Is Vaginal Laxity Associated with Vaginal Parity and Mode of Delivery?

INTRODUCTION AND HYPOTHESIS: Vaginal laxity (VL) is a common symptom of pelvic floor dysfunction. Although VL has become a frequent topic for research in the last decade, its pathogenesis is still not well understood. The objective was to determine whether vaginal parity or mode of delivery is associated with vaginal laxity. METHODS: This was a retrospective observational study involving women seen in a tertiary urogynecology clinic between May 2016 and November 2018 with symptoms of pelvic floor dysfunction. Patients underwent a standardized interview, clinical examination (POP-Q), and four-dimensional (4D) pelvic floor ultrasound (PFUS). Data regarding vaginal parity and the mode of delivery were based on patient-reported information. Archived 4D-PFUS volumes were analyzed offline to evaluate levator hiatal area on Valsalva. RESULTS: Data from 1,051 patients were analyzed. VL was reported by 236 women (23%) who were younger on average (mean age 54 vs 59 years, p < 0.001) and less likely to be menopausal (530 out of 815 [65.0%] vs.129 out of 236 [54.7%]), p = 0.004]. Symptoms of prolapse were much more common in the VL group (214 out of 236 [91%] vs 316 out of 815 [39%], p = < 0.001) and on imaging mean levator hiatal area (HA) on Valsalva was larger (31 vs 26 cm2, p = 0.01). Vaginal parity was associated with VL symptoms (235 out of 236 [99%] vs 767 out of 815 [94%], p = 0.008), but neither VL prevalence nor bother increased with higher parity. Women who delivered vaginally were three times more likely to complain of VL than those who delivered only by cesarean section. CONCLUSIONS: Vaginal laxity was found to be more prevalent in vaginally parous women. This effect seems to be largely attributable to the first delivery. Instrumental delivery was not shown to increase association with VL compared with normal vaginal delivery.

Assessment of Perineal Scars on Translabial Pelvic Floor Ultrasound: A Pilot Study.

OBJECTIVE: We aimed to describe a method for identifying and evaluating perineal scars using translabial pelvic floor ultrasound. We hypothesized that translabial ultrasound can identify a perineal scar and can differentiate episiotomies from spontaneous tears. METHODS: This pilot study is a secondary analysis of data obtained in the Epi-No® trial. Perineal integrity was assessed using volumes acquired on pelvic floor muscle contraction according to the method previously described for anal sphincter imaging. A scar was diagnosed if a hypoechoic distortion in the perineum was noted. We postulated that an episiotomy would result in a linear scar visible on four dimensional translabial ultrasound whereas nonlinear scars were considered the result of spontaneous perineal tear of grade 2 or higher. The results of this assessment were compared with data retrieved from electronic medical records. RESULTS: A scar was identified in 79/120 women (66%): 42 (35%) linear and 37 (31%) nonlinear. Sonographic and clinical diagnosis agreed on the presence or absence of perineal trauma in 66%. Agreement for the type of laceration was 50%. CONCLUSION: In this retrospective pilot study, a blinded assessment of translabial ultrasound volume data showed agreement between clinical data and sonographic assessment of perineal integrity in 66% and of type of laceration in 50%. More work is needed to optimize the method in assessment of perineal scars to improve its performance before it can be used in clinical audit and research.

Twisting of transobturator midurethral slings: does it matter?

INTRODUCTION AND HYPOTHESIS: We assessed rotation/twisting of transobturator midurethral slings (TOTs) and tested for associations with de novo chronic pain and voiding dysfunction. METHODS: A retrospective pilot study including patients seen after Monarc™ TOT surgery at a single tertiary hospital in 2005-2016. Patients underwent an interview, clinical examination, uroflowmetry and 4D pelvic floor ultrasound. Volume datasets were analyzed blinded against all other data. Sling rotation/twisting was evaluated in volumes obtained at rest. The sling axis was measured relative to the vertical in the midline and in the most lateral parasagittal slice. Total sling rotation was calculated by summation of absolute angle differences between midline and lateral angles. "Corkscrew" rotation was noted when direction of rotation was opposite on the contra-lateral side. RESULTS: The study included 215 patients. Fifty-two (24%) were excluded, leaving 163. Mean age was 57 years (28-87; SD 12), mean BMI 29.4 kg/m2 (18.3-47.4, SD 6). Follow-up was at a median of 17 months (IQR 11-27). Chronic de novo pain was reported by 15 women (9%; dyspareunia by 11 and pelvic/vaginal pain by 4). On imaging, mean total sling rotation was 144° (12-335, SD 56). In the majority (n = 103, 63%) it rotated counter-clockwise from its midline position and in 30 (18%) it rotated clockwise. "Corkscrew" rotation was noted in 30 (18%). De novo chronic pain was associated with lower BMI and vaginal sling exposure but not with sling rotation. The latter was not found to be associated with voiding dysfunction either. CONCLUSIONS: Monarc™ TOTs rotate considerably throughout their course. The degree of twisting or rotation and its direction was not found to be associated with de novo postoperative chronic pain or voiding dysfunction.

Abdominal pressure and pelvic organ prolapse: is there an association?

INTRODUCTION AND HYPOTHESIS: The primary aim of this study was to assess for an association between maximal intra-abdominal pressure reached on Valsalva (MAP) and pelvic organ prolapse (POP) on subjective, clinical and sonographic evaluation. Another objective was to test for association between MAP and body mass index (BMI). METHODS: A retrospective cross-sectional study was carried out on 504 archived datasets of women seen for pelvic floor dysfunction symptoms between January 2017 to September 2019 at a tertiary urogynaecology clinic. Patients underwent a standardized interview including use of visual analogue scores (VAS) to evaluate bother of pelvic floor symptoms, examination using the International Continence Society Pelvic Organ Prolapse Quantification (ICS POP-Q), dual-channel filling and voiding cystometry, test for MAP as well as four-dimensional (4D) transperineal ultrasound. Analysis of ultrasound volumes for pelvic organ descent was performed blinded against all other data. RESULTS: There was a weak but significant association between MAP and sonographic rectal ampulla descent (p = 0.0275). There was also a significant association between MAP and bother of obstructed defecation symptoms (p = 0.0065). Symptomatic, clinical and sonographic POP in the anterior or the apical compartments was not significantly associated with MAP. On multivariate analysis, the association between MAP and rectal descent remained significant (p = 0.01). There was no significant association between BMI and MAP. CONCLUSIONS: This cross-sectional study showed an association between MAP and posterior compartment prolapse on imaging as well as between MAP and the bother score of obstructed defecation symptoms. There was no association between BMI and MAP.

Is vaginal laxity an early symptom of pelvic organ prolapse?

INTRODUCTION AND HYPOTHESIS: We aimed to determine whether vaginal laxity (VL) may be an early symptom of pelvic organ prolapse (POP). METHODS: This was a retrospective observational study including patients referred to a urogynecological clinic owing to symptoms of pelvic floor dysfunction. The interview included inquiry about VL and POP symptoms. Clinical examination and translabial ultrasound (TLUS) were performed to assess for POP and levator hiatal ballooning. Analysis of ultrasound datasets was performed with the reviewer blinded against all other data. Linear regression models were used to evaluate associations between age categories and symptoms. RESULTS: The analysis included 531 patients. Mean age was 59.7 years (range 17.6-92.0), mean body mass index (BMI) was 29.5 kg/m2 (14.7-53.5) and 458 (86%) were vaginally parous. Overall, POP symptoms were reported by 258 patients (49%) and VL by 118 (22%). Significant clinical POP was diagnosed in 384 patients (72%). Levator hiatal ballooning on TLUS was noticed in 56%. VL and POP symptoms showed a divergent association with age (p = 0.02). This trend remained significant after controlling for clinically significant POP (p = 0.01) and levator hiatal ballooning (p = 0.002). Of 384 women with significant objective POP, 231 (60%) reported symptoms of POP. Of 153 women with asymptomatic POP, 7 (5%) reported symptoms of VL. CONCLUSIONS: With increasing age the prevalence of VL decreases, whereas symptoms of POP increase. However, only very few women with objective POP perceive VL as their only symptom, independent of age at presentation. Therefore, vaginal laxity does not seem to be an early symptom of prolapse.

Predictors of occult stress urinary incontinence.

BACKGROUND: Causes for occult stress urinary incontinence (SUI) are poorly recognised. AIMS: To explore the mechanisms behind occult SUI. We hypothesised that cystocele type affects the risk of occult SUI. MATERIALS AND METHODS: We conducted a retrospective, cross-sectional study on 878 consecutive women assessed at a tertiary urogynaecologic clinic between July 2016 and November 2018. The population of this study consisted of 424 women with urodynamic stress incontinence. Women with previous anti-incontinence surgery were excluded. All women underwent a standardised interview, clinical examination and urodynamic testing. Translabial ultrasound was used to categorise cystoceles into Green type II (cystocele with open retrovesical angle) and Green type III (cystocele with intact retrovesical angle). We compared women with overt SUI to those with occult SUI (defined as stress incontinence only observed after prolapse reduction) for demographic characteristics, urodynamic findings and functional anatomy. Predictors for occult SUI were identified with a multivariable logistic regression model. RESULTS: Of 424 women, 362 (85%) had overt, and 62 (15%) occult SUI. There were 136 (32%) women who had a significant cystocele on imaging; 57 (42%) were classified as type II and 79 (58%) as type III. On multivariable regression, age and cystocele type were significantly associated with occult SUI. Odds for occult SUI was 10.9 times higher with type III (cystocele with an intact retrovesical angle) than with type II cystocele (cystocele with an open retrovesical angle; 95% CI 1.3-90.9). CONCLUSIONS: Cystocele type affects the risk of occult SUI. Type III cystocele (intact retrovesical angle) associates with occult SUI.

Is the Visual Analogue Scale inferior to the Pelvic Organ Prolapse Distress Inventory for assessing symptom bother of pelvic organ prolapse?

AIMS: Disease-specific validated questionnaires are used to quantify symptom severity, but they are time consuming to complete and evaluate. A Visual Analogue Scale (VAS) assessment of bother is simpler and faster. The aim of this study is to compare VAS with individual and composite pelvic floor disability index-short form 20 items in predicting significant pelvic organ prolapse (POP). METHODS: A retrospective analysis of data was obtained at a tertiary urogynaecological clinic between February 2017 and August 2018. All women filled out the PFDI-20 and underwent a standardised physician-directed interview, POP-Q and translabial ultrasound. Women with symptoms of POP were asked to indicate the degree of bother using a VAS. Receiver operating characteristic curves were used to evaluate the performance of individual Pelvic Organ Prolapse Distress Inventory (POPDI)-6 items, the six-item composite POPDI-6 score, and VAS in predicting significant POP on clinical and ultrasound examination. RESULTS: The complete data sets of 231 women were analysed. Median VAS for POP was 2.9 (range 0-10). Median POPDI-6 individual and composite scores for items one to six were 2, 2, 2, 1, 2, 0 (all range 0-4) and 9 (range 0-22), respectively. The majority had significant prolapse on clinical examination (n = 195, 84%) and on ultrasound (n = 192, 83%). The composite POPDI-6 prolapse score provided areas under the curve of 0.68 and 0.64 for the prediction of clinical and sonographic POP, compared to 0.74 and 0.69, respectively, for VAS. The difference was not significant (P = 0.3 and 0.8, respectively). CONCLUSIONS: The VAS score was not inferior to the POPDI-6 in predicting significant POP. It has the potential to simplify the assessment of symptom severity.

A long-term comparative study of Uphold™ transvaginal mesh kit against anterior colporrhaphy.

INTRODUCTION AND HYPOTHESIS: Cystocele recurrence remains a major challenge. Anterior colporrhaphy (AC) offers variable success rates that are mostly poorer than native-tissue repairs in other compartments. We compared outcomes after the use of Uphold™ transvaginal mesh kit and AC. METHODS: A retrospective external audit including patients after Uphold™ mesh implantation (2010-2016) analysed against previously published data obtained in identical fashion after AC at the same hospital (2002-2005). Patients underwent an interview, clinical examination and 4D-translabial ultrasound. Offline analysis was performed blinded against all other data. RESULTS: Of 264 patients after mesh and 242 patients after AC, we saw 82 (31%) and 83 (34%), after a median interval of 3.9 years (range 0.4-7.3). Mean age was 64 years (34-86), mean body mass index was 27.7 kg/m2 (15-56) and median vaginal parity 3 deliveries (1-9). AC and mesh groups significantly differed with regard to median follow-up interval (4.3 vs 3.2 years), mean age (61.3 ± 12 vs 67.2 ± 7.5 years), vaginal parity (3 vs 2), past instrumental delivery (20 out of 83 vs 36 out of 82) and concurrent hysterectomy, other prolapse repair or midurethral sling (35 out of 83 vs 1 out of 82, 58 out of 83 vs 76 out of 82 and 12 out of 83 vs 29 out of 82 respectively). The mesh group had 9 cases of dyspareunia, 4 of chronic pelvic pain and 4 vaginal mesh exposures. Univariate comparison between groups for satisfaction and sonographic cystocele favoured mesh. However, point Ba, symptoms of prolapse and reoperation for prolapse were not significantly different. Associations were confirmed on multivariate analysis. CONCLUSIONS: This analysis of two audit projects suggests that the transvaginal Uphold™ mesh kit may confer some advantages over AC for cystocele repair.

Persistent levator co-activation is not associated with symptoms or bother of obstructed defecation.

INTRODUCTION AND HYPOTHESIS: The objective was to determine whether levator contraction during Valsalva (i.e. levator co-activation), as visualised on four-dimensional translabial ultrasound (4D-TLUS), is associated with obstructed defecation (OD). METHODS: This was a retrospective study including patients attending a tertiary urogynecological unit. All underwent an interview, clinical examination and 4D-TLUS. Frequent straining at stool, digitation or sensation of incomplete emptying during defecation were considered symptoms of OD. Archived 4D-TLUS volumes were evaluated, blinded to all other data. Hiatal anteroposterior (AP) diameter (cm) and hiatal area (cm2) were measured at rest and on maximal Valsalva. Patients with anatomical explanations for OD (rectocele, enterocele and/or rectal intussusception) were excluded. Reduction of hiatal AP diameter and hiatal area on Valsalva were tested for association with symptoms of OD. RESULTS: Overall, 1,383 women attended the unit from May 2013 to July 2016. Seven hundred and eight were excluded because of rectocele (n = 645), enterocele (n = 116) or rectal intussusception (n = 41) and 10 owing to missing data, leaving 665. Mean age was 53 years (16-89), mean body mass index (BMI) 27.3 kg/m2 (15.7-64.4). OD symptoms were reported by 368 (55.3%) patients. On imaging, a reduction in hiatal AP diameter on Valsalva was seen in 11.7% cases (78 out of 665) with a mean reduction of 3.5 mm (0.2-17). A mean reduction in hiatal area of 1.9cm2 (0.03-14.8) on Valsalva was seen in 9.8% cases (65 out of 665). These sonographic measures of levator co-activation were not associated with OD symptoms or bother caused by them (p > 0.05). CONCLUSIONS: Levator co-activation seen on TLUS is not associated with obstructed defecation.

Is vaginal flatus related to pelvic floor functional anatomy?

INTRODUCTION AND HYPOTHESIS: Vaginal flatus is an embarrassing condition that can impair women's quality of life. The underlying pathophysiology is unclear. We aimed to evaluate the association between vaginal flatus and pelvic floor anatomy. METHODS: Retrospective observational study on women seen in a tertiary urogynaecological service. All had undergone a standardised interview, clinical examination and four-dimensional transperineal ultrasound. Offline analysis of volume data was performed blinded against clinical data. RESULTS: Datasets of 570 women were analysed. Five hundred twelve (90%) were vaginally parous. Vaginal flatus was reported by 190 (33%). Mean bother score was 4.2 (SD 3.4, range 0-10). One hundred eighty-five reported frequency of vaginal flatus: it occurred < once a month in 25 (14%), once a month in 70 (38%), once a week in 47 (25%), once daily in 28 (15%) and > once daily in 15 (8%). One hundred two women identified the following precipitating factors: intercourse in 72 (71%), postural change in 22 (22%) and physical activities in 9 (9%). Vaginal birth, central and posterior compartment prolapse, anal incontinence, higher levator resting tone and younger age were associated with vaginal flatus. The latter was moderately correlated with symptom bother (correlation coefficient - 0.21). CONCLUSIONS: Vaginal flatus is a prevalent and bothersome condition affecting one-third of our study population. The condition is associated with pelvic floor functional anatomy. A higher resting tone may confer a higher resistance against which trapped air is expelled during physical activities. Younger age was moderately correlated with symptom bother.

Long-term subjective, clinical and sonographic outcomes after native-tissue and mesh-augmented posterior colporrhaphy.

INTRODUCTION AND HYPOTHESIS: Our primary objective was to describe long-term outcomes after posterior colporrhaphy with and without mesh augmentation. METHODS: This was a retrospective study including 93 patients after posterior colporrhaphy (native tissue in 39 and synthetic mesh augmented in 54). The indication was symptoms of prolapse with clinical posterior vaginal wall prolapse. Mesh augmentation and concomitant prolapse operations were performed at the surgeon's discretion. Patients underwent interview, clinical examination and 4D pelvic floor ultrasound. Imaging analysis was done with the reviewer blinded against all other data. Generalized linear modeling was used to compare groups with logistic regression for binary and linear regression for continuous outcomes. RESULTS: Patients were seen on average 5.3 years after surgery and described persistent symptoms of prolapse in 32% and of obstructed defecation in 33%. Clinical recurrence (Bp ≥ -1) was seen in 20%, while sonographic recurrence (rectal ampulla descent to ≥ 15 mm below the symphysis pubis) was noted in 12%. A true rectocele was diagnosed in 33% of patients. No major differences in outcomes were found between those who underwent native tissue and those who had a mesh-augmented repair. CONCLUSIONS: Mesh augmentation was not superior to native tissue posterior colporrhaphy, and both were only moderately effective in eliminating a true rectocele and symptoms of obstructed defecation 5 years after reconstructive surgery.

Does flatus incontinence matter?

INTRODUCTION AND HYPOTHESIS: This study aimed to determine whether incontinence to flatus is associated with women's bother in a symptomatic population and with sonographically diagnosed external anal sphincter (EAS) trauma. METHODS: This is a retrospective study of women attending a tertiary urogynecological unit between May 2013 and November 2015. Baseline evaluation included a standardized interview with St. Mark's Incontinence Score (SMIS) and visual analog scale (VAS) assessment for bother, as well as a physical examination and translabial pelvic floor ultrasound. At least one volume obtained covered the entire length of the EAS. These volumes were analyzed with the reviewer blinded to all clinical data. RESULTS: During the inclusion period, 1104 patients visited the unit. Fifty-three patients were excluded from the study for missing data, leaving 1051 for final analysis. Mean age was 57 years (56-58) and mean body mass index (BMI) 29.1 kg/m2 (28.8-29.6). The prevalence of any anal incontinence (AI) and flatus incontinence were 16.4% (172/1051) and 13.9% (146/1051), respectively. In the group of patients with AI, mean SMIS was 11.8 (11.0-12.6), and mean VAS for AI bother was 5.4 (5.0-5.9). Significant EAS trauma was detected in 9.8% (103/1051) of patients and was associated with flatus incontinence (p = 0.002). Including a flatus incontinence question in the SMIS questionnaire improved the prediction of patient bother from AI (R2 87.8% versus R2 86.3%, p = 0.04). CONCLUSIONS: Flatus incontinence is associated with ultrasound findings of EAS trauma and with higher patient bother from AI.

The Routine Use of Intrapartum Ultrasound in Clinical Decision-Making during the Second Stage of Labor - Does It Have Any Impact on Delivery Outcomes?

BACKGROUND/AIMS: The study aimed to assess whether the use of intrapartum transperineal ultrasound (US) can reduce the rate of failed vacuum extraction (VE). METHODS: This is a retrospective cohort study including all women delivering at term with the diagnosis of protracted second stage of labor. The mode of delivery and rate of failed VE were compared between women who underwent a US examination prior to the decision on obstetrical interventions ("+US" group) and those in whom clinical decisions were based upon digital assessment only ("no-US" group). RESULTS: The study included 635 women. Among the "no-US" group (536), there were 13 failed VE attempts (3.6%) vs. none in the "+US" group (99, p = 0.1). There was a significant difference between the groups regarding mode of delivery (p = 0.001), with a lower cesarean section (CS) rate (20.2 vs. 27.8%) among the "+US" group. Maternal age, body mass index, nulliparity, gestational age at delivery, and birth weight, as well as neonatal short-term outcome did not differ significantly between the 2 groups. CONCLUSIONS: We demonstrate that among women who had the addition of intrapartum US during the second stage of labor there was a trend toward a lower rate of failed VE (although not reaching statistical significance), with a lower rate of CS but not affecting neonatal outcome.

Can assessing the angle of progression before labor onset assist to predict vaginal birth after cesarean?: A prospective cohort observational study.

OBJECTIVE: To assess whether pre-labor measurement of the angle of progression (AOP) can assist in predicting a successful vaginal birth after cesarean in women without a previous vaginal birth. METHODS: A prospective observational cohort study performed in a single tertiary center including women at term with a single previous cesarean delivery (CD), without prior vaginal births, who desire a trial of labor. Transperineal ultrasound was used to measure the AOP before the onset of labor. The managing staff in the delivery suite was blinded to the ultrasound measurements. Clinical data and delivery outcome were retrieved from medical records. The study was approved by the institutional ethics committee (KMC 0117-10). RESULTS: Of the 111 women included in the study, 67 (60.4%) had a successful vaginal birth after CD. Women were sonographically assessed at a median of 3 days [interquartile range (IQR) 1-3 days] prior to delivery. The median AOP was significantly narrower in women who eventually underwent a CD than in those who delivered vaginally (88°, IQR 78-96° vs. 99°, IQR 89-107°, respectively; p < .001). An AOP >98° (derived from a receiver operating characteristic curve) was associated with a successful vaginal birth after CD in 87.5% of women. Multivariable regression analysis demonstrated that each additional 1° in the AOP increases the chance for a successful vaginal birth after CD by 6%. CONCLUSIONS: Pre-labor AOP may be a useful sonographic tool for predicting vaginal birth after CD and can assist in consulting primiparous women with a prior CD opting for a trial of labor.

Demographic risk factors for pelvic organ prolapse: Do smoking, asthma, heavy lifting or family history matter?

OBJECTIVE: Our objective was to identify non-obstetric risk factors for pelvic organ prolapse in women attending a urogynecology clinic. STUDY DESIGN: A retrospective study of 662 women referred for pelvic floor dysfunction between January 2017 and August 2018. Participants underwent a standardized interview, clinical exam including Pelvic Organ Prolapse Qualification (POP-Q) assessment, and four-dimensional transperineal ultrasound. They were questioned about smoking, asthma, heavy lifting and family history of pelvic organ prolapse, as well as prolapse symptoms. Significant clinical prolapse was defined as POP-Q stage ≥2 for anterior and posterior compartments and stage ≥1 for apical prolapse. Offline analysis of volume data was performed blinded against all other data. Statistical analysis included logistic regression with multivariable models adjusted for age, body mass index, vaginal parity, levator hiatal area and levator avulsion. RESULTS: Participating women were assessed at a mean age of 58 (SD 13.3) years with a mean body mass index of 28.93 kg/m2 (standard deviation 5.98). The vast majority were vaginally parous (88.2 %) with a median of two vaginal deliveries (range 0-7). Previous hysterectomy was reported by 29.3 % of women (n = 194) and previous prolapse repair by 17.2 % (n = 114). Past or current smoking was reported by 300 (45.6 %) women, 113 (17.2 %) reported asthma, 246 (37.6 %) heavy lifting and 186 (28.6 %) a family history of pelvic organ prolapse. Heavy lifting was associated with sonographic prolapse (odds ratio 1.71, 95 % confidence interval 1.2-2.4), confirmed on multivariable analysis (P = 0.046). Heavy lifting was positively associated with symptoms (P = 0.053) and clinical signs of pelvic organ prolapse (P = 0.056) on univariate analysis; however, this became non-significant on multivariable analysis. No associations were found for individual compartments except for a trend towards more posterior compartment prolapse with heavy lifting. CONCLUSIONS: Smoking, asthma and family history of prolapse were not found to be a significant risk factor for prolapse in our study population. Heavy lifting may be a potential risk factor, in particular for posterior compartment prolapse.

Molecular Mechanism of AMPA Receptor Modulation by TARP/Stargazin.

AMPA receptors (AMPARs) mediate the majority of fast excitatory transmission in the brain and critically contribute to synaptic plasticity and pathology. AMPAR trafficking and gating are tightly controlled by auxiliary transmembrane AMPAR regulatory proteins (TARPs). Here, using systematic domain swaps with the TARP-insensitive kainate receptor GluK2, we show that AMPAR interaction with the prototypical TARP stargazin/γ2 primarily involves the AMPAR membrane domains M1 and M4 of neighboring subunits, initiated or stabilized by the AMPAR C-tail, and that these interactions are sufficient to enable full receptor modulation. Moreover, employing TARP chimeras disclosed a key role in this process also for the TARP transmembrane domains TM3 and TM4 and extracellular loop 2. Mechanistically, our data support a two-step action in which binding of TARP to the AMPAR membrane domains destabilizes the channel closed state, thereby enabling an efficient opening upon agonist binding, which then stabilizes the open state via subsequent interactions.

Stargazin reduces desensitization and slows deactivation of the AMPA-type glutamate receptors.

The AMPA-type glutamate receptors mediate the majority of the fast excitatory synaptic transmission and critically contribute to synaptic plasticity in the brain, hence the existence of numerous trafficking proteins dedicated to regulation of their synaptic delivery and turnover. Stargazin (also termed gamma2) is a member of a recently identified protein family termed transmembrane AMPA receptor regulatory proteins (TARPs). TARPs physically associate with AMPA receptors and participate in their surface delivery and anchoring at the postsynaptic membrane. Here, we report that next to its trafficking roles, stargazin may also act as a positive allosteric modulator of AMPA receptor ion channel function. Coexpression of stargazin with AMPA receptor subunits, either in Xenopus oocytes or in human embryonic kidney 293 cells, significantly reduced receptor desensitization in response to glutamate. Receptor deactivation rates were also slowed, and the recovery from desensitization was accelerated. Structurally, based on the data showing a tight correlation between desensitization and the stability of the AMPA receptor intradimer interface, we propose that binding of stargazin may stabilize the receptor conformation. Functionally, our data suggest that AMPA receptors complexed with stargazin (and possibly also with other TARPs) at the postsynaptic membrane are significantly more responsive to synaptically released glutamate compared with AMPA receptors lacking stargazin/TARP interaction. The putative existence of such two states of synaptic AMPA receptors, with and without stargazin/TARP binding, may provide a novel mechanism for regulation of excitatory synaptic strength during development and/or in synaptic plasticity in the adult brain.