Attenuation of opioid analgesic tolerance in p75 neurotrophin receptor null mutant mice

Repeated exposure to opioid drugs can lead to the development of tolerance, which manifests as a reduction in analgesic potency, and physical dependence, a response indicated by a withdrawal syndrome. Accumulating evidence suggests that the nerve growth factor (NGF) family of neurotrophins may have an important modulatory role in the induction of opioid analgesia and opioid addiction. Because neurotrophins universally bind the p75 neurotrophin receptor (p75NTR), we investigated whether the activity of this receptor is involved in the development of opioid analgesic tolerance and physical dependence. We found that in both the wild-type and p75NTR−/− mice an acute systemic (i.p.) injection of morphine produced a maximal analgesic response as measured by the thermal tail-immersion test. Repeated injection of morphine over 5 days in wild-type mice resulted in a progressive decline of the analgesic effect and a concomitant loss of the agonist potency, reflecting development of morphine tolerance. However, the loss of morphine analgesia was not observed in p75NTR−/− mice. In the second part of this study, mice were given escalating doses of systemic (i.p.) morphine over 5 days and subsequently challenged with the opioid receptor antagonist naloxone. This challenge precipitated a robust withdrawal syndrome that was comparable in wild-type mice and p75NTR−/− mice. The findings suggest that p75NTR activity plays a critical role in the development of opioid analgesic tolerance but not in the induction or the expression of opioid physical dependence.     

Fading memories in aging and neurodegeneration: Is p75 neurotrophin receptor a culprit?

Aging and age-related neurodegenerative diseases have become one of the major concerns in modern times as cognitive abilities tend to decline when we get older. It is well known that the main cause of this age-related cognitive deficit is due to aberrant changes in cellular, molecular circuitry and signaling pathways underlying synaptic plasticity and neuronal connections. The p75 neurotrophin receptor (p75^NTR) is one of the important mediators regulating the fate of the neurons in the nervous system. Its importance in neuronal apoptosis is well documented. However, the mechanisms involving the regulation of p75^NTR in synaptic plasticity and cognitive function remain obscure, although cognitive impairment has been associated with a higher expression of p75^NTR in neurons. In this review, we discuss the current understanding of how neurons are influenced by p75^NTR function to maintain normal neuronal synaptic strength and connectivity, particularly to support learning and memory in the hippocampus. We then discuss the age-associated alterations in neurophysiological mechanisms of synaptic plasticity and cognitive function. Furthermore, we also describe current evidence that has begun to elucidate how p75^NTR regulates synaptic changes in aging and age-related neurodegenerative diseases, focusing on the hippocampus. Elucidating the role that p75^NTR signaling plays in regulating synaptic plasticity will contribute to a better understanding of cognitive processes and pathological conditions. This will in turn provide novel approaches to improve therapies for the treatment of neurological diseases in which p75^NTR dysfunction has been demonstrated.     

P75NTR的研究进展

P75NTR作为神经营养因子的受体之一,在神经系统的发育过程中发挥着至关重要的生物学作用。随着s-p75NTR, proNGF, proBDNF和sortilin等的发现,P75NTR在凋亡信号转导通路中的重要作用成为研究的热点。     

The p75 neurotrophin receptor is widely expressed in conventional papillary thyroid carcinoma

Papillary thyroid carcinomas (PTCs) are associated with alterations in several proto-oncogenes related with nervous system development and function, such as TrkA and RET, which are commonly rearranged in these carcinomas. The other oncogenic event recently identified in PTC is the BRAF V600E mutation. Because the role of TrkA was not completely elucidated in thyroid cancer ethiopathogenesis, we decided to study the expression of active, phosphorylated TrkA and of its coreceptor p75 neurotrophin receptor (p75 NTR) in a series of 92 PTC (37 lesions of conventional PTC, 28 of follicular variant of PTC [FVPTC], and 27 of other variants of PTC) as well as in 21 samples of normal thyroid and nonneoplastic thyroid lesions used as a controls. We observed neoexpression of p75 NTR in PTC, particularly in conventional PTC and in other variants of PTC displaying a papillary growth pattern, rather than in FVPTC. No immunoexpression of p75 NTR was observed in normal thyroid nor in nonneoplastic thyroid lesions. The cellular localization of p75 NTR immunoexpression was also significantly associated with the growth pattern of PTC, being much more frequently detected in an apical localization in PTC with papillary architecture than in PTC with a follicular or solid growth pattern. This apical localization of p75 NTR was significantly associated with the presence of BRAF V600E. No significant differences were detected between normal thyroid, nonneoplastic lesions, and PTC (or any PTC variant) regarding expression/activation of TrkA, thus suggesting that by itself and in contrast to p75 NTR, TrkA is not altered during PTC development.     

