Prevention of glomerular hyperfiltration in rats with streptozotocin-induced diabetes by an atrial natriuretic peptide receptor antagonist

Summary The contribution of atrial natriuretic peptide (ANP) to the development of glomerular hyperfiltration in diabetes was investigated by examining the effects of HS-142-1, a non-peptide antagonist of biological receptors for ANP, on glomerular filtration rate (GFR) and renal plasma flow (RPF) in rats with streptozotocin-induced diabetes. Three to four weeks after streptozotocin injection, the plasma concentration of ANP, urinary cyclic GMP excretion rate, GFR, and RPF were significantly higher in diabetic rats than in control rats. The increase in GFR and RPF in diabetic rats was significantly reduced, in a dose-dependent manner, by a single intravenous injection of HS-142-1; the maximal effect was apparent at a dose of 10 mg per kg of body weight. Continuous subcutaneous administration of HS-142-1 with an osmotic minipump for 3 to 4 weeks, beginning 2 days after streptozotocin injection, prevented the increases in urinary cyclic GMP excretion rate, GFR, and RPF observed in untreated diabetic rats. These results highlight the importance of ANP in the development of diabetic glomerular hyperfiltration and indicate that this condition can be prevented by continuous inhibition of the action of ANP.Abbreviations ANP Atrial natriuretic peptide - GFR glomerular filtration rate - RPF renal plasma flow - PAH para-aminohippurate     

Calcitonin gene-related peptide receptor antagonist ubrogepant for the treatment of acute migraine: A meta-analysis

Background:The objective of this study is to systematically evaluate the efficacy and safety of the calcitonin gene-related peptide (CGRP) receptor antagonist ubrogepant for the treatment of acute migraine.Methods:Randomized controlled trials (RCTs) of ubrogepant for treatment of acute migraine were identified in PubMed, MEDLINE, EMBASE, and the Cochrane Library from database establishment to June 2020; we also searched ClinicalTrials.gov manually during the same period. Then, RevMan 5.3 software was used to perform a meta-analysis on each outcome measure.Results:A total of 5 RCTs involving 4903 patients were included; there were 3358 cases in the ubrogepant group and 1545 cases in the placebo group. The meta-analysis showed the following results: at 2 hours postdose, the percentages of participants reporting pain relief and the absence of photophobia, nausea, and phonophobia were significantly higher in the ubrogepant group than in the placebo group (odds ratio [OR] = 1.71, 95%CI: 1.48–1.97, P < .00001; OR = 1.33, 95%CI: 1.22–1.45, P < .00001; OR = 1.07, 95%CI: 1.03–1.11, P = .0006; OR = 1.21, 95%CI: 1.14–1.28, P < .00001). The incidence of common adverse events was similar between the 2 groups (P > .05).Conclusion:Ubrogepant is effective and safe for the treatment of acute migraine.Registration number:PROSPERO CRD42019145286.     

Opposing effects on infarction of delta and kappa opioid receptor activation in the isolated rat heart: implications for ischemic preconditioning

δ-Opioid receptors are known to participate in the protection found following ischemic preconditioning (IPC), but the role of κ-receptors in IPC is currently controversial. Langendorff-perfused rat hearts received 35 min regional ischemia and 2 h reperfusion. PC (2 cycles 5 min global ischemia) substantially reduced infarct size. Pharmacological PC with the δ-agonist DADLE (10 nmol/L) had similar protective effects. However, higher dose DADLE (1 μmol/L) had a less beneficial effect, and in conjunction with the δ-antagonist naltrindole unexpectedly increased infarct size (61.5 ± 2.0%, p < 0.05 v 45.9 ± 2.3% in controls) sugggesting a non-δ effect. The universal κ-opioid agonist bremazocine (30 nmol/L) increased infarct size (61.3 ± 1.6%, p < 0.05 v controls), an effect abrogated by the selective κ₁-antagonist nor-binaltorphimine (BNI). Since opiates are known to have anti-adrenergic effects, which hypothetically may help to mediate IPC, cyclic AMP levels were measured in DADLE and in bremazocine-treated hearts. Decreased levels of cyclic AMP at the start of the regional ischemic period were found in low dose DADLE hearts (0.485 ± 0/020, n = 8, vs controls, 0.654 ± 0.025 nmol/g wet weight, p < 0.001), but not in high dose DADLE nor in bremazocine treated hearts. Thus, in the isolated rat heart κ₁-opioid receptor activation exacerbates infarct size through an as yet unknown mechanism, suggesting that there could be an “anti-preconditioned state”. In contrast, δ-activity mediates protection which may be associated with a reduction of tissue cyclic AMP levels. Received: 16 November 1999, Accepted: 7 December 1999     

