TRAP150 interacts with the RNA-binding domain of PSF and antagonizes splicing of numerous PSF-target genes in T cells

PSF (a.k.a. SFPQ) is a ubiquitously expressed, essential nuclear protein with important roles in DNA damage repair and RNA biogenesis. In stimulated T cells, PSF binds to and suppresses the inclusion of CD45 exon 4 in the final mRNA; however, in resting cells, TRAP150 binds PSF and prevents access to the CD45 RNA, though the mechanism for this inhibition has remained unclear. Here, we show that TRAP150 binds a region encompassing the RNA recognition motifs (RRMs) of PSF using a previously uncharacterized, 70 residue region we have termed the PSF-interacting domain (PID). TRAP150's PID directly inhibits the interaction of PSF RRMs with RNA, which is mediated through RRM2. However, interaction of PSF with TRAP150 does not appear to inhibit the dimerization of PSF with other Drosophila Behavior, Human Splicing (DBHS) proteins, which is also dependent on RRM2. Finally, we use RASL-Seq to identify ∼40 T cell splicing events sensitive to PSF knockdown, and show that for the majority of these, PSF's effect is antagonized by TRAP150. Together these data suggest a model in which TRAP150 interacts with dimeric PSF to block access of RNA to RRM2, thereby regulating the activity of PSF toward a broad set of splicing events in T cells.

PSF: nuclear busy-body or nuclear facilitator?

PTB-associated splicing factor (PSF) is an abundant and essential nucleic acid-binding protein that participates in a wide range of gene regulatory processes and cellular response pathways. At the protein level, PSF consists of multiple domains, many of which remain poorly characterized. Although grouped in a family with the proteins p54nrb/NONO and PSPC1 based on sequence homology, PSF contains additional protein sequence not included in other family members. Consistently, PSF has also been implicated in functions not ascribed to p54nrb/NONO or PSPC1. Here, we provide a review of the cellular activities in which PSF has been implicated and what is known regarding the mechanisms by which PSF functions in each case. We propose that the complex domain arrangement of PSF allows for its diversity of function and integration of activities. Finally, we discuss recent evidence that individual activities of PSF can be regulated independently from one another through the activity of domain-specific co-factors.

Alternative splicing networks regulated by signaling in human T cells

The formation and execution of a productive immune response requires the maturation of competent T cells and a robust change in cellular activity upon antigen challenge. Such changes in cellular function depend on regulated alterations to protein expression. Previous research has focused on defining transcriptional changes that regulate protein expression during T-cell maturation and antigen stimulation. Here, we globally analyze another critical process in gene regulation during T-cell stimulation, alternative splicing. Specifically, we use RNA-seq profiling to identify 178 exons in 168 genes that exhibit robust changes in inclusion in response to stimulation of a human T-cell line. Supporting an important role for the global coordination of alternative splicing following T-cell stimulation, these signal-responsive exons are significantly enriched in genes with functional annotations specifically related to immune response. The vast majority of these genes also exhibit differential alternative splicing between naive and activated primary T cells. Comparison of the responsiveness of splicing to various stimuli in the cultured and primary T cells further reveals at least three distinct networks of signal-induced alternative splicing events. Importantly, we find that each regulatory network is specifically associated with distinct sequence features, suggesting that they are controlled by independent regulatory mechanisms. These results thus provide a basis for elucidating mechanisms of signal pathway-specific regulation of alternative splicing during T-cell stimulation.

Differential effects of the muscarinic M2 antagonists, AF-DX 116 and gallamine, on single neurons of rabbit sympathetic ganglia

Intracellular recording techniques were used to compare the effects of the M2 muscarinic antagonists, AF-DX 116 and gallamine, on membrane potential (Vm), input resistance (Ri), responses induced by methacholine, muscarinic slow postsynaptic potentials and action potentials in the superior cervical ganglion of the rabbit. Gallamine or AF-DX 116 antagonized methacholine-induced or synaptically-evoked muscarinic hyperpolarization, without having significant effect on depolarization induced by methacholine or synaptically. The drug AF-DX 116 reduced evoked muscarinic hyperpolarizing potentials, without significant change in Vm or Ri, recorded in the absence of muscarinic stimulation. In contrast to AF-DX 116, gallamine elicited a concentration-dependent depolarization of the membrane, with a corresponding increase in Ri, when tested in the absence of muscarinic stimulation. These effects of gallamine were accompanied by an increase in duration and decrease in the slope of the descending phase of the action potential. Blockade by gallamine of evoked hyperpolarization was independent of membrane depolarization and readily occurred when gallamine-induced depolarization was prevented by clamping Vm at its pre-gallamine level. The effects of gallamine were maintained during its presence and reversed upon washing with gallamine-free physiological solution. These results indicate that AF-DX 116 and gallamine have a specificity for antagonism of muscarinic responses, mediated by receptors of the M2 type in the superior cervical ganglion. However, gallamine, while an effective antagonist of M2 responses, also has the ability to modify the electrical characteristics of ganglion cells and thus may modify ganglionic transmission by mechanisms other than antagonism of receptors.

