Multiplexin: Biological Overview | References
Gene name - Multiplexin
Synonyms - Endostatin
Cytological map position - 65E1-65E2
Function - ligand, extracellular matrix
Symbol - Mp
FlyBase ID: FBgn0260660
Genetic map position - chr3L:6,992,000-7,046,849
Classification - Endostatin-like, LamininG, Collagen triple helix repeat
Cellular location - extracellular
At synapses in organisms ranging from fly to human, a decrease in postsynaptic neurotransmitter receptor function elicits a homeostatic increase in presynaptic release that restores baseline synaptic efficacy. This process, termed presynaptic homeostasis, requires a retrograde, trans-synaptic signal of unknown identity. Multiplexin was identified in a forward genetic screen for homeostatic plasticity genes. Multiplexin is the Drosophila homolog of Collagen XV/XVIII, a matrix protein that can be proteolytically cleaved to release Endostatin, an antiangiogenesis signaling factor. This study demonstrates that Multiplexin is required for normal calcium channel abundance, presynaptic calcium influx, and neurotransmitter release. Remarkably, Endostatin has a specific activity, independent of baseline synapse development, that is required for the homeostatic modulation of presynaptic calcium influx and neurotransmitter release. These data support a model in which proteolytic release of Endostatin signals trans-synaptically, acting in concert with the presynaptic CaV2.1 calcium channel, to promote presynaptic homeostasis (Wang, 2014).
The nervous system is continually modified by experience. Given the tremendous complexity of the nervous system, it is astounding that robust and reproducible neural function can be sustained throughout life. It is now apparent that homeostatic signaling systems stabilize the excitable properties of nerve and muscle and, thereby, constrain how the nervous system can be altered by experience or crippled by disease. The Drosophila neuromuscular junction (NMJ) has emerged as a powerful model system to dissect the underlying mechanisms that achieve the homeostatic modulation of presynaptic neurotransmitter release. At the Drosophila NMJ, inhibition of postsynaptic glutamate receptor function causes a homeostatic increase in presynaptic neurotransmitter release that precisely restores muscle excitation to baseline levels. This phenomenon is conserved from fly to human. Importantly, presynaptic homeostasis has also been observed at mammalian central synapses in vitro in response to differences in target innervation and altered postsynaptic excitability and following chronic inhibition of neural activity (Wang, 2014).
Despite progress in identifying presynaptic effector proteins that are required for the expression of presynaptic homeostasis (Davis, 2013), the identity of the retrograde signaling system remains unknown. Numerous neurotrophic factors, such as nerve growth factor; brain-derived neurotrophic factor (BDNF); and glia-derived neurotrophic factor, as well as nitric oxide, endocannabinoids, and adhesion molecules, are identified as retrograde signals that regulate presynaptic cell survival, differentiation, and biophysical properties in an activity-dependent manner. Among these molecules, BDNF has been implicated in the trans-synaptic control of presynaptic release in cultured hippocampal neurons. Previous work demonstrated that a bone morphogenetic protein (BMP) ligand (Glass bottom boat) is released from muscle, activates a type II BMP receptor at the presynaptic terminal, and is required for the growth of the presynaptic nerve terminal. This BMP signaling system is also necessary for presynaptic homeostasis. However, the BMP signaling system is a permissive signal that acts at the motoneuron cell body (Wang, 2014).