The p75 neurotrophin receptor is essential for neuronal cell survival and improvement of functional recovery after spinal cord injury

The mechanisms initiating post-spinal cord injury (SCI) apoptotic cell death remain incompletely understood. The p75 neurotrophin receptor (p75(NTR)) has been shown to exert both pro-survival and pro-apoptotic effects on neural cells in vitro. While a previous study had shown that there is decreased oligodendrocyte apoptosis distal to a clean partial transection injury of the cord in mice with nonfunctional p75(NTR), most human spinal cord injuries do not involve partial transections but are rather due to compression/contusion injuries with significant perilesional ischemia. Therefore, we sought to examine the role of the p75(NTR) in a clinically relevant clip compression model of SCI in p75 null mice with an exon III mutation. Mice with a functional p75(NTR) had increased caspase-9 activation at 3 days after SCI in comparison to the functionally deficient p75(NTR) mice. However, at 7 days following SCI there was no difference in the activation of the effector caspases (caspase-3 and caspase-6) at the spinal cord lesion. Moreover, at 7 days after injury, there was increased terminal deoxynucleotidyl transferase-mediated dUTP nick-end (TUNEL) positive cell death at the injury site in the functionally deficient p75(NTR) mice. Using double labeling with TUNEL and cell specific markers we showed that the absence of p75(NTR) function increased the extent of neuronal but not oligodendroglial cell death at the injury site. This selective loss of neuronal cells after SCI was confirmed with a decrease in levels of microtubule-associated protein 2 in the p75 null mice. Furthermore, the wild-type animals had dramatically improved survival and enhanced locomotor recovery at 8 weeks after SCI when compared with the p75(NTR) null mice. Also at 8 weeks, there were significantly more neurons present at the injury site of wild-type mice when compared with p75 null mice. We conclude that the p75(NTR) receptor is integral to neuronal cell survival and endogenous reparative mechanisms after compressive/contusive SCI.     

Immunohistochemical analysis of TrkA neurotrophin receptor expression in human non-neuronal carcinomas

The Trk family of tyrosine protein kinase receptors plays a significant role in the development and maintenance of neural tissues. It has been recently shown that Trk receptors are also expressed by a wide range of normal non-neuronal tissues in humans in a cell type-specific manner. In the present study, the expression patterns of TrkA in 337 non-neuronal invasive carcinomas of 15 different human tissues were investigated immunohistochemically. Overall, 133 (39%), 101 (30%) and 103 (31%) tumors exhibited strong, moderate and no TrkA Immunoreactivity, respectively. Esophageal and thyroid carcinomas expressed high levels of TrkA, whereas the levels in gastric and colon cancers were low. TrkA expression was detected not only in carcinomas originating from TrkA-positive normal counterpart tissues, Including the esophagus, breast, lung and uterus, but also in those from TrkA-negative tissues/cells of the thyroid, liver and ovary. Immunostaining for nerve growth factor-β, the specific ligand for TrkA, in esophageal and breast carcinomas demonstrated its immunoreactivity in stromal fibroblasts and some TrkA-expressing tumor cells. These results suggest that paracrine/autocrine regulation via stromal/tumoral NGF-tumoral TrkA interaction may be involved In the growth of certain non-neuronal carcinomas.     

Status epilepticus induces a TrkB to p75 neurotrophin receptor switch and increases brain-derived neurotrophic factor interaction with p75 neurotrophin receptor: an initial event in neuronal injury induction

In neuronal cultures it has been demonstrated that neurotrophins can elicit neuronal death through the p75 neurotrophic receptor (p75ntr) in the absence of concomitant Trk signaling. However, it was suggested that p75ntr induces neuronal death after status epilepticus (SE) in neuronal populations that express relatively high quantities of tropomyosin receptor kinase B (TrkB). Here, using Western blot and immunohistochemistry analyses in the hippocampus, we found that 3-h SE caused a remarkable decrease in TrkB expression and phosphorylation, and a significant increase in p75ntr. TrkB modification occurs before the overexpression of the tumor suppressor protein p53, accompanies the cell damage taking place in the dentate gyrus, and precedes the CA1 neuronal injury as assessed by Fluoro-Jade B and Nissl staining. Co-immunoprecipitation of brain-derived neurotrophic factor (BDNF) or its immature form proBDNF showed increased interaction with p75ntr after its binding to TrkB was reduced. Interestingly, proBDNF also increases its binding with p75ntr after seizures that do not cause neuronal death (animals injected with pilocarpine that fail to enter SE). However, in those animals, TrkB protein levels remained unchanged and its phosphorylation increased. Our results indicate an intrinsic capacity of neurons in vivo to modify final neurotrophin output by changing the proportion of their receptors' expression and the receptors' interaction with their ligands. These early events support the idea that neurotrophins may be involved in the induction of neuronal death in vivo under pathological conditions.     

