pathways singularly and in combination, we have shown that, as in pyramidal neurons, these signalling pathways work in an interconnected and interdependent network to regulate the neurite growth, but their functions, relationships, and interdependencies are different from those observed in pyramidal neurons. Furthermore, immunoprecipitation studies showed that stimulation of CD40L reverse signalling recruits the catalytic fragment of Syk tyrosine kinase, but in contrast to pyramidal neurons, PKC does not participate in this recruitment. Our findings show that distinctive networks of three signalling pathways mediate the opposite effects of CD40L reverse signalling on neurite growth in excitatory and inhibitory neurons.

How CD40L reverse signaling regulates axon and dendrite growth.

CD40-activated CD40L reverse signaling is a major physiological regulator of axon and dendrite growth from developing hippocampal pyramidal neurons. Here we have studied how CD40L-mediated reverse signaling promotes the growth of these processes. Cultures of hippocampal pyramidal neurons were established from Cd40-/- mouse embryos to eliminate endogenous CD40/CD40L signaling, and CD40L reverse signaling was stimulated by a CD40-Fc chimera. CD40L reverse signaling increased phosphorylation and hence activation of proteins in the PKC, ERK, and JNK signaling pathways. Pharmacological activators and inhibitors of these pathways revealed that whereas activation of JNK inhibited growth, activation of PKC and ERK1/ERK2 enhanced growth. Experiments using combinations of pharmacological reagents revealed that these signaling pathways regulate growth by functioning as an interconnected and interdependent network rather than acting in a simple linear sequence. Immunoprecipitation studies suggested that stimulation of CD40L reverse signaling generated a receptor complex comprising CD40L, PKCß, and the Syk tyrosine kinase. Our studies have begun to elucidate the molecular network and interactions that promote axon and dendrite growth from developing hippocampal neurons following activation of CD40L reverse signaling.

CD40 forward signalling is a physiological regulator of early sensory axon growth.

Multiple members of the tumour necrosis factor superfamily (TNFSF) regulate the growth and branching of neural processes late in development, when neurons are establishing and refining connections. Here, we present the first evidence that a TNFSF member acts much earlier in development, when axons are growing to their targets. CD40L transiently enhanced axon growth from embryonic mouse DRG neurons cultured at this early stage. Early spinal nerves of embryos lacking the CD40L receptor (Cd40-/- mice) were significantly shorter in vivo than those of Cd40+/+ littermates. CD40L was synthesized in early DRG targets and was co-expressed with CD40 in early DRG neurons. Whereas CD40L enhanced early axon growth independently of neurotrophins, disruption of a CD40L/CD40 autocrine loop impaired early neurotrophin-promoted axon growth. In marked contrast to the widespread regulation of axon and dendrite growth by CD40L reverse signalling later in development, CD40-Fc, which activates reverse signalling, had no effect on early sensory axon growth. These results suggest that CD40 forward signalling is a novel physiological regulator of early axon growth that acts by target-derived and autocrine mechanisms.

Regional Differences in the Contributions of TNF Reverse and Forward Signaling to the Establishment of Sympathetic Innervation.

Members of the TNF and TNF receptor superfamilies acting by both forward and reverse signaling are increasingly recognized as major physiological regulators of axon growth and tissue innervation in development. Studies of the experimentally tractable superior cervical ganglion (SCG) neurons and their targets have shown that only TNF reverse signaling, not forward signaling, is a physiological regulator of sympathetic innervation. Here, we compared SCG neurons and their targets with prevertebral ganglion (PVG) neurons and their targets. Whereas all SCG targets were markedly hypoinnervated in both TNF-deficient and TNFR1-deficient mice, PVG targets were not hypoinnervated in these mice and one PVG target, the spleen, was significantly hyperinnervated. These in vivo regional differences in innervation density were related to in vitro differences in the responses of SCG and PVG neurons to TNF reverse and forward signaling. Though TNF reverse signaling enhanced SCG axon growth, it did not affect PVG axon growth. Whereas activation of TNF forward signaling in PVG axons inhibited growth, TNF forward signaling could not be activated in SCG axons. These latter differences in the response of SCG and PVG axons to TNF forward signaling were related to TNFR1 expression, whereas PVG axons expressed TNFR1, SCG axons did not. These results show that both TNF reverse and forward signaling are physiological regulators of sympathetic innervation in different tissues.