A large-scale, electrophysiology-based forward genetic screen for mutations that block presynaptic homeostasis (Dickman, 2009 and Müller, 2011) identified multiplexin as a candidate homeostatic plasticity gene. Drosophila Multiplexin is the homolog of human Collagen XV and XVIII, matrix molecules that are expressed ubiquitously in various vascular and epithelial basement membranes throughout the body (Seppinen, 2011). Mutations in the human COL18A1 gene cause Knobloch syndrome, characterized by retinal detachment, macular abnormalities, and occipital encephalocele. Patients with Knobloch syndrome are also predisposed to epilepsy, highlighting the critical function of Collagen XVIII in the central nervous system. Moreover, the C-terminal of Collagen XVIII, encoding an Endostatin domain, can be cleaved proteolytically (Chang, 2005, Felbor, 2000; Heljasvaara, 2005) and functions as an antiangiogenesis factor to inhibit tumor progression (Dhanabal, 1999, O'Reilly, 1997; Yamaguchi, 1999). Endostatin inhibits angiogenesis by interacting with various downstream signaling factors, including vascular endothelial growth factor receptors (Kim, 2002), integrins (Wickström, 2002), and Wnt signaling molecules (Hanai, 2002). Little is known regarding the function of multiplexin in the nervous system. This study provides evidence that Endostatin, a proteolytic cleavage product of Drosophila Multiplexin, functions as a trans-synaptic signaling molecule that is essential for the homeostatic modulation of presynaptic neurotransmitter release at the Drosophila NMJ (Wang, 2014).
Loss of Endostatin blocks the homeostatic modulation of presynaptic calcium influx and presynaptic neurotransmitter release. This activity is remarkably specific to presynaptic homeostasis, since loss of Endostatin has no effect on baseline neurotransmission or synapse morphology. Endostatin also interacts genetically with the pore-forming subunit of the CaV2.1 calcium channel and is required for the homeostatic increase of presynaptic calcium influx during synaptic homeostatic plasticity. Finally, transgenic overexpression of Endostatin is sufficient to rescue synaptic homeostasis and baseline neurotransmitter release when it is supplied to either the presynaptic or postsynaptic side of the synapse. Although deletion of Endostatin does not impair baseline transmission, overexpression of Endostatin in the dmpf07253 mutant is sufficient to restore baseline transmission release even in the absence of the Thrombospondin-like domain. As a working model, it is proposed that inhibition of postsynaptic glutamate receptors initiates the proteolytic cleavage of Multiplexin, which resides in the synaptic cleft. It is further proposed that release of Endostatin acts upon presynaptic calcium channels, directly or indirectly, to potentiate calcium influx and presynaptic neurotransmitter release. This model is consistent with data from other systems demonstrating that activation of Endostatin requires proteolytic cleavage of Collagen XVIII by matrix metalloproteases (MMPs) and cysteine cathepsins. Moreover, only free Endostatin released by cleavage functions as an antiangiogenesis factor (Heljasvaara, 2005; Wang, 2014).
The means by which presynaptic calcium channel function is modulated by Endostatin remains to be elucidated. Recently, it has been shown that a presynaptic Deg/ENaC channel is also necessary for the homeostatic modulation of presynaptic release (Younger, 2013). In this previous study, a model is presented in which ENaC channel insertion causes a sodium leak and modest depolarization of the presynaptic resting membrane potential that, in turn, potentiates presynaptic calcium influx. One possibility is that the interaction of Endostatin with the presynaptic CaV2.1 channels enables the channels to respond to low-voltage modulation. This would be consistent with both Endostatin and the ENaC channel being strictly necessary for presynaptic homeostasis. It remains formally possible that Endostatin stabilizes presynaptic ENaC channels and, thereby, influences presynaptic calcium influx. For example, it was demonstrated that the interaction between ENaC channels and extracellular collagens mediates the mechanosensory transduction in the touch reception systems (Chalfie, 2009; Liu, 1996; Wang, 2014).
