“Smell perception changes the brain”. Proust evoked his infancy when the baker’s fragrance went through his nose. The memories triggered by an odorant reflect the structural changes that occurred at the time when the external stimuli associated with the particular functional state of the brain during the first impression. We use the olfactory system of Drosophila to monitor the functional consequences of altering the number of synapses and, conversely, we monitor the structural changes that olfactory perception causes in the corresponding brain centers. So far, we detected that an increment in the number of synapses that the sensory neurons establish with their partners in the olfactory glomeruli, even if this increment occurs only in one antenna, is enough to cause higher olfactory indexes to several odorants, either at attractive or repulsive concentrations (Acebes and Ferrús, 2001). That is, more sensory synapses elicit stronger behavioural responses allowing a higher sensitivity to odorant perception. In the opposite direction, odorant habituation leads to reduced number of synapses in selected glomeruli. These structural effects of adaptation are odorant, concentration, time and glomerulus specific (Devaud et al., 2001). Finally, odorant perception is also modified by neuropeptides such as tachykinin reflecting the abundance of peptidergic terminals in the olfactory glomeruli (Winther et al., 2006). Current efforts in this project aim to dissect the structural effects on excitatory versus inhibitory synapses upon olfactory perception and habituation.

“Troponin I, more than just a muscle protein”. The contraction in striated muscles is mediated by the sliding of the actin-containing thin filaments over the myosin-containing thick filaments. The regulation of this sliding is achieved by a complex of four proteins: Tropomyosin, Troponin I, Troponin C and Troponin T. At rest, Troponin I inhibits (hence its name) the interaction between actin and myosin heads. When calcium rises in the vicinity, Troponin C captures it changing its conformation. This change moves the entire complex and Troponin I can no longer inhibit, allowing actin and myosin to interact, hydrolyze ATP and, thus, move.

We have studied for a long time the Troponin I encoding gene in Drosophila, wings up A (wupA) [a.k.a. held up (hdp)], and identified its family of encoded products and some mechanisms that control its expression (Barbas et al., 1991, 1993; Prado et al., 1995; Kronnert et al., 1999; Ferrús, 2001; Naimi et al., 2001; Marin et al., 2004). While flies have only one gene for Troponin I, mammals have three, slow twitch (TNNI1), fast twitch (TNNI2) and cardiac (TNNI3). In humans, mutations in TNNI2 and TNNI3 cause distal arthrogryposis type 2B and familial hypertrophic cardiomyopathy, respectively. Troponins are also reported as overexpressed in several types of cancer. It appears that the diversity of functions that the three genes in mammals can provide, is represented in flies by one gene, albeit yielding at least ten different RNA isoforms. It was intriguing, however, that the wupA mutations in flies result in two types of phenotypes: a) collapse of flight muscles causing the abnormal upheld position of wings, yet allowing viability, and b) embryonic lethal. While the first type is easily traced to the well known role of Troponin I in muscles, the second type is not because the lethality is observed when the muscle primordia are not even formed. This unexpected finding prompted us to ask: what could be the early function of Troponin I?

A recent report from our lab illustrates unexpected roles of Troponin I and Tropomyosin in cell divisions to maintain chromosomal integrity and, also, to locate cell-polarity proteins on the cell surface. This nuclear function requires sumoylation of Troponin I to allow its translocation to the nucleus. Considering these novel functions, Troponin I and its associated Tropomyosin might be at the origin of several types of cancers. At this time, we propose that the Troponin-Tropomyosin complex regulates a multipurpose actin-based force-generating motor. Sahota et al., 2009

“A code within a code: Novel components and novel functions for transcription and chromatin complexes”. The transcription factor TFIID is a multiprotein complex that includes the TATA-box binding protein TBP and a number of associated factors, TAF_II. Studies on the components of this and other transcription complexes have been based on biochemical purification methods. Through a genetic approach, we identified a novel TFIID component, dTAF_II8A, encoded in the gene prodos (pds) (Hernández 2001). In yeast two-hybrid tests using dTAFII8A as bait, we cloned the Drosophila TAF_II, dTAF_II16, as a specific dTAF_II8A target. dTAF_II16 is closely related to human hTAF_II30 and to another recently discovered Drosophila TAF, dTAF_II24. dTAF_II8A and dTAF_II24 do not interact, however, thus establishing a functional difference between these dTAFs. Finally, we show that dTAF_II8A function is required for cell viability in somatic mosaics. It is worth noting that dTAF_II8A is particularly abundant in embryo stem cells.

For transcription complexes to operate, however, other complexes need to modify the chromatin structure previously. Chromatin remodeling complexes actually share several components with the transcription complexes in such a way that the dynamics of the whole process can be better understood as a gradual transformation of chromatin-remodeling into transcription complexes. We studied one of these components, ADA3, and the phenotypes associated to its mutants. ADA3 is a subunit of GCN5-containing histone acetyltransferase complexes (HAT) that modify chromatin structure. The fly dADA3 is a major contributor to oogenesis, and it is also required for somatic cell viability. dADA3 localizes to chromosomes and it is significantly reduced in dGcn5 and dAda2a, but not in dAda2b, mutant backgrounds. In dAda3 mutants, acetylation at histone H3 K9 and K14, but not K18, and at histone H4 K12, but not K5, K8 and K16, is significantly reduced. Also, phosphorylation at H3 S10 is reduced in dAda3 and dGcn5 mutants. We concluded that dADA3 plays a role in HAT complexes which acetylate H3 and H4 at specific residues. In turn, this results in chromatin structural effects of certain rearrangements, and transcription of specific genes (Grau 2008). Surprisingly, we did not observe acetylation defects in H4 K5 in dAda3 mutants, while in dAda2a and dGcn5 mutants, this site is affected. As it seems, the so called Histone code for modification of specific K residues, is based on an ADA-code which determines which HAT complex will acetylate which histone K residue.

“Pulling the thread of Ariadne”. We identified the Ariadne gene in Drosophila and named it according to the abnormal path of mutant axons (Aguilera et al., 2000). The encoded protein has become the founder member of a new conserved family of ubiquitin ligases that include the Parkinson related member Parkin (Marin and Ferrus 2002; Marin et al., 2004). E3-Ubiquitin ligases bind to specific substrates, ubiquitin is thenconjugated to the target protein by the formation of an isopeptidebond. This process is best known for its role in targeting proteins for degradation by the proteasome. However depending on the type of ubiquitin-chain linkage, polyubiquitylation of proteins might activate kinases or provide a scaffold for the nucleation of diverse signalling processes. The ubiquitin ligases have emerged as key regulators of cell signalling processes. Ariadne 1a mutants have defects in metamorphosis. Current studies have identified the ubiquitin conjugating partner, UbcD10, and are now aimed to identify the substrate target for this enzymatic activity that may link the protein with the metamorphosis pathway.