AC Lab :: Membrane and Receptor Biology

Membrane proteins mediate a wide range of essential cellular processes such as signaling across the membrane, cell-cell recognition, and membrane transport. About 30% of all open reading frames (ORFs) are predicted to encode membrane proteins and almost 50% of all proteins encoded by eukaryotic genomes are membrane proteins. Importantly, membrane proteins represent prime candidates for the generation of novel drugs in all clinical areas.

Since a significant portion of integral membrane proteins remains in contact with the membrane, the structure and function of membrane proteins depend on their interactions with the surrounding lipids. The serotonin1A (5-HT1A) receptor, an important neurotransmitter receptor, is a member of a superfamily of seven transmembrane domain receptors that couple to GTP-binding regulatory proteins (G-proteins). Although G-protein coupled receptors (GPCRs) represent ~50% of current drug targets, only a small fraction of all GPCRs are presently targeted by drugs. Serotonergic signaling plays a key role in the generation and modulation of various cognitive, behavioral and developmental functions. Disruptions in serotonergic systems have been implicated in the etiology of mental disorders such as schizophrenia, migraine, infantile autism, eating disorders, and obsessive compulsive disorder (Pucadyil et al. (2005) Cell. Mol. Neurobiol.; Kalipatnapu and Chattopadhyay (2007) Cell. Mol. Neurobiol.). Interestingly, mutant (knockout) mice lacking the serotonin1A receptor exhibit enhanced anxiety-related behavior, and represent an important animal model for genetic vulnerability to complex traits such as anxiety disorders and aggression in higher animals. Although none of the serotonin receptors have been purified to homogeneity from native sources yet, our group has been able to partially purify and solubilize functional serotonin1A receptors (Kalipatnapu and Chattopadhyay (2005) IUBMB Life).

Seminal work from our laboratory has comprehensively demonstrated the requirement of membrane cholesterol in the function of the serotonin1A receptor (Pucadyil and Chattopadhyay (2004) Biochim. Biophys. Acta ; Pucadyil and Chattopadhyay (2006) Prog. Lipid Res.). We have recently generated a cellular model for the Smith-Lemli-Opitz Syndrome (SLOS), a disease associated with defective cholesterol biosynthesis. SLOS is an autosomal recessive disorder characterized clinically by mental retardation, physical deformities, failure to thrive and multiple congenital anomalies. Our results show that ligand binding activity, G-protein coupling and downstream signaling of serotonin1A receptors are impaired in the cellular model of SLOS (Paila et al. (2008) Biochim. Biophys. Acta). The potential clinical relevance of these results stems from the fact that defective cholesterol biosynthesis constitutes a common theme for a number of genetically inherited disorders. Our results could be potentially useful in understanding the molecular basis that underlie the pathophysiology of SLOS and could provide novel insight in formulating future treatment for the disease. Further, we have shown ; that 7-dehydrocholesterol (7-DHC), the immediate biosynthetic precursor of cholesterol, could not support the function of the serotonin1A receptor (Singh et al. (2007) Biochem. Biophys. Res. Commun.; Chattopadhyay et al. (2007) Biochem. Biophys. Res. Commun.). Our results comprehensively demonstrate the specific requirement of membrane cholesterol for the function of serotonin1A receptor, although global membrane effects cannot be completely ruled out (Paila and Chattopadhyay (2009) Glycoconj. J.). Based on these results, we envisage that there could be specific/nonannular cholesterol binding site(s) in the serotonin1A receptor (Paila et al. (2009) Biochim. Biophys. Acta). Keeping in mind the pharmacological relevance of the serotonin1A receptor, the interaction of this transmembrane protein with the lipid environment assumes greater significance in its function in healthy and diseased states.

In an interesting and developing aspect of this work, we have addressed the issue of stringency criteria of the molecular structure of cholesterol in supporting the function of the serotonin1A receptor. Cholesterol is the end product of the long and multistep sterol biosynthetic pathway. It is a unique molecule in terms of high level of in-built stringency, fine tuned by natural evolution for its ability to optimize properties of eukaryotic cell membranes in relation to biological functions. We have taken a two-prong approach: (i) monitoring the effect of cholesterol biosynthetic precursors on the organization and dynamics of membranes using a variety of fluorescence-based approaches (Shrivastava et al. (2008) Biochemistry), and (ii) utilizing the function of a representative G-protein coupled receptor ( the serotonin1A receptor) as a ‘readout’ for the suitability of a sterol in supporting receptor function (Singh et al. (2007) Biochem. Biophys. Res. Commun.; Chattopadhyay et al. (2007) Biochem. Biophys. Res. Commun.; Singh et al. (2009) Biochim. Biophys. Acta).