Synchrony between neurons with similar muscle fields in monkey motor cortex.

Synchronous firing of motor cortex cells exhibiting postspike facilitation (PSF) or suppression (PSS) of hand muscle EMG was examined to investigate the relationship between synchrony and output connectivity. Recordings were made in macaque monkeys performing a precision grip task. Synchronization was assessed with cross-correlation histograms of the activity from 144 pairs of simultaneously recorded neurons, while spike-triggered averages of EMG defined the muscle field for each cell. Cell pairs with similar muscle fields showed greater synchronization than pairs with nonoverlapping fields. Furthermore, cells with opposing effects in the same muscles exhibited negative synchronization. We conclude that synchrony in motor cortex engages networks of neurons directly controlling the same muscle set, while inhibitory connections exist between neuronal populations with opposing output effects.

Host-cell specific effects of the nicotinic acetylcholine receptor chaperone RIC-3 revealed by a comparison of human and Drosophila RIC-3 homologues.

RIC-3 is a transmembrane protein which enhances maturation (folding and assembly) of neuronal nicotinic acetylcholine receptors (nAChRs). In this study, we report the cloning and characterisation of 11 alternatively spliced isoforms of Drosophila melanogaster RIC-3 (DmRIC-3). Heterologous expression studies of alternatively spliced DmRIC-3 isoforms demonstrate that nAChR chaperone activity does not require a predicted coiled-coil domain which is located entirely within exon 7. In contrast, isoforms containing an additional exon (exon 2), which is located within a proline-rich N-terminal region, have a greatly reduced ability to enhance nAChR maturation. The ability of DmRIC-3 to influence nAChR maturation was examined in co-expression studies with human alpha7 nAChRs and with hybrid nAChRs containing both Drosophila and rat nAChR subunits. When expressed in a Drosophila cell line, several of the DmRIC-3 splice variants enhanced nAChR maturation to a significantly greater extent than observed with human RIC-3. In contrast, when expressed in a human cell line, human RIC-3 enhanced nAChR maturation more efficiently than DmRIC-3. The cloning and characterisation of 11 alternatively spliced DmRIC-3 isoforms has helped to identify domains influencing RIC-3 chaperone activity. In addition, studies conducted in different expression systems suggest that additional host cell factors may modulate the chaperone activity of RIC-3.

RIC-3 enhances functional expression of multiple nicotinic acetylcholine receptor subtypes in mammalian cells.

Recent studies have shown that RIC-3, originally identified in Caenorhabditis elegans as the protein encoded by the gene resistance to inhibitors of cholinesterase (ric-3), can enhance functional expression of alpha7 nicotinic acetylcholine receptors (nAChRs). In the present study, the influence of C. elegans and human RIC-3 upon multiple homomeric (alpha7, alpha8, and alpha9) and heteromeric (alpha3beta2, alpha3beta4, alpha4beta2, alpha4beta4, and alpha9alpha10) nAChR subtypes has been examined in transfected mammalian cells by radioligand binding and functional characterization. Coexpression of RIC-3 facilitates a dramatic enhancement of the ability of alpha7 (and the closely related alpha8 subunit) to generate functional nAChRs in otherwise nonpermissive mammalian cells. In contrast, coexpression of RIC-3 did not facilitate functional expression of either homomeric alpha9 or heteromeric alpha9alpha10 nAChRs in mammalian cell lines. It is noteworthy that whereas RIC-3 has been reported to cause a marked functional inhibition of heteromeric nAChRs such as alpha3beta4 and alpha4beta2 expressed in Xenopus laevis oocytes, RIC-3 significantly enhances levels of functional expression of these and other (alpha3beta2 and alpha4beta4) heteromeric nAChRs when expressed in mammalian cell lines. In addition, the interaction of multiple nAChR subunits (alpha3, alpha4, alpha7, beta2, and beta4) with RIC-3 has been demonstrated by coimmunoprecipitation from metabolically labeled transfected cells. It is significant that coimmunoprecipitation experiments have provided evidence that RIC-3 associates with unassembled nAChR subunits, a finding that is consistent with previous suggestions that RIC-3 may act by enhancing the maturation (subunit folding and assembly) of nAChRs. We conclude that RIC-3 is an nAChR-associated protein that can enhance functional expression of multiple nAChR subtypes in transfected mammalian cells.