Light-induced tyrosine phosphorylation of BIT in the rat suprachiasmatic nucleus

Tyrosine phosphorylation of the transmembrane protein SIRPα: Sensing synaptic activity and regulating ectodomain cleavage for synapse maturation

Synapse maturation is a neural activity-dependent process during brain development, in which active synapses preferentially undergo maturation to establish efficient neural circuits in the brain. Defects in this process are implicated in various neuropsychiatric disorders. We have previously reported that a postsynaptic transmembrane protein, signal regulatory protein-α (SIRPα), plays an important role in activity-dependently directing synapse maturation. In the presence of synaptic activity, the ectodomain of SIRPα is cleaved and released and then acts as a retrograde signal to induce presynaptic maturation. However, how SIRPα detects synaptic activity to promote its ectodomain cleavage and synapse maturation is unknown. Here, we show that activity-dependent tyrosine phosphorylation of SIRPα is critical for SIRPα cleavage and synapse maturation. We found that during synapse maturation and in response to neural activity, SIRPα is highly phosphorylated on its tyrosine residues in the hippocampus, a structure critical for learning and memory. Tyrosine phosphorylation of SIRPα was necessary for SIRPα cleavage and presynaptic maturation, as indicated by the fact that a phosphorylation-deficient SIRPα variant underwent much less cleavage and could not drive presynaptic maturation. However, SIRPα phosphorylation did not affect its synaptic localization. Finally, we show that inhibitors of the Src and JAK kinase family suppress neural activity-dependent SIRPα phosphorylation and cleavage. Together, our results indicate that SIRPα phosphorylation serves as a mechanism for detecting synaptic activity and linking it to the ectodomain cleavage of SIRPα, which in turn drives synapse maturation in an activity-dependent manner.

Functions and molecular mechanisms of the CD47-SIRPalpha signalling pathway

Signal regulatory protein (SIRP)alpha, also known as SHPS-1 or SIRPA, is a transmembrane protein that binds to the protein tyrosine phosphatases SHP-1 and SHP-2 through its cytoplasmic region and is predominantly expressed in neurons, dendritic cells and macrophages. CD47, a widely expressed transmembrane protein, is a ligand for SIRPalpha, with the two proteins constituting a cell-cell communication system. The interaction of SIRPalpha with CD47 is important for the regulation of migration and phagocytosis. Recent studies have implicated the CD47-SIRPalpha signalling pathway in immune homeostasis and in regulation of neuronal networks. Advances in the structural and functional analyses of the CD47-SIRPalpha signalling pathway now provide exciting hints of the therapeutic benefits of manipulating this signalling system in autoimmune diseases and neurological disorders.

Effects of central injection of l-carnosine on sympathetic nerve activity innervating brown adipose tissue and body temperature in rats

In the present study, using urethane-anesthetized rats, we examined the effects of intralateral cerebral ventricular (LCV) injection of various doses of L-carnosine on neural activity innervating brown adipose tissue (BAT-SNA) and body temperature (BT). We found that injection of a low dose of L-carnosine (0.01 microg) suppressed BAT-SNA significantly. Conversely, a high dose (100 microg) of L-carnosine significantly elevated BAT-SNA. In the light period (14:00), brown adipose tissue temperature (BAT-T) and BT were suppressed after low and elevated after high dose injection of L-carnosine whereas in the dark period (2:00), these parameters remained unchanged with L-carnosine treatment. Bilateral lesions of the hypothalamic suprachiasmatic nucleus (SCN) abolished the effects of low and high doses of L-carnosine on BAT-SNA, BAT-T and BT. Furthermore, high dose treatment with L-carnosine altered c-Fos induction in the SCN and the PVN. These results suggest that l-carnosine affects BAT-SNA, BAT-T and BT in a dose-dependent manner in the rat, and that the SCN may be involved in these effects.

Structural insight into the specific interaction between murine SHPS-1/SIRP alpha and its ligand CD47

SRC homology 2 domain-containing protein tyrosine phosphatase substrate 1 (SHPS-1 or SIRP alpha/BIT) is an immunoglobulin (Ig) superfamily transmembrane receptor and a member of the signal regulatory protein (SIRP) family involved in cell-cell interaction. SHPS-1 binds to its ligand CD47 to relay an inhibitory signal for cellular responses, whereas SIRPbeta, an activating member of the same family, does not bind to CD47 despite sharing a highly homologous ligand-binding domain with SHPS-1. To address the molecular basis for specific CD47 recognition by SHPS-1, we present the crystal structure of the ligand-binding domain of murine SHPS-1 (mSHPS-1). Folding topology revealed that mSHPS-1 adopts an I2-set Ig fold, but its overall structure resembles IgV domains of antigen receptors, although it has an extended loop structure (C'E loop), which forms a dimer interface in the crystal. Site-directed mutagenesis studies of mSHPS-1 identified critical residues for CD47 binding including sites in the C'E loop and regions corresponding to complementarity-determining regions of antigen receptors. The structural and functional features of mSHPS-1 are consistent with the human SHPS-1 structure except that human SHPS-1 has an additional beta-strand D. These results suggest that the variable complementarity-determining region-like loop structures in the binding surface of SHPS-1 are generally required for ligand recognition in a manner similar to that of antigen receptors, which may explain the diverse ligand-binding specificities of SIRP family receptors.