Dynorphin A, kappa opioid receptors and the antinociceptive efficacy of asimadoline in streptozotocin-induced diabetic rats

Aims/hypothesis We investigated spinal and peripheral kappa opioid systems in diabetic rats.Materials and methods Dynorphin A, N-methyl-d-aspartate (NMDA) and kappa opioid receptor (KOR) were measured in spinal cord, dorsal root ganglia, peripheral nerves and foot skin of control and streptozotocin-induced diabetic rats by immunoassay and Western blotting. Behavioural assessments of paw tactile sensitivity and formalin-evoked hyperalgesia were performed in normal and diabetic rats before and after treatment with asimadoline.Results Dynorphin A protein levels were significantly increased in peripheral nerves and footpad skin of diabetic rats. Dynorphin A exhibits both anti- and pro-nociceptive properties depending on activation of either KOR or NMDA receptors. Spinal protein levels of these receptors were not changed by diabetes, while KOR levels in the sciatic and peroneal nerves were significantly increased. Exploiting the presence and elevated levels of KOR in the periphery, we investigated the effect of the peripheral KOR agonist asimadoline on formalin-evoked hyperalgesia and tactile allodynia in diabetic rats. Both formalin-evoked hyperalgesia and tactile allodynia in diabetic rats were acutely ameliorated by asimadoline. To confirm that the effect of asimadoline was related to its property as KOR agonist, diabetic rats were pretreated with the selective KOR antagonist nor-binaltorphimine. Intraplantar nor-binaltorphimine abolished the ability of asimadoline to alleviate tactile allodynia in diabetic rats. Systemic and intrathecal nor-binaltorphimine partially inhibited the effect of asimadoline against formalin-evoked hyperalgesia in diabetic rats.Conclusions/interpretation Using selective peripheral KOR agonists to take advantage of elevated peripheral KOR expression may provide a novel therapeutic approach for painful diabetic neuropathy.     

Effects of H2-receptor antagonist and selective muscarinic receptor antagonist on early gastric stagnation after pylorus-preserving pancreaticoduodenectomy: Results of a randomized controlled study

The suppressive effects of an H₂-receptor antagonist, ranitidine, and a selective muscarinic receptor antagonist, pirenzepine, in improving gastric stasis during the early postoperative period after pylorus-preserving pancreaticoduodenectomy (PPPD) were assessed. Thirty-two PPPD patients were divided into four groups of 8 patients each. Group 1 served as controls and were given no medication. Group 2 received i.v. ranitidine alone, group 3 received i.v. pirenzepine alone, and group 4 received i.v. ranitidine and pirenzepine combined. The daily volume and the total acidity of the gastric juice, aspirated via a nasogastric tube, were measured on post-PPPD days 1–13. One patient in group 1 was withdrawn from this study due to severe gastric bleeding. The mean daily aspirated volume of gastric juice in group 1 (n=7) was 1043±362 ml on the 3rd postoperative day, and it gradually fell after the 5th postoperative day, but still exceeded 500 ml on day 13. In group 2 (n=8), the mean maximum volume, 614±251ml, was reached on the 4th postoperative day, and in group 3 (n=8), the mean maximum volume, 680 + 391 ml, was reached on the 3rd postoperative day. In contrast, in group 4 (n=8), the gastric juice output appeared to be suppressed and the mean maximum volume was 380±218 ml, this being reached on the 6th postoperative day. Gastric juice secretion was significantly higher in group 1 than in the other three groups (P<0.01). The total acidity was significantly lower in groups 3 and 4 than in group 1 (P<0.01). These results indicate that the postoperative administration of the combination of ranitidine and pirenzepine suppresses the volume and acidity of the gastric juice after a PPPD.     