AF-DX 116: a selective antagonist of the slow inhibitory postsynaptic potential and methacholine-induced hyperpolarization in superior cervical ganglion of the rabbit

AF-DX 116 [11-([2-[(diethylamino)methyl]-1-piperdinyl]acetyl)-5, 11-dihydro-6H-pyrido[2,3-b][1,4]benzodiaze pine-6-one], a muscarinic receptor antagonist that divides the M2-type muscarinic receptor into additional functional classes, modified muscarinic responses recorded from the superior cervical ganglion of the rabbit with sucrose or air gap techniques. Incubation of ganglia with AF-DX 116 suppressed the amplitude of the slow-inhibitory postsynaptic potential (s-IPSP) in a concentration-dependent and highly specific manner. At concentrations which reduced the amplitude of the s-IPSP by 80 to 90%, there was no significant reduction of the amplitudes of the muscarinic slow-excitatory postsynaptic potential or the nicotinic fast-excitatory postsynaptic potential. In addition, superfusion of ganglia with AF-DX 116 resulted in the concentration-dependent suppression of ganglionic hyperpolarization induced by methacholine without suppression of methacholine-induced depolarization. Ganglionic hyperpolarization that was produced by norepinephrine was unaffected by AF-DX 116. Increasing the level of acetylcholine available for interaction with muscarinic receptors by increasing the number of stimulus volleys that were applied to the preganglionic nerve resulted in a parallel shift, to the right, of the concentration-response curve for suppression of the s-IPSP by AF-DX 116. Similarly, incubation of ganglia with the specific antiacetylcholinesterase, BW 284 (1-5-bis[4-allyl dimethylammonium phenyl]pentan-3 one dibromide), increased the concentration of AF-DX 116 that was required to produce a comparable suppression of the s-IPSP. These results indicate that the s-IPSP in mammalian superior cervical ganglion involves an action of acetylcholine at the M2 type receptor that is preferentially blocked by AF-DX 116.

Modification of nicotinic ganglionic transmission by muscarinic slow postsynaptic potentials in the in vitro rabbit superior cervical ganglion

The influence of slow muscarinic postsynaptic potentials, i.e., the s-IPSP and s-EPSP, on synaptic transmission mediated through nicotinic receptors was studied in the superior cervical ganglion of the rabbit. Postganglionic spikes and synaptic potentials were elicited by delivery of conditioning and test stimulus pulses to afferent fibers. When paired stimulus volleys were separated by brief intervals (20-100 msec) or long intervals (1,000-8,000 msec), the population spike elicited by the test stimulus was larger in amplitude than that elicited by the conditioning volley. When paired stimulus volleys were separated by 250-500 msec, the amplitude of the population spike elicited by the test volley was smaller than that elicited by the conditioning stimulus. Gallamine, which selectively blocks the s-IPSP, reduced the suppression of the test spike which occurred when stimulus IPIs ranged between 250-500 msec. Pirenzepine, which selectively blocks the s-EPSP, reduced the late facilitation of test postganglionic spikes which occurred with stimulus IPIs greater than 1,000 msec. The non-selective muscarinic antagonist QNB, produced changes in postganglionic spike amplitude that were similar to the combined effects of gallamine and pirenzepine. The evidence indicates that the s-IPSP and s-EPSP modified the excitability state of the ganglionic neurons and subsequent synaptic transmission that was mediated through nicotinic receptors.

