Hibernating Bears Defying Osteoporosis

Transcription

1 Bone Physiology: Hibernating Bears Defying Osteoporosis by Emily, Tori, Carolyn, Aine, and Hannah

2 Introduction What is disuse bone osteoporosis? bone loss that results from not enough stress or pressure on the bones, whether it's in one specific area or throughout the entire body. Who can it affect? Patients with spinal cord injuries Patients confined to bedrest Astronauts

3 Why are we looking at black bears? Black bears have similar lower limb morphology to humans and walk plantigrade But bears sleep 5-7 months of the year why do they not get disuse osteoporosis Normal recovery period for humans and animals is 2-3x as long as the period of disuse but bears not?

4 Proposed Questions 1. If bears do lose bone during disuse, how can they recover sufficient bone when their active and disuse periods are approximately equal in duration? 2. If they are able to prevent or mitigate bone loss during disuse, what are the biological mechanisms?

5 What is Hibernation? Survival response to food shortages In bears: Sleep 5-7 months Don t urinate or defecate Heart rate drops Body temp drops Lose 37% of body weight Imagine if you lost 37% of your body weight! Skeletal muscle protein concentration decreases *Bone formation, volume, and mineral content do not decrease during hibernation

6 Mechanics: Shivering Low magnitude / high frequency stimulation Not total state of disuse Mechanism of maintaining bone mass? Article: NO à consider astronauts More likely mechanism of maintaining body temperature

7 If not that, then what? More likely biological factors responsible for maintaining bone mass, metabolism, strength, etc. Blood samples (serum) from both wild and captive bears à look for markers that might explain this phenomenon in bears Purpose: measure bone formation against bone resorption during hibernation

8 Introduction: Biological Mechanisms In serum samples, found two markers: PICP = bone formation ICTP = bone resorption Levels of these markers indicate state of bone metabolism What happens during hibernation?

9 The Data: ICTP vs. PICP ICTP levels increased during hibernation increased resorption PICP levels spiked shortly after hibernation increased bone formation MAINTENANCE

10 PICP Levels No significant difference between PICP levels pre-hibernation and during hibernation Significant difference between hibernation and post-hibernation Interesting: the serum marker for bone formation did not decrease during disuse

11 Hypothesis 1: Bone is not lost during disuse because bone formation is linked to resorption even though turnover increases. Hibernating bears don t urinate or defecate leading to constant blood calcium concentrations Calcium levels may upregulate bone formation

12 Hypothesis 2: Some bone is lost during disuse because resorption increases faster than formation, but is recovered soon after the bear awakens for the spring. Spike in PICP after hibernation When bears awaken their bones perceive the bear s weight as high and activate a bone formation response

13 Age-Related Changes in Bone Strength Cortical bone Testing those tibias Gender neutral

14 Ultimate Stress/ Age Graph Bear and human tibial samples Bending and twisting the tibias Bear bone>>human bone

15 Porosity/ Age Graph Where do humans and bears differ? Both increase bone mineral content over time Black bear bone porosity Human bone porosity

16 Humans vs. Bears Cortical bone strength increases in humans and bears Osteoblastic activity don t give a $#*@ about disuse So who wins the boxing match? (The bear, duh)

17 Osteopenia What is osteopenia? decrease in total amount of bone skeleton after a period of disuse The Cause? decreased osteoblastic formation (cells that synthesize bone) increased osteoclastic resorption (cells that break down bone tissue) How can this be quantified? cross-sectional area of bones and intracortical pore size

18 Why Bears? Both immobilization of dogs for 32 weeks, and turkeys for 4 weeks: significant relative bone loss What about the bears? During hibernation: there is an increase in size of the resorption cavity BUT: there are fewer resorption cavities overall and there is less of an increase relative to other animals in disuse

19 What does this mean?

20 Why should you care? Bone formation and maintenance in most animals is hindered by disuse yet bears are not similarly affected during hibernation Bears must have unique traits that allow this If we can understand these mechanisms it could help with the development of treatments for human osteoporosis