Therapeutic Nanobodies Targeting Cell Plasma Membrane Transport Proteins: A High-Risk/High-Gain Endeavor

Abstract

Cell plasma membrane proteins are considered as gatekeepers of the cell and play a major role in regulating various processes. Transport proteins constitute a subclass of cell plasma membrane proteins enabling the exchange of molecules and ions between the extracellular environment and the cytosol. A plethora of human pathologies are associated with the altered expression or dysfunction of cell plasma membrane transport proteins, making them interesting therapeutic drug targets. However, the search for therapeutics is challenging, since many drug candidates targeting cell plasma membrane proteins fail in (pre)clinical testing due to inadequate selectivity, specificity, potency or stability. These latter characteristics are met by nanobodies, which potentially renders them eligible therapeutics targeting cell plasma membrane proteins. Therefore, a therapeutic nanobody-based strategy seems a valid approach to target and modulate the activity of cell plasma membrane transport proteins. This review paper focuses on methodologies to generate cell plasma membrane transport protein-targeting nanobodies, and the advantages and pitfalls while generating these small antibody-derivatives, and discusses several therapeutic nanobodies directed towards transmembrane proteins, including channels and pores, adenosine triphosphate-powered pumps and porters.

Keywords: cell plasma membrane transport proteins; drug target; nanobodies; therapy.

Figure 1

Classification of cell plasma membrane proteins. (A) Cell plasma membrane proteins can be divided in 3 general classes, namely integral membrane proteins, peripheral membrane proteins and lipid anchored proteins. (B) Integral membrane proteins can be further divided into 2 groups, namely integral monotopic proteins and transmembrane proteins. (C) Based on their topology, the family of transmembrane proteins can be classified in type I, type II, type III, type IV, β-barrel and multi-pass transmembrane proteins. (D) Multi-pass transmembrane proteins can be divided into different classes based on their function, namely transport proteins and receptors. (E) Transport proteins include channels and pores, ATP-powered pumps and 3 types of porters, namely uniporters, symporters and antiporters.

Figure 2

Conventional antibodies and nanobodies. (A) Immunoglobulin-G (IgG) with 2 heavy (H) and 2 light (L) chains. The L chain comprises 1 variable (VL) and 1 conserved (CL) domain, whereas the H chain contains 1 variable (VH) and 3 constant (CH1, CH2 and CH3) domains. The paired VH and VL domains form the variable fragment (Fv) and bind to the antigen. The L chain and the first half of the H chain (VH and CH1) are known as the antigen binding fragment (Fab). The CH2 and CH3 domains of the 2 H chains form the crystallizable fragment (Fc). (B) Heavy chain-only antibodies (HCAbs) are smaller than conventional antibodies. They are devoid of L chains and the H chain lacks a CH1 domain. HCAbs recognize antigens via the variable domain of the H chain of HCAbs (VHH), also known as a nanobody.