Nerve growth factor and neural oncology

The brain-tumor related protein podoplanin regulates synaptic plasticity and hippocampus-dependent learning and memory

INTRODUCTION

Podoplanin is a cell-surface glycoprotein constitutively expressed in the brain and implicated in human brain tumorigenesis. The intrinsic function of podoplanin in brain neurons remains however uncharacterized.

MATERIALS AND METHODS

Using an established podoplanin-knockout mouse model and electrophysiological, biochemical, and behavioral approaches, we investigated the brain neuronal role of podoplanin.

RESULTS

Ex-vivo electrophysiology showed that podoplanin deletion impairs dentate gyrus synaptic strengthening. In vivo, podoplanin deletion selectively impaired hippocampus-dependent spatial learning and memory without affecting amygdala-dependent cued fear conditioning. In vitro, neuronal overexpression of podoplanin promoted synaptic activity and neuritic outgrowth whereas podoplanin-deficient neurons exhibited stunted outgrowth and lower levels of p-Ezrin, TrkA, and CREB in response to nerve growth factor (NGF). Surface Plasmon Resonance data further indicated a physical interaction between podoplanin and NGF.

DISCUSSION

This work proposes podoplanin as a novel component of the neuronal machinery underlying neuritogenesis, synaptic plasticity, and hippocampus-dependent memory functions. The existence of a relevant cross-talk between podoplanin and the NGF/TrkA signaling pathway is also for the first time proposed here, thus providing a novel molecular complex as a target for future multidisciplinary studies of the brain function in the physiology and the pathology. Key messages Podoplanin, a protein linked to the promotion of human brain tumors, is required in vivo for proper hippocampus-dependent learning and memory functions. Deletion of podoplanin selectively impairs activity-dependent synaptic strengthening at the neurogenic dentate-gyrus and hampers neuritogenesis and phospho Ezrin, TrkA and CREB protein levels upon NGF stimulation. Surface plasmon resonance data indicates a physical interaction between podoplanin and NGF. On these grounds, a relevant cross-talk between podoplanin and NGF as well as a role for podoplanin in plasticity-related brain neuronal functions is here proposed.

Neurotrophic Effects of Mu Bie Zi (Momordica cochinchinensis) Seed Elucidated by High-Throughput Screening of Natural Products for NGF Mimetic Effects in PC-12 Cells

Post-mitotic central nervous system (CNS) neurons have limited capacity for regeneration, creating a challenge in the development of effective therapeutics for spinal cord injury or neurodegenerative diseases. Furthermore, therapeutic use of human neurotrophic agents such as nerve growth factor (NGF) are limited due to hampered transport across the blood brain barrier (BBB) and a large number of peripheral side effects (e.g. neuro-inflammatory pain/tissue degeneration etc.). Therefore, there is a continued need for discovery of small molecule NGF mimetics that can penetrate the BBB and initiate CNS neuronal outgrowth/regeneration. In the current study, we conduct an exploratory high-through-put (HTP) screening of 1144 predominantly natural/herb products (947 natural herbs/plants/spices, 29 polyphenolics and 168 synthetic drugs) for ability to induce neurite outgrowth in PC12 dopaminergic cells grown on rat tail collagen, over 7 days. The data indicate a remarkably rare event-low hit ratio with only 1/1144 tested substances (<111.25 µg/mL) being capable of inducing neurite outgrowth in a dose dependent manner, identified as; Mu Bie Zi, Momordica cochinchinensis seed extract (MCS). To quantify the neurotrophic effects of MCS, 36 images (n = 6) (average of 340 cells per image), were numerically assessed for neurite length, neurite count/cell and min/max neurite length in microns (µm) using Image J software. The data show neurite elongation from 0.07 ± 0.02 µm (controls) to 5.5 ± 0.62 µm (NGF 0.5 μg/mL) and 3.39 ± 0.45 µm (138 μg/mL) in MCS, where the average maximum length per group extended from 3.58 ± 0.42 µm (controls) to 41.93 ± 3.14 µm (NGF) and 40.20 ± 2.72 µm (MCS). Imaging analysis using immunocytochemistry (ICC) confirmed that NGF and MCS had similar influence on 3-D orientation/expression of 160/200 kD neurofilament, tubulin and F-actin. These latent changes were associated with early rise in phosphorylated extracellular signal-regulated kinase (ERK) p-Erk1 (T202/Y204)/p-Erk2 (T185/Y187) at 60 min with mild changes in pAKT peaking at 5 min, and no indication of pMEK involvement. These findings demonstrate a remarkable infrequency of natural products or polyphenolic constituents to exert neurotrophic effects at low concentrations, and elucidate a unique property of MCS extract to do so. Future research will be required to delineate in depth mechanism of action of MCS, constituents responsible and potential for therapeutic application in CNS degenerative disease or injury.