Rational design of a selective antagonist of epsilon protein kinase C derived from the selective allosteric agonist, pseudo-RACK peptide

We have previously shown that domains involved in binding of protein kinase C (PKC) isozymes to their respective anchoring proteins (RACKs) and short peptides derived from these domains are PKC isozyme-selective antagonists. We also identified PKC isozyme-selective agonists, named psiRACK peptides, derived from a sequence within each PKC with high homology to its respective RACK. We noted that all the psiRACK sequences within each PKC isozyme have at least one non-homologous amino acid difference from their corresponding RACK that constitutes a charge change. Based on this information, we have devised here a new approach to design an isozyme-selective PKC antagonist, derived from the psiRACK sequence. We focused on epsilonPKC psiRACK peptide, where the pseudo-epsilonRACK sequence (psiepsilonRACK; HDAPIGYD; corresponding to epsilonPKC85-92) is different in charge from the homologous RACK-derived sequence (NNVALGYD; corresponding to epsilonRACK285-292) in the second amino acid. Here we show that changing the charge of the psiepsilonRACK peptide through a substitution of only one amino acid (aspartate to asparagine) resulted in a peptide with an opposite activity on the same cell function and a substitution for aspartate with an alanine resulted in an inactive peptide. These data support our hypothesis regarding the mechanism by which pseudo-RACK peptide activates PKC in heart cells and suggest that this approach is applicable to other signaling proteins with inducible protein-protein interactions.     

Olmesartan, a novel angiotensin II type 1 receptor antagonist, reduces severity of atherosclerosis in apolipoprotein E deficient mice associated with reducing superoxide production

Background and aim

Oxidative stress may play an important role in the development of atherosclerosis. Some angiotensin II type 1 (AT₁) receptor antagonists have the capacity of reducing oxidative stress in addition to the hemodynamic actions. Accordingly, we assessed the hypothesis that olmesartan, a novel AT₁ receptor antagonist, reduced the severity of atherosclerosis in apolipoprotein (apo) E-deficient mice associated with reducing oxidative stress.

Methods and results

Atherosclerosis was induced in apo E-deficient mice fed a high fat diet. Mice were intraperitoneally treated with an injection of olmesartan (1 mg/kg/day) daily over 8 weeks, and were compared with the untreated controls. Blood pressure was not changed significantly by the olmesartan treatment. Fatty streak plaque developed in apo E-deficient mice, and was suppressed in mice that received olmesartan. In addition, olmesartan reduced not only superoxide production but the overload of oxidative stress in aortic walls. There were no significant differences in serum lipid levels between olmesartan-treated and -untreated groups. In vitro study showed that both olmesartan and its active metabolite RNH-6270, an enantiomer of olmesartan, suppressed interferon-γ, macrophage inflammatory protein-2, and thioredoxin (a marker of oxidative stress) concentrations in cultured cells.

Conclusion

Olmesartan may suppress atherosclerosis via reducing not only superoxide production but also the overload of oxidative stress in this animal model.     

Methylnaltrexone: a novel approach for the management of opioid-induced constipation in patients with advanced illness

In April 2008, the US FDA granted approval to methylnaltrexone (Relistor^®), the first peripheral µ-opioid-receptor antagonist for the treatment of opioid-induced constipation in advanced-illness patients receiving palliative care and for whom other laxative therapies failed to achieve adequate results. Methylnaltrexone, a quaternary derivative of naltrexone, introduces a novel mechanism of action that selectively antagonizes the peripheral µ-receptors in the GI tract without effects on the CNS. In clinical trials, subcutaneous methylnaltrexone reversed opioid-induced constipation after the first dose in approximately 50–60% of the patients. In most of the cases, effective laxation occurred within 1 h. The therapeutic benefit was sustained in multiple-dose studies. Owing to the nature of the population studied, safety data are available for approximately 4 months of use. Although it is not the focus of this article, methylnaltrexone’s mechanism of action suggests it could be beneficial for other peripheral, opioid-induced adverse effects, such as opioid-related nausea, vomiting, urinary retention, pruritus or postoperative ileus.     

