Ibrahim Zakhary

Sodium-dependent vitamin C transporter SVCT2: Expression and function in bone marrow stromal cells and in osteogenesis

Ascorbic acid (Vitamin C) has a critical role in bone formation and osteoblast differentiation, but very little is known about the molecular mechanisms of ascorbic acid entry into bone marrow stromal cells (BMSCs). To address this gap in knowledge, we investigated the identity of the transport system that is responsible for the uptake of ascorbic acid into bone marrow stromal cells (BMSCs). First, we examined the expression of the two known isoforms of the sodium-coupled ascorbic acid transporter, namely SVCT1 and SVCT2, in BMSCs (Lin-ve Sca1+ve) and bone at the mRNA level. Only SVCT2 mRNA was detected in BMSCs and bone. Uptake of ascorbic acid in BMSCs was Na(+)-dependent and saturable. In order to define the role of SVCT2 in BMSC differentiation into osteoblasts, BMSCs were stimulated with osteogenic media for different time intervals, and the activity of SVCT2 was monitored by ascorbic acid uptake. SVCT2 expression was up-regulated during the osteogenic differentiation of BMSCs; the expression was maximal at the earliest phase of differentiation. Subsequently, osteogenesis was inhibited in BMSCs upon knock-down of SVCT2 by lentivirus shRNA. We also found that the expression of the SVCT2 could be negatively or positively modulated by the presence of oxidant (Sin-1) or antioxidant (Ascorbic acid) compounds, respectively, in BMSCs. Furthermore, we found that this transporter is also regulated with age in mouse bone. These data show that SVCT2 plays a vital role in the osteogenic differentiation of BMSCs and that its expression is altered under conditions associated with redox reaction. Our findings could be relevant to bone tissue engineering and bone related diseases such as osteoporosis in which oxidative stress and aging plays important role.

A Model for Osteonecrosis of the Jaw with Zoledronate Treatment following Repeated Major Trauma

This study aims to develop a reproducible rat model for post-traumatic bisphosphonate-related osteonecrosis of the jaw (BRONJ). In our previous studies using dental extraction as an inducing factor, only 30% - 60% of zoledronate-treated animals fulfilled the definition of clinical BRONJ. We modified the zoledronate regimen and introduced repeated surgical extraction to illicit quantifiable BRONJ in all animals. Eighty retired-breeder female Sprague-Dawley rats were divided between the treatment (IV zoledronate; 80 μg/kg/week for 13 weeks) and control (saline) groups. On week 13, the left mandibular first molar was surgically extracted, followed by the second molar a week later. Animals were euthanized at 1-week, 2-weeks, and 8-weeks following extraction. The occurrence and severity of BRONJ were scored in each animal based on gross and MicroCT analysis. Parameters of bone formation and osteoclast functions at the extraction site were compared between groups. All zoledronate-treated animals developed a severe case of BRONJ that fulfilled the clinical definition of the condition in humans. Osteoclast attachment continued to be defective eight weeks after stopping the treatment. There were no signs of kidney or liver toxicity. Our data confirmed that repeated surgical extraction (major trauma) by itself consistently precipitated massive bone necrosis in ZA-treated animals, eliminating the need to induce pre-existing infection or comorbidity. These results will be the basis for further studies examining the in-vivo pathogenesis and prevention of BRONJ.

Vascular alterations in the sprague-dawley rat mandible during intravenous bisphosphonate therapy

Long-term use of intravenous bisphosphonates, such as zoledronic acid (zoledronate), has been linked to bisphosphonate-related osteonecrosis of the jaw (BRONJ). Invasive dental surgery seems to trigger the bone necrosis in most cases. To determine the effects of zoledronic acid on the vascular structure of the rat mandible. Extracted of the mandibular first molar in rats that received 2 IV injections of zoledronate (20 μg/kg), 4 weeks apart. Zoledronate-treated rats (n = 18) were then compared to a control group of untreated rats (n = 18). At the fourth, eighth, and 12th week after molar extraction, 8 rat mandibles from each group were perfused with 35% radiopaque triphenylbismuth in methyl methacrylate via carotid artery perfusion. Mandibles were harvested and examined by micro-CT to assess the spatial and dimensional changes of the vasculature as a result of zoledronate treatment. The micro-CT analysis showed that zoledronic acid-treated rats had blood vessels that were thicker, less connected, and less ordered than control rats that were not exposed to zoledronic acid. This study demonstrated that treatment with zoledronic acid in rats is associated with vascular changes in alveolar bone. Further studies are underway to explore whether these vascular changes contribute to the pathogenesis of BRONJ.

Effect of Metformin on Periimplant Wound Healing in a Rat Model of Type 2 Diabetes

Purpose:To investigate the effects of hyperglycemia and metformin (a popular biguanide antidiabetic) on periimplant healing. Methods:Thirty-six male rats were assigned to 3 groups: (1) nondiabetic Wistar-Kyoto rats (controls), (2) Goto-Kakizaki (GK) spontaneously diabetic rats (GK group), and (3) GK rats were fed metformin (100 mg/kg body weight per day) in their water for 4 weeks (GK + Met group). The right maxillary first molars were extracted and sites were allowed 1 month to heal. Titanium implants (1 × 3 mm) were placed in healed extraction sites. Six rats from each group were analyzed at weeks 1 and 4 by micro computed tomography for bone/implant contact ratio, percent bone volume, trabecular number, and bone mineral density. Blood was also analyzed for glucose, HbA1c, and pyridinoline (PYD). Results:At week 1, glucose levels in the GK-Met rats were high, and all bone parameters were similar to GK rats (lower bone parameters and higher PYD than controls). At week 4, glucose levels in the GK-Met rats and all parameters were similar to controls. Conclusions:Hyperglycemic GK type 2 diabetic rats showed improved blood glucose and wound healing around oral implants after metformin administration.

