The hyaluronan and proteoglycan link proteins: Organizers of the brain extracellular matrix and key molecules for neuronal function and plasticity

• Hapln is a key molecule in the formation and control of perineuronal matrix in the adult brain.
• Two distinct types of hyaluronan-condensed perineuronal matrices exist: perineuronal net and perinodal extracellular matrix.
• Perineuronal nets play a key role in the control of plasticity in the adult CNS.
• Physiological and developmental roles of perinodal extracellular matrices are summarized.

The hyaluronan and proteoglycanbinding link protein (Hapln) is a key molecule in the formation and control of hyaluronan-based condensed perineuronal matrix in the adult brain. This review summarizes the recent advances in understanding the role of Haplns in the formation and control of two distinct types of perineuronal matrices, one for “classical” PNN and the other for the specialized extracellular matrix (ECM) at the node of Ranvier in the central nervous system (CNS). We introduce the structural components of each ECM organization including the basic concept of supramolecular structure named “HLT model”. We furthermore summarize the developmental and physiological role of perineuronal ECMs from the studies of Haplns and related molecules. Finally, we also discuss the potential mechanism modulating PNNs in the adult CNS. This layer of organized matrices may exert a direct effect via core protein or sugar moiety from the structure or by acting as a binding site for biologically active molecules, which are important for neuronal plasticity and saltatory conduction.