Understanding the Physics of Kidney Development

Abstract image of tubules repelling each other and shifting around.
The model of tubule packing developed by the Hughes Lab shows the tubules repelling each other and shifting around.

A recent study by Penn Bioengineering researchers sheds new light on the role of physics in kidney development. The kidney uses structures called nephrons and tubules to filter blood and pass urine to the bladder. Nephron number is set at birth and can vary over an order of magnitude (anywhere from 100,000 to over a million nephrons in an individual kidney). While the reasons for this variability remain unclear, low numbers of nephrons predispose patients to hypertension and chronic kidney disease. 

Now, research published in Developmental Cell led by Alex J. Hughes, Assistant Professor in the Department of Bioengineering, demonstrates a new physics-driven approach to better visualize and understand how a healthy kidney develops to avoid organizational defects that would impair its function. While previous efforts have typically approached this problem using molecular genetics and mouse models, the Hughes Lab’s physics-based approach could link particular types of defects to this genetic information and possibly highlight new treatments to prevent or fix congenital defects.

During embryonic development, kidney tubules grow and the tips divide to make a branched tree with clusters of nephron stem cells surrounding each branch tip. In order to build more nephrons, the tree needs to grow more branches. To keep the branches from overlapping, the kidney’s surface grows more crowded as the number of branches increase. “At this point, it’s like adding more people to a crowded elevator,” says Louis Prahl, first author of the paper and Postdoctoral Fellow in the Hughes Lab. “The branches need to keep rearranging to accommodate more until organ growth stops.”

To understand this process, Hughes, Prahl and their team investigated branch organization in mouse kidneys as well as using computer models and a 3D printed model of tubules. Their results show that tubules have to actively restructure – essentially divide at narrower angles – to accommodate more tubules. Computer simulations also identified ‘defective’ packing, in which the simulation parameters caused tubules to either overlap or be forced beneath the kidney surface. The team’s experimentation and analysis of published studies of genetic mouse models of kidney disease confirmed that these defects do occur.

This study represents a unique synthesis of different fields to understand congenital kidney disease. Mathematicians have studied geometric packing problems for decades in other contexts, but the structural features of the kidney present new applications for these models. Previous models of kidney branching have approached these problems from the perspective of individual branches or using purely geometric models that don’t account for tissue mechanics. By contrast, The Hughes Lab’s computer model demonstrates the physics of how tubule families interact with each other, allowing them to identify ‘phases’ of kidney organization that either relate to normal kidney development or organizational defects. Their 3D printed model of tubules shows that these effects can occur even when one sets the biology aside.

Hughes has been widely recognized for his research in the understanding of kidney development. This new publication is the first fruit of his 2021 CAREER Award from the National Science Foundation (NSF) and he was recently named a 2023 Rising Star by the Cellular and Molecular Bioengineering (CMBE) Special Interest Group. In 2020 he became the first Penn Engineering faculty member to receive the Maximizing Investigators’ Research Award (MIRA) from the National Institutes of Health (NIH) for his forward-thinking work in the creation of new tools for tissue engineering.

Pediatric nephrologists have long worked to understand the cause of these childhood kidney defects. These efforts are often confounded by a lack of evidence for a single causative mutation. The Hughes Lab’s approach presents a new and different application of the packing problem and could help answer some of these unsolved questions and open doors to prevention of these diseases. Following this study, Hughes and his lab members will continue to explore the physics of kidney tubule packing, looking for interesting connections between packing organization, mechanical stresses between neighboring tubule tips, and nephron formation while attempting to copy these principles to build stem cell derived tissues to replace damaged or diseased kidney tissue. Mechanical forces play an important role in developmental biology and there is much scope for Hughes, Prahl and their colleagues to learn about these properties in relation to the kidney.

Read The developing murine kidney actively negotiates geometric packing conflicts to avoid defects” in Developmental Cell.

Other authors include Bioengineering Ph.D. students and Hughes Lab members John Viola and Jiageng Liu.

This work was supported by NSF CAREER 2047271, NIH MIRA R35GM133380, Predoctoral Training Program in Developmental Biology T32HD083185, and NIH F32 fellowship DK126385.

2022 Career Award Recipient: Michael Mitchell

by Melissa Pappas

Michael Mitchell (Illustration by Melissa Pappas)

Michael Mitchell, J. Peter and Geri Skirkanich Assistant Professor of Innovation in the Department of Bioengineering, is one of this year’s recipients of the National Science Foundation’s CAREER Award. The award is given to early-career faculty researchers who demonstrate the potential to be role models in their field and invest in the outreach and education of their work.

Mitchell’s award will fund research on techniques for “immunoengineering” macrophages. By providing new instructions to these cells via nanoparticles laden with mRNA and DNA sequences, the immune system could be trained to target and eliminate solid tumors. The award will also support graduate students and postdoctoral fellows in his lab over the next five years.

The project aligns with Mitchell’s larger research goals and the current explosion of interest in therapies that use mRNA, thanks to the technological breakthroughs that enabled the development of COVID-19 vaccines.

“The development of the COVID vaccine using mRNA has opened doors for other cell therapies,” says Mitchell. “The high-priority area of research that we are focusing on is oncological therapies, and there are multiple applications for mRNA engineering in the fight against cancer.”

A new wave of remarkably effective cancer treatments incorporates chimeric antigen receptor T-cell (CAR-T) therapy. There, a patient’s T-cells, a type of white blood cell that fights infections, are genetically engineered to identify, target and kill individual cancer cells that accumulate in the circulatory system.

However, despite CART-T therapy’s success in treating certain blood cancers, the approach is not effective against cancers that form solid tumors. Because T-cells are not able to penetrate tumors’ fibrous barriers, Mitchell and his colleagues have turned to another part of the immune system for help.

