InteractiveFly: GeneBrief

SLC22A family member: Biological Overview | References


Gene name - SLC22A family member

Synonyms -

Cytological map position - 79A4-79A4

Function - plasma membrane transporter

Keywords - a plasma membrane transporter - localizes in the dendrites of mushroom body neurons - terminates synaptic transmission from cholinergic projection neurons through uptake of the released neurotransmitter acetylcholine - enhances olfactory memory by allowing the neurotransmitter signal from projection neurons to be more persistent

Symbol - SLC22A

FlyBase ID: FBgn0037140

Genetic map position - chr3L:21,934,704-21,938,636

NCBI classification - Sugar phosphate permease [Carbohydrate transport and metabolism] - Major Facilitator Superfamily

Cellular location - surface transmembrane



NCBI links: EntrezGene, Nucleotide, Protein
BIOLOGICAL OVERVIEW

The mechanisms that constrain memory formation are of special interest because they provide insights into the brain's memory management systems and potential avenues for correcting cognitive disorders. RNAi knockdown in the Drosophila mushroom body neurons (MBn) of a newly discovered memory suppressor gene, Solute Carrier DmSLC22A, a member of the organic cation transporter family, enhances olfactory memory expression, while overexpression inhibits it. The protein localizes to the dendrites of the MBn, surrounding the presynaptic terminals of cholinergic afferent fibers from projection neurons (Pn). Cell-based expression assays show that this plasma membrane protein transports cholinergic compounds with the highest affinity among several in vitro substrates. Feeding flies choline or inhibiting acetylcholinesterase in Pn enhances memory, an effect blocked by overexpression of the transporter in the MBn. The data argue that DmSLC22A is a memory suppressor protein that limits memory formation by helping to terminate cholinergic neurotransmission at the Pn:MBn synapse (Gai, 2016).

Genetic studies have now identified hundreds of genes required for normal memory formation. Some of these genes regulate the development of the cells and circuits required for learning; some mediate the physiological changes that occur with acquisition and storage. Of particular interest are gene functions that suppress normal memory formation and, by analogy with tumor suppressor genes, are referred to as memory suppressor genes. These genes and their products can, in principle, suppress memory formation by antagonizing the process of acquisition, limiting memory consolidation, promoting active forgetting, or inhibiting retrieval. Recently, a large RNAi screen of ∼3,500 Drosophila genes has bee carried out, and several dozen new memory suppressor genes were identified (Walkinshaw, 2015), identified as such, because RNAi knockdown produces an enhancement in memory performance after olfactory conditioning (Gai, 2016).

Aversive olfactory classical conditioning is a well-studied type of learning in Drosophila and consists of learning a contingency between an odor conditioned stimulus (CS) and most often an unconditioned stimulus (US) of electric shock. Many cell types in the olfactory nervous system are engaged in this type of learning, including antennal lobe projection neurons (Pn), several different types of mushroom body neurons (MBn), dopamine neurons (DAn), and others, but a focused model of olfactory memory formation holds that MBn are integrators of CS and US information with the CS being conveyed to the MBn dendrites by the axons of cholinergic, excitatory Pn of the antennal lobe, and the US conveyed to the MBn by DAn Gai, 2016).

A memory suppressor gene identified and describe in this report encodes a member of the SLC22A transporter family. The Solute Carrier (SLC) family of transporters in humans consists of 395 different, membrane-spanning transporters that have been organized into 52 different families. Some of these are localized pre-synaptically and involved in neurotransmitter recycling, others localize to glia for clearance of neurotransmitter from the synapse. In addition, glutamate transporters can be localized post-synaptically to regulate neurotransmission strength via clearance mechanisms. Some of these SLC transporters have prominent roles in neurological and psychiatric disorders and in drug design, including SLC1A family members that are responsible for glutamate uptake and clearance of this neurotransmitter from the synaptic cleft and SLC6A2-4 proteins that transport monoamines into cells. Inhibitors of these proteins, which include the serotonin-specific reuptake inhibitors (SSRIs) and serotonin-noradrenaline reuptake inhibitors (SNRIs), increase monoamine dwell time at the synapse and are used to treat depression and several other neuropsychiatric disorders (Gai, 2016).