Abeta(1-42) stimulates adult SVZ neurogenesis through the p75 neurotrophin receptor

The generation of amyloid-beta peptide (Aβ) and its accumulation in amyloid plaques are generally recognized as key characteristics of Alzheimer's disease. A number of reports have indicated that Aβ can regulate the proliferation of neural precursor cells and adult neurogenesis, suggesting that this may underpin the cognitive decline and compromised olfaction also associated with the condition. Here we report that Aβ_1–42 treatment both in vitro and in vivo, as well as endogenous generation of Aβ in C100 and APP/PS1 transgenic models of Alzheimer's disease, stimulate neurogenesis of young adult subventricular zone precursors. The neurogenic effect of Aβ_1–42 was found to require expression of the p75 neurotrophin receptor (p75^NTR) by the precursor cells, and activation of p75^NTR by metalloprotease cleavage. However, precursors from 12-month-old APP/PS1 mice failed to respond to Aβ_1–42. Our results suggest that overstimulation of p75^NTR-positive progenitors during early life might result in depletion of the stem cell pool and thus a more rapid decline in basal neurogenesis. This, in turn, could lead to impaired neurogenic function in later life.     

Neurotrophin signaling through the p75 neurotrophin receptor

The neurotrophins are growth factors that play critical roles in the development, maintenance, survival, and death of the nervous system. The signal transducing systems that mediate the diverse biological functions of the neurotrophins are initiated by their interactions with two categories of cell surface receptors, the Trk family of tyrosine kinases and the p75 neurotrophin receptor (p75NTR). While the Trk receptors are responsible for most of the survival and growth properties of the neurotrophins, the actions of p75NTR fall into two categories. First, p75NTR is a Trk co-receptor that can enhance or suppress neurotrophin-mediated Trk receptor activity. Second, p75NTR autonomously activates signaling cascades that result in the induction of apoptosis or in the promotion of survival. The signaling cascades activated by p75NTR remain elusive, but structural and functional differences between p75NTR and other tumor necrosis factor receptor (TNFR) superfamily members suggest that p75NTR employs distinct signaling pathways. p75NTR has been shown to activate the NF-kappaB, Akt, and JNK pathways and interacts with several adaptor proteins. Of these, NRAGE, NADE, and NRIF have been associated with the induction of apoptosis, and FAP-1, RIP2, and TRAF6 appear to promote cellular survival. It remains a major challenge to link the various p75NTR binding proteins to specific p75NTR-dependent functions, but the identification of p75NTR interactors and signaling pathways has sparked new directions in p75NTR research, and will provide a better understanding of this enigmatic receptor.     

Constitutive expression of the low-affinity neurotrophin receptor and changes during axotomy-induced death of sensory neurones in the neonatal rat dorsal root ganglion

Sensory neurones in the dorsal root ganglion (DRG) of the neonatal rat express the 75-kDa low-affinity neurotrophin receptor (p75NTR) and these neurones degenerate rapidly after axotomy. p75NTR belongs to the tumour necrosis factor superfamily, several members of which have a role in cell death and it is constitutively expressed within a subpopulation of DRG neurones. p75NTR has been implicated in mediating the degeneration of these neurones after axotomy. In this study, we characterize the expression of p75NTR in sensory neurones of the newborn rat DRG using immunohistochemistry. Furthermore, we investigate the change in constitutive expression pattern of p75NTR in these neurones following axotomy. In the C7 and C8 DRG of the newborn rat, p75NTR is expressed in approximately 70% of DRG neurones. Those expressing p75NTR can be classified into subpopulations with moderate or intense p75NTR expression, each present in approximately equal proportions. Whilst p75NTR expression is observed in neurones throughout the entire neuronal diameter range, a correlation exists between neuronal diameter and p75NTR expression intensity. We also found that the most vulnerable population following axotomy were those sensory neurones which constitutively express the highest levels of p75NTR, i.e. the large-diameter neurones.     

