Effect of linker substitution on the binding of butorphan univalent and bivalent ligands to opioid receptors

A series of bivalent hydroxy ether butorphan ligands were prepared and their binding affinities at the opioid receptors determined. Addition of a hydroxy group to a hydrocarbon chain can potentiate binding affinity up to 27- and 86-fold at the mu and kappa opioid receptors, respectively. Two bivalent ligands with sub-nanomolar binding affinity at the mu and kappa opioid receptors were discovered.

Synthesis and opioid receptor binding affinities of 2-substituted and 3-aminomorphinans: ligands for mu, kappa, and delta opioid receptors

The phenolic group of the potent mu and kappa opioid morphinan agonist/antagonists cyclorphan and butorphan was replaced by phenylamino and benzylamino groups including compounds with para-substituents in the benzene ring. These compounds are highly potent mu and kappa ligands, e.g., p-methoxyphenylaminocyclorphan showing a K(i) of 0.026 nM at the mu receptor and a K(i) of 0.03 nM at the kappa receptor. Phenyl carbamates and phenylureas were synthesized and investigated. Selective o-formylation of butorphan and levorphanol was achieved. This reaction opened the way to a large set of 2-substituted 3-hydroxymorphinans, including 2-hydroxymethyl-, 2-aminomethyl-, and N-substituted 2-aminomethyl-3-hydroxymorphinans. Bivalent ligands bridged in the 2-position were also synthesized and connected with secondary and tertiary aminomethyl groups, amide bonds, and hydroxymethylene groups, respectively. Although most of the 2-substituted morphinans showed considerably lower affinities compared to their parent compounds, the bivalent ligand approach led to significantly higher affinities compared to the univalent 2-substituted morphinans.

Univalent and bivalent ligands of butorphan: characteristics of the linking chain determine the affinity and potency of such opioid ligands

Bivalent morphinan compounds containing ester linkers were synthesized and their binding affinities at the mu, delta, and kappa opioid receptors determined. Addition of methyl groups adjacent to the hydrolytically labile ester linkage increased stability while only partially affecting binding affinity. The resulting bivalent ligands with optimized spacer length and structure show potent binding profiles with the most potent compound (4b), having K(i) values of 0.47 nM for both the mu and kappa opioid receptors, and 4a, having K(i) values of 0.95 and 0.62 nM for the mu and kappa receptors, respectively. Both 4a and 4b were partial agonists at the kappa and micro receptors in the [(35)S]GTPgammaS binding assay.

Syntheses of novel high affinity ligands for opioid receptors

A series of novel high affinity opioid receptor ligands have been made whereby the phenolic-OH group of nalbuphine, naltrexone methiodide, 6-desoxonaltrexone, hydromorphone and naltrindole was replaced by a carboxamido group and the furan ring was opened to the corresponding 4-OH derivatives. These furan ring 'open' derivatives display very high affinity for mu and kappa receptors and much less affinity for delta. The observation that these target compounds have much higher receptor affinity than the corresponding ring 'closed' carboxamides significantly strengthens our underlying pharmacophore hypothesis concerning the bioactive conformation of the carboxamide group.

Redefining the structure-activity relationships of 2,6-methano-3-benzazocines. Part 7: syntheses and opioid receptor properties of cyclic variants of cyclazocine

A series of 7,8- and 8,9-fused triazole and imidazole analogues of cyclazocine have been made and characterized in opioid receptor binding and [(35)S]GTPgammaS assays. Target compounds were designed to explore the SAR surrounding our lead molecule for this study, namely the 8,9-fused pyrrolo analogue 2 of cyclazocine. Compared to 2, many of the new compounds in this study displayed very high affinity for opioid receptors.

Syntheses and opioid receptor binding properties of carboxamido-substituted opioids

A series of 15 novel opioid derivatives were made where the prototypic phenolic-OH group of traditional opioids was replaced by a carboxamido (CONH(2)) group. For 2,6-methano-3-benzazocines and morphinans similar or, in a few instances, enhanced affinity for mu, delta and kappa opioid receptors was observed when the OH-->CONH(2) switch was applied. For 4,5alpha-epoxymorphinans, binding affinities for the corresponding carboxamide derivatives were much lower than the OH partner consistent with our pharmacophore hypothesis concerning carboxamide bioactive conformation. The active metabolite of tramadol and its carboxamide counterpart had comparable affinities for the three receptors.

