The varitint–waddler mouse phenotypes and the TRPML3 ion channel mutation: cause and consequence

Abstract

The transient receptor potential mucolipins (TRPMLs) are the most recently discovered subfamily of TRP ion channel proteins. Positional cloning approach has identified two mutations in the TRPML3 (Mcoln3) gene that cause the varitint-waddler mouse phenotypes. Short for variable tint (diluted coat color), the varitint-waddler consists two phenotypes Va and Va ( J ). The mutation associated with the Va phenotype is an alanine to proline substitution at position 419 (A419P) within the predicted fifth transmembrane (TM5) domain of TRPML3. The second Va ( J ) mouse phenotype arose spontaneously from an isoleucine to threonine substitution at position 362 (I362T) that is proximal to the predicted TM3 domain in addition to the existing A419P mutation on TM5. Mice with the Va and Va ( J ) mutations exhibit a spectrum of disease phenotypes from diluted coat color to auditory and vestibular problems, depending on which alleles are present. It has been over 5 years since the discovery of these TRPML3 mutations, and it was just recently that the nature of these mutations has been characterized. In this review, we discuss the molecular and cell physiological effects of the two distinct TRPML3 mutations. We reveal the effects of proline substitution on transmembrane domain structure and channel function and discuss how the Va mutation confers its cytotoxicity, while the Va ( J ) mutation results in an apparent rescue phenotype. Finally, we briefly tackle molecular strategies that have been employed to neutralize the cytotoxic effect and constitutive channel activity of the Va mutation.

The Role of Transient Receptor Potential Cation Channels in Ca2+ Signaling

“…TRPML3 is the only TRPML channel found in the plasma membrane, making it easier to characterize the channel properties via whole-cell current measurements. This revealed that TRPML3 is an inwardly rectifying, Ca 2þ -permeable cation channel (Cuajungco and Samie 2008). The current is inhibited by an acid extracytosolic (analogous to the luminal side) pH (Kim et al 2008).…”

“…Notably, elimination of TRPML3 regulation by extracytosolic pH has the same functional and cellular phenotype as the A419P (Va) mutation, a gain-of-function mutation that causes the varitint-waddler phenotype (a disease characterized by deafness, circling behavior, and pigmentation defects) (Di Palma et al 2002). This mutation is likely to disrupt channel-gating by locking the channel in an open state, making the channel constitutively active and yielding much larger currents (Cuajungco and Samie 2008); although basic properties, such as I-V characteristics, single channel conductance, and ion selectivity have not changed Nagata et al 2008). The same mutation has been made in TRPML1 and TRPML2 channels in order to effectively characterize the pore properties of these channels using whole-cell recordings.…”

Heteromultimeric TRPML channel assemblies play a crucial role in the regulation of cell viability models and starvation-induced autophagy

“…Loss-of-function mutations in TRPML1 are implicated in mucolipidosis type IV (MLIV; MIM# 252650) pathogenesis, whereas a spontaneous gain-of-function mutation in TRPML3 is principally implicated in the varitint-waddler mouse mutant (Cuajungco and Samie, 2008;Zeevi et al, 2007). The varitint-waddler mouse features hearing loss, vestibular dysfunction, and abnormal pigmentation (Di Palma et al, 2002).…”

Activating Mutations of the TRPML1 Channel Revealed by Proline-scanning Mutagenesis

“…Va channel in an unregulated and "open" state (25). Moreover, a Pro scan of the whole TM5 of TRPML3 revealed multiple additional GOF Pro substitutions (16).…”

“…Va is referred to as a gain-of-function (GOF) mutation (25). Furthermore, because Pro introduction into a transmembrane ␣-helix often causes kinks, hinges, or swivels (26), the "helix-breaking effect" of Pro was proposed to lock the TRPML3…”