Changes in cochlear PMCA2 expression correlate with the maturation of auditory sensitivity

The long and short: Non-coding RNAs in the mammalian inner ear

Non-coding RNAs (ncRNAs) play a critical role in the entire body, and their mis-regulation is often associated with disease. In parallel with the advances in high-throughput sequencing technologies, there is a great deal of focus on this broad class of RNAs. Although these molecules are not translated into proteins, they are now well established as significant regulatory components in many biological pathways and pathological conditions. ncRNAs can be roughly divided into two main sub-groups based on the length of the transcript, with both the small and long non-coding RNAs having diverse regulatory functions. The smaller length group includes ribosomal RNAs (rRNA), transfer RNAs (tRNA), small nuclear RNAs (snRNA), small nucleolar RNAs (snoRNA), microRNAs (miRNA), small interfering RNAs (siRNA), and PIWI-associated RNAs (piRNA). The longer length group includes linear long non-coding RNAs (lncRNA) and circular RNAs (circRNA). This review is designed to present the different classes of small and long ncRNA molecules and describe some of their known roles in physiological and pathological conditions, as well as methods used to assess the validity and function of miRNAs and lncRNAs, with a focus on their role and functions in the inner ear, hearing and deafness.

The Genomics of Auditory Function and Disease

Current estimates suggest that nearly half a billion people worldwide are affected by hearing loss. Because of the major psychological, social, economic, and health ramifications, considerable efforts have been invested in identifying the genes and molecular pathways involved in hearing loss, whether genetic or environmental, to promote prevention, improve rehabilitation, and develop therapeutics. Genomic sequencing technologies have led to the discovery of genes associated with hearing loss. Studies of the transcriptome and epigenome of the inner ear have characterized key regulators and pathways involved in the development of the inner ear and have paved the way for their use in regenerative medicine. In parallel, the immense preclinical success of using viral vectors for gene delivery in animal models of hearing loss has motivated the industry to work on translating such approaches into the clinic. Here, we review the recent advances in the genomics of auditory function and dysfunction, from patient diagnostics to epigenetics and gene therapy.

The noncoding genome and hearing loss

The age of sequencing has provided unprecedented insights into the human genome. The coding region of the genome comprises nearly 20,000 genes, of which approximately 4000 are associated with human disease. Beyond the protein-coding genome, which accounts for only 3% of the genome, lies a vast pool of regulatory elements in the form of promoters, enhancers, RNA species, and other intricate elements. These features undoubtably influence human health and disease, and as a result, a great deal of effort is currently being invested in deciphering their identity and mechanism. While a paucity of material has caused a lag in identifying these elements in the inner ear, the emergence of technologies for dealing with a minimal number of cells now has the field working overtime to catch up. Studies on microRNAs (miRNAs), long non-coding RNAs (lncRNAs), methylation, histone modifications, and more are ongoing. A number of microRNAs and other noncoding elements are known to be associated with hearing impairment and there is promise that regulatory elements will serve as future tools and targets of therapeutics and diagnostics. This review covers the current state of the field and considers future directions for the noncoding genome and implications for hearing loss.

Prestin Contributes to Membrane Compartmentalization and Is Required for Normal Innervation of Outer Hair Cells

Outer hair cells (OHC) act as amplifiers and their function is modified by medial olivocochlear (MOC) efferents. The unique OHC motor protein, prestin, provides the molecular basis for somatic electromotility, which is required for sensitivity and frequency selectivity, the hallmarks of mammalian hearing. Prestin proteins are the major component of the lateral membrane of mature OHCs, which separates apical and basal domains. To investigate the contribution of prestin to this unique arrangement, we compared the distribution of membrane proteins in OHCs of wildtype (WT) and prestin-knockout (KO) mice. In WT, the apical protein PMCA2 was exclusively localized to the hair bundles, while it was also found at the lateral membrane in KOs. Similarly, a basal protein KCNQ4 did not coalesce at the base of OHCs but was widely dispersed in mice lacking prestin. Since the expression levels of PMCA2 and KCNQ4 remained unchanged in KOs, the data indicate that prestin is required for the normal distribution of apical and basal membrane proteins in OHCs. Since OHC synapses predominate in the basal subnuclear region, we also examined the synaptic architecture in prestin-KO mice. Although neurite densities were not affected, MOC efferent terminals in prestin-KO mice were no longer constrained to the basal pole as in WT. This trend was evident as early as at postnatal day 12. Furthermore, terminals were often enlarged and frequently appeared as singlets when compared to the multiple clusters of individual terminals in WT. This abnormality in MOC synaptic morphology in prestin-KO mice is similar to defects in mice lacking MOC pathway proteins such as α9/α10 nicotinic acetylcholine receptors and BK channels, indicating a role for prestin in the proper establishment of MOC synapses. To investigate the contribution of prestin’s electromotility, we also examined OHCs from a mouse model that expresses non-functional prestin (499-prestin). We found no changes in PMCA2 localization and MOC synaptic morphology in OHCs from 499-prestin mice. Taken together, these results indicate that prestin, independent of its motile function, plays an important structural role in membrane compartmentalization, which is required for the formation of normal efferent-OHC synapses in mature OHCs.

Contribution of Plasma Membrane Calcium ATPases to neuronal maladaptive responses: Focus on spinal nociceptive mechanisms and neurodegeneration

Plasma membrane calcium ATPases (PMCAs) are ion pumps that expel Ca²⁺ from cells and maintain Ca²⁺ homeostasis. Four isoforms and multiple splice variants play important and non-overlapping roles in cellular function and integrity and have been implicated in diseases including disorders of the central nervous system (CNS). In particular, one of these isoforms, PMCA2, is critical for spinal cord (SC) neuronal function. PMCA2 expression is decreased in SC neurons at onset of symptoms in animal models of multiple sclerosis. Decreased PMCA2 expression affects the function and viability of SC neurons, with motor neurons being the most vulnerable population. Recent studies have also shown that PMCA2 could be an important contributor to pain processing in the dorsal horn (DH) of the SC. Pain sensitivity was altered in female, but not male, PMCA2^+/- mice compared to PMCA2^+/+ littermates in a modality-dependent manner. Changes in pain responsiveness in the female PMCA2^+/- mice were paralleled by female-specific alterations in the expression of effectors, which have been implicated in the excitability of DH neurons, in mechanisms governing nociception and in the transmission of pain signals. Other PMCA isoforms and in particular, PMCA4, also contribute to the excitability of neurons in the dorsal root ganglia (DRG), which contain the first-order sensory neurons that convey nociceptive information from the periphery to the DH. These findings suggest that specific PMCA isoforms play specialized functions in neurons that mediate pain processing. Further investigations are necessary to unravel the precise contribution of PMCAs to mechanisms governing pathological pain in models of injury and disease.