Adipocyte-secreted BMP8b mediates adrenergic-induced remodeling of the neuro-vascular network in adipose tissue.

Activation of brown adipose tissue-mediated thermogenesis is a strategy for tackling obesity and promoting metabolic health. BMP8b is secreted by brown/beige adipocytes and enhances energy dissipation. Here we show that adipocyte-secreted BMP8b contributes to adrenergic-induced remodeling of the neuro-vascular network in adipose tissue (AT). Overexpression of bmp8b in AT enhances browning of the subcutaneous depot and maximal thermogenic capacity. Moreover, BMP8b-induced browning, increased sympathetic innervation and vascularization of AT were maintained at 28 °C, a condition of low adrenergic output. This reinforces the local trophic effect of BMP8b. Innervation and vascular remodeling effects required BMP8b signaling through the adipocytes to 1) secrete neuregulin-4 (NRG4), which promotes sympathetic axon growth and branching in vitro, and 2) induce a pro-angiogenic transcriptional and secretory profile that promotes vascular sprouting. Thus, BMP8b and NRG4 can be considered as interconnected regulators of neuro-vascular remodeling in AT and are potential therapeutic targets in obesity.

TWE-PRIL reverse signalling suppresses sympathetic axon growth and tissue innervation.

TWE-PRIL is a naturally occurring fusion protein of components of two TNF superfamily members: the extracellular domain of APRIL; and the intracellular and transmembrane domains of TWEAK with no known function. Here, we show that April-/- mice (which lack APRIL and TWE-PRIL) exhibited overgrowth of sympathetic fibres in vivo, and sympathetic neurons cultured from these mice had significantly longer axons than neurons cultured from wild-type littermates. Enhanced axon growth from sympathetic neurons cultured from April-/- mice was prevented by expressing full-length TWE-PRIL in these neurons but not by treating them with soluble APRIL. Soluble APRIL, however, enhanced axon growth from the sympathetic neurons of wild-type mice. siRNA knockdown of TWE-PRIL but not siRNA knockdown of APRIL alone also enhanced axon growth from wild-type sympathetic neurons. Our work reveals the first and physiologically relevant role for TWE-PRIL and suggests that it mediates reverse signalling.

An essential role for neuregulin-4 in the growth and elaboration of developing neocortical pyramidal dendrites.

Neuregulins, with the exception of neuregulin-4 (NRG4), have been shown to be extensively involved in many aspects of neural development and function and are implicated in several neurological disorders, including schizophrenia, depression and bipolar disorder. Here we provide the first evidence that NRG4 has a crucial function in the developing brain. We show that both the apical and basal dendrites of neocortical pyramidal neurons are markedly stunted in Nrg4-/- neonates in vivo compared with Nrg4+/+ littermates. Neocortical pyramidal neurons cultured from Nrg4-/- embryos had significantly shorter and less branched neurites than those cultured from Nrg4+/+ littermates. Recombinant NRG4 rescued the stunted phenotype of embryonic neocortical pyramidal neurons cultured from Nrg4-/- mice. The majority of cultured wild type embryonic cortical pyramidal neurons co-expressed NRG4 and its receptor ErbB4. The difference between neocortical pyramidal dendrites of Nrg4-/- and Nrg4+/+ mice was less pronounced, though still significant, in juvenile mice. However, by adult stages, the pyramidal dendrite arbors of Nrg4-/- and Nrg4+/+ mice were similar, suggesting that compensatory changes in Nrg4-/- mice occur with age. Our findings show that NRG4 is a major novel regulator of dendritic arborisation in the developing cerebral cortex and suggest that it exerts its effects by an autocrine/paracrine mechanism.

CD40 is a major regulator of dendrite growth from developing excitatory and inhibitory neurons.

Dendrite size and morphology are key determinants of the functional properties of neurons and neural circuits. Here we show that CD40, a member of the TNF receptor superfamily, is a major regulator of dendrite growth and elaboration in the developing brain. The dendrites of hippocampal excitatory neurons were markedly stunted in Cd40-/- mice, whereas those of striatal inhibitory neurons were much more exuberant. These striking and opposite phenotypic changes were also observed in excitatory and inhibitory neurons cultured from Cd40-/- mice and were rescued by soluble CD40. The changes in excitatory and inhibitory neurons cultured from Cd40-/- mice were mimicked in neurons of Cd40+/+ mice by treatment with soluble CD40L and were dependent on PKC-ß and PKC-γ, respectively. These results suggest that CD40-activated CD40L reverse signalling has striking and opposite effects on the growth and elaboration of dendrites among major classes of brain neurons by PKC-dependent mechanisms.

