Mini-Retinas 2.0: New Model for Studying Human Eye Disease Reveals Previously-Unknown Mechanism of Vision Loss

Scientists from Dresden developed a new generation of human mini-retinas and used them to create a new model for incurable human eye disease and discovered an unknown mechanism of vision loss.

Scientists from Dresden developed a research model that opens up new opportunities for studying vision loss. Their lab-grown mini-retinas are the first ones to include key features of the macula. Using the new system, the team found a novel way to induce complex pathological changes in the retina and described a previously unknown mechanism of vision loss. The mini-retinas offer a potential first model to study age-related macular degeneration (AMD) in human retina tissue. The work led by Prof. Mike O. Karl at the Center for Regenerative Therapies Dresden (CRTD) at TU Dresden and Deutsches Zentrum für Neurodegenerative Erkrankungen (DZNE) was published in the journal Nature Communications.

The secret to the high-resolution vision of humans lies in a small region in the back of our eyes, known as the macula. This tiny part of our retina is unique to humans. It contains a large number of cone photoreceptor cells that make the high resolution color vision possible for us. Photoreceptor loss is the key symptom of incurable blinding diseases such as age-related macular degeneration (AMD) and other inherited diseases. Once photoreceptors are lost, they cannot be replaced or regrown. The vision is gradually lost.

“According to estimations, every fourth person over the age of 60 suffers from AMD, with currently about 67 million patients in Europe,” says Prof. Mike O. Karl, research group leader at the Center for Regenerative Therapies Dresden (CRTD) at TU Dresden and German Center for Neurodegenerative Diseases (DZNE).  Despite that, AMD is still not understood well enough to develop an effective therapy. Because the macula is unique to humans, studying macular diseases in animal models is less than optimal. The lack of other research systems that represent the human macula more closely is the main challenge to overcome.

“Together with my team, we now developed a new human retina system that reproduces several features of a key region within the human macula. We show for the first time that such a system allows studies of pathological changes that occur in diseases such as AMD,” adds Prof. Karl.

New Generation of Mini-Retinas

Organoids are mini-organs grown in a laboratory that can be created from virtually any human cell through a technology called reprogramming. A lab-grown organoid contains multiple cell types and reproduces characteristic features of the organ it mimics. This allows the researchers to focus on the cell-cell interactions and map complex mechanisms of disease in a human setting.

The new generation of retinal organoids developed by Prof. Karl and his group are rich in cone photoreceptors and reproduce several key parameters within a subregion of the macula, called parafovea, where AMD starts. “For the first time, we have a model with some features of a crucial region of the human retina. We show that these organoids can be used to study more complex pathologies of vision loss, like the ones observed in AMD.”

 “So far, retinal organoids mostly corresponded to peripheral parts of the human retina. They did not feature parts of the macula – the key region responsible for high resolution vision,” explains Prof. Karl. “Compared to other parts of the retina, the macula has a lot of specialized features. We expect that pathological changes may develop differently there.”

A Robust New System to Advance AMD Research

Research shows that photoreceptors are not the only type of cells that are dysfunctional in vision loss. “Modeling the interaction between multiple different cell types is what makes vision loss challenging to study, and – ultimately – understand and cure,” explains Prof. Karl.

To induce several pathological changes like those observed in AMD patients, the researchers used two proteins, HBEGF and TNF, together referred to as HT. The HT proteins were previously shown to be involved in neurodegenerative diseases, but not known to be sufficient to induce a pathology.  

When HT proteins were applied to mini-retinas, they induced complex changes that resembled the ones known from the AMD patients. “We’ve observed a progressive loss of photoreceptor cells. On top of that, we’ve seen parallel changes in other retinal cells, specifically Müller glia. They started to divide and form scar-like lesions that replaced lost photoreceptors,” says the lead author of the study Dr. Manuela Völkner.

“All these are features of retinal degeneration in AMD and the damage observed in late stages of most other retinal diseases. While these processes take years to develop in patients, we’ve reproduced comparable changes in photoreceptor and glial cells within a single month,” adds Dr. Völkner. The team believes that this change in time frame is beneficial to study disease mechanisms. It can speed up the observation of the dynamics of pathological changes and the results of potential interventions.

Tossed Out to Die - Vision Loss by Photoreceptor Degeneration

Within 10 days of stimulation with HT proteins, already half of all photoreceptors in the mini-retinas had been lost. The team expected to find a lot of dead cells in the retina. However, they could hardly find any. “It looked as if they disappeared. We were truly puzzled by this observation,” says Prof. Karl.

The team tracked the cells over time and used a variety of sensitive techniques to find that the photoreceptors do not actually die inside the retina. They are forced out of the retina before they die. “So far, nobody has shown such a mechanism in neurodegenerative diseases,” explains Prof. Karl.

