Phenylketonuria: Direct and indirect effects of phenylalanine

Exp Neurol. 2016 Jul:281:28-36. doi: 10.1016/j.expneurol.2016.04.013. Epub 2016 Apr 14.


High phenylalanine concentrations in the brain due to dysfunctional phenylalanine hydroxylase (Pah) are considered to account for mental retardation in phenylketonuria (PKU). In this study, we treated hippocampal cultures with the amino acid in order to determine the role of elevated levels of phenylalanine in PKU-related mental retardation. Synapse density and dendritic length were dramatically reduced in hippocampal cultures treated with phenylalanine. Changes in cofilin expression and phosphorylation status, which were restored by NMDA, as well as reduced activation of the small GTPase Rac1, likely underlie these structural alterations. In the Pah(enu2) mouse, which carries a mutated Pah gene, we previously found higher synaptic density due to delayed synaptic pruning in response to insufficient microglia function. Microglia activity and C3 complement expression, both of which were reduced in the Pah(enu2) mouse, however, were unaffected in hippocampal cultures treated with phenylalanine. The lack of a direct effect of phenylalanine on microglia is the key to the opposite effects regarding synapse stability in vitro and in the Pah(enu2) mouse. Judging from our data, it appears that another player is required for the inactivation of microglia in the Pah(enu2) mouse, rather than high concentrations of phenylalanine alone. Altogether, the data underscore the necessity of a lifelong phenylalanine-restricted diet.

Keywords: C3; Cofilin; Dendritic arborisation; Hippocampus; Microglia; Phenylketonuria; Synapse.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Cells, Cultured
  • Cofilin 1 / metabolism
  • Dendrites / drug effects
  • Dendrites / physiology
  • Disease Models, Animal
  • Embryo, Mammalian
  • Entorhinal Cortex / pathology
  • Excitatory Amino Acid Agonists / pharmacology
  • Gene Expression Regulation / drug effects
  • Gene Expression Regulation / genetics
  • Hippocampus / pathology
  • In Vitro Techniques
  • Mice
  • Mice, Inbred C57BL
  • Mice, Transgenic
  • Microglia / drug effects
  • Microglia / ultrastructure
  • Mutation / genetics
  • N-Methylaspartate / pharmacology
  • Neurons / drug effects
  • Neurons / ultrastructure
  • Phagocytosis / drug effects
  • Phagocytosis / genetics
  • Phenylalanine / metabolism*
  • Phenylalanine / pharmacology
  • Phenylalanine Hydroxylase / genetics
  • Phenylalanine Hydroxylase / metabolism
  • Phenylketonurias* / genetics
  • Phenylketonurias* / metabolism
  • Phenylketonurias* / pathology
  • Synapses / drug effects
  • Synapses / pathology
  • Synapses / ultrastructure
  • rac1 GTP-Binding Protein / metabolism


  • Cofilin 1
  • Excitatory Amino Acid Agonists
  • Phenylalanine
  • N-Methylaspartate
  • Phenylalanine Hydroxylase
  • rac1 GTP-Binding Protein