The fibronectin leucine-rich repeat transmembrane protein Flrt2 is required in the epicardium to promote heart morphogenesis

Abstract

SUMMARYThe epicardium, the outermost tissue layer that envelops the developing heart and provides essential trophic signals for the myocardium, derives from the pro-epicardial organ (PEO). Two of the three members of the Flrt family of transmembrane glycoproteins, Flrt2 and Flrt3, are strongly co-expressed in the PEO. However, beginning at around day 10 of mouse development, following attachment and outgrowth, Flrt3 is selectively downregulated, and only Flrt2 is exclusively expressed in the fully delaminated epicardium. The present gene-targeting experiments demonstrate that mouse embryos lacking Flrt2 expression arrest at mid-gestation owing to cardiac insufficiency. The defects in integrity of the epicardial sheet and disturbed organization of the underlying basement membrane closely resemble those described in Flrt3-deficient embryos that fail to maintain cell-cell contacts in the anterior visceral endoderm (AVE) signalling centre that normally establishes the A-P axis. Using in vitro and in vivo reconstitution assays, we demonstrate that Flrt2 and Flrt3 are functionally interchangeable. When acting alone, either of these proteins is sufficient to rescue functional activities in the AVE and the developing epicardium.

Structural Basis of Latrophilin-FLRT-UNC5 Interaction in Cell Adhesion

“…Fibronectin leucine-rich repeat transmembrane (FLRTs) proteins are cell-surface molecules that contribute to early embryonic, vascular, and neural development (Egea et al, 2008; Leyva-Diaz et al, 2014; Maretto et al, 2008; Muller et al, 2011; O’Sullivan et al, 2012; Yamagishi et al, 2011). FLRTs are type-I membrane proteins with extracellular regions consisting of a leucine-rich repeat (LRR) domain with ten LRR repeats, a fibronectin type 3 domain followed by a juxtamembrane linker (Lacy et al, 1999) (Figure 1A).…”

Latrophilins Function as Heterophilic Cell-adhesion Molecules by Binding to Teneurins

“…FLRTs were implicated in FGF signaling (22) and cell adhesion during development (23). Knock-out experiments established that both FLRT2 and FLRT3 are essential for embryonic development and likely required for normal development of the heart (23,24). In neurons, FLRT3 mRNA is up-regulated after nerve injury (25)(26)(27).…”

Neural Cell Adhesion Molecules Belonging to the Family of Leucine-Rich Repeat Proteins

“…In contrast, the primary sequences of human FLRT1, -2, and -3 exhibit similarities of only 16-20 % (Lacy et al 1999 ). Despite these moderate homologies FLRT2 and -3 are reported to be functionally interchangeable (Muller et al 2011 ).…”

“…In mouse brain, FLRT3 is around P14 highly expressed in the dentate gyrus (DG) and the CA3 region, but not the CA1 region, of the hippocampus (O'Sullivan et al 2012 ). For detailed studies of FLRT3 expression during chicken limb development and FLRT2 during mouse heart and craniofacial development, see Smith and Tickle ( 2006 ), Tomas et al ( 2011 ) and Muller et al ( 2011), Maretto et al ( 2008 and Gong et al ( 2009 ), respectively. In adult rat brain FLRT3 is ubiquitously expressed.…”

“…Overexpression of the protein promotes neuritogenesis in, e.g., cerebellar granule and DRG neurons (Tsuji et al 2004 ;Robinson et al 2004 ), and during chicken development the protein plays a role in the regulation of cell adhesion and establishment of the dorsal-ventral position of the apical ectodermal ridge (Smith and Tickle 2006 ). In mouse, knockout of FLRT2 or -3 during development leads to a disorganized basement membrane in the anterior visceral endoderm region and defects in, e.g., headfold fusion and heart development (Muller et al 2011 ;Maretto et al 2008 ;Egea et al 2008 ). Moreover, studies in mouse and rat have shown that FLRT3 expression is increased considerably in response to nerve injury, suggesting that FLRT3 plays a part in neuronal recovery and regeneration (Tsuji et al 2004 ;Tanabe et al 2003 ).…”