In “The Last of Us,” a video game and recently-wrapped HBO series, giant mutant fungi turn much of humanity into zombies. In…
Halloween Evokes Thoughts on Organoids
When the jello brains and gummy organs of Halloween come out, my thoughts turn to organoids. These are tiny organs, or parts of them, grown in lab dishes or transplanted into rodents, so we can watch a disease begin and maybe even test a candidate drug. Organoid technology isn’t a headline hog like CRISPR, but it’s intriguing, and certainly easier to envision.
Organoids that appear from dividing stem cells offer a landscape of early development – the process of organogenesis. A heart, liver, or kidney takes form from dividing, folding, and interacting cells, a little like watching a photographic image emerge and sharpen in a pan of developer, for those who remember that technology.
The most valuable organoids are those that arise from induced pluripotent stem (iPS) cells. These aren’t plucked from human embryos – there’s nothing political about organoids – but are typically derived from a fibroblast skin cell from a patient with a specific genetic disease. The cells are nurtured in a complex broth of growth factors, cytokines, and hormones and more. CRISPR might be deployed to introduce a specific mutation into a stem cell, or snip out or replace one that the patient has.
As the programmed body parts of an organelle unfurl, like the characteristic tubules of a kidney or the islets of a pancreas, researchers monitor protein markers that festoon the cell surfaces. These topographies indicate the gene expression profiles, patterns, and programs that guide organ formation – in the dish as a model for a human body.
Organoids are a standard tool of preclinical drug development. They’re widely used to test candidate compounds, perhaps revealing toxicity that could stall or halt progress towards a clinical trial. Organoids from patients can be used to model their pathologies, from the start. And organoids derived from several patients might reveal why some cases are more severe than others.
Here’s a look at three intriguing new organoids.
Brains dominate Halloween candies, their coils more recognizable than, say, a flat spleen or a lumpy thyroid. They are perhaps the most valuable miniaturized body part, because many disorders arise from brain processes gone awry, and the blood-brain barrier in a body keeps out many molecules that could be candidate drugs. Researchers can tweak tiny brain-parts-in-a-dish, like the cerebrum, to see if CRISPR can correct a rare genetic defect. But brain organoids are useful to investigate more common conditions too.
Researchers at Nationwide Children’s Hospital are growing cerebral organoids from stem cells taken from a patient who has a very small head (microcephaly), epilepsy, profound intellectual disability, autism spectrum disorder, and unusual facial features. Genome sequencing revealed a pathogenic variant (mutation) of the gene AUTS2, one of more than 100 lying behind autism spectrum disorders. The study appears in Brain.
After identifying the AUTS2 variant, the team nurtured mini-brains representing the cerebrum to illuminate the first inklings of microcephaly. They saw reduced growth, deficits in the proliferation of neural progenitor cells, yet nerve cells specializing too soon, compared to brain organoids made from the patient’s parent who had not passed on the variant. Gene editing corrected the genetic glitch, after which the organoids grew normally.
Then the team dug deeper. They plucked individual cells from the mini-brains and applied single-cell RNA sequencing to identify which genes were active in which cells. And they pinpointed dampening of specific genes needed to generate the signals that enable neural progenitor cells to divide and occupy the cerebral cortex, where they continue specializing into mature neurons. The gene variant halts that choreography.
When I wrote human anatomy and physiology textbooks, I found the urinary system to be the most complicated. Following the concentrations of various molecules inside, outside, and flitting across the torturous tubules, tufts, and loops packed into each kidney’s million or so nephrons, aligned like a hunk of string cheese, required my brain to recall too much chemistry. Nowadays I edit a nephrology journal, and that’s how I learned about the minuscule bits of kidneys that researchers are growing in dishes or tucking under the capsules that hug the kidneys of mice and rats.
Kidney organoids are more like the real deal – organs in bodies – than older techniques of culturing disembodied cells or creating transgenic mice, which have some human genes. But kidney organoids have a few shortcomings. They tend to remain in an immature state resembling fetal kidneys, have too few blood vessels, and may harbor a few muscle, nerve, or skin cells, nestled among the highly specialized kidney cell types.
One experiment grew 400 tiny kidneys from a patient who has polycystic kidney disease. Investigators applied drugs called protein kinase inhibitors to the nascent kidney organoids to see if cyst formation slowed. Other kidney diseases modeled with organoids include tuberous sclerosis, Alport syndrome, cystinosis, and congenital nephrotic syndrome.
Researchers at the University Medical Center Groningen describe developing patient-derived parathyroid organoids, or PTOs, in Stem Cell Reports.
The four parathyroid glands that dot the thyroid gland in the neck are each about the size of a grain of rice. The tiny glands secrete parathyroid hormone, which controls calcium and phosphorus metabolism in the blood and bones. Hormone balance from the parathyroids is important, as it is for any gland.
Overactive parathyroids (hyperparathyroidism) can cause osteoporosis and elevate risk of developing kidney stones, heart disease, and high blood pressure. The list of associated symptoms is long and general – achy joints and bones, weak muscles, fatigue, depression, loss of appetite, nausea, vomiting, constipation, increased thirst and urination, confusion, and impaired thinking and memory. Or, overactive parathyroids might not cause symptoms at all.
Underactive parathyroids (hypoparathyroidism) produce too little hormone, lowering blood calcium. It can lead to tingling fingertips, toes and lips; twitching facial muscles; cataracts; leg cramps; fatigue; dry, rough skin; brittle hair and nails; and anxiety and depression. Autoimmunity or damage from neck surgery can cause it.
The researchers isolated stem cells from patients having parathyroid surgery, then let the cells divide in lab dishes and tested ability to form PTOs. The miniscule structures resembled parathyroid glands and produced the right proteins, indicating normal parathyroid gene expression. And the organoids responded with hormone secretion ups and downs in sync to changes in calcium concentration and parathyroid hormone-lowering drugs.
“These organoids can be used to test future parathyroid-targeted drugs and imaging tracers. This technique could be used to culture healthy parathyroid gland organoids to treat patients with hypoparathyroidism,” said author Schelto Kruijff.
Next Halloween I’ll check in with more variations on the organoid them.