Unplugging the Donut of Drainage

Malfunction in the trabecular meshwork (TM), leading to blockage of Schlemm’s canal (SC), often leads to a dangerously increased IOP in primary open angle glaucoma (POAG) – and yet we don’t even know why this problem occurs. Although surgical removal of the blockage through trabeculotomy is an option, it’s common for the blockage to reappear down the line and for IOP to jump back up. Glaucoma is already the most common cause of blindness globally and the number of people affected is projected to increase (thanks to our aging population), so we can surely no longer ignore an important question: Why does this donut shaped tissue become dysfunctional for so many people? 

It’s true that there have been plenty of attempts to investigate the root cause of this pathological factor in the past with various in vitro and animal models for glaucoma – but nothing gleaned from these studies has been effectively translated into preventing or treating the tissue malfunction. 

Put simply, we need to use modern biotechnology to create better tissues in the lab to discover exactly what is going wrong in aqueous humor drainage – and how we can fix it (1).

Unveiling the problem

Our aim, within Lisa J. Hill’s lab at the University of Birmingham, UK, is to create a human 3D model of the primary site of POAG pathology – the blockage at the juxtacanalicular TM layer and SC inner wall, to be precise – and dig into the root cause of TM dysfunction. 

The main challenge? As far as drainage systems go, the TM and SC are remarkably complex – from both a biological and mechanical perspective. It is not simply a biochemical issue (which would still not be simple), but a combination of biological, physical, and mechanical properties that affect the bulk function of the tissue.

The cellular variability in TM tissue (described in Donut of Drainage) is yet to be effectively presented when developing TM and POAG models in vitro – I believe this is because there needs to be a shift in focus when researching this tissue. We need to move away from the research question, “How do we make a TM model that replicates human physiology in the lab?” Instead, we must ask, “What piece of the TM are we trying to mimic? What type of TM cells are we creating? And are we producing all the genetic and functional outputs needed to represent the sections we want to research?”

Frankly, the current models used to research this system struggle to encompass the complementary biological and physical attributes that form a functional in vivo tissue. Typically, these models contain one aspect or the other, without ever combining the two to create a wholly biomimetic system. And though every model has advantages and limitations, we need to ensure that any model has the complexity and relevance to effectively answer the research questions we want to answer. A 2D cell model with single cell types on plastic is alien compared with living tissue and so can probably only answer very limited questions. And when looking at animal models, which of course have the necessary 3D architecture and multi-cellular interactions, you cannot guarantee that discoveries will have relevance to human disease. We’re trying to fill this big gap with a human cellular model that mirrors the complexities of living tissue.

The future is 3D

There are myriad biological and physical factors that work in synchronicity to regulate fluid outflow. To develop the complexity of the TM and SC in the lab requires the adoption of tissue engineering principles to produce both biological and mechanical aspects of the human tissue. This complexity is what has taken our project from solely investigating the TM, to the co-modular culture of both TM and SC that can recreate fluid outflow regulation. The next step is to create POAG models that also assess how optic nerve and retinal ganglion cell death occurs during increased pressure scenarios.

Creating 3D, humanized in vitro models that can replicate the complexity of these tissues in a controlled, reproducible manner would allow thorough investigations into basic mechanisms of the pathological processes as well as facilitate pre-clinical drug testing. As a byproduct of developing more sophisticated models of POAG, with TM and SC endothelial co-culture, we can start to fill gaps within the understanding of fluid flow regulation and pinpoint how physical cues complement the biological outputs of these cells within the fluid outflow pathway. Crucially, it will enable us to start tackling the high levels of POAG by treating the underlying causes rather than just alleviating the symptoms. A deeper understanding of tissue functionality may also help guide us to regenerative capabilities. Wherever we end up, we hope we are able to limit the prevalence and reduce the burden of current eye diseases for patients and healthcare systems.