PNNs..

Right, time to talk glycan chemistry because your brain refuses to like simple things.

1. What “CS sulfation” actually is

CS = chondroitin sulfate, the sugar chains (GAGs) attached to proteoglycans in PNNs (like aggrecan, brevican, etc.).

Each repeat unit has a GalNAc sugar that can be sulfated at different positions:

• C4S = chondroitin-4-sulfate
• C6S = chondroitin-6-sulfate

So “CS sulfation” just means where the sulfate group is attached on the sugar ring. That stupid little positional detail radically changes how the ECM behaves.

2. Functional difference: C4S vs C6S

Empirically:

• C4S (4-sulfated CS)
  • More inhibitory to:
    • axon growth
    • regeneration
    • plasticity
  • Think: “no entry” sign.
• C6S (6-sulfated CS)
  • More permissive to:
    • sprouting
    • synapse remodeling
    • plasticity
  • Think: “you may pass if you behave.”

PNNs are loaded with these chains. The C4S/C6S ratio is basically a chemical “plasticity dial” for the net.

3. What aging does to that dial

Key findings:

• With age, C6S drops hard in PNNs; C4S dominates.
  • Foscarin et al: aged rats show increased C4S/C6S ratio, PNNs become more inhibitory, and memory performance drops.
  • Fawcett/Yang: in very old brains, C6S becomes almost undetectable in PNNs.