Tni

TnI, short for troponin I, is one of the three subunits of the troponin complex that regulates contraction in striated muscle. Along with troponin T (TnT) and troponin C (TnC), troponin I plays a key inhibitory role that prevents actomyosin interaction in the relaxed state and participates in the calcium-dependent activation of contraction in cardiac and skeletal muscle.

Structure and isoforms

The troponin complex consists of three subunits: TnI, TnT, and TnC. TnI serves as the inhibitory subunit that binds to actin and inhibits actin–myosin interaction. There are distinct TnI isoforms encoded by different genes: cardiac troponin I (cTnI) is encoded by TNNI3, slow skeletal troponin I by TNNI1, and fast skeletal troponin I by TNNI2. The cardiac isoform, which contains a unique N-terminal extension, has phosphorylation sites (notably Ser23/Ser24) that modulate calcium sensitivity and contractile regulation via protein kinase A signaling.

Function

In the presence of calcium, calcium-bound TnC undergoes a conformational change that reduces the inhibitory effect of TnI, allowing actin–myosin cross-bridge cycling and muscle contraction. Removal of calcium returns the complex to its inhibitory state, promoting relaxation. This regulation of actin–myosin interaction by the troponin complex is essential for heart muscle contraction and, to varying degrees, skeletal muscle contraction.

Clinical significance

Troponin I is widely used clinically as a biomarker of myocardial injury. High-sensitivity assays for cardiac troponin I (hs-cTnI) detect small elevations in blood that indicate myocardial damage, such as during acute myocardial infarction, injury from other cardiac conditions, or even certain non-cardiac illnesses. A diagnosis often relies on a rising or falling concentration pattern, rather than a single measurement, and interpretation considers the clinical context and assay specifics.

Notes

Because TnI has cardiac- and skeletal-muscle–specific isoforms, its measurement helps distinguish cardiac from skeletal muscle injury in appropriate clinical settings.