The taming of the cell penetrating domain of the HIV Tat: Myths and realities

Abstract

Protein transduction with cell penetrating peptides over the past several years has been shown to be an effective way of delivering proteins in vitro and now several reports have also shown valuable in vivo applications in correcting disease states. An impressive bioinspired phenomenon of crossing biological barriers came from HIV transactivator Tat protein. Specifically, the protein transduction domain of HIV Tat has been shown to be a potent pleiotropic peptide in protein delivery. Various approaches such as molecular modeling, arginine guanidinium head group structural strategy, multimerization of PTD sequence and phage display system have been applied for taming of the PTD. This has resulted in identification of PTD variants which are efficient in cell membrane penetration and cytoplasmic delivery. In spite of these state of the art technologies, the dilemma of low protein transduction efficiency and target specific delivery of PTD fusion proteins remains unsolved. Moreover, some misconceptions about PTD of Tat in the literature require considerations. We have assembled critical information on secretory, plasma membrane penetration and transcellular properties of Tat and PTD using molecular analysis and available experimental evidences.

Introduction

The past decade has witnessed tremendous advances in the field of protein transduction, aiming to correct defects for proteins involved in a variety of disease processes. At present, it is possible to produce a given protein molecule by recombinant DNA technology for in vivo therapeutic applications. Nevertheless, it still remains a challenge to deliver the recombinant proteins to desired targets in vivo, although small molecules or peptides capable of crossing cellular membranes have been successfully designed to deliver small or moderately large proteins. Despite developments in the area of protein transduction peptides, the classical delivery methods of protein coding genes via adeno-associated virus (AAV) [1], [2], adenovirus (AV) [3], [4], lentivirus [5], herpes virus (HSV) [6], [7] vectors, and plasmid expression vectors [8], [9] remain the preferred choice for expression of proteins.

Because of their natural abilities in delivering the specific genes to permissive cells, viral vector-mediated gene expression is considered the most efficient and reliable approach for expressing functional proteins de novo in mitotically active or post-mitotically blocked cell types (HIV viral vectors). Nonetheless, viral vectors invariably are required in large doses to achieve therapeutic expression levels of intended protein (s). Moreover, viral vectors integrate with the host chromatin material. These properties may have consequences from long term effects on host genetic systems, and therefore, safety remains a serious concern for their ultimate clinical application [10], [11], [12], [13].

An alternative approach that appears to be the safest is to produce recombinant proteins exogenously and then deliver them systemically or by localized injections into the target organs. The delivery and bioavailability of recombinant proteins into cells or tissues need further improvements, however. Discovery of the HIV Tat protein transduction domain (PTD) has opened avenues for directing in vitro and in vivo delivery of proteins into cells. Several studies have shown the potential of PTD in drug delivery [14], [15] and transduction of proteins as large as 110 kDa into different cells [16]. In vivo injection of fusion proteins systemically has demonstrated the effectiveness of the PTD in protein delivery [15], [16]. In the present review we discuss the current status of the protein transduction focusing mainly on the PTD domain of HIV Tat.