Maintaining intact myelin sheaths is vital for PNS function, yet the molecular players involved in regulating this process are poorly understood. Now Charnay and colleagues show that the transcription factor KROX20, known to be involved in the onset of myelination, is also integral to its maintenance.

A role for KROX20 in preserving myelin integrity was suggested by its expression throughout adulthood in Schwann cells and by links between mutations in KROX20 and congenital and late-onset human myelinopathies. However, previously generated Krox20 mutant mice died at early perinatal stages (owing to the importance of Krox20 in hindbrain development), and could not provide answers about later aspects of myelin development and maintenance. The researchers therefore developed two conditional mouse mutants that evade early lethality by delaying the onset of Krox20 inactivation.

First, compound heterozygous mutants were generated, in which Cre recombinase was inserted into one allele of the Krox20 locus. The second allele contained a Krox20 exon flanked by loxP sites, marking it for excision by Cre recombinase. This resulted in the inactivation of Krox20 by 4 days after birth, as Cre recombinase levels built up in Krox20-expressing cells. The complete absence of myelination in these animals, which survive for 6 weeks, indicated that continued Krox20 expression is essential for the completion of myelination.

To investigate Krox20's role in adult myelin maintenance, a second model was created using a Cre recombinase transgene that could be induced at a specific timepoint by tamoxifen treatment. Inactivation of Krox20 at 3 months after birth resulted in a rapid breakdown of the myelin sheath, showing that continual expression of this gene is of key importance for myelin maintenance.

In response to Krox20 inactivation, mature Schwann cells seemed to return to an immature state, indicated by the upregulation of markers of less differentiated Schwann cells, such as Sox2, and made abortive attempts to initiate re-myelination. KROX20 has previously been suggested to suppress Sox2, indicating that, in addition to its established role in activating myelination, KROX20 might maintain myelin by repressing Schwann cell dedifferentiation.

This study shows that KROX20 is crucial for myelin maintenance, and introduces two models of delayed Krox20 inactivation that might aid investigations into human myelinopathies. In particular, the inducible mutant replicates many features of late-onset myelinopathies such as Charcot-Marie-Tooth disease. Moving forward, it will be important to understand the mechanisms by which Krox20 governs Schwann cell fate and to investigate the relationship to similar processes in the CNS.