Archives

  • 2018-07
  • 2018-10
  • 2018-11
  • 2019-04
  • 2019-05
  • 2019-06
  • 2019-07
  • 2019-08
  • 2019-09
  • 2019-10
  • 2019-11
  • 2019-12
  • 2020-01
  • 2020-02
  • 2020-03
  • 2020-04
  • 2020-05
  • 2020-06
  • 2020-07
  • 2020-08
  • 2020-09
  • 2020-10
  • 2020-11
  • 2020-12
  • 2021-01
  • 2021-02
  • 2021-03
  • 2021-04
  • 2021-05
  • 2021-06
  • 2021-07
  • 2021-08
  • 2021-09
  • 2021-10
  • 2021-11
  • 2021-12
  • 2022-01
  • 2022-02
  • 2022-03
  • 2022-04
  • 2022-05
  • 2022-06
  • 2022-07
  • 2022-08
  • 2022-09
  • 2022-10
  • 2022-11
  • 2022-12
  • 2023-01
  • 2023-02
  • 2023-03
  • 2023-04
  • 2023-05
  • 2023-06
  • 2023-07
  • 2023-08
  • 2023-09
  • 2023-10
  • 2023-11
  • 2023-12
  • 2024-01
  • 2024-02
  • 2024-03
  • 2024-04

    • We also examined Smad as

    2018-11-07

    We also examined Smad7 as an important regulator of the TGF-β signaling pathway as it inhibits phosphorylation of Smad2 by the TGF-β receptors, thereby acting as a negative feedback loop in this pathway to prevent deregulation. Without Smad7, TGF-β receptor activation cannot be inhibited -explaining for example increased pSmad2- but downstream transcriptional activation remains impaired since Smad3 is lacking (See Fig. 8, left panel for a schematic overview). In agreement, we found no upregulation of Smad7 in Smad3−/− aortas, or of other downstream genes such as PAI-1. When we now compare the findings between Smad3−/− and Fibulin-4 animals, although in both mouse models aneurysms are formed, the underlying cause and phenotypical consequences are quite different (Fig. 8). In Fibulin-4 aortas it is thought that TGF-β is released from the matrix, resulting in activation of TGF-β signaling pathway, which eventually also results in activated downstream transcription. However, in Smad3−/− animals, although there can be activation of TGF-β receptors, as also shown by increased pSmad2 signal, transcriptional activation downstream of Smad3 is impaired. One important downstream gene that is normally transcribed is SMAD7, which is a potent inhibitor of the TGF-β signaling pathway. We find decreased Smad7 transcripts, which might explain the continued activation of the TGF-β receptor. Furthermore, this altered signaling and transcriptional pattern might also lead to an altered ‘appearance’ of the VSMCs for the immune system, thereby attracting immune cells. In accord, Smad3−/− animals suffered from so-called ‘sterile’ infections in the aortic wall, as derived from the HE stained sections showing infiltrations of immune cells. As these infiltrations were only observed after dilatation of the ML355 became apparent, this might suggest that this immune reaction is triggered after the aneurysm is formed. Indeed, Ye et al. showed that administration of anti-GM-CSF antibody to Smad3−/− mice reduced inflammation and also diminished aorta dilation (Ye et al., 2013). This would also be in agreement with the fact that we only observed MMP activity in aortas with an aneurysm that could be traced back to the immune cells adjacent to the adventitia of the aortic wall. Moreover, the fact that here the MMP activation is derived from immune cells rather than from VSMCs, and is only present at late stages of aneurysm formation, after the immune infiltrates are present, would argue that MMP activity is a good marker for aneurysm formation caused by ECM deficiencies, but not for those due to SMAD3 mutations. The longitudinal echocardiograms showed dilatations with a rapid increase of the aneurysm within a very short period of time. It could be that the increased upstream TGF-β receptor activation, together with the lack of collagen and ECM accumulation results in dilatations; the structural integrity fails progressively. This, together with a possibly altered appearance of the VSMC both in structure as well as transcriptional profile, could attract immune cells, which start cleaning up the detected ‘vascular damage’ at the expense of macrophage-induced deterioration of the already fragile aortic wall. This would then explain the rapid and aggressive aneurysmal growth. Most genetic studies have been focusing on delineating the clinical phenotype associated with SMAD3 mutations (Hilhorst-Hofstee et al., 2013; Aubart et al., 2014; Fitzgerald et al., 2014; Wischmeijer et al., 2013; van der Linde et al., 2012). Despite the increasing number of reported mutations, functional studies indicating the pathological effect of these mutations are lacking. Existing experimental data and molecular predictions suggest that SMAD3 mutations are mainly loss of function (van de Laar et al., 2011; Aubart et al., 2014). Many reported mutations lead to frame shifts, deletions and likely nonsense mediated decay. Others are perturbing the heterodimer formation SMAD3/4 or leading to nonfunctional complexes. Yet, a dominant-negative effect of some mutations cannot be excluded.