TNF superfamily member APRIL enhances midbrain dopaminergic axon growth and contributes to the nigrostriatal projection in vivo.

We have studied the role of the tumor necrosis factor superfamily member APRIL in the development of embryonic mouse midbrain dopaminergic neurons in vitro and in vivo. In culture, soluble APRIL enhanced axon growth during a window of development between E12 and E14 when nigrostriatal axons are growing to their targets in the striatum in vivo. April transcripts were detected in both the striatum and midbrain during this period and at later stages. The axon growth-enhancing effect of APRIL was similar to that of glial cell-derived neurotrophic factor (GDNF), but in contrast to GDNF, APRIL did not promote the survival of midbrain dopaminergic neurons. The effect of APRIL on axon growth was prevented by function-blocking antibodies to one of its receptors, BCMA (TNFRSF13A), but not by function-blocking antibodies to the other APRIL receptor, TACI (TNFRSF13B), suggesting that the effects of APRIL on axon growth are mediated by BCMA. In vivo, there was a significant reduction in the density of midbrain dopaminergic projections to the striatum in April-/- embryos compared with wild type littermates at E14. These findings demonstrate that APRIL is a physiologically relevant factor for the nigrostriatal projection. Given the importance of the degeneration of dopaminergic nigrostriatal connections in the pathogenesis and progression of Parkinson's disease, our findings contribute to our understanding of the factors that establish nigrostriatal integrity.

T-type Ca2+ channels are required for enhanced sympathetic axon growth by TNFα reverse signalling.

Tumour necrosis factor receptor 1 (TNFR1)-activated TNFα reverse signalling, in which membrane-integrated TNFα functions as a receptor for TNFR1, enhances axon growth from developing sympathetic neurons and plays a crucial role in establishing sympathetic innervation. Here, we have investigated the link between TNFα reverse signalling and axon growth in cultured sympathetic neurons. TNFR1-activated TNFα reverse signalling promotes Ca2+ influx, and highly selective T-type Ca2+ channel inhibitors, but not pharmacological inhibitors of L-type, N-type and P/Q-type Ca2+ channels, prevented enhanced axon growth. T-type Ca2+ channel-specific inhibitors eliminated Ca2+ spikes promoted by TNFα reverse signalling in axons and prevented enhanced axon growth when applied locally to axons, but not when applied to cell somata. Blocking action potential generation did not affect the effect of TNFα reverse signalling on axon growth, suggesting that propagated action potentials are not required for enhanced axon growth. TNFα reverse signalling enhanced protein kinase C (PKC) activation, and pharmacological inhibition of PKC prevented the axon growth response. These results suggest that TNFα reverse signalling promotes opening of T-type Ca2+ channels along sympathetic axons, which is required for enhanced axon growth.

Selective regulation of axonal growth from developing hippocampal neurons by tumor necrosis factor superfamily member APRIL.

APRIL (A Proliferation-Inducing Ligand, TNFSF13) is a member of the tumor necrosis factor superfamily that regulates lymphocyte survival and activation and has been implicated in tumorigenesis and autoimmune diseases. Here we report the expression and first known activity of APRIL in the nervous system. APRIL and one of its receptors, BCMA (B-Cell Maturation Antigen, TNFRSF17), are expressed by hippocampal pyramidal cells of fetal and postnatal mice. In culture, these neurons secreted APRIL, and function-blocking antibodies to either APRIL or BCMA reduced axonal elongation. Recombinant APRIL enhanced axonal elongation, but did not influence dendrite elongation. The effect of APRIL on axon elongation was inhibited by anti-BCMA and the expression of a signaling-defective BCMA mutant in these neurons, suggesting that the axon growth-promoting effect of APRIL is mediated by BCMA. APRIL promoted phosphorylation and activation of ERK1, ERK2 and Akt and serine phosphorylation and inactivation of GSK-3ß in cultured hippocampal pyramidal cells. Inhibition of MEK1/MEK2 (activators of ERK1/ERK2), PI3-kinase (activator of Akt) or Akt inhibited the axon growth-promoting action of APRIL, as did pharmacological activation of GSK-3ß and the expression of a constitutively active form of GSK-3ß. These findings suggest that APRIL promotes axon elongation by a mechanism that depends both on ERK signaling and PI3-kinase/Akt/GSK-3ß signaling.