Activation of Endostatinin in other systems requires proteolytical cleavage of Collagen XVIII by MMPs and cysteine cathepsins. This raises an intriguing possibility that, during synaptic homeostasis, Multiplexin could be cleaved by synaptic MMPs, releasing Endostatin to trigger a homeostatic change in presynaptic release. In this model, inhibition of postsynaptic glutamate receptors would lead to the activation of MMPs within the synaptic cleft. Thus, the retrograde signal would be a multistage system, providing opportunity for both amplification and multilevel control of the signaling event. At glutamatergic synapses in hippocampal neurons, proteolytic cleavage of neuroligin-1, a synaptic adhesion molecule residing in postsynaptic terminals, is triggered by postsynaptic NMDA receptor activation. Cleavage of neuroligin-1 depresses presynaptic transmission by reducing presynaptic release probability in a trans-synaptic manner (Peixoto, 2012). Thus, the activity-dependent cleavage of cell adhesion and extracellular matrix proteins could provide a robust and evolutionarily conserved feedback paradigm for trans-synaptic signaling to regulate synaptic efficacy in diverse neuronal circuits (Wang, 2014).
Multiplexins are multidomain collagens typically composed of an N-terminal thrombospondin-related domain, an interrupted triple helix and a C-terminal endostatin domain. They feature a clear regulatory function in the development of different tissues, which is chiefly conveyed by the endostatin domain. This domain can be found in proteolytically released monomeric and trimeric versions, and their diverse and opposed effects on the migratory behavior of epithelial and endothelial cell types have been demonstrated in cell culture experiments. The only Drosophila multiplexin displays specific features of both vertebrate multiplexins, collagens XV and XVIII. This study characterized the Drosophila multiplexin (dmp) gene and found that three main isoforms are expressed from it, one of which is the monomeric endostatin version. Generation of dmp deletion alleles revealed that Dmp plays a role in motor axon pathfinding, as the mutants exhibit ventral bypass defects of the intersegmental nerve b (ISNb) similar to other motor axon guidance mutants. Transgenic overexpression of monomeric endostatin as well as of full-length Dmp, but not trimeric endostatin, were able to rescue these defects. In contrast, trimeric endostatin increased axon pathfinding accuracy in wild type background. It is concluded that Dmp plays a modulating role in motor axon pathfinding and may be part of a buffering system that functions to avoid innervation errors (Meyer, 2009).
Axonal pathfinding is a complex process depending on the balance of various attractive and repulsive guidance cues that act on an individual growth cone. The C-terminal domains of vertebrate multiplexins have received ample attention as activators and inhibitors of endothelial and epithelial cell migration, and some evidence also exists for a role of multiplexins as modulators of neuronal migratory behavior. Multiplexins are multidomain proteins, containing an N-terminal domain with homology to the N-terminal domain of thrombospondin (TSPN), an interrupted collagen triple helix domain and a C-terminal non-collagenous domain (NC1), that subdivides into a trimerization region, a protease-sensitive hinge region and a globular endostatin (ES) domain. One of the two vertebrate multiplexins, collagen XVIII, and C. elegans collagen XVIII occur in three different isoforms that can contain further N-terminal domains. This study found that the Drosophila multiplexin (Dmp) is encoded by a complex locus, comprising CG8647 and CG33171, that encodes a TSPN domain, but no further N-terminal moieties (Meyer, 2009).
Two deletions were generated within the dmp gene, and both deletion strains show pathfinding deficiencies of the same quality. ISNb growth cones fail to detach from the main nerve tract and steer into the ventral muscle field but continue to grow on dorsally. Additionally, ISNb as well as ISN nerves sometimes disrespect segment boundaries and fuse to nerves of adjacent segments. The two deletion strains differ by the incidence of pathfinding errors, as dmpΔC12–25, lacking the larger part of the gene, including part of the triple helix repeat region and the NC1-encoding region, shows greater penetrance than dmpΔN4–11, where the second alternative N-terminus and the following triple helical repeat regions are missing. dmpΔN4–11 is not likely to be a null mutation since there is wild type-like transcript present from the NC1-encoding region of the gene. This situation suggests that a low dosage of a truncated Dmp version leads to an improved phenotype compared with dmpΔC12–25. However, an effect caused by the absence of the protein's N-terminal portion cannot be completely ruled out. For example, one can speculate about a function of a putative integrin-interacting RGD motif present therein (Meyer, 2009).
