Tyrosinase and ocular diseases: some novel thoughts on the molecular basis of oculocutaneous albinism type 1

Tyrosinase (TYR) is a multifunctional copper-containing glycoenzyme (approximately 80 kDa), which plays a key role in the rate-limiting steps of the melanin biosynthetic pathway. This membrane-bound protein, possibly evolved by the fusion of two different copper-binding proteins, is mainly expressed in epidermal, ocular and follicular melanocytes. In the melanocytes, TYR functions as an integrated unit with other TYR-related proteins (TYRP1, TYRP2), lysosome-associated membrane protein 1 (LAMP1) and melanocyte-stimulating hormone receptors; thus forming a melanogenic complex. Mutations in the TYR gene (TYR, 11q14-21, MIM 606933) cause oculocutaneous albinism type 1 (OCA1, MIM 203100), a developmental disorder having an autosomal recessive mode of inheritance. In addition, TYR can act as a modifier locus for primary congenital glaucoma (PCG) and it also contributes significantly in the eye developmental process. Expression of TYR during neuroblast division helps in later pathfinding by retinal ganglion cells from retina to the dorsal lateral geniculate nucleus. However, mutation screening of TYR is complicated by the presence of a pseudogene-TYR like segment (TYRL, 11p11.2, MIM 191270), sharing approximately 98% sequence identity with the 3' region of TYR. Thus, in absence of a full-proof strategy, any nucleotide variants identified in the 3' region of TYR could actually be present in TYRL. Interestingly, despite extensive search, the second TYR mutation in 15% of the OCA1 cases remains unidentified. Several possible locations of these "uncharacterized mutations" (UCMs) have been speculated so far. Based on the structure of TYR gene, its sequence context and some experimental evidences, we propose two additional possibilities, which on further investigations might shed light on the molecular basis of UCMs in TYR of OCA1 patients; (i) partial deletion of the exons 4 and 5 region of TYR that is homologous with TYRL and (ii) variations in the polymorphic GA complex repeat located between distal and proximal elements of the human TYR promoter that can modulate the expression of the gene leading to disease pathogenesis.     

A Review on Joint Carotid Intima-Media Thickness and Plaque Area Measurement in Ultrasound for Cardiovascular/Stroke Risk Monitoring: Artificial Intelligence Framework

Journal of Digital Imaging - Cardiovascular diseases (CVDs) are the top ten leading causes of death worldwide. Atherosclerosis disease in the arteries is the main cause of the CVD, leading to...     

Devastating salt‐wasting crisis in a four‐month‐old male child with congenital adrenal hyperplasia,highlighting the essence of neonatal screening

Congenital adrenal hyperplasia (CAH) is a rare condition usually referred to as a group of genetic disorders resulting due to a deficiency of steroid enzymes required by adrenal glands to produce cortisol and mineralocorticoid hormones. It has an autosomal recessive mode of inheritance and is further categorized into two types—Classic and Non‐Classic. Non‐Classic CAH is a more common milder form that presents late after puberty. Classic CAH, although more severe, is rare and detected at birth and is associated with the life‐threatening adrenal crisis in both sexes and virilization of the external genitalia in females (46, XX) patients, whereas in males, no overt abnormality of the external genitalia is present. We present a case of a four‐month‐old male child with the classic form of CAH who was brought with complaints of loose stools, projectile non bilious vomiting, decreased urine output, and failure to feed for 3 days. The child had a clinical presentation of salt wasting with hypoglycemia and hyperpigmentation of his genitalia. The USG findings revealed increased anteroposterior diameter of renal pelvis indicative of a growth in the suprarenal area. 17‐hydroxyprogesterone (17‐OHP) was found to be elevated confirming the diagnosis. He was treated with hydrocortisone with gradual improvement in his glucose and electrolytes. The patient was discharged home on replacement therapy consisting of oral prednisolone and fludrocortisone acetate and followed up as outpatient with significant improvement in the clinical findings. The fact that the child was not screened for CAH at birth led to the critical consequences of the disease in this case. To prevent life‐threatening adrenal crisis and help perform appropriate sex assignments for affected female patients, newborn screening (NBS) programs for the classical form of CAH should be made mandatory even in low‐ and middle‐income countries.     

Mapping the TYR gene reveals novel and previously reported variants in Eastern Indian patients highlighting preponderance of the same changes in multiple unrelated ethnicities

