Ca2+-dependent secretory granule fusion with the plasma membrane is the final step for the exocytic release of inflammatory mediators, neuropeptides, and peptide hormones. Secretory cells use a similar protein machinery at late steps in the regulated secretory pathway, employing protein isoforms from the Rab, Sec1/Munc18, Munc13/CAPS, SNARE, and synaptotagmin protein families. However, no small-molecule inhibitors of secretory granule exocytosis that target these proteins are currently available but could have clinical utility. Here we utilized a high-throughput screen of a 25,000-compound library that identified 129 small-molecule inhibitors of Ca2+-triggered secretory granule exocytosis in RBL-2H3 mast cells. These inhibitors broadly fell into six different chemical classes, and follow-up permeable cell and liposome fusion assays identified the target for one class of these inhibitors. A family of 2-aminobenzothiazoles (termed benzothiazole exocytosis inhibitors or bexins) was found to inhibit mast cell secretory granule fusion by acting on a Ca2+-dependent, C2 domain–containing priming factor, Munc13-4. Our findings further indicated that bexins interfere with Munc13-4–membrane interactions and thereby inhibit Munc13-4–dependent membrane fusion. We conclude that bexins represent a class of specific secretory pathway inhibitors with potential as therapeutic agents.

Myxedema coma is a severe form of hypothyroidism representing a endocrinologic emergency. It requires prompt identification and management, as mortality rates exceed 50%. Its rarity stems from early recognition and thyroid medication availability. Its presentation can be non-specific, making it a challenging diagnosis. This is a 67-year-old male inmate who was brought to the ED due to hypoactivity. He had a long-standing history of bipolar disorder, and hypothyroidism receiving oral levothyroxine. On evaluation, patient had slowed mentation, GCS 14/15, sluggish reactive pupils, macroglossia, diffuse non-pitting edema, and delayed relaxation of the deep tendon reflexes in the extremities. Vital signs were abnormal; T: 35.2 °C, RR: 10 rpm, SpO2: 84 %, BP: 137/89 mmHg and HR: 42 bpm without chronotropism. 12-lead ECG revealed a complete atrioventricular block (AV block), with non-conductive P waves and idioventricular rhythm. Patient became hemodynamically unstable, transcutaneous pacemaker was placed. Dopamine infusion was initiated for adequate mean arterial pressure. Subsequently, a femoral transvenous pacemaker was performed. However, neurological deterioration prompted mechanical ventilation. Exploring reversible AV block etiologies, laboratory results were markedly elevated for TSH at 184.775 ng/mL and decreased T4 at 1.5 ng/mL. Lithium levels were therapeutic. Myxedema coma was identified and timely treatment was provided with intravenous thyroid hormone replacement, intravenous hydrocortisone, and supportive care. Patient was transferred to an ICU where TSH was monitored. After 5 days of receiving IV thyroid hormone replacement therapy, TSH improved. However, patient remained dependent on transvenous pacemaker, for which permanent pacemaker had to be placed. With further therapy, patient’s neurological status improved leading to extubation, and subsequent discharge. Thyroid hormones play a vital role in the electrical current of the heart; hence, disturbances may potentiate cardiac arrhythmias. Sinus bradycardia and QT interval prolongation are commonly seen. As the severity of hypothyroidism progresses, high-grade AV block may be encountered, being third degree AV block the most challenging and severe. Patients with high-degree AV block in the setting of reversible etiologies, commonly, do not need a permanent pacemaker. On the contrary, our patient developed complete dependence of the pacemaker for adequate cardiac synchrony, despite adequate replacement therapy. With this case, we illustrate the importance of a thorough evaluation in patients with AV block of unknown origin, with special attention to reversible etiologies. Thyroid function abnormalities should be promptly identified and managed for better outcomes. Furthermore, it may decrease cardiac death risk and the need for invasive procedures, such as permanent pacemaker placement.