InteractiveFly: GeneBrief

Secreted Wg-interacting molecule: Biological Overview | References

Recruitment of stem cells is crucial for tissue repair. Although stem cell niches can provide important signals, little is known about mechanisms that coordinate the engagement of disseminated stem cells across an injured tissue. In Drosophila, adult brain lesions trigger local recruitment of scattered dormant neural stem cells suggesting a mechanism for creating a transient stem cell activation zone. This study found that injury triggers a coordinated response in neuro-glial clusters that promotes the spread of a neuron-derived stem cell factor via glial secretion of the lipocalin-like transporter Swim. Strikingly, swim is induced in a Hif1-α-dependent manner in response to brain hypoxia. Mammalian Swim (Lcn7) is also upregulated in glia of the mouse hippocampus upon brain injury. These results identify a central role of neuro-glial clusters in promoting neural stem cell activation at a distance, suggesting a conserved function of the HIF1-α/Swim/Wnt module in connecting injury-sensing and regenerative outcomes (Simoes, 2022).

Injury is known to stimulate diverse forms of plasticity, which serve to restore organ function. Many tissues harbor a small number of undifferentiated adult stem cells that are engaged in tissue turnover or become activated following injury to replace damaged cells. Some tissues, such as muscle or brain, contain mainly dormant stem cells that are not dividing and reside in a reversible state of quiescence. Niche cells in intimate contact with quiescent stem cells have been found to provide activating cues upon tissue damage. However, little is known how the activation of multiple dispersed stem cell units is coordinated to establish an adequate stem cell response zone across an injured tissue (Simoes, 2022).

Quiescent progenitor cells in muscle and the brain respond to injury in mammals, but also in fruit flies (Drosophila). This allows to harness the extensive genetic tools available in Drosophila to dissect injury-dependent stem cell activation. Although still unclear, the presence of dormant stem cells in short-lived insects indicates that these cells may play a beneficial role for tissue plasticity or repair upon predator attacks or inter-species aggressions (Simoes, 2022).

In the adult fly brain, experimental stab lesions to the optic lobes (OLs) or the central brain trigger a proliferative response resulting in local neurogenesis several days after injury (AI), which has been linked to activation of normally quiescent neural progenitor cells (qNPs). qNPs have also been found to promote adult brain plasticity in contexts unrelated to injury. On the other hand, stab lesions can also trigger glial divisions shortly after injury (Simoes, 2022).

Despite extensive knowledge on neural stem cell proliferation during fly development, the signals governing qNP activation in response to injury are unknown. A ubiquitous pulse of Drosophila Myc (dMyc) overexpression has been previously shown to promote qNP division, but the signals detected by qNPs remained enigmatic (Simoes, 2022).

In mammals, a wide variety of signals are known to regulate quiescent neural stem cells (qNSCs) in homeostatic conditions, whereas their response to tissue damage is less well understood. qNSCs are located in two main niches, the subventricular zone and the dentate gyrus of the hippocampus, buried within the brain. Upon brain injury, qNSCs only partially enter an activated state, and neuroblast recruitment to infarcted brain regions and local neurogenesis is limited (Simoes, 2022).

Strikingly, the initial consequences triggered by brain injury, which include neural cell death, upregulation of antioxidant defense, and c-Jun N-terminal kinase (JNK) stress signaling, are very conserved in flies and mice suggesting that injury sensing of qNSCs/qNPs may rely on common principles. In this work, injury-induced changes were studied in the adult fly brain leading to recruitment of isolated qNPs near the injury site. A crucial role was identified of damage-responsive neuro-glial clusters (DNGCs), which enable proliferation of distant qNPs by promoting an enlarged stem cell activation zone. Evidence is provided that these multicellular units orchestrate the spatial and temporal availability of an essential, but localized stem cell factor for qNPs via injury-stimulated secretion of the transport protein Swim. As Swim production is dependent on the injury-sensitive transcription factor HIF1-α, the identified mechanism may serve to spatially and temporary adjust the stem cell activation zone to the extent of damage suffered in a given tissue area, resulting in locally calibrated pulses of stem cell activity (Simoes, 2022).

How tissue damage is sensed and how the recruitment of multiple stem cell units is coordinated in response to local, heterogeneous tissue damage represents a fundamental question. By investigating how dispersed qNPs are locally recruited to injury, we have identified a mechanism that creates a defined zone of stem cell activation in the adult fly brain. The process is dependent on DNGCs, which depending on their size and possibly composition may regulate the extent by which a localized stem cell factor such as Wg/Wnt can travel to rare qNPs in the vicinity. Whereas the neuronal cells provide Wg/Wnt, the glial component supplies the carrier protein Swim, thereby promoting the dispersion of the signal. This cooperative interaction of two different cell types to gain long range function of Wg/Wnt is rather unique (Simoes, 2022).

