Synthetic Biology for In-Situ Synthesis of Neurotransmitters: Engineering Commensal Bacteria for the Production of Dopamine Precursors in the Enteric Environment as a Therapeutic Strategy for Parkinson’s Disease

The relationship between the gastrointestinal environment and neurological function has emerged as one of the most significant areas of biomedical research in the twenty-first century. This connection, now formally recognized as the gut-brain axis, encompasses bidirectional communication pathways that integrate neural, hormonal, and immunological signaling between the enteric nervous system and the central nervous system. The implications of this axis for understanding and treating neurological disorders have profound consequences for clinical practice and therapeutic development. Parkinson’s disease, affecting approximately 6.1 million individuals globally according to the Global Burden of Disease Study 2016, represents a neurodegenerative disorder characterized by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta. The motor symptoms of Parkinson’s disease, including bradykinesia, rigidity, resting tremor, and postural instability, result primarily from this dopamine deficiency in the striatum. The standard pharmacological intervention, levodopa administration, has remained relatively unchanged since its introduction in the 1960s despite well-documented limitations including peripheral metabolism, fluctuating plasma concentrations, and the development of levodopa-induced dyskinesias in approximately 40 percent of patients after four to six years of treatment. These limitations have motivated extensive research into alternative delivery strategies and novel therapeutic approaches that might provide more consistent dopaminergic stimulation with fewer side effects. The enteric nervous system, containing between 200 and 600 million neurons, represents the largest and most complex division of the peripheral nervous system. This neural network maintains bidirectional communication with the central nervous system through the vagus nerve, which comprises approximately 80 percent afferent fibers transmitting sensory information from the gastrointestinal tract to the brainstem.