MTB, the murine homolog of condensin II subunit CAP-G2, represses transcription and promotes erythroid cell differentiation.

Chromosome condensation is essential for proper segregation of duplicated sister chromatids in mitosis. Mammalian erythroid maturation is also associated with gradual nuclear condensation. However, few proteins that are directly involved in chromosome condensation during erythropoiesis have been identified. In this report, we show that MTB (more than blood), which was initially isolated in a yeast two-hybrid screen for proteins that interact with the basic helix-loop-helix (bHLH) protein stem cell leukemia (SCL), and later identified as the murine homolog of the condensin II subunit CAP-G2, participates in erythroid cell development. MTB interacts with SCL and another hematopoietic bHLH protein, E12, and is recruited to the nucleus by SCL and E12. In addition, MTB can repress SCL/E12-mediated transcriptional activation. Consistent with the model that MTB may function together with SCL/E12 heterodimer during erythroid cell development, MTB is highly expressed in the erythroid lineage and is upregulated upon erythroid differentiation. Moreover, overexpression of MTB promotes the terminal differentiation of the murine erythroleukemia erythroid cell line. Together, these findings demonstrate that the condensin II subunit MTB/mCAP-G2 plays a novel function during erythropoiesis and suggest that key hematopoietic transcription factors such as SCL and E12 may regulate the terminal differentiation of hematopoietic cells through the interaction with condensin complexes.

Physiological properties of raphe magnus neurons during sleep and waking.

Neurons in the medullary raphe magnus (RM) that are important in the descending modulation of nociceptive transmission are classified by their response to noxious tail heat as ON, OFF, or NEUTRAL cells. Experiments in anesthetized animals demonstrate that RM ON cells facilitate and OFF cells inhibit nociceptive transmission. Yet little is known of the physiology of these cells in the unanesthetized animal. The first aim of the present experiments was to determine whether cells with ON- and OFF-like responses to noxious heat exist in the unanesthetized rat. Second, to determine if RM cells have state-dependent discharge, the activity of RM neurons was recorded during waking and sleeping states. Noxious heat applied during waking and slow wave sleep excited one group of cells (ON-U) in unanesthetized rats. Other cells were inhibited by noxious heat (OFF-U) applied during waking and slow wave sleep states in unanesthetized rats. NEUTRAL-U cells did not respond to noxious thermal stimulation applied during either slow wave sleep or waking. ON-U and OFF-U cells were more likely to respond to noxious heat during slow wave sleep than during waking and were least likely to respond when the animal was eating or drinking. Although RM cells rarely respond to innocuous stimulation applied during anesthesia, ON-U and OFF-U cells were excited and inhibited, respectively, by innocuous somatosensory stimulation in the unanesthetized rat. The spontaneous activity of >90% of the RM neurons recorded in the unanesthetized rat was influenced by behavioral state. OFF-U cells discharged sporadically during waking but were continuously active during slow wave sleep. By contrast, ON-U and NEUTRAL-U cells discharged in bursts during waking and either ceased to discharge entirely or discharged at a low rate during slow wave sleep. We suggest that OFF cell discharge functions to suppress pain-evoked reactions during sleep, whereas ON cell discharge facilitates pain-evoked responses during waking.

Physiological survey of medullary raphe and magnocellular reticular neurons in the anesthetized rat.