The spatially and temporally very restricted expression of dmp contrasts with the landmarks of the two collagen IV molecules, that are the only other conserved collagens in Drosophila and general important ECM constituents. In Drosophila, their expression begins in embryonic stage 12 or even earlier and hemocytes produce and disperse them all over the embryo while circulating in the hemolymph throughout the rest of embryonic development. During late embryogenesis, specific hemocytes also migrate along the ventral midline between the CNS and the epidermis, but these midline hemocytes are dispersed very asymmetrically, and it is not believed that any of the dmp expression observed originates from these cells. Hence, Dmp production is largely different from collagen IV expression, since it is restricted to limited domains of immobile cells and occurs markedly later in development (Meyer, 2009).
Both dmp deletion strains were homozygous viable. This is true for collagen XV and XVIII single knockouts in mice and even with the double knockout (Ylikarppa, 2003), as well as the C. elegans cle-1 mutation (Ackley, 2001), none of which are lethal. However, the dmp gene and two type IV collagen genes are the only collagen genes in the Drosophila genome, whereas vertebrates and nematodes possess a far greater number of collagen types and genes. This renders the dispensability of Dmp in flies even more surprising than in other organisms, and underlines that its role is very different from that of collagen IV (Meyer, 2009).
It was possible to rescue the dmpΔC12–25 ISNb phenotype by overexpressing the Dmp3hNC1 isoform in dmpΔC12–25 mutant background, while its overexpression in wild type had no effect on axon pathfinding. This indicates that the protein product from the transgene functions in a fashion similar to the endogenous protein. Monomeric Endostatin (ES) had a rescue effect on the dmpΔC12–25 ISNb phenotype that is indistinguishable from the effect of UAS-3hNC1, whereas UAS-NC1, which is expected to lead to the formation of trimeric ES, did not improve the phenotype. It is concluded from this that a beneficial activity resides specifically within the monomeric ES version, which is necessary and sufficient for ensuring ISNb pathfinding accuracy. Biochemical work on vertebrate collagen XVIII has identified a number of proteases capable of releasing monomeric ES. In the dmp mutants, a proteolytic release of monomeric ES might account for the rescue effect of UAS-3hNC1. This implies that ES release is not possible from UAS-NC1, since this construct does not improve the mutant phenotype. A possible explanation for this is that the triple helix region of the molecule is needed for specific enzyme-substrate recognition (Meyer, 2009).
The overexpression effect of ES in wild type background is qualitatively different from its impact on the dmp mutant phenotype. While ES overexpression was largely beneficial in the mutant, it caused a variety of phenotypes in wild type background, ranging from a defective body morphology giving the impression that morphogenesis was hampered on a general level, to wild type looking individuals and such that appeared to exhibit a specific motor axon guidance phenotype. It is suggested that the phenotypic differences between the two genotypes are due to an ES dosage effect. ES overexpression in dmp mutant background produces an ES quantity that can be fully absorbed by the standard interaction partners, whereas the ES excess in the wild type overexpression situation leads to an ES overcharge of standard or non-standard interaction partners that inhibits vital interactions with other molecules. In an alternative scenario, the detrimental effect of ES in wild type background would arise from an interaction between ectopic ES and the trunk of the Dmp molecule (Meyer, 2009).
When overexpressing Dmp3hNC1, no detrimental effect is observed, so the full-length protein dosage increase over wild type level apparently does not pose a problem. This can be due to the endogenous mechanisms that balance proteolytic generation of monomeric ES, such as the characteristics of the proteases involved, which can include their spatial and temporal distribution, quantity, substrate affinity and the resulting turnover numbers (Meyer, 2009).
