Towards understanding the role of the negatively charged sugar sialic acid on embryonic development and kidney function
Biochemistry
Developmental Biology
Nephrology
Final Report Abstract
Cell surface sialylation of glycoproteins and glycolipids is pivotal for embryonic development and indispensable for kidney function. To unravel the underlying molecular mechanisms we interfered with sialylation by targeting a key enzyme in the biosynthesis, the CMP-sialic acid synthetase (Cmas). Ubiquitous depletion of Cmas in a knock-out mouse model was generated to investigate embryonic development of asialo embryos (Cmas-/-). We bypassed embryonic lethality by use of a conditional approach (podocyte-specific Cmas depletion, PCmas-/-) to address the function of sialylation for renal blood filtration. Embryonic development of asialo embryos: First, we ensured that Cmas-deficient cell lines showed asialo cell surfaces, to confirm that no redundant enzyme exists. While mice with an ubiquitous depletion of CMAS (Cmas-/-) die around embryonic day E9.5 in utero, asialo CMAS-deficient (Cmas-/-) murine embryonic stem cells (mESC) were indistinguishable from wildtype mESC in cell culture in terms of morphology and histology. No differences in viability, proliferation and differentiation into the three germ layers were observed in vitro. In line with this observation, the asialo embryo proper was able to develop somites, heart and neural structures in vivo, indicating that early embryonic development is independent from sialylation. However, intrauterine growth restriction with a marked variability in developmental deficits in the asialo-embryo proper, together with placental deficits and abnormalities in extraembryonic tissues (tissue derived from embryo, forms placenta and surrounding membranes) dominated the Cmas-/- phenotype. We could show that these alterations are caused by a complement 3 (C3) driven maternal attack against asialo-extraembryonic tissues, which are in direct contact with maternal blood at the fetal maternal interface. These tissues are heavily sialylated in the wildtype and protect the embryo (which due to its paternal components represents a semiallograft to the mother) from such a maternal attack. Thus, we could identify sialylation as an important component for self-recognition during pregnancy. Function of sialylation for renal blood filtration: Loss of cell surface sialylation on podocytes in PCmas-/- mice resulted in disturbance of podocyte homeostasis, podocyte loss a breakdown of the blood filtration barrier and finally in kidney failure. The phenotype of PCmas-/- mice was first reminiscent to human Minimal Change Disease (MCD) and then to Focal Segmental Glomerulosclerosis (FSGS), indicating that both might represent different stages of the same disease. Astonishingly we were not able to isolate asialo-podocytes from PCmas-/- mice, but could instead generate PCmas-/- podocytes by using CRISPR/Cas9 technology and showed that sialylation per se is not essential for podocyte viability, proliferation or differentiation. However, impaired podocyte adhesion to the glomerular basement membrane most likely contributed to disease development. Preliminary studies on human biopsies indicated segmental glomerular hyposialylation in FSGS patients. Our findings strongly implicate that loss of sialylation might be part of the complex process causing FSGS, a disease with a variety of underlying genetic or acquired causes that are still unknown in the majority of cases. Sialylation might therefore serve as both a diagnostic marker and a therapeutic target. A dietary sialic acid supplementation therapy, which has successfully improved muscle function in patients suffering from hereditary inclusion body myopathy, might provide a new therapeutic strategy to cure or delay FSGS and potentially other glomerulopathies.
Publications
- (2017) Sialylation is dispensable for early murine embryonic development in vitro. ChemBioChem. 18(13):1305-1316
Abeln M, Borst KM, Cajic S, Thiesler H, Kats E, Albers I, Kuhn M, Kaever V, Rapp E, Münster- Kühnel AK, Weinhold B
(See online at https://doi.org/10.1002/cbic.201700083) - (2019) Podocyte-Specific Sialylation-Deficient Mice Serve as a Model for Human FSGS. J Am Soc Nephrol. 30(6):1021-1035
Niculovic KM, Blume L, Wedekind H, Kats E, Albers I, Groos S, Abeln M, Schmitz J, Beuke E, Bräsen JH, Melk A, Schiffer M, Weinhold B, Münster-Kühnel AK
(See online at https://doi.org/10.1681/ASN.2018090951) - (2019) Sialic acid is a critical fetal defense against maternal complement attack. J Clin Invest. 129(1):422-436
Abeln M, Albers I, Peters-Bernard U, Flächsig-Schulz K, Kats E, Kispert A, Tomlinson S, Gerardy- Schahn R, Münster-Kühnel AK, Weinhold B
(See online at https://doi.org/10.1172/JCI99945)
