Except as herein provided, no trustee and no employee of the board shall have any direct interest in the gains or profits of any investment made by the board of trustees, nor as such receive any pay or emolument for his services. No trustee or employee of the board of trustees shall directly or indirectly for himself or as an agent in any manner use the assets of the retirement system except to make such current and necessary payments as are authorized by the board of trustees, nor shall any trustee or employee of the board become an endorser or surety or become in any manner an obligor for moneys loaned by or borrowed from the board of trustees.