The SLC22A family of transporters is distinguished into two major classes that carry either organic cations (SLC22A1-5, 15, 16, and 21) or anions (SLC22A6-13 and 20) across the plasma membrane, with generally low substrate binding affinity and high capacity. They transport numerous molecules with diverse structures, including drugs, acetylcholine, dopamine, histamine, serotonin, and glycine among others. Ergothioneine has been identified as a high-affinity substrate for SLC22A4 and spermidine for SLC22A16. Mice mutant in the two organic cation transporters, SLC22A2 and SLC22A3, exhibit behavioral phenotypes suggestive of functions in anxiety, stress, and depression. These observations point out the importance of the SLC22A family for brain function and cognition. Recently, a Drosophila SLC22A family member, CarT (CG9317) was identified and found to transport carcinine into photoreceptor neurons for the recovery of essential visual neurotransmitter histamine (Gai, 2016).

This study shows that the Drosophila gene, CG7442, functions as a memory suppressor gene and is a member of the SLC22A family. This transporter is expressed most abundantly in the dendrites of the MBn, at the synapses with the cholinergic antennal lobe Pn. Cell-based expression assays show that Drosophila SLC22A transports choline and acetylcholine with the highest affinity among several substrates. Pharmacological and genetic data support the model that Drosophila SLC22A functions at the Pn:MBn synapse to terminate cholinergic neurotransmission, differing from well-characterized presynaptic choline transporters for neurotransmitter recycling, and mechanistically explaining its role in behavioral memory suppression (Gai, 2016).

These data connect the SLC22A family of transporters and memory suppression. DmSLC22A, located on the dendrites of the adult α/β and α'/β' MBn, removes ACh from the Pn:MBn synapses in the calyx. The normal expression level of this plasma membrane transporter limits the transference of olfactory information to the MBn by removing neurotransmitter from the synapse. Overexpression of DmSLC22A hardens this limit, weakening the CS representation and weakening memory formation. Reducing DmSLC22A expression has the opposite effect of softening the limit, producing a stronger CS representation and stronger memory formation. Thus, the data indicate that acetylcholinesterase and postsynaptic SLC22A transporter function jointly to regulate neurotransmitter persistence at the synapse. This conclusion is notable, given the longstanding emphasis on ACh degradation as the primary route for termination of the cholinergic synaptic signal. Although the evidence is strong for the proposed mechanism, the transporter exhibits broad substrate specificity and expression outside of the Pn:MBn synapse. Alternative or additional mechanisms of action in memory suppression thus remain a possibility (Gai, 2016).

The current data are consistent with the model that ACh persistence at the Pn:MBn synapse is a surrogate for the strength of the CS and therefore a primary effector of olfactory memory strength. Other data similarly point to the strength of stimulation of MBn as an important variable for regulating memory strength. The MBn also receive inhibitory input through GABAA receptors expressed on the MBn. Overexpressing the MBn-expressed GABAA receptor Rdl impairs learning, while RNAi knockdown of this receptor in the MBn enhances memory formation. This regulation of memory strength is independent of the US pathway involved in classical conditioning, functioning similarly for both aversive and appetitive USs. However, it is noted that the in vivo functions for the SLC22A class of transporters must be broader than the focused model presented above. For instance, the data indicate that the Drosophila SLC22A protein transports both acetylcholine and dopamine in ex vivo preparations. Moreover, the protein's memory suppressor function maps to both MBn and the DAn. How DmSLC22A might function in DAn to suppress memory formation has not been explored, but one reasonable hypothesis is that DmSLC22A transports acetylcholine at the synapse between upstream and putative cholinergic neurons that provide input to the DAn that convey the US in classical condition. Testing this hypothesis requires identifying the presynaptic neurons to the DAn that carry the US information (Gai, 2016).