TNFα reverse signaling promotes sympathetic axon growth and target innervation.

Reverse signaling via members of the tumor necrosis factor (TNF) superfamily controls multiple aspects of immune function. Here we document TNFα reverse signaling in the nervous system to our knowledge for the first time and show that it has a crucial role in establishing sympathetic innervation. During postnatal development, sympathetic axons express TNFα as they grow and branch in their target tissues, which in turn express TNF receptor 1 (TNFR1). In culture, soluble forms of TNFR1 act directly on postnatal sympathetic axons to promote growth and branching by a mechanism that depends on membrane-integrated TNFα and on downstream activation of ERK. Sympathetic innervation density is substantially lower in several tissues in postnatal and adult mice lacking either TNFα or TNFR1. These findings reveal that target-derived TNFR1 acts as a reverse-signaling ligand for membrane-integrated TNFα to promote growth and branching of sympathetic axons.

ProNGF promotes neurite growth from a subset of NGF-dependent neurons by a p75NTR-dependent mechanism.

The somatosensory and sympathetic innervation of the vertebrate head is derived principally from the neurons of trigeminal and superior cervical ganglia (SCG), respectively. During development, the survival of both populations of neurons and the terminal growth and branching of their axons in the tissues they innervate is regulated by the supply of nerve growth factor (NGF) produced by these tissues. NGF is derived by proteolytic cleavage of a large precursor protein, proNGF, which is recognised to possess distinctive biological functions. Here, we show that proNGF promotes profuse neurite growth and branching from cultured postnatal mouse SCG neurons. In marked contrast, proNGF does not promote the growth of trigeminal neurites. Studies using compartment cultures demonstrated that proNGF acts locally on SCG neurites to promote growth. The neurite growth-promoting effect of proNGF is not observed in SCG neurons cultured from p75(NTR)-deficient mice, and proNGF does not phosphorylate the NGF receptor tyrosine kinase TrkA. These findings suggest that proNGF selectively promotes the growth of neurites from a subset of NGF-responsive neurons by a p75(NTR)-dependent mechanism during postnatal development when the axons of these neurons are ramifying within their targets in vivo.

Protein tyrosine phosphatase receptor type O inhibits trigeminal axon growth and branching by repressing TrkB and Ret signaling.

Axonal branches of the trigeminal ganglion (TG) display characteristic growth and arborization patterns during development. Subsets of TG neurons express different receptors for growth factors, but these are unlikely to explain the unique patterns of axonal arborizations. Intrinsic modulators may restrict or enhance cellular responses to specific ligands and thereby contribute to the development of axon growth patterns. Protein tyrosine phosphatase receptor type O (PTPRO), which is required for Eph receptor-dependent retinotectal development in chick and for development of subsets of trunk sensory neurons in mouse, may be such an intrinsic modulator of TG neuron development. PTPRO is expressed mainly in TrkB-expressing (TrkB(+)) and Ret(+) mechanoreceptors within the TG during embryogenesis. In PTPRO mutant mice, subsets of TG neurons grow longer and more elaborate axonal branches. Cultured PTPRO(-/-) TG neurons display enhanced axonal outgrowth and branching in response to BDNF and GDNF compared with control neurons, indicating that PTPRO negatively controls the activity of BDNF/TrkB and GDNF/Ret signaling. Mouse PTPRO fails to regulate Eph signaling in retinocollicular development and in hindlimb motor axon guidance, suggesting that chick and mouse PTPRO have different substrate specificities. PTPRO has evolved to fine tune growth factor signaling in a cell-type-specific manner and to thereby increase the diversity of signaling output of a limited number of receptor tyrosine kinases to control the branch morphology of developing sensory neurons. The regulation of Eph receptor-mediated developmental processes by protein tyrosine phosphatases has diverged between chick and mouse.

