Study links life's milestones to a non-circadian biological rhythm in teeth

January 6, 2016, New York University
The top canonical pathways identified by IPA are:5-day growth rhythm* Proline Biosynthesis II (from Arginine)* tRNA Charging* Citrulline Biosynthesis* Glycine Biosynthesis III* Superpathway of Citrulline Metabolism5-day degradation rhythm* Adenine and Adenosine Salvage III* Sucrose Degradation V (Mammalian)* Purine Ribonucleosides Degradation to Ribose-1-phosphate* Adenosine Nucleotides Degradation II* Purine Nucleotides Degradation II (Aerobic) Credit: Dr. Timothy Bromage

The circadian rhythm, or 'daily biological clock,' controls much of an organism's regular pace of development, and this growth paradigm has been the focus of intense molecular, cellular, pharmacological, and behavioral, research for decades. But then, why do rats mature faster than humans?

"It is impossible to explain enormous variations in age at maturity and other developmental milestones just by looking at in this daily rhythm," said Dr. Timothy Bromage, a professor of Biomaterials & and of Basic Science & Craniofacial Biology at the New York University College of Dentistry. "This suggests that another biological timing mechanism is at work."

Through metabolomic analysis of blood plasma, Dr. Bromage and his team, have for the first time, linked these variations to another biological timing mechanism operating on multi-day (multidien) rhythms of growth and degradation. The findings were published today in the online journal PLOS ONE.

This research builds upon earlier studies by Dr. Bromage that observed multi-day biological rhythms within incremental growth lines in tooth enamel and skeletal bone tissue first published in the February, 2009 issue of Calcified Tissue International.

"These rhythms, originating in the hypothalamus, a region of the brain that functions as the main control center for the autonomic nervous system, affect bone, body size, and many metabolic processes, including heart and respiration rates," Dr. Bromage hypothesized. "The rhythms affect an organism's overall pace of life and its lifespan, so a rat that grows teeth and bone in a fraction of the time of a , in fact also lives faster and dies at a much younger age."

Top: Daily (circadian) growth lines, or cross-striations (short arrows) are observed between adjacent long-period (multidien) striae of Retzius (long arrows) (distance between the adjacent striae of Retzius shown = 30 μm). Bottom: Striae of Retzius may be seen to course across the horizontal field of view (FW = 450 μm). The number of cross-striations between adjacent striae of Retzius is termed the "repeat period" (RP). Credit: Dr. Timothy Bromage

In his current research, Dr. Bromage and his team further characterized these rhythms through metabolome and genome analysis of blood plasma from a medium-sized mammal, the domestic pig. The study, "The Swine Plasma Metabolome Chronicles "Many Days" Biological Timing and Functions Linked to Growth," is the first ever use of metabolomics to address a question in evolutionary biology.

The researchers found that metabolites and RNA drawn from 33 domestic pigs over a two-week period oscillate on a five-day rhythm. Using microscopic analysis, the investigators also observed a corresponding five-day rhythm in the pigs' tooth enamel.

Further study revealed two five-day rhythms in tandem—one controlling tissue growth and a second one beginning three days later for degradation of growth-related molecular compounds back to their basic biological entities for use in the next growth round.

"These findings provide new insight into biological processes regulating growth and body size and controlling gestation length, weaning, age at maturity and other ," said Dr. Bromage. "We believe this to be a key component to what regulates species' life history evolution."

Explore further: NYU dental professor discovers biological clock

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