A Brain Tumor/Organotypic Slice Co-culture System for Studying Tumor Microenvironment and Targeted Drug Therapies

Brain tumors are a major cause of cancer-related morbidity and mortality. Developing new therapeutics for these cancers is difficult, as many of these ...

Dana-Farber Cancer Institute

Hebrew University of Jerusalem

During development, trigeminal nerve fibers navigate and establish their axonal projections to the developing tooth in a highly spatiotemporally controlled manner. By analyzing Sema3a and its receptor Npn1 knockout mouse embryos, we found that Sema3a regulates dental trigeminal axon navigation and patterning, as well as the timing of the first mandibular molar innervation, and that the effects of Sema3a appear to be mediated by Npn1 present in the axons. By performing tissue recombinant experiments and analyzing the effects of signaling molecules, we found that early oral and dental epithelia, which instruct tooth formation, and epithelial Wnt4 induce Sema3a expression in the presumptive dental mesenchyme before the arrival of the first dental nerve fibers. Later, at the bud stage, epithelial Wnt4 and Tgfbeta1 regulate Sema3a expression in the dental mesenchyme. In addition, Wnt4 stimulates mesenchymal expression of Msx1 transcription factor, which is essential for tooth formation, and Tgfbeta1 proliferation of the dental mesenchymal cells. Thus, epithelial-mesenchymal interactions control Sema3a expression and may coordinate axon navigation and patterning with tooth formation. Moreover, our results suggest that the odontogenic epithelium possesses the instructive information to control the formation of tooth nerve supply.

Coordination of trigeminal axon navigation and patterning with tooth organ formation: epithelial-mesenchymal interactions, and epithelial Wnt4 and Tgfbeta1 regulate semaphorin 3a expression in the dental mesenchyme.

Large numbers of neurons are eliminated by apoptosis during nervous system development. For instance, in the mouse dorsal root ganglion (DRG), the highest incidence of cell death occurs between embryonic days 12 and 14 (E12-E14). While the cause of cell death and its biological significance in the nervous system is not entirely understood, it is generally believed that limiting quantities of neurotrophins are responsible for neuronal death. Between E12 and E14, developing DRG neurons pass through tissues expressing high levels of axonal guidance molecules such as Semaphorin 3A (Sema3A) while navigating to their targets. Here, we demonstrate that Sema3A acts as a death-inducing molecule in neurotrophin-3 (NT-3)-, brain-derived neurotrophic factor (BDNF)- and nerve growth factor (NGF)-dependent E12 and E13 cultured DRG neurons. We show that Sema3A most probably induces cell death through activation of the c-Jun N-terminal kinase (JNK)/c-Jun signaling pathway, and that this cell death is blocked by a moderate increase in NGF concentration. Interestingly, increasing concentrations of other neurotrophic factors, such as NT-3 or BDNF, do not elicit similar effects. Our data suggest that the number of DRG neurons is determined by a fine balance between neurotrophins and Semaphorin 3A, and not only by neurotrophin levels.

Semaphorin 3A and neurotrophins: a balance between apoptosis and survival signaling in embryonic DRG neurons.

Axon guidance cues are critical for neuronal circuitry formation. Guidance molecules may repel or attract axons directly by effecting growth cone motility, or by impinging on neuronal polarity. In Semaphorin3A null mice, many axonal errors are detected, most prominently in DRG neurons. It has been generally assumed the repellent properties of Semaphorin3A are the cause of these erroneous axonal projections. Here we show that, in semaphorin3A-null mice, the initial trajectory of neurons in the DRG is abnormal, suggesting that Semaphorin3A may instruct neuronal polarity. In corroboration, in vitro Semaphorin3A dramatically increases neuronal polarization, as indicated by GSK3beta and Rac1 sub-cellular localization in DRG neurons. Polarization effects of Semaphorin3A are regulated by activated MAPK, as indicated by p-MAPK 42/44 polarization and the need for its activity for Rac1 and GSK3beta polarization. Taken together, our findings suggest that Semaphorin3A plays a role in the formation of neuronal polarity, in addition to its classic repellent role.

Semaphorin3A accelerates neuronal polarity in vitro and in its absence the orientation of DRG neuronal polarity in vivo is distorted.

The p75 neurotrophin receptor (p75(NTR)) interacts with multiple ligands and coreceptors. It is thought to mediate myelin growth inhibition as part of the Nogo receptor complex, in addition to its other roles. Paradoxically, however, peripheral axons of p75(ExonIII-/-) mutant embryos are severely stunted. This inhibition of axon growth may be a result of neurite elongation defects in p75(NTR) mutant neurons. Here, we show that p75(ExonIII-/-) DRG neurons are hypersensitive to the repellent molecule Semaphorin3A (Sema3A). NGF modulates Sema3A activity equally well in both the p75(NTR) mutant and wild-type neurons, indicating that the hypersensitivity of p75(NTR) mutant neurons is probably not related to their NGF receptor activity. Neuropilin1 and p75(NTR) partially colocalize in DRG growth cones. After Sema3A stimulation, the degree of colocalization is dramatically increased, particularly in clusters associated with Sema3A receptor complex activation. Coimmunoprecipitation studies show that p75(NTR) interacts directly with the Sema3A receptors Neuropilin1 and PlexinA4. When coexpressed with both Neuropilin1 and PlexinA4, p75(NTR) reduces the interaction between these two receptor components. Finally, p75(NTR)/Sema3A double-mutant embryos show growth similar to that observed in Sema3A-null mice. These data indicate that p75(NTR) is an important functional modulator of Sema3A activity and that, in the absence of p75(NTR), oversensitivity to Sema3A leads to severe reduction in sensory innervation. Our results also suggest that while inhibition of p75(NTR) in CNS injury may enhance nerve regeneration resulting from the inhibition of myelin-associated protein, it may also inhibit nerve regeneration through its modulation of Sema3A.