The p75 neurotrophin receptor and neuronal apoptosis

Although evidence continues to accumulate for the apoptosis-inducing role of the p75 neurotrophin receptor, several outstanding questions remain. One of these concerns the signal transduction pathway of p75, which continues to be elusive. The evidence for the roles of ceramide, c-jun kinase and NF-kappaB is discussed: none of these are able to account satisfactorily for p75 death signalling. Negative modulation of Trk signalling by p75 could account for part of the pro-apoptotic effect, but is unlikely to be a major component. Although recent evidence indicates that the juxtamembrane region is critical for causing cell death, p75 has a well-conserved death domain. This may be important for functions other than killing. In glial cells and some neurons that express p75 but not TrkA, p75 causes cell death in response to nerve growth factor (NGF) binding. In sensory neurons and PC12 cells, p75 appears to signal constitutively. In cholinergic forebrain neurons, p75 expression leads to atrophy and downregulation of cholinergic markers, rather than cell death. The major challenges in p75 research are to define its signalling pathways, and particularly the intracellular proteins with which it interacts. Another major challenge is to develop a model that reconciles the different facets of p75, such as its ability in some situations to assist TrkA to rescue NGF-dependent neurons, but to stimulate apoptosis in others.     

P162对食管癌细胞株Eca109的放射增敏作用及其对p75NTR表达的影响

 目的：研究靶向Ras-GTP酶激活蛋白SH3功能区结合蛋白(G3BP)的新药P162对人食管癌细胞株Eca109的放射增敏作用及其对p75神经营养因子受体(p75NTR)表达的影响。方法：CCK-8法检测P162对食管癌细胞株Eca109增殖抑制的影响;集落形成实验检测P162对Eca109细胞的放射增敏效应,单击多靶模型拟合细胞存活曲线并计算放射增敏比;倒置显微镜观察细胞形态学改变;流式细胞术检测p75NTR的表达。结果：P162对食管癌细胞株Eca109有增殖抑制作用,且呈时间和剂量依赖性,2.5、5.0、10 μmol/L P162对Eca109细胞的放射增敏比分别为1.54、2.35和2.33。随着照射剂量的增加,食管癌细胞中p75NTR的表达增加,经5 μmol/L P162处理的实验组中p75NTR的表达明显低于未经处理的对照组。结论：P162对Eca109细胞有放射增敏作用,并且能抑制食管癌干细胞p75NTR的表达。P162的增敏作用可能与抑制食管癌干细胞有关。     

过表达P75神经生长因子受体负向调控骨髓间充质干细胞的成骨分化

文题释义： P75神经生长因子受体：即P75^NTR,神经生长因子的受体之一,以三聚体和单体的形式镶嵌于细胞表面,主要包含胞外区、跨膜区以及胞内区3个结构区域。低聚状态决定了其对细胞的主要调节功能,与配体结合后通过改变构象来进行信号传导,进而发挥调节细胞的作用。 成骨分化：多向分化干细胞向单能成骨细胞转变,并最终向骨组织发展的一系列生理过程称为成骨分化,对调节骨形成和骨再生具有重要意义,其成骨分化过程中,碱性磷酸酶的活力逐渐增加,细胞内或者胞外基质的钙等矿物质沉积,形成钙结节,骨钙素及Ⅰ型胶原蛋白表达增加。背景：P75^NTR广泛表达于神经组织及细胞中,并发挥着促进或抑制分化的双重作用;同时在骨折不愈合局部组织中也发现了P75^NTR存在着过表达,因此研究P75^NTR对骨髓间充质干细胞成骨分化的作用具有重要的意义,为临床上提高骨折不愈合修复的疗效提供重要靶点。 目的：观察P75^NTR过表达对大鼠骨髓间充质干细胞体外诱导成骨分化的影响。 方法：选取SD大鼠双侧股骨,用全骨髓分离贴壁法提取大鼠骨髓间充质干细胞并进行体外传代培养;构建表达EGFP的P75^NTR过表达质粒GV358-P75^NTR,并用空慢病毒载体包装收集P75^NTR过表达慢病毒载体;选取体外培养至原代10 d的大鼠骨髓间充质干细胞,消化后种板,加入P75^NTR过表达慢病毒及相关感染试剂进行感染实验,感染7 d后用荧光显微镜观察绿色荧光蛋白的表达,Western blot检测P75^NTR蛋白的过表达情况;感染后用常规培养基培养7 d,更换成骨诱导分化培养基培养,诱导培养后第7,10,14天用酶标法定量检测碱性磷酸酶活力,诱导培养后第7,14天用茜素红染色观察矿化结节形成情况。 结果与结论：①P75^NTR过表达慢病毒感染骨髓间充质干细胞后能表达出绿色荧光蛋白(感染效率约90%),且P75^NTR蛋白表达量明显增多,与阴性病毒对照组比较差异有显著性意义(P < 0.05),过表达P75^NTR细胞模型构建成功;②与阴性病毒对照组及未转染组相比,P75^NTR过表达组细胞诱导分化后相应时间点的碱性磷酸酶活力明显降低,矿化结节形成减少,细胞聚集分布减弱,差异有显著性意义(P < 0.05);③结果表明,P75^NTR过表达负向调控了大鼠骨髓间充质干细胞的成骨分化。局部组织中P75^NTR过表达抑制周围骨髓间充质干细胞成骨分化可能是骨缺损或骨折不愈合的重要因素。 ORCID: 0000-0001-6923-3177(朱伦井) 中国组织工程研究杂志出版内容重点：干细胞;骨髓干细胞;造血干细胞;脂肪干细胞;肿瘤干细胞;胚胎干细胞;脐带脐血干细胞;干细胞诱导;干细胞分化;组织工程     