INAUGURAL ARTICLE by a Recently Elected Academy Member:Click-to-lead design of a picomolar ABA receptor antagonist with potent activity in vivo

Abscisic acid (ABA) is a key plant hormone that mediates both plant biotic and abiotic stress responses and many other developmental processes. ABA receptor antagonists are useful for dissecting and manipulating ABA’s physiological roles in vivo. We set out to design antagonists that block receptor–PP2C interactions by modifying the agonist opabactin (OP), a synthetically accessible, high-affinity scaffold. Click chemistry was used to create an ∼4,000-member library of C4-diversified opabactin derivatives that were screened for receptor antagonism in vitro. This revealed a peptidotriazole motif shared among hits, which we optimized to yield antabactin (ANT), a pan-receptor antagonist. An X-ray crystal structure of an ANT–PYL10 complex (1.86 Å) reveals that ANT’s peptidotriazole headgroup is positioned to sterically block receptor–PP2C interactions in the 4′ tunnel and stabilizes a noncanonical closed-gate receptor conformer that partially opens to accommodate ANT binding. To facilitate binding-affinity studies using fluorescence polarization, we synthesized TAMRA–ANT. Equilibrium dissociation constants for TAMRA–ANT binding to Arabidopsis receptors range from ∼400 to 1,700 pM. ANT displays improved activity in vivo and disrupts ABA-mediated processes in multiple species. ANT is able to accelerate seed germination in Arabidopsis, tomato, and barley, suggesting that it could be useful as a germination stimulant in species where endogenous ABA signaling limits seed germination. Thus, click-based diversification of a synthetic agonist scaffold allowed us to rapidly develop a high-affinity probe of ABA–receptor function for dissecting and manipulating ABA signaling.

The phytohormone abscisic acid (ABA) controls numerous physiological processes in plants ranging from seed development, germination, and dormancy to responses for countering biotic and abiotic stresses (1). ABA binds to the PYR/PYL/RCAR (Pyrabactin Resistance 1/PYR1-like/Regulatory Component of ABA Receptor) soluble receptor proteins (2, 3) and triggers a conformational change in a flexible “gate” loop flanking the ligand-binding pocket such that the ABA–receptor complex can then bind to and inhibit clade A type II C protein phosphatases (PP2Cs), which normally dephosphorylate and inactivate SNF1-related protein kinase 2 (SnRK2). This, in turn, leads to SnRK2 activation, phosphorylation of downstream targets, and multiple cellular outputs (4, 5).Chemical modulators of ABA perception have been sought as both research tools for dissecting ABA’s role in plant physiology and for their potential agricultural utility (6, 7). Dozens of ABA receptor agonists, which reduce transpiration and water use by inducing guard cell closure, have been developed and are being explored as chemical tools for mitigating the effects of drought on crop yields (7–23), most of them either being analogs of ABA or sulfonamides similar to quinabactin (24). ABA receptor antagonists could conceivably be useful in cases where water is not limiting, for example, to increase transpiration and gas exchange under elevated CO₂ in glasshouse agriculture, as germination stimulators, and for studying the ABA dependence of physiological processes, among other applications (25–31). Thus, both ABA receptor agonists and antagonists have potential uses as research tools and for plant biotechnology.In principle, there are at least two mechanisms for blocking ABA receptor activation: by preventing gate closure, which is necessary for PP2C binding, or by sterically disrupting the activated, closed-gate receptor conformer from binding to PP2Cs. Prior efforts to design antagonists have focused on the latter strategy and include multiple ABA-derived ligands such as AS6 (25), PanMe (26), 3′-alkyl ABA (30–32), 3′-(phenyl alkynyl) ABA (33), or ligands derived from tetralone ABA (34) with varying degrees of conformational restriction (27, 28, 35). With the exception of PanMe, these antagonists have linkers attached to the 3′ carbon of ABA or 11′ carbon of tetralone ABA, which is positioned to disrupt receptor–PP2C interactions by protruding through the 3′ tunnel. PanMe was created by modifying ABA’s C4′ (Fig. 1) with a toluylpropynyl ether substituent designed to occupy the 4′ tunnel, a site of close receptor–PP2C contact (26). Structural studies showed that this 4′ moiety adopts two conformations, one that resides in the 4′ tunnel and another that occupies the adjacent 3′ tunnel (26). Collectively, these elegant studies have demonstrated that antagonists of receptor–PP2C interactions can be designed by modifying agonists at sites situated proximal to the 3′ or 4′ tunnels. Despite these advances, current antagonists have limitations. For example, PanMe, which has low nanomolar affinity for the subfamily II receptor PYL5, is limited by relatively low activity on subfamily I and III ABA receptors, and as we show here, the ABA antagonist AA1 (36) (Fig. 1) lacks detectable antagonist activity in vitro and is, therefore, unlikely to be a true ABA receptor antagonist. Together, these data suggest that higher-affinity pan-antagonists and/or molecules with increased bioavailability will be necessary to more efficiently block endogenous ABA signaling. We set out to address these limitations by modifying the scaffold of the synthetic ABA agonist opabactin (OP), which has an approximately sevenfold increase in both affinity and bioactivity relative to ABA (21). We describe an OP derivative called antabactin (ANT) and show that it is a high-affinity binder and antagonist of ABA receptors that disrupts ABA-mediated signaling in vivo.Open in a separate windowFig. 1.Structures of ABA, PanMe, and AA1.     

