Retinal Pericytes: Characterization of Vascular Development-Dependent Induction Time Points in an Inducible NG2 Reporter Mouse Model

Retinal astrocyte morphology predicts integration of vascular and neuronal architecture

Astrocytes are important regulators of blood flow and play a key role in the response to injury and disease in the central nervous system (CNS). Despite having an understanding that structural changes to these cells have consequences for local neurovascular physiology, individual astrocyte morphology remains largely unexplored in the retina. Here, we used MORF3 mice to capture full membranous morphology for over fifteen hundred individual astrocytes in the mouse retina, a highly metabolically active component of the CNS. We demonstrate that retinal astrocytes have been misrepresented as stellate in morphology due to marker use like GFAP and S100β which underestimates cell complexity. We also find that astrocytes contain recurring morphological motifs which are predictive of the underlying neurovascular architecture of the inner retina and suggestive of function. These motifs predict fine sampling and integration of retinal ganglion cell electrical activity with consequences for blood flow regulation. Additionally, our data shows that astrocytes participate in neurovascular interactions to a much greater degree than currently reported. 100% of cells contact the vasculature through one of three mutually exclusive classes of connections. Similarly, 100% of cells contact some neuronal element, be it an RGC axon or soma. Finally, we report that astrocyte morphology depends on retinal eccentricity, with cells appearing compressed near the nerve head and in the periphery. These results reveal a large degree of astrocyte morphological complexity that informs their contribution to neurovascular coupling in the retina.

Pericyte-derived cells participate in optic nerve scar formation

Introduction: Pericytes (PCs) are specialized cells located abluminal of endothelial cells on capillaries, fulfilling numerous important functions. Their potential involvement in wound healing and scar formation is achieving increasing attention since years. Thus, many studies investigated the participation of PCs following brain and spinal cord (SC) injury, however, lacking in-depth analysis of lesioned optic nerve (ON) tissue. Further, due to the lack of a unique PC marker and uniform definition of PCs, contradicting results are published. Methods: In the present study the inducible PDGFRβ-P2A-CreER^T2-tdTomato lineage tracing reporter mouse was used to investigate the participation and trans-differentiation of endogenous PC-derived cells in an ON crush (ONC) injury model, analyzing five different post lesion time points up to 8 weeks post lesion. Results: PC-specific labeling of the reporter was evaluated and confirmed in the unlesioned ON of the reporter mouse. After ONC, we detected PC-derived tdTomato⁺ cells in the lesion, whereof the majority is not associated with vascular structures. The number of PC-derived tdTomato⁺ cells within the lesion increased over time, accounting for 60-90% of all PDGFRβ⁺ cells in the lesion. The presence of PDGFRβ⁺tdTomato^- cells in the ON scar suggests the existence of fibrotic cell subpopulations of different origins. Discussion: Our results clearly demonstrate the presence of non-vascular associated tdTomato⁺ cells in the lesion core, indicating the participation of PC-derived cells in fibrotic scar formation following ONC. Thus, these PC-derived cells represent promising target cells for therapeutic treatment strategies to modulate fibrotic scar formation to improve axonal regeneration.

Characterization of the two inducible Cre recombinase-based mouse models NG2- CreERTM and PDGFRb-P2A-CreERT2 for pericyte labeling in the retina

PURPOSE/AIM OF THE STUDY

Pericytes (PCs), located abluminal of endothelial cells on capillaries, are essential for vascular development and stability. They display a heterogeneous morphology depending on organ localization, differentiation state and function. Consequently, PCs show a diverse gene expression profile, impeding the usage of a unique PC marker and therefore the distinct identification of PCs. Inducible reporter mouse models represent an important tool for investigating the fate of PCs under physiological and pathophysiological conditions. PC-specific expression efficiency of the fluorescence reporter tdTomato following tamoxifen induction was analyzed and compared in two inducible Cre recombinase-expressing mouse models under control of the NG2 and PDGFRb promotor.

MATERIAL AND METHODS

The NG2-CreER^TM-tdTomato and the PDGFRb-P2A-CreER^T2-tdTomato mice were treated with tamoxifen at three defining time points of retinal vascular development: postnatal days (P)5, P10/11/12 and P48/49/50/51. TdTomato reporter induction efficiency was determined by analyzing retinal whole mounts utilizing confocal microscopy, using the antibodies Anti-neural/glial antigen 2 (PCs), Anti-Collagen IV (basement membrane) and Anti-Glutamine Synthetase (Müller glial cells).

RESULTS

Tamoxifen induction at the three different time points resulted in PC-specific expression of tdTomato in both reporter models. In the NG2-CreER^TM-tdTomato mouse, the induction efficiency ranged from 21.9 to 35.5%. In the PDGFRb-P2A-CreER^T2-tdTomato mouse an induction efficiency between 78.9 and 94.1% was achieved. TdTomato expression in the retina was restricted to PCs and vascular smooth muscle cells in the NG2-CreER^TM-tdTomato mouse, however, in the PDGFRb-P2A-CreER^T2-tdTomato mouse, tdTomato was also expressed in Müller glial cells.

CONCLUSION

Both reporter mouse models represent promising tools for fate mapping studies of PCs. While the NG2-CreER^TM-tdTomato mouse reveals very specific labeling of PCs in the retina, its induction efficiency is lower compared to the PDGFRb-P2A-CreER^T2-tdTomato mouse. Although the latter revealed a high percentage of tdTomato-positive PCs in the retina, additional labeling of Müller cells potentially hampers analysis of reporter-positive PCs.

Pericytes cross-talks within the tumor microenvironment

Cancer cells are embedded within the tumor microenvironment and interact dynamically with its components during tumor progression. Understanding the molecular mechanisms by which the tumor microenvironment components communicate is crucial for the success of therapeutic applications. Recent studies show, by using state-of-the-art technologies, including sophisticated in vivo inducible Cre/loxP mediated systems and CRISPR-Cas9 gene editing, that pericytes communicate with cancer cells. The arising knowledge on cross-talks within the tumor microenvironment will be essential for the development of new therapies against cancer. Here, we review recent progress in our understanding of pericytes roles within tumors.