Scoliosis, tiny beating cilia and spinal fluid flow

Why do 3% of kids spontaneously develop curvature of the spine during adolescence? This condition of unknown cause is called idiopathic scoliosis. An unexpected disease mechanism has been revealed recently in the journal Science. Developmental biologists from University of Toronto and Princeton University have modelled scoliosis in fish showing that disruptions in the tiny beating projections from the cells lining the spinal canal that help fluid flow through these narrow passages underlie this disease. Theories have traditionally focused on bone, cartilage or neuromuscular activity. This new biological mechanism may provide new directions to investigate less invasive therapies than the current treatments that involve bracing and corrective surgery.

Because of research in developmental biology, the study of how an embryo grows, changes shape and decides which side will be heads or tails or where kidneys will grow, we know a lot about when different genes are turned on in a coordinated fashion to make an animal. Zebrafish have been particularly useful for studying development. It is easy to make genetic changes, grow lots of them in a relatively small space and monitor their development with their transparent bodies.

One of the ways developmental biologists figure out which genes are needed for which developmental process is by generating animals with mutated genes and then seeing if something looks different or goes wrong as the animal develops. A few years ago Dr. Brian Ciruna’s lab at University of Toronto found that mutating the gene ptk7 in zebrafish produced curvature of the spine in a developmental timeframe similar to adolescence in humans. His team searched for mutations in the human form of the gene, PTK7, in idiopathic scoliosis patients and were lucky enough to find one mutation in a patient that disrupted the function of the protein it codes for.

Researchers in Rebecca Burdine’s lab at Princeton had noticed that many of their zebrafish with mutated genes involved in cilia movement had curved spines. In their Science paper, together with the Ciruna lab they showed that ptk7 specifically made the cilia sparse and irregular in direction, and that disruptions in four other cilia motility genes also caused spinal curves. They also defined a critical window when motile cilia are particularly important when the spine is growing. In the Princeton University press release, Ciruna says, “This window appears to be not during embryogenesis and not in adulthood, but specifically when fish are growing rapidly, in other words, fish adolescence.”

Tracing fluorescent microspheres injected into the area of the brain at the top of the spinal cord in ptk7 mutants showed slow and irregular flow. The team rescued the spinal curvature by adding back a functional version of ptk7. Since ptk7 is turned on in other tissues and has other effects that could muddy the waters, the team added on genetic instructions so that the rescue ptk7 would only be turned on in sites where motile cilia normally are present but nowhere else. This showed that functional ptk7  specifically in the motile cilia was needed to rescue the curved spines of the ptk7 mutants.

To investigate the window in development when cilia motility is relevant to idiopathic scioliosis, the team took advantage of temperature sensitive mutants that affects a gene involved in cilia motility. The mutation is only expressed when the fish are kept in a 30^oC tank but not at 25^oC. Since cilia motility is needed for numerous processes during development, keeping the mutants at 25^oC for a period allowed the fish to get over the embryogenesis period to avoid developing defects unrelated to scoliosis. After finding that the stage of development most sensitive for spinal curvature was similar to idiopathic scoliosis in humans, the team showed they could prevent scoliosis from progression after it had already begun by moving the fish back into 25^oC where motile cilia were again functional. The authors described this as a proof-of-principle that idiopathic scoliosis could potentially be managed without surgery.

Despite the obvious differences between humans and fish, when it comes to scoliosis animal models zebrafish perform swimmingly. Another fish, the guppy is the only animal where a scoliosis-like condition occurs naturally. Like humans with idiopathic scoliosis guppies aren’t born with it but develop curvature of the spine when they are growing. Forces on the spine in both humans and fish run parallel to the head to ‘tail’ axis. We have the weight of the head, gravity and impact from footsteps. Fish spines have the head on force of swimming through water and beating of the tail. Gravity acts very different on the spines of quadrupeds like rodents which are not susceptible to spinal curves unless they are forced to be bipedal.

Adolescence is awkward enough without having to wear a brace or have metal rods surgically inserted into your back. The majority of people with idiopathic scoliosis will not need such drastic treatments but it’s hard for physicians to judge who needs these interventions to prevent future disfigurement, heart and lung problems and chronic pain, and who will be fine without them. Future investigations into motile cilia and spinal fluid flow and how these affect the spin as it grows could provide new avenues to look for less invasive, non-surgical treatments for this condition.

Extra tidbit

I first became aware of scoliosis when I read the Judy Blume book Deenie as a preteen. But why don’t I remember seeing anyone like Deenie wearing a brace when I was in school? It turns out the US has historically screened for and treated scoliosis much more aggressively than Canada and other countries. Whether school screening is effective and whether the available treatment options are beneficial seem to be controversial topics.

Images

Left: Internet archive book The anatomy of the nervous system, from the standpoint of development and function (1920) via Wikimedia Commons. "A" points to ependymal cells that line the spinal cord and have cilia projecting into the spinal canal.

Right: An anatomical illustration from the 1921 German edition of Anatomie des Menschen: ein Lehrbuch für Studierende und Ärzte with latin terminology. via Wikimedia Commons.

References

Beauséjour, Marie, Lise Goulet, Stefan Parent, Debbie Feldman, Isabelle Turgeon, Marjolaine Roy-Beaudry, Jose Sosa, and Hubert Labelle. "The Effectiveness of Scoliosis Screening Programs: Methods for Systematic Review and Expert Panel Recommendations Formulation." Scoliosis 8.1 (2013): 12. Web.    

Gorman, Kristen F., and Felix Breden. "Idiopathic-type Scoliosis Is Not Exclusive to Bipedalism." Medical Hypotheses 72.3 (2009): 348-52. Web.            

 Grimes, D. T., C. W. Boswell, N. F. C. Morante, R. M. Henkelman, R. D. Burdine, and B. Ciruna. "Zebrafish Models of Idiopathic Scoliosis Link Cerebrospinal Fluid Flow Defects to Spine Curvature." Science 352.6291 (2016): 1341-344. Web. 

Hayes, Madeline, Xiaochong Gao, Lisa X. Yu, Nandina Paria, R. Mark Henkelman, Carol A. Wise, and Brian Ciruna. "Ptk7 Mutant Zebrafish Models of Congenital and Idiopathic Scoliosis Implicate Dysregulated Wnt Signalling in Disease." Nature Communications Nat Comms 5 (2014): 4777. Web.            

Linker, Beth. "A Dangerous Curve: The Role of History in America's Scoliosis Screening Programs." Am J Public Health American Journal of Public Health 102.4 (2012): 606-16. Web.            

"Patient Support – National Scoliosis Foundation." National Scoliosis Foundation.  http://www.scoliosis.org/patient-support/ Web. 14 June 2016.

Scoliosis linked to disruptions in spinal fluid flow. Princeton University news release. Retrieved June 15, 2016, from http://www.eurekalert.org/pub_releases/2016-06/pu-slt061016.php