Ongoing Research at Liu Laboratory
There are two main classes of treatments for osteoporosis, anti-catabolic agents, such as bisphosphonates, and anabolic agents, such as intermittent parathyroid hormone (PTH) treatment. There are many types of anti-catabolic agents, which block osteoclast action to prevent further bone loss. PTH is the only FDA approved anabolic agent for postmenopausal osteoporosis treatment. Our lab has several ongoing studies regarding the benefits and use of PTH as a treatment either alone or in conjunction with other drugs.
Combined PTH and Bisphosphonate Therapy
One ongoing study focuses on the combination of PTH and bisphosphonate therapy. The literature has shown conflicting results regarding combined therapy. Some groups have found that bisphosphonates blunt the effects of PTH, while others show an additive effect. Our work in this area has shown promising results indicating an additive effect of combined therapy (Fig 1). However, the mechanisms through which this additive effect occurs is unclear. One hypothesis we have proposed is that PTH can promote modeling-based bone formation (Fig 2). Our lab is interested in further identifying the mechanisms through which this additive effect is achieved, and elucidating potential modeling-based formation events by in vivo micro-computed tomography (µCT) and advanced in vivo dynamic imaging techniques.
Fig 1. 3D bone structure of control (VEH), bisphosphonate (ALN), parathyroid hormone (PTH), and combined (PTH+ALN) treated before and after a 12 day treatment period.
Fig 2. Typically, bone remodeling is thought to be initiated by resorption (red), followed by formation (green) as shown on the bottom panel. Modeling-based formation is formation without prior resorption (top panel).
Advanced In Vivo Dynamic Imaging Techniques
Our lab focuses on longitudinal µCT imaging studies in both humans and rodents which allow us to track the same segment of bone over time. In doing this we have developed techniques to measure precise changes in the bone's micro architecture, down to the individual trabecula level. Our lab is interested in using these imaging and computational tools to gain insight into the complex physiological responses to treatment.
Fig 3. 3D individual trabecular dynamics (ITD) analysis isolates individual trabecular structures and follows them over time to quantify incidences of structural deterioration and repair.
Fig 4. 3D dynamic in vivo histomorphometry identifies areas of new bone in green and lost bone in red to quantify measures such as bone formation rate (BFR/BS) and bone resorption rate (BRR/BS).
Cyclic Treatment Regime with repeated cycles of on and off daily injection of PTH
In clinical practice the recommended PTH treatment duration is limited to 18-24 months. Despite its potent effect of promoting new bone formation, the literature has shown that bone mineral density rapidly decreases upon withdrawal from PTH treatment. However, postmenopausal osteoporosis is a life-long chronic condition. Thus, a more sustained and long-term treatment effect is needed. In order to maximize the efficacy of PTH, we are currently investigating the mechanisms behind the adverse effect of PTH withdrawal in an ovariectomized (OVX) rat model and to test the efficacy of a cyclic PTH treatment regime on rescuing PTH's withdrawal effect.
Fig 5. 3D rendering of trabecular bone microarchitecture at week 0 (4-wk post OVX), week 3 (end of PTH treatment), and week 6 and week 12 (3 and 9 weeks after discontinuation of PTH)
Fig 6. 3D rendering of trabecular bone microarchitecture of P9V9 (9 week PTH and 9 week discontinuation), VEH (saline injected for 18 week), C3P3V3 (3 repeated cycles of 3 weeks of PTH followed by 3 weeks of saline injection, for a total of 18 weeks over 3 cycles), C3P3V2 (3 repeated cycles of 3 weeks of PTH followed by 2 weeks of saline injection, for a total of 15 weeks over 3 cycles)
Effects of Pregnancy, Lactation, and Weaning on Maternal Bone
During pregnancy and lactation, the increased calcium demand caused by fetal/infant growth induces substantial changes in maternal calcium metabolism. This affects a variety of physiological processes, including remodeling of the skeleton. In particular, lactation is known to result in dramatic maternal bone loss. This bone loss is partially recovered following weaning; however, even long after weaning, the extent of recovery remains incomplete, and structural and/or mechanical deficits remain. At the same time, multiple clinical studies indicate that a history of pregnancy and lactation does not have any negative effects on future risk of osteoporosis or fracture. Thus, the long-term effects of pregnancy- and lactation-associated bone loss remain unclear. Using a rat model, combined with advanced in vivo µCT imaging techniques, biological assays, and mechanical and material testing procedures, our lab is working to answer some of these questions. We are currently investigating the ways in which the maternal bone compensates for reproductive bone losses and incomplete recovery post-weaning, through modified bone microstructure, material properties, and altered bone loss patterns post-menopause. We hope that a better understanding of reproductive bone loss and recovery, as well as its effects on post-menopausal bone loss, will lead to improved women's health, both during the reproductive years and post-menopause.
Fig 7. Longitudinal changes in the trabecular bone in the proximal tibia of a rat as a result of pregnancy, lactation, and weaning.
Fig 8. Bone remodeling taking place during pregnancy, lactation, and weaning. Newly formed bone is shown in blue, while lost bone is shown in red.
Effects of Reproduction and Lactation on Osteocyte Microenvieonment
Bone tissue is constantly renewed by bone remodeling processes that involves different types of cells, such as osteoblasts, osteoclasts and osteocytes. As a long-lived mechanosensitive cell, osteocyte plays an important role in maintaining bone tissue quality and mechanical integrity. Our work by nanoindentation and backscattered scanning electron microscopy (BSEM) has shown promising results, indicating that osteocyte and its process can actively mediate its peri-lacunar bone matrix during reproduction and lactation, which may lead to increased lacunar size post-OVX, thus affecting the local bone tissue composition and the skeletal mechano-sensitivity. Our working hypothesis is that reproduction and lactation prime the microenvironment of osteocytes, resulting in an elevated mechano-sensitivity, which may be protective to estrogen-deficiency-induced bone loss later in life. This research on osteocyte microenvironment could provide novel and critical clinical insights for osteoporosis prevention and treatment for postmenopausal women with different reproductive histories.
Fig 9. BSEM images: Differences in lacunar area between control and OVX in virgin and reproductive rats.
3D Vascular Imaging within the rat tibiae
The bone remodeling process works in conjunction with the microvascular network within the bone marrow to regulate calcium and phosphate homeostasis. Because of their drastic effects on bone remodeling, it is likely that the bone vasculature will undergo major alterations during periods of bone loss and bone gain. However, simultaneous visualization of the effect of bone remodeling in the trabecular and vascular microstructures remains challenging. Our lab focuses on establishing a novel technique that simultaneously visualizes the 3D microstructure of bone and microvasculature using standard µCT. We can simulate bone loss due to aging using ovariectomy-induced osteoporosis (OVX) and bone gain via intermittent PTH. By combining the bone microvascular network with measures of bone formation and resorption, it can improve our understanding of the effects of bone remodeling on the bone-blood vessel functional unit.
Fig 10. (Top) 3D representative bone and blood vessel images of the proximal tibia of OVX, control, and PTH groups. (Bottom) The 3D trabecular bone and blood vessel images with bone formation and resorption labeled in green and purple, respectively.