Genetic approaches to access cell types in mammalian nervous systems

Abstract

Understanding brain circuit organization and function requires systematic dissection of its cellular components. With vast cell number and diversity, mammalian nervous systems present a daunting challenge for achieving specific and comprehensive cell type access-prerequisite to circuit analysis. Genetic approaches in the mouse have relied on germline engineering to access marker-defined cell populations. Combinatorial strategies that engage marker intersection, anatomy and projection pattern (e.g. antero-grade and retro-grade viral vectors), and developmental lineage substantially increase the specificity of cell type targeting. While increasing number of mouse cell types are becoming experimentally accessible, comprehensive coverage requires larger coordinated efforts with strategic infrastructural and fiscal planning. CRISPR-based genome editing may enable cell type access in other species, but issues of time, cost and ethics remain, especially for primates. Novel approaches that bypass the germline, such as somatic cell engineering and cell surface-based gene delivery, may reduce the barrier of genetic access to mammalian cell types.

Figure 1

Six cardinal types of GABAergic neurons (left) are delineated by their transcription profiles mainly consisting of 6 functional gene categories encoding a molecular scaffold that mediate synaptic input-output communication (right). Modified from Paul et al 2017.

Figure 2

Two major approaches to access cell types. A. Utilizing gene expression regulatory mechanisms. Toolgene (“effector”) can be inserted into host genome via genetic engineering, or delivered into cells on a viral vector or plasmids. Its expression can be regulated on the DNA level by a binary system such as Cre-loxP(①), on the transcriptional level promoter and enhancers(②), on the post-transcription level by miRNAs (③), or on the protein level through modulating protein stability or localization(④). B. Recognizing cell surface molecules. Viral infection is mediated by interaction between capsid protein and membrane receptors (⑤). Nanobody recognizing cell surface antigen, or engineered ligands recognizing membrane receptors, can be used to decorate virus or non-viral vehicles such as nanoparticles which modulates neuron activity or delivers drugs (⑥).

Figure 3

Genetic access to cell types in the mouse brain. A. Binary system for cell type targeting using single driver. B. Intersectional targeting paradigms. C. Besides marker gene expression, other cellular features including developmental lineage, anatomical location and connectivity can also be utilized to target specific cell types within broad classes of neural stem cells, neurons and glia. For example, retrograde virus can be combined with Cre driver mice to label cells expressing a specific marker and also projecting to a specific target brain region.