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Although neurotrophins have traditionally been regarded as neuronal survival factors, recent work has suggested a role for these factors in synaptic plasticity. In particular, brain-derived neurotrophic factor (BDNF) rapidly enhances synaptic transmission in hippocampal neurons through trkB receptor stimulation and postsynaptic phosphorylation mechanisms. Activation of trkB also modulates hippocampal long-term potentiation, in which postsynaptic N-methyl-D-aspartate glutamate receptors play a key role. However, the final common pathway through which BDNF increases postsynaptic responsiveness is unknown. We now report that BDNF, within 5 min of exposure, elicits a dosedependent increase in phosphorylation of the N-methyl-Daspartate receptor subunit 1. This acute effect occurred in hippocampal synaptoneurosomes, which contain pre-and postsynaptic elements, and in isolated hippocampal postsynaptic densities. Nerve growth factor, in contrast, caused no enhancement of phosphorylation. These results suggest a potential mechanism for trophin-induced potentiation of synaptic transmission.Neurotrophins play important roles in the survival and differentiation of specific neuronal populations during development and adulthood (1-3). Four members of the mammalian neurotrophin gene family have been identified: nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and NT-4͞5 (4, 5). The neurotrophins bind to receptor tyrosine kinases of the trk protooncogene family with high affinity. trkA is a high-affinity receptor for NGF (6, 7), trkB for BDNF and NT-4͞5 (8, 9) as well as NT-3 (10), and trkC for .In addition to classical trophic activities, we and others have shown that neurotrophins acutely modulate synaptic transmission (12-19) and long-term potentiation (20). Experiments with dissociated cultures of hippocampal neurons indicate that BDNF and NT-4͞5 rapidly and selectively potentiate excitatory synaptic currents (17-19), and this trkB-mediated synaptic enhancement involves phosphorylation-dependent modulation of postsynaptic responsiveness (18). The exact mechanisms by which trkB activation modulates synaptic transmission remain to be established.We recently found that trkB is a functionally active, intrinsic component of the adult rat postsynaptic density (PSD) isolated from cerebral cortex and hippocampus (21). The PSD is a proteinaceous disc-shaped subcellular organelle apposed to the inner surface of the postsynaptic membrane of chemical synapses (22). Neurotransmitter receptors, protein kinases, and ion channel proteins are anchored to the PSD (22), suggesting that the PSD participates in signal transduction and even receptor regulation. In fact, our recent studies revealed that N-methyl-D-aspartate (NMDA) receptors can be phosphorylated by intrinsic PSD kinases (23,24). Phosphorylation of NMDA receptors is known to enhance receptor binding, channel conductance, and synaptic transmission (25-27). Because trkB activation enhances synaptic activity via postsynaptic phosphor...

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Although neurotrophins are critical for neuronal survival and differentiation, recent studies suggest that they also regulate synaptic plasticity. Brain-derived neurotrophic factor (BDNF) rapidly increases synaptic transmission in hippocampal neurons, and enhances long-term potentiation (LTP), a cellular and molecular model of learning and memory. Loci and precise mechanisms of BDNF action remain to be defined: evidence supports both pre- and postsynaptic sites of action. To help elucidate the synaptic mechanisms of BDNF action, we used antisera directed against the extracellular and intracellular domains of trkB receptors, anti-trkBout and anti-trkBin, respectively, to localize the receptors in relation to synapses. Synaptic localization of BDNF was examined in parallel using anti-BDNF antisera. By light microscopy, trkBin and trkBout immunoreactivities were localized to hippocampal neurons and all layers of the overlying visual cortex. Immunoelectron microscopic analysis of the cerebral cortex revealed that trkB and BDNF localize discretely to postsynaptic densities (PSD) of axo-spinous asymmetric synaptic junctions, that are the morphological correlates of excitatory, glutamatergic synapses. TrkB immunoreactivity was also detected in the nucleoplasm by light and electron microscopy. Western blot analysis indicated that both anti-trkBout and anti-trkBin antisera react with a protein band in the PSD corresponding to the molecular weight expected for trkB; however, molecular species distinct from that for trkB were recognized in the nuclear fraction by both anti-trkBin and anti-trkBout antisera, indicating that the nuclear immunoreactivity, seen by immunocytochemistry, reflects cross-reactivity with proteins closely related to, but distinct from, trkB. The PSD localization of both BDNF and trkB supports the contention that this receptor/ligand pair participates in postsynaptic plasticity.

The antitumor effects of a novel antibody drug conjugate (ADC) was tested against human solid tumor cell lines and against human triple negative breast cancer (TNBC) xenografts in immunosuppressed mice. The ADC targeting activated matriptase of tumor cells was synthesized by using the potent anti-tubulin toxin, monomethyl auristatin-E linked to the activated matriptase-specific monoclonal antibody (M69) via a lysosomal protease-cleavable dipeptide linker. This ADC was found to be cytotoxic against multiple activated matriptase-positive epithelial carcinoma cell lines in vitro and markedly inhibited growth of triple negative breast cancer xenografts and a primary human TNBC (PDX) in vivo. Overexpression of activated matriptase may be a biomarker for response to this ADC. The ADC had potent anti-tumor activity, while the unconjugated M69 antibody was ineffective in a mouse model study using MDA-MB-231 xenografts in mice. Treatment of a human TNBC (MDA-MB-231) showed potent anti-tumor effects in combination with cisplatin in mice. This ADC alone or in combination with cisplatin has the potential to improve the treatment outcomes of patients with TNBC as well as other tumors overexpressing activated matriptase.

Mesenchymal stromal cells (MSCs) are multipotent fibroblast-like cells located in the bone marrow that localize to areas of tissue damage including wounds and solid tumors. Within the tumor microenvironment, MSCs adopt the phenotype of carcinoma-associated fibroblasts (CAFs) and stimulate tumor growth. Production of the chemokine CXCL12, also known as stromal cell-derived factor 1 (SDF-1), by MSCs is required for their in vitro migration in response to tumor cells and has also been implicated in stimulation of tumor growth. The tumor suppressor p53 regulates cellular migration, CXCL12 production and the promotion of tumor growth by carcinoma-associated fibroblasts (CAFs). We investigated the role of p53 in MSC migration to tumors. P53 inhibits the migration of MSCs in response to tumor cells in conjunction with a decrease in CXCL12 transcription. Conversely, decreased p53 activity leads to enhanced MSC migration. Interestingly, increased p53 activity inhibits MSC migration even in the context of high concentrations of exogenous CXCL12. These data show that stromal p53 status impacts the recruitment of MSCs to solid tumors through both regulation of CXCL12 production as well as other mechanisms. Stromal p53 may influence other important aspects of tumor biology such as tumor growth and metastasis through mechanisms distinct from CXCL12.