Cell death in neurodegenerative diseases is often thought to be governed by apoptosis; however, an increasing body of evidence suggests the involvement of alternative cell death mechanisms in neuronal degeneration. We studied retinal neurodegeneration using 10 different animal models, covering all major groups of hereditary human blindness (rd1, rd2, rd10, Cngb1 KO, Rho KO, S334ter, P23H, Cnga3 KO, cpfl1, Rpe65 KO), by investigating metabolic processes relevant for different forms of cell death. We show that apoptosis plays only a minor role in the inherited forms of retinal neurodegeneration studied, where instead, a non-apoptotic degenerative mechanism common to all mutants is of major importance. Hallmark features of this pathway are activation of histone deacetylase, poly-ADP-ribose-polymerase, and calpain, as well as accumulation of cyclic guanosine monophosphate and poly-ADP-ribose. Our work thus demonstrates the prevalence of alternative cell death mechanisms in inherited retinal degeneration and provides a rational basis for the design of mutation-independent treatments.

KCNQ4 is an M-type K þ channel expressed in sensory hair cells of the inner ear and in the central auditory pathway. KCNQ4 mutations underlie human DFNA2 dominant progressive hearing loss. We now generated mice in which the KCNQ4 gene was disrupted or carried a dominant negative DFNA2 mutation. Although KCNQ4 is strongly expressed in vestibular hair cells, vestibular function appeared normal. Auditory function was only slightly impaired initially. It then declined over several weeks in Kcnq4 À/À mice and over several months in mice carrying the dominant negative allele. This progressive hearing loss was paralleled by a selective degeneration of outer hair cells (OHCs). KCNQ4 disruption abolished the I K,n current of OHCs. The ensuing depolarization of OHCs impaired sound amplification. Inner hair cells and their afferent synapses remained mostly intact. These cells were only slightly depolarized and showed near-normal presynaptic function. We conclude that the hearing loss in DFNA2 is predominantly caused by a slow degeneration of OHCs resulting from chronic depolarization.

KCNQ2 and KCNQ3 are two homologous K ؉ channel subunits that can combine to form heterotetrameric channels with properties of neuronal M channels. Loss-of-function mutations in either subunit can lead to benign familial neonatal convulsions (BFNC), a generalized, idiopathic epilepsy of the newborn. We now describe a syndrome in which BFNC is followed later in life by myokymia, involuntary contractions of skeletal muscles. All affected members of the myokymia͞BFNC family carried a mutation (R207W) that neutralized a charged amino acid in the S4 voltage-sensor segment of KCNQ2. This substitution led to a shift of voltage-dependent activation of KCNQ2 and a dramatic slowing of activation upon depolarization. Myokymia is thought to result from hyperexcitability of the lower motoneuron, and indeed both KCNQ2 and KCNQ3 mRNAs were detected in the anterior horn of the spinal cord where the cells of the lower motoneurons arise. We propose that a difference in firing patterns between motoneurons and central neurons, combined with the drastically slowed voltage activation of the R207W mutant, explains why this particular KCNQ2 mutant causes myokymia in addition to BFNC.

Connexin45 (Cx45) is known to be expressed in the retina, but its functional analysis was problematic because general deletion of Cx45 coding DNA resulted in cardiovascular defects and embryonic lethality at embryonic day 10.5. We generated mice with neuron-directed deletion of Cx45 and concomitant activation of the enhanced green fluorescent protein (EGFP). EGFP labeling was observed in bipolar, amacrine, and ganglion cell populations. Intracellular microinjection of fluorescent dyes in EGFP-labeled somata combined with immunohistological markers revealed Cx45 expression in both ON and OFF cone bipolar cells. The scotopic electroretinogram of mutant mice revealed a normal a-wave but a 40% reduction in the b-wave amplitude, similar to that found in Cx36-deficient animals, suggesting a possible defect in the rod pathway of visual transmission. Indeed, neurotransmitter coupling between AII amacrine cells and Cx45-expressing cone bipolar cells was disrupted in Cx45-deficient mice. These data suggest that both Cx45 and Cx36 participate in the formation of functional heterotypic electrical synapses between these two types of retinal neurons that make up the major rod pathway.

The KCNQI potassium channel alpha-subunit can associate with various KCNE beta-subunits that drastically influence channel gating. Here we show that in the mouse gastrointestinal tract KCNQ1 is prominently expressed in stomach, small intestine and colon, while KCNE3 is expressed in the colon and to a lesser extent in small intestine. Immunostaining revealed that KCNQ1 colocalizes with KCNE3 in the basolateral membranes of crypt cells of the colon and small intestine. Together with the previously shown electrophysiological properties of KCNQ1/KCNE3 channels, this strongly suggests that they form the basolateral potassium conductance that is required for transepithelial cAMP-stimulated chloride secretion. In the stomach, KCNQ1 is expressed together with the H+/K+-ATPase in the luminal membrane of acid-secreting parietal cells of gastric glands. KCNE2, but neither KCNE1 nor KCNE3 was detected in the stomach by Northern analysis. Similar to KCNQ1, KCNE2 was present in gastric glands in only a subset of cells that probably represent parietal cells. The coexpression of KCNQ1 and KCNE2 in HEK293 cells yielded potassium currents that were open at resting voltages, suggesting that these heteromeric channels may underlie the apical potassium conductance in acid-secreting parietal cells that is necessary for the recycling of potassium ions during acid secretion via the H+/K+-ATPase.