Phenobarbital withdrawal seizures may occur over several weeks before remitting: human data and hypothetical mechanism

This case is the first report of a patient who had phenobarbital (PB) withdrawal seizures after having been seizure-free for 3 years following temporal lobe surgery. The patient had been taking PB for 14 years when a gradual taper of PB was started. When PB was at 60 mg/d, a titration of lamotrigine (LTG) was started. However, typical complex seizures occurred when the patient was on PB 60 mg/d, along with LTG 25mg/d. PB was increased back to 90 mg/d and levetiracetam (LEV) was titrated. Seizures appeared when the patient was on PB 30 mg/d and LEV 750 mg BID and continued for 3 weeks after PB was stopped and the patient was on LEV 1,000 mg BID. For the following 6 months, her aura frequency remained elevated in comparison to her baseline aura of two auras per month for the previous year before the start of the PB taper. She was followed for 24 months after her last PB withdrawal seizure. During the last 8 months, her aura frequency returned to her baseline. As suggested by animal studies, the PB withdrawal seizures and increase in aura frequency in this patient may be explained by changes in her levels of GABA(A) receptor subunits.

Synthesis and pharmacological evaluation of hydrophobic esters and ethers of butorphanol at opioid receptors

We synthesized several hydrophobic esters and ethers of butorphanol and assessed their affinities at opioid receptors in CHO cell membranes. Tested compounds displayed moderate to high affinities to the mu and kappa receptors. The findings accord with previous evidence of a lipophilic binding pocket in the opioid receptors that can be accessed to afford good binding affinity without the need for a phenolic hydrogen-bond donor group. The most potent (K(i)=61 pM at mu and 48 pM at kappa) novel agent was (-)-N-cyclobutylmethylmorphinan-3-yl-14-ol phenoxyacetate (4d).

In vivo characterization of (-)(-)MCL-144 and (+)(-)MCL-193: isomeric, bivalent ligands with mu/kappa agonist properties

Once opioid receptor dimers were postulated, a goal has been to synthesize and screen novel opioids, with the hope of furthering our knowledge of the structure-activity relationship of opioid ligands with the opioid receptors. The aim of the current study was to address whether two isomeric bivalent ligands would have pharmacological differences after central administration, in vivo. The two compounds, (-) bis(N-cyclobutylmethyl-morphinan-3-yl) sebacoylate dihydrochloride (MCL-144) and 1-((+)N-cyclobutylmethylmorphinan-3-yl)-10-((-) N-cyclobutylmethylmorphinan-3-yl)sebacolyate (MCL-193) are each linked by a 10-carbon chain ester. The active (-) enantiomer for both ligands is 3-hydroxy-N-cyclobutylmethyl morphinan ((-)MCL-101), a N-cyclobutylmethyl analogue of cyclorphan (J Med Chem 43:114-122, 2000). MCL-144 contains two active levo rotatory (-)(-) pharmacophores, while MCL-193 contains one active (-) and one inactive (+) pharmacophore of MCL-101. In vitro analysis demonstrated that all three compounds, (-)(-)MCL-144, (+)(-)MCL-193 and (-)MCL-101 were kappa agonists and mu partial agonists. (-)(-)MCL-144 and (-)MCL-101 had much higher affinity for both the mu and kappa opioid receptors compared to (+)(-)MCL-193. In vivo, (-)(-)MCL-144 and (+)(-)MCL-193 produced full dose-response curves, in the 55 degrees C tail-flick test, with each compound having an ED(50) value of 3.0 nmol after intracerebroventricular (i.c.v.) administration. The analgesic properties of both compounds were antagonized by the mu-selective antagonist, beta-funaltrexamine and the kappa-selective antagonist nor-binaltorphimine. Concomitant, i.c.v., administration of either (-)(-)MCL-144 or (+)(-)MCL-193 with morphine, did not significantly antagonize morphine-induced antinociception at any dose tested. In antinociceptive tests, (-)(-)MCL-144 and (+)(-)MCL-193 had the same pharmacological properties, demonstrating that having two active pharmacophores separated by a 10-carbon spacer group did not increase the antinociceptive efficacy of the compound. Additionally, it was also of interest to compare (-)(-)MCL-145 and (-)(-)MCL-144, as the only difference between these bivalent ligands is the spacer region connecting the two pharmacophores, yet (-)(-)MCL-145 produced an ED(50) value 10-fold lower than (-)(-)MCL-144 (ED(50) values = 0.3 nmol and 3.0 nmol, respectively).

