Osteoblastic expansion induced by parathyroid hormone receptor signaling in murine osteocytes is not sufficient to increase hematopoietic stem cells.

Microenvironmental expansion of hematopoietic stem cells (HSCs) is induced by treatment with parathyroid hormone (PTH) or activation of the PTH receptor (PTH1R) in osteoblastic cells; however, the osteoblastic subset mediating this action of PTH is unknown. Osteocytes are terminally differentiated osteoblasts embedded in mineralized bone matrix but are connected with the BM. Activation of PTH1R in osteocytes increases osteoblastic number and bone mass. To establish whether osteocyte-mediated PTH1R signaling expands HSCs, we studied mice expressing a constitutively active PTH1R in osteocytes (TG mice). Osteoblasts, osteoclasts, and trabecular bone were increased in TG mice without changes in BM phenotypic HSCs or HSC function. TG mice had progressively increased trabecular bone but decreased HSC function. In severely affected TG mice, phenotypic HSCs were decreased in the BM but increased in the spleen. TG osteocytes had no increase in signals associated with microenvironmental HSC support, and the spindle-shaped osteoblastic cells that increased with PTH treatment were not present in TG bones. These findings demonstrate that activation of PTH1R signaling in osteocytes does not expand BM HSCs, which are instead decreased in TG mice. Therefore, osteocytes do not mediate the HSC expansion induced by PTH1R signaling. Further, osteoblastic expansion is not sufficient to increase HSCs.

Overview of serotonin syndrome.

BACKGROUND: Serotonin syndrome (SS) is a rare and potentially life-threatening toxic state caused by an adverse drug reaction that leads to excessive central and peripheral serotonergic activity. This excessive serotonin hyperstimulation may be secondary to 1 standard therapeutic dose of a single agent, inadvertent interactions between various drugs, intentionally or unintentionally excessive use of particular drugs, deliberate self-harm, or recreational use of certain drugs. This review article serves as an overview of the epidemiology, pathophysiology, clinical features, diagnosis, differential diagnosis, management, and prevention of SS. METHODS: The authors conducted a MEDLINE search for the period covering 1955 to 2011. RESULTS: SS commonly occurs after the use of serotonergic agents alone or in combination with monoamine oxidase inhibitors. SS classically consists of a triad of signs and symptoms broadly characterized as alteration of mental status, abnormalities of neuromuscular tone, and autonomic hyperactivity. However, all 3 triads of SS may not occur simultaneously. Clinical manifestations are diverse and nonspecific, which may lead to misdiagnosis. SS can range in severity from mild to life-threatening. Most cases of SS are mild and resolve with prompt recognition and supportive care. Management of SS involves withdrawal of the offending agent(s), aggressive supportive care to treat hyperthermia and autonomic dysfunction, and occasionally the administration of serotonin antagonists--cyproheptadine or chlorpromazine. Patients with moderate and severe cases of SS require inpatient hospitalization. CONCLUSIONS: Psychiatrists, clinicians, and general practitioners must develop increased awareness of SS due to the current increase in the use of serotonergic agents in clinical practice. As SS is a manifestation of adverse pharmacology, it is not considered an idiosyncratic drug reaction, making it predictable and highly preventable. Most cases of SS are mild and easily managed. With prompt recognition and supportive care, more severe cases of SS have a favorable prognosis.