The present study was designed to provide a detailed and quantitative description of the physiological characteristics of neurons in the medullary raphe magnus (RM) and adjacent nucleus reticularis magnocellularis (NRMC) under anesthetized conditions. The background discharge and noxious stimulus-evoked responses of RM and NRMC neurons were recorded in rats lightly anesthetized with isoflurane. All cells that were isolated successfully were studied. After recording background discharge, the neuronal response to repeated noxious thermal and noxious mechanical stimulation of the tail was recorded. Most cells were identified as nonserotonergic by their irregular or rapid background discharge pattern. Because the spontaneous discharge of most RM nonserotonergic cells contained pauses and bursts, a comparison between the change in rate evoked by tail heat and the range of rate changes that occur spontaneously was used to classify cells. The mean responses of ON and OFF cells were more than four times the standard deviation of the changes in rate observed spontaneously. ON cells were excited in 86% of the tail heat trials tested. Similarly, OFF cells were inhibited in 97% of the noxious tail heat trials tested. The heat-evoked changes in ON and OFF cell discharge varied over more than two orders of magnitude and were greater in cells with greater rates of background discharge. The heat-evoked responses of and cells had durations of tens of seconds to minutes and were always sustained beyond the visible motor response. Most ON and OFF cells responded to noxious tail clamp in a manner that was similar to their response to noxious heat. More than half of the NEUTRAL cells that were unresponsive to noxious heat were responsive to noxious tail clamp. A minority of ON, OFF, and NEUTRAL cells responded to innocuous brush stimulation with weak, transient responses. Although many cells discharged too infrequently to be classified, units with physiological properties that were different from those described above were rare. In conclusion, most RM and NRMC cells belong to three nonserotonergic physiological cell classes that can be distinguished from each other by the consistency, not the magnitude, of their responses to repeated noxious thermal stimulation. Because most of the heat-evoked change in and cell discharge occurs after the conclusion of the initial motor withdrawal, ON and OFF cells are likely to principally modulate the response to subsequent noxious insults.

Spectral analysis of arterial blood pressure and raphe magnus neuronal activity in anesthetized rats.

Recent evidence suggests that nociceptive modulatory cells in the nucleus raphe magnus (RM) and adjacent nucleus reticularis magnocellularis (NRMC) may participate in the modulation of autonomic processing. Therefore, spectral analyses were used to determine component frequencies common to both arterial blood pressure and the activity of RM/NRMC neurons in rats lightly anesthetized with isoflurane. These analyses detected powerful, extremely low frequency (period length: 6.4-18.5 min) oscillations in arterial blood pressure and in the activity of two classes of RM/NRMC neurons, ON and OFF cells. All ON cells discharged during periods of low blood pressure, whereas all OFF cells discharged during periods of high blood pressure. In contrast, the discharge of NEUTRAL and REGULAR cells did not have a consistent relationship to blood pressure. The role of these cells in nociceptive modulation is also unclear. The results presented indicate that ON and OFF cells may participate in the modulation of both autonomic and nociceptive processing.

Effects of isoflurane concentration on the activity of pontomedullary raphe and medial reticular neurons in the rat.

Neurons in the pontomedullary raphe magnus (RM) and adjacent nucleus reticularis paragigantocellularis pars alpha (NRPG alpha) are thought to participate in the modulation of spinal nociceptive transmission. In order to determine whether these cells also contribute to the suppression of nocifensive reflexes produced by general anesthetics, the spontaneous activity of RM/NRPG alpha cells was recorded in rats anesthetized with isoflurane (IF) at several steady state concentrations, corresponding to depths which are below, equal to, or above the threshold for blocking the motor response to noxious stimuli (minimum alveolar concentration, MAC). Neurons were classified by their spontaneous activity patterns and their responses to noxious stimulation as OFF, ON, REGULAR or NEUTRAL cells. After cell classification, unit activity, arterial blood pressure, heart rate, and EEG activity were simultaneously recorded, in the absence of somatic stimulation, for 1 h at each of two or three concentrations of IF. The concentrations tested were low (1.05-1.25%), medium (1.30-1.45%) and high (1.70-1.90%). ON, OFF and some NEUTRAL cells exhibited alternating periods of inactivity and activity when recorded during periods of low and medium anesthetic concentrations. At high steady state anesthetic concentrations, the mean discharge of most OFF, ON and NEUTRAL cells decreased by greater than 25% from their mean discharge rate at the low concentration. REGULAR cells maintained a uniform firing rate at all steady state anesthetic concentrations studied. Since high concentrations of IF do not activate OFF cells, the putative inhibitory output neuron of the RM/NRPG alpha, it is unlikely that the activity of RM/NRPG alpha neurons contributes to the suppression of nocifensive movement by the general anesthetic, IF.