NGF blockade at early times during bone cancer development attenuates bone destruction and increases limb use

Studies in animals and humans show that blockade of nerve growth factor (NGF) attenuates both malignant and nonmalignant skeletal pain. While reduction of pain is important, a largely unanswered question is what other benefits NGF blockade might confer in patients with bone cancer. Using a mouse graft model of bone sarcoma, we demonstrate that early treatment with an NGF antibody reduced tumor-induced bone destruction, delayed time to bone fracture, and increased the use of the tumor-bearing limb. Consistent with animal studies in osteoarthritis and head and neck cancer, early blockade of NGF reduced weight loss in mice with bone sarcoma. In terms of the extent and time course of pain relief, NGF blockade also reduced pain 40% to 70%, depending on the metric assessed. Importantly, this analgesic effect was maintained even in animals with late-stage disease. Our results suggest that NGF blockade immediately upon detection of tumor metastasis to bone may help preserve the integrity and use, delay the time to tumor-induced bone fracture, and maintain body weight.

Nerve growth factor signaling in prostate health and disease

The prostate is one of the most abundant sources of nerve growth factor (NGF) in different species, including humans. NGF and its receptors are implicated in the control of prostate cell proliferation and apoptosis and it can either support or suppress cell growth. The co-expression of both NGF receptors, p75(NGFR) and tropomyosin-related kinase A (trkA), represents a crucial condition for the antiproliferative effect of NGF; indeed, p75(NGFR) is progressively lost during prostate tumorigenesis and its disappearance represents a malignancy marker of prostate adenocarcinoma (PCa). Interestingly, a dysregulation of NGF signal transduction was found in a number of human tumors. This review summarizes the current knowledge on the role of NGF and its receptors in prostate and in PCa. Conclusions bring to the hypothesis that the NGF network could be a candidate for future pharmacological manipulation in the PCa therapy: in particular the re-expression of p75(NTR) and/or the negative modulation of trkA could represent a target to induce apoptosis and to reduce proliferation and invasiveness of PCa.

Pathophysiology of glial growth factor receptors

Activation and proliferation of glial cells are common events in the pathology of the nervous system. Although we are only beginning to understand the molecular signals leading to glial activation in vivo, there is increasing evidence that growth factors and their receptors may play an important part. In this paper we summarize the data on the pathophysiology of glial growth factor receptors and their ligands in the central and peripheral nervous systems.

The effect of nerve growth factor on DNA synthesis, cyclic AMP and cyclic GMP accumulation by mouse spleen lymphocytes

Nerve growth factor (NGF), a trophic neuropeptide, is known to stimulate development, and to be important in the maintenance and survival of sympathetic and sensory neurons. Considering the presence of specific receptors on the surface of spleen cells, the effect of 2.5s nerve growth factor on 3H-thymidine uptake, cAMP and cGMP accumulation in mouse spleen lymphocytes has been studied. It was found that NGF added in vitro at the concentrations between 4 x 10(-7) and 4 x 10(-8) M significantly inhibited the incorporation of 3H-thymidine into lymphocytes DNA and increased cAMP levels in a dose-dependent manner but had no effect on cGMP levels. The maximal stimulation of cAMP synthesis occurred between 5 and 30 min after the NGF addition to the culture medium. When NGF was administered in vivo a significant dose-dependent inhibition of the lymphocytes proliferation was observed. These results indicate that an early increase of cAMP concentration is responsible for the antiproliferative action of NGF on mouse spleen lymphocytes and suggest that NGF could play an important role in the regulation of immune system function.

Characterization of functional nerve growth factor-receptors in a CNS glial cell line: monoclonal antibody 217c recognizes the nerve growth factor-receptor on C6 glioma cells