Bone regeneration and docking site healing after bone transport distraction osteogenesis in the canine mandible.

PURPOSE Bone transport distraction osteogenesis provides a promising alternative to traditional grafting techniques. However, existing bone transport distraction osteogenesis devices have many limitations. The purpose of this research was to test a new device, the mandibular bone transport reconstruction plate, in an animal model with comparable mandible size to humans and to histologically and mechanically examine the regenerate bone. MATERIALS AND METHODS Eleven adult foxhounds were divided into an unreconstructed control group of 5 animals and an experimental group of 6 animals. In each animal, a 34-mm segmental defect was created in the mandible. The defect was reconstructed with a bone transport reconstruction plate. Histologic and biomechanical characteristics of the regenerate and unrepaired defect were analyzed and compared with bone on the contralateral side of the mandible after 4 weeks of consolidation. RESULTS The reconstructed defect was bridged with new bone, with little bone in the control defect. Regenerate density and microhardness were 22.3% and 42.6%, respectively, lower than the contralateral normal bone. Likewise, the anisotropy of the experimental group was statistically lower than in the contralateral bone. Half the experimental animals showed nonunion at the docking site. CONCLUSION The device was very stable and easy to install and activate. After 1 month of consolidation, the defect was bridged with new bone, with evidence of active bone formation. Regenerate bone was less mature than the control bone. Studies are underway to identify when the regenerate properties compare with normal bone and to identify methods to augment bone union at the docking site.

Rare incidence of Gorham disease with limited involvement of the maxilla: case report

Gorham disease is a rare disease of the bony skeleton that is haracterised by osteolysis. The cause is unknown. The axial keleton and mandible are the most commonly affected, and n rare incidences, the maxilla. We know of only three cases in he maxilla alone. Limited knowledge about the origins and athophysiology of the disease make diagnosis challenging. o increase awareness, we describe a rare case, which was imited to the maxilla. A 54-year-old woman presented for evaluation of hyperlastic tissue after an unexplained loss of alveolar bone in the ight maxilla. The history of increased mobility and subseuent loss of teeth 1-8 was relevant. She had ulcerated tissue round teeth 3 and 4. Radiographic examination showed xtensive bone loss, erosion, and thinning of the right maxilla, paring the nasopalatine canal (Fig. 1). An excisional biopsy as recommended, and histopathological examination was onsistent with focal gingival fibrous hyperplasia. There was o sign of active disease, and she was followed up to ensure ealing and resolution of the lesion. The course of the disease is unpredictable and bony resorpion may halt spontaneously.1,2 The diagnosis can be difficult nd is done by exclusion, which is based on history, and radioraphic, histological, and laboratory findings.3 Presentation

Osseointegration of dental implants placed into canine mandibular bone regenerated by bone transport distraction osteogenesis.

PURPOSE The purpose of this study was to compare the osseointegration of dental implants placed in canine mandibular bone and in regenerated bone produced by bone transport distraction osteogenesis. MATERIALS AND METHODS Ten adult foxhounds were divided into two groups of five animals each. In all animals, a 40-mm defect was created on one side of the mandible. A bone transport reconstruction plate was used to stabilize the mandible and regenerate bone. Six weeks after the distraction period was finished, dental implants were placed in regenerated and native mandibular bone. The animals were sacrificed after another 6 and 12 weeks of healing, respectively. RESULTS Microcomputed tomographic evaluation showed that bone volume fraction (BV/TV) was greater at the coronal regions of the implants and decreased toward the apical regions. There was an increase in BV/TV around implants placed in regenerated bone from 6 to 12 weeks of healing. The regenerated group showed lower BV/TV at 6 weeks versus implants placed in native bone but had reached the same levels as the native bone at 12 weeks. Histology showed that direct bone-to-implant contact was greater for implants placed in native bone than for those placed in regenerated bone for both time periods. The removal torque of the implants placed in native bone was higher at 6 weeks than that of implants placed in regenerated bone. At 12 weeks, there were no statistically significant differences in removal torque between the groups. CONCLUSIONS Bone was successfully regenerated in all animals. The implants placed entirely in regenerated bone were osseointegrated. The regenerated bone around the implants became denser over time. This finding suggests that implants placed entirely in regenerated bone will be as well integrated as implants in native mandibular bone by 12 weeks after placement.

Characterization of primary osteocyte-like cells from rat mandibles

OBJECTIVE The mandible is continuously undergoing remodeling as a result of mechanobiologic factors, such as chewing forces, tooth loss, orthodontic forces, and periodontitis. The effects of mechanical stress and biologic signals in bone homeostasis have been the focus of many investigations. However, much of this research utilized osteocytes derived from long bones, but little is known about the mandible-derived osteocytes. This study tests a protocol to isolate and grow osteocytes from rat mandible. STUDY DESIGN Rat mandibles were harvested, sectioned into small pieces, and subjected to a sequence chemical treatment and enzymatic digestion. The treated tissues were cultured for a few weeks while cells emerged. Cells were sorted by using the osteocyte marker podoplanin, an early marker for osteocyte differentiation. The cells were then characterized according to morphology, biochemical markers (osteocalcin, podoplanin, and sclerostin), and alkaline phosphatase activity and compared with an isotype cell line MLO-Y4 cells. RESULTS The mandibular osteocytic cells had stellate shape and were positive for osteocalcin, podoplanin, and sclerostin and lower alkaline phosphatase activity compared with MLO-Y4 osteocyte-like cells. CONCLUSIONS The protocol to isolate osteocyte-like cells will allow the investigators to investigate the mechanobiologic differences in biomechanical response between these mandibular and long bone osteocyte-like cells under various conditions.