Ischemic insults to the heart and brain, i.e., myocardial and cerebral infarction, respectively, are amongst the leading causes of death worldwide. While there are therapeutic options to allow reperfusion of ischemic myocardial and brain tissue by reopening obstructed vessels, mitigating primary tissue damage, post-infarction inflammation and tissue remodeling can lead to secondary tissue damage. Similarly, ischemia in retinal tissue is the driving force in the progression of neovascular eye diseases such as diabetic retinopathy (DR) and age-related macular degeneration (AMD), which eventually lead to functional blindness, if left untreated. Intriguingly, the easily observable retinal blood vessels can be used as a window to the heart and brain to allow judgement of microvascular damages in diseases such as diabetes or hypertension. The complex neuronal and endocrine interactions between heart, retina and brain have also been appreciated in myocardial infarction, ischemic stroke, and retinal diseases. To describe the intimate relationship between the individual tissues, we use the terms heart-brain and brain-retina axis in this review and focus on the role of transforming growth factor β (TGFβ) and neurotrophins in regulation of these axes under physiologic and pathologic conditions. Moreover, we particularly discuss their roles in inflammation and repair following ischemic/neovascular insults. As there is evidence that TGFβ signaling has the potential to regulate expression of neurotrophins, it is tempting to speculate, and is discussed here, that cross-talk between TGFβ and neurotrophin signaling protects cells from harmful and/or damaging events in the heart, retina, and brain.

Transforming growth factor β (TGFβ) signaling has manifold functions such as regulation of cell growth, differentiation, migration, and apoptosis. Moreover, there is increasing evidence that it also acts in a neuroprotective manner. We recently showed that TGFβ receptor type 2 (Tgfbr2) is upregulated in retinal neurons and Müller cells during retinal degeneration. In this study we investigated if this upregulation of TGFβ signaling would have functional consequences in protecting retinal neurons. To this end, we analyzed the impact of TGFβ signaling on photoreceptor viability using mice with cell type-specific deletion of Tgfbr2 in retinal neurons and Müller cells (Tgfbr2^ΔOC) in combination with a genetic model of photoreceptor degeneration (VPP). We examined retinal morphology and the degree of photoreceptor degeneration, as well as alterations of the retinal transcriptome. In summary, retinal morphology was not altered due to TGFβ signaling deficiency. In contrast, VPP-induced photoreceptor degeneration was drastically exacerbated in double mutant mice (Tgfbr2^ΔOC; VPP) by induction of pro-apoptotic genes and dysregulation of the MAP kinase pathway. Therefore, TGFβ signaling in retinal neurons and Müller cells exhibits a neuroprotective effect and might pose promising therapeutic options to attenuate photoreceptor degeneration in humans.

Hereditary retinal degenerations like retinitis pigmentosa (RP) are among the leading causes of blindness in younger patients. To enable in vivo investigation of cellular and molecular mechanisms responsible for photoreceptor cell death and to allow testing of therapeutic strategies that could prevent retinal degeneration, animal models have been created. In this study, we deeply characterized the transcriptional profile of mice carrying the transgene rhodopsin V20G/P23H/P27L (VPP), which is a model for autosomal dominant RP. We examined the degree of photoreceptor degeneration and studied the impact of the VPP transgene-induced retinal degeneration on the transcriptome level of the retina using next generation RNA sequencing (RNASeq) analyses followed by weighted correlation network analysis (WGCNA). We furthermore identified cellular subpopulations responsible for some of the observed dysregulations using in situ hybridizations, immunofluorescence staining, and 3D reconstruction. Using RNASeq analysis, we identified 9256 dysregulated genes and six significantly associated gene modules in the subsequently performed WGCNA. Gene ontology enrichment showed, among others, dysregulation of genes involved in TGF-β regulated extracellular matrix organization, the (ocular) immune system/response, and cellular homeostasis. Moreover, heatmaps confirmed clustering of significantly dysregulated genes coding for components of the TGF-β, G-protein activated, and VEGF signaling pathway. 3D reconstructions of immunostained/in situ hybridized sections revealed retinal neurons and Müller cells as the major cellular population expressing representative components of these signaling pathways. The predominant effect of VPP-induced photoreceptor degeneration pointed towards induction of neuroinflammation and the upregulation of neuroprotective pathways like TGF-β, G-protein activated, and VEGF signaling. Thus, modulation of these processes and signaling pathways might represent new therapeutic options to delay the degeneration of photoreceptors in diseases like RP.

