Structural basis for endosomal trafficking of diverse transmembrane cargos by PX-FERM proteins

Transit of proteins through the endosomal organelle following endocytosis is critical for regulating the homeostasis of cell-surface proteins and controlling signal transduction pathways. However, the mechanisms that control these membrane-transport processes are poorly understood. The Phox-homology (PX) domain-containing proteins sorting nexin (SNX) 17, SNX27, and SNX31 have emerged recently as key regulators of endosomal recycling and bind conserved Asn-Pro-Xaa-Tyr-sorting signals in transmembrane cargos via an atypical band, 4.1/ezrin/radixin/moesin (FERM) domain. Here we present the crystal structure of the SNX17 FERM domain bound to the sorting motif of the P-selectin adhesion protein, revealing both the architecture of the atypical FERM domain and the molecular basis for recognition of these essential sorting sequences. We further show that the PX-FERM proteins share a promiscuous ability to bind a wide array of putative cargo molecules, including receptor tyrosine kinases, and propose a model for their coordinated molecular interactions with membrane, cargo, and regulatory proteins.

Incidence of tracheal stenosis and other late complications after percutaneous tracheostomy

OBJECTIVE

To determine the incidence of tracheal stenosis, voice and breathing changes, and stomal complications after percutaneous dilatational tracheostomy (PDT).

METHODS

From December 1992 through June 1999, 420 critically ill patients underwent 422 PDTs. There were 340 (81%) long-term survivors, 100 (29%) of whom were interviewed and offered further evaluation by fiberoptic laryngotracheoscopy (FOL) and tracheal computed tomography (CT). Tracheal stenosis was defined as more than 10% tracheal narrowing on transaxial sections or coronal and sagittal reconstruction views. Forty-eight patients agreed to CT evaluation; 38 patients also underwent FOL. CT and FOL evaluations occurred at 30 +/- 25 (mean +/- standard deviation) months after PDT.

RESULTS

Twenty-seven (27%) patients reported voice changes and 2 (2%) reported persistent severe hoarseness. Vocal cord abnormalities occurred in 4/38 (11%) patients, laryngeal granuloma in 1 (3%) patient, focal tracheal mucosal erythema in 2 (5%) patients, and severe tracheomalacia/stenosis in 1 (2.6%) patient. CT identified mild (11-25%) stenosis in 10 (21%) asymptomatic patients, moderate (26-50%) stenosis in 4 (8.3%) patients, 2 who were symptomatic, and severe (>50%) stenosis in 1 (2%) symptomatic patient. Ten patients (10%) reported persistent respiratory problems after tracheal decannulation, but only four agreed to be studied. Two patients had moderate stenosis, and one had severe stenosis. One patient's CT scan was normal. No long-term stomal complications were identified or reported.

CONCLUSIONS

Subjective voice changes and tracheal abnormalities are common after endotracheal intubation followed by PDT. Long-term follow-up of critically ill patients identified a 31% rate of more than 10% tracheal stenosis after PDT. Symptomatic stenosis manifested by subjective respiratory symptoms after decannulation was found in 3 of 48 (6%) patients.

Rab8a interacts directly with PI3Kγ to modulate TLR4-driven PI3K and mTOR signalling

Toll-like receptor 4 (TLR4) is activated by bacterial lipopolysaccharide (LPS) to mount innate immune responses. The TLR4-induced release of pro- and anti-inflammatory cytokines generates robust inflammatory responses, which must then be restrained to avoid disease. New mechanisms for the critical regulation of TLR-induced cytokine responses are still emerging. Here we find TLR4 complexes localized in LPS-induced dorsal ruffles on the surface of macrophages. We discover that the small GTPase Rab8a is enriched in these ruffles and recruits phosphatidylinositol 3-kinase (PI3Kγ) as an effector by interacting directly through its Ras-binding domain. Rab8a and PI3Kγ function to regulate Akt signalling generated by surface TLR4. Rab8a and PI3Kγ do not affect TLR4 endocytosis, but instead regulate mammalian target of rapamycin signalling as a mechanism for biasing the cytokine profile to constrain inflammation in innate immunity.

Classification of the human phox homology (PX) domains based on their phosphoinositide binding specificities

Phox homology (PX) domains are membrane interacting domains that bind to phosphatidylinositol phospholipids or phosphoinositides, markers of organelle identity in the endocytic system. Although many PX domains bind the canonical endosome-enriched lipid PtdIns3P, others interact with alternative phosphoinositides, and a precise understanding of how these specificities arise has remained elusive. Here we systematically screen all human PX domains for their phospholipid preferences using liposome binding assays, biolayer interferometry and isothermal titration calorimetry. These analyses define four distinct classes of human PX domains that either bind specifically to PtdIns3P, non-specifically to various di- and tri-phosphorylated phosphoinositides, bind both PtdIns3P and other phosphoinositides, or associate with none of the lipids tested. A comprehensive evaluation of PX domain structures reveals two distinct binding sites that explain these specificities, providing a basis for defining and predicting the functional membrane interactions of the entire PX domain protein family.

Temporary intravenous bag silo closure in severe abdominal trauma

Several temporary abdominal wall closure techniques have been described in the literature. We present our experience with an inexpensive and efficient method of temporary abdominal closure when bowel edema and distension preclude safe primary closure. Our technique is a variation of the silon (silo) closure used in the repair of gastroschisis and omphalocele, using a pre-gas-sterilized, soft 3-L plastic cystoscopy fluid irrigation bag cut to an oval shape and stapled or sutured to the skin edges of the wound.

Bedside percutaneous tracheostomy with bronchoscopic guidance in critically ill patients

BACKGROUND

Bedside percutaneous dilational tracheostomy, a relatively new method of tracheal cannulation, provides safe and ready access to the trachea to relieve airway obstruction and tracheopulmonary secretions. The dilational technique has undergone various modifications during the past decade. Complications of this procedure are primarily related to the lack of direct visualization during tracheostomy tube placement and to poor patient selection.

OBJECTIVE

To report the utility of percutaneous dilational tracheostomy with bronchoscopic guidance in 162 critically ill patients.

MAIN OUTCOME MEASURES

Mortality rates and complications associated with this technique.

RESULTS

Twenty-five patients (15.4%) died while hospitalized. No deaths were related to tracheostomy. There were four (2.5%) major complications: one pneumothorax and three posterior tracheal tears, which healed spontaneously. There were five (3.1%) minor complications: one posterior mucosal disruption, one minor bleeding episode, and three minor episodes of cellulitis. One hundred thirty-seven patients (84.6%) were discharged. Twenty-nine patients (21.2%) were available for follow-up and were experiencing no significant problems or complications following the procedure. Compared with standard open tracheostomy, charges were reduced by $1628.20 per patient ($263,768.40 total savings).

CONCLUSIONS

Bedside percutaneous tracheostomy with bronchoscopic guidance is safe and cost-effective. Complications compare favorably with that of open tracheostomy. Major complications should be avoided with continuous bronchoscopic observation during the procedure.

Assembly and solution structure of the core retromer protein complex

Retromer is a peripheral membrane protein complex that has pleiotropic roles in endosomal membrane trafficking. The core of retromer possesses three subunits, VPS35, VPS29 and VPS26, that play different roles in binding to cargo, regulatory proteins and complex stabilization. We have performed an investigation of the thermodynamics of core retromer assembly using isothermal titration calorimetry (ITC) demonstrating that VPS35 acts as the central subunit to which VPS29 and VPS26 bind independently. Furthermore, we confirm that the conserved PRLYL motif of the large VPS35 subunit is critical for direct VPS26 interaction. Heat capacity measurements of VPS29 and VPS26 binding to VPS35 indicate extensive binding interfaces and suggest conformational alterations in VPS29 or VPS35 upon complex formation. Solution studies of the retromer core using small-angle X-ray scattering allow us to propose a model whereby VPS35 forms an extended platform with VPS29 and VPS26 bound at distal ends, with the potential for forming dimeric assemblies.

Parkinson Disease-linked Vps35 R524W Mutation Impairs the Endosomal Association of Retromer and Induces α-Synuclein Aggregation

Endosomal sorting is a highly orchestrated cellular process. Retromer is a heterotrimeric complex that associates with endosomal membranes and facilitates the retrograde sorting of multiple receptors, including the cation-independent mannose 6-phosphate receptor for lysosomal enzymes. The cycling of retromer on and off the endosomal membrane is regulated by a network of retromer-interacting proteins. Here, we find that Parkinson disease-associated Vps35 variant, R524W, but not P316S, is a loss-of-function mutation as marked by a reduced association with this regulatory network and dysregulation of endosomal receptor sorting. Expression of Vps35 R524W-containing retromer results in the accumulation of intracellular α-synuclein-positive aggregates, a hallmark of Parkinson disease. Overall, the Vps35 R524W-containing retromer has a decreased endosomal association, which can be partially rescued by R55, a small molecule previously shown to stabilize the retromer complex, supporting the potential for future targeting of the retromer complex in the treatment of Parkinson disease.

Mechanisms and patterns of injuries related to large animals

BACKGROUND

Injuries from encounters with large animals represent a significant health risk for rural communities. We evaluated our regional trauma centers' experience with large-animal injuries to determine whether certain mechanisms and patterns of injury predicted either major head/craniofacial or torso (chest/abdomen/pelvis) trauma.

METHODS

The hospital courses of 145 patients with injuries related to large animals were reviewed retrospectively to determine patterns of injury, specific injury mechanisms, species-specific injuries, and predictors of multiple body region trauma.

RESULTS

Seventy-nine patients (55%) were injured by horses, 47 patients (32%) by bulls, 16 patients (11%) by cows, and 3 patients (2%) by wild animal attacks. The predominant species-specific mechanisms of injury were falls (horses), tramplings (bulls), and kicks (cows). Brain/craniofacial injuries were most common from horse-related encounters (32%), whereas bull and cow encounters usually resulted in torso injuries (45% and 56%, respectively). Multiple body region injuries occurred in 32% of patients. Fractures of the upper extremities were more often associated with torso and head/craniofacial injuries (48%) than lower extremity injuries (17%) (p = 0.02).

CONCLUSION

Large animal injuries frequently involve multiple body regions with species-specific mechanisms. Upper extremity injuries are associated with a significantly higher percentage of torso and head/craniofacial injuries, which may have implications for field triage.

VPS29 is not an active metallo-phosphatase but is a rigid scaffold required for retromer interaction with accessory proteins

VPS29 is a key component of the cargo-binding core complex of retromer, a protein assembly with diverse roles in transport of receptors within the endosomal system. VPS29 has a fold related to metal-binding phosphatases and mediates interactions between retromer and other regulatory proteins. In this study we examine the functional interactions of mammalian VPS29, using X-ray crystallography and NMR spectroscopy. We find that although VPS29 can coordinate metal ions Mn²⁺ and Zn²⁺ in both the putative active site and at other locations, the affinity for metals is low, and lack of activity in phosphatase assays using a putative peptide substrate support the conclusion that VPS29 is not a functional metalloenzyme. There is evidence that structural elements of VPS29 critical for binding the retromer subunit VPS35 may undergo both metal-dependent and independent conformational changes regulating complex formation, however studies using ITC and NMR residual dipolar coupling (RDC) measurements show that this is not the case. Finally, NMR chemical shift mapping indicates that VPS29 is able to associate with SNX1 via a conserved hydrophobic surface, but with a low affinity that suggests additional interactions will be required to stabilise the complex in vivo. Our conclusion is that VPS29 is a metal ion-independent, rigid scaffolding domain, which is essential but not sufficient for incorporation of retromer into functional endosomal transport assemblies.

Structural basis for different phosphoinositide specificities of the PX domains of sorting nexins regulating G-protein signaling

Sorting nexins (SNXs) or phox homology (PX) domain containing proteins are central regulators of cell trafficking and signaling. A subfamily of PX domain proteins possesses two unique PX-associated domains, as well as a regulator of G protein-coupled receptor signaling (RGS) domain that attenuates Gαs-coupled G protein-coupled receptor signaling. Here we delineate the structural organization of these RGS-PX proteins, revealing a protein family with a modular architecture that is conserved in all eukaryotes. The one exception to this is mammalian SNX19, which lacks the typical RGS structure but preserves all other domains. The PX domain is a sensor of membrane phosphoinositide lipids and we find that specific sequence alterations in the PX domains of the mammalian RGS-PX proteins, SNX13, SNX14, SNX19, and SNX25, confer differential phosphoinositide binding preferences. Although SNX13 and SNX19 PX domains bind the early endosomal lipid phosphatidylinositol 3-phosphate, SNX14 shows no membrane binding at all. Crystal structures of the SNX19 and SNX14 PX domains reveal key differences, with alterations in SNX14 leading to closure of the binding pocket to prevent phosphoinositide association. Our findings suggest a role for alternative membrane interactions in spatial control of RGS-PX proteins in cell signaling and trafficking.

Infectious disease consultation and microbiologic surveillance for intensive care unit trauma patients: a pilot study

Infection remains a major cause of posttrauma morbidity. We retrospectively reviewed 2 cohorts of trauma patients admitted to a regional trauma center before and after a policy change integrating prospective microbiologic surveillance and infectious disease (ID) consultation into management of trauma admissions. Primary interests were effects of this policy change on antimicrobial use and diagnostic precision (particularly differentiation of infection from colonization). Associated costs, microflora, survival, and disability were also compared. Patients were stratified for risk of infection. ID consultation was associated with a 49% increased odds that an infection diagnosis was microbiologically based (P=.006) and 57% reduction of antibiotics costs per hospitalized day (P=.0008). Costs of consultation and an 86% increase (P<10(-6)) in total cultures combined to minimally exceed that financial saving. The observed improvements in diagnostic precision and antimicrobial usage, however, suggest consideration of prospective microbiologic surveillance and multidisciplinary physician teams including ID physicians for high-risk trauma patients.

The early effects of implementing American College of Surgeons level II criteria on transfer and survival rates at a rurally based community hospital

We conducted a retrospective review to determine the early effects of implementing the American College of Surgeons (ACS) level II criteria on the number of transferrals and survival rates of trauma patients in a rurally based hospital. Data were collected from time period "B" (13 months before) and time period "A" (14 months after) implementing ACS criteria. Patient data parameters included age, sex, Revised Trauma Score, Glasgow Coma Scale score, Injury Severity Score, number of days hospitalized, diagnoses, place of injury (i.e., local county or transfer from another county), outcome, and probability of survival. There was a significant increase in the number of patients with Injury Severity Score > or = 15 from period B to period A (189 vs. 297, p = 0.002). A much higher percentage of these patients were transfers from out of county (period B = 33% vs. period A = 59.5%, p = 0.0001). Despite a higher percentage of transferred patients with probability of survival < or = 25% (period B = 25% vs. period A = 58%, p = 0.002), the survival rate in this group improved from 7.5% during time period B to 25.5% after implementing level II criteria (p = 0.0303). This data suggest that implementing level II ACS guidelines has the early beneficial effects of increasing transfers of seriously injured patients and improving survival in the most critically injured group.

Phosphoinositide binding by the SNX27 FERM domain regulates its localization at the immune synapse of activated T-cells

Sorting nexin 27 (SNX27) controls the endosomal-to-cell-surface recycling of diverse transmembrane protein cargos. Crucial to this function is the recruitment of SNX27 to endosomes which is mediated by the binding of phosphatidylinositol-3-phosphate (PtdIns3P) by its phox homology (PX) domain. In T-cells, SNX27 localizes to the immunological synapse in an activation-dependent manner, but the molecular mechanisms underlying SNX27 translocation remain to be clarified. Here, we examined the phosphoinositide-lipid-binding capabilities of full-length SNX27, and discovered a new PtdInsP-binding site within the C-terminal 4.1, ezrin, radixin, moesin (FERM) domain. This binding site showed a clear preference for bi- and tri-phosphorylated phophoinositides, and the interaction was confirmed through biophysical, mutagenesis and modeling approaches. At the immunological synapse of activated T-cells, cell signaling regulates phosphoinositide dynamics, and we find that perturbing phosphoinositide binding by the SNX27 FERM domain alters the SNX27 distribution in both endosomal recycling compartments and PtdIns(3,4,5)P3-enriched domains of the plasma membrane during synapse formation. Our results suggest that SNX27 undergoes dynamic partitioning between different membrane domains during immunological synapse assembly, and underscore the contribution of unique lipid interactions for SNX27 orchestration of cargo trafficking.

The quality of forensic psychological assessments, reports, and testimony: acknowledging the gap between promise and practice

During the past decade, the field of forensic psychological assessment entered a period of standard setting, reflected in the publication of specialty guidelines for practice and in the proliferation of educational opportunities, training programs, and credentialing and certification procedures for forensic examiners. Representing significant efforts to advance the quality of psychological assessments in legal contexts, these developments foreshadow the promise of forensic assessment. During this same time period, new evidence emerged regarding the quality of forensic practice. This article reviews this evidence and evaluates current practice against the promise of forensic assessment. Forensic reports appear to be of higher quality than those described by commentators in the 1970s and early 1980s; nevertheless, the level of practice falls far short of professional aspirations for the field. The review identifies significant areas of weakness that demand the attention of professional organizations, accrediting agencies, educators, lawmakers, practitioners, and consumers.

Outcomes following injury in a predominantly rural-population-based trauma center

OBJECTIVE

To determine the incidence, severity, and outcomes of injury in patients treated in a rural-based level I trauma center and to compare the outcomes with a nationally indexed patient population--the Major Trauma Outcome Study.

DESIGN

Retrospective evaluation of trauma registry data.

SETTING

State of Illinois designated level I trauma center located in Urbana.

PATIENTS

A total of 2246 trauma patients admitted from August 1989 through August 1992, with a mortality cohort of 158 patients. There were 1735 patients (77%) with Injury Severity Scores less than or equal to 19 and 511 patients (23%) with more severe injuries (Injury Severity Scores > or = 20).

MAIN OUTCOME MEASURES

Mortality rates using the TRISS method, the Major Trauma Outcome Study, and final patient dispositions.

RESULTS

The overall mortality rate, excluding those patients who were pronounced dead on arrival, was 125/2213 (5.6%). Eighty-six (69%) of these 125 patients had neurological Abbreviated Injury Scores of 3 or greater, with neurotrauma being a major contributor to their deaths. The m-statistic was 0.99 and the z-statistic was -3.30 for the entire group. The observed probability of survival met or exceeded the expected probability of survival when compared with the Major Trauma Outcome Study in all categories.

CONCLUSION

Acceptable mortality rates compared with the Major Trauma Outcome Study can be achieved in a rural-community-based level I trauma center despite relatively small numbers of critically injured patients. Such outcomes may assist in justifying resource allocation for trauma centers in rural areas.

An evaluation of triple-lumen catheter infections using a guidewire exchange technique

One hundred fifty-six triple-lumen central venous catheters placed into 65 different sites in 52 surgical or trauma patients were prospectively evaluated to determine the frequency of catheter-related infection and the efficacy of multiple guidewire exchanges. Thirty-four anatomic sites in 33 patients without clinical signs of sepsis were compared to 31 anatomic sites in 19 patients with a clinical diagnosis of sepsis based on clinical, microbiologic, and radiologic evaluation. Anatomic sites were used indefinitely unless a positive semiquantitative catheter culture (greater than or equal to 15 cfu) or catheter-associated bacteremia developed. Seven catheter-related infections developed in five septic patients (26.3%) while no infections developed in the nonseptic group (p = 0.004). Three of 31 anatomic sites (9.6%) were associated with bacteremia. The number of hospital days to initial catheter insertion and the length of catheter site use were considerably higher in the septic patients compared to the nonseptic group. We conclude that triple-lumen catheter-related infections are uncommon in patients with no evidence of infection and can be safely guidewire-exchanged to prolong site use. Catheter-related infections in septic patients are much more frequent and unpredictable. Although guidewire exchange has a role in septic patients, multiple exchanges cannot routinely be recommended.

Laparoscopic diaphragm rupture repair

Diaphragm rupture is an infrequently encountered but well-documented injury in the multiply injured patient. Only a few cases in which minimally invasive techniques were used for repair have been reported thus far. Herein we describe the repair of a diaphragm rupture in a 36-year-old man who was injured in a motor vehicle accident. In a 10-year review of the literature, we were able to locate seven journal articles reporting 10 patients. We conclude that in appropriate stable patients with diaphragm rupture, minimally invasive techniques offer a reasonable alternative to open laparotomy or thoracotomy.

The safety of prolonging the use of central venous catheters: a prospective analysis of the effects of using antiseptic-bonded catheters with daily site care

OBJECTIVE

To determine rates of catheter colonization and catheter-related bloodstream infection (CRBSI) when antiseptic-bonded central venous catheters (CVCs) and standardized daily site care are used with no predetermined interval for removal.

DESIGN

Prospective observational study.

SETTING

Two major trauma centers.

PATIENTS

All trauma patients admitted to two major trauma centers that received a CVC from May 1996 through May 1998.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Catheters were semiquantitatively cultured to identify bacterial colonization and CRBSI. Monitored variables included total catheter days, anatomical site of catheter insertion, and area in hospital of catheter insertion. CVC tips and intracutaneous segments were semiquantitatively cultured. A total of 460 (92%) of 501 catheters placed in 324 trauma patients were evaluable, representing 95.5% of all catheter days during the study period. Rates of catheter colonization and CRBSI were 5% (5/1000 catheter days) and 1.5% (1.511000 catheter days), respectively. Subclavian catheters were in place longer than femoral or internal jugular catheters (p < .0001), but the colonization rate was significantly lower (p = .03; relative risk, 0.34; 95% confidence interval, 0.15-0.77). No differences in CRBSI rates among anatomical sites or between catheters used < or =14 days and those used >14 days were identified.

CONCLUSION

Femoral and internal jugular antiseptic-bonded CVCs develop bacterial colonization earlier than subclavian CVCs. Subclavian antiseptic-bonded CVCs combined with standardized daily site care may be safely used >14 days in trauma patients.

De novo macrocyclic peptides for inhibiting, stabilizing, and probing the function of the retromer endosomal trafficking complex

The retromer complex (Vps35-Vps26-Vps29) is essential for endosomal membrane trafficking and signaling. Mutation of the retromer subunit Vps35 causes late-onset Parkinson’s disease, while viral and bacterial pathogens can hijack the complex during cellular infection. To modulate and probe its function, we have created a novel series of macrocyclic peptides that bind retromer with high affinity and specificity. Crystal structures show that most of the cyclic peptides bind to Vps29 via a Pro-Leu–containing sequence, structurally mimicking known interactors such as TBC1D5 and blocking their interaction with retromer in vitro and in cells. By contrast, macrocyclic peptide RT-L4 binds retromer at the Vps35-Vps26 interface and is a more effective molecular chaperone than reported small molecules, suggesting a new therapeutic avenue for targeting retromer. Last, tagged peptides can be used to probe the cellular localization of retromer and its functional interactions in cells, providing novel tools for studying retromer function.

Structure and Membrane Binding Properties of the Endosomal Tetratricopeptide Repeat (TPR) Domain-containing Sorting Nexins SNX20 and SNX21

Sorting nexins (SNX) orchestrate membrane trafficking and signaling events required for the proper distribution of proteins within the endosomal network. Their phox homology (PX) domain acts as a phosphoinositide (PI) recognition module that targets them to specific endocytic membrane domains. The modularity of SNX proteins confers a wide variety of functions from signaling to membrane deformation and cargo binding, and many SNXs are crucial modulators of endosome dynamics and are involved in a myriad of physiological and pathological processes such as neurodegenerative diseases, cancer, and inflammation. Here, we have studied the poorly characterized SNX20 and its paralogue SNX21, which contain an N-terminal PX domain and a C-terminal PX-associated B (PXB) domain of unknown function. The two proteins share similar PI-binding properties and are recruited to early endosomal compartments by their PX domain. The crystal structure of the SNX21 PXB domain reveals a tetratricopeptide repeat (TPR)-fold, a module that typically binds short peptide motifs, with three TPR α-helical repeats. However, the C-terminal capping helix adopts a highly unusual and potentially self-inhibitory topology. SAXS solution structures of SNX20 and SNX21 show that these proteins adopt a compact globular architecture, and membrane interaction analyses indicate the presence of overlapping PI-binding sites that may regulate their intracellular localization. This study provides the first structural analysis of this poorly characterized subfamily of SNX proteins, highlighting a likely role as endosome-associated scaffolds.