Clinical effects of pranlukast, an oral leukotriene receptor antagonist, in mild-to-moderate asthma: a 4 week randomized multicentre controlled trial

OBJECTIVE: Leukotriene antagonists are increasingly used in asthma management. Pranlukast is a new, orally active, selective inhibitor of CysLt1 leukotriene receptor. The present clinical trial was performed to study the effect and safety of pranlukast in mild-to-moderate asthma. METHODOLOGY: A randomized, double-blind, placebo-controlled, parallel group study was performed in eight medical centres in Korea. Mild-to-moderate asthma patients who had been treated with beta2-agonists and/or inhaled corticosteroids were studied. The patients' symptoms were evaluated by asthma diary and twice-daily peak flow monitoring. RESULTS: Of the 206 patients enrolled, 197 were eligible for analysis. The pranlukast group (n = 98) showed statistically significant improvement in asthma symptoms, including asthma attack rate, daily living score, and morning and evening asthma scores. Pranlukast significantly reduced the consumption of beta2-agonist. Compared with the placebo group, forced vital capacity (FVC) and forced expiratory volume in 1 s (FEV1) were not significantly higher in the pranlukast group. Morning and evening peak expiratory flow (PEF) were significantly increased after pranlukast treatment at weeks 2 and 4 (380.8 +/- 10.1 L/min at baseline, 394.5 +/- 10.1 at week 2, 396.3 +/- 10.4 at week 4). There were no serious adverse reactions. CONCLUSION: Pranlukast, an oral leukotriene antagonist, was well tolerated and was effective for the management of mild-to-moderate asthma.     

Levels of adiponectin, C-reactive protein and interleukin-1 receptor antagonist are associated with insulin sensitivity: a population-based study

BACKGROUND: We evaluated the relationship of insulin sensitivity (assessed with the quantitative insulin sensitivity check index, QUICKI) to adiponectin and pro-inflammatory markers, levels of high-sensitivity C-reactive protein (hs-CRP) and interleukin-1 receptor antagonist (IL-1 Ra). METHODS: Cross-sectional study. Study population (N=923, i.e 411 men and 512 women) included five different population-based age groups (born in 1942, 1947, 1952, 1957 and 1962), [mean age 46 years and mean body mass index (BMI) 26 kg/m(2)]. Study protocol included an interview and measurements of anthropometric parameters and glucose, insulin, adiponectin, hs-CRP and IL-1 Ra. RESULTS: Correlation (r) between QUICKI and adiponectin level was 0.334 [95% confidence intervals (CI), 0.275-0.392] and partial correlation adjusted for gender, BMI, smoking status, physical activity and age was 0.247 (95% CI, 0.185-0.308). There was negative correlation between QUICKI and IL-1 Ra (r= -0.385; 95% CI, -0.440 to -0.328) which remained statistically significant after the adjustment for confounding factors (r= -0.178; 95% CI, -0.240 to -0.113). Similarly, QUICKI was negatively correlated with hs-CRP (r= -0.241; 95% CI, -0.302 to -0.178), but after the adjustment it lost its statistical significance. There was a statistically significant gender difference (p=0.018) in correlation between QUICKI and IL-1 Ra levels (men: r= -0.348; 95% CI, -0.436 to - 0.261; women r= -0.500; 95% CI, -0.537 to -0.398). CONCLUSIONS: Our results show that adiponectin level and markers of low-grade inflammation are related to insulin sensitivity. Adiponectin and IL-1 Ra levels might be better markers of the risk of obesity and type 2 diabetes than hs-CRP.     

