Comment on "Tequila, a neurotrypsin ortholog, regulates long-term memory formation in Drosophila"

Genes and circuits for olfactory-associated long-term memory in Drosophila

One of the formidable challenges in modern neuroscience is to identify the physical basis of long-term memory (LTM) storage−the engram. Cellular and molecular experiments have suggested that the engram for a particular behavioral task is encoded as changes in synaptic structure and function, yet distributed in an unknown fashion across an ill-defined neural circuit or network. Accumulating genetic and circuitry information has provided some clues toward resolving this engram puzzle.This review will focus on recent discoveries of genes and circuits involved in the formation of olfactory-associated LTM in Drosophila.

Agrin binds BMP2, BMP4 and TGFbeta1

The C-terminal 95 kDa fragment of some isoforms of vertebrate agrins is sufficient to induce clustering of acetylcholine receptors but despite two decades of intense agrin research very little is known about the function of the other isoforms and the function of the larger, N-terminal part of agrins that is common to all isoforms. Since the N-terminal part of agrins contains several follistatin-domains, a domain type that is frequently implicated in binding TGFβs, we have explored the interaction of the N-terminal part of rat agrin (Agrin-Nterm) with members of the TGFβ family using surface plasmon resonance spectroscopy and reporter assays. Here we show that agrin binds BMP2, BMP4 and TGFβ1 with relatively high affinity, the K_D values of the interactions calculated from SPR experiments fall in the 10⁻⁸ M–10⁻⁷ M range. In reporter assays Agrin-Nterm inhibited the activities of BMP2 and BMP4, half maximal inhibition being achieved at ∼5×10⁻⁷ M. Paradoxically, in the case of TGFβ1 Agrin N-term caused a slight increase in activity in reporter assays. Our finding that agrin binds members of the TGFβ family may have important implications for the role of these growth factors in the regulation of synaptogenesis as well as for the role of agrin isoforms that are unable to induce clustering of acetylcholine receptors. We suggest that binding of these TGFβ family members to agrin may have a dual function: agrin may serve as a reservoir for these growth factors and may also inhibit their growth promoting activity. Based on analysis of the evolutionary history of agrin we suggest that agrin's growth factor binding function is more ancient than its involvement in acetylcholine receptor clustering.

Rapid consolidation to a radish and protein synthesis-dependent long-term memory after single-session appetitive olfactory conditioning in Drosophila

In Drosophila, formation of aversive olfactory long-term memory (LTM) requires multiple training sessions pairing odor and electric shock punishment with rest intervals. In contrast, here we show that a single 2 min training session pairing odor with a more ethologically relevant sugar reinforcement forms long-term appetitive memory that lasts for days. Appetitive LTM has some mechanistic similarity to aversive LTM in that it can be disrupted by cycloheximide, the dCreb2-b transcriptional repressor, and the crammer and tequila LTM-specific mutations. However, appetitive LTM is completely disrupted by the radish mutation that apparently represents a distinct mechanistic phase of consolidated aversive memory. Furthermore, appetitive LTM requires activity in the dorsal paired medial neuron and mushroom body alpha'beta' neuron circuit during the first hour after training and mushroom body alphabeta neuron output during retrieval, suggesting that appetitive middle-term memory and LTM are mechanistically linked. Last, experiments feeding and/or starving flies after training reveals a critical motivational drive that enables appetitive LTM retrieval.

Response to Comment on "Tequila, a Neurotrypsin Ortholog, Regulates Long-Term Memory Formation in Drosophila "

Sonderegger and Patthy argue that the trypsin catalytic domains of Drosophila Tequila and human neurotrypsin are not linked by an orthology relationship. We present analyses based both on BLAST (basic local alignment search tool) comparisons and on phylogenetic relationships, which show that these two proteases do share an orthologous region that includes the trypsin domain.