Extracellular biotransformation of beta-endorphin in rat striatum and cerebrospinal fluid

Numerous studies have investigated the behavioural effects of beta-endorphin, both endogenous and exogenously applied. However, the potential for biotransformation of beta-endorphin in the extracellular space of the brain has not been previously directly addressed in vivo. Utilising microinfusion/microdialysis and matrix-assisted laser desorption/ionisation mass spectrometry, we investigated beta-endorphin biotransformation in the striatum of rats. We infused 1.0 nmol beta-endorphin into the striatum of adult male Fischer rats and observed rapid cleavage resulting in beta-endorphin 1-18, as well as several fragments resulting from further N-terminal degradation. In vitro studies with incubation of full-length beta-endorphin, with and without protease inhibitors, in the incubation fluid of isolated striatal slices indicate that beta-endorphin is initially cleaved predominantly at the Phe(18)-Lys(19), position, as well as at the Leu(17)-Phe(18) position. Investigations of cerebrospinal fluid revealed similar enzymatic cleavage of beta-endorphin. The observed pattern of cleavage sites (Phe(18)-Lys(19) and Leu(17)-Phe(18)) is consistent with published in vitro studies of purified insulin-degrading enzyme cleavage of beta-endorphin. The binding affinities of full-length beta-endorphin, as well as previously identified beta-endorphin fragments alpha-endorphin (beta-endorphin 1-16) and gamma-endorphin (beta-endorphin 1-17), and the fragment identified in the present study, beta-endorphin 1-18, at heterologously expressed mu, delta and kappa-opioid receptors, respectively, were determined; the affinity of the truncation fragments is reduced at each of the receptors compared to the affinity of full length beta-endorphin.

Redefining the structure-activity relationships of 2,6-methano-3-benzazocines. 5. Opioid receptor binding properties of N-((4'-phenyl)-phenethyl) analogues of 8-CAC

A series of aryl-containing N-monosubstituted analogues of the lead compound 8-[N-((4'-phenyl)-phenethyl)]-carboxamidocyclazocine were synthesized and evaluated to probe a putative hydrophobic binding pocket of opioid receptors. Very high binding affinity to the mu opioid receptor was achieved though the N-(2-(4'-methoxybiphenyl-4-yl)ethyl) analogue of 8-CAC. High binding affinity to mu and very high binding affinity to kappa opioid receptors was observed for the N-(3-bromophenethyl) analogue of 8-CAC. High binding affinity to all three opioid receptors were observed for the N-(2-naphthylethyl) analogue of 8-CAC.

Synthesis and pharmacological evaluation of 6,7-indolo/thiazolo-morphinans--further SAR of levorphanol

To further extend the structure-activity relationships of levorphanol, two series of novel morphinans were prepared by incorporation of an indole or aminothiazole fragment to the hexyl ring (ring C) in levorphanol. Such morphinans differed from previously reported ligands in that such indole- or aminothiazole-containing morphinans displayed enhanced binding affinity to the delta opioid receptor, while the affinity to kappa and micro receptors was slightly reduced.

Pharmacological properties of bivalent ligands containing butorphan linked to nalbuphine, naltrexone, and naloxone at mu, delta, and kappa opioid receptors

Our investigation of bivalent ligands at mu, delta, and kappa opioid receptors is focused on the preparation of ligands containing kappa agonist and mu agonist/antagonist pharmacophores at one end joined by a chain containing the mu antagonist pharmacophores (naltrexone, naloxone, or nalbuphine) at the other end. These ligands were evaluated in vitro by their binding affinity at mu, delta, and kappa opioid receptors and their relative efficacy in the [35S]GTPgammaS assay.