A small molecule p75 ligand prevents cognitive deficits and neurite degeneration in an Alzheimer's mouse model

The p75 neurotrophin receptor (p75^NTR) is associated with multiple mechanisms linked to Alzheimer's disease (AD); hence, modulating its function might confer therapeutic effects. In previous in vitro work, we developed small molecule p75^NTR ligands that inhibited amyloid-β-induced degenerative signaling and prevented neurite degeneration. In the present study, a prototype p75^NTR ligand, LM11A-31, was administered orally to the Thy-1 hAPP^Lond/Swe (APP^L/S) AD mouse model. LM11A-31 reached brain concentrations known to inhibit degenerative signaling without toxicity or induction of hyperalgesia. It prevented deficits in novel object recognition after 2.5 months and, in a separate cohort, deficits in Y-maze performance after 3 months of treatment. Stereology studies found that the number and size of basal forebrain cholinergic neurons, which are normal in APP^L/S mice, were unaffected. Neuritic dystrophy, however, was readily apparent in the basal forebrain, hippocampus and cortex, and was significantly reduced by LM11A-31, with no effect on amyloid levels. These studies reveal that p75^NTR is an important and tractable in vivo drug target for AD, with LM11A-31 representing a novel class of therapeutic candidates.     

p75真核表达载体的构建及其在PC-3细胞中的表达

目的：构建人神经生长因子低亲和力受体p75真核表达质粒,并在前列腺癌细胞PC-3中表达。方法：运用RT-PCR技术从人脑胶质细胞瘤组织中分离扩增编码人p75完整基因,与pcDNA载体重组,并通过酶切和测序鉴定;用脂质体介导法将重组质粒转染到PC-3细胞中,经G418筛选后,用RT-PCR、间接免疫荧光化学和细胞免疫组织化学法在转录和翻译两个水平上检测表达产物。结果：酶切和测序证实所获得的重组质粒包含p75完整基因;在转染pcDNA3．1( )-p75的PG3细胞中检测到转录和翻译产物。结论：成功地构建了p75真核表达质粒并在PC-3细胞中表达,为深入研究p75在前列腺癌中的生物学功能奠定了基础。     

Structure of the functional interleukin-2 receptor: Evidence for the association of human p55 and murine p75 molecules in a mouse T cell line

The structural basis of the high affinity interleukin-2 receptorwhich was previously reconstituted in a cultured murine T cellline, EL4 by expressing either wild-type Tac antigen complementaryDNA (cDNA) or a chimeric cDNA was characterized. The chimericcDNA encodes a membrane portion whose extracellular portionconsists of that of Tac antigen whereas transmembrane and cytoplasmicportions consists of those the human insulin beta chain. TheTac antigen/anti-Tac antibody complex was treated by chemicalcrosslinking reagents, purified by goat anti-mouse immunoglobulin(lg), and was analysed by SDS–PAGE. We here demonstrated the presence in mouse EL4 transfectantsof a novel membrane protein which is closely associated withthe products of transfected cDNAs in the absence of interleukln-2.The protein is 75 kDa in size and is detected in cells whichexpress high affinity interieukln-2 receptor but not in cellswhich only express low affinity interleukin-2 receptor. Thetransmembrane region and the cytoplasmic region of Tac antigenis not necessary for the formation of the complex consistingof Tac antigen and 75 kDa molecule, indicating that a murine75 kDa molecule associates with Tac antigen extra-cellularly.     

