SFRPs act as negative modulators of ADAM10 to regulate retinal neurogenesis

Abstract

It is well established that embryonic mouse retinal neurogenesis requiresNotch and Wnt signaling are also required for the development of vertebrate neural retina.This structure develops from a neuroepithelium composed of multipotent progenitors, which undergo a series of competence states to give rise to six neuronal and one glial cell types 2 . As progenitor cells produce the various cell types, Notch through lateral inhibition, maintains neighboring cells in a multipotent, proliferative state, ensuring that sufficient numbers of progenitors are retained for consecutive waves of neurogenesis. Thus, downregulation of Notch is a prerequisite for retinal neuronal differentiation 2 .Wnt/βcatenin signaling has also been implicated in the proliferation of vertebrate retinal precursors. However, in the mouse embryonic neural retina this function is limited to progenitor cells located in the periphery 3, 4 . In contrast, Wnt/βcatenin signaling is not active in the central retina and cell proliferation and differentiation proceed normally in mice with conditional deletion of βcatenin in the neural retina, although retinal lamination is altered 5 .Similarly, retinal specific inactivation of Fzd5, a non-canonical Wnt receptor mostly impacts on retinal vasculature formation but has no effect on neurogenesis 6 By analyzing the functional consequences of Sfrp1 and Sfrp2 compound inactivation during mouse retinal neurogenesis, we demonstrate here a novel and Wnt-independent role of Sfrps in the regulation of Notch signaling. We explain this finding by demonstrating that Sfrps can bind and downregulate the activity of ADAM10, a metalloprotease with multiple substrates including Notch, N-cadherin and APP. 4 RESULTS Sfrp1 and Sfrp2 are essential for proper eye developmentSfrp1 and Sfrp2 are expressed during murine eye development with a complementary pattern that includes all eye structures 7 . Sfrp1 transcripts are localized to the optic cup periphery and the retina pigmented epithelium from E10.5, while Sfrp2 is predominant in the neural retina (Fig. S1). Despite restricted mRNA expression, Sfrp proteins efficiently diffuse in the extracellular space 17 and Sfrp1 was immunodetected, albeit at low levels, also in the neural retina (Fig. S1), supporting the proposed Sfrp functional redundancy 9, 11, 12 .Accordingly, the eye of Sfrp1 and Sfrp2 single null embryos appeared histologically normal.In contrast at E16.5, the latest viable stage, the eyes of Sfrp1 -/-;Sfrp2 -/-compound mutants (n=20) were smaller than those of control littermates (n=30) with morphological visible alterations, including dorsal peripheral defects, reduction of the lens size, abnormal cornea and eye lid formation, increased thickness of the neural retina and abnormal vitreal accumulation of mesenchyme-derived angioblasts that normally form the hyaloid artery, the major vascular structure of the embryonic eye (Fig. S2). Inactivation of Sfrp1/2 alters retinal neurogenesisMultipotent progenitors in the neural retina generates neuronal and one glial cells wi...

Meis1 coordinates a network of genes implicated in eye development and microphthalmia

“…Nevertheless, whether direct or not, the poor Notch pathway activation, in conjunction with decreased expression of Ccnd1 and other microphthalmia-associated genes, could explain the Meis1 −/− microphthalmic phenotype. Indeed, Notch signaling controls the number of progenitors entering retinal differentiation: loss of Notch function forestalls retinal neurogenesis (Jadhav et al, 2006), whereas abnormal Notch receptor activation transiently increases retinal proliferation and differentiation (Esteve et al, 2011). Notably, Meis1 action on the Notch pathway and on microphthalmia-related genes could be linked since Sox1, Sox2 and Notch signaling have been shown to regulate each other's activity in various contexts (Genethliou et al, 2009;Kan et al, 2004;Neves et al, 2011).…”

“…Furthermore, the number of Otx2 + retinal progenitors (supplementary material Fig. S1G,I) (Bovolenta et al, 1997) and that of Tuj1 + or islet 1/2 (Isl1/2) + differentiating neurons (Esteve et al, 2011) was reduced in E12/E13 Meis1 −/− retinas (Fig. 2J,L,M,O).…”

Secreted and Transmembrane Wnt Inhibitors and Activators

“…sFRP1, for example, binds to RANKL, a member of the tumor necrosis factor family, thereby inhibiting osteoclast formation (Hausler et al 2004). sFRP1 also interacts with and inhibits the metalloprotease ADAM10, thus acting as a negative modulator of Notch signaling to regulate retinal neurogenesis (Esteve et al 2011). Sizzled binds to and inhibits the activity of BMP1/Tolloid, a metalloprotease that cleaves the BMP antagonist chordin, thereby inhibiting BMP signaling (Lee et al 2006).…”

Notch signaling: simplicity in design, versatility in function

“…MAML represents another nexus between Notch and Wnt signaling, and, in addition to its role in stabilizing Notch ICD-CSL interactions, MAML has now been shown to bind to both GSK3 (Saint Just Ribeiro et al, 2009) and Notch signaling; sFRPs bind to ADAM10, downregulating its activity and thus inhibiting Notch signaling. This has consequences for retinal neurogenesis, a process known to be Notch dependent but Wnt independent (Esteve et al, 2011). …”