In-vitro investigation of oxazol and urea analogues of morphinan at opioid receptors

A series of 2-amino-oxazole (7 and 8) analogs and 2-one-oxazole analogs (9 and 10) were synthesized from cyclorphan (1) or butorphan (2) and evaluated in-vitro by their binding affinity at mu, delta, and kappa opioid receptors and compared with their 2-aminothiozole analogs 5 and 6. Ligands 7-10 showed decreased affinities at kappa and mu receptors. Urea analogs (11-14) were also prepared from 2-aminocyclorphan (3) or 2-aminobutorphan (4) and evaluated in-vitro by their binding affinity at mu, delta, and kappa opioid receptors. The urea derived opioids retained their affinities at mu receptors while showing increased affinities at delta receptors and decreased affinities at kappa receptors. Functional activities of these compounds were measured in the [35S]GTPgammaS binding assay, illustrating that all of these ligands were kappa agonists. At the mu receptor, compounds 11 and 12 were mu agonist/antagonists.

Glycosylation Improves the Central Effects of DAMGO

A series of mu-agonist DAMGO analogs were synthesized and pharmacologically characterized to test the 'biousian' hypothesis of membrane hopping. DAMGO was altered by incorporating moieties of increasing water solubility into the C-terminus via carboxamide and simple glycoside additions. The hydrophilic C-terminal moieties were varied from glycinol in DAMGO (1) to l-serine amide (2), l-serine amide beta-d-xyloside (3), l-serine amide beta-d-glucoside (4), and finally to l-serine amide beta-lactoside (5). Opioid binding and mouse tail-flick studies were performed. Antinociceptive potency (intravenous) increased, passing through a maximum (A(50) approximately 0.2 micromol/kg) for 2 and 3 as membrane affinity versus water solubility became optimal, and dropped off (A(50) approximately 1.0 micromol/kg) for 4 and 5 as water solubility dominated molecular behavior. Intravenous A(50) values were plotted versus hydrodynamic values (glucose units, g.u.) for the glycoside moieties, or the hydrophilic/hydrophobic Connolly surface areas (A(50) versus e(-Awater/Alipid)), and provided either a V-shaped or a U-shaped curve, as predicted by the 'biousian' hypothesis. The mu-selective receptor profile was maintained (K(i)'s = 0.66-1.3 nm) upon modifications at the C-terminus. The optimal 'degree of glycosylation' for the DAMGO peptide message appears to be between 1.25 and 1.75 g.u. (hydrodynamic g.u.), or 0.75 and 0.90 in terms of the surface-derived amphipathicity values.

New opioid designed multiple ligand from Dmt-Tic and morphinan pharmacophores

Here, we report the synthesis of a designed multi-pharmacophore ligand derived from the linkage of a delta selective peptide antagonist (Dmt-Tic) and a mu/kappa morphinan agonist butorphan (MCL 101) through a two methylene spacer. The new compound MCL 450 maintains the same characteristics as those the two reference compounds. MCL 450 represents a useful starting point for the synthesis of other multiple opioid ligands endowed with analgesic properties with low tolerance and dependence.

Redefining the structure-activity relationships of 2,6-methano-3-benzazocines. 4. Opioid receptor binding properties of 8-[N-(4'-phenyl)-phenethyl)carboxamido] analogues of cyclazocine and ethylketocycalzocine

The synthesis and evaluation of a series of aryl-containing N-monosubstituted analogues of the lead compound 8-carboxamidocyclazocine were performed to probe a putative hydrophobic binding pocket of opioid receptors. High binding affinity to mu, kappa, and delta opioid receptors was observed for the 8-[N-(4'-phenyl)-phenethyl)carboxamido] analogue.

Opioid receptors and signaling on cells from the immune system

This review discusses the criteria for determining whether a binding site or functional response is directly mediated by either the mu, delta, or kappa opioid receptors. In 1988, Sibinga and Goldstein published the first review that addressed whether cells from the immune system express opioid receptors. The criteria that they used, namely, structure-activity relationships, stereoselectivity, dose- and concentration-dependence, and saturability are still relevant criteria today for determining if an immunological response is mediated by either the mu, delta or kappa opioid receptors. Radioligand receptor binding studies and functional studies that clearly show the presence of an opioid receptor on immunocytes are presented. Selective agonists and antagonists for the mu, delta, and kappa opioid receptors are discussed, and the need for their use in experiments is emphasized. Conditions used in functional assays are very important. Receptor desensitization and downregulation occur within minutes after the application of an agonist. However, many immunological assays are applying an agonist for days before measuring an immunological effect. The results obtained may reflect changes that are results of receptor desensitization and/or downregulation instead of changes that are observed with acute activation of the receptor. The future of receptor pharmacology lies in the crosstalk and dimerization of G protein-coupled receptors. In transfected systems, opioid receptors have been shown to dimerize with chemokine and cannabinoid receptors, resulting in crosstalk between different types of receptors.

