Myosin storage myopathy mutations yield defective myosin filament assembly in vitro and disrupted myofibrillar structure and function in vivo.

Myosin storage myopathy (MSM) is a congenital skeletal muscle disorder caused by missense mutations in the ß-cardiac/slow skeletal muscle myosin heavy chain rod. It is characterized by subsarcolemmal accumulations of myosin that have a hyaline appearance. MSM mutations map near or within the assembly competence domain known to be crucial for thick filament formation. Drosophila MSM models were generated for comprehensive physiological, structural, and biochemical assessment of the mutations' consequences on muscle and myosin structure and function. L1793P, R1845W, and E1883K MSM mutant myosins were expressed in an indirect flight (IFM) and jump muscle myosin null background to study the effects of these variants without confounding influences from wild-type myosin. Mutant animals displayed highly compromised jump and flight ability, disrupted muscle proteostasis, and severely perturbed IFM structure. Electron microscopy revealed myofibrillar disarray and degeneration with hyaline-like inclusions. In vitro assembly assays demonstrated a decreased ability of mutant myosin to polymerize, with L1793P filaments exhibiting shorter lengths. In addition, limited proteolysis experiments showed a reduced stability of L1793P and E1883K filaments. We conclude that the disrupted hydropathy or charge of residues in the heptad repeat of the mutant myosin rods likely alters interactions that stabilize coiled-coil dimers and thick filaments, causing disruption in ordered myofibrillogenesis and/or myofibrillar integrity, and the consequent myosin aggregation. Our Drosophila models are the first to recapitulate the human MSM phenotype with ultrastructural inclusions, suggesting that the diminished ability of the mutant myosin to form stable thick filaments contributes to the dystrophic phenotype observed in afflicted subjects.

A novel postsynaptic group II metabotropic glutamate receptor role in modulating baroreceptor signal transmission.

The nucleus tractus solitarius (NTS) is essential for orchestrating baroreflex control of blood pressure. When a change in blood pressure occurs, the information is transmitted by baroreceptor afferent fibers to the central network by glutamate binding to ionotropic glutamate receptors on second-order baroreceptor neurons. Glutamate also activates presynaptic group II and III metabotropic glutamate receptors (mGluRs), depressing both glutamate and GABA release to modulate baroreceptor signal transmission. Here we present a novel role for postsynaptic group II mGluRs to further fine-tune baroreceptor signal transmission at the first central synapses. In a brainstem slice with ionotropic glutamate and GABA receptors blocked, whole-cell patch-clamp recordings of second-order baroreceptor neurons revealed that two group II mGluR agonists evoked concentration-dependent membrane hyperpolarizations. The hyperpolarization remained when a presynaptic contribution was prevented with Cd(2+), was blocked by a postsynaptic intervention of intracellular dialysis of the G-protein signaling inhibitor, was mimicked by endogenous release of glutamate by tractus solitarius stimulation, and was prevented by a group II mGluR antagonist. Postsynaptic localization of group II mGluRs was confirmed by fluorescent confocal immunohistochemistry and light microscopy. Group II mGluR induced-currents consisted of voltage-dependent outward and inward components, prevented by tetraethylammonium chloride and tetrodotoxin, respectively. In contrast to group II mGluR-induced hyperpolarization, there was no effect on intrinsic excitability as determined by action potential shape or firing in response to depolarizing current injections. The data suggest a novel mechanism for postsynaptic group II mGluRs to fine-tune baroreceptor signal transmission in the NTS.

House-dust mite allergen and ozone exposure decreases histamine H3 receptors in the brainstem respiratory nuclei.

Allergic airway diseases in children are a common and a growing health problem. Changes in the central nervous system (CNS) have been implicated in contributing to some of the symptoms. We hypothesized that airway allergic diseases are associated with altered histamine H3 receptor expression in the nucleus tractus solitarius (NTS) and caudal spinal trigeminal nucleus, where lung/airway and nasal sensory afferents terminate, respectively. Immunohistochemistry for histamine H3 receptors was performed on brainstem sections containing the NTS and the caudal spinal trigeminal nucleus from 6- and 12-month-old rhesus monkeys who had been exposed for 5 months to house dust mite allergen (HDMA)+O3 or to filtered air (FA). While histamine H3 receptors were found exclusively in astrocytes in the caudal spinal trigeminal nucleus, they were localized to both neuronal terminals and processes in the NTS. HDMA+O3 exposure significantly decreased histamine H3 receptor immunoreactivity in the NTS at 6 months and in the caudal spinal trigeminal nucleus at 12 months of age. In conclusion, exposing young primates to HDMA+O3 changed histamine H3 receptor expression in CNS pathways involving lung and nasal afferent nerves in an age-related manner. Histamine H3 receptors may be a therapeutic target for allergic asthma and rhinitis in children.