Oculocutaneous albinism (OCA) is a group of congenital autosomal recessive disorders with seven known subtypes (OCA1–OCA7) characterized by loss or absence of pigmentation in the skin, hair, and eyes. OCA1, caused by pathogenic variations in the tyrosinase (TYR) gene, has been documented to be the most prevalent subtype across the world including India. In the present study, we recruited 53 OCA-affected individuals from 45 unrelated families belonging to 20 different marriage groups/ethnicities of 15 different districts of West Bengal. We took a targeted sequencing-based approach to find the causal variations in the TYR gene. We report here identification of two novel potentially pathogenic variations [NM_000372.4:c.614C>T, NP_000363.1:p.(Pro205Leu), and NM_000372.4:c.1036+1=/G>T], one novel synonymous TYR variant [NM_000372.4:c.204=/A>G, NP_000363.1:p.(Gln68=)], two pathogenic variations documented for the first time in Indian OCA cases [NM_000372.4:c.1147G>A, NP_000363.1:p.(Asp383Asn), and NM_000372.4:c.585G>A, NP_000363.1:p.(Trp195*)], along with nine previously reported pathogenic variants in 36 out of 53 (∼68%) patients recruited. We report common haplotype backgrounds for the two most prevalent variations [NM_000372.4:c.124G>A, NM_000372.4:c.832C>T] in cases belonging to different marriage/ethnic groups, suggesting a possible founder effect. To our knowledge, this is the most comprehensive genetic study on OCA1 from India, firmly establishing OCA1 as the commonest form of albinism in this part of the world.     

Temporal neurotransmitter conditioning restores the functional activity of adult spinal cord neurons in long-term culture

The ability to culture functional adult mammalian spinal cord neurons represents an important step in the understanding and treatment of a spectrum of neurological disorders including spinal cord injury. Previously, the limited functional recovery of these cells, as characterized by a diminished ability to initiate action potentials and to exhibit repetitive firing patterns, has arisen as a major impediment to their physiological relevance. In this report, we demonstrate that single temporal doses of the neurotransmitters serotonin, glutamate (N-acetyl-DL-glutamic acid) and acetylcholine-chloride lead to the full electrophysiological functional recovery of adult mammalian spinal cord neurons, when they are cultured under defined serum-free conditions. Approximately 60% of the neurons treated regained their electrophysiological signature, often firing single, double and, most importantly, multiple action potentials.     

Electrophysiological characterization of embryonic hippocampal neurons cultured in a 3D collagen hydrogel

Rat embryonic hippocampal neurons were cultured in (1) 3D collagen hydrogels as ‘entrapped’ evenly distributed cells, (2) at the interface of two collagen layers (sandwich model), and (3) on the surface of collagen coated coverslips (2D model). In the ‘entrapment’ model the neuronal processes grew out of the plane of the cell body and extended into the collagen matrix, in contrast to the sandwich model where the cells and their processes rarely left the plane in which they were seeded. Hippocampal neurons ‘entrapped’ in the 3D collagen gel grew the same number, but shorter, processes and exhibited improved survival compared to neurons cultured in the 2D model. There was no difference in the electrophysiological properties of the neurons cultured in the 3D compared to the 2D model except in the resting membrane potential and in the duration of the after-hyperpolarization. Spontaneous postsynaptic currents were recorded in 14- and 21-day-old 3D cultures evidencing functional synapse formation. Our results indicate that the physiological characteristics of 3D neuronal cultures are similar to traditional 2D cultures. However, functional 3D networks of hippocampal neurons will be necessary for multi-level circuit formation, which could be essential for understanding the basis of physiological learning and memory.     

Node of Ranvier formation on motoneurons in vitro

One of the most significant interactions between Schwann cells and neurons is myelin sheath formation. Myelination is a vertebrate adaptation that enables rapid conduction of action potentials without a commensurate increase in axon diameter. In vitro neuronal systems provide a unique modality to study both factors influencing myelination and diseases associated with myelination. Currently, no in vitro system for motoneuron myelination by Schwann cells has been demonstrated. This work details the myelination of motoneuron axons by Schwann cells, with complete Node of Ranvier formation, in a defined in vitro culture system. This defined system utilizes a novel serum-free medium in combination with the non-biological substrate, N-1[3 (trimethoxysilyl) propyl] diethylenetriamine (DETA). The myelinated segments and nodal proteins were visualized and quantified using confocal microscopy. This defined system provides a highly controlled, reproducible model for studying Schwann cell interactions with motoneurons as well as the myelination process and its effect on neuronal plasticity. Furthermore, an in vitro system that would allow studies of motoneuron myelination would be beneficial for understanding peripheral demyelinating neuropathies such as diabetes induced peripheral neuropathy and could lead to a better understanding of CNS demyelinating diseases like multiple sclerosis, as well as neuromuscular junction maturation and maintenance.     

Regeneration and characterization of adult mouse hippocampal neurons in a defined in vitro system

Although the majority of human illnesses occur during adulthood, most of the available in vitro disease models are based upon cells obtained from embryonic/fetal tissues because of the difficulties involved with culturing adult cells. Development of adult mouse neuronal cultures has a special significance because of the abundance of transgenic disease models that use this species. In this study a novel cell culture method has been developed that supports the long-term survival and physiological regeneration of adult mouse hippocampal cells in a serum-free defined environment. In this well-defined, controlled system, adult mouse hippocampal cells survived for up to 21 days in culture. The cultured cells exhibited typical hippocampal neuronal morphology and electrophysiological properties after recovery from the trauma of dissociation, and stained positive for the expected neuronal markers. This system has great potential as an investigative tool for in vitro studies of adult diseases, the aging brain or transgenic models of age-associated disorders.