Most neurons have one or two long processes called axons that conduct electrical information to their targets, including other neurons, muscles, and organs. The process occurs mainly in the prenatal period. In that stage, axons travel long distances by using attractive and repulsive guidance cues to reach their proper targets and form synapses.

Once neural circuits are established and matured, neuronal axons lose their ability to elongate (Ramon y Cajal, 1928). Thus, axonal injury to the adult central nervous system (CNS) results in irreversible neural circuit disconnection. After injury, the distal ends of dissected neurons in the adult CNS are transformed into abnormal spherical structures called dystrophic endballs, and axonal regeneration halts in glial scars that develop after trauma (Sakamoto et al., 2019; Silver and Miller, 2004; Tom et al., 2004). This depressive structural change in axon terminals is caused by the activation of the receptor-type tyrosine phosphatase PTPσ, which is located on the neuronal plasma membrane, by exposure to chondroitin sulfate proteoglycans (CSPG) that accumulate in the glial scar (Coles et al., 2011a; Shen et al., 2009). CS acts on PTPσ and inhibits the axonal regeneration of neurons, whereas heparan sulfate (HS) competitively interacts with the receptor and promotes axonal elongation (Coles et al., 2011b; Sakamoto et al., 2021a; Sakamoto et al., 2021b). Consistently, HSPGs, which contain HS chains, are abundantly expressed in the cell-surface neurons and play essential roles in the development of the CNS by promoting growth cone development, axonal elongation, and synapse formation (Cole et al., 1986; Inatani et al., 2003; Sarrazin et al., 2011; Wang and Denburg, 1992). Moreover, we have shown that enoxaparin, a moiety of low-molecular-weight HS, can enhance axonal regeneration and functional recovery in a rat model of spinal cord injury (Ito et al., 2021).

In contrast to the case of adult CNS, it is well known that axons in the juvenile CNS still have regenerative activity after axonal injury. Indeed, several reports have demonstrated that embryonic axons can elongate even in the inhibitory environment, in which adult axons fail to regenerate (Condic, 2001; Forehand and Farel, 1982; Shimizu et al., 1990). While the matrix of the CNS varies between embryonic and mature stages (Miyata et al., 2012), the above strong growth potential of embryonic neurons cannot be explained without addressing their autonomous abilities. Moreover, the genetic profiles of these juvenile neurons including GAP-43, KLF7, Sox11, and Cacna2d2 are advantageous in axonal regeneration (Blackmore et al., 2012; Doster et al., 1991; Tedeschi et al., 2016; Wang et al., 2015), as evidenced by a study showing that the genetic profiles of neurons during axonal regeneration using neural progenitor cell transplantation to a spinal cord injury model were similar to the profiles of embryonic neurons (Poplawski et al., 2020).

In the present study, we hypothesized that embryonic neurons have a specific mechanism in their HSPG profiles that differs from that of adult neurons and that it competes with the effect of CSPG on PTPσ to enable axonal outgrowth in a CSPG environment. Here, we used dorsal root ganglion (DRG) neurons from embryonic mice to investigate HSPG composition and axonal behavior in a CSPG environment and compared them with adult neurons to elucidate the mechanisms by which embryonic nerves enable robust axonal outgrowth. This elucidation could provide new molecular insights useful for the development of therapies to counteract inhibited nerve regeneration after CNS injury.