Nowadays Learning Management Systems (LMS) are not restricted to distant learning. Nevertheless, the pedagogical expressiveness of courses designed by teachers is strongly dependent on their knowledge and level of expertise on the LMS they use. The GraphiT project aims to help teachers design pedagogically sound and technically executable learning designs. To this end, we propose to support teachers by providing them with an LMS-specific Visual Instructional Design Language, according to a Domain Specific Modeling approach and tooling. This paper focuses on the abstract syntax of such language. We propose a specific LMS-centered approach for raising the pedagogical expressiveness of their implicit learning design semantics. We discussed how the LMS low-level parameterisations could be abstracted in order to build higher-level building blocks. Based on the Moodle LMS, we present and verify our meta-modeling approach by formalising the abstract syntax of a Moodle-dedicated instructional design language.