The biological effects of nerve growth factor (NGF) have been shown to be mediated by the high-affinity form of the nerve growth factor receptor (NGF-R) in sympathetic and sensory neurons, and in PC12 cells. We report here that the central nervous system C6 rat glioma cell line likewise expresses functional high-affinity NGF-Rs. The expression of NGF-R mRNA in C6 cells can be up-regulated by cycloheximide and its own ligand, NGF; and it can be rapidly down-regulated by epidermal growth factor (EGF). Furthermore, C6 cells display NGF responsiveness by expressing c-fos mRNA within 30 minutes of treatment with NGF; and after 4-5 days of NGF exposure, C6 cells cease dividing as measured by [3H]-thymidine uptake, change shape, and reveal neurite-like processes. Scatchard analysis of [125I]-labelled NGF bound to solubilized C6 cells confirms the presence of both high- and low-affinity receptor protein. Crosslinking radiolabeled NGF to its receptor in the presence or absence of excess unlabeled NGF, followed by immunoprecipitation with monoclonal antibody (mAb) 192-IgG (a known anti-NGF-R antibody) and SDS-PAGE reveals a 100 kD band corresponding to the NGF/NGF-R complex. An identical band is observed when the immunoprecipitation is carried out with mAb 217c, suggesting that the 217c epitope is related to NGF-R. The 217c antibody was generated against C6 cells and shown to be a cell surface antibody (Peng et al., Science 215:1102-4, 1982); several investigators have used it subsequently as an immunocytochemical marker for Schwann cells. The significance of NGF-Rs in a CNS glial cell line is unclear, but association of NGF with the control of proliferation and/or differentiation of primitive glial cells is suggested.

Embryonal central neuroepithelial tumors: current concepts and future challenges

While the embryonal central neuroepithelial tumors present complex conceptual and clinical problems, advances in cell type identification by special neurohistological, immunohisto- and immunocytochemical techniques have permitted discrimination of distinct cytomorphogenetic entities. These are based in part on their resemblance to the normal phases of neurocytogenesis. Four of these tumors, medulloepithelioma, desmoplastic infantile ganglioglioma, pineoblastoma and medulloblastoma, are designated as multipotential in light of their capacity to undergo divergent differentiation. Cytomorphogenetic, clinical and experimental data implicate fetal neural cell targets for transformation and raise the possibility that aberrant developmental regulatory mechanisms may contribute to the biologic behavior of these tumors. Growth factors and some neuroregulatory neurotransmitters (such as serotonin) are known to act as modulators of normal neuromorphogenesis. They could play a regulatory role in central neuroepithelial tumors on the hypothesis that the aberrant behavior of the embryonal neoplasms could either be modified by functional receptor responses or result from abnormal receptor responses to these substances. Future challenges include the definition of new cytomorphogenetic entities and subgroups of the currently defined forms of embryonal CNS tumors based on the presence of specific growth factors and neuroregulatory neurotransmitters, or their receptors, the characterization of neoplastic receptor responses mediating any modulatory role of the presently known growth factors or neuroregulatory neurotransmitters on the growth and maturation potential of the embryonal central neuroepithelial tumors and the further definition of developmental, stage-specific modulators that might be operative in these tumors.

Nerve growth factor modification of the ethylnitrosourea model for multiple schwannomas

The administration of ethylnitrosourea (ENU) to pregnant rats late in gestation or to neonatal rats results in the induction of Schwann cell tumors in a high percentage of perinatally exposed animals. Exogenous administration of nerve growth factor (NGF) significantly reduces the number of Schwann cell tumors and other neurogenic tumors developing in ENU-treated rats. Administration of antibodies directed against NGF prior to neonatal ENU exposure results in a substantial increase in the incidence of Schwann cell tumors, particularly in the trigeminal nerves of both rats and mice. Transplacental ENU treatment causes early neoplastic proliferation (ENP) at 90 days of age in the Schwann cell population of trigeminal nerves in nearly all exposed rats. A variety of NGF treatment protocols (single or multiple inoculations or microinfusion prior to or following ENU exposure) resulted in a significant reduction in ENU-induced ENP in trigeminal nerves. These results indicate that NGF may convey protection either directly or indirectly, by an unknown mechanism, to Schwann cells and other supportive neural cells by reducing their sensitivity to ENU-induced neoplastic transformation.

The effect of nerve growth factor and antibodies to nerve growth factor on ethylnitrosourea-induced neoplastic proliferation in rat trigeminal nerves

Sprague-Dawley (CD) rats were injected intravenously with ethylnitrosourea at a dose of 20 mg/kg on day 20 of gestation. This exposure resulted in early neoplastic proliferation or development of a neurinoma of the trigeminal nerve in 58% of the offspring at 90 days of age. Implantation of osmotic microinfusion pumps containing 2.5S nerve growth factor prior to ethylnitrosourea administration significantly reduced the incidence of early neoplastic proliferation. Postnatal implantation of microinfusion pumps containing 2.5S nerve growth factor also resulted in a significant but less pronounced reduction of early neoplastic proliferation. Immunoglobulin G directed against nerve growth factor (anti-nerve growth factor) did not influence the incidence of early neoplastic proliferation when administered via microinfusion pumps implanted on day 15 postnatally. These findings suggest that nerve growth factor has a protective effect on the developing nervous system against ethylnitrosourea-induced carcinogenesis.