“Looking back at the studies that analyze patient retinas, we can now see a lot of changes that are reminiscent of this mechanism of photoreceptor displacement. This suggests that this process might actually occur in aging, AMD, and some acquired, as well as inherited, retinal diseases” explains Prof. Karl. It remains to be seen if the new observation can be used as a potential clinical biomarker and therapeutic target to prevent vision loss.

A Bridge Between the Model Organisms and Humans

“Organoids offer the unique chance to watch and understand the dynamics of degeneration of the human retina” says Felix Wagner, co-author of the study who record videos of human photoreceptor loss in action by live microscopy.

“We have developed the first human model that can reproduce complex and progressive hallmarks of neurodegenerative diseases like in AMD. The mini-retinas offer new avenues to facilitate fundamental and translational medicine research. It provides a new starting point for the development of drug-, gene-, and cell-based therapies and possibly even a complete model of the complex macula in the future,” summarizes Prof. Karl.

Collaborative Work

The team emphasizes that the work would not have been possible without expert collaborators and specialized research facilities. The long-standing collaboration with the team of Prof. Jörg Hackermüller at Leipzig University and Helmholtz Centre for Environmental Research (UFZ) was key for the study.

The CRTD and DZNE in Dresden offer the opportunity of using cutting-edge resources. “By working together with experts in stem cell engineering, flow cytometry, next-generation sequencing, electron and light microscopy, and image analysis we were able to make huge leaps forward with this project,” says Prof. Karl.

Funding

The research was supported by a grant from the German Research Foundation (DFG) within the framework of the priority program SPP2127: Gene- and cell-based therapies to counteract neuroretinal degeneration. It was also funded through the ERA-NET Neuron research consortium of the Federal Ministry of Education and Research in Germany (BMBF) ReDiMoAMD and supported by the CRTD at TU Dresden, DZNE Dresden, and others.

Original Publication

Manuela Völkner, Felix Wagner, Lisa Maria Steinheuer, Madalena Carido, Thomas Kurth, Ali Yazbeck, Jana Schor, Stephanie Wieneke, Lynn J. A. Ebner, Claudia Del Toro Runzer, David Taborsky, Katja Zoschke, Marlen Vogt, Sebastian Canzler, Andreas Hermann, Shahryar Khattak, Jörg Hackermüller & Mike O. Karl: HBEGF-TNF induce a complex outer retinal pathology with photoreceptor cell extrusion in human organoids.
Nature Communications (October 2022)
Link: https://doi.org/10.1038/s41467-022-33848-y

About the Center for Regenerative Therapies Dresden (CRTD)

The Center for Regenerative Therapies Dresden (CRTD) of TU Dresden is an academic home for scientists from more than 30 nations. Their mission is to discover the principles of cell and tissue regeneration and leverage this for the recognition, treatment, and reversal of diseases. The CRTD links the bench to the clinic, scientists to clinicians to pool expertise in stem cells, developmental biology, gene-editing, and regeneration towards innovative therapies for neurodegenerative diseases such as Alzheimer's and Parkinson's disease, hematological diseases such as leukemia, metabolic diseases such as diabetes, bone and retina diseases.
The CRTD was founded in 2006 as a research center of the German Research Foundation (DFG) and funded until 2018 as a DFG Research Center, as well as a Cluster of Excellence. Since 2019, the CRTD is funded by the TU Dresden and the Free State of Saxony.
The CRTD is one of three institutes of the central scientific facility Center for Molecular and Cellular Bioengineering (CMCB) of the TU Dresden.
Web: www.tu-dresden.de/cmcb/crtd
Web: www.tu-dresden.de/cmcb

About the Deutsches Zentrum für Neurodegenerative Erkrankungen, DZNE (German Center for Neurodegenerative Diseases)

The DZNE is a research institute funded by the German federal and state governments, comprising ten sites across Germany. It is dedicated to diseases of the brain and nervous system, such as Alzheimer’s, Parkinson’s, and ALS, which are associated with dementia, movement disorders and other serious health impairments. To date, there are no cures for these diseases, which represent an enormous burden for countless affected individuals, their families, and the healthcare system. The aim of DZNE is to develop novel strategies for prevention, diagnosis, care, as well as treatment, and to transfer them into practice. To this end, DZNE cooperates with universities, university hospitals, research centers and other institutions in Germany and abroad. The institute is a member of the Helmholtz Association and belongs to the German Centers for Health Research.

Resources:

Website of Prof. Karl’s group (MOKALAB): https://tud.de/cmcb/crtd/karl           
Full resolution pictures: https://tud.link/vwqq  

Scientific contact:

Prof. Dr. med. Mike O. Karl
Center for Regenerative Therapies Dresden (CRTD)
Technische Universität Dresden
Fetscherstr. 105, 01307 Dresden
Tel: +49 (0) 351 210463-604
Email: