Structure-function analysis of ceTIR-1/hSARM1 explains the lack of Wallerian axonal degeneration in C. elegans.

Wallerian axonal degeneration (WD) does not occur in the nematode C. elegans, in contrast to other model animals. However, WD depends on the NADase activity of SARM1, a protein that is also expressed in C. elegans (ceSARM/ceTIR-1). We hypothesized that differences in SARM between species might exist and account for the divergence in WD. We first show that expression of the human (h)SARM1, but not ceTIR-1, in C. elegans neurons is sufficient to confer axon degeneration after nerve injury. Next, we determined the cryoelectron microscopy structure of ceTIR-1 and found that, unlike hSARM1, which exists as an auto-inhibited ring octamer, ceTIR-1 forms a readily active 9-mer. Enzymatically, the NADase activity of ceTIR-1 is substantially weaker (10-fold higher Km) than that of hSARM1, and even when fully active, it falls short of consuming all cellular NAD+. Our experiments provide insight into the molecular mechanisms and evolution of SARM orthologs and WD across species.

A duplex structure of SARM1 octamers stabilized by a new inhibitor.

In recent years, there has been growing interest in SARM1 as a potential breakthrough drug target for treating various pathologies of axon degeneration. SARM1-mediated axon degeneration relies on its TIR domain NADase activity, but recent structural data suggest that the non-catalytic ARM domain could also serve as a pharmacological site as it has an allosteric inhibitory function. Here, we screened for synthetic small molecules that inhibit SARM1, and tested a selected set of these compounds in a DRG axon degeneration assay. Using cryo-EM, we found that one of the newly discovered inhibitors, a calmidazolium designated TK106, not only stabilizes the previously reported inhibited conformation of the octamer, but also a meta-stable structure: a duplex of octamers (16 protomers), which we have now determined to 4.0 Å resolution. In the duplex, each ARM domain protomer is engaged in lateral interactions with neighboring protomers, and is further stabilized by contralateral contacts with the opposing octamer ring. Mutagenesis of the duplex contact sites leads to a moderate increase in SARM1 activation in cultured cells. Based on our data we propose that the duplex assembly constitutes an additional auto-inhibition mechanism that tightly prevents pre-mature activation and axon degeneration.

Comparison of 1-Day Emergency Department Observation and Inpatient Ward for 1-Day Admissions in Syncope Patients.

BACKGROUND: In an era of increasing health care costs, the need for hospitalization is being scrutinized. In particular, 1-day hospitalizations are thought to be especially costly and unnecessary, and, increasingly, emergency department observation units (EDOUs) are being used as alternatives. OBJECTIVE: Our aim was to determine the differences in outcomes and diagnoses between 1-day inpatient and EDOU stays for syncope. METHODS: We retrospectively reviewed a cohort of patients with syncope who were seen in an urban ED with 1-day admission to an inpatient ward, EDOU, or full hospitalization. Etiology of syncope was classified as benign (vasovagal, dehydration), serious (dysrhythmia, sepsis, stroke/intracranial bleed, hemorrhage, valvular, ischemia, pulmonary embolism), or unknown. Data were analyzed using Fisher's exact test and t-test. RESULTS: One hundred and seventy-two of 351 patients were >1-day admissions, 152 (85%) were admitted for 1 day, and 27 (15%) were admitted to EDOU. The mean (standard deviation [SD]) age when admitted to the hospital was significantly higher at 72 (18.4) years for > 1-day admissions and 68.8 (19.6) years for 1-day admissions vs. 53.0 (18.9) years for EDOU patients (p < 0.01). For fully admitted patients, 36% had benign etiologies of syncope and 38% had serious causes of syncope; in 1-day admitted patients, 48% had benign etiologies and 14% had serious causes. Among EDOU patients, 44% had benign etiologies and none were serious. One-day patients were more likely to have unknown causes of syncope at discharge (36%; 95% confidence interval 0.28 to 0.43) when compared with admitted patients (26%; 95% CI 0.2 to 0.33); similarly, observation patients were more likely to be discharged without a diagnosis (56%; 95% CI 0.37 to 0.74; p ≤ 0.05). CONCLUSIONS: EDOU patients were less likely than patients admitted to the hospital to be discharged with an etiology of their syncope. Future EDOU protocols can benefit from set admission criteria and standardized evaluation protocols to facilitate maximal use of EDOU for syncope.