The expression of prodynorphin gene is down-regulated by activation with lipopolysaccharide in U-937 macrophage cells

Dynorphins are endogenous kappa opioid peptides widely distributed in the central nervous system. A number of recent reports have established their roles in modulating immunological functions. In this study, we investigated the expression of prodynorphin (PDYN) gene, which encodes the precursor of dynorphin peptides, in human macrophage U-937 cell line. PDYN mRNA was expressed at a detectable level, measured with a standard RT-PCR method in U-937 cells, but not in Jurkat and Raji cells, which are human T and B lymphocytes, respectively. Further analyses with RT-PCR assays by using primers covering each exon of the PDYN gene showed that U-937 cells expressed the adult brain-type PDYN mRNA. Most interestingly, activation of U-937 cells with lipopolysaccharide (LPS) led to a decrease in PDYN mRNA levels. This decrease was dependent on both the concentration of LPS and the duration of LPS treatment. In order to test the role of transcription in LPS-mediated down-regulation of PDYN gene expression in U937 cells, the proximal PDYN gene promoter with a length of either approximately 300 base pairs or approximately 900 base pairs was cloned and inserted upstream of luciferase reporter gene. Results from transient transfections and dual luciferase assays showed that PDYN gene promoter activity was decreased by LPS treatment. Taken together, our results suggested the U-937 cells expressed adult brain-type PDYN mRNA which was down-regulated by activation of the cells with LPS due to an inhibition of PDYN gene transcription.

Differential Targeting of G -Subunit Signaling with Small Molecules

G protein betagamma subunits have potential as a target for therapeutic treatment of a number of diseases. We performed virtual docking of a small-molecule library to a site on Gbetagamma subunits that mediates protein interactions. We hypothesized that differential targeting of this surface could allow for selective modulation of Gbetagamma subunit functions. Several compounds bound to Gbetagamma subunits with affinities from 0.1 to 60 muM and selectively modulated functional Gbetagamma-protein-protein interactions in vitro, chemotactic peptide signaling pathways in HL-60 leukocytes, and opioid receptor-dependent analgesia in vivo. These data demonstrate an approach for modulation of G protein-coupled receptor signaling that may represent an important therapeutic strategy.

Reduction of lipopolysaccharide-induced interleukin-6 production by the kappa opioid U50,488 in a mouse monocyte-like cell line

Several studies demonstrate that opioids modulate the immune response via opioid receptors expressed directly on the immune cells themselves. Recently, it has been suggested that the kappa opioid system has a modulatory role in various inflammatory diseases including rheumatoid arthritis. This modulation may occur via changes in cytokine secretion by monocyte-derived cells. To further study this opioid-immune relationship, we stimulated P388D1 cells, a mouse monocyte-like cell line, with lipopolysaccharide (LPS) in the presence or absence of the kappa opioid-selective ligand, U50,488. Pretreatment with U50,488 significantly reduced LPS-stimulated interleukin-6 (IL-6) production as measured by ELISA. This effect was mediated by the kappa opioid receptor, because nor-binaltorphimine (nor-BNI), a kappa-selective antagonist, blocked this inhibition. It is likely that this reduction of IL-6 protein by U50,488 treatment is attributed to decreases in IL-6 mRNA. RT-PCR experiments demonstrated that U50,488 treatment significantly reduced the LPS-mediated increase in IL-6 mRNA and that this effect was also blocked by nor-BNI. Understanding the mechanism behind the reduction of proinflammatory cytokine production by opioids may lead to the development of more effective therapeutics for inflammatory diseases.

Synthesis and Preliminary In vitro Investigation of Bivalent Ligands Containing Homo- and Heterodimeric Pharmacophores at μ, δ, and κ Opioid Receptors

A series of homo- and heterodimeric ligands containing kappa agonist and mu agonist/antagonist pharmacophores joined by a linker chain of varying lengths was synthesized and evaluated in vitro by their binding affinity at mu, delta, and kappa opioid receptors. The functional activities of these compounds were measured in the [(35)S]GTPgammaS binding assay. The data suggest that the stereochemistry of the pharmacophores, the N-substituents of the pharmacophore, ester linkages, and the spacer length were crucial factors for optimum interactions of such ligands at opioid receptor binding sites. These novel ligands as well as their pharmacological properties will serve as the basis for our continuing investigation of such bivalent ligands as probes of the opioid receptor oligomerization phenomena and for in vivo studies as analgesics.

