µ/ω-TRTX-Tap1a

µ/ω-TRTX-Tap1a (Tap1a) is a 35 amino acid peptidyl toxin originally isolated from the venom of the tarantula, Theraphosa apophysis. Tap1a inhibits voltage-gated sodium (Na_V) and voltage-gated calcium (Ca_V)3 channels by inducing a hyperpolarizing shift in both voltage-dependent activation and steady state inactivation¹. Tap1a specifically inhibits Na_V1.7, Na_V1.2, and Ca_V3.1 with nanomolar potency and Na_V1.3, Na_V1.6, Na_V1.1, and Ca_V3.2 at low micromolar concentrations¹. These ion channels participate in neuron polarization, transmission of somatosensory signals, as well as neuronal cell differentiation, death, and survival. Thus, they are involved in many diseases, including pain disorders, epilepsy, and age-related neurodegeneration.

Spider peptides modulate an array of ion channels and receptor proteins. Knottins, which are a subtype of spider peptides, are also referred to as inhibitor cystine knot (ICK) peptides. ICK peptides harbor a disulfide-rich structural motif that forms a “knot”, which confers high structural, thermal, and proteolytic stability. The modelled structure of Tap1a revealed an ICK fold typical of spider venom peptides, as well as a hydrophobic patch involved in the binding of spider venom peptides to Ca_V3 and Na_V channels¹.

Ca_V3 are T-type, low voltage-gated calcium channels. Their electrophysiological properties include low voltage thresholds for activation and inactivation, rapid inactivation, and rebound bursting. These properties are responsible for the Ca_V3-mediated fine-tuned regulation of neuronal excitability in both the central nervous system (CNS) and peripheral nervous system (PNS)².

Ca_V3.1 is highly expressed in the brain amygdala, subthalamic nuclei, cerebellum, and thalamus. In contrast, Ca_V3.1 is only moderately expressed in the heart. Ca_V3.1 participates in neuron polarization, synaptic transmission, as well as neuronal cell differentiation, death, and survival. Ca_V3.1 was implicated in the process of age-related neurodegeneration, Parkinson’s disease, and Alzheimer’s disease³. Moreover, mutations in Ca_V3.1 have been shown to induce cerebellar ataxia. Ca_V3.2 channels are expressed in the thalamus where they play a role in the pathophysiology of epilepsy. In addition, the constitutive deletion of the Ca_V3.2 gene alleviates acute pain, inflammatory pain, and chronic visceral pain in mice⁴.

Na_V1.1–1.9 are voltage-gated sodium channels. They open upon depolarization of the membrane and inactivate rapidly before returning to the closed state upon membrane hyperpolarization. The rapid influx of Na+ ions is vital to the generation and propagation of action potential (AP) as well as the transmission of somatosensory signals.

Na_V1.7 is expressed in the PNS, dorsal root ganglia neurons, visceral sensory neurons, olfactory sensory neurons, trigeminal ganglia, and sympathetic neurons. Na_V1.7 gain-of-function mutations have been identified in patients with various pain disorders, such as inherited erythromelalgia (IEM), paroxysmal extreme pain disorder (PEPD), small fiber neuropathy (SFN), and painful diabetic peripheral neuropathy⁵.

Na_V1.2 is abundantly expressed at the nodes of Ranvier and in the axon initial segment (AIS) during early development. Na_V1.2 plays a dominant role in the initiation and propagation of APs. In mature neurons, Na_V1.6 takes the role of AP initiation, and   Na_V1.2 merely augments APs. Pathogenic variants of Na_V1.2 are common causes of neurodevelopmental disorders such as episodic ataxia, schizophrenia, autism spectrum disorder, and intellectual disability with and without seizures⁶.