Heparan sulfate endosulfatases, Sulf1 and Sulf2, are required for formation of the medial habenula-interpeduncular nucleus circuit.

The medial habenula (MHb), a medial subdivision of the habenula located in the dorsal diencephalon, sends axons towards the interpeduncular nucleus (IPN) through the fasciculus retroflexus. Upon reaching the IPN, MHb axons cross the midline multiple times. Although heparan sulfate proteoglycans have previously been shown to play a role in MHb axon guidance, the role of heparan sulfate 6-O-endosulfatases, Sulfatase 1 (Sulf1) and Sulfatase 2 (Sulf2), has to date not been investigated. We thus examined MHb axon trajectories in Sulf1 and Sulf2 knockout mice and found that Sulf1/2 double knockout (DKO) mice showed several types of pathfinding errors after reaching the ventral surface of the brain. The abnormalities included aberrant anterior extension into the mammillary body, abnormal lateral projection on the brain surface, and reduced midline crossing within the IPN. Furthermore, the DKO mice exhibited altered outcomes in social conflict in a tube test assay: they showed a higher win rate against wild-type mice. This behavioral phenotype may reflect functional impairment of the MHb-IPN circuit resulting from its developmental abnormalities. Taken together, these findings suggest that Sulf1/2 are required for proper MHb-IPN circuit formation during development, which is in turn critical for its normal function in adulthood.