R FVIE W N. Mons and D. Cooper- Adenylatecyclases in neuronal signaling Adenylate cyclases: critical loci in neuronal signaling Nicole Mons and Dermot M.F. Cooper Current findings show that adenylate cyclases comprise a heterogeneous multigene family, members of which are variously regulated by the eLand 13~lsubunits of G proteins, by Ca z+ and by protein kinases. In the CNS, individual isoforms of adenylate cyclase are expressed discretely in select regions of the brain. At the subcellular level, adenylate cyclases can be concentrated into dendritic spines, thereby increasing their susceptibility to multiple regulatory influences. Altogether, such findings greatly expand knowledge of the potential role of this archetypical signaling system in the modulation of neuronal function. Trends Neurosci. (1995) 18, 536-542 Nicole Mons is at URA-CNRS339, Universi O,of Bordeaux I, Talence F-33405, France, and Dennot M.F. Cooperis at the Dept of Phannacolo~, Universityof ColoradoHealth Sciences Center, Dem'::r, CO 80262, USA. E ARLY BIOCHEMICAL STUDIES had concluded that 'brain' adenylate cyclase, which could be stimu- lated by Ca2+/calmodulin, differed from 'peripheral' adenylate cyclase, which was insensitive to Ca2+(Refs 1,2). However, following the cloning of the first adenylate cyclase in 1989 (Ref. 3), a Pandora's box of adenylate cyclases that were differently regulated and distributed was discovered. Subsequent cloning stud- ies, some of which aimed at identifying either a par- ticular functional form of adenylate cyclase or the type of adenylate cyclase expressed in a particular tis- sue, led to the identification of eight isoforms3-~6. A quite unexpected feature of the inferred topography of these isoforms is a tantalizing resemblance to mem- brane transporters. All eight isoforms share a common double motif with six membrane-spanning domains, and two large cytoplasmic segments, which closely resembles the structure proposed for molecules such as the P-glycoprotein and the cystic-fibrosis trans- membrane conductance-regulator (CFTR) chloride channel '7. The membrane-spanning domains are not highly conserved in the isoforms - unlike ion chan- nels ~. Nevertheless, the gross resemblance to ion channels, coupled with the sensitivity to local rises in the intracellular concentration ([Ca2÷]~) (see later) nur- tures the thought that this structure has not arisen by accident. The two cytosolic segments are most highly conserved in adenylate cyclases (up to 92% similarity) and are likely to represent catalytic and regulatory domains ~9'2°. Based on sequence similarities, the iso- forms have been classified into three subfamilies: the type I-like group (I, III and VIII); the type II-like group (types II, IV and VII); and the type V-like group (types V and VI) (Refs 13,19-21). Functional properties are shared within these subfamilies, although individual isoforms display unique responses. As the regulatory properties of tile isofo~ms of adenylate cyclase have been discussed in a number of recent reviews, these data will be described only briefly here to assist in the consideration of their function in discrete areas in the CNS (Refs 21,22). Multiple modes of regulation Along with their complex structure, an unantici- pated feature of the isoforms of adenylate cyclase that are known currently is that they are all multiply regu- lated 2~-24.Broadly speaking, each adenylate cyclase can be regulated by a subunits of a G protein j9'2°, although type I, in particular, is quite refractory to stimulation by Gs~ (Ref. 25) and the type II-like group is refractory to inhibition by G~ (Ref. 26). However, apart from idiosyncratic responses to Gs, subunits, individual iso- forms show a range of responses to factors such as Ca 2+,protein kinase C (PKC) and [3~ subunits of G pro- teins. Table 1 summarizes the signaling pattern that can be observed for each isoform, when its cDNA is expressed in intact HEK 293 cells, and subjected to either physiological elevation in [CaZ+]l, hormonal activation of Gs, or activation of PKC by phorbol esters. These diverse modes of regulation fuel the desire to understand where the adenylate cyclases are expressed in the CNS, whether the various regulatory options are utilized and how the expression of the adenylate cyclases is regulated. Differential expressionof isoforms of adenylate cyclase mRNA The distinct regulatory properties of the eight known isoforms of adenylate cyclase are comple- mented by distinct patterns of expression of mRNA in mammalian tissues 4'6'~-~°,13,29,3°. However, in the brain, although transcripts of all isoforms of adenyl- ate cyclase can be detected by the polymerase chain reaction (PCR), only three (I, II and V) are expressed strongly, as determined by analysis by in situ hybridization 1~'31-33(Fig. 1); two others (VII and VIII) are represented at lower levels 13'~6'34.Each isoform has a unique pattern of expression, although some regions of the brain express more than one form of mRNA that encodes adenylate cyclase (Fig. 1A-C). Type-I mRNA occurs prominently in areas that have been implicated in processes of learning and memory, which include the CA1-CA2 pyramidal and granular cell layers of the hippocampus, and the cerebral cortex and cerebellum (Fig. 1A) 31'3z. Type-V mRNA is ex- pressed strongly, but is restricted to the caudate putamen, the nucleus accumbens and the olfactory tubercle (Fig. 1C) H,33. In the caudate putamen, type-V mRNA is expressed homogeneously in the abundant 536 TINS VoL 18, No. 12, 1995 © 199.5, Elsevier Science Ltd 0166 - 2236/95/$09.50