Transgenic expression of trimeric ES, as formed by the NC1 transgene product, and monomeric ES had exactly converse effects on the ISNb phenotypes of wild type and dmpΔC12–25 mutants. Free ES was able to rescue the pathfinding defects observed in dmpΔC12–25 and had a detrimental effect in wild type background. In contrast, the NC1 transgene, while having no impact in the mutant background, significantly reduced the wild type ISNb error rate of 7.2% to a virtually flawless ISNb guidance with 0.75% errors. These observations indicate that the function of NC1 depends on the presence of endogenous Dmp. In one scenario, endogenous Dmp plays a role in creating a microenvironment or architecture in the basement membrane that is permissive for NC1 activity. Conversely, a basement membrane without Dmp deteriorates in quality resulting in an inactive form of NC1. Alternatively, a direct interaction between Dmp or parts of it and NC1 may be required for NC1 function. Each feature of Dmp may potentially convey the relevant interactions, including the collagen triple helix region, the TSPN-1 domain, the NC1 portion, or several of these domains cooperate. Since both the Dmp triple helix region and the ES domain contain putative glycosaminoglycan attachment attachment sites, options for functional moieties include heparan sulfate side chains (Meyer, 2009).
Another question concerns the mode of action of NC1. One possibility is that it acts, if supported by the presence of functional Dmp, by releasing ES. In this scenario, NC1 simply is an inhibited version of ES. This option, however, is contradicted by the observation that neither overexpression of ES nor of 3hNC1 improved pathfinding in wild type background. An alternative idea is that ES monomers and NC1 trimers play distinct roles. In this setting, ES cannot be released from NC1, but NC1 exerts its own action as a trimer when Dmp is present. One option for NC1 activity is that its trivalence is used to crosslink different ECM components. This might stabilize structures that contain guidance information and eventually enhance the stringency of guidance interactions (Meyer, 2009).
The results show that the wild type ISNb error rate of 7.2% can be reduced by 10-fold by the expression of additional trimeric endostatin from the UAS-NC1 transgene. This illustrates that Dmp possesses a capacity to influence pathfinding accuracy that is not fully taken advantage of in the normal wild type situation. This could be explained in that a low rate of statistically no more than one error per individual does not decrease the affected individual's fitness and hence does not exert any selective pressure (Meyer, 2009).
Another possible reason is that increasing the fidelity of guidance interactions would interfere with the system's error prevention and/or correction mechanisms. As already described, correct pathfinding depends on the appropriate balance of attractive and repulsive forces, and this system is susceptible to perturbations. The observation that nerves are occasionally misrouted in wild type is a reflection of this fact. An ability to correct errors would therefore convey a certain degree of robustness in dealing with such perturbations. Indeed, error correction has been described in the Drosophila motor axon guidance system for SNa misrouting phenotypes that occur due to axonal overexpression of FasII. In third instar larvae, incidence of this phenotype is much lower than in embryos, suggesting that errors can get corrected during larval stages. For zebrafish retinal axons, it has also been stated that errors occur in wild type, but get corrected. In the example of ISNb pathfinding, excess trimeric endostatin might impair the system's plasticity by overly stabilizing misleading guidance interactions, and thus impair correction mechanisms, and by consequence reduces plasticity of the system (Meyer, 2009).
The observation that dmp mutations lead to pathfinding defects shows that certain features of Dmp do normally act to improve guidance accuracy. Hence, Dmp seems to play a dual modulating role, and as detailed above, these roles are likely exerted by different mechanisms. On the one hand, Dmp helps leading nerves where they belong, but on the other hand, does normally not make full use of its capacity to do so. This opens up the possibility that Dmp is an agent that has multiple means to modulate axon guidance accuracy by buffering axonal navigation against perturbations. It will be interesting to see if excess NC1 is able to improve guidance phenotypes of mutants other than Dmp (Meyer, 2009).
Different effects of monomeric versus oligomeric endostatin have also been observed in C. elegans (Ackley, 2001), but in contrast the the current results, the C. elegans cle-1 neuronal migratory phenotype gets rescued by the putatively trimeric NC1 domain, whereas the ES domain does not provide rescue activity (Meyer, 2009).