Antagonist discrimination of two muscarinic responses elicited by applied agonists and orthodromic stimuli in superior cervical ganglion of rabbit

Pirenzepine and gallamine selectively and differentially antagonized two muscarinic responses, in the superior cervical ganglion of the rabbit, whether elicited by the muscarinic agonist methacholine or by orthodromic stimulation. Methacholine elicited a biphasic ganglionic response, consisting of hyperpolarizing and depolarizing components that were the agonist-induced equivalents of the slow-inhibitory and slow-excitatory postsynaptic potentials elicited by orthodromic stimulation. Superfusion of ganglia with pirenzepine resulted in a concentration-dependent suppression of depolarization induced by methacholine with no suppressant action on ganglionic hyperpolarization. In contrast, superfusion of ganglia with gallamine resulted in a concentration-dependent suppression of ganglionic hyperpolarization and the slow-inhibitory postsynaptic potential. These effects occurred without appreciable suppression of ganglionic depolarization or the slow-excitatory postsynaptic potential. The action of gallamine was specific for muscarinic hyperpolarization. Hyperpolarizations produced by superfusion with dopamine or norepinephrine were unaffected by gallamine, at concentrations that suppressed the muscarinic slow-inhibitory post-synaptic potential. Incubation with anti-cholinesterases produced a parallel shift, to the right, of concentration-response curves for suppression by gallamine of the slow-inhibitory postsynaptic potential. This was presumably the consequence of an increase in the acetylcholine available for interaction with the muscarinic receptor. The evidence suggests that the ability of gallamine and pirenzepine to suppress selectively the slow-inhibitory and slow-excitatory postsynaptic potentials, as previously demonstrated, is through an action at muscarinic receptors. Furthermore, the data suggest that these pharmacological agents produce their effects by interaction at different muscarinic recognition sites.

Differential and selective antagonism of the slow-inhibitory postsynaptic potential and slow-excitatory postsynaptic potential by gallamine and pirenzepine in the superior cervical ganglion of the rabbit

Two cholinergic antagonists, gallamine and pirenzepine, agents that have been shown to bind selectively to different subpopulations of the muscarinic receptor, were found to antagonize selectively and differentially the amplitudes of the slow-inhibitory and slow-excitatory postsynaptic potentials in the superior cervical ganglion of the rabbit. Incubation of ganglia with gallamine resulted in a concentration-dependent suppression of the slow-inhibitory postsynaptic potential. The pharmacological action of gallamine was highly specific. At concentrations which reduced the amplitude of the slow-inhibitory postsynaptic potential by as much as 70-90%, there was no reduction of the amplitudes of the muscarinic slow-excitatory postsynaptic potential, the nicotinic fast-excitatory postsynaptic potential, noncholinergic slow-slow-excitatory postsynaptic potential, or post-stimulus hyperpolarizing afterpotentials. The amplitude of the slow-excitatory postsynaptic potential was actually facilitated in the presence of gallamine, presumably as a result of suppression of the overlapping slow-inhibitory postsynaptic potential. In contrast to the action of gallamine, pirenzepine produced a selective suppression of the amplitude of the slow-excitatory postsynaptic potential. Pirenzepine had very little influence on the amplitude of the slow-inhibitory postsynaptic potential at concentrations sufficient to reduce the amplitude of the slow-excitatory postsynaptic potential by as much as 50%, and had no effect on the amplitudes of the nicotinic fast-excitatory postsynaptic potential or noncholinergic slow-slow-excitatory postsynaptic potential. The evidence presented suggests that multiple muscarinic recognition sites, previously identified by studies of the affinities of pharmacological agents for the muscarinic receptor, may actually be involved in synaptic transmission and functionally coupled to cellular effector mechanisms.

Ganglionic slow postsynaptic potentials and muscarinic asynchronous discharge in postganglionic nerve elicited by orthodromic stimulation

Simultaneous recordings of the slow postsynaptic potentials of in vitro rabbit superior cervical ganglion and the muscarinic asynchronous discharge in the postganglionic nerve were obtained in order to examine their relationship. Analyses for linear correlation indicated that the peak amplitude of the S-EPSP cannot fully determine the maximum rate of muscarinic asynchronous discharge. The rate of asynchronous discharge in the postganglionic nerve systematically increased with increases in either the frequency or duration of preganglionic stimulation even after the S-EPSP was at asymptotic levels. We suggest that this deviation from linearity is the result of repetitive spike discharge elicited by the S-EPSP. The S-IPSP had no apparent influence on generation of asynchronous afterdischarge, but may have had an influence on asynchronous discharge occurring during a train of stimuli. Incubation of ganglia with gallamine, an antagonist of the ganglionic muscarinic receptors mediating hyperpolarization, resulted in the selective blockade of the S-IPSP. Neither the S-EPSP nor the muscarinic afterdischarge were suppressed by muscarinic receptor blockade by gallamine, nor did gallamine produce any effect on the nicotinic F-EPSP or the noncholinergic SS-EPSP. Temporary exposure of ganglia to dopamine, in the presence of an inhibitor of catechol-O-methyltransferase, was followed by a potentiation of the muscarinic afterdischarge in accordance with the long-term enhancement reported for the S-EPSP.