At the cellular level, a model is proposed whereby injury-sensitive HIF1-α directs Swim synthesis in glial cells. Swim transporters diffuse and facilitate the spread of localized neural-derived Wg ligands, probably by binding to and shielding the lipid-residues of Wg/Wnt in the aqueous extracellular space. Mobile Wg-Swim complexes are consequently able to reach and activate qNPs in the injured brain domain. Wg/Wnt signal transduction and downstream upregulation of dmyc is shown to be crucial for the proliferation of this novel cell type. Overall, it is proposed that the described mechanism provides a means to match recruited stem cell activity to the spatial and temporal persistence of damage in the injured brain. Activation of dormant neural progenitors by high levels of Wg/Wnt Wg/Wnt signaling is probably one of the most universal pathways driving stem cell proliferation. Nevertheless, an understanding of Wg/Wnt signals for dormant stem cells has only recently emerged. Dormant muscle stem cells, for example, maintain quiescence by raising their threshold for Wnt transduction via cytoplasmic sequestration of &betal-catenin, and qNSC in the hippocampus do not rely on Wnt signaling under homeostasis but display a high capability to respond to Wnt in a graded manner when exposed. Similarly, the results demonstrate that qNPs start proliferating when high Wg/Wnt levels are provided in an autocrine fashion (Simoes, 2022).

Overall, the results suggest that activation of qNPs in the adult fly brain is mainly prevented by the low availability of Wg/Wnt ligands under homeostatic conditions. Although Wnt signaling normally occurs between adjacent cells, this study provides evidence that Wg functions at a tissue scale in the injured fly brain (Simoes, 2022).

This study describes the property of Swim to extend the signaling range of Wg/Wnt. Further research will be required to determine whether other stem cell-relevant factors can be transported by Swim. In zebrafish, reduced levels of Swim/Lcn7 produce craniofacial defects due to compromised Wnt3 signaling, highlighting a different context of Wnt/Swim interaction. A Wg/Swim interaction has previously been proposed in developing epithelia in flies, although the effect was not observed in a more recent study (Simoes, 2022).

Swim::mCherry is strongly expressed in the adult ovary germline of flies, in agreement with data from the recently published Fly Cell Atlas. Remarkably, swim KO flies showed reduced fertility, a phenotype which has also been reported for lcn7/tinagl1 KO mice (Takahashi, 2016). Interestingly, Swim expression in the germarium strongly overlapped with Wg::GFP, in line with previous findings describing a requirement of extensive Wg travel from the niche to distant follicular stem cells (Simoes, 2022).

Finally, this study elucidated how the Swim/Wg stem cell-activating signal is connected to damage sensing in the injured brain. Both in flies and mice, swim/lcn7 induction occurs in glial cells in response to brain injury. Remarkably, stroke-induced lcn7 induction is not observed in mouse brains, in which Hif1-α has been deleted from mature neurons and glial cells. This suggests that HIF1-α-dependent swim regulation is conserved in mammals (Simoes, 2022).

According to the current model, the damage responsiveness of stem cells is strongly gated by the availability of stable HIF1-α during acute hypoxia. Such a limited activation pulse would effectively restrict the mitotic effect of Swim/Wg complexes to the acute phase of repair, acting as a safeguard mechanism against overgrowth. Moreover, the hypoxia-dependent secretion of Swim would allow to temporally and locally fine-tune the realm of the stem cell activation zone to injury. Local oxygen concentrations modulate the activity of adult stem cells in different niches. In the fly larval OL, Dpn-expressing neural progenitors proliferate in a pronounced hypoxic environment, which bears parallels to the situation following brain injury (Simoes, 2022).

In the mammalian brain, injury-induced Wnt ligands may not efficiently reach qNSCs in distant neurogenic niches, resulting in poor stem cell activation. As such, Wnt pathway stimulating approaches hold promise as possible treatment for brain injury as they are known to support regeneration at several levels including qNSC activation, neurogenesis, and axon outgrowth. Increasing the mobility or stability of Wg/Wnts by Swim-like transporters may therefore represent a successful strategy to engage endogenous progenitors into regeneration. Given the fact that Wg/Wnts can support tissue renewal and regeneration in numerous tissues, the properties of Swim to transform a restricted tissue area into a temporary stem cell-activating zone, uncovered in this study may have important applications in regenerative medicine (Simoes, 2022).

Although the current experiments have revealed an impaired distribution of Wg in the injured brain in the absence of Swim transporters, it cannot be completely rule out that Swim may alter Wg function by other means than physical binding and direct transport. Ideally, the injury-induced formation of Wg-Swim complexes should be observable in the extracellular space. Although colocalization of Swim and Wg signals was detected, it was not possible to image Wg-Swim complexes at high resolution due to elevated background of Wg and mCherry antibodies when performing extracellular stainings. Overcoming these current limitations with overexpression systems or optimized immunodetection should allow to capture the dynamics of Wg-Swim interactions in injured brain tissue in the future (Simoes, 2022).