Regulation of neurite growth by tumour necrosis superfamily member RANKL.

RANKL (receptor-activator of NF-κB ligand, TNFSF11) is a member of the TNF superfamily that regulates bone remodelling and the development of the thymus, lymph nodes and mammary glands. While RANKL and its membrane bound receptor RANK (TNFRSF11A) are expressed in the adult central nervous system and have been implicated in thermoregulation, the potential function of RANK signalling in the developing nervous system remains unexplored. Here, we show that RANK is expressed by sympathetic and sensory neurons of the developing mouse peripheral nervous system and that activating RANK signalling in these neurons during perinatal development by either treating cultured neurons with soluble RANKL or overexpressing RANK in the neurons inhibited neurotrophin-promoted neurite growth without affecting neurotrophin-promoted neuronal survival. RANKL is expressed in tissues innervated by these neurons, and studies in compartment cultures demonstrated that RANKL is capable of acting directly on neurites to inhibit growth locally. Enhancing RANK signalling in cultured neurons resulted in NF-κB activation and phosphorylation of the p65 NF-κB subunit on serine 536. Transfecting neurons with a series of mutated signalling proteins showed that NF-κB activation and p65 phosphorylation occurred by an IKKß-dependent mechanism and that blockade of this signalling pathway prevented neurite growth inhibition by RANKL. These findings reveal that RANKL is a novel negative regulator of neurite growth from developing PNS neurons and that it exerts its effects by IKKß-dependent activation of NF-κB.

The intracellular portion of GITR enhances NGF-promoted neurite growth through an inverse modulation of Erk and NF-κB signalling.

NF-κB transcription factors play a key role in regulating the growth of neural processes in the developing PNS. Although several secreted proteins have been shown to activate NF-κB to inhibit the growth of developing sympathetic neurons, it is unknown how the endogenous level of NF-κB activity present in these neurons is restricted to allow neurite growth to occur during their normal development. Here we show that activation of the glucocorticoid-induced tumour necrosis factor receptor (GITR) inhibits NF-κB activation while promoting the activation of Erk in developing sympathetic neurons. Conversely, inhibition of GITR results in an increase in NF-κB dependent gene transcription and a decrease in Erk activation leading to a reduction in neurite growth. These findings show that GITR signalling can regulate the extent of sympathetic neurite growth through an inverse modulation of Erk and NF-κB signalling, which provides an optimal environment for NGF-promoted growth.

Selective regulation of nerve growth factor expression in developing cutaneous tissue by early sensory innervation.

BACKGROUND: In the developing vertebrate peripheral nervous system, the survival of sympathetic neurons and the majority of sensory neurons depends on a supply of nerve growth factor (NGF) from tissues they innervate. Although neurotrophic theory presupposes, and the available evidence suggests, that the level of NGF expression is completely independent of innervation, the possibility that innervation may regulate the timing or level of NGF expression has not been rigorously investigated in a sufficiently well-characterized developing system. RESULTS: To address this important question, we studied the influence of innervation on the regulation of NGF mRNA expression in the embryonic mouse maxillary process in vitro and in vivo. The maxillary process receives its innervation from predominantly NGF-dependent sensory neurons of the trigeminal ganglion and is the most densely innervated cutaneous territory with the highest levels of NGF in the embryo. When early, uninnervated maxillary processes were cultured alone, the level of NGF mRNA rose more slowly than in maxillary processes cultured with attached trigeminal ganglia. In contrast to the positive influence of early innervation on NGF mRNA expression, the levels of brain-derived neurotrophic factor (BDNF) mRNA and neurotrophin-3 (NT3) mRNA rose to the same extent in early maxillary processes grown with and without trigeminal ganglia. The level of NGF mRNA, but not BDNF mRNA or NT3 mRNA, was also significantly lower in the maxillary processes of erbB3-/- mice, which have substantially fewer trigeminal neurons than wild-type mice. CONCLUSIONS: This selective effect of initial innervation on target field NGF mRNA expression provokes a re-evaluation of a key assertion of neurotrophic theory that the level of NGF expression is independent of innervation.

The death receptor antagonist FLIP-L interacts with Trk and is necessary for neurite outgrowth induced by neurotrophins.