Palonosetron and hydroxyzine pre-treatment reduces the objective signs of experimentally-induced acute opioid withdrawal in humans: a double-blinded,randomized, placebo-controlled crossover study

Background: Treatments for reducing opioid withdrawal are limited and prone to problematic side effects. Laboratory studies, clinical observations, and limited human trial data suggest 5-HT3-receptor antagonists and antihistamines may be effective. Objectives: This double-blind, crossover, placebo-controlled study employing an acute physical dependence model evaluated whether (i) treatment with a 5-HT3-receptor antagonist (palonosetron) would reduce opioid withdrawal symptoms, and (ii) co-administration of an antihistamine (hydroxyzine) would enhance any treatment effect. Methods: At timepoint T = 0, healthy (non-opioid dependent, non-substance abuser) male volunteers (N = 10) were pre-treated with either a) placebo, b) palonosetron IV (0.75 mg), or c) palonosetron IV (0.75 mg) and hydroxyzine PO (100 mg) in a crossover study design. This was followed at T = 30 by intravenous morphine (10 mg/70kg). At T = 165, 10 mg/70kg naloxone IV was given to precipitate opioid withdrawal. The objective opioid withdrawal score (OOWS) and subjective opioid withdrawal score (SOWS) were determined 5 and 15 minutes after naloxone administration (T = 170, 180, respectively). Baseline measurements were recorded at T = ?30 and T = ?15. Results: Comparison of average baseline OOWS scores with OOWS scores obtained 15 minutes after naloxone was significant (p = 0.0001). Scores from 15 minutes post-naloxone infusion showed significant differences in OOWS scores between treatment groups: placebo, 3.7 ± 2.4; palonosetron, 1.5 ± 0.97; and palonosetron with hydroxyzine, 0.2 ± 0.1333. Conclusions: Pretreatment with palonosetron significantly reduced many signs of experimentally-induced opioid withdrawal. Co-administration with hydroxyzine further reduced opioid withdrawal severity. These results suggest that 5-HT3 receptor antagonists, alone or in combination with an antihistamine, may be useful in the treatment of opioid withdrawal.     

Regulation of gastric function by endogenous gastrin releasing peptide in humans: studies with a specific gastrin releasing peptide receptor antagonist

BACKGROUND AND AIMS: The main goal of our study was to characterise the activity of BIM26226 as a peripheral gastrin releasing peptide (GRP) receptor antagonist in healthy human subjects and to determine if endogenous GRP is a physiological regulator of gastric acid secretion and gastrin release. METHODS: Our study consisted of three parts. In part I, subjects received saline or BIM26226 followed by graded doses of intravenous human GRP in a four period crossover design. In part II, subjects received BIM26226 or saline during oral meal ingestion or modified sham feeding. In part III, subjects received an acidified meal in the presence and absence of BIM26226 in a two period crossover design. In addition, gastrin and somatostatin mRNA were measured in biopsy specimens during saline and BIM26226 infusion. RESULTS: BIM26226 dose dependently inhibited GRP induced acid output. Acid secretion after oral liquid meal intake and sham feeding was significantly inhibited by BIM26226 (p<0.01) whereas plasma gastrin release remained unchanged. Gastrin and somatostatin mRNAs were not significantly different after saline or BIM26226. CONCLUSIONS: BIM26226 is a potent GRP antagonist in humans. Endogenous GRP may be a physiological regulator of gastric acid secretion. Gastrin release does not seem to be under the control of GRP.     

