Heat-stress and light-stress induce different cellular pathologies in the symbiotic dinoflagellate during coral bleaching.

Coral bleaching is a significant contributor to the worldwide degradation of coral reefs and is indicative of the termination of symbiosis between the coral host and its symbiotic algae (dinoflagellate; Symbiodinium sp. complex), usually by expulsion or xenophagy (symbiophagy) of its dinoflagellates. Herein, we provide evidence that during the earliest stages of environmentally induced bleaching, heat stress and light stress generate distinctly different pathomorphological changes in the chloroplasts, while a combined heat- and light-stress exposure induces both pathomorphologies; suggesting that these stressors act on the dinoflagellate by different mechanisms. Within the first 48 hours of a heat stress (32°C) under low-light conditions, heat stress induced decomposition of thylakoid structures before observation of extensive oxidative damage; thus it is the disorganization of the thylakoids that creates the conditions allowing photo-oxidative-stress. Conversely, during the first 48 hours of a light stress (2007 µmoles m(-2) s(-1) PAR) at 25°C, condensation or fusion of multiple thylakoid lamellae occurred coincidently with levels of oxidative damage products, implying that photo-oxidative stress causes the structural membrane damage within the chloroplasts. Exposure to combined heat- and light-stresses induced both pathomorphologies, confirming that these stressors acted on the dinoflagellate via different mechanisms. Within 72 hours of exposure to heat and/or light stresses, homeostatic processes (e.g., heat-shock protein and anti-oxidant enzyme response) were evident in the remaining intact dinoflagellates, regardless of the initiating stressor. Understanding the sequence of events during bleaching when triggered by different environmental stressors is important for predicting both severity and consequences of coral bleaching.

Impaired growth plate function in bmp-6 null mice.

Bone morphogenetic protein 6 (BMP-6) is expressed by different skeletal cells including osteoblasts and growth plate chondrocytes, suggesting roles in bone formation and growth regulation. To address these possibilities, we examined whether cancellous and cortical bone parameters, or indices of growth plate function, are altered in bmp-6 null mice as assessed under basal conditions, and following stimulation of bone formation and suppression of growth by estrogen treatment. Ten-week-old female littermate bmp-6 null and wild-type (WT) mice were administered vehicle or E(2) 4, 40, 400 or 4,000 microg/kg/day by daily sc injection for 28 days (6-8 per group). Tibias were removed, and detailed histomorphometric analysis of the proximal metaphysis and growth plates, and tibial diaphysis were performed on longitudinal and transverse sections respectively. Long bone area as measured by DXA was reduced in vehicle-treated bmp-6 null mice compared with WT littermate controls. In addition, vehicle-treated bmp-6 null mice had a reduced cross-sectional area at the tibial mid-diaphysis as assessed by histomorphometry, whereas cancellous bone indices were unaffected. Histomorphometry of the proximal tibial metaphysis demonstrated a defect in bone formation immediately adjacent to the growth plate in bmp-6 null mice compared to WT mice following E(2) treatment. E(2) administration was also associated with a dose-responsive decrease in longitudinal growth rate, and proliferative and hypertrophic zone parameters of the growth plate (p<0.0001). Significantly greater reductions following E(2) treatment were observed in longitudinal growth rate (p<0.01), proliferating and hypertorphic zone widths (p<0.001), and proliferating (p<0.0002) and hypertrophic (p<0.002) cells per column of bmp-6 null mice compared to WT mice. Our observation that long bones are reduced in size compared to wild-type mice primarily through a decrease in cortical cross-sectional area, whilst cancellous bone mass is unaltered, suggests a non-redundant role for BMP-6 in periosteal but not trabecular bone formation. Moreover, growth plate function was reduced in bmp-6 null mice receiving estrogen, leading to an impaired cancellous bone response to estrogen at the highest dose, suggesting that BMP-6 also plays a physiological role in maintaining growth plate function.

Estrogen receptor-alpha dependency of estrogen's stimulatory action on cancellous bone formation in male mice.

We examined whether estrogen receptor (ER)alpha is required for estrogen to stimulate cancellous bone formation in long bones of male mice. 17 beta-Estradiol (E(2)) was administered to ER alpha(-/-) male mice or wild-type (WT) littermate controls at 40, 400, or 4000 microg/kg by daily sc injection for 28 d and histomorphometric analysis performed at the distal femoral metaphysis. In WT mice, treatment with E(2) (40 microg/kg per d) increased the proportion of cancellous bone surfaces undergoing mineralization and stimulated mineral apposition rate. In addition, higher doses of E(2) induced the formation of new cancellous bone formation surfaces in WT mice. In contrast, E(2) had little effect on any of these parameters in ER alpha(-/-) mice. Immunohistochemistry was subsequently performed using an ER alpha-specific C-terminal polyclonal antibody. In WT mice, ER alpha was expressed both by cancellous osteoblasts and a significant proportion of mononuclear bone marrow cells. Immunoreactivity was also observed in cancellous osteoblasts of ER alpha(-/-) mice, resulting from expression of the activation function-1-deficient 46-kDa ER alpha isoform previously reported to be expressed in normal osteoblasts and bones of ER alpha(-/-) mice. Taken together, our results suggest that estrogen stimulates bone formation in mouse long bones via a mechanism that requires the presence of full-length ER alpha possessing activation function-1.