FLICE-inhibitory protein (FLIP) is an endogenous inhibitor of the signaling pathway triggered by the activation of death receptors. Here, we reveal a novel biological function for the long form of FLIP (FLIP-L) in neuronal differentiation, which can be dissociated from its antiapoptotic role. We show that FLIP-L is expressed in different regions of the mouse embryonic nervous system. Immunohistochemistry of mouse brain sections at different stages reveals that, in neurons, FLIP is expressed early during the embryonic neuronal development (embryonic day 16) and decreases at later stages (postnatal days 5-15), when its expression is essentially detected in glial cells. FLIP-L overexpression significantly enhances neurotrophin-induced neurite outgrowth in motoneurons, superior cervical ganglion neurons, and PC12 cells. Conversely, the downregulation of FLIP-L protein levels by specific RNA interference significantly reduces neurite outgrowth, even in the presence of the appropriate neurotrophin stimulus. Moreover, NGF-dependent activation of two main intracellular pathways involved in the regulation of neurite outgrowth, extracellular signal-regulated kinases (ERKs) and nuclear factor kappaB (NF-kappaB), is impaired when endogenous FLIP-L is downregulated, although TrkA remains activated. Finally, we demonstrate that FLIP-L interacts with TrkA, and not with p75(NTR), in an NGF-dependent manner, and endogenous FLIP-L interacts with TrkB in whole-brain lysates from embryonic day 15 mice embryos. Altogether, we uncover a new role for FLIP-L as an unexpected critical player in neurotrophin-induced mitogen-activated protein kinase/ERK- and NF-kappaB-mediated control of neurite growth in developing neurons.

Macrophage stimulating protein is a neurotrophic factor for a sub-population of adult nociceptive sensory neurons.

Macrophage stimulating protein (MSP) is a pleiotropic growth factor that signals via the RON receptor tyrosine kinase. Here we demonstrate that MSP increases the proportion of cultured adult mouse DRG neurons displaying discernable neuritic processes and promotes the elongation and branching of these processes in a dose dependent manner. RON expression in adult DRG is largely restricted to nerve growth factor (NGF)-responsive nociceptive neurons, and MSP mimics the effects of NGF by increasing the expression of several mRNAs that encode functionally important proteins that are characteristically expressed by this neuronal sub-population. MSP mRNA is expressed at high levels in the peripheral target fields of DRG somatic afferents, but is undetectable in DRG, spinal cord or freshly dissected sciatic nerve. These results suggest that MSP is a peripheral target-derived neurotrophic factor for NGF-responsive adult DRG neurons.

Developmental regulation of sensory neurite growth by the tumor necrosis factor superfamily member LIGHT.

In a PCR screen to identify novel cytokine candidates involved in neuronal development, we identified transcripts for the tumor necrosis factor superfamily member 14 (TNFSF14), generally known as LIGHT (lymphotoxin-related inducible ligand that competes for glycoprotein D binding to herpesvirus entry mediator on T cells), together with its receptors, lymphotoxin-beta receptor (LTbetaR) and TNF family receptor herpesvirus entry mediator (HVEM), in the experimentally tractable sensory neurons of the mouse nodose ganglion. Immunocytochemistry revealed coexpression of LIGHT and its receptors in all nodose ganglion neurons in neonates. Enhancing LIGHT signaling in these neurons by overexpressing LIGHT inhibited BDNF-promoted neurite growth during a narrow window of development in the immediate perinatal period without affecting neuronal survival. Overexpressing a LIGHT mutant that selectively activates HVEM, but not one that selectively activates LTbetaR, also inhibited BDNF-promoted growth, suggesting that neurite growth inhibition is mediated via HVEM. Blocking HVEM signaling by a function-blocking anti-HVEM antibody significantly enhanced neurite growth from nodose neurons grown both with and without BDNF. Likewise, neurons from LIGHT-deficient neonates exhibited significantly greater neurite growth than neurons from wild-type littermates in both the presence and absence of BDNF. LIGHT overexpression significantly inhibited NF-kappaB activity, while preventing LIGHT-induced NF-kappaB inhibition by overexpressing the p65 and p50 NF-kappaB subunits prevented LIGHT-mediated growth inhibition. Together, these findings show that LIGHT/HVEM signaling negatively regulates neurite growth from developing sensory neurons via NF-kappaB inhibition.