Read the full story in Penn Engineering Today.

2022 CAREER Award Recipient: Lukasz Bugaj

by Melissa Pappas

Lukasz Bugaj (illustration by Melissa Pappas)

Therapies that use engineered cells to treat diseases, infections and chronic illnesses are opening doors to solutions for longstanding medical challenges. Lukasz Bugaj, Assistant Professor in Bioengineering, has been awarded a National Science Foundation CAREER Award for research that may be key to opening some of those doors.

Such cellular therapies take advantage of the complex molecular mechanisms that cells naturally use to interact with one another, enabling them to be more precise and less toxic than traditional pharmaceutical drugs, which are based on simpler small molecules. Cellular therapies that use engineered immune system cells, for example, have recently been shown to be highly successful in treating certain cancers and protecting against viral infections.

However, there is still a need to further fine-tune the behavior of cells in these targeted therapies. Bugaj and colleagues are addressing that need by developing new ways to communicate with engineered cells once they are in the body, such as turning molecular events on and off at specific times.

The research team recently discovered that both temperature and light can act as triggers of a specific fungal protein, dynamically controlling its location within a mammalian cell. By using light or temperature to instruct that protein to migrate toward or away from the cell’s membrane, Bugaj and his colleagues showed how it could serve as a key component in controlling the behavior of human cells.

Read the full story in Penn Engineering Today.

2021 CAREER Award recipient: Alex Hughes, Assistant Professor in Bioengineering

by Melissa Pappas

Alex Hughes (illustration by Melissa Pappas)

The National Science Foundation’s CAREER Award is given to early-career researchers in order to kickstart their careers in innovative and pivotal research while giving back to the community in the form of outreach and education. Alex Hughes, Assistant Professor in Bioengineering and in Cell and Developmental Biology, is among the Penn Engineering faculty members who have received the CAREER Award this year.

Hughes plans to use the funds to develop a human kidney model to better understand how the development of cells and tissues influences congenital diseases of the kidney and urinary tract.

The model, known as an “organoid,” is a lab-grown piece of human kidney tissue on the scale of millimeters to centimeters, grown from cultured human cells.

“We want to create a human organoid structure that has nephrons, the filters of the kidney, that are properly ‘plumbed’ or connected to the ureteric epithelium, the tubules that direct urine towards the bladder,” says Hughes. “To achieve that, we have to first understand how to guide the formation of the ureteric tubule networks, and then stimulate early nephrons to fuse with those networks. In the end, the structures will look like ‘kidney subunits’ that could potentially be injected and fused to existing kidneys.”

The field of bioengineering has touched on questions similar to those posed by Hughes, focusing on drug testing and disease treatment. Some of these questions can be answered with the “organ-on-a-chip” approach, while others need an even more realistic model of the organ. The fundamentals of kidney development and questions such as “how does the development of nephrons affect congenital kidney and urinary tract anomalies?” require an organoid in an environment as similar to the human body as possible.

“We decided to start with the kidney for a few reasons,” says Hughes. “First, because its development is a beautiful process; the tubule growth is similar to that of a tree, splitting into branches. It’s a complex yet understudied organ that hosts incredibly common developmental defects.

“Second,” he says, “the question of how things form and develop in the kidney has major medical implications, and we cannot answer that with the ‘organ-on-a-chip’ approach. We need to create a model that mimics the chemical and mechanical properties of the kidney to watch these tissues develop.”

The fundamental development of the kidney can also answer other questions related to efficiency and the evolution of this biological filtration system.

“We have the tendency to believe that systems in the human body are the most evolved and thus the most efficient, but that is not necessarily true,” says Hughes. “If we can better understand the development of a system, such as the kidney, then we may be able to make the system better.”

Hughes’ kidney research will lay the foundation for broader goals within regenerative medicine and organ transplantation.

Read the full story in Penn Engineering Today.

A Career of Accomplishments: Daniel Bogen

Daniel K. Bogen,, MD, PhD
Department chair David Meaney toasts Daniel Bogen

Daniel K. Bogen, MD, PhD, a professor in Penn’s Department of Bioengineering, is retiring. A Harvard alumnus (AB, 1972;  PhD, 1977; MD, 1979). Dr. Bogen was the the first MD/PhD hired by the department in its history.  Starting at Penn in 1982, Dr. Bogen spent his entire career on the faculty.

Early in his career, Dr. Bogen focused on cardiac tissue mechanics and understanding the functional changes that occur to heart tissue after ischemic insult.  These publications were among the first to use finite element techniques to address the critical problem of how heart wall contraction changes after injury.  Some of these papers are continually cited even today. Motivated to work on practical and applied clinical bioengineering-based problems, Dr. Bogen transformed his research to build items that patients would use.  Rather than a timescale from discovery to application that can last decades for most academic researchers, Dr. Bogen’s new direction allowed him to put items in the hands of patients within months.  In addition, Dr. Bogen’s led the PENNToys program, a nationally known program designing toys for children with disabilities.

The passion for impact also extended into the classroom. Reimagining the laboratory education in bioengineering, he used NSF-sponsored funding to create a discovery-based educational experience for undergraduates. This laboratory educational experience became an international model program, copied by many highly ranked bioengineering/biomedical engineering programs. This educational program was the cornerstone of the proposal funded by the Whitaker Foundation, leading to the construction of Skirkanich Hall, the current home of the Department of Bioengineering, in 2006. As a testament to his gifts as an education, Dr. Bogen’s teaching excellence was rewarded in 2005 with the Christian R. and Mary F. Lindback Award, which is the highest university teaching award bestowed by Penn.

Dr. Bogen will remain active in his retirement, and always enjoys hearing from alumni and students. Feel free to send him a congratulatory note — dan@seas.upenn.edu.