SMAD4: A critical regulator of cardiac neural crest cell fate and vascular smooth muscle development

BACKGROUND

During embryogenesis, cardiac neural crest-derived cells (NCs) migrate into the pharyngeal arches and give rise to the vascular smooth muscle cells (vSMCs) of the pharyngeal arch arteries (PAAs). vSMCs are critical for the remodeling of the PAAs into their final adult configuration, giving rise to the aortic arch and its arteries (AAAs).

RESULTS

We investigated the role of SMAD4 in NC-to-vSMC differentiation using lineage-specific inducible mouse strains. We found that the expression of SMAD4 in the NC is indelible for regulating the survival of cardiac NCs. Although the ablation of SMAD4 at E9.5 in the NC lineage led to a near-complete absence of NCs in the pharyngeal arches, PAAs became invested with vSMCs derived from a compensatory source. Analysis of AAA development at E16.5 showed that the alternative vSMC source compensated for the lack of NC-derived vSMCs and rescued AAA morphogenesis.

CONCLUSIONS

Our studies uncovered the requisite role of SMAD4 in the contribution of the NC to the pharyngeal arch mesenchyme. We found that in the absence of SMAD4+ NCs, vSMCs around the PAAs arose from a different progenitor source, rescuing AAA morphogenesis. These findings shed light on the remarkable plasticity of developmental mechanisms governing AAA development.

SMAD4: A Critical Regulator of Cardiac Neural Crest Cell Fate and Vascular Smooth Muscle Differentiation

Background

The pharyngeal arch arteries (PAAs) are precursor vessels which remodel into the aortic arch arteries (AAAs) during embryonic cardiovascular development. Cardiac neural crest cells (NCs) populate the PAAs and differentiate into vascular smooth muscle cells (vSMCs), which is critical for successful PAA-to-AAA remodeling. SMAD4, the central mediator of canonical TGFβ signaling, has been implicated in NC-to-vSMC differentiation; however, its distinct roles in vSMC differentiation and NC survival are unclear.

Results

Here, we investigated the role of SMAD4 in cardiac NC differentiation to vSMCs using lineage-specific inducible mouse strains in an attempt to avoid early embryonic lethality and NC cell death. We found that with global SMAD4 loss, its role in smooth muscle differentiation could be uncoupled from its role in the survival of the cardiac NC in vivo . Moreover, we found that SMAD4 may regulate the induction of fibronectin, a known mediator of NC-to-vSMC differentiation. Finally, we found that SMAD4 is required in NCs cell-autonomously for NC-to-vSMC differentiation and for NC contribution to and persistence in the pharyngeal arch mesenchyme.

Conclusions

Overall, this study demonstrates the critical role of SMAD4 in the survival of cardiac NCs, their differentiation to vSMCs, and their contribution to the developing pharyngeal arches.

A new mechanism of fibronectin fibril assembly revealed by live imaging and super-resolution microscopy

Fibronectin (Fn1) fibrils have long been viewed as continuous fibers composed of extended, periodically aligned Fn1 molecules. However, our live-imaging and single-molecule localization microscopy data are inconsistent with this traditional view and show that Fn1 fibrils are composed of roughly spherical nanodomains containing six to eleven Fn1 dimers. As they move toward the cell center, Fn1 nanodomains become organized into linear arrays, in which nanodomains are spaced with an average periodicity of 105±17 nm. Periodical Fn1 nanodomain arrays can be visualized between cells in culture and within tissues; they are resistant to deoxycholate treatment and retain nanodomain periodicity in the absence of cells. The nanodomain periodicity in fibrils remained constant when probed with antibodies recognizing distinct Fn1 epitopes or combinations of antibodies recognizing epitopes spanning the length of Fn1. Treatment with FUD, a peptide that binds the Fn1 N-terminus and disrupts Fn1 fibrillogenesis, blocked the organization of Fn1 nanodomains into periodical arrays. These studies establish a new paradigm of Fn1 fibrillogenesis. This article has an associated First Person interview with the first author of the paper.

[Peripheral benzodiazepine receptors and their possible biological role]

While specific BZD receptors in the central nervous system were identified using [3H]diazepam as radioligand, additional binding sites have been found also in peripheral tissues. These so-called "peripheral-type benzodiazepine receptors" occur ubiquitous in various tissues. Different actions on biological processes have been described to be connected with peripheral-type benzodiazepine receptors, but their definite physiological role has not yet been clarified. this review covers basic findings of this second binding site for benzodiazepines, trying to present a survey about those binding sites and their putative biological role.

Characterization of peripheral-type benzodiazepine binding sites on human lymphocytes and lymphoma cell lines and their role in cell growth

Peripheral-type benzodiazepine binding sites (PBRs) are ubiquitous in mammalian tissues. However, the physiological role of PBRs has not yet been clarified. In this study we characterized a saturable and high affinity binding site for [3H]Pk 11195 (isoquinoline carboxamide derivative) on human lymphocytes and different lymphoma cell lines. Binding parameters of the human T-lymphoma cell line CCRF-CEM came closest to values for lymphocyte binding. Thus, the CCRF-CEM cell line appears to be a suitable lymphocyte cell model for further study of PBRs. To evaluate the pharmacological specificity of binding to human lymphocytes and CCRF-CEM cells we investigated the potency of different ligands to displace [3H]Pk 11195 from its binding site. Pk 11195 was found to be the most potent inhibitor followed by 4'-chlorodiazepan (Ro5-4864) and diazepam (range of inhibition constants from 6.7 x 10(-9) M to 3.6 x 10(-7) M), whereas ligands specific for the central-type receptor like clonazepam and flumazenil had no displacing potency in the tested concentration range (10(-10)-10(-4) M). Since it was assumed that PBRs might be involved in the regulation of cell growth and differentiation, we studied the influence of PBR ligands on cell growth and survival using a quantitative colorimetric assay (MTT). Ligands which bind selectively to PBRs inhibited cell multiplication in vitro. However, half-effective concentrations (EC50) were in the micromolar range and above therapeutic in vivo concentrations (range of EC50 values from 2.4 x 10(-5) M to 1.5 x 10(-4) M). Clonazepam and flumazenil had no inhibiting potency in the tested concentration range (10(-10)-10(-4) M). Although the difference between values for displacing potency and ability to inhibit cell multiplication cannot be explained as yet, it is interesting that all PBR-ligands followed the same sequence in displacing [3H]Pk 11195 and inhibiting cell multiplication and that central type ligands were ineffective in both assays. This association suggest a mediating role of PBR binding in cell growth.