The lens has the highest protein concentration of any tissue within the human body, accounting for 38 % of its total wet weight [14]. Protein concentration is not uniform; higher concentrations exist in the lens nucleus creating a high refractive index (RI). A gradient of protein concentration, and corresponding RI, corrects for the ellipsoid shape of the lens and allows accurate light focussing across the entire structure. The cytoplasm of lens cells contains high concentrations (~0.2–0.4 g ml⁻¹) of crystallin proteins. Lens crystallins are polydisperse and exist in a dense, short-range spatial order that does not alter with the increasing protein concentration range seen from the lens cortex, to the nucleus [14]. It is this specialized arrangement that allows crystallins to exist at high concentration whilst establishing a high level of transparency and protein stability, and to ensure the longevity of these proteins in the absence of cell turn-over, a vascular supply or innervation. There are two main families of crystallin proteins that exist in the human lens; the βγ-crystallin superfamily that function as structural proteins and the α-crystallins, which function both as structural proteins and as molecular chaperones. As chaperones, α-crystallins recognize and bind non-native and unfolded proteins to prevent accumulation as insoluble light scattering aggregates [15]. The beaded filament of the vertebrate lens is important for establishing the precise cellular architecture that is also critical for transparency. It has been suggested that the beaded filament may facilitate lens protein stability by providing a scaffold for, and thereby optimizing, protein chaperone function [16]. As a form of intermediate filament, the beaded filament may also support tissue integrity during the mechanical stresses of accommodation [17]. Within the human lens, the beaded filament assembles by the binding of BFSP1 (filensin) with BFSP2 (phakinin) [18].

The lens is a non-passive tissue. Since organelle degradation alters the physiology of lens fibre cells – to leave them incapable of generating energy and replacing damaged proteins – lens homeostasis is critical. Within the lens, a specialized internal microcirculatory system exists that delivers essential nutrients throughout the lens mass, whilst maintaining constant cellular volume and preserving tissue architecture. The vertebrate lens relies on the surrounding eye components to provide the nutrients it requires- the aqueous humor is fundamental in this, supplying the lens with metabolites such as glucose and amino acids at the anterior surface. It is also thought that the anterior lens epithelium is proactive in the take up of these molecules and ions for their subsequent diffusion in to the peripheral fibre cells [19]. The microcirculatory current travels through the lens via gap junctions comprised of connexin proteins that are present in the cell membranes of lens fibres. Gap junctions create an intracellular pathway, connecting cells at the surface of the lens mass with central cells. They are responsible for electrically coupling the anterior epithelial cells [20] and for the circulation of nutrients and ions throughout fibres of the lens mass, and the removal of metabolic waste products [21]. Water is circulated through membrane channels comprised of aquaporin 1 (AQP1) in lens epithelial cells [22], and major intrinsic protein (MIP) in fibre cells [23]. The breakdown of glucose by anaerobic metabolism is the main source of energy for both growth and homeostasis in the lens. Uptake of glucose from the aqueous humour appears to be mediated by the glucose transporter, GLUT1 in the lens epithelium, for circulation throughout the structure. Lens fibre cell membranes are incredibly cholesterol saturated such that bilayers form to create areas of pure cholesterol [24] that are thought to facilitate lens transparency by smoothing and maintaining the physical properties of cell membranes to prevent light scattering [25, 26]. The importance of cholesterol in the maintenance of lens clarity is apparent from genetic or therapeutic inhibition of cholesterol biosynthesis which leads to cataractogenesis [27].

Disruption of lens protein function can lead to cataract and other serious developmental abnormalities as highlighted in Table 3.2. A summary of the structural components of the lens and the features that establish and maintain its transparency can be found in Table 3.3.

Lens opacities develop when the short-range order of crystallin protein is disturbed or the highly regular cellular organisation is disrupted, leading to fluctuation of protein density throughout the dimensions of the lens and resulting in regional differences in protein concentration [28]. At a molecular level, this may occur as a result of protein misfolding, instability or insolubility, leading to aggregation and altered interactions of proteins. Alterations in the homeostatic mechanisms or the physiological environment of the lens may indirectly impact upon crystallin proteins and also lead to cataractogenesis.

Aberrant protein folding results in the development of cataract when the accumulation of damage occurs at a faster rate than the lens chaperone α-crystallin, can manage, resulting in the formation of protein aggregates [28]. α-crystallin can identify partially or fully unfolded protein and segregate it, thus preventing its accumulative aggregation [29]. This mechanism is sufficient in the maintenance of lens clarity over an individual’s lifetime, where protein damage only accrues gradually with age. However, mutation of any one of the highly expressed lens crystallins will result in rapidly accumulating protein damage that is likely to overwhelm the chaperoning system [30]. Likewise, mutation of α-crystallin may reduce or abolish chaperone function, also resulting in the accumulation of damaged crystallin proteins [31]. In young fibre cells of the lens cortex, physiological stress caused by the accumulation of damaged protein can prevent differentiation in to mature fibre cells [32]. Therefore, crystallin mutations can cause cataract as a result of aberrant protein folding and where the effect is particularly detrimental to the developing lens, may also cause anterior segment defects such as microphthalmia and microcornea secondary to microphakia (Fig. 3.2).

Cataract can also occur in the absence of protein misfolding. Within lens fibres, extremely soluble and stable crystallin proteins exist at high concentration. They are tightly and regularly packaged in a highly ordered manner to create an even protein distribution [28]. Given that crystallins also vary in size, they are said to exist in polydisperse, short-range order. To achieve this arrangement, crystallins display repulsive charges on their protein surface [33]. Crystallin mutations that alter protein surface charges lead to altered interactions and reduced solubility that cannot be resolved by the α-crystallin chaperone [34, 35]. Such alterations are disruptive to the normal short-range protein order and subsequently result in the co-existence of pockets of high and low protein concentration. Protein-rich areas have a higher refractive index than protein-poor areas of the lens, thus causing light scattering that appears as opacity within the lens [36].