A neuronal subpopulation in the mammalian enteric nervous system expresses TrkA and TrkC neurotrophin receptor-like proteins

Increasing evidence suggests that, in addition to peripheral sensory and sympathetic neurons, the enteric neurons are also under the control of neurotrophins. Recently, neurotrophin receptors have been detected in the developing and adult mammalian enteric nervous system (ENS). Nevertheless, it remains to be established whether neurotrophin receptors are expressed in all enteric neurons and/or in glial cells and whether expression is a common feature in the enteric nervous system of all mammals or if interspecific differences exist. Rabbit polyclonal antibodies against Trk proteins (regarded as essential constituents of the high-affinity signal-transducing neurotrophin receptors) and p75 protein (considered as a low-affinity pan-neurotrophin receptor) were used to investigate the cell localization of these proteins in the ENS of adult man, horse, cow, sheep, pig, rabbit, and rat. Moreover, the percentage of neurons displaying immunoreactivity (IR) for each neurotrophin receptor protein was determined. TrkA-like IR and TrkC-like IR were observed in a neuronal subpopulation in both the myenteric and submucous plexuses, from esophagus to rectum in humans, and in the jejunum-ileum of the other species. Many neurons, and apparently all glial cells, in the human and rat enteric nervous system also displayed p75 IR. TrkB-like IR was found restricted to the glial cells of all species studied, with the exception of humans, in whom IR was mainly in glial cells and a small percentage of enteric neurons (about 5%). These findings indicate that the ENS of adult mammals express neuronal TrkA and TrkC, glial TrkB, and neuronal-glial p75, this pattern of distribution being similar in all examined species. Thus, influence of specific neurotrophins on their cognate receptors may be considered in the physiology and/or pathology of the adult ENS. Anat. Rec. 251:360–370, 1998. © 1998 Wiley-Liss, Inc.     

Expression of p75<Superscript>NTR</Superscript> and Trk neurotrophin receptors in the enteric nervous system of human adults

Recent evidence suggests that the expression of p75NTR and Trk neurotrophin receptors is essential for neuronal survival, not only during development, but also in adulthood. The aim of the present study was to investigate the cell localization and distribution of p75NTR and Trk receptors in the normal adult human enteric nervous system (ENS) using double-label immunohistochemistry. Immunoreactivity for p75NTR was observed in a few neurons, whereas Trk immunoreactivity was present in a higher percentage of neurons. Strong expression of both types of neurotrophin receptors was found in the enteric glia. In addition, Trk immunoreactivity was localized to nerve endings in the lamina propria, muscularis mucosa and along or between circular and longitudinal smooth muscle layers, as well as to the enteric epithelium. Furthermore, polynuclear cells, mast cells and folliculoreticular cells in the germinal layer of lymph nodes displayed p75NTR and/or Trk immunopositivity. Expression of neurotrophin receptors in an inflammatory tissue sample was much more intense compared with that of normal tissue samples. These results suggest that neurotrophin receptors, through interactions with neurotrophins, may play a critical role in functional integrity of the ENS during adulthood.     

Thymocyte depletion affects neurotrophin receptor expression in thymic stromal cells

Thymocytes and thymic stromal cells cross-talk in a bidirectional manner within the thymus, thus contributing to the generation of mature T-cells. The thymic stromal cells in the rat express the high- (TrkA, TrkB) and low-affinity (p75NTR) receptors for neurotrophins. In this study we analysed the regulation of TrkA, TrkB and p75NTR expression in the rat thymus by thymocytes. We induced thymocyte apoptosis by administration of corticoids in rats, and then analysed the expression and distribution of these receptors 1, 4 and 10 days later. Thymocyte death was assessed by the activation of caspase-3 in cells undergoing apoptosis. We observed massive thymocyte apoptosis 1 day after injection and, to a lesser extent, after 4 days, which was parallel with a reduction in the density of thymic epithelial cells normally expressing TrkA and p75NTR. Furthermore, TrkA expression was found in cortical thymic epithelial cells, which normally lack this receptor. The expression of TrkB was restricted to a subset of macrophage-dendritic cells, and remained unchanged with treatment. The normal pattern of neurotrophin receptor expression was almost completely restored by day 10. The results demonstrate that the expression of neurotrophin receptors by thymic epithelial cells, but not by macrophage-dendritic cells, is regulated by thymocytes.