Evidence of Swim secretion and association with extracellular matrix in the Drosophila embryo

Secreted wingless-interacting protein (Swim) is the Drosophila ortholog gene of the mammalian Tubulointerstitial Nephritis Antigen Like 1 (TINAGL1). Swim and TINAGL1 proteins share a significant homology, including the somatomedin B and the predictive inactive C1 cysteine peptidase domains. In mammals, both TINAGL1 and its closely related homolog TINAG have been identified in basement membranes, where they may function as modulators of integrin-mediated adhesion. In Drosophila, Swim was initially identified in the eggshell matrix. Further biochemical analysis indicated that Swim binds to wingless (wg) in a lipid-dependent manner. This observation together with RNAi knockdown studies suggested that Swim is an essential cofactor of Wg-signalling. However, recent elegant genetic studies ruled out the possibility that Swim is required alone to facilitate Wg signalling in Drosophila, because flies without Swim are viable and fertile. This study used the UAS/Gal4 expression system together with confocal imaging to analyze the in vivo localization of a chimeric Swim-GFP in the developing Drosophila embryo. The data fully support the notion that Swim is an extracellular matrix component that upon ectopic expression is secreted and preferentially associates with the basement membranes of various organs and with the specialized tendon matrix at the muscle attachment sites (MAS). In conclusion, Swim is an extracellular matrix component, and it is possible that Swim exhibits overlapping functions in concert with other undefined components (Kaltezioti, 2021).

Secreted Wingless-interacting molecule (Swim) promotes long-range signaling by maintaining Wingless solubility

Lipid-modified Wnt/Wingless (Wg) proteins can signal to their target cells in a short- or long-range manner. How these hydrophobic proteins travel through the extracellular environment remains an outstanding question. This study reports on a Wg binding protein, Secreted Wg-interacting molecule (Swim), that facilitates Wg diffusion through the extracellular matrix. Swim, a putative member of the Lipocalin family of extracellular transport proteins, binds to Wg with nanomolar affinity in a lipid-dependent manner. In quantitative signaling assays, Swim is sufficient to maintain the solubility and activity of purified Wg. In Drosophila, swim RNAi phenotypes resemble wg loss-of-function phenotypes in long-range signaling. It is proposed that Swim is a cofactor that promotes long-range Wg signaling in vivo by maintaining the solubility of Wg (Mulligan, 2012).

Search PubMed for articles about Drosophila Swim

Brown, L. J., Alawoki, M., Crawford, M. E., Reida, T., Sears, A., Torma, T. and Albig, A. R. (2010). Lipocalin-7 is a matricellular regulator of angiogenesis. PLoS One 5(11): e13905. PubMed ID: 21085487

Kaltezioti, V., Vakaloglou, K. M., Charonis, A. S. and Zervas, C. G. (2021). Evidence of Swim secretion and association with extracellular matrix in the Drosophila embryo. Int J Dev Biol. PubMed ID: 34881800

Lucas, K. J., Roy, S., Ha, J., Gervaise, A. L., Kokoza, V. A. and Raikhel, A. S. (2015). MicroRNA-8 targets the Wingless signaling pathway in the female mosquito fat body to regulate reproductive processes. Proc Natl Acad Sci U S A 112(5): 1440-1445. PubMed ID: 25605933

Mulligan, K. A., Fuerer, C., Ching, W., Fish, M., Willert, K. and Nusse, R. (2012). Secreted Wingless-interacting molecule (Swim) promotes long-range signaling by maintaining Wingless solubility. Proc Natl Acad Sci U S A 109(2): 370-377. PubMed ID: 22203956

Simoes, A. R., Neto, M., Alves, C. S., Santos, M. B., Fernandez-Hernandez, I., Veiga-Fernandes, H., Brea, D., Dur, I., Encinas, J. M. and Rhiner, C. (2022). Damage-responsive neuro-glial clusters coordinate the recruitment of dormant neural stem cells in Drosophila. Dev Cell. PubMed ID: 35716661

Takahashi, A., Rahim, A., Takeuchi, M., Fukui, E., Yoshizawa, M., Mukai, K., Suematsu, M., Hasuwa, H., Okabe, M. and Matsumoto, H. (2016). Impaired female fertility in tubulointerstitial antigen-like 1-deficient mice. J Reprod Dev 62(1): 43-49. PubMed ID: 26522507

Wang, X. and Page-McCaw, A. (2014). A matrix metalloproteinase mediates long-distance attenuation of stem cell proliferation. J Cell Biol 206(7): 923-936. PubMed ID: 25267296

date revised: 16 November, 2022

Home page: The Interactive Fly © 2011 Thomas Brody, Ph.D.