First person – Kevin Zhang

Kevin Zhang

How would you explain the main findings of your paper to non-scientific family and friends?

Age-related macular degeneration (AMD) is the leading cause of blindness in older adults. Unfortunately, the mechanism by which AMD develops is unknown. Iron overload and oxidative stress have been implicated in AMD, but we don't understand how these damage the retina. Therefore, we wanted to find out the ways through which iron might lead to retinal damage. We treated retinal pigment epithelium (RPE) cells with iron and found that they had increased numbers of lysosomes, organelles that function as cellular recycling centers. However, these lysosomes were dysfunctional, with impaired enzyme activity. We also studied a mouse with RPE iron accumulation and discovered that they similarly have increased lysosomal number in their RPEs. These lysosomes not only had dysfunctional enzymes, but also had leaky membranes, which could lead to cell death. RPE cells had many metabolic defects, including the accumulation of lipids and toxic lipid peroxidation products. Our findings indicated that iron induces generation of toxic lipids that lead to lysosomal dysfunction and eventual cell death, suggesting a mechanism that may also be relevant to the development of AMD.

What are the potential implications of these results for your field of research?

There is currently no treatment for dry AMD, the most common type of AMD. Treatment development has been hindered by poor understanding of the mechanistic basis for AMD. Our findings suggest that targeting iron metabolism, such as through reduced dietary intake and iron chelation, could slow AMD progression. Furthermore, because we were able to link iron to disruptions in lysosome function and lipid metabolism, our work suggests that targeting these systems could also lead to a potential cure.

What are the main advantages and drawbacks of the experimental system you have used as it relates to the disease you are investigating?

We used both RPE cells in cell culture and a mouse model that develops progressive retinal iron overload. Cell culture allowed us to modulate experimental variables more precisely and quickly, and furthermore had the benefit of enabling us to measure lysosomal enzyme activity easily. Although our cells displayed characteristics very similar to native RPE cells, by virtue of being grown in isolation, they could not fully recapitulate the in vivo system. Our mouse model provided a much better representation of what iron overload in humans could do, although the pathology took a year to develop and we were limited by the number of mice we had. However, we used non-invasive imaging techniques to track disease progression over time and incorporated multiomics approaches to gain a comprehensive understanding of cellular metabolism. These methods allowed us to learn a ton of information despite having limited numbers of mice.

What has surprised you the most while conducting your research?

I was very impressed with the degree of lysosomal accumulation in our mouse model. When we looked with electron microscopy, we saw that entire RPE cells were packed with these organelles. This finding was supported by our fluorescence microscopy images; when we used an antibody targeting lysosomal membranes, the entire cell lit up with these fluorescent bubble-like structures. It was beautiful! It was amazing that the cells could look so abnormal yet still retain some function and survive for so long.

What do you think is the most significant challenge impacting your research at this time and how will this be addressed over the next 10 years?

Many biochemical assays use tissue or cell lysates and then generate a reading based on these lysates. Unfortunately, both tissues and cells are heterogeneous, so the assays we use don't have enough resolution to determine whether specific cells or organelles are dysfunctional. For example, if an assay reports decreased enzyme activity, we don't know whether the enzyme activity is decreased in every cell or just a subset of cells. These ambiguities could lead to incorrect conclusions and lead researchers to prematurely dismiss some avenues of research. Newer technologies like single-cell sequencing are addressing this issue in genetics research, but protein and metabolism research is lagging a bit behind. However, mass spectrometry imaging and single-cell proteomics have recently come into development, which I think could greatly change the landscape of molecular biology research.

What changes do you think could improve the professional lives of scientists?

Scientific research has become so detailed that it's impossible to do everything yourself. I believe that greater interdisciplinary collaboration would not only accelerate research, but also make the lives of individual scientists better. Answers to countless biological questions would be more quickly and accurately found if interrogated from a chemistry/physics/mathematics perspective. By establishing interdisciplinary collaborations, scientists can explore new directions and avoid going down wrong paths.

What's next for you?

I am finishing up my MD/PhD at the University of Pennsylvania and will soon be applying to ophthalmology residency. My goal is to become an academic ophthalmologist, treat patients and work on currently untreatable problems in the lab.