Modulation of semaphorin3A activity by p75 neurotrophin receptor influences peripheral axon patterning.

Extensive neuronal cell death during development is believed to be due to a limiting supply of neurotrophic factors. In vitro studies suggest that axon guidance molecules directly regulate neuronal survival, raising the possibility that they play a direct role in neuronal cell death in vivo. However, guidance errors may also influence survival indirectly due to loss of target-derived neurotrophic support. The role of guidance molecules in neuronal death in vivo has thus been difficult to decipher. Semaphorin3A, a repulsive guidance cue for sensory neurons, can induce sensory neuron death in vitro. Null mice studies of the Semaphorin3A coreceptors showed that guidance activity is mediated by PlexinA4, but PlexinA3 partially compensates in PlexinA4(-/-) mice. Here we demonstrate that both Plexins contribute to Sema3A-induced cell death in vitro, albeit in a different hierarchy. PlexinA3 is absolutely required, while PlexinA4 makes a smaller contribution to cell death. We found that PlexinA3(-/-) mice, which, unlike PlexinA4(-/-) mice, do not exhibit sensory axon patterning defects, show reduced neuronal apoptosis and an increased number of DRG neurons. Semaphorin3A involvement in neuronal death in vivo was demonstrated by a sensitization experiment using the proapoptotic effector Bax. Our results identify Plexins as mediators of Semaphorin-induced cell death in vitro, and provide the first evidence implicating Semaphorin/Plexin signaling in neuronal survival independent of its role in axon guidance. The results also support the idea that naturally occurring neuronal cell death reflects not only competition for target-derived trophic factors, but also the action of proapoptotic signaling via a Semaphorin/Plexin pathway.

The Semaphorin receptor PlexinA3 mediates neuronal apoptosis during dorsal root ganglia development.

Axon guidance molecules determine the pattern of neuronal circuits. Accuracy of the process is ensured by unknown mechanisms that correct early guidance errors. Since the time frame of error correction in Sema3A null mice partly overlaps with the period of naturally occurring cell death in dorsal root ganglia (DRG) development, we tested the hypothesis that apoptosis of misguided neurons enables error correction. We crossed BAX null mice, in which DRG apoptosis is blocked, with Sema3A null mice to induce errors. Analyses of these double-null mouse embryos showed that the elimination of abnormal projections is not blocked in the absence of BAX. Surprisingly however, there are fewer surviving neurons in Sema3A null or Sema3A/BAX double-null newborn mice than in wild-type mice. These results suggest that guidance errors are corrected by a BAX-independent cell death mechanism. Thus, aberrant axonal guidance may lead to reductions in neuronal numbers to suboptimal levels, perhaps increasing the likelihood of neuropathological consequences later in life.

Elimination of aberrant DRG circuitries in Sema3A mutant mice leads to extensive neuronal deficits.

The biological activity of a recombinant protein is routinely measured using a bioassay such as an enzyme assay. However, many proteins have no enzymatic activity and in many cases it is difficult to devise a simple and reliable approach to test their activity. Semaphorins, Ephrins, Slits, Netrins or amylin-assisted proteins have numerous activities affecting many systems and cell types in the human body. Most of them are also able to induce rapid cytoskeleton changes at least in some cell types. We assumed therefore, that such proteins might be tested based on their ability to modulate the cytoskeleton. Here we tested a number of semaphorins in an impedance based label-free platform that allows for dynamic monitoring of subtle morphological and adhesive changes. This system has proved to be a very fast, sensitive and effective way to monitor and determine the activity of such proteins. Furthermore we showed that it is possible to customize a cell-protein system by transfecting the cells with specific receptors and test the cell response following the addition of the recombinant ligand protein. Since other protein families such as Ephrins and Netrins can also influence the cytoskeleton of some cells, this approach may be applicable to a large number of proteins.

A new impedance based approach to test the activity of recombinant protein - Semaphorins as a test case.

Injury to the CNS induces astrogliosis, an astrocyte-mediated response that has both beneficial and detrimental impacts on surrounding neural and non-neural cells. The precise signaling events underlying astrogliosis are not fully characterized. Here, we show that astrocyte activation was altered and proliferation was reduced in Semaphorin 4B (Sema4B)-deficient mice following injury. Proliferation of cultured Sema4B(-/-) astrocytes was also significantly reduced. In contrast to its expected role as a ligand, the Sema4B ectodomain was not able to rescue Sema4B(-/-) astrocyte proliferation but instead acted as an antagonist against Sema4B(+/-) astrocytes. Furthermore, the effects of Sema4B on astrocyte proliferation were dependent on phosphorylation of the intracellular domain at Ser825. Our results suggest that Sema4B functions as an astrocyte receptor, defining a novel signaling pathway that regulates astrogliosis after CNS injury.

Oded Behar

Department of Cancer Biology

Dana-Farber Cancer Institute

A Brain Tumor/Organotypic Slice Co-culture System for Studying Tumor Microenvironment and Targeted Drug Therapies

Oded Behar

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Dana-Farber Cancer Institute

A Brain Tumor/Organotypic Slice Co-culture System for Studying Tumor Microenvironment and Targeted Drug Therapies

Department of Cancer Biology