RESUMO
The functions of different G-protein αßγ subunit combinations are traditionally ascribed to their various α components. However, the discovery of similarly diverse γ subtypes raises the possibility that they may also contribute to specificity. To test this possibility, we used a gene targeting approach to determine whether the closely related γ(3) and γ(7) subunits can perform functionally interchangeable roles in mice. In contrast to single knock-out mice that show normal survival, Gng3(-/-)Gng7(-/-) double knock-out mice display a progressive seizure disorder that dramatically reduces their median life span to only 75 days. Biochemical analyses reveal that the severe phenotype is not due to redundant roles for the two γ subunits in the same signaling pathway but rather is attributed to their unique actions in different signaling pathways. The results suggest that the γ(3) subunit is a component of a G(i/o) protein that is required for γ-aminobutyric acid, type B, receptor-regulated neuronal excitability, whereas the γ(7) subunit is a component of a G(olf) protein that is responsible for A(2A) adenosine or D(1) dopamine receptor-induced neuro-protective response. The development of this mouse model offers a novel experimental framework for exploring how signaling pathways integrate to produce normal brain function and how their combined dysfunction leads to spontaneous seizures and premature death. The results underscore the critical role of the γ subunit in this process.
Assuntos
Encéfalo/enzimologia , Epilepsia/enzimologia , Subunidades gama da Proteína de Ligação ao GTP/metabolismo , Transdução de Sinais , Animais , Encéfalo/patologia , Epilepsia/genética , Epilepsia/patologia , Subunidades alfa Gi-Go de Proteínas de Ligação ao GTP/genética , Subunidades alfa Gi-Go de Proteínas de Ligação ao GTP/metabolismo , Subunidades gama da Proteína de Ligação ao GTP/genética , Predisposição Genética para Doença , Camundongos , Camundongos Knockout , Receptor A2A de Adenosina/genética , Receptor A2A de Adenosina/metabolismo , Receptores Dopaminérgicos/genética , Receptores Dopaminérgicos/metabolismo , Receptores de GABA-B/genética , Receptores de GABA-B/metabolismoRESUMO
Sarcoglycans are originally identified in muscle for their involvement in limb-girdle muscular dystrophies. They form a multi-meric complex (alpha-, beta-, gamma-, delta-sarcoglycan) that associates with dystrophin, dystroglycan and other proteins to constitute the larger dystrophin-glycoprotein complex at the muscle membrane. Three sarcoglycan subunits (epsilon-, beta-, delta-sarcoglycan) were previously identified in Schwann cells and shown to associate with dystroglycan and a Schwann cell-specific dystrophin isoform (Dp116) at the outermost Schwann cell membrane. Currently, little is known about the exact composition and function of the sarcoglycan complex in the peripheral nervous system. In this study, we showed that the Schwann cell sarcoglycan complex consists of epsilon-, beta-, delta-sarcoglycan and the newly identified zeta-sarcoglycan subunit. The expression of sarcoglycans precedes the onset of myelination and is induced by neurons. In sarcoglycan-deficient BIO14.6 hamsters, loss of the Schwann cell sarcoglycan complex reduces the steady state levels of alpha-dystroglycan and Dp116. Ultrastructural analysis of sciatic nerves from the mutant animals revealed altered myelin sheaths and disorganized Schmidt-Lanterman incisures indicative of myelin instability. The disruption in myelin structure increased in severity with age. Nerve conduction studies also showed subtle electrophysiological abnormalities in the BIO14.6 hamsters consistent with reduced myelin stability. Together, these findings suggest an important role of sarcoglycans in the stability of peripheral nerve myelin.