Mono- and oligomeric ES have also been tested in vertebrate tissue culture systems for their effects on tubule formation and angiogenesis. Oligomeric ES is an inhibitor of tube morphogenesis in HUVEC (human umbilical vein endothelial cell) cultures, since an addition of oligomeric ES to the culture before tube formation inhibited the process, which is motility-dependent, whereas after completion of tube formation, oligomeric ES had a motogenic activity leading to dispersal of the tubular structures. Monomeric ES alone did not have any effect on cell behavior, but pre-incubation with ES inhibited the motogenic effect of oligomeric ES. In a CAM (chorioallantoic membrane) angiogenesis assay system, mono- and oligomeric ES of collagen XV and XVIII are converse in their inhibitory activity, that also depends on the cytokine used for stimulating angiogenesis. In another context, using IBE cells (intrahepatic biliary epithelial), monomeric ES even enhanced tubule formation. Hence, the action of mono- and trimeric ES seems to greatly depend on the biological context, on the cell types involved and additional signals they receive from their environment. The only common theme seems to be an antagonistic action of monomeric versus oligomeric ES. In this light, it is not surprising that the rescue activity that was observed on the Drosophila motor axon guidance phenotype is exactly opposite to the results for mechanosensory neuron (MSN) migration in C. elegans (Ackley, 2001). Notably, the biological process of axonal growth cone steering depends on directional cues and their correct interpretation, which is a fundamental difference to all other experimental systems that compared the action of mono- and oligomeric ES so far, as these systems all focused on the presence or absence of migratory activity per se. This includes the observations made for C. elegans MSN migration, as the phenotypes described by Ackley (2001) mainly consist of errors in migration distance of whole neurons and not direction. So the observations of ES effects in the Drosophila system are found in a novel biological context, and differences compared with other contexts are not surprising (Meyer, 2009).
For murine multiplexins biochemical interactions with several other ECM components have been observed. Together with the variety of biological effects described for multiplexin fragments, it seems likely that its exact mechanisms of action depends very much on the biological context. This study has provided the starting point for functionally integrating Drosophila Multiplexin into the biological process of motor axon pathfinding (Meyer, 2009).
Search PubMed for articles about Drosophila Multiplexin
Ackley, B. D., Crew, J. R., Elamaa, H., Pihlajaniemi, T., Kuo, C. J. and Kramer, J. M. (2001). The NC1/endostatin domain of Caenorhabditis elegans type XVIII collagen affects cell migration and axon guidance. J Cell Biol 152: 1219-1232. PubMed ID: 11257122
Chalfie, M. (2009). Neurosensory mechanotransduction. Nat Rev Mol Cell Biol 10: 44-52. PubMed ID: 19197331
Chang, J. H., Javier, J. A., Chang, G. Y., Oliveira, H. B. and Azar, D. T. (2005). Functional characterization of neostatins, the MMP-derived, enzymatic cleavage products of type XVIII collagen. FEBS Lett 579: 3601-3606. PubMed ID: 15978592
Davis, G. W. (2013). Homeostatic signaling and the stabilization of neural function. Neuron 80: 718-728. PubMed ID: 24183022abilization of neural function
Dhanabal, M., Ramchandran, R., Waterman, M. J., Lu, H., Knebelmann, B., Segal, M. and Sukhatme, V. P. (1999). Endostatin induces endothelial cell apoptosis. J Biol Chem 274: 11721-11726. PubMed ID: 10206987
