For issuers with their own ISIN the share price is being maintained by the issuer agent, i.e., your bank responsible for the share emission. In case your shares are listed on an exchange or market, the market will define the share price. Otherwise, for non-listed issuers, the issuer agent can update the share price based on your latest valuation. This can be based on a capital increase or a secondary transaction in which a significant number of shares is created or transferred. A common threshold used for significant is 10 percent, but on discretion of your issuer agent.