From the Cover: A stable antimicrobial peptide with dual functions of treating and preventing citrus Huanglongbing

Citrus Huanglongbing (HLB), caused by a vector-transmitted phloem-limited bacterium Candidatus Liberibacter asiaticus (CLas), is the most devastating citrus disease worldwide. Currently, there are no effective strategies to prevent infection or to cure HLB-positive trees. Here, using comparative analysis between HLB-sensitive citrus cultivars and HLB-tolerant citrus hybrids and relatives, we identified a novel class of stable antimicrobial peptides (SAMPs). The SAMP from Microcitrus australiasica can rapidly kill Liberibacter crescens (Lcr), a culturable Liberibacter strain, and inhibit infections of CLas and CL. solanacearum in plants. In controlled greenhouse trials, SAMP not only effectively reduced CLas titer and disease symptoms in HLB-positive trees but also induced innate immunity to prevent and inhibit infections. Importantly, unlike antibiotics, SAMP is heat stable, making it better suited for field applications. Spray-applied SAMP was taken up by citrus leaves, stayed stable inside the plants for at least a week, and moved systemically through the vascular system where CLas is located. We further demonstrate that SAMP is most effective on α-proteobacteria and causes rapid cytosol leakage and cell lysis. The α-helix-2 domain of SAMP is sufficient to kill Lcr. Future field trials will help determine the efficacy of SAMP in controlling HLB and the ideal mode of application.

Citrus Huanglongbing (HLB), also known as citrus greening, is caused by the vector-transmitted phloem-limited bacterium Candidatus Liberibacter asiaticus (CLas). It is the most destructive disease threatening citrus industries worldwide (1, 2), and thus far, no cure has been discovered. Current management strategies include insecticide application to control the transmission vector Asian citrus psyllids (ACP) and antibiotics treatment to inhibit CLas (3), but neither of these could control HLB effectively. Since the first report of HLB in Florida in 2005, citrus acreage and production in Florida decreased by 38% and 74%, respectively (2, 4). The disease has spread to most citrus-producing states, including Texas and California. Along with drastic losses in fruit production, increasing chemical applications to control the vector and the bacteria have raised costs significantly, making citrus production for growers unsustainable. In severely affected areas, such as Florida, effective therapy is demanded because disease eradication is impractical. In recently impacted areas, such as California, the focus remains on preventing new infections. Hence, innovative therapeutic and preventive strategies to combat this lethal citrus disease are urgently needed to ensure the survival of the citrus industry.One of the most effective and ecofriendly strategies to combat pathogen infection is to utilize existing plant innate immunity–related genes from disease resistant or tolerant varieties for plant protection. Upon pathogen infection, plant defense response genes undergo expression reprogramming to trigger plant innate immunity. Plant endogenous small RNAs play a pivotal role in this regulatory process (5, 6). In addition, primary pathogen infection or application of some phytohormone analogs and chemicals, such as salicylic acid (SA) analogs, could induce systemic acquired resistance or defense priming in plants, which can promote faster and stronger host immune responses upon subsequent pathogen challenges (7, 8).Although all commercially important citrus varieties are susceptible to HLB (9, 10), HLB tolerance has been observed in some hybrids [e.g., US-942 and Sydney hybrid 72 (11, 12)] and close citrus relatives (e.g., Microcitrus australiasica, Eremocitrus glauca, and Poncirus trifoliata) (13). By comparative expression analysis of small RNAs and messenger RNAs (mRNAs) between HLB-sensitive cultivars and HLB-tolerant citrus hybrids and relatives (11, 12), we identified a list of candidate natural defense genes potentially responsible for HLB tolerance (14). One of the candidate regulators is a novel antimicrobial peptide (AMP), which we named “stable antimicrobial peptide” (SAMP). Here, we demonstrate that SAMP not only has the antimicrobial activity but also has the priming activity and can induce citrus systemic defense responses. This dual-functional SAMP can reduce CLas titer and suppress disease symptoms in HLB-positive trees and activate plant systemic defense responses against new infection.