Dickman, D. K. and Davis, G. W. (2009). The schizophrenia susceptibility gene dysbindin controls synaptic homeostasis. Science 326: 1127-1130. PubMed ID: 19965435
Felbor, U., Dreier, L., Bryant, R. A., Ploegh, H. L., Olsen, B. R. and Mothes, W. (2000). Secreted cathepsin L generates endostatin from collagen XVIII. EMBO J 19: 1187-1194. PubMed ID: 10716919
Hanai, J., Gloy, J., Karumanchi, S. A., Kale, S., Tang, J., Hu, G., Chan, B., Ramchandran, R., Jha, V., Sukhatme, V. P. and Sokol, S. (2002). Endostatin is a potential inhibitor of Wnt signaling. J Cell Biol 158: 529-539. PubMed ID: 12147676
Heljasvaara, R., Nyberg, P., Luostarinen, J., Parikka, M., Heikkila, P., Rehn, M., Sorsa, T., Salo, T. and Pihlajaniemi, T. (2005). Generation of biologically active endostatin fragments from human collagen XVIII by distinct matrix metalloproteases. Exp Cell Res 307: 292-304. PubMed ID: 15950618
Kim, Y. M., Hwang, S., Kim, Y. M., Pyun, B. J., Kim, T. Y., Lee, S. T., Gho, Y. S. and Kwon, Y. G. (2002). Endostatin blocks vascular endothelial growth factor-mediated signaling via direct interaction with KDR/Flk-1. J Biol Chem 277: 27872-27879. PubMed ID: 12029087
Liu, J., Schrank, B. and Waterston, R. H. (1996). Interaction between a putative mechanosensory membrane channel and a collagen. Science 273: 361-364. PubMed ID: 8662524
Meyer, F. and Moussian, B. (2009). Drosophila multiplexin (Dmp) modulates motor axon pathfinding accuracy. Dev Growth Differ 51: 483-498. PubMed ID: 19469789
Muller, M. and Davis, G. W. (2012). Transsynaptic control of presynaptic Ca(2)(+) influx achieves homeostatic potentiation of neurotransmitter release. Curr Biol 22: 1102-1108. PubMed ID: 22633807
O'Reilly, M. S., Boehm, T., Shing, Y., Fukai, N., Vasios, G., Lane, W. S., Flynn, E., Birkhead, J. R., Olsen, B. R. and Folkman, J. (1997). Endostatin: an endogenous inhibitor of angiogenesis and tumor growth. Cell 88: 277-285. PubMed ID: 9008168
Peixoto, R. T., Kunz, P. A., Kwon, H., Mabb, A. M., Sabatini, B. L., Philpot, B. D. and Ehlers, M. D. (2012). Transsynaptic signaling by activity-dependent cleavage of neuroligin-1. Neuron 76: 396-409. PubMed ID: 23083741
Seppinen, L. and Pihlajaniemi, T. (2011). The multiple functions of collagen XVIII in development and disease. Matrix Biol 30: 83-92. PubMed ID: 21163348
Wang, T., Hauswirth, A. G., Tong, A., Dickman, D. K. and Davis, G. W. (2014). Endostatin is a trans-synaptic signal for homeostatic synaptic plasticity. Neuron 83: 616-629. PubMed ID: 25066085
Wickstrom, S. A., Alitalo, K. and Keski-Oja, J. (2002). Endostatin associates with integrin alpha5beta1 and caveolin-1, and activates Src via a tyrosyl phosphatase-dependent pathway in human endothelial cells. Cancer Res 62: 5580-5589. PubMed ID: 12359771
Yamaguchi, N., Anand-Apte, B., Lee, M., Sasaki, T., Fukai, N., Shapiro, R., Que, I., Lowik, C., Timpl, R. and Olsen, B. R. (1999). Endostatin inhibits VEGF-induced endothelial cell migration and tumor growth independently of zinc binding. EMBO J 18: 4414-4423. PubMed ID: 10449407
Ylikarppa, R., Eklund, L., Sormunen, R., Muona, A., Fukai, N., Olsen, B. R. and Pihlajaniemi, T. (2003). Double knockout mice reveal a lack of major functional compensation between collagens XV and XVIII. Matrix Biol 22: 443-448. PubMed ID: 14614990
Younger, M. A., Muller, M., Tong, A., Pym, E. C. and Davis, G. W. (2013). A presynaptic ENaC channel drives homeostatic plasticity. Neuron 79: 1183-1196. PubMed ID: 23973209
date revised: 5 November 2014
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