Nalmefene induced elevation in serum prolactin in normal human volunteers: partial kappa opioid agonist activity?

In humans, mu- and kappa-opioid receptor agonists lower tuberoinfundibular dopamine, which tonically inhibits prolactin release. Serum prolactin is, therefore, a useful biomarker for tuberoinfundibular dopamine. The current study evaluated the unexpected finding that the relative mu- and kappa-opioid receptor selective antagonist nalmefene increases serum prolactin, indicating possible kappa-opioid receptor agonist activity. In all, 33 healthy human volunteers (14 female) with no history of psychiatric or substance use disorders received placebo, nalmefene 3 mg, and nalmefene 10 mg in a double-blind manner. Drugs were administered between 0900 and 1000 on separate days via 2-min intravenous infusion. Serial blood specimens were analyzed for serum levels of prolactin. Additional in vitro studies of nalmefene binding to cloned human kappa-opioid receptors transfected into Chinese hamster ovary cells were performed. Compared to placebo, both doses of nalmefene caused significant elevations in serum prolactin (p<0.002 for nalmefene 3 mg and p<0.0005 for nalmefene 10 mg). There was no difference in prolactin response between the 3 and 10 mg doses. Binding assays confirmed nalmefene's affinity at kappa-opioid receptors and antagonism of mu-opioid receptors. [(35)S]GTPgammaS binding studies demonstrated that nalmefene is a full antagonist at mu-opioid receptors and has partial agonist properties at kappa-opioid receptors. Elevations in serum prolactin following nalmefene are consistent with this partial agonist effect at kappa-opioid receptors. As kappa-opioid receptor activation can lower dopamine in brain regions important to the persistence of alcohol and cocaine dependence, the partial kappa agonist effect of nalmefene may enhance its therapeutic efficacy in selected addictive diseases.

Redefining the structure-activity relationships of 2,6-methano-3-benzazocines. Part 3: 8-Thiocarboxamido and 8-thioformamido derivatives of cyclazocine

8-Position variants of cyclazocine have been made where the phenolic 8-OH was replaced by thioamide, amidine, guanidine, urea and thiourea groups. High affinity for opioid receptors was observed for the 8-CSNH2 and 8-NHCHS analogues indicating that these groups are isosteric with not only the 8-OH but with the previously synthesized 8-CONH2 and 8-NHCHO cyclazocine derivatives.

The human herpesvirus 6 G protein-coupled receptor homolog U51 positively regulates virus replication and enhances cell-cell fusion in vitro

Human herpesvirus 6 (HHV-6) is a ubiquitous T-lymphotropic betaherpesvirus that encodes two G protein-coupled receptor homologs, U12 and U51. HHV-6A U51 has been reported to bind to CC chemokines including RANTES, but the biological function of U51 remains uncertain. In this report, we stably expressed short interfering RNAs (siRNAs) specific for U51 in human T cells and then infected these cells with HHV-6. Viral DNA replication was reduced 50-fold by the U51 siRNA, and virally induced cytopathic effects were also inhibited. In contrast, viral replication and syncytium formation were unaltered in cells that expressed a scrambled derivative of the siRNA or an irrelevant siRNA and were restored to normal when a human codon-optimized derivative of U51 was introduced into cells containing the U51 siRNA. To examine the mechanism whereby U51 might contribute to viral replication, we explored the signaling characteristics of U51. None of the chemokines and opioids tested was able to induce G protein coupling by U51, and no evidence for opioid ligand binding by U51 was obtained. The effect of U51 on cell-cell fusion was also evaluated; these studies showed that U51 enhanced cell fusion mediated by the G protein of vesicular stomatitis virus. However, a U51-specific antiserum had no virus-neutralizing activity, suggesting that U51 may not be involved in the initial interaction between the virus particle and host cell. Overall, these studies suggest that HHV-6 U51 is a positive regulator of virus replication in vitro, perhaps because it may promote membrane fusion and facilitates cell-cell spread of this highly cell-associated virus.