L-proline and glutamatergic neurotransmission: clarifying the modulatory role of neuronal proline transporter in cognition.
Zusammenfassung der Projektergebnisse
L-proline shares several properties with neurotransmitters. It has heterogeneous regional distribution in brain, a neuronal biosynthetic pathway, Ca2+ dependent release upon depolarization, and high-affinity uptake mediated by the neuronal L-proline transporter PROT, which is expected to control extracellular levels of L-proline. Because there is no evidence for specific L-proline receptors in the central nervous system (CNS) and PROT localizes exclusively in a subset of glutamatergic synapses, it has been proposed that L-proline may modulate transmission at glutamate synapses. Consistent with this hypothesis, exogenous L-proline activates NMDA receptors and potentiates excitatory transmission in hippocampal slices in vitro. However, the specific functional role of PROT in the CNS remains unknown. The main goal of our project is to determine the in vivo function of PROT in brain. Toward this objective, we first generated a PROT deficient mouse line and subsequently performed a battery of behavioural and biochemical analyses. Our study revealed that, unexpectedly, PROT knockout mice do not show symptoms similar to those previously identified in mouse models of metabolic hyperprolinemia. PROT deficient mice display a distinctive phenotype characterized by increased cognitive inflexibility, which could compromise adaptive responses to changing environments. These deficits correlate with alterations of glutamatergic synapse biochemistry in thalamus, a brain region reported to influence behavioural flexibility. In addition, by using cultured cortical neurons, we observed that inhibition of PROT leads to reduced surface expression of GluA1, supporting a role for neuronal PROT in modulating AMPAR synaptic trafficking, a molecular event known to influence mental flexibility. Reduced cognitive flexibility is characteristic of several psychiatric and neurologic disorders, including schizophrenia and autism. Hyperprolinemia and high levels of L-proline in cerebrospinal fluid have been associated with schizophrenia and transcriptomic analyses have recently revealed a decreased expression of PROT in post-mortem autistic brains. Our results suggest that some behavioural symptoms observed in human brain disorders may involve mutations in the human PROT gene or alterations in its expression levels. Thus, the PROT gene is a candidate disease gene, and our PROT -/- mice might constitute a new valuable model to study one particular dimension or endophenotype of human psychiatric disorders.