WNT7A and WNT7B, secreted by neural cells, are essential regulators of developmental brain angiogenesis and blood-brain barrier integrity. In brain endothelial cells, WNT7 proteins activate β-catenin signaling through the ligand-specific receptor complex GPR124-RECK and classical WNT receptors of the FZD and LRP families. Previous studies suggested that WNT7 isoforms assemble a GPR124-RECK-FZD-LRP5/6 multi-receptor complex for signaling. However, direct biochemical evidence for this complex and its signaling mechanisms remains elusive. Here, we investigated the formation and signaling mechanisms of WNT7 co-receptor complexes in brain endothelial cells using CRISPR/Cas9, biochemical analyses, and cell-based assays. Unexpectedly, cells with knockout of all FZD isoforms retained ∼25% of WNT7 responsiveness, whereas classical WNT3A signaling was completely abolished. Similarly, knockout of all Dvl paralogs, key mediators of FZD signaling, preserved ∼50% of WNT7 signaling activity but fully blocked WNT3A responses. In contrast, knockout of Gpr124, Reck, or Lrp5/6 completely abrogated WNT7 signaling. While both WNT7A and WNT3A triggered LRP6 phosphorylation, only WNT3A induced DVL2 phosphorylation. Biochemical analyses revealed WNT7-dependent recruitment of LRP5/6, but not FZD, to the GPR124-RECK heterodimer, forming a GPR124-RECK-WNT7-LRP5/6 complex. In GPR124-deficient cells, WNT7 proteins still assembled a RECK-WNT7-LRP5/6 core complex, yet this complex lacked signaling activity and LRP6 phosphorylation. Clustering of RECK-WNT7-LRP5/6 complexes with recombinant dimeric GPR124 ectodomain or a RECK antibody partially restored signaling, suggesting that GPR124 mediates formation of higher-order complexes. Our findings indicate that WNT7 signaling in brain endothelial cells is driven by distinct co-receptor complexes: a FZD-independent GPR124-RECK-LRP5/6 complex and FZD-dependent complexes that likely act in synergy.

Transition from the transient hyaloid vasculature to the mature retinal vascular network is a critical step in mammalian eye development. While WNT7B signaling in endothelial cells regulates programmed hyaloid vessel regression, the specific receptors and downstream mechanisms remain poorly defined. Here, we identify GPR124, a ligand-specific co-receptor for WNT7A/B, as a key regulator of this process. Using an endothelial-specific, tamoxifen-inducible Gpr124 knockout mouse model, we show that loss of GPR124 in hyaloid endothelium markedly impairs postnatal hyaloid vessel clearance. Notably, GPR124 function in the developing eye was highly compartment-specific: although crucial for hyaloid vessel regression, it was dispensable for retinal angiogenesis, despite being expressed in both vascular beds. Gpr124 deletion led to downregulation of several established WNT target genes in hyaloid endothelial cells, consistent with a role for GPR124 in mediating WNT/β-catenin signaling, as previously shown in brain endothelium. While GPR124 was not required for induction of isolated apoptotic events in hyaloid vessels, it promoted segmental vessel regression, characterized by linear clusters of apoptotic endothelial cells. Single-cell RNA sequencing of hyaloid endothelial cells, combined with immunostaining, revealed transcriptional changes consistent with partial endothelial-mesenchymal transition (EndMT). Our findings identify GPR124 as a key mediator of hyaloid vessel regression and suggest that GPR124-dependent WNT/β-catenin signaling and EndMT may contribute to vascular remodeling during ocular development.