Regulation of sympathetic and parasympathetic nerve activities by BIT/SHPS-1

The hypothalamus plays a central role in the homeostatic regulation of internal physiological conditions such as body temperature and energy balance. We have previously shown that cold exposure enhances tyrosine phosphorylation of BIT/SHPS-1 (brain immunoglobulin-like molecule with tyrosine-based activation motifs/SHP substrate-1) in hypothalamic nuclei including the suprachiasmatic nucleus. In order to elucidate the function of BIT/SHPS-1 in the hypothalamus, we stimulated BIT/SHPS-1 in vivo by using the anti-BIT monoclonal antibody (mAb) 1D4, which reacts with the extracellular domain of BIT/SHPS-1 and induces its tyrosine phosphorylation. Administration of mAb 1D4 into the third cerebral ventricle enhanced the electrical activity of the renal sympathetic nerves, while it suppressed that of the gastric parasympathetic nerves. Similarly, blood pressure increased in response to the mAb 1D4 injection, and additionally, temperatures of the abdomen and brown adipose tissue increased. These results indicate that BIT/SHPS-1 is involved in the hypothalamic regulation of thermogenesis via the autonomic nervous system.

SIRPα negatively regulates differentiation of PC12 cell

Signal regulatory protein alpha (SIRPalpha) is an Ig superfamily protein whose cytoplasmic region contains immunoreceptor tyrosine-based inhibitory motif (ITIM), which when tyrosine phosphorylated binds the SH2-domain containing phosphatase 2 (SHP-2). Both SIRPalpha and SHP-2 are highly expressed in brain. Murine cerebellar cells cultured on SIRPalpha-coated surface exhibit enhanced neurite outgrowth and SIRPalpha is localized at sites of synaptogenesis in postnatal mouse brain. In this study, we show that nerve growth factor (NGF) stimulation resulted in elevated SIRPalpha expression during PC12 differentiation. We also show that NGF-induced morphological differentiation, but not growth arrest response, was inhibited by ectopic SIRPalpha expression. PC12 cells stably expressing SIRPalpha proliferated more rapidly than mock-transfected cells. The activity of c-jun N-terminal kinase (JNK) decreased in SIRPalpha-transfected PC12 cells, whereas nuclear factor-kappaB (NF-kappaB) activity increased. Collectively, our results suggest that SIRPalpha may stabilize synaptic connections by inhibiting improper neurite outgrowth and might realize its neuronal function, at least in part, by modulating JNK and NF-kappaB activity.

Cold exposure induces tyrosine phosphorylation of BIT through NMDA receptors in the rat hypothalamus

The hypothalamus has a central role in maintaining homeostases of physiological conditions including body temperature and energy balance. To examine molecular responses to cold exposure in the hypothalamus, we examined changes in protein tyrosine phosphorylation in the suprachiasmatic nucleus of the hypothalamus after acute cold exposure in rats. It was found that brain immunoglobulin-like molecule with tyrosine-based inhibitory motifs (BIT, also called SHPS-1, SIRPalpha or p84), a transmembrane glycoprotein with two ITIM motifs, showed enhanced tyrosine phosphorylation after cold exposure. Its tyrosine phosphorylation induced by cold exposure was also found in other hypothalamic nuclei including the paraventricular nucleus, lateral hypothalamic area, ventromedial hypothalamus, and arcuate nucleus. This phosphorylation was blocked by AP-5, an NMDA receptor antagonist, indicating that it was mediated by NMDA receptors. These results suggest that BIT is involved in the mechanism of neuronal responses to cold exposure in the hypothalamus.

Tyrosine phosphorylation of BIT on photic stimulation in the rat retina

BIT is a transmembrane glycoprotein with three immunoglobulin-like domains in its extracellular region and tyrosine phosphorylation sites in its cytosolic region. We have previously shown that BIT was tyrosine phosphorylated in the hypothalamic suprachiasmatic nucleus in response to light exposure during the dark period, and suggested that it was involved in the light entrainment of the circadian clock. To further investigate the function of BIT in the nervous system, we examined the effect of photic stimulation on its tyrosine phosphorylation in the rat retina. It was found that the tyrosine phosphorylation level of BIT in the retina was higher in the light period than in the dark period. In addition, a light stimulation during the dark period resulted in a rapid phosphorylation of BIT and a subsequent association of BIT with SHP-2. The phosphorylation state was quickly reverted when the light was turned off. The light-dependent phosphorylation of BIT was also observed in isolated cultured retinas, and this was blocked by a specific Src-family inhibitor, PP-2. Immunohistochemical study showed that BIT was highly enriched in the inner and outer plexiform layers in the retina, where the immunoreactivity to anti-SHP-2 antibody was also detected. These results suggest that tyrosine phosphorylation of BIT is involved in neuronal transmission in the retina.

Resetting Mechanism of Central and Peripheral Circadian Clocks in Mammals

Almost all organisms on earth exhibit diurnal rhythms in physiology and behavior under the control of autonomous time-measuring system called circadian clock. The circadian clock is generally reset by environmental time cues, such as light, in order to synchronize with the external 24-h cycles. In mammals, the core oscillator of the circadian clock is composed of transcription/translation-based negative feedback loops regulating the cyclic expression of a limited number of clock genes (such as Per, Cry, Bmal1, etc.) and hundreds of output genes in a well-concerted manner. The central clock controlling the behavioral rhythm is localized in the hypothalamic suprachiasmatic nucleus (SCN), and peripheral clocks are present in other various tissues. The phase of the central clock is amenable to ambient light signal captured by the visual rod-cone photoreceptors and non-visual melanopsin in the retina. These light signals are transmitted to the SCN through the retinohypothalamic tract, and transduced therein by mitogen-activated protein kinase and other signaling molecules to induce Per gene expression, which eventually elicits phase-dependent phase shifts of the clock. The central clock controls peripheral clocks directly and indirectly by virtue of neural, humoral, and other signals in a coordinated manner. The change in feeding time resets the peripheral clocks in a SCN-independent manner, possibly by food metabolites and body temperature rhythms. In this article, we will provide an overview of recent molecular and genetic studies on the resetting mechanism of the central and peripheral circadian clocks in mammals.

Stimulation of BIT induces a circadian phase shift of locomotor activity in rats

Circadian rhythms of mammals are generated by a circadian oscillation of master pacemaker genes in the suprachiasmatic nucleus of the hypothalamus (SCN), and entrained by environmental factors such as 24-h light-dark cycles. We have previously shown that light exposure during the dark period enhanced tyrosine phosphorylation of brain immunoglobulin-like molecule with tyrosine-based activation motifs (BIT) in the rat SCN. To elucidate the functional roles of BIT in the circadian clock, we stimulated BIT using an anti-BIT monoclonal antibody (mAb) 1D4, which reacts with its extracellular region and induces phosphorylation of its intracellular tyrosine residues. Administration of mAb 1D4 into the third cerebral ventricle induced tyrosine phosphorylation of BIT in the SCN. Behavioral analyses showed that the SCN-injection of the antibody at CT15 induced a phase delay of the circadian rhythm of locomotor activity, and that at CT20 induced a phase advance. Pretreatment with MK801, a non-competitive antagonist of NMDARs, diminished the 1D4-induced phase shift at CT20, but not at CT15. These results suggest that BIT is involved in the entrainment of circadian rhythms through the function of NMDARs and non-NMDARs.

In vivo regulation of phosphoinositide 3-kinase in retina through light-induced tyrosine phosphorylation of the insulin receptor beta-subunit

Recently, we have shown that phosphoinositide 3-kinase (PI3K) in bovine rod outer segment (ROS) is activated in vitro by tyrosine phosphorylation of the C-terminal tail of the insulin receptor (Rajala, R. V. S., and Anderson, R. E. (2001) Invest. Ophthal. Vis. Sci. 42, 3110-3117). In this study, we have investigated the in vivo mechanism of PI3K activation in the rodent retina and report the novel finding that light stimulates tyrosine phosphorylation of the beta-subunit of the insulin receptor (IRbeta) in ROS membranes, which leads to the association of PI3K enzyme activity with IRbeta. Retinas from light- or dark-adapted mice and rats were homogenized and immunoprecipitated with antibodies against phosphotyrosine, IRbeta, or the p85 regulatory subunit of PI3K, and PI3K activity was measured using PI-4,5-P(2) as substrate. We observed a light-dependent increase in tyrosine phosphorylation of IRbeta and an increase in PI3K enzyme activity in isolated ROS and in anti-phosphotyrosine and anti-IRbeta immunoprecipitates of retinal homogenates. The light effect was localized to photoreceptor neurons and is independent of insulin secretion. Our results suggest that light induces tyrosine phosphorylation of IRbeta in outer segment membranes, which leads to the binding of p85 through its N-terminal Src homology 2 domain and the generation of PI-3,4,5-P(3). We suggest that the physiological role of this process may be to provide neuroprotection of the retina against light damage by activating proteins that protect against stress-induced apoptosis.

SHPS-1, a multifunctional transmembrane glycoprotein

Src homology 2 (SH2) domain-containing protein tyrosine phosphatase substrate 1 (SHPS-1) is a member of the signal regulatory protein (SIRP) family. The amino-terminal immunoglobulin-like domain of SHPS-1 is necessary for interaction with CD47, a ligand for SHPS-1, which plays an important role in cell-cell interaction. The intracellular region of SHPS-1, on the other hand, may act as a scaffold protein, binding to various adapter proteins. Interestingly, increasing evidence has shown that SHPS-1 is involved in various biological phenomena, including suppression of anchorage-independent cell growth, negative regulation of immune cells, self-recognition of red blood cells, mediation of macrophage multinucleation, skeletal muscle differentiation, entrainment of circadian clock, neuronal survival and synaptogenesis. Recent progress has been made in attributing these novel exciting functions. Here we discuss how this interesting molecule works and consider its true role in biology.