Everyone's kidneys is different as these organs don't develop according to a masterplan, which makes it much harder for scientists to figure out how to grow artificial kidneys. — Vecteezy
To Dr Alex Hughes, assistant professor in bioengineering at the University of Pennsylvania’s School of Engineering and Applied Science in the United States, the kidney is a work of art.
“I find the development of the kidney to be a really beautiful process,” he says.
Most people only ever see the organ in cross-section, through textbooks or by dissecting animal kidneys in high (secondary) school biology class: a bean-shaped slice with lots of tiny tubes.
“I think that really undersells how amazing the structure is,” he says, pointing out that kidneys grow in utero like forests of pipes, branching exponentially.
Densely packed with tubules clustered in units known as nephrons, kidneys cleanse the blood, maintain the body’s fluid and electrolyte balance, while also regulating blood pressure.
The organ played a crucial role in vertebrates emerging from the ocean: as one paper puts it, kidneys preserve the primordial ocean in all of us.
Unfortunately, kidneys struggle in the modern world.
Excessively salty food, being overweight, not exercising enough, drinking too much and smoking can all raise blood pressure, which damages the kidney’s tiny blood vessels, as does diabetes.
In some cases, damage to the kidney’s nephrons can be slowed with lifestyle changes, but unlike the liver, bones and skin, which can regrow damaged tissue, kidneys have a limited capacity to regenerate.
At present, without a transplant, the nephrons we have at birth must last a lifetime.
A huge disease burden
Today, one in ten people worldwide – more than 850 million in all – suffers from chronic kidney disease (CKD).
The condition is hard to detect initially; it is also progressive and incurable.
By 2040, CKD is expected to be the fifth-leading cause of years-of-life lost globally.
Eventually, CKD leads to kidney failure, at which point there are only two treatments: dialysis – which costs tens of thousands of dollars per year, frequently causes pain and requires patients to spend hours each week hooked up to machines that filter the blood – or kidney transplantation.
The waiting list to receive a new kidney in the US is roughly 100,000 people, and three to five years long.
Even if everyone born today adopted healthier lifestyles, millions would still suffer from the disease.
The most common prenatal developmental abnormalities involve the kidneys and urinary tract, impacting 2% of all births, or nearly three million babies each year.
“There is a huge clinical burden of kidney disease,” says Assist Prof Hughes.
“And there are relatively few engineers trying to come up with new solutions.”
To that end, the his lab focuses on elucidating the mechanisms behind kidney development and using those insights to create kidney tissue from scratch, which could reduce the need for both dialysis and transplantation.
“I think there’s just enormous opportunity to think about synthetically reconstituting kidney tissues for regenerative medicine,” he says.
Building without a blueprint
To grow artificial kidneys, researchers like Assist Prof Hughes first need to understand how nature builds the organ.
This is harder than it sounds.
Everyone’s heart and circulatory system look more or less the same, but no two pairs of kidneys are exactly alike.
Kidneys form as their tubules branch, a variable process that leads to some people’s kidneys having nine times as many nephrons as others – and potentially many more times the filtration power and lifespan.
“There’s a lot of variability in how many nephrons we have,” Assist Prof Hughes points out, referring to the kidney’s tiny, functional unit.
“If you have fewer nephrons, does that mean that you have a higher chance of CKD?
"The research seems to support this.”
The mechanisms that govern the branching process and nephron formation have long been poorly understood.
“It’s like a city’s water distribution network,” he says, “but it’s being built by these cells that somehow collectively know what to build and where their neighbours are and what junctions to make – all without a blueprint.”
Rhythm of the kidneys
In a recent paper in the journal Nature Materials, Assist Prof Hughes and his lab reported discovering a potential governor of kidney growth: tiny mechanical stress waves, which occur when the kidney’s densely-packed tubules bump into one another.
“Imagine being in an elevator and the elevator’s packed with people already,” he says.
“If you keep adding people, it will create this mechanical stress – you’d literally be pushing people away with your elbows.”
Assist Prof Hughes and his collaborators – including co-first authors postdoctoral scholar Dr Louis Prahl, bioengineering doctoral student Liu Jiageng and bioengineer Dr John Viola – carefully analysed microscopic images of developing animal kidneys at different times to determine their geometry.
They also pressed on the organs with tiny tools to measure their rigidity.
They found that the more tightly packed the tubules – which increases over time – the stiffer the tissue.
As tubule branching continues, each additional branch caused a pulse of mechanical stress, which the team believes may constitute one of the signals for nephron formation.
Each tubule, his group concluded, essentially competes for space with its neighbours.
In other words, there’s no master plan the kidneys follow, which helps explain why the number of nephrons in mature kidneys differs from person to person.
The finding suggests that kidney development is something like an improvised dance, with each tubule reacting to the touch of its neighbours.
In videos created by the Hughes Lab to visualise the process, adjacent nephrons form one after the other, as if they were following a beat.
“It’s still a hypothesis,” adds Assist Prof Hughes, “but we think that the stem cells that are around these tubules are effectively listening for these mechanical stress waves to guide their decision-making about when to form a nephron or when not to.”
If researchers can simulate that rhythm, they might be able to guide the development of artificial kidneys, which would represent a tremendous leap forward in treating CKD.
A matter of ratios
At the moment, artificial kidney tissue, in the form of clusters of cells known as organoids, is far from clinical usefulness.
Whereas normal kidneys involve an ordered collection of different cell types, organoids typically wind up as chaotic masses of cells in the wrong places.
“You can create the right cell types,” says Assist Prof Hughes, “but their spatial organisation is incorrect for the most part.”
Kidneys’ spatial organisation is crucial – a water filtration plant can’t work if the pipes don’t line up.
Unfortunately, the tubules in organoids typically display insufficient branching and fail to drain into a single exit point.
In other words, they can’t fulfil the kidney’s most crucial functions: filtering waste from the blood and ensuring that waste exits the body.
“There needs to be a lot of engineering innovation in how we guide those tissues to be more lifelike,” he says.
Part of the problem is that kidney organoids require at least three different types of stem cells: one for the tubules, one for the nephrons and one for support structures like blood vessels.
Unlike, say, gut organoids, which model intestinal tissue and can be grown from a single type of stem cell, kidney organoids are inherently more complicated.
In a second recent paper published in the journal Cell Systems, the Hughes Lab proposed a novel solution: create tiny communities of the various cell types patterned in a mosaic.
By adjusting the ratios of each stem cell type, the researchers were able to influence the composition of the organoid.
Assist Prof Hughes and his co-authors – first author Catherine Porter, Samuel Grindel, and University of California Berkeley and University of California San Francisco’s Grace Qian, all bioengineering doctoral students – developed custom microwells, in which they grew a variety of different combinations of kidney stem cells, almost like bakers trying out different recipes.
As the ratios changed, the researchers noticed a “peak” in tubule formation, suggesting an optimal composition for growing kidney tissue, which they termed the “goldilocks” ratio.
“If we change the ratio, we see quite different compositions of the organoid,” says Assist Prof Hughes.
“So you can treat these as designer organoids where you have control over the outcome.”
Filling the gap
Ultimately, he hopes to combine these dual insights – i.e. the mechanical stress waves that influence kidney development and the ratios that shape organoid formation – into clinical applications.
“You can imagine as these organoids are differentiating,” he says, “you could simulate that rhythmic process and see if suddenly, you can kick off a larger- scale outcome.”
The urgency of developing alternatives to transplantation and dialysis is hard to overstate.
At present rates, there will never be enough kidneys for transplantation.
“I think that’s a big gap that engineers can hope to fill,” he says.
In his office, Assist Prof Hughes keeps his great-grandfather’s pocket watch, a reminder of how function and form go hand in hand when it comes to designing intricate mechanical objects.
The watch still runs.