One unexplained observation is that although DmSLC22A knockdown enhances the duration of memory produced from stronger memory traces instilled at acquisition, it slows the rate of acquisition as measured by acquisition curves. However, this observation has been made with another memory suppressor gene as well. A knockdown of the pre- and post-synaptic scaffolding protein, Scribble, has the same effect of producing more enduring memories but slowing acquisition. In addition, similar observations have been made in mouse: injection of muscarinic acetylcholine receptor antagonists impairs memory acquisition but enhances retention (Gai, 2016).

These studies bring a focus on the SLC22A family of plasma membrane transporters as potential targets for neurotherapeutics. Of the 24 members of this family, only a few have been studied in some detail in the nervous system. RNA expression experiments have shown that SLC22A1-5 are all expressed in the brain, with SLC22A3 and A4 being the most abundant, and immunohistochemistry experiments have revealed that SLC22A4-5 are localized at dendrites within the hippocampus. Mammalian members of this family of transporters and, by extension, probably DmSLC22A, are subject to regulation by multiple signaling molecules including protein kinase A, calcium/calmodulin-dependent protein kinase II, and the mitogen-activated protein kinases. Knockout mice for SLC22A2 and A3 show reduced basal level of several neurotransmitters in a region-dependent manner and decreased anxiety-related behaviors, although the effects of SLC22A3 on anxiety-related behaviors is debated. In addition, the knockouts or antisense insults reveal behavioral changes in depression-related tasks, with SLC22A2 knockouts exhibiting increased behavioral despair, and SLC22A3 antisense-treated animals exhibiting decreased behavioral despair. Little is known about the biological or behavioral functions of the other members of the SLC22A family. The current results show that the SLC22A family of transporters is also involved in memory suppression (Gai, 2016).

DmSLC22A is a unique and new type of memory suppressor gene. There are, to date, about two dozen memory suppressor genes identified in the mouse and about three dozen such genes in Drosophila. The mechanisms by which all of these genes suppress memory formation are not yet known, but a few themes have emerged. For instance, several of the genes suppress memory formation by limiting excitatory neurotransmitter release and function, or the expression and function of post-synaptic receptors. DmSLC22A appears to fall into this category. Another example is Cdk5, which negatively influences the expression of NR2B and limits memory formation. Knockouts of some GABA receptors reduce inhibitory tone of learning circuitry so as to facilitate memory formation. Several of the known memory suppressor genes are known to function in active forgetting processes. These include damb, a dopamine receptor involved in forgetting mechanisms; scribble, a pre- and post-synaptic scaffolding gene; and rac, a small G protein involved in the biochemistry of active forgetting. Memory suppressor genes can also encode signaling molecules that negatively regulate transcription factors required for long-term memory and the transcription factors themselves, such as repressing isoforms of Aplysia Creb; ATF4, a transcription factor homolgous to ApCreb-2, and protein phosphatase I. Elucidating all of the genetic constraints on memory formation and their mechanisms will have profound consequences for understanding of how the brain forms and stores memories and for the development of cognitive therapeutics (Gai, 2016).


REFERENCES

Search PubMed for articles about Drosophila Slc22a

Gai, Y., Liu, Z., Cervantes-Sandoval, I. and Davis, R.L. (2016). Drosophila SLC22A transporter is a memory suppressor gene that influences cholinergic neurotransmission to the mushroom bodies. Neuron 90: 581-595. PubMed ID: 27146270

Walkinshaw, E., Gai, Y., Farkas, C., Richter, D., Nicholas, E., Keleman, K. and Davis, R. L. (2015). Identification of genes that promote or inhibit olfactory memory formation in Drosophila. Genetics 199(4): 1173-1182. PubMed ID: 